Multiphase Science and Technology

POOL BOILING HEAT TRANSFER ON HORIZONTAL STEEL TUBES WITH DIFFERENT DIAMETERS

ABSTRACT

Compared with the many experimental investigations on nucleate boiling heat transfer in the literature, the papers devoted to the influence of the tube diameter are scarce, and very often the variations of either tube diameter or saturation pressure or heat flux are limited, or other important features of the heating surface, like wall material or roughness, have not been kept constant. Therefore, former pool boiling experiments on horizontal steel tubes with diameters of 7.6 mm and 88.4 mm have been extended to diameters of 4 mm and 30 mm. The tubes were made of mild steel (St35.8 or St37.8), with similar inner structure and their surfaces were emeried in the same manner, resulting in similar values of the mean roughness parameters. Propane and n-Hexane were used as boiling liquids. The saturation pressure varied between approx. 1 and 80 per cent of the critical pressure and the heat flux between 0.1 and 100 kW/m².
The results show that the heat transfer coefficients α at constant normalized pressure p* = p_s /p_c (p_c = critical pressure) do not vary much with tube diameter at intermediate to high heat fluxes; only at small heat fluxes near beginning nucleation, the differences in a may amount up to approx. ±25%, particularly at intermediate pressures (0.4 < p* < 0.04). The relative increase of the heat transfer coefficient with pressure or heat flux, respectively, is somewhat more pronounced for the small tubes than for the bigger ones. Both tendencies can be explained as effects of the bubbles which are produced on the lower parts of the tube and slide a certain distance along the tube surface on their way upwards. As a whole, the influence of the tube diameter found in these experiments is significantly smaller than according to correlations in the literature.