Ignition and Extinction Behavior of Fuels in a Microcombustor
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x CHAPTER 1 MOTIVATION AND APPROACH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.
Direct numerical simulation of micro combustion in a narrow circular channel with a detailed kinetic mechanism
Abstract Direct Numerical Simulation (DNS) of micro combustion in a narrow circular channel is conducted for a methane–air mixture. The narrow circular channel with a radius of 0.5 mm is supposed to have a wall temperature gradient in the streamwise direction. To represent the methane–air reaction, a detailed kinetic mechanism (GRI-Mech 3.0) including 53 species and 325 elementary reactions is adopted. Four series of DNS are conducted with different inflow temperature and wall temperature gradient to investigate the unsteady phenomena in micro combustion. Similar to the previously reported experiment, unsteady behaviors of the flame such as the oscillating flame and the flame repetitive extinction and ignition (FREI) are obtained depending on the inflow temperature and the wall temperature gradient. To investigate the FREI mechanism, the total heat transfer rate through the wall of the mixture is introduced and evaluated in one cycle of both FREI and oscillating flame. It has been shown that FREI possesses the negative phase in the total heat transfer rate through the wall. Therefore it suggests that the definition of the origin of FREI is given by this total heat transfer rate. It is also presented that the amplitude and period of FREI exhibit linear relationships with the wall thermal thickness based on the wall temperature gradient when they are scaled by the inflow mass flux of the mixture and the thermal thickness of the steady flame at the same inflow and outflow temperature conditions.
Effect of wall surface reaction on a methane-air premixed flame in narrow channels with different wall materials
Abstract In order to elucidate effect of wall material on chemical quenching behavior, a methane-air premixed flame formed in 5-mm-wide channel is investigated. In the present study, platinum, quartz, alumina and chromium are chosen as the wall materials. Platinum, chromium and alumina thin films ∼100 nm in thickness are deposited on quartz substrates using sputtering, vacuum arc plasma gun or atomic layer deposition techniques to establish equivalent thermal boundary condition with different wall chemical reactions. OH-PLIF/micro-OH-PLIF and numerical simulation with detailed reaction mechanisms are employed to examine interaction between the gas-phase and the wall surface reactions. It is clearly shown through the PLIF measurements that OH ∗ mole fraction in the vicinity of the wall is the highest for alumina, while it is decreased in order of quartz, chromium, and platinum. On the platinum surface, the gas-phase combustion is suppressed due to fast consumption of the reactants by the catalytic reaction. On the other hand, on the other surfaces, radical quenching cause the reduction of OH ∗ near the wall. By using a radical quenching model, the initial sticking coefficient associated with radical adsorption is evaluated. It is found that radical quenching does exist on the quartz wall, while the alumina surface works as an inert surface.