Journal Article Reviews and Analyzes Methods of Separating Heat Transfer Modes in Fire
The peer-reviewed journal article “Separation of Heat Transfer Modes in Fire: Review and Analysis” has been published in the Fire Safety Journal. Key findings from this study support the Heat Transfer and Fire Damage Patterns on Walls for Fire Model Validation research project, led by the Fire Safety Research Institute (FSRI), part of UL Research Institutes. The article was co-authored by FSRI research engineers Jason Floyd and Matt DiDomizio, as well as Jonathan Hodges from Jensen Hughes.
Understanding Different Heat Transfer Modes
Heat transfer from a fire-adjacent object to a surface includes both convection and radiation heat transfer. Computational Fluid Dynamics (CFD) fire models independently predict each of these heat transfer modes; however, experimental measurements of heat transfer are typically limited to measurements of the total heat transfer rather than the separation of individual components. Each heat transfer mode has different components which can contribute to the uncertainty of variation in heat transfer modes. One of the difficulties in improving CFD fire model predictions of heat flux from a fire to a surface is the high uncertainty in separating an experimentally-measured heat flux into its convective and radiative components. Without this information, it’s difficult to identify the specific gaps in understanding which require further research to improve model predictions. Over the years, researchers have proposed several methods for separating the heat transfer components; however, each approach comes with its own assumptions, limitations, and uncertainties.
Conducting Literature Surveys
Different studies have used a variety of methods to separate heat transfer modes. Since there is no consensus on the accuracy associated with these different approaches, five categories of methods used to separate the fluxes have been identified to review prior work:
- Single measurement augmented with analysis
- Isolating the transmission of the radiative heat flux
- Disrupting the convective boundary layer
- Varying the absorbed area of radiation
- Varying the surface temperature
The review of prior work focused on approaches that have used experimental methods to separate the heat transfer modes without relying on analytical, empirical, or computational calculations of radiation or convection. This study also reviewed four different principles employed in the literature, summarizing the existing studies, the mathematical formulations employed in the methods, applied engineering analysis to the assumptions made in each method, and estimated the expanded uncertainty associated with each approach.
“It was surprising how large uncertainties in mode separation could be for conditions found in typical fire experiments.”
—Jason Floyd, principal research engineer, FSRI
Reducing Uncertainty in Heat Transfer Modes
After reviewing prior work, it was determined that under ideal conditions, the uncertainty of the separated heat transfer mode is approximately 15% of the total gauge heat flux for each method. Under less-than-ideal conditions, this uncertainty can be significantly higher. General recommendations to reduce these uncertainties in future studies are provided below:
Calibration: Use the same water temperature, sensor mounting, and substrate configuration in the calculation of the sensor sensitivity in calibration that is planned for use in testing.
Cooling Water: Use hot water for water-cooling when possible to reduce the potential for condensation on the gauge or window.
Geometric Similarity: When geometric similarity is assumed through test replicates or symmetry using diverse sensors for separation, verify that the same thermal environment is achieved with additional sensors.
Geometric Proximity: When two gauges are placed next to each other for the purposes of separating heat transfer modes, align the two gauges in the direction where the gradient in heat flux is expected to be the lowest (i.e., vertically aligned in a wall fire). The separation distance should be minimized as much as practically possible, ideally 12 mm or less.
Soot Deposition: Minimize the exposure of low emissivity or windowed sensors until the target measurement period. Prevent further accumulation of soot on the sensor after measurement. Characterize the quantity of soot at the end of testing.
Sensor Temperature: The heat transfer coefficient between low and high-temperature scissors is likely to be different. Consider separating heat transfer modes at both high temperatures and low temperatures to characterize the change in convection with temperature.
Time-Averaging: Selection of the time-averaging window is important in these measurements. There are competing requirements between minimizing the duration of the exposure to prevent fouling of the sensor from soot and a larger averaging window to reduce uncertainties due to the intermittence of the flame. The analysis in this work indicates a 10-second averaging window removes most of the effects of local intermittence in the flaming region; however, longer averaging windows of 20–30 seconds may be needed outside the continuous flaming region. In addition, longer averaging windows may be required at all locations depending on the stability of the laboratory conditions.
Transpiration Radiometers: The uncertainty in these devices for local fire exposures is high due to local disruptions in the flow field affecting the incident radiation from the flame to the surface. Focus measurements with these devices on scenarios in large flames where other sensors would not withstand the exposure.
View Angle: Minimize the difference in view angles between gauges when separating the heat transfer modes. The difference between a 150-180 degree angle was not significant in the cases evaluated in this work; however, differences were observed once a narrower view angle was used.
Window Materials: The uncertainty using ZnSe windows is significantly lower than with other window materials due to the flat transmissivity spectra. However, these windows may begin to oxidize after 1-2 minutes of direct flame exposure. Consider limiting the exposure duration or adjusting the sensor design to improve contact of the window with the heat sink to reduce the temperatures reached. If other window materials are used with more variable transmission spectra, verify that the spectra used in calibration are similar to those expected during testing.
About Fire Safety Journal
Fire Safety Journal is the leading publication dealing with all aspects of fire safety engineering. Its scope is purposefully wide, as it is deemed important to encourage papers from all sources within this multidisciplinary subject, thus providing a forum for its further development as a distinct engineering discipline. This is an essential step towards gaining a status equal to that enjoyed by the other engineering disciplines.
