The micro-combustor described in this report is developed to enable the realisation of microscale power device with low-weight and long life as an alternative technology to traditional battery systems. A graphite micro-combustor based on the concept of thermophotovoltaic (TPV) system is designed and fabricated in this research. Establishment of experimental
setup with design of plenum (gas/ air mixer) and ignition system are also developed in this research work to facilitate the investigation of sustaining stable combustion in the graphite micro-combustor.
Concept and principles of thermophotovoltaic is firstly introduced along with its major components and commercial applications. A detail review of studying the effects of wall thermal conductivity, wall thickness and inlet fuel flow rate on combustion characteristics and flame stability at micro-scale is carried out to understand the combustion phenomenon. It is observed that extinction and blowout of flame are prevented if the wall thermal conductivity is in the optimal range of 16-100 W/mK while stable combustion is achieved with wall thickness of 600 µm (or greater) and inlet velocity of 0.5 m/s. Thus, these parameters have provided good design parameters for the proposed micro-combustor.
The graphite micro-combustor is designed in a rectangular shape with a 24 mm3 flat rectangular combustion chamber. This micro-combustor is capable of accommodating high temperature during micro-combustion due to its high melting temperature (3900 K) which is conceivable that the inside wall of the combustor may reach temperature in excess of 1500 K during experiments. Other material properties of graphite such as thermal conductivity of 25 W/mK, excellent machinability and good corrosion resistance suggest that it is an ideal material for the micro-combustor. Due to its simplicity in design, the micro-fabricated portion consists of 4 graphite stacks and a pyrex glass are fabricated using in-house conventional CNC machinery in the University of Queensland, Mechanical Workshop. Metallic composites (DurabondTM 954) are selected as the adhesive to assemble the 5 made parts of the microcombustor due to its high thermal expansion and safety factor. In this research, hydrogen is employed as the combustion fuel for the micro-combustor as it has high ignition energy, heating value, flammability limits and quenching distance which best suited to facilitate the study of sustaining stable combustion.
A detail experimental setup is designed in this research for the feasibility study of sustaining stable combustion in the graphite micro-combustor. The setup consists of (1) ignition system, (2) plenum, (3) two set of mass flow rate controllers, (4) micro-scale thermocouples and (5) data logger. A plenum capable of mixing hydrogen and air to an appropriate ratio to facilitate stable combustion in the experiment is designed, fabricated and assembled in the University of Queensland, Mechanical Workshop using convectional CNC machinery and welding methods. Likewise, an ignition system which is made up of 3 main components mainly the ignition coil, ignition circuit and power supply unit is constructed in the University of Queensland, Electronic Workshop. The system is designed with capability of generating spark energy of 9 watts with an electrode gap of ~5 mm for initiating the micro-combustion during future experimental study.
The overall research works in this thesis represent a critical step in the direction of microscale power device for replacing traditional batteries. The designed graphite micro-combustor is still in its development phase and future necessary works such as numerical and experimental study can be performed before it can be employed in commercial applications.