Numerical analysis of the radiant heating effectiveness of a continuous glass annealing furnace

Radiative heat transfer is the dominant mode of heat transfer in glass annealing furnaces. Especially in electrically heated furnaces due to the nonparticipating nature of the furnace atmosphere the radiative heat transfer predominantly occurs between surfaces. In this regard, in container glass manufacturing the layout of the container glasses inside the furnace can considerably affect the radiative heating effectiveness of the furnace. In this numerical study, the effect of the spacing between the bottle rows on the radiant heating effectiveness of the furnace was numerically investigated. An industrial scale continuous annealing furnace model was used and the combined conduction - radiation heat transfer modes in the furnace were solved with the in-house developed transient solver. The convective heat transfer inside the furnace was not taken into account due to its relatively lower contribution in heat transfer. The computational cost of the numerical model is reduced with the use of a computational domain consisting of 7 bottle rows instead of full 29 bottle rows. Reverse Monte Carlo Ray Tracing method is used to solve for the surface-to-surface radiative heat transfer inside the radiant heating zone and integrated into GPU to reduce its computational cost. Further computational speed-up was achieved with the use of a 2nd order accurate radiation boundary condition implementation. It allows a time step increase of 2.5x in comparison with the first order accurate implementation for the same level of accuracy. Finally, a parametric study on the effect of the spacing between the bottle rows on the radiative heating of the bottles have been conducted. In this regard, the spacing between the bottle rows and the conveyor speed have been altered in a way that the throughput of the furnace, the number bottles processed per unit time, is kept constant. The study shows that there is an optimum spacing value between the bottle rows. For the particular bottle geometry considered in this study, the optimum row spacing value is found to be 35% of the bottle diameter. Running the furnace at this optimum value, the length of the heating zone could be reduced by 8.5% compared to the case when the furnace was operated with the bottles almost in touch with each other.