Jiayao Zhang

Bio: Jiayao Zhang is an academic researcher from Sandia National Laboratories. The author has contributed to research in topics: Jet (fluid) & Combustion. The author has an hindex of 5, co-authored 9 publications receiving 80 citations.

Design of "model-friendly" turbulent non-premixed jet burners for C2+ hydrocarbon fuels.

Abstract: Experimental measurements in laboratory-scale turbulent burners with well-controlled boundary and flow configurations can provide valuable data for validating models of turbulence-chemistry interactions applicable to the design and analysis of practical combustors. This paper reports on the design of two canonical nonpremixed turbulent jet burners for use with undiluted gaseous and liquid hydrocarbon fuels, respectively. Previous burners of this type have only been developed for fuels composed of H2, CO, and/or methane, often with substantial dilution. While both new burners are composed of concentric tubes with annular pilot flames, the liquid-fuel burner has an additional fuel vaporization step and an electrically heated fuel vapor delivery system. The performance of these burners is demonstrated by interrogating four ethylene flames and one flame fueled by a simple JP-8 surrogate. Through visual observation, it is found that the visible flame lengths show good agreement with standard empirical correlat...

Understanding and predicting soot generation in turbulent non-premixed jet flames.

Abstract: This report documents the results of a project funded by DoD's Strategic Environmental Research and Development Program (SERDP) on the science behind development of predictive models for soot emission from gas turbine engines. Measurements of soot formation were performed in laminar flat premixed flames and turbulent non-premixed jet flames at 1 atm pressure and in turbulent liquid spray flames under representative conditions for takeoff in a gas turbine engine. The laminar flames and open jet flames used both ethylene and a prevaporized JP-8 surrogate fuel composed of n-dodecane and m-xylene. The pressurized turbulent jet flame measurements used the JP-8 surrogate fuel and compared its combustion and sooting characteristics to a world-average JP-8 fuel sample. The pressurized jet flame measurements demonstrated that the surrogate was representative of JP-8, with a somewhat higher tendency to soot formation. The premixed flame measurements revealed that flame temperature has a strong impact on the rate of soot nucleation and particle coagulation, but little sensitivity in the overall trends was found with different fuels. An extensive array of non-intrusive optical and laser-based measurements was performed in turbulent non-premixed jet flames established on specially designed piloted burners. Soot concentration data was collected throughout the flames, together with instantaneous images showing the relationship between soot and the OH radical and soot and PAH. A detailed chemical kinetic mechanism for ethylene combustion, including fuel-rich chemistry and benzene formation steps, was compiled, validated, and reduced. The reduced ethylene mechanism was incorporated into a high-fidelity LES code, together with a moment-based soot model and models for thermal radiation, to evaluate the ability of the chemistry and soot models to predict soot formation in the jet diffusion flame. The LES results highlight the importance of including an optically-thick radiation model to accurately predict gas temperatures and thus soot formation rates. When including such a radiation model, the LES model predicts mean soot concentrations within 30% in the ethylene jet flame.

Joint Temperature-Volume Fraction Statistics of Soot in Turbulent Non-Premixed Jet Flames of Ethylene and a JP-8 Surrogate.

Abstract: Soot emissions from internal combustion engines and aviation gas turbine engines face increasingly stringent regulation, but available experimental datasets for sooting turbulent combustion model development and validation are largely lacking, in part due to the difficulty of making quantitative spaceand time-resolved measurements in this type of flame. To address this deficiency, we have performed a number of different laser and optical diagnostic measurements in sooting, nonpremixed jet flames fueled by ethylene or a prevaporized JP-8 surrogate. Most laser diagnostic techniques inherently lose their quantitative rigor when significant laser beam and signal attenuation occur in sooting flames. However, the ‘3-line’ approach to simultaneous measurement of soot concentration (on the basis of laser extinction) and soot temperature (on the basis of 2-color pyrometry) actually relies on the presence of significant laser attenuation to yield accurate measurements. In addition, the 3-line approach yields complete time-resolved information. In the work reported here, we have implemented the 3-line diagnostic in well-controlled non-premixed ethylene and JP-8 jet flames with a fuel exit Reynolds number of 20,000 using tapered, uncooled alumina refractory probes with a 10 mm probe end separation. Bandpass filters with center wavelengths of 850 nm and 1000 nm were used for the pyrometry measurement, with calibration provided by a hightemperature blackbody source. Extinction of a 635 nm red diode laser beam was used to determine soot volume fraction. Data were collected along the flame centerline at many different heights and radial traverses were performed at selected heights. A data sampling rate of 5 kHz was used to resolve the turbulent motion of the soot. The results for the ethylene flame show a mean soot volume fraction of 0.4 ppm at mid-height of the flame, with a mean temperature of 1450 K. At any given instant, the soot volume fraction typically falls between 0.2 and 0.6 ppm with a temperature between 1300 and 1650 K. At greater heights in the flame, the soot intermittency increases and its mean concentration decreases while its mean temperature increases. In the JP-8 surrogate flame, the soot concentration reaches a mean value of 1.3 ppm at mid-height of the flame, but the mean soot temperature is only 1270 K. Elevated soot concentrations persist for a range of heights in the JP-8 flame, with a rise in mean temperature to 1360 K, before both soot volume fraction and temperature tail off at the top of this smoking flame.

Applied Combustion Diagnostics

Abstract: The editors have assembled a world-class group of contributors who address the questions the combustion diagnostic community faces. They are chemists who identify the species to be measured and the interfering substances that may be present; physicists, who push the limits of laser spectroscopy and laser devices and who conceive suitable measuremen

Modeling of Fine-Particle Formation in Turbulent Flames

Abstract: The generation of nanostructured particles in high-temperature flames is important both for the control of emissions from combustion devices and for the synthesis of high-value chemicals for a variety of applications. The physiochemical processes that lead to the production of fine particles in turbulent flames are highly sensitive to the flow physics and, in particular, the history of thermochemical compositions and turbulent features they encounter. Consequently, it is possible to change the characteristic size, structure, composition, and yield of the fine particles by altering the flow configuration. This review describes the complex multiscale interactions among turbulent fluid flow, gas-phase chemical reactions, and solid-phase particle evolution. The focus is on modeling the generation of soot particles, an unwanted pollutant from automobile and aircraft engines, as well as metal oxides, a class of high-value chemicals sought for specialized applications, including emissions control. Issues arising due to the numerical methods used to approximate the particle number density function, the modeling of turbulence-chemistry interactions, and model validation are also discussed.

Investigation of soot formation in pressurized swirl flames by laser measurements of temperature, flame structures and soot concentrations

Abstract: Soot formation and oxidation were investigated in swirl flames operated with ethylene/air at elevated pressure in a gas turbine model combustor with optical access. Coherent anti-Stokes Raman scattering was used for temperature measurements, laser-induced incandescence for soot concentration and laser-induced fluorescence for the determination of OH radical distributions. A major focus of the experiments was the investigation of the influence of the injection of secondary oxidation air into the fuel-rich product gas of the primary combustion zone. Soot is mainly present in tiny filament-like regions left without OH signal. In the 3 bar flame with oxidation air injection these are found in a region separating the primary combustion zone, fed by combustion air and ethylene, and the secondary combustion induced by oxidation air and unburned hydrocarbons (UHC) that are transported into the inner recirculation zone. The different behavior of flames with and without oxidation air is most pronounced in the inner recirculation zone that is strongly influenced by the oxidation air admixture. This is reflected by changed OH distributions, mean temperatures and the shape of the temperature pdfs and results in significantly different soot distributions. The combined temperature statistics and correlated OH and soot distributions acquired at 3 and 5 bar are well suited to support the understanding of soot formation and oxidation and are expected to be a valuable input to soot model validation.