This paper presents a synthesized review on the recent literature concerning micro-scale flow boiling. The topics covered are macro- to micro-scale transition, flow patterns, pressure drop, heat transfer coefficient, critical heat flux, superficial void fraction and liquid entrainment. The analyses revealed some characteristics common to micro-scale two-phase flow, i.e. absence of stratified flow, predominance of annular flow over all saturated region, uniform liquid film thickness during horizontal flows, reduced liquid entrainment, high heat transfer coefficients and pressure drops. Despite the importance of liquid entrainment and void fraction in predictive methods, there is a lack of experimental results for these parameters in the micro-scale literature. Important accomplishments concerning the investigation of micro-scale flow boiling have been obtained over the last two decades, but some aspects, including local physical mechanisms related to heat transfer, onset of dryout and flow boiling instabilities still remain unclear.

Flow boiling in micro-scale channels – Synthesized literature review

Applications of ANNs in flow and heat transfer problems in nuclear engineering: A review work

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1325&context=coolingpubs

Characterization of Hierarchical Manifold Microchannel Heat Sink Arrays under Simultaneous Background and Hotspot Heating Conditions

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1311&context=coolingpubs

Predicting two-phase flow distribution and stability in systems with many parallel heated channels

Two-phase flow regime identification in a horizontal pipe was realized based on the liquid phase velocity information and the machine learning method. Ultrasound Doppler velocimetry was employed to measure the liquid velocity. Statistical features extracted from the velocity time series data, such as mean, root mean square, and power spectral density, were used to realize real-time flow regime identification. In addition, two novel parameters—maximum velocity ratio and maximum velocity difference ratio—were proposed to identify plug and decaying slug flow. Different classification algorithms were employed to achieve a high identification accuracy. Moreover, transient flow regime identification with a fast response was realized based on two classification algorithms—long–short term memory and convolutional neural network. The results show that the accuracy of real-time flow regime identification based on a flow regime map can reach up to 93.1% using support vector machine, the maximum velocity ratio and maximum velocity difference ratio are effective in identifying plug and decaying slug flow, and transient flow regime identification under slug flow condition can be realized with an accuracy of 94% based on a convolutional neural network (CNN). Decaying slugs with long lengths confuse the CNN and are responsible for the error in identification. The results presented herein are expected to expand the available knowledge on two-phase flow regime identification.

Two-phase flow regime identification based on the liquid-phase velocity information and machine learning

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1167&context=coolingpubs

Electrical impedance-based void fraction measurement and flow regime identification in microchannel flows under adiabatic conditions

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1206&context=coolingpubs

Local measurement of flow boiling heat transfer in an array of non-uniformly heated microchannels

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.

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

Decreasing form factors and diminishing numbers of thermal interfaces and spreading layers in modern, compact electronic packages result in non-uniform heat generation profiles at the chip level being transmitted directly to the heat sinks. An improved understanding of the effects of non-uniform heating on the heat dissipation limits in microchannel heat sinks has become essential. An experimental investigation is conducted to measure the location and magnitude of critical heat flux (CHF) in a microchannel heat sink exposed to a range of non-uniform heating profiles. A 12.7 mm X 12.7 mm silicon microchannel heat sink with an embedded 5 X 5 array of individually controllable heaters is used in the experiments. The microchannels in the heat sink are 240 mm wide and 370 μm deep, and are separated by 110 mm wide fins. The dielectric fluid HFE-7100 is used as the coolant, with an average mass flux in the heat sink of approximately 800 kg/m2s. High-speed visualizations of the flow are recorded to capture the CH...

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1253&context=coolingpubs

Effects of Non-Uniform Heating on the Location and Magnitude of Critical Heat Flux in a Microchannel Heat Sink

Electrical impedance of a two-phase mixture is a function of void fraction and phase distribution. The difference in the electrical conductance and permittivity of the two phases can be exploited to measure electrical impedance for obtaining void fraction and flow regime characteristics. An electrical impedance meter is constructed for the measurement of void fraction in microchannel two-phase flow. The experiments are conducted in air-water two-phase flow under adiabatic conditions. A transparent acrylic test section of hydraulic diameter 780 micrometer is used in the experimental investigation. The impedance void meter is calibrated against the void fraction measured using analysis of images obtained with a high-speed camera. Based on these measurements, a methodology utilizing the statistical characteristics of the void fraction signals is employed for identification of microchannel flow regimes.Copyright © 2011 by ASME

Susan N. Ritchey

Papers

Electrical impedance-based void fraction measurement and flow regime identification in microchannel flows under adiabatic conditions

Local measurement of flow boiling heat transfer in an array of non-uniformly heated microchannels

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

Effects of Non-Uniform Heating on the Location and Magnitude of Critical Heat Flux in a Microchannel Heat Sink

Impedance-Based Void Fraction Measurement and Flow Regime Identification in Microchannel Flows