A lattice Boltzmann model for heat transfer in porous media

Porous media are commonly found not only in the nature but also in industries. Furthermore, porous media is an important research prototype for a diversity of disciplines. So far a REV (representative elementary volume) scale lattice Boltzmann (LB) model has been proposed and popularly used for investigation on heat transfer in porous media. Unfortunately, such model suffers from a serious drawback that it cannot address an investigated domain where the heat capacitance (the product of density and specific heat capacity) of porous media varies spatially obviously. Such deficit restricts dramatically its applicable range. The purpose of the present work is to remedy such serious shortcoming in a simple way. Numerical validation demonstrates the capability and reliability of the present model. In order to clearly show the advantage of the present model, here a single-relaxation-time LB model is taken as an example to illustrate how to remedy the shortcoming of previous models. Its multiple-relaxation-time counterpart can be established straightforwardly in the same way

Methane combustion in MILD oxyfuel regime: influences of dilution atmosphere in co-flow configuration

MILD (moderate or intense low oxygen dilution) oxyfuel combustion is a recently proposed clean combustion mode which can remedy the shortcomings of the standard oxyfuel combustion technology. Nowadays most available studies on MILD oxyfuel combustion focus on how to realize this new combustion regime in O2/CO2 atmosphere. The open research on methane MILD oxyfuel combustion in O2/H2O atmosphere is quite sparse. In the present work, we carry out a comprehensive comparison study on methane MILD oxyfuel combustion in different dilution atmosphere for the first time. The JHC (jet in hot co-flow) burner is adopted as a research prototype. The investigation is based on numerical simulation, so firstly the adopted numerical approach is validated by some experimental data in open literature. The numerical comparison is conducted by varying the mass fraction of oxygen in the co-flow and the temperature of the hot co-flow, two key parameters affecting fine reaction structures in JHC. Through the present investigation, a number of findings are reported for the first time and some conclusions presented in previous publications are checked with analyses, especially on some conflicted claims between the previous publications. In addition, several new questions are raised, which may inspire further research activities in future

Mesoscopic Methods in Engineering and Science

Matter, conceptually classified into fluids and solids, can be completely described by the microscopic physics of its constituent atoms or molecules. However, for most engineering applications a macroscopic or continuum description has usually been sufficient, because of the large disparity between the spatial and temporal scales relevant to these applications and the scales of the underlying molecular dynamics. In this case, the microscopic physics merely determines material properties such as the viscosity of a fluid or the elastic constants of a solid. These material properties cannot be derived within the macroscopic framework, but the qualitative nature of the macroscopic dynamics is usually insensitive to the details of the underlying microscopic interactions

On a highly reactive Fe2O3/Al2O3 oxygen carrier for in situ gasification chemical looping combustion

Interest in the direct use of solid fuel in chemical looping combustion (CLC) technology makes the in-situ Gasification Chemical Looping Combustion (iG-CLC) an attractive approach for the low-cost capture of CO2. Highly reactive material is required in iG-CLC in order to achievea fast reaction between the fuel and oxygen carrier. In this work, a material,Fe2O3/Al2O3 synthesized by sol-gel,was evaluated in a fluidized-bed reactor by reaction with lignite. This is the first time sol-gel-derived Fe2O3/Al2O3 material has been used in an iG-CLC process. Operation conditions, including steam content in the fluidization gas, temperature and potential oxygen ratio were investigated to explore their influence on combustion and char conversion. The results show that a higher steam concentration can greatly enhance the rate of char gasification and hence the reaction between the lignite and the oxygen carrier, where as a negligible effect of the steam content was noticed on volatile combustion. In addition, the use of the highly reactive Fe-based material prepared by the sol-gel method significantly increased the char gasification rate as compared to other previously evaluated materials. Moreover, the combustion efficiencies of volatiles and char from the lignite,respectively,were studied. Using the Fe2O3/Al2O3 material enabled a low oxygen carrier inventory of 600 kg/MWth to be reached in order to achieve 99% char combustion, which is much lower than that reported in other works. These findings suggest that Fe2O3/Al2O3 prepared by sol-gel is a highly reactive oxygen carrier for iG-CLC.This work was supported by “National Natural Science Foundation of China (51390494)”, and “National Basic Research and Development Program (2011CB707300)”. Daofeng Mei is grateful for the support provided by the Chinese Scholar Council (CSC201306160054). Appreciation is also shown to the staff of the Analytical and Testing Center, Huazhong University of Science and Technology for the related experimental analysis.National Natural Science Foundation of ChinaChinese Scholar CouncilPeer reviewe

A lattice Boltzmann model for heat transfer in heterogeneous media

So far the lattice Boltzmann (LB) method has matured as a powerful tool to address a diversity of heat and mass transfer challenges. For most practical applications, the variation of thermophysical properties of working media will influence the performance of industrial systems substantially. However, nowadays the efforts to improve the LB method to consider variable thermophysical properties of working media are quite sparse. In the present work we firstly analyze the shortcomings of the available LB approaches for modeling working fluid with variable thermophysical properties. Based on the analysis, a simple LB model is proposed to overcome these shortcomings. The feasibility and reliability of the new LB model have been validated by three simple but nontrivial benchmark tests. Although it is originally proposed to simulate fluid flow with variable thermophysical properties, the present model can be extended directly to some other research areas where variation of thermophysical properties of working media should be considered, such as conjugate heat transfer between solid materials

Double diffusion natural convection in a square cavity filled with nanofluid

Double diffusive natural convection of nanofluid is commonly found in renewable energy engineering. However, nowadays our understanding on its fundamental characteristics is still limited. Especially, three crucial questions on its fundament have not been answered yet: (1) its performance not only in laminar regimes but also beyond laminar regimes, (2) the influence of the ratio of buoyancy forces on heat and mass transfer, (3) the correlation among the dimonsionless quantities which describe the features of this kind of convection. The present work tries to reveal the characteristics of double diffusive natural convection of nanofluid over a wide range, from laminar regimes to turbulent regimes, with the aid of numerical experiments. It is observed that the behavior of nanofluid in the laminar regimes is different from that in the turbulent regimes. Some conclusions presented in previous literature for laminar double diffusive of nanofluid may be invalid in its turbulent counterpart. The effect of the ratio of buoyancy forces on heat and mass transfer of nanofluid possesses some similarities with the pure base fluid as well as some obvious differences. Especially, a power-like correlation among the Nusselt number, Sherwood number, Rayleigh number, ratio of buoyancy forces and nanoparticle volume fraction has been extracted for the first time through our numerical experiments

Size and thermal effects on sedimentation behaviors of two spheres

Gas-solid flows are commonly found in nature, as well as in industries. In such flows the size of the solid particles generally is not uniform. In addition, usually there is heat transfer between solid particles and gas flows. The hydrodynamics and heat transfer both make the behavior of gas-solid flows extremely complicated. In order to reveal these effects, in this paper three cases: (1) two isothermal, (2) two hot and (3) two cold spherical particles with various size ratios are investigated using lattice Boltzmann method-immersed boundary (LB-IB). It is observed that, for the first time, the tumbling duration of both two hot particles and two cold particles settling in vertical channel, is prolonged with size ratio increasing. The differences of threshold size ratio among the three cases are significant and the threshold size ratio of two hot particles is the largest one. Especially, it is found that heat transfer affects critically the interaction of two hot particles with low size ratios. In addition, against particle size ratio increasing, heat transfer effects on the interaction between two non-identical particles become weak

Simulation of double diffusive convection in fluid-saturated porous media by lattice Boltzmann method

The research on double diffusive convection in porous media is important to deepen our insights into sustainable development and environment protection. A lattice Boltzmann (LB) model for REV (representative elementary volume) scale simulation of double diffusive convection in fluid-saturated porous media is proposed in the present work. It can work well not only for porous media with uniform porosity but also for non-uniform porous media. Several benchmark tests are adopted to validate its capability. The good agreement with previous publications demonstrates its applicability. It can provide an alternative numerical tool for modelling complex heat and mass transfer in fluid-saturated porous media beyond double diffusive convection, such as heat and moisture transfer in multi-layer building materials