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Old   August 1, 2005, 23:28
Default negative heat transfer coeffecient
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hello all, I am trying to simulate natural convection over a vertical plate, but i am getting a negative surface heat transfer coeffecient. why is this?
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Old   August 2, 2005, 00:26
Default Re: negative heat transfer coeffecient
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heat transfer coefficient will be calculated by using heat flux divided by the temperature diff which is equal to (Twall - Treference). if ur Treference is greater than Twall u get a negetive h. check for the reference values and see if ur plate is g etting heated up or is it getting cooled....
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Old   August 3, 2005, 21:57
Default Re: negative heat transfer coeffecient
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Thanx rajani,

Inspite of having my wall temperature greater than the reference temp value the heat tranfer coefficient values are negative.

I obained negative heat transfer coeffecient for pr = 0.72 and the temperatures being below 0 degrees celcius (wall temp = 257K (-16C), air temp = 247 (-26C))

however for temperatures in the region of 900K, where pr = 0.72 (from internet), I obtained positive heat transfer coeffecients, but some how my results dont appear to correlate properly with the theoritical results.

I need to know the exact thermal properties based on prandtl number for pr = 0.69 and pr = 0.72.

Unfortunately, i browsed the internet as well as many books but founds different answers.

Can any of you give the thermal properties of air at pr=0.72.I need air temp,desity, thermal conductivity,viscosity and specific heat properties.


Sireesha Pasari
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Old   August 3, 2005, 23:19
Default Re: negative heat transfer coeffecient
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try Perry's Chemical Engineers' Handbook. you should look at it carefully though. it is the properties that will decide Pr value and not vice versa and there are three of them involved in its definition. for all substances, you will find properties as a function of temperature.

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