Advanced Thermo and Prop

What are the elements of a Joule Cycle

- Closed cycle (closed system) - Repeated circulation - Also known as a Brayton Cycle

What are the assumptions of the ideal cycle?

- Compression and expansion processes are reversable and adiabatic (i.e isentropic) - Change in KE between inlet and outlet of each component is negligable - No pressure losses through equipment - Working fluid can be treated as a perfect gas with constant specific heats - Mass flow of gas throughout the cycle - Heat transfer in any HE is complete

Give 3 advantages of the Integrated Electric Populsion (IEP) over more traditional marine propulsion systems

- Decoupled from the shafts (more efficient use of space) - Fuel savings - compact electric motor (can achieve higher power/weight ratios for marine vessels)

What are the two methods to thrust augmentation

- Liquid injection (no-longer in common use) - Afterburning

Why is the intake treated as an adiabatic duct?

- To account for the effect of friction - assume that there is no heat or work transfer through the duct, and therefore the stagnation temperature will be constant, but there will be a loss of stagnation pressure due to friction and due to shock waves at supersonic flight conditions.

How does a turbofan improve propulsive efficiency?

- decreasing the mean jet velocity. • Lower jet velocities also quieter. • A portion of the total mass flow bypasses part of the compressor, the combustion chamber, the turbine and possibly the core nozzle. • Thrust now has components from a cold and a hot stream.

What are the processes in a diesel cycle

1-2 Isentropic Compression 2-3 Heat addition at constant pressure 3-4 Isentropic Expansion 4-1 Heat Rejection

What are the processes in an otto cycle

1-2 Isentropic compression from BDC to TDC 2-3 Combustion (const. v heat transfer) 3-4 Power stroke (Isentropic expansion) 4-1 Exhaust (Heat rejection to the surroundings)

What are the two intake efficiencies?

1. Isentropic efficiency ηi defined in terms of temperature rises. - intake efficiency will be high for subsonic aircraft. - supersonic flight speeds the intake efficiency will decrease with Mach number 2. Ram efficiency ηr defined in terms of pressure rises.

How is propulsion generated from a Turboshaft?

All of the usable hot gas energy is extracted and converted to shaft power using a free power turbine. e.g. to drive the rotor blades

What is involved in the afterburning method to thrust augmentation?

Also known as Reheat. • Injection of fuel between the turbine exit and the propelling nozzle where it burns using unburnt oxygen in the turbine exhaust flow. • A diffuser is used to decrease the gas velocity across the burners. • Temperatures following afterburning can be much higher than at turbine inlet (up to ~ 2000 K). • Stoichiometric combustion is desirable for maximum thrust augmentation. • Increase in the total temperature at the nozzle inlet will increase the sonic speed at exit and thus the velocity through the nozzle and thus the thrust. • Much greater fuel flow results in large increase in SFC. • Will slightly decrease the dry thrust and overall dry performance due to increase in and flow path complexity and engine mass. • Assuming choked operation of nozzle. • Jet velocity is equal to the sonic velocity at the nozzle exit which will depend on the temperature at exit, i.e. T5 or T7 which will be the critical temperature at those conditions.

What are the advantages/disadvantages of an axial compressor?

Axial-flow compressors have the following advantages: • High peak efficiency. • Small frontal area forgiven airflow. • Straight-through flow, allowing high ram efficiency. • Increased pressure rise due to increased number of stages with negligible losses. They have the following disadvantages: • Good efficiency over narrow rotational speed range. • Difficulty of manufacture and high cost. • Relatively high weight. • High starting power requirements (this has been partially overcome by split compressors).

What are the advantages/disadvantages of a centrefugal compressor?

Centrifugal-flow compressors have the following advantages: • High pressure rise per stage. • Efficiency over wide rotational speed range. • Simplicity of manufacture with resulting low cost. • Low weight. • Low starting power requirements. They have the following disadvantages: • Large frontal area for given airflow. • Impracticality if more than two stages because of losses in turns between stages

How is propulsion generated from a Turboprop?

Energy of the hot gas is used to drive an additional, separate turbine which provides shaft power to drive a propeller. A small amount of energy remains for the generation of thrust from a nozzle.

Draw a schematic of a propelling nozzle

Photo slide 5

How is propulsion generated from a Turbojet & Turbofan?

Propulsion from reaction forces generated by exhaust gas (thrust from nozzle(s))

For the same range of operating conditions, which is cycle is more efficient?

Spark ignition cycle

When is the specific work output of a simple gas turbine at a maximum?

T2=T4 Specific Work OUtput is constrained by the Metallurgical Limit Temperature

Why are intercoolers seldom used on gas turbines?

The large amounts of cooling require bulky heat exchangers supplied with large quantities of coolant such as water

T/F: The propeller efficiency will decrease the power available for propulsion

The power of the engine is multiplied with the efficiency to get the power available. The efficiency is between zero and one, hence it decreases the power available for propulsion.

What is the purpose of a propelling nozzle?

The propelling nozzle is used to expand the engine exhaust to give a high-velocity jet. • It converts the high p & T at the turbine exit plane into a high propelling velocity at the exit plane of the nozzle. • This is opposite to the function of the inlet in which KE is converted into pressure energy. • Energy can be assumed to be conserved in both. Hence the stagnation temperature T0 is conserved.

In the coefficient of drag equation, what does the CDo term represent?

The zero lift drag coefficient

A simple convergent nozzle eases the integration of....

Variable area for afterburning. • Thrust reverser. • Noise suppression.

What is the relationship between the bypass ratio and Fan Pressure Ratio (FPR)?

a) Bypass ratio (B) and fan pressure ratio (FPR) must be specified in addition to the overall pressure ratio and the turbine inlet temperature. b) The temperature and pressure in the bypass duct is determined from the inlet conditions and FPR. c) The mass flow through the bypass duct is determined from the total mass flow and B. d) A calculation of the cold stream thrust is made across the cold nozzle (NB with air as the working fluid). The critical pressure ratio must be evaluated to check for choking. Pressure thrust must be calculated if nozzle is choked. e) In a two-spool configuration (Fig. 3.15) the fan is driven by the LP turbine. First calculations for the inner spool (HP compressor, combustor and HP turbine) are made. This gives the conditions at the entry to the LP turbine. f) use T05, ht and (T05 - T06) to calculate the LPT pressure ratio and thus the conditions at the entry to the hot nozzle can be calculated. The hot stream thrust is then calculated in the normal way. g) If the two stream are mixed before being exhausted through a single nozzle then the mixed condition at nozzle entry (7) must be determined. This is done via an enthalpy and momentum balance.

How is the Specific thrust coefficient Kf calculated?

actual specific gross thrust/isentropic gross thrust actual jet velocity/isentropic jet velocity (for complete expansion)

To maintain steady level flight, the angle of attack must increase ...

as the velocity decreases As the velocity decreases, dynamic pressure actually decreases. This requires a larger lift coefficient for the same lift force, thus a smaller angle of attack.

What ratio represents r in an Otto cycle?

r = compression ratio = Volume at BDC/Volume at TDC

What is the temperature ratio of a simple gas turbine?

t = T3/T1

What is a bypass ratio?

the ratio of the mass flow through the bypass duct (cold stream) to that through the engine core (hot stream).

How is the nozzle area affected by afterburning?

• Afterburning requires an increase in nozzle area to maintain the mass flow, as although the velocity is increased the density of the gas is decreased. • Area must increase to prevent back pressure on cycle. • Originally two-position nozzles used. Now variable area is more common. • Additional increase in pressure thrust.

What is the effect supersonic flow has on the nozzle pressure ratio?

• At high supersonic speeds the large ram pressure rise in the intake results in a very high nozzle pressure ratio. • For Ma = 2 - 3 the value of p04 /pa may be as high as 10 - 20, many times higher than the critical pressure ratio. • In this case variable exit/throat area is critical to avoid shock losses over the operating range. • This must be done within the following limitations: Saravanamuttoo et al 2001 a) Exit diameter must be less than max engine diameter to avoid drag increase. b) The included angle of the divergence must be less than ~ 30° as the loss in thrust associated with divergence of the jet increases sharply at greater angles. This involves a length and weight penalty for the nozzle.

What is the effect a converging/diverging nozzle has on Thrust output?

• For an isentropic expansion, the thrust produced is maximum when complete expansion to pa occurs in the nozzle. • The pressure thrust from incomplete expansion A5(p5 - pa) does NOT fully compensate for the loss in momentum thrust due to decreased jet velocity. • For an isentropic expansion, the thrust produced is maximum when complete expansion to pa occurs in the nozzle. • The pressure thrust from incomplete expansion A5(p5 - pa) does NOT fully compensate for the loss in momentum thrust due to decreased jet velocity. • This is NOT the case when friction is taken into account in a real engine because the theoretical jet velocity cannot be achieved. • The addition of a convergent-divergent nozzle would result in increases in engine weight, length and diameter and complexity of installation. • Experiments have shown that for values of p04 /pa up to 3, there is no gain from using a convergent-divergent nozzle even when the area ratio has been optimised.

What are the attributes of centrefugal compressors?

• Large pressure ratio per stage • Reduced number of stages (shorter, compact engine) • Good overall efficiencies for smaller mass flows • Reduced manufacturing cost

What does Isentropic nozzle efficiency depend on?

• Length of jet pipe • Presence of intrusive geometries (i.e. afterburners, thrust reversers, mixers) • Amount of swirl at the turbine exit.

All intakes for airbreathing gas turbines should provide......

• Maximum pressure recovery • Stable airflow that is insensitive to yaw and velocity changes • Correct and uniform mass flow across the engine compressor face • Minimum drag • Minimum noise from the fan and compressor

What is involved in the liquid injection method to thrust augmentation?

• Often referred to as Water Injection. • Widely used in civil aviation up to the 1970s. • Mixture of water and m/ethanol often used to prevent freezing. • Liquid injection into or upstream of the compressor will cool the incoming air and increase the density, and thus the mass flow. • Alternatively, liquid injection directly into the combustion chamber (where it is vaporised) will increase the mass flow through the turbine relative to the compressor, thus decreasing the required pressure and temperature drop, which is beneficial to thrust. • Especially useful at hot and high airports. • Gives an increase in thrust but at the expense of complexity, increased wear and tear and the need to carry the additional mass. • Primarily used to increase take-off thrust.

What are the attributes of axial compressors?

• Small cross-sectional area per unit mass flow • Easier to multistage (good for large overall pressure ratios) • Good component efficiencies (though diminish at small scale) • Able to handle large mass flow rates

What is the purpose of an exhaust cone in propelling nozzle?

• The exhaust cone enables the desired area increase in the diffuser and protects the rear face of the turbine disc. • It is usual to hold the velocity at the diffuser exit to approx. M = 0.5 (~290 m.s-1). • The turbine rear support struts are design to straighten out the exhaust gas flow into the jet pipe as residual whirl velocity will result in losses.

Decribe what processes are happening at the throat and exit of a convergind/divering nozzle

• The exit of the convergent section becomes a throat with the actual exit diverging. • The subsonic flow in the convergent section accelerates as the static pressure falls. • At the throat the velocity reaches the sonic speed of the gas. • The now supersonic flow continues to expand through the divergent nozzle section, continuing to increase in velocity. • Pressure thrust on the wall of the nozzle results as a reaction to this increase in momentum of the flow. • The ratio of throat size to exit size controls the expansion pressure ratio

What is the primary purpose of an inlet?

• There is a static pressure rise as the incoming flow is decelerated due to the intake geometry. Intake Design • Prime design requirement is to minimise this pressure loss up to the face of the compressor (with a uniform pressure and velocity) at all flight conditions.