Numerical analysis of heat transfer enhancement in a straight tube heat exchanger in the presence of non-boiling air-water flow
Abstract
Cooling of water with high temperature in a straight tube under condition of having non-boiling multi-phase air–water flow has been studied numerically and various flow rates of water and air have been introduced to study formation of various patterns of flow and their effects on heat transfer. This study is devoted to studying effect of air injection on heat transfer enhancement in macro-scale heat exchangers in which the dimensions of the tube is above micron which is applicable in many industrial heat exchangers. FVM-VOF method has been considered to solve the governing equations. A variety of flow patterns such as bubbly, slug and annular flow were observed. Higher air flow rate is associated with formation of larger bubbles while higher flow rate of water would decrease the bubble size. The obtained demonstrated that presence of the air phase would enhance the heat transfer, however, at lower water velocity with formation of larger bubbles a higher value for Nusselt enhancement ratio is obtained. Formation of bubbly flow and annular flow are both associated with higher heat transfer in comparison with associated single phase flow resulting in Nusselt enhancement ratio of 1.36 and 3.06 respectively, however the highest enhancement ratio was observed in slug flow to be 4.66. It was revealed that presence of bubbles would enhance transfer of energy when being compared with single phase flow and the various flow patterns associated with various flow rates of air and water remained its own distinct impact on the heat transfer augmentation.