This paper shows that a combination of near wall laseroptical and high resolution heat transfer measurements can provide deeper insight into the complex interaction between fluid flow and heat transfer in heat exchanger geometries. It also presents a comparison between steady-state and transient calculations of the air-side flow and heat transfer in fin-and-tube heat exchangers.
The velocity field in a staggered tube configuration is examined at Reynolds numbers of Re_dh=6000 by means of 2D particle image velocimetry (PIV) as well as laser doppler anemometry (LDA). The PIV vector maps are taken in planes parallel to the fin (x−y plane) of an upscaled heat exchanger model. Measurements are made at different normal distances between the measuring plane and the fin wall. This allows the study of the change of the flow field due to the influence of the wall. In selected regions the mean velocity component in y−direction of the flow is determined by LDA.
The results are compared to measurements of the local heat transfer coefficient that are made by means of the ammonia absorption method (AAM). The regions of enhanced heat transfer due to the presence of horseshoe vortices are correlated to certain flow structures that appear close to the wall. It is found that an increased heat transfer goes along with significant changes of the V-component including frequent changes of the sign. It is concluded that patterns of 3D longitudinal horseshoe vortices that cause a local heat transfer enhancement can be inferred from an appropriate combination of 1D and 2D flow measurements.
The comparison of the steady-state and transient numerical simulations of louvered fin-and-tube heat exchangers shows that the unsteady formulation is necessary to be able to use second order discretization schemes for the momentum equation but that the discretization rather than the time formulation has the dominating effect on the solution.