Session: 01-07 Thermal Management Systems and Heat Exchangers

Paper Number: 153194

Aero-Thermo Analysis of a Waste Heat Recovery Heat Exchanger 

The aero engine industry, as all other industry, is tasked with reducing environmental impact, becoming more sustainable and transition into carbon-neutral solutions. Propulsion without greenhouse gases and contrails are however some years ahead of us, and in anticipation of that, multiple research programs has been launched.
The EU, Clean Aviation, is such a programme, within EU’s research and innovation programme Horizon Europe.
Part of the programme is the project SWITCH which utilize the WET (Water Enhanced Turbofan) architecture for environmental solutions for the “ultra-efficient / short medium range” aircraft.

Ideal thermal efficiency of gas turbines, as defined by the Brayton cycle, is a key metric in evaluating performance.
Modern aero-engines operate at high pressure ratios, with a theoretical thermal efficiencies of well above 60%, however, practical efficiencies remain sub-40%, due to thermal and mechanical losses, and unrecoverable exhaust heat.
To further improve the efficiency, and thereby meeting the sustainability targets, a new approach is required. Studies of alternatives cycles, e.g. integrating the Bryton & Rankine cycle is such an approach. The employment of a waste heat recovery system will substantially increase efficiency and thereby reduced CO2 footprint and NOx emissions.

This paper focuses on the design and analysis of a tubular HEX located within the exhaust stream of a medium sized geared turbofan. The chosen concept consists of a multitude of tubes, in a cylindrical shaped HEX with radial cross-flow to maximize heat transfer for a marginal weight penalty. The optimization of HEX represents a significant engineering challenges in aviation, as any increase in system weight and volume directly impacts system efficiency. The primary side (water/steam) is set in counter-flow condition while the exhaust gases is turned 90 degree to achieve the cross-flow condition.

Although the selected HEX design resembles a multiple-pass shell-n’-tube steam generator, the turning and radial flowing exhaust gases makes any simplistic analyze approach futile. Traditional methods of assessing HEX performance, such as Log Mean Temperature Difference (LMTD) and ϵ-NTU, rely on idealized conditions and consequently standard correlations cannot be used to access  heat transfer and pressure drop.

Instead this study employs detailed 3D-CFD simulations using ANSYS/Fluent to model the exhaust gas flow around the HEX tubes, incorporating temperature-dependent fluid properties and transient flow models. The water/steam flow inside the tubes is modeled through a heuristic 1D thermally coupled Bernoulli equation, utilizing correlations for heat transfer and pressure loss.

This study explores the impact of non-ideal gas distribution on HEX performance, focusing on pressure losses and heat transfer degradation due to transient flow variation and mass-flow mal-distribution. Experimental data based on a full-scale rig are used to validate the predicted data and models used. The rig is operating in reverse condition, i.e. cold flow around heated tubes, however it is argued that the validated CFD models can be used with accuracy at elevated temperatures using the advanced flow models, and temperature dependent fluid properties. The primarily results from the study is to quantifying knock-down factors on performance, heat transfer rate, and pressure loss, due to the non-ideal flow condition. Modification to standard correlations for tube matrixes performance is proposed.

Some interesting results are; i) the overall heat transfer performance is marginally penalized due to the uneven flow across the HEX, ii) although the large-scale flow oscillation* in the distributing channel  the condition through the tube matrix is markedly stable and can be estimated with the modified standard correlation.  

*) Advanced and transient accurate simulation; e.g. hybrid RANS/LES and V-LES has been used to address these conditions.

Future work will further examine the coupled interactions between the primary and secondary sides through e.g. using transient phase-change multiphase simulation to characterize and resolve boiling phenomena’s, including possible instabilities.

Presenting Author: Jonas Bredberg GKN Aerospace Sweden AB

Presenting Author Biography: Jonas holds a Ph.D. in turbulence and heat transfer modelling and has 25 years of experience using various CFD tools

Authors:

Isak Jonsson Chalmers University of Technology
Jonas Bredberg GKN Aerospace Sweden AB
Valentin Vikhorev Chalmers University of Technology
Yasser Alrifai GKN Aerospace Sweden AB
Jonathan Bergh GKN Aerospace Sweden AB