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

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89