CFD Optimization Of Flow Dynamics And Heat Transfer In Mechanically Stirred Reactor

As a crucial equipment in the chemical industry,the stirred tank reactor is widely used in the processes of hydrolysis,polymerization and so on.Related reactions are usually intensive exothermic,which brings challenges to the regular operation of the equipment.For the sake of operating safety and process control,it is necessary to optimize the design and operation of the reactor to achieve favorable mixing and desired heat transfer performance.In the present work,systemic Computational Fluid Dynamic simulations were carried out to analyze the flow dynamics and heat transfer in the stirred tank,and corresponding optimal structural design are conducted as well.In terms of framework,it’s mainly composed of three sections.Firstly,the influence of blade inclination and its combination on the flow dynamics and heat transfer processes in the stirred tank was investigated;secondly,comparative studies were conducted to investigate the influence of the coil diameter on the flow dynamics and heat transfer in the stirred tank;finally,the effects of operating parameters on the heat transfer were investigated in detail,including the impeller rotating speed,the flow rate and temperature of the cooling water.The main contents of this paper are given as follows:1.First of all,both the upper and the lower impeller have the same blade angle.The flow dynamics and heat transfer in the stirred tank are compared under different blade inclination angles of 30?,45?,60?,and 90?.The results show that,with the increasing blade inclination angle,the larger the high-speed area of the flow field was obtained and the convective heat transfer coefficient on the outer surface of the cooling coil rose too.The stirring power increased for the larger blade inclination angle.The 45?blade impeller and the 60?blade impeller had the best comprehensive performance of flow dynamics and heat transfer performance.Afterward,two kinds of combination of the upper and lower impellers were investigated,with one combination having a 45?blade angle for the upper impeller and a 60?blade angle for the lower and another combination having a 60?blade angle for the upper impeller and a 45?blade angle for the lower.The results showed that the upper impeller had a more notable influence on the flow field distribution and heat transfer performance in the tank.And the change in the order of blade angle for the upper and lower impellers had remarkable impact on the heat flux and heat transfer coefficient of coil,but lead to few alternations in the stirring power.Taking full account of the mixing and heat transfer performance in the tank,it was considered that the combination of upper-45?-lower-60?agitator was preferred.2.With the same total exchange area of the cooling coil remaining constant,five kinds of coil diameters were studied through CFD simulations.The results indicated,the flow behavior in the vicinity of the cooling coil alternated remarkably although weak changes happened to the global flow field.In detail,the coil not only promoted the axial flow near the tank wall,but also inhibited the tangential flow through the gap.For different diameters,there are intensive changes in turbulence inside the coil,which had a greater impact on the surface heat flux.It’s also found that the inner convective heat transfer coefficient of the coil decreased with the increase of coil diameter,and the outer convective heat transfer coefficient is also slightly decreased.The coil with(9=0.09m exhibited the best overall performance of flow and heat transfer effects.3.Based on the reactor structure optimized above,the influence of the operating parameters on the flow dynamics and heat transfer in the stirred tank was analyzed as well,including the impeller rotating speed,the flow rate and inlet temperature of cooling water.The results manifested that the convective heat transfer coefficient on outer surface of coil increased with the increase of impeller speed.The increasing flow rate strengthened the secondary flow inside the coil,and there was a significant rise in the convective heat transfer coefficient on the inner surface of the coil.The inlet temperature of cooling water had negligible effect on the convective heat transfer coefficient on both sides of the coil,but the low-temperature cooling water enhanced the heat flux and improved the heat transfer efficiency.In practical industrial applications,a comprehensive trade-off among various structures and operating parameters was necessary,according to the specific applications,material characteristics,reaction characteristics,cost and so on.