When applying more than one heat transfer enhancement technique, a comparison can be done by calculating the performance index. But there are many forms for it; sometimes it can be calculated as:
(h1/h2)/(f1/f2)^1/3
or
(Nu1/Nu2)/(f1/f2)^1/3
or
(h1/h2)/(f1/f2)
or
(h1/h2)/(dP1/dP2)
What is preferred and why?
I want to thank Prof. Dr. Abdulmajeed Mohamad
I asked him this question in private message, and He kindly answered as follows:
In general, heat transfer enhancement comes by the price of pressure drop (pumping power), "no free lunch".
The best way is to compare entropy generation for all the cases you have. As you know the best one with less entropy generation.
The reason I am suggesting that because entropy generation takes into account the heat transfer and pressure drop on the same bases.
As you mentioned, some people use the formula you suggested or j parameter. I think entropy generation has more physical meaning rather than combining heat transfer coefficient with friction factor or pressure difference.
If you look book "Heat Exchanger" by Shah and others. There is section on entropy generation calculations for heat exchanger...
So, great thanks Sir
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