Heat transfer from glass melt to cold cap: Effect of heating rate

Abstract

Conversion of melter feed to glass occurs in the cold cap that floats on the melt pool in a nuclear waste glass melter. The conversion rate (the melting rate or the glass production rate) is controlled by the heat flux delivered to the cold cap from the molten glass. In an attempt to analyze the intricate relationship between the rate of heating, the feed foaming response, and the rate of melting, we measured the change in feed volume at different heating rates by using several melter feeds known to exhibit a wide range of melting rates under identical melter operating conditions. As expected, the maximum foam porosity increased as the heating rate increased.However, contrary to expectation, the temperature at which the foam reached maximum volume either decreased or increased with the heating rate, depending on the feed composition. A change in maximum foam temperature from the feed volume expansion test indicates a similar change of the cold-cap bottom temperature, which influences the heat flow to the cold cap, and thus the rate of melting. K E Y W O R D Sfeed-to-glass conversion, foaming, heat flux, melting rate 402 | LEE Et aL.

Glass production rate in electric furnaces for radioactive waste vitrification

“…An interesting consequence of a virtual constancy of ξ 0 and little variability of λ G is that, by the relationship d B = ( λ G / Q )( T MO – T B ), the thickness of the TBL depends on the heat flux, Q , delivered from the melt pool . The same conclusion can be made about the primary foam layer thickness that also must enable the incoming heat to be transferred to the upper portion of the cold cap.…”

Viscosity of glass‐forming melt at the bottom of high‐level waste melter‐feed cold caps: Effects of temperature and incorporation of solid components

“…The FET curves allow obtaining the minimum volume, V FO , to which the feed shrinks as foaming starts at the foam onset temperature, T FO , and the maximum volume, V FM , reached at the foam collapsing temperature, T FM . The solid green data points in Figure mark the FET curve for a foaming segment to which we fitted a third‐order polynomial function of the form V / V G = a 0 + a 1 ( T /1000) + a 2 ( T /1000) 2 + a 3 ( T /1000) 3 , where V G is the final glass melt volume ( V / V G is the normalized volume), and a 0 , a 1 , a 2 , and a 3 are fitted coefficients . The values of T FO and T FM then are T FO = and T FM = , and the maximum porosity is ψ M = 1 − V G / V FM .…”

“…Because primary foam is complex, inaccessible, and difficult to simulate in the laboratory, its rheology has not been fully investigated yet. Several attempts have been made to measure the viscosity of foaming melter feed at the stage where the glass‐forming melt has connected, but so far they only succeeded when the foam was collapsing …”

Modeling batch melting: Roles of heat transfer and reaction kinetics

“…The effect of the reaction kinetics on batch properties is shown in Figure , which displays the normalized volume of a simple soda‐lime‐silica batch (Figure A) and a borosilicate nuclear waste melter batch (Figure B) as a function of temperature and heating rate, Φ. As discussed by Lee et al, the foam onset temperature and the maximum foam volume tend to increase as Φ increases, because they are influenced by reaction kinetics that shifts conversion extent to higher temperatures in response to faster heating rates. During the foam collapse, at temperatures above the foam maximum temperature , the foam level is higher at any given temperature when is higher.…”

“…Along with the kinetics of gas‐evolving reactions and melt viscosity, foam stability is also affected by the presence of dissolving solid particles and their dissolution rate, by foam porosity, or by the mechanical disturbances associated with bubble coalescence . As discussed by Lee et al, these factors are the reason T FM shows irregular response to for different batch compositions, and why the decay of primary foam tends to be more erratic than the decay of a typical surface foam.…”