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SAFIR Midterm Seminar 20.-21.2.2005, Espoo, Finland. Experiments and modelling on vertical flame spread. Olavi Keski-Rahkonen, Johan Mangs & Simo Hostikka VTT Building and Transport. Contents. Experimental Goals Research tactics Cone calorimeter experiments Flame spread experiments
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SAFIR Midterm Seminar 20.-21.2.2005, Espoo, Finland Experiments and modelling on vertical flame spread Olavi Keski-Rahkonen, Johan Mangs & Simo Hostikka VTT Building and Transport
Contents • Experimental • Goals • Research tactics • Cone calorimeter experiments • Flame spread experiments • Heated sample experiments • Heating and autoignition experiments • DNS simulation • Conclusions
Goals • Create from an accurate but still practical model for vertical flame spread to be implemented in CFD numerical fire simulaton tools like LES-model FDS • Create prototype concepts and models for testing instruments needed for measuring model parameters from industrial objects and materials like cables and building parts • Carry out the task to implementation during SAFIR-program
Research tactics The problem is so complicated, and the goals so demanding, that all possible means has to be used in parallel and in close interaction with each other • Previous reasearch: Literature studies • Ongoing and planned research: Personal contacts to major operators worldwide • Experimental: for economy smallest possible scale for screening, and modelling the major physics, then scale up gradually • Modelling: From basic physics to practical validated algorithms • Analytical models, even crude for understanding major physics • Numerical models and simulation for testing use in practise • Computational physics to find brute force ’right’ solutions using axisymmetric forced 2D DNS-like simulation at scales possible to calculate • Use modelling results to design experiments, use experimental data to select models • Design of testing equipment: Minimum set of variables measurable well enough • Economy principles: • Solve easy problems first, and use them for generalization • Use quickiest, cheapest information in Bayesian sense, improve later
RHR using cone calorimeter • Cone calorimeter time window tolerable • Cylindrical samples solve the background problem
Cone calorimeter modelling • Cone calorimeter to cylinder calorimeter? • Double cone calorimeter using planar samples better for signal to noise ratio? • Radiative heat transfer a basic part of flame spread mechanism
Flame spread experiment W15 • Digital photographing a viable screening method • A flame sheet model worth of trying
Flame spread experiment W13 • Pine stick 8 mm dia 2 m long • Ignited from below • Video recording • Digital photographic recording • Digital editing of photographs • Thermocouples close to sample for temperature recordings • Thrmocouples a reasonable way to extract essential information
Temperature modellig using a view factor • A view factor model a reasonable starting point • However, needs further refinements
Flame spread velocity measurements • Long vertical round sticks a good idea to determine flame spread velocity • Digital photographing and editing works satisfactorily for screening • Labor intensive for routine production • Nonlocal burning possible due to ducts in wood
Timber as modeling material • Timber easy to shape target material • Charring material; good model for cables • Density dependence roughly exponential • Species a minor factor
Flame spread on heated samples • Initial heating a quick and cheap way to vary ambient conditions • Roughly exponential dependence on temperature • Data needed up to autoignition temperature
Sample heating and humidity • Two first terms of the series describe heating well • Humidity significant for wood • Water effective for cable flame reterdancy
DNS simulation • Sample 4 mm diameter birch stick • Fixed pyrolysis temperature assumed • Pyrolysis takes place in a zone travelling within the sample • Axi-symmetric Direct Numerical Simulaton: • brute force solving of Navier-Stokes equations • possible for physically small systems • impossible for real size objects • gives good guidance for simpler modelling • Simulations by FDS code in 2D DNS-like mode
Conclusions • Screening carried out for flame spread: • Literature study • 103 experiments • simple modeling • DNS simulation • Timber a good sample material for modelling • Contributed physical phenomena identified • Simplified modelling started • DNS simulations allow admiring ’right’ solutions • Multimethod problem solving speeds progress • New testing instruments identified and drafted • Modified cone (cylinder) calorimeter • Vertical flame spread test rig with initial heating capability • Time to proceed from screening to modeling, algorithm building and small scale validation