# Optimization of Heat Exchangers for Intercooled Recuperated Aero Engines

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## Abstract

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## 1. Introduction

_{2}and 80% in NO

_{x}emissions, compared to the year 2000 levels, as mentioned also in [1,2].

## 2. Optimization of Heat Exchanger Concepts

#### 2.1. The Heat Exchanger–Recuperator Porosity Model Approach

#### 2.2. Hot Side Pressure Loss Model

#### 2.3. Cold Side Pressure Loss Model

#### 2.4. Heat Transfer Model

^{2}/m

^{3}), calculated as the ratio of the total outer surface of the heat exchanger tubes to the occupied volume of the heat exchanger, while ${U}_{overall}$ corresponds to the overall heat transfer coefficient and is calculated by Equation (5),

## 3. Results of the Recuperation Concepts

## 4. Discussion

## 5. Conclusions

- The optimization procedure of the reference MTU heat exhcanger was based on the development of a customizable numerical tool which can efficiently model the heat exchanger performance regarding heat transfer enhancement and pressure losses minimization, specially designed for aero engine applications.
- The described numerical tool is based on an advanced porosity model approach where the heat exchanger core is modeled as a porous media of predifined heat transfer and pressure losses behavior. Additionally, the derived porosity model is able to provide accurate results to a wide range of conditions (from laboratory to flight conditions) and to incorporate the major critical recuperator design decisions in the CFD computations, through the coupling of the flow momentum and energy transport equations.
- The optimization efforts resulted in two completely new innovative recuperator design concepts, named as CORN (COnical Recuperative Nozzle) and STARTREC (STraight AnnulaR Thermal RECuperator). These concepts were following an annular tubes recuperator axisymetric core design inside the hot-gas exhaust nozzle.
- The proposed recuperators were assesed in a thermodynamic cycle analysis providing improved SFC and significant benefits in terms of pollutants emission and weight reduction in comparison to the original NEWAC recuperator installation of the IRA engine, thus promoting the fullfillment of ACARE goals towards greener and environmentally friendly aero engines.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## Nomenclature

${a}_{0},{a}_{1}$ | Viscous pressure loss coefficients |

${b}_{0},{b}_{1},{b}_{2}$ | Inertial pressure loss coefficients |

$C,m,n$ | Nusselt number coefficients |

${C}_{1},{C}_{2}$ | Calibration constants for inner pressure losses |

$f$ | Pressure loss per tube length coefficient |

${h}_{outer},{h}_{inner}$ | Outer and inner heat transfer coefficients, W·m^{−2}·K^{−1} |

${h}_{total},{h}_{static}$ | Total and static specific enthalpy, J·kg^{−1} |

$L$ | Length of the heat exchanger, m |

${\dot{m}}_{f}$ | Mass flow of the consumed fuel |

$\overline{Nu}$ | Nusselt number |

${P}_{tot\_inner}$ | Total pressure of the inner flow, Pa |

$Pr$ | Prandtl number |

$Re$ | Reynolds number |

${S}_{exchange}$ | Heat exchange surface per unit volume of the heat exchanger, m^{2}·m^{−3} |

$T$ | Temperature, K |

${T}_{inner}$ | Temperature of the inner flow, K |

$v$ | Kinematic viscosity, m^{2}·s^{−1} |

${U}_{i}$ | Cartesian velocity vector, m·s^{−1} |

${U}_{overall}$ | Overall heat transfer coefficient, W·m^{−2}·K^{−1} |

$\overline{{u}_{i}{u}_{j}}$ | Reynolds stresses, m^{2}·s^{−2} |

${u}_{inner}$ | Inner “cold” air velocity, m·s^{−1} |

${x}_{i}$ | Cartesian coordinates |

$\mathrm{\Delta}P$ | Hot-gas pressure losses, Pa |

$\mu $ | Molecular viscosity, kg·m^{−1}·s^{−1} |

$\rho $ | Fluid density, kg·m^{−3} |

CORN | COnical Recuperative Nozzle |

HEX | Heat EXchanger |

IRA | Intercooled Recuperative Aero-engine |

LEMCOTEC | Low Emissions Core-Engine Technologies |

NEWAC | NEW Aero engine Core concepts |

SFC | Specific Fuel Consumption |

STARTREC | STraight AnnulaR Thermal RECuperator |

ULTIMATE | Ultra Low emission Technology Innovations for Mid-century Aircraft Turbine Engines |

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**Figure 2.**Implementation of pressure loss and heat transfer source terms in the momentum and energy transport equations.

**Figure 3.**(

**a**) NEWAC nozzle configuration; (

**b**) computational grid of the nozzle; (

**c**,

**d**) swirl effects in the NEWAC nozzle, different colors indicate velocity magnitude value configuration (red: high, blue: low).

**Figure 4.**NEWAC nozzle configuration (

**a**) velocity; (

**b**) static temperature non-dimensional distributions.

**Figure 6.**STARTREC (STraight AnnulaR Thermal RECuperator) (

**a**) geometry; (

**b**) flow currents per 90° sector.

**Figure 7.**CORN (

**a**) computational grid (45° sector); (

**b**) static temperature non-dimensional distribution; (

**c**) velocity streamlines.

**Figure 8.**STARTREC (

**a**) computational grid (90° sector); (

**b**) static temperature non-dimensional distribution; (

**c**) velocity streamlines.

**Figure 9.**Thermodynamic cycle in Gas Turb 11 for IRA Engine Average Cruise conditions (

**a**) and schematic illustration of IRA engine basic components (

**b**). The stations numbering is following the standard GasTurb numeration.

Case | SFC Reduction (in Relation to a Conventional Aero Engine) | Heat Exchangers‘ Weight Reduction (in Relation to NEWAC) |
---|---|---|

NEWAC nozzle | 12.3% | 0 |

CORN | 13.1% | ~5% |

STARTREC | 9.1% | ~50% |

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**MDPI and ACS Style**

Misirlis, D.; Vlahostergios, Z.; Flouros, M.; Salpingidou, C.; Donnerhack, S.; Goulas, A.; Yakinthos, K.
Optimization of Heat Exchangers for Intercooled Recuperated Aero Engines. *Aerospace* **2017**, *4*, 14.
https://doi.org/10.3390/aerospace4010014

**AMA Style**

Misirlis D, Vlahostergios Z, Flouros M, Salpingidou C, Donnerhack S, Goulas A, Yakinthos K.
Optimization of Heat Exchangers for Intercooled Recuperated Aero Engines. *Aerospace*. 2017; 4(1):14.
https://doi.org/10.3390/aerospace4010014

**Chicago/Turabian Style**

Misirlis, Dimitrios, Zinon Vlahostergios, Michael Flouros, Christina Salpingidou, Stefan Donnerhack, Apostolos Goulas, and Kyros Yakinthos.
2017. "Optimization of Heat Exchangers for Intercooled Recuperated Aero Engines" *Aerospace* 4, no. 1: 14.
https://doi.org/10.3390/aerospace4010014