PPD - Operative Temperature

Forum for DesignBuilder CFD users to exchange views and ask each other questions

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jean marais
Posts: 175

PPD - Operative Temperature

Postby jean marais » Thu Aug 22, 2013 6:22 pm

1) does DB calculate the radiative temperature at each cell concidering
a) view factor weighting to radiative wall, ceiling, floor, roof, window, (other?) surfaces?
2) Why do the PPD contours that I plot differ so much from when I read off the graph in ASHRAE 55 using the Operative Temps at same point and the Relative Humidity which is a constant input, even when the velocities are very low as same location? (Read of graph = 10%; PPD contour shows = 20%)
3) When will DB include the DR draft risk calculation?
Posts: 7

Postby kpx » Mon Sep 08, 2014 10:47 am

1) a) good question, I asked me the same thing. I've not yet found a clear answer, but I think probably the radiant temp shown on a slice is simply the cells face temps as resulting form CFD solutions, so not based on view-factors considerations, that
is as we usually intened. But thinking about CFD process is solved the resulting radiant temps are meaningful anyway and in some way related to 'view factor' because they depend on cells positions and their relative distance from the zone surfaces over wich boundary conditions (that are surf temps) are applied.
If anyone knows more about it, please reply
Steve Potter
Posts: 646
Location: newcastle

Postby Steve Potter » Fri Sep 26, 2014 9:04 am

Hi Jean,

DB CFD does incorporate a simplified method for deriving view factors for each cell from which the cell MRT is calculated. The method involves constructing a small sphere at the centre of each cell, the surface of which is divided into a number of contiguous segments. Rays are then cast from the cell centres through the centre of each associated sphere segment to identify intersection with the first 'visible' domain surface in order to derive a view factor.


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