INFLUENCE OF CRITICAL PHENOMENA ON THE HEAT TRANSFER FROM A FLAT PLATE
Free convective heat transfer to a mixture of ethane and n-heptane in the critical region was studied experimentally. The experimental apparatus consisted of a cylindrical high pressure cell with parallel heating and cooling plates inside. On the heated plate the heat transfer experiments were carried out. By controlling the flow rate of a cooling fluid through the cooling plate, bulk temperature and bulk pressure in the cell could be kept constant. The complete pressure cell could be turned by 90 degrees allowing the investigation of heat transfer from a vertical and from a horizontal plate to the fluid above. During the measurements the pressure was kept above the critical pressure, the bulk temperature below the critical temperature of the mixture. The observed heat transfer coefficients reached values of 2000 W/m2 for a temperature difference of around 5K for both the vertical and horizontal orientation although different convective patterns in the cell develop. For the vertical plate a recirculating velocity boundary layer along the plate, for the horizontal plate roll cells exist. With the onset of retrograde condensation the heat transfer coefficient increased twofold to values over 4000 W/m2K. For the horizontal plate this augmentation due to retrograde condensation occurs as long as the surface temperature is inside the retrograde region of the mixture. For the vertical plate an augmentation for all temperature differences occurs.