In April of this year, the Queensland division of the Australia Institute of Refrigeration, Air-conditioning and Heating (AIRAH) held an education session on the design and compliance of car park ventilation using jet fans.

Now even though this seems to be a very specific example of a performance solution, this case highlights an aspect of achieving compliance with performance-based solutions that needs further discussion and direction from the Australian Building Codes Board (ABCB) to ensure a harmonised approach Australia-wide.

The car park ventilation system using jet fans may impact on the life safety systems for fire suppression and smoke control. These critical fire safety systems may be adversely affected by the ventilation system, and hence a detailed assessment of the ventilation system design is required to meet the performance requirements of the NCC. The NCC performance requirements are EP1.4 and EP2.2.

While there are professional designers with the capabilities to develop these performance solutions, it is difficult for inexperienced individuals without a specialty in this area to assess that a design complies with the requirements. The guidelines provided by Fire & Rescue New South Wales (FRNSW) are a step in the right direction.

The guidance within the NCC is insufficient and therefore the FRNSW government department has provided a Fire Safety Guideline – Guidance for impulse fans in car parks V1 – Oct 2014 to assist with providing a more consistent approach to the design and compliance in accordance with the performance provisions. The concern with the use of jet fans is the impact on the deactivation of the jet fan, and the impact of the jet fan airstream on fire system sprinkler heads.

While the FRNSW guidelines are a good document, they are not part of the NCC, or referenced within the NCC. To date, only FRNSW have developed a set of guidelines, and whilst other states such as Queensland are looking at adopting the FRNSW guidelines, they are going through their own process of assessing the requirements and understanding the impacts of this performance solution. This results in a significant delay, increased costs and uncertainty for building developers and designers.

This is important because, in some instances, the jet fans can provide an acceptable solution at a much reduced cost and reduced impact on structural and architectural car park design when compared to a traditional design. The assessment of the jet fan solution is currently an industry-driven process which is typically accepted by the building certifier/surveyor once sufficient evidence has been provided by the designer. This highlights other concerns with the CFD method used for verification of the performance of the physics of these complex engineering designs.

Looking in more detail at CFD analysis, we find it can be a highly inaccurate method of assessment. Computational fluid dynamics (CFD) is the use of applied mathematics, physics and computational software to visualize how a gas or liquid flows, as well as how the gas or liquid affects objects as it flows past. Computational fluid dynamics is based on the Navier-Stokes equations.

When using CFD, make sure that an appropriate method is used for the particular configuration you are investigating. Used properly, modern CFD solvers (using a wide variety of methods) can yield remarkably accurate predictions. The more you know about the strengths and limitations about the computational fluid dynamics modelling methods at your disposal, the better your ability to get good results. The use of CFD should be restricted to highly skilled individuals with in-depth knowledge of not only the mathematics, but also the real world verification of the CFD analysis.

This raises two questions:

  • How does anyone outside the specialised engineering field of CFD analysis assess whether the CFD analysis has been applied appropriately to the situation?
  • How do you ensure the person doing the CFD analysis has the required level of understanding of the software, the mathematics and practical applications of CFD to the solution represented?

After all, fire safety systems are lifesaving systems, and while cheaper alternative solutions for carpark ventilation are available and may be implemented, the verification of their performance impacts on fire safety systems is still, I think, unclear.

  • Hi Ken,
    Thank you for the well written article. You raise some very good points that I am sure are expressed by others in the industry.

    The fire engineering discipline has been performing CFD for analysis on fires for 20-30 years. CFD analysis is accepted by almost all approval authorities worldwide in all engineering disciplines. In fact CFD is accepted for much higher risk fire scenarios than would be expected by using jet fans.

    While there is always a variance in knowledge throughout different engineering practitioners, there is a substantial amount of empirical evidence from on-site testing of jet fans in car parks that is available to validate CFD analysis for this scenario.

    One could argue that the amount of physical testing of jet fans in car parks has already shown the safety of these systems. In reality most jet fan car park installations are similar and have produced comparable results to date in real-life tests. However CFD analysis is a good safety net for the time being until the industry gains more experience in these systems.

    • Ken, that's a great "heads up". Indeed the AIRAH Queensland presentation was a great example of why more people should make sure and get to these.

      Of real interest, and as you mentioned – there are life safety aspects, and the Fire / Rescue services take them very seriously. Interestingly there are a number of conditions on the application – eg positions in aisles, local smoke sensing / shutdown and more. One must be AWARE of these things .. it is not something we would want to find out after an event.

      THanks for the link and keep on writing and spreading good info!

  • Hi Ken, with respect to the jet fan issue, the first important part of the CFD simulation is to have a reasonably accurate flow characteristics of the particular jet fan model to be used in the car park as an input parameter. What most people did was to ask the fan manufacturer about the flow rate, dimensions of the outlet and angle of the outlet of the jet fan, and then use them as the CFD input, period. There were no validations of the simulated air flow profile based on such input against the actual air flow profile, and even if people wanted to do the validation they couldn't because there were no field test results provided by the fan manufacturers.

  • Hi to all.
    I am happy to find such a brilliant topic. Impulse systems appeared in Ukrainian Fire Market only a couple years ago. I understand its benefits as install cost, less structural requirrments to usual duct systems, diferrent velocities for vent, smoke or CO2 control,etc. However, as a sprinkler system designer I have asked all importers how they can explaine its probable negative effect on sprinkler heat activation on the one hand and water discharge on smoke transfer on the another hand. Unfortunately, the only their replies were about its usage in europe that, in turn, has to be a reasonable proof.

    Only now, the topic author gave me a link of FRNSW guidelines where we can find a methodology of how to determine such influence.

    My question is wether anyone has done or seen such test results. I am looking forward to finding any data about this matter.

    Many thanks to anyone who have managed to read my english and understand my ideas!)

  • Ken,
    I get a sense of hesitation and scepticism from your article. And it is probably warranted. Modern fire based CFD codes are fairly simple use, plug some numbers in, run a model and get and answer with some pretty pictures that look right, However it takes a high level understanding of that does on inside the CFD code to be able to say with a suitable level of confidence that the results are an accurate representation of reality.

    As you say "it is difficult for inexperienced individuals without a speciality in this area to assess that a design complies with the requirements". I agree entirely, which is why you need to engage a suitably qualified Fire Engineer with provide experience and or qualifications in CFD fire modelling.

    You rightly question how you can be sure someone has the correct level of understanding to undertake such complex CFD modelling or how someone such as a certifying authority can expect to have sufficient understanding to assess the Performance Solution… the answers are: you can't really know for sure and assessors may not have the knowledge. However the solution to both of these problems is to require a peer review of the performance solution (or even just the CFD modelling) by a secondary Fire Engineer who has (or appears to have) a suitable level of understanding of CFD fire modelling.