Heat transfer from glass melt to cold cap: Computational fluid dynamics study of cavities beneath cold cap

Abstract

Efficient glass production depends on the continuous supply of heat from the glass melt to the floating layer of batch, or cold cap. Computational fluid dynamics (CFD) are employed to investigate the formation and behavior of gas cavities that form beneath the batch by gases released from the collapsing primary foam bubbles, ascending secondary bubbles, and in the case of forced bubbling, from the rising bubbling gas. The gas phase fraction, temperature, and velocity distributions below the cold cap are used to calculate local and average heat transfer rates as a function of the bubbling rate. It is shown that the thickness of the cavities is nearly independent of the cold cap shape and the amount of foam evolved during batch conversion. It is ~7 mm and up to ~15 mm for the cases without and with forced bubbling used to promote circulation within the melt, respectively. Using computed velocity and temperature profiles, the melting rate of the simulated high‐level nuclear waste glass batch was estimated to increase with the bubbling rate to the power of ~0.3 to 0.9, depending on the flow pattern. The simulation results are in good agreement with experimental data from laboratory‐ and pilot‐scale melter tests.

Conversion degree and heat transfer in the cold cap and their effect on glass production rate in an electric melter

“…The volume of fluid method was used to track the melt–atmosphere interface 29 . Simulations with resolved bubbling were performed to determine the buoyancy momentum source 30 and volume fraction of air bubbles that build up underneath the cold cap, approximately $$\varepsilon =0.75$$ 31 . To account for the effect of these bubbles on the heat transfer to the cold cap, the thermal conductivity below the cold cap was calculated as $$\lambda \; = \;\varepsilon {\lambda _a} + ( {1 - \varepsilon } ){\lambda _g}$$, where $${\lambda _a}$$ and $${\lambda _g}$$ are the thermal conductivities of air and glass, respectively.…”

“…29 Simulations with resolved bubbling were performed to determine the buoyancy momentum source 30 and volume fraction of air bubbles that build up underneath the cold cap, approximately 𝜀 = 0.75. 31 To account for the effect of these bubbles on the heat transfer to the cold cap, the thermal conductivity below the cold cap was calculated as 𝜆 = 𝜀𝜆 𝑎 + (1 − 𝜀)𝜆 𝑔 , where 𝜆 𝑎 and 𝜆 𝑔 are the thermal conductivities of air and glass, respectively.…”

Cold‐cap structure in a slurry‐fed electric melter

“…As the glass-forming melt fraction increases, open pores are closing at the foam onset temperature, T FO , trapping evolving gases and turning the bottom part of the cold cap into primary foam. [44][45][46][47] The primary foam bubbles grow, coalesce, and collapse, releasing gas into large bubbles/cavities that move horizontally with the melt below the foam layer, 48 eventually escaping to the atmosphere 49,50 around the cold cap edges or through vent holes (openings in the cold cap).…”

Effect of material properties on batch‐to‐glass conversion kinetics

“…37 Above the foam onset temperature, T FO , gases are trapped in the glass-forming melt, creating a layer of primary foam. The primary foam bubbles grow, coalesce, and collapse into gas cavities that move horizontally, being carried by the melt circulating under the cold cap and eventually escaping into the atmosphere 38,39 around the cold-cap edges.…”