Comparison between Using Different Fluids

In most electronic devices, the CPU temperature should not exceed 85℃ (Ramaswamy et al 2002), therefore many researchers have immersed the electronic chips in dielectric liquids such as fluorocarbon liquids. The main disadvantage for

these liquids is the high price and the low thermal conductivity that is the main

reason for reducing the applying power on the CPU. Water has high thermal

conductivity and cheaper than dielectric liquids, therefore it is preferred to use as

working fluid.

There are some studies that have been done to compare between the effects of using

water or air as working fluid on the heat transfer behaviour within enclosures.

Neymark et al. (1989) investigated experimentally the effect of the internal vertical

partitions on the natural heat transfer within the enclosure. The partition position was

at the centre of the enclosure and it had doorway geometry with the height fixed at

one-half the enclosure height. One of the vertical walls of the enclosure was

differentially heated and the opposite wall was isothermal. The other surfaces were

considered to be adiabatic. Two different working fluids were used in their study,

namely, water and air. To achieve the similarity between two cases of natural

convection of water and air the experiments requires matching geometry, the thermal boundary conditions, 𝑅𝑎 and 𝑃𝑟.

The similarity of Rayleigh number between air and water can be achieved by

appropriate choice of length scale and temperature difference but it is not possible to

match the Prandtl numbers of air and water.

They concluded that, the aperture size has a great effect on the temperature across

the aperture. With decreasing the size of aperture, the temperature across the aperture

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aperture temperature difference of air is greater than the water as illustrated in Figure

(2.3).

Bar-Cohen (1993) described the features and performance of different cooling mode

for a number of fluids in his survey. It was found that natural convection in liquids

has the added advantage of higher cooling rates compared to those in air for a given

temperature difference. He also noticed that water appears to be the best coolant

fluid because it has large specific heat and high thermal conductivity compared to

others. Moreover, water is an effective coolant and offers greater uniformity of chip

temperatures than air-cooling but the use of liquid necessitates the presence of an

enclosure. In many applications involving the packaging of high dissipation

equipment such as power supplies, hermetically sealed units may be employed.

Hsieh et al. (1994) presented an experimental study of natural convection inside a

rectangular enclosure where it was heated at one of the vertical wall and cooled from

the opposite wall. The other walls remained adiabatic. The effects of different values Figure (2.3) Temperature difference across aperture in air and water

vs aperture width at 𝑅_𝑎 = 2 × 1012_{, Neymark et al. (1989)}

Aperture width Air

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of Rayleigh number, aspect ratio (the ratio of height to length of enclosure) and three

different working fluids (air, water and silicone oil) were investigated.

Their results showed that the circulation pattern within the enclosure became clear as

Rayleigh number and aspect ratio increased. Moreover, the effects of Aspect ratio on

heat transfer were not clearly noted for all the cases that they studied. However, the

Prandtl number effects were notable for the thinner thermal boundary layers for

higher Prandtl numbers.

Steady and 2-D natural convection heat transfer in an enclosure heated from below

and symmetrically cooled from the sides was studied numerically by Ganzarolli and

Milanez (1995). The boundary condition for the bottom wall of the cavity was at a

uniform temperature or uniform heat flux while the two vertical walls were at a

uniform temperature. The top wall was isolated and two different working fluid (air

and water) were examined. In addition, the effect of aspect ratio (the ratio of length

to height of enclosure) and Rayleigh number were also investigated.

From their results, for the square cavity, different boundary conditions at the cavity

bottom wall did not strongly affect the flow. For a shallow cavity, the flow and

thermal fields within the enclosure depend on the boundary condition that applied to

the heated wall. In the case of uniform temperature, the cavity was not always

thermally active along its whole extension and the flow was not uniform. For the

uniform heat flux boundary condition, even for low values of Rayleigh number, the

streamlines and the isotherms occupied more uniformly the whole enclosure. Water

gave more uniform results than air and had low temperature values.

Turan et al. (2012) have studied numerically the effect of constant wall temperature

and constant wall heat flux boundary conditions at the vertical side walls for laminar,

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aspect ratios 𝐴𝑅 (height to length of enclosure) while the horizontal walls assumed to be adiabatic. The range of the Rayleigh number was between 104 to 106 where the enclosure was filled with two different fluids (air and water).

Their results shows that for both constant wall temperature and constant heat flux

boundary conditions, when the Rayleigh number is increased the mean Nusselt

number also increased. Further, In the case of constant wall temperature boundary

condition, the mean Nusselt number increased up to a certain value of the 𝐴𝑅𝑚𝑎𝑥

then after this value the mean Nusselt number started to decrease but in the constant wall heat flux the mean Nusselt number increased as 𝐴𝑅 increased. The authors concluded that, water has better thermal management than air.