Chih Yung Wu

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.

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.

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.

CHEMILUMINESCENCE MEASUREMENTS OF LOCAL EQUIVALENCE RATIO IN A PARTIALLY PREMIXED FLAME

Spatially resolved, time-averaged, multipoint measurements of flame emission spectra using two Cassegrain mirrors and two spectrometers are performed and the results are used to obtain the correlation of the intensity ratio of OH*/CH* and C2*/OH* to the equivalence ratio in the laminar flames over an equivalence ratio range of 0.8–1.4. Results show that a strong correlation exists between the intensity ratio and equivalence ratio. The calibration equations obtained from the laminar flame measurements are then applied to obtain the local equivalence ratio in a partially premixed swirling flame. Experimental results demonstrate that multipoint measurement of local equivalence ratio in the partially premixed swirling methane flames is feasible. However, this non-laser based chemiluminescence technique can only be applied to determine the local flame stoichiometry in the reaction zone of the flames. Further improvement of the measurement system and possibility of simultaneous measurements of equivalence ratio and temperature are discussed.

EFFECTS OF DILUTION ON BLOWOUT LIMITS OF TURBULENT JET FLAMES

An extended database on blowout velocities of inert-diluted methane, propane, and hydrogen jet flames in the turbulent regime was experimentally established and used to examine and verify existing theories of blowout velocity estimation. Helium, argon, nitrogen, and carbon dioxide were used as the inert diluents to generate different initial properties at the jet exit. The theories of blowout velocity estimation by Kalghatgi (Combust. Sci. Technol., vol. 26, pp. 233–239, 1981) and Broadwell et al. (Proc. Combust. Instit., vol. 20, pp. 303–310, 1984) in the highly diluted regime were carefully examined using jet flames of different fuels diluted with inerts of different gas properties. The results showed that among the theories the blowout velocity estimation of Kalghatgi is more reliable in the extended region. On the other hand, the blowout velocity estimation of Broadwell et al. can do as well after proper modification by including the Reynolds number effect Re f. Based on the experimental results, modifications to the theories are proposed to accommodate for the deviation when they are used in the diluted regime. In addition, different from laminar jet flames, diffusive properties in terms of mass and thermal diffusivities are not the dominant parameters of blowout velocity in turbulent jet flames.

DEVELOPMENT OF A CATALYTIC HYDROGEN MICRO-PROPULSION SYSTEM

In the present study, the feasibility of a catalytic hydrogen micro-propulsion system suitable for providing thrust for micro-satellite attitude control and orbit transfer is demonstrated and examined experimentally and numerically. The effects of major design and operational parameters of fuel/air flow rate, equivalence ratio and nozzle contraction ratio on the thruster performance are investigated. For ease of observation and numerical comparison, a platinum catalytic tube with an inner diameter of 500 µm and length 1 cm is used as the reactor and is tightly inserted into a quartz tube with a convergent nozzle made of quartz of different contraction ratios. For most of the cases tested in experiments, catalytic surface reaction occurs near entrance of the platinum tube due to the high diffusivity and high surface reaction rate of hydrogen. With increasing fuel concentration, the transition point from kinetic-controlled reaction to diffusion-controlled reaction moves further downstream. When the fuel velocity is increased, the hydrogen may not be consumed completely because of insufficient residence time. However, the unburned hydrogen has been heated, by upstream surface reactions, such that gas phase autoignition may occur in the recirculation zone just down stream of the platinum tube. The catalytic hydrogen micro-thruster developed in this study can self-sustain and provide a maximum thrust of 7 mN, which is suitable for precision attitude control of micro-and pico-satellites.

A study of the interaction between a jet flame and a lateral wall

Abstract The dynamic process of the interaction between a jet flame and a lateral wall is experimentally studied. The evolution of the outer buoyant vortices, which are involved in the jet flame bulge and flame tip-cutting phenomena, is found to play the central role in the flame-wall interaction process for low speed jet flames. The flame response as the lateral wall approaches from infinity can be categorized into five characteristic stages based on the ratio of the flame bulge size on both sides. As the wall approaches, the flame is observed to first increase in the size of the flame bulge on the wall side. Then, flame is wiggling with off-set flame bulges on both sides. The flame is then seen to incline and attach to the wall, and finally flame on the wall side is complete quenched as the separation distance is further reduced. These variations in flame structure are closely related to the retarded evolution of the buoyant vortices due to the wall and are scaled and explained based on similarity as well as the induced strain rate due to the variation of the outer vortices.

Measurements of a high-luminosity flame structure by a shuttered PIV system

It is difficult to measure the velocity distribution inside a high-luminosity flame by using the particle image velocimetry (PIV) system with a double-shutter mode CCD camera. The second raw image of the PIV image pair is usually contaminated by flame emission. The main cause of the problem is an excess exposure time which lets the flame emission overlap the particle image in the second frame. If the flame-contamination problem is not significant, for example in faint flames, digital image processing can improve the image to an acceptable level. Nevertheless, when the PIV technique is applied to high-luminosity flames, the second raw particle image would be contaminated by flame emission. In this paper, incorporating a mechanical shutter in the PIV system with a double-shutter CCD camera is proposed to improve PIV measurements in high-luminosity flames. Measurements in faint, high-luminosity as well as very bright flames were tested. The results show that the present setup can accurately resolve the flow velocity field inside the flame cone, through the flame and in the post flame zone for all the flame conditions analyzed. The velocity distributions and streamline patterns measured by the present equipment are reasonable and meaningful.

The Physical Characteristics of Bio-Oil from Fast Pyrolysis of Rice Straw

Rice straw is one of the main renewable energy sources in central and south Taiwan. In this study, bio-oil was produced from rice straw using a bench-scale plant that included a fluidized bed, a char removal system, and an oil collection system using an oil-recycling spray condenser. We investigated the effects of pyrolysis temperature and carrier gas flow rate on the distribution of products and on the properties of the bio-oil obtained. Experiments were conducted at reactor temperatures of 350–500 °C with carrier gas flow rates of 7.5–15 L/min and a feed rate of 1 kg/h. The results indicated that the optimum reaction temperature and carrier gas flow rate for the production of bio-oil were 450 °C and 10 L/min, respectively. The highest percentage of bio-oil in the products in these experiments was 41.3 wt%. The pH value of the bio-oil was ~4.1 and the viscosity was ~9 cSt (at 25 °C), depending on the storage time, temperature, and char content. This study establishes the operating parameters of a biomass fast pyrolysis system and provides some properties of rice straw bio-oil relevant to storage and use.

The Performance of a Diesel Engine Blended with Coffee Bean Residue Pyrolysis Oil

In recent years, global economic development has led to rises in crude oil prices and the greenhouse effect. Developing renewable energy has become a major issue internationally. Biomass energy is one form of renewable energy. Biomass enery as a transportation fuel can be deliverd from fats, starch biomass fuel or biofuel, and cellulose biofuels. Because concerns exist that fats could lead to a food crisis, most nations have focused on researching and developing cellulose. In this study, we used coffee bean residue as a fuel source. We employed the pyrolysis process to produce pyrloysis oil products. We also used appropriate emulsion technology and suitable amounts of petroleum diesel for adding coffee bean residue to investigate the influence that coffee bean residue pyrolysis oil with low-add ratios (5 vol. %) has on diesel engine performance. The results indicated that coffee bean residue pyrlosis oil with low-add ratios resulted in an earlier diesel engine ignition time. Additionally, at low rpms, the diesel engine’s indicated specific fuel consumption (ISFC) was relatively low. However, ISFC was superior at higher rpms. In summary, when adding a portion of coffee bean residue pyrlosis oil to diesel engines, performance was superior during high-rpm operations compared to lower rpm. Pollutant emissions were also significantly improved. The results of this study can provide a reference for future high-add ratios and high-load engine tests.