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Smoke spread calculations for fires in underground mines
Mälardalen University, School of Sustainable Development of Society and Technology. (MERO)ORCID iD: 0000-0002-8326-2860
2010 (English)Report (Other academic)
Abstract [en]

This report is part of the research project “Concept for fire and smoke spread prevention in mines”, conducted by a research group at Mälardalen University.The project is aimed at improving fire safety in mines in order to obtain a safer working environment for the people working for the mining companies in Sweden or for visitors in mines open to the public.This report deals with the issue on smoke spread calculations for fires in underground mines.The main purposes of the report are:- Using both one- or two-dimensional calculation models as well as three-dimensional CFD calculation models.- Positioning the design fires (a pool fire, a fire in a loader, a fire in a loader in a sprinklered drift, a cable fire and a bus fire) at various sites with respect to for example the ventilation system.- Investigating the complexities of the various models, their limitations and deficiencies etc.- Comparing the results from the calculation models with each other and with experimental data where applicable and available.The work in this report started with describing smoke spread in underground mines in general and then continuing with describing three types of calculation models used in this report. After that calculations and simulations were conducted – using the three models – for the five designated design fire scenarios and the results were presented for strategic sites with respect to egress safety and the intervention of the fire and rescue services. The results from the CFD simulations were thereafter validated with respect to flame temperature and grid size convergence. It is misleading to fully compare the outputs of the three calculation models with each other without considering the differences and limitations of the three models as they are based upon different assumptions that differ considerably. For example the hand calculation expressions and the mine ventilation network simulation program assume unidirectional flow in the drifts compared with FDS that account for multi directional flows in the drifts. Also the hand calculation expressions and the mine ventilation network simulation program both assume complete mixture of air and fire gases while FDS does not make that assumption. Visibility is not one of the output parameters of the mine ventilation network simulation program, thus limiting the available data for comparison. But one of the purposes of this report was to investigate the complexities, limitations and deficiencies of the involved models.The advantage of the one- and two-dimensional models is the fact that the computational requirements are considerably lower than compared with a CFD model. Also with Ventgraph it is possible to obtain fast and transient solutions even though the simulated system of mine drifts is vast and complex. The advantage of FDS is the fact that it will model the area closest to the fire most accurately of the three models, for example accounting for multi directional flows in the near area of the fire. The disadvantages of the models are for example that it is not possible to fully account for the highly variable heat release rate of for example a fire in a tyre or in a vehicle when using a mine ventilation network program, as the ramp up to the maximum heat release rate is assumed to be linear and that every fire in a branch is assumed to be constant after reaching the maximum heat release rate. This does not apply to heat release rate curves that are uniform in shape such as a pool fire; in this case the heat release rate values used will be practically the same in all three types of calculation models.Also the time periods of the simulations differ between the three models as the simulations in FDS will become impractically long as the time to simulate increases due to higher computational requirements. Thus only the first 10-20 minutes were simulated in FDS and so the chance of comparison in for example the case of the fire in the loader is strongly limited.When comparing the results of the three calculation models the following conclusions can be made:- The mine ventilation network simulation program generally shows higher temperatures at the measuring points compared with the outputs of the other two models. One probable reason for this is that the heat release rate could not be represented as adequately as for the other two models; in all cases the heat release rate levels were higher for the mine ventilation network simulation program.- Generally the hand calculations showed much lower visibility figures than the CFDmodel. One reason for this – besides the fact that we are dealing with two models withvastly different approaches - is most likely in the difference in the types of smokecharacteristic factors used in the two types of models.- The FDS simulations generally showed small changes in temperature when comparingwith the other two models. This could be attributed to the fact that the measuring pointsare positioned at fairly large distances from the fire and thus the fire gases will coolconsiderably. Also the maximum heat release rate of some of the fires was small - ~1MW – and thus the impact on the nearby environment will be limited.- The results of the hand calculations with respect to the visibility is a good approximationfor the design fires with fairly low maximum heat release rate as the stratification in thiscase will be almost nonexistent and thus the smoke spread can be assumed to be equal tothe ventilation velocity in a drift (one dimensional smoke spread).- With respect to the egress safety the visibility will be the critical factor. In the case of thepool fire (design fire) the visibility will start to decrease at an early stage of the fire bothaccording to the hand calculations and the FDS simulation. After approximately a fewminutes the visibility in a large section of the connecting main ramp will be affected dueto the open nature of the area. Thus the egress will have to take place at an early stage inorder to ensure safety. Regarding the fire in the loader, the fire in the parking drift andthe cable fire the hand calculations indicate a sharp decline in visibility after a fewminutes, but the facts that the FDS simulation showed no differences in visibility and thatthe heat release rate is relatively small in all three cases (<1 MW) would indicate that thevisibility would be affected but in a limited manner and thus the egress safety will not belargely affected during the first 10-20 minutes due to for example the large spaces in themine drifts. With respect to the bus fire the same conclusions are drawn as in the case ofthe pool fire except that the FDS simulation predicts a much slower smoke spread thanthe results from the hand calculations.- With respect to the intervention from the fire and rescue services the visibility will also bethe critical factor as for the egress safety. The loader fire in the sprinklered drift and thecable fire should generally not pose any large problem to the intervention of the fire andrescue service, as the maximum heat release is small and the smoke spread largely limited.But the pool fire, the loader fire and the bus fire will be problematic to the interventionof the fire and rescue service as the maximum heat release is large and the smoke spreadis extensive affecting a large area before the arrival of the fire and rescue service (>30minutes). Thus the fire and rescue service will have to start the intervention at a largedistance from the site of the fire and work its way towards the fire. This will take a longtime and will decrease the chance of rescuing any personnel left in the area.As no data from conducted full-scale fire experiments were found that were applicable to any ofthe five design fire scenarios, future work should deal with validating the results of the threemodels with experimental results from conducted full scale fire tests corresponding to any of thefive design fire scenarios. In this case more profound comparisons and conclusions can bedrawn. The work and reflections from this report can be used when working on the full-scale fireexperiment.Measuring points should be placed in the near vicinity to the fire as well as sites further awayfrom the fire (> 50 m), this in order to effectively investigate and compare the results of one- andtwo-dimensional models versus a three dimensional model.Also further and deeper studies of the applied mine ventilation network simulation programshould be performed, investigating for example the assumptions and calculation models behindthe specific software.

Place, publisher, year, edition, pages
Västerås: Mälardalens högskola , 2010. , p. 58
Series
Studies in Sustainable Technology / Arbetsrapport ; 2010:07
National Category
Other Environmental Engineering Engineering and Technology Energy Engineering
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-9869OAI: oai:DiVA.org:mdh-9869DiVA, id: diva2:325849
Projects
GRUVANAvailable from: 2010-06-21 Created: 2010-06-21 Last updated: 2014-01-07Bibliographically approved

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