Apparatus and methods

Apparatus 2 comprises a plurality of co-axial shells 10 which are spaced apart from each other and each shell having a cross-sectional area narrowing in an axial direction, and the apparatus having an entrance 4 and exit 6 for a medium that passes between the shells. The shells may be spaced laterally and axially, may comprise of truncated hollow cones, spheres (fig 3), pyramids (fig 2), polygons (fig 4), domes (fig 5), ovals (fig 7), or vault (fig 6) shapes. The apparatus may be a catalytic converter, a heat exchanger (fig 10), an electrical resistance heater (fig 9), a thermoelectric device or a solar absorber (fig 11). Each shell may be made from a catalyst or comprise of a catalyst coating or comprise of radiation absorbing material. The shells may provide combustion chambers, their walls may be perforated (22, fig 10) or corrugated. The medium may be a fluid such as water and/or air that may be mixed together and heated as it flows through the spaces.