Analytical Solution
Analytical calculation of the coefficient of heat transfer from a mould with homogeneous temperature under conditions of natural convection was performed for shell mould tempera- tures of 500°C (932°F) and 1000ºC (1832°F). The results show that the convective coefficient of heat transfer αconv
is
relatively low, only slightly dependent on the shell mould temperature, and reaches the values:
• αconv = 7.7 W/m2 500°C (932°F)
• αconv = 8.3 W/m2 1000°C (1832°F)
.K for a mould temperature of .K for a mould temperature of
Unlike the convection component, the radiation coefficient of heat transfer is strongly dependent on the mould wall tem- perature and on the emissivity of the mould material (Figure 4). The values of emission coefficient for different ceramic
materials εm are usually given in a relatively wide range of 0.4 to 0.9.
confirmed by our own measurements and by comparing the results established by thermocouples and by thermocamera measurement.
The actual radiation flow depends on the effective emissiv- ity values, the radiation view factor, which gives the effect of mutual irradiation of walls (the size of surfaces and gaps between the castings), and on the emissivity of the thermal cover around the shell. Analytical calculation of the radia- tion heat transfer can only be performed for simple geomet- rical configurations, and it cannot be solved for the model under consideration.
It is evident that the width of this emissivity interval leads to substantially different results. Data in the literature indi- cate that the real value of emissivity of ceramic molochite (hard abrasion resistant alumino silicate) mould can be found in the range εm
= 0.8 to 0.9. These values were also
Figure 5. Values of the total heat transfer coefficient in the into-axis and out-of cluster directions are shown in this spatial map.
Figure 6. Distribution of total and convective heat transfer coefficients along the periphery of castings (the difference in values is the transfer of radiation) is shown.
International Journal of Metalcasting/Spring 10 73
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