1. Introduction

Heat pipes were developed especially for space applications during the early 60´ by the NASA. One main problem in space applications was to transport the temperature from the inside to the outside, because the heat conduction in a vacuum is very limited. Hence there was a necessity to develop a fast and effective way to transport heat, without having the effect of gravity force. The idea behind is to create a flow field which transports heat energy from one spot to another by means of convection, because convective heat transfer is much faster than heat transfer due to conduction. Nowadays heat pipes are used in severalapplications, where one has limited space and the necessity of a high heat flux. Of course it is still in use in space applications, but it is also used in heat transfer systems, cooling of computers, cell phones and cooling of solar collectors.Figure 1.Shows the schematic diagram of heat pipe.

Heat pipes and their applications in thermal management have been studied for decades. They constitute an efficient, compact tool to dissipate substantial amount of heat from various engineering systems including electroniccomponents. Heat pipes are able to dissipate substantial amount of heat with a relatively small temperature drop along the heat pipe while providing a self-pumping ability due to an embedded porous material in their structure. A limiting factor for the heat transfer capability of a heat pipe is related to theworking fluid transport properties. In order to overcome this limitation, the thermophysical properties of the fluid can be improved. An innovative way to enhance liquid thermal conductivity is the dispersion of highly conductive solid nanoparticles within the base fluid.

Figure 1. Schematic view of heat pipe

2. Literature Review

The heat transfer characteristics of the heat pipe have been studied by a number of researchers. Lin et al.[1]experimentally analyzed the two-phase flow and heat transfer of R141b in a small tube. Song et al.[2] discussed about the heat transfer performance of axial rotating heat pipes at steady state conditions. Xuan et al.[3] performed experiment on a flat heat pipe under different heat fluxes, inclinations and theamount of the working fluid. Liu et al.[4] considered the effects of the length of the evaporator, vapour temperature on the critical values of the upper and lower boundaries.Due to the useful features of a nanofluid, various research groups [5–16] have tried to engage it within a heat pipe and study the subsequent thermal enhancement experimentally. Different nanoparticles such as silver[5,10,13,15], CuO [7,11], diamond[8,14], titanium[6,9], nickel oxide[12], and gold[16] have been utilized within the heat pipe working fluid. Theimproved thermal performance is observed through a reduction in thermal resistance[5,8–11,13,15,16], a drop in thetemperature gradient along the heat pipe[5,14,15], an increase in the heat pipe efficiency[6], and an enhancement in the overall heat transfer coefficient[12]. In some studies[7,11], the existence of an optimum amount of nanoparticle mass concentration providing the highestthermal performance has been established. Liu et al.[11] have shown that the heat pipe operating pressure has a significant effect on the thermal performance. In another study, Riehl [12] has observed that a higher heat transfer coefficient can be seen when using nanoparticles in water under low heat input conditions. Tsaia[16] investigated theinfluence of particle size on the heat pipe thermalperformance. From the analysis, it has been found that the nanofluids have the higher heat transfer capacity than the conventional working fluids and also it reduces the overall thermal resistance between the evaporator and the condenser. It is in this direction, a working fluid which is the combination of copper nanoparticle and the aqueous solution of long chain alcohols has been explored in order to enhance the thermal performance of the heat pipe.

All the types of heat pipes have a common problem of heat transfer limitation. These limitations determine themaximum heat transfer rate for a particular heat pipe under the normal working conditions. In heat pipe design, the capillary limit and the boiling limits are important under the normal working conditions. The surface tension is an important key factor for capillary limit. The surface tension of all the pure liquids is normally decreasing with increase in temperature, as the liquid moves along the interface towards the cooler condenser zone. The working fluids currently available for heat pipes have negative surface-tension gradients with temperature, that are unfavourable for the spreading or re-wetting on a heated surface, and therefore, the operating temperature and the heat load of heat pipe systems are limited. The heat pipe systems also suffer from operational instability problems because of thecharacteristics of the negative surface tension gradient withtemperature[17]. For medium temperature applications, water is used as a working fluid, due to its availability, cost, safety, easiness to handle and high surface tension. At elevated temperatures the surface tension of the water also decreases rapidly with increase in temperature. The heat transfer capability of the all heat transfer devices including heat pipe is limited by the working fluid transport properties. To overcome these limitations, the thermo physical properties of the working fluid have to be improved. The heat transfer rate of heat transfer devices can be improved by adding additives to the working fluids to change the fluid transport properties and flow features. One of the methods is to use the aqueous solutions of alcohols, with chain lengths longer than four carbon atoms. Vochten and Petre[18] experimentally revealed that the surface tension of aqueous solutions of alcohols, with chain lengths longer than four carbon atoms, have a positivegradient with temperature when the temperature exceeds a certain value. The stable working fluid circulation in a heat pipe is achieved through the capillary pressure head developed by the wick structure. The maximum capillary head developed in the heat pipe must be more than the various pressure losses along the vapour-liquid path which includes liquid pressure drop, vapour pressure drop, gravity pressure drop and pressure

drop in wick structure. It is sufficient that only a small amount of the long-chain alcohols, in the order of 10^-3 mole per liter, is required to change the surface tension characteristics of water without affecting the other bulk properties of the water[19].

Figure 2. Schematic diagram of experimental setup

Some experimentalinvestigations have revealed that nano-fluids haveremarkablyhigher thermal conductivity and greater heat transfercharacteristics than conventional pure fluids[20-23]. Theconvective heat transfer feature and flow performance ofcopper–water nanofluids in a tube were experimentallyinvestigated by Xuan and Li[24]

In the present analysis, heat pipe of copper container and the stainless steel wrapped screen is used as a wick material. The TiO₂nanofluid with a size of 50 nm and the aqueous solution of TiO₂nanofluid prepared by adding the 2 ml/lit of n- Butanol is used as working fluid. The concentration of TiO₂nanofluid in the DI water is 100 mg/lit. The nanofluids are prepared using the ultrasonic homogenizer for TiO₂nanofluid and copper particle in the aqueous solution of n- Butanol. The experiments are conducted for various inclinations of heat pipe to the horizontal with different heat inputs. The objective of this work is to study about the thermal efficiency improvement of heat pipe using TiO₂nanofluid and the aqueous solution of n-Butanol in TiO₂nanofluid.

3. Experimental Setup

The schematic diagram of the heat pipe under consideration and the thermocouple locations are shown in Figure. 2. The experimental set up was made from a copper tube with an inner diameter of 20.8 mm and outer diameter 22 mm Two layers of stainless steel screen mesh wick has been used to improve the capillary action. The length of the evaporator, adiabatic and condenser sections are 150,300 and 150 mm respectively. The heat pipe is charged with 30 ml of working fluid, which approximately corresponds to the amount required to fill the evaporator. The wall temperature distribution of the heat pipe in adiabatic zone is measured using six evenly spaced copper constantan (k-type) thermocouples with an uncertainty of ±0.1℃, at an equal distance from the evaporator. In addition the thermocouples are also located in evaporator surface (three locations), condenser surface (three locations), inlet and outlet of the condenser jacket. The electrical power input is applied at the evaporator section using cylindrical electric heater attached to it with proper electrical insulation and the heater is energized with 230V AC supply using a variac and measured using a power transducer with an uncertainty of ±1 W. Water jacket has been used at the condenser end to remove the heat from the pipe. The heat pipe has the ability to transfer the heat through the internal structure. As a result, a sudden rise in wall temperature occurs which could damage the heat pipe if the heat is not released at the condenser properly. Therefore, the cooling water is circulated first through the condenser jacket, before the heat is supplied to the evaporator. The condenser section of the heat pipe is cooled using water flow through a 150 mm long jacket with an inner diameter of 30 mm and outer diameter of 36 mm. The water flow rate is measured using a rotameter on the inlet line to the jacket that has an uncertainty of ±1% and the flow rate is kept constant at 0.08 kg/min. The inlet and outlet temperatures of the cooling water are measured using two copper constantan thermocouples. The adiabatic section of the heat pipe is completely insulated with the glass wool. The amount of heat loss from the evaporator and condenser surface is negligible.

4. Experimental Procedure

The experiments are conducted using three identical heatpipes which are manufactured as per mentioned dimensions. One of the heat pipes is filled with de-ionized water, secondone with TiO₂nanofluid and third one with the aqueoussolution of n-Butanol with TiO₂nanofluid. The power inputto the heat pipe is gradually raised to the desired power level. The surface temperatures at six different locations along the adiabatic section of heat pipe are measured at regular time intervals until the heat pipe reaches the steady state condition. Simultaneously the evaporator wall temperatures, condenser wall temperatures, water inlet and outlet temperatures in the condenser zone are measured. Once the steady state is reached, the input power is turned off and cooling water is allowed to flow through the condenser to cool the heat pipe and to make it ready for further experimental purpose. Then the power is increased to the next level and the heat pipe is tested for its performance. Experimental procedure is repeated for different heat inputs (30, 40, 50, 60 and 70 W) and different inclinations of pipe (0^o, 15°, 30°, 45°, 60°, 75° and 90°) to the horizontal and observations are recorded. The output heat transfer rate from the condenser is computed by applying an energy balance to the condenser flow. The vacuum pressure in the inner side of the heat pipe is monitored by vacuum gauge, which is attached in the condenser end of the heat pipe.

5. Results and Discussions

5.1. Effect of Heat Pipe Inclination in Efficiency

Figure 3. Efficiency for different inclinations at 30W heat input

Figure.3 – 7 shows the variations of heat pipe thermal efficiency for DI water, TiO₂nanofluid and TiO₂nanofluid with aqueous solution of n-Butanol with various tilt angles for 30 W, 40 W, 50 W, 60 W and 70 W heat inputs respectively. From all the figures, it has been observed that the efficiency of heat pipe increases with increasing values of the tilt angle. This is due to the fact that the gravitational force has a significant effect on the flow of working fluid between the evaporator section and the condenser section along with the capillary action of wick. However, when the heat pipe inclination angle exceeds 30° for de-ionic water and 45° for TiO₂nanofluid and TiO₂nanofluid with aqueous solution of n-Butanol, the heat pipe thermal efficiency tends to decrease. The efficiency of the heat pipe seems to decrease since the formation of the liquid film is at higher rate inside the condenser which results in the increased value of the thermal resistance between the vapour of the working fluid and the cooling medium in the condenser. Therefore the thermal efficiency decreases when the angle exceeds 30° for DI water and 45° for TiO₂ nanofluid and TiO₂nanoparticle in the aqueous solution of n-Butanol. The heat pipe thermal efficiency can be calculated from the ratio between the cooling capacity rate of water at the condenser section to the power supplied at the evaporator section. Generally, the nanoparticles suspension in the fluid has significant effect on the enhancement of heat transfer due to its higher heat capacity and higher thermal conductivity of working fluid. Therefore, the heat pipe thermal efficiency heatpipe increases with nanofluids as compared to that of the base working fluid.

Figure 4. Efficiency for different inclinations at 40W heat input

Figure 5. Efficiency for different inclinations at 50W heat input

In this analysis, the thermal efficiency of heat pipeincreases about 14 % with TiO₂nanofluid as a working fluid when compared with the DI water. Besides, the heat pipe which uses TiO₂nanofluid with aqueous solution of n-Butanolincreases the thermal efficiency to nearly about 20% as compared that of DI water due to the positive surfacetension gradient possessed by n-Butanol.

Figure 6. Efficiency for different inclinations at 60W heat input

Figure 7. Efficiency for different inclinations at 70W heat input

5.2. Effect of Thermal Resistance

Figure 8. Thermal resistance of heat pipe for 0° inclination

Figure.8-14 shows the comparative results of thermal resistance of heat pipe with DI water, TiO₂nanofluid and TiO₂nanofluid with aqueous solution of n-Butanol. The thermal resistance (R) of the heat pipe is defined as where Te and Tc are average values of temperatures at the evaporator and condenser sections respectively and Q is the heat supplied to the heat pipe. From all the figures, it is clear that the thermal resistance of heat pipe decreases for all the three working fluids with increasing values of angle ofinclination and the heat input. However, the thermal resistances of heat pipes using both the base fluid and nanofluids are comparatively high at low heat loads for thereason that a relatively solid liquid film resides in the evaporator section. On the other hand, these thermal resistances condense quickly to its minimum value when theheat load is increased. At higher inclinations of heat pipe (above 60^o with horizontal) and higher heat input (more than 50 W) the difference between the thermal resistance of DI water &TiO₂nanofluid is nearly 10%. The thermal resistance of the TiO₂nanofluid with aqueous solution of n- Butanolis less than the DI water and TiO₂nanofluid for all the variables and the value is nearly 28% less than of the DI water.

Figure 9. Thermal resistance of heat pipe for 15° inclination

Figure 10. Thermal resistance of heat pipe for 30° inclination

Figure 11. Thermal resistance of heat pipe for 45° inclination

Figure 12. Thermal resistance of heat pipe for 60° inclination

Figure 13. Thermal resistance of heat pipe for 75° inclination

Figure 14. Thermal resistance of heat pipe for 90° inclination

5.3. Variation of Temperature Difference between Evaporator and Condenser

Figure 15. Variation of (Te-Tc) for 0° inclination

Figure 16. Variation of (Te-Tc) for 15° inclination

The temperature difference between the evaporator and the condenser for various heat loads is shown in Figure.15-21. From the figures, it is observed that the temperature difference between the evaporator and the condenser increases for increasing values of heat input. It is clear that the temperature difference between the condenser and the evaporator is lesser for TiO₂nanofluid with aqueous solution of n-Butanol than the other working fluids. As the n-Butanol has the positive surface tension gradient with temperature, more amount of condensate is moved to the evaporator from the condenser at higher rate. Moreover, the surface temperature of the evaporator section when the TiO₂ nanofluid with aqueous solution of n-Butanol is used as the working fluid is 24% less than DI water and 19 % less than the TiO₂nanofluid. The reason behind that is the aqueous solution of n-Butanol increases the rate of heat transfer.

Figure 17. Variation of (Te-Tc) for 30° inclination

Figure 18. Variation of (Te-Tc) for 45° inclination

Figure 19. Variation of (Te-Tc) for 60° inclination

Figure 20. Variation of (Te-Tc) for 75° inclination

Figure 21. Variation of (Te-Tc) for 90° inclination

6. Conclusions

In this study, the thermal performances of the cylindrical heat pipe with wire mesh are experimentally investigated using the DI water, TiO₂nanofluid and TiO₂nanofluid with aqueous solution of n-Butanol as working fluids. To evaluate the thermal performance of the heat pipe using the TiO₂ nanofluid and TiO₂nanofluid with aqueous solution of n-Butanol, the thermal efficiency and the thermal resistances between the evaporator and the condenser regions are measured and compared with the heat pipe using DI water. Based on the experimental results, it is found that the thermal efficiency of TiO₂nanofluid with aqueous solution of n-Butanol is higher than the base fluid DI water and TiO₂nanofluid and the thermal resistance also reduces to three fourth of base fluid. The results show that the thermal performances of the heat pipe may be enhanced by adding a very small amount of long chain alcohol which gives the better performance than the conventional working fluid and nanofluid. This may be due to the reason that, the dilute aqueous solution of n-Butanol which is having a positive surface tension gradient with temperature gives rise to an increased value of the capillary limit and the boiling limit of the heat pipe along with the nanofluid.