Nusselt number in impingent jet experiment

Naren

I am trying to get a good mesh for impingement jet on semi-cylinderical plate with holes in it which simulates the effects of leading edge(318K) cooling by impingement jet(293K). There is a high temperature difference between just few cells near heated semicircular wall which is happening due to coarse mesh cells. My basic calculation in experiment is for Nusselt number on the bottom heated wall. It is coming too low. I have no idea why it is happening. Any ideas. Now introducing size function has controlled yplus on heated wall but there is no affect on Nusselt number which is supposed to be pretty high.