Nucleate boiling and critical heat flux of HFE-7100 in horizontal narrow spaces

The pool boiling heat transfer and critical heat flux CHF of saturated HFE-7100 at atmospheric pressure on a confined smooth copper surface were experimentally studied. The horizontal upward boiling surface was confined by a face-to-face parallel unheated surface. We analysed the effects obtained by changing the diameter of the unheated surface and the gap between the boiling surface and the adiabatic surface. The gap values investigated were s= 0.5, 1.0, 2.0, 3.5. mm. To confine the circular boiling surface (d= 30. mm), two different Plexiglas discs were used: one with a diameter D= 30. mm, equal to that of the copper boiling surface, and the other with a diameter D= 60. mm, equal to that of the overall test section support. For each configuration, boiling curves were obtained up to the thermal crisis. For both configurations, it was observed that, at low wall superheat, the effect of confinement was not significant if Bo> 1, while for Bo≤ 1 the heat transfer coefficient increased as the channel width s decreased. By contrast, at high wall superheat, a drastic reduction in both heat transfer and CHF was seen when the channel width s decreased; this reduction was less pronounced when the smaller confinement disc (D= 30. mm) was used. CHF data were also compared with the values predicted by literature correlations.