1st Nordic Drying Conference - NDC'01, Trondheim, Norway, June 27-29, 2001

INFLUENCE OF THERMAL RADIATION ON THE TOTAL HEAT TRANSFER COEFFICIENT AT HIGH IMPINGEMENT TEMPERATURES Pertti Heikkilä and Nenad Milosavljevic Metso Paper, Inc., Air Systems FIN-20240 Turku, Finland Keywords: impingement heat transfer, thermal radiation, tissue, paper drying, Yankee ABSTRACT The objective of this study is to present the experimental results of impingement heat transfer covering a large impingement air temperature range, from 100 to 700 C, and different nozzle surface emissivities. The main target of the work has been to measure impingement heat transfer in similar impingement conditions as used in real industrial dryers in the paper industry, because such high temperature data are not available in the literature. Most existing heat transfer data are based on measurements at very low temperatures. The heat transfer measurements were carried out on a laboratory-scale test rig. The main parts of the rig were a fan, a gas burner for air heating, a heavily insulated nozzle array with 300 x 500 mm impingement surface, a 40 mm thick and 300 x 500 mm sized aluminum plate for determination of heat transfer, and a data acquisition system. The heat transfer quantity was determined from the heating-up rate of the aluminum plate. The influence of thermal radiation on heat transfer was studied so that in one set of experiments the nozzle surface and the aluminum plate were bright metal surfaces, and in another set of experiments both surfaces were coated by a black color with emissivity close to 1. The experimental heat transfer coefficients were compared with the heat transfer coefficient obtained from the most common empirical correlation in the literature (Martin, 1977).

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