A new heat pipe cooling device.

Comanescu, Dinu ; Comanescu, Adriana ; Filipoiu, Ioan Dan 等

1. INTRODUCTION

The fractals are well studied from mathematical point of view. Their simulation and their assimilation with various forms existing in the nature are also presented in the literature. By using proper design software the Peano, Hilbert and Koch profile parameters are used to obtain optimal constructions (Barnsley & Demko, 1986).

The paper presents a specific application of fractals, which are used in a heat pipe cooling device. This new device type was developed for some years in Politehnica University of Bucharest by a group of multi-disciplinary group researchers lead by this paper authors. The research found some original aspects unfound in the technical literature. Firstly mentioned on this occasion there are also specified new development research directions, which may be achieved on adequate projects.

2. ABOUT HEAT PIPES

The heat pipe is composed of three basic components (Zaghdoudi et al. 2004):

* the container;

* the working fluid;

* the wick or capillary structure which has a fractal form (two plates: superior and inferior) which can be placed in mobile computer structures. (Fig. 1).

Working fluid is vaporized in the evaporator and flows toward the condenser where it deposits its heat by condensation. Capillary forces in the porous wick return the condensed working fluid to the evaporator. The function of the container is to isolate the working fluid from the outside environment. It has to therefore be leak-proof, maintain the pressure differential across its walls, and enable transfer of heat to take place from and into the working fluid (Sobhan, 2005).

[FIGURE 1 OMITTED]

Selection of the container material depends on many factors (Shankara Narayanan 2006). These are as follows:

* Compatibility (both with working fluid and external environment);

* Strength to weight ratio;

* Thermal conductivity;

* Ease of fabrication, including welding, machine ability and ductility;

* Porosity;

* Wet ability. A high thermal conductivity ensures a minimum

temperature drop between the heat source and the wick.

3. FRACTALS USED IN A HEAT PIPE COOLING DEVICE

Having in view the geometrical characteristics, the Peano curve fractal may be used for heat pipe device. In this situation the section of such a system are chosen in function of other requirements (heat transfer, flow, technological manufacturing, etc).

The main property of this curve, which its fractal dimension is two (Voinea & Stroe, 2000), is that a square may be filled with it (Fig. 2). For example a Peano curve is built by Hilbert (Fig. 2a). By using the property of its auto-similarity one may determine its fractal dimension, which it is also two as in the previous case (Voinea & Stroe, 2000).

The auto-similarity is a characteristic property of the mathematical fractals. This is the main difference between a mathematical fractal and a natural one, which it is governed by the dynamic phenomena, as such of growing.

By assumption the Hutchinson operator (Fig. 2b) as a succession of affine transformation operators (translation, rotation, reducing by similitude, affine reducing, mirror, cutting) a fractal appears as a fixed point of this operator.

In the case of the Koch curve (Fig. 2c) the Hutchinson operator (W) is composed successively by a 1/3 contraction for the OB segment (W1), followed by a 600 rotation and a 1/3 translation for the BC segment (W2), a similar transformation for the CD segment (W3) and a 1/3 contraction with a translation for the DK segment (W4), so that

W = [W.sub.1] [union] [W.sub.2] [union] [W.sub.3] [union] [W.sub.4] (2)

The W operator previously defined generating by successive iterations the Koch curve.

[FIGURE 2 OMITTED]

The previous units are inserted in Peano type networks, Fig. 3. The structures based on the Hilbert (Fig. 3a) and Hutchinson (Fig. 3b) curve and with different sections are possible to be used for units or heat transfer pipes and networks.

The last constructions (Fig. 3c) are based on the Koch curve. Even if they have rather strange image these are the first steps to attempt new field of applications for other fractals as Barnsley fern.

4. TESTING DATA RESULTS

For validation, basic tests have been conducted. In Fig. 4 is presented the experimental test equipment. Inside the container, placed on the heating system, is a liquid. Under its own pressure the liquid enters in the pores of the capillary material and is wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of vaporization. The gas has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. This entire cycle usually happens with less than a 5[degrees]C differential from one end of the pipe to another. We used in the experiments the simplest type of wick structure, double-layer fractal mesh screen wick. The geometric and thermophysical properties of the wick have been selected are presented in the Table 1. Based on hereinbefore fractal capillary structure we obtained the following heat pipe specifications:

* Horizontal orientation

* Maximum heat transfer: 30 W;

* Nominal operating temperature: 40[degrees]C;

* Pipe diameter: 0.08 m;

* Heat Pipe length: Evaporator: 0.04 m; Adiabatic: 0.05 m; Condenser: 0.07 m.

5. CONCLUSIONS

The main purpose of this paper is that to find some new structures for the laptops and notebooks cooling systems. In order to increase its transfer efficiency and shape minimization the authors firstly use various fractals configuration as thin plates with large cooling surfaces. Because they are thin, the plates can be used in cooling device where the dimensions are critical.

[TABLE 1 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The design was validated through basic experimental tests which demonstrate the cooling capability of this structure (maximum heat transfer is 30 W). All of the heat transfer limitations, with the exception of the boiling limitation, exceed the specified heat transfer rate of 30W. The low value of 0.131W for the boiling limitation strongly suggests that the liquid will boil in the evaporator and possibly cause local dry spots to develop. The reason the liquid boils is because the effective thermal conductivity of the wick is equal to the conductivity of the liquid, which is very low in this case. Because the liquid is saturated at toe vapor-liquid interface, a low effective thermal conductivity requires a large amount of wall superheat which, in turn, causes the liquid to boil. This problem can be circumvented by using a high conductivity wire mesh or sintered metal wick, which greatly increases the effective conductivity. It should be noted, however, that because porous wicks have lower permeabilities, the capillary limitation should be lower as well.

The future researches include: optimizing the structure, researches regarding different cooling fluid using and investigation of full cooling characteristics of this device.

6. REFERENCES

Barnsley, M. F. & Demko, S. (1986). Chaotic Dynamics and Fractals, Academic Press ISBN 0120790602

Sobhan B. C. (2005). Modeling of the Flow and Heat Transfer in Micro Heat Pipes. Available from: http://www.rpi.edu/tphtl/research/mfht/mfht.html Accessed: 2006-07-20

Shankara Narayanan K.R. (2006). What's A Heat Pipe? Available from: http://www.cheresources.com/htpipes.shtml Accessed: 2006-06-16

Zaghdoudi M. C.; Tantolin C. & Godet C. (2004). Use Of Heat Pipe Cooling Systems In The Electronics Industry. Available from: http://www.electronics cooling.com/html/2004_nov_a1.html Accessed: 2005-06-20

Voinea, R. & Stroe, I. (2000). Introduction in theory of dynamical systems, Ed.Academiei Romane, ISBN 973-270739-9, Bucharest