Answer Happy

a) An object, with characteristic length d and constant surface temperature To, is placed in a stream of air with velocity u, constant temperature Ta, density p, viscosity u, specific heat Cp and thermal conductivity k. If q is the heat flux between the object and the air, then the process can be described by the following dimensionless groups: Nu = f(Re, Pr) = where: hd Nu k Re = pud Pr ucp k > u and h is the heat transfer coefficient between the object and air, h = q AT with AT=T.-Ta What is the significance of each of the groups? [4] b) When an object with characteristic length 1m and constant surface temperature of 200°C was placed in an air stream with velocity 100 m s-1 and temperature 20°C, it was found that the heat flux was 15000 W m2. A geometrically similar object with characteristic length 2m is placed in an air stream. The temperatures of the object and the air are the same in both cases. Calculate the air velocity and the heat transfer coefficient, h, for the object with characteristic length of 2 m so that the two processes are physically similar. [13] c) What is the heat flux for the geometrically similar process with characteristic length 2 m ? [3]