Showing papers on "Nusselt number published in 2011"
TL;DR: In this article, the boundary layer flow induced in a nanofluid due to a linearly stretching sheet is studied numerically and the transport equations include the effects of Brownian motion and thermophoresis.
1,086 citations
TL;DR: In this article, the authors reported the flow of a nanofluid near a stagnation point towards a stretching surface and the effects of Brownian motion and thermophoresis are further taken into account.
298 citations
TL;DR: In this paper, the convective flow and heat transfer of an incompressible viscous nanofluid past a semi-infinite vertical stretching sheet in the presence of a magnetic field are examined.
292 citations
TL;DR: In this article, a review summarizes the correlations development for fluid flow and heat transfer characteristics of nanofluids in forced and free convection flows, and shows that most of the investigations recommended conventional friction factor correlation of base fluid for pressure drop prediction of the nano-fluid for both laminar and turbulent flows in minichannel as well as in microchannel.
Abstract: Nanofluids are engineered colloids made of a base fluid and nanoparticles, which become potential candidate for next generation heat transfer medium. Nanofluids have higher thermal conductivity and single-phase heat transfer coefficients than their base fluids. The heat transfer coefficient increases appear to go beyond the mere thermal conductivity effect, and cannot be predicted by traditional pure fluid correlations. This review summarizes the correlations development for fluid flow and heat transfer characteristics of nanofluids in forced and free convection flows. The review shows that most of the investigations recommended conventional friction factor correlation of base fluid for pressure drop prediction of the nanofluids for both laminar and turbulent flows in minichannel as well as in microchannel. However, the conventional correlation is not suitable for heat transfer coefficient of nanofluid and hence various correlations have been suggested for the Nusselt number for both laminar and turbulent flow. However, the large deviation of predicted values for proposed correlations has been observed may be due to strong influence of particle properties and nanofluid composition on flow and heat transfer characteristics, lack of common understanding on basic mechanism of nanofluid flow and insufficient experimental data on nanofluid heat transfer. Hence, a general framework for heat transfer correlation needs to be developed.
272 citations
TL;DR: In this article, the authors extended the unifying theory of thermal convection to the very large Ra regime where the kinetic boundary-layer is turbulent, and obtained effective scaling laws of about Nu∝Ra 0.14, Nu ∝Ra0.22, and Nu√ Ra 0.38, respectively.
Abstract: Very different types of scaling of the Nusselt number Nu with the Rayleigh number Ra have experimentally been found in the very large Ra regime beyond 1011. We understand and interpret these results by extending the unifying theory of thermal convection [Grossmann and Lohse, Phys. Rev. Lett. 86, 3316 (2001)] to the very large Ra regime where the kinetic boundary-layer is turbulent. The central idea is that the spatial extension of this turbulent boundary-layer with a logarithmic velocity profile is comparable to the size of the cell. Depending on whether the thermal transport is plume dominated, dominated by the background thermal fluctuations, or whether also the thermal boundary-layer is fully turbulent (leading to a logarithmic temperature profile), we obtain effective scaling laws of about Nu∝Ra0.14, Nu∝Ra0.22, and Nu∝Ra0.38, respectively. Depending on the initial conditions or random fluctuations, one or the other of these states may be realized. Since the theory is for both the heat flux Nu and the ...
268 citations
TL;DR: In this paper, an Eulerian two-fluid model is considered to simulate the nanofluid flow inside the microchannel and the governing mass, momentum and energy equations for both phases are solved using the finite volume method.
259 citations
TL;DR: In this paper, the authors proposed a center-cleared twisted tape aiming at acxhieving good thermohydraulic performance in laminar convective heat transfer, which is a widely used technique for heat transfer enhancement.
250 citations
TL;DR: In this article, an experimental investigation has been conducted on the flow friction and heat transfer in sinusoidal microchannels with rectangular cross sections, and the experimental results, mainly the overall Nusselt number and friction factor, for wavy micro-channels are compared with those of straight baseline channels with the same cross section and footprint length.
247 citations
TL;DR: In this article, a lower bound for the minimum number of computational mesh nodes, required to conduct accurate numerical simulations of moderately high (boundary layer dominated) turbulent Rayleigh-B\'enard convection, in the thermal and kinetic boundary layers close to bottom and top plates was derived.
Abstract: Results on the Prandtl-Blasius type kinetic and thermal boundary layer thicknesses in turbulent Rayleigh-B\'enard convection in a broad range of Prandtl numbers are presented. By solving the laminar Prandtl-Blasius boundary layer equations, we calculate the ratio of the thermal and kinetic boundary layer thicknesses, which depends on the Prandtl number Pr only. It is approximated as $0.588Pr^{-1/2}$ for $Pr\ll Pr^*$ and as $0.982 Pr^{-1/3}$ for $Pr^*\ll\Pr$, with $Pr^*= 0.046$. Comparison of the Prandtl--Blasius velocity boundary layer thickness with that evaluated in the direct numerical simulations by Stevens, Verzicco, and Lohse (J. Fluid Mech. 643, 495 (2010)) gives very good agreement. Based on the Prandtl--Blasius type considerations, we derive a lower-bound estimate for the minimum number of the computational mesh nodes, required to conduct accurate numerical simulations of moderately high (boundary layer dominated) turbulent Rayleigh-B\'enard convection, in the thermal and kinetic boundary layers close to bottom and top plates. It is shown that the number of required nodes within each boundary layer depends on Nu and Pr and grows with the Rayleigh number Ra not slower than $\sim\Ra^{0.15}$. This estimate agrees excellently with empirical results, which were based on the convergence of the Nusselt number in numerical simulations.
240 citations
TL;DR: In this paper, turbulent forced convection flow of water-Al2O3 nanofluid in a circular tube, subjected to a constant and uniform heat flux at the wall, is numerically analyzed.
240 citations
TL;DR: In this paper, the authors discussed the unsteady flow and heat transfer of a Casson fluid over a moving flat plate with a parallel free stream, and the analytic solutions of the system of nonlinear partial differential equations valid for all times in the whole spatial domain are constructed in the series form by a homotopic approach.
Abstract: This paper discusses the unsteady flow and heat transfer of a Casson fluid over a moving flat plate with a parallel free stream. The analytic solutions of the system of nonlinear partial differential equations valid for all times in the whole spatial domain are constructed in the series form by a homotopic approach. The influences of the governing parameters on the velocity, temperature, skin friction coefficient, and local Nusselt number are thoroughly investigated. It is revealed that an increase in the dimensionless time decreases the velocity and enhances the temperature. The surface shear stress and surface heat transfer are enhanced by increasing the Casson fluid parameter (β) and Eckert number (Ec), respectively. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.20358
TL;DR: In this article, a new correlation of the Nusselt number was presented using the results of the experiments with titanium dioxide nanoparticles dispersed in water, where the volume fraction of nanoparticles in the base fluid was less than 0.25%.
TL;DR: In this paper, the effect of roughness parameters on Nusselt number (Nu) and friction factor (f) has been determined and the results obtained were compared with those of smooth duct.
TL;DR: In this paper, the effect of V-cut twisted tape insert on heat transfer, friction factor and thermal performance factor characteristics in a circular tube were investigated for three twist ratios (y, = 2.0, 4.4 and 6.0) and three different combinations of depth and width ratios (DR, = 0.43, DR,= 0.34 and WR,=Â0.34).
TL;DR: In this article, the authors investigated the effect of helically corrugated tubes on convective heat transfer in a single-phase turbulent flow by using them in a concentric tube heat exchanger.
TL;DR: In this paper, the authors investigated the effects of the nanoparticle volume fraction on the flow and heat transfer characteristics under the influence of thermal buoyancy and temperature dependent internal heat generation or absorption.
TL;DR: In this article, the double-diffusive natural convective boundary-layer flow of a nanofluid past a vertical plate is studied analytically, and a similarity solution is presented.
TL;DR: In this article, the steady two-dimensional boundary layer flow past a static or a moving wedge immersed in nanofluids is investigated numerically using an implicit finite difference scheme known as the Keller-box method and the NAG routine DO2HAF.
TL;DR: In this article, the effects of pertinent parameters such as the Reynolds number (0≤ Re ≤ 1000), the solid volume fraction ( 0≤ ϕ ≤ 0.04), and the Hartmann number on the flow and temperature fields and the heat transfer performance of the microchannel were examined against numerical predictions.
Abstract: This paper numerically examines the laminar forced convection of a water–Al 2 O 3 nanofluid flowing through a horizontal microchannel. The middle section of the microchannel is heated with a constant and uniform heat flux. The middle section is also influenced by a transverse magnetic field with a uniform strength. The effects of pertinent parameters such as the Reynolds number (0≤ Re ≤1000), the solid volume fraction (0≤ ϕ ≤0.04) and the Hartmann number (0≤ Ha ≤100) on the flow and temperature fields and the heat transfer performance of the microchannel are examined against numerical predictions. The results show that the microchannel performs better heat transfers at higher values of the Reynolds and Hartmann numbers. For all values of the Reynolds and Hartmann numbers considered in this study, the average Nusselt number on the middle section surface of the microchannel increases as the solid volume fraction increases. The rate of this increase is considerably more at higher values of the Reynolds number and at lower values of the Hartmann number.
TL;DR: In this paper, the magnetohydrodynamic (MHD) flow and heat transfer characteristics for the boundary layer flow over a permeable stretching sheet are considered, and the problem formulation is developed in the presence of thermal radiation.
Abstract: The magnetohydrodynamic (MHD) flow and heat transfer characteristics for the boundary layer flow over a permeable stretching sheet are considered. Velocity and thermal slip conditions are taken into account. Problem formulation is developed in the presence of thermal radiation. Governing non-linear problem is solved by a homotopy analysis method. Convergence of the derived solutions is studied. Numerical values of skin-friction coefficient and local Nusselt number are tabulated. Effects of pertinent parameters on the velocity and temperature profiles are discussed. Comparison between the present and previous limiting results is shown. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this paper, a top-sided lid-driven cavity heated by a corner heater is considered under isothermal boundary conditions with different length in bottom and right vertical walls. But the results showed some very interesting results.
TL;DR: In this paper, the authors investigated the stability of double-diffusive convection for viscoelastic fluid with Soret effect in a porous medium using a modified-Maxwell-Darcy model.
TL;DR: In this paper, the mean Nusselt number Nu ¯ increases with increasing values of Rayleigh number for both Newtonian and power-law fluids, and with increasing shear-thickening Nu ¯ settles to unity ( Nu ¯ = 1.0 ) as heat transfer takes place principally due to thermal conduction.
Abstract: Two-dimensional steady-state simulations of laminar natural convection in square enclosures with differentially heated sidewalls subjected to constant wall temperatures have been carried out where the enclosures are considered to be completely filled with non-Newtonian fluids obeying the power-law model. The effects of power-law index n in the range 0.6 ⩽ n ⩽ 1.8 on heat and momentum transport are investigated for nominal values of Rayleigh number (Ra) in the range 103–106 and a Prandtl number (Pr) range of 10–105. It is found that the mean Nusselt number Nu ¯ increases with increasing values of Rayleigh number for both Newtonian and power-law fluids. However, Nu ¯ values obtained for power-law fluids with n 1 ( n > 1 ) are greater (smaller) than that obtained in the case of Newtonian fluids with the same nominal value of Rayleigh number Ra due to strengthening (weakening) of convective transport. With increasing shear-thickening (i.e. n > 1) the mean Nusselt number Nu ¯ settles to unity ( Nu ¯ = 1.0 ) as heat transfer takes place principally due to thermal conduction. The effects of Prandtl number have also been investigated in detail and physical explanations are provided for the observed behaviour. New correlations are proposed for the mean Nusselt number Nu ¯ for both Newtonian and power-law fluids which are shown to satisfactorily capture the correct qualitative and quantitative behaviour of Nu ¯ in response to changes in Ra, Pr and n.
TL;DR: The thermohydraulic characteristics of the circular tubes equipped with alternate clockwise and counter-clockwise twisted-tapes (TA) for the Reynolds number ranging from 830 to 1990, are reported in this paper.
TL;DR: In this paper, an experimental investigation on the convective heat transfer and friction factor characteristics in the plain and dimpled tube under laminar flow with constant heat flux is carried out with distilled water and CuO/water nanofluids.
TL;DR: In this article, the effects of Grashof number, inclination angle of the sloping wall, volume fraction of nanoparticles on flow and temperature patterns as well as the heat transfer rate within the trapezoidal enclosure were investigated for various pertinent parameters.
TL;DR: The transport is most efficient for the counterrotating case along the diagonal in phase space with ω(o) ≈ -0.4ω(i) and the exponent 0.38 corresponds to the ultimate regime scaling for the analogous Rayleigh-Bénard system.
Abstract: We analyze the global transport properties of turbulent Taylor-Couette flow in the strongly turbulent regime for independently rotating outer and inner cylinders, reaching Reynolds numbers of the inner and outer cylinders of Rei=2×106 and Reo=±1.4×106, respectively. For all Rei, Reo, the dimensionless torque G scales as a function of the Taylor number Ta (which is proportional to the square of the difference between the angular velocities of the inner and outer cylinders) with a universal effective scaling law G∝Ta0.88, corresponding to Nuω∝Ta0.38 for the Nusselt number characterizing the angular velocity transport between the inner and outer cylinders. The exponent 0.38 corresponds to the ultimate regime scaling for the analogous Rayleigh-Benard system. The transport is most efficient for the counterrotating case along the diagonal in phase space with ωo≈-0.4ωi.
TL;DR: In this article, the effect of Brownian motion and thermophoresis on the boundary layer flow characteristics of a nanofluid over a vertical plate with a constant surface heat flux is investigated numerically, following a similarity analysis of the transport equations.
TL;DR: In this article, the Lattice Boltzmann method (LBM) was used to simulate the convection in enclosures using water/SiO2 nanofluid, and the results showed that the average Nusselt number increases with volume fraction for the whole range of Rayleigh numbers and aspect ratios.
TL;DR: In this paper, a review of the literature on convective heat transfer on internal separated flows has been extensively conducted in the past decades, and the influence of several parameters such as the geometrical specifications, boundary conditions, type of fluids, and inclination angle on the hydrodynamic and thermal characteristics using (BFS).
Abstract: Research in convective heat transfer on internal separated flows has been extensively conducted in the past decades. This review summarizes numerous researches on two topics. The first section focuses on studying the fluid flow and heat transfer behavior of different types of single-phase fluid flows over backward facing step (BFS) at different orientations. The second section concentrates on everything related to nanofluids; its preparation, properties, behavior, applications, and many others. The purpose of this article is to get a clear view and detailed summary of the influence of several parameters such as the geometrical specifications, boundary conditions, type of fluids, and inclination angle on the hydrodynamic and thermal characteristics using (BFS). The reattachment length and maximum Nusselt number are the main target of such research where correlation equations were developed and reported in experimental and numerical studies. The heat transfer enhancement of nanofluids along with the nanofluids preparation technique, types and shapes of nanoparticles, base fluids and additives, transport mechanisms, and stability of the suspension are also discussed.