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.     

Effect of full length twisted tape inserts on heat transfer and friction factor enhancement with Fe3O4 magnetic nanofluid inside a plain tube: An experimental study

Turbulent convective heat transfer and friction factor characteristics of magnetic Fe₃O₄ nanofluid flowing through a uniformly heated horizontal circular tube with and without twisted tape inserts are estimated experimentally. Experiments are conducted in the particle volume concentration range of 0 < φ < 0.6%,twisted tape inserts of twist ratio in the range of 0 < H/D < 15and Reynolds number range of 3000 < Re < 22000. Heat transfer and friction factor enhancement of 0.6% volume concentration of Fe₃O₄ nanofluid in a plain tube with twisted tape insert of twist ratio H/D = 5 is 51.88% and 1.231 times compared to water flowing in a plain tube under same Reynolds number. Generalized regression equation is presented for the estimation of Nusselt number and friction factor for both water and Fe₃O₄ nanofluid in a plain tube and with twisted tape inserts under turbulent flow condition.     

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.     

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.     

Assessment of thermoelectric module with nanofluid heat exchanger

For applications such as cooling of electronic devices, it is a common practice to sandwich the thermoelectric module between an integrated chip and a heat exchanger, with the cold-side of the module attached to the chip. This configuration results thermal contact resistances in series between the chip, module, and heat exchanger. In this paper, an appraisal of thermal augmentation of thermoelectric module using nanofluid-based heat exchanger is presented. The system under consideration uses commercially available thermoelectric module, 27 nm Al₂O₃–H₂O nanofluid, and a heat source to replicate the chip. The volume fraction of nanofluid is varied between 0% and 2%. At optimum input current conditions, experimental simulations were performed to measure the transient and steady-state thermal response of the module to imposed isoflux conditions. Data collected from the nanofluid-based exchanger is compared with that of deionized water.Results show that there exist a lag-time in thermal response between the module and the heat exchanger. This is attributed to thermal contact resistance between the two components. A comparison of nanofluid and deionized water data reveals that the temperature difference between the hot- and cold-side, ΔT = T_h ? T_c ≈ 0, is almost zero for nanofluid whereas ΔT > 0 for water. When ΔT ≈ 0, the contribution of Fourier effect to the overall heating is approximately zero hence enhancing the module cooling capacity. Experimental evidence further shows that temperature gradient across the thermal paste that bonds the chip and heat exchanger is much lower for the nanofluid than for deionized water. Low temperature gradient results in low resistance to the flow of heat across the thermal paste. The average thermal contact resistance, R = ΔT/Q, is 0.18 and 0.12 °C/W, respectively for the deionized water and nanofluid. For the range of optimum current, 1.2 ? current ? 4.1 A, considered in this study, the COP ranges between 1.96 and 0.68.     

Flow-boiling heat transfer of R-134a-based nanofluids in a horizontal tube

The influence of nanoparticles on the flow-boiling of R-134a and R-134a/polyolester mixtures is quantified for flows of low vapor quality (x < 20%) over a range of mass fluxes (100 < G < 400 kg/m² s). With direct dispersion of SiO₂ nanoparticles in R-134a, the heat transfer coefficient decreases (as much as 55%) in comparison to pure R-134a. This degradation is, in part, due to difficulties in obtaining a stable dispersion. However, excellent dispersion is achieved for a mixture of R-134a and polyolester oil with CuO nanoparticles, and the heat transfer coefficient increases more than 100% over baseline R-134a/polyolester results. In the range of these experiments, nanoparticles have an insignificant effect on the flow pressure drop with the R-134a/POE/CuO nanofluid.     

Heat transfer augmentation in a two-sided lid-driven differentially heated square cavity utilizing nanofluids

The behaviour of nanofluids is investigated numerically inside a two-sided lid-driven differentially heated square cavity to gain insight into convective recirculation and flow processes induced by a nanofluid. A model is developed to analyze the behaviour of nanofluids taking into account the solid volume fraction χ. The transport equations are solved numerically with finite volume approach using SIMPLE algorithm. Comparisons with previously published work on the basis of special cases are performed and found to be in excellent agreement. The left and the right moving walls are maintained at different constant temperatures while the upper and the bottom walls are thermally insulated. Three case were considered depending on the direction of the moving walls. Governing parameters were 0.01 < Ri < 100 but due to space constraints only the results for 0.1 < Ri < 10 are presented. It is found that both the Richardson number and the direction of the moving walls affect the fluid flow and heat transfer in the cavity. Copper–Water nanofluid is used with Pr = 6.2 and solid volume fraction χ is varied as 0.0%, 8%, 16% and 20%. Detailed results are presented for flow pattern and heat transfer curves.     

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.     

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.     

The boundary layers of an unsteady stagnation-point flow in a nanofluid

The boundary layer of an unsteady two-dimensional stagnation-point flow of a nanofluid is further investigated. The similarity equations are solved numerically for three types of nanoparticles, namely copper (Cu), alumina (Al₂O₃), and titania (TiO₂) in the water based fluid with Prandtl number Pr = 6.2. The skin friction coefficient, the local Nusselt number, and the velocity and temperature profiles are presented and discussed. Effects of the solid volume fraction parameter φ on the fluid flow and heat transfer characteristics are thoroughly examined. Interesting observation is that there are dual solutions seen for negative values of the unsteadiness parameter A (decelerating flow with A < 0).     

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.     

Natural convection in a rectangular enclosure containing an oval-shaped heat source and filled with Fe3O4/water nanofluid

This work concerns with the study of natural convection heat transfer in rectangular cavities with an inside oval-shaped heat source filled with Fe₃O₄/water nanofluid. The finite element method is employed to solve the governing equations for this problem. Average Nusselt numbers are presented for a wide range of Rayleigh number (10³ ≤ Ra ≤ 10⁵), volume fraction of nanoparticles (0 ≤ ϕ ≤ 14%), and four different size and shapes of the heat source. Depending on concentration of the nanoparticle, geometry of the heat source, and the value of Rayleigh number different behaviors are monitored for average Nusselt numbers. Configuration of the heat source dictates a significant change on the behavior of the average Nusselt number, while addition of the nanoparticles has a negative effect on the magnitude of Nusselt number for this problem.     

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.     

Conjugated shear stress and Prandtl number effects on reflux condensation heat transfer inside a vertical tube

Local reflux condensation heat transfer coefficients have been measured inside a vertical tube with water (2.6 < Pr_liq < 4.5), ethanol (12.4 < Pr_liq < 18.4) and isopropanol (23 < Pr_liq < 55) as the test fluids. A counter current flow situation is established with the liquid film (0.68 < Re_film < 2000) and the vapour (1000 < Re_vap < 16,500) flowing downward and upward, respectively. The heat transfer has found to be impeded by the shear stress only in cases of a very thin film, i.e. in the smooth laminar range, and it can well be correlated by a simple analytical model. In the laminar-wavy range including developing turbulence the heat transfer coefficients are found to increase with the shear stress, an effect which proved to be enhanced with rising Pr_liq numbers. This has been correlated with very good agreement.     

Flow over and forced convection heat transfer in Newtonian fluids from a semi-circular cylinder

Momentum and heat transfer characteristics of a semi-circular cylinder immersed in unconfined flowing Newtonian fluids have been investigated numerically. The governing equations, namely, continuity, Navier–Stokes and energy, have been solved in the steady flow regime over wide ranges of the Reynolds number (0.01 ? Re ? 39.5) and Prandtl number (Pr ? 100). Prior to the investigation of drag and heat transfer phenomena, the critical values of the Reynolds number for wake formation (0.55 < Re^c < 0.6) and for the onset of vortex shedding (39.5 < Re_c < 40) have been identified. The corresponding values of the lift coefficient, drag coefficient, and Strouhal number are also presented. After establishing the limit of the steady flow regime, the influence of the Reynolds number (0.01 ? Re ? 39.5) and Prandtl number (Pr = 0.72, 1, 10, 50 and 100) on the global flow and heat transfer characteristics have been elucidated. Detailed kinematics of the flow is investigated in terms of the streamline and vorticity profiles and the variation of pressure coefficient in the vicinity of the cylinder. The functional dependence of the individual and total drag coefficients on the Reynolds number is explored. The Nusselt number shows an additional dependence on the Prandtl number. In addition, the isotherm profiles, local Nusselt number (Nu_L) and average Nusselt number (Nu) are also presented to analyze the heat transfer characteristic of a semi-circular cylinder in Newtonian media.     

Investigation effect of nanoparticle mean diameter on mixed convection Al2O3-water nanofluid flow in an annulus by two phase mixture model

In this paper, laminar mixed convection of nanofluid (Al₂O₃–water) in horizontal concentric annulus with constant heat flux boundary condition has been studied. Two thermal boundary conditions were investigated, one in which a uniform heat flux at the inner wall and an adiabatic at the other wall, and the other inner and outer walls were heated in a same heat flux. Two phase mixture model employed to investigate effect of mean diameter of nanoparticle on the hydrodynamics and thermal characteristic. The fluid flow properties are assumed constant except for the density in the body force, which varies linearly with the temperature (Boussinesq's hypothesis), thus the fluid flow characteristics are affected by the buoyancy force. Three dimensional elliptical governing equations have been discretized using the finite volume approach (FVM) using SIMPELC algorithm to investigate fluid flow throughout of an annulus duct. Numerical simulations have been carried out for the nanoparticle volume fraction (ϕ = 0.02) and various mean diameters of nanoparticles (d_p) between 13 and 72 nm and different values of the Grashof and Reynolds numbers. The calculated results demonstrate that Nusselt number decreases with increasing nanoparticle mean diameter while it does not influence significantly the hydrodynamic parameters. Also this results show that nanoparticle distribution at the annuluses cross section is non-uniformity.     

Laminar flow and heat transfer in a periodic serpentine channel with semi-circular cross-section

Computational fluid dynamics (CFD) has been used to study fully developed laminar flow and heat transfer behaviour in periodic serpentine channels with a semi-circular cross-section. The serpentine elements are characterised by their wavelength (2L), channel diameter (d) and radius of curvature of bends (R_c), with results reported for Reynolds numbers (Re) up to 450, as well as for a range of geometric configurations (3 < L/d < 12.5, 0.525 < R_c/d < 2.25) at Re = 110. The flow in these channels is characterised by the formation of Dean vortices following each bend. As the Reynolds number is increased, more complex vortical flow patterns emerge and the flow domain becomes increasingly dominated by these vortices. Alignment of flow with vorticity leads to efficient fluid mixing and high rates of heat transfer.Constant wall heat flux (H2) and constant wall temperature (T) boundary conditions and a range of fluid Prandtl numbers (0.7 < Pr < 100) have been examined. High rates of heat transfer and low pressure loss are found relative to fully-developed flow in a straight pipe, with heat transfer enhancements greater than 10 for a Prandtl number of 100.As part of this work, we also obtain an accurate value for the Nusselt number for fully-developed flows in straight semi-circular passages with constant wall temperature, Nu_T = 3.323(±0.001).     

Experimental investigation of mixed convection heat transfer from longitudinal fins in a horizontal rectangular channel

Mixed convection heat transfer from longitudinal fins inside a horizontal channel has been investigated for a wide range of modified Rayleigh numbers and different fin heights and spacings. An experimental parametric study was made to investigate effects of fin spacing, fin height and magnitude of heat flux on mixed convection heat transfer from rectangular fin arrays heated from below in a horizontal channel. The optimum fin spacing to obtain maximum heat transfer has also been investigated. During the experiments constant heat flux boundary condition was realized and air was used as the working fluid. The velocity of fluid entering channel was kept nearly constant (0.15 ? w_in ? 0.16 m/s) using a flow rate control valve so that Reynolds number was always about Re = 1500. Experiments were conducted for modified Rayleigh numbers 3 × 10⁷ < Ra¹ < 8 × 10⁸ and Richardson number 0.4 < Ri < 5. Dimensionless fin spacing was varied from S/H = 0.04 to S/H = 0.018 and fin height was varied from H_f/H = 0.25 to H_f/H = 0.80. For mixed convection heat transfer, the results obtained from experimental study show that the optimum fin spacing which yields the maximum heat transfer is S = 8–9 mm and optimum fin spacing depends on the value of Ra¹.     

ReD-dependence of dynamic thermal conductivities of nanofluids

The dynamic thermal conductivity of nanofluids is examined under fully developed laminar flow conditions (440 < Re_D < 818) in a circular tube (1.753-mm ID) subjected to a uniform heat flux. The experiment has been conducted using nanofluids containing Al₂O₃ nanoparticles of 45-nm nominal diameter dispersed in DI water at two volume concentrations of 2% and 4%. At the small Reynolds numbers, the measured dynamic thermal conductivity is lower by as much as 16% in comparison with the corresponding static thermal conductivity at Re_D = 0. The dynamic thermal conductivity gradually increases with increasing Re_D, but never exceeds the static conductivity level up to the highest tested Reynolds number of 818. The hot wall-to-fluid temperature gradient drives thermophoretic depletion of nanoparticles from the wall to the tube center. The lower dynamic thermal conductivity at the small Reynolds number is believed to be attributed to the reduced effective conduction near the wall where the nanoparticle concentration is relatively lower than in the core of the tube. In contrast, examination of the heat transfer between the suspended nanoparticles and the surrounding water molecules shows that the nanoparticle thermophoretic velocities increase with increasing Reynolds number. The increased thermophoretic velocities inside the nanofluid tend to compensate for the aforementioned reduced thermal conductivity near the wall, and this is conjectured to account for the recovery of the dynamic thermal conductivity to the static level at the higher Reynolds number.     

The performance evaluation of Al2O3/water nanofluid flow and heat transfer in microchannel heat sink

The numerical modeling of the conjugate heat transfer and fluid flow of Al₂O₃/water nanofluid through the microchannel heat sink is presented in the paper. The laminar flow regime was considered along with viscous dissipation effect. The microchannel heat sink with square microchannels and D_h = 50 μm is considered. The heat flux was fixed to q = 35 W/m² with heating and cooling cases. The water based Al₂O₃ nanofluid was encountered with various volume concentrations of Al₂O₃ particles $?=1''–''9\%$ and three diameters of the particle d_p = 13, 28 and 47 nm. The analysis is performed on the results obtained for the local heat transfer coefficients based on a fixed pumping power. The results reveal a different local heat transfer behavior compared to the analysis made on a basis of the constant Re.