Computational Fluid Dyna mics (CFD) fire modelling has been used in fire hazard assessment over 50 years. However, m ost of the CFD models are not properly validated by large -scale fire tests due to high cost and other environmental difficulties. There were many cr iticisms on having many adjustable parameters and they will be discussed in this article . Application of CFD in fire modelling should be watched with problems rarely discussed in the literature. Improper uses gave wrong predictions and f ull-scale burning tests are needed to justify the results . Further , using CFD in big fire investigation has to be very careful with support by appropriate burning tests. There is a long way to go in applying CFD to simulate big room fires. 2 1. Introduction Computational Fluid Dynamics (CFD) (or known as Numerical Heat Transfer and Fluid Flow before ) has been used [1 -5] in fire modelling over the past 50 years. The fire model was used in smoke extraction design and fire hazard asse ssment in performa nce-based design (PBD) or Fire Engineering Approach (FEA) in many places [6-8] in the Asia -Oceania region . There were numerous problems from day one because there are many adjustable parameters . Reporting long Available Safe Egress Time (ASET) [ 9,10] based on CFD predictions without considering smoke toxicit y is a good example, giving higher responsibility to safety management staff af terward . Previous approved projects with fire safety provisions deduced from PBD -FEA using CFD results must be reviewed for checking the building use matching with assum ptions m ade in those hazard assessment report. Fire safety provisions have to be upgrad ed if necessary. Installing additional sprinkler in surveyed ai rport terminals is a good example . Fire safety management [11] must be inspected at the very least . Recent increase in penalty [12] on safety to HK$10 million is a good move. Most of the CFD models are not validated systematically at the moment . Very limited developing works were reported and th ey were done almost 20 years ago. The reason is high cost and other dif ficulties such as environmental pollution in conducting large -scale fire tests. The free CFD software Fire Dynamics Simulator FDS [13] is a common design tool on smoke ma nage ment system . PBD -FEA reports throu gh CFD are always criticized. In Hon g Kong, hot smoke tests are required [14,15] in halls of irregular shape or buildings taller than 12 m in order to evaluat e the performance of smoke exhaust syst ems. In fact, many government parties would n ot consider PB D-FEA if there is no new architecture. The a ccuracy of CFD predictions depends on three groups of parameters in physical models [1,16] ; numerical parameters such as convergence criteria ; and parameters in the simulati on including the boundary condition . With out such clear justification on using CFD in PBD -FEA, it is difficult to be approved by the Authority who has to take safety responsibility . Penalty was very low punishment on violating codes before, but much higher now. That explains why PBD -FEA is a voide d in many new projects without innovative architectures . The recent increase of safety punishment to HK$ 10 million [12] might alert the top management to care more on investing safety. The safety culture [11,18,19] on mainland might be a good practi ce. Detaining all senior staff managing a hospital in the recent big fire at Bei jing [20] should be a good lesson to learn. They must keep approp riate safety management reports, at least, to demonstrate that they are 3 handling safety seriously. In recent incide nts of collapse of concrete [ 21-23], the government starts to arrest owner a nd chairman of building’s owners’ corporation of safety failure. 2. General Concerns Three-dimensionality and the instability of the flow fields should be included in simu lating three -dimensional unstable turbulent flow . Very little validation work s had been done, with some good and bad exa mples reported in the literature [24-28]. Ability of CFD to resolve the flow in the turbulent fire plume only is in doubt . Uncertai nty in the a bility of CFD to resolve the turbulent exchange flow across the opening is another concern a s reported be fore [5,28] . The common challenges are: • Air pressure and turbulence parameters are seldom presented, and only the velocity vector pattern s and tempe rature distribution are shown. • Stability criteria should be monitored [17,29] and reported properly . Grid size • Grid sensitivity criteria [17,29] are only deduced f rom temperatur e and velocity predictions, but not pressure or turbulence paramet ers. • There is no justification on fire phenomena by scale -models or full -scale burning tests . Perhaps CFD is reliable for practical smoke control design only after addressing these concerns . Application of CFD to simulate big room fire storing high amoun t of combus tibles should be careful . Full-scale burning tes ts on typical scenarios with similar conditions are still necessary for justifying the CFD predictions . Boundary conditions including b i-directional flow on windows and doors in a building should be specifie d properly. Extending the computing domain outside was proposed by Galea and associates [30]. There were other example s in using such suggestion [5,31,32]. 3. Using CFD in Simulating Big Room Fire There are general concerns on applying CF D for simulating e ven small fires in big rooms . A question asked: How to han dle big building fires [ 3,28]? There had been CFD simulations of fire in the literature. 4 CFD investigation of inclined ceiling jets was made to validate the FireFOAM code [33-35] before applic ation in large -scale, rack -storage fire driven sp rinkler act ivation simulations. The study provides a detailed analysis of temperature and velocity characteristics from the simulation results [35]. Predictions were compared against experi mental data from the literature for unconfined, smooth ceilings. FireFOAM is an unsteady solver for buoyancy driven turbulent reacting flows based on a C ++ code that employs the Open -FOAM platform [ 33,34]. The FireFOAM code solves the Navier –Stokes equat ions using a F avre-filtered fully compressible flow formulation. In addition, transport equations are considered for species mass fractions and sensible enthalpy as in [ 36]. The diffusivity of different species is assumed to be equal and computed using Pressure-Implicit Split -Operator (PISO) based algorithm [ 37,38]. The default sub -model for pyrolysis in FireFOAM 2.2.x [ 33,34] is applied, which is only by a single -step decomposition model . As reviewed by Ren et al. 2017 [3 9] was reported on the solid phase pyrolysis. Chaos et al. [40,41] studied fire properties of corrugated cardboa rd and wood using data from Fire Propagation Apparatus with bench -scale tests [4 2]. Results on such studies would be difficult to handle for big ministorage fire. The workshop o n Measurement and Computation of Fire Phenomena was held i n 2017 [43]. Discrep ancies between experimental data and computational results m ight be attributed to modeling errors. Measurements and modeling of pyrolysis phenomena are very compl icated and depen d on thermal radiatio n [44-49]. As reported by Zeinali et al. [ 50], distribut ion of mass density through the thickness of medium density fiberboard panels is known to be non -uniform. Material property on flame spread simulations of Single Burning Item cor ner fire tests. Simul ations are performed using FireFOAM 2.2.x [33,34], consid ering both uniform and non -uniform material density profiles, using model - effective material properties determined from bench -scale fire propagation apparatus. Applying CFD in fire protection engi neering applications , even the flame -to-wall heat fluxes (convection and radiation), which couple the gas -phase combustion with the solid -phase pyrolysis was not estimated properly [ 51]. For fully developed large -scale fires, radiative heat transfer is a k ey [52]. Only at the fire development and suppression sta ges, the fire size is small (a t 1 MW), and convection is comparable to radiation. There were challenges as expected . A wall model for convective heat transfer has been developed for under -resolved 5 engineering applications. Good agreement was found with the measurements at early stage, but again, not fully -developed fire. There is also another CFD package developed [52 -56]. CFD -based pyrolysis studies were discussed [52]. Reaction kinetics suggested t o go through molecular dynamics simulation . Non-premixed mixture fraction combustion model was applied to stud y fire whirls [5 3] through reaction at flame sheet only. Such approach cannot simulate big fires where flame sheets were diff icult to define and s imulate. There is still a long way to go before CFD can be used to study complicated combustion involving many types of combustibles in high quantity . The fire scenario of wooden village buildings in Hunan was investigated by Zhang et . al [57] using CFD based simulation. Temperature, smoke spread, CO concentrat ion and visibility were predicted . The fire was simulated by using a 0.5 m fire source . The boundary conditions were set to match the external environment, with an ambie nt temper ature of 20 ◦C. The fire simulation time was 1200 s. Fixed and movable fire lo ads were considered. Fixed fire loads were used for each room including the timber structure, whereas movable fire loads were employed for all the furniture. The doors a nd window s of the residential houses were kept open throughout the experiment. Wood was modelled separately from furniture. A single -step mixing controlled combustion method was adopted for the simulation. By default, the reaction of fuel and oxygen is inf initely f ast and controlled only by mixing . It was claimed that the method can be used to assist fire investigation. However, combustion of wood is complicated as reviewed before [ 58-65]. 4. Importance in Simulating Combustion in Big Room Fires A big mini storage fire lasting for 108 hours occurred on 21 June 2016 in Hong Kong , which killed two firefighters. Several aspects of this big fire has been reported [29]. Fire investigation should be supported by full-scale burning tests in the fire invest igation report, not just by CFD because of storing so much combustibles in limited spac e. Free CFD software is commonly used and there is no guarantee that the sof tware is appropriate for simulating such big room fires. Problems identified in the applicati on of CFD are well -known . Combustion of b urning the stored materials ha s to be included . Even burning Poly(methyl methacrylate) (PMMA) with simple kinetics of thermal decomposition [ 66], there are three stages in the combustion process: • PMMA decompose s to prod uce monomer methylmethacrylate (MMA); 6 • monomer MMA decomposes to gene rate smal l gaseous molecules; and • these small molecules undergo combustion. Intermediate chemical reactions are very complex with 7 group reactions: • thermal decomposition; • thermal o xidative decomposition; • decomposition of monomer MMA; • methane combu stion; • methanol combustion; • formaldehyde combustion; and • acetylene combustion. There are thousands of reactions to model. For example, 26 species and 77 elementary reactions in metha ne combustion. The reaction mechanism of acetylene has 103 elementary reaction s and 38 species. As combustion chemistry is too complicated to be in cluded and th e CFD studies in PBD -FEA projects are basically on simulating turbulent flow for estima ting the possible flaw concept of ASET . The fire source is taken as a heat sou rce with specified thermal power [67]. Storage fire is a combustion system [ 68,69 ] with many physical and chemical processes. Thermal feedback from the fire to the fuel surface gasifies more fuel vapors to burn in the flame for sustaining the combustion p rocess. R oom fires are even more complicated than those fires burning in open air because the heat and smoke generated will be trapped by the walls and ceiling. Thermal radiativ e feedbac k from the enclosure had been identified to be a significant factor in determi ning the fuel gasification rate. It is the primary cause of instability in growing to a big fire. But the gas temperature next to the ceiling at the onset of instability varies f rom 450 to 798oC in most cases. There might be some other phenomena, say mixin g of the gasified fuel vapors to appropriate ratios. Further, combustibles with different ignition temperatures were observed to be ignited at roughly the same time und er some e nclosure fires. The downward thermal radiative heat flux might not in crease so fast to ignite combustibles of significantly different ignition temperatures together, though the heat flux itself is increasing rapidly. 7 In addition to thermal instab ility of the combustibles due to thermal radiation from the ceiling and the la yer, ‘bri dge-mixing’ of fuel vapors due to fire -induced indoor aerodynamics might be another possible reason why different combustibles can be ignited simultaneously. 5. Bridge -Mixing ‘Bridge -mixing’ of igniting fuel vapors gasified from different combusti bles by thermal radiation during flashover was observed [ 68]. Combustible items of different ignition temperatures ignited under some mixing conditions. Mixing of gasifi ed fuel v apors by the fire-induced airflow in rooms of different geometry is a key fact or in igniting each combustible item. This releases more heat and generates flames to give conditions satisfying the flashover criteria. The ‘bridge -mixing’ concept is verified by preliminary simultaneous cone calorimetry on different polymer mat erials wi th and without mixing of their decomposited products. The “bridge -mixing” concept [ 68,69 ] was proposed on igniting combustibles of different ignition temperatures at t he same t ime in a room fire to give flashover and subsequent burning. The gasi fied fuel vapors liberated from fuels might mix thoroughly to give a “premixed state” inside the enclosure as in Fig. 1. That depends on whether the airflow induced by the fire c ould enha nce mixing of the fuel vapors from different combustibles. On ignitin g the gas ified fuel mixture to form a flame above the combustibles, thermal feedback might be strong enough to ignite the materials underneath. Al solid combustibles are ignited at the sa me time. The room is suddenly full of flame, which spreads out from a ll openin gs, being an indicator of flashover [ 70-74]. The bridge -mixing concept was verified earlier by bench -scale tests with a cone calorimeter under pilot ignition [ 68,69 ]. Based on c one bench -scale tests [ 68,69 ] as in Fig. 1 on varying the separation distance of the two plastic cubes, the proposed ‘bridge -mixing’ concept can explain why almost all combustibles were ignited simultaneously during flashover as observed for some scenarios . This should be applied to study combustion process in room fires storing hug e amount combustibles as in ministorage areas but with inadequate air due to limited ventilation. In particular, there are hundreds of intermediate chemical reactions, w ith some key reactions involving short -lived intermediate chemical radicals ge nerating huge heat and stable toxic products such as carbon monoxide. Putting in firefighting water will suppress many intermediate chemical reactions and give much more toxic gases. O perating positive ventilators would drive in more air to make the 8 situation mu ch more complicated. Hot fuel gases would be burnt to give out more heat and additional smoke. Cool fuel gas due to water application would mix with the air driven in. The firef ighting time can never be safe as surveyed years ago before [75]. Tha t is why the firefighters have to be equipped with personal protective equipment (PPV) which the author has evaluated those portable breathing apparatus in 2007. This supported t he Allowe d Firefighting Time (AFFT) (Might use Allowed Rescue Time ART [29]) around 30 minutes total, in sending Compartment Fire Attack CFA task force to dangerous fire sites. This is entirely different from the Available Safe Egre ss Time ASET fire scienc e approac h in fire safety man agement of normal occupants without PPE, but no h aving mob ility difficulties. However, doing this kind of full -scale burning tests requires resources. Those experiments in 2007 spent over HK$0.8 million (US$0.1 million) illegal diesel c ontributed by the Customs Department. Only a small room was burnt, no t a minis torage area! 6. Evaluation of CFD Predictions CFD predictions should be evaluated properly [76]. Longer -term research on the three groups of parameters in t he CFD mod els should be carried out to develop the model , but it would take a lo ng time [ 77-81]. At the moment, it is only good for designing smoke extraction system. Even so, officers should justify CFD simulations in PBD -FEA project submission , at least on : • The c onver gence and stability criteria, t urbulent parameters. • Free boundaries. • Presentation of predicted pressure and t urbulent para meters . • Comparing with e xperimental data from several s cenarios (out of many up to hundre ds of scenarios studied in hazard assessment) full-scale burning tests. • In-situ field tests to evaluat e the system performance on agreed scenario up to 2 MW design fire with hot smoke tests in hall . It is unknown yet on what to evaluate more in simulating ministorage fires by C FD involving combustio n chemist ry. However, the above are required as minimum. 7. PPV Operation Simulation Positive pressure ventilation (PPV) was employed in the firefighting with criticisms [32] that operating PPV led to a more hazardous fire e nvironment rather than i mprov ing the v isibil ity or reducing the temperature inside. 9 Firstly, different views on operating PPV for firefighting and rescue were summarized [32]. Ventilation arrangements includ ing the locations of PPV and exhaust openings in a room should be caref ully in vestigate d for better understanding the efficiency of PPV in firefighting . Secondly, a ction of operating P PV in a room fire studied by physical scale modelling had been discussed [32]. Temperature changes and fire spre ad patterns were captured. It is found that the presence of an opposite vent resulted in fire of smaller size and shorter duration. The CFD study is much more complicated with PPV and easy to be challenged [28,32] . Expe rts in computer simulation are always joking : Garbage in, gar bage out . 8. Conclus ion Application of CFD in fire modelling should be watched with problems discussed by users but not reported thoroughly in the literature. More experimental work s should be done to justify the predictions . Cutting res earch fundi ng in th e past 30 years limit ed the develop ment of CFD. Trained fire researchers are working as PBD -FEA engineer s to get higher pay . No doub t, using CFD in fire investigation report has to be very careful and predictions must be supported by appropriat e burning tests. Further, some FEA engineers used few fire tests with CFD results to argue code specification. J oking that only HK$3 million FEA consultancy fee can waive HK$ 1 billion fire s ervice installations . But question s come: What are the fire scen arios an d their consequences ? Who is the Authority to shoulder the responsibility ? What is the cost of Fie S afety Management to keep the desig ned scenario following FEA report ? 10 Acknow ledge ment The work described in this paper was supporte d by a gran t from t he Research Grants Council of the Hong Kong Special Admini strative Regi on, China for the project “A Stud y on Operating P ositive Pressure Venti lator in Roo m Fires” (P roject No. CityU 1121 2422).