Measurement of Heat Transfer and Fire Damage Patterns on Walls for Fire Model Validation

Abstract

Fire models are presently employed by fire investigators to make predictions of fire dynamics within structures. Predictions include the evolution of gas temperatures and velocities, smoke movement, fire growth and spread, and thermal exposures to surrounding objects, such as walls. Heat flux varies spatially over exposed walls based on the complex thermal interactions within the fire environment, and is the driving factor for thermally induced fire damage. A fire model predicts the temperature and heat transfer through walls based on field predictions, such as radiative and convective heat flux, and is also subject to the boundary condition represen-tation, which is at the discretion of model practitioners. At the time of writing, Fire Dynamics Simulator can represent in-depth heat transfer through walls, and transverse heat transfer is in a preliminary development stage. Critically, limited suitable data exists for validation of heat trans-fer through walls exposed to fires. Mass loss and discoloration fire effects are directly related to the heat transfer and thermal decomposition of walls, therefore it is crucial that the representation of transverse heat transfer in walls in fire models be validated to ensure that fire investigators can produce accurate simulations and reconstructions with these tools. The purpose of this study was to conduct a series of experiments to obtain data that addresses three validation spaces: 1) thermal exposure to walls from fires; 2) heat transfer within walls exposed to fires; and 3) fire damage patterns arising on walls exposed to fires. Fire Safety Research Institute, part of UL Research Institutes, in collaboration with the Bureau of Alcohol, Tobacco, Firearms and Explosives Fire Research Laboratory, led this novel research endeavor. Experiments were performed on three types of walls to address the needs in this validation space: 1. Steel sheet (304 stainless steel, 0.793 mm thick, coated in high-emissivity high-temperature paint on both sides). This wall type was used to support the heat flux validation objective. By combining measurements of gas temperatures near the wall with surface temperatures obtained using infrared thermography, estimates of the incident heat flux to the wall were produced. 2. Calcium silicate board (BNZ Marinite I, 12.7 mm thick). This wall type was used to support the heat transfer validation objective. Since calcium silicate board is a noncombustible material with well-characterized thermophysical properties at elevated temperatures, measurements of surface temperature may be used to validate transverse heat transfer in a fire model without the need to account for a decomposition mechanism. 3. Gypsum wallboard (USG Sheetrock Ultralight, 12.7 mm thick, coated in white latex paint on the exposed side). This wall type was used to support the fire damage patterns validation objective. Two types of fire effects were considered: 1) discoloration and charring of the painted paper facing of the gypsum wallboard; and 2) mass loss of the gypsum wallboard (which is related to the calcination of the core material). In addition to temperature and heat flux measurements, high resolution photographs of fire patterns were recorded, and mass loss over the entirety of the wall was measured by cutting the wall into smaller samples and measuring the mass of each individual sample. A total of 63 experiments were conducted, encompassing seven fire sources and three wall types (each combination conducted in triplicate). Fire sources included a natural gas burner, gasoline and heptane pools, wood cribs, and upholstered furniture. A methodology was developed for obtaining estimates of field heat flux to a wall using a large plate heat flux sensor. This included a numerical optimization scheme to account for convection heat transfer. These data characterized the incident heat flux received by calcium silicate board and gypsum wallboard in subsequent experiments. Fire damage patterns on the gypsum wallboard, attributed to discoloration and mass loss fire effects, were measured. It was found that heat flux and mass loss fields were similar for a given fire type, but the relationship between these measurements was not consistent across all fire types. Therefore, it was concluded that cumulative heat flux does not adequately describe the mass loss fire effect. Fire damage patterns attributed to the discoloration fire effect were defined as the line of demarcation separating charred and uncharred regions of the wall. It was found that the average values of cumulative heat flux and mass loss ratio coinciding with the fire damage patterns were 10.41 ± 1.51 MJ m−2 and 14.86 ± 2.08 %, respectively. These damage metrics may have utility in predicting char delineation damage patterns in gypsum wallboard using a fire model, with the mass loss ratio metric being overall the best fit over all exposures considered. The dataset produced in this study has been published to a public repository, and may be accessed from the following URL: <https://doi.org/10.5281/zenodo.10543089>.