In the research of intube flow boiling, a widely accepted factor is that there are two main mechanisms participating in the heat transfer. One is nucleate boiling, which is dependent on the presence of active nuclei on the heated wall and the heat transfer coefficient is much influenced by the heat flux, much similar to pool boiling. In the other heat transfer mode, the boiling nuclei are fully suppressed and heat is transferred by the liquid evaporation through the interface of liquid film and vapor core. This is also called forced convective evaporation or convective boiling. In the evaporation region, the dependence of heat transfer on heat flux is not distinct and governed mainly by the mass flux and vapor quality. In the open literature on convective boiling heat transfer of refrigerant researches have been extensively conducted in straight tube. The studies, however, on two-phase flow boiling heat transfer in helically coiled tube are far less than that in straight tube. Because of the high efficiency in heat transfer and compactness in volume, helically coiled tubes are used extensively in practical industries. Therefore, it is important for application to obtain the correlations of flow boiling heat transfer coefficient in helically coiled tube. A new kind of micro-finned helically coiled tube was developed by the present authors and the flow boiling heat transfer characteristics are experimental studied in this paper, using R134a, an environment-friendly refrigerant as experimental fluid. By introducing convective boiling number NCB, as suggested by V. V. Klimenko (in Ref. [8]), the transition boundary between nucleate boiling and forced convective boiling in helically coiled tube is examined, which is much different with that in straight tube. Based on the analysis of the mechanisms of flow boiling, heat transfer correlation of the specific micro-finned helically coiled tubes is obtained, which has a mean absolute deviation of 13.8%.
Flow Boiling Heat Transfer of Tap Water in Micro-Channels (2nd Report, Heat Transfer Correlation for Boiling Region in Straight Channels)