Contact between hot glass and wet porous material : Part 1. Theoretical considerations and experimental results

Abstract

The contact behaviour between hot glass and a porous material containing water has been investigated. The mathematical equations which describe the one-dimensional problem have been derived. A test rig has been developed to measure the time-dependent contact temperature and pressure for contact materials such as sintered stainless steel, alumina, zirconia and sintered glass. It could be shown that the contact temperature is dependent on the saturation pressure of the generated steam. This is valid for the tight system. If the system is leaky and steam escapes laterally, the boiling front migrates into the porous medium and the contact temperature rises rapidly above the saturation temperature. The heat flux density originating from the thermal contact has been calculated using the mathematical equations and the experimental data. In the first second of contact, the heat flux density is nearly constant. With a pressure of 3 bar and a porosity of 25% of the medium the heat flux density ( in W/m²) is 140*10³ for stainless steel, 120*10^ for alumina, 115-10³ for zirconia ( ZrO₂ ) and 70*10³ for sintered glass. The leaky system shows higher values of heat flux density of 260* 10³ W/m² for the sintered stainless steel which is dependent upon the mass flow of the escaping steam. After release o f the pressure, the hot water (ϑhw > 100 °C) reboils. Only some of the water boils, the rest being ejected in form of small droplets or foam. In the case of the leaky system, the reboiling and ejection of water droplets are more violent. The tight system develops a steam layer, which prevents direct contact between the glass and the porous material. The resulting glass surface is very smooth. Under certain not yet fully clarified conditions contact with sintered alumina and zirconia produces glass surfaces covered by numerous rounded pits with a diameter of ≈ 5 µ m . These pits are probably generated by the ejected water droplets.