Thermal effects on the diffusive layer convection instability of an exothermic acid-base reaction front

Chemically Enhanced Convective Dissolution of CO_{2}

Convective dissolution, one of the main mechanisms for geological storage of CO_{2}, occurs when supercritical or gas CO_{2} dissolves partially into an aqueous solution, thus triggering downward convection of the denser CO_{2}-enriched liquid. Chemical reaction in the liquid can greatly enhance the process. Here, experimental measurements of convective flow inside a cylinder filled with a sodium hydroxide (NaOH) solution show that the plume's velocity can be increased tenfold as compared to a situation with no NaOH. This tremendous effect is predicted by a model with no adjusting parameters.

Fluctuating hydrodynamics of reactive liquid mixtures

Fluctuating hydrodynamics (FHD) provides a framework for modeling microscopic fluctuations in a manner consistent with statistical mechanics and nonequilibrium thermodynamics. This paper presents an FHD formulation for isothermal reactive incompressible liquid mixtures with stochastic chemistry. Fluctuating multispecies mass diffusion is formulated using a Maxwell-Stefan description without assuming a dilute solution, and momentum dynamics is described by a stochastic Navier-Stokes equation for the fluid velocity. We consider a thermodynamically consistent generalization for the law of mass action for non-dilute mixtures and use it in the chemical master equation (CME) to model reactions as a Poisson process. The FHD approach provides remarkable computational efficiency over traditional reaction-diffusion master equation methods when the number of reactive molecules is large, while also retaining accuracy even when there are as few as ten reactive molecules per hydrodynamic cell. We present a numerical algorithm to solve the coupled FHD and CME equations and validate it on both equilibrium and nonequilibrium problems. We simulate a diffusively driven gravitational instability in the presence of an acid-base neutralization reaction, starting from a perfectly flat interface. We demonstrate that the coupling between velocity and concentration fluctuations dominates the initial growth of the instability.

Control of chemical chaos through medium viscosity in a batch ferroin-catalysed Belousov–Zhabotinsky reaction

A macroscopic parameter, such as medium viscosity, can be used to fine tune chemical chaos in a reaction–diffusion–convection system.

Chemical control of dissolution-driven convection in partially miscible systems: nonlinear simulations and experiments

Numerical simulations combined with experimental results from two laboratory-scale model systems show how to control convective dissolution by chemical reactions.

Acceleration of convective dissolution by chemical reaction in a Hele-Shaw cell

New laboratory experiments quantify the destabilising effect of a second-order chemical reaction on the fingering instability of a diffusive boundary layer in a Hele-Shaw cell. We show that, for a given chemical system, the dynamics of such a reactive boundary layer is fully determined by two dimensionless groups, Da/Ra(2), which measures the timescale for convection compared to those for reaction and diffusion, and CBo', which reflects the excess of the environmental reactant species relative to the diffusing solute. Results of a systematic study varying CBo' in the range 0-0.1 are presented. It is shown that the chemical reaction increases the growth rate of a perturbation and favours small wavelengths compared to the inert system. A higher concentration of CBo' not only accelerates the onset of convection, but crucially also increases the transport of the solute by up to 150% compared to the inert system. This increase in solute transfer has important practical implications, such as in the storage of carbon dioxide in saline aquifers.

Chemical control of dissolution-driven convection in partially miscible systems: theoretical classification

Dissolution-driven convection occurs in the host phase of a partially miscible system when a buoyantly unstable density stratification develops upon dissolution. Reactions can impact such convection by changing the composition and thus the density of the host phase. Here we study the influence of A + B → C reactions on such convective dissolution when A is the dissolving species and B a reactant initially in solution in the host phase. We perform a linear stability analysis of related reaction-diffusion density profiles to compare the growth rate of the instability in the reactive case to its non reactive counterpart when all species diffuse at the same rate. We classify the stabilizing or destabilizing influence of reactions on the buoyancy-driven convection in a parameter space spanned by the solutal Rayleigh numbers RA,B,C of chemical species A, B, C and by the ratio β of initial concentrations of the reactants. For RA > 0, the non reactive dissolution of A in the host phase is buoyantly unstable. In that case, we show that the reaction is enhancing convection provided C is sufficiently denser than B. Increasing the ratio β of initial reactant concentrations increases the effect of chemistry but does not significantly impact the stabilizing/destabilizing classification. When the non reactive case is buoyantly stable (RA≤ 0), reactions can create in time an unstable density stratification and trigger convection if RC > RB. Our theoretical approach allows classifying previous results in a unifying picture and developing strategies for the chemical control of convective dissolution.

Impact of pressure, salt concentration, and temperature on the convective dissolution of carbon dioxide in aqueous solutions

The convective dissolution of carbon dioxide (CO2) in salted water is theoretically studied to determine how parameters such as CO2 pressure, salt concentration, and temperature impact the short-time characteristics of the buoyancy-driven instability. On the basis of a parameter-free dimensionless model, we perform a linear stability analysis of the time-dependent concentration profiles of CO2 diffusing into the aqueous solution. We explicit the procedure to transform the predicted dimensionless growth rate and wavelength of the convective pattern into dimensional ones for typical laboratory-scale experiments in conditions close to room temperature and atmospheric pressure. This allows to investigate the implicit influence of the experimental parameters on the characteristic length and time scales of the instability. We predict that increasing CO2 pressure, or decreasing salt concentration or temperature destabilizes the system with regard to convection, leading to a faster dissolution of CO2 into salted water.

Control of convective dissolution by chemical reactions: general classification and application to CO(2) dissolution in reactive aqueous solutions

In partially miscible two-layer systems within a gravity field, buoyancy-driven convective motions can appear when one phase dissolves with a finite solubility into the other one. We investigate the influence of chemical reactions on such convective dissolution by a linear stability analysis of a reaction-diffusion-convection model. We show theoretically that a chemical reaction can either enhance or decrease the onset time of the convection, depending on the type of density profile building up in time in the reactive solution. We classify the stabilizing and destabilizing scenarios in a parameter space spanned by the solutal Rayleigh numbers. As an example, we experimentally demonstrate the possibility to enhance the convective dissolution of gaseous CO_{2} in aqueous solutions by a classical acid-base reaction.

Chemical Control of Hydrodynamic Instabilities in Partially Miscible Two-Layer Systems

Hydrodynamic instabilities at the interface between two partially miscible liquids impact numerous applications, including CO2 sequestration in saline aquifers. We introduce here a new laboratory-scale model system on which buoyancy- and Marangoni-driven convective instabilities of such partially miscible two-layer systems can easily be studied. This system consists of the stratification of a pure alkyl formate on top of a denser aqueous solution in the gravitational field. A rich spectrum of convective dynamics is obtained upon partial dissolution of the ester into the water followed by its hydrolysis. The properties of the convective patterns are controlled by the miscibility of the ester in water, the feedback of the dissolved species on its own miscibility, as well as the reactivity of given chemicals in the aqueous solution with the solubilized ester.

Hydrodynamic instability of thermal fronts in reactive porous media: spinning patterns

The interaction of convection and reaction front propagation is known to exhibit a nonlinear phenomenons in the case of an adverse flow, i.e., when the gas velocity (V=[u,v]') and the front velocity (Vf) are of opposite directions. This was demonstrated both experimentally and analytically under isothermal conditions with Vf constant independent of V. Here we analyze thermal reaction front propagation in a porous medium in which an exothermic reaction of Arrhenius kinetics occurs. Numerical simulations of a packed-bed reactor model accounting for variable temperature, concentration, and hydrodynamic fields following the Euler-Darcy equations revealed emergence of spinning transversal patterns. Such solution cannot emerge in a two-variable (C,T) model, assuming "frozen" hydrodynamics, within a feasible domain of parameters. Two models are employed for analysis: a qualitative model based on a learning one-tube and two-tube consideration and an extended Landau-Darries instability analysis to account for the momentum losses and for a variable front propagation velocity. Both models revealed the important role of the Vf(u) dependence which can be presented as Vf=u-Vch, where the chemical component of the front velocity (Vch) depends on the main governing parameters such as the adiabatic temperature rise (ΔTa) and the inlet velocity (uin). The instability takes place if the parameter β=∂Vch/∂u<1. The effect of ΔTa, uin on the instability domain obtained in simulations can be translated to the effect of the parameter β.

Convective dynamics of traveling autocatalytic fronts in a modulated gravity field

Modulation of the gravity field, spanning from the hyper-gravity to micro-gravity of a parabolic flight, reveals the contribution of Marangoni flow in a propagating reaction front with an open air–liquid interface.