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Proceedings of the ASME 2011 9th International Conference on Nanochannels, Microchannels, and Minichannels ICNMM2011 June 19-22, 2011 Edmonton, Alberta, CANADA
ICNMM2011-58168 EXPERIMENTAL STUDY ON THE EFFECT OF SYNTHETIC JET ON FLOW BOILING INSTABILITY IN A MICROCHANNEL Ruixian Fang, Wei Jiang, Jamil Khan Department of Mechanical Engineering University of South Carolina, Columbia, SC, USA
stabilizing effects are studied alone and in conjunction with upstream throttling. Flow instabilities are characterized by visual observations and the pressure fluctuations measured between the inlet and outlet manifolds. Two-phase flow boiling in microchannels is different with that of conventional size channels. For bubble ebullition in microchannels, the surface tension becomes predominant and significantly reduces the vapour-liquid velocity slip. Thus the two-phase flow hydrodynamic characteristics are affected [1]. Bubble growth is limited in the radial direction by the narrow channel wall, and grows rapidly in the longitudinal direction, both upstream and downstream. Flow reversal may occur if the net force due to the evaporation momentum overcomes the surface tension and the inertia forces. Experimental studies on the onset of nucleate boiling [2-4] and flow patterns [5-6] have confirmed these differences. Flow boiling instabilities [1, 7-11] in microchannels have received increasing interest in recent years. Many types of flow instability modes, which have been investigated extensively and well identified in macroscale channels, are also presented in small-diameter channels, as discussed by Bergles and Kandlikar [11]. The flow excursion or Ledinegg instability is the most common static instability mentioned in the microchannel flow boiling works [1, 7]. It is resulted from the unique pressure drop characteristic of a boiling channel (the demand curve). When the heated channel is part of a forced circulation loop, operation at the portion of the demand curve with negative slope can be unstable. A well defined minimum point on the demand curve closely relates to the onset of flow instability, and it is a crucial operational threshold [1, 12]. Dynamic instabilities such as density wave oscillations and pressure drop oscillations may also occur in microchannel flow boiling. They are characterized by typical frequencies, amplitudes and physical mechanisms. In
ABSTRACT Two-phase flow instability in microchannel flow boiling exhibits pressure fluctuations with different frequencies and amplitudes. One possible way to suppress the dynamic instability is to introduce synthetic jet near the inlet of the channel. An important feature of synthetic jet is that it allows momentum transfer into the microchannel flow without net mass injection into it. The strength and frequency of the jet can be controlled by changing the driving voltage and frequency of the piezoelectric driven jet actuator. The effects of the synthetic jet together with upstream throttling are evaluated as a means of stabilizing the flow boiling instability. The results are compared with the case without synthetic jet. The pressure dynamics of the microchannel flow caused by the synthetic jet are also analyzed. Keywords: synthetic jet, microchannel, flow boiling instability, flow boiling, two phase flow 1. INTRODUCTION Flow boiling heat transfer in microchannels has shown great potential for removal of high heat flux in the thermal management of microsystems, micro-processors, high power density electronics, etc. However in practical applications, one great concern is the thermally induced two-phase flow instabilities. Oscillations of flow rate and system pressure are undesirable as they may cause problems of system control and in extreme circumstances, premature of critical heat flux condition so that the device may be overheated. This paper addresses one possible method to suppress the instabilities during flow boiling in microchannels by introducing synthetic jet into the channel flow. The jet
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which, the pressure drop oscillations are closely related to the interaction between the channel and upstream compressible volume such as entrained air bubble or flexible hose [11]. Density wave oscillations are the most common dynamic instabilities encountered in two-phase flow systems. It results from the feedback between the flow rate, the vapour generation rate and the pressure drop in the heated channel [13]. Those two types of dynamic instabilities are identified in the microchannel flow boiling work of Qu and Mudawar [14]. To suppress the flow boiling instability, inlet flow restrictions are frequently employed in both conventional channels and microchannels. The upstream throttling of the test section can effectively isolate the boiling channels from compressible volume. While upstream throttling of each individual microchannel may cure the excursive instability. Wallis and Heasley [15] studied the effect of inlet restrictions for conventional channels. They found that an inlet restriction, which increased single-phase flow friction, had a stabilizing effect. For microchannels, Qu and Mudawar [14] eliminated the severe pressure drop oscillation by throttling the flow immediately upstream of the test module. But the parallel channel instability still presents. Kandlikar et al. [16] experimentally investigated two methods to stabilize the flow boiling instability and avoid the reverse flow phenomena. They placed pressure drop elements at the inlet of each microchannel to introduce a significant increase in the flow resistance in the reversed flow direction. Meanwhile, artificial nucleation cavities were also drilled to help lower the wall superheat during boiling. They found that fabricated nucleation sites in conjunction with the 4% area pressure drop elements completely eliminated the instabilities associated with the reverse flow, but with a very high pressure drop penalty. Similarly, Kosar et al. [17] placed 20 µm wide 400 µm long flow orifice in the inlet of 200 µm wide microchannels to eradicate flow oscillations. Throttling in each individual channel may result in very large pressure penalty. Other methods that can alter the pressure field of the channel flow or lower the wall superheat may also have the effects to reduce the instabilities. A synthetic jet has the ability to import net momentum into the channel flow and affect the bubble ebullition process. In addition, the jet strength can be adjusted and its frequency can also be controlled. Those features render it as a potential active method to suppress the instabilities. Synthetic jet is mainly used for active flow control in areas including jet vectoring [18], separation control of both external and internal flows [19-20]. The method uses a small actuator which synthesizes a jet from the flow that is
being controlled without the need for mass injection. It requires no input piping and complex fluidic packaging. A synthetic jet actuator, in general, consists of an enclosed cavity with one side of the cavity having an orifice while a flexible membrane located on the opposite side to the orifice. The jet is generated at the orifice by oscillating the membrane. Working fluid is sucked into the cavity and then rapidly expelled out. As the outgoing flow passes the sharp edges of the orifice, the flow separates forming a vortex ring, which propagates into the ambient fluid. An important feature of synthetic jets is that they are zero-net-mass-flux in nature (Glezer and Amitay, [21]), since they are synthesized from ambient fluid. As such, synthetic jets allow momentum transfer into the surrounding fluid without net mass injection into the overall system. This attribute makes synthetic jets ideally suited for fabrication using micromachining techniques that enable low cost fabrication, realization of large arrays, and the potential for integration of control electronics. For thermal management applications, synthetic jets impinging directly on electronics have been investigated for many years. Recently, several works studied the thermal effects of synthetic jet interacting in a pre-existing flow [2224]. In all those applications, the working fluids are air, or single-phase water. In this study, the synthetic jet will be used in two-phase flow boiling the first time. The objective of the present work is to study the effects of synthetic jet on the microchannel flow boiling instabilities. The effects of jet introduced pressure dynamics, jet combining with channel upstream throttling, and jet driving frequency are investigated in this study. 2. EXPERIMENTAL APARATUS The experimental setup, as shown in Figure 1, consists of a water supply loop, a microchannel heat sink test section integrated with a synthetic jet actuator, a data acquisition system, a signal generation system to drive the jet actuator, and a high-speed digital camera system. 2.1. Water flow loop The micro gear pump provides constant flow rate of deionized water to the microchannel test section. The flow rate of the pump ranges from 0.9 mL/min to 80 mL/min. A 2 µm inline filter is installed at the exit of the pump. After the filter, water is a degassed through a membrane vacuum degasser. The flow rate is measured by a rotameter type of flow meter. Following the rotameter, a precision flow control valve is used for throttling purpose. Water then flows through the microchannel test section. Water/steam drained out from the test section is condensed and ducted
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Figure 1
Schematic of flow loop and test section
into a container put on a high precision balance, which is employed to further calibrate the mean flow rate. 2.2. Test Section The test section assembly is illustrated in Figure 2. It consists of a cover plate with a synthetic jet actuator on it, a housing, a microchannel heat sink, a cartridge heater, insulation blocks, a support plate and a piezoelectric disk bender actuator. The microchannel heat sink is fabricated on a single copper block. The copper is an Oxygen-Free Electronic alloy number C10100. It has a thermal conductivity of 391 W/m K at room temperature. The top surface of the copper block measures 5 mm wide and 26 mm long. A single microchannel is machined into the copper block top surface. The channel is in the middle of the top surface and has a cross-sectional dimension of 550 µm wide and 500 µm deep. 3 mm below the top surface of the heat sink, five holes with diameter of 0.85 mm are drilled into the side wall up to the half width of the copper block. Five type-K thermocouples with a 0.8 mm bead diameter are inserted into these holes to measure the heat sink’s stream-wise temperature distribution. The thermocouples are denoted in Figure 1 as T1 to T5 from upstream to downstream. The locations, as measured from the inlet of the microchannel and along its length, are 2 mm, 6.5 mm, 13 mm, 18.5 mm, and 24 mm. Below the thermocouple holes, a small protruding platform is machined around the periphery of the Figure 2
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Test section exploded view
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heat sink to both facilitate accurate positioning the heat sink in the housing and to ensure adequate area for sealing. Below the platform, a 6.35 mm diameter through hole is drilled along the length of the cooper block to accommodate the cartridge heater. The resistive cartridge heater is powered by an Agilent DC power supply and provides a constant heat flux to the copper block. Power supplied to the cartridge heater is calculated based on the voltage and current readings from the DC power supply. The housing part is made from high temperature polycarbonate plastic. The central part of the housing is removed where the heat sink can be inserted. The protruding portion of the heat sink ensured that the top surface of the heat sink is flush with the top surface of the housing. RTV silicon rubber is applied along the interface between the housing and the heat sink to prevent leakage. Two absolute pressure transducers are connected to the deep portion of inlet and outlet plenums via pressure ports to measure the inlet and outlet pressure, respectively. Two type-K thermocouples are located at the inlet and outlet plenums to measure the channel inlet/outlet water temperatures.
Figure 4 Enlarged view of synthetic jet assembly In this study, a commercially available Unimorph piezoelectric disk bender is employed as the vibrating element of the actuator. The Unimorph disk is made of two other disks bonded together, one is a piezoelectric ceramic disc and the other is a metal substrate. The metal substrate makes it much less fragile than the ceramic alone. In this case, the metal substrate is a thin brass disc, which has a diameter of 12 mm and a thickness of 100 µm, the piezoelectric ceramic disk has a diameter of 9 mm and a thickness of 100 µm. Silver electrode is coated on the piezoelectric ceramic surface. The piezoelectric disk bows up or down as a voltage is applied between the metal substrate and the silver electrode. The resonance frequency of the piezoelectric disk bender itself is 9 kHz. During assembly, the piezoelectric disk bender is placed on top of the cylindrical cavity coaxially to seal it. The periphery of the disk bender is tightly fixed by a hollow screw nut. After assembly, the synthetic jet actuator resonance frequency is lowered because of the damping effect of water enclosed in the cavity.
Figure 3 Test section assembly
2.3. Data acquisition and signal generation systems
The cover plate made from transparent polycarbonate plastic is bolted atop the housing. The cover plate and the micro-slot in the heat sink form a closed microchannel as shown in Figure 3. An O-ring in the housing maintains a leak-proof assembly. The synthetic jet actuator is located right above the microchannel and 5 mm downstream from the channel inlet, as shown in Figure 3 and Figure 4. It is formed by a cylindrical cavity, a 400 µm diameter orifice on the cavity bottom surface, and a vibration membrane on the cavity top surface. The cylindrical jet cavity has a diameter of 9.6 mm and 1.5 mm in depth. It is machined into the cover plate. The orifice is drilled through the bottom surface of the cavity, which is 0.5 mm in thickness.
A NI CompactDAQ-9172 data acquisition system is employed to record signals from the pressure transducers and thermocouples. The system communicates with a computer via a USB interface. A program written in LabVIEW software is used for all data acquisition. The sample rate for the thermocouples is 10 S/s and 1 KHz for the two pressure transducers. The signal generation system is designed to supply sinusoidal signal to drive the piezoelectric disk actuator. It includes a direct digital synthesis function generator, a wide band power amplifier. The system is capable of generating sine wave with the frequency from 1 Hz to 7 MHz, and with the amplitude up to 200 volts (peak).
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2.4. Measurement uncertainty
Case (b) With synthetic jet only Case (c) With upstream throttling only Case (d) With upstream throttling and synthetic jet Before reporting the jet stabilizing effects, the controllable ability of synthetic jet is introduced first.
The accuracy of pressure transducers, thermocouples, and Agilent E3612A DC power supply are 0.25%, ±0.2oC, ±0.01V, and 0.5% respectively. The rotameter has an advised flow accuracy of 2.0%. However, the measured accuracy of the flow rate is actually less than 1%. The sine wave flatness of the function generator is 1.0% and the output peak voltage accuracy of the amplifier is given by ±0.5V.
4.1. Pressure dynamics caused by synthetic jet When there is fluid flowing through the microchannel test section, the pressure pulsations generated by the synthetic jet are imposed onto the pressure field of the main stream in the microchannel, whereby controllable disturbance can be generated. The pressure dynamics of the synthetic jet on the microchannel flow field have been studied in our previous single-phase work using the same setup [25]. Several characteristics of the synthetic jet are briefly summarized here. The scale and frequency of the periodic fluctuation can be controlled by changing the driving voltage and frequency of the piezoelectric driven synthetic jet actuator. Change in the voltage will change the deformation of the piezoelectric disk bender directly. In general, there is a linear relationship between the voltage and the deformation for piezoelectric materials. Another way to effectively control the synthetic jet is to change its driving frequency. Increasing the frequency will increase the strength of the synthetic jet as more electrical energy is transformed into mechanical energy by the piezo actuator per unit of time, and as the result, the momentum transferred into the microchannel flow is increased. Figure 5 shows the channel inlet pressure variations for a test case with synthetic jet. The average channel stream velocity is at 0.32 m/s, the jet is operated at 80 Hz with a driving voltage of 60 Vp.
3. EXPERIMENTAL PROCEDURE Once the test loop is built and well insulated, heat loss experiments without synthetic jet are performed at steady state for each specified flow rate. Keep the flow rate constant, the single-phase heat loss can be easily determined from the measurements of water temperature differences between the inlet and outlet. The heat loss is found to increase approximately linearly with the microchannel surface temperature. Such a linear relationship is extrapolated to those temperatures with boiling in microchannel. A calibration chart is constructed by plotting the heat loss versus the temperature of the microchannel surface. During the actual saturation boiling experiments, this chart is used to calculate the actual heat carried away by the microchannel. First, tests are performed without synthetic jet. For a constant flow rate and heating power input, steady state is achieved when the surface temperature of the microchannel remains constant over a 15 min time interval. At steady state, temperature and pressure readings are recorded for 3 minutes. Then the heating power is stepped up, and a new state is investigated. This procedure is repeated until flow reversal is detected with the aid of the inlet pressure and temperature signals. For tests with synthetic jet, the jet actuator is driving at desired frequency and amplitude by setting the function generator and the amplifier. Repeat the above procedure until flow reversal is detected for the same flow rate.
Absolute pressure (KPa)
126.5
4. RESULTS AND DISCUSSION The evaluation of synthetic jet for stabilizing flow boiling in microchannel is presented in this section. Flow instability is determined through high-speed visual observations, measurements of pressure drop fluctuations and the inlet water temperature fluctuations. Based on our observation, the unsteady state is defined when the flow reversal is observed, and a characteristic oscillation frequency can be identified, and the fluctuations amplitude of inlet temperature > 1 oC. Typically, the inlet temperature fluctuation amplitude is around ±0.2 oC at stable state because of the measurement uncertainties. The results are reported for the following four cases: Case (a) Base case, no synthetic jet, no upstream throttling
126.0
without jet
with jet
125.5 125.0 124.5 124.0 123.5 123.0 122.5 122.0 1517.552
1517.577
1517.602
1517.627
1517.652
Time (sec) Figure 5 Inlet pressure variations with/without synthetic jet. Channel flow Re = 188, jet V = 60 Vp, f = 80 Hz The pressure dynamics for 0.1 seconds are plotted in the figure. Totally 8 cycles are detected in this time frame. It
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corresponds to the 80 Hz driving frequency. For comparison, the inlet pressure for the same test case without jet is also measured and plotted as dash line in Figure 5. When the jet is in operation, the inlet pressure fluctuates around the dash line with the peak to peak amplitude of 3.5 kPa and a characteristic oscillation frequency. In the present study, the jet is operated from 80Hz to 150Hz, and the operating voltage is kept constant at 60V peak (Vp) for all the test cases with synthetic jet.
data points are marked as squares as shown in Figure 6. The jet actuator is operated at 60 Vp and 80 Hz. As shown in the figure, for single-phase heat transfer with synthetic jet, the data points shift to the left by around 4 oC, indicating a higher heat transfer coefficient. This is because the disturbances generated by the synthetic jet disrupt the laminar boundary layer in the channel flow, as the result, microchannel heat transfer performance is enhanced [25]. For two-phase flow boiling, the synthetic jet delays the onset of flow instability to a slightly higher input heat power as it can be seen from Figure 6. And the wall superheat is reduced by around 1 oC with synthetic jet. It is also observed that the amplitude of the pressure fluctuations is reduced with synthetic jet. However, it seems that the synthetic jet has not much improvement on stabilizing the flow instability. From the point view of the nucleating bubble force balance, for the case without jet, forces are balancing between the surface tension, inertia force, and the net force due to the evaporation momentum. When the jet is applied, the channel pressure field is changed in a periodic manner, and net momentum is introduced into the channel flow. The jet introduced momentum may help to push the nucleating bubble to downstream during the expelling stroke, and attract the bubble to upstream during the suction stroke. In either way, it causes the bubble detaching at a smaller size before it blocks the channel. As a result, flow reversal is prevented. Nevertheless, if the input heat power is increased to a certain value, at which the evaporation momentum force overcomes the opposing forces, including the jet introduced forces, the flow reversal will still happen. The severe sustainable fluctuations of the pressure drop throughout the test channel are predominant over the jet introduced pressure changing, so the jet effects are minimized in this case. The upstream compressible volume may responsible for this early onset of flow instability since the loop is built with soft rubber tubes. Upstream throttling will partially cure this type of instability as discussed earlier in the introduction section.
4.2. Case (a) Base case Flow boiling test for the base case is performed with a constant mass flow rate of 82 kg/m2s. The channel wall temperature versus input heat power is plotted in Figure 6. The data points are marked as filled triangles as shown in the figure. The term OUB in the figure means the onset of unstable boiling. 11
Heat Power (W)
without jet
OUB
with jet
10.5 OUB
10 9.5 9 8.5 8 80
85
90
95
100
105
Temperature (oC) Figure 6 Average surface temperature of the channel. G= 82 kg/m2s. For the case with jet, V = 60Vp and f = 80Hz As expected, the heat sink temperature increases linearly with the input heat power in the single-phase region. It represents constant heat transfer coefficient. The change of slope starts at around 102 oC, which indicates the start of two-phase flow boiling. Further increasing the heater power, flow instability is observed at a wall temperature around 103 o C. For this specific flow rate, the onset of unstable boiling is near the point of onset of flow boiling. The measured outlet water temperature is around 92 oC, which means that the flow boiling is actually in the subcooled boiling region. At the onset of unstable boiling, severe sustainable pressure drop fluctuations and flow reversal were observed. The loop flow rate also exhibits sustainable fluctuations, which can be observed by the fluctuations of the float in the rotameter.
4.4. Case (c) with upstream throttling only In this test case, the upstream flow control valve is partially closed to isolate the test section from upstream compressible volume. As a result, the flow rate is kept almost constant at the channel inlet during the flow boiling experiment. Test results for the channel mass flow rate at 82 kg/m2s are plotted in Figure 7. The data points are marked as filled triangle as shown in the figure.
4.3. Case (b) with synthetic jet only For comparison, the test results with synthetic jet are plotted on the same figure together with the base case. The
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Heat Power (W)
25
without jet with jet 120Hz with jet 150Hz
20
OUB
OUB 15 OUB 10 5 60
70
80
90
100
110
120
Temperature (oC) Figure 7 Average surface temperature of the channel for G= 82 kg/m2s. For the cases with jet, V = 60Vp
(a)
Comparing with the base case as shown in Figure 6, with a lower level upstream throttling the onset of flow instability is slightly delayed to a higher input heat power. The stabilizing effect of upstream throttling depends on the degree of the throttling. For the single channel setup, further throttling the upstream control valve will completely eliminate the flow reversal, but the pressure penalty for the pump is very high as indicated in the work of Kandlikar et al. [16]. Techniques such as artificial drilled nucleate sites [16], or synthetic jet in the present study, may help to prevent the instability while keeping a lower level of throttling. The temperature and pressure signals for input heat power at 11.5 W, which is slightly above the heat power for the onset of flow instability, are plotted in Figure 8. As shown in Figure 8(a), the channel inlet water temperature fluctuates from 34 oC to 42 oC. This is one of the evidences of the reversing flow boiling. In the same figure, the temperature signals for three thermocouples, namely T1, T3, and T5, are also plotted. It shows that the heat sink temperature doesn’t experience visible fluctuations. The pressure signals for the channel inlet and outlet are plotted in Figure 8(b). The amplitude of the inlet pressure fluctuation is around 6 kPa. And the oscillation frequency is around 1.2Hz for this specific operating condition. The outlet pressure signal seems in phase with the inlet pressure signal, but the fluctuating amplitude is negligible comparing to its noise amplitude.
(b) Figure 8 The unsteady temperature (a) and pressure (b) signals for G= 82 kg/m2s and heat power Q = 11.5 W rhombus with green boarder, as shown in the figure. It is observed that the jet effects are improved with upstream throttling. As shown in Figure 7, the onset of unstable boiling is substantially delayed with synthetic jet. The 120 Hz case improves the heat power from 11.31 W (throttling but no jet) to 16.59 W with stable operations. This corresponds to around 47% improvement in terms of heat power. For the 120 Hz case, further increase the input heat power to above 16.59 W will result in flow instability again. Whereas, increasing the jet operating frequency to 150 Hz will stabilize this instability and further improve the heat power to 22.12 W with stable operations. For this case, the improvement is around 96% comparing with the case without jet. To further analysis the stabilizing effects of the synthetic jet, the temperature and pressure signals for input heat power at 11.5 W and jet driving frequency at 120 Hz are plotted in Figure 9. Comparing with the temperature and pressure signals in Figure 8, it can be seen that the flow is stabilized with synthetic jet. As shown in Figure 9(a), the inlet temperature is in steady state. There is no flow reversal detected in this case.
4.5. Case (d) with upstream throttling and synthetic jet Keeping the upstream throttling level unchanged as in test case (c), and applying the synthetic jet, the results with synthetic jet are also plotted in Figure 7 for comparison. The jet is operating at a driving voltage of 60 Vp, and a frequency of 120Hz and 150Hz respectively. The data points for f = 120 Hz are marked as square with black boarder , and the data points for f = 150 Hz are marked as
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Hz. Thus the characteristic oscillation frequency of the inlet pressure signal is found to be the jet driving frequency. The combination of lower level upstream throttling and synthetic jet may not totally eradicate the flow instability. For the case with jet operating at 150 Hz, flow instability still shows up if the input heat power is further increased to above 22.12 W. Further increase the jet frequency up to 180 Hz cannot stabilize the flow again. It is observed that the two-phase portion has moved upstream to the point where the jet orifice located. Steam bubbles begin to appear inside of the jet cavity. The piezoelectric driven synthetic jet actuator does not tolerate air bubbles well. The performance of the jet actuator will be degraded greatly. To avoid this, any methods that can keep the jet operating in the singlephase portion of the channel flow boiling will help. Such possible methods include moving the jet position further upstream, providing larger inlet subcooling, or further throttling the upstream valve.
(a)
5. CONCLUSION The present work experimentally investigated the effects of synthetic jet on the instability observed during flow boiling in a microchannel. The pressure dynamic of the microchannel flow caused by the synthetic jet were analyzed. Test cases with and without synthetic jet were studied experimentally and compared with each other. All cases were investigated with the same mass flow condition. For the case with synthetic jet only, the onset of unstable flow boiling is slightly delayed compared with the case without jet. However it seems that the synthetic jet alone has not much improvement on stabilizing the flow instability. Combination of the synthetic jet with a lower level upstream throttling can substantially stabilize the flow instability, but not totally eliminate it. It is also found that changing the operating frequency of the jet can change its ability of flow stabilization. Further research will extend to multi-channel heat sink to evaluate the synthetic jet effects on stability of the flow boiling phenomena.
(b)
Presssure (kPa)
112 108
P INLET
104 100 96 P OUTLET
92 0
0.05
0.1 Time (s)
0.15
0.2 6. ACKNOWLEDGMENTS
(c) Figure 9 The temperature (a) and pressure (b) (c) signals for G= 82 kg/m2s, heat power Q = 11.5 W and jet f = 120 Hz
The authors acknowledge financial support from the US Office of Naval Research under grant number N00014-08-10080.
Synthetic jet also changes the pressure signals. Figure 9(b) shows the pressure signals at the channel inlet and outlet. It shows that the severe fluctuations of the pressure signals appeared in Figure 8(b) are eliminated. If the pressure signals of Figure 9(b) are plotted in a 0.2 s time frame, as shown in Figure 9(c), 24 cycles are counted in this time interval. It corresponds to a frequency of 120
7. NOMENCLATURE frequency, Hz mass velocity, kg/m2s onset of unstable boiling input heat power, W Reynolds number voltage, volt
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peak voltage, volt
[13] L. Tadrist, Review on two-phase flow instabilities in narrow spaces, Int. J. Heat Fluid Flow 28 (2007), pp. 54–62. [14] Qu, W., and Mudawar, I., Measurement and Prediction of Pressure Drop in Two-Phase Microchannel Heat Sinks, Int. J. Heat Mass Transfer2003,, 46(15), pp. 2737–2753. [15] G.B. Wallis, J.H. Heasley, Oscillations in two-phase flow systems, J. Heat Transfer, Trans. ASME, Ser. C 83 (1961) 363–369. [16] S. Kandlikar, W.K. Kuan, D.A. Willistein and J. Borelli, Stabilization of flow boiling in microchannels using pressure drop elements and fabricated nucleation sites, ASME J. Heat Transfer 128 (2006), pp. 389–396.
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ICNMM2011-58168 EXPERIMENTAL STUDY ON THE EFFECT OF SYNTHETIC JET ON FLOW BOILING INSTABILITY IN A MICROCHANNEL Ruixian Fang, Wei Jiang, Jamil Khan Department of Mechanical Engineering University of South Carolina, Columbia, SC, USA
stabilizing effects are studied alone and in conjunction with upstream throttling. Flow instabilities are characterized by visual observations and the pressure fluctuations measured between the inlet and outlet manifolds. Two-phase flow boiling in microchannels is different with that of conventional size channels. For bubble ebullition in microchannels, the surface tension becomes predominant and significantly reduces the vapour-liquid velocity slip. Thus the two-phase flow hydrodynamic characteristics are affected [1]. Bubble growth is limited in the radial direction by the narrow channel wall, and grows rapidly in the longitudinal direction, both upstream and downstream. Flow reversal may occur if the net force due to the evaporation momentum overcomes the surface tension and the inertia forces. Experimental studies on the onset of nucleate boiling [2-4] and flow patterns [5-6] have confirmed these differences. Flow boiling instabilities [1, 7-11] in microchannels have received increasing interest in recent years. Many types of flow instability modes, which have been investigated extensively and well identified in macroscale channels, are also presented in small-diameter channels, as discussed by Bergles and Kandlikar [11]. The flow excursion or Ledinegg instability is the most common static instability mentioned in the microchannel flow boiling works [1, 7]. It is resulted from the unique pressure drop characteristic of a boiling channel (the demand curve). When the heated channel is part of a forced circulation loop, operation at the portion of the demand curve with negative slope can be unstable. A well defined minimum point on the demand curve closely relates to the onset of flow instability, and it is a crucial operational threshold [1, 12]. Dynamic instabilities such as density wave oscillations and pressure drop oscillations may also occur in microchannel flow boiling. They are characterized by typical frequencies, amplitudes and physical mechanisms. In
ABSTRACT Two-phase flow instability in microchannel flow boiling exhibits pressure fluctuations with different frequencies and amplitudes. One possible way to suppress the dynamic instability is to introduce synthetic jet near the inlet of the channel. An important feature of synthetic jet is that it allows momentum transfer into the microchannel flow without net mass injection into it. The strength and frequency of the jet can be controlled by changing the driving voltage and frequency of the piezoelectric driven jet actuator. The effects of the synthetic jet together with upstream throttling are evaluated as a means of stabilizing the flow boiling instability. The results are compared with the case without synthetic jet. The pressure dynamics of the microchannel flow caused by the synthetic jet are also analyzed. Keywords: synthetic jet, microchannel, flow boiling instability, flow boiling, two phase flow 1. INTRODUCTION Flow boiling heat transfer in microchannels has shown great potential for removal of high heat flux in the thermal management of microsystems, micro-processors, high power density electronics, etc. However in practical applications, one great concern is the thermally induced two-phase flow instabilities. Oscillations of flow rate and system pressure are undesirable as they may cause problems of system control and in extreme circumstances, premature of critical heat flux condition so that the device may be overheated. This paper addresses one possible method to suppress the instabilities during flow boiling in microchannels by introducing synthetic jet into the channel flow. The jet
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which, the pressure drop oscillations are closely related to the interaction between the channel and upstream compressible volume such as entrained air bubble or flexible hose [11]. Density wave oscillations are the most common dynamic instabilities encountered in two-phase flow systems. It results from the feedback between the flow rate, the vapour generation rate and the pressure drop in the heated channel [13]. Those two types of dynamic instabilities are identified in the microchannel flow boiling work of Qu and Mudawar [14]. To suppress the flow boiling instability, inlet flow restrictions are frequently employed in both conventional channels and microchannels. The upstream throttling of the test section can effectively isolate the boiling channels from compressible volume. While upstream throttling of each individual microchannel may cure the excursive instability. Wallis and Heasley [15] studied the effect of inlet restrictions for conventional channels. They found that an inlet restriction, which increased single-phase flow friction, had a stabilizing effect. For microchannels, Qu and Mudawar [14] eliminated the severe pressure drop oscillation by throttling the flow immediately upstream of the test module. But the parallel channel instability still presents. Kandlikar et al. [16] experimentally investigated two methods to stabilize the flow boiling instability and avoid the reverse flow phenomena. They placed pressure drop elements at the inlet of each microchannel to introduce a significant increase in the flow resistance in the reversed flow direction. Meanwhile, artificial nucleation cavities were also drilled to help lower the wall superheat during boiling. They found that fabricated nucleation sites in conjunction with the 4% area pressure drop elements completely eliminated the instabilities associated with the reverse flow, but with a very high pressure drop penalty. Similarly, Kosar et al. [17] placed 20 µm wide 400 µm long flow orifice in the inlet of 200 µm wide microchannels to eradicate flow oscillations. Throttling in each individual channel may result in very large pressure penalty. Other methods that can alter the pressure field of the channel flow or lower the wall superheat may also have the effects to reduce the instabilities. A synthetic jet has the ability to import net momentum into the channel flow and affect the bubble ebullition process. In addition, the jet strength can be adjusted and its frequency can also be controlled. Those features render it as a potential active method to suppress the instabilities. Synthetic jet is mainly used for active flow control in areas including jet vectoring [18], separation control of both external and internal flows [19-20]. The method uses a small actuator which synthesizes a jet from the flow that is
being controlled without the need for mass injection. It requires no input piping and complex fluidic packaging. A synthetic jet actuator, in general, consists of an enclosed cavity with one side of the cavity having an orifice while a flexible membrane located on the opposite side to the orifice. The jet is generated at the orifice by oscillating the membrane. Working fluid is sucked into the cavity and then rapidly expelled out. As the outgoing flow passes the sharp edges of the orifice, the flow separates forming a vortex ring, which propagates into the ambient fluid. An important feature of synthetic jets is that they are zero-net-mass-flux in nature (Glezer and Amitay, [21]), since they are synthesized from ambient fluid. As such, synthetic jets allow momentum transfer into the surrounding fluid without net mass injection into the overall system. This attribute makes synthetic jets ideally suited for fabrication using micromachining techniques that enable low cost fabrication, realization of large arrays, and the potential for integration of control electronics. For thermal management applications, synthetic jets impinging directly on electronics have been investigated for many years. Recently, several works studied the thermal effects of synthetic jet interacting in a pre-existing flow [2224]. In all those applications, the working fluids are air, or single-phase water. In this study, the synthetic jet will be used in two-phase flow boiling the first time. The objective of the present work is to study the effects of synthetic jet on the microchannel flow boiling instabilities. The effects of jet introduced pressure dynamics, jet combining with channel upstream throttling, and jet driving frequency are investigated in this study. 2. EXPERIMENTAL APARATUS The experimental setup, as shown in Figure 1, consists of a water supply loop, a microchannel heat sink test section integrated with a synthetic jet actuator, a data acquisition system, a signal generation system to drive the jet actuator, and a high-speed digital camera system. 2.1. Water flow loop The micro gear pump provides constant flow rate of deionized water to the microchannel test section. The flow rate of the pump ranges from 0.9 mL/min to 80 mL/min. A 2 µm inline filter is installed at the exit of the pump. After the filter, water is a degassed through a membrane vacuum degasser. The flow rate is measured by a rotameter type of flow meter. Following the rotameter, a precision flow control valve is used for throttling purpose. Water then flows through the microchannel test section. Water/steam drained out from the test section is condensed and ducted
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Figure 1
Schematic of flow loop and test section
into a container put on a high precision balance, which is employed to further calibrate the mean flow rate. 2.2. Test Section The test section assembly is illustrated in Figure 2. It consists of a cover plate with a synthetic jet actuator on it, a housing, a microchannel heat sink, a cartridge heater, insulation blocks, a support plate and a piezoelectric disk bender actuator. The microchannel heat sink is fabricated on a single copper block. The copper is an Oxygen-Free Electronic alloy number C10100. It has a thermal conductivity of 391 W/m K at room temperature. The top surface of the copper block measures 5 mm wide and 26 mm long. A single microchannel is machined into the copper block top surface. The channel is in the middle of the top surface and has a cross-sectional dimension of 550 µm wide and 500 µm deep. 3 mm below the top surface of the heat sink, five holes with diameter of 0.85 mm are drilled into the side wall up to the half width of the copper block. Five type-K thermocouples with a 0.8 mm bead diameter are inserted into these holes to measure the heat sink’s stream-wise temperature distribution. The thermocouples are denoted in Figure 1 as T1 to T5 from upstream to downstream. The locations, as measured from the inlet of the microchannel and along its length, are 2 mm, 6.5 mm, 13 mm, 18.5 mm, and 24 mm. Below the thermocouple holes, a small protruding platform is machined around the periphery of the Figure 2
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Test section exploded view
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heat sink to both facilitate accurate positioning the heat sink in the housing and to ensure adequate area for sealing. Below the platform, a 6.35 mm diameter through hole is drilled along the length of the cooper block to accommodate the cartridge heater. The resistive cartridge heater is powered by an Agilent DC power supply and provides a constant heat flux to the copper block. Power supplied to the cartridge heater is calculated based on the voltage and current readings from the DC power supply. The housing part is made from high temperature polycarbonate plastic. The central part of the housing is removed where the heat sink can be inserted. The protruding portion of the heat sink ensured that the top surface of the heat sink is flush with the top surface of the housing. RTV silicon rubber is applied along the interface between the housing and the heat sink to prevent leakage. Two absolute pressure transducers are connected to the deep portion of inlet and outlet plenums via pressure ports to measure the inlet and outlet pressure, respectively. Two type-K thermocouples are located at the inlet and outlet plenums to measure the channel inlet/outlet water temperatures.
Figure 4 Enlarged view of synthetic jet assembly In this study, a commercially available Unimorph piezoelectric disk bender is employed as the vibrating element of the actuator. The Unimorph disk is made of two other disks bonded together, one is a piezoelectric ceramic disc and the other is a metal substrate. The metal substrate makes it much less fragile than the ceramic alone. In this case, the metal substrate is a thin brass disc, which has a diameter of 12 mm and a thickness of 100 µm, the piezoelectric ceramic disk has a diameter of 9 mm and a thickness of 100 µm. Silver electrode is coated on the piezoelectric ceramic surface. The piezoelectric disk bows up or down as a voltage is applied between the metal substrate and the silver electrode. The resonance frequency of the piezoelectric disk bender itself is 9 kHz. During assembly, the piezoelectric disk bender is placed on top of the cylindrical cavity coaxially to seal it. The periphery of the disk bender is tightly fixed by a hollow screw nut. After assembly, the synthetic jet actuator resonance frequency is lowered because of the damping effect of water enclosed in the cavity.
Figure 3 Test section assembly
2.3. Data acquisition and signal generation systems
The cover plate made from transparent polycarbonate plastic is bolted atop the housing. The cover plate and the micro-slot in the heat sink form a closed microchannel as shown in Figure 3. An O-ring in the housing maintains a leak-proof assembly. The synthetic jet actuator is located right above the microchannel and 5 mm downstream from the channel inlet, as shown in Figure 3 and Figure 4. It is formed by a cylindrical cavity, a 400 µm diameter orifice on the cavity bottom surface, and a vibration membrane on the cavity top surface. The cylindrical jet cavity has a diameter of 9.6 mm and 1.5 mm in depth. It is machined into the cover plate. The orifice is drilled through the bottom surface of the cavity, which is 0.5 mm in thickness.
A NI CompactDAQ-9172 data acquisition system is employed to record signals from the pressure transducers and thermocouples. The system communicates with a computer via a USB interface. A program written in LabVIEW software is used for all data acquisition. The sample rate for the thermocouples is 10 S/s and 1 KHz for the two pressure transducers. The signal generation system is designed to supply sinusoidal signal to drive the piezoelectric disk actuator. It includes a direct digital synthesis function generator, a wide band power amplifier. The system is capable of generating sine wave with the frequency from 1 Hz to 7 MHz, and with the amplitude up to 200 volts (peak).
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2.4. Measurement uncertainty
Case (b) With synthetic jet only Case (c) With upstream throttling only Case (d) With upstream throttling and synthetic jet Before reporting the jet stabilizing effects, the controllable ability of synthetic jet is introduced first.
The accuracy of pressure transducers, thermocouples, and Agilent E3612A DC power supply are 0.25%, ±0.2oC, ±0.01V, and 0.5% respectively. The rotameter has an advised flow accuracy of 2.0%. However, the measured accuracy of the flow rate is actually less than 1%. The sine wave flatness of the function generator is 1.0% and the output peak voltage accuracy of the amplifier is given by ±0.5V.
4.1. Pressure dynamics caused by synthetic jet When there is fluid flowing through the microchannel test section, the pressure pulsations generated by the synthetic jet are imposed onto the pressure field of the main stream in the microchannel, whereby controllable disturbance can be generated. The pressure dynamics of the synthetic jet on the microchannel flow field have been studied in our previous single-phase work using the same setup [25]. Several characteristics of the synthetic jet are briefly summarized here. The scale and frequency of the periodic fluctuation can be controlled by changing the driving voltage and frequency of the piezoelectric driven synthetic jet actuator. Change in the voltage will change the deformation of the piezoelectric disk bender directly. In general, there is a linear relationship between the voltage and the deformation for piezoelectric materials. Another way to effectively control the synthetic jet is to change its driving frequency. Increasing the frequency will increase the strength of the synthetic jet as more electrical energy is transformed into mechanical energy by the piezo actuator per unit of time, and as the result, the momentum transferred into the microchannel flow is increased. Figure 5 shows the channel inlet pressure variations for a test case with synthetic jet. The average channel stream velocity is at 0.32 m/s, the jet is operated at 80 Hz with a driving voltage of 60 Vp.
3. EXPERIMENTAL PROCEDURE Once the test loop is built and well insulated, heat loss experiments without synthetic jet are performed at steady state for each specified flow rate. Keep the flow rate constant, the single-phase heat loss can be easily determined from the measurements of water temperature differences between the inlet and outlet. The heat loss is found to increase approximately linearly with the microchannel surface temperature. Such a linear relationship is extrapolated to those temperatures with boiling in microchannel. A calibration chart is constructed by plotting the heat loss versus the temperature of the microchannel surface. During the actual saturation boiling experiments, this chart is used to calculate the actual heat carried away by the microchannel. First, tests are performed without synthetic jet. For a constant flow rate and heating power input, steady state is achieved when the surface temperature of the microchannel remains constant over a 15 min time interval. At steady state, temperature and pressure readings are recorded for 3 minutes. Then the heating power is stepped up, and a new state is investigated. This procedure is repeated until flow reversal is detected with the aid of the inlet pressure and temperature signals. For tests with synthetic jet, the jet actuator is driving at desired frequency and amplitude by setting the function generator and the amplifier. Repeat the above procedure until flow reversal is detected for the same flow rate.
Absolute pressure (KPa)
126.5
4. RESULTS AND DISCUSSION The evaluation of synthetic jet for stabilizing flow boiling in microchannel is presented in this section. Flow instability is determined through high-speed visual observations, measurements of pressure drop fluctuations and the inlet water temperature fluctuations. Based on our observation, the unsteady state is defined when the flow reversal is observed, and a characteristic oscillation frequency can be identified, and the fluctuations amplitude of inlet temperature > 1 oC. Typically, the inlet temperature fluctuation amplitude is around ±0.2 oC at stable state because of the measurement uncertainties. The results are reported for the following four cases: Case (a) Base case, no synthetic jet, no upstream throttling
126.0
without jet
with jet
125.5 125.0 124.5 124.0 123.5 123.0 122.5 122.0 1517.552
1517.577
1517.602
1517.627
1517.652
Time (sec) Figure 5 Inlet pressure variations with/without synthetic jet. Channel flow Re = 188, jet V = 60 Vp, f = 80 Hz The pressure dynamics for 0.1 seconds are plotted in the figure. Totally 8 cycles are detected in this time frame. It
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corresponds to the 80 Hz driving frequency. For comparison, the inlet pressure for the same test case without jet is also measured and plotted as dash line in Figure 5. When the jet is in operation, the inlet pressure fluctuates around the dash line with the peak to peak amplitude of 3.5 kPa and a characteristic oscillation frequency. In the present study, the jet is operated from 80Hz to 150Hz, and the operating voltage is kept constant at 60V peak (Vp) for all the test cases with synthetic jet.
data points are marked as squares as shown in Figure 6. The jet actuator is operated at 60 Vp and 80 Hz. As shown in the figure, for single-phase heat transfer with synthetic jet, the data points shift to the left by around 4 oC, indicating a higher heat transfer coefficient. This is because the disturbances generated by the synthetic jet disrupt the laminar boundary layer in the channel flow, as the result, microchannel heat transfer performance is enhanced [25]. For two-phase flow boiling, the synthetic jet delays the onset of flow instability to a slightly higher input heat power as it can be seen from Figure 6. And the wall superheat is reduced by around 1 oC with synthetic jet. It is also observed that the amplitude of the pressure fluctuations is reduced with synthetic jet. However, it seems that the synthetic jet has not much improvement on stabilizing the flow instability. From the point view of the nucleating bubble force balance, for the case without jet, forces are balancing between the surface tension, inertia force, and the net force due to the evaporation momentum. When the jet is applied, the channel pressure field is changed in a periodic manner, and net momentum is introduced into the channel flow. The jet introduced momentum may help to push the nucleating bubble to downstream during the expelling stroke, and attract the bubble to upstream during the suction stroke. In either way, it causes the bubble detaching at a smaller size before it blocks the channel. As a result, flow reversal is prevented. Nevertheless, if the input heat power is increased to a certain value, at which the evaporation momentum force overcomes the opposing forces, including the jet introduced forces, the flow reversal will still happen. The severe sustainable fluctuations of the pressure drop throughout the test channel are predominant over the jet introduced pressure changing, so the jet effects are minimized in this case. The upstream compressible volume may responsible for this early onset of flow instability since the loop is built with soft rubber tubes. Upstream throttling will partially cure this type of instability as discussed earlier in the introduction section.
4.2. Case (a) Base case Flow boiling test for the base case is performed with a constant mass flow rate of 82 kg/m2s. The channel wall temperature versus input heat power is plotted in Figure 6. The data points are marked as filled triangles as shown in the figure. The term OUB in the figure means the onset of unstable boiling. 11
Heat Power (W)
without jet
OUB
with jet
10.5 OUB
10 9.5 9 8.5 8 80
85
90
95
100
105
Temperature (oC) Figure 6 Average surface temperature of the channel. G= 82 kg/m2s. For the case with jet, V = 60Vp and f = 80Hz As expected, the heat sink temperature increases linearly with the input heat power in the single-phase region. It represents constant heat transfer coefficient. The change of slope starts at around 102 oC, which indicates the start of two-phase flow boiling. Further increasing the heater power, flow instability is observed at a wall temperature around 103 o C. For this specific flow rate, the onset of unstable boiling is near the point of onset of flow boiling. The measured outlet water temperature is around 92 oC, which means that the flow boiling is actually in the subcooled boiling region. At the onset of unstable boiling, severe sustainable pressure drop fluctuations and flow reversal were observed. The loop flow rate also exhibits sustainable fluctuations, which can be observed by the fluctuations of the float in the rotameter.
4.4. Case (c) with upstream throttling only In this test case, the upstream flow control valve is partially closed to isolate the test section from upstream compressible volume. As a result, the flow rate is kept almost constant at the channel inlet during the flow boiling experiment. Test results for the channel mass flow rate at 82 kg/m2s are plotted in Figure 7. The data points are marked as filled triangle as shown in the figure.
4.3. Case (b) with synthetic jet only For comparison, the test results with synthetic jet are plotted on the same figure together with the base case. The
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Heat Power (W)
25
without jet with jet 120Hz with jet 150Hz
20
OUB
OUB 15 OUB 10 5 60
70
80
90
100
110
120
Temperature (oC) Figure 7 Average surface temperature of the channel for G= 82 kg/m2s. For the cases with jet, V = 60Vp
(a)
Comparing with the base case as shown in Figure 6, with a lower level upstream throttling the onset of flow instability is slightly delayed to a higher input heat power. The stabilizing effect of upstream throttling depends on the degree of the throttling. For the single channel setup, further throttling the upstream control valve will completely eliminate the flow reversal, but the pressure penalty for the pump is very high as indicated in the work of Kandlikar et al. [16]. Techniques such as artificial drilled nucleate sites [16], or synthetic jet in the present study, may help to prevent the instability while keeping a lower level of throttling. The temperature and pressure signals for input heat power at 11.5 W, which is slightly above the heat power for the onset of flow instability, are plotted in Figure 8. As shown in Figure 8(a), the channel inlet water temperature fluctuates from 34 oC to 42 oC. This is one of the evidences of the reversing flow boiling. In the same figure, the temperature signals for three thermocouples, namely T1, T3, and T5, are also plotted. It shows that the heat sink temperature doesn’t experience visible fluctuations. The pressure signals for the channel inlet and outlet are plotted in Figure 8(b). The amplitude of the inlet pressure fluctuation is around 6 kPa. And the oscillation frequency is around 1.2Hz for this specific operating condition. The outlet pressure signal seems in phase with the inlet pressure signal, but the fluctuating amplitude is negligible comparing to its noise amplitude.
(b) Figure 8 The unsteady temperature (a) and pressure (b) signals for G= 82 kg/m2s and heat power Q = 11.5 W rhombus with green boarder, as shown in the figure. It is observed that the jet effects are improved with upstream throttling. As shown in Figure 7, the onset of unstable boiling is substantially delayed with synthetic jet. The 120 Hz case improves the heat power from 11.31 W (throttling but no jet) to 16.59 W with stable operations. This corresponds to around 47% improvement in terms of heat power. For the 120 Hz case, further increase the input heat power to above 16.59 W will result in flow instability again. Whereas, increasing the jet operating frequency to 150 Hz will stabilize this instability and further improve the heat power to 22.12 W with stable operations. For this case, the improvement is around 96% comparing with the case without jet. To further analysis the stabilizing effects of the synthetic jet, the temperature and pressure signals for input heat power at 11.5 W and jet driving frequency at 120 Hz are plotted in Figure 9. Comparing with the temperature and pressure signals in Figure 8, it can be seen that the flow is stabilized with synthetic jet. As shown in Figure 9(a), the inlet temperature is in steady state. There is no flow reversal detected in this case.
4.5. Case (d) with upstream throttling and synthetic jet Keeping the upstream throttling level unchanged as in test case (c), and applying the synthetic jet, the results with synthetic jet are also plotted in Figure 7 for comparison. The jet is operating at a driving voltage of 60 Vp, and a frequency of 120Hz and 150Hz respectively. The data points for f = 120 Hz are marked as square with black boarder , and the data points for f = 150 Hz are marked as
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Hz. Thus the characteristic oscillation frequency of the inlet pressure signal is found to be the jet driving frequency. The combination of lower level upstream throttling and synthetic jet may not totally eradicate the flow instability. For the case with jet operating at 150 Hz, flow instability still shows up if the input heat power is further increased to above 22.12 W. Further increase the jet frequency up to 180 Hz cannot stabilize the flow again. It is observed that the two-phase portion has moved upstream to the point where the jet orifice located. Steam bubbles begin to appear inside of the jet cavity. The piezoelectric driven synthetic jet actuator does not tolerate air bubbles well. The performance of the jet actuator will be degraded greatly. To avoid this, any methods that can keep the jet operating in the singlephase portion of the channel flow boiling will help. Such possible methods include moving the jet position further upstream, providing larger inlet subcooling, or further throttling the upstream valve.
(a)
5. CONCLUSION The present work experimentally investigated the effects of synthetic jet on the instability observed during flow boiling in a microchannel. The pressure dynamic of the microchannel flow caused by the synthetic jet were analyzed. Test cases with and without synthetic jet were studied experimentally and compared with each other. All cases were investigated with the same mass flow condition. For the case with synthetic jet only, the onset of unstable flow boiling is slightly delayed compared with the case without jet. However it seems that the synthetic jet alone has not much improvement on stabilizing the flow instability. Combination of the synthetic jet with a lower level upstream throttling can substantially stabilize the flow instability, but not totally eliminate it. It is also found that changing the operating frequency of the jet can change its ability of flow stabilization. Further research will extend to multi-channel heat sink to evaluate the synthetic jet effects on stability of the flow boiling phenomena.
(b)
Presssure (kPa)
112 108
P INLET
104 100 96 P OUTLET
92 0
0.05
0.1 Time (s)
0.15
0.2 6. ACKNOWLEDGMENTS
(c) Figure 9 The temperature (a) and pressure (b) (c) signals for G= 82 kg/m2s, heat power Q = 11.5 W and jet f = 120 Hz
The authors acknowledge financial support from the US Office of Naval Research under grant number N00014-08-10080.
Synthetic jet also changes the pressure signals. Figure 9(b) shows the pressure signals at the channel inlet and outlet. It shows that the severe fluctuations of the pressure signals appeared in Figure 8(b) are eliminated. If the pressure signals of Figure 9(b) are plotted in a 0.2 s time frame, as shown in Figure 9(c), 24 cycles are counted in this time interval. It corresponds to a frequency of 120
7. NOMENCLATURE frequency, Hz mass velocity, kg/m2s onset of unstable boiling input heat power, W Reynolds number voltage, volt
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peak voltage, volt
[13] L. Tadrist, Review on two-phase flow instabilities in narrow spaces, Int. J. Heat Fluid Flow 28 (2007), pp. 54–62. [14] Qu, W., and Mudawar, I., Measurement and Prediction of Pressure Drop in Two-Phase Microchannel Heat Sinks, Int. J. Heat Mass Transfer2003,, 46(15), pp. 2737–2753. [15] G.B. Wallis, J.H. Heasley, Oscillations in two-phase flow systems, J. Heat Transfer, Trans. ASME, Ser. C 83 (1961) 363–369. [16] S. Kandlikar, W.K. Kuan, D.A. Willistein and J. Borelli, Stabilization of flow boiling in microchannels using pressure drop elements and fabricated nucleation sites, ASME J. Heat Transfer 128 (2006), pp. 389–396.
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