Vortex-enhanced heat transfer by a new delta-winglet array

Liquid-to-air and phase-change heat exchanger performance is crucial to meet efficiency standards with low cost and environmental impact in numerous end-use energy applications. Research on performance improvement of heat exchangers often focuses on the air side because transport coefficients are inherently lower for air than for liquid or two-phase flow. Vortex generation has emerged as a promising method for enhancing air-side heat transfer. Compared to fin interruption, this technique has the advantage of low cost and ease of implementation, and is usually accompanied by a modest pressure drop penalty. In this study, a new vortex-generator array deployed in a “V” is proposed, aiming to utilize favorable interaction between generators and produce strong vortices even at low Reynolds numbers. A preliminary investigation of vortex strength in a water tunnel shows that boosting effects occur in a V-array which makes the design superior to an offset arrangement. The strongest vortex is measured for a two-row zero-spacing V-array deployed at 30º. The impact of V-formation arrays on heat transfer is assessed in a developing channel flow using infrared thermography. Over a Reynolds number range based on channel height of 340 to 940, the two-pair V-array yields the largest augmentation of 14-32% in heat transfer, as compared to 7-19% and 15-27% obtained from the conventional two-row pairs and the three-pair V-array, respectively. CFD analysis is also conducted to illustrate the evolving counter-rotating longitudinal vortices and to provide pressure drop data. Based on the heat transfer and pressure drop performance and taking other factors such as manufacturing cost and spatial constraints into consideration, the two-pair V-array deployed at 30º is recommended as an appropriate design for implementation in prototype plain-fin heat exchangers. The proposed design is finally examined in a prototype plain-fin-and-tube heat exchanger through full-scale wind-tunnel testing under dry surface conditions. Overall performance evaluation using the criteria of the modified area goodness factor and the volume goodness factor indicates superiority of the heat exchanger enhanced by the V-array. The average enhancement ratio of 1.21 in the modified area goodness factor could lead to a 25% reduction in the required heat transfer area. As a result, the heat exchanger can be manufactured with less material, which allows more compactness and reduced cost. The VG array is found more effective at comparatively low Reynolds numbers, representative of many HVAC&R applications and compact heat exchanger designs