CFD modelling and exergy analysis of a heat pump cycle with Tesla turbine using CO2 as a working fluid

Abstract

The transcritical carbon dioxide heat pump cycle has been drawing much research interest due to its environmental friendliness and the thermodynamic features of carbon dioxide. However, there is one concerning issue, which is the huge exergy loss associated with the isenthalpic process in the expansion valve. In the current study, a new transcritical carbon dioxide heat pump cycle is proposed, where a Tesla turbine replaces the expansion valve. The Tesla turbine is a bladeless turbine that works with any two-phase fluid, which is the case for the expansion of supercritical CO2. A 3D computational fluid dynamics model is first developed to simulate the flow of carbon dioxide within a Tesla turbine, and then the extracted results are used as data for subsequent thermodynamic modeling of the heat pump cycle. The Tesla turbine power production and exergy losses, as well as the proposed heat pump cycle coefficient of performance are investigated in terms of the turbine rotor angular velocity and the gas cooler and evaporator pressures. It is demonstrated that the coefficient of performance of the cycle where a Tesla Turbine is integrated is up to 16.3% higher than the traditional cycle with the expansion valve. In addition, at rotor angular velocity equals to 1000 rad/s, the turbine power is maximum and increasing the inlet pressure leads to the higher torque and consequently higher turbine power. At lower inlet pressure, the coefficient of performance of the heat pump cycle is higher. A thermodynamic trade-off is illustrated between the power production from the Tesla turbine and the vapor quality at the outlet of the Tesla turbine, as a function of rotor angular velocity. It is numerically proven that the optimum rotor angular velocity corresponds to the maximum exergy efficiency of the Tesla turbine, which in turn leads to the maximum coefficient of the performance of the whole cycle.