Research on Heat Transfer Performance of Coaxial Mixer with Inner Combined Impeller

Abstract

A novel coaxial mixer was proposed, which had an outer wall-scraping frame and a combined inner impeller consisting of a four-pitched-blade turbine paddle (PBT-4) and a Rushton turbine paddle (RT-6). Its heat transfer performance in Newtonian fluid under different operating modes (single rotation of inner impeller, corotation and counter-rotation) was experimentally investigated in a stainless steel agitated vessel with semicircle spiral jacket and ellipsoidal head by measuring the average heat transfer coefficient of the inner wall. Meanwhile, the wall temperature, the heat flow rate and the local heat transfer coefficient in the coaxial agitated vessel were determined with the help of numerical simulation. The results show that the heat transfer coefficient near the inner wall surface of the agitated vessel will increase with the increasing speed of the inner or outer impeller whether under single-shaft agitation or under coaxial agitation mode, whereas the effect of speed of the inner impeller on heat transfer is more obvious and efficient. Under the same conditions, the heat transfer of coaxial agitation is more efficient than that of single-shaft agitation, especially the local heat transfer coefficient on the upper and lower part of the agitated vessel, which is increased greatly. In addition, the local heat transfer coefficient varies with the axial position of the agitated vessel, and the heat transfer coefficient in zones between the upper and lower impeller is relatively high. The effect of the corotation mode and counter-rotation mode on heat transfer is of little difference, but the corotation mode is recommended owing to its advantage of low power consumption and good mixing performance.