(399f) Research on Separation Performance of Supersonic Separator with a Forward Helical Guide Blade

Introduction: The natural gas collected from the gas wells generally contain large amounts of saturated water vapor, while the presence of moisture will undoubtedly cause clogging and corrosion to natural gas transportation pipelines, valves and other auxiliary equipment. And not only that the residual vapor will seriously affect the power and quality of the supply natural gas, reducing the calorific value utilization of the natural gas. Therefore it is an indispensable intermediate link to effectively remove the free water and saturated water during the natural gas treatment process. Supersonic separator is a new type of gas-liquid separation equipment, with low consumption, safe and efficient and pollution-free advantages, not only for dehydration drying of saturated natural gas, but also for hydrocarbons gas removal of original natural gas and liquefied natural gas. In this paper, the basic model of supersonic separator was established by using Fluent numerical simulation software, and the helical guide blade was arranged near the entrance of Laval nozzle. The flow field characteristics inside the device were simulated and analyzed. The influence of the geometric parameters of the helical guide blade on the separation performance of the device was evaluated systematically.

Methods /Analysis: In this paper, a three-dimensional physical model was established for the supersonic separation device, the flow region was meshed, and the RNG k-Ɛ turbulence model was selected. The second-order upwind scheme was used to discretize and the appropriate boundary conditions were set. The internal flow field of the supersonic separation device was simulated and analyzed. It can be concluded that the process of air flow through the Laval nozzle is adiabatic expansion, and the temperature and pressure in the diffuser tube significantly decreased. The extremely low temperature makes it pausible that the moisture of the gas phase condenses into droplets. The inlet tangential velocity of the separation straight pipe is increased, but the uniformity of the swirling field is reduced, and the right half of the fluid field is mainly axial flow. The droplets are tossed to the pipe wall by centrifugal force in the separation straight pipe. The outlet tangential velocity of the straight pipe is reduced. In short, the overall tangential velocity of the separation straight pipe is small, which is not conducive to the efficient separation of the droplets. Therefore, it is necessary to improve the tangential speed in the separation pipe to ensure the high separation performance.

Two parameters were under consideration in order to optimum the separation performance. With other parameters fixed, the pitch of the basic model was firstly changed to study the effect of helical guide blade on the flow field performance. On comparison of the temperature, velocity and pressure changes along the axis based on three different pitch of 30mm,60mm and 90mm, it can be seen that, when the air flows through the spiral guide blade, with the increase in pitch, the temperature change is more gentle, the flow rate increases slightly, and the pressure drop decreases. In the Laval nozzle, the temperature drops rapidly, reaching the valley at the outlet of the diffuser, and the temperature drop is inversely proportional to the pitch. The temperature and pressure rise suddenly near the entrance of the diffuser because the conversion from the supersonic flow to the subsonic flow generates a shock, and with the pitch decreases, the flow rate, temperature and pressure change more gentle after the shock. Therefore, the flow field performance of the model with a 30mm pitch is more stable. The tangential velocity distribution of supersonic separator with three kinds of pitch is compared. With the increase of the pitch, the vortex of the tail edge of the guide blade gradually deviates from the centerline of the straight pipe, and the outlet tangential velocity of the straight pipe increases first and then decreases in the radial direction. The slope of the tangential velocity change decreases with the increase of the pitch, and the maximum tangential velocity near the wall is getting smaller and smaller. When the pitch is 30mm, the swirling vortex is centered, the tangential velocity and the centrifugal acceleration are highest, and the tangential velocity is symmetrical in the radial direction, which can ensure the separation straight pipe with the better centrifugal force. Based on the influence of the above three kinds of pitch on the flow field stability and swirling effect of the supersonic separator, it was found that the model with the pitch of 30mm is the best in both stability and separation performance.

Then with other parameters fixed, the length of the blade of the basic model was changed to study the effect of helical guide blade on the flow field performance. On comparison of the temperature, velocity and pressure changes along the axial direction based on three different blade length of 90mm, 120mm and 150mm, it indicated that with the blade length increase, the change of the gas flow rate and temperature is not obvious in the swirl pipe, the Laval nozzle and the separated straight pipe, and the variation range is very similar. In the separated straight pipe, with the increase of the length of the blade, the pressure drop goes up because the collision area between the fluid and the wall increases. When the fluid flows into the diffuser, there is a shock near the entrance and the flow field change is distinct. When the blade length is 150mm, there are many jumps in the changes of the flow velocity, temperature and pressure in the diffuser tube, which demonstrated that too long blade increases the instability of the diffuser flow field. In contrast, for both models with 90mm and 120mm blades, their flow field changes are more gentle and similar. The outlet tangential velocity variation along the radial direction in the separated straight pipe for the three kinds of guide blades are compared. With the increase of the length of the blade, the tangential velocity of the same radial point increases, and the changes in the tangential velocity for both 120mm and 150mm models are similar. In conclusion, the model with a blade length of 120 mm has a relatively stable flow field and a better swirling strength than others.

Results: The swirl flow field with a small tangential velocity was firstly formed in the swirl tube, and the swirl flow direction is same in the blade tail. When the fluid passes through the Laval tube, the axial velocity of the fluid rapidly accelerates to supersonic velocity and the tangential velocity increases with the flow of the fluid. The fluid can keep the diffuser and the separation straight pipe in a low temperature state, which can avoid the secondary evaporation of the droplets and reduce the loss of the blade shock. The flow field of the supersonic separator was analyzed by adjusting the pitch. With the decrease of the pitch, the centrifugal acceleration in the straight pipe is increased, and the separation performance of the outlet of the separation pipe was improved, and the flow field in the downstream expansion tube is stable. According to the comparison, the separation performance of the model with 30mm pitch is best, and the centrifugal acceleration is highest among three choices. It was found that the model with 120mm length had a more stable flow field and better swirl separation ability than others.