Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - START (inveSTigation of an ultrA compact Reverse flow combusTor)

Teaser

The main objective of START project is to carry out a series of advanced investigations on a prototypical reverse flow, ultra-compact, combustor designed and manufactured by GE-Avio for turboprop engine as a part of the SAT ITD MAESTRO. The aim is to support the validation of...

Summary

The main objective of START project is to carry out a series of advanced investigations on a prototypical reverse flow, ultra-compact, combustor designed and manufactured by GE-Avio for turboprop engine as a part of the SAT ITD MAESTRO. The aim is to support the validation of the developed technologies and design rules by means of full annular combustion tests and high fidelity numerical simulations.
Targeted objectives of START can be summarized as follows:
• Verify a full additive combustor at real engine conditions in terms of combustor performance, by the measurement of emissions, gas exit temperature and liner metal temperature, through extensive full annular tests. Data will also permit validation of numerical modelling results.
• Improve the knowledge of combustor metal temperature and validation of aero-thermal predictions by gathering 2D temperature maps using InfraRed techniques across dedicated optical access on the full annular rig.
• Improve and further validate existent aero-thermal CFD modelling based on a two-step approach: RANS based CHT calculations for metal temperature and flow split predictions and LES (or Hybrid RANS-LES) calculations of the flame domain for combustor performance evaluation.
• Development of an innovative CFD approach based on unsteady CHT based on Hybrid RANS-LES, to allow direct calculation of aero-thermal and combustion performance behavior of the combustor. The methodology will also exploit and further validate dedicated strategy to model multi-hole liners.

Work performed

Most part of the work carried out in the 1st period of the START project deals with the development and assessment of a multiphysics tool, named U-THERM3D, for high-fidelity predictions of metal temperature using the code ANSYS Fluent in the context of Scale Resolving Simulations. Such activity is in charge to UNIFI. Due to a delay in the design and manufacturing process of the additive combustor by the Topic Manager (AvioAero), the activity planned for WP2 and related to the full annular test at real scale conditions of the combustor, have been delayed and postponed to the 2nd period.
In the past years, a 3D coupled approach for the thermal design of combustor liners, called THERM3D, was developed by UNIFI and has been used as reference and starting point for the development of the new U-THERM3D approach. The basic idea behind U-THERM3D procedure is a desynchronisation of time steps in the solution of the involved phenomena, that can be summarized in convection (including several sub-phenomena as combustion, spray evolution etc.), conduction in the solid and radiation. Each of them is solved in a dedicated simulation, running with a parallel coupling strategy. As in reported in similar studies in literature (see for instance Duchaine et al., 2009), instantaneous values are exchanged at the coupling iteration and consist of surface quantities for the solid-fluid and solid-radiation interactions whereas volume quantities for the fluid-radiation coupling. The procedure used in U-THERM3D is depicted in Figure 1.
The CFD and conduction solvers advance in time with their own time-step. As far as radiation is concerned a steady solver is exploited because of the extremely small time scales. Convective and radiative wall heat fluxes are manipulated before sending them to the conduction solver.
In addition to the three solvers, a dedicated tool for the calculation of effusion holes is coupled, relying on the imprinted technique. The orifices are replaced by inlet/outlet boundaries as sketched in Figure 2.
Before testing the new methodology on the START full additive combustor, it was decided to prove and assess the U-THERM3D tool on the AvioAero LEMCOTEC combustor.
The test case consists in the prototype of a single annular combustor developed in the context of the LEMCOTEC European project. Some details about combustor and injection system geometry are reported in Figure 3. Experimental results and steady state numerical modelling of such test case have been already published by UNIFI and AvioAero as part of the dissemination of LEMCOTEC project (see papers D. Bertini et al., «Numerical and experimental investigation on an effusion-cooled lean burn aeronautical combustor: aerothermal field and metal temperature», in Proceedings of ASME Turbo Expo, 2018 and L. Mazzei et al., «Numerical and experimental investigation on an effusion-cooled lean burn aeronautical combustor: aerothermal field and emissions», in Proceedings of ASME Turbo Expo, 2018.).
An example of the results obtained by the investigation of an Approach operating condition are reported in Figures 4 an5. In particular Figure 4 shows two contours of instantaneous (left) and time averaged (right) gas temperature in a meridian plane of the combustor. Figure 5 is showing a comparison of measured metal temperature along combustor centerline on inner and outer liner (with thermocouples), compared with numerical results obtained with steady and unsteady calculations.

Final results

The expected impact of the results produced by START project can be found in the general outcomes of the SAT initiative of the Clean Sky 2 programme where the program MAESTRO (WP8) is funded. The goal is to improve and strengthen the European aeronautics industry in the field of small aircraft (up to 19 passengers). This branch of aircraft industry is particularly relevant in the New Member States, counting more than 40 companies (mainly SME) and several universities and research centers. The support to SAT sector of the aeronautics will have positive impacts on the following societal aspects:
• Improving an environmental friendly multi-modal transport system at European level
• Improving more safe, more reliable and more efficient small aircraft operations
• Reducing the environmental impact from small aircraft in terms of noise and pollutant emissions
• Improving of the supply chain for reducing costs and time to market
MAESTRO goals concerning the thermal efficiency of turboprop engine aim at reducing SFC of -11% with respect to a 2014 reference engine: the achievement of this objective will certainly require an increase of compressor pressure ratio and turbine entry temperature. The final outcome will be a set of TRL6 technologies that could also potentially used in other applications. Besides the reduction of SFC, MAESTRO is also working at significantly reducing the total operating cost of the engine (up to -10%). Combined achievement of SFC and costs targets is pushing towards the introduction of Additive Manufacturing (AM) for some parts of the engine. AM may allow to draw out an optimization of shapes and weight together with the opportunity to explore, for instance, non-conventional solutions for cooling geometries and secondary flow paths in general.
The innovation and knowledge improvement brought by START project in the development of technologies for a mature and reliable application of additive manufactured hot gas parts, will greatly impact on the competitiveness of European aeronautic industry, especially in the sector of small aircraft, where reduction of size, weight and costs is a cutting-edge aspect. This will not only impact on the growth in the specific market sector, up to now dominated by US and Canada, but will give a positive incentive to the development of small aircraft traffic at European level, which is a key contribution of the multi-modal transport target of Clean Sky 2.