SE-2408 (RP 155) -- Heat, Mass and Momentum Transfer in a Parallel Plate Dehumidifying Exchanger PDF

The process of combined heat, mass, and momentum transfer in a mixture of water vapor and air flowing over a cold surface where the velocity, temperature, and vapor concentration profiles all under simultaneous development was investigated.

When condensation occurs, droplets or a film of condensate collects on the surface. Practically all work in the area of pure vapor condensation pertains to laminar film condensation, while applications in dehumidification seem to involve a combination of both drops and film or a wavy film. The presence of the water on the surface does affect the transfer phenomena, and the rate at which water is deposited may be a factor. To circumvent the unknowns, the industry depends almost entirely on test data from full scale heat transfer surfaces operating under actual dehumidifying conditions. Data obtained in this way generally obey the t1j_factor " analogy. However, when applied to relate dry surface heat transfer coefficients (or j-factors) to wet surface heat and mass transfer coefficients (or j-factors), the j-factor analogy is not always valid. The present state-of-the-art does not afford a clear relationship between sensible heat transfer coefficients and combined heat and mass transfer coefficients; and it is not known when the analogy is true and when it is not. Work discussed herein had the basic objective of discovering as much as possible about basic mechanism of combined heat and mass transfer. A simple parallel plate surface used so that some variables could be eliminated and so that flow conditions could be variable and controllable.

At the outset it was thought that free stream turbulence intensity was an important variable connection with the water layer on the heat exchange surface and that the water on the surwas probably in the form of a wavy like film.

The results of the study indicate that the quantity of water on the surface is important; however free stream turbulence intensity does not appear to be an important factor. Results the visualization study showed that the condensation was dropwise on aluminum, constantan, copper over a Reynolds number range of 600 to 4000.