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Publication Title | A computational study of the influence of the injection characteristics on micro-turbine combustion

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16th Australasian Fluid Mechanics Conference Crown Plaza, Gold Coast, Australia

2-7 December 2007

A computational study of the influence of the injection characteristics on micro-turbine combustion

C.A. Gonzalez1, K.C. Wong1 and S. Armfield1 1School of Aerospace, Mechanical & Mechatronic Engineering

The University of Sydney, AUSTRALIA

Abstract

Micro-turbines have been lately recognized as promising alter- natives for powering unmanned aerial vehicles (UAVs), hybrid transport and small scale electricity generation. Due to their traditional use in military and recreational applications, a good deal of empirical and general data is available but little technical and scientific information about their behaviour, and in particu- lar about their combustion characteristics can be obtained.

Injection is widely recognized as a major controller of the com- bustion process in thermal machines such as diesel engines and gas turbines. In this paper a computational study is undertaken to identify the influence of the injection characteristics on the thermodynamic variables inside a commercial micro-turbine. Large eddy simulation is used for describing the turbulence. Statistical design of experiments is used to evaluate the influ- ence of each factor and their interactions as well as for reduc- ing the amount of simulations. Results indicate that changes in droplet size and injection velocity can improve the conditions at the outlet of the combustor.

Introduction

The micro-turbine industry has grown dramatically in the last decade. This is because new applications for these engines, such as small scale electricity generation, unmanned aerial vehicle propulsion and hybrid transport, have been developed.

Micro-turbines, when compared with the state of the art recip- rocating engines, promise better power to weight ratio, more flexibility, lower emissions and the possibility of flying faster and at higher altitudes. However, their fuel efficiency is still low. This is the main reason why they have not found yet a more widespread use in those areas. A further understanding of the behaviour of the engine can yield improvements in their performance. This is the aim of this work, focusing on the com- bustion chamber.

The combustion process is a key factor in the operation of the turbine. A better understanding of this process can not only benefit the efficiency of the engine at given conditions but also make it possible to expand the area of operation of the engine. Several studies have been undertaken analysing pressure loss [1], implementing lean combustion for low NOx emissions [2], comparing different configurations [3] and analysing the com- bustion process through computational fluid dynamic simula- tions [4, 5].

The injection process plays an important role in the combustion, yet the extent of the influence of the main injection variables, such as droplet diameter, spray diameter and injection veloc- ity on the combustion process of micro turbine engines, using vaporisers at this scale, is still unclear.

This paper focuses on the influence of three variables of the injection process on the combustion inside micro turbines using CFD. The initial droplet diameter, the outlet velocity and the spray angle are studied. All these variables can be modified in a

straight forward manner, for example by changing the injection pressure or changing the injector type using air blast, simplex or plain orifice atomizers. This paper can serve as a guide to determine if the implementation of more advanced atomizers is useful for micro-turbine combustion.

In micro-turbine engines, the optimal condition for a combustor is to have the highest and most homogenious temperature at the outlet with the minimum pressure drop. Given a constant mass flow rate and outlet area, the pressure can be easily related with the velocity at the outlet. In this study, only the velocity and average temperature at the outlet are considered in a high load condition.

In the following section, the methodology of the study is thus described. The details of the turbine and combustor, together with the model and mesh details are presented first. The turbu- lence and combustion models are then described, followed by the boundary and operating conditions and a description of the design of experiments and test plan. Results are presented in terms of the temperature and velocity fields. Finally the conclu- sions are detailed.

Methodology

Turbine and combustor

The case considered here is the KJ66 micro-turbine [7]. This turbine has been created for small aircraft propulsion and is specially designed for easy manufacture. It is readily obtained and there is plenty of empirical information available, making it ideal for this research i.e. [6, 7].

The KJ66 combustor features direct injection of the fuel with six vaporising sticks for achieving complete combustion before the turbine stage. Typical Reynolds numbers for the air inflow at the inlet are around 54,000. A diagram of this component can be seen in Figure 1. This is a 60 degrees section cut, as employed for the model explained in the following paragraphs. The fuel injector has a 0.7 mm diameter nozzle and uses a standard 12V pump.

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Figure 1: 60 degrees section cut of the combustor

Image | A computational study of the influence of the injection characteristics on micro-turbine combustion



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