the mold-metal interface the heat input into the wall drops which accounts for the overall drop in pyrolysis speed and pyrolysis rate.
Transport of Core Gas
The gases generated through pyrolysis are transported through the core with an associated pressure loss. For the range of flow speeds expected during gas transport, Darcy’s law is adequate to predict gas velocities:
Equation 5
the core gas pressure. K is the intrinsic sand permeability found to be of order 10-10
Here g
u is the gas flow velocity and Pg
is m2
in room temperature air flow experiments14 (with the actual value varying inversely with the square of the sand grain size)12
is core gas viscosity which should increase with temperature as
µg core gas pressure. ∝ T ,11 and Pg is the
The density of the core gas simultaneously satisfies the mass transport equation and the ideal gas equation of state:
Equation 6 Equation 7 where Pg and P are microscopic core
gas and macroscopic core binder densities and T is the gas tempera- ture. The specific form of the right hand side in Equation (6) is given by Equation (4). The gas density is expected to increase significantly as the gas is transported from the hot pyrolysis zone to the colder venting (e.g. print) surfaces of the core. Fur- ther, a smaller change (drop) in gas density is expected as the gas moves in the direction of lower pressure.
The thermal contact between the gas and the sand is expected to be very good given the large surface per unit volume of sand and the relatively small gas transport velocity expect- ed. Thus we use a one-temperature model with the gas temperature tak- en at the temperature of the sand. The
60
energy advection and expansion work effects are also expected to be small. Further, as described in the previous sections, the molar content of the condensable species is small and both the effects of possible condensation/evaporation on gas density and on gas specific energy and temperature are ignored.
At the boundaries of the core two possibilities exist: either the gas is free to exit, or the gas is sealed in by the surrounding metal. This is decided with the aid of Equation (2). The bubble
Table 2. First Order Decomposition Reactions Active During Pyrolysis of a PUCB binder.3
Fraction of binder volatilized by a
given reaction–column 2, Arrhenius parameters for a given reaction–columns 3 and 4, and approximate temperature range over which a given reaction is active–columns 5 and 6
Figure 1. Left panel: Pyrolysis speed vs. time in a PUCB bonded flat core wall immersed in Al at 720C (1328F). Right panel: The binder fraction in the mold at 7, 15 and 30 seconds. The origin is at the metal-mold interface.
International Journal of Metalcasting/Summer 2011
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