AGT100 COMBUSTOR BASIC INFORMATION AND TUTORIALS

What is Allison AGT100 Combustor?

The main features of this combustor, shown schematically in have been described by Rizk and Mongia. It comprises a prechamber in which the fuel is vaporized and mixed with air, a pilot and ignition chamber, and the main cylindrical chamber.

Variable geometry is employed to control the stoichiometry in the primary zone. The prechamber contains a centerbody that houses both the main fuel injector and a pilot nozzle, which is employed only for lightup and acceleration to engine idle speed.

The main fuel is introduced from a manifold surrounding the prechamber, just downstream of the prechamber axial swirler. Uniform filming of the fuel is achieved by spraying it through eight tangential holes onto the etched surface of the prechamber.

The swirling air assists in the prefilming process. The high temperatures of the inlet air and the prechamber walls combine to promote rapid vaporization of the fuel within the prechamber.

At power modes higher than idle, additional air is admitted into the prechamber through a radial swirler to merge and mix with the air flowing through the axial swirler.

Engine lightup is initiated in a small pilot chamber located on the side of the main combustion chamber. This piloting device also serves as a sustainer source when the combustor is operating at low inlet air temperatures or at conditions that lie outside the normal lean blowout limits.

The swirling vaporized fuel–air mixture flows into the main chamber through a round opening in the center of the dome. At high-power settings, additional air is injected into the main chamber through eight holes that are drilled in a manner designed to impart a swirling motion to the flowing air.

Four simple rectangular dilution holes were chosen to ease fabrication of the ceramic liner. Variable geometry, in the form of sliding bands, is used to vary and control the flow areas of the dilution holes and the radial swirler in the prechamber.

At low-power modes, most of the air flows through the dilution holes. As the fuel flow rate is increased above idle, the variable geometry is moved to increase the airflow through the radial swirler and to reduce, by a corresponding amount, the airflow through the dilution holes.

The use of variable geometry enabled the AGT100 combustor to meet the program goals of 5.0 and 37 g/kg fuel for NOx and CO, respectively. Moreover, the experimental data acquired in the course of this investigation was used by Rizk and Mongia to develop a model for calculating NOx formation in
LPP combustors.

This model takes into account the effects of pressure, residence time, and air distribution between different combustion zones. It also provides useful insight into the contribution of the pilot chamber to the total NOx emissions.

GE (GENERAL ELECTRIC) LM6000 COMBUSTOR BASIC INFORMATION AND TUTORIALS

What is General Electric LM6000 Combustor?

Another important aeroderivative gas turbine is General Electric’s LM6000. This premix combustor employs about twice the volume of the conventional annular combustor it replaces in order to maintain low levels of CO and UHC while greatly reducing the emissions of NOx. Part of the air used in combustion, which at maximum power is around 80% of the total combustor airflow, flows into the combustion zone through three annular rings of premixers.

The two outer rings each have 30 fuel–air premixers, whereas the inner ring has 15. This arrangement of premixers facilitates fuel staging at part-load operation. The total of 75 fuel nozzles is formed by having 15 stems with three premixers on each stem, plus 15 stems with two premixers on each stem.

Each stem incorporates two or three separate fuel circuits for independently fueling the premixers. A short annular liner was selected to minimize the amount of air needed for wall cooling. Only backside cooling is used, so a thermal barrier coating is applied to both the liner and in the dome area to keep the metal temperatures within acceptable limits.

The use of a multipass diffuser also permits further reduction in overall combustor length. Of special importance to the attainment of low emissions is the design of the premixers. The double annular counter rotating swirler (DACRS) was conceived to satisfy the restraints of autoignition and size. The duct diameter is reduced toward the exit in order to create an accelerating flow, thereby preventing flashback.

The conical centerbody located along the centerline of the premixer can be used to supply liquid fuel to an atomizer at its tip, and gas passages for diffusion burning at low-power conditions. The objective with this type of mixing device is to produce a completely homogeneous mixture of fuel and air at the premixer exit.

As the total area of the fuel-injection holes is fixed by the flow rate and the available fuel njection pressure, the design procedure is essentially one of finding the best compromise between the desire for small injection holes to give a large number of fuel-injection points, and the equally important requirement of large injection holes to allow the fuel jets to penetrate across the airstream.

A big advantage of the premixer module concept is that, once developed, it has broad applications to a wide range of combustor sizes and configurations, as discussed above in connection with the ABB-EV burner.

The basic module remains the same regardless of combustor size; only the number and
arrangement varies. Thus, according to Joshi et al,, the DACRS II and DACRS III mixers could be applied to a range of GE engines, including the LM1600, LM2500, and LM6000, because single digit NOx emissions have been attained with both these mixers at test conditions encompassing the operating ranges of these engines.

COMPRESSOR SURGE BASIC INFORMATION AND TUTORIALS

What is Compression Surge?

Compressor surge is a phenomenon of considerable interest; yet it is not fully understood. It is a form of unstable operation and should be avoided. It is a phenomenon that, unfortunately, occurs frequently, sometimes with damaging results. Surge has been traditionally defined as the lower limit of stable operation in a compressor, and it involves the reversal of flow.

This reversal of flow occurs because of some kind of aerodynamic instability within the system. Usually, a part of the compressor is the cause of the aerodynamic instability, although it is possible for the system arrangement to be capable of augmenting this instability.

Compressors usually are operated at a working line, separated by some safety margin from the surge line. Extensive investigations have been conducted on surge.

Poor quantitative universality or aerodynamic loading capacities of different blades and stators, and an inexact knowledge of boundary-layer behavior make the exact prediction of flow in the compressor at the off-design stage
difficult.

A decrease in the mass flow rate, an increase in the rotational speed of the impeller, or both can cause the compressor to surge. Whether surge is caused by a decrease in flow velocity or an increase in rotational speeds, the blades or the stators can stall.

Note that operating at higher efficiency implies operation closer to surge. It should be noted here that total pressure increases occur only in the rotational part of the compressor, the blades.

The surge line slope on multistage compressors can range from a simple single parabolic relationship to a complex curve containing several break-points or even "notches." The complexity of the surge line shape depends on whether or not the flow limiting stage changes with operating speed from one compression stage to another; in particular, very closely matched stage combinations frequently exhibit complex surge lines. In the case of compressors with variable inlet guide vanes, the surge line tends to bend more at higher flows than with units that are speed controlled.

Usually surge is linked with excessive vibration and an audible sound; yet, there have been cases where surge not accompanied by audible sound has caused failures. Usually, operation in surge and, often, near surge is accompanied by several indications, including general and pulsating noise level increases, axial shaft position changes, discharge temperature excursions, compressor differential
pressure fluctuations, and lateral vibration amplitude increases.

Frequently, with high-pressure compressors, operation in the incipient surge range is accompanied by the emergence of a low frequency, asynchronous vibration signal that can reach predominant amplitudes, as well as excitation of various harmonics of blade passing frequencies. Extended operation in surge causes thrust and journal beating failures.

Failures of blades and stators are also experienced due to axial movement of the shaft causing contact of blades and stators. Due to the large flow instabilities experienced, severe aerodynamic stimulation at one of the blade natural response frequencies is caused, leading to blade failure.

JUMO 004 (GERMANY) HISTORICAL ENGINE TURBINE MACHINE

This engine is of great historical interest because it was the world’s first mass produced turbojet and one that saw extensive service in World War II. It was among the first engines to employ axial flow turbo machinery and straight through combustors.

Each of the six tubular combustors was supplied with fuel at pressures up to 5.2 MPa (750 psi) from a pressure-swirl atomizer, which sprayed the fuel upstream into the primary combustion zone. The primary air flowed into the liner through six swirl vanes, the amount of air being sufficient to achieve near-stoichiometric combustion at the engine design point.

Mixing between combustion products and dilution air was achieved using an assembly of stub pipes that were welded to a ring at their upstream end and to the outer perimeter of a 10-cm diameter dished baffle at their downstream end. The hot combustion products flowed radially outward through the gaps between the stub pipes to meet and mix with part of the cold secondary air.

The remaining secondary air flowed through the stub pipes, incidentally serving to protect them from burnout because of their immersion in the hot combustion gases, to provide further mixing of hot and cold gases in the recirculation zone created by the presence of the baffle.