Investigation of the effects of preheating on the characteristics of micro combustion

Abstract

The requirement for efficient power sources for portable electronics and miniature mechanical devices, such as laptops, micro robots or micro aerial vehicles, has led to research on ultra micro gas turbine (UMGT). The ultra micro gas turbine is one of the most promising power sources for small scale applications due to its higher power and energy densities compared to currently used batteries. In order to realise UMGT as a viable power source, its individual components have to be developed, since downscaling introduces new problems for each component. Since the micro combustor is one of the key components of UMGT, it is an obvious area for investigation and improvement. Until now, it has been very difficult for micro combustors to achieve wide flame stability, high combustion efficiency and clean combustion with low pressure loss, due to the associated downscaling problems, such as high heat loss ratio and small residence time. In order to overcome the issues associated with smaller residence time, preheating the reactants was investigated in the present study, both experimentally and numerically. Two different domains were used in the two different methods. The effects of preheating the reactants on the flame stability and combustion characteristics were investigated experimentally on a 46 mm inner diameter quartz-walled flat flame combustor. The preheating was provided with external heaters. The flame shape and behaviour were observed with different air mass flow rates and equivalence ratios. The experimental results showed that preheating widened the flame stability limit. From the experimental results a new correlation was developed between mass flow rate, reactant temperature, air to fuel ratio and the diameter of the micro combustor. With this correlation, it is possible to determine the minimum combustor diameter required for stable flat flame combustion. Also, in order to have a stable flat flame at higher mass flow rates, the correlation enables the calculation of the required reactants temperature, and is a major contribution to the design of micro combustors. Furthermore, it was found that with the flat flame combustor, very clean, that is, almost complete combustion was achieved with both natural gas and methane as fuels. In addition, different flame holder materials were investigated for their pressure loss characteristics and usability for UMGT applications. On the other hand, the effects of preheating on the flame stability and species concentrations were investigated numerically within a 2 mm diameter micro combustor tube, over two-dimensions. The simulations were performed with both adiabatic walls and nonadiabatic walls. At high incoming flow velocities, preheating was shown to widen the flame stability limit. On the other hand, there is no advantage in preheating for low incoming flow velocities or low mass flow rates. A comparison of employing different skeletal and detailed reaction mechanisms was made. Furthermore, in the non-adiabatic case the effect of heat loss through the outer wall via convection and radiation were investigated with different heat transfer coefficients. In conclusion, preheating widens the flame stability limit and provides flame sustainability even at very small scales. The information generated in this research will help the development of UMGT as a viable alternative to conventional power sources at small scale.