Real-Time Monitoring of Dynamical State Changes in Staged Combustion

We describe techniques for diagnosing the state of coal-fired utility burners using dynamic characteristics of the output of optical flame scanner signals. The analysis techniques are optimized for targeting dynamical features associated with nonlinear instabilities that develop as burner operating parameters are changed. Various specific instability indicators are used, including shifts in probability distributions, temporal asymmetry and coarsed-grained descriptions of unstable periodicities. We show that careful application of such methods can accurately characterize a range of different flame states. Specifically, transitions through bifurcation points between attached and lifted flames are targeted, giving insight into causes of instability such as stoichiometry or feed and flow variations. We demonstrate results from the application of these methods to utility-scale staged pulverized-coal burners in a real-time software package. The Flame Doctor™ burner-monitoring software is presently undergoing plant-implementation trials in a program sponsored by the Electric Power Research Institute and participating utilities. In a practical application, we show how real-time monitoring and intervention can significantly mitigate adverse combustion conditions.

The Onset of Gas Entrainment From a Stratified Two-Phase Region Through a Single Side-Oriented Branch

A new theoretical investigation has been conducted for the prediction of the critical height at the onset of gas entrainment during single discharge from a stratified two-phase region through a branch installed on an inclined flat wall. Two models have been developed; a simplified point-sink model and a more-accurate finite-branch. The predicted critical height at the onset of gas entrainment was proven to be a function of Froude number (Fr) and density ratio of the interface fluids. The results of the predicted critical height at the onset of gas entrainment, at low values of Fr (<10), were found to be more accurate when using the finite-branch analysis compared to the results found using the pink-sink analysis. Whereas, with increasing Fr, the predicted values of both models converged to the same value. Furthermore, the point-sink analysis was demonstrated to be independent of wall inclination angle, while the finite-branch analysis showed a slight decrease in the value of the critical height with increasing wall inclination angle. Three different experimental data sets at wall inclination angles of zero, 45 and 90 degrees (i.e. side, inclined and bottom branches) were used in the following study for the comparisons between the experimental and theoretically predicted results. A good concurrence was illustrated between the experimental and theoretical values.

Digital Recording and Numerical Reconstruction of Holograms: A New Optical Diagnostic for Combustion

Digital holographic interferometry (DHI) is a relatively newer imaging and measurement technique that electronically records a hologram (e.g., on a CCD) and reconstructs it using a numerical method. Cumbersome chemical processing of the hologram is avoided in DHI, thereby providing greater flexibility, speed, and the potential for real time processing. In conventional holography fringes that are neither bright nor dark on a hologram cannot be accurately resolved. The DHI technique reported so far has not yet been used for combustion applications. Herein, we will evaluate its efficacy for making temperature measurements in flames and assess its applicability through a simulation. The double exposures associated with the holographic technique are each considered recorded by a hypothetical CCD sensor at separate times. We have applied the principles of Fourier optics to develop a numerical method for hologram reconstruction.

An Experimental Investigation of Soot Formation During Nonpremixed Turbulent Combustion of Hydrocarbon Fuels

Although practical combustion devices involve turbulent conditions, crucial soot investigations have generally been based on the data obtained from laminar flames with relatively limited number of studies in the literature on turbulent flames. Motivated by the need for data that can allow proper characterization of soot properties within the fuel-rich regions of turbulent flames, optical experiments were carried out within hydrocarbon-fueled nonpremixed turbulent jet flames. Specifically, two gaseous fuels, ethylene and acetylene, were burned at relatively high Reynolds numbers in air at atmospheric pressure. In-situ diagnostics included laser scattering and extinction techniques to determine the soot field at various axial and radial positions in these flames. The findings are relevant not only to developing advanced computational models for accurate predictions of radiative transfer but also to controlling and predicting performance and pollutant emissions in combustion systems.

Experimental and Numerical Studies of Short Pulse Propagation in Model Systems

In this paper experimental and numerical studies of the propagation of short-pulsed lasers through scattering and absorbing media are presented. Experimental results of a 60 ps pulse laser transmission in tissue phantoms are obtained and compared with Monte Carlo simulations. Good agreement between the Monte Carlo simulation and experimental measurement is found. Three models are developed for the simulation of short pulse transport. Benchmark comparisons among the Monte Carlo (MC), transient discrete ordinates method (TDOM) and transient radiation element method (TREM) are conducted.

Micro Combustion Research for Micro Thermophotovoltaic Systems

A research program is currently underway with the purpose of developing a novel, MEMS-sized thermophotovoltaic (TPV) system, which would use hydrogen as fuel and would be capable of delivering power on the order of watts in a package less than one cubic centimeter in volume. High surface to volume ratio is very favorable to the output power density per unit volume, though it tends to suppress ignition and quench the reaction in micro devices. This is the most attractive feature of micro TPV system. As part of an effort to develop such a micro power system, numerical and experimental works are carried to test feasibility of combustion in micro devices and determine relevant factors affecting micro combustion. Results indicate micro flame tube combustor is favorable in keeping the uniform of temperature along the wall, and stable combustion could be achieved in a tube with a sudden step within wider flow rate and wider hydrogen/air ratio than in straight tube. High and uniform temperature has been achieved along the wall of flame tube, which is very important to the efficiency of micro TPV system.

Local Heat Transfer Coefficients and Pressure Drop During Evaporation in a Smooth Horizontal Tube

R22 is the most widely employed HCFC working fluid in vapour compression plant. HCFCs must be replaced within 2020. Major problems arise with the substitution of the working fluids, related to the decrease in performance of the plant. Therefore, extremely accurate design procedures are needed. The relative sizing of each of the components of the plant is crucial for cycle performance. For this reason, the knowledge of the new fluids heat transfer characteristics in condensers and evaporators is required. The local heat transfer coefficients and pressure drop of pure R22 and of the azeotropic mixture R507 (R125-R143a 50%/50% in weight) have been measured during convective boiling. The test section is a smooth horizontal tube made of a with a 6 mm I.D. stainless steel tube, 6 m length, uniformly heated by Joule effect. The effects of heat flux, mass flux and evaporation pressure on the heat transfer coefficients are investigated. The evaporating pressure varies within the range 3 ÷10 bar, the refrigerant mass flux within the range 200 ÷ 1000 kg/m2s, the heat flux within 0 ÷ 44 kW/m2. A comparison have been carried out between the experimental data and those predicted by means of the most credited literature relationships.

An Experimental Study of Microscopic Explosive Boiling Induced by Pulsed-Laser Irradiation

Microscopic explosive boiling due to rapid heating has found many applications in modern technologies such as thermal ink jet printing, laser cleaning, and laser surgery. It is a nonequilibrium process involving an extremely high liquid superheating. This paper presents an experimental study of such an explosive vaporization process induced by firing a microsecond pulsed laser beam on a thin Pt film deposited on a quartz substrate. The temperature variation of the Pt film is measured by recording the electric resistance of the film during laser heating and subsequent cooling. A high-speed photographic technique is employed to visualize the bubble formation and the explosive evaporation process. Explosive boiling experiments have been carried out in either a pool of acetone liquid or a thin acetone film covering the Pt film. The heating rate achieved ranges from 8.0×106 K/s to 9.0×107 K/s. Violent explosive boiling was observed in the case of a liquid film and the vapor bubbles together with liquid droplets were expelled from the Pt film. While in the case of a liquid pool, only a large cluster of bubbles was formed on the Pt film during laser heating. A close examination of the temperature curves reveals a sudden reduction in the heating rate during laser heating, and an apparent bubble nucleation temperature can be defined. Experimental data show that this apparent bubble nucleation temperature is a strong function of the heating rate. It is close to the equilibrium boiling point at low heating rates while approaches the homogeneous nucleation temperature at high heating rates.

Photographic Study of Orientation Effects on Near-Saturated Flow Boiling

Experiments were performed to examine the effects of body force on flow boiling CHF. FC-72 was boiled along one wall of a transparent rectangular flow channel that permitted photographic study of the vapor-liquid interface just prior to CHF. High-speed video imaging techniques were used to identify dominant CHF mechanisms. Six different CHF regimes were identified: Wavy Vapor Layer, Pool Boiling, Stratification, Vapor Counterflow, Vapor Stagnation, and Separated Concurrent Vapor Flow. CHF showed significant sensitivity to orientation for flow velocities under 0.2 m/s, where extremely low CHF values where measured, especially with a downward-facing heated wall and downflow orientations. High flow velocities dampened the effects of orientation considerably. The CHF data were used to assess the suitability of previous CHF models and correlations for different orientations and velocities. It is shown the Interfacial Lift-off Model is very effective at predicting CHF for high velocities at all orientations. The flooding limit, on the other hand, is useful at estimating CHF at low velocities and downflow orientations.

Turbulent Friction Factor for a Rod Bundle in Consideration of a Subchannel Geometry

A generalized turbulent friction factor for a rod bundle was developed based on “Law of the Wall” for a tube. It was included two parameters which are one parameter of hydraulic diameter and flow area of a subchannel and rod bundle and another parameter (called geometry parameter hereinafter) of subchannel configuration and pitch-to-diameter ratio (P/D) for a single subchannel. The turbulent geometry parameter for a single subchannel has been used as a constant on the previous works but it was found to be dependent on subchannel shapes and P/D from the present work. Hence, it was modeled as a function of the subchannel shapes and P/D from 1.0 to 1.5. The turbulent geometry parameters for single subchannels were validated by the theoretical derivation of a triangular and square subchannel. Those are compared and agreed well with the previous measurement data for 4 kinds of subchannel types such as a triangular, a square, a wall and a corner subchannel. The present model of turbulent friction factor for a rod bundle included the turbulent geometry parameter has been compared with the various experimental results for circular tubes and hexagonal tubes with various rod numbers. The predicted turbulent friction factors for those rod bundles were agreed excellently with experimental results.