hi i am working on mixed convection and radiation through ducts by using flunet...it is a square duct(3d) all 4 walls r maintained at constant wall temp(1000K),inlet condition as velocity inlet and outlet as pressure outlet(atm)...for mixed covectin Gr/Re*2 =1,Gr = g*bita*(Twall-Tinlet)*D*3/(mu*2)...so i fixed D=5 cm,Tinlet=300K,Twall=1000K,g=-9.81....on this basis i can find Gr Number and then Reynolds no....from this reynolds no i can fix velocity at inlet...all properties set as constant expect density which is set as incompresble ideal gas...my problem is (1)what shall i put operating and backflow temp? (2)how to get nusselt no....when i did as plot-->wall fluxes-->surface nusselt number and then selecting all 4 walls iam getting so many values(clustered on graph), so i tried to draw one line on the walls and took nusselt no distribution...values seems to b right..am i right?? as iam taking incompresible ideal gas then what value fluent will take for coeffient of thermal expansion(Bita)?

after this when i included radiation then i don;t know wat values i hav to put for absorbtion and scattering coeffint..default i put zero(as it is air), bt for this case iam surface nusselt no values zero on the walls....even i tried by changing absorbtion coffeint values it did not work.....plzz can any one help me out?? please plzzzzzz frends.......iam really very thankful to all