Estimating Thermal Radiation Fields from 3D Flame Reconstruction

The dominant mechanism for the spread of a fire in a building is radiative heat transfer from flames. Therefore, in order to understand how a fire might develop it is necessary to determine the thermal radiation field surrounding a fire. The heat flux at any target location depends upon the flame’s emissive power and the shape factor between the flame’s surface and the target. To determine the shape factor, it is necessary to define the geometry of the flame surface, and develop a method for obtaining the shape factor for that geometry. In this article we describe a method for determining the flame geometry using image-processing techniques. Images of the fire are recorded using two cameras and the flame is reconstructed in three dimensions. Once the surface of the flame has been defined it is possible to calculate the shape factor at a given target. Using the shape factor and an estimate of the emissive power of the flame, the heat flux at the target can be determined. Repeating this process for multiple targets builds a thermal radiation field. Results from applying the method to a controlled flame produced by a diffusion burner show a high correlation between estimated and measured heat flux values.     

Background signals from fire detectors: Measurement, analysis, application

A research programme has been running in excess of three years monitoring and recording the signals produced by individual heat and smoke sensors in a medium sized analogue addressable fire detector installation. The data has been recorded on 40 Mbyte Winchester discs which have been connected via electrically isolating circuitry to the same scanner interface as the Automatic Fire Detection Systems computer. One 40 Mbyte capacity disc is capable of collecting and recording all of the scan data continuously, at the system scan rate, for a period of 5 days. A second smaller analogue addressable system has been monitored by a microcomputer event logging system for a period of approximately 4 years.

Various forms of statistical treatment analysis have been carried out on the data. The preferred form appears to be ‘return period analysis’—a cumulative logarithmic frequency analysis showing the number of times threshold signal levels have been exceeded. The latter technique enables a relationship between threshold analogue signal levels and frequency of false alarms to be predicted. The measurement and analysis techniques developed are applicable to the commissioning of both new installations and extensions to existing installations. The techniques enable potentially troublesome sensor points to be identified before they can cause false alarms. The approach can reduce the number of false alarms and may thus enable the sensitivity of parts of the system to be enhanced.     

The J-Value and Its Role in Evaluating Investments in Fire Safety Schemes

Fire safety engineers endeavour to ensure that a design achieves an adequate level of fire safety. For uncommon buildings, adequate safety cannot be based on precedent and an explicit evaluation of the adequacy of proposed safety features may be required. Commonly, this requires demonstration that the residual risk associated with the design is as low as is reasonably practicable. In those situations, a measure for a safety scheme’s benefit relative to its cost is required, as more efficient safety schemes should be preferred over less efficient ones to maximize the number of lives saved under societal resource constraints. To this end, the J-value has been introduced in other engineering fields as a decision support indicator for assessing the efficacy of safety features. The J-value has been derived from societal welfare considerations (the Life Quality Index) and is adopted in the current paper for applications in fire safety engineering. It is demonstrated herein how the J-value can inform decisions on fire safety, and how it can provide a basis for assessing whether or not a proposed fire safety scheme should be implemented. Future work will focus on its implementation as a tool for assessing the benefit of real life fire safety scheme implementations, such as sprinkler installations.     

Performance of a light timber‐framed compartment in natural fire subjected to lateral load

A common approach for designing buildings for lateral stability during and post‐fire in New Zealand is to ensure that a fire‐rated structure does not collapse when subjected to a nominal horizontal force. For external walls of residential buildings, which are required to resist a lateral load of 0.5 kPa, it is hypothesised that the adjacent unrated construction could provide sufficient support. A natural fire experiment has been conducted to evaluate the fire performance of a laterally loaded light timber‐framed compartment, with external dimensions of 4.33 m × 3.35 m and a stud height of 2.4 m constructed with a timber truss roof and plasterboard ceiling. During the experiment, the ceiling collapsed at 12 to 13 minutes, and the bottom chord of the roof truss failed in tension after 28 minutes which resulted in the fire‐rated wall losing its lateral stability at 28 minutes. The fire severity experienced in the compartment has been estimated to correspond to an equivalent time of 33‐minute exposure to a standard furnace time‐temperature. It is concluded that there is no need to provide nominal (additional) moment‐resisting fixity at the base of the fire‐rated wall when exposed to the standard fire for no more than 30 minutes.     

Thermophysical properties of polyurethane foams and their melts

The thermal conductivity (λ) and the specific heat (c_p) of seven polyurethane foam formulations and their melts are obtained using a transient plane source technique called the Hot Disk experiment. In the experiment, the Hot Disk sensor is sandwiched by the samples and acts as both a heat source and a temperature sensor. The fundamental assumption is that throughout the experiment, the heat from the sensor does not penetrate beyond the boundaries of the sample. The suitable sample dimensions and sensor radius are determined from the preliminary calculations. Through sensitivity analysis, the appropriate measuring time and output power for the experiments are established. For polyurethane foams, λ ranges from 0.048 to 0.050 W/m K, and c_p ranges from 2359 to 2996 J/kg K. For melts, λ ranges from 0.186 to 0.200 W/m K, and c_p ranges from 1958 to 2076 J/kg K. When foam decomposes into melts, the changes in thermophysical properties shows λ increases by approximately 300%, whereas c_p decreases by approximately 20%. On the basis of these changes, the collapse of the foam structure into melt appears to improve the heat transfer through the material. Copyright © 2013 John Wiley & Sons, Ltd.     

Data Structures for Fire Test Information Exchange Using XML

This paper describes a database schema to manage information from a range of standard fire tests. The schema uses eXtensible Markup Language (XML) which has been widely adopted as a means of data management. The structure of the XML schema was based on the Fire Database Management System (FDMS) and other relevant work. The current schema can store data from five internationally recognised fire test standards but is sufficiently flexible to extend to other standard tests or ad-hoc experiments.     

Low-cost wool-based fire blocking inter-liners for upholstered furniture

New Zealand is reluctant to implement mandatory fire safety regulations for domestic furniture because of the cost/benefit even though up to 30% of household deaths and injuries over the past 20 or more years can be attributed to soft furnishings. Work has been carried out to develop a low-cost inter-liner using fire retardant treated wool with altered proportions of different fire resistant synthetic fibres that may be attractive for voluntary inclusion by manufacturers. Bench- and full-scale tests have been conducted to characterise the fire performance of composites of polypropylene upholstery fabric, different inter-liner materials and a polyurethane foam core. Inter-liner blends that reduced the peak rate of heat release, extended the time to peak rate of heat release and reduced other combustion product output were identified. A blend of 75% Zirpro treated fire retardant wool and 25% Panox fibre was found to be the best candidate material when fire performance, cost and practical application were considered.     

Analysis of Vehicle Fire Statistics in New Zealand Parking Buildings

The risk of fire in parking buildings is dependant on the probability of a fire occurring and the severity of the fire. This paper reviews the research data available on vehicle fires and suggests the severity of such a fire for a risk analysis. The paper then examines the historical data for vehicle fires in New Zealand parking buildings from 1995 to 2003 to determine the probability of the occurrence of such fires and the likelihood of multiple vehicle involvement. It is found that annual vehicle fire frequencies in parking buildings are generally lower than those in buildings of other occupancies but increases with the annual usage ratio.     

Design Fire Characteristics for Probabilistic Assessments of Dwellings in England

In England, there are no fixed requirements on the parameters adopted when considering residential design fires, and analyses undertaken are often deterministic with limited consideration given to probabilistic assessments and the sensitivity of parameters. The Home Office dwelling fires dataset has been analysed, considering the fire damage area and the time from ignition to fire and rescue service arrival. From this, lognormal distributions for the maximum heat release rate (HRR) and fire growth rate of residential fires have been approximated. The mean maximum HRR ranges from 900 kW to 1900 kW, with a standard deviation ranging from 2000 kW to 3700 kW, depending on property type and room of fire origin. The mean growth rate, assuming a t2 relationship, ranges from 0.0022 kW/s2 to 0.0034 kW/s2, with a standard deviation ranging from 0.0071 kW/s2 to 0.0132 kW/s2. When considering incidents which result in immediate fire and rescue service call out following ignition, the mean growth rate increases to a range of 0.0058 kW/s2 to 0.0088 kW/s2. As a result of the analyses, design fire distributions are provided which can be adopted for probabilistic assessments. For deterministic analyses, it is proposed that an approximate 95th percentile fire may be adopted, aligning with a medium growth rate of 0.0117 kW/s2 and a maximum fuel-limited HRR in the region of 3800 kW to 4400 kW, depending on whether the dwelling is a house or an apartment. A 95th percentile design fire broadly aligns with values already specified in guidance, helping to substantiate the existing recommendations.