Semi-analytical Modeling Approach for Flow Boiling Heat Transfer in Uniformly Heated Horizontal Micro-Channels

Two-phase flow boiling in mini/micro-channels is an essential topic for studying and developing miniature cooling systems which have a wide range of industrial, automotive, nuclear, and aerospace applications, since they insure providing higher heat dissipation and ability to work at surface temperature higher than those provided by conventional channel sizes. Therefore, it has gained much more attention recently and several extensive studies have been carried out for predicting the essential design parameters such as heat transfer coefficient, pressure drop, void fraction, and critical heat flux. However, most of the available predictive methods are mainly empirical correlations and few analytical models. Moreover, several essential and fundamental flow boiling phenomena, such as heat transfer mechanisms, liquid film dry-out, and bubble dynamics, are still unclear and need to be investigated and clarified. The main objective of the present thesis is to investigate the characteristics of two basic flow patterns, namely, annular flow and slug flow for flow boiling in a single horizontal mini/micro-channel under uniform distribution of heat flux. A one-dimensional semi-analytical model has been developed based on the principle of the separated flow model. It predicts heat transfer coefficient, pressure drop, void fraction, and critical heat flux in the annular flow regime, which is usually observed at medium and high vapor qualities. The basic model equations are derived by applying the mass, momentum, and energy equations for each phase. The obtained equations are initial value differential equations, which are solved numerically by Runge-Kutta method. Furthermore, the influences of interfacial parameters on the main dependent variables have been addressed. Liquid film, trapped between the vapor core and tube inner surface in the annular flow regime, is an essential parameter since the evaporation of liquid film is the dominant heat transfer mechanism in this regime and the critical heat flux (CHF) is identified based on the dry-out of the liquid film. Therefore, the liquid film was fairly predicted, to estimate heat transfer coefficient and CHF in annular flow regime. Moreover, a semi-analytical model for slug flow has been developed based on the principle of drift flux model to investigate the characteristics of slug flow pattern such as bubble velocity, bubble length, liquid slug velocity, liquid slug length, and bubble frequency. The effects of working fluid properties, mass flux, heat flux, and channel size on the modeled parameters have been taken into account. As a result, it has been found that in the annular flow regime, the CHF increases with the mass flux and channel size, while it is inversely proportional to the critical vapor quality. Furthermore, the results of two-phase flow boiling heat transfer coefficient showed that heat transfer coefficient increases with mass flux and vapor quality, while it decreases when the channel size increases. Regarding the pressure drop results, it has been observed that the pressure drop is inversely proportional to the channel size, and increases with the mass flux and vapor quality. Additionally, it has been shown that void fraction increases with vapor quality and decreases with channel size. Also, the bubble frequency in slug flow regime has been predicted, and it is shown that the bubble frequency increases with vapor quality and mass flux. Furthermore, it has been noticed that the peak of the bubble frequency increases with mass flux. The proposed flow boiling models have been validated against extensive two-phase experimental data in literature for the critical heat flux (CHF), heat transfer coefficient (h_tp) , pressure drop (dP_tp), and void fraction (α), respectively, for different working fluids (Water, CO2, LN2, FC72, R134a, R1234ze, R245f2, R236fa, R410a, and R113) in horizontal micro-channels with a range of hydraulic diameters of 0.244≤D≤3.1 mm, and a range of the Reynolds numbers of 1,900≤Re_eq≤48,000. The annular flow model predicted the tested experimental data of the CHF, h_tp, dP_tp, and α, with mean absolute error of 20%, 18%, 23%, and 3%, respectively. Furthermore, a good agreement with the experimental data of bubble lengths and bubble frequency has been obtained with MAE of 27.83% and 29.55 %, respectively.