In designing heat exchangers, thermal engineers rely on research into the fundamentals of heat exchange and the correlations that are developed from experimentation and analysis. Heat exchangers are widely used in thermal power generation facilities, integrated heating and cooling systems, and process plants. Heat exchangers in which phase-change (boiling and condensation) occurs are critical components of the thermodynamic cycles of many clean energy facilities such as concentrated solar thermal power plants, where steam is used to drive turbines during electrical power generation. This ground-breaking experimental study was conducted 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 apply a uniform surface temperature boundary condition.

In the past, it was generally assumed that boiling or phase-change conditions provided a uniform surface temperature boundary condition that would equal to the saturation temperature. However, the temperature measurements indicated that the surface temperatures on the inside of the test section increased along the tube length before approaching a constant temperature. The effects of Reynolds number, the inlet fluid temperatures and heat input rate on the surface temperatures’ uniformity were investigated through further experimental and numerical analyses. It was found out that the degree of surface temperature nonuniformity was affected by these changes, but a uniform temperature along the entire tube length could not obtained. Ultimately it was concluded that the high heat transfer coefficients associated with developing flow significantly affected the surface temperature uniformity as these were found to be dominant in the entrance region. 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 and would only be valid for fully developed flow.

This research was conducted by Rudaviro Munzara, under the supervision of Dr Marilize Everts, and was jointly funded by the DAAD Germany Academic Exchange Programme and the Clean Energy Research Group at the University of Pretoria.  Rudaviro graduated with an MSc. in Applied Science Mechanics and is registered as a Candidate Mechanical Engineer with the Engineering Council of South Africa (ECSA).  As a proud UP Alumni, he is actively involved in renewable energies, such as the design and buildout of commercial and industrial scale Solar PV and CSP plants around South Africa.