Surface temperature uniformity for single-phase developing laminar flow through horizontal tubes submerged in boiling water

Abstract

An experimental study was done to investigate the heat transfer characteristics of simultaneously hydrodynamically and thermally developing laminar flow of water through a horizontal tube, which was submerged in boiling water to keep its surface temperature uniform. A water bath with two compartments was used to house the water at saturation whose boiling was brought about by heater elements that covered the base area of the water bath. The inside and outside diameters of the test section were 11.2 mm and 12.5 mm, respectively, and the heated length was 1 m. Experiments were conducted between Reynolds numbers of 500 and 3 000 considering different heat input levels and inlet temperatures that varied between 20 °C and 80 °C. Although nucleate pool boiling was expected to provide a uniform surface temperature, thermocouple measurements on the test section showed that the wetted surface temperatures on the inside of the test section were not uniform when the flow was still thermally developing. The surface temperatures increased and approached a constant along the length of the test section. The effects of varying the Reynolds number, the tube inlet temperatures and the heat input into the system on the surface temperatures’ uniformity were investigated. In none of the tests taken were the surface temperatures sufficiently uniform although the degrees of surface temperature uniformity changed with the variation of these factors. For the first time, it has been established that the assumption of a uniform surface temperature during phase-change conditions is not valid for developing flow. From the several analyses done, it was concluded that the high heat transfer coefficients associated with developing flow significantly affected the surface temperatures’ uniformity as these were found to be dominant in the entrance region. Therefore, the general assumption of a uniform surface temperature boundary condition during phase-change conditions would only be valid for fully developed flow.