Design of a non-intrusive electrical impedance-based void fraction sensor for microchannel two-phase flows

TLDR: In this paper, a nonintrusive electrical impedance-based sensor is developed for measurement of local void fraction in air-water adiabatic flow through rectangular microchannels.

Abstract: A non-intrusive electrical impedance-based sensor is developed for measurement of local void fraction in air–water adiabatic flow through rectangular microchannels. Measurement of the void fraction in microchannels is essential for the formulation of two-phase flow heat transfer and pressure drop correlations, and may enable real-time flow regime control and performance prediction in the thermal regulation of high-heat-flux devices. The impedance response of the sensor to a range of flow regimes is investigated for a configuration with two aligned electrodes flush-mounted on opposing microchannel walls. Numerical simulations performed on a multi-phase domain constructed from three-dimensional reconstruction of experimentally observed phase boundaries along with the corresponding experimental results serve to establish the relationship between void fraction and dimensionless impedance for this geometric configuration. A reduced-order analytical model developed based on an assumption of stratified gas–liquid flow allows ready extension of these calibration results to different working fluids of interest.

Figures

Table 1. Comparison of the overall impedance obtained analytically and by numerical 501 simulations for crosswise electrodes and a domain flow length equal to one electrode width. 502

Table 2. Analytical expressions for the liquid and gas phase impedances under stratified flow; 504 the expressions assume the liquid and gas behave respectively as a pure resistor and a pure 505 capacitor. 506

Figure 11. Analytical model prediction using the stratified flow hypothesis for a dielectric fluid, 544 FC-72 (𝜌0 = 1013 Ω m; 𝜀𝑟0 = 1.8 [44]). 545

Figure 9. Theoretical stratified flow impedance given by equation (10) as a function of void 537 fraction for frequencies of 1, 20 and 100 kHz and the properties given in Table 1. 538

Figure 7. (a) Electric field in the crosswise direction and (b) envelopes of the percentage of the 530 total current passing through planes parallel to the electrodes in the crosswise configuration. 531

Figure 1. Schematic diagram of (a) crosswise and (b) streamwise electrode pair configurations. 511 The electrodes and microchannel have square cross-sections of side length d and area A. 512

Frequently asked questions

Q1. What are the contributions in "Design of a non-intrusive electrical impedance-based void fraction sensor for microchannel two-phase flows" ?

In this paper, the authors focus on void fraction measurement using electrodes flush-mounted in the wall of a microchannel due to the simplicity of incorporation into the channel geometries. 

Q2. How many pipe diameters did the electrodes provide a localized measurement?

An electrode separation distance of 1.5 to 2.5 74 pipe diameters was found to provide a localized measurement at sufficiently close spacing while 75 still providing a reading independent of the flow regime. 

Q3. What is the void fraction in 149 the region of interest?

To determine the volumetric void fraction in 149 the region of interest, three-dimensional air volumes are generated from the two-dimensional 150 phase boundaries assuming that the air regions are axisymmetric about their major axis. 

Q4. How was the Canny 147 algorithm implemented in MATLAB?

The air-water interfaces in the binary images of the frames were detected using the Canny 147 algorithm implemented in MATLAB [38]. 

Q5. What is the void fraction in the homogeneous flow model?

In the 158 case of homogeneous flow, i.e., under the assumptions of uniform phase distribution in the flow 159 cross-section and equal phase velocities, the void fraction is given by 160βα = (homogeneous flow model). 

Q6. What is the simplest way to calculate the impedance of a gas phase?

The overall 280 impedance is governed by the resistive behavior of the liquid, and is largely independent of the 281 gas phase capacitance. 

Q7. How has the thermal and hydraulic behavior of fluid flow in microchannels been investigated?

The thermal and hydraulic behavior of fluid 29 flow in microchannels has therefore been extensively investigated [1–3] as a means to address 30 the increasing cooling demands of high-performance electronics. 

Q8. How can the measured impedance be compared to the numerical and theoretical models?

Once extraneous circuit impedance is accounted for, the experimental data can be directly 294 compared to the numerical and theoretical models. 

Q9. What is the simplest way to measure the flow impedance?

The overall impedance is measured for a microchannel filled with a liquid of known 285 resistance (up to 200 kΩ) to assess the circuit impedance. 

Q10. How does the liquid film thickness in a gas-liquid flow be measured?

The use of wire probes for the measurement 478 of liquid film thickness in annular gas-liquid flows Can. J. Chem. Eng. 56 754–7 479[34] 

Q11. What was the prediction of the experimental void fractions?

In [41], the 169 homogeneous flow model was also found to provide the best prediction of the experimental void 170 fractions in bubbly and slug flow patterns; it was noted that the homogeneous flow assumptions 171 did not apply for annular flow with high void fractions.