Tony Yuan

Effects of fuel-air mixing on flame structures and NOx emissions in swirling methane jet flames

An experimental investigation is performed to study the effects of initial fuel-air mixing on NOx and CO emissions in swirling methane jet flames. The major parameters used to modify the initial fuel-air mixing ahead of the swirling flame are the swirl number, the fuel-air momentum flux ratio, and the fuel injection location. Two characteristic swirling combustion modes, the fuel jet-dominated (type-1) and the strongly recirculating (type-2) flames, are identified from flame visualization and 2-D laser-induced predissociative fluorescence imaging of OH by varying the fuel-air momentum flux ratio. Laser Doppler velocimetry (LDV) measurements show that the shear layer between the edge of the swirling recirculation zone and the external flow is a highly turbulent and rapid mixing region. The maximum mean flame temperature is located at the edge of the recirculation zone, indicating violent combustion and strong mixing of fuel, air, and hot products in this region. Strong and rapid mixing of the strongly recirculating flame, which increases mixture homogeneity and shortens the characteristic time for NOx formation, results in a lower NOx emission index than that in the fuel jet-dominated flame. Excess cold air entrained by the swirling flow may quench the combustion and the hot products, resulting in an increase of CO emission, indicating poor combustion efficiency. By modifying the fuel injection pattern with the annular fuel injector, which changes the fuel-air mixing pattern and properly smooths the rapid mixing leading to a higher flame temperature, the NOx emission level can further be reduced with a significant decrease in CO emission.

Effects of flame lifting and acoustic excitation on the reduction of NOX emissions

Abstract The effect of the combustion fluid dynamics of a lifted jet flame with and without acoustic excitation on the control of NOx formation in the flame is investigated. A partially premixed jet was used, operated under lift-off flame bifurcation condition in the hysteresis region. Results show that flame lifting and acoustic excitation are effective in reducing the emission index of NOx (EINOx). A lean premixed condition, achieved by the strong upstream mixing of the lifted flame base, can further reduce NOx formation without increasing CO emissions. The prompt NO mechanisms of super-equilibrium OH concentrations and three-body recombination through N20 intermediates dominate in the initial region close to the flame base of the lean premixed, lifted flame. A lower initial prompt NOx and shorter flame length with reduced flame temperature, caused by the enhanced upstream mixing due to flame lifting and acoustic excitation, result in the low NOx and CO emissions in the present lean premixed jet flame.

Effects of partial premixing on pollutant emissions in swirling methane jet flames

Abstract Experimental measurements of visible flame heights, temperatures, and pollutant emission indices in partially premixed swirling flames with a broad range of central fuel tube equivalence ratios (Φ F ) are reported. Two cases of partially premixed swirling flames are studied; one with a constant fuel tube exit velocity and the other with an increased exit velocity. With increasing partial premixing, the visible flame height decreases and the overall flame color changes from yellow to blue. Temperature measurements indicate that the flame structures become thinner and temperatures increase continuously with increasing partial premixing. Emission index, EINO x and EICO, values decrease with increasing levels of partial premixing and reach a minimum value at Φ F ≈ 3, followed by an increase as Φ F approaches the blowoff limits. The reduction in EINO x and EICO at optimum partial premixing (Φ F ≈ 3), as compared with that for the non-premixed swirling flame is at least 23% and 77%, respectively. The emission index for NO x scales very well with the fuel mass fraction and the fuel-air momentum flux ratio. Good agreement is achieved between the predictions and measured results.

Measurements of the stabilization zone of a lifted jet flame under acoustic excitation

The turbulence and concentration characteristics in the stabilization zone of a lifted jet flame with and without acoustic excitation are measured by a time-resolved Rayleigh scattering, a LDV, and a hot vire anemometry system together with other probes. Both amplification and suppression of the flow can be achieved by acoustic excitation. By careful comparison of the turbulence and concentration characteristics in the stabilization zone for the natural, amplification and suppression cases, it is found that the key parameters of lifted flame stabilization in the stabilization zone are the integral length scale, the F probability of the presence of a flammable premixture, and the G probability of the presence of a fluid with a temperature reaching the ignition temperature. Amplification excitation enhances the large-scale coherent vortices and the vortical entrainment, thus enhancing the length scale, theF probability, and the G probability of having a combustibltoe premixture in the stabilization zone. In this case, the flame shifts upstream to a higher gas speed location and restabilizes there. Suppression excitation shows the opposite results. Practical need calls for a new model capable of predicting the stabilization zone structure of excited lifted flames where the large length scale, theF andG probabilities are suggested to be important parameters.

Optical observation of the impingements of nitrogen tetroxide/ monomethylhydrazine simulants

The spray phenomena of doublet and triplet impingements of nitrogen tetroxide (NTO) and monomethylhydrazine (MMH) simulants were studied by the planar laser induced fluorescence technique. The total flow rates of the simulants were controlled at ∼8.00 g/s to simulate the operations of a 5-lb f rocket, and the ratios of the mass flow rates (O/F) of NTO and MMH simulants were varied from 1.0 to 2.4. Statistical analysis was employed to examine the probability distributions of the mass of individual simulants at 10 mm downstream from the impinging point. The distributions of local mixture ratios and flame temperatures were deduced, and the estimation of characteristic exhaust velocity was performed. The results revealed that the breakup and mixing of the impinging jets were closely related to the momentum flux ratio of the jets and the surface tension of the liquids. The triplet impingement was superior in symmetry and uniformity of the spray than that of the doublet impingement, and its mixing was less sensitive to the momentum flux ratio and showed a better mixing effect than doublet impingement at higher O/F ratio conditions. With the detailed distribution information provided by the optical technique, the optimum C* occurred at O/F = 1.18 with doublet impingements for the operation of a 5-lb f NTO/MMH rocket.

The application of spontaneous vibrational Raman scattering for temperature measurements in high pressure laminar flames

Ultraviolet (UV) spontaneous vibrational Raman scattering is used for temperature measurements in premixed methane/air flat flames burning at 1-5 bar. The measured spectra indicate that the peak and integrated intensities of the O 2 and N 2 Stokes Raman signals are linearly proportional to pressure at room temperature. Temperature measurements are made by spectral-fit of the theoretical spectra to the measured N 2 Q -branch spectra. The Raman-measured temperatures agree well with the thermocouple-measured temperatures. The line widths (full width at half maximum; FWHM) of the trapezoidal slit function in flames at 1-5 bar are used to extrapolate the line widths for pressures up to 30 bar. The calculated theoretical spectra indicate that the spectral-fit method can be used to determine flame temperature at conditions close to adiabatic and 30 bar with an adequate accuracy.

Comparison of Hot-Fire and Cold-Flow Observations of Nitrogen Tetroxide/Monomethylhydrazine Impinging Combustion

The combustion phenomena of doublet impingements of nitrogen tetroxide and monomethylhydrazine were observed in this research. The total flow rates of the propellants were controlled at 8:00 g=s to simulate the operation of a 5 lbf rocket, and the ratios of the mass flow rates (O=F) of nitrogen tetroxide and monomethylhydrazine were varied from 1.0 to 2.5. With a two-axis translation module, a C-type thermocouple was used to measure the two-dimensional temperature distributions of the flames at 20 mm downstream of their impinging points. The observations showed that induction distances always appeared from the positions of propellants’ impingement to ignition, and the ignition was induced by the local concentrations of gaseous monomethylhydrazine.Themaximum flamelengthof80 mmappearedatthebestmixingconditionofO=F 1:2, and diffusion-type flames at higher O=F showed shorter flame lengths for poor mixing after the flames had been ignited. Comparing to the previous cold-flow observations by the planar laser induced fluorescence technique with simulants of water- and chloroform-based solutions to match the densities, viscosities, and surface tensions of monomethylhydrazine and nitrogen tetroxide, the locations of high-temperature zones in hot-fire experiments were adequately described by the calculated two-dimensional temperature distributions from the observed local mixture ratio data at the corresponding ignition positions in the cold-flow experiments. These results inferred that the mixture ratio profile was almost conserved after ignition. That is, by properly analyzing the mass distribution of the cold-flow observation with simulants, the temperature distribution of the impinging combustion of nitrogen tetroxide/monomethylhydrazine can be predicted.

Design, fabrication, and test of a microelectromechanical-system-based millinewton-level hydrazine thruster

T HE miniaturization of space systems, such as microsatellites, has become an important development trend. Using a cluster of microspacecraft with a constellational architecture to replace a traditional spacecraft can greatly reduce the costs of production and launch, increase flexibility, and disperse the risks of a mission. Miniaturized spacecraft are classified based on mass, power, and dimensions. Spacecraft with amass of less than 20 kg are classified as class I microspacecraft [1] and require millinewton-level thrusts for spacecraft control. For microspacecraft, the onboard thrusters must be extremely small and lightweight; microelectromechanical systems (MEMS) are thus employed in microthruster design and fabrication [2]. A number ofmicropropulsion systems have been proposed.Micro cold-gas systems have been constructed and used in practice [3,4]; however, a rather low specific impulse (60–80 s) limits their usage. Micro electric-type thrusters provide a high specific impulse [5], but the requirement of high power for operation limits them to larger spacecraft. Miniaturized solid-propellant thrusters have a simple structure and a high specific impulse [6], but their relatively high thrust level (10–10 mN) and single use restrict their application. Monopropellant thrusters are appropriate for miniaturization due to their simplicity and acceptable working temperatures [7]. The catalytic reaction of monopropellant systems mitigates the constraints of radical quenching and mixing prohibition found in the microcombustion of bipropellant systems. Although hydrogen peroxide/silver systems have been tested and effectively reacted in microreactors, hydrazine is considered a better monopropellant for actual microthruster design and operation [7]. A millinewton hydrazine (N2H4) monopropellant thruster is presented in this work. MEMS technologies are employed in the design. The design considerations and component fabrication are discussed. The vacuum thrust of the designed thruster was measured and its propulsive performance was analyzed.

A cold-flow experimental observation of the two-stage impinging type injector for rocket propulsion

Green (low toxicity) liquid rocket propellants have become attractive in recent years due to the features of the low cost and less environmental impact. However, the green propellants, such as kerosene/H2O2, usually have different operational conditions (i.e. relatively high O/F ratio) compared to conventional propellants because of their chemical properties. In this research, a new concept of the two-stage impinging type injector (O-F-F-O) is adopted for investigating the spray mixing at high O/F ratios between 3.75 and 6.25. The impinging distance, jet velocity and impinging angle for the two-stage impinging type injector are design parameters examined, where the impinging angle is more effective at spray atomization and droplet distribution. The PLIF technique is used to measure the droplet distribution so as to identify the spray characteristics. In order to simplify the development process of the injector, the predicted mixture ratio distribution from the individual fuel (F-F) and oxidizer (O-O) sprays by overlapping their averaged images is used to compare with the actual distribution from the two-stage impinging spray (O-F-F-O). At a constant total mass flow rate, results indicate that tendencies towards the variations of the average characteristic velocity (C) with increasing O/F ratios are similar for outcomes of the prediction and actual measurement. Also, there is obvious flow fields interaction between the fuel and oxidizer sprays and coordinating their relative intensities of sprays well can optimize the mixture ratio distribution of the two-stage impinging spray. Better mixing occurs when the fuel and oxidizer sprays have more similar and uniform distributions.

The impinging-type injector design of MMH/NTO liquid rocket engine

By the PLIF optical technique, this study focused on the development of the impinging type injector design of the 100-lbf level monomethylhydrazine/nitrogen tetroxide liquid rocket. The design of the multi-pair impinging injector plate is based on the cold flow and hot fire observations of unlike-doublet impinging jets of monomethylhydrazine/nitrogen tetroxide and their simulants. The mass distributions of the impinging jets from the cold flow observations are used to arrange the paired orifices of the impinging unit on the injector plate through a image projection method. For the most uniform distribution and the best overall estimated C* of the overall spray, the orifice size of the impinging element and the configuration of the paired orifices of an injector plate was determined for the prototype injector plate design. Static tests of the ground-test liquid rocket engine with the designed injector plate are performed. The measured sea-level thrust of the engine was 62.1 lb, and the characteristic exhaust velocity reached 1429.2 m/s at a total propellant mass flow rate of 153.4 g/s. The estimated vacuum thrust was 99.8 lb, and the vacuum Isp was 295.8 s as well. The combustor pressure of the liquid rocket engine was 153.2 psi and remained stable during operation. During the 17 seconds of the hot fire test, the highest wall temperature was measured at the nozzle throat, which was about 1000K.