Pavel Hrma

Thermal healing of cracks in glass

Abstract Vickers indented glass samples were subjected to heat treatments from 525 to 725°C. Subsequently, they were examined microscopically at room temperature. Blunting and pinching of crack tips, rounding and grooving of radial cracks, receding and breaking up of lateral and median cracks, and cylinderization and spheroidization of closed cavities were observed. The morphology change pattern indicates that viscous flow driven by capillarity was operating. When tested for strength in bending at room temperature, samples fractured through the indent if they had experienced only the early stages of healing. Otherwise, fracture occurred elsewhere. Both indented and as-received samples respond to elevated temperature by strengthening followed by weakening. Indented samples strengthened to the level of as-received samples within a narrow time interval, probably due to crack bridging resulting from grooving. As-received samples strengthened gradually with time, suggesting that processes other than viscous flow were responsible for their heating. It is proposed that pre-existing flaws are similar to the reversibly closed macroscopic cracks with hydrogen or cationic bonds. Weakening is rationalized by new flaws development at elevated temperatures.

Model for a steady state foam blanket

Abstract A model relating foam height to bubble size and gas flux has been established based on the assumption that the foam height is determined by gravitational drainage and the survival time of a critically thin film separating a top bubble from the atmosphere. The foam behavior is described in terms of two limiting gas fluxes: the threshold flux and the critical flux. If the critical flux is smaller than the flux at which bulk foam occurs and the actual gas flux approaches the critical flux, foam height becomes unbounded, and the foam volume increases until vent holes begin to develop. If bubbles in the liquid grow much faster than they rise to the surface, bulk foam is created rather than surface foam. An example of foam from refining gases in a continuous glassmelting furnace is discussed in detail. Bulk foam is common in allectrical glassmelting.

Interaction between solid, liquid and gas during glass batch melting

Abstract A set of data on dissolution of monodispersed silicate sand in a glass batch suddenly exposed to a constant temperature within the range from 1250 to 1410°C was analyzed. It was found that (a) the fraction of sand dissolved in the stage of vigorous melting reactions increases rather irregularly with increasing temperature and linearly with decreasing initial grain radius, and (b) the mass transfer coefficient does not change with time in the second stage of melting, but increases with increasing grain radius and (surprisingly) decreases with increasing temperature for T ⩽ 1350°C and increases for T > 1350°C. An attempt is made to explain this behavior taking into consideration the initial heating rate, formation of intermediate crystalline compounds, evolution of batch gases and refining gases, and buoyant segregation of undissolved grains.

Boundary condition for mass transfer across a moving interface

Abstract The boundary condition for mass transfer through a phase interface in a multicomponent mixture with diffusion on both sides of the interface has been developed from first principles. If applied to a binary mixture with diffusion on only one side of the interface, this condition becomes identical with that used by Bird, Stewart, and Lightfoot. However, the approximate formulation which neglects density differences between phases can lead to absurd results if the actual density differences are non-zero. It is also shown that Coopers modified Noyes-Nernst equation is equivalent to the exact formula.

Glassmelting in 2004

Abstract Throughout its long history, glassmaking has been a healthy field of human activity and there is no reason to doubt that this will also be true in 2004. It can be expected that glassmelting will approach the stage of sophisticated simplicity and full controllability. A high degree of integration of experimental research and industrial practice will be accomplished by progress in theoretical analysis and mathematical modeling. Hopefully, this development will be backed at universities by a high level of education in glass engineering.