Flow boiling in microchannels of a pumped two-phase loop : experimental and numerical analysis focusing on two-phase flow instability mitigation
Authors:   , Kokate Rohan Uday 
Published: September 2024

High-power electronics exceed the heat dissipation capability of conventional air or liquid-based forced convection cooling systems. A pumped two-phase loop (P2PL) utilizing phase change heat transfer can effectively transport and dissipate high heat fluxes generated from the electronics at very small thermal resistance while consuming negligible pumping power. The objective of this dissertation is to systematically investigate flow boiling in microchannels of a pumped two-phase loop using both numerical and experimental methods. In Chapter 2, a comprehensive thermal-hydraulic flow network model of the P2PL was developed to study the system performance characteristics. While previous studies have focused on individual components (e.g., evaporator and condenser) within the two-phase cooling system, this study is dedicated to system-level analysis. The model considers pressure variations due to the compressibility of the vapor, providing accurate predictions of system performance under various operating conditions. The study also highlights the trade-off between the total system thermal resistance reduction and the increased pressure drop (pumping power) with an increase in mass flow rate. In Chapter 3, the numerical model established in Chapter 2 was used to investigate Ledinegg instability within the P2PL. The study highlights the interactions between the components within P2PL in response to abrupt perturbations in thermal and hydraulic operating conditions, simulating real-world scenarios. As a key outcome, the chapter presents a flow stability map which serves as a valuable tool for system designers and operators, allowing them to select operating points that ensure stable system behavior. In Chapter 4, an experimental investigation of subcooled flow boiling of R134a in microchannels of a P2PL is presented. Unlike prior research focusing on standalone evaporators with fixed boundary conditions, neglecting the interactions between the evaporator and other components within the P2PL, this study investigates flow boiling in a closed loop system. The dominant flow boiling mechanisms and its transitions in the microchannel were identified based on the varying slopes of the boiling curves. Notably, the heat dissipation capability increased with mass flow rate in the convective boiling regime but remained unaffected in the nucleate boiling regime. In Chapter 5, pressure drop oscillations (PDOs) in P2PL system with microchannel evaporator, a critical factor influencing thermal management performance, was experimentally investigated. Importantly, PDOs originating in the evaporator propagate throughout the entire P2PL, impacting the heat source with significant temperature oscillations. Finally, the study demonstrates successful mitigation of PDOs and associated temperature oscillations using an active control strategy. In Chapter 6, high-speed visualization with simultaneous temperature and pressure measurements was used to investigate the local flow boiling regimes and their impact on the thermal characteristics of R134a in parallel microchannels. The results reveal that flow separation at the microchannel inlet promoted rapid bubble formation due to both pressure-driven flashing and thermal-driven nucleation on channel walls. The local heat transfer coefficient served as an indicator, demonstrating the significant influence of flow regime transitions on the thermal characteristics within the microchannels. In Chapter 7, a U-Net based deep learning tool for automated detection and segmentation of vapor bubbles was developed to measure critical local flow boiling characteristics like nucleation site density for a range of operating conditions. The influence of heat and mass flux on nucleation site density of R134a during subcooled flow boiling in microchannels was investigated. The wall superheat was identified as the primary driver of nucleation at the channel inlet with limited influence of flow velocity.