The melting of raw materials in industrial glass furnace leads to bubble generation. Bubble density and size distribution are initially correlated to the granulometry of raw materials [1]. The carbonate decomposition leads to CO2 release which is a source of large quantity of bubbles due to the low solubility of CO2 [2]. The chemical reaction on silica seeds with precursor liquid leads also to a bubble generation. These phenomena are the origin of the presence of foam above raw materials generally called primary foam. To remove the bubble, glass makers add fining agents. The out-gassing occurring generally at high temperature, T around [1300-1500]°C can lead to a foam creation. A layer of few centimeters of foam can spread over a large part of the glass bath.

From the point of view of the heat transfer, the presence of a foam layer is harmful to the energetic efficiency. According to Fedorov and Viskanta [3], few centimeters of foam reduces dramatically the transmittance. By corollary, the temperature in combustion space increases which is detrimental to the refractory materials.

The creation of a steady-state thickness of foam is a balance of source of bubbles rising from the liquid bath and death of the bubble at the top of the foam. According to Pilon and Viskanta [4], three situations can exist for which the control parameter is the superficial gas velocity jg resulting from a fluid mechanics analysis. Below a threshold jm, the foam is unstable. When jg is equal to jm, a foam appears. Above this threshold, a foam is created.

In this lecture, the origin of gas source resulting to fining process will be first presented to determine the main characteristic of the superficial gas velocity. After a review of the onset of foaming, the mechanisms leading to the death of foam will be presented based on film stability as already studied in [5]. A simple model to describe the creation of foam will be also presented.

References:

[1] D. Boloré, M. Gibilaro, L. Massot, P. Chamelot, E. Cid, O. Masbernat, and F. Pigeonneau. X-ray imaging of a high temperature furnace applied to glass melting. J. Am. Ceram. Soc., 103:979–992, 2020.
[2] J. van der Schaaf and R. G. C. Beerkens. A model for foam formation, stability, and breakdown in glass-melting furnaces. J. Colloid Interface Sci., 295:218–229, 2006.
[3] A. G. Fedorov and R. Viskanta. Radiation characteristics of glass foam. J. Am. Ceram. Soc., 83:2769–2776, 2000.
[4] L. Pilon and R. Viskanta. Minimun superficial gas velocity for onset of foaming. Chem. Eng. Process., 43:149–160, 2004.
[5] F. Pigeonneau, H. Kočárková, and F. Rouyer. Stability of vertical films of molten glass due to evaporation. Colloids Surf., A, 408:8–16, 2012.