Thermodynamic behaviour and heat transfer characteristics of air vessels in water delivery systems

Air vessels can effectively prevent water hammer caused by pump failure; however, the thermodynamic behaviour and accurate model of air in air vessels are rarely discussed, and an empirical polytropic index of 1.20 is commonly used for air vessel design. This study calculated the polytropic index based on the improved rational heat transfer (RHT) model using the multiphase volume of fluid (VOF) and k-& epsilon; turbulence model. The influence of dynamic airflow and heat transfer in the air vessel on the water delivery system pressure was studied. The effects of inlet and outlet flows, initial pressure, and initial air ratio on heat transfer were investigated. The polytropic indices of the expansion and compression processes in this study were between 1.10 and 1.40 and between 1.05 and 1.40, respectively. Curve fitting was applied to analyse the polytropic indices and substituted into the method of characteristics (MOC) for air vessel design. A hydraulic transient simulation of a practical water delivery project was conducted to compare the variable and fixed polytropic indices. The results showed that variable polytropic indices can better describe the dynamic processes of air. Using a fixed index of 1.20 for a low flow rate is risky. By contrast, the system can be optimised at high flow rates.