Effects of Preferential Concentration on Heat Transfer in Particle-Based Solar Receivers

2016 ◽  
Vol 139 (2) ◽  
Author(s):  
Hadi Pouransari ◽  
Ali Mani
Keyword(s):  
Heat Transfer ◽  
Turbulent Flows ◽  
Particle Transport ◽  
Three Dimensional ◽  
Stokes Number ◽  
Radiative Heating ◽  
Radiation Absorption ◽  
Order Unity ◽  
Carrier Fluid ◽  
Preferential Concentration

The working principle of particle-based solar receivers is to utilize the absorptivity of a dispersed particle phase in an otherwise optically transparent carrier fluid. In comparison to their traditional counterparts, which use a solid surface for radiation absorption, particle-based receivers offer a number of opportunities for improved efficiency and heat transfer uniformity. The physical phenomena at the core of such receivers involve coupling between particle transport, fluid turbulence, and radiative heat transfer. Previous analyses of particle-based solar receivers ignored delicate aspects associated with this three-way coupling. Namely, these investigations considered the flow fields only in the mean sense and ignored turbulent fluctuations and the consequent particle preferential concentration. In the present work, we have performed three-dimensional direct numerical simulations of turbulent flows coupled with radiative heating and particle transport over a range of particle Stokes numbers. Our study demonstrates that the particle preferential concentration has strong implications on the heat transfer statistics. We demonstrate that “for a typical setting” the preferential concentration of particles reduces the effective heat transfer between particles and the gas by as much as 25%. Therefore, we conclude that a regime with Stokes number of order unity is the least preferred for heat transfer to the carrier fluid. We also provide a 1D model to capture the effect of particle spatial distribution in heat transfer.

scholarly journals Concentration and velocity statistics of inertial particles in upward and downward pipe flow

2017 ◽  
Vol 822 ◽  
pp. 640-663 ◽  
Author(s):  
J. L. G. Oliveira ◽  
C. W. M. van der Geld ◽  
J. G. M. Kuerten
Keyword(s):  
Liquid Velocity ◽  
Fluid Velocity ◽  
Three Dimensional ◽  
Stokes Number ◽  
Terminal Velocity ◽  
Point Particle ◽  
Inertial Particles ◽  
Carrier Fluid ◽  
Velocity Statistics ◽  
The Difference

Three-dimensional particle tracking velocimetry is applied to particle-laden turbulent pipe flows at a Reynolds number of 10 300, based on the bulk velocity and the pipe diameter, for developed fluid flow and not fully developed flow of inertial particles, which favours assessment of the radial migration of the inertial particles. Inertial particles with Stokes number ranging from 0.35 to 1.11, based on the particle relaxation time and the radial-dependent Kolmogorov time scale, and a ratio of the root-mean-square fluid velocity to the terminal velocity of order 1 have been used. Core peaking of the concentration of inertial particles in up-flow and wall peaking in down-flow have been found. The difference in mean particle and Eulerian mean liquid velocity is found to decrease to approximately zero near the wall in both flow directions. Although the carrier fluid has all of the characteristics of the corresponding turbulent single-phase flow, the Reynolds stress of the inertial particles is different near the wall in up-flow. These findings are explained from the preferential location of the inertial particles with the aid of direct numerical simulations with the point-particle approach.

Heat Transfer in Discretely Heated Side-Vented Compact Enclosures by Combined Conduction, Natural Convection, and Radiation

1999 ◽  
Vol 121 (4) ◽  
pp. 1002-1010 ◽  
Author(s):  
E. Yu ◽  
Y. K. Joshi
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Numerical Solutions ◽  
Vertical Wall ◽  
Three Dimensional ◽  
Radiative Heating ◽  
Computational Domain ◽  
Flow Recirculation ◽  
Discrete Heat Source ◽  
Rayleigh Numbers

A three-dimensional investigation of combined conduction, natural convection, and radiation in a side-vented compact enclosure is carried out. The focus of the study is on the enhancement of overall heat transfer through the opening, and the roles of the various modes in achieving it. A discrete heat source, flush-mounted centrally on a vertical substrate, is placed in the enclosure with a single rectangular opening on the opposite vertical wall. Steady-state computations are carried out for Rayleigh numbers, Ra, at 2.6 × 106 and 2.0 × 107. The results show that radiation plays a significant role in the overall heat transfer, and the radiative transport is even more pronounced for lower Ra. It is found that natural convection is weakened by radiation, however, contrary to the existing studies on top vented enclosures, the overall heat transfer is enhanced when radiation is included in the computations. Flow recirculation by radiative heating of enclosure walls is predicted, and is also observed experimentally. Heat spreading in the substrate is found to effect both convection and radiation. The numerical solutions on an extended computational domain are found in good agreement with the experimental data, when the conjugate effects are accounted for.

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF HEAT TRANSFER AUGMENTATION USING AL2O3-ETHYLENE GLYCOL NANOFLUIDS UNDER TURBULENT FLOWS IN A FLAT TUBE

2016 ◽  
Vol 78 (9-2) ◽  
Author(s):  
M. Kh. Abdolbaqi ◽  
Nor Azwadi Che Sidik ◽  
Muhammad Noor Afiq Witri Muhammad Yazid ◽  
Rizalman Mamat ◽  
W. H. Azmi ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Ethylene Glycol ◽  
Friction Factor ◽  
Turbulent Flows ◽  
Volume Concentration ◽  
Three Dimensional ◽  
Hydrodynamic Performance ◽  
Homogeneous Fluid ◽  
Flat Tube ◽  
Fluid Behaviour

A study of computational fluid dynamics has been conducted to study the characteristics of the heat transfer and friction factor of Al2O3/Ethylene glycol-water nanofluid flowing inside flat tube. The three dimensional realizable k–e turbulent model with an enhanced wall treatment was utilized. The evaluation of the overall performance of the tested tube was predicated on the thermo-hydrodynamic performance index. The obtained results showed that the difference in behaviour depending on the parameter that has been selected to compare the nanofluid with the base fluid. In addition, the friction factor and the heat transfer coefficient increases with an increase of the nanoparticles volume concentration at the same Reynolds number. The penalty of pressure drop is negligible with an increase of the volume concentration of nanoparticles. Conventional correlations that have been used in turbulent flow regime to predict average heat transfer and friction factor are Dittus-Boelter and Blasius correlations, for tubes are also valid for the tested nanofluids which consider that the nanofluids have a homogeneous fluid behaviour.

scholarly journals Effects of Blade Fillet Structures on Flow Field and Surface Heat Transfer in a Large Meridional Expansion Turbine

Energies ◽  
2019 ◽  
Vol 12 (15) ◽  
pp. 3035
Author(s):  
Fusheng Meng ◽  
Qun Zheng ◽  
Jian Zhang
Keyword(s):  
Heat Transfer ◽  
Turbulent Flows ◽  
Thermal Load ◽  
Three Dimensional ◽  
Leading Edge ◽  
Flow State ◽  
Aerodynamic Loss ◽  
Expansion Turbine ◽  
Large Expansion ◽  
High Thermal Load

This paper is a continuation of the previous work, aiming to explore the influence of fillet configurations on flow and heat transfer in a large meridional expansion turbine. The endwall of large meridional expansion turbine stator has a large expansion angle, which leads to early separation of the endwall boundary layer, resulting in excessive aerodynamic loss and local thermal load. In order to improve the flow state and reduce the local high thermal load, five typical fillet distribution rules are designed. The three-dimensional Reynolds-Averaged Navier-Stokes (RANS) solver for viscous turbulent flows was used to investigate the different fillet configurations of the second stage stator blades of a 1.5-stage turbine, and which fillet distribution is suitable for large meridional expansion turbines. The influence of fillet structures on the vortex system and loss characteristics was analyzed, and its impact on wall thermal load was studied in detail. The fillet structure mainly affects the formation of horseshoe vortexes at the leading edge of the blade so as to reduce the loss caused by horseshoe vortexes and passage vortexes. The fillet structure suitable for the large meridional expansion turbine was obtained through the research. Reasonable fillet structure distribution can not only improve the flow state but also reduce the high thermal load on the wall surface of the meridional expansion turbine. It has a positive engineering guiding value.

scholarly journals Numerical Simulation of the Process of Combustion of a Stoichiometric Hydrogen-Oxygen Mixture in a Steam Generator

2021 ◽  
Vol 9 (2) ◽  
pp. 34-51
Author(s):  
Andrii Avramenko ◽  
Keyword(s):  
Heat Transfer ◽  
Numerical Methods ◽  
Turbulent Flows ◽  
Steam Generator ◽  
Nuclear Power ◽  
Combustion Process ◽  
Three Dimensional ◽  
Radiant Heat ◽  
Oxygen Mixture ◽  
Radiant Heat Transfer

Numerical methods are used to study the process of combustion of a stoichiometric hydrogen-oxygen mixture. The mathematical models were validated using experimental data. The combustion process is modelled in the three-dimensional unsteady formulation. With account of the recommendations of other authors, the turbulent flows are described in the paper using the standard k-ε turbulence model. The Eddy Dissipation Model (EDM) is used to describe the process of combustion of the hydrogen-oxygen mixture. The description of the complex heat transfer between the gas, flame and walls in the paper accounts for radiant heat transfer by using the P1 model. The paper deals with combustion processes in a burner and a model steam generator. Numerical methods were used to evaluate the effect of inlet flow turbulisation, and the flow rate and the method of feeding extra water to the combustion chamber on the process of combustion of the stoichiometric hydrogen-oxygen mixture. The influence of the design and operating mode factors on the alteration of the flame-steam interface and on the flame extinguishing conditions were studied. The results obtained can be used in future in designing equipment that uses hydrogen as a fuel to increase nuclear power plant (NPP) manoeuvrability.

scholarly journals Intermittency in the relative separations of tracers and of heavy particles in turbulent flows

2014 ◽  
Vol 757 ◽  
pp. 550-572 ◽  
Author(s):  
L. Biferale ◽  
A. S. Lanotte ◽  
R. Scatamacchia ◽  
F. Toschi
Keyword(s):  
Reynolds Number ◽  
Turbulent Flows ◽  
Isotropic Turbulence ◽  
Exit Time ◽  
Three Dimensional ◽  
Stokes Number ◽  
Inertial Range ◽  
Relative Dispersion ◽  
Heavy Particles ◽  
Self Similar

AbstractResults from direct numerical simulations (DNS) of particle relative dispersion in three-dimensional homogeneous and isotropic turbulence at Reynolds number $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{\mathit{Re}}_{\lambda } \sim 300$ are presented. We study point-like passive tracers and heavy particles, at Stokes number $\mathit{St}=0.6,1$ and 5. Particles are emitted from localised sources, in bunches of thousands, periodically in time, allowing an unprecedented statistical accuracy to be reached, with a total number of events for two-point observables of the order of ${10^{11}}$. The right tail of the probability density function (PDF) for tracers develops a clear deviation from Richardson’s self-similar prediction, pointing to the intermittent nature of the dispersion process. In our numerical experiment, such deviations are manifest once the probability to measure an event becomes of the order of – or rarer than – one part over one million, hence the crucial importance of a large dataset. The role of finite-Reynolds-number effects and the related fluctuations when pair separations cross the boundary between viscous and inertial range scales are discussed. An asymptotic prediction based on the multifractal theory for inertial range intermittency and valid for large Reynolds numbers is found to agree with the data better than the Richardson theory. The agreement is improved when considering heavy particles, whose inertia filters out viscous scale fluctuations. By using the exit-time statistics we also show that events associated with pairs experiencing unusually slow inertial range separations have a non-self-similar PDF.

Computational Fluid Dynamics and Heat Transfer Analysis for a Novel Heat Exchanger

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
Haolin Ma ◽  
Dennis E. Oztekin ◽  
Seyfettin Bayraktar ◽  
Sedat Yayla ◽  
Alparslan Oztekin
Keyword(s):  
Heat Transfer ◽  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Heat Exchanger ◽  
Turbulent Flows ◽  
Flow Structure ◽  
Three Dimensional ◽  
Flow Characteristics ◽  
Heat Transfer Analysis ◽  
Two Dimensional

Computational fluid dynamics (CFD) and heat transfer simulations are conducted for a novel heat exchanger. The heat exchanger consists of semi-circle cross-sectioned tubes that create narrow slots oriented in the streamwise direction. Numerical simulations are conducted for Reynolds numbers (Re) ranging from 700 to 30,000. Three-dimensional turbulent flows and heat transfer characteristics in the tube bank region are modeled by the k-ε Reynolds-averaged Navier–Stokes (RANS) method. The flow structure predicted by the two-dimensional and three-dimensional simulations is compared against that observed by the particle image velocimetry (PIV) for Re of 1500 and 4000. The adequate agreement between the predicted and observed flow characteristics validates the numerical method and the turbulent model employed here. The three-dimensional and the two-dimensional steady flow simulations are compared to determine the effects of the wall on the flow structure. The wall influences the spatial structure of the vortices formed in the wake of the tubes and near the exit of the slots. The heat transfer coefficient of the slotted tubes improved by more than 40% compare to the traditional nonslotted tubes.

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