Crystallization fouling of turbo-structured tubes during nucleate pool boiling

Abstract

The superiority of enhanced boiling on structured heat transfer surfaces over plain surfaces is based on excessive bubble generation and behavior as well as the latent heat of evaporation. The generated bubbles with turbulence of large scale would give rise to profoundly higher heat transfer coefficients. Meanwhile, the very same reasons of bubble formation and behavior may lead to accelerate the fouling of these surfaces for aqueous solutions with inverse solubility. The present study characterizes the fouling behaviour of turbo structured tubes under nucleate pool boiling. Several experiments were conducted for heat fluxes ranging from 50 to 250 kW/m2 and a concentration of 1.6 g/L of CaSO4 aqueous solution. The investigated structured tubes were re-entrant cavity tubes referred to as the Turbo-B type which were compared with plain tubes and low finned tubes of different fin heights and densities. Of these investigated tubes, the Turbo-B showed preferable anti-fouling behaviour. Based on a comparison of anti-fouling performance of the test tubes, the Turbo-B reduced fouling resistance up to ∼27 % and ∼83 % than the finned test tubes of 19 fpi (fins per inch) and 40 fpi, respectively. Most interestingly, the deposition of turbo tubes was of powdery and frail structure which could easily be cleaned. Moreover, the deposit was only formed on top of these tubes while no sign of deposition was observed within the cavities. The deposit structure for plain tubes, on the contrary, was a hard, thick and homogeneous layer under similar operating conditions as those for the turbo tubes.