Günter Weidmann

Chaotic mixing in a Joule-heated glass melt

A numerical study of two-dimensional thermal convection of a highly viscous fluid driven by volumetric heating has been carried out to investigate the effect of time-dependent thermal forcing on the degree of mixing. The problem is relevant to electrically heated glass melting furnaces which are traditionally operated with time-independent heating. The numerical computations carried out are for two model fluids representing semitransparent and opaque melts, respectively. Lagrangian motion of passive tracers is numerically simulated and degree of mixing in the glass melt is quantified in terms of mixing entropy. The computed flow patterns indicate that time-dependent streamlines intersect transversely and, thereby, result in well-mixed regions. The stirring mechanism involves the rotational stretching and folding produced by two oscillating vortices with varying size and circulation. The model predictions indicate that the chaotic mixing is strongly dependent on the period of electrode firing cycle. The pr...

Shaping at Low Viscosities

The technological control of all subprocesses in glass making (melting, hot forming, annealing) requires a correct description of the heat transport within the glass and the heat transfer between glass and vessels (crucibles, tanks), molds, or cooling equipment (lehrs, vans). Especially in hot forming, the heat transfer from the glass into the molds is of decisive importance. The heat balance of the glass-mold system has considerable influence on the process control and, finally, on the product quality.

Selection considerations between ZERODUR® and silicon carbide for dimensionally-stable spaceborne optical telescopes in two-earth-orbits

A key consideration in defining a space telescope mission is definition of the optical materials. This selection defines both the performance of the system and system complexity and cost. Optimal material selection for system stability must consider the thermal environment and its variation. Via numerical simulations, we compare the thermal and structural-mechanical behavior of ZERODUR® and SiC as mirror substrates for telescope assemblies in space. SiC has significantly larger CTE values then ZERODUR®, but also its thermal diffusivity k/(ρcp) is larger, and that helps to homogenize thermal gradients in the mirror. Therefore it is not obvious at first glance which material performs with better dimensional stability under realistic unsteady, inhomogeneous thermal loads. We specifically examine the telescope response to transient, gradient driving, thermal environments representative of low- and high-earth- orbits.

Homogenizing and Conditioning

Tracer particles in high-viscosity fluids, such as polymer or glass melts, exhibit complex kinematics during mixing processes. At first glance, this is surprising because high viscosities are usually associated with “simple” flows. Nevertheless, a more thorough analysis shows that deterministic chaos is at work and that this is synonymous with good mixing.