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Transcritical working cycles of refrigerants led to increased interest in heat transfer near the Critical State. In general, experimental results for this region differ significantly from those far from it because some fluid properties vary much more there than at a greater distance. In this paper, two-phase and single-phase free convective heat transfer to refrigerant R125 is discussed for fluid states very close to the Critical Point and far from it. In Part II, circumferential variation of the wall superheat is related to the motion of the fluid within the superheated boundary layer. In nucleate boiling, a minimum in the superheat develops on the lower parts of the wall that can be related to additional evaporation into the bubbles sliding upwards in close contact with the heated wall. In film boiling and supercritical free convection, a slight maximum superheat is discovered on the lower parts of the wall, the relative size of which increases close to the Critical Point, while it does not exist in far sub-critical free convection. |
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