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Foam formation in glass furnaces
Postdoc: John van der Schaaf Phone: +31 (0) 40 247 4712Supervision: Ruud Beerkens Fax: +31 (0) 40 244 6653Cooperation: TNO-TPD, Rexam, EET E-mail: [email protected]
IntroductionIn a glass-melting furnace, fining of the glass-melt may cause foam formation. If the gas-bubble flux to the glass-melt surface is highenough, a layer of foam is formed. This foamlayer insulates the glass-melt from heatradiation generated in the combustion spaceabove the glass-melt. Effectively, heat transferis severely restricted towards the glass-meltand a large amount of heat is reflected, thusincreasing crown temperatures (increasedrefractory attack hazard) and reducing glass-melt temperatures.Consequently, maintaining glass-melttemperatures at specified values by burner-control becomes increasingly difficult withincreasing foam-layer thickness. Thus, in orderto be able to predict and prevent foamformation, a foam-model, based on firstprinciples, is developed in this project.
Model DescriptionThe transient foam-layer height, HF, is givenby: )t(j)t(jH bininF τ−−=with τb the average lifetime of a gas bubble inthe foam-layer and j in the gas bubble to theglass-melt surface. The average lifetime of agas-bubble at the glass-melt surface can bederived from the momentum balances and theequation of continuity, using partially immobilegas-bubble walls. The degree of immobilizationdepends on the surface activity, represented byparameter ψ. The relationship derived for τb is:
δψ
+
δδτ
=τcc
02b 2
lnR1
3R
The lifetime of a gas-bubble in a foam-layer willincrease because of down-flowing liquid fromthe top of the foam. This effect is described by:
∑=
−
ττ
−τ
τ−
ττ−+τ
=δδ N
1i
1i
fN
0
cfN
e)1i(
e
The lifetime of a gas bubble in a N-layer foam τN,is thus determined implicitly.
ResultsExperiments In the figure below, the transientfoam height is shown for different constant gasfluxes in a glass-melt at 1400oC. At low gasfluxes a finite foam-height is rapidly attained(slope equals gas flux), which increases onlyslowly in time. At higher fluxes, a steady,unbounded, increase in foam height is observed.When the gas flow is stopped, the foam heightrapidly decreases.
Model In the figure below, the simulatedtransient foam height is displayed. The samebehavior is observed as in the experiments. Atlow gas fluxes a bounded foam height isobtained, at higher gas fluxes the foam heightincreases indefinitely.
020406080
100
0 500 1000
Hf(mm)
time(s)
