Heat transfer in thin fibrous materials under high heat flux conditions

Despite the existence of standardized bench top tests for evaluating fabrics for thermal protective clothing for flash fire and other high heat flux exposures, there are still many questions about the thermal response of these thin fibrous materials under high heat flux conditions. While others have developed analytical and numerical models of these materials, these models are difficult to use and have not been overly successful in predicting fabric temperatures.

The finite element method was used to develop a model of the heat transfer in two common inherently flame resistant fabrics, Nomex$\sp{\circler}$ IIIA and Kevlar$\sp{\circler}$/PBI, subjected to the high heat fluxes used in bench top tests, such as the thermal protective performance (TPP) test. The apparent heat capacity method was used to model thermochemical reactions in these materials using information from thermal gravimetric analysis (TGA) and differential scanning calorimeter (DSC) tests. In-depth absorption of radiation, variable thermal properties, and the heat transfer across an air space between the fabric and a test sensor were also included. The absolute temperatures predicted by this relatively simple model are within 4% of those measured by an infrared thermometer. Estimated times to the Stoll second degree burn criterion are within 6% of those from actual tests. Flow visualization was also used to describe the natural convection flow patterns in the air space.

A parametric study conducted using this numerical model indicated that the most important parameters are the boundary conditions used to describe the high heat flux exposure. Effects of varying other parameters. such as fabric thickness and moisture regain were also demonstrated. These results were used to explain differences in the performance of the two fabrics.

Possible modifications to existing bench top test methods were also investigated. From the heat transfer perspective planar tests are shown to be adequate representations of the human body. Fixed duration flame exposure tests provide more information than existing tests. Advantages and disadvantages of using copper disk and skin simulant heat flux sensors were outlined. Different methods used to treat data from these test sensors, and to make skin burn predictions, were also compared.