Role Of Convective Mixing Research Articles

A numerical study on combustion mode characterization for locally stratified dual-fuel mixtures

Combustion modes in locally stratified dual-fuel (DF) mixtures are numerically investigated for methanol/n-dodecane blends under engine-relevant pressures. In the studied constant-volume numerical setup, methanol acts as a background low-reactivity fuel (LRF) while n-dodecane serves as high-reactivity fuel (HRF), controlling local ignition delay time. The spatial distribution of n-dodecane is modeled as a sinusoidal function parametrized by stratification amplitude (Y′) and wavelength (0.01 mm<λ<15 mm). In contrast, methanol is assumed to be fully premixed with air at equivalence ratio 0.8. First, one-dimensional setup is investigated by hundreds of chemical kinetics simulations in (Y′, λ) parameter space. Further, the concepts by Sankaran et al. (2005, Proceedings of the Combustion Institute) and Zeldovich (1980, Combustion and Flame) on ignition front propagation speed are applied to develop a theoretical analysis of the time-dependent diffusion–reaction problem. The theoretical analysis predicts two combustion modes, (1) spontaneous ignition and (2) deflagrative propagation, and leads to an analytical expression for the border curve called β-curve herein. One-dimensional chemical kinetics simulations confirm the presence of two combustion modes in (Y′, λ) parameter space while the β-curve explains consistently the position of phase border observed in the simulations. Finally, the role of convective mixing is incorporated to the theoretical expression for the β-curve. The effect of convection on combustion mode is assessed by carrying out two-dimensional fully-resolved simulations with different turbulence levels. Two-dimensional numerical simulation results give evidence on combustion mode switching, which is consistent with predictions of the modified β-curve for turbulent cases. The practical output of the paper is the β-curve which is proposed as a predictive tool to estimate combustion modes for various fuels or fuel combinations.

Growth and Decay of High-Biomass Algal Bloom in the Northern Arabian Sea

The annual episodes of high-biomass algal bloom are a known phenomenon in the northern Arabian Sea. In the present study, we demonstrate growth and decay of bloom using long-term satellite-retrieved parameters such as chlorophyll-a (chl-a), sea surface temperature (SST), phytoplankton absorption (aph), detritus–coloured dissolved organic matter absorption (adg) and model-derived mixed layer depth (MLD). Chl-a, the index of phytoplankton biomass, showed an inverse relationship with SST with a lag of 2–8 days between lowest SST and highest chl-a. The deepening of MLD associated with high chl-a and low SST indicated a role of convective mixing in the formation of bloom. The average SST of ~ 25 °C was found to be a favourable condition for such open ocean bloom. The lag was more towards eastern boundary of the basin, indicating variable intensities of convective mixing influencing spatial extent of bloom. The positive correlation between aph and adg indicated simultaneous production and degradation showing delay of 3–10 days from peak production to degradation.

A simple and self-consistent geostrophic-force-balance model of the thermohaline circulation with boundary mixing

Abstract. A simple model of the thermohaline circulation (THC) is formulated, with the objective to represent explicitly the geostrophic force balance of the basinwide THC. The model comprises advective-diffusive density balances in two meridional-vertical planes located at the eastern and the western walls of a hemispheric sector basin. Boundary mixing constrains vertical motion to lateral boundary layers along these walls. Interior, along-boundary, and zonally integrated meridional flows are in thermal-wind balance. Rossby waves and the absence of interior mixing render isopycnals zonally flat except near the western boundary, constraining meridional flow to the western boundary layer. The model is forced by a prescribed meridional surface density profile. This two-plane model reproduces both steady-state density and steady-state THC structures of a primitive-equation model. The solution shows narrow deep sinking at the eastern high latitudes, distributed upwelling at both boundaries, and a western boundary current with poleward surface and equatorward deep flow. The overturning strength has a 2/3-power-law dependence on vertical diffusivity and a 1/3-power-law dependence on the imposed meridional surface density difference. Convective mixing plays an essential role in the two-plane model, ensuring that deep sinking is located at high latitudes. This role of convective mixing is consistent with that in three-dimensional models and marks a sharp contrast with previous two-dimensional models. Overall, the two-plane model reproduces crucial features of the THC as simulated in simple-geometry three-dimensional models. At the same time, the model self-consistently makes quantitative a conceptual picture of the three-dimensional THC that hitherto has been expressed either purely qualitatively or not self-consistently.

Role of Convective Mixing in the Long-Term Storage of Carbon Dioxide in Deep Saline Formations

Summary Storage of carbon dioxide in deep formations is being actively considered for the reduction of greenhouse gas emissions. Relevant experience in the petroleum industry comes from natural gas storage and enhanced recovery using carbon dioxide, but this experience is over a time scale less than the hundreds or thousands of years required for carbon dioxide storage. On these long time scales, different mechanisms need to be considered. In the long term, the dominant mechanism for dissolution of carbon dioxide in formation water is convective mixing rather than pure diffusion. This arises because the density of formation water increases upon dissolution of carbon dioxide, creating a density instability. Linear stability analysis has been used to estimate the time required for this instability to occur in anisotropic systems. For sufficiently thick formations with moderate vertical permeability, this time ranges from less than a year up to a few hundred years. Further approximate analysis shows that the time needed for the injected gas to dissolve completely is typically much longer, on the order of hundreds of years to tens of thousands of years, depending on the vertical permeability. This theoretical analysis is compared with the results of numerical simulations.

The role of convective mixing in degassing the Earth's mantle

The chemical evolution of the mantle depends on the way rocks are processed at the surface of the Earth and on the efficiency of convective mixing. In order to quantify how mixing and processing depend on convection parameters (Rayleigh number, heating mode and viscosity stratification), I conducted an extensive set of simple 2D convection simulations with passive tracers and compute (a) bulk Lagrangian strain rates to evaluate mixing efficiency and (b) processing histories. At high Rayleigh number, the strain rate is only a function of the velocity of the flow, whatever the heating mode or viscosity stratification. This is not the case in low Rayleigh number experiments with basal heating where a transition regime is observed with inefficient mixing though chaotic. The simulations show that the processing efficiency depends a lot on the heating mode: the probability of sampling primordial rocks is larger with internal heating than with basal heating. Scaling laws are proposed to parameterize mantle processing histories and it is predicted that in a 300 K hotter Earth, convection can account for the early degassing of the mantle (more than 90% in less than 100 Myr).

Convective Mixing in the Mantle: The Role of Viscosity Differences

SUMMARY The fate of the subducted oceanic crust has been a subject of controversy since the hypothesis of plate tectonics was first proposed. There is now considerable evidence to support the mantle-mixing hypothesis according to which the subducted oceanic lithosphere is heated and softened and then becomes entrained in the convective flows in the mantle. Normal strains stretch and thin the oceanic crust until diffusion processes complete the homogenization. Since solid-state diffusion processes are effective only on the scale of centimetres, then 6 km thick oceanic crust must be stretched by a factor of lo5 or more before losing its identity. Direct observational evidence for this hypothesis comes from studies of high-temperature peridotite massifs. In these massifs, pyroxenite layers with a basaltic composition are embedded in a lherzolitic matrix; the pyroxenite layers with thicknesses varying from centimetres to a few metres may be identified with the stretched, subducted oceanic crust. With the aim of improving our understanding of the role of convective mixing, we have carried out a study of the influence of viscosity differences on the stretching process. We have obtained equations that describe the stretching of a symmetrical two-dimensional body of highly viscous fluid embedded in a normal strain flow. These can be solved in closed form when the body is elliptical, and asymptotically for a general shape. We find that for mantle conditions, viscosity differences are unlikely to inhibit the stretching of the subducted oceanic crust significantly.