The role of heat exchange on the behaviour of an oscillatory two-phase low-grade heat engine

A particular type of oscillatory low-grade heat engine is examined with specific attention to the two-phase heat transfer process undergone in the device during operation. Three linear and spatially lumped models of this device are investigated, which incorporate: (a) a linear temperature profile (LTP) in the heat exchangers; (b) a constant temperature difference (CTD) between the heat exchangers and working fluid; and (c) a dynamic ability of the heat exchanger (DHX) blocks to store and release heat periodically. The LTP model has been presented in earlier work [10], where it was shown that including the effects of liquid flow inertia was important in the modelling of the device. In the present paper, the CTD and DHX models are developed and probed in order to study the effects of alternative heat exchange assumptions and descriptions on the operation and performance of the device. The condition for which continuous, sustained oscillations occur in the three models (i.e., marginal stability) and their associated exergetic efficiencies at this condition are calculated and compared. It is found that all three models predict oscillation frequencies in the same order of magnitude, but that the CTD model predicts unrealistically high efficiencies. Interestingly, when parameters associated with the power cylinder of the device are varied, a discontinuity in the oscillation frequency of the system is observed with all models. This feature has not been reported previously and is important for the better understanding and, ultimately, the improved design of such devices.