Enhancement of subcooled flow boiling heat transfer on cylinders using interference sleeves

This paper describes a experimental study of subcooled flow boiling of water on a horizontal stainless steel tube with an interference sleeve (a sleeve with perforations). The flow of water is in the annular section between the sleeve surface and a outer glass tube. Six sleeves were used with different hole geometries (hole diameters and the pitch of holes) and the clearance between the sleeve and the boiling tube. Results show that sleeves with larger values of clearances and pitch needed lower temperature driving forces for a given heat flux, but with the disadvantage of an early transition to film boiling. However, interference sleeves with hole diameters of 1 mm and those with a combination of 1 mm and 2 mm holes on alternate rows required high heat fluxes for transition to film boiling. The level of inlet sub-cooling showed a marked effect of the two phase heat transfer coefficient. A subcooling of 20 K gave heat transfer coefficients that were two to three times lower than that obtained with a sub-cooling of 5 K. An increase in the inlet mass velocity of the liquid showed a significant increase in the wall superheat accompanied by a greatly increased heat flux required for transition to film boiling. A correlation has been proposed for the two phase heat transfer coefficient and the importance of the parameters in the correlation is discussed in terms of the physics of the process.