Natural convection in horizontal layers of water cooled from above to near freezing was studied analytically. The water was confined laterally and underneath by rigid insulators, and the upper horizontal surface was subjected to: (1) a constant 0C temperature, rigid conducting boundary, and (2) a free, water to air convection boundary condition, in which the convective heat transfer coefficient was held constant at values of 5.68 W/m2 · K and 284 W/m2 · K (1.0 and 50.0 Btu/hr ft2F) and the temperature of the ambient air was maintained at 0C. The ratios of the width to the depth of the rectangular water layers under consideration were W/D = 1, 3, and 6. Initially the water is assumed to be at a uniform temperature of either 4C or 8C, and then the upper surface boundary condition was suddenly applied. It was observed in all cases for which the initial water temperature was 4C, that the water remained stagnant and became thermally stratified. Heat transfer application of either of the surface boundary conditions to water initially at 8C produced large convective eddies extending from the bottom to the top of the layer of water. As the liquid layer cooled further, two distinct horizontal regions appeared, the 4C isothermal line separating the two. This produces a region of hydrodynamic instability in the fluid since the maximum density fluid (4C) is physically located above the less dense fluid in the lower portion of the cavity. The large eddies which appeared initially were confined to the hydrodynamically unstable region bounded by the 4C isotherm and the bottom of the cavity. The action of viscous shearing forces upon the stable water above the 4C isotherm produced a second “layer” of eddies. An alternating direction implicit finite difference method was used to solve the coupled system of partial differential equations. The paper presents transient isotherms and streamlines and a discussion of the effect of maximum density on the flow patterns.

Soldiers, first responders and other high risk occupations such as power line technicians are routinely exposed to dangerous situations where severe burn injuries are possible. Standard flame resistant (FR) fabrics provide minimal burn protection when exposed to a flash flame incident. As a result, improvement in thermal protection is desperately needed and remains an ongoing subject of research and development. A simplified one dimensional physical model composed of a muscle layer, skin/fat layer, air gap(s) and fabric layer(s) is used to model heat transfer entering the body covered by a garment that is exposed to a flash flame. Heat transfer within the skin and muscle layers is modeled by combined conduction, metabolic heat generation and blood perfusion by a recently developed modification to the heat equation termed the bio-heat equation. Boundary conditions include a fixed temperature (core body temperature) at the inside of the muscle layer and combined convection and radiation from the flame on the outside of the fabric. The heat equation is solved by discretizing the domain in one dimension and using a finite volume approach to derive the finite difference equations. This model is an initial step to be used to provide an assessment of common FR garments with respect to both comfort in ambient conditions and protection during a flash flame. It also provides for parametric analysis to determine ideal thermo-physical properties, fabric thicknesses and layering for better protection during flash flame incidents. Estimates for time to burn injury from the numerical model is presented with experimental results using live mannequin flame tests (ASTMF-1930), standard vertical flame tests (ISO-17492) and a non-standard flame test with combined convection and radiation heat fluxes up to 85 kW/m2. The main effort of this study revolves around an initial working design for a dynamic garment termed On Demand Thermal Protection (ODTP). The primary focus of the design is the development of a thermistor circuit embedded in a protective garment to act as an electric sensor for rapidly deploying the necessary thermal protection that is needed as predicted by the numerical model instantaneously in the event of a flash flame incident. An initial prototype is being developed with a focus on designing the thermistor circuit to mechanically actuate protective components in a flash-flame environment. Concepts include rapidly releasing a pressurized flame retardant fluid through vinyl tubing sewn into a garment and deploying a protective barrier around the face and neck when the thermistor circuit detects a sudden change in heat transfer. A summary of the prototype along with experimental testing to date compared to the theoretical predictions from the model described above is presented.