We investigate how the shape of a heat source may enhance global heat transfer at short time. An experiment is described that allows us to obtain a direct visualization of heat propagation from a prefractal radiator. We show, both experimentally and numerically, that irregularly shaped passive coolers rapidly dissipate at short times, but their efficiency decreases with time. The de Gennes scaling argument is shown to be only a large scale approximation, which is not sufficient to describe adequately the temperature distribution close to the irregular frontier. This work shows that radiators with irregular surfaces permit increased cooling of pulsed heat sources.