Physical meaning of GAUGE_HEAT_FLUX and INCIDENT HEAT FLUX

[CK Lam]

Dear All,

(1) GAUGE_HEAT_FLUX is the amount of energy that would be absorbed if
the surface were cold (or at some specific temperature). Then we
should only have positive value on this quantity, shouldn’t we?
Otherwise, if we have –ve GAUGE_HEAT_FLUX, then the meaning would be
“the amount of energy that would be EMITTED if the surface were cold
(or at some specific temperature).” I think it sounds rather strange…

(2) Moreover, I would like to ask what the physical meaning of
“INCIDENT_HEAT_FLUX” is. From the manual Section 12.3.5, it is stated
that for diagnostic purpose, it is sometimes convenient to output the
'INCIDENT_HEAT_FUX'.

I do not understand the difference between the GAUGE_HEAT_FLUX and
“INCIDENT_HEAT_FLUX”, and the physics when this “INCIDENT_HEAT_FLUX”
is negative.

Best Regards,
Lam

[dr_jfloyd]

GAUGE_HEAT_FLUX is the heat flux as measured to a fixed temperature
surface. If the surface temperature is warmer than the environment it
is exposed to then the heat flux will be negative.

INCIDENT_HEAT_FLUX is the incident radiative flux plus the convective
flux. So again, if the gauge is warmer than the surrounding this will
be negative. The difference between GAUGE and INCIDENT is that
incident does not account for radiation leaving the surface and that
the temperature used for the convective portion is the actual surface
temperature as opposed to a user defined gauge temperature.

[CK Lam]

Thanks for your clarification.

Moreover, I have a little question on how to measure the radiant heat
flux to a body. I wonder if I am correct.

METHOD 1: As I read through the discussion forum, “RADIANT_INTENSITY”
of the gas phase is to the integrated quantity over 2*pi solid angles
at a point. When we want to consider the radiant heat flux using the
slice data, we can take “radiant heat flux ~ RADIANT_INTENSITY / 4”,
provided that the radiation is uniform.

METHOD 2: An alternative method is to create an object, and we can
look into the “boundary” quantities, such as “HEAT_FLUX”,
“GAUGE_HEAT_FLUX”, or “INCIDENT_HEAT_FLUX” on the object surface we
are interested. Furthermore, if we want numerical outputs, instead of
only “visual” data, we can place a devc on the surface of the object
and set “IOR” to specify which direction the object surface is facing.

Am I correct?

Thanks,
Lam

On Dec 23, 10:20 pm, dr_jfloyd <drjfl...@gmail.com> wrote:
> GAUGE_HEAT_FLUX is the heat flux as measured to a fixed temperature
> surface.  If the surface temperature is warmer than the environment it
> is exposed to then the heat flux will be negative.
>
> INCIDENT_HEAT_FLUX is the incident radiative flux plus the convective
> flux.  So again, if the gauge is warmer than the surrounding this will
> be negative.  The difference between GAUGE and INCIDENT is that
> incident does not account for radiation leaving the surface and that
> the temperature used for the convective portion is the actual surface
> temperature as opposed to a user defined gauge temperature.
>
> On Dec 22, 9:48 pm, CK Lam <iamkitc...@gmail.com> wrote:
>
>
>
> > Dear All,
>
> > (1) GAUGE_HEAT_FLUX is the amount of energy that would be absorbed if
> > the surface were cold (or at some specific temperature). Then we
> > should only have positive value on this quantity, shouldn’t we?

> > Otherwise, if we have –ve GAUGE_HEAT_FLUX, then themeaningwould be

> > “the amount of energy that would be EMITTED if the surface were cold
> > (or at some specific temperature).” I think it sounds rather strange…
>
> > (2) Moreover, I would like to ask what thephysicalmeaningof
> > “INCIDENT_HEAT_FLUX” is. From the manual Section 12.3.5, it is stated
> > that for diagnostic purpose, it is sometimes convenient to output the
> > 'INCIDENT_HEAT_FUX'.
>
> > I do not understand the difference between the GAUGE_HEAT_FLUX and
> > “INCIDENT_HEAT_FLUX”, and the physics when this “INCIDENT_HEAT_FLUX”
> > is negative.
>
> > Best Regards,

> > Lam- Hide quoted text -
>
> - Show quoted text -

[dr_jfloyd]

INTEGRATED INTENSITY (RADIANT_INTENSITY in older versions) is the
integral of the radiant flux through a grid cell over 4 pi solid
angles, unless you anticipate an isotropic flux, this is probably not
the best method to obtain the hazard to a person.

One question to ask yourself is how do you plan on using this heat
flux? Are you using it as input to a skin burn model, are you using
it to compare to values on a plot of burn level vs. flux and time?
What you will do with the flux will drive what flux you need (i.e.
net, incident, etc).

METHOD2 will give you a flux over 2pi, but 1) only along the grid axes
and 2) it means having to put a solid OBST in the flowfield.

There is a third method. RADIATIVE HEAT FLUX GAS. It is a DEVC that
does not need to be attached to a surface that will integrate over 2
pi solid angles the incident radiative flux along a user defined
ORIENTATION.

[Nikolai Ortiz]

Hi.. I was looking for info about this INTEGRATED INTENSITY and HEAT FLUX discussion.

I see this is a old topic but as well I see the thing is not clear yet.

In my Case I get this nice Output Radiation IsoCurves from PHAST, for example 7 kW/m2 25 kW/m2 etc...

If I want to get this kind of info what output quantity must I use in FDS?

Must use GAUGE HEAT FLUX in the floor surface? (with this option I only can get the floor radiation, or I have to put many floating obstacles in the domain to get HRRPUA?

or by other hand , using  INTEGRATED INTENSITY in multiples elevations? 

My goal is to see the difference in the radiation curves between a model that don't take in to count obstacles in the radiation (PHAST) and another model with many kinds of  obstacles and floor forms (FDS).

pdta:

I really know that FDS models and PHAST models are different and the results must be different but still are good initials points to compare.

[dr_jfloyd]

Option 1)  Attempt to determine from the PHAST documentation exactly what PHAST is outputting (contacting the vendor for PHAST if needed) and pick the equivalent FDS output.

Option 2)  Setup the same simple problem in FDS and PHAST (something simple enough that PHAST is intended to do it well) and see what FDS output gives you the same results as PHAST.