Optimization of high performance evaporator tubes through structural modifications for pool boiling heat transfer

Abstract

The present study provides a factorial experiment on the optimization of horizontally oriented reentrant type evaporator tubes with respect to pool boiling heat transfer through surface topology modifications. During this experiment the pool boiling heat transfer performance of three tubes with reentrant structures of 0.4 mm, 0.5 mm and 0.6 mm channel height were investigated in comparison to a plain reference tube. The used working fluids were R134a and FC-72 under saturation conditions at a reduced pressure of p \star = 0.086. The investigated heat flux range was 1 kW/m2 to 240 kW/m2 in case of R134a and 1 kW/m2 to to critical heat flux of each tested surface in case of FC-72. Based on the obtained experimental results it was found that heat transfer enhancement of the reentrant type test tubes is strongly affected by the channel height, the applied heat flux and the used working fluid. Furthermore it was found that apparently different heat transfer enhancement mechanisms take place at the reentrant structures in different ranges of the applied heat flux. In order to evaluate the combined effects of the variation parameters on heat transfer performance a factorial data reduction method was applied to the experimental data. As a result of this approach it was possible to identify the combination of investigated variation parameters that leads to the best pool boiling heat transfer performance. Using the data from the factorial data processing method it was additionally possible to draw preliminary conclusions about the dominant heat transfer enhancement mechanism in each investigated heat flux range. Especially the observed heat transfer enhancement in the heat flux range above 100 kW/m2 is discussed under the assumption of a two phase flow in the reentrant channels, that is driven by pressure gradients due to impulsive working fluid evaporation in the channels