Engines 1

Engines

Total Pressure = Static Pressure + Dynamic Pressure

Total Pressure = Velocity + Pressure

Bernoulli’s Theorem states that as a subsonic fluid passes through a converging opening, its velocity increases and its pressure decreases while the total pressure does not change

Subsonic Airflow in a nozzle increases in velocity and decreases in pressure. In a diffuser, the pressure increases and the velocity decreases. The nozzle is convergent and the diffuser is divergent at subsonic speeds

Supersonic Airflow in a nozzle increase velocity and decrease pressure. In a diffuser, pressure increases and velocity decreases. With supersonic airflow, a nozzle is divergent and a diffuser is convergent.

All gas generators have compressors, combustion chambers and turbines.

The Brayton cycle for a jet engine consist of the simultaneous firing of the Intake, Compression, Combustion and exhaust cycles.

The compressor, compresses the air by decreasing the volume of the unit. +P, +V, +T
The diffuser than effects the air. +P, -V
The combustion chamber ignites gas which expands and further increases the air’s volume. -P, +V
The Turbine extracts energy to run the compressor. -P, +V
The exhaust extracts the air and moves the engine. -P, +V

	Inlet	Compressor	Diffuser	Burner	Turbine	Exhaust
Temperature	+	+	NC	+	-	-
Pressure	+	+	+	-	-	-
Velocity	-	+	-	+	+	+

Gross thrust does not take into account inlet airflow produced by the engine moving through the air. Net thrust takes the initial airflow into account and give a more realistic synonymous with thrust
As the temperature increase, density decrease, and the thrust decreases. Air Temp is inverse to Thrust.
If pressure increases, density increase and thrust will increase.
As altitude increases, P and T will drop. The rate thrust decrease due to pressure drop is greater than the increase in thrust from T dropping. The engine produces less thrust as altitude increases. After 36,000’ the T stabilizes and the thrust will drop dramatically. 36,000 is the optimum cruise altitude because thrust available, plus low fuel flow and diminished drag provide optimum performance!
As inlet velocity approaches the exhaust velocity thrust is reduced. If mass and air are held constant thrust will decrease as airspeed increases.
The ram air effect forces more and more air into the inlet as airspeeds increase. This increases the mass and pressure of the air. This offsets the decrease in acceleration and produces a neutral or slight gain in thrust at subsonic speeds.
Thrust is measured in the following ways:
Jet: EPR (TPDI)
Fan: EPR
Prop: Torquemeter (SHP)
Shaft: Torquemeter
Inlet ducts act as diffuser by always decreasing velocity and increasing pressure. Velocity is decreased to stabilize airflow before it reaches the diffuser or compressor. This will provide the proper amount of high pressure, turbulence free air to the compressor. It must operate efficiently from ground idle to supersonic speeds at high altitude.
Single Entrance is the simplest and most effective. It has smooth airflow. Divided entrance allows the pilot to sit lower and reduce friction due to shorter duct length. Must be offset from the fuselage so as not to upset the boundary layer. It also curves which can cause turbulence.
Subsonic and Supersonic Airflow inlet ducts always act as diffusers by increasing pressure and decreasing velocity. Subsonic air has a divergent opening while supersonic air has a convergent/divergent opening.
Variable Geometry inlet ducts use ramps, wedges or cones to change the shape of the inlet as the aircraft speed varies between sub/supersonic.
The compressor section must supply enough air to satisfy the requirements of the combustion section. The pressure and velocity is increased with this air being directed to the burner section. A secondary purpose is to supply compressor bleed air to operate various components throughout the aircraft and engine.
Centrifugal Flow uses a impeller, diffuser and manifold to compress the air. Air is ingested near the center and accelerated outward toward the diffuser adding potential and kinetic energy. This is done by passing the air through the divergent passages of the impeller; which increase both velocity and pressure. The high pressure air then enters the diffuser which is stationary and converts the velocity into more pressure. The air then goes to the compressor manifold which directs the air into the combustion chamber.
Axial Flow uses rotor blades and stator veins. Each pair of rotor-stators form a stage. Turbine drives the rotors which increases the pressure and the velocity of the air. Then the air is pushed through the static stator vanes which act as diffuser (increase pressure, and decrease velocity). This continues through each stage of the compressor. Compressors are set up as velocity remains about constant throughout. As pressure increases, the air becomes more compressed. The cross sectional area of each stage decreases so that velocity remains constant as the pressure increases.
Axial-Centrifugal uses a compressor inlet case, compressor and a diffuser. The compressor directs air to the outside air compressor. The air is compressed 7:1 by a three stage axial and single stage centrifugal compressor that is run as one integral unit by the turbine. The air leaves the compressor via diffuser pipes and enters the diffuser where P is further increased.
The burner section is the combustion chamber and provides the means for proper mixing of the fuel and air. It must deliver combustion gases to the turbine section at the appropriate temperature. It must also add sufficient heat energy to the gases to accelerate their mass and produce the desired thrust for the engine and power the turbines. It must minimize the pressure decrease to allow the pressure to turn the turbines, keep combustion efficiency high, not blow out the flame and complete the burning before the gases enter the turbine stage.
The can burner has individual cans around the circumference of burner section. Each has its own fuel nozzle, liner and casing. Primary air is burned with the secondary air cooling the liner and casing. Strong and durable, easy to maintain, and individual units can be replaced. It provides a poor use of space, pressure loss, and uneven turbine heating.
The annular burner provides continuous, circular, inner and outer shroud around the outside of the compressor drive shaft. The liner has holes to cool the inside. Fuel is introduced through a series of nozzles where it is mixed with air and ignited. It provides uniform heat, better air mixing and great use of space, but is difficult to repair, has structural problems with its large diameter and thin wall design.
The can-annular burner is used on larger high performance engines. Cans at the front mix and burn the air and fuel. The hot gases then pass to the annular chamber where they are further mixed. It is easy to maintain, has great uniform heat, is structurally sound, lower pressure loss, efficient, but is considerably more expensive.
The turbine section is comprised of stators and rotors. The turbine section drives the compressor and accessories. It is designed to increase velocity. Converts the heat of the expanding gases to mechanical energy. Approximately 75% of the total P is converted to mechanical energy, while the remaining 25% is used for thrust.
The components of a turbine engine are effected by thermal stress. The repeated heating and cooling results in creep. The turbine uses an EGT Exhaust Gas Temp gauge and an ITT Interstage Turbine Temp gauge to measure thermal changes. The turbine section is exposed to the greatest amount of thermal stress.
Creep is the gradual elongation of blades as they are heated and can lead to catastrophic failures.
The fir tree method is used to cool turbine blades and prevent them from deforming. Blades are attached to the turbine shaft to improve ability to withstand high heat and repeated heating and cooling.
Heat is a result of the combustion of the fuel air mixture. High pressure is a result and used to turn the turbine.
The exhaust section is used to vent the gases rearward at a high exit velocity with minimized turbulence. It is usually composed of an outer duct, inner core and radial hollow struts. The struts support the inner core and straighten out the whirling gasses. The cone converts the gases to a solid jet. The initial divergent shape between the outer duct and the inner core allow for smooth expansion of the gases. Toward the rear, the exhaust section becomes progressively narrower converting pressure to velocity.
Subsonic uses a convergent nozzle and is usually fixed.
Supersonic turbine air is high sonic not supersonic. Convergent/Divergent nozzles are sued to decrease pressure and increase velocity. Air speed is very high sonic and makes air compressible.
The afterburner is used by turbofans and jets to increases thrust available by 50% or more. Fuel consumption increase by 300%. Secondary air from the burner is used in the turbojets. Secondary air and bypass air is used in turbofans. Two methods are used to ignite the afterburner. Hot streak and Spark/Torch. Hot streak uses an extra quantity of fuel which is injected into one of the combustion chamber. Thus streaks of hot gases ignite the afterburner fuel. The spark/torch method uses a pilot light or igniter plug located near the spray bars and flameholder to ignite the afterburner fuel/air mixture.
The afterburner is comprised of spray bars, flameholder, screech liners, and variable exhaust nozzles. The spray bars spray fuel into the stream of hot gasses. The flameholder provides a region of turbulent eddies to reduce the gas V. The screech line is a sleeve with holes that reduces the violent pressure fluctuations. The variable exhaust nozzle provides the convergent shape for subsonic flows and convergent divergent for supersonic afterburner flows. This prevents a pressure buildup which will stall the engine.
The relative wind is formed by the inlet airflow and the compressor rotation vectors
A compressor is designed to provide an optimum compression ratio. If the AOA of the compressor blades is too low, the compression is inefficient. If it is too high, then a compressor stall may result. A stall occurs when airflow over an airfoil brakes away causing the airfoil to lose lift due to excesssive AOA.
The rotors are fixed to the rotor disk, the change in AOA occurs when the RBM changes or the inlet airflow V changes. A decrease in inlet airflow or decrease in RPM will increase the AOA of the rotor blades.
A compressor stall is characterized by a surge. This surge results in a reduction of airflow to the turbines which means the more fuel is needed to maintain current thrust. This will increase burner and turbine temperatures. If PCL is constant, RPM decay and ITT rise along with possible noises will be present.
Compressor stalls are caused by airflow distortions and mechanical failures.
Mechanical problems that can lead to compressor stalls include variable inlet guide vane and strator vane failures which result in too much or too little airflow at low engine speeds. Fuel Control Unit failure which cause rich blow outs (too much fuel with combustion reaching back into the compressor) or lean die outs (airflow is too high to allow fuel to ignite in compressor). FOD causes damage to the blades and alters their aerodynamic properties. And Variable exhaust nozzle failure if it fails open at supersonic speeds could cause back pressure which will result in a compressor stall.
To avoid compressor stalls avoid erratic or abrupt PCL changes, especially at slow speeds or high angles of attack. Maintain a prescribed minimum airspeed and avoid abrupt changes in altitude.
To minimize compressor stalls, use the following: Variable inlet guide vanes and stator vanes so that AOA is changed at low speeds. These are controlled by the stator vane actuator using fuel pressure via the FCU. Dual/Twin/Split Spool Axial flow compressors allow the front rotor to turn at a lower speed than the rear rotor so that the front rotor wont be choked by low airflow. Bleed valves near the middle or rear of the compressor vent air and increase airflow in the front compressor at low engine speeds. Variable exhaust nozzle is used to unload the pressure after the afterburner.
If a stall occurs, reduce attitude of aircraft so AOA is less, retard the PCL just below the stall threshold to let the engine catch up with inlet airflow, and once engine returns to normal, slowly advance the PCL to desired setting.
The turbojet consist of a Inlet, Compressor, Burner, Turbine and Exhaust.
Thrust Specific Fuel Consumption is the amount of fuel required to produce one pound of thrust. The propulsive force of the turbojet relies on the amount of fuel added to the air mass. More air requires more fuel.
Turbojets rely on the thrust to produce 25% of the total energy for its propulsion while the other 75% goes into mechanical force required to power the compressor. They are not efficient
More air require more fuel. Since density of air decreases with altitude, less fuel is required at higher altitudes. Turbojets are inefficient at producing thrust at low speeds. Low-mass coupled with high velocity exhaust is wasteful compared to high mass, low V airflow from turboprop. This changes as speed increases. At high speeds, the difference between V of the exhaust and the surrounding atmosphere is lessened so it is more efficient than when a turbojet is at low speeds
Turbojets have higher TSFC than Turbofans because it is inefficient at producing thrust at low speeds and requires more fuel.
Turbojets have low propulsive efficiency at low forward airspeeds, relatively high TSFC at low altitudes and low airspeeds, long takeoff roll is required and they have the lightest specific weight (wt per pound of thrust produced)
Turbofans have a gas generator (compressor, burner and turbine) and a duct enclosed fan. It is like a turbojet with a fan surrounding it.

Hosted by www.Geocities.ws