Experiments and modelling on vertical flame spread

1. 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

2. Contents • Experimental • Goals • Research tactics • Cone calorimeter experiments • Flame spread experiments • Heated sample experiments • Heating and autoignition experiments • DNS simulation • Conclusions

3. Experiments on vertical flame spread and ignition

4. Examples of experimental set-ups

5. 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

6. 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

7. RHR using cone calorimeter • Cone calorimeter time window tolerable • Cylindrical samples solve the background problem

8. 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

9. Flame spread experiment W15 • Digital photographing a viable screening method • A flame sheet model worth of trying

10. 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

11. Temperature modellig using a view factor • A view factor model a reasonable starting point • However, needs further refinements

12. 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

13. Timber as modeling material • Timber easy to shape target material • Charring material; good model for cables • Density dependence roughly exponential • Species a minor factor

14. 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

15. Sample heating and humidity • Two first terms of the series describe heating well • Humidity significant for wood • Water effective for cable flame reterdancy

16. 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

18. 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