Combustion.pdf

Combustors and Combustion Airflow Combustion Section The combustion section is typically located directly between the compressor diffuser and turbine section. All combustion sections contain the same basic elements: One or more combustion chambers (combustors or cans)...

Combustors and Combustion Airflow Combustion Section The combustion section is typically located directly between the compressor diffuser and turbine section. All combustion sections contain the same basic elements: One or more combustion chambers (combustors or cans) A fuel injection system An ignition source A fuel drainage system. Image by Jeff Dahl licensed under Creative Commons Licence. Adapted for training use by Aviation Australia. Turbine engine with combustion section labelled The combustion chamber, or combustor, is where the fuel and air are mixed and burned. The fuel injection system supplies the fuel through the fuel nozzles into the combustors. A typical ignition source is the high-energy capacitor discharge system. A fuel drainage system accomplishes the important task of draining the unburned fuel after engine shutdown. The combustion section, or burner as it is called, consists basically of an outer casing, an inner perforated liner, a fuel injection system and a starting ignition system. The function of this section is to add heat energy to the flowing gases, thereby expanding and accelerating the gases into the turbine section. The perforations are various sizes and shapes, all having a specific effect on the flame propagation within the liner. 2022-08-24 B1-15a Gas Turbine Engine Page 127 of 244 CASA Part 66 - Training Materials Only  One way to think about combustion is that when fuel heat is added, the volume of the gas increases, and with flow area remaining the same, this causes an acceleration of gases to occur. This process is referred to as combustion at constant pressure: the pressure at the exit does not alter significantly from the entry pressure. The fuel injection system meters the appropriate amount of fuel through the fuel nozzles into the combustors. Fuel nozzles are located in the combustion chamber case or in the compressor outlet elbows. Fuel is delivered through the nozzles into the liners in a finely atomised spray to ensure thorough mixing with the incoming air. A more rapid and efficient combustion process is achieved with a finer spray. A typical ignition source for gas turbine engines is the high-energy capacitor discharge system, consisting of an exciter unit, two high-tension cables and two spark ignitors. This ignition system produces 60 to 100 sparks per minute, resulting in a ball of fire at the igniter electrodes. Some of these systems produce enough energy to shoot sparks several inches, so care must be taken to avoid a lethal shock during maintenance tests. A fuel drainage system accomplishes the important task of draining the unburned fuel after engine shutdown. Draining accumulated fuel reduces the possibility of exceeding tailpipe or turbine inlet temperature limits due to an engine fire after shutdown. In addition, draining the unburned fuel helps to prevent gum deposits in the fuel manifold, nozzles and combustion chambers which are caused by fuel residue. 2022-08-24 B1-15a Gas Turbine Engine Page 128 of 244 CASA Part 66 - Training Materials Only  Combustion Chambers The combustion chamber (combustor) has the task of burning large quantities of fuel supplied through fuel burners with extensive volumes of air supplied by the compressor, then releasing the heat so that the air expands and accelerates to give a smooth stream of uniformly heated gas to the turbine under all conditions. To efficiently burn the fuel/air mixture, a combustion chamber must: Mix fuel and air effectively in the best ratio for good combustion Burn the mixture as efficiently and quickly as possible Cool the hot combustion gases to a temperature the turbine blades can tolerate Distribute hot gases evenly to the turbine section. Combustion chamber This task must be accomplished with the minimum loss in pressure and with the maximum heat release for the limited space available. There are currently three basic types of combustion chambers: Multiple-can Can/annular Annular. Functionally they are the same, but their design and construction are different. 2022-08-24 B1-15a Gas Turbine Engine Page 129 of 244 CASA Part 66 - Training Materials Only  Combustion chamber in a turbine engine Multiple-Can Combustor Multiple-can combustor 2022-08-24 B1-15a Gas Turbine Engine Page 130 of 244 CASA Part 66 - Training Materials Only  The multiple-can type combustion chamber consists of a series of individual combustor cans which act as individual burner units. It is well suited to centrifugal compressor engines because of the way compressor discharge air is equally divided at the diffuser. Each can is constructed with a perforated stainless steel liner inside the outer case. The inner liner is highly heat resistant and is easily removed for inspection once the combustion can is removed from the engine. Each combustion can has a large degree of curvature which provides high resistance to wear. However, the shape is inefficient in terms of the amount of space required and the added weight. Combustion chamber components with interconnector propagation tubes labelled The individual combustors in a typical multiple-can combustion chamber are interconnected with small flame propagation tubes (known as interconnectors). The combustion starts in the two cans equipped with igniter plugs, and then the flame travels through the tubes and ignites the fuel/air mixture in the other cans. Each interconnector tube is actually a small tube surrounded by a larger tube or jacket. The small inner tube carries the flame between the cans, and the outer tube carries airflow between the cans that cools and insulates. There are eight or 10 cans in a typical multiple-can combustion section. The cans are numbered clockwise when looking from the rear of the engine on most engines, with the number one can on the top. All the combustor cans discharge exhaust gases into an open area at the turbine nozzle inlet. 2022-08-24 B1-15a Gas Turbine Engine Page 131 of 244 CASA Part 66 - Training Materials Only  Can-Annular Combustor The can-annular combustor is more common to older commercial aircraft. This design consists of an outer case containing multiple liners located radially about the axis of the engine. The liners take air in at the front and discharge it at the rear. Interconnector tubes are utilised to connect the liners and provision is made for two igniter plugs in the lower cans. Can-annular combustor In the illustration above, eight liners are used. Each liner has its own fuel nozzle cluster supporting the liner at the front end and a device with eight apertures, called an outlet duct, supporting the liner at the back. Each combustor liner is annular in shape. Interconnector tubes are utilised to connect the liners. Ignition is provided in two liners by igniter plugs. An advantage of this combustor is that it is designed for ease of on-the-wing maintenance. The outer case is made to slide back to facilitate liner inspection. The liner may be removed for inspection and replacement without splitting the engine. Its short length provides lower pressure drop. Combining the gases from all of the cans provides a uniform temperature at the turbine. 2022-08-24 B1-15a Gas Turbine Engine Page 132 of 244 CASA Part 66 - Training Materials Only  Can-annular combustion chamber components 2022-08-24 B1-15a Gas Turbine Engine Page 133 of 244 CASA Part 66 - Training Materials Only  Annular Combustor Today, annular combustors are commonly used in both small and large engines. The reason for this is that, for the same power output, the length of the chamber can be 75% or less of that of a multiple- can system for an engine of the same diameter, making it the most efficient. An annular combustion chamber consists of a housing and a perforated inner liner, or basket. The liner is a single unit that encircles the outside of the turbine shaft housing. The shroud can be shaped to contain one or more concentric baskets. An annular combustor with two baskets is known as a double annular combustion chamber. Normally, the ignition source consists of two spark igniters similar to the type found in multiple-can combustors. Annular combustion chamber In a conventional annular combustor, airflow enters at the front and is discharged at the rear with primary and secondary airflow, much the same as in the multiple-can design. However, unlike the can type combustors, an annular combustor must be removed as a single unit for repair or replacement. This usually involves complete separation of the engine at a major flange. 2022-08-24 B1-15a Gas Turbine Engine Page 134 of 244 CASA Part 66 - Training Materials Only  Annular combustor components Reverse-Flow Annular Combustor Some annular combustors are designed so the airflow can reverse direction. Reverse flow annular combustion chamber 2022-08-24 B1-15a Gas Turbine Engine Page 135 of 244 CASA Part 66 - Training Materials Only  These reverse-flow combustors serve the same function as the conventional flow type, except the air flows around the chamber and enters from the rear. This results in the combustion gases flowing in the opposite direction of the normal airflow through the engine. In a typical reverse-flow annular combustor, the turbine wheels are inside the combustor area rather than downstream, as with the conventional flow designs. This allows for a shorter and lighter engine. Relevant Youtube link: Combustion Cans (Video) Combustion Airflow In order to allow the combustion section to mix the incoming fuel and air, ignite the mixture, and cool the combustion gases, airflow through a combustor is divided into primary and secondary paths. Approximately 20%–30% of the incoming air is designated as primary, while 70%–80% becomes secondary (depending on engine design). Combustion air flow Primary Combustion Air Primary, or combustion, air is directed inside the liner in the front end of a combustor. As this air enters the combustor, it passes through a set of swirl vanes, which gives the air a radial motion and slows down its axial velocity to 5–6 ft/s. The reduction in airflow velocity is very important because kerosene type fuels have a slow flame-propagation rate. An excessive velocity airflow could literally blow the flame out of the engine. This malfunction is known as a flameout. A toroidal vortex (similar to a smoke ring) created in the flame area provides the turbulence required to properly mix the fuel and air. Once mixed, the combustion process is complete in the first third of a combustor. 2022-08-24 B1-15a Gas Turbine Engine Page 136 of 244 CASA Part 66 - Training Materials Only  Secondary Airflow The secondary airflow in the combustion section flows at a velocity of several hundred feet per second around the combustor’s periphery. This flow of air forms a cooling air blanket on both sides of the liner and centres the combustion flames so they do not contact the liner. Some secondary air is slowed and metered into the combustor through the perforations in the liner, where it ensures combustion of any remaining unburned fuel. Primary and secondary flow inside the combustor Finally, secondary air mixes with the burned gases and cools the air to evenly distribute energy to the turbine nozzle at a temperature that the turbine section can withstand. Combustion Air / Fuel Ratio Liquid fuels must be converted from their liquid state to a vapour before they will burn. In addition, the ratio of fuel vapour to oxygen in the air must be chemically correct for complete combustion. A stoichiometric mixture is a perfectly balanced air-fuel mixture of 15 parts air to one part fuel by weight. An air-fuel mixture that is leaner than 15:1 has less fuel in the mixture, while a rich mixture has more fuel. Combustible air-fuel ratios range from 8:1 to 22:1. Often one can see the air-fuel ratio expressed as 60:1. When this occurs, the writer is expressing it in terms of the total airflow rather than of primary combustor airflow. If primary airflow is approximately 25% of total airflow, then 15:1 is 25% of 60:1. 2022-08-24 B1-15a Gas Turbine Engine Page 137 of 244 CASA Part 66 - Training Materials Only  Flame Stabilisation It is desirable to anchor the flame as close as possible to the fuel nozzle. It must have a region of low- velocity air in the combustor. The flame in the combustor is stabilised by reducing axial velocity of the air. The swirl vanes slow the gas, and the fuel is introduced in fine particles. Air exits the compressor at about 200 m/s (700 ft/s), where it is then diffused. Its axial velocity drops to approximately 150 m/s (500 ft/s). The air velocity is further reduced by using swirl vanes to approximately 2 m/s (6 ft/s) in the combustion chamber. The fuel nozzle imparts a spin to the fuel by inducing a swirl into the fuel and air entering the combustor. The swirl also assists atomisation of the fuel, and air holes in the liner primary zone are shaped to induce a toroidal vortex, which stabilises and anchors the flame. Combustion chamber swirl vanes 2022-08-24 B1-15a Gas Turbine Engine Page 138 of 244 CASA Part 66 - Training Materials Only  Flame Temperatures The primary combustion temperature is 1800-2000 °C. Flame stabilisation, cooling and dilution air keep the gas temperatures within the tolerance of the turbine materials at approximately 800- 1100°C, depending on engine design. © RR The Jet Engine Combustion flame temperature graph For information only: At the time of writing, engine manufacturers have implemented some major design changes to annular combustors and air flow paths to achieve the emission levels set by regulators. One such example is from GE, where the combustion liner has no dilution holes, is 3D-printed with ceramic matrix composites (CMC) and has cooling air (30% secondary) going only around the outside of the liner. The fuel is premixed with air before it enters the combustion zone and primary air is now 70%, giving a cooler, leaner fuel burn. This system is known as TAPS (Twin Annular Premix Swirler). These unique combustion systems will be explored in more detail during the engine type course for that particular aircraft. 2022-08-24 B1-15a Gas Turbine Engine Page 139 of 244 CASA Part 66 - Training Materials Only  Twin Annular Premix Swirler Relevant Youtube link: 3D Printed Fuel Nozzle (Video) 2022-08-24 B1-15a Gas Turbine Engine Page 140 of 244 CASA Part 66 - Training Materials Only  Combustor Drain The combustor drain valve shown below is a mechanical device located in the low point of a combustion case. It is closed by gas pressure within the combustor during engine operation and opened by spring pressure when the engine is not in operation. This valve prevents fuel accumulation in the combustor after a false start, shutdown or any other time fuel might tend to puddle at the low point. Draining fuel in this manner prevents such safety hazards as after-fires and hot starts. This drain also removes un-atomised fuel which could ignite near the lower turbine stator vanes, causing serious local overheating during starting, when cooling airflow is at the lowest flow rate. Combustor drain valve Relevant Youtube link: Combustion Chamber Part 1 (Video) Relevant Youtube link: Combustion Chamber Part 2 (Video) 2022-08-24 B1-15a Gas Turbine Engine Page 141 of 244 CASA Part 66 - Training Materials Only