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Why Eplus use CHTC=1000 W/mK for raining

asked 2018-10-02 09:32:33 -0500

Long's avatar

When I was doing Eplus simulation, I have found the outside surface CHTC values are set by Eplus to 1000 W/mK during the rain event. But there is not any explanation in the documentation. Can anybody give some comments on why Eplus set 1000 W/mK for CHTC during raining? Is there any supporting paper?

Thanks & regards Zheng Long

In Eplus documentation:

''if it is raining indicated by epw file, exterior surfaces exposed to wind are assumed to be wet. The convection coefficient is set to a very high number - 1000 and the outside temperature used for the surface will be the wet-bulb temperature."

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answered 2018-10-02 23:26:21 -0500

Wet surfaces that are evaporating freely are at wet bulb temperature. It is just a psychrometric characteristic. EnergyPlus set this as the outside boundary condition. As far as I know, the CHTC is not used in the calculations and it is just an index calculated for reporting purposes.

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Thanks for your comments. From my perspective, it is true wet surfaces are at wet bulb temperature. However, even if the surfaces are considered wet, it seems a big assumption that the CHTC values are 1000. I am very curious that the reason behind the value of 1000.

In terms of whether CHTC is involved in the calculation, my understanding is CHTC is used to calculate external surface heat balance — solar radiation + longwave radiation + convection - conduction = 0. As convection is involved in external surface heat balance, it seems CHTC is used to calculate convection heat transfer.

Long's avatar Long  ( 2018-10-03 02:45:06 -0500 )edit

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Asked: 2018-10-02 09:32:33 -0500

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Last updated: Oct 02 '18