Experimental and computational analysis of ribbing structure modification effect in tube and fin cross-flow heat exchangers operating at non-uniform inflow of media

Abstract

Distributions of media streams flowing in a cross-flow tube and fin heat exchanger are usually non-uniform. This could be an effect of the heat exchanger construction, its installation method, design of a flowing channel or all those factors combined.The problem of the non-uniform media flow in heat exchangers of different types is not new, and it has been investigated by many researchers. Early results were sometimes ambiguous. More recent outcomes indicate that the effect of the nonuniform inflow of heat carriers to the heat exchanger could be significant—it may adversely affect the device’s efficiency to a large extent. Investigations of tube and fin cross-flow heat exchangers carried out for almost twenty years at the Institute of Thermal Technology of the Silesian University of Technology, by way of experiments and numerical simulations, also confirm these latest conclusions. The reduction in overall heat exchanger capacity, comparing to the uniform inflow of media, may reach up to 18%. This work presents results of experimental and computational investigations of tube, fin, cross-flow, double row heat exchangers air-water. The heat exchangers under consideration are built in the form of two rows of elliptic tubes with rectangular fins. The ribbing structure of the first heat exchanger is uniform. This device was investigated primarily in order to determine its efficiency but also the range and the form of non-uniform inflow of air. The air flow distribution was tested on a special test station during a series of measurements. The results of the analysis of this heat exchanger were used to design a second heat exchanger with a non-uniform structure of fins on individual tubes. It was assumed that by changing the heat transfer surface (thickening the fins) in the region of high air speed, the efficiency of modified heat exchangers could be enhanced. Testing this hypothesis is the main aim of this work. The experimental results generally confirm the hypothesis, showing a rise in efficiency of up to 8%. However, it should be noted that the design of the modified ribbing structure is not optimal and changing this structure impacts the hydraulic resistance and distribution of air mass flow rate at the heat exchanger inflow. This effect should be considered when evaluating the results.