  The accidental ignition of combustible atmospheres by hot surfaces is of
great concern for chemical and process plant safety. In this paper, we present
our research regarding the evolution of thermal plumes originating from hot
hemispheres and discs. In particular, we focus on the effect of the orientation
of the surface on the ignition process. The auto-ignition temperatures and
ignition locations were studied experimentally. To get further insight, we
conducted detailed numerical simulations and validated them with measurements.
Three-dimensional simulations were performed on hot hemispheres and hot discs
for different orientations ranging from 0{\deg} to 180{\deg}. The solver
employs a transient, implicit scheme which is based on the coupled heat
transfer and flow equations. The mesh in the vicinity of the hot surfaces is
refined to resolve the steep temperature gradients and to capture the boundary
layer separation. The influence of the orientation on critical hot spots in the
gas mixture is analysed by examining the flow structures and the temperature
evolution of the buoyancy-driven flow. Using the obtained results, we discuss
the change of the onset and location of the ignition.
