Uday G. Hegde

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.

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 uf061 = thermal diffusivity uf062 = coefficient of thermal expansion uf064 = shift in grid point location uf067 = specific heat ratio uf072 = density uf079 = 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.