Description of heat flux measurement methods used in hydrocarbon and propellant fuel fires at Sandia.

Abstract

The purpose of this report is to describe the methods commonly used to measure heat flux in fire applications at Sandia National Laboratories in both hydrocarbon (JP-8 jet fuel, diesel fuel, etc.) and propellant fires. Because these environments are very severe, many commercially available heat flux gauges do not survive the test, so alternative methods had to be developed. Specially built sensors include "calorimeters" that use a temperature measurement to infer heat flux by use of a model (heat balance on the sensing surface) or by using an inverse heat conduction method. These specialty-built sensors are made rugged so they will survive the environment, so are not optimally designed for ease of use or accuracy. Other methods include radiometers, co-axial thermocouples, directional flame thermometers (DFTs), Sandia "heat flux gauges", transpiration radiometers, and transverse Seebeck coefficient heat flux gauges. Typical applications are described and pros and cons of each method are listed. Heat Flux Measurement Methods in Fires of 62 AcknowledgementsThe author would like to acknowledge the support of numerous colleagues who have developed these heat flux measurement methods and their use over the years. Some of these colleagues are (in alphabetical order) Bennie Blackwell, Tom Blanchat, Tom Diller, Victor Figueroa, Walt Gill, Chuck Hanks, and Ned Keltner. Thanks to Allen Ricks and Ben Blackwell for their peer review. Heat Flux Measurement Methods in Fires5 of 62 Heat Flux Measurement Methods in Fires7 of 62 List of Tables IntroductionHeat flux measurements are important because, in heat transfer applications, providing only a temperature measurement is sometimes not sufficient to fully specify the environment. Using the electrical analogy (Ohm's Law), one needs two of the three variables (current, potential difference, or resistance) to fully specify an electrical circuit. Similarly, in a heat transfer application, one needs two of the three variables (heat flux, temperature difference, and thermal resistance) to fully specify the problem of interest. The temperature difference is similar to a potential difference, heat flux is similar to current, and thermal resistance is similar to electrical resistance.Most often the metric of interest is the response of an item in a fire. This metric is more related to the heat flux than just to the temperature because the thermal resistances to radiation and convection are not captured when only the temperature is specified.The measurement of heat flux in hydrocarbon fuel fires (e.g., diesel, JP-8, etc.) is difficult due to the high temperatures (e.g., 1000qC or higher), the sooty and chemically reactive environment, and due to large temperature gradients which produce thermal stresses. Many commercially available sensors do not work well in fires because they are water-cooled. Soot builds up on the sensing surface due to thermophoresis and this layer of soot changes the gage sensitivity. Post-test soot deposit thicknesses of ¼" have been measured after fir...

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