Session: 04-13 Combustor Design I

Paper Number: 151634

Design by Large-Eddy Simulations of a Rich Burn – Quick Mix – Lean Burn Lab-Scale Combustion Chamber Operating in High-Pressure Conditions 

The Rich burn - Quick mix - Lean burn (RQL) concept is a staged combustion process that exhibits a fuel-rich primary region to ensure a stable flame in all operating conditions. This rich flame which promotes soot, Unburned HydroCarbons (UHCs) and CO formation is then quickly fed by an injection of a large quantity of air in order to decrease the flame temperature and dilute the burnt gases to finally generate a lean combustion providing an oxidation of soot and a lower temperature that reduces the NOx production. The aim of this article is therefore to detail the design of an optimized RQL combustion module equiped with a new-generation fuel injector from the French engine manufacturer Safran Helicopter Engines and intended to study soot oxidation and NOx reduction processes under high-pressure conditions (up to 14 bar) by simultaneously invistigating the Rich, Quick-mix and Lean regions using various laser-based diagnostics. The results will serve as a reference to validate soot oxidation and NOx modelling in realistic operating conditions. The design of the RQL module is first ensured by performing LES simulations of the reactive flow produced by a monophasic kerosene/air mixture with the AVBP solver integrating the Globally Reduced Chemistry mechanism 2S-KERO-BFER and the Dynamically Thickened Flame combustion model. Various geometric configurations of this module were tested at the nominal regime to gradually improve its performance while seeking a suitable solution optimizing the following flow, mechanical and thermal constraints: mixing and dilution efficiency, imposed distribution of mass flow rates / pressure drop / equivalent ratio, manufacturability, as well as walls and optical windows thermal resistance. As a result, the principal versions of the module are first presented to show the progress made to well separate the RQL regions of the combustor while preserving the ability to perform a detailed analysis by advanced laser diagnostics. A final two-phase LES simulation with liquid fuel kerosene incorporating an Analytically Reduced Chemistry mechanism for NOx formation is performed to guarantee the best geometry of the RQL module. The adopted design consists of the incorporation of upper and lower dilution channels in which preheated and compressed air flow coming from the plenum is circulated and then injected into the combustion chamber. The height of the combustor is comparable to those of current chambers while the dilution air tubes are mainly oriented towards the central area of the reactive flow where measurements will be performed. The laser sheets are introduced through an optical window located in the upper inter-channel space and directed towards the lower channel where the central dilution hole is slightly shifted downwards with respect to the other dilution holes to minimize light interferences in the measurement plane. This arrangement will enable an optical study of the scalar parameters governing the reactive flow, as well as the formation and oxidation of various polluants. A second air flow circulating in the dilution channels will produce impinging jets designed to cool the walls in contact with the flame area. This air flow will be extracted through small holes designed to cool the upper and side optical windows, as well as through slots and channels oriented towards the combustor exit. Another part of the air flow issued from the plenum is used to cool the wall supporting the injection system by means of impinging jets. It is then evacuated into the combustion chamber through slits serving to cool part of the walls of the dilution channels and the optical windows. A detailed analysis of the LES results reveals a significant dependence of mixing and dilution efficiency on the location, size and inclination of the dilution tubes. The optimisation of these parameters enabled the production of a well-organised rich swirled flame, the improvement of mixing and dilution quality and the development of a lean combustion after mixing by the dilution jets, while respecting the constraints imposed in the design of a combustion chamber representative of a helicopter engine combustor.

Presenting Author: Afaf Karrouk CORIA - CNRS UMR 6614

Presenting Author Biography: I am a second-year PhD student at the CORIA laboratory in Rouen, France. I am currently working with Safran Helicopter Engines on the Rich burn - Quick mix - Lean burn (RQL) combustion technology.

Authors:

Afaf Karrouk CORIA - CNRS UMR 6614
Benjamin Quevreux CORIA - CNRS UMR 6614
Clément Brunet SAFRAN HELICOPTER ENGINES
Stéphane Richard SAFRAN HELICOPTER ENGINES
Gilles Cabot CORIA - CNRS UMR 6614
Frédéric Grisch CORIA - CNRS UMR 6614