INVESTIGATION OF LOCAL HEAT-TRANSFER IN COMPACT HEAT-EXCHANGERS BY HOLOGRAPHIC-INTERFEROMETRY

Compact heat exchangers are key components for the development of futu re 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 tr ansfer. Holographic interferometry was applied to visualize the temper ature field. This optical measuring method offers the advantage of del ivering information about the temperature distribution without disturb ing the flow pattern. A digital image processing system was used far e valuation of the interferograms. The local Nusselt number was determin ed from the isotherms at the wall. Two types of geometries for compact plate heat exchangers were investigated using air as test fluid: plai n fin arrangements of plate-fin heat exchangers and circular segment s haped turbulence promoters in plate heat exchangers. The test section was heated by hot water to obtain a constant wall temperature as a the rmal boundary condition. During the experiments Re was varied between 500 and 3000, a range where low pressure losses occurred. Investigatio ns 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 wi th radii of 5 mm. In the case of the circular segment shaped turbulenc e promoters, a staggered, a nonstaggered, and an inclined arrangement were investigated. The overall Nusselt number shows that the nonstagge red ribs lead to the best heat transfer. To compare the results with t he heat transfer in a flat duct, the Nusselt number distribution betwe en parallel plates was calculated by equations taken from the literatu re. The present results show that an enhancement of heat transfer of 1 00% for low Reynolds numbers and approximately 300% for Re = 2500 can be achieved.