Hot spot locations and temperature distributions in a forced convection photochemical reactor

We have computed steady axisymmetric temperature distributions in a tubular photoreactor in which fluid in laminar flow absorbs light from an azimuthally uniform, radially incident light source. In addition to the primary on-axis temperature maximum, a second hot spot forms off the centerline as the optical density γ increases. This results from a shift of light absorption toward the wall, which is held at a fixed temperature (e.g. by forced convection cooling on the outside). At still larger γ the centerline hot spot disappears, leading to a decrease in the maximum temperature and accompanied by an increase in the temperature near the wall. There is a critical value of γ, depending on the Peclet number Pe, below which the hot spot is located on the centerline and above which a second hot spot forms and moves off the centerline. The reduction in maximum centerline temperature may be advantageous for product selectivity and yield in the central core of the reactor, but may be disadvantageous with respect to the formation of light-absorbing deposits on the interior of the tube wall. Axial heat conduction has significant effects for Pe < 50, including displacement of the hot spot upstream from the exit plane of the illuminated section, as well as an increase in the maximum temperature.