Numerical Simulation Of Sludge Drying Process Under The Action Of Steady And Pulsating Flow

The sewage sludge in China is increasing year by year,and hot air drying is an important part of sludge pretreatment.It is of great significance to improve drying efficiency to reduce the size of heat exchanger and reduce the cost.Pulsating air flow has some advantages in improving heat exchange and drying efficiency,and has a wide application prospect in drying field.Therefore,the paper will use pulsating air flow to study the sludge drying process,and also use stable air flow to study the sludge drying process.The inlet air flow is hot air.The sludge was simplified into porous medium by CFD simulation technology,and the drying mathematical model was established to predict the water loss process of the thin layer of sludge.In this thesis,the sludge drying process under the action of steady flow is studied by numerical simulation.At the same time,the research on sludge drying process under pulsating flow is laid a foundation for the study of sludge drying process.The results show that the sludge drying process can be divided into the rising drying stage,the constant speed drying stage and the low speed drying stage.The change rule of the sludge moisture ratio is consistent in each working condition;When the air temperature rises from 100 ℃ to 180 ℃,the drying efficiency increases by 47%,the air speed rises from 0.5m/s to 8m/s,the drying efficiency increases by about 40%,the sludge thickness decreases from 15 mm to 5mm,and the drying efficiency increases by 38%.In addition,the relationship between drying time and variable was analyzed.It was found that the drying efficiency could not be improved by increasing the sludge temperature and wind speed indefinitely.When the sludge is between 5-15 mm,the thicker the sludge is,the more economical it is.The thesis also studies the sludge drying process under the action of pulsating flow.The velocity of inlet air flow is periodic change,and it is transferred into FLUENT software in the form of UDF.The drying characteristics,flow field characteristics and vorticity of sludge are analyzed by changing the frequency,amplitude and waveform of pulsating flow.The wave forms include square wave,sine wave and reciprocating square wave.The results show that the drying process of sludge can be strengthened under the action of pulsating flow.The results show that the drying time is saved by about 4.13% when the frequency increases to 5Hz;The higher the frequency,the larger the vortex volume at the back edge and the top of the sludge,the larger the speed of the sludge tail wall attachment,the more uniform the fluid temperature field,and the greater the drying efficiency,especially the obvious improvement of drying efficiency at the back side of sludge.However,in the range of 0-5hz,the higher the frequency increases,the smaller the increase range.The results show that the drying process can be enhanced by three waveforms,the drying efficiency of square wave is similar to that of reciprocating square wave,the drying efficiency of square wave is slightly higher than that of reciprocating square wave,and the sine wave drying efficiency is between square wave and steady flow;The vortex produced by square wave is larger than sine wave,and the reciprocating square wave can make the sludge dry more evenly.The results show that the larger the amplitude is,the larger the vortex volume at the top and the tail of sludge,the higher the drying efficiency,especially the drying efficiency of the sludge rear side is obviously improved;The drying efficiency was increased obviously when the amplitude was 1.5;When the amplitude is greater than 1,the outlet air flow is mixed back and there is repeated oscillation,which is conducive to the drying process.