| Discharge method is one of methods to measure the flow rate characteristics of pneumatic components. Being simple, energy-saving and efficient, it is also highly applicable. In this internationally coordinated research project, theoretical and experimental study on the discharge method were carried out in order to provide International Organization for Standardizaion (ISO) with a novel and general-purpose method for measuring flow rate characteristics of pneumatic components. By combining finite volume simulation, theoretical analysis along with experimental study, the following specifics were systematically studied in this paper: the distribution and evolution of the velocity and temperature fields during discharge, the isothermal chamber characteristics and isothermal discharge method, the polytropic exponent, the thermodynamic model during discharge as well as their applications. The objective of this study is to accurately and roundly understand the process of discharge and improve the measurement accuracy and repeatability, which are of great theoretical and practical significance.At first, the physical models for empty and isothermal chambers have been established under the assumption that the stuffers in the isothermal chamber serve as porous media, numerical simulations were thus carried out with Fluent software. The velocity and temperature distribution and their profiles are obtained. Simulation results indicate that the velocity gradient locates primarily at the vicinity of discharge orifice while the velocity in the chamber is almost uniformly zero. The temperature distribution in the chamber is associated with the shape of the chamber. The lowest temperature is located at the center of chamber, with increasing temperature gradient during the process of discharge. The temperature drop of isothermal chamber is about 20 times less than that of empty tank, which indicate that the discharge process of isothermal chambers is very close to isothermal process. To validify the simulation results, the experimental pressure curves in the chambers agree well with the simulations. Despite a certain error, simulations of the average air temperature profiles inside the chamber during discharge exhibit the same evoluting trend and norm to those by experiments.Secondly, the temperature change of isothermal chambers is critical to their usability and it is therefore systematically studied under various conditions. The results show that the metal thread stuffed in chamber can greatly enhance heat transfer during discharge, which can dramatically decrease the temperature drop so that the discharge process can be regarded as an isothermal process. The isothermal characteristic of isothermal chamber is influenced by many factors under fixed charge pressure and discharge orifice. The material has little effect on isothermal characteristic as long as the stuffer is metal. The finer metal thread and the higher stuff density yield better isothermal characteristic. The higher discharge velocity, the worse isothermal characteristic is. Under the condition of an initial pressure 700 kPa and a stuff density 0.3kg/L of copper threads, the ratio of chamber volume (L) and the sonic conductance (×10-8m3/(s·Pa)) should be greater than or equal to 6 in order to be eligibly treated as an isothermal chamber. The isothermal chamber discharge method can be used to measure the sonic conductance and critical pressure ratio of the discharge valve. Although neglecting temperature change might lead to some error, the accurate results can be obtained with global optimization.Next, the polytropic exponent during sonic discharge and the whole discharge is studied experimentally. The polytropic exponent during discharge keeps varying from 1.4 to 1, indicating the existence of heat transfer as the heat flux enhances with the progression of the discharge. Furthermore, by appling the polytropic exponent to the data processing of discharge method in order to improve the accuracy of constant volume discharge method, partial polytropic exponent method and complete polytropic exponent method are proposed. The accuracy and stability of sonic conductance obtained with complete polytropic exponent are very high, at a standard deviation of less than 1.2% throughout the whole measurement process.In addition, the thermodynamic model for tank discharge is founded with lumped parameter. In order to determine the heat transfer coefficient which is difficult to obtain experimentally, two kinds of heat transfer models were given: fixed heat transfer coefficient model and natural convection model. In the fixed heat transfer coefficient model, the coefficient is determined with the pressure curve after stopped discharge. The discharge process was simulated with this model, and the result is very close to that of the experiments when compared with the simulation results of the adiabatic model. The heat transfer coefficient in the natural convection model is determined on the basis of relationship of natural-convection, and it changes with the discharge process. The pressure and temperature curve obtained with simulation based on this model match well with those of the experiments, which indicated that natural convection model can reflect the real discharge quite well.At last, by combining the discharge thermodynamic model base on natural convection with the discharge pressure curve, the temperature and polytropic exponent during discharge can be obtained from initial state,which were quite closed to those from experiments. Therefore,it can be concluded that this method can be a very good substitute to the"stop method"in determining the average temperature during discharge. The sonic conductance and critical pressure ratio of discharge valve can be identified by optimization based on discharge thermodynamic model and discharge pressure curve, where the results are basically the same as those by ISO 6358. This identification method is simpler and more energy-saving than ISO 6358, and its accuracy is equal or higher than that the A grade in ISO 6358.
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