Determination of Intrinsic Material Flammability Properties from Material Tests assisted by Numerical Modelling

Computational Fluid Dynamics (CFD) codes are being increasingly used in the field of fire safety engineering. They provide, amongst other things, velocity, species and heat flux distributions throughout the computational domain. The various sub-models associated with these have been developed sufficiently to reduce the errors below 10%-15%, and work continues on reducing these errors yet further. However, the uncertainties introduced by using material properties as an input for these models are considerably larger than those from the other sub-models, yet little work is being done to improve these. Most of the data for these material properties comes from traditional (standard) tests. It is known that these properties are not intrinsic, but are test-specific. Thus, it can be expected that the errors incurred when using these in computations can be significant. Research has been held back by a lack of understanding of the basic factors that determine material flammability. The term “flammability” is currently used to encompass a number of definitions and “properties” that are linked to standardised test methodologies. In almost all cases, the quantitative manifestations of “flammability” are a combination of material properties and environmental conditions associated with the particular test method from which they were derived but are not always representative of parameters linked intrinsically with the tested material. The result is that even the best-defined parameters associated with flammability cannot be successfully introduced into fire models to predict ignition or fire growth. The aim of this work is to develop a new approach to the interpretation of standard flammability tests in order to derive the (intrinsic) material properties; specifically, those properties controlling ignition. This approach combines solid phase and gas modelling together with standard tests using computational fluid dynamics (CFD), mass fraction of flammable gases and lean flammability limits (LFL). The back boundary condition is also better defined by introducing a heat sink with a high thermal conductivity and a temperature dependant convective heat transfer coefficient. The intrinsic material properties can then be used to rank materials based on their susceptibility to ignition and, furthermore, can be used as input data for fire models. Experiments in a standard test apparatus (FPA) were performed and the resulting data fitted to a complex pyrolysis model to estimate the (intrinsic) material properties. With these properties, it should be possible to model the heating process, pyrolysis, ignition and related material behaviour for any adequately defined heating scenario. This was achieved, within bounds, during validation of the approach in the Cone Calorimeter and under ramped heating conditions in the Fire Propagation Apparatus (FPA). This work demonstrates that standard flammability and material tests have been proven inadequate for the purpose of obtaining the “intrinsic” material properties required for pyrolysis models. A significant step has been made towards the development of a technique to obtain these material properties using test apparatuses, and to predict ignition of the tested materials under any heating scenario. This work has successfully demonstrated the ability to predict the driving force (in-depth temperature distribution) in the ignition process. The results obtained are very promising and serve to demonstrate the feasibility of the methodology. The essential outcomes are the “lessons learnt”, which themselves are of great importance to the understanding and further development of this technique. One of these lessons is that complex modelling in conjunction with current standard flammability test cannot currently provide all required parameters. The uncertainty of the results is significantly reduced when using independently determined parameters in the model. The intrinsic values of the material properties depend significantly on the accuracy of the model and precision of the data

Evaluation of the Thermophysical Properties of Poly(MethylMethacrylate): A Reference Material for the Development of a flammability Test for Micro-Gravity Environments

Masters of Science Thesis, The University of Maryland, Department of Fire Protection Engineering, 1999.A study has been conducted using PMMA (Poly(methyl methacrylate)) as a reference material in the development process of the Forced Flow and flame Spread Test (FIST). This test attempts to establish different criteria for material flammability for micro-gravity environments. The FIST consists of two tests, ignition and flame spread tests, that provide a series of material “fire” properties that jointly provide important information on the flammability of a material. This work deals with the former. PMMA was chosen as a reference material mainly because of its well characterized properties. Evaluation of the ignition delay time as a function of a suddenly imposed external heat flux can be described by a known relationship with the minimum surface temperature at which piloted ignition can occur. The ignition temperature can be obtained from the experimental determination of a critical heat flux for ignition and the total convective heat transfer coefficient. And by assuming the absorptivity to be approximately unity, a material constant can be found, and is often referred to as the thermal inertia. The ignition temperature is addressed by splitting the ignition process into the time required to initiate thermal decomposition of the material and the ignition delay time. The present work provides an independent evaluation of the evolution of the thermal properties of PMMA, as a function of temperature. The thermophysical properties were determined by using the time to ignition and time to pyrolysis approached as obtained from the FIST. Discrepancies between these two approaches were resolved by defining a mixing time and a minimum average fuel concentration for ignition. Independent evaluation of the density, thermal conductivity and specific heat serve to correlate the property values of obtained from the FIST

Fire Safety in High-rise Buildings, Lessons Learned from the WTC

This paper addresses the tragic events of September 11th, 2001. Provides a brief background on the philosophy of fire protection for high-rise buildings and the behavior of a fire within a compartment. It further describes the events and the particular scenario corresponding to the World Trade Center. No attempt is made of providing a description of what caused the collapse but the objective is more to illustrate the characteristics of the fire and highlight the possible uncertainties. The paper concludes with a list of lessons learned and questions yet to be answer but fundamentally, with a plea for a detailed analysis of this event and a subsequent plan for fire research. Understanding the mechanisms that led to the collapse of the World Trade Center will enable engineers to provide a safer environment for the users of similar and other types of buildings but also for the firemen that sacrifice their lives trying to save the lives of other people

Large-scale pool fires

A review of research into the burning behaviour of large pool fires and fuel spill fires is presented. The features which distinguish such fires from smaller pool fires are mainly associated with the fire dynamics at low source Froude numbers and the radiative interaction with the fire source. In hydrocarbon fires, higher soot levels at increased diameters result in radiation blockage effects around the perimeter of large fire plumes; this yields lower emissive powers and a drastic reduction in the radiative loss fraction; whilst there are simplifying factors with these phenomena, arising from the fact that soot yield can saturate, there are other complications deriving from the intermittency of the behaviour, with luminous regions of efficient combustion appearing randomly in the outer surface of the fire according the turbulent fluctuations in the fire plume. Knowledge of the fluid flow instabilities, which lead to the formation of large eddies, is also key to understanding the behaviour of large-scale fires. Here modelling tools can be effectively exploited in order to investigate the fluid flow phenomena, and LES codes, in particular, provide an avenue for further research

Ignition Performance of New and Used Motor Vehicle Upholstery Fabrics

This paper examines the standards for fire safety in transport systems and in particular the test method for the flammability of materials within passenger compartments of motor vehicles. The paper compares data from ignition tests conducted in the Cone Calorimeter and the FIST apparatus with tests conducted using the FMVSS 302 horizontal flame spread apparatus. Ten materials were selected as representative of those used as seat coverings of private and commercial passenger vehicles. The time to ignition of new and used materials subject to exposure heat fluxes between 20 kW/m2 and 40 kW/m2 was measured. The results from the ignition tests were analyzed using thermally thick and thermally thin theoretical models. The critical heat flux for sustained piloted ignition was determined from the time to ignition data using the thermally thin approach. Derived ignition temperatures from both the thermally thick and thermally thin methods were compared with measurements using a thermocouple attached to the back surface of materials in selected tests. The flame spread rates in the FMVSS 302 apparatus were determined and a comparison was made between the performance of the materials in the flame spread apparatus, the Cone Calorimeter and the FIST. The results suggests that a critical heat flux criterion could be used to provide an equivalent pass/fail performance requirement to that specified by the horizontal flame spread test although further testing is needed to support this

The Role of Pressure in Inverse Design for Assembly

Isotropic pairwise interactions that promote the self assembly of complex particle morphologies have been discovered by inverse design strategies derived from the molecular coarse-graining literature. While such approaches provide an avenue to reproduce structural correlations, thermodynamic quantities such as the pressure have typically not been considered in self-assembly applications. In this work, we demonstrate that relative entropy optimization can be used to discover potentials that self-assemble into targeted cluster morphologies with a prescribed pressure when the iterative simulations are performed in the isothermal-isobaric ensemble. By tuning the pressure in the optimization, we generate a family of simple pair potentials that all self-assemble the same structure. Selecting an appropriate simulation ensemble to control the thermodynamic properties of interest is a general design strategy that could also be used to discover interaction potentials that self-assemble structures having, for example, a specified chemical potential.Comment: 29 pages, 8 figure

A Study of Fire Durability for a Road Tunnel: Comparing CFD and Simple Analytical Models

The durability of various typical tunnel sections in the event of a prescribed 100 MW fire has been assessed. Cast-iron sections, pre-cast concrete sections and in-situ concrete cut and cover sections are all considered to be part of a 1 km long road tunnel. An analysis of the tunnel constructions and surrounding geology (based on a real tunnel) has led to the estimation of failure temperatures for the structural elements, internal cladding systems, jet fans and their fixings. A commercial computational fluid dynamics (CFD) code was used to simulate various fire scenarios and calculate the times to failure of tunnel elements. Simulations were carried out for fires in different locations for the three section types. In parallel to the CFD study, an analytical model was devised to predict gas temperatures in the tunnel. Both models used the same input variables and general assumptions and great attention was given to establish the highest possible accuracy for all input variables and general assumptions. Comparing the predicted gas phase temperatures shows that there is less than a 20% difference between the complex CFD and the simple analytical model; this is well within the bounds of uncertainty inherent in either model and to the input parameters. Using both sets of gas phase temperatures, a detailed heat transfer study was carried out to calculate the temperature evolution of each of the tunnel elements. The differences in gas temperatures between the two modelling methods did not alter the conclusions regarding the time to failure of any tunnel elements. It is found that fire durability can be better analyzed by separating the fire environment into two zones, a near field close to the flames, where accuracy is defined by the assumptions, and a far field where the precision of the results is linked to the modelling method. This approach allows establishing that, for this particular case, failure of structural elements can only occur in the near field. This study shows that the detail of the calculations needs to be consistent with the accuracy of the input parameters and assumptions. Although CFD models can give highly detailed results, the implied accuracy of the results is defined by the assumptions inherent in the model setup, thus, there is the potential of a very costly and refined computation that leads to results of comparable accuracy to simple, less costly, models

Hyperdynamic left ventricular ejection fraction in the intensive care unit

Introduction: Limited information exists on the etiology, prevalence, and significance of hyperdynamic left ventricular ejection fraction (HDLVEF) in the intensive care unit (ICU). Our aim in the present study was to compare characteristics and outcomes of patients with HDLVEF with those of patients with normal left ventricular ejection fraction in the ICU using a large, public, deidentified critical care database. Methods: We conducted a longitudinal, single-center, retrospective cohort study of adult patients who underwent echocardiography during a medical or surgical ICU admission at the Beth Israel Deaconess Medical Center using the Multiparameter Intelligent Monitoring in Intensive Care II database. The final cohort had 2867 patients, of whom 324 had HDLVEF, defined as an ejection fraction >70 %. Patients with an ejection fraction <55 % were excluded. Results: Compared with critically ill patients with normal left ventricular ejection fraction, the finding of HDLVEF in critically ill patients was associated with female sex, increased age, and the diagnoses of hypertension and cancer. Patients with HDLVEF had increased 28-day mortality compared with those with normal ejection fraction in multivariate logistic regression analysis adjusted for age, sex, Sequential Organ Failure Assessment score, Elixhauser score for comorbidities, vasopressor use, and mechanical ventilation use (odds ratio 1.38, 95 % confidence interval 1.039–1.842, p =0.02). Conclusions: The presence of HDLVEF portended increased 28-day mortality, and may be helpful as a gravity marker for prognosis in patients admitted to the ICU. Further research is warranted to gain a better understanding of how these patients respond to common interventions in the ICU and to determine if pharmacologic modulation of HDLVEF improves outcomes

Calculation Methods for the Heat Release Rate of Materials of Unknown Composition

The Heat Release Rate (HRR) is a critical parameter to characterise a fire. Different methods have been developed to estimate it. The most widespread techniques are based on mass balance. If the heat of combustion of the fuel is known, the measure of the mass loss allows its evaluation. If the burning material can not be identified, calorimetric principles can be used. They rely on oxygen consumption (OC) or carbon dioxide and carbon monoxide generation (CDG) measurements. Their asset comes from the observation that the amount of energy release per unit mass of O2 consumed or per unit mass of CO2 produced is relatively constant for a large number of materials. Thus, an accurate HRR can be obtained without knowing the composition of the burning fuel. The aim of this work is to assess this last statement and define how essential the knowledge of the chemistry to calculate HRR for complex materials such as polymers including fire retardants and/or nanocomposites, energetic materials or pine needles is. This assessment ends in an OC and CDG calorimetry comparison of several materials in order to investigate the propensity to determine whether converging or diverging HRR results when average energy constants are used

Experimental Layout and Description of the Building

Chapter 2 in the book: The Dalmarnock Fire Tests: Experiments and Modelling, Edited by G. Rein, C. Abecassis Empis and R. Carvel, Published by the School of Engineering and Electronics, University of Edinburgh, 2007. ISBN 978-0-9557497-0-4This chapter describes the experimental set-up of Dalmarnock Fire Tests One and Two. While Test One was planned to allow a fire to develop freely to post-flashover conditions, Test Two was designed to allow for ventilation management. A detailed account of the building layout, the set-up of the different experiments and measurements carried out during the full-scale tests is given, together with the specifics of the instrumentation installed in the building. A description of the fuel load and the ventilation conditions during both tests is also presented. Due to the large amount of data presented, the chapter is organised so that the most relevant information is shown in the main body of the text, and additional detail, such as the instrument coordinates, is appended in an annex at the end of the chapter