Modelling of the thermal tempering of flat glass : determination of radiative and convective heat transfer enabling prediction and control of the residual stresses

The thermal tempering of flat glass in a laboratory facility and an industrial unit with single-jet and multi-jet coolings have been modelled. Firstly the influence of radiation on the transient and residual stresses through the glass thickness was considered. 3D computational fluid dynamic (CFD) models were then developed to analyse the airflow and determine the convective heat transfer at the glass surface during tempering. By coupling these models with 3D finite element models taking both structural and stress relaxation into account, the residual tempering stresses were computed. Photoelastic measurements of the stresses on the surface and through the thickness of the tempered glass plates were used to validate the numerical approach. The homogeneity of the temperature distribution and the residual stresses in the multi-jet configuration were analysed. Finally, a new ultrasonic method to control the residual tempering stresses is proposed. The measurements made on glass plates tempered using a single-jet configuration were in agreement with the numerical predictions.