Concentration Analyses on Stagnation-Point Spray Flames and Low-Reynolds-Number Jets

• Main Authors: An-Kuo Hsu, 許安國
• Other Authors: Ta-Hui Lin
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/23023319609893290804

博士 === 國立成功大學 === 機械工程學系碩博士班 === 91 === The concentration analysis is the main objective in this thesis. It contains two parts: the analysis of the stagnation-point spray flame and the analysis of a low-Reynolds-number slot jet. The O2, CO2, CH4 and CO concentrations were measured in a stagnation-point gasoline spray flame by using a set of stagnation-point spray combustion system, a set of gas extracting system with a quartz probe, and a gas chromatograph in the study of the first part. Then, the above-mentioned apparatus along with a PDPA and a thermocouple were used to measure the gas-phase concentrations, the droplet sizes, the number densities, the axial velocities, and the gas-phase temperatures in an ethanol-gasoline spray premixed flame. As for the second part, the concentration fields were determined experimentally and numerically for a low-Reynolds-number slot jet. Experimentally, the fluid containing oxygen and nitrogen was issued from a slot jet into the quiescent atmosphere. The oxygen concentrations were measured by using the above-mentioned apparatus of concentration measurement. Furthermore, smoke particles were used to visualize the flow field. Numerically, a set of governing equations of the conservation of momentum, mass and species were solved by finite-difference method to obtain the velocity and concentration fields. As for the study of stagnation-point gasoline spray flame, the measured species concentrations can be combined with the results of gas-phase temperature, the droplet size, the number density, and the axial velocity to understand the interactions between the droplets, flamelets, and the flow; hence we can comprehend the exchanges of mass, momentum, and energy in a stagnation-point spray premixed flame. As for the study of ethanol-gasoline spray flame, we conclude that the CO reduction is apparent when more than 10% ethanol was added to the gasoline. As for the study of second part: due to the interactions among momentum transfer, mass transfer and gravitational force, the flow and concentration fields at low Reynolds numbers with considerable mass transfer are found very different from those at high Reynolds numbers. The flow field is greatly influenced by the gravitational force due to concentration-induced density change; and the flow field in turn affects the concentration distribution. For an upward jet of density higher than air, at Re = 39.4, one vortex in the far region exists. However, at Re = 78.8 and 152.2, two vortices appear; one attached at the slot exit and one in the far region. At Re = 157.7, only one vortex attached at the slot exit. As the slot exit velocities increase, the positions of the recirculation vortices in y direction will increase. Due to the influence of the recirculation vortex, some down-up-down concentration profiles which are called saddle-shaped curves occur at x direction near the slot exit. Furthermore, as the slot exit velocities increase, the distances that can reach will increase. The calculated concentration field is in fairly good agreement with the experimental result at Re=39.4 and Re=78.8; therefore, the numerical model can simulate a low-Reynolds-number slot jet well.