ABSTRACT
This Paper tackles the effect of heat transmission changes in heat pipes on the thermal as well as mechanical properties of heat pipes. According to Zohuri (2016, p.5), the current rate at which people develop high tech electronic devices with good speed is high, especially when considering devices with high processing rates like personal computers and communication devices.
Zohuri (2016, p.5) further states that, normally, these devices are aesthetically pleasing, which limits the design of the space structure, which is a great challenge for thermodynamic engineers. Without affecting the high-efficiency output of electronic products, engineers are not only resolving how to control the heat output generated by the output power, and also protect another department to work properly (Fraas 1989, p.7).
The heat pipe remains to be an efficient and effective heat transfer device as it utilizes efficiently the phase changes of the working substance (Thang.et.al 2009, p.3). Fraas (1989,p.9) states that, because it has an excellent thermal conductivity, a simple structure design, and uniform temperature changes. According to Fraas (1989, p.7), heat pipes are used for heat transfer, heat flux conversion and thermal control. Heat exchangers made of heat pipes have characteristics such as small size, light weight, small heat transfer temperature difference, and a long life, therefore heat pipes are considered not only as the most cost-effective thermal design but also the most reliable in regards to the technology. Heat pipe technology is relatively new, reliability and life test information is quite scarce (Thang.et.al. 1989, p.5)
Therefore, the purpose of this study is conducted life tests and accelerated life tests on heat pipes to obtain thermal properties data for heat pipes (Zohuri 2011, p.10). Zohuri (2011, p.11) states that, heat pipes are generally considered as reliable and durable as it can be used for more than 10 years with a steady performance without a decrease in the performance. The setup experiment was aimed at testing more heat pipes simultaneously and analysing the thermal resistance between the test values(Zohuri 2011, p.11).
Keywords: Performance, Heat transferability and reliability.
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INTRODUCTION & BACKGROUND
According to Fraas (1989, p.6), the heating pipe is a significantly effective heat transfer element that transfers heat from one place to another under a small temperature gradient. The heat pipe is also an important cooling component of modern electronic devices because heat pipe has a good thermal conductivity and isothermal property (Fraas 1989, p.7). Thang.et.al (2009, p.6) mentions that, presently, copper water pipes are widely used in spacecraft temperature control, electrical and electronic equipment cooling areas. Most of the heat pipes applied to the computer industries are made by copper because the water is a safety liquid to the environment. The most common form of the heat pipe is copper as a fluid container and water as a working fluid. The most significant essence of a heat pipe that is made of compared to other heat pipes is that it can carry much more heat per unit in the range of temperatures associated with the cooling of an electronic device(Zohuri 2016, p.15). Most literature describes copper as compatible with working fluid water, but there are an infrequently available data results for life testing. Therefore, this project mainly conducts accelerated thermostatic hot water experiments on various types of heat pipes to obtain their life data. Sixty copper heat pipes are being tested.
Operation Principle of Heat Pipe
The heat pipes contains a container that is sealed with pipe walls entailing end caps, a structure of wick as well as a few fluids for working that are balanced with their vapour. The heat pipes lengths are portioned into 3 distinctive portion namely: the condenser, evaporator, together with the adiabatic section.Fraas (1989, p.18) points out that, in different applications and designs, heat pipes may have multiple heat sources, and they may sink with or without adiabatic section. When external evaporator section is heated, the liquid is converted into vapour (Fraas 1989, p.19). The vapour’s pressure then leads vapour to the adiabatic part and then towards the condenser.Then condensation of vapours occurs some vapour condenses and releases its suppressed heat of evaporation unto the heat sink, and others liquefy to the liquid, flowing towards the evaporator section via the gravity action.
Figure 1- Heat Pipe Structure Design.
Usually, the main cause of heat pipe degradation is creation of the gases that are non-condensable, that accumulate in the heat pipe condenser [1]. The reason for heat pipe performance degradation is due to (1) chemical reaction generates the non-condensable vapour (2) corrosion and erosion of the container as well as the wick. Thang.et.al (2009, p.12) mention that, although the heat pipe is a rare serious fault, the amount of vapour which is non-condensable accumulated in the condenser is becoming more and more as time goes on, forming a barrier to the vapour flow at the point of diffusion, therefore, reducing the available condenser working area (Thang.et.al. 2009, p.12). The erosion as well as corrosion of both the wick as well as the container establishes as the modification of wetting as a working fluid Angle of working fluid, together with the change of porosity or permeability, and the wick capillary pore diameter. The solid sediments produced by erosion as well as corrosion are carried away by moving fluid until it reaches the evaporator section where generated the deposit when the fluid vaporizes. This results in rising of resistance towards fluids flowing within an evaporator, leading to a decreased of heat volume.
EXPERIMENTAL METHOD
The experiment will enable conducting of heat, as well as the cooling and the reheating using sixty water heating pipes made of copper. The heating pipes will then undergo testing when divided in high temperature endurance test and temperature cycle test. 6 pieces heat pipe will be tested in hot water dipping test.
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- High-Temperature Endurance Test
This experiment is meant to test the heat pipe for endurance as it is subjected to a temperature of +130C and a humidity of 50%.
Figure 2-
| Main parameter | Test specification |
| Test Type | High Temperature |
| Temperature | +130 ºC |
| Humidity | 50% |
| Test time | 1500 hours |
| Orientation | Horizontal |
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- Temperature Cycle Test
This experiment is meant to test the pipes through different temperature cycle from -40C-> +130C with a humidity of 100%
Figure 3-
| Main parameter | Test specification |
| Test Type | Temperature Cycle |
| Temperature | -40 ºC-> 130 ºC |
| Humidity | 100% |
| Number of cycles | 1000 cycles |
| Orientation | Horizontal & Vertical |
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- Hot water Dipping Test
6 different type samples of commercially available oxygen-free copper/water heat pipes are used for the life test. They are flat pipes having sintered fibre, non-sintered fibre and sintered copper powder as wicks, lining circumferentially in the inside wall of the pipe. All 6 heat pipes tested are 3 mm in thickness and 200 mm in length.
6 heat pipes divided into 6 sets with each set of experiments needs to be completed independently. Each set was tested at one fixed constant temperature. Fig.4 showed the test set up.
Figure 4- Hot water dipping test.
Heat pipe types
| Type | Description |
| F | Sintered fibre wick attached to only one side tube wall |
| C | Non-sintered fibre wick |
| N | Sintered copper powder wick |
Sintered fibre wick properties
Sintered fibre has only recently started to be embraced as wick materials utilised in heat pipes.According to Zohuri (2011, p.26),a sintered fibre as a wick structure provides significantly smaller capillaries’ channels. Moreover, sintered fibre has minimal effects on the permeability of a heat pipe (Zohuri 2011,p.26). Sintered fibre is also characterised by its substantially low thermal resistance.
Non-sintered fibre wick properties
Non-sintered fibre when utilized within a wick structure presents substantially larger capillaries’ channels as compared to sintered fibre within a wick structure (Zohuri 2011,p.27). Furthermore Zohuri (2011,p.27) states that, non-sintered fibre has a number of effects in relation to the heat pipe’s permeability, as compared to sintered fibre.
Sintered copper powder wick properties
According to Fraas (1989, p.32), sintered copper powder presents favourable characteristics in regards to utilisation for applications of substantially high heat flux; this is as a result of significantly high capillary pressure factor. A high capillary pressure factor is achieved by the radius of the small pore (Fraas 1989, p.33). However when compared to screen wicks sintered copper powder provides substantially low thermal resistance, as well as high heat flux capacities; together with significantly better resistance against gravity that may affect the length of heat pipes.
EXPERIMENTAL SET UP & TESTING
Water heat pipes made of copper are treasured because of their effective thermal managing of their ability to provide tremendously minimal resisting of thermal transportation;1 -200°C (Thang.et.al. 2009, p.22).The following figure shows the heat pipe to be tested in this experiment.
Figure 5- Copper-water heat pipe
Core parameter of the heat pipe:
| Core parameters | Specification |
| Pipe Material | Copper |
| Pipe Height | 200 mm |
| Pipe Thickness | 3 mm |
| Working Fluid | Water |
Fig.6 illustrates the real structure. The setup consists of two aluminium at the handle. This setup consists of two aluminium handles, an aluminium urn as an evaporator, two foam rubbers, three plastic nuts four steel wire hooks, three bolts, as well as four copper heat pipes. foams are placed to prevent movements that is affected by fluid vapour. Moreover, It reduces heat transferring which is direct between the heat pipes as well as aluminium handles (Thang.et.al. 2009,p.25).
Figure 6- Aluminium handles, steel hook, plastic nuts & nitrile foam rubber
designed to hold the heat pipe.
This new design allows us to easily remove and upload heat pipe samples and save time because of its simplicity and the ability to test three or four heat pipe samples at a time. The stability of the heat pipe on the aluminium handle should be completed within 1 minute. The aluminium handle is made by cutting the aluminium bar with the mitre sawing machine. Then we drill three holes with the drill in our design. We polish the aluminium handle with a sander to get a smooth surface. The foam rubber is the same width as the aluminium alloy handle. The following figure shows the setting of the fixed heat pipe for the aluminium handle of the test program.
Figure 7- Aluminium handles manufactured
The main parameter of the aluminium handles:
| Main Parameter of aluminium handles | Specification |
| Handle Length | 200 mm |
| Handle Width | 19 mm |
| Handle Thickness | 3 mm |
References List
FRAAS, A. P. (1989). Heat exchanger design. New York, Wiley.
THANG ,N., MASATAKA, M., AKIHIRO, T., MITSURU, K., RENYA, I., (2009). Prediction of Long-Term Performance of Miniature Heat Pipes from Accelerated Life Tests.Fujikura Ltd.
ZOHURI, B. (2011). Heat pipe design and technology: a practical approach. Boca Raton, FL, CRC Press. http://www.crcnetbase.com/isbn/9781439845240.
ZOHURI, BAHMAN. (2016). Heat Pipe Design and Technology A Practical Approach: Modern Applications for Practical Thermal Management + Ereference. Springer Verlag.
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