Small diameter, optimization, experimental validation
The literature has extensively demonstrated the benefits of tube diameter reduction for enhancing airside thermal performance, increasing overall compactness and potential refrigerant charge reduction in air-to-refrigerant heat exchangers. The first part of this work consisted of a numerical design optimization of an air-to-water heat exchanger using 3mm and 5mm diameter tubes with same face and surface areas. The purpose is to demonstrate that when all geometric characteristics are kept constant, the smaller tube diameter has always greater thermal-hydraulic performance. The results showed that for the same air pressure drop, the 3mm diameter tube heat exchanger has up to 15% more capacity, while for same capacity the 5mm diameter tube heat exchanger has more than 60% greater pressure drop. The two selected designs, in addition to a reference 3mm design with greater fin density, were prototyped and tested in a temperature and humidity-controlled wind-tunnel. The prototypes were not an exact representation of the designs since the 3mm had reduced fin density, while the 5mm had increased fin density. These changes gave advantage to the 5mm in terms of capacity however, the experimental results clearly illustrated the superior overall thermal-hydraulic performance of the 3mm design which, even with reduced surface area, resulted in 5-10% better thermal-hydraulic ratio. The numerical prediction deviated from test data on average by 2.8% on heat load, and 14% on airside pressure drop.