Nikolaos I. Prasianakis

Experimental and Numerical Investigation of Fuel-Lean H2/CO/Air and H2/CH4/Air Catalytic Microreactors

ABSTRACT The catalytic combustion of fuel-lean H2/CO/air and H2/CH4/air mixtures (equivalence ratios φ = 0.3–0.5) was investigated experimentally and numerically in a 30 × 30 × 4 mm3 microreactor made of SiC and equipped with six 1.5-mm internal diameter platinum tubes. The goal was to demonstrate high surface temperatures (>1200 K) with good spatial uniformity, for power generation applications in conjunction with thermophotovoltaic devices. Surface temperatures were measured with an infrared camera while exhaust gas compositions were assessed with a micro gas chromatograph. Three-dimensional simulations with detailed hetero-/homogeneous chemistry, conjugate heat transfer in the solid, and external heat losses complemented the measurements. The diverse transport (Lewis number), kinetic (catalytic reactivity), and thermodynamic (volumetric heat release rate) properties of the H2, CO, and CH4 fuels gave rise to rich combustion phenomena. Optimization of the channel flow directions mitigated the high spatial non-uniformities of temperature, which were induced by the low Lewis number of H2. Measured surface temperature distributions had mean values as high as 1261 K, with standard deviations as low as 10.6 K. Syngas or biogas (H2/CO mixtures) yielded lower wall temperatures compared to undiluted H2, even for small volumetric CO:H2 ratios (1:9 and 2:8). Although CO had a high catalytic reactivity when combusting in H2/CO mixtures, its larger than unity Lewis number did not allow for the attainment of high surface temperatures. Mixtures of H2/CH4 (such as fuels produced by natural gas decarbonization) were the least attractive due to the substantially lower catalytic reactivity of CH4.

Deciphering pore-level precipitation mechanisms

Mineral precipitation and dissolution in aqueous solutions has a significant effect on solute transport and structural properties of porous media. The understanding of the involved physical mechanisms, which cover a large range of spatial and temporal scales, plays a key role in several geochemical and industrial processes. Here, by coupling pore scale reactive transport simulations with classical nucleation theory, we demonstrate how the interplay between homogeneous and heterogeneous precipitation kinetics along with the non-linear dependence on solute concentration affects the evolution of the system. Such phenomena are usually neglected in pure macroscopic modelling. Comprehensive parametric analysis and comparison with laboratory experiments confirm that incorporation of detailed microscale physical processes in the models is compulsory. This sheds light on the inherent coupling mechanisms and bridges the gap between atomistic processes and macroscopic observations.

Benchmark computations for 3D two-phase flows

Following our previous work on the application of the diffuse interface coupled lattice Boltzmann-level set (LB-LS) approach to benchmark computations for 2D rising bubble simulations, this paper investigates the performance of the coupled scheme in 3D two-phase flows. In particular, the use of different lattice stencils, e.g., D3Q15, D3Q19 and D3Q27 is studied and the results for 3D rising bubble simulations are compared with regard to isotropy and accuracy against those obtained by finite element and finite difference solutions of the NavierStokes equations. It is shown that the method can eventually recover the benchmark solutions, provided that the interface region is aptly refined by the underlying lattice. Following the benchmark simulations, the application of the method in solving other numerically subtle problems, e.g., binary droplet collision and droplet splashing on wet surface under high Re and We numbers is presented. Moreover, implementations on general purpose GPUs are pursued, where the computations are adaptively refined around the critical parts of the flow.

Upscaling Strategies of Porosity-Permeability Correlations in Reacting Environments from Pore-Scale Simulations

In geochemically reacting environments, the mineral dissolution and precipitation alters the structural and transport properties of the media of interest. The chemical and structural heterogeneities of the porous media affect the temporal evolution of the permeability with respect to porosity. Such correlations follow a nonlinear trend, which is difficult to estimate a priori and without knowledge of the microstructure itself, especially under the presence of strong chemical gradients. Macroscopic field-scale codes require such an input, and in the absence of exact descriptions, simplified correlations are used. After highlighting the diversity of microstructural evolution paths, due to dissolution, we discuss possible upscaling strategies.

Quasi-equilibrium lattice Boltzmann method

Abstract.A general lattice Boltzmann method for simulation ofnfluids with tailored transport coefficients is presented. It isnbased on the recently introduced quasi-equilibrium kinetic models,nand a general lattice Boltzmann implementation is developed.nLattice Boltzmann models for isothermal binary mixtures with angiven Schmidt number, and for a weakly compressible flow with angiven Prandtl number are derived and validated.n