HEAT TRANSFER IN GLASS QUENCHING FOR GLASS TEMPERING

This paper presents the results of an experimental study of heat transfer characteristics in single-phase and two-phase stagnation point flows pertinent to quenching of glass in the tempering process. Two-phase flows were generated by injecting water mist into the stagnation flow air far upstream of the nozzle exit. This resulted in a temporal and spatially invariant size distribution of the droplets that were carried toward the hot test plate by the air flow. PIV measurements were made at the nozzle exit to determine the mainitude and uniformity of air velocity profile in both single-phase and two-phase flows. The two-phase flows were also characterized by measurements of drop size distribution and number density using images of droplets resulting from laser induced fluorescence. The ratio of nozzle to-plate distance and the nozzle diameter was maintained at 0.5 throughout the experiments. Steady state experiments were performed for plate heat fluxes ranging from 10 to 50kW/m(2), and Reynolds numbers ranging from 2,000 to 122,000 and water/air mass flow ratios up to 4.75%. Single-phase flow results indicate that the Reynolds number dependence of the Nusselt number is similar to Re-0.68. Two-phase flow results show a maximum heat transfer enhancement of 26% for water/air mass flow ratio of 4.75%. It was visually determined that for plate temperatures above 200 degrees C and for the drop size distribution tested, the water droplets do not impinge on the plate surface. Therefore, the heat transfer enhancement was attributed to the evaporation of water droplets within the thermal boundary layer. This is an important condition to prevent spatially non -uniform quenching and the resulting shattering of glass. Transient characteristics of single-phase and two-phase flows were also analyzed and compared. By changing the water/air mass flow ratio, the cooling curve for a two-phase flow can be adjusted to meet the requirements of the industrial process.