A dynamic model for predicting condensation heat and mass transfer characteristics in falling film condenser

A mathematical model of falling film condenser is developed to predict the dynamic response of shell-and-tube condenser having R1233zd(E) as refrigerant for high temperature heat pump systems. The model incorporates a vapor-mass-flux ratio, which includes the relative velocity slip between vapor and liquid to examine the performance of tube bundles in falling film condensers. Discretization of governing equations and control-volumes method are applied to construct the dynamic model which is first validated with the well-known HTRI software in steady-state performance. The mean relative deviations are within 14.8% for temperature and 2.0% for heating capacity in steady state. The established model can be used to predict the dynamic heat transfer response to the change of operating conditions and geometric configurations of tubing. When compared to the change of inlet pressure of vapor refrigerant, the response of the condensation temperature is much slower, yielding some 1.7 times longer time to reach stable state than that of pressure. Further, various heat exchange tubes (smooth tubes and low-fin tubes with 3 different fin pitches) are examined. It is found that the heating capacity and water outlet temperature increase with the rise of fin density. Yet higher fin density also tends to reach stable state earlier. The heat transfer enhancement ratio ranges from 2.7 to 2.0 for low-fin tube with 1.3 mm fin pitch, and the enhanced tubes are more effective with the low subcooling temperature.