benard convection

samik

i am doing the simulation of benard convection in FLUENT. i am facing a problem and need a way out. i tried to plot the nusselt no variation with rayleigh no. to do that i first solved the energy equation only and found the total heat transfer rate ( report/fluxes/total heat tranfer rate /wall) . as expected it came exactly as a pure conduction heat tranfer rate. later i solved both flow and energy but contrary to expectation the heat transfer rate came much low. i expected it to be more than earlier as after rolls form Nu>1. i am consulting a book by E.L.Koschmieder which says for 1700<Ra<3000 , Nu~ 0.24 (Ra)^0.25. now i cant find a way to extract this data from fluent.now plotting surface nusselt no did not gave me the expected result( i got a periodic plot with the min value 30 and max value 110 for Ra 2500,aspect ratio 2 case).