Evaporation and heat transfer coefficient

[kcmharish]

Hai

I have a problem with heat transfer coefficient estimation. Please help

Here is the problem: Two experimental scenarios are compared.

Case 1: I have a well defined rectangular area in a silicon wafer. Water is completely spread over it. The mass of water is known. Air is blown over the liquid to enhance the evaporation rate. At steady state, the wet bulb temperature is reached. My calculated heat transfer coefficient is very similar to the one computed using the heat and mass transfer analogy.

Case 2: Now, the area is halved.But the mass of liquid is same. Am still getting the same mass transfer coefficient, but my heat transfer coefficeint twice in this case. This is clearly against the Chilton-Colburn heat and mass transfer analogy.

Only if i take the complete area of wafer, i am able to validate the heat and mass transfer analogy.

One possible reason, i can think of in this case is the lateral heat diffusion along the perimeter of the wafer so that the complete wafer is at uniform temperature during steady state. Is that a right reason?

Please help me with this.

 

[Aaron Barnard]

Yes, it is possible that lateral heat diffusion is affecting the heat transfer coefficient. This phenomenon is known as "thermal boundary layer" or "thermal creep". In this case, the heat transfer coefficient is affected by the ratio of the exposed surface area to the total surface area. If the exposed surface area is small compared to the total surface area, the heat flux is lower due to the thermal boundary layer effect.