David A. Torvi

Influence of Air Gaps On Bench-Top Test Results of Flame Resistant Fabrics

One of the primary differences among various test methods used to evaluate fabrics for thermal protective clothing is the presence or absence of a horizontal air gap between the fabric to be tested and the test sensor. Numerical modeling and flow visualization experiments were used to study the effect of the size of this air space on bench-top test results. The relative magnitudes of conduction, convection and radiation heat transfer in the air gap are shown, and photographs of the flow patterns in these enclosures are included. Applications of this work to other areas of fire protection engineering are dis cussed.

Heat Transfer in Thin Fibrous Materials Under High Heat Flux

A heat-transfer model has been developed for two common, inherently flame-resistant fabrics, Nomex® IIIA and Kevlar®/PBI, when subjected to the high heat fluxes used in bench top tests, such as the thermal protective performance (TPP) test, ASTM D 4108. The apparent heat capacity method was used to model thermochemical reactions in these materials with information from thermal gravimetric analysis (TGA) and differential scanning calorimeter (DSC) tests. Also included were in-depth radiation absorption, variable thermal properties, and heat transfer across an air space from the fabric to a test sensor. The finite element method was used to solve the resulting equations. Absolute temperatures predicted by this relatively simple model fall within 4% of those measured by an infrared thermometer. Estimated times to the Stoll second-degree burn criterion are within 6% of those derived from actual tests.

Improving Heat Transfer Models of Air Gaps in Bench Top Tests of Thermal Protective Fabrics

An improved model has been developed to simulate heat transfer in horizontal air spaces between thermal protective fabrics and test sensors in bench top tests, such as the thermal protective performance test. This model calculates the radiation and convection heat transfer from the test specimen to the test sensor. Radiation heat transfer is calculated by treating the bottom boundary of the enclosure as a series of isothermal rectangular pieces. Convection heat transfer is calculated using an empirical correlation and by assuming that convection only occurs over a portion of the cross-section of the enclosure. Predicted times required to exceed the Stoll second degree burn criterion were found to be within 3 % of those measured during actual bench top tests of steel shimstock using air gaps from 6.4 mm (1/4 in.) to 19.1 mm (3/4 in.).

Research in Protective Clothing for Firefighters: State of the Art and Future Directions

This paper presents a summary of documented research into protective clothing for firefighters. Particular emphasis is placed on research into estimating the useful life of this clothing and the development of test standards for evaluating its thermal protection. Other areas covered in this review include moisture transfer in clothing, heat stress, design criteria, chemical protective clothing, and heat transfer modeling of protective clothing. Recommendations for future protective clothing research are also presented.

Effects of Variations in Thermal Properties on the Performance of Flame Resistant Fabrics for Flash Fires

A numerical model of heat transfer in thermal protective fabrics under high heat flux conditions is used to determine the effects of varying individual thermal properties and boundary conditions on the predicted performance of single layer fabrics during bench top tests simulating flash fire conditions. The fabric thermal properties with the largest effects are thermal conductivity and specific heat. The boundary conditions, i.e., flame temperature and emissivity and convective heat transfer coefficient, have an even larger effect on predicted bench top test results. The results of the parametric studies are described in this paper, along with a discussion about how these results may be used to design thermal protective fabrics.

FIERAsystem: A Fire Risk Assessment Tool to Evaluate Fire Safety in Industrial Buildings and Large Spaces:

FIERAsystem is a computer model for evaluating fire protection systems in industrial buildings. The model has been developed as a tool to assist fire protection engineers, building officials, fire service personnel and researchers in performing fire safety engineering calculations, and can be used to conduct hazard and risk analyses, as well as to evaluate whether a selected design satisfies established fire safety objectives. While the model is primarily designed for use in warehouses and aircraft hangars, it can be modified for application to other industrial buildings. This paper describes the framework for FIERAsystem, along with its capabilities and flexibility. Individual models used to perform calculations are discussed, particularly those that calculate fire development and life hazard. A hazard analysis of an aircraft hangar is then described in detail, as an example of the types of calculations this model can perform. Methods used by the model to conduct risk assessments are also briefly described.

Characterization of the Combustion Process of Flame Resistant Thermal Protective Textiles in the Presence of Oily Contaminants: Effects of Contamination and Decontamination:

This paper reports two experimental studies wherein the combustion process of flame resistant (FR) thermal protective textiles is characterized in terms of thermal decomposition and heat release parameters before and after contamination and in terms of heat release parameters after contamination and decontamination. Aramid and FR cotton/nylon decomposed at higher and aramid/FR viscose at lower temperature in the presence of oil. Oil interferes with thermally induced interactions between aramid and FR viscose, altering the thermal decomposition rates and formation of char, and thereby increasing the effectiveness of the flame retardant present in the viscose. It is apparent that oily contaminants present in FR fabrics affect the initiation of the thermal degradation and formation of char. All contaminated FR fabrics showed significantly higher peak heat release rate (PHRR), total heat release (THR) and effective heat of combustion (EHC) compared to uncontaminated ones. Oily specimens laundered with no detergent or prewash product had higher PHRR, THR and EHC compared to other treatments regardless of the fabric type or number of contamination/decontamination cycles. Heat release increased with increased number of contamination/decontamination cycles for most laundry treatments for all FR fabrics. FR cotton/nylon had the highest and aramid had the lowest PHRR and THR whether specimens were uncontaminated, contaminated or decontaminated. In this study heat release from FR fabrics increased with increased oily contamination.

Estimating Water Requirements for Firefighting Operations Using FIERAsystem

A new computer model for estimating water requirements for firefighting purposes has been developed by the Fire Risk Management Program of the National Research Council of Canada. This work was done in partnership with the Canadian Department of National Defence, as part of the development of a computer model to evaluate fire protection systems in light industrial buildings (FIERAsystem). The new model considers the geometry of the building, possible fire scenarios that may occur in the building, fire detector locations and characteristics, the effect of automatic suppression systems on the fire, the locations of adjacent buildings and the response and effectiveness of the fire department. The program calculates the required flow rates of water at the time of fire department intervention for suppression of the fire and for exposure protection for each side of the building. These flow rates can then be compared to the total capacity of the fire engines available to determine if existing resources are sufficient. The program has been designed to be interactive, so that the user can immediately see the effects of various parameters on the required water flow rate. Descriptions of case studies are also included to demonstrate the use of this model.

A variable property heat transfer model for predicting soil temperature profiles during simulated wildland fire conditions

A numerical model of heat transfer in dry soil was developed to predict temperatures and depth of lethal heat penetration during cone calorimeter tests used to simulate wildland fire exposures. The model was used to compare predictions made using constant and temperature-dependent thermal properties with experimental results for samples of dry sand exposed to heat fluxes of 25, 50 and 75 kW m–2. Depths of lethal heat penetration predicted using temperature-dependent properties were within 2 to 10% of the values determined using measured temperatures, while predictions made using constant properties were within 10 to 21% of the experimental values. In both cases, predictions made by the model were within the 1-cm accuracy with which the depth of seeds and plant shoots in the soil can be determined in practice. The model generally over-predicted the depth of lethal heat penetration in dry or moist soil when temperature-dependent properties were used, and over-predicted the depth of lethal heat penetration in soils with a moisture content of greater than 10% if constant thermal properties were used.

An analysis of two-heater active thermal control technology for device class testing

A novel technology for controlling temperature rise in the class testing is described in this article. This technology is based on two active heater sources and is called a two-heater active thermal control (2H-ATC) system. From a point of control, a lumped analytical model for representing the whole class testing process is very important, and is developed in this article. The model was validated by comparing the simulated result with the measured result on a commercial tester. Based on this model, we have studied the issue of optimization of the performance of the testing process, in particular examining effects of test system parameters on system performance. We have also observed a concept called critical heater power, which is important in achieving a minimum overshoot at the transition from the preheating stage to the testing stage. The outcome of this study has already been applied in practical process control during the whole class testing.