Influence of wall material on nucleate pool boiling of liquid nitrogen

Kurzfassung

The wall material thermal conductivity k is one of the surface characteristics that influences the pool boiling heat transfer coefficient (HTC). It is believed that growth rates of vapor bubbles and their frequency are mechanisms affected by the thermal conductivity. In order to investigate this effect, different surface materials (copper, aluminum, stainless steel) are tested in a horizontal surface geometry. From a physical point of view, the redistribution of local heat fluxes between the free surface (wet by the liquid) and the spot covered with nucleation centers is hindered in the case of low values of k. But the wall material k affects the nucleate boiling curve only in the case of a very limited number of nucleation sites. Previous experiments in fact concluded that for a rough surface that has a larger number of vapor generating centers, the wall thermal conductivity does not significantly affect the boiling HTC; therefore the tested surfaces are mechanically polished to avoid any roughness effect. In this paper, nucleate pool boiling curves of saturated nitrogen under ambient pressure are experimentally obtained on a horizontal flat surface and the effect of three different wall materials is evaluated. Hysteresis behaviour has been observed for the copper and aluminum surfaces, while the results for stainless steel are contradictory. The critical heat flux (CHF) is also compared and analyzed by taking into account the thermal effusivity of the different materials. Finally, experimental results are compared against the most common empirical correlations for nucleate pool boiling heat transfer.