抄録
Transient characteristics of an evaporator installed in a vapor compression heat pump system using R 22 as a working fluid have been experimentally investigated in the case of step change in the inlet refrigerant flow rate. The test evaporator is a double-tube heat exchanger in which the refrigerant flows inside the inner tube and the heating water flows counter currently in the surrounding annulus. The inner tube is an internally grooved copper tube of a 9.52 mm o.d. and an 8.72 mm mean i.d. The refrigerant flow rate was regulated by the expansion valve. The results are: (1) The transient change of local refrigerant flow pattern was observed and graphically demonstrated. (2) The changes of inlet refrigerant flow rate, local vapor pressure, local wall temperature and local heating water temperature were measured. (3) The local values of heat exchange rate, coefficient of evaporation heat transfer, vapor quality, void fraction, mean density and refrigerant flow rate were evaluated by solving unsteady energy equation of heating water and unsteady continuity and energy equations of refrigerant using the measured values mentioned above. (4) The local hold up of refrigerant changes corresponding to the change of refrigerant flow rate. (5) In the case of step decrease, the appearance of maximum values of mean density in the evaporation region corresponds to the minimum values of heat exchange rate, vapor quality and void fraction in 10 to 20 sec. after step change. (6) In the case of step increase, the earlier changes in wall temperature, heating water temperature and heat exchange rate at the more upper stream correspond to the shift of the dry out point from the upper to lower stream. (7) Dynamic stability time in the case of the step increase is longer than in the case of the step decrease.
