An experimental study of forced convection effectiveness of Al2O3-water nanofluid flowing in circular tubes

In the present study, experimental efforts have been performed to explore the forced convection heat transfer using water-based suspension of Al₂O₃ nanoparticles (nanofluid) to replace the pure water as the working fluids in circular tubes. The nanofluid was prepared as a functional forced convection fluid and the thermal properties including the density, thermal conductivity, and dynamic viscosity were investigated experimentally. Besides, forced convection heat transfer in circular tubes was investigated with water-based nanofluid containing various mass fractions of the Al₂O₃ nanoparticles (2, 5, and 10 wt%) under the following operating conditions: the volume flow rate Q_f = 23.6–183.5 cm³/min (the Reynolds number Re_f,0 = 188–2095), the heating power applied at the outer wall of the tube q_o , eff.^″ = 1908–7362 W/m², and the inlet fluid temperature T_in = 24.5–25.5 °C or 49.5–50.5 °C. Measured data showed that the dispersion of increasing mass fraction of Al₂O₃ nanoparticles can effectively improve the thermal conductivity relative to the pure water. Besides, higher average heat transfer effectiveness ${\stackrel{-}{\epsilon }}_{h,\mathit{btd}}$ and figure of merit FOM are noted for the cases with higher inlet fluid temperature T_in.     

Numerical study of microchannel heat sink performance using nanofluids

This study presents the numerical simulation of three-dimensional incompressible steady and laminar and turbulent fluid flow of a trapezoidal micro-channel heat sink (MCHS) using CuO/water nanofluid as a cooling fluid. Navier–Stokes equations with conjugate energy equation are discretized by the finite-volume method. CFD predictions of laminar and turbulent forced convection of CuO/water nanofluids by single-phase and two-phase models (mixture model) are compared. The parameters studied include the particle volume fraction (ϕ = 0.204 %, 0.256%, 0.294% and 0.4%), and the volumetric flow rate ($\dot{\mathit{V}}=10\mathrm{mL}/min$, 15 mL/min and 20 mL/min). Comparisons of the thermal resistance predicted by the single-phase and two-phase models with corresponding experimental results show that the two-phase model is more accurate than the single-phase model. In the laminar flow, the thermal resistance of nanofluids is smaller than that of the water, which decreases as the particle volume fraction and the volumetric flow rate increase. In addition, the pressure drop of both nanofluid-cooled MCHS and pure water-cooled MCHS is discussed. For the laminar flow case, the pressure drop increases slightly for nanofluid-cooled MCHS.     

An experimental investigation of forced convective cooling performance of a microchannel heat sink with Al2O3/water nanofluid

Experiments were conducted to investigate forced convective cooling performance of a copper microchannel heat sink with Al₂O₃/water nanofluid as the coolant. The microchannel heat sink fabricated consists of 25 parallel rectangular microchannels of length 50 mm with a cross-sectional area of 283 μm in width by 800 μm in height for each microchannel. Hydraulic and thermal performances of the nanofluid-cooled microchannel heat sink have been assessed from the results obtained for the friction factor, the pumping power, the averaged heat transfer coefficient, the thermal resistance, and the maximum wall temperature, with the Reynolds number ranging from 226 to 1676. Results show that the nanofluid-cooled heat sink outperforms the water-cooled one, having significantly higher average heat transfer coefficient and thereby markedly lower thermal resistance and wall temperature at high pumping power, in particular. Despite the marked increase in dynamic viscosity due to dispersing the alumina nanoparticles in water, the friction factor for the nanofluid-cooled heat sink was found slightly increased only.     

Flow over and forced convection heat transfer around a semi-circular cylinder at incidence

Wake dynamics and forced convective heat transfer characteristics past a semi-circular cylinder at incidence have been investigated numerically. Utilizing air as an operating fluid computations are carried out for wide ranges of the Reynolds number (80 ? Re ? 180) and angle of incidences (0 ? α ? 180°). Angle of incidence reveals three flow separation zones. Structure properties of shear layer and vortex motions on each flow separation zones are analyzed critically. Functional dependence of drag (C_D), lift (C_L), and moment (C_M) coefficients on the angle of incidence is explored and analyzed in detail. Increase in angle of incidence increases streamline curvature. A structural similarity is observed between the contours of vorticity and the corresponding isotherms. Strouhal number shows a decreasing trend up to certain values of α and thereafter it increases marginally. A new correlation of Strouhal number as a function of Re and α has been established for the present range of Reynolds numbers. At the singularity points a sudden jump in local Nusselt number distribution is observed. The trend of variation of average Nusselt number with α is similar to that of Strouhal number variation. The average Nusselt number is found to vary as ${\mathit{Re}}^{0.529}(1+\alpha {)}^{-0.0476}$.     

Numerical study of hydrodynamic and heat transfer of nanofluid flow in microchannels containing micromixer

In this study heat transfer and fluid flow of Al₂O₃/water nanofluid in two dimensional parallel plate microchannel without and with micromixers have been investigated for nanoparticle volume fractions of ϕ = 0, ϕ = 4%  and base fluid Reynolds numbers of Re_f = 5, 20, 50. One baffle on the bottom wall and another on the top wall work as a micromixer and heat transfer enhancement device. A single-phase finite difference FORTRAN code using Projection method has been written to solve governing equations with constant wall temperature boundary condition. The effect of various parameters such as nanoparticle volume fraction, base fluid Reynolds number, baffle distance, height and order of arrangement have been studied. Results showed that the presence of baffles and also increasing the Re number and nanoparticle volume fraction increase the local and averaged heat transfer and friction coefficients. Also, the effect of nanoparticle volume fraction on heat transfer coefficient is more than the friction coefficient in most of the cases. It was found that the main mechanism of enhancing heat transfer or mixing is the recirculation zones that are created behind the baffles. The size of these zones increases with Reynolds number and baffle height. The fluid pushing toward the wall by the opposed wall baffle and reattaching of separated flow are the locations of local maximum heat transfer and friction coefficients.     

Turbulent heat transfer and friction factor of Al2O3 Nanofluid in circular tube with twisted tape inserts

The thermophysical properties like thermal conductivity and viscosity of Al₂O₃ nanofluid is determined through experiments at different volume concentrations and temperatures and validated. Convective heat transfer coefficient and friction factor data at various volume concentrations for flow in a plain tube and with twisted tape insert is determined experimentally for Al₂O₃ nanofluid. Experiments are conducted in the Reynolds number range of 10,000–22,000 with tapes of different twist ratios in the range of 0 < H/D < 83. The heat transfer coefficient and friction factor of 0.5% volume concentration of Al₂O₃ nanofluid with twist ratio of five is 33.51% and 1.096 times respectively higher compared to flow of water in a tube. A generalized regression equation is developed for the estimation of Nusselt number and friction factor valid for both water and nanofluid in plain tube and with inserts under turbulent flow conditions.     

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.     

Nanofluid stabilizes and enhances convective boiling heat transfer in a single microchannel

The flow boiling heat transfer in a single microchannel was investigated with pure water and nanofluid as the working fluids. The microchannel had a size of 7500 × 100 × 250 μm, which was formed by two pyrex glasses and a silicon wafer. A platinum film with a length of 3500 μm and a width of 80 μm was deposited at the bottom channel surface, acting as the heater and temperature sensor. The nanofluid had a low weight concentration of 0.2%, consisting of de-ionized water and 40 nm Al₂O₃ nanoparticles. The nanoparticle deposition phenomenon was not observed. The boiling flow displays chaotic behavior due to the random bubble coalescence and breakup in the milliseconds timescale at moderate heat fluxes for pure water. The flow instability with large oscillation amplitudes and long cycle periods was observed with further increases in heat fluxes. The flow patterns are switched between the elongated bubbles and isolated miniature bubbles in the timescale of 100 s. It is found that nanofluid significantly mitigate the flow instability without nanoparticle deposition effect. The boiling flow is always stable or quasi-stable with significantly reduced pressure drop and enhanced heat transfer. Miniature bubbles are the major flow pattern in the microchannel. Elongated bubbles temporarily appear in the milliseconds timescale but isolated miniature bubbles will occupy the channel shortly. The decreased surface tension force acting on the bubble accounts for the smaller bubble size before the bubble departure. The inhibition of the dry patch development by the structural disjoining pressure, and the enlarged percentage of liquid film evaporation heat transfer region with nanoparticles, may account for the heat transfer enhancement compared to pure water.     

CFD modeling (comparing single and two-phase approaches) on thermal performance of Al2o3/water nanofluid in mini-channel heat sink

CFD modeling of laminar forced convection on Al₂O₃ nanofluid with size particles equal to 33 nm and particle concentrations of 0.5, 1 and 6 wt.% within 130 < Re < 1600 in mini-channel heat sink is executed by four individual models (single phase, VOF, mixture, Eulerian). Three-dimensional steady-state governing partial differential equations was discretized using finite volume method.Influences of some important parameters such as nanoparticle concentration and Reynolds number on the enhancement of nanofluid heat transfer have been investigated. The difference between the two-phase models results was marginal, and they were more precise by comparison with experimental reference data than single phase model. Besides with regard to the most precise and less CPU usage and run time, mixture model was chosen to obtain a correlation based on dimensionless numbers for the Nusselt number and friction factor estimation.     

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.