Author Topic: Effect of Nanofluid Concentration on the Performance of Circular Heat Pipe  (Read 2870 times)

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Author : M. G. Mousa
International Journal of Scientific & Engineering Research, IJSER - Volume 2, Issue 4, April-2011
ISSN 2229-5518
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Abstract The goal of this paper is to experimentally study the behavior of nanofluid to improve the performance of a circular heat pipe.  Pure water and Al2O3-water based nanofluid are used as working fluids. An experimental setup is designed and constructed to study the heat pipe performance under different operating conditions. The effect of filling ratio, volume fraction of nano-particle in the base fluid, and heat input rate on the thermal resistance is investigated.  Total thermal resistance of the heat pipe for pure water and Al2O3-water based nanofluid is also predicted.  An experimental correlation is obtained to predict the influence of Prandtl number and dimensionless heat transfer rate, Kq on thermal resistance.  Thermal resistance decreases with increasing Al2O3-water based nanofluid compared to that of pure water.  The experimental data is compared to the available data from previous work.  The agreement is found to be fairly good.

Index Terms Heat Pipe, Thermal Performance, Nanofluids

To solve the growing problem of heat generation by electronic equipment, two-phase change devices such as heat pipe and thermosyohon cooling systems are now used in electronic industry.  Heat pipes are passive devices that transport heat from a heat source to a heat sink over relatively long distances via the latent heat of vaporization of a working fluid. The heat pipe generally consists of three sections; evaporator, adiabatic section and condenser.  In the evaporator, the working fluid evaporates as it absorbs an amount of heat equivalent to the latent heat of vaporization.  The working fluid vapor condenses in the condenser and then, returns back to the evaporator.  Nanofluids, produced by suspending nano-particles with average sizes below 100 nm in traditional heat transfer fluids such as water and ethylene glycol provide new working fluids that can be used in heat pipes. A very small amount of guest nano-particles, when uniformly and suspended stably in host fluids, can provide dramatic improvement in working fluid thermal properties. The goal of using nanofluids is to achieve the highest possible thermal properties using the smallest possible volume fraction of the nano-particles (preferably < 1% and with particle size<50 nm) in the host fluid.
  Kaya et al. [1] developed a numerical model to simulate the transient performance characteristics of a loop heat pipe.  Kang et al. [2], investigated experimentally, the performance of a conventional circular heat pipe provided with deep grooves using nanofluid. The nanofluid used in their study was aqueous solution of 35 nm diameter silver nano-particles.  It is reported that, the thermal resistance decreased by 10-80% compared to that of pure water. 
Pastukhov et al. [3], experimentally, investigated the performance of a loop heat pipe in which the heat sink was an external air-cooled radiator. The study showed that the use of additional active cooling in combination with loop heat pipe increases the value of dissipated heat up to 180 W and decreases the system thermal resistance down to 0.29 K/W. 
Chang et al. [4] investigated, experimentally, the thermal performance of a heat pipe cooling system with thermal resistance model.  An experimental investigation of thermosyphon thermal performance considering water and dielectric heat transfer liquids as the working fluids was performed by Jouhara et al. [5]. The copper thermosyphon was 200 mm long with an inner diameter of 6 mm. Each thermosyphon was charged with 1.8 ml of working fluid and tested with an evaporator length of 40 mm and a condenser length of 60 mm. The thermal performance of the water charged thermosyphon is compared with the three other working fluids (FC-84, FC-77 and FC-3283). The parameters considered were the effective thermal resistance as well as the maximum heat transport.  These fluids have the advantage of being dielectric which may be better suited for sensitive electronics cooling applications. Furthermore, they provide adequate thermal performance up to approximately 50 W, after which liquid entrainment compromises the thermosyphon performance. 
Lips et al. [6], studied experimentally, the performance of plate heat pipe (FPHP).  Temperature fields in the heat pipe were measured for different filling ratios, heat fluxes and vapor space thicknesses.  Experimental results showed that the liquid distribution in the FPHP and consequently its thermal performance depends strongly on both the filling ratio and the vapor space thickness. A small vapor space thickness induces liquid retention and thus reduces the thermal resistance of the system. Nevertheless, the vapor space thickness influences the level of the meniscus curvature radii in the grooves and hence reduces the maximum capillary pressure. Thus, it must be, carefully, optimized to improve the performance of the FPHP.  In all the cases, the optimum filling ratio obtained, was in the range of one to two times the total volume of the grooves. A theoretical approach, in non-working conditions, was developed to model the distribution of the liquid inside the FPHP as a function of the filling ratio and the vapor space thickness. 
Das et al. [7-8] and Lee et al. [9]) found great enhancement of thermal conductivity (5-60%) over the volume fraction range of 0.1 to 5%.     
All these features indicate the potential of nanofluids in applications involving heat removal.  Issues, concerning stability of nanofluids, have to be addressed before they can be put to use.  Ironically, nanofluids of oxide particles are more stable but less effective in enhancing thermal conductivity in comparison with nanofluids of metal particles.   
 The aim of the present work is to investigate, experimentally, the thermal performance of a heat pipe. The affecting parameters on thermal performance of heat pipe are studied. The type of working fluid (pure water and Al2O3-water based nanofluid), filling ratio of the working fluid, volume fraction of nano-particles in the base fluid, and heat input rate are considered as experimental parameters.  Empirical correlation for heat pipe thermal performance, taking into account the various operating parameters, is presented.
2. Experimental Setup and Procedure
            A schematic layout of the experimental test rig is shown in Fig.1.  This research adopts pure water and Al2O3-water based nanofluid as working fluids.  The size of nano-particles is 40 nm. The test nanofluid is obtained by dispersing the nano-particles in pure water.  The working fluid is charged through the charging line (6). In the heat pipe, heat is generated using an electric heater (12). The vapor generated in the evaporator section (8) is moved towards the condenser section (4) via an adiabatic tube (5) whose diameter and length are 20 mm and 40 mm, respectively. Both   evaporator and condenser sections have the size of 40 mm-diameter and 60 mm-height. The condensate is allowed to return back to evaporator section by capillary action "wick structure" through the adiabatic tube. The surfaces of the evaporator section, adiabatic section, and condenser section sides are covered with 25 mm-thick glass wool insulation (3). Seventeen calibrated cooper-constantan thermocouples (T-type) are glued to the heat pipe surface and distributed along its length to measure the local temperatures (Fig. 2). Two thermocouples are used to measure ambient temperature. All thermocouples are connected to a digital temperature recorder via a multi-point switch.  The non condensable gases are evacuated by a vacuum pump.  The heat pipe is evacuated u to 0.01 bar via the vacuum line (10). The power supplied to the electric heater (12) is measured by a multi-meter (13).  The input voltage was adjusted, using an autotransformer (2). The voltage drops across the heater were varied from 5 to 45 Volts. The A.C. voltage stabilizer (1) is used to ensure that there is no voltage fluctuation during experiments. The pressure inside the evaporator was measured by a pressure gage with a resolution of 0.01 bar.
Thermocouples (with the uncertainty lower than 0.20 oC) are distributed along the surfaces of the heat pipe sections as follows: six thermocouples are attached to the evaporator section, two thermocouples are attached to the adiabatic section, and nine thermocouples are attached to the condenser section. The obtained data for temperatures and input heat rate are used to calculate the thermal resistance.
        One can define the filling ratio, FR, as the volume of charged fluid to the total evaporator volume.  The working fluid is charged at 30 oC.   
The effects of working fluid type, filling ratio, volume fraction of nano-particles in the base fluid, and heat input rate on the thermal performance of the heat pipe are investigated in the experimental work.  The experimental runs are executed according to the following steps:       
1. The heat pipe is evacuated and charged with a certain amount of working fluid
2. The supplied electrical power is adjusted manually at the desired rate using the autotransformer.
3. The steady state condition is achieved after, approximately, one hour of running time using necessary adjustments to the input heat rate. After reaching the steady state condition, the readings of thermocouples are recorded, sequentially, using the selector switch. The voltage of the heater is measured to determine the value of applied heat flux. Finally, the pressure inside the evaporator is recorded.
4. At the end of each run, power is changed and  step 3 is repeated.
5. Steps 1 through 4 are repeated using with another adjusted amount of working fluid. The filling ratios, FR, used are 0.2, 0.4, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.80 and 1.0.
 Pure water and Al2O3-water based nanofluid are used as working fluids. Steps 1 through 5 are repeated for Al2O3-water based nanofluid using several values of volume fractions of nano-particles. The volume fractions used are 0.25%, 0.4%, 0.5%, 0.6%, 0.75%, 1.0% and 1.5%, respectively.

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