Numerical and Experimental Investigations into Thermo-fluid Characteristics of Convective Boiling in Counter Flow Mini/Microchannel Heat Sinks

Abstract

Boiling Heat Transfer (BHT), known for its high heat transfer capability, is widely used in various fields like power plants, chemical processing, nuclear power plants, electronics cooling, etc. It employs latent heat of vaporization of the working fluid and enhanced single-phase convection, resulting in a much higher rate of heat transfer with a lower wall temperature. The combination of convective and nucleate boiling phenomena exhibits complex physical mechanisms of heat transfer and fluid flow. It is important to thoroughly understand the intricate thermo-fluid mechanisms, which are the key to designing new strategies to enhance the flow boiling heat transfer performance. There is abundant literature on experimental and numerical approaches to explore flow boiling. These studies have facilitated the exploration of innovative solutions, including geometrical and surface modifications, which enhance the performance of flow boiling heat transfer.

The present study investigates the thermo-hydrodynamics characteristics of convective boiling in counter flow mini/microchannels. For numerical simulations, we have employed the Lattice Boltzmann Method (LBM) to investigate the bubble dynamics and heat transfer performance of flow boiling in a vertical straight channel and then in a vertical diverging channel to reveal its effects on heat transfer and flow stability. Following a similar numerical approach, we have studied the flow boiling phenomenon inside the Counter Flow Straight Microchannels (CFSM) and compared with that in the Co-Current Straight Microchannel (CCSM), providing a detailed analysis of bubble dynamics and the heat transfer performance. Lastly, the convective boiling in a Counter Flow Diverging Minichannel (CFDMn) heat sink with a miniaturized size of 3 cm² is investigated experimentally to thoroughly explore the flow boiling phenomenon and mechanism inside this CFDMn. Moreover, we have introduced two robust microstructures incorporated onto the bottom surface of the minichannels and investigated their effects on the overall flow boiling performance of the respective heat sinks.

The LBM study aims to numerically investigate the flow boiling phenomenon inside a vertical diverging channel adopting the multi-relaxation time (MRT) based Pseudopotential Lattice Boltzmann method (LBM). The effects of operational and geometrical parameters on the flow boiling characteristics in the diverging channel are explored in detail and compared with that in the straight channel with the same inlet width. Bubbles coalescence and its effects on heat transfer under variable pitch distances between two discrete microheaters in the flow direction are examined thoroughly. The evolution of the flow pattern from bubbly to slug flow under variable wall superheats are performed by using multiple discrete microheaters to determine the critical wall superheat for the initiation of slug bubble in both channels. The results indicate a lower critical superheat for the initiation of slug bubble formation in the straight channel than that in the diverging channel. The simulation results suggest that the diverging channel exhibits a higher bubble growth rate with better heat transfer capability than the straight channel does and, for the range of diverging angles studied, the effects are more pronounced with an increase in the diverging angle. The bubble departure frequency and heat transfer are enhanced by increasing the wall superheat and flow rate. The current study also revealed that there is an optimum pitch distance for the maximum average heat flux under a given condition. The findings of the current study help to understand the evolution of thermo-hydrodynamics characteristics of flow boiling under variable geometrical and operational modifications which helps to develop a better design strategy for cooling applications.

The LBM approach is subsequently extended to simulate the flow boiling heat transfer inside a CFSM pair and compared with that in a CCSM one using the MRT-based Pseudopotential model combined with a thermal phase change LB model. The simulation results reveal that the CFSM pair acquires a lower void fraction and demonstrates better heat transfer with a higher critical heat flux (CHF) than that of the CCSM one. The simulation results on CHF for a CCSM pair agree well with the predictions form a well-established correlation in the literature. The simulations quantitatively provide the channel-to-channel heat transfer distribution in the axial direction. The results reveal that better overall heat transfer performance can be achieved with a higher CHF in the CFSM pair by increasing the Reynolds number and inlet subcooling through enhanced channel-to-channel heat transfer.

Lastly, we investigate the convective boiling in a CFDMn heat sink in a compact size of 3 cm², i.e., 1.5 cm × 2 cm, with or without integrated microstructures. The two microstructures have been developed and integrated into the bottom surfaces of the CFDMn heat sinks: one with gradient micro pin-fins (PF-CFDMn) and another combining gradient microcavities with micro pin-fins (Cav+PF-CFDMn). Comprehensive flow boiling experiments are conducted under various operating conditions to identify the best configuration with the most appropriate operating condition to dissipate the highest heat flux with minimal pressure drop and stable two-phase flow patterns. The current study reveals that the synergistic effects of micro pin-fins and optimized microcavities in the Cav+PF-CFDMn configuration achieve a very high effective heat flux (q_b″) of 746.14 W/cm² and a two-phase heat transfer coefficient (h_tp) of 28.18 W/cm²K with wall temperature below 132 °C without reaching the CHF. Significantly, it maintains an exceptionally low pressure drop of 2.35 kPa with stable two-phase flow under the optimal operating condition. Compared to the CFDMn with the bare surface (BS-CFDMn), Cav+PF-CFDMn demonstrates excellent enhancements of 201% in q_b″ and 124% in h_tp.

Date of Award	26 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chin PAN (Supervisor) & Steven WANG (Co-supervisor)