Simulative And Experimental Studies Of Gas Thermodynamic Processes In The Twin Screw Vacuum Pump

Twin crew vacuum pump(TSVP)is widely used in aerospace,semiconductor,petroleum,chemical and other fields.It has the advantages of wide pumping speed,low vibration and noise,simple and compact structure,no friction in pumping components,long life,low energy consumption and so on.So,it stands out in various vacuum pumps.The thermodynamic processes of the gas in TSVP directly reflects the mechanism of the pumping,affecting the pumping efficiency and exhaust temperature of the pump,which fundamentally determines the performance of the pump.Furthermore,they have important guidingsignificance on the structural design,manufacturing and performance of the TSVP.Therefore,this paper mainly studies the thermodynamic processes of the TSVP.First of all,this paper analyzes the mechanism of the pumping processes of TSVP,and the leakage model of gas backflow in TSVP is established.Based on the leakage model,the analytical calculation model of gas thermodynamic processes in the pumping process is constructed.The TSVP is the dry vacuum pump.In the working processes,the backflow and leakage,between the screws and between the screw and the pump cavity,is one of the factors that have the greatest influence on the performance indexes of the TSVP,such as the limit vacuum and the pumping efficiency.Establishing a correct calculation model of the leakage model between the gas stages in the pump is the basic premise for accurately describing the gas thermodynamic processes in the pump.Based on the deep analysis and experimental experience of the backflow and leakage in the TSVP,this paper breaks through the mode of characterizing the backflow leakage with the rotor structural parameters,and tries to directly give the distribution of the backflow and leakage between gas stages along the way in the pump(also with the change of the transportation time).According to the different characteristics of each stage of the pumping processes in the TSVP,the pumping processes are decomposed into sucking process,compression process,transferring a process,transferring b process,backflashing process and exhausting process,so this paper established a more accurate leakage model for TSVP,and the self consistency conditions of suction stage and transportation stage verifies for the established leakage model.Based on the established gas leakage model,this paper deeply analyzes the change of gas from the inlet to the outlet in the working chamber.The paper establishes a general calculation model of differential form of gas thermodynamic processes in the TSVP,and then deduces the gas thermodynamic processes model of analytical form in different(equal pitch screw,single-stage gradational lead screw,dual-stage constant lead screw and multi-stage constant lead screw)structure pumps,In this paper,the gas thermodynamic processes model in these kinds of pumps is programmed with MATLAB,and the law of the change of the gas thermodynamic parameters in different types of pumps is obtained.The advantages and disadvantages of these kinds of screw forms are compared.The conclusion is that the multi-stage constant lead screw has the advantages of lower exhaust power consumption compared with other structures,which is an ideal structure form among these forms.Secondly,a novel computational fluid dynamics(CFD)simulation method is established to simulate the TSVP,the geometry model of the half moon piston suction working chamber is carried out to calulation the gas thermodynamic processes in the pump.On the basis of summing up different methods of various mature CFD simulation software in the market,comparing their advantages and disadvantages,the paper verifies the feasibility of several theoretically feasible and fast calculation methods to simulate the TSVP,proving that immersion solid method and other methods are not suitable for the simulation of TSVP,and providing reference experience for other researchers to simulate the TSVP.In this paper,a new method to simulate the thermodynamic processes of screw vacuum pump is tried.The geometric model of half moon working chamber in the thermodynamic processes is taken as the research object,and the surface of the working chamber is added with the same rotation speed and moving speed as in the actual working processes.At the same time,the peripheral surface of the working chamber is defined as the same leakage channel as in the actual processes,In order to avoid the problem that the screw rotor gap is too small to simulate accurately,the leakage channel is assigned to the leakage model in the simulation process.And the structural characteristics of different kinds of variable pitch rotors by piston compression.The simulation results show that the thermodynamic parameters of the gas varying with the time.And the flow law of gas in the working chamber is obtained.Then,in view of the lack of published experimental tests on the leakage flow and thermodynamic processes in the TSVP,an experimental test platform for the TSVP is specially established in this paper.In order to measure the pressure distribution along the pump cavity during the pumping process of the TSVP,a test sample pump with multi-stage screw rotor is specially selected,which is divided on both sides of the pump wall.There are five holes to connect the pressure sensor and other five holes to connected the temperature sensor respectively.The experimental test platform can test the pressure and temperature along the TSVP,and the pumping speed curve of the pump can also be tested.In this paper,the pressure distribution along the pump is measured experimentally,which indirectly verifies the leakage model and thermodynamic theoretical model established in this paper.Finally,the results of theoretical analysis model and CFD simulation are compared with the experimental results.The differences between the results of the model with leakage and the model with zero leakage are compared.And the results of different leakage models are also compared.A large number of general laws of the pumping processes in the TSVP are obtained.