Calculating the HRR with equating the HRR to the brandlast

[Katharina Michaelis]

Now I reached the point that it's my task to calculate the correct HRR according to the mobile brand last MJ. We decided that the brand last should be 1357200 MJ in the 1rst case, 2667600 in the 2nd case. The time in both cases is from 0 until 1800 seconds.

The area is 6700m^2.

So we have the equations:

Int (x) dx  = 1357200 MJ (0, 1800 s)

Int(x) dx = 2667600 MJ (0, 1800 s)

1800s*x = 137200 MJ is the 1st equation,

1800s*x = 2667600 MJ is the 2nd equation.

The solutions are 137200MJ / 1800s and 1800s / 2667600.

Is it correct that my result for the HRR is in kW?

[dr_jfloyd]

If the integral over time is giving y Joules and the time is x seconds, then dividing y by x gives the average value of the heat release rate. 

If for your speficic application, the appropriate fire scenario is a constant fire over time ,then that average heat release rate is the fire you would put into CFAST.

If for your specific application, the appropriate fire scenarios is time dependent (e.g. t^2), then the average heat release rate is not the same as the heat release rate you would put in CFAST.

What is an approrirate time dependence of the heat release rate is not something we can decide for you. This is something that should be based on the types fuel present or their storage arrangement, the end goal of the analysis, requirements imposed by local codes and standards, requirements imposed by stakeholders (building owner, building tenant, the authority having juridiction), etc.

[Katharina Michaelis]

Thank you,  so I guess for the average HRR I would need to take J, not MJ...

[dr_jfloyd]

If you divide MJ by s you get MW

If you divide kJ by s you get kW

If you divide J by s you get W

In each case you are dividing energy by time to get power.

What matters is having the correct units for the values you input into CFAST.  

[Katharina Michaelis]

Now I got HRR 209,44 kW in the 1st case, HRR is 411,67 kW in the 2nd case. Area is 6700m². How can I get the Heat of combustion with that data? Is there a formula? 

Thank you.

--
You received this message because you are subscribed to a topic in the Google Groups "CFAST" group.
To unsubscribe from this topic, visit https://groups.google.com/d/topic/cfast/FbViWCErM8s/unsubscribe.
To unsubscribe from this group and all its topics, send an email to cfast+un...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/cfast/698ead03-2158-4afa-9db2-fa4be0b4d0e3n%40googlegroups.com.

[Katharina Michaelis]

And the mobile brand last is 377MJ/m² and 741MJ/m². The HRR 209,44kW, 411,56kW, area 6700m². How to get a HoC with that data?

[dr_jfloyd]

That's something for you to decide based on the types of fuels involved in your fire scenario. Textbooks, handbooks, journal papers, etc. are all possible sources of information.

[Katharina Michaelis]

Can't I calculate it with the integral anyhow? We decided that this is what I should calculate. 

You received this message because you are subscribed to the Google Groups "CFAST" group.
To unsubscribe from this group and stop receiving emails from it, send an email to cfast+un...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/cfast/68a14212-c915-4e35-b5b9-60f1ce2e3e40n%40googlegroups.com.

[Katharina Michaelis]

Or should I take 50000 kJ/kg as heat of combustion as it is a default value?

[Katharina Michaelis]

Is the following correct: If the mobile fire load is 377 MJ/m² and the area 6700m². Then the integral of the heat release rate is 377*6700MJ? Is the result then in MW and I have to change to kW? Or is it already kW?

[Günter Becker]

Dear Katharina,

as I mentioned in an earlier email, heat of combustion is a material property. You find a number of values for various materials in wikipedia (https://en.wikipedia.org/wiki/Heat_of_combustion); the tables are starting approximately in the middle of that article.

So, I suggest, you first visit the compartment which you want to analyse using cfast. Bring a piece of paper and a pen to write down the amounts of combustible equipment. You might need a folding rule or a more sophisticated metering device; sometimes, you have to calculate the mass from the volume. When it comes to assess speed of fire propagation or alike, the geometry may be helpful; so possibly, take your mobile phone with you to take some photos of the combustible material. Also, there may be hidden material, e.g. lubricating oil inside a mechanical device.

In a later stage, fire alarms as installed, or fire fighting devices like sprinklers or alike may be important. Some fires get extinguished before they become interesting. Also, dampers or fire protection doors are of interest. So, to avoid a second visit or a third one, you may take an inventory of these components, too, once you are there.

When you have got this list, ask someone who has experience in fire protection, lest there be any open questions.

Best regards

Günter

To view this discussion on the web visit https://groups.google.com/d/msgid/cfast/CALS3Whab9%3D6Zxo_GT2-SE-21gMG1AJ%3DUYw7WSQq%3DJayUek8BHQ%40mail.gmail.com.

--
==============================================================
Dr.-Ing. Günter Becker
RISA Sicherheitsanalysen GmbH
Xantener Str. 11
10707 Berlin
Deutschland / Germany
Tel.: ++49 30 315706-0
FAX.: ++49 30 315706-21
Email: Guenter...@risa.de
Homepage: www.risa.de
Geschäftsführer Prof. Dr.-Ing. Leonidas Camarinopoulos
Eingetragen beim Amtsgericht Charlottenburg HRB 33234
ISO 9001:2015 zertifiziert durch den TÜV Rheinland