Heat transfer across sheared suspensions: role of the shear-induced diffusion

2013 ◽  
Vol 724 ◽  
pp. 527-552 ◽  
Author(s):  
Bloen Metzger ◽  
Ouamar Rahli ◽  
Xiaolong Yin
Keyword(s):  
Thermal Diffusivity ◽  
Volume Fraction ◽  
Spherical Particles ◽  
Transient Temperature ◽  
Particle Diffusion ◽  
Driving Mechanism ◽  
Average Particle ◽  
Particle Rotation ◽  
Effective Thermal Diffusivity ◽  
Couette Cell

AbstractSuspensions of non-Brownian spherical particles undergoing shear provide a unique system where mixing occurs spontaneously at low Reynolds numbers. Through a combination of experiments and simulations, we investigate the effect of shear-induced particle diffusion on the transfer of heat across suspensions. The influence of particle size, particle volume fraction and applied shear are examined. By applying a heat pulse to the inner copper wall of a Couette cell and analysing its transient temperature decay, the effective thermal diffusivity of the suspension, $\alpha $, is obtained. Using index matching and laser-induced fluorescence imaging, we measured individual particle trajectories and calculated their diffusion coefficients. Simulations that combined a lattice Boltzmann technique to solve for the flow and a passive Brownian tracer algorithm to solve for the transfer of heat are in very good agreement with experiments. Fluctuations induced by the presence of particles within the fluid cause a significant enhancement (${\gt }200\hspace{0.167em} \% $) of the suspension transport properties. The effective thermal diffusivity was found to be linear with respect to both the Péclet number ($\mathit{Pe}= \dot {\gamma } {d}^{2} / {\alpha }_{0} \leq 100$) and the solid volume fraction ($\phi \leq 40\hspace{0.167em} \% $), leading to a simple correlation $\alpha / {\alpha }_{0} = 1+ \beta \phi \mathit{Pe}$ where $\beta = 0. 046$ and ${\alpha }_{0} $ is the thermal diffusivity of the suspension at rest. In our Couette cell, the enhancement was found to be optimum for a volume fraction, $\phi \approx 40\hspace{0.167em} \% $, above which, due to steric effects, both the particle diffusion motion and of the effective thermal diffusion dramatically decrease. No such correlation was found between the average particle rotation and the thermal diffusivity of the suspension, suggesting that the driving mechanism for enhanced transport is the translational particle diffusivity. Movies are available with the online version of the paper.

Suspensions with a tunable effective viscosity: a numerical study

2012 ◽  
Vol 693 ◽  
pp. 345-366 ◽  
Author(s):  
L. Jibuti ◽  
S. Rafaï ◽  
P. Peyla
Keyword(s):  
Effective Viscosity ◽  
Volume Fraction ◽  
Numerical Study ◽  
Spherical Particles ◽  
Dimensionless Number ◽  
Particle Rotation ◽  
Concentrated Suspensions ◽  
Sheared Suspensions ◽  
Neutrally Buoyant ◽  
Universal Behaviour

AbstractIn this paper, we conduct a numerical investigation of sheared suspensions of non-colloidal spherical particles on which a torque is applied. Particles are mono-dispersed and neutrally buoyant. Since the torque modifies particle rotation, we show that it can indeed strongly change the effective viscosity of semi-dilute or even more concentrated suspensions. We perform our calculations up to a volume fraction of 28 %. And we compare our results to data obtained at 40 % by Yeo and Maxey (Phys. Rev. E, vol. 81, 2010, p. 62501) with a totally different numerical method. Depending on the torque orientation, one can increase (decrease) the rotation of the particles. This results in a strong enhancement (reduction) of the effective shear viscosity of the suspension. We construct a dimensionless number $\Theta $ which represents the average relative angular velocity of the particles divided by the vorticity of the fluid generated by the shear flow. We show that the contribution of the particles to the effective viscosity can be suppressed for a given and unique value of $\Theta $ independently of the volume fraction. In addition, we obtain a universal behaviour (i.e. independent of the volume fraction) when we plot the relative effective viscosity divided by the relative effective viscosity without torque as a function of $\Theta $. Finally, we show that a modified Faxén law can be equivalently established for large concentrations.

Effective Thermal Diffusivity of Porous Media in the Wall Vicinity

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
H. Sakamoto ◽  
F. A. Kulacki
Keyword(s):  
Heat Transfer ◽  
Porous Media ◽  
Thermal Diffusivity ◽  
Saturated Porous Medium ◽  
Heat Transfer Analysis ◽  
Transient Temperature ◽  
Bulk Property ◽  
Wall Region ◽  
Effective Thermal Diffusivity ◽  
Near Wall

Transient heat transfer from an impulsively heated vertical constant heat flux plate embedded in a stationary saturated porous medium is studied experimentally and analytically to determine near-wall thermal diffusivity. The effective diffusivity is shown to depend on the properties of the constituent materials and the near-wall particle morphology. For porous media comprising randomly stacked spheres, the near-wall region is characterized by fewer particle contacts with the wall than in the bulk medium, and this difference is the source of larger thermal diffusivity in the context of volume-averaged values, which apply to the bulk property far from the wall. For combinations of different spherical solids and interstitial fluids, which give a range of fluid:solid conductivity ratio from 0.5 to 2400, early-time transient temperature profiles can be predicted using the thermal conductivity of the interstitial fluid. A conjugate heat transfer analysis accurately predicts the time the conductive front takes to travel through the impermeable wall and quantifies the effect of conduction along the wall on the local and overall Nusselt numbers. The present results raise the possibility of reinterpretation of much of the porous media heat transfer experiments in the literature.

Shape effects on turbulent modulation by large nearly neutrally buoyant particles

2012 ◽  
Vol 712 ◽  
pp. 41-60 ◽  
Author(s):  
Gabriele Bellani ◽  
Margaret L. Byron ◽  
Audric G. Collignon ◽  
Colin R. Meyer ◽  
Evan A. Variano
Keyword(s):  
Single Phase ◽  
Volume Fraction ◽  
Spatial Scales ◽  
Spherical Particles ◽  
Phase Flow ◽  
Single Phase Flow ◽  
Particle Rotation ◽  
Turbulent Statistics ◽  
Small Scales ◽  
Shape Effects

AbstractWe investigate dilute suspensions of Taylor-microscale-sized particles in homogeneous isotropic turbulence. In particular, we focus on the effect of particle shape on particle–fluid interaction. We conduct laboratory experiments using a novel experimental technique to simultaneously measure the kinematics of fluid and particle phases. This uses transparent particles having the same refractive index as water, whose motion we track via embedded optical tracers. We compare the turbulent statistics of a single-phase flow to the turbulent statistics of the fluid phase in a particle–laden suspension. Two suspensions are compared, one in which the particles are spheres and the other in which they are prolate ellipsoids with aspect ratio 2. We find that spherical particles at volume fraction ${\phi }_{v} = 0. 14\hspace{0.167em} \% $ reduce the turbulent kinetic energy (TKE) by 15 % relative to the single-phase flow. At the same volume fraction (and slightly smaller total surface area), ellipsoidal particles have a much smaller effect: they reduce the TKE by 3 % relative to the single-phase flow. Spectral analysis shows the details of TKE reduction and redistribution across spatial scales: spherical particles remove energy from large scales and reinsert it at small scales, while ellipsoids remove relatively less TKE from large scales and reinsert relatively more at small scales. Shape effects are far less evident in the statistics of particle rotation, which are very similar for ellipsoids and spheres. Comparing these with fluid enstrophy statistics, we find that particle rotation is dominated by velocity gradients on scales much larger than the particle characteristic length scales.

Simultaneous Measurement of the Effective Thermal Conductivity and Effective Thermal Diffusivity of Nanofluids

2008 ◽  
Author(s):  
S. M. Sohel Murshed ◽  
Kai Choong Leong ◽  
Chun Yang
Keyword(s):  
Thermal Conductivity ◽  
Thermal Diffusivity ◽  
Effective Thermal Conductivity ◽  
Simultaneous Measurement ◽  
Volume Fraction ◽  
Particle Volume Fraction ◽  
Maximum Increase ◽  
Effective Thermal Diffusivity ◽  
Thermal Diffusivities ◽  
Volumetric Loading

A transient double hot-wire technique was developed for precise and simultaneous measurement of the effective thermal conductivity and effective thermal diffusivity of nanofluids. The measured effective thermal conductivities and effective thermal diffusivities of nanofluids were found to be higher than those of base fluids and they increase significantly with increasing volume fraction of nanoparticles. The increments of the thermal diffusivities were found to be slightly larger compared to the thermal conductivity values. For example, at 5% volumetric loading of TiO2 nanoparticles of 15 nm and 10 × 40 nm in ethylene glycol, the maximum increase in effective thermal conductivity was found to be 17% and 20%, whereas the maximum increase in effective thermal diffusivity was 25% and 29%, respectively. Besides particle volume fraction, particle material, particle size and the nature of the base fluid were found to have influence on the effective thermal conductivity and diffusivity of nanofluids. Based on the calibration results obtained for the base fluids the measurement error was estimated to be within 1.2 to 2%.

Prediction of the Effective Thermal Diffusivity of Discretely Inhomogeneous Media

2009 ◽  
Author(s):  
Daniel W. Mackowski ◽  
Mario Ramos
Keyword(s):  
Wave Propagation ◽  
Thermal Diffusivity ◽  
Addition Theorem ◽  
Inhomogeneous Media ◽  
Spherical Particles ◽  
Effective Thermal Diffusivity ◽  
Effective Medium Model ◽  
The Matrix ◽  
Sphere Radius ◽  
Definition Of

An extended definition of the effective thermal diffusivity is posed via an analogy to acoustic and EM wave propagation in discretely inhomogeneous media. Specifically, the propagation of a periodic, plane thermal wave of frequency ω, through an inhomogeneous medium consisting of spherical particles embedded in a continuous matrix, is theoretically examined. An exact solution for the time–harmonic conduction equation, for the multiple sphere system, is developed by use of the scalar wave harmonic functions and the addition theorem for the harmonics. An effective medium model, which is based on the Quasi–Crystalline approximation (QCA) for acoustic and EM wave propagation, is developed, and a formulation for the frequency–dependent effective thermal diffusivity is derived. In the limit of x = Rω/α0→0, where R is the sphere radius and α0 the matrix thermal diffusivity, it is shown that formulation reduces to that derived from a static model.

Thermal Diffusivity of Dispersed Materials

1978 ◽  
Vol 100 (4) ◽  
pp. 720-724 ◽  
Author(s):  
T. Y. R. Lee ◽  
R. E. Taylor
Keyword(s):  
Thermal Conductivity ◽  
Thermal Diffusivity ◽  
Room Temperature ◽  
Volume Fraction ◽  
Transient State ◽  
Second Phase ◽  
Effective Thermal Diffusivity ◽  
Variable Dispersion ◽  
Flash Technique ◽  
Effective Diffusivities

Measurements have been made of the effective thermal diffusivity at room temperature of composites consisting of one phase randomly dispersed in a second phase. The method is based on the flash technique. Data are presented for four types of composites ranging in particle-to-matrix diffusivity ratios from 0.48 to 1137, in volume specific heat ratios 0.04 to 1.16, and in volume fraction of dispersed particle from zero up to 34 percent. The results show that the limitations of the concept of an effective thermal diffusivity are far beyond the situations to which it is currently applied in the transient state heat conduction problems. Values of effective diffusivities derived from values of the effective thermal conductivity calculated from the Bruggeman variable-dispersion equation are found to agree well with the measured diffusivity values.

Adsorption of safranine T from aqueous medium by nano mesoporous MCM-41: adsorption equilibrium, kinetics, and thermodynamics

2020 ◽  
Vol 10 ◽  
Author(s):  
Xiao-Dong Li ◽  
Qing-Zhou Zhai
Keyword(s):  
Electron Microscopy ◽  
Ph Value ◽  
Particle Diameter ◽  
Spherical Particles ◽  
Practical Significance ◽  
Average Particle ◽  
Average Pore ◽  
Kinetics And Thermodynamics ◽  
Safranine T ◽  
Mcm 41

Introduction: In industrial production, a small amount of saffron T emissions will cause increase of water color and increase of chemical oxygen consumption, so study of the decolorization of saffron T wastewater has an important practical significance. Methods: MCM (Mobil Composition of Matter)-41 molecular sieve was synthesized by hydrothermal method. Power Xray diffraction and scanning electron microscopy were used to characterize the sample. Safranine T dye was adsorbed from water by the MCM-41 prepared. Kinetics and thermodynamics of the adsorption were studied. Results: The MCM-41 sample presented spherical particles and regular. The BET (Brunner-Emmett-Teller) specific surface area of the sample determined by 77 K low temperature nitrogen adsorption-desorption isotherm was 932 m2 /g. Its average particle diameter was 110 nm. TEM (transmission electron microscopy) results showed that the sample structure presented a honeycomb pore structure and the average pore diameter was 3.0 nm. The results showed that when room temperature was 20 ± 1 ℃, adsorbate safranine T: adsorbent MCM-41 = 20 : 1，the optimum pH value of adsorption was 4.0 and contact time was 20 min, the adsorption rate reached 98.29% and the adsorption capacity was 19.66 mg/g. The entropy change and enthalpy change of the adsorption system are respectively ΔS0 = 157.5 J/(mol·K); ΔH0 = 21.544 kJ/mol. When temperature was 277.15, 293.15, 303.15 K，the free energy change was respectively △G1 0 = -22.107 kJ/mol, △G2 0 = -24.627 kJ/mol, △G3 0 = -26.202 kJ/mol. Conclusion: The adsorption of safranine T by MCM-41 belongs to a pseudo-second-order adsorption. This adsorption accords with the Freundlich equation and belongs to a heterogeneous adsorption. The adsorption is an endothermic reaction of entropy increase, being spontaneous.

A Parametric Numerical Study of Radiative Transfer in Thermotropic Materials

2013 ◽  
Author(s):  
Adam C. Gladen ◽  
Susan C. Mantell ◽  
Jane H. Davidson
Keyword(s):  
Particle Size ◽  
Radiative Transfer ◽  
Optical Thickness ◽  
Parametric Study ◽  
Volume Fraction ◽  
Numerical Study ◽  
Anisotropic Scattering ◽  
Index Of Refraction ◽  
Spherical Particles ◽  
Size Parameter

A thermotropic material is modeled as an absorbing, thin slab containing anisotropic scattering, monodisperse, spherical particles. Monte Carlo ray tracing is used to solve the governing equation of radiative transfer. Predicted results are validated by comparison to the measured normal-hemispherical reflectance and transmittance of samples with various volume fraction and relative index of refraction. A parametric study elucidates the effects of particle size parameter, scattering albedo, and optical thickness on the normal-hemispherical transmittance, reflectance, and absorptance. The results are interpreted for a thermotropic material used for overheat protection of a polymer solar absorber. For the preferred particle size parameter of 2, the optical thickness should be less than 0.3 to ensure high transmittance in the clear state. To significantly reduce the transmittance and increase the reflectance in the translucent state, the optical thickness should be greater than 2.5 and the scattering albedo should be greater than 0.995. For optical thickness greater than 5, the reflectance is asymptotic and any further reduction in transmittance is through increased absorptance. A case study is used to illustrate how the parametric study can be used to guide the design of thermotropic materials. Low molecular weighted polyethylene in poly(methyl methacrylate) is identified as a potential thermotropic material. For this material and a particle radius of 200 nm, it is determined that the volume fraction and thickness should equal 10% and 1 mm, respectively.

Effects of thermal oxidation on the effective thermal diffusivity of titanium alloys

2014 ◽  
Vol 47 (38) ◽  
pp. 385306 ◽  
Author(s):  
A Somer ◽  
F Camilotti ◽  
G F Costa ◽  
A R Jurelo ◽  
A Assmann ◽  
...  
Keyword(s):  
Thermal Diffusivity ◽  
Titanium Alloys ◽  
Thermal Oxidation ◽  
Effective Thermal Diffusivity

An Efficient Immersed Boundary Method Based on Penalized Direct Forcing for Simulating Flows Through Real Porous Media

2012 ◽  
Author(s):  
Wim-Paul Breugem ◽  
Vincent van Dijk ◽  
René Delfos
Keyword(s):  
Porous Medium ◽  
Ct Scan ◽  
Immersed Boundary Method ◽  
Volume Fraction ◽  
Solid Volume Fraction ◽  
Particle Diameter ◽  
Immersed Boundary ◽  
Average Particle ◽  
Boundary Method ◽  
Direct Forcing

A computationally efficient Immersed Boundary Method (IBM) based on penalized direct forcing was employed to determine the permeability of a real porous medium. The porous medium was composed of about 9000 glass beads with an average particle diameter of 1.93 mm and a porosity of 0.367. The forcing of the IBM depends on the local solid volume fraction within a computational grid cell. The latter could be obtained from a high-resolution X-ray Computed Tomography (CT) scan of the packing. An experimental facility was built to determine the permeability of the packing experimentally. Numerical simulations were performed for the same packing based on the data from the CT scan. For a scan resolution of 0.1 mm the numerical value for the permeability was nearly 70% larger than the experimental value. An error analysis indicated that the scan resolution of 0.1 mm was too coarse for this packing.

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