What is the importance of thermal entry length in case of closed internal fluid flow loop?
There are many more fluid flow experiments for unusual geometries (e.g., entrance and exit regions) than ones including heat transfer. This means you are much more likely to find an empirical relationship for the pressure drop than the heat transfer coefficient. This is where Reynolds Analogy is so helpful: St=f/2 Double the pressure drop = double the friction factor = double the Stanton number.
This is the region of highest heat transfer coefficient and also, usually where the heat flux is highest. In the case of shell boilers this area is where failures are most common due to overheating or cyclic stresses. In tubular condensers this zone can be quite important too. In conventional (liquid / liquid) shell and tube heat exchangers the effect is not often significant.
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