Experimental investigation of nanofluids on sintered heat pipe thermal performance

Dilute dispersion of silver nano-particles in pure water was employed as the working fluid for conventional 1 mm wick-thickness sintered circular heat pipe. The nanofluid used in present study is an aqueous solution of 10 and 35 nm diameter silver nano-particles.The experiment was performed to measure the temperature distribution and compare the heat pipe temperature difference using nanofluid and DI-water. The tested nano-particle concentrations ranged from 1, 10 and 100 mg/l. The condenser section of the heat pipe was attached to a heat sink that was cooled by water supplied from a constant temperature bath maintained at 40 °C.At a same charge volume, the measured nanofluids filled heat pipe temperature distribution demonstrated that the temperature difference decreased 0.56–0.65 °C compared to DI-water at an input power of 30–50 W. In addition, the nanofluid as working medium in heat pipe can up to 70 W and is higher than pure water about 20 W.     

Experimental thermal conductivity of ethylene glycol and water mixture based low volume concentration of Al2O3 and CuO nanofluids

Thermal conductivity of ethylene glycol and water mixture based Al₂O₃ and CuO nanofluids has been estimated experimentally at different volume concentrations and temperatures. The base fluid is a mixture of 50:50% (by weight) of ethylene glycol and water (EG/W). The particle concentration up to 0.8% and temperature range from 15 °C–50 °C were considered. Both the nanofluids are exhibiting higher thermal conductivity compared to base fluid. Under same volume concentration and temperature, CuO nanofluid thermal conductivity is more compared to Al₂O₃ nanofluid. A new correlation was developed based on the experimental data for the estimation of thermal conductivity of both the nanofluids.     

Experimental investigation of heat transfer and friction factor with water–propylene glycol based CuO nanofluid in a tube with twisted tape inserts

Convective heat transfer and friction factor characteristics of water/propylene glycol (70:30% by volume) based CuO nanofluids flowing in a plain tube are investigated experimentally under constant heat flux boundary condition. Glycols are normally used as an anti-freezing heat transfer fluids in cold climatic regions. Nanofluids are prepared by dispersing 50 nm diameter of CuO nanoparticles in the base fluid. Experiments are conducted using CuO nanofluids with 0.025%, 0.1% and 0.5% volume concentration in the Reynolds numbers ranging from 1000 < Re < 10000 and considerable heat transfer enhancement in CuO nanofluids is observed. The effect of twisted tape inserts with twist ratios in the range of 0 < H/D < 15 on nanofluids is studied and further heat transfer augmentation is noticed. The increment in the pressure drop in the CuO nanofluids over the base fluid is negligible but the experimental results have shown a significant increment in the convective heat transfer coefficient of CuO nanofluids. The convective heat transfer coefficient increased up to 27.95% in the 0.5% CuO nanofluid in plain tube and with a twisted tape insert of H/D = 5 it is further increased to 76.06% over the base fluid at a particular Reynolds number. The friction factor enhancement of 10.08% is noticed and increased to 26.57% with the same twisted tape, when compared with the base fluid friction factor at the same Reynolds number. Based on the experimental data obtained, generalized regression equations are developed to predict Nusselt number and friction factor.     

Thermal resistance of screen mesh wick heat pipes using the water-based Al2O3 nanofluids

In the present study, the effect of nanofluids on the thermal performance of heat pipes is experimentally investigated by testing circular screen mesh wick heat pipes using water-based Al₂O₃ nanofluids with the volume fraction of 1.0 and 3.0 Vol.%. The wall temperature distributions and the thermal resistances between the evaporator and the adiabatic sections are measured and compared with those for the heat pipe using DI water. The averaged evaporator wall temperatures of the heat pipes using the water-based Al₂O₃ nanofluids are much lower than those of the heat pipe using DI water. The thermal resistance of the heat pipe using the water-based Al₂O₃ nanofluids with the volume fraction of 3.0 Vol.% is significantly reduced by about 40% at the evaporator-adiabatic section. Also, the experimentally results implicitly show that the water-based Al₂O₃ nanofluids as the working fluid instead of DI water can enhance the maximum heat transport rate of the heat pipe. Based on the two clear evidences, we conclude that the major reason which can not only improve the maximum heat transport rate but also significantly reduce the thermal resistance of the heat pipe using nanofluids is not the enhancement of the effective thermal conductivity which most of previous researchers presented. Especially, we experimentally first observe the thin porous coating layer formed by nanoparticles suspended in nanofluids at wick structures. Based on the observation, it is first shown that the primary mechanism on the enhancement of the thermal performance for the heat pipe is the coating layer formed by nanoparticles at the evaporator section because the layer can not only extend the evaporation surface with high heat transfer performance but also improve the surface wettability and capillary wicking performance.     

Two-phase flow patterns of nitrogen and nanofluids in a vertically capillary tube

Two-phase flow patterns of nitrogen gas and aqueous CuO nanofluids in a vertically capillary tube were investigated experimentally. The capillary tube had an inner diameter of 1.6 mm and a length of 500 mm. Water-based CuO nanofluid was a suspension consisted of water, CuO nanoparticles and sodium dodecyl benzol sulphate solution (SDBS). The mass concentration of CuO nanoparticles varied from 0.2 wt% to 2 wt%, while the volume concentration of SDBS varied from 0.5% to 2%. The gas superficial velocity varied from 0.1 m/s to 40 m/s, while the liquid superficial velocity varied from 0.04 m/s to 4 m/s. Experiments were carried out under atmospheric pressure and at a set temperature of 30 °C. Compared with conventional tubes, flow pattern transition lines occur at relatively lower water and gas flow velocities for gas–water flow in the capillary tube. While, flow pattern transition lines for gas–nanofluid flow occur at lower liquid and gas flow velocities than those for gas–water in the capillary tube. The effect of nanofluids on the two-phase flow patterns results mainly from the change of the gas–liquid surface tension. Concentrations of nanoparticles and SDBS have no effects on the flow patterns in the present concentration ranges.     

Transport properties of ultra-low concentration CuO–water nanofluids containing non-spherical nanoparticles

CuO–water nanofluids were prepared from non-spherical CuO nanoparticles by dispersing them in water through the aid of ultrasonication along with the use of Tiron as dispersant. Thermal conductivity enhancements of 13% and 44% have been obtained with 0.016 vol% CuO–water nanofluids at 28 °C and 55 °C respectively, which could be attributed to the high aspect ratio and Brownian motion of nanoparticles. Correlations have been developed to predict the influence of temperature (28–55 °C) and nanoparticles volume concentration (<0.016 vol%) on relative viscosity and thermal conductivity ratio. The results indicate the potential of this nanofluid for thermal management applications.     

Experimental investigation of oxide nanofluids laminar flow convective heat transfer

In the present investigation nanofluids containing CuO and Al₂O₃ oxide nanoparticles in water as base fluid in different concentrations produced and the laminar flow convective heat transfer through circular tube with constant wall temperature boundary condition were examined. The experimental results emphasize that the single phase correlation with nanofluids properties (Homogeneous Model) is not able to predict heat transfer coefficient enhancement of nanofluids. The comparison between experimental results obtained for CuO / water and Al₂O₃ / water nanofluids indicates that heat transfer coefficient ratios for nanofluid to homogeneous model in low concentration are close to each other but by increasing the volume fraction, higher heat transfer enhancement for Al₂O₃ / water can be observed.     

Heat transfer enhancement of nanofluids in a double pipe heat exchanger with louvered strip inserts

The effect of using louvered strip inserts placed in a circular double pipe heat exchanger on the thermal and flow fields utilizing various types of nanofluids is studied numerically. The continuity, momentum and energy equations are solved by means of a finite volume method (FVM). The top and the bottom walls of the pipe are heated with a uniform heat flux boundary condition. Two different louvered strip insert arrangements (forward and backward) are used in this study with a Reynolds number range of 10,000 to 50,000. The effects of various louvered strip slant angles and pitches are also investigated. Four different types of nanoparticles, Al₂O₃, CuO, SiO₂, and ZnO with different volume fractions in the range of 1% to 4% and different nanoparticle diameters in the range of 20 nm to 50 nm, dispersed in a base fluid (water) are used. The numerical results indicate that the forward louvered strip arrangement can promote the heat transfer by approximately 367% to 411% at the highest slant angle of α = 30° and lowest pitch of S = 30 mm. The maximal skin friction coefficient of the enhanced tube is around 10 times than that of the smooth tube and the value of performance evaluation criterion (PEC) lies in the range of 1.28–1.56. It is found that SiO₂ nanofluid has the highest Nusselt number value, followed by Al₂O₃, ZnO, and CuO while pure water has the lowest Nusselt number. The results show that the Nusselt number increases with decreasing the nanoparticle diameter and it increases slightly with increasing the volume fraction of nanoparticles. The results reveal that there is a slight change in the skin friction coefficient when nanoparticle diameters of SiO₂ nanofluid are varied.     

Thermal performance of screen mesh wick heat pipes using water-based copper nanofluids

Fairly stable surfactant free copper–distilled water nanofluids are prepared using prolonged sonication and homogenization. Thermal conductivity of the prepared nanofluid displays a maximum enhancement of ～15% for 0.5 wt% of Cu loading in distilled water at 30 °C. The wall temperature distributions and the thermal resistances between the evaporator and the condenser sections of a commercial screen mesh wick heat pipe containing nanofluids are investigated for three different angular position of the heat pipe. The results are compared with those for the same heat pipe with water as the working fluid. The wall temperatures of the heat pipes decrease along the test section from the evaporator section to the condenser section and increase with input power. The average evaporator wall temperatures of the heat pipe with nanofluids are much lower than those of the heat pipe with distilled water. The thermal resistance of the heat pipe using both distilled water and nanofluids is high at low heat loads and reduces rapidly to a minimum value as the applied heat load is increased. The thermal resistance of the vertically mounted heat pipe with 0.5 wt% of Cu–distilled water nanofluid is reduced by ～27%. The observed enhanced thermal performance is explained in light of the deposited Cu layer on the screen mesh wick in the evaporator section of the heat pipe.     

Experimental investigation and development of new correlation for thermal conductivity and viscosity of BioGlycol/water based SiO2 nanofluids

Nanofluids are a new class of engineered heat transfer fluids which exhibit superior thermophysical properties and have potential applications in numerous important fields. In this study, nanofluids have been prepared by dispersing SiO₂ nanoparticles in different base fluids such as 20:80% and 30:70% by volume of BioGlycol (BG)/water (W) mixtures. Thermal conductivity and viscosity experiments have been conducted in temperatures between 30 °C and 80 °C and in volume concentrations between 0.5% and 2.0%. Results show that thermal conductivity of nanofluids increases with increase of volume concentrations and temperatures. Similarly, viscosity of nanofluid increases with increase of volume concentrations but decreases with increase of temperatures. The maximum thermal conductivity enhancement among all the nanofluids was observed for 20:80% BG/W nanofluid about 7.2% in the volume concentration of 2.0% at a temperature of 70 °C. Correspondingly among all the nanofluids maximum viscosity enhancement was observed for 30:70% BG/W nanofluid about 1.38-times in the volume concentration of 2.0% at a temperature of 70 °C. The classical models and semi-empirical correlations failed to predict the thermal conductivity and viscosity of nanofluids with effect of volume concentration and temperatures. Therefore, nonlinear correlations have been proposed with 3% maximum deviation for the estimation of thermal conductivity and viscosity of nanofluids.     

Influence of nanofluid on heat transfer in a loop heat pipe

Experiments are conducted to investigate heat transfer characteristics of using nanofluid in a Loop Heat Pipe (LHP) as a working medium for heat input range from 20 W to 100 W. The experiments are carried out by manufacturing the LHP, in which the setup consists of a water tank with pump, a flat evaporator, condenser installed with two pieces of fans, two transportation lines (vapor and liquid lines), copper pipe sections for attachment of the thermocouples and power supply. The uniqueness of the current experimental setup is the vapor and liquid lines of LHP which are made of transparent plastic tube to visualize the fluid flow patterns. In this study, the LHP performance using silica (SiO₂–H₂O) nanofluid with particle volume fraction of 3% which was used as a coolant is examined. The experimental results are verified by simulation using Finite Element Method (FEM). The LHP performance is evaluated in terms of transient temperature distribution and total thermal resistance (R_t). R_t is estimated for both LHP using SiO₂–H₂O nanofluid and pure water cases under a steady state condition. The results reveal the average decrease of 28%–44% at heat input ranging from 20 W to 100 W in total thermal resistance of LHP using SiO₂–H₂O nanofluid as compared with pure water. Therefore, the presence of nanoparticles could greatly enhance the cooling of LHP. The experimental and simulation results are found in good agreement.     

Numerical investigation on heat transfer and friction factor characteristics of laminar and turbulent flow in an elliptic annulus utilizing nanofluid

In this paper, a numerical investigation on heat transfer performance and flow fields of different nanofluids flows through elliptic annulus in a laminar and turbulent flow regimes. The three-dimensional continuity, Navier–Stokes and energy equations are solved by using finite volume method (FVM) and the SIMPLE algorithm scheme is applied to examine the effects of laminar and turbulent flow on heat transfer characteristics. This study evaluates the effects of four different types of nanoparticles, Al₂O₃, CuO, SiO₂ and ZnO, with different volume fractions (0.5–4%) and diameters (25–80 nm) under constant heat flux boundary condition using water as a base fluid were used. The Reynolds number of laminar flow was in the range of 200 ≤ Re ≤ 1500, while for turbulent flow it was in the range of 4000 ≤ Re ≤ 10,000. The results have shown that SiO₂–water nanofluid has the highest Nusselt number, followed by ZnO–water, CuO–water, Al₂O₃–water, and lastly pure water. The Nusselt number for all cases increases with the volume fraction but it decreases with the rise in the diameter of nanoparticles. In all configurations, the Nusselt number increases with Reynolds number. It is found that the glycerine–SiO₂ shows the best heat transfer enhancement compared with other tested base fluids.     

High heat flux phase change on porous carbon nanotube structures

Carbon nanotube (CNT) forests are investigated as porous wick structures for chip-scale heat pipe cooling systems. An analytical model is developed to demonstrate the merits of phase change heat transfer on nanoscale porous structures, compared to that on microscale porous wick. Results indicate that nanoscale porous structures increase the thin-film evaporation surface area by one order of magnitude, which can significantly increase phase change heat transfer efficiency. The pertinent wick structure properties of the CNT forest are experimentally measured. Results show that the CNT forest is highly porous (～95% porosity), and possesses large variations in effective thermal conductivity ranging from 0.8 to 180 W/m K. Effective pore size of the CNT wick structure varies between 50 and 180 nm, which can generate capillary pressure up to two orders of magnitude higher than the microscale wick structure. However, its low permeability, about three to four orders of magnitude lower than the traditional wicks, underscores the necessity of bi-porous CNT wick structures. The bi-porous CNT wick structures are composed of nanoscale porous CNT clusters, separated by microscale (～50 μm wide) passages. Experimental results show a maximum heat flux of 770 W/cm² over a 2 mm × 2 mm heating area. With enhanced thin-film evaporation, heat transfer coefficients are improved by up to 100%, compared to the microscale wick. In contrast, the low CHF ～140 W/cm² over a 10 × 10 mm² heating area is caused by vapor occupation of the microscale pores and the reduction of wick permeability.     

Operational characteristics of two biporous wicks used in loop heat pipe with flat evaporator

Two special biporous wicks are adopted in stainless-steel–ammonia loop heat pipes (LHPs) with flat evaporator to enhance their heat transfer performances. The experimental results demonstrate that thermal and hydraulic characteristics of the wick with porosity of 69% (in LHP 2) are better than that of the wick with porosity of 65% (in LHP 1). The maximum heat loads of LHP 1 and LHP 2 could, respectively, reach 120 W (heat flux 11.8 W/cm²) and 130 W (12.8 W/cm²) at the allowable evaporator temperature below 60 °C. Meanwhile, they can start up at heat load as low as 2.5 W. The LHPs show very fast and smooth response to heat load and operate stably without obvious temperature oscillation. The total thermal resistances of the LHPs vary between 1.47 and 0.33 °C/W at heat load ranging from 10 to 130 W.     

Heat transfer characteristics in double tube helical heat exchangers using nanofluids

In the present work a three-dimensional analysis is used to study the heat transfer characteristics of a double-tube helical heat exchangers using nanofluids under laminar flow conditions. CuO and TiO₂ nanoparticles with diameters of 24 nm dispersed in water with volume concentrations of 0.5–3 vol.% are used as the working fluid. The mass flow rate of the nanofluid from the inner tube was kept and the mass flow rate of the water from the annulus was set at either half, full, or double the value. The variations of the nanofluids and water temperatures, heat transfer rates and heat transfer coefficients along inner and outer tubes are shown in the paper. Effects of nanoparticles concentration level and of the Dean number on the heat transfer rates and heat transfer coefficients are presented. The results show that for 2% CuO nanoparticles in water and same mass flow rate in inner tube and annulus, the heat transfer rate of the nanofluid was approximately 14% greater than of pure water and the heat transfer rate of water from annulus than through the inner tube flowing nanofluids was approximately 19% greater than for the case which through the inner and outer tubes flow water. The results also show that the convective heat transfer coefficients of the nanofluids and water increased with increasing of the mass flow rate and with the Dean number. The results have been validated by comparison of simulations with the data computed by empirical equations.     

Experimental comparative study of heat pipe performance using CuO and TiO2 nanofluids

In recent years, developing an energy efficient conventional heat pipe is more important because of the development of electronics and computer industries. To enhance the thermal performance of heat pipe, different nanofluids have been widely used. In this paper, an experimental investigation of heat transfer performance of heat pipe has been conducted using three different working fluids such as DI water, CuO nanofluid and TiO₂ nanofluid. The heat pipe used in this study is made up of copper layered with two layers of screen mesh wick for better capillary action. The Parameters considered in this study are heat input, angle of inclination and evaporator fill ratio. The concentration of nanoparticle used in this study is of 1.0 wt.%. From the experimental results, comparisons of thermal performance were made between the heat pipes using various working fluids. Among various fill ratio charged, the heat pipe shows good thermal performance when it is operated at 75% fill ratio for all working fluids. However, the heat pipe operated with CuO nanofluid showed higher results compared with TiO₂ nanofluid and DI water. Copyright © 2013 John Wiley & Sons, Ltd.     

An experimental study on the heat transfer performance and pressure drop of TiO2-water nanofluids flowing under a turbulent flow regime

Nanofluid is a new class of heat transfer fluids engineered by dispersing metallic or non-metallic nanoparticles with a typical size of less than 100 nm in the conventional heat transfer fluids. Their use remarkably augments the heat transfer potential of the base liquids. This article presents the heat transfer coefficient and friction factor of the TiO₂-water nanofluids flowing in a horizontal double tube counter-flow heat exchanger under turbulent flow conditions, experimentally. TiO₂ nanoparticles with diameters of 21 nm dispersed in water with volume concentrations of 0.2–2 vol.% are used as the test fluid. The results show that the heat transfer coefficient of nanofluid is higher than that of the base liquid and increased with increasing the Reynolds number and particle concentrations. The heat transfer coefficient of nanofluids was approximately 26% greater than that of pure vol.%, and the results also show that the heat transfer coefficient of the nanofluids at a volume concentration of 2.0 vol.% was approximately 14% lower than that of base fluids for given conditions. For the pressure drop, the results show that the pressure drop of nanofluids was slightly higher than the base fluid and increases with increasing the volume concentrations. Finally, the new correlations were proposed for predicting the Nusselt number and friction factor of the nanofluids, especially.     

Visualization and thermal resistance measurement for the sintered mesh-wick evaporator in operating flat-plate heat pipes

This work presents visualization and measurement of the evaporation resistance for operating flat-plate heat pipes with sintered multi-layer copper-mesh wick. A glass plate was adopted as the top wall for visualization. The multi-layer copper-mesh wick was sintered on the copper bottom plate. With different combinations of 100 and 200 mesh screens, the wick thickness ranged from 0.26 mm to 0.8 mm. Uniform heating was applied to the base plate near one end with a heated surface of 1.1 × 1.1 cm². At the other end was a cooling water jacket. At various water charges, the evaporation resistances were measured with evaporation behavior visualized for heat fluxes of 16–100 W/cm². Quiescent surface evaporation without nucleate boiling was observed for all test conditions. With heat flux increased, the water film receded and the evaporation resistance reduced. The minimum evaporation resistances were found when a thin water film was sustained in the bottom mesh layer. With heat flux further increased, partial dryout appeared with dry patches in the bottom mesh holes, first at the upstream end of the heated area and then expanded across the evaporator. The evaporation resistance re-rose in response to the appearance and expansion of partial dryout. When the fine 200 mesh screen was used as the bottom layer, its thinner thickness and stronger capillarity led to smaller minimum evaporation resistances.     

Performance of LHPs with a novel design evaporator

In order to realize an excellent heat transfer performance of the LHPs, including the fast start-up and high heat transfer capacity, a new connection design between the evaporator envelope and the wick surface without the clearance was proposed. The LHPs with a cylindrical evaporator, 22 mm diameter and 80 mm long, were fabricated with water as the working fluid and an 70% inventory.Copper wicks made of different particle sizes were used in both the start-up and heat transfer capacity tests. It was experimentally observed that the sintered wick with 139 μm diameter particles had the best heat transfer performance. It achieved a start-up time of only 150 s under 30 W heat load, a heat transfer capacity of 500 W under the allowable evaporator temperature of 85 °C, and a low thermal resistance of 0.070–0.165 °C/W. Internal temperature measurements were also conducted to determine the mechanism of the heat leak, to identify the heat pipe effect, and to compare the heat leak with different wicks corresponding to the change of the heat load during the operation     

Experimental study on the thermo-physical properties of car engine coolant (water/ethylene glycol mixture type) based SiC nanofluids

The engine coolant (water/ethylene glycol mixture type) becomes one of the most commonly used commercial fluids in cooling system of automobiles. However, the heat transfer coefficient of this kind of engine coolant is limited. The rapid developments of nanotechnology have led to emerging of a relatively new class of fluids called nanofluids, which could offer the enhanced thermal conductivity (TC) compared with the conventional coolants. The present study reports the new findings on the thermal conductivity and viscosity of car engine coolants based silicon carbide (SiC) nanofluids. The homogeneous and stable nanofluids with volume fraction up to 0.5 vol.% were prepared by the two-step method with the addition of surfactant (oleic acid). It was found that the thermal conductivity of nanofluids increased with the volume fraction and temperature (10–50 °C), and the highest thermal conductivity enhancement was found to be 53.81% for 0.5 vol.% nanofluid at 50 °C. In addition, the overall effectiveness of the current nanofluids (0.2 vol.%) was found to be ~ 1.6, which indicated that the car engine coolant-based SiC nanofluid prepared in this paper was better compared to the car engine coolant used as base liquid in this study.