Experimental investigation of liquid R-134a and water forced convection heat transfer in small circular tubes

This study provides heat transfer experimental results for water and R-134a heated as well as R-134a cooled in tubes with diameter from 1.1 to 4.0 mm. Developing and fully developed heat transfer performances are of interest and discussed. The experimental results show that the laminar water developing heat transfer coefficients can be reasonably well predicted by the Shah and Bhatti [1] empirical correlation while the turbulent water flow heat transfer coefficients perfectly agree with those predicted by the Gnielinski [21] correlation. The R-l34a heating and cooling Nusselt numbers are approximately 10% higher than those predicted by Petukhov [11]. There is no significant difference between heating and cooling heat transfer performance. Compared to the water heat transfer performance results, the predicting deviations of the existing heat transfer correlations for R-134a are higher than those for water but still within the experimental uncertainty ranges. While comparing the present heat transfer test results with the friction test results by Yang et al. [22], we may conclude that the mechanisms of flow shear forces acted in the current tested small tubes are exactly the same as those acted in larger tubes for water and R-134a. However, the Prandtl number dependency in traditional larger tubes equations may not be entirely exact for flow in small tubes.