Research On Law Of Sewage Separation Using Hydrocyclone Enhanced By Ejection

In recent years,China’s sewage source heat pump has developed rapidly in engineering applications,but its theoretical research has significantly lagged,especially the anti-blocking and anti-fouling problems of sewage heat exchangers in sewage source heat pump.Although researchers have developed many sewage de-foulant devices,their performance reliability,cost,and universality are in urgent need of further improvement.Thus,to solve the anti-blocking and anti-fouling problems that has limited the development of sewage source heat pump for a long time,in this paper,the hydrocyclone-separation technology was applied to the sewage de-foulant device in sewage source heat pump.The sewage hydrocyclone enhanced by ejection was proposed,which used the sewage after heat exchange to take away the foulants separated by swirling flow and took the heat from clean water by temporarily separating foulants.In this paper,experiments and numerical simulations were conducted to study the strengthening effect of ejection on the separation performance of sewage hydrocyclone,explain continuous-phase flow field and particle behavior characteristics in the sewage hydrocyclone enhanced by ejection,and make clear effects of the operating conditions and structural parameters on the separation performance and flow field.As a result,the law of sewage separation using hydrocyclone enhanced by ejection was revealed from both the macroscopic and microscopic perspectives.Firstly,this paper analyzes the limitations of existing de-foulant technologies used in sewage source heat pump and hydrocyclone-separation technologies and proposes the sewage-separation technology using hydrocyclone enhanced by ejection,sewage hydrocyclone enhanced by ejection,and the sewage source heat pump using the sewage hydrocyclone enhanced by ejection.Then,the concept of comprehensive separation efficiency,which is more suitable for evaluating the separation performance of sewage hydrocyclone enhanced by ejection used in sewage source heat pump,was proposed based on the reduced separation efficiency proposed by Kelsall in 1953.Secondly,the experimental unit of sewage hydrocyclone enhanced by ejection was built,and experimental researches under different feed flow rates,underflow-pipe diameters,vortex-finder lengths,and feed pressures were carried out.Results showed that the sewage hydrocyclone enhanced by ejection had high and stable separation efficiency(>90%)of foulant(<4 mm)and sand(75~250 μm),low foulant concentration,small split ratio(<10%),and small total static pressure loss(<20 kPa);the more foulants,the higher the separation efficiency.Compared with hydrocyclones with underflow-pipe diameter of 0(Similar to conventional hydrocyclones that usually operate with a closed ‘‘grit pot” under the underflow orifice and always discharge the foulant by an automatically or manually operated drain valve),the novel hydrocyclone greatly reduced the occurrence of “entrainment of coarse particles in overflow”,improved the separation efficiency by 41.6%~46.2%,and achieved the continuous discharge of foulants with only a slight total static pressure drop.Compared with traditional hydrocyclones with underflow opening to atmosphere,the ejection function eliminated the air core,which would reduce the separation efficiency,with water seal.To further reveal mechanism of the aforementioned experimental results from microscopic view of the continuous-phase flow field and particle behavior characteristics,and make up for the lack of experimental conditions,in this paper,the computational-fluid-dynamics method was applied to establish the numerical model of the sewage hydrocyclone enhanced by ejection using the Fluent software.Reynolds Stress Model was utilized to capture the 3-D strong swirling anisotropic turbulent flow in the sewage hydrocyclone enhanced by ejection.Discrete Phase Model was employed to predict the particle behavior characteristics.The split ratios obtained by experiments and simulations under different underflow-pipe diameters were used to validate the above-mentioned numerical simulations.Effects of underflow-pipe diameter,vortex-finder length,forward ejection velocity,and reverse ejection velocity on the continuous-phase flow field,particle behavior characteristic,and total separation performance were investigated by numerical simulations.Simulation results suggested that,when the underflow-pipe diameter was smaller than the underflow-orifice diameter,the underflow-pipe diameter was proportional to the separation efficiency and split ratio.This relationship was similar to the corresponding relationship of conventional hydrocyclones and was consistent with the experimental results in this paper.When the underflow-pipe diameter was larger than the underflow-orifice diameter,circulation flow that promoted the “entrainment of coarse particles in overflow” was generated in the underflow pipe.The intensity of the inner circulation flow was proportional to the underflow-pipe diameter.As a result,the underflow-pipe diameter was inversely proportional to the separation efficiency and split ratio.When the vortex-finder length was close to the cylindrical-section length,the separation efficiency of particles with a sufficiently large density was always 100%.Whereas when the vortex-finder length was slightly longer than the cylindrical-section length,the separation efficiency first decreased and then increased.On the contrary,when the vortex-finder length was long enough,the separation efficiency slightly decreased with the increase of the vortex-finder length.Compared with the design of variable underflow-pipe diameter,the design of adjusting the forward or reverse ejection velocity could simultaneously achieve the high separation efficiency and acceptable low split ratio without increasing the risk of blocking the underflow orifice and underflow pipe(i.e.,without decreasing the carrying capacity of the underflow orifice and underflow pipe).Compared with adjusting forward ejection velocity,adjusting reverse ejection velocity could decrease the optimum ejection velocity and hence decrease the operating cost of the sewage hydrocyclone enhanced by ejection and make it more economical and practical.In this paper,the law of the sewage separation using hydrocyclone enhanced by ejection was revealed from both macroscopic separation performance and microscopic flow field and particle behavior characteristic in the sewage hydrocyclone enhanced by ejection.The research work has important theoretical significance and also provides a new idea for solving the anti-blocking and anti-fouling problems of sewage heat exchangers that has limited the development of sewage source heat pumps for a long time and the problem of blocking the underflow pipe that has limited the development of hydrocyclone-separation technologies for a long time.Furthermore,this research also has significant practical value for further improving the performance of sewage source heat pumps and hydrocyclones and expanding the the application fields of the two.