The ideal CSTR model assumes that the fluid in the reactor is perfectly mixed, and that there are no concentration gradients inside the reactor. However, in a real-world stirred tank reactor, you won't have perfect mixing and there will be concentration gradients present. The baffles contribute additional disturbance to the flow created by the mixer, and provide more effective mixing. So, including the baffles brings you closer to the ideal of perfect mixing
1.Biot No. is considered only for heat transfer from solids whereas Nusselt No. is heat transfer to or from fluids.
2. Biot is (conductive resistance)/(convective resistance), whereas Nu is convective H.T. over conductive H.T.
COP of heat pump is always 1 more than that of refrigetator.
The main difference between both lies in their desired effects.
For refrigerator it is transfer of heat Q1 I.e. from refrigerator to outside whereas for heat pump it is to transfer Q2 amount of heat to hotter body or reservoir.