The convective stability associated with carbon sequestration is usually investigated by adopting an unsteady diffusive basic profile to account for the space and time development of the carbon saturated boundary layer instability. The method of normal modes is not applicable due to the time dependence of the nonlinear base profile. Therefore, the instability is quantified either in terms of critical times at which the boundary layer instability sets in or in terms of long time evolution of initial disturbances. This paper adopts an unstably stratified basic profile having a step function density with top heavy carbon saturated layer (boundary layer) overlying a lighter carbon free layer (ambient brine). The resulting configuration resembles that of the Rayleigh-Taylor problem with buoyancy diffusion at the interface separating the two layers. The discontinuous reference state satisfies the governing system of equations and boundary conditions and pertains to an unstably stratified motionless state. Our model accounts for anisotropy in both diffusion and permeability and chemical reaction between the carbon dioxide rich brine and host mineralogy. We consider two cases for the boundary conditions, namely an impervious lower boundary with either a permeable (one-sided model) or poorly permeable upper boundary. These two cases posses neither steady nor unsteady unstably stratified equilibrium states. We proceed by supposing that the carbon dioxide that has accumulated below the top cap rock forms a layer of carbon saturated brine of some thickness that overlies a carbon-free brine layer. The resulting stratification remains stable until the thickness, and by the same token the density, of the carbon saturated layer is sufficient to induce the fluid to overturn. The existence of a finite threshold value for the thickness is due to the stabilizing influence of buoyancy diffusion at the interface between the two layers. With this formulation for the reference state, the stability calculations will be in terms of critical boundary layer thickness instead of critical times, although the two formulations are homologous. This approach is tractable by the classical normal mode analysis. Even though it yields only conservative threshold instability conditions, it offers the advantage for an analytically tractable study that puts forth expressions for the carbon concentration convective flux at the interface and explores the flow patterns through both linear and weakly nonlinear analyses.

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The rainbow schlieren deflectometry (RSD) technique is used to determine the liquid boundary and the fuel volume fraction distributions in the vapor region of a high-pressure fuel spray. Experiments were conducted in a constant pressure flow vessel, whereby a customized single-hole common-rail diesel injector is used to introduce n-heptane fuel into a coflow of low-speed ambient air at two different test conditions. Only the quasi-steady period of the fuel spray is considered, and multiple injections are performed to acquire statistically significant data at an image acquisition rate of 20 kHz. An algorithm to identify the liquid boundary using intensity recorded by the RSD images is presented. The results are compared against measurements obtained by the Mie scattering technique. Results demonstrate that the RSD can be a powerful optical diagnostics technique to simultaneously quantify both the vapor and liquid regions in the high-pressure fuel sprays.

In support of efforts to develop improved models of turbulent spray behavior and combustion in diesel engines, experimental data and analysis must be obtained for guidance and validation. For RANS-based CFD modeling approaches, representative ensemble average experimental results are important. For high-fidelity models such as LES-based CFD simulations, precise individual experimental results are desirable. However, making comparisons between a given experiment and LES simulation is a challenge since local parameters cannot be directly compared. In this work, an optically accessible constant pressure flow rig (CPFR) is utilized to acquire diesel-like fuel injection and reaction behavior simultaneously with three optical diagnostic techniques: rainbow schlieren deflectometry (RSD), OH* chemiluminescence (OH*), and two-color pyrometry (2CP). The CPFR allows a large number of repeated injection experiments to be performed for statistical analysis and convergence using ensemble-averaging techniques, while maintaining highly repeatable test conditions. Even for stable test conditions, variations in local turbulent fuel-air mixing introduce variability which manifests as significant differences in OH* and 2CP results. Experimental measurements of characteristic parameters including liquid and vapor jet penetration, lift-off length, soot temperature and concentration, and turbulent flame speed, along with the shot-to-shot variability of each data set, are presented and discussed. A statistical method is utilized to analyze the extent of this variability, and to identify superlative injections within the data set for discussion and analysis of shot-to-shot variations.

It has long been understood that phase changes are one way to accomplish separations of mixtures, and for liquid mixtures, the process of freezing followed by a thaw results in a high degree of purification for the final component in the mixture to melt. This thaw, into a liquid mixture, occurs at a lower temperature than the freeze/melting point of the highest-freeze-point component. As such, the freeze point is dependent on the mixture, and therefore, it is generally unknown for arbitrary mixtures along with associated phase change properties such as the enthalpy and entropy of fusion. In this work, forty-one freeze points of mixtures containing either bicyclohexyl or n-tridecane are measured and discussed. These molecules, neat have freeze points of 6.4 and −3.6 °C, respectively, which are well above the allowable maximum freeze point of jet-a fuel, but in lower concentration, these molecules may help to achieve compliance with all the requirements of ASTM D7566 fuel specification for sustainable aviation fuel. A predictive freeze point model for hydrocarbon mixtures is developed here using a Hess cycle, 1st law, and 2nd law analysis to determine the enthalpy and entropy of fusion, and thus, the freeze point of the mixture through the Gibbs free energy equilibrium condition for the solid−liquid phase changes. The first-principle-developed model is validated against the experimental freeze point measurements. It captures the nonlinearity with the mole fraction of the highest-freeze-point component (bicyclohexyl or ntridecane) and provides a conservative estimate for the freeze point at mole fractions between 0.065 and 0.25�where the freeze point varies considerably with changes in mole fraction.