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Japan Aerospace Exploration Agency

Advanced combustion technologies for small, high-output core engines

Technological development needs to conform to the ever-tightening environmental standards on aircraft engines. Reducing fuel consumption through high bypass ratio engines means using a larger fan driven by a smaller core engine. A small, high-output core can be achieved by increasing the combustor's pressure and temperature. To do that, it is necessary to develop low-NOx (nitrogen oxide) technology that is viable in high-pressure, high-temperature conditions.

JAXA has researched combustors that use "lean-premixed combustion," an effective approach to cutting down on NOx emissions. By making sure that fuel and air mix at an optimal ratio, lean-premixed combustion can help limit NOx emissions to a low level. However, lean-premixed combustors are prone to significant instability-related issues like oscillating combustion (strong pressure oscillation that resonates inside the combustion chamber) and flashback (a phenomenon in which flame travels back into the mixing chamber). Achieving stable combustion is an essential element for achieving low-NOx combustion. JAXA will take a complementary, three-pronged approach that combines advanced combustion technologies, combustion stabilization technologies, and combustion measurement technologies to solve these problems and establish research results as basic technologies for next-generation jet engines.

Using its findings, JAXA also plans to develop the technologies for application to higher-pressure conditions (around 40 atm), multi-sector annular combustors, and other combustors produced by engine manufacturers, including stationary gas turbine combustors.

Advanced combustion technologies: Using fluidic devices to control the airflow distribution of an airblast fuel nozzle

Lean-premixed combustion is one promising approach to reducing NOx emissions from aircraft jet engines and other gas turbines, but premixed combustion has a smaller equivalence ratio range than diffusion combustion and can lead to problems like reduced combustion efficiency and oscillating combustion. In hopes of achieving stable combustion across a wide range of engine operations, JAXA has researched and developed a staged fuel nozzle that combines a pilot burner for ensuring stable diffusion combustion and a main burner for performing lean-premixed, low-NOx combustion. This staged fuel nozzle, which has already gone into practical use in industrial gas turbines, adjusts the flow distribution of the fuel supply to the burners based on load conditions and thereby achieves both low-NOx performance and combustion stability. In industrial gas turbines, in addition to controlling fuel flow distribution, controlling the airflow distribution helps to keep the air-fuel ratio in the combustion zone within the appropriate range. Airflow distribution is controlled by establishing a bypass channel in the combustor's transition piece or airflow-limiting mechanisms in some of the burners. However, these types of airflow-control mechanisms have never been implemented in aircraft jet engine combustors.

An airflow-control mechanism needs to be placed in the high-temperature, high-pressure conditions of a combustion air channel. Consequently, unlike fuel flow-control mechanisms which can be installed in outside air channels, airflow-control mechanisms must function in harsh environments. To control airflow in an aircraft jet engine combustor, the control mechanism has to be operational under these demanding conditions. The control mechanism must also be reliable, compact, and lightweight. With these points in mind, fluidic devices are being tested as control mechanisms. Fluidic devices have simple structures without any mechanical moving parts inside the combustion air channel, enabling high reliability in harsh environments.

As it continues to move ahead with research on using fluidic devices to control airflow inside airblast fuel nozzles, the Propulsion Systems Research Group is incorporating water flow test-based visualized flow field measurements (PIV) and atmospheric pressure combustion tests to assess changes in flow fields and combustion conditions.

A concept for using fluidic devices to control
the airflow distribution of an airblast fuel nozzle

Measuring local equivalence ratios using a laser-induced plasma spectroscopy (LIPS) system

Figure 1: Laser-induced plasma spectroscopy (LIPS) system

Figure 2: The optical spectrum of a methane-gas mixture
□: Equivalence ratio = 0.52; –: Equivalence ratio = 0.92

Figure 3: An injector (overhead view, left) and a schematic showing the injector combined with a burner (right)

Figure 4: Radial distribution of local equivalence ratio in the wake flow of a double tube injector
■: 10 mm downstream; □: 35 mm downstream

With the laser-induced plasma spectroscopy (LIPS) method, the elemental composition of substances can be investigated by first using a lens to focus a pulsed laser beam and induce plasma, and then performing a spectral analysis on the light that the resulting plasma creates as it cools. Traditionally, LIPS has often served as a method for the elemental analysis of solid samples, but recent years have seen the method draw attention as an approach to gas composition analysis.

JAXA is currently using the LIPS method in its research on measuring the mixture concentration (equivalence ratios) of fuel and air in aircraft engine combustors. Figure 1 depicts a LIPS measurement system, which is comprised of equipment for generating laser-induced plasma and equipment for receiving light and performing spectroscopy. Figure 2, meanwhile, shows an optical spectrum for a mixture of gases with two different equivalence ratios. The emission wavelengths vary according to element type and energy relaxation process, while a higher equivalence ratio means that the hydrogen atom emissions peak at higher wavelengths. These conditions stem from an increase in the proportion of methane (CH4), the fuel component. By using these properties, it is possible to measure the equivalence ratio distribution inside an engine combustor. One example appears in Figure 3, which lists measurements for the process of mixing ambient airflow with methane fuel injected via a fuel injector (Figure 3) through a double tube slit. The results correspond to the local equivalence ratio distribution levels in the radial direction at two locations downstream from the burner. At the location 10 mm downstream from the burner, the distribution peaks at a radial distance of ±15 mm without any fuel in the center. On the other hand, the distribution at the location 35 mm downstream from the burner shows the presence of fuel in the center and a peak position located further out. These findings suggest that the mixture of fuel and air progresses as the flow develops.

The advantages of the LIPS method are that it helps reduce the size of measurement equipment and makes it relatively easy to configure optical systems. Due to these benefits, the method is poised to serve as a valuable tool in measuring and gathering data from inside jet engines under hard-to-measure conditions marked by high temperature and pressure levels.

Combustion instability detection technology

The operating range of conventional combustors was decided in advance. However, by using measurement values from new combustion instability detection technology to determine the operating range in real-time, it is possible to eliminate margins which had been installed to ensure safety and to enlarge the operating range. This leads to increased performance.

In research up to this point, it has been possible to detect combustion instability one second prior to occurrence. JAXA is working to apply the technology to the detection of combustion instability phenomena which occur for a variety of reasons.