Hot air of 200deg C is passed inside a box with certain mass flow rate and exits at same mass flow rate, it heats up content (metal object) in the box, what will be the heat dissipated through the walls, also heat flow inside the box is given.
You don´t give enough details to answer the question. Assuming that the box is in a room with external air with the temperature t and that the hot air at 200ºC passes through the inside, the equations to calculate the film coefficients of the walls of the box, h(Kcal/h-m2 -ºC), according to the book "Heat Transmission" of Alan J. Chapman, for plates are:
Vertical walls of the box: h = 1.22[(200 - t)/L]0.25
Bottom wall of the box: h = 1.13[(200 - t)/L]0.25
Upper wall of the box: h = 0.5[(200 - t)/L]0.25
L is the dimension (length or width) in m, of the wall.
If one wall, has the surface A, the dissipated heat of it, is q = Ah(200 - t)
There is convection as well as conduction heat transfer was takes place. for that you have to find heat coefficient and calculate the heat transfer rate using suitable properties of air and metal box thermal conductivity.
It all depends on the flow rate, the size of the metal object, the thermal conductivity of the metal object, the thermal conductivity of the box material, the thickness of the box material, the temperature of the outer surface of the box. With this information not provided, hard to say
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