Study On Flow And Heat Transfer Characteristics Of Polyethylene Granules

The bluk flow heat exchanger has been widely used because of its high heat transfer efficiency,low pollution,and small occupied area.The wavy plate moving bed heat exchanger is a new type exchanger whichi is developed on the basis of it.The granular flow behavior in the wavy plate moving bed is affected by the shape of the wavy plate.The heat transfer process between the wavy plate and granules is related to the granular flow behavior closely.However,the research on the granular flow behavior and heat transfer in the wavy plate moving bed is still not perfect.In accordance with this situation,the following work has been done:(1)In order to analyze the dense-phase flow of the granules in the wavy plate moving bed at the particle scale,the discrete element method(DEM)was used to simulate the flow law,porosity,the number of particles contacted with the wavy plate and the contact force in the wavy plate and the single-side wavy plate flow channel.In the process of granular flow,the motion trajectory of the particles in the direction of the vertical plate was influenced by the shape of the surface of the surface of the wave plate,that is to say,the mixture of particles mainly occurs in this direction significantly.In the scope of this study,the number of particles contacted with the wavy plate decreased with the increase of the granular velocity,and the normal contact force changed a little,and it had nothing to do with the shape of the plate surface.(2)The hot water at constant temperature was used to heat the polyethylene granules at room temperature in the wavy plate moving bed heat exchanger.The influence of the distance between paltes(25 mm、30 mm、38 mm),the flux of hot water(1~3 L/min)and the granular flow rate(9 ~38 g/s)on the heat transfer process was investigated.Through the treatment of the experimental data,it was found that the change of the flux of hot water had no great influence on the temperature difference of the granules at the same distance of plates.But the granular flow velocity had larger influence on the granular temperature difference.It was found that the trend of the exponential decreased with the increase of the granular velocity through fitting analysis.The temperature difference of the granules was larger when the distance of plates was smaller.(3)At the same distance of plates,the increase of granular velocity would lead to an increase in heat transfer rate and total heat transfer coefficient within a certain range.However,,the heat transfer rate increaseed gradually and the total heat transfer coefficient tended to remain unchanged as the granular velocity increases.Within the range of granular velocity in this study,the heat transfer by convection between the granular and the wall could be promoted as the increase of the granular velocity at the beginning.However,the number of particles contacted with the wall decreased as the velocity continued to increase,which would reduce the erosion between the particles and the wall and cause the increase of the thickness of the film near the wall.These made the total heat transfer coefficient constant.The correlations between Nu,Fr,Pe and the dimensionless temperature Ct were obtained through the analysis of the experimental parameters and the arrangement of the experimental data,,which can provide a certain basis for the design of the wavy plate moving bed heat exchanger.