Yei Chin Chao

An experimental investigation of the blowout process of a jet flame

An experimental study was performed to investigate the roles of triple flames and flame front instabilities in the blowout transient process. Two-dimensional laser-induced predissociative fluorescence (LIPF) OH and particle image velocimetry (PIV) diagnostic techniques were used for measurements of instantaneous flame structure and velcotiy data. Initial conditions were aligned by external acoustic excitation and triggering. The blowout transient process can be divided into four regions: the pulsating, onset of receding, receding, and extinction regions, according to the dynamic characteristics of the flame. In the pulsating region, the flame base is basically pulsating at two specific heights with jittering. Flame from instability may play a role in leading to the onset of blowout process. Both LIPF OH image and PIV results show the possible existence of the triple- (or edge-) flame structures in the flame base in the pulsating and onset regions. High strain rate, higher than the extinction strain rate, encountered by the flame base in the onset region should be cousidered as a prominent factor for the blowout process.

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.

Behavior of the lifted jet flame under acoustic excitation

The importance of large coherent vortical structures on lifted jet flame stability under acoustic excitation was extensively demonstrated in this investigation. By employing visualization and digital image processing methods, the most probable stabilization locations of a lifted flame base in the hysteresis region were found to be located at the roll-up and pairing positions of the coherent vortices. The flame stabilization mechanism in the hysteresis region was identified by phase-averaged concentration and velocity measurements. Phase-averaged results show that the strong entrainment induced by the large vortices during roll-up and pairing processes plays a key role in preventing the lifted flame from propagating upstream and causes the hysteresis phenomenon. Modification of lifted flame stability by acoustic excitation was demonstrated to be feasible. Acoustic excitation at frequencies in the turbulence amplification region of the cold jet is helpful in enhancing the flame stability in the high exit velocity region where the flame base zone is stabilized downstream of the end of the potential core; such excitation could extend the blowout limit by more than 25%. Excitation at frequencies in the turbulence suppression region is helpful when the flame is stabilized upstream of the end of the potential core.

Development and Ground Tests of a 100-Millinewton Hydrogen Peroxide Monopropellant Microthruster

3, with a throat diameter of 0.5-mm packing, with silver flake as the catalyst, and 92% hydrogen peroxide is adopted with a flow rate of 0:18 g=s. Measurements of the ignition delay at different catalyst-bed preheating temperatures are compared to define the operating parameters for practical applications. Test results show that the hydrogen peroxide microthruster gives a good c efficiency and a short ignition delay compared with the performance of large-scale systems. Under atmospheric pressure at sea level, the developed microthruster can produce 182 mN with a specific impulse of 101 s.

Downstream boundary effects on the spectral characteristics of a swirling flowfield

The spectral characteristics and the structural response of a swirling flowfield are investigated subject to a non-axisymmetric disturbance and a contraction imposed downstream. Two natural frequencies are noted in different regions of the undisturbed swirling flowfield, one is due to a precessing vortex core and the other to the most amplified downstream azimuthal instability. The downstream contraction usually causes compression of the central recirculation zone into two side-lobes, increases the dominant frequencies and forms a straight central vortex core with a high axial velocity. The dominant downstream instability frequency depends linearly on the inlet Reynolds number and on the mode of the breakdown. For the downstream non-axisymmetric disturbance, such as the passing of the turbine blades, the fundamental frequency is not altered by the disturbance and the oscillation strength of the downstream instability is greatly reduced as the excitation frequency remains unmatched with the dominant downstream natural frequency. Downstream azimuthal instability promotes the breakdown recirculation.

The stabilization mechanism of the lifted jet diffusion flame in the hysteresis region

Recent experimental efforts focused on near-field coherent vortex dynamics, and their impact on stabilization of a lifted jet diffusion flame in the hysteresis region are reported. Simultaneous jet flow and flame visualizations are conducted first to obtain a global feature of flow/flame interaction. The statistical liftoff heights are calculated by a DIP (digital image processing) method. The gas concentration and velocity distributions induced by the vortex evolution as well as the corresponding flame front motion are deduced from phase-averaged measurements of planar Mie-scattering gas concentration images, LDV and ion-signals, respectively. The planar gas concentration technique employed here extends our previous work (Chao et al. 1990, 1991 a) to include phase-averaging. Results of the experiments show that the most probable flame base locations in the hysteresis region are at the coherent vortex roll-up and pairing locations. The deeply entrained air lump caused by large-scale vortices during roll-up and pairing is the main obstruction to flame propagation back to the nozzle exit and causes the hysteresis phenomenon.

OPERATIONAL CHARACTERISTICS OF CATALYTIC COMBUSTION IN A PLATINUM MICROTUBE

Catalytic combustion of hydrogen in a platinum microtube, or subquenching diameter tube, is studied via theoretical analysis, experiments, and numerical simulation in terms of the major operation and design parameters. Fine-thermocouple, laser-induced fluorescence (LIF) and Raman scattering are used to measure the temperature and major species and OH concentration data at the tube exit. The experimental results show that the tube-exit temperature increases with fuel concentration, velocity, and tube size. For high fuel concentration and velocity cases in the 1000- and 500-µm tubes, an obvious gas-phase reaction behind the exit can be detected by thermocouple and LIF-OH images. Numerical simulation results show that smaller tube sizes and lower velocities would enhance the conversion ratio on the catalytic surface due to the enhanced diffusion of surface species of H2 and O2. Based on the current results and analysis, the characteristic operation regions of hydrogen catalytic combustion in microtubes are quantitatively identified in terms of parameters related to heat generation and heat loss characteristics, competition among the timescales, and tube size. Decreasing the tube size will shift the operation region toward the high-concentration and high-velocity portion of the domain with a smaller operation area.

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.

Characteristics of microjet methane diffusion flames

Characteristics of microjet methane diffusion flames stabilized on top of the vertically oriented, stainless-steel tubes with an inner diameter ranging from 186 to 778 μ m are investigated experimentally, theoretically and numerically. Of particular interest are the flame shape, flame length and quenching limit, as they may be related to the minimum size and power of the devices in which such flames would be used for future micro-power generation. Experimental measurements of the flame shape, flame length and quenching velocity are compared with theoretical predictions as well as detailed numerical simulations. Comparisons of the theoretical predictions with measured results show that only Ropers model can satisfactorily predict the flame height and quenching velocity of microjet methane flames. Detailed numerical simulations, using skeletal chemical kinetic mechanism, of the flames stabilized at the tip of d = 186, 324 and 529 μ m tubes are performed to investigate the flame structures and the effects of burner materials on the standoff distance near extinction limit. The computed flame shape and flame length for the d = 186 μm flame are in excellent agreement with experimental results. Numerical predictions of the flame structures strongly suggest that the flame burns in a diffusion mode near the extinction limit. The calculated OH mass fraction isopleths indicate that different tube materials have a minor effect on the standoff distance, but influence the quenching gap between the flame and the tube.