We studied the boiling heat transfer in a small smooth channel, to increase the heat transfer coefficient , we have insert a porous medium (metallic foams), but the results for metallic foams showed an increase in the saturation temperature difference (that is mean a decrease in the heat transfer coefficient) and an increase in the diphasic length.
How can we interpret this phenomena?
Heat transfer surface and method for producing such surface
US 4246057 A
ABSTRACT
The invention relates to improving the heat transfer properties in boiling liquids of tubes and other surfaces by applying a very porous reticulated organic foam layer in contact with the tube surface, and then plating a thin metal coating on the foam substrate. Preferably, the tube is copper and the plating consists of a first electrolessly applied layer of copper and a second electroplated layer. The foam is preferably in the form of a thin tape which is spirally wound about the tube. Pyrolization of the foam after plating improves the heat transfer performance of the tube.
Boiling produces large scale mixing in the channel hence the temperature gradient across the channel remains low. The porous foam reduces large scale mixing and heat transfer across the channel occurs almost exclsively through the liquid conductivity and foam conductivity. Probably the foam equivalent conductivity, which is solid material conductivity x (relative foam density)/3 is too low to compensate for the large scale mixing.
Thank you Daniel Alecu
another study is current to determine the effective conductivity of the foam.
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