Research of hydrodynamics and heat transfer during the transverse air flow of a row of cylinders with screw grooves

Abstract

Cylinder cross-flow is a common phenomenon in many fields of technology. Technological simplicity of tubular structures makes them attractive, especially when using working bodies that are under different pressure values. However, the cylinders belong to the category of «poorly streamlined» bodies, and there are many opportunities to improve their hydrodynamics and heat transfer. For a circular cylinder, there is a speed range in which its hydraulic resistance can decrease due to the deformation of the cylinder surface. This phenomenon can be used for the rational design of heat exchangers. In the open-type wind tunnel, heat transfer coefficients and hydraulic resistances of single-row cylinder bundles with several types of spiral grooves on the outer surface have been determined. The largest increase in heat transfer (64 %) was shown by the cylinder with the smallest pitch of the groove (10 mm), the second place was taken by the cylinder with a relatively large step – 40 mm. Using the best spiral groove allowed reducing the hydraulic resistance by 19 %. Visualization and computer simulation have been used to explain the effects. The conformity of computer simulations to the experimental results was determined by comparing the average heat transfer coefficient (calculated and determined using an ice calorimeter). As a result, the turbulence model RNG_ke has been chosen, which provides a better fit of the experimental model. Computer simulations have explained the physical picture of the flow around cylinders with spiral grooves, including their mutual influence with a different axial orientation in the bundle. It has been shown that the presence of a spiral groove, which on the one hand increases heat transfer and on the other hand reduces hydraulic resistance, can significantly increase thermohydraulic efficiency (Reynolds analogy factor).