HRR - Burner Surface Temperature

[oliver gresslehner]

When you define the parameter HRRPUA for a burner surface do you also need to define the surface temperature and if what effect do this have ?

Thanks in advance. 

[Kevin McGrattan]

That is up to you. Is the burning surface a liquid pool? A pile of charcoal briquettes? What effect does it have? Try it both ways and find out.

[oliver gresslehner]

Thanks for your fast reply. 

The burning surface is simply an obstacle without material properties. 

I just ran two simple test cases with different surface temperatures. 

HRRPUA = 1000 kW/m² (default)

Ramp-up-time= 1 s (default)

Case 1: Surface Temperature is set with default value (300°C)

Case 2: Surface Temperaature is set with 600°C

An optical comparison with temperature-slices looks very similiar. 

I also added THCP's above the obst. There I found small differences but not in the same size than initial differnce between both surface temperatures (600°C and 300°C). 

These results looks like a difference within random variables during the initialization of the simulation and not based on the different surface temperature. 

Am I wrong about that ?. 

Thanks additionally in advance and best regards from Austria

[dr_jfloyd]

This is a type of question where doing a simple hand calculation can provide valuable insights.  

Flame temperatures in the continuous flame region of fire are ~1000 C.  Assuming a blackbody for the flame volume (It isn't. Some grid cells will still see the cold ambient through the flame.) and and some reasonable upper bound for the flame convective heat transfer coefficient (say 50 W/m2/K), you can compute an upper bound for heat transfer from the fire to the object's surface.  This winds up being would be ~125 kW/m2 (for 600 C) or ~165 kW/m2 ( for 300 C).  This is a difference of 40 kW/m2 or 4 % of the 1000 kW/m2 HRRPUA of the fire.  This is a conservative upper bound. The actual amount is going to be much less. The flame on the sides of the obstacle is only a few grid cells thick at most and as a result the radiative component is going to be much less than that for a blackbody.  The same would apply for the cells along the top edge of the obstacle.  That 4 % is going to be more like 1 - 2 % at most. Would you expect to easily see a difference like that in the noisy temperature signal you get when trying to measure temperature? 

[oliver gresslehner]

Dear dr_jfloyd, 

Thanks for your reply and your explanation. 

Am I right that the surface temperature stands similiar to a flash point  temperature / ignition temperature ofthe potential fire load and the energy consumption between flames and heated up burning surface of the fire load ?. 

Thanks in advance. 

[dr_jfloyd]

The surface temperature of a burning object is whatever temperature balances the net heat flux at the surface with heat being conducted through the object and any energy required to pyrolyze or vaporize the fuel. 

For a gas burner or liquid spray burner, it will depend on the construction of the burner and the degree to which that construction will preheat the fuel before it leaves the burner. 

The flash point for a liquid fuel is the fuel temperature where the vapor pressure can result in a flammable mixture. The actual surface temperature for a burning pool of liquid is going to be somewhere between the flashpoint and the boiling point and will depend on the pool thickness, if it is a single spill of liquid or a continuous spill, and the substrate beneath the liquid.