Heat transfer and flow characteristics of the cooling system of an industrial glass tempering unit

This paper deals with impinging air jets cooling applied to thermal glass tempering. The static tempering of a square flat glass sample in an industrial unit is modelled to assess the convective heat transfer from the glass surface to the cooling jets. The flow in the air supply nozzles is first investigated to verify the uniformity of the flow conditions at the jet outlets. The numerical simulation of the static tempering of a flat glass sample is validated by comparison with measurements of the residual stresses by photoelasticity and with surface temperature measurements by pyrometry. Heat transfer coefficients are calculated for four different jets Reynolds numbers. It is found that the relative deviation of the local heat transfer from the average heat transfer is independent of the Reynolds number and that the average Nusselt number correlates to the Reynolds number by a power law in good agreement with those commonly found in the literature. The heat transfer is low compared to other correlations, because of the relatively large H/D ratio and of the strong cross flow induced by the large amount of spent air.