An experimental investigation on the effects of surfactants on the thermal performance of hybrid nanofluids in helical coil heat exchangers

In the present study, the effects of surfactants on the thermal performance of the hybrid nanofluid (Alumina–Silver) at constant wall temperature and laminar flow have been experimentally studied in a helical coil heat exchanger. Different surfactants such as anionic Sodium Dodecyl Sulfate (SDS) and nonionic Poly Vinyl Pyrrolidone (PVP) in the concentration of range of 0.1–0.4 wt.% are employed. It is found that the thermal performance can be maximized by using the 0.2 vol.% hybrid nanofluid and 0.1 wt.% SDS anionic surfactant in the helical coil. The maximum thermal performance in the presence of hybrid Alumina–Silver nanofluid and SDS anionic surfactant is 16% higher than that of the pure distilled water. The presented results can have potential application in process intensification and optimum design of heat exchangers.     

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.     

Heat transfer enhancement using Al2O3–water nanofluid for an electronic liquid cooling system

We have experimentally investigated the behaviour and heat transfer enhancement of a particular nanofluid, Al₂O₃ nanoparticle–water mixture, flowing inside a closed system that is destined for cooling of microprocessors or other electronic components. Experimental data, obtained for turbulent flow regime, have clearly shown that the inclusion of nanoparticles into distilled water has produced a considerable enhancement of the cooling block convective heat transfer coefficient. For a particular nanofluid with 6.8% particle volume concentration, heat transfer coefficient has been found to increase as much as 40% compared to that of the base fluid. It has also been found that an increase of particle concentration has produced a clear decrease of the heated component temperature. Experimental data have clearly shown that nanofluid with 36 nm particle diameter provides higher heat transfer coefficients than the ones of nanofluid with 47 nm particle size.     

Boiling enhancement by TiO2 nanoparticle deposition

TiO₂ nanoparticle-coated nickel wires were produced by electrical heating in various nanofluid concentrations ranging from 0.01 to 1 wt.% with various processing heat fluxes from 0 to 1000 kW/m². The experimental results demonstrated up to 82.7% enhancement on critical heat flux (CHF) in condition of coated nickel wire (processed in 1 wt.% with 1000 kW/m²) boiling in pure water. The contact angle measurement revealed that the hydrophilic porous coating formed by vigorous vaporization of TiO₂ nanofluid in nucleate boiling regime enormously modified the wettability of heating surface consequently improving the CHF. Besides, it is evident that the coverage of nanoparticle deposition tended to become more complete as concentration and processing heat flux increased based on SEM and EDS analysis. The nanoparticles dispersed in base fluid exhibited little effect on CHF enhancement and could even hinder the percentage of CHF augmentation from boosting, which demonstrated that one could enhance CHF by using only small amount of nanoparticles just adequate to form surface coatings instead of preparing working fluid with great bulk. However, according to the boiling curves in all cases of coated nickel wires, it is supposed that the nucleate boiling heat transfer coefficient deteriorates as a result of thermal resistance resulted from the occurrence of nanoparticle deposition. In summary, the coated porous structure of nanoparticles leads to enhance CHF and to decrease boiling heat transfer coefficient.     

Cooling performance investigation of electronics cooling system using Al2O3–H2O nanofluid

In this study, the cooling performance of Al₂O₃–H₂O nanofluid was experimentally investigated as a much better developed alternative for the conventional coolant. For this purpose the nanofluid was passed through the custom-made copper minichannel heat sink which is normally attached with the electronic heat source. The thermal performance of the Al₂O₃–H₂O nanofluid was evaluated at different volume fraction of the nanoparticle as well as at different volume flow rate of the nanofluid. The volume fraction of the nanoparticle varied from 0.05 vol.% to 0.2 vol.% whereas the volume flow rate was increased from 0.50 L/min to 1.25 L/min. The experimental results showed that the nanofluid successfully has minimized the heat sink temperature compared to the conventional coolant. It was noticed also that the thermal entropy generation rate was reduced via using nanofluid instead of the normal water. Among the other functions of the nanofluid are to increase the frictional entropy generation rate and to drop the pressure which are insignificant compared to the normal coolant. Given the improved performance of the nanofluid, especially for high heat transportation capacity and low thermal entropy generation rate, it could be used as a better alternative coolant for the electronic cooling system instead of conventional pure water.     

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.     

Estimation local convective boiling heat transfer coefficient in mini channel

The aim of this paper is to present an inverse heat conduction method used for determining the local convective boiling heat transfer coefficient in mini channel for pure water, copper nanofluid with using three different concentrations of nanoparticles: 5 mg/L, 10 mg/L and 50 mg/L. Sequential specification function method is used to solve the IHCP and estimate the space-variable convective heat transfer coefficient. The uncertainties in the estimated in heat transfer coefficient are calculated using Bias and Variance errors. The technique is used in a series of numerical experiments to provide the optimum experimental design for a boiling heat transfer investigation.     

Effect of nanoparticles size on thermal performance of nanofluid in a trapezoidal microchannel-heat-sink

This paper deals with spherical nanoparticles size effects on thermal performance and pressure drop of a nanofluid in a trapezoidal microchannel-heat-sink (MCHS). Eulerian–Eulerian two-phase numerical approach is utilized for forced convection laminar, incompressible and steady three dimensional flow of copper-oxide nanoparticles with water as base fluid at 100 to 200 nm diameter and 1% to 4% volume concentration range. Continuity, momentum, energy and volume conservation equations are solved at whole of the computational domain via finite volume method. Obtained results signify that pressure drop increases 15% at Re = 500 and 1% volume concentration while nanoparticles diameter increases from 100 to 200 nm. By increasing volume concentration, nanoparticles size effect becomes more prominent and it is observed that increment rate of pressure drop is intensified for above 150 nm particles diameter. Unlike the pressure drop, heat transfer decreases with an increase in nanoparticles diameter. Also, it is observed that with an increase in nanoparticles diameter, average Nusselt number of base fluid decreases more than that of the nanoparticles and this signifies that base fluid has more efficacy on thermal performance of copper-oxide nanofluid.     

Heat transfer enhancement and pressure drop characteristics of TiO2–water nanofluid in a double-tube counter flow heat exchanger

This article reports an experimental study on the forced convective heat transfer and flow characteristics of a nanofluid consisting of water and 0.2 vol.% TiO₂ nanoparticles. The heat transfer coefficient and friction factor of the TiO₂–water nanofluid flowing in a horizontal double-tube counter flow heat exchanger under turbulent flow conditions are investigated. The Degussa P25 TiO₂ nanoparticles of about 21 nm diameter are used in the present study. The results show that the convective heat transfer coefficient of nanofluid is slightly higher than that of the base liquid by about 6–11%. The heat transfer coefficient of the nanofluid increases with an increase in the mass flow rate of the hot water and nanofluid, and increases with a decrease in the nanofluid temperature, and the temperature of the heating fluid has no significant effect on the heat transfer coefficient of the nanofluid. It is also seen that the Gnielinski equation failed to predict the heat transfer coefficient of the nanofluid. Finally, the use of the nanofluid has a little penalty in pressure drop.     

Turbulent forced convection effectiveness of alumina–water nanofluid in a circular tube with elevated inlet fluid temperatures: An experimental study

The present study aims to explore experimentally the influence of elevated inlet fluid temperature on the turbulent forced convective heat transfer effectiveness of using alumina–water nanofluid over pure water in an iso-flux heated horizontal circular tube at a fixed heating power. A copper circular pipe of inner diameter 3.4 mm was used in the forced convection experiments undertaken for the pertinent parameters in the following ranges: the inlet fluid temperature, T_in = 25 °C, 37 °C and 50 °C; the Reynolds number, Re_bf = 3000–13,000; the mass fraction of the alumina nanoparticles in the water-based nanofluid formulated, ω_np = 0, 2, 5, and 10 wt.%; and the heating flux, q_o^″ = 57.8–63.1 kW/m². The experimental results clearly indicate that the turbulent forced convection heat transfer effectiveness of the alumina–water nanofluid over that of the pure water can be further uplifted by elevating its inlet temperature entering the circular tube well above the ambient, thereby manifesting its potential as an effective warm functional coolant. Specifically, an increase in the averaged heat transfer enhancement of more than 44% arises for the nanofluid of ω_np = 2 wt.% as the inlet fluid temperature is increased from 25 °C to 50 °C.     

Thermal performance augmentation using water based Al2O3-gamma nanofluid in a horizontal shell and tube heat exchanger under forced circulation

Shell and tube heat exchanger is one of the most prevalent heat exchangers with a wide variety of industrial applications, i.e., power plants, chemical processes, marine industries, HVAC systems, cooling of hydraulic fluid and engine oil in heavy duty diesel engines and the like specifically where a need to heat or cool a large fluid volume exist and also higher-pressure use. In the present study, the effect of using Al₂O₃-water nanofluid on thermal performance of a commercial shell and tube heat exchanger with segmental baffles is assessed experimentally. For this purpose, Al₂O₃-gamma nanoparticles with 15 nm mean diameter (99.5% purity) and Sodium Dodecyl Benzene Sulphonate (SDBS) as surfactant are used to make aqueous Al₂O₃ nanofluid at three various volume fractions of nanoparticles (φ = 0.03, 0.14 and 0.3%). Indeed, in this paper the effect of some parameters of hot working fluid such as Reynolds number and volume concentration of nanoparticles on heat transfer characteristics, friction factor and thermal performance factor of a shell and tube heat exchanger under laminar flow regime is investigated. The results indicate a substantial increment in Nusselt number as well as the overall heat transfer coefficient of heat exchanger by enhancement of Reynolds number and it can be seen that, at a certain Reynolds number, heat transfer characteristics of heat exchanger increase as the nanoparticles volume concentration increases. Outcomes of the heat transfer evaluation demonstrate that applying nanofluids instead of base fluid lead to increment of Nusselt number up to 9.7, 20.9 and 29.8% at 0.03, 0.14 and 0.3 vol%, respectively. Likewise it is seen that at mentioned nanoparticles volume fractions, overall heat transfer coefficient of heat exchanger enhances around 5.4, 10.3 and 19.1%, respectively. In term of pressure drop, a little penalty is found by using nanofluid in the test section. Eventually a thermal performance assessment on the heat exchanger was conducted. According to the analysis results, utilizing nanofluid at minimum and maximum nanoparticles volume fractions (φ = 0.03 and 0.3%) results in average augmentation of around 6.5% and 18.9% in thermal performance factor (η) of the heat exchanger compared to the base liquid, respectively.     

Heat transfer,friction factor and effectiveness analysis of Fe3O4/water nanofluid flow in a double pipe heat exchanger with return bend

The convective heat transfer, friction factor and effectiveness of different volume concentrations of Fe₃O₄ nanofluid flow in an inner tube of double pipe heat exchanger with return bend has been estimated experimentally and turbulent flow conditions. The test section used in this study is of double pipe type in which the inner tube diameter is 0.019 m, the annulus tube diameter is 0.05 m and the total length of inner tube is 5 m. At a distance of 2.2 m from the inlet of the inner tube the return bend is provided. The hot Fe₃O₄ nanofluid flows through an inner tube, where as the cold water flows through an annulus tube. The volume concentrations of the nanoparticles used in this study are 0.005%, 0.01%, 0.03% and 0.06% with Reynolds number range from 15,000 to 30,000. Based on the results, the Nusselt number enhancement is 14.7% for 0.06% volume concentration of nanofluid flow in an inner tube of heat exchanger at a Reynolds number of 30,000 when compared to base fluid data; the pumping penalty of nanofluid is < 10%. The effectiveness of heat exchanger for water and nanofluid flow is explained in terms of number of transfer units (NTU) in order to estimate the overall performance of the double pipe heat exchanger. New correlations for Nusselt number and friction factor have been developed based on the experimental data.     

Pool boiling heat transfer on the microheater surface with and without nanoparticles by pulse heating

We study the pool boiling heat transfer on the microheater surface with and without nanoparticles by pulse heating. Nanofluids are the mixture of de-ionized water and Al₂O₃ particles with 0.1%, 0.2%, 0.5% and 1.0% weight concentrations. The microheater is a platinum surface by 50 × 20 μm. Three types of bubble dynamics were identified. The first type of bubble dynamics is for the boiling in pure water, referring to a sharp microheater temperature increase once a new pulse cycle begins, followed by a continuous temperature increase during the pulse duration stage. Large bubble is observed on the microheater surface and it does not disappear during the pulse off stage. The second type of bubble dynamics is for the nanofluids with 0.1% and 0.2% weight concentrations. The microheater surface temperature has a sharp increase at the start of a new pulse cycle, followed by a slight decrease during the pulse duration stage. Miniature bubble has oscillation movement along the microheater length direction, and it disappears during the pulse off stage. The third type of bubble dynamics occurs at the nanofluid weight concentration of 0.5% and 1.0%. The bubble behavior is similar to that in pure water, but the microheater temperatures are much lower than that in pure water. A structural disjoining pressure causes the smaller contact area between the dry vapor and the heater surface, decreasing the surface tension effect and resulting in the easy departure of miniature bubbles for the 0.1% and 0.2% nanofluid weight concentrations. For the 0.5% weight concentration of nanofluids, coalescence of nanoparticles to form larger particles is responsible for the large bubble formation on the heater surface. The microlayer evaporation heat transfer and the heat transfer mechanisms during the bubble departure process account for the higher heat transfer coefficients for the 0.1% and 0.2% nanofluid weight concentrations. The shortened dry area between the bubble and the heater surface, and the additional thin nanofluid liquid film evaporation heat transfer, account for the higher heat transfer coefficient for the 0.5% nanofluid weight concentration, compared with the pure water runs.     

Heat transfer enhancements of low volume concentration Al2O3 nanofluid and with longitudinal strip inserts in a circular tube

The turbulent convective heat transfer and friction factor behavior of Al₂O₃ nanofluid in a circular tube with different aspect ratios of longitudinal strip inserts are studied experimentally. Experiments are conducted with water and nanofluid in the range of 3000 < Re < 22,000, particle volume concentration 0 < φ < 0.5% and longitudinal strip aspect ratios of 0 < AR < 18. The agreement between the values of Nusselt number obtained with water is satisfactory when compared with the data of Heish and Huang. Results indicate that heat transfer coefficients increase with nanofluid volume concentration and decrease with aspect ratio.     

Heat transfer and flow characteristics of AL2O3–water nanofluid in a double tube heat exchanger

An experimental study was carried out in order to find out the effects of Al₂O₃ nanofluid with a mean diameter of 20 nm on heat transfer, pressure drop and thermal performance of a double tubes heat exchanger. The effective viscosity of nanofluid was measured in various temperatures ranging from 27 °C to 55 °C. Experiments were carried out at different Reynolds numbers ranging from 5000 to 20,000, approximately, and in various nanoparticles concentration up to 1% by volume. Results indicate that there is a good potential in promoting the thermal performance of heat exchanger by adding nanoparticles in the investigated ranges where there is not a severe pressure drop penalty. The empirical correlation was created for Nusselt number variation based on the Reynolds number and nanoparticles concentration.     

Effect of particle size on the convective heat transfer in nanofluid in the developing region

An experimental investigation on the convective heat transfer characteristics in the developing region of tube flow with constant heat flux is carried out with alumina–water nanofluids. The primary objective is to evaluate the effect of particle size on convective heat transfer in laminar developing region. Two particle sizes were used, one with average particle size off 45 nm and the other with 150 nm. It was observed that both nanofluids showed higher heat transfer characteristics than the base fluid and the nanofluid with 45 nm particles showed higher heat transfer coefficient than that with 150 nm particles. It was also observed that in the developing region, the heat transfer coefficients show higher enhancement than in the developed region. Based on the experimental results a correlation for heat transfer in the developing region has been proposed for the present range of nanofluids.     

Heat transfer enhancement in a hybrid microchannel-photovoltaic cell using Boehmite nanofluid

Experiments were conducted to investigate the cooling performance of water-based Boehmite (AlOOH · xH₂O) nanofluid in a hybrid photovoltaic (PV) cell. A Perspex plate consists of 40 parallel rectangular microchannels with a hydraulic diameter of 783 μm, a length of 24 cm, a width of 1.8 mm and a depth of 500 μm attached to the back of the cell. Cooling performances of water, as the base fluid, and three different concentrations of nanofluid (0.01, 0.1 and 0.3 wt.%) were compared. The nanofluid thermal performance has been assessed from the obtained results for outlet flow temperature and the average PV surface temperature. The average PV surface temperature decreased from 62.29 °C to 32.5 °C at zero and 300 ml/min of flow rate for 0.01 wt.% nanofluid, respectively. Moreover, the highest improving in the electrical efficiency was achieved about 27% for 0.01 wt.% concentration of the nanofluid at this flow rate.     

Comparative study of the effect of hybrid nanoparticle on the thermal performance of cylindrical screen mesh heat pipe

The thermal performance of a cylindrical screen mesh heat pipe with hybrid nanofluid was experimentally investigated. The hybrid nanofluid was prepared by mixing Al2O3 and CuO nanoparticles with deionised water. The heat pipe was fabricated with straight copper tube of dimensions 300 mm length, 12.5 mm outer diameter and 1 mm thickness. The wick structure in the heat pipe was created by a three layer copper screen mesh of 100 mesh size. The heat input to the heat pipe was varied from 50 W to 250 W in five equal steps. The heat pipe was tested with three hybrid nanofluids made with combinations of Al2O3 and CuO nanoparticle in DI water (Al2O3 75%–CuO 25%, Al2O3 50%–CuO 50% and Al2O3 25%–CuO 75%). The tested hybrid nanofluids were made with 0.1% volume concentration of Al2O3 and CuO nanoparticle combination in deionised water. The results of the investigation showed that for the maximum heat load of 250 W considered in this work, the thermal resistance of the hybrid nanofluid with combination, Al2O3 25%–CuO 75%, showed 44.25% reduction compared to deionised water. The reduction in thermal resistance is due to the formation of porous coating on the wick surface which increases the wettability and surface roughness thereby creating more nucleation sites as seen in the SEM images. From the experimental investigation, it was observed that hybrid nanofluids are alternative to the conventional working fluids in heat pipes for electronic cooling applications.     

Numerical simulation of natural convection of nanofluid in a square enclosure: Effects due to uncertainties of viscosity and thermal conductivity

The present study aims to identify effects due to uncertainties in effective dynamic viscosity and thermal conductivity of nanofluid on laminar natural convection heat transfer in a square enclosure. Numerical simulations have been undertaken incorporating a homogeneous solid–liquid mixture formulation for the two-dimensional buoyancy-driven convection in the enclosure filled with alumina–water nanofluid. Two different formulas from the literature are each considered for the effective viscosity and thermal conductivity of the nanofluid. Simulations have been carried out for the pertinent parameters in the following ranges: the Rayleigh number, Ra_f = 10³–10⁶ and the volumetric fraction of alumina nanoparticles, ? = 0–4%. Significant difference in the effective dynamic viscosity enhancement of the nanofluid calculated from the two adopted formulas, other than that in the thermal conductivity enhancement, was found to play as a major factor, thereby leading to contradictory results concerning the heat transfer efficacy of using nanofluid in the enclosure.     

Pool boiling characteristics of nanofluid on flat plate based on heater surface analysis

Nucleate pool boiling of Al₂O₃ based aqueous nanofluid on flat plate heater has been studied experimentally. For boiling of nanofluid (< 0.1 vol.%) on heating surface with ratio of average surface roughness to average diameter of particles much less than unity when boiling continue to CHF, the heat transfer coefficient of nanofluid boiling reduces while critical heat flux (CHF) increases. CHF enhancement increased with volume fraction of nanoparticles. Atomic force microscope (AFM) images from boiling surface showed that after boiling of nanofluid the surface roughness increases or decreases depending on initial condition of heater surface. Changes in boiling surface topology during different regions of boiling, wettability and thermal resistance of heater surface owing to nanoparticles deposition cause to variations in nanofluids boiling performance.