Trench effect

[SamFIRT]

http://fire-dynamics.info/page4/page4.html

Trench effect was known about before the Kings Cross Fire as it was known that flame deflects towards the slope of a hill when crop fires etc bun up the slope.

Discuss.

[wee brian]

A hill isn't a trench.

[John Webb]

The size of the model used in the report linked to by SamFIRT is significantly smaller than the models used by the Fire Research Station and the HSE labs at Buxton when looking at the causes of the Kings Cross fire. I don't have the detailed reports from either establishment so can't make comments re the 'critical angle' in the linked report and how this compares with the findings from the earlier work.

There is a difference between an open hillside and a staircase - in the first case there is just the plain surface to which the fire spreads- besides the slope of the hill the density of vegetation and its dryness will be a factor. In the second case the confining nature of the walls strongly influences the speed of the development of the fire because of the nearness of the fuel.

What would be an interesting comparison between the 'trench' effect and outdoor 'wildfires' would be the case where a wildfire enters a deep ravine or gulch where fire spread might become more like the trench effect. I recall from reads long ago of American fire journals, which often looked at wildfires, that this aspect of wildfires was recognised but it takes a bit of a jump in scale and context to think in terms of a similar effect 'indoors'.

[CivvyFSO]

I think the report itself explains what I would have considered to be a driving factor, the entrainment of air. In the trench less air can come in from the sides, meaning there is less cooling of the plume, meaning a hotter fire, meaning more pyrolysis in the unburnt area, meaning a quicker progressing fire. Since watching Backdraft 10 times back to back in response to Sam's comment about not seeing the red belly of the beast or something, and now knowing that fire is a living breathing thing, I would also expect that the combustion/flame will progress forward more rapidly into the areas where more oxygen is available.

If this is true then it doesn't take much to imagine the tunnel scenario, with any incline ensuring that gravity helps the flames move upwards and across, and the distinct lack of entrainment will ensure a particularly hot fire, assisting the upwards movement even more, and the combustion moving forward rapidly towards the areas where more oxygen is available.

Or something else.

I don't know.

[SamFIRT]

Let us consider the airflow above the flame contained in the trench, how does that affect the flame height, propagation, HSG and direction of burn?

Oh and

Since watching Backdraft 10 times back to back in response to Sam's comment about not seeing the red belly of the beast

Nice to see you are catching up on training videos Civvy 

[BLEVE]

The point being? 

[SamFIRT]

A free and frank discussion to enhance learning... nothing more

I have heard that …

Trench effect was known about before the Kings Cross Fire as it was known that flame deflects towards the slope of a hill when crop fires etc bun up the slope.

...is being taught in a well known institution.

I don’t agree that it is the same effect for many of the reasons already posted.

I wondered what other people’s opinion was.

That is the point of a discussion forum…. is it not? 

[Clevelandfire 3]

I dont know to answer your question. Im not convinced the trench effect was known before the Kings X fire. I can understand the theory and why fire could effect anything above it on a slope of any kind whether in the open or enclosed. Just dunno.

[John Webb]

I don't think the original statement that "Trench effect was known about before the Kings Cross fire as it was known that flame deflects towards the slope of a hill when crop fires etc burn up the slope." is correct. The open air hillside phenomena almost certainly is caused only by the 'Coanda' effect where a moving fluid (gas or liquid) tends to 'stick' to a surface.

The 'Trench effect' includes not only the 'Coanda' effect but greater pre-heating of the fuel and other factors which are not present in the original example.

[BLEVE]

For once I agree, it is not the same effect. As others have stated a hill is not a trench.

[Tom Sutton]

Video of a laboratory experiment showing the trench effect, no walls, http://youtu.be/BJ6VSOkpDYs (read the comments below) plus another http://youtu.be/V8gcFX9jLGc. Not sure if you needed the experiment to prove the point.