Heat transfer to water droplets on a flat plate in the film boiling regime /
by Kenneth Joseph Baumeister.
Description
- Language(s)
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English
- Published
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1964.
- Summary
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The mass evaporation rates and overall heat transfer coefficients are determined both theoretically and experimentally for water droplets which are supported by their own superheated vapor over a flat hot plate. The theoretical and experimental mass evaporation rates are found to agree within 20 per cent over a droplet volume range of 0.05 cc to 1 cc and overa temperature range of 600 F to 1000 F. In this parameter range, the mass evaporation rate varies from 0.001 to 0.01 (g/sec), and the steam gap thickens and mass evaporation rate increase for increased volume and temperature. The overall heat transfer coefficient ranges between 70 (BTU/hr ft2 F) for 0.05 cc droplets and 40 for 1 cc droplets in the temperature range considered. Also, the theoretical analysis yields the axial and radial velocity distribution under the droplet and a velociety correction factor which is applied to Fourier's equation for one-dimensional steady state heat conduction across the steam gap. The water droplets are approximated by a flat spheroidal geometry with a uniform steam gap beneath the drop let and a saturated steam vapor cover on the top surface of the droplet. The shape of the droplet and the average droplet thickness are determined analytically. The analytical results compare favorable to experimental measures. The assumptions are made that the bottom of the spheroid is at the saturdation temperature and that the evaporation takes place uniformly beneath the spheroid. The flow is shown to have a Reynolds number of approximately 10; consequently, the flow is treated as incompressible and laminar with negligible energy dissipation. in addition, the constant fluid property assumption is made, and because of the large amount of time required for the evaporation of the droplet, the droplet at any instant is assumed to be in a pseudo steady state condition' that is, the flow is approximated by a steady state solution at any instant of time. The analytical method of attack is to solve the momentum, continuity, and energy equations simultaneously. The partial differential momentum and continuity equations are reduced to ordinary non-linear differential equations by the method of combination of variables. Possible solutions to the non-linear equations are mapped by means of an analytical computer. Then, these physically acceptable solutions are combined in a graphical manner with the solution of the macroscopic energy equation, which is solved explicitly, to yield the mass evaporation rate and steam gap thickness of the droplet as a function of droplet size, plate temperature, and gravitational potential. the effect of the gravitational potential on the mass evaporation rate is considered in detail in the theoretical development. a reduction in the gravitational potential from 1 (earth) to 0.16 (moon) is shown to reduce the mass evaporation rate by approximately half.
- Note
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Vita.
Manuscript copy.
- Physical Description
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xiv, 131 leaves :
ill. ;
28 cm.
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