Feasibility Study of a Conventional Turbofan With a Constant Volume Combustor

The conventional Brayton CPC (constant pressure combustion) cycle engines have almost reached maturity in terms of thermal efficiency and SFC (specific fuel consumption). With environmental norms taking the front seat in civilian applications, the goal is to think of innovative concepts that are faster, cheaper, cleaner and quieter. Among many proposed concepts, the patented wave rotor – PDE (pulse detonation engine) combined combustor has generated much interest of producing a workable Humphrey CVC (constant volume combustion) cycle engine, with the potential to replace the conventional CP combustor with its inherently higher thermal efficiency, which translates to lower SFC, fuel burn and thus lower CO₂ emissions. However, with the higher flame temperatures and pressure gain (i.e.: pressure rise minus pressure losses) in the combustor, the NO_x emissions are expected to increase, but at the same time limited by the lower residence times on account of detonative combustion. This study seeks to investigate if the NO_x will indeed rise even with shorter residence times. ACARE (Advisory Council for Aeronautics Research in Europe) has set ambitious goals to reduce the LTO (landing and take-off) NO_x by 80% by 2020. This study mainly considers the overall NO_x emissions besides the LTO NO_x, which is not currently regulated, but steps are being taken towards covering this in future revisions of the regulations. The approach used is to develop a performance-based emissions model that could be used for the CVC engine, and also for the conventional CPC baseline engine, in order to compare both using a multidisciplinary design tool (PMDF) for a medium-haul flight of 2500 nm. PMDF is coupled to an optimiser in order to assess and optimise the CVC for best fuel burn and NO_x emissions without compromising on thrust. The results of this preliminary study does indeed show an increase in NO_x emissions index, but interestingly, it also shows that the overall emissions are reduced when comparing the two engines at bypass ratios higher than 7. Moreover, there are strategies to reduce NO_x formation in parallel to techniques used today.