An experimental and numerical investigation into compact heat exchangers
Experimental and numerical experiments were carried out on different heat exchanger section types to determine the performance of vortex generators. The heat exchanger sections investigated were plain channel fin types with delta and rectangular winglet pairs, rectangular wings and embossed vortex generators. Fin-tube heat exchangers were also investigated in both in-line and staggered arrangements with and without vortex generators over the Reynolds number range 65 - 653 and angle of attack of vortex generators 15° - 60°. With the need for improved heat exchanger performance fin modifications are normally used to enhance the gas side heat transfer coefficient. However, with many of these fin modifications a significant pressure drop penalty can result. Vortex generators enhance heat transfer with a minimal increase in pressure drop. The effect of vortex generators in heat exchanger sections at low Reynolds numbers need to be assessed so that optimal positioning can be determined. To do this local heat transfer coefficients need to be measured. Steady steady state physical tests with a constant heat flux boundary condition were used to measure local and average heat transfer values and thereby measure the effect of vortex generators on heat transfer and pressure drop. Numerical modelling allows a detailed picture of the flow field and local heat transfer to be seen. Such numerical modelling by Computational Fluid Dynamics (CFD) is still in its infancy and comparisons against detailed experimental data are still needed before simulations can be used without physical testing. The advantage of CFD is that a large number of simulations can be completed in a short time span when compared to full size physical tests. At low Reynolds numbers, it was found that the inclusion of vortex generators in all heat exchanger types had the effect of increasing the average heat transfer coefficient and pressure drop. In all but one of the cases investigated, the in increase average heat transfer coefficient was larger than the increase in pressure drop. For the case of a staggered tube arrangement, with vortex generators at 60°, there was a reduction in pressure drop when compared to the case without vortex generators. This was due to delayed separation and reduced wake region behind the tube. This specific vortex generator position is considered to be near optimal. It is shown that CFD can successfully reproduce data from physical tests on heat exchanger sections with and without vortex generators. The procedures given can be generalised to optimise further geometries. The benefits of including vortex generators in heat exchangers will enable smaller heat exchangers to be utilised or an increase in effectiveness for the user.