Modelling of a two-phase thermofluidic oscillator for low-grade heat utilisation

Abstract

The Non-Inertive-Feedback Thermofluidic Engine (NIFTE) is a two-phase thermofluidic oscillator which, by means of persistent periodic thermal-fluid oscillations when placed across a steady temperature difference, is capable of utilising low-grade (i.e., low temperature) heat to induce a fluid motion. Innovative devices which comprise no or few moving parts and that can operate utilising low-grade heat for fluid pumping and/or pressurisation are currently under development based on the NIFTE concept to: (i) understand the fundamental principle of operation of this novel technology; (ii) construct reliable, simple models that capture the first-order dominant underlying processes that govern its operation and performance for the purpose of early-stage engineering design; and (iii) to investigate the potential of this technology in specific fluid-pumping applications. Three spatially lumped linearised models of the NIFTE are developed through the use of electrical analogies. The first model (LTP) imposes a static (i.e. steady) linear temperature profile along the surface of the heat exchangers, the second model (CTD) imposes a constant-temperature difference between the surface of the heat exchanger and the working fluid, the third model (DHX) allows the solid heat exchanger blocks to store and release heat dynamically as they interact thermally with the working fluid. Through carrying out a parametric study on the LTP model, with and without inertial effects in the liquid phase it is shown that the inclusion of inertia has a significant effect on the trends and magnitudes of key performance indicators, namely the temperature gradient along the heat exchangers, oscillation frequency and exegetic efficiency. In addition, much improved predictions of the oscillation frequency and temperature gradient are possible when using the inertive LTP model. Following from this, a parametric study on the three models, all including inertia, is used to show that the CTD model predicts unrealistically high exergetic efficiencies, and as such is omitted from any further studies. A dissipative thermal loss parameter that can account for the exergetic losses due to the parasitic, cyclic phase change and heat exchange within the device is included in the LTP and DHX models in an effort to make realistic predictions of the exergetic efficiencies. A parametric study on the LTP and DHX models, including and excluding the thermal loss parameter is carried out and the results are compared to experimental data. It is found that the inclusion of the thermal loss parameter greatly improves the prediction of the exergetic efficiency in both the LTP and DHX models, both in trend and approximate magnitude. From the results it is concluded that, on accounting for thermal losses, the DHX model achieves the best predictions of the key performance indicators of the NIFTE, that is, of the oscillation frequency and exergetic efficiency of the device. An investigation on the applicability of different working fluids for the NIFTE, based on the dynamic heat exchanger model including thermal losses, with emphasis on the effects of key thermodynamic properties on the maximum thermal efficiency of an idealised cycle and the predicted exergetic efficiency of the device is also carried out. The change in specific volume due to vaporisation and the maximum saturation pressure of the working fluid in the cycle are found to have a dominant role in determining these efficiencies. Thirty one pure working fluids are studied, under a given set of scenarios, each representing a different practical application for the NIFTE device. For the scenario where the maximum pressure of the engine is defined by the pumping application, higher efficiencies are predicted for wet and isentropic fluids. For the scenario where the hot and cold heat exchanger temperatures are set by the external heat source and sink, higher efficiencies are predicted by dry and isentropic fluids. In this work, it is estimated that, with optimised designs and well-selected working fluids, the NIFTE may be capable of thermal efficiencies in the range 1 – 5 % when operating with low-grade heat at temperatures from 50 to 100◦C, with current best performance of 1.5% at 80◦C.