Study of the cooling of a uniformly heated vertical tube by an ascending flow of air and a falling water film

An experimental study is reported of the cooling of an electrically heated tube by a flow of air ascending within it in the presence of a thin falling water film. This was produced by spraying jets onto the inside surface of the tube at the top and allowing the water to run down the inside surface. Experiments were performed for a range of values of power input with both n aturally induced and forced airflow to examine the influence of water injec tion temperature and water flow rate on the distribution of temperature alo ng the tube. The results obtained show that when the water is supplied at a temperature of about 20 degrees C, the dominant heat removal mechanism is convective cooling by the falling film. In contrast, when the water is supp lied at temperatures of 55 degrees C and above the dominant mechanism is ev aporation of water from the surface of the film. A semi-empirical model dev eloped for the purpose of aiding the interpretation of the experimental res ults reproduced observed behaviour very satisfactorily and enabled a clear picture to be obtained of the extent to which the various mechanisms of hea t transfer involved contribute to the removal of heat. (C) 1999 Elsevier Sc ience Inc. All rights reserved.