Characterization of Two-Phase Flow and Boiling Heat Transfer in Mini- and Micro-Channels

Abstract

This thesis focuses on two main studies to understand fluid flow and heat transfer phenomena at the mini- and micro-scale. In the first study, experimental investigations are conducted to determine the effect of channel curvature on the two-phase frictional pressure drop and thermal entrance length for flow of refrigerant R134a in a helically coiled mini-tube of 2 mm inner diameter. The two-phase flow characteristics are investigated through flow boiling of the refrigerant under uniform wall temperature boundary conditions. A generalized correlation is proposed to predict the two-phase frictional pressure drop through the coiled tube for design of efficient compact helical tube heat exchangers. A large set of experimental data is collected to evaluate the prediction performance of the proposed two-phase correlation. The two-phase predictions agree fairly well with experimental results. A complementary numerical investigation is also carried out to compare with the heat transfer study from experiments. The experimental and numerical results are seen to be in good agreement. It is found that the thermal entrance length for the coiled tube is similar to that for a straight tube at low Reynolds numbers and curvatures. The increase in heat transfer coefficient is also found to be proportional to the increase in curvature of the tube. This result is relevant for optimum geometry considerations in design of efficient systems for applications such as space cooling and thermal desalination. In the second part of this work, adiabatic two-phase flow experiments were conducted on a PDMS microfluidic device with different fluid combinations to investigate the intrinsic interfacial instabilities that cause flow transition from one regime to another in microchannels. A theoretical model, which is an extended form of the Kelvin-Helmholtz and Rayleigh instabilities, is developed for the characterization of the two-phase flow dynamics in the micro-channel. The model is used to determine conditions for maintaining stable parallel air and water flows and predict conditions where interfacial instability between fluid phases takes place. The experimental results are used to validate these theoretical conditions. The model prediction capability is also tested on visualization data from another study, with decent agreement. The influence of parameters, such as fluid velocity, surface wettability and other properties on the two-phase flow characteristics are also investigated. The results of this study are useful for carbon capture and sequestration applications, where carbon dioxide (CO2) is injected into the sub-surface rocks to control CO2 emissions. This study is also relevant for predicting and controlling interfacial instability in various energy applications, emulsification processes, and for improved mixing in enhanced oil recovery processes.

Date of Award	May 2014
Original language	American English
Supervisor	TJ Zhang (Supervisor)