will locally increase on small surfaces to a maximum of approximately 50 to 55 in contrast to 20-25 W/m2
Due to forced convection at air velocities of 2 m/s the coeffi- cient αconv
.K in the
case of natural convection. This increase is thus of no funda- mental effect on the total cooling intensity of the mould. Forced convection is of negligible effect on castings in “leeward” posi- tions of the shell mould. The distribution of total heat transfer coefficient at individual positions of the model system and the effect of forced convection are clear to see in the polar plot in Figure 9. Higher air-flow velocities on the casting bed than 2–3 m/s are not very likely standard foundry conditions.
Experimental Measurement of Temperature Fields in a System of Cylindrical Castings
The actual values of temperature fields in the metal and in the shell mould were measured experimentally on moulds with a system of cylindrical castings of 50 mm (1.96 in), 30 mm (1.18 in), 20 mm (0.78 in), and 10 mm (0.39 in) in diameter, which were arranged symmetrically on a pitch circle of 170
mm (6.69 in) in diameter. There are always two castings of identical diameter in the cluster, positioned radially opposite each other. The length of dia. 50 mm (1.96 in) and 30 mm (1.18 in) castings is 200 mm (7.87 in) while that of dia. 20 (0.78 in) and 10 mm (0.39 in) castings is 120 mm (4.72 in). A schematic diagram of the cluster is shown in Figure 10.
The alloy cast is chromium-nickel-cobalt steel, designation N155. Prior to casting, the shell moulds are heated to 1050ºC (1922°F); they are removed from the furnace and placed on a sand bed and filled in 3 to 5 minutes (within this period, measuring thermocouples are positioned in the mould). The mould parameters and the casting regime are given in Table 1.
Temperature Measurement When Casting into Experimental Shell Moulds
The metal temperature is measured in the longitudinal axis of test bars at a distance of 60 mm (2.36 in) from the upper edge of the bar. The measuring tips of thermocouples are at
Table 1. Experimental Setup
Figure 9. Total heat transfer coefficient αtot air-flow velocities of 0 and 2 m/s is shown.
International Journal of Metalcasting/Spring 10
for
Figure 10. A schematic diagram of a test mould layout.
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