Appa
Jan2-12, 11:52 AM
I was searching this forum for a post like mine but couldn't find anything suitable. The problem is as follows:
There is an electronics cabinet with a double wall, and between the walls there is an air-flow that cools down the cabinet. The walls are isothermal and at different temperatures, both temperatures being higher than the ambient one. What I had to do first was to find the heat flow to the fluid when the temperatures were given, and I was able to do that quite easily with a few correlations that I found in a book called "Handbook of heat transfer fundamentals".
Nu=\frac{qs}{2HW(T_{w}-T_{∞})k}
where T_{w} = \frac{1}{2}(T_{1}+T_{2}), s the distance between the plates, H the hight of the plates and W their width. q is the heat thransfer to the fluid from both plates.
Nu_{fd}=\frac{4T*^{2}+7T*+4}{90(1+T*)^{2}}Ra
where Ra=\frac{gβ(T_{w}-T_{∞})s^{3}}{\nu \alpha}\frac{s}{H} and T*= \frac{T_{2}-T_{∞}}{T_{1}-T_{∞}}, T_{1}≥T_{2}
Nu= [(Nu_{fd})^{m}+(cRa^{1/4})^{m}]^{1/m}
I know all the constant values and can thus find q. Now to the problem itself:
After having found q I should find how much energy flows through wall no 2, if there is a constant heat flux from the environment toward wall 1, called q_{1}.
This I need to know in order to find the wall temperatures in another phase of the problem. I know I will have to iterate the temperatures but I am having trouble understanding the physics of this whole problem.
So, What I know is the Nusselt number and the h (from h=\frac{k}{s}Nu) but how can I find the temperatures? What I don't understand is whether the h I already found is only the convection heat transfer from both walls to the fluid or if I can use it in the expression q=hA(T_1-T_2), or the heat flow from wall 1 to wall 2, as well. If this is so, I guess I should only find suitable energy balances for both points and then iterate woth some initial temperatures.
Could someone please help me with this? I am a bit stuck as I don't understand the physics of what I am doing quite clearly. Cheers.
There is an electronics cabinet with a double wall, and between the walls there is an air-flow that cools down the cabinet. The walls are isothermal and at different temperatures, both temperatures being higher than the ambient one. What I had to do first was to find the heat flow to the fluid when the temperatures were given, and I was able to do that quite easily with a few correlations that I found in a book called "Handbook of heat transfer fundamentals".
Nu=\frac{qs}{2HW(T_{w}-T_{∞})k}
where T_{w} = \frac{1}{2}(T_{1}+T_{2}), s the distance between the plates, H the hight of the plates and W their width. q is the heat thransfer to the fluid from both plates.
Nu_{fd}=\frac{4T*^{2}+7T*+4}{90(1+T*)^{2}}Ra
where Ra=\frac{gβ(T_{w}-T_{∞})s^{3}}{\nu \alpha}\frac{s}{H} and T*= \frac{T_{2}-T_{∞}}{T_{1}-T_{∞}}, T_{1}≥T_{2}
Nu= [(Nu_{fd})^{m}+(cRa^{1/4})^{m}]^{1/m}
I know all the constant values and can thus find q. Now to the problem itself:
After having found q I should find how much energy flows through wall no 2, if there is a constant heat flux from the environment toward wall 1, called q_{1}.
This I need to know in order to find the wall temperatures in another phase of the problem. I know I will have to iterate the temperatures but I am having trouble understanding the physics of this whole problem.
So, What I know is the Nusselt number and the h (from h=\frac{k}{s}Nu) but how can I find the temperatures? What I don't understand is whether the h I already found is only the convection heat transfer from both walls to the fluid or if I can use it in the expression q=hA(T_1-T_2), or the heat flow from wall 1 to wall 2, as well. If this is so, I guess I should only find suitable energy balances for both points and then iterate woth some initial temperatures.
Could someone please help me with this? I am a bit stuck as I don't understand the physics of what I am doing quite clearly. Cheers.