Numerical investigation of the heat and mass transfer in a vertical tube evaporator with the three-zone analysis

A numerical study for the flow, heat and mass transfer characteristics near the inflow region of the vertical evaporating tube with the films flowing down on both the inside and outside tube walls has been carried out. Condensation occurs along the outside wall and evaporation at the free surface of the inside film. The transport equations for momentum and energy are parabolized by the boundary-layer approximation and solved by using the marching technique. In this kind of numerical approach, the accurately predicting the early stage is really important because a small error at the previous step can produce the amplified big error at the next step. To accurately predict the flow at the inflow region of the vertical evaporating tube, the calculation domain of two film flow regions and tube wall is solved simultaneously. The interesting heat transfer characteristics revealed through this three-zone simulation, such as the evaporation delay and the temperature inflection at the very near inflow region are found and discussed along the discrepancy between the inner film inlet temperature and the saturation temperature. The case that the inner film comes in with the saturation temperature shows a good performance. The velocity and temperature fields as well as the amounts of the condensed and evaporated mass in both inner and outer films are predicted for the various conditions.