Study Of Quenching Distance And Lean-burn Extinction Limit In Mini-scale Combustion

Micro-thruster, miniature energy or power devices require high-density energy source, which can be obtained by mini-scale combustion. Via the review of related work at home and abroad, the importance of the mini-scale combustion for the future requirements of micro structure energy and power equipments was summarized, and its problems and challenges were also pointed out. Meanwhile, the catalytic combustion, combustion in porous media and a combination of both technologies were the most main and the most effective methods to reduce the quenching distance and expand lean extinction limit. Study on mini-scale combustion is very important for both engineering applications and academic research.The 2D numerical method for catalytic combustion was developed based on CHEMKIN and FLUENT platform. Using detailed chemical kinetic mechanism, the catalytic combustion of CH₄/Air on Pt was numerical studied and the catalytic combustion models were validated. Firstly, on the platform of stagnation flow, the catalytic temperatures of the stagnation point calculated by 1D program Spin were compared with the Deutschmann's calculations. Introducing gas phase reactions, the 2D axisymmetric simulation was compared with 1D simulation by analysising the flame structure, and the validation of the 2D numerical model and the applicability of 1D assumption were investigated. By comparion of the 1D and 2D calculations at several stretch rates (SR), it can be found the radial diffusion must be taken into consideration at low SR while the temperature and species distribution agreed well with 1D assumption at high SR. Finally, in order to study the effection of catalytic combustion on the quenching distance, combustion in the mini-scale 2D expanding channel was simulated. The results showed that the surface catalytic combustion stabilized the flame in a smaller space and significantly reduced the extinction distance. In addition, the surface catalytic combustion can reduce the local high temperature, reduce heat loss and improve the fuel conversion rate. It is conducive to organizing a more stable combustion within mini-scale space and reducing pollutant emissions.Using porous medium combustion technology, a hollow and non-hollow side-face radiator and top-face radiator were designed as heat sources of TPV system. The side-face radiator has larger radiation surface area. The radiation temperature was above 1000℃at the firing power about 4kW. When the power was 4.3kW, the maximum radiation density reached up to 9kW/m². Combustion in porous media can be organized in large area with better temperature uniformity. For a given firing power, the better uniformity of surface radiation temperature can be obtained at low equivalence ratio. When the equivalence ratio was 0.5, the surface radiation temperature gradient was 1.7℃/mm at power of 4.3kW. Heat recuperation can improve the radiation temperature and the radiation efficiency. However, it easily led to flashing back. In contrast with the side-face radiator, the top-face radiator can obtain higher and more uniform radiative surface temperature and it is easily installed with PV cells for systems integration. However, the radiative surface area was smaller. Experimental results of the top-face radiator showed that the maximum relative temperature difference was less than 3% and a self-sustainable combustion was achieved at the lowest equivalence ratio of 0.33. For all experimental cases, the NO_x emissions in both radiators were less than 25 PPM; the CO emissions are lower than 10 PPM at the equivalence ratio higher than 0.45.A mini-scale porous combustor with heat recuperation was designed. Both methane and propane combustion are experimentally investigated in terms of small scale combustion characteristic, lean extinction limit and low pollutant emissions. Research was assistanted by numerical method to study combustion temperature and heat recovery efficiency. Experimental results showed that a long time stable combustion can be maintained with the firing power in the range of 0.2 3.8kW. The lean extinction limit was 0.40 for the methane combustion, while 0.39 for the propane combustion. In most cases, the NO_x emission was about 20PPM and the CO emission was lower than 100PPM. Using two temperature model, effective thermal conductivity model, the established numerical method can well predict the porous media temperatures and the wall temperatures. Numerical results showed that the combustion zone temperatures reached up to 2000K, while the exhausts temperatures at the exit were lower than 900K. The 130mm-long annular channel achieved 40% of the thermal recycle efficiency, which made the ultra-lean combustion extremely stable.