Enhanced Heat Transfer for Boiling of a Refrigerant on a Micro-Structured Cylindrical Surface and Effect of Saturation Temperature

Boiling on the outside surface of cylindrical tubes is an important heat transfer process widely used in industry applications. It is known that boiling heat transfer coefficient increases with increasing saturation temperature. However, a quantitative measure of saturation temperature effect on boiling heat transfer is not readily available, especially for boiling on surfaces of microstructures. This work was motivated by the need to predict evaporator performance in a chiller while taking into account the effect of saturation temperature on boiling heat transfer coefficient. Experiments of boiling of refrigerant R123 on the micro-structured outside surface of an evaporator tube have been performed at three saturation temperatures in the range of 4.4 to 17.8 degrees C. Water flows inside the test tubes and provides heat to the refrigerant for boiling. In addition, experiments of R123 boiling on smooth cylindrical tubes have been performed at the saturation temperature 4.4 degrees C to provide a baseline to quantify the enhancement in boiling heat transfer due to microstructures on the test tubes. For boiling on the micro-structured surface, the boiling heat transfer coefficient increases by nearly 15% for the temperature range considered in this work. Measurements also showed that heat transfer coefficient for boiling on the test tubes of micro-structures is 12.3 times higher than boiling on the smooth surface. The Cooper correlation over-predicted by 40% the boiling heat transfer coefficient on the smooth cylindrical surface, but significantly under-predicted the performance for boiling on the tubes of micro-structures. It is found that the prediction of Cooper correlation multiplied by an enhancement factor 7.9 has a good agreement with measured heat transfer coefficient for boiling on the tubes of micro-structures at all the three saturation temperatures. Visual observations indicated that bubble departure characteristics on the micro-structured surface are different from those on the smooth surface. In addition to promoted bubble nucleation by re-entrant cavities on the micro-structured surface, the different bubble departure characteristics also contribute to the enhancement of boiling performance.