Arquitectura de Red GSM

Apart from ambient temperature Ta, the effect of combined heat transfer coefficient (hcr)

due to convection and radiation is analyzed. In this case, Ta is maintained at 25o C and

the value of hcr is varied from 9 to 13 W/m2 0C. Figure 4.8a shows the temperature

distribution when the combined heat transfer coefficient is equal to 9 W/m2 0C. It is seen that the temperature varies from minimum 33.740C to maximum of 37.030C in the various layers of the human head. It can be clearly seen from figure 4.8a that the inner most layer of brain has an area maintained at constant temperature around 370C at the center and gradually decreases to the skin layer. The skin tissue has a lowest temperature around 33.740C. A considerable variation also observed in the bone layer from 34.840C to 35.930C. The fat layer has a temperature 34.100C. The lowest temperature is observed in the skin layer 33.740C. The effect of increased value of

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It is observed that the effect of hcr is more prominent on the outer layers of head than the

brain area. The maximum temperature which persists in the brain segment has changed negligibly when hcr is increased from 9 to 13 W/m2 0C. However, the outer skin

temperature decreased by 0.8 0C due to change in combined heat transfer coefficient.

a b

c d

e f

Figure 4.8: Temperature variation in human head at different values of hcr

a) 9 W/m20C b) 10 W/m20C c) 11 W/m20C d) 12 W/m20C e) 13 W/m20C f) temperature from innermost to outer layer of head

0.04 0.06 0.08 0.1 0.12 0.14 33 33.5 34 34.5 35 35.5 36 36.5 37 37.5 radial distance(m) te m p e ra tu re (C ) 9 10 11 12 13

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of Malaya

The following section describes the temperature variation along with the thermal gradient and heat flux as a function of variation in heat transfer coefficient. It may be noted that the geometry of the head along with the thickness of brain, bone, fat and skin layer remains same as that of the previous case. The only exception is that the convective coefficient varies which can represent the effect of air velocity across the head. The ambient temperature was considered to be constant (i.e. 250C). Figure 4.9 illustrates the behavior of heat transfer when combined convective coefficient is 8.25W/m2 0C. It is seen that the skin temperature remains within the acceptable range and the maximum heat flux is found at a distance roughly 2 cm to 4 cm radius of the brain layer.

a b

c

Figure 4.9: convective and raidative heat transfer coefficient at 8.25W/m2 0C a) Nodal temperature b) total thermal gradient c) total thermal flux

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The figure 4.10a shows the temperature distribution from 33.620C to 37.030C in the various layers of the human head for the combined (convective and radiative) heat transfer coefficients at 9.5 W/m20C. It can be seen clearly from the figure 4.10a that the inner most layer brain has an elliptical region maintained at constant temperature around 370C at the center and gradually decreases to the skin layer. The skin tissue has a lowest temperature around 33.620C. A considerable variation also observed in the bone layer from 34.00C to 35.90C. The fat layer has a temperature 34 0C. The lowest temperature is observed in the skin layer 33.620C.

The figure 4.10b indicates the distribution of thermal gradient in the different layers of the human head for the combined (convective and radiative) heat transfer coefficient at 9.5 W/m2 0C. It is clearly observed from the figure 4.10b that a circular region of the brain layer at the center has large thermal gradient around 426.430C/m, which gradually decreases to the lowest at the end of the brain. The bone layer has lowest thermal gradient compared to the other layers. And a significant largest thermal gradient was observed in fat tissue, which shows the heat loss is more from the fat tissue compared to the other layers. By comparing with the previous figure 4.9b, a slight increase in the largest thermal gradient is found. The skin layer has thermal gradient around 140.550C/m.

The figure 4.10c explains the distribution of thermal flux in the different layers of the human head for the combined (convective and radiative) heat transfer coefficient at 9.5W/m20C. It is clearly seen that the brain tissue at the center has the largest thermal flux (i.e 216.48W/m2) and decreases uniformly to the lowest (i.e 12.83W/m2) at the boundary of the brain layer. The brain tissue losses highest heat from the center to the end. The bone and fat and skin tissue maintained almost equal thermal flux around 80.75W/m2.

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a b

c

Figure 4.10: convective and raidative heat transfer coefficient at 9.5W/m2 0C a) nodal temperature, b) total thermal gradient c) total thermal flux

Figure 4.11 to figure 4.13 illustrate the effect of combined heat transfer coefficient when its value is varied from 10.5W/m20C, 11.5W/m2 0C and 12.5 W/m2 0C respectively. It is worth mentioning that the higher the heat transfer coefficient the better would be the exchange of heat either from head to outside environment or vice- versa depending on the temperature difference between the head and the environment. It can be seen that the skin temperature decreases from 33.41 deg C (fig4.11) to 33 deg C when convective and radiative heat transfer coefficient is increased to 12.5 W/m2 0C. the maximum thermal gradient which is found across the fat layer increased from 583.180C/m to 661.73 0C/m that itself indicates the heat transfer rate should increase, since heat transfer rate is directly proportional to the thermal gradient across given section of an object.

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a b

c

Figure 4.11: convective and raidative heat transfer coefficient at 10.5W/m2 0C. a) nodal temperature b) total thermal gradient c) total thermal flux

a b

c

Figure 4.12: convective and raidative heat transfer coefficient at 11.5W/m2 0C. a) nodal temperature b) total thermal gradient c) total thermal flux

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a b

c

Figure 4.13: convective and raidative heat transfer coefficient at 12.5W/m2 0C. a) nodal temperature, b) total thermal gradient c) total thermal flux

Figure 4.14 illustrates the change of temperature across different layers of human head as a function of combined convective and radiative heat transfer coefficient which is varied in the steps of 8.25 W/m2 0C, 9 W/m2 0C, 9.5 W/m2 0C, 10 W/m2 0C, 10.5 W/m2 0

C, 11 W/m2 0C, 11.5 W/m2 0C, 12 W/m2 0C, 12.5 W/m2 0C, and 13 W/m2 0C. It can be clearly seen that as the heat transfer coefficient increases the skin temperature decreases where as there is a negligible change in temperature was observed in the brain layer. The bone and fat layers also shows the decrease in the temperature with the increase in the heat transfer coefficients. Thus it can be said that the increase in heat transfer coefficient which is strongly dependent on wind velocity is desirable to keep the skin temperature at acceptable range.

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Figure 4.14: Effect of combined heat transfer coefficient on temperature variation across human head