Investigation of Semi‐Constant Volume Combustion for Micro Gas Turbines

Abstract

An efficient micro gas turbine could be used to power long range high altitude unmanned aerial vehicles or alternatively as a small portable electrical power generator. However, the smaller the gas turbine, the harder it is to achieve a competitive efficiency. Current micro gas turbine systems use a standard “constant pressure” combustor. These combustors have many practical benefits. However, their use results in a small pressure loss. Attempts have been made to implement constant volume combustion into a gas turbine. Such systems would increase the pressure ratio across the combustor and hence the efficiency. This thesis investigates a semi-constant volume combustion chamber for micro gas turbines. The system under investigation is implemented by choking the nozzle guide vane of the first turbine stage. This restricts the mass flow rate out of the combustor, resulting in a pressure rise. Along with this a mechanical valve is placed upstream of the combustor to ensure reverse flow is avoided during high pressure phases in the combustion chamber. Investigations examining various aspects of this combustor type have been carried out previously. However, analysis of the combustor inlet valve and its influence are scarce. This component is not found in conventional gas turbine systems, and as such its incorporation into the system is one of the key challenges for successful implementation of a semi-constant volume combustor. This thesis will focus on the inlet valve design by analysing various configurations. In addition to this, the effect of changes in combustor and nozzle geometry, combustion frequency and heat loss will be explored. This investigation is carried out by developing a model which simulates the semi-constant volume combustion cycle. This analysis found that a combustor pressure ratio of 1.27 may be possible. Cycle calculations show that this combustor pressure ratio could reduce specific fuel consumption by over 30% and increase specific thrust by 50%.