[en] This report deals with the experimental study of forced heat convection in annular spaces through which flow of air is passing when a uniform heat flux is dissipated across the inner wall. These observations took place chiefly in the region where thermal equilibrium are not yet established. Amongst other things it became apparent that, both in the region where thermal equilibrium conditions are on the way to establishment and where they are already established, the following relationship held good: the longitudinal temperature gradient, either on the wall or in the fluid stream, is proportional to the heat flux dissipated q, and inversely proportional to the average flow rate V: dT/dx = B (q/V). From this result the next step is to express the variations of the local convection coefficient α (or of the Margoulis number M) in a relationship of the form: 1/M = ψ(V) + F(x). If this relationship is compared with the classical empirical relationship α = KVⁿ (where n is close to 0.8), the relationship: 1/M = ξV^1-n + F(x) is obtained (ξ is a constant for a given annular space); from this it was possible to coordinate the whole set of experimental results. (author)