Post on 29-May-2020
Hand Calculations, Zone Models and CFD
Areas of Disagreement and Limits of Application in practical Fire Protection
Engineering
Wojciech WęgrzyńskiBuilding Research Institute (ITB)
SFPE Polish Chapter
Piotr TofiłoThe Main School of Fire Service
SFPE Polish Chapter
Rafał PorowskiThe Main School of Fire Service
SFPE Polish Chapter
Presentation• Introduction
• Illustration of the issue – plume theory
• Strengths & Weakness
• Practical examples
• Opportunities & Threats
• Correct workflow of an engineering work:
Analysis of theoretical
boundaries of the study
Choice of applicable tools for the design
Preliminary studywith simple tools
Verification of the assumptions and
the tools
Proposal of a solution
Advanced studyof the solution
Verification of the study with simple
tools
SolutionProblem
Introduction
Redesign?
Introduction• Realistic workflow of an engineering work…
What doesengineer feel,
that should work
CFDSolutionProblem Plenty of pictures
to support the first case
If it does not work, it is a problem…
The Knowledge• Fire Safety Engineers should have a reasonable background, to
judge on basic solutions, know the boundaries of their tools andhow to use them.
• Engineers are responsible for their own V&V• Other participants of the process may not be that competent in this
field – they need a simple tool for verification of the engineers work• Education falls short in providing solution to this issue – there are
far more people involved on different stages of the design, than wecan educate!
The Knowledge
Why lack of knowledge is a problem?
One man can ask more stupid questions, than twenty wiseman can answer…
The Knowledge
Why lack of knowledge is a problem?• With insufficient ability to verify the design on the other side, it
sometimes feels like a waste of time to explain the solution• Engineer takes a risk, that the solution will not be understood, and
possibly reverted• Engineer may be forced into a sub-optimal solution or unnecessary
simulations, required only for explanation but not for the solutionitself
Toolbox
So what is in our toolbox?• Simple „hand calculation” models that origin from experimental
studies and relate to particular phenomena;• Zone (or lumped) models that relate to a global view on the fire, but
in a simplified geometry and discretization• CFD and other numerical tools, that may give the most detailed
answer but are time and resource consuming
• Often the CFD is viewed as „oracle” for the design – and due to itscost is done at the end, becoming a tool of acceptance and not thedesign;
• Engineers may be forced to do the CFD because it is the easiesttool to prove your solution for AHJ
• Tools are misused or used beyond their scope of application, butuser is not aware, as he is not verifying his solution with otherapproach
Toolbox
Is There a Problem with the Choice of Tools?
To illustrate that, we have chosenthe axisymmetric plume theory,which is one of most investigatedfire related phenomena, and onethat is used in almost any firerelated design.
Is There a Problem with the Choice of Tools?
What Are the Differences between Models?
Q = 2500 kW, Q” = 250 kW/m² Q = 5000 kW, Q” = 250 kW/m²
What Are the Differences between Models?
Q = 2500 kW, Q” = 1000 kW/m² Q = 5000 kW, Q” = 1000 kW/m²
What Are the Differences between Models?
What Are the Differences between Models?
The Explanation• Hand models are based on different experiments, with their own
limitations. The closer to the experiment, the better the results;• In some areas, the results of CFD are matching the hand-calc closely,
but there are areas where the differences are enormous;• Choosing a tool may be a choice of model, user is not aware of;• Can a hand model be a verification tool for more complex tool, given the
error?• Will anyone beside a Fire Safety Engineer be aware of these issues?
Our Idea• We need a good guideline on how to choose our models, where are
their limits and what is their scope of application• We have good guidelines on V&V (SFPE, NIST, NUREG etc.), but some
parties will not go into a complex methodology, as they cannot afford thetime or lack the knowledge
• The first step for this – what are the strengths, weakness, opportunitiesand threats connected with our tools?
Strong Points of Our ToolsHand calculations/simple models• steady state calculations unless the time is directly connected to the investigated
phenomena (i.e. sprinkler activation, surface flame spread);• finding realistic limits for a phenomena (i.e. maximum temperature that can occur);• initial check, preliminary studies in which instant result is required;• spatial domain directly connected to the size of phenomena investigated; • level of details described by the method is directly connected to the size of the
domain.
Strong Points of Our ToolsZone Fire Models• transient simulations;• situations in which the zone averaged output is enough and the detailed local
conditions are not in the main scope of the analysis;• quick computational time makes it a suitable tool for probabilistic studies;• model made with modular geometry of compartments, with hard limitations in their
size;• flows between compartments may be analyzed;• possible use of sub models (plume, ceiling jet), but have the same limitations as
the hand calculations.
Strong Points of Our ToolsCFD• transient or steady state simulations of complex flows of heat and mass in almost any geometry;• location of features within the model influences the results – can be used in assessment of complex
ventilation system and their optimization;• the level of details that can be investigated is limited mostly by the time and space discretization;• overall size and complexity of the model is limited only by the preparation time and computational
power available;• investigation of far field results without focusing on details of complex phenomena (i.e. to assess
smoke movement we substitute pyrolysis and flamelet model into volumetric source of heat and smoke)
Weak Points of Our Tools
• The weak points we present in the article can be attributed to themost of user errors in the modelling process,
• Moving out of the scope of model application (geometry, size offire, air velocity, turbulent flow etc.)
• Trying to model phenomena too complex for the model – eachadditional „complication” into the model increases the uncertaintyand resources require to process it!
A Good Practice• An illustration on how to benefit from knowledge of strength and
weakness of each model is shown on four different examples• At times, a single tool may not be enough to solve the issue, and at
other times using more tools may actually simplify the wholeprocess
• It is always good to take a look back and verify your results with adifferent, even not ideal, tool
Example 1
Flow out of a small compartment, estimationof the temperature:• possible to do both with a CFD• estimation of the flow – Zone model• estimation of the temperature – hand model
Example 1
Scenario Mass flow (CFD) Mass flow (B-Risk)B1 13,8
13,39
B2 13,3B3 13,15B4 13,6B5 13,6B6 13,5
Flow out of a small compartment, estimation of the temperature
Example 2Flow out of a small compartment through a very small opening
Example 2
• Modelling the compartment fire a priori with a CFD is a difficult andvery resource consuming task, with the results being burdened withlarge, unknown, error…
• Modelling the flow our of small opening with a zone model isimpossible…
• Solution – use both, transpone the mass flow estimated with Zonemodel through the opening into a boundary condition in the CFD
Example 3Jet fan system design in a car park
Example 3Jet fan system design in a car park:• CFD is too expensive to use as a general design tool for simple car
parks, while it is the best tool for this case• Simple hand models can allow first dimensioning of the system, but
are unable to provide a proof to AHJ that system works. Thesemodels may seem difficult so the designers often go with ownjudgment based on experience, as these projects have a „feel” ofrepeatability, while this routine may lead to a fault design!
A solution is to pre-design with hand-calc, and verify with CFD
Example 4Tunnel ventilation system design
Example 4Tunnel ventilation system design:• boundaries of the system will strongly influence the results of the
analysis;• It is extensive effort to make a 3-D model of a whole system in order
to resolve the pressure at the boundaries• There are 1-D models that may be used for a tunnel ventilation
design, in order to simplify this problem and provide the designer withcorrect values of pressure loss at the tunnel ends
The results of this study on applicability of various models is summarizedthe best by presenting opportunities and threats connected to eachmodel.
A user is responsible for his V&V, but there is a need for morecomprehensive guidelines on applicability of the models, which can beused by not only engineers, but also AHJ, investors, third party orstudents.
Summary
OpportunitiesHand calculations/simple models Zone Fire Models CFD• methods presented in standards
are usually hand calculation/simple models – these methods are usually straightforward accepted by AHJ;
• quickest method, can be used to find initial assumptions for further studies and thus reduce amount of complex analysis to carry (i.e. calculation of the thrust of jet-fans required in a tunnel);
• solution of complex small scale phenomena to become a boundary condition for CFD modeling;
• probabilistic search for worst case scenario for further research;
• great academic tool to present visualization of the theoretical aspects;
• almost limitless creation of numerical domain, including many details hidden in different approach - solution suiting the project;
• cost optimization studies due to possibility of using smaller safety margins (i.e. combination of smoke and wind simulation may lead to optimization of NSHEV);
ThreatsHand calculations/simple models Zone Fire Models CFD• while in their scope of application, that
usually is limited by the original experimental validation, expected results should be correct, using these methods outside of their range may lead to large error;
• relying on maximum theoretical values of parameter in the design leads to unnecessary overdesign, far bigger than required margin of safety (i.e. temperature of steel calculation from EC-1);
• going over limits of the model may yield reliable results, but the real area of application depends on the case and generally remains unknown;
• local turbulence in the flow of smoke may be critical for mixing air and smoke, and lead to failure of ventilation system – it is a phenomena not possible to witness in zone model;
• due to high cost of the method, long computational time and difficulties in verification, CFD simulations require high work ethics and great knowledge – the computation is as good, as the engineer behind it;
• user error may cause fault results, that can be overlooked if not checked with additional means;
• often used as the “confirmation” and not the “design” study – errors shown by analysis at late stage of the project may be intentionally hidden or neglected;
Thank You for Your Attention!Piotr Tofiło
The Main School of Fire ServiceSFPE Polish Chapterpiotr.tofilo@gmail.com
+48 728 109 723
Rafał PorowskiThe Main School of Fire Service
SFPE Polish Chapterrporowski@gmail.com
Wojciech WęgrzyńskiInstytut Techniki Budowlanej
w.wegrzynski@itb.plfire@itb.pl
+48 696 061 589