A test facility is described that has been constructed to investigate local heat transfer and pressure drop for evaporating or condensing refrigerants. The empirical method of B. Pierre [1] for correlating the average heat-transfer coefficients of refrigerants evaporating in horizontal tubes is presented in conjunction with the data of several authors [3–6]. Data on local heat-transfer coefficients and pressure drop are presented for Refrigerant-22 evaporating in two 4-ft-long, 0.343-in-ID straight horizontal tubes, and are correlated by a refinement of the curve proposed in [1]. The procedure of Martinelli-Nelson [9] correlated the data for local pressure drop within 15 per cent.
The free-convection heat-transfer coefficients were measured, as a function of spacing, for two parallel, electrically heated, vertical plates. The top of the rectangular space between the plates was left open, and during most of the tests the bottom and sides were closed. The heat input per unit area was substantially uniform, and the Grashof number, based on plate height, was of the order of 1010. The results demonstrate how the surface-temperature rise increases, or the local Nusselt number decreases, as either of the cross-section dimensions of the free-convection space is reduced. This decrease in Nusselt number is less rapid than was expected from purely two-dimensional flow considerations. This is ascribed to the effect of an asymmetrical flow pattern observed in the space. In some cases, a periodic reversal of this asymmetrical flow also was observed. When the space between plates was opened sufficiently at the bottom, the results were reasonably consistent with the correlation proposed by Jakob for a single vertical plate with a turbulent boundary layer; these results were almost independent of spacing between the heated surfaces down to the minimum spacing tested.
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