Heat Transfer Mechanisms of Propane Boiling on Horizontal Steel Tubes With Smooth and Enhanced Surfaces

The trend towards a better understanding of the fundamentals of nucleate boiling in re-entrant cavities is supported by the variation of the heating surface’s characteristics and the identification of parameters influencing the heat transfer at enhanced tubes. The optimized surface of enhanced evaporator tubes supports the bubble formation by providing stable nucleation sites, which are cavities that trapped the necessary amount of vapor to generate the next bubble. The optimal size of the cavities for bubble formation depends on various thermodynamic properties of the fluid and the wall material. The knowledge of these physical mechanisms is important for the further optimization. The influence of micro- and macrostructures on the overall heat transfer coefficient is investigated with the refrigerant R134a and the hydrocarbon propane (R290) boiling in a wide range of reduced pressures (p* = p_s/p_c = 0.03 to 0.5) and heat fluxes (0.05 to 100 kW/m²). The measurements are carried out using a standard apparatus and a horizontally positioned, electrically heated surface with various wall materials. Two different materials — copper and mild steel — with the same surface preparation by polishing are investigated. Furthermore, heat transfer measurements are carried out on a plain mild steel tube and on an industrially manufactured surface of the GEWA-PB type. The polished surfaces demonstrate a deterministic microstructure, the roughness parameters depends strongly on the measurement direction. The heat transfer coefficient as function of the heat flux of the polished copper tube can be described by the correlation of the VDI Heat Atlas, while the mild steel surface differ from former investigations due to the deep re-entrant cavities remaining from the drawn surface. The onset of boiling is nearly the same of both materials because of these cavities on the mild steel surface. As presented in the recent years, the heat transfer of nucleate boiling at tubes with subsurface channels can be divided into different domains, each influenced by different parameters like wettability, the product of vapor density and evaporation enthalpy. The identification of parameters influencing the bubble formation is done by heat transfer measurements, high-speed-video recording and photographic documentation. The experimental results of this work are compared to results of the polished surfaces. The heat transfer coefficient increases drastically for the enhanced tube, especially for beginning nucleation. The same α-q-relationship as on plain tubes is observed for higher pressures and heat fluxes but for three times higher values of the heat transfer coefficient α.