View-factor and emitter shielding

[bluegem]

Ive been having discussions with a colleague of mine about the most accurate method of calculating a view-factor when there is an object between the receiver and emitter. I was hoping someone could provide some advice on the matter.

Basically I am trying to model the radiant heat flux of a wildfire on a particular point on a house. Assuming that the wildfire is a 100m wide by 10m high box, I am able to calculate the view-factor however, when an object is placed between the the emitter and receiver, it is hard to calculate the effects it will have on the view-factor.

Assuming the object is shielding 100% of its size of the emitter, there are two methods that i can think of however both produce far different results. Which do you think is more accurate?

So, let's say we have two surfaces. the one on the left is radiating heat and the one on the left is purely receiving (see 1).

Lets now assume that a barrier is installed (red line) in front of the emitter. This barrier is the same width as the emitter. (see 2)

From here we can take two paths.

a) calculate the view-factor for the portion that is shielded and then minus it from the original view-factor.

b) minus the portion of the emitter that is shielded and recalculate the view-factor.

Im at a loss why they produce vastly different results... Any ideas?

http://img854.imageshack.us/img854/1181/viewfactor.jpg

 

[Valerie Park]

I would say that method (a) is more accurate in this case, since it takes into account the fact that the barrier is shielding a portion of the emitter. In method (b), you are essentially assuming that the barrier is not there at all, and so the view-factor will be calculated as if there were no barrier. This could lead to highly inaccurate results.