Session F3: Reacting Flows: General II

Front tracking velocimetry for chemical reaction fronts in a flow

Chemical or biological quantities that react create fronts which separate reacted from unreacted regions. These fronts have non-trivial dynamics, especially when advected by fluid flows. To understand the motion of fronts, we have created a tracking method which measures reaction front speed at many points in space and time. We use a simultaneous measurement of reaction and flow which allows us to separate dynamics due to advection, and those due to reaction. In the case of sharp fronts, we will demonstrate front tracking in simulation results, as well as tracking in experiments using the Belousov-Zhabotinsky reaction. We will present the measurements, examine their statistics, and discuss future studies in fundamental properties of reactive mixing that are enabled by this tool.   

Optimal Stretching for Advection-Reaction-Diffusion in a Bluff Body Wake

We study front growth of the excitable Belousov-Zhabotinsky (BZ) reaction in the wake region behind a triangular bluff body in a water channel. We analyze the reaction propagation based on a recent paper\footnote{T. D. Nevins and D. H. Kelley, Phys. Rev. Lett. \textbf{117}, 164502 (2016).} that correlates reaction state with Lagrangian stretching. We measure the probability of a region being reacted, conditioned on the local stretching, and show that an optimal range of stretching enhances reaction propagation. The optimal range we measure is similar to the range found previously in a very different flow$^{1}$, an array of vortices. We hypothesize that an optimal stretching range exists in many advection-reaction-diffusion systems with excitable chemistry, and that its numerical value is largely dependent on the chemistry rather than advection variety. Our experiments may also give insight into the dynamics of plankton blooms behind islands in ocean currents.   

Calibration of X-ray computed tomography (XCT) using a flat flame burner

As a non-invasive, high-resolution technique, X-ray computed tomography (XCT) enables interrogation of three-dimensional field data, such as temperature and density variations, in a combustion context. The objective of this research is the calibration and uncertainty quantification of X-ray based diagnostics using a well-characterized, stable flame, where temperature, concentration, and flow speed can be predictably controlled. To this end, a flat-flame burner is designed and used for the calibration of a tabletop X-ray system consisting of a source, collimator, and flat-panel detector. A premixed methane/air flame, operated from fuel-lean to fuel-rich conditions, is used to characterize features of the scanner, such as drift, attenuation, and noise. Implied temperature fields based on X-ray attenuation are compared to thermocouple measurements. This work furthers the development of XCT as a combustion diagnostic capable of yielding non-intrusive 3D temperature datasets in optically inaccessible environments.   

Assessment of PLIF-Based Heat Release Rate Markers using DNS of Highly Turbulent Premixed Flames

Planar Laser Induced Fluorescence (PLIF) remains the most common measurement tool for describing turbulent flame topologies. However, the interpretation of the images obtained from such experiments can be obscured due to various experimental constraints, such as the finite laser thickness, the application of intensifier, etc. Synthetic-PLIF images are constructed in this study to understand the effects of various experimental reality using direct numerical simulations. Two DNS databases of highly turbulent premixed methane flames are employed, to generate the synthetic PLIF images. The thickness of the laser sheet and optical blur parameters are systematically varied to study their effects on the implied reactive layer thickness, topological correspondence with heat release rates, as well as the resolved scales of the flames. It is found that the optical blur can have a significant influence on the measured layer thickness, and significant discrepancy between the DNS and the synthetic PLIF arises when the laser thickness is approximately twice the size of the reactive layers.   

OH PLIF measurement in a spark ignition engine with a tumble flow

Under lean conditions, high compression ratio and strong tumble flow; cycle-to-cycle variations of combustion in spark ignition (SI) engines is prominent, therefore, relation between flame propagation characteristics and increase of pressure needs to be clarified. The present study is aimed at exploring the spatial and temporal development of the flame kernel using OH planar laser-induced fluorescence (OH PLIF) in an optical SI engine. Equivalence ratio is changed at a fixed indicated mean effective pressure of 400 kPa. From the measurements taken at different crank angle degrees (CAD) after ignition, characteristics of flame behavior were investigated considering temporal evolution of in-cylinder pressure, and factors causing cycle-to-cycle variations are discussed. In addition, the effects of tumble flow intensity on flame propagation behavior were also investigated.   

Two-Color Pyrometry with Diesel Combustion

Diesel combustion lasts only milliseconds and takes place inside a closed engine cylinder. Because of this, the mixing and subsequent combustion processes are still not completely understood. Using optically accessible experimental apparatuses and various highspeed optical diagnostic techniques can give insight into the effects of different types of fuels on their subsequent combustion. Two-color pyrometry is an example of such techniques, and has been proven to give accurate temperature measurements of a flame while requiring no physical contact with the surface of interest. A two-color pyrometer has been designed, built, and tested with a Bunsen burner, with the intent of applying the pyrometer to a combustion spray chamber in the future. Initial testing has been made at various fuel rates using a controlled Bunsen burner flame. Temperature maps have been generated from the pyrometer images showing trends that flames with higher fuel flow rates burned at lower mean temperatures. A preliminary video of diesel spray has been captured, showing that future application to diesel combustion is possible with the pyrometer setup.   

A chemical reaction for mixing

We introduce a new, versatile chemical reaction between two transparent reactants producing a fluorescent product in water. The kinetic of this second order reaction, besides depending on the reactants concentration and on temperature, can be adjusted by varying the pH of the substrate, in a way that the reaction time spans over several decades (from a fraction of a second, to hours). The fluorescence intensity is directly proportional to the product concentration, allowing to measure molecular mixing in a variety of situations. We will describe in particular the interplay between molecular diffusion, reaction kinetics and substrate deformation for a blob deposited in a stirred medium in which it reacts as it mixes.   

Modeling of Dissipation Element Statistics in Turbulent Non-Premixed Jet Flames

The dissipation element (DE) analysis is a method for analyzing and compartmentalizing turbulent scalar fields. DEs can be described by two parameters, namely the Euclidean distance $\ell$ between their extremal points and the scalar difference in the respective points $\Delta\phi$. The joint probability density function (jPDF) of these two parameters $P(\Delta\phi,\ell)$ is expected to suffice for a statistical reconstruction of the scalar field. In addition, reacting scalars show a strong correlation with these DE parameters in both premixed and non-premixed flames. Normalized DE statistics show a remarkable invariance towards changes in Reynolds numbers. This feature of DE statistics was exploited in a Boltzmann-type evolution equation based model for the probability density function (PDF) of the distance between the extremal points $P(\ell)$ in isotropic turbulence. Later, this model was extended for the jPDF $P(\Delta\phi,\ell)$ and then adapted for the use in free shear flows. The effect of heat release on the scalar scales and DE statistics is investigated and an extended model for non-premixed jet flames is introduced, which accounts for the presence of chemical reactions. This new model is validated against a series of DNS of temporally evolving jet flames.   

A Mixing Length Scale of Unlike Impinging Jets

Bi-propellant thrusters in space propulsion systems often utilize unlike-doublet or triplet injectors. The impingement of hypergolic liquid jet streams of fuel and oxidizer involves the expanding sheet, droplet fragmentation, mixing, evaporation, and chemical reactions in liquid and gas phases, in which the rate controlling phenomenon is the mixing step. In this study, a defined length scale demonstrates the distribution of fuel and oxidizer, and therefore, represents their mixing states, allowing for providing a physical meaning of widely accepted practical indicator, so called Rupe factor, over half a century of injector design history. We concisely formulate the characteristic velocity in a consistent manner for doublet and triplet injectors as a function of propellant injection conditions. The validity of the present formulation is convinced by comparing with hot firing tests.   

Basics of advection-diffusion: still blurry?

We investigate experimentally the most basic case of advection diffusion: a lamella of fluorescent scalar advected by a simple shear flow. This simple configuration illustrates the advection-diffusion coupling which leads to an initial advection-dominated decay of the transverse dimension of the blob, $s(t)/s_0\sim t^{-1}$, followed by a diffusion-dominated broadening $s(t)/s_0\sim t^{\frac{1}{2}}$. The minimum transverse dimension of the blob, $i.e$ the Batchelor scale, is directly observed and systematically investigated at different Péclet number. A coarsening protocol is used to determine the minimal spatial resolution needed to resolve at all times the evolution of the concentration distribution. We also investigate the coalescence between two nearby lamellae and its implication on the evolution of the concentration distribution.