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Inside Heat Transfer Coefficient - Radiation

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#1 Neelakandan

Neelakandan

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Posted 07 December 2018 - 01:15 AM

Hello All,

 

I happened to work on Heat transfer calculation recently and have a doubt on the heat transfer through radiation.

 

Considering an insulated wall/pipe having heat source from inside (gas), we know that the heat transfer happens from inside through convection+radiation then conduction through wall and finally convection+radiation outside to the environment. But when I went through the example calculations and spreadsheets I noticed that the radiative heat transfer co-efficient from inside is often neglected and only the convective heat transfer co-efficient considered.  However for the outside I can see both convection and radiation heat transfer co-effts considered in all cases.

 

Could some on throw some light on this. Thank you so much.

 

Regards

Neel

Mechanical Engineer



#2 Dazzler

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Posted 03 January 2019 - 11:18 PM

Hi Neel,

Reason is you probably have fast moving gas in pipe inside and so it is turbulent regime flow, well mixed, even temperature of the gas from centre of pipe to the wall.  Convective heat transfer and conductive heat transfer through the gas "film" at the inside wall would be the governing heat transfer methods or terms, and radiation insignificant.

Dazzler



#3 anand1194

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Posted 08 February 2019 - 05:51 AM

The reason for that might be the case that, convective heat transfer coefficient is much higher than radiation heat transfer. For a significant amount of heat transfer through radiation, temperature difference between gas and metal surface from inside should be ample. In case of fired heater flue gas temperature in radiation section is very high (1500 F and above), which allows heat transfer through radiation to become dominant. Also, for the radiation from gas, it depends on emissivity. The emissivity of diatomic gases such as oxygen, nitrogen etc, which forms huge part of flue gas are very poor. In flue gas carbon dioxide and water vapor have good emissivity and hence only they contribute in radiation. So its important you take a look at the components of the gas and whether they emit radiation in thermal range. Solid materials on the other hand has very high emissivity and that might be the reason for significant heat transfer through radiation from metal outside surface.

If you want to understand thoroughly, you can read "Process Heat Transfer" by D.Q. Kern.

 

Anand



#4 Neelakandan

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Posted 12 February 2019 - 06:57 AM

Thank you Dazzler and Anand for the response. I also did some readings in between and understood that the Radiation heat transfer predominantly depend on the temperature difference between surfaces (observe T^4 in the Radiation heat transfer formula). Mostly the temperature between the gas to the inside wall is same hence the radiation heat transfer makes insignificant impact on the overall heat transfer.

 

Thanks Anand for the information regarding the emissivity of flue gases.

 

Neel






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