Yueh Heng Li

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.

Combustion in a Meso-Scale Liquid-Fuel-Film Combustor with Central-Porous Fuel Inlet

Utilizing a metal-porous medium for a liquid fuel film combustor is an effective method to increase the contact surface area and conduction heat transfer for liquid fuel vaporization and flame stabilization. Based on this concept, a meso-scale liquid fuel film combustor with a central porous inlet is developed and tested. The effects of porous material type and bead size on the flame structures and combustion characteristics are examined. Porous media made of stainless steel and bronze are tested in the meso-scale combustor with different fuel and air flow rates, equivalence ratios, and bead sizes. The flame structure and its corresponding stabilization mechanism are different between the stainless steel and the bronze porous media combustor. In the stainless steel case, the high specific heat capacity enhances fuel vaporization and fuel-air mixing, and the flame anchor locates on the surface of the porous cap. In the bronze case, due to its low heat capacity, the flame is swept downstream where the recirculation zone above the porous cap offers a low velocity field to help anchor the flame. The flame structure and stabilization mechanism in the chamber can be related to a tribrachial flame. Chemiluminescence measurement and Abel deconvolution are performed to verify the flame structure in the vicinity of the porous cap. In addition, temperature measurements and exhaust gas analysis highlight the differences in combustion characteristics between the two kinds of porous media. As regards bead size effects, results indicate that there is no obvious difference in flame structure and flame anchoring position, but the stable operating ranges of a porous combustor decrease with decreasing bead size due mainly to the concomitant increase in thermal conductivity.

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.

A tubular-flame combustor for thermophotovoltaic power systems

The two approaches are addressed in the study for improving the efficiency of combustion-driven thermophotovoltaic(TPV) power system instead of material modification in photovoltaic cell and emitter. One is attempting to integrate visible wavelength from flames and near-infrared wavelength from the silicon carbide emitter for matching the quantum efficiency of nowadays PV cells. The other is coating metal-oxide-deposited layer on a quartz tube and acting as en emitter. Then, the metal-oxide-deposited emitter has advantages of high thermal diffusivity and easy-machining characteristics compare with the traditional emitter. Concept, design and demonstration on both tubular combustors are addressed and discussed in this paper. Results show that the two cost-effective strategies can significantly ameliorate the shortcomings of present PV cell and emitter, and apparently increase the radiation and quantum efficiency.

A parametric study on the effects of displacer-cylinder-circumferential-wall thermal conditions on the performance of a γ-type LTD Stirling engine

ABSTRACT Regarding a Stirling engine’s heat source and heat sink, most of the studies in the literature focus only on the magnitude of temperature difference between them. However, different Stirling engines adopt very different heat-source and heat-sink configurations. This study is aimed at understanding the effects of different displacer-cylinder-wall thermal conditions on engine performance using computational fluid dynamics (CFD). Results include p–V diagrams, heat flux distributions, temperature variations, and effects of three displacer-cylinder-wall parameters on indicated power and efficiency. It is found that the thermal conditions on the displacer-cylinder-circumferential wall (DCCW) impose significant effects on engine performance. Within the ranges of parameters investigated in this study, extending the coverage of heat source and heat sink on this wall improves up to 28% in indicated power at the cost of losing about 10% in efficiency, proving the significance of DCCW conditions on engine performance.