Global warming is a world-wide problem caused by the increasing concentration of CO2 and other products of combustion in the atmosphere. The world community is striving to stabilise, or even reduce, the output of CO2, and therefore every source of this greenhouse gas must be accounted for. The commercial aviation industry is in an especially unenviable position, as the present annual growth of 5% is greater than that of any other sector contributing to the world-wide CO2 output. It is necessary therefore to consider and investigate every considerable measure which would have the potential to reduce the atmospheric emission of CO2, or indeed aircraft fuel consumption in general.
One such measure is the development of more fuel efficient aircraft engines. Considerable progress has been made in this direction in the past;
however, the continual increase in the pressure ratio and combustor exit temperatures of gas turbine engines needed for improved fuel efficiency has been accompanied by an undesirable increase in the NOx emissions. The inter-cooled recuperative gas turbine offers the best possibility of solving the emissions problem, as it is designed to substantially lower fuel consumption at low pressure ratio. For this reason, the inter-cooled recuperative engine can simultaneously reduce both CO2 and NOx emissions.
The exhaust gas recuperator (hex), whose design and development was the content of this project, is one of the key components of the Inter-cooled Recuperative Aero-engine (IRA).
The primary objective of the present project was to develop an optimal design concept of a hex for application in the IRA-engine with respect to efficiency, operational life, weight and cost. It was an intended goal to check whether these requirements can be attained, whether constraints or restrictions on the requirements may be necessary, and if so, what effects such constraints will have on the performance of the engine.
To achieve the goals, a complete design and construction process has been carried out which included also the thermal and structure-mechanical analyses for the purpose of attaining an estimated operational lifetime. Flow tests and CFD modelling have been performed in order to optimise the orientation of the various elements of the heat exchanger in the exhaust duct of the IRA-engine and to make realistic assessments of pressure losses. Furthermore, manufacturing investigations have been undertaken with the goal of reducing the high expenditure costs, but with a minimal influence on the performance and life of the heat exchanger.
The objectives have been widely achieved. The heat exchanger designed for and integrated into the IRA-engine meets the desired weight. Due to the introduction of new manufacturing techniques the manufacturing costs could be reduced by more than 50%. The life of the heat exchanger exceeded the life requirements of the engine, although some further tests are necessary to validate the assessed life. It was demonstrated that pressure drop and heat exchange rates depend strongly on how the heat exchanger elements are arranged in the exhaust duct of the IRA engine.
The originally designed arrangement led to a hot gas side pressure drop which was up to 100% higher than the target. Also the air side pressure loss was higher than designed. The latter can be decreased by relatively simple measures (increase the cross-section of the pipes connecting the heat exchanger with the combustor). The reduction of the hot gas pressure drop to the targeted magnitude requires a careful analysis of the pressure drop sources which can be done only via CFD. Very important is a flow distributed as homogeneous as possible over the eight heat exchanger elements in the exhaust duct of the IRA engine which again can only be achieved via suitable design tools.
Therefore the most important output of this project is the acquisition of the design philosophy, technology and tools needed for the engineering development of an exhaust gas recuperator of this type. The tools developed and validated in the present project are the key to select out of a number of alternatives that specific heat exchanger configuration which meets the targets.
Additionally the use of an exhaust gas recuperator in an industrial gas turbine has been investigated with consideration of performance improvements and the specific demands on integration of this application.