Numerical Solution of Heat Transfer in Model Mould
. It is obvious that the intensity of cooling is substantially greater in the radial direction out from the cluster axis than in the direction of the cluster axis. (In the lower part of the curve the effect of the sand bed is shown.)
Numerical solution of heat transfer on a model mould was performed using the FLUENT software. Considered in the calculation was a shell mould with a homogeneous surface temperature of 1000°C (1832°F), with the mould resting on a sand bed and with natural air convection. Figure 5 is a spatial map of the values of total heat transfer coefficient αtot
Figure 6 gives the distribution of the total and the convective heat transfer along the periphery of individual castings. The largest share in the heat transfer in the outward direction is that of radiation into the surroundings. Due to the mutual irradiation of individual castings, the amount of heat radi- ated in the direction of the cluster is minimal, and cooling mostly proceeds by convection. Heat transfer by convection is roughly uniform along the whole periphery, and the value αconv
is approximately 20 to 25 W/m2 .K.
The intensity of cooling along the casting periphery is very inhomogeneous, and the ratio of total heat transfer coeffi- cients in the axial direction of the cluster axis αint the direction out of the mould αex
to those in reaches values of up to 1:5.
This results in a temperature field asymmetry of the casting, a difference between the cooling velocity of inner and outer walls, and a shift of the temperature axis out of the geomet- ric axis in the direction of the cluster axis. On the basis of the temperature field asymmetry, asymmetries in the casting structure and in the properties can be expected.
Cooling Under Forced Convection
Shell moulds are usually filled under conditions of relatively calm surroundings. In spite of this it may happen that due to
Figure 7. Trajectory of air flow in a shell mould with forced convection and heating of air is illustrated.
ventilation or for some other reasons there will be forced air convection. The effect of forced convection was therefore ana- lyzed on the model mould via numerical simulation. The mould was cooled with a current of air perpendicular to the axis of castings alternately in 5 velocity values from 0.7 to 5 m/s. The air flow trajectory is affected by the layout of castings in the cluster (Figure 7) and there are different cooling intensities on the “windward” and “leeward” sides and in the gaps between castings. The change in the intensity of total heat flow at indi- vidual points only depends on the local increase in convective heat transfer value since the heat transfer by radiation is in no way influenced by forced convection. It is evident that cooling asymmetry appears not only along the periphery of individual castings but also along individual positions of the casting.
Figure 8 gives a comparison of heat transfer by natural con- vection and heat transfer by air flowing at a velocity of 2 m/s in position “l”, which is indicated on the right of the figure.
Figure 8. A comparison of the coefficient of convective heat transfer is made in two situations: a) natural convection and b) air flowing at 2 m/s.
74 International Journal of Metalcasting/Spring 10
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