Z. Vlahostergios

Modelling Operation of System of Recuperative Heat Exchangers for Aero Engine with Combined Use of Porosity Model and Thermo-Mechanical Model

Abstract The present work describes an effort to model the operation of a system of recuperative heat exchangers of an aero engine for real engine operating conditions. The modelling was performed with the combined use of a porous medium model and a thermo mechanical model. The porous medium model was taking into account the heat transfer and pressure loss behaviour of the heat exchangers while the thermo mechanical one was used for the calculation of the wall temperature distribution of the elliptic tubes of the heat exchangers. As it is presented, the combined use of these models can provide a useful tool which can help in the prediction of the macroscopic behaviour of the system of recuperative heat exchangers of the aero engine which can be used for optimization purposes and numerical studies.

Best Strategies for the Development of a Holistic Porosity Model of a Heat Exchanger for Aero Engine Applications

In an attempt to manage CFD computations in aero engine heat exchanger design, this work presents the best strategies and the methodology used to develop a holistic porosity model, describing the heat transfer and pressure drop behavior of a complex profiled tubular heat exchanger for aero engine applications. Due to the complexity of the profile tube heat exchanger geometry and the very large number of tubes, detailed CFD computations require very high CPU and memory resources. For this reason the complex heat exchanger geometry is replaced in the CFD computations by a simpler porous medium geometry with predefined pressure loss and heat transfer.The present work presents a strategy for developing a holistic porosity model adapted for heat exchangers, which is capable to describe their macroscopic heat transfer and pressure loss average performance. For the derivation of the appropriate pressure loss and heat transfer correlations, CFD computations and experimental measurements are combined. The developed porosity model is taking into consideration both streams of the heat exchanger (hot and cold side) in order to accurately calculate the inner and outer pressure losses, in relation to the achieved heat transfer and in conjunction with the selected heat exchanger geometry, weight and operational parameters. For the same heat exchanger, RAM and CPU requirement reductions were demonstrated for a characteristic flow passage of the heat exchanger, as the porosity model required more than 80 times less computational points than the detailed CFD model. The proposed porosity model can be adapted for recuperation systems with varying heat exchanger designs having different core arrangements and tubes sizes and configurations, providing an efficient tool for the optimization of the heat exchangers design and leading to an increase of the overall aero engine performance.Copyright

Conceptual design study of a geared turbofan and an open rotor aero engine with intercooled recuperated core

The development of more efficient aero engines is becoming a matter of great importance due to the need for pollutant emissions reduction (e.g. CO2, NOx). Toward this direction, two of the most promising aero engine architectures that have been proposed are the ultrahigh by-pass geared turbofan and the open rotor configurations, both of which are targeting the low thrust-specific fuel consumption and reduced NOx production. In the current study, investigations are performed in order to determine the improvements in thrust-specific fuel consumption for these configurations. More specifically, on the basic geared turbofan and open rotor configurations an intercooler and a recuperator are implemented between the compressors and the exhaust nozzle, respectively. The intercooler is installed in order to reduce the high pressure compressor work demand, while the recuperator is used in order to preheat the compressor discharge air by exploiting the otherwise wasted increased enthalpy content of the exhaust hot gas. The recuperator consists of elliptically profiled tubes and its design is based on the innovative tubular heat exchanger core arrangement that has been invented and developed by MTU Aero engines AG. The intercooled recuperative geared turbofan is evaluated against a nonintercooled recuperative geared turbofan, while the intercooled recuperative open rotor is evaluated against a nonintercooled recuperative open rotor. The results showed that the implementation of intercoolers and recuperators can further improve specific fuel consumption and can also lead to NOx emission reduction.

Development of surrogate models for the prediction of the flow around an aircraft propeller

ABSTRACT In the present work, the derivation of two surrogate models (SMs) for modelling the flow around a propeller for small aircrafts is presented. Both methodologies use derived functions based on computations with the detailed propeller geometry. The computations were performed using k–ω shear stress transport for modelling turbulence. In the SMs, the modelling of the propeller was performed in a computational domain of disk-like geometry, where source terms were introduced in the momentum equations. In the first SM, the source terms were polynomial functions of swirl and thrust, mainly related to the propeller radius. In the second SM, regression analysis was used to correlate the source terms with the velocity distribution through the propeller. The proposed SMs achieved faster convergence, in relation to the detail model, by providing also results closer to the available operational data. The regression-based model was the most accurate and required less computational time for convergence.

Efforts to improve aero engine performance through the optimal design of heat recuperation systems targeting fuel consumption and pollutant emissions reduction

The present work is focused on the conceptual development and numerical assessment of various new heat recuperation system configurations, specifically designed and optimized for a state-of-the-art turbofan application developed by MTU Aero Engines AG. The optimization efforts were performed through CFD computations, experimental measurements and aero engine thermodynamic cycle analysis. A critical part of the optimization phase was conducted using a customizable numerical tool modelling the recuperation system heat transfer and pressure loss characteristics and including the effect of important heat exchanger design decisions. The numerical tool was based on an advanced porosity model approach in which the heat exchangers macroscopic behaviour was included through the integration of predefined heat transfer and pressure loss correlations, calibrated through CFD computations and experimental measurements. The optimization led to two new recuperation configurations with significant improvements regarding the aero engine fuel consumption and pollutant emissions reduction providing direct environmental and economic benefits.

Thermodynamics Cycle Analysis, Pressure Loss and Heat Transfer Assessment of a Recuperative System for Aero Engines

Aiming in the direction of designing more efficient aero engines, various concepts have been developed in recent years, among which is the concept of an intercooled and recuperative aero engine. Particularly in the area of recuperation, MTU Aero Engines has been driving research activities in the last decade. This concept is based on the use of a system of heat exchangers mounted inside the hot-gas exhaust nozzle (recuperator). Through the operation of the system of heat exchangers, the heat from the exhaust gas, downstream the LP turbine of the jet engine is driven back to the combustion chamber. Thus, the preheated air enters the engine combustion chamber with increased enthalpy, providing improved combustion and by consequence, increased fuel economy and low-level emissions. If additionally an intercooler is placed between the compressor stages of the aero engine, the compressed air is then cooled by the intercooler thus, less compression work is required to reach the compressor target pressure.In this paper an overall assessment of the system is presented with particular focus on the recuperative system and the heat exchangers mounted into the aero engine’s exhaust nozzle. The herein presented results were based on the combined use of CFD computations, experimental measurements and thermodynamic cycle analysis. They focus on the effects of total pressure losses and heat exchanger efficiency on the aero engine performance especially the engine’s overall efficiency and the specific fuel consumption. More specifically, two different hot-gas exhaust nozzle configurations incorporating modifications in the system of heat exchangers are examined. The results show that significant improvements can be achieved in overall efficiency and specific fuel consumption hence contributing into the reduction of CO2 and NOx emissions.The design of a more sophisticated recuperation system can lead to further improvements in the aero engine efficiency in the reduction of fuel consumption.This work is part of the European funded research program LEMCOTEC (Low Emissions Core engine Technologies).Copyright

Modeling Separation: Induced Transition Using a Non-Linear Three Equation Turbulence Model and a Reynolds Stress Turbulence Model

This paper presents an effort to model separation-induced transition on a flat plate with a semi-circular leading edge, by using two advanced turbulence models, the three equation non-linear model k-e-A2 of Craft et al. [16] and the Reynolds-stress model of Craft [13]. The mechanism of the transition is governed by the different inlet velocity and turbulence intensity conditions, which lead to different recirculation bubbles and different transition onset points for each case. The use of advanced turbulence models in predicting the development of transitional flows has shown, in past studies, good perspectives. The k-e-A2 model uses an additional transport equation for the A2 Reynolds stress invariant and it is an improvement of Craft et al. [12] non-linear eddy viscosity model. The use of the third transport equation gives improved results in the prediction of the longitudinal Reynolds stress distributions and especially, in flows where transitional phenomena may occur. Although this model is a pure eddy-viscosity model, it borrows many aspects from the more complex Reynolds-stress models. On the other hand, the use of an advanced Reynolds-stress turbulence model, such as the one of Craft [13], can predict many complex flows and there are indications that it can be applied to transitional flows also, since the crucial terms of Reynolds stress generation are computed exactly and normal stress anisotropy is resolved. The model of Craft [13], overcomes the drawbacks of the common used Reynolds-stress models regarding the computation of wall-normal distances and vectors in order to account for wall proximity effects. Instead of these quantities, it employs “normalized turbulence lengthscale gradients” which give the ability to identify the presence of strong inhomogeneity in a flow development, in an easier way. The final results of both turbulence models showed acceptable agreement with the experimental data. In this work it is shown that there is a good potential to model separation-induced transitional flows, with advanced turbulence modeling without any additional use of ad-hoc modifications or additional equations, based on various transition models.Copyright