Experimental and numerical flow visualization in rectangular cross-section channels with smooth expansion and contraction

Currently, one of the most common types of heat exchangers is plate heat exchangers, including microchannel heat exchangers. To improve the efficiency of heat exchangers, optimization of the plate geometry is necessary which is achieved through numerical flow and heat transfer modeling. This requires validation of the results not only for integral characteristics such as pressure drop or heat transfer coefficient, but also for the flow structure. In this paper, we experimentally and numerically investigate the flow structure in slotted channels with a one-sided smooth expansion and contraction with a minimum (33 % of the slot height), average (53 % of the slot height), and maximum (200 % of the slot height) expansion at a Reynolds number of approximately 300. For numerical modeling, we used a steady state three-dimensional incompressible formulation based on the Reynolds-averaged Navier-Stokes equations with their closure using the k-ε Realizable turbulence model. During the experimental studies, we employed the optical method of particle image velocimetry which consists of cross-correlation analysis of successive images of the flow seeded with tracer particles, which allows us to determine their average displacement over the time between frames, and, consequently, the two-dimensional velocity field. To implement this method, a setup was created, including the development, manufacture, and testing of a droplet generator based on a Laskin nozzle. Using numerical modeling and experiments, instantaneous and averaged velocity fields in the mid-longitudinal cross-section of the channels were obtained. The numerical modeling results show good qualitative and quantitative agreement with the experimental data, with deviations not exceeding 8 %. It was also established that, in the case of significant expansion, a vortex flow can occur in the wide section of the channel. The vortex is not stationary, despite the low Reynolds number, but moves both left and right and up and down. This, when averaged, leads to differences in the velocity vector fields between the experimental and modeled results. The models and approaches used in this study were found to provide acceptable accuracy in reproducing flow structures. Due to the presence of vortex structures at significant expansion, which can be formed by fins with significant fin heights, low- and medium-expansion channels are recommended for use in heat exchange channels as they ensure a flow without separation.

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