Numerical Investigation On The Intensity Of Horseshoe Vortex Systems Developed In The Fin-and-tube Heat Exchanger And Its Tube Shapes Optimization

Fin-and-tube heat exchangers are widely used in industry and civilian fields such as power plants,HVAC instruments,and wind tunnels.Improvement in the thermalhydraulic performance of heat exchangers can have significant contributions to environmental protection and reduction of carbon emissions.In most cases,the fluid flow outside the tubes is air,and most of the thermal resistance in fin-and-tube heat exchangers occurs in the air-side convective heat transfer process.Thus,the air-side thermalhydraulic performance of fin-and-tube heat exchangers has been widely investigated.Recently,several researchers have focused on using tubes with irregular shapes to further improve the thermalhydraulic performance of the heat exchangers.Besides,few researchers have investigated the horseshoe vortex systems developed in fin-and-tube heat exchangers,which are known to have significant influences on the heat transfer capability and the flow resistance characteristic of the heat exchangers.However,the effect of important parameters such as tube shapes or the Reynolds number on the intensity of horseshoe vortex systems hasn’t been revealed in-depth yet.Hence,the present study investigates the influence of these parameters on the special development of horseshoe vortex systems and further optimizes the irregular tube shapes to improve the thermalhydraulic performance of heat exchangers.In the fin-and-tube heat exchangers using staggered column arrangement,tubes in back rows are affected by the wake region of the frontal rows of tubes,and the air flows toward tubes in the first row hasn’t been accelerated by the narrow flow passage like in other rows.Thus,the difference in the spatial development of intensity of horseshoe vortex systems in different rows is investigated.The result of numerical simulations shows that in the heat exchangers investigated,the strongest horseshoe vortex system appears in the second row,and the horseshoe vortex systems in the back rows are significantly strengthened by the jet-like main flow with high speed.The influence of Reynolds numbers,tube ellipticity,and fin pitch on the intensity of horseshoe vortex systems and its effect on the local heat transfer enhancement is also investigated.It is found that the intensity of horseshoe vortex systems drops significantly as the Reynolds number and the tube ellipticity drop,but the intensity of weaker vortexes in the horseshoe vortex system drop more rapidly.Besides,When the Reynolds number and the tube ellipticity vary,the local average Nusselt number on fin surfaces is remarkably linearly dependent on the product of Reynolds numbers and dimensionless velocity circulation of primary vortexes.What’s more,the fin pitch has significant influences on the size and the intensity of horseshoe vortex systems and the distance between vortex and tube surface.As a result,the horseshoe vortex system strengthens the local heat transfer capacity on the fin surface more efficiently when the fin pitch is larger.Base on these observations,the windward and leeward tube ellipticity of the irregular tubes are optimized simultaneously to further improve the thermalhydraulic performance of heat exchangers.The numerical results show that the optimal tube shape has its windward ellipticity equal to 0.5 and its leeward ellipticity equal to 0.2.Compared with the optimal elliptical tube and fin heat exchangers investigated,the case using the optimal irregular tube has 2.8% higher average heat transfer coefficients and 9.1% lower pressure drops.This result shows the potential of the practical application of irregular tubes in heat exchangers.