Heat transfer and fouling characteristics during falling film evaporation in a vertical sintered tube

Heat transfer and fouling performance of water in a sintered porous stainless-steel tube were investigated experimentally. The test tube has a porous tube wall composed of stainless-steel powder with a typical diameter of 0.3 mu m, and the special wall was designed to improve the thermal performance during falling film evaporation by increasing the nucleation site densities and relieve the foul processing through vapor transmission. Heat transfer tests were implemented at a saturation temperature of 373.2 K, over a mass velocity range of 042-1.47 kg/ (m s), with a subcooled degree of 0.5 K at inlet, and the temperature, pressure and mass flux of steam fixed, while the fouling tests were carried out using high-rigidity water as work fluid at a fixed mass velocity of 0.84 kg/ (m s). Heat transfer coefficient of the sintered tube is about 30% lower than that of the plain tube, and the inferior effect of the porous structure is caused by the weak thermal conductivity steam filled in the wall void and possible vapor film formed in the inner surface of tube. There is a flatten relationship between the heat transfer coefficient of the plain and the mass velocity, while that of the porous tube increases as the mass flux increases. As for the fouling process, the fouling resistance for the porous tube seems to make little difference on the overall thermal performance of the test section, contrary to the heat transfer deterioration for the smooth tube during the fouling deposition process.