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Thermal neutrality

Thermal neutrality defines temperatures on a manikin in an optimum state that guarantees overall thermal comfort. Each real or simulated thermal state can be compared to this state to demonstrate local or global temperature differences according to the state of optimum thermal comfort.

The typical way to derive thermal comfort indices in simulations is based on temperatures or heat fluxes on human skin sectors. This implies to comparisons between the actual thermal state and the state of thermal neutrality. E.g. for Zhang's local comfort and sensation indices the model requires the difference T_sk-T_sk,0, actual skin temperature minus neutrality temperature. Aside from that, core temperatures might also be important, but are often negligible because of moderate thermal conditions, e.g. in automotive cabins.

In terms of pre-processing, the state of neutrality with all corresponding local skin temperatures is essential. Therefore a steady state simulation for the un-clothed, resting manikin is required at certain boundary conditions: at 40% humidity and low air speed the environmental air/wall temperature is 30°C. Those boundary conditions are typically "thermally comfortable", the human thermal sensation is "neutral", i.e. neither warm nor cold.

The FIALA-FE simulation leads to a thermal equilibrium at 37°C core and 34.4°C mean skin temperature without any active system regulations like shivering, vasomotion or sweating. Temperatures reach steady state conditions and internal metabolic heat production is fully conducted via boundary conditions, like convection, radiation, evaporation and respiration. Plotting the simulated temperatures on the human skin shows maximum values (35-36°C) for head and thorax. As shown in the figure skin temperatures decline for peripheric regions, except for hands. Lowest temperatures of about 32°C appear at the feet. Obviously all human temperatures are higher than the environment temperature of 30°C. This allowes internal metabolic heat to be conducted to the environment.