Nucleate Pool Boiling of CaSO_4 Solutions on Enhanced Tubes

摘要

Finned tubes are widely utilized in refrigerant applications where the nucleate pool boiling is the dominant heat transfer mode. There are nevertheless potential applications for boiling processes at higher heat fluxes where the working fluids are aqueous solutions. Perhaps the most spectacular examples are pulp mills, reformer boilers, and desalination units. Nonetheless, the boiling characteristics of aqueous Solutions are different from those of distilled water due to the presence of inverse solubility salts in water. The present work aims at experimentally investigating the heat transfer coefficient during nucleate pool boiling of aqueous CaSO_4 Solutions on horizontal finned tubes. The test finned tubes are low finned tube type with various fins density of 19, 26 and 40 fins per inch. For the sake of comparison, the results will also be compared with piain tubes as well as distilled water. Two types of tube materials also were used copper-nickel alloy and copper as Substrate to investigate the effect of tube material in boiling characteristics. The test runs were carried out at atmospheric pressure for different heat fluxes ranging from 100 to 300 kW/m~2 at saturated CaSO_4 concentration of 1.6 g/L. To avoid fouling during boiling of CaSG_4, the clean heat transfer coefficient was measured within the first three minutes. A set of high speed video records were taken during the experiments to observe the bubbles behaviour during boiling of CaSO_4 and distilled water. Experimental results show that: 1) for saturated CaSO_4 concentrations, a significant reduction in heat transfer coefficient occurred compared with distilled water; 2) the higher the fin density is the lower reduction in heat transfer coefficients; 3) copper-nickel finned tubes showed lower reduction in heat transfer coefficient than copper finned tubes.