Michael C. Hicks

Ignition of hydrothermal flames

Supercritical water oxidation is one of the most promising technologies for complete oxidation of complex organic compounds. Flames in supercritical water, often referred to as hydrothermal flames, improve the oxidation rates of reactants in an organic waste stream. The ignition and control of flames in supercritical water could potentially be used to reduce the reaction time (from seconds to milliseconds) and enhance the thermochemical decomposition rates of recalcitrant molecules without the release of any harmful intermediates. This provides a platform to design compact reactors for processing complex organic waste followed by their conversion to valuable compounds. This paper reviews some notable work focused on the ignition and qualitative observations of hydrothermal flames as an environmentally friendly technology. More specifically, the review highlights the classification and characterization of hydrothermal flames with several demonstrations of laboratory scale (e.g., visual flame cell) and pilot scale (e.g., transpiring wall reactor) reactor configurations. The process parameters such as feed concentration, reaction temperature, oxidant temperature, oxidant flow rate, and transpiration flow properties (in the case of transpiring reactors) are comprehensively discussed for their influence on the ignition and stability of hydrothermal flames, and total organic carbon removal. In addition, the impact of these parameters on the performance of various flame reactors is presented. Finally, the paper also outlines some wide-ranging applications and challenges concerning the industrial utilization of hydrothermal flames.

An elementary model for autoignition of laminar jets.

In this paper, we formulate and analyse an elementary model for autoignition of cylindrical laminar jets of fuel injected into an oxidizing ambient at rest. This study is motivated by renewed interest in analysis of hydrothermal flames for which such configuration is common. As a result of our analysis, we obtain a sharp characterization of the autoignition position in terms of the principal physical and geometrical parameters of the problem.

Experimental Study of Initial Diameter Effects on Convection-free Droplet Combustion in the Standard Atmosphere for n-Heptane, n-Octane, and n-Decane: International Space Station and Ground-based Experiments

A comprehensive investigation is reported on varying the initial droplet diameter (Do) over a wide range on the burning characteristics of three normal alkane fuels that are representative of components found in practical fuel systems. The droplet burning characteristics of n-heptane, n-octane and n-decane, were studied experimentally in a low gravity ambience to minimize the influence of convection and promote spherical droplet flames as well as formation of a shell-like structure of soot aggregates that reside between the droplet and flame. Initial droplet diameters ranged from about 0.5 mm to 5.0 mm, and the experiments were carried out in the standard atmosphere (room temperature and normal atmospheric pressure) in a ground-based (drop tower) and a spaced-based (the International Space Station) facility. The range of Do investigated influences mechanisms related to radiative transport and sooting dynamics on the droplet burning process that determine the droplet burning rate, sooting dynamics and flame extinction mechanisms. The results show that the burning rate monotonically decreases with increasing Do. Varying Do over the range investigated promotes a transition from a soot-dominated process, with a minimal influence of luminous radiative affects, for small droplets to increased radiative losses for larger droplets that reduce heat transfer to the droplet surface. At a given time after ignition, the relative position of the flame to the droplet decreased with increasing Do. A rather abrupt increase in flame diameter was noted for Do ~ 1 mm followed by a monotonic decrease with further increases of Do for all of the fuels examined. The relative position of the soot shell to the droplet increased with time, while it also increased with Do for a given time after ignition. A three-staged burning process was found for Do > 3 mm suggesting several extinction modes. An early extinction mechanism is speculated to be the result of radiation losses from the flame rather than more diffusively controlled processes. Evaporation continues after the first extinction until reaching a second limit with a rather abrupt decrease in the droplet burning rate – which is speculated to be a “cool-flame” extinction. The morphology of the extinction process showed an oscillatory dynamic in which the flame would peel away from the droplet then re-appear before completely disappearing.

An Elementary Model for Autoignition of Free Round Turbulent Jets

This paper is concerned with the study of autoignition of fully developed free round turbulent jets consisting of oxidizing and chemically reacting components. We derive an elementary, still experimentally feasible, model for autoignition of such jets and present analysis of this model. The derivation of the model is based on the well-established experimental fact that the fully developed free round turbulent jets, in a first approximation, have the shape of a conical frustum. Moreover, the velocity as well as concentration fields within such jets, prior to autoignition, assume self-similar profiles and can be viewed as prescribed. Using these facts as well as the appropriately modified Semenov--Frank-Kamenetskii theory of thermal explosion we derive an equation that describes the initial stage of evolution of the temperature field within the jet. We provide detailed analysis of the model that results in a sharp condition for autoignition of free round turbulent jets in terms of principal physical and geo...

On Autoignition of Co-Flow Laminar Jets

This paper is concerned with the derivation and mathematical analysis of a model for autoignition of laminar co-flow jets. Such jets consist of two parts: an inner part with oxidizer that is surrounded by an outer part with fuel, or the reverse. To derive a model we use a combination of Burke--Schumann theory of diffusion flames and Semenov--Frank-Kamenerskii theory of thermal explosion. The main advantage of our model is that it gives a well-defined condition for autoignition of a jet. We provide detailed analysis of the model that reveals dependency of the autoignition position on principal physical and geometric parameters involved. Moreover, we give explicit expressions for autoignition position in asymptotic regimes relevant to applications.

Influence of a Temperature Gradient on the Behavior of Near-Critical Water in Microgravity

Super-Critical Water Oxidation (SCWO) is an attractive candidate technology for processing solid and liquid wastes for long duration space and extraterrestrial planetary missions. However, an experimental database for critical transition of water as well as SCWO under the microgravity conditions relevant to space and extra-terrestrial environments is currently lacking. The first building block for this database is the behavior of the critical transition of water from a two phase liquid-vapor system to a supercritical fluid. To this end, a collaborative experiment between NASA and the French space agency, CNES, was recently conducted on the International Space Station. This experiment studied the effects of microgravity on phase distribution, heat addition, evolution of bubbles, and hysteresis effects in a small constant volume system during the critical transition process. This paper describes the results from key test sequences from the experiment which focused on the behavior of water under the influence of a temperature gradient near the critical point. Experimental data consisted of images of the fluid, temperature measurements in the cell body, and images from a grid-displacement technique in the supercritical regime at near-critical conditions. Nomenclature Cp = specific heat at constant pressure k = thermal conductivity K = constant in Eq. (6) n = refractive index P = pressure r = fluid cell radius t = time T = temperature x, y = coordinates  = thermal diffusivity  = coefficient of thermal expansion  = shift in grid point location  = specific heat ratio  = density  = constant in Eq. (5) = spatial average

Salt Precipitation and Transport in Near-Critical and Super-Critical Water

*† The presence of salts can severely limit the lifetime of supercritical water oxidation system components due to corrosive behavior arising from their deposition on active surfaces. A series of experiments is being conducted at NASA Glenn to elucidate salt distribution and transport in liquid, vapor, and supercritical fluid phases. This paper describes the objectives, apparatus, and some of the experimental and modeling results obtained utilizing sodium sulfate-water solutions at different concentrations. Experiments are carried out isochorically with backlit imaging of the fluid cell, temperature measurements, and pressure measurements as the primary diagnostics. Of particular interest are the onset of precipitation, size of the particulates, their spatial distribution in the fluid, transport of salt into the vapor phase at elevated temperatures, and salt deposition levels on the surfaces of the test section.

Combustion of Droplets in Steady and Unsteady Slow Flows in Microgravity

Experimental observations of methanol and heptane droplets burning in air at atmospheric pressure in the presence of steady and unsteady slow flows in microgravity are presented. Velocities varied from 0 cm/s to 5 cm/s with corresponding Reynolds number from 0 to about 1.3. For steady flow, the fractional change in burning rate was found to vary linearly with Reynolds number over the range tested. Accelerations of -10, -4/3, -1, 2/3 and 1 cm/s 2 were also investigated. Burning rates were found to be nearly constant with the size of the acceleration having little effect on their values. Flame shapes were strongly influenced by decelerating flows for heptane while flow acceleration had little effect on both fuels. The measured radiant output for heptane seemed to lag the changes in flame shapes during flow deceleration indicating that both the flame shape as well as the flame temperature may cause the observed behavior.