Calculating Jet Fires: Length, Angle, & Heat Flux

[neorich]

I am trying to calculate the effects of a Jet fire from a pipe rupture where the pipe contains flammable gas. I have come across a model by D.A. Carter which predicts the heat flux at a distance x from the fire. The problem that I have is that I want to also calculate the length of the flame and the angle of tilt of the flame due to any wind.

I have found some references to calculate flame length but they give different answers and some are only valid for vertical releases with no wind.

I see refeences to the API 521 standards and a calculation involving:

Length=0.00604*Q^0.4776 (taken from ChE forums website)...(1)
(Assuming Q in BTU/hr and Length in feet)
and another one given as:

Height = 0.0042*Q^0.478 again from ChE forums although credited to Milten R. Beychok in "Fundamentals of Stack Gas Dispersion." ...(2)
Note here Q is in BTU/hr and Height is in feet.

Another one was found again linked to API 521 as:

Length=0.00326*(Q)^0.478 ...(3)
(Vertical release with no wind)
Q is the mass flow rate * heat of combustion. The numbers I am working with are 43.7kg/s flow rate and 47710000J/kg.

Results:
Equation1 - 93m
Equation2 - 65m
Equation3 - 93m

It would seem 1 & 3 are simply for different unit systems.

The problem I have is that neither of these match the value given by Carter of 39m for flame length.

Can anyone please explain why this discrepancy exists (2.3x) and help me to rectify it? Also How can the tilt angle be calculated? If the above methods are the appropriate onesI should be using, how do I take account of the wind's effect on flame length? I have found the resources for this problem to be very sparse when related to jet fires. Help would be much appreciated.

Kind Regards

neorich

 

[Navin A S]

boyThe discrepancy in the flame lengths is likely due to the difference in the assumptions that these equations are based on. The equations you have listed (1, 2, and 3) are all based on the assumption of a vertical release with no wind, while Carter's equation takes into account the effects of wind. If you want to calculate the length of the flame and the angle of tilt of the flame due to any wind, then you will need to use Carter's equation. Carter's equation assumes that the flame length is proportional to the square root of the heat flux, and that the flame is tilted by an angle equal to the wind speed. To calculate the flame length and angle of tilt, you will need to know the wind speed at the location of the pipe rupture and the heat flux at the same location. Once you have this information, you can use Carter's equation to calculate the flame length and tilt angle.

 

[quicknote]

ard

Dear neorichard,

Thank you for reaching out with your question about calculating the effects of a jet fire. As a scientist, it is important to ensure accuracy and consistency in our calculations. I understand your frustration with the discrepancies in the equations you have found and I will do my best to provide some insight and potential solutions.

Firstly, it is important to note that different equations may be used for different scenarios and assumptions. The Carter model you mentioned may have been developed specifically for a certain type of jet fire, while the API 521 standard equations may apply to a broader range of scenarios. It is also possible that the equations you have found are based on different assumptions and therefore give different results.

To address your specific question about the discrepancy between the equations and the Carter model, it would be helpful to compare the assumptions and inputs used in each calculation. For example, the Carter model may have been developed for a specific gas composition and release rate, while the equations you found may have been based on different values. It is also possible that the Carter model takes into account wind effects, while the other equations do not.

In terms of calculating the angle of tilt of the flame, this may require more advanced calculations and modeling. Factors such as wind speed and direction, gas release rate, and terrain topography can all affect the tilt angle of a jet fire. It may be helpful to consult with experts in the field or use specialized software to accurately calculate this angle.

In conclusion, it is important to carefully consider the assumptions and inputs used in each equation to understand the discrepancies and choose the most appropriate one for your specific scenario. Additionally, seeking guidance from experts and utilizing specialized software may help in accurately calculating the angle of tilt and other parameters of a jet fire. I hope this helps and wish you the best of luck in your calculations.

Sincerely,