heat detectors in lower layer

[julia_f...@hotmail.com]

Hello,

i am working with CFAST version 7 for my masterthesis...i have some questions...

The Room hight is 5meter. in my lower layer are some heat detectors...they are located ervery 50 centimeters (so at 1,0m, 1,5m 2,0m, 2,5m 3,0m 3,5m) and their activation temperature is 50 degree. In the output-excel all of the detectors have the same values / graph. 

How is the calculation for the lower layer? why have all of the detectors the same value, i should know how is my temperature in a hight of 2,50m. How does it work?

Anybody an idea?

Thx Ju. Braun

[Richard Peacock]

I have two thoughts, a general one and a more specific one that I can't fully answer without looking at your actual test case.

In general, CFAST, like all zone models, represents the environment in a compartment with two temperatures, one for the relatively hot upper gases and one for the relatively cooler lower gases.  Thus all else being equal, all the detectors see the same lower layer temperature and thus react the same.

More specifically, CFAST does include algorithms to estimate the plume temperature when the location of a detector or target is closer to a fire location.  I've attached a sample CFAST output the shows the calculated local temperatures in a slice across the compartment. 

Far away from the fire in the middle of the compartment, the temperatures all take the value of the layer temperature.  So, depending on how big your fire is and how far the detectors are from the fire, they could see elevated temperatures or just the layer temperatures. To understand more, we'd have to see your individual CFAST input file.

Richard Peacock

[julia_f...@hotmail.com]

Hello 

thanks for the quick answer.

My test room is in the data file, so maybe you can look inside.

I would like to know when (point in time) my tmperature is rising up to 50 degree at a height of 2,50m, but not close to the fire location. In my test room are a lot of heat detectors at a height of 2,50n horizontal and vertically. 

I deduce from your answer that the programm CFSAT can´t resolve my problem? 

One thing is seldom realting to your answer....my lower layer temperature is not the same temperature of my heat detectors at a height of 2,50m? 

Do you have an idea what else i can try to get the information.

Thanks

Ju Braun

[Richard Peacock]

Hello 

thanks for the quick answer.

My test room is in the data file, so maybe you can look inside.

I would like to know when (point in time) my tmperature is rising up to 50 degree at a height of 2,50m, but not close to the fire location. In my test room are a lot of heat detectors at a height of 2,50n horizontal and vertically. 

I deduce from your answer that the programm CFSAT can´t resolve my problem? 

This depends on  number of variables including the fire size/location, compartment size, ventilation size/location, detector location, etc.  For the single compartment test case with open vents and no mechanical ventilation, a zone model is likely to do quite well in estimating the temperatures.  If the compartment is smaller with smaller vents and mechanical ventilation, the zone model assumptions of a distinct two-layer environment would be less appropriate for truly local conditions due to mixing from the mechanical ventilation.  If you truly need local conditions, you need to use a more complex CFD code such as FDS (http://fire.nist.gov/fds), for example.

 

One thing is seldom realting to your answer....my lower layer temperature is not the same temperature of my heat detectors at a height of 2,50m? 

For detector and targets, there are (at least) two temperature results provided, a local gas temperature and a link temperature (for detectors) or surface temperature (for targets).  The gas temperature should be the layer temperature for detectors/targets far removed from a fire (nearer the fire, the presence of the fire plume will increase the local gas temperature).  The link/surface temperature will be different since it is calculated by heat transfer from the surroundings. As the fire is growing, the link/surface temperature typically lags the gas temperature.