Dynamic response of gravity-driven flow in multi-section absorption heat pump

Abstract

The multi-section absorption heat pump (AHP) is applied in the novel district heating (DH) system of China. This new type of AHP exhibits improved performance in DH systems, but involves a more complicated structural design and the risk of internal flow instability problems such as reversed pressure difference and liquid accumulation. The flow circulations of the solution and refrigerant consist of several gravity-driven flow processes that are rarely observed in conventional AHPs, and these processes have not been addressed in previous studies. The present work studies the dynamic response of gravity-driven flow inside a multi-section AHP in a DH system. A dynamic model capable of calculating the liquid level, flow rate, and pressure change of various gravity-driven flow processes is developed and validated. The steady-state conditions calculated by the dynamic model are compared with a validated model in the literature. The gap between the temperature and pressure calculations is less than 0.3 K and 0.05 kPa, respectively. The dynamic simulation results are compared with the start-up process test results of a multi-section AHP. The pressure values and trend change fit the test results well. The standard deviation (SD) and root-mean-square error (RMSE) of the pressure calculation errors are less than 5% and 5.5%, respectively. The model is used to study typical transient processes of multi-section AHPs in a DH system. In the start-up process, the pressure of the generator is 1–2 kPa lower than that of the absorber after the solution circulation begins. This reversed pressure difference leads to solution accumulation at the bottom of the generator. The solution flow rate into the absorber is 20% lower than the designed value. After the heat source is input into the generator, the reversed pressure difference disappears. Solution accumulation expedites the flow of the solution into the absorber. The flow rate is 75% higher than that of the designed value, leading to solution accumulation in the absorber. In the heat source sudden drop process, reversed pressure difference and solution accumulation problems also occur. The reasons for flow instability in each process and possible methods for preventing them are discussed.