Study on quasi-isothermal expansion process of compressed air based on spray heat transfer

Abstract

The piston expander with compressed air as working medium has been widely studied for its advantages of cleanliness, reliability and safety, but its application is limited by low exhaust temperature and low output power/efficiency. Theoretically the isothermal expansion process is the best method to solve the above problems. Spraying into the cylinder can greatly improve the heat transfer rate, which is an effective way to realize the isothermal expansion process. In order to realize the isothermal expansion process of piston expander,we propose a method to inject micron water mist at high temperature into the cylinder. Firstly, the nonlinear transient mathematical model of the adiabatic expansion process is established by using energy equation, gas state equation and Lagrange dynamics equation. The experimental results verify the effectiveness of the model. Secondly,based on the adiabatic expansion model, the heat transfer equation of small water droplet and air in the cylinder is introduced, and the model is extended to the nonlinear transient mathematical model of quasi-isothermal expansion process. Simulation results showed that under the given boundary conditions, compared with the adiabatic expansion model, the quasi isothermal expansion model significantly higher temperature of the air inside the cylinder, the average air temperature in cylinder increased by 24.7%, because the pressure increase in the inlet and expansion stages of the quasi-isothermal expansion model is less than that in the exhaust stage, the output power/ efficiency of the quasi isothermal expansion model is reduced by 7.7%/12.3%. Based on the analysis of the above results and the calculation of the air power in the exhaust stage of the two expansion models, a new type of two-stage piston expander is proposed. This study will provide theoretical support for the experimental study of quasi-isothermal expansion process of spray heat transfer.