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Psychrometric studies Henry, Herbert Clarke

Abstract

The local and macroscopic wet-bulb temperature depressions have been measured for naphthalene, p-dichlorobenzene and d-camphor particles sublimating into an air or helium gas stream. The organic chemicals were moulded into cylindrical, spherical and flat plate 'wet-bulbs'. A simple model based on the analogy between heat and mass transport postulates the dependence of the macroscopic psychrometric ratio of a cylinder in crossflow, a sphere or a flat plate, and the local psychrometric ratio of a plate, on the Lewis number of the system. A second model is developed which enables the calculation, by means of a Biot number correction, of the local psychrometric ratio of a cylinder or a sphere when heat is conducted internally through the body. This local psychrometric ratio can also be related to the system Lewis number. Two models are also developed to predict the 'limiting' wet-bulb temperature depression for a cylinder, sphere or flat plate under conditions of pure molecular diffusion of heat and mass. The measured local and macroscopic psychrometric ratios for all wet-bulb shapes studied were found to be independent of the gas temperature level, gas velocity, free stream turbulence intensity and integral scale of turbulence, in accordance with the models. For spheres and cylinders, the particle diameter did not affect the measured local and macroscopic psychrometric ratios. The experimental wet-bulb temperature depressions were found to vary linearly with the measuring thermocouple wire diameter to the three-halves power. The macroscopic psychrometric ratio, β₀, of spheres and cylinders has been correlated with the film Lewis No., Le[subscript f] , for eighteen gas-vapor systems by β₀= Le[subscript f] ⁻º‧⁵⁷. The author's data have extended the upper limit of Lewis number for correlation of macroscopic psychrometric ratio from 3.7 to 7.2. The author's measurements in helium indicate that the semiempirical correlation of Bedingfield and Drew is the best previous relationship for predicting the macroscopic psychrometric ratio for a cylinder. Under conditions of forced convection, the local psychrometric ratio, ββ, at the front stagnation point of spheres and cylinders can be correlated by ββ = Le[subscript f]⁻º‧⁶⁹ This result verifies the Chilton - Colburn analogy for attached laminar boundary layer flow. The local psychrometric ratio at the rear stagnation point of spheres and cylinders can be predicted by ββ = Le[subscript f]⁻º‧⁴⁶ when a wake exists behind the particle. This result is suggestive that a transport mechanism such as that proposed by Danckwerts may be controlling the transport processes in the wake. The local psychrometric ratios on a flat plate under conditions of laminar boundary layer flow are in close agreement with the Chilton - Colburn analogy. The intensity and longitudinal scale of turbulence down stream of screens in a four-inch square wind tunnel are presented.

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