Fluorescent Imaging of Void Fraction in Two-phase ... - Semantic Scholar
3rd International Symposium on Two-Phase Flow Modeling and Experimentation Pisa, 22-24 September 2004
FLUORESCENT IMAGING OF VOID FRACTION IN TWO-PHASE MICROCHANNELS David Fogg, Roger Flynn, Carlos Hidrovo, Lian Zhang, Kenneth Goodson Department of Mechanical Engineering, Stanford University, Stanford, CA 94305
ABSTRACT Two-phase flows in micro-geometries have received increasing attention in recent years for cooling of electronic devices. However, due to the constraints imposed by the geometry, metrology for liquid-vapor flows in MEMS devices is limited primarily to wall temperature distributions and inlet and exit pressures. New experimental techniques need to be developed to quantify parameters such as local liquid temperature, pressure, and void or liquid fraction. In this work, a fluorescent technique is developed to measure local liquid fraction in two-phase micro-flows. Utilizing a photodiode and single fluorescent dye, high speed transient measurements of isothermal flows are obtained in devices with appropriate optical access. Analysis indicates the technique can be extended to boiling and condensation when two photodiodes and dyes are used. For the current study, the theory for both isothermal and boiling flows is developed and the technique is applied to isothermal flows involving air and DI water using fluorescein as the fluorescent dye.
flows have begun to appear. Jacobi and Thome modeled the heat transfer from elongated bubbles in microchannel flows [10], Qu and Mudawar analyzed the heat transfer in annular two phase microchannel flows [11], and Yarin et. al studied two-phase capillary flow with phase change at the meniscus [12]. Koo et. al developed a one-dimensional steady-state numerical model for boiling microchannel flows [13], which Fogg et. al are extending to describe transients [14]. However, due to the lack of data and metrology methods, numerical and analytical models are difficult to verify. To construct accurate models of microchannel flows, the metrology needs to be developed for local parameters such as void fraction, pressure, and liquid temperature. Void is particularly important as it is indicative of the flow regimes and heat and mass transfer. Traditional void fraction measurement techniques are difficult to implement in microchannels. Due to the small dimensions of the microchannels, invasive techniques such as hot wire anemometry are not applicable because the probes will significantly influence the flow. Non-invasive techniques such as gamma densitometry or X-ray tomography are impractical due to cost, space, and/or lab safety constraints. Capacitive sensors, which take advantage of the significant differences between the dielectric constants of the two phases, are promising but are difficult to calibrate due to dependences on both overall void fraction and flow regime [15]. Void fractions have been reported in triangular micro heat pipes using capacitive sensors [16], but the behavior of capacitive sensors for highly transient flow has yet to be characterized. Fluorescence has been used for optical measurements of temperature in micro-geometries [17,18]. The intensity of the emitted light is dependent on the temperature of the fluorescent dye allowing measurements accurate to within 1oC. Fluorescence has also been used to measure void fraction. Angelini demonstrated such a technique for macroscale dispersed droplet flows [19]. This technique has also been used in fuel vaporization studies [20]. In the present work, Angelini’s technique is applied to isothermal
INTRODUCTION The study of two-phase flows in micro-sized geometries has received increasing attention over the past few years. Micro-sized heat pipes [1], jets [2], and heat exchangers [3] are being studied as cooling solutions for a wide variety of electronic devices including integrated circuits and radiation sources. Due to the size of the microchannels, metrology in past experimental work has been restricted to wall temperatures, inlet and exit pressures, and digital images. To support the development of analytical models, measurement techniques for local void fraction, pressure, and liquid temperature need to be developed. Many of the past studies have been experimental in nature. Peng et al [4,5] studied flow transition and heat transfer for water and methanol in V-shaped microchannels with hydraulic diameters from 200 to 600 µm and found no bubbles formed even though the wall superheat suggested nucleate boiling should occur. Stanley [6] introduced inert gases into water flowing through rectangular microchannels ranging from 56 to 256 µm in hydraulic diameter. The pressure drop data was consistent with a homogeneous flow model. Bowers and Mudawar [7] examined the two-phase pressure drop of R-113 in microchannels and also found their data consistent with a homogeneous flow model. With nitrogen and water in triangular silicon/glass microchannels with hydraulic diameters of 40 to 80 µm, Jiang [8] observed an immediate transition from single phase to annular flow without observing bubbly flow. Balasubramanian and Kandlikar [9] used high speed photography to observe slug and annular flow patterns in microchannels. However, despite the wealth of experimental work, the studies have been limited to wall temperatures using thermocouples or integrated resistive thermometers, inlet and exit pressures using commercial pressure sensors, and flow visualization using high speed digital imaging. Analytical and numerical models for microscale two-phase
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Figure 2: The quantum efficiency of fluorescein as a function of temperature. The intensity is normalized by the value at 20oC.
Figure 1: The absorption and emission spectra of fluorescein [21].
microflows and the theory to extend the method to liquid fraction in boiling flows is developed.
channel, β is the volume averaged liquid fraction, and c is the molar concentration of the dye. If āλiDβ
FLUORESCENT IMAGING OF VOID FRACTION IN TWO-PHASE MICROCHANNELS David Fogg, Roger Flynn, Carlos Hidrovo, Lian Zhang, Kenneth Goodson Department of Mechanical Engineering, Stanford University, Stanford, CA 94305
ABSTRACT Two-phase flows in micro-geometries have received increasing attention in recent years for cooling of electronic devices. However, due to the constraints imposed by the geometry, metrology for liquid-vapor flows in MEMS devices is limited primarily to wall temperature distributions and inlet and exit pressures. New experimental techniques need to be developed to quantify parameters such as local liquid temperature, pressure, and void or liquid fraction. In this work, a fluorescent technique is developed to measure local liquid fraction in two-phase micro-flows. Utilizing a photodiode and single fluorescent dye, high speed transient measurements of isothermal flows are obtained in devices with appropriate optical access. Analysis indicates the technique can be extended to boiling and condensation when two photodiodes and dyes are used. For the current study, the theory for both isothermal and boiling flows is developed and the technique is applied to isothermal flows involving air and DI water using fluorescein as the fluorescent dye.
flows have begun to appear. Jacobi and Thome modeled the heat transfer from elongated bubbles in microchannel flows [10], Qu and Mudawar analyzed the heat transfer in annular two phase microchannel flows [11], and Yarin et. al studied two-phase capillary flow with phase change at the meniscus [12]. Koo et. al developed a one-dimensional steady-state numerical model for boiling microchannel flows [13], which Fogg et. al are extending to describe transients [14]. However, due to the lack of data and metrology methods, numerical and analytical models are difficult to verify. To construct accurate models of microchannel flows, the metrology needs to be developed for local parameters such as void fraction, pressure, and liquid temperature. Void is particularly important as it is indicative of the flow regimes and heat and mass transfer. Traditional void fraction measurement techniques are difficult to implement in microchannels. Due to the small dimensions of the microchannels, invasive techniques such as hot wire anemometry are not applicable because the probes will significantly influence the flow. Non-invasive techniques such as gamma densitometry or X-ray tomography are impractical due to cost, space, and/or lab safety constraints. Capacitive sensors, which take advantage of the significant differences between the dielectric constants of the two phases, are promising but are difficult to calibrate due to dependences on both overall void fraction and flow regime [15]. Void fractions have been reported in triangular micro heat pipes using capacitive sensors [16], but the behavior of capacitive sensors for highly transient flow has yet to be characterized. Fluorescence has been used for optical measurements of temperature in micro-geometries [17,18]. The intensity of the emitted light is dependent on the temperature of the fluorescent dye allowing measurements accurate to within 1oC. Fluorescence has also been used to measure void fraction. Angelini demonstrated such a technique for macroscale dispersed droplet flows [19]. This technique has also been used in fuel vaporization studies [20]. In the present work, Angelini’s technique is applied to isothermal
INTRODUCTION The study of two-phase flows in micro-sized geometries has received increasing attention over the past few years. Micro-sized heat pipes [1], jets [2], and heat exchangers [3] are being studied as cooling solutions for a wide variety of electronic devices including integrated circuits and radiation sources. Due to the size of the microchannels, metrology in past experimental work has been restricted to wall temperatures, inlet and exit pressures, and digital images. To support the development of analytical models, measurement techniques for local void fraction, pressure, and liquid temperature need to be developed. Many of the past studies have been experimental in nature. Peng et al [4,5] studied flow transition and heat transfer for water and methanol in V-shaped microchannels with hydraulic diameters from 200 to 600 µm and found no bubbles formed even though the wall superheat suggested nucleate boiling should occur. Stanley [6] introduced inert gases into water flowing through rectangular microchannels ranging from 56 to 256 µm in hydraulic diameter. The pressure drop data was consistent with a homogeneous flow model. Bowers and Mudawar [7] examined the two-phase pressure drop of R-113 in microchannels and also found their data consistent with a homogeneous flow model. With nitrogen and water in triangular silicon/glass microchannels with hydraulic diameters of 40 to 80 µm, Jiang [8] observed an immediate transition from single phase to annular flow without observing bubbly flow. Balasubramanian and Kandlikar [9] used high speed photography to observe slug and annular flow patterns in microchannels. However, despite the wealth of experimental work, the studies have been limited to wall temperatures using thermocouples or integrated resistive thermometers, inlet and exit pressures using commercial pressure sensors, and flow visualization using high speed digital imaging. Analytical and numerical models for microscale two-phase
1
1.05
120
Absorption Emission
1.00 Normalized Intensity
N o rm a liz e d In te n s ity .
100 80 60 40 20
0.95
0.90
0.85
0 300
350
400
450 500 Wavelength (nm)
550
0.80
600
20
30
40
50
60
70
80
90
100
Temperature (C)
Figure 2: The quantum efficiency of fluorescein as a function of temperature. The intensity is normalized by the value at 20oC.
Figure 1: The absorption and emission spectra of fluorescein [21].
microflows and the theory to extend the method to liquid fraction in boiling flows is developed.
channel, β is the volume averaged liquid fraction, and c is the molar concentration of the dye. If āλiDβ
