Heat Transfer Dependence on Pressure
Heat Transfer Dependence on Pressure
(OP)
Hello,
Here is my situation:
A hollow multi layed concentric cylinder that is placed inside a vacuum. There is a heating element in the middle of the cylinder.
Temp initial is room temp and the pressure initial is atm.
Temp is increased to 350 degs C. Pressure is reduced to about 40 milliTorr.
I have calculated the Heat Transfer for atmospheric pressure but can not seem to find enough information about thermal conductivity of different materials at vacuum pressures.
Could anyone tell me an equation for thermal conductivity of materials at vacuum pressures.
Basically, im trying to figure out an ideal pressure for my system, that is still in a vacuum, and to determine the differences between heat transfer if 1) the heating element is placed on the inside of the cylinder or 2) at the outside around the cylinder.
Thanks for any responses,
DoubleEngineer
Basic Problem--
Here is my situation:
A hollow multi layed concentric cylinder that is placed inside a vacuum. There is a heating element in the middle of the cylinder.
Temp initial is room temp and the pressure initial is atm.
Temp is increased to 350 degs C. Pressure is reduced to about 40 milliTorr.
I have calculated the Heat Transfer for atmospheric pressure but can not seem to find enough information about thermal conductivity of different materials at vacuum pressures.
Could anyone tell me an equation for thermal conductivity of materials at vacuum pressures.
Basically, im trying to figure out an ideal pressure for my system, that is still in a vacuum, and to determine the differences between heat transfer if 1) the heating element is placed on the inside of the cylinder or 2) at the outside around the cylinder.
Thanks for any responses,
DoubleEngineer
Basic Problem--





RE: Heat Transfer Dependence on Pressure
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RE: Heat Transfer Dependence on Pressure
RE: Heat Transfer Dependence on Pressure
I was trying to determine if thermal energy transfered (mostly conduction) throughout my system is effected by pressue. Knowing that convective heat transfer is pressure dependent.
RE: Heat Transfer Dependence on Pressure
thread391-210603: Thermal conductivity of a "vacuum"
thread391-231539: Modeling thermal conductivity of a vacuum space
Also, just to be sure, this isn't a school problem, right?
Patricia Lougheed
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RE: Heat Transfer Dependence on Pressure
im trying to model the total heat transfer of all three transports for the system and how it changes with pressure.
RE: Heat Transfer Dependence on Pressure
corus
RE: Heat Transfer Dependence on Pressure
i do have miniscule gaps between layers that are very difficult to determine because of the multiple layers around the cylinder.
i have used grey body radition equations(and assumptions) to figure out the net transfer of a cylinder inside a rectangle.
the problem lies in the geometries that are inside the chamber. im not sure of an appropriate model or equation for the view factors of the radition
i believe i need a more accurate model for this but am having difficulties in deciding an appropriate model based on the equations and my designs/parameters.
is there a way to accurately test this? we have thermal couples placed inside the device that gives accurate readings, but im looking as of right now to understand more of the math behind it.
RE: Heat Transfer Dependence on Pressure
q(i) = E(bi) - J(i) / (1 - e(i) / e(i)*A(i)
and
q(i) =
sum of n surfaces with j=1 (J(i)-J(j)) / (A(i)*F(ij)^-1)
and that q= convection (till a certain pressure) + conduction(changes slighlty due to roughness and spacing with varying pressure) + radition (of mulitple surfaces)
RE: Heat Transfer Dependence on Pressure
Then, the heat flow within the object is strictly conduction. The heat flow out of the object into the chamber is convection plus radiation. AND, under ALL conditions the heat flow from the heater equals the heat flow leaving the object.
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RE: Heat Transfer Dependence on Pressure
heat flow thru object is conduction, thats obvious, but radiation is still involved because the heater is bouncing off heat from the walls of the chamber.
RE: Heat Transfer Dependence on Pressure
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RE: Heat Transfer Dependence on Pressure
http://eetd.lbl.gov/ECS/aerogels/sa-thermal.html
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RE: Heat Transfer Dependence on Pressure
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