Skip to main content
European Commission logo print header
Contenu archivé le 2022-12-23

Improvement of combustion processes by swirling flows and turbulent recirculation flames

Objectif

- The project is aimed at improving understanding of swirling flows with applications to practical combusting flows. A range of instrumentation based upon laser-light scattering techniques complemented by probe methods has been developed and used to quantify the turbulent nature of swirling jet flows, including those with a dispersed liquid phase, and to extend the knowledge of turbulent transport processes in combustor-related flows. Attention was focused on the atomisation of liquids in swirling jets, with application on the development of low-emission combustion technologies;
- The advanced instrumentation used throughout the work has included the development of a new hot-wire probe and system capable of yielding three instantaneous velocity components at a point in a 3-D flowfield. In addition, the work considered techniques such as flow visualisation, and forms of laser-Doppler and phase-Doppler velocimetry, as well as sampling probes for gas species concentrations;
- In particular, the use of phase-Doppler velocimetry to study two-phase flows with practical relevance was evaluated in a purpose-built rig in terms of the mechanisms of polydispersion in regions of large variations of flow time scales.
- The results obtained in swirling jets issuing into a coflowing stream provide evidence of the lack of flow similarities in near-nozzle regions and are important to assess physical models to be used to extrapolate the experimentally-acquired information. In addition, the atomisation of a liquid film in co-axial swirling jets was studied in detail and showed that the disintegration of the liquid film close to the atomising edge of the nozzle is associated with a periodic process. The importance of the atomisation process in the development of low-emission combustor technologies was assessed in a combusting laboratory environment, based on detailed measurement of pollutant emissions.
- Main innovative aspects of the work performed in the reporting period :
- development of a new hot-wire probe for simultaneous measurements of the three velocity components in swirling jets;
- analysis of the response of a phase-Doppler velocimeter in polydisperse, two phase jet flows;
- detailed analysis of the break-up of an annular liquid sheet downstream of an air-blast atomiser, as a function of the swirling level of the flow and of the liquid film characteristics.
The work programme over the reporting period comprised several tasks, which can be conveniently summarised as follows :

- Development and evaluation of instrumentation
- A new hot-wire probe was developed, together with a data processing system, in order to measure instantaneously these velocity components at a point in a three-dimensional flowfield. The technique was tested in a well-defined turbulent pipe flow and then, applied to the swirling jet flow issuing into a coflowing stream reported previously;
- A dedicated optical system for the visualisation of the processes of liquid atomisation in swirling jets was developed including three basic techniques, namely :
- direct visualisation, making use of laser light sheet illumination;
- laser shadowgraphy;
- white lighting of the flow, with sequential images acquired by a fast CCD camera.
- A phase-Doppler velocimeter was assembled to measure simultaneously the velocity, size and mass flux of the air and particle phases in dispersed jets. The velocimeter was tested in a polydisperse particle laden turbulent jet (Re = 15.000) making use of glass beads with a size distribution centred at 50um and with a standard deviation of 15um. The results quantify the extent to which the system is able to detect the effects of particle polydispersion in flow regions of large variation of the flow time scales.


- Measurement programme
- A turbulent swirling jet issuing into a co-flowing steam was experimentally investigated following the previously reported results. It is shown that the concept of flow similarity does not apply when the distance from the jet exit plane is less than 30 nozzle diameters. In this zone, the effect of swirl on the Reynolds stress is shown to be small;
- The atomisation processes and spray quality typical of the swirling flows found in practical combustors was studied in detail making us of a laboratory model of a prefilming airblast atomiser. The tests were carried out at atmospheric pressure and using water for a liquid film thickness among 0.2 and 0.7mm a liquid mass flow rate up to 11g/s. primary air velocities up to 200m/s and swirl numbers in the range 0 < S <2.5. The results quantify the various atomisation regimes, from the Rayleigh mode up to prompt atomisation, and show that the disintegration of the liquid film close to the atomising edge of the nozzle is associated with a periodic process mainly dependant on the primary air velocity. The Sauter mean diameter of the liquid phase is shown to be independent of the liquid film velocity for coaxial swirling flows, providing that the primary air velocity is kept above 120m/s. The related drop size velocity distribution is shown to contribute to the optimisation of combustion efficiency in practical combustors;
- The efficiency of the concept of lean-premixed-prevaporised, LPP, combustion technology was studied in two laboratory combustors. An axisymmetric combustion chamber was used downstream of a premixing duct, where liquid gasoline was prevaporized and mixed with a swirling air stream. Measurements of pollutant emissions at the exit of the combustion chamber quantify the performance of the technology in terms of the combustion of gaseous propane. In addition, the results were extended through the use of a rectangular sector combustor, which have allowed the analysis of flame interaction , as in annular combustion chambers.

Thème(s)

Data not available

Appel à propositions

Data not available

Régime de financement

CSC - Cost-sharing contracts

Coordinateur

INSTITUTO SUPERIOR TECNICO
Contribution de l’UE
Aucune donnée
Adresse
Avenida Rovisco Pais, Pav. Mecanica 1-2°
1049-001 LISBOA
Portugal

Voir sur la carte

Coût total
Aucune donnée