A Study On The Characteristic Of Nonequilibrium Phase Change, Supersonic Flow And Mass Transfer Of Condensing Gases

Supersonic dehydration separation technology has broad application prospects in the natural gas pretreatment. It has been already used in some real engineering projects in some countries. Due to its performance is far from the theoretical ideal expectation, it has not applied widely in China.At present, there has not been an acceptable theoretical mechanism description for the two phase condensation flow inside the supersonic separator, and this is why it is still very difficult to provide generalization design guidelines. Therefore, it is necessary to carry out a systematic, deeply and accurately investigation to explore the mechanism inside the separator. It is urgent to establish a reasonable and reliable theoretical model for compressible supersonic flow, develop relative thorough and verified calculation method and explore new design standard. In the current research, based on the survey of the technology development, both experimental and numerical studies are carried out.According to the work principle of the supersonic separator, the throat size is calculated under the condition of low and high pressure by using the ideal gas state equation and PR state equation, respectively.The different shapes of the convergent section combined with divergent section of Laval nozzle structure are stuied and analyzed using the numerical method, which prove that Witozinsky curve is a better choice for the supersonic separator.One-dimensional flow inside the straight tube is investigated by using thermodynamic theory for adiabatic friction compressible flows, and the limitation length of the straight tube is obtained for the design of straight tube section. The relationship between the divergence angle of the diffuser part and the friction loss is also studied. And some valuable conclusions has been obtained.A new type supersonic separator based on the provided principle is designed and manufactured.Theexperimental rig of the supersonic separator is constructed and the systematic experimental investigations are carried out. Choosing moist air as the working medium, the influences of the pressure loss ratio and relative humidity on the dew-point decrease are analysed. The results show that at given inlet temperature the big pressure loss ratio lead to a large dew-point decrease.When the pressure loss ratio is 72.6%, the dew-point decrease will be 22℃.Through the analysis of the content of the separated water from the tube, it can be concluded that the pressure drop can also lead to dew-point decrease. So in evaluation of the performance of the supersonic separator, the dew-point decrease resulting from pressure drop must also be considered.Based on the experimental data of supersonic separation, thermodynamicanalysis of the adiabatic expansion is carried out. The results show that too large pressure loss ratio will lead to more availability loss. To calculate the separated content of liquid from multicomponent natural gas, the phase equilibrium is added.Based on the flow behavior, the RSM turbulent model is recommended for modeling flow inside the supersonic separator and a three dimensional numerical model is established. Using this model, the flow pattern and shock wave location can be predicted and other parameter such as pressure, temperature, and mach number inside the flow can be obtained. The influence of the pressure loss ratio on the performance of the supersonic separator is also investigated. Through the study, one can conclude that for a given supersonic separator, reducing the back pressure can improve the performance in limit extent. When the pressure loss ratio is small, reducing back pressure can make the shock wave location move to the exit of the divergent section. But when the pressure loss ratio exceeds the limit, lower back pressure cannot change the shock wave location. These indicate that when the back pressure is small enough, the friction resistance downstream the divergent section is the dominant factor to affect the shock wave location. So it is an important method to reduce the friction resistance to improve the performance of the supersonic separator. To conquer the shock wave problem, a mass flow rate-adjustable separator is provided, which is able to improve the performance of the supersonic separator.Besides, a comprehensive three-dimensional fluid numerical model to study the flow behavior and separation efficiency in a supersonic separator is established coupled with the discrete particle model(DPM). The mixture of air and water droplets was chosen as working fluid. The gas phase was modeled with compressible Navier–Stokes equations and the RSM turbulence model wasused. The droplet phase was modeled with the discrete particle model(DPM), in which the droplets are assumed to have the same sphere shape and the phase transition and nucleation process are neglected. Furthermore, based on the proposed numerical approach, the gas-droplet turbulent flow structures were predicted. The effects of different structure parameters and operation conditions on the separation efficiency were also investigated.In order to explore the separation mechanism and heat transfer inside the separator, a numerical model with consideration of such effects as strong swirling turbulent and non-equilibrium condensation process is established. The numerical model is conducted to determine the dominant factors on the separation performance. The numerical model results are verified through comparison with experimental data obtained from the literature and the results demonstrated good agreement between the two ways. By using the proposed numerical model, the distribution of pressure, temperature, velocity, droplet nucleation rate and Mach number are investigated. Furthermore, the simulation results can provide a foundation for the analysis of flow characteristic caused by the supersonic speed and nucleation phenomenon and also a basis for the design and geometrical optimization of the supersonic separator.