Using Directional Flame Thermometers for Measuring Thermal Exposure

Abstract

One of the recommendations that came from the NIST investigation of the World Trade Center disaster was the need for quantitative heat flux measurements in larger scale fire safety tests. These heat flux data are needed to support the development of engineering models to predict the performance of fire protection materials and systems. Current standardized fire resistance tests such as ASTM E119 or ISO 834 or IMO A754 are all qualitative tests. The furnace temperature is controlled to a standard time-temperature curve. Implicit assumptions are made that (1) the thermal exposure can be described solely by the measured furnace temperature history and (2) that exposure will be repeatable. Historical variations of 50 % or more in the qualitative fire protection ratings, such as a 1 h fire barrier, between different furnaces or laboratories indicate that these two assumptions are not well founded. This paper describes the use of a proven type of sensor called a directional flame thermometer (DFT) for making quantitative heat flux measurements in fire resistance tests. DFTs have been used for over 20 years to characterize the thermal environment in both large pool fires and in furnaces, to monitor flashover in structure fires, and in many other fire environments. DFTs are passive thermocouple-based sensors. They do not require calibration. Instead, the designs and materials with known thermal properties are fixed to provide a repeatable response. Using inverse heat conduction analysis techniques, heat fluxes are calculated using a heat conduction model of the DFT with temperature-dependent thermal properties and two or more thermocouple temperature measurements in a DFT. A fully nonlinear inverse heat conduction code is used for detailed post-test data analysis. A new data analysis tool for DFTs, called an inverse heat conduction-digital filter functions (IHC-DFF) has been developed for specific DFT designs to provide heat flux measurements in real-time, much like a calibration curve. IHC-DFFs are convolution-type digital filters that are used to provide real-time heat flux readouts during a test or for a quick-look capability for large sets of data. Simpler models are also used for analyzing early (<5–10 min) and late-time DFT data (>15 min). The current work demonstrates that DFT measurements can provide the quantitative data needed to support the development of performance models and improve our understanding of the thermal exposure in fire resistance tests.

Performance testing of wildland fire chemicals using a custom-built heat flux sensor

“…Several alternative robust devices have been studied and developed to measure heat flux in fire conditions. [10][11][12][13] However, these alternative devices were not without fault, and it was identified that there were various issues such as inconsistencies in the heat flux estimates, slow response times, and various miscellaneous design flaws. [10][11][12][13] To resolve some of these adverse issues, Sullivan and McDonald 14 developed a robust and inexpensive custom-built sensor, capable of measuring high heat loads in wildland fire scenarios for extended periods of time, without the use of external cooling.…”

“…[10][11][12][13] However, these alternative devices were not without fault, and it was identified that there were various issues such as inconsistencies in the heat flux estimates, slow response times, and various miscellaneous design flaws. [10][11][12][13] To resolve some of these adverse issues, Sullivan and McDonald 14 developed a robust and inexpensive custom-built sensor, capable of measuring high heat loads in wildland fire scenarios for extended periods of time, without the use of external cooling. The sensor estimated the total heat flux emitted from burning vegetative fuels to within one standard deviation of the values measured by a commercial Schmidt-Boelter heat flux gauge and within a comparable response time.…”

“…Several alternative robust devices have been studied and developed to measure heat flux in fire conditions. 10–13 However, these alternative devices were not without fault, and it was identified that there were various issues such as inconsistencies in the heat flux estimates, slow response times, and various miscellaneous design flaws. 10–13…”

Evaluation of a new device for simultaneous measurement of heat flux and gas velocity in a diffusion flame

“…The directional flame thermometers (DFT) can be used to measure heat flux. 1 DFTs essentially consist of two parallel Inconel plates separated by ceramic fiber insulation. Temperature is measured at the interior surface of the two plates.…”

Steady‐state heat flux measurements in radiative and mixed radiative–convective environments

“…‡ These sensors are not normally intended for use in situations where convection is significant [4,5]. Other gauges that have been developed for research applications, such as the directional flame thermometer (DFT) [6] and the hemispherical heat flux gauge (HFG) [7], have been tested mostly in radiative applications and have not yet been characterized extensively in mixed radiative-convective conditions. The purpose of this paper is to provide a systematic comparison of the responses of four different heat flux gauges in a variety of radiative and mixed radiative-convective environments.…”