Longitudinal vortex generation to enhance the performance of compact heat exchangers: Flow and heat transfer behavior for plain and interrupted fins.

摘要

Streamwise vortex generation as a method to enhance the thermal-hydraulic performance of compact heat exchangers is explored. Plain-fin-round-tube, offset-strip-fin-flat-tube, and louvered-fin-flat-tube heat exchangers are considered, with the goal of enhancing heat transfer for conditions typical to air-conditioning and refrigeration.; In an application of vortex generation (VG) to a plain-fin-and-tube heat exchanger, three winglet-array configurations are considered. Numerical results, assuming a constant tube-wall temperature, show the impingement of winglet-redirected flow on the tube surface an important heat transfer mechanism. The predictions indicate that at Re=850 an inline winglet array achieves enhancements up to 32% in total heat flux and 74% in j-factor over the baseline case, with an associated pressure-drop increase of about 41%. Wind tunnel experiments confirm the numerical results and support the promise of VG arrays as an enhancement strategy.; Numerical simulations of flow and heat transfer in an offset-strip fin array are compared to local convective mass transfer data from the literature, in order to validate the methods for highly interrupted fins. The modeling is then extended to solve for the three-dimensional, unsteady laminar flow and conjugate heat transfer in a louver array. The numerical results are further validated by comparing numerical predictions to surface-averaged heat transfer and core pressure drop data from the literature. Simulations are performed for Re≤1330 and disclose that streamwise vortices are convected through the louver gaps by the bulk flow. However, before entering a louver gap the vortex interacts with the boundary layer on the neighboring louver, leading to the generation of secondary flows, a reduced size of the longitudinal vortices in the louver gap, and enhanced mixing. The vortex-induced surface-normal flows and mixing in the louver wake region result in more than a 30% enhancement in heat transfer at Re=1330.; The numerical results are used to guide the first full-scale implementation of a VG-enhanced multi-louver heat exchanger. The heat transfer and pressure drop performance under both dry- and wet-surface conditions were determined through wind-tunnel experiments. Average heat transfer enhancements of 21% for dry conditions and 23% for wet conditions are achieved for 200≤ Re≤550, with a pressure drop penalty smaller than 7%.