Numerical analysis of the flow and heat transfer in cylindrical clothing microclimates – Influence of the microclimate thickness ratio

•Natural convection can play a central role on the heat transfer in clothing microclimates.•Natural convection should not be ignored for microclimate thickness ratio above 0.1.•Convective fluxes vary along the skin by up to 90% for a thickness ratio of 0.25.•Heat transfer strongly varies along the microclimate so space-averaged results can be misleading. Clothing microclimates, i.e. the space between the skin and the clothing, can play a central role in the heat and mass exchanges from or to the body. This is especially true for protective clothing, where microclimates are generally thicker and natural convection is more likely to occur. We used a computational fluid dynamics approach to perform numerical studies of fluid flow and heat transfer across cylindrical clothing microclimates for Reynolds number of 3900. Transient simulations were performed for three different values of microclimate thickness to diameter ratio (0.05, 0.10 and 0.25), considering a two-dimensional cross-section of a human limb surrounded by a porous fabric and exposed to cool external air (10°C). The obtained local heat transfer along the skin shows that increasing the microclimate thickness ratio from 0.05 to 0.25 decreases the convective heat fluxes by up to 100% in the upstream regions of the microclimate, and increases them up to 190% in the downstream regions. This asymmetry, which indicates an increasingly important role of natural convection as the microclimate thickness ratio is increased, is often overlooked in space-averaged approaches due to the opposite changes in the different regions of the microclimate. Local variations in temperature along the outer fabric and in convective fluxes along the skin were significant, reaching up to 14K and 90%, respectively. The critical thickness ratio above which natural convection should not be ignored was found to be 0.1 (e.g. corresponding to a microclimate thicknesses of 11 mm or 8 mm, around an upper arm or forearm, respectively).