Investigation of local heat transfer in compact heat exchangers by holographic interferometry

Abstract

Compact heat exchangers are key components for the development of future aircraft devices. An enhancement of the heat transfer results in a decrease in the heat exchanger size and thus in lower weight and lower investment costs. Exact knowledge of the temperature distribution in the boundary layer is necessary for a specific augmentation of heat transfer. Holographic interferometry was applied to visualize the temperature field. This optical measuring method offers the advantage of delivering information about the temperature distribution without disturbing the flow pattern. A digital image processing system was used for evaluation of the interferograms. The local Nusselt number was determined from the isotherms at the wall. Two types of geometries for compact plate heat exchangers were investigated using air as test fluid: plain fin arrangements of plate-fin heat exchangers and circular segment shaped turbulence promoters in plate heat exchangers. The test section was heated by hot water to obtain a constant wall temperature as a thermal boundary condition. During the experiments Re was varied between 500 and 3000, a range where low pressure losses occurred. Investigations of the plate-fin arrangements show that the Nusselt number for the geometry with 1-mm radii is about 15% higher than that for the duct with radii of 5 mm. In the case of the circular segment shaped turbulence promoters, a staggered, a nonstaggered, and an inclined arrangement were investigated. The overall Nusselt number shows that the nonstaggered ribs lead to the best heat transfer. To compare the results with the heat transfer in a flat duct, the Nusselt number distribution between parallel plates was calculated by equations taken from the literature. The present results show that an enhancement of heat transfer of 100% for low Reynolds numbers and approximately 300% for Re = 2500 can be achieved.