Numerical Investigation On Heat Transfer Performance Of Heated Twin Cylinders In Crossflow

Fluid flow around cylindrical structures in groups is ubiquitous in various fields of engineering i.e.heat exchanger tube bundles,high-rise buildings,chemical reaction towers,chimneys,electronics components on boards,bridge piers,etc.Among them,the Reynolds number of air-cooled systems for electronic chips ranges from 10~2 to 10~4.However,the experimental investigation at a low Reynolds number is very hard to implement.Thus,the flow structures and heat transfer around multiple circular cylinders in crossflow at a low Reynolds number should be understood by numerical simulations to design various components in air-cooled systems for electronic engineering.This thesis aims to investigate the effect of the configurations of two heated identical cylinders on the Strouhal number(St),flow structures,and surface forced heat convection with a diameter D over P~*=P/D=1.2～6 andα=0°～90°,where P is the center-to-center distance between the two cylinders andαis the alignment angle of the line through the centers of the two cylinders with respect to the direction of free stream incoming flow.The investigation is conducted by numerical simulation with finite volume method at Re=150 and Pr=0.71.The temperature of the incoming airflow is T_∞=298.15 K.The twin cylinders are kept with a constant temperature of T_w=299.15 K.Seven flow structure regimes are identified based on the flow patterns and Strouhal numbers:a steady flow mode(SS),two single-vortex-street modes(S-Ⅰ and S-Ⅱ),and two two-vortex-street modes(T-Ⅰ and T-Ⅱ).The vortex evolution is identified at x~*=x/D>15(x is the downstream distance from the center of the upstream cylinder).Based on the vortex motion and strength,Mode S-Ⅰ and Mode T-Ⅱ are further divided into two sub-regimes S-Ⅰ(a),S-Ⅰ(b)and T-Ⅱ(a),T-Ⅱ(b),respectively.The time-and-surface-averaged Nusselt number,temperature field,and time-averaged streamwise heat flux are analyzed to discover the surface forced heat convection performance and wake heat transport characteristics for different flow regimes.The relationship of flow structure regimes with shear layer interactions,vortex structure,and heat transfer in wake is thoroughly discussed.It is found that the global forced convection for downstream cylinder is enhanced for Mode S-Ⅱ and Mode T-Ⅰ.