Part I. Effect of Shape on Evaporation of Drops of n-Heptane. Part ll. Thermal and Material Transport from Spheres into a Turbulently Flowing Air Stream. Part Ill. Temperature Gradients in Turbulent Gas Streams Recovery Factors in Steady, Uniform Flow

Citation

Sato, Kazuhiko (1955) Part I. Effect of Shape on Evaporation of Drops of n-Heptane. Part ll. Thermal and Material Transport from Spheres into a Turbulently Flowing Air Stream. Part Ill. Temperature Gradients in Turbulent Gas Streams Recovery Factors in Steady, Uniform Flow. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/QGB7-3769. https://resolver.caltech.edu/CaltechETD:etd-01212004-095601

Abstract

I. Measurements of the rate of evaporation of drops of varying shapes are required in order to determine the influence of such a variable on this important industrial transport process. Few data, if any, are available concerning the evaporation of drops of hydrocarbons. Measurements were made of the evaporation of drops of n-heptane at air velocities from 2 to 8 feet per second, corresponding to Reynolds numbers as high as 200. The transport rate was established for drops under approximately 30 different conditions, and for each drop the shape, surface area, and volume were determined in detail. The measurements were made at an air temperature of 100[degrees] F. and at a low level of turbulence, with negligible radiant transfer to the drop. The temperature of the evaporating drop was measured with small thermocouples. The results indicate a marked influence of the shape of the drop on its evaporating rate, and the Sherwood number was found to increase by as much as 30 percent for drops which deviate significantly from spherical forms. The present results, when reduced to the behavior of spheres, are in excellent agreement with the early measurements of Frossling and in fair agreement with those of Ranz and Marshall. II. An investigation was made on the effect of turbulence level on the thermal and material transport from spherical surfaces in a turbulently flowing air stream. A high level of turbulence was produced at the location of the test sphere by inserting a perforated plate in the flowing air stream. Measurements were made of the rate of heat flow from a polished silver sphere, 0.500 inch in diameter, at air velocities from 4 to 32 feet per second. The measurements were made at a bulk air temperature of 100[degrees] F. Measurements were made of the rate of evaporation of n-hexane and n-heptane from a porous sphere, 0.500 inch in diameter, at air velocities from 2 to 35 feet per second. The measurements were made at bulk air temperatures of 85[degrees] f., 100[degrees] F. and 115[degrees] F. The results indicate that the turbulence level in the flowing air stream has a marked influence on the magnitude of the transfer coefficients. It was found that, in an air stream flowing at a bulk velocity of 16 feet per second with a turbulence level of 13 per cent, the Nusselt number increased by 11 per cent over the value obtained in the undisturbed air stream. The Sherwood number was found to increase by 14 per cent under the same flow condition. The data obtained in this investigation were compared where possible with published information of similar nature. III. At high velocities the temperature variation across a flowing stream is appreciable under steady conditions for fluids having a high Prandtl number. A knowledge of such temperature distributions within steady, uniform streams is needed in certain thermal transport calculations. This information is not readily available in the literature. By utilizing available information concerning the dissipation of kinetic energy and the thermal transport in laminar turbulently flowing streams, expressions were derived for the temperature distribution in fluids flowing between infinite parallel plates and in circular conduits. The calculations included both adiabatic and isothermal conditions at the wall. These expressions differ somewhat from the relationships which have been derived for the temberature distribution in fluids flowing along a plate in an infinite stream under isobaric conditions. Recovery factors are directly proportional to the Prandtl number and not to the square root of the Prandtl number as found for the case of isobaric flow. Only the value of the Prandtl number is required in order to establish the temperature distribution under known conditions of laminar flow, whereas supplemental information is required in order to determine the behavior in turbulent flow.

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