Modelling and Evaluation of the Transient Behaviour of Evaporators in Aircraft Engine Preliminary Design

Kurzfassung

New concepts for aero engines are investigated in the aviation industry for reaching carbon neutral climate goals and finding cost-efficient solutions. The Water-Enhanced Turbofan (WET) concept is one of the proposed solutions for increasing the overall efficiency of an aircraft engine, decreasing contrail formation and reducing nitrogen oxide emission. Heat exchangers, like evaporators, must be installed in aero engines to realise the WET concept or other promising concepts, such as hydrogen combustion. Space and mass constraints in aero-engines compel designers to test complex geometries, and at a preliminary design level, results of such evaporator configurations should be quickly attained. Since evaporators in an aircraft engine operate in dynamic conditions, a design tool is needed to examine the transient behaviour of evaporators to understand possible operational problems during the preliminary design level. There is no such model to quickly predict the transient response on a preliminary design level for complex geometries and multiple parallel changes in the inlet boundary conditions. Therefore, in the scope of this thesis, a transient model is developed and implemented in the design tool to analyse the dynamic response of evaporators in an aero-engine environment. The developed method considers the effect of the thermal inertia of the heat exchanger wall on the transient evaluation of temperature profiles and uses the residence time information while calculating fluid properties. For a two-fluid evaporator, the transient model can handle a simultaneous change of the inlet boundary conditions such as temperature, pressure, mass flow rate, water-to-air ratio, and fuel-to-air ratio. The computational cost of the implemented code is significantly low and accurate enough for a preliminary design. The transient model is validated for a single-phase heat exchanger with a numerical benchmark in a concentric, double-pipe counterflow geometry. Three validation cases are demonstrated, including simultaneous changes in inlet temperatures and mass flow rates on both fluid sides. Temperature results from the developed model align well with the numerical benchmark model in all three cases. Then, the transient model is used to evaluate temperature propagation in a spiral-shaped crossflow evaporator, which might be integrated as a heat recovery steam generator in the WET concept. Two transient cases are investigated. First, a start-up scenario is simulated, and the inlet boundary conditions of the evaporator vary from the switch-off operating point to the idle operating point. Second, a go-around scenario is simulated, and the inlet boundary conditions of the evaporator vary from the approach operating point to the end-of-field operating point. Compared to the start-up scenario, the evaporator reaches its steady state much faster in the go-around case. In the start-up scenario, the temperature profiles of the transient model converge perfectly to the final operating point temperatures. However, in the go-around scenario, the temperature profiles of the transient model show a slight deviation from the final operating point temperatures at the end of the transient simulation. Another investigation is conducted to determine the effect of ramp time on the dynamic response. Increasing the ramp time causes a delay in reaching a steady state. However, the effect is not directly linear. In the start-up case, increasing the ramp time by 40 seconds causes a delay of 27 seconds in reaching a steady state. In the go-around case, increasing the ramp time by 19 seconds causes a delay of 8 seconds in reaching a steady state. This work forms a basis for future research on the dynamic characteristics of the aero-engine heat exchangers.