Effect of off-bottom clearance of Lightnin impeller on heat transfer performance in the stirred tank coupled with draft tube

The heat transfer issue in stirred tanks is a prevalent concern in the industry. This article aims to investigate the fluid flow and heat transfer within a stirred tank equipped with a Lightnin A320 impeller coupled with a draft tube (DT) structure. The heat transfer performance of a 37 L jacket stirred tank is experimentally measured with the rotation speeds of 50 - 300 RPM. The effects of off-bottom clearances and rotational speeds on mixing and heat transfer performance are explored using Computational Fluid Dynamics (CFD) by flow patterns, local velocity, turbulent kinetic energy (TKE), helicity, vorticity, heat transfer coefficient, synergy angle, extensional efficiency, and coefficient of variation (CoV), respectively. The numerically predicted temperature curves have a good agreement with the experimental results. An optimal off-bottom clearance with C/D = 0.43 is found in this stirred tank. Furthermore, empirical correlations are developed to predict heat transfer performance by analyzing the relationship between wall heat transfer coefficient, power consumption, and heat transfer rate. By evaluating the variation of the temperature gradient synergy angle with time and rotational speed of the stirred tank, it was determined that a stabilization time of 120 s is required for the synergy angle. In the area outside the DT, 80 % of the region exhibits extensional efficiency values greater than 0.5. For 100 - 300 RPM, achieving a CoV of 0.01 requires 48.36 %, 67.26 %, 76.81 %, 82.34 %, and 85.67 % less time compared to that of 50 RPM, respectively.