Calculation of Convective Boiling in a Vertical Tube at Sub-atmospheric Pressures

The convective heat transfer coefficient and the wall temperature in a uniform heated tube with upward flow regarding the pressure drop along the tube are calculated. Heat transfer capacity of the flow is implicit function of its velocity and its heat transfer coefficient; hence studying flow boiling heat transfer is prerequisite of calculating heat capacity of the flow. While studying two-phase flow in the vertical tube at sub-atmospheric pressures, we should note that the pressure drop effect on the flow and fluid properties cannot be neglected, because the pressure drop is comparable to operating pressures. Two-phase flow pressure drop and two-phase properties such as void fraction are also investigated in this study. In order to imply the pressure drop effect on the flow along the tube, its length is divided into small cells and the flow and fluid properties in each cell are assumed to be uniform. A model based on Kandlikar observation which is based on two main thermal regions and three sub regions is proposed to calculate the heat transfer coefficient. A computer program has been written to divide the tube length to small equal length cells and calculate pressure drop of each cell and update the flow properties in them. It also uses the appropriate heat transfer correlation to calculate heat transfer coefficient. The heat transfer coefficient is used to calculate the tube wall temperature distribution. The wall temperature distribution at 14.7 and 22.7 kPa, are numerically calculated and compared to experimental results