I am trying to calculate heat transfer from an inclined flat plate under free convection conditions. I am referring to Fundamentals of heat and mass transfer (Incropera / DeWitt). The equations and a spreadsheet which can be used to find a solution can be found at: http://www.brighthubengineering.com...transfer-coefficient-estimation-calculations/ "Natural Convection Heat Transfer from an Inclined Plate" These calculations involve calculating the Rayleigh number and then using a correlation to find the Nussult number and thus the heat transfer coefficient. The problem is that i don't understand what happens when the plate is cooled (at lower temperature than the surrounding air). The plate i am trying to calculate this for is an evaporator. The because the temperature between the plate and air is negative this makes the Rayleigh number negative and i don't think this is correct. The correlation is then erroneous because you need to take Ra^(1/4), which clearly is incorrect for a negative number. If you make the surface temperature of the plate lower than the air in the example spreadsheet reference above it goes into error so the spreadsheet has not addressed the problem. I am presuming that the Rayleigh number is negative because the air is being cooled rather than heated, or related to that cooling is occurring. I am thinking I should just times the Rayleigh number by -1 to make it positive and calculate the heat transfer co-efficient from this but i have no idea if this is valid. The book i am referring to has a diagram of an inclined plate being cooled in the same chapter, so it would be strange if this cannot be made to work. My values are as follows: Air temp = -0.4 deg C, plate temp = -50 deg C, Tf = -25.2 deg C, g = 9.81cos60 = 4.905, β=1/(-25.2+273) = 0.00404, L^3 = 4.913, v = 1.132 * 10^-5 (m^2/s), α = 15.9*10^-6 (m^2/s), pr = 0.72. Any ideas?