Implicit Continuous-fluid Eulerian method.13
The asymp-
totic expansion of the integral of Equation (4), the pyrolysis rate equation, was found to be accurate. It was used to update binder variables every time step for every rate in Table 2.
In Table 4, a comparison is made between the two observed seal heights and the computed values. The bubbling from the core is predicted to stop with 12.1 cm of metal over the jacket in good agreement with the observed value of 12.5 cm. The needed metal head is built up at 30 seconds after the beginning of fill. At this point gas flux to metal goes to zero (Figure 4) and the gas pressure in the jacket (Figure 3) is such that at the geometric core peak the gas pressure is balanced by a combination of metal and surface tension pressures. The latter is taken to be 450 Pa, a value appropri- ate for the measured bubble size and a high temperature Al surface tension of 0.85 N/m.
The locations of gas blow to metal are also well captured. At 10 seconds when only the lower portion of the jacket is submerged (see Figure 4 for the timing of the submersion
and seal events) the gas blows to metal along the lower rim of the core with the largest gas flux at lower core peaks. Gas also escapes ahead of the metal front in the upper portion of the jacket (Figure 3). At 15 seconds the core submerges completely and significant gas blow is correctly predicted along its upper rim.
In the simulation the gas in the lower portion of the core seals at 17.5 seconds, with 9.4 cm of metal over lower core peaks. This is higher than the observed value of 7.5 cm and this discrepancy remains unexplained.
The outgassing rate of the jacket core rises rapidly as the jacket is submerged and is very close to its peak value at submersion time. At about 22 seconds the thinner, 8 mm, sections of the core begin to pyrolyze through and the total outgassing rate begins to drop (Figure 4). The normalization adopted for the pyrolysis rate allows comparison of differ- ent cores. Per unit area and per unit binder the water jacket outgasses less than an instantaneously submerged thick flat core slab (compare Figures 1 and 4).
Table 4. Comparison of Computed and Observed Bubble-Stop Metal Heights
Figure 4. Left panel: Metal height in mold during fill. Right panel: Normalized core outgassing (upper curve) and gas-to-metal blow vs. time. The mass rate is normalized by the core surface area and the bulk density of pyrolyzable binder. Fine dashed lines mark the submersion times and coarse dashed lines mark the gas seal times of the lower and upper parts of the water jacket core.
International Journal of Metalcasting/Summer 2011 63
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