Experimental heat transfer, pressure drop, and flow visualization of R134a in vertical mini/micro tubes. Doctoral thesis.

For the application of minichannel heat exchangers, it is necessary to have accurate design tools for predicting heat transfer and pressure drop. Until recently, this type of heat exchangers was not well studied, and in the scientific literature there were large discrepancies between results reported by different investigators. This doctoral thesis aims to add to the knowledge of the fundamentals of single- and two-phase flow heat transfer and pressure drop in narrow channels, thereby aiding in the development of this new, interesting technology with the possibility of decreasing the size of electronics through better cooling, and of increasing the energy efficiency of thermal processes and thermodynamic cycles through enhanced heat transfer. The heat transfer and pressure drop results were compared both to conventional correlations developed for larger diameter channels and to correlations developed specifically for microscale geometries. Compared to many previous investigations, this study shows that the test data agree well with single-phase heat transfer and friction factor correlations known to be accurate for larger channels, thus expanding their ranges to cover mini/microchannel geometries. The test data for boiling heat transfer were compared to several correlations from the literature, both for macro- and mini-channels. A new correlation for saturated flow boiling heat transfer of refrigerant R134a correlation was obtained based on the present experimental data. This correlation predicts the presented data with a mean absolute deviation of ±8%.