Thermal mixing enhancement of a free cooling/heating system with a 2D space-filling plate

Free-cooling/heating system has been incorporated with heating, ventilating and air-conditioning (HVAC) systems to provide low-cost cooling. The present work investigates numerically with the use of 2D space-filling plates to enhance the thermal mixing between the drawn in surrounding cooler air and the recycled warm air within a T-duct to strengthen the heat transfer at HVAC heat exchanger. Three inserts are used to generate turbulence at Reh = 2.19 × 104, i.e. positive square-fractal grid (PSFG), negative square-fractal grid (NSFG), and circular orifice (CO). Two-equation turbulence model is employed for turbulence kinetic energy κ predictions in insert induced thermal mixing. In particular, the effects of κ on space-filling geometry and tilted angle for each insert are discussed. Results show that for 45° tilted inserts, the thermal mixing performance of CO is about 2134%, 1382%, and 374% higher than the empty channel, PSFG, and NSFG, respectively, at x/H = 4.2. Tilted inserts thermal mixing are significantly better than the non-tilted cases. Plate tilting allows the production of larger scales of flow recirculation, of which accompany by higher flow fluctuations leeward from each insert. Therefore, with CO inserts being fully encompassed by the larger and wider scales of κ, the thermal mixing outperforms the space-filling fractal inserts.