Improvement on thermal performance of a disk-shaped miniature heat pipe with nanofluid
© Tsai et al; licensee Springer. 2011
Received: 21 June 2011
Accepted: 14 November 2011
Published: 14 November 2011
The present study aims to investigate the effect of suspended nanoparticles in base fluids, namely nanofluids, on the thermal resistance of a disk-shaped miniature heat pipe [DMHP]. In this study, two types of nanoparticles, gold and carbon, in aqueous solution are used respectively. An experimental system was set up to measure the thermal resistance of the DMHP with both nanofluids and deionized [DI] water as the working medium. The measured results show that the thermal resistance of DMHP varies with the charge volume and the type of working medium. At the same charge volume, a significant reduction in thermal resistance of DMHP can be found if nanofluid is used instead of DI water.
Keywordsheat pipe heat spreader electronic packaging nanofluid
The demand for low cost and efficient cooling packaging has been increasing in recent years due to the large power density generated by electronic and optical devices. One of the choices is to use a heat pipe to spread the generated heat. A novel packaging base with a disk-shaped miniature heat pipe [DMHP] is proposed to replace the conventional copper base of the transmitter outline [TO] can package for a laser diode . DMHP consists of multiple micro-grooves that radiate from the center of the base. The thermal performance of DMHP depends on the charge volume of the working fluid. It was found that the optimal volumetric fluid charge for the minimum thermal resistance is about 55%. In order to further increase the thermal performance of DMHP, a nanofluid was selected to replace deionized [DI] water as the working medium in the heat pipe.
Nanofluid has drawn the attention of researchers in the heat transfer community for heat transfer enhancement. Several previous studies showed that the thermal conductivity of a fluid could be significantly enhanced by adding suspended metal or nonmetal nanoparticles [2–6]. Xuan and Li  showed that the effective thermal conductivity of water-copper nanofluid is 75% greater than that of the base fluid (water in this case) even with only 8% volumetric fraction of particles in the base fluid. Besides, an experimental system was set up by Xuan and Li  to investigate the convective heat transfer phenomena of water-copper nanofluid in a tube. They found that the convective heat transfer coefficient in a tube could be increased by the addition of nanoparticles to the fluid when the volumetric fraction of the suspended nanoparticles was low.
Nanofluids have also been used in heat pipes in recent years [8–10], and the thermal enhancements of nanofluids on heat pipes were shown in these studies. There is no surprise that suspended particles in a fluid can affect the boiling heat transfer phenomenon at the solid-liquid interface. Huang et al.  showed that the pool boiling heat transfer of a heated stainless steel horizontal plate was significantly enhanced by adding glass, copper, and stainless steel microparticles into DI water. However, fluids with suspended microparticles may cause some problems such as abrasion and clogging . Thus, they are not suitable for the applications of miniature heat pipes in which the pore size of the porous medium or the hydraulic diameter of the microchannel is of the order of the micrometer.
Therefore, the present study proposes to employ a nanofluid as a working medium of the DMHP. Two types of suspended nanoparticles were used, namely gold nanoparticles and carbon nanoparticles. A measuring system is also set up to investigate the effect of added nanoparticles in the fluid on the thermal resistance of DMHP.
Preparation of nanoparticles
Because the silicon rubber is elastic, it was used to seal the top of the aluminum base with vacuum grease and to keep the chamber airtight. An ultra-thin syringe needle was used to insert into the chamber and to pump the chamber down. Then, a syringe pumping controller is used to pump a proper quantity of working fluid into the chamber. For the present study, DI water and nanofluid at five different charges with 18%, 37%, 55%, 74%, and 92%, respectively, of the total void volume were used.
A laser diode was used as the applied heat source in the measurement. The heating power of the laser diode was measured by an optical power meter (Vector H410, Scientech, Inc., Boulder, CO, USA) with a resolution of 0.001 W. The laser beam was focused on the center region (4 mm in diameter) of the aluminum base which was painted black with an aborptivity of α λ = 0.95. The applied heat loads were ranged from 0.1 to 0.6 W, and the heat fluxes were ranged from 4.7 to 28.2 KW/m2. Once both the heating load (Q) and the temperature difference (dT = T evap - T cond) were measured, the thermal resistance (R) could then be evaluated from the equation, R = dT/Q. The thermal resistance at each heat load could be calculated by the same process. The thermal resistances were averaged for all heat loads to be an averaged thermal resistance (R av) at each charge volume. The room temperature was kept at 20°C, and the measured temperature range is about 20°C to approximately 40°C. Based on the measurement error of the thermocouples and the power meter, the mean deviation of thermal resistance is about 13.9%.
For validation of basic properties of the working media, viscosity and thermal conductivity were measured. The viscosities of DI water and nanofluid were measured by a disk-type rotating viscometer (Brookfield RVTCP, Brookfield Engineering Lab., Middleboro, MA, USA). The uncertainty in viscosity measurement is about ± 3%. The thermal conductivity of DI water and nanofluid was measured by a transient hot wire method. The uncertainty in thermal conductivity measurement is about ± 2.3%.
Results and discussion
Measured dynamic viscosities of nanofluid and DI water
Viscosity at 20°C
Viscosity measured in present study
Viscosity from Cengelat 20°C (mPa·s)
Thermal conductivity measured in the present study (W/mK)
Thermal conductivity from Cengelat 10°C (W/mK)
Nanofluid (Au nanoparticles)
Nanofluid (carbon nanoparticles)
The viscosity of DI water is almost the same as that in the data in the Heat Transfer textbook . The data show that the viscosity of nanofluid with gold nanoparticles is close to that of DI water. Since the volume fraction of the gold nanoparticles is only 0.17% in this study, such a low concentration cannot have a large effect on the viscosity of the base fluid.
The present measured data show that the viscosity of the nanofluid with carbon nanoparticles is about 12% higher than that of the DI water. The volume fraction of carbon nanoparticles in the nanofluid is about 9.7%. As compared with the nanofluid with gold nanoparticles, the higher volume fraction of the carbon nanoparticles in the base fluid results in a greater viscosity of the nanofluid.
The measured values of the thermal conductivity of nanofluids and DI water are also listed in Table 1. The thermal conductivity of nanofluid with gold nanoparticles is only about 8.5% higher than that of DI water, which is within the uncertainty range of the measuring device. This increase in thermal conductivity with suspended gold nanoparticles is almost negligible when the volumetric fraction of nanoparticles in nanofluid is small. Based on the measured viscosity and thermal conductivity of the nanofluids, the physical properties of gold nanofluid are almost the same as those of DI water due to the low volumetric fraction of the nanoparticles in nanofluid.
Although the reductions of thermal resistances for nanofluids are not guaranteed for all charge volumes, the nanofluids somehow present a better thermal performance. There are several possible explanations for the enhanced heat transfer by the nanofluid. First, the nanofluids have larger convective heat transfer coefficients than those of pure fluids . Second, the nanofluids have larger thermal conductivities than those of the pure fluids . However, the above effects are only obvious for large volumetric fractions of the nanoparticles and not suitable for the present cases due to the low volumetric fractions. Xuan and Li  proposed one more possible explanation that the movement of nanoparticles improves the energy exchange process in the fluid. Tsai et al.  employed nanofluids as working mediums for a conventional circular heat pipe. Their results showed that the major reduction in the thermal resistance of the heat pipe is on the thermal resistance from the evaporator to the adiabatic section. The major thermal resistance occurring at the evaporator side is caused by the vapor bubble formation at the liquid-solid interface. Thus, the reduction of the thermal resistance may be related with the influence of nanofluid on the bubble formation at the evaporator side of the DMHP. The larger the nucleation size of a vapor bubble that will block the transfer of heat from the solid surface to the liquid, the higher the thermal resistance at the evaporator will be . The suspended nanoparticles tend to bombard the vapor bubble during bubble formation. Therefore, it is expected that the nucleation size of a vapor bubble is much smaller for a fluid with suspended nanoparticles than that without them. Thus, a lower thermal resistance can occur at the solid-liquid interface for a fluid with suspended nanoparticles.
Due to the more uniform dispersion and smaller diameter of the gold nanoparticles in the base fluid, the gold nanofluid has a comparable thermal performance with carbon nanofluid of higher volume fraction.
Summary and conclusions
The results showed that the dynamic viscosity of nanofluid with gold nanoparticles is close to that of DI water. The viscosity of nanofluid with carbon nanoparticles is 9% higher than that with gold nanoparticles.
As compared to a DMHP with DI water, the present measured data verify that the tested DMHP with gold nanoparticles and carbon nanoparticles do not have an obvious reduction of thermal resistance for all charge volumes. These are due to the low volumetric fraction of gold nanoparticles and the non-uniform dispersion and large diameter of carbon nanoparticles. It is also noted that the best charge volume is about 55% for all three working fluids.
For further enhancement of the thermal performance of the DMHP, the nanofluids of higher volumetric fraction and more uniform dispersion should be considered to be used as working fluids.
The financial support of this work was provided by the KAUST award with a project number of KUK-C1-014-12.
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