JET PROPULSION AND ROCKETS

Transcription

1 JET PROPULSION AND ROCKETS By Mr. K.Venugopal Assistant professor MLRI T

2 The principle of jet propulsion involves imparting momentum to a mass of fluid in such a manner that the reaction of imparted momentum provides a propulsive force. It may be achieved by expanding the gas, which is at high temperature and pressure, through a nozzle due to which high velocity jet of hot gases is produced (in the atmosphere) that gives a propulsive force (in opposite direction due to its reaction).

3 The propulsion system may be classified as follows : 1. Air stream jet engines (Air breathing engines) a. steady combustion system (continuous air flow) i. Turbo jet ii. Turbo prop iii. Ram jet b. Intermittent combustion system (intermittent flow) i. pulse jet or flying bomb 2. Self contained rocket engines(non air breathing engines) i. Liquid propellant ii. solid propellant

4 In air breathing engines, the oxygen necessary for the combustion is taken from the surroundings atmosphere whereas in a rocket engine the fuel and the oxidiser are contained in the body of the unit which is to be propelled.

5 Turbo jet engine

6 Different components of turbo jet are 1-2 is compressor 2-3 is combustion chamber 3-4 turbine 4-5 is nozzle

7 Advantages of turbo-jet engines Construction is much simpler Engine vibrations absent Much higher speeds possible power supply is uninterrupted and smooth weight to power ratio superior frontal area small

8 Disadvantages of turbo jet engines Less efficient Life of unit comparatively shorter More noisy Materials required are quite expensive inefficient below 550 km/h

9 Turbo prop Engine

10 The turboprop entails the advantage of turbojet (low specific weight and simplicity in design) and propeller (high power takeoff and high propulsion efficiency at speeds below 600 km/h). The overall efficiency of turbo prop is improved by providing the diffuser before compressor. the pressure rise take in the diffuser. this pressure rise takes place due to conversion of kinetic energy of the incoming air (equal to the air craft velocity)into pressure energy by the diffuser. the type of compression is known as 'ram effect'.

11 Ram jet

12 Ram jet is also called as athodyd, lorin tube or flying stovepipe. these are having capability to fly at supersonic speeds. compressor and turbine are not necessary as the entire compression depends only on the ram compression. Ram jet engine consists of a diffuser, combustion chamber and nozzle. The air enters the ram jet with supersonic speed and is slowed down to the sonic velocity in the supersonic diffuser, consequently the pressure suddenly increases in the supersonic diffuser to the formation of shock wave. the pressure of air further increased in the subsonic diffuser increasing the temperature of air above the ignition temperature.

13 Advantages of ram-jet engines no moving parts light in weight wide variety of fuels can be used limitations of ramjet engines fuel consumption is too large at low and moderate speeds it cannot be started by its own diffuser need to be designed carefully

14 Pulse jet engine

15 A pulse jet engine is an intermittent combustion engine and it operates on a cycle similar to a reciprocating engine, where as the turbo jet and ram jet engines are continuous in operation and are based on brayton cycle. pulse jet engine is like an athodyd, develops thrust by a high velocity of jet of exhaust gases without the aid of compressor or turbine. Its development is primarily due to the inability of the ram jet to be a self starting.

16 Advantages of pulse-jet engines simple in construction well adopted to pilotless aircrafts capable of producing static thrust thrust in excess of drag at much low speeds. Disadvantages of pulse-jet engines high intensity of noise severe vibrations high rate of fuel consumption and low thermodynamic efficiency the operating altitude is limited by air density consideration

17 ROCKETS Similar to jet propulsion, the thrust required for rocket propulsion is produced by the high velocity jet of gases passing through the nozzle. But the main difference is that in case of jet propulsion the oxygen required for combustion is taken from the atmosphere and fuel is stored whereas for rocket engine, the fuel and oxidiser both are contained in a propelling body and as such it can function in vacuum.

18 The rockets may be classified as follows 1. According to the type of propellant i. Liquid propellant rocket ii. solid propellant rocket 2. According to the number of motors i. single stage rocket (consists of one rocket motor) ii. multi stage rocket (consists of more than one rocket motor)

19

20 Requirements of an ideal rocket propellant An ideal rocket propellant should have the following characteristics / properties High heat valve Reliable smooth ignition Stability and ease of handling and storing low toxicity and corrosiveness Highest possible density so that it occupies less space

21 Applications of Rockets The fields of application of rockets are as follows Long range artillery Lethal weapons Signalling and firework display Jet assisted take-off For satellites For space ships Research

22 Rankine cycle By Mr. K.Venugopal Assistant professor MLRI T

23 BASIC COMPONENTS OF POWER PLANTS: 1.Boiler (to generate steam) 2.Turbine (convert heat into mechanical work) 3.Condenser (convert steam to water ejecting the heat present in steam to other medium) 4.Pump (transfer water from condenser to boiler by increasing pressure) 5.Generator (mechanical work to electrical work)

24 CARNOT CYCLE: Carnot cycle is an ideal cycle.it is not possible in actual process. The cycle consists of 2 reversible isothermal process (PV=C (very slow process)) and 2 reversible adiabatic process (PV γ =C(very quick process)).the maximum thermal efficiency of an heat engine is obtained when heat is supplied at source temperature and is rejected at sink temperature.carnot cycle has the maximum efficiency because heat is added isothermal at source temperature at source temperature and rejected isothermally at sink temperature. The carnot cycle with steam as working fluid is shown in fig. In this case heat supply and reduction at constant temperature can be supply obtained by maintaining constant pressure while fluid is in wet region.

25 The sequence of operation: 1. Isothermal compression (3-4) 2. Isothermal heating (4-1) 3. Isentropic expansion or reversible adiabatic expansion (1-2) 4. Isothermal cooling (2-3) Area under P-V diagram gives work done and area under T-S diagram gives heat transfer.

26 1. Process 4-1: 1 kg of boiling water at temperature T 1 is heated to form wet steam or dryness fraction.thus heat is absorbed at constant temperature T 1 and pressure P 1 during this operation. 2. Process 1-2: During this operation steam is expanded isentropically to temperature T 2 and pressure P 2.the point 2 represents the condition of steam after expansion. 3. Process 2-3: During this operation heat is rejected at constant pressure P 2 and temperature T Process 3-4: In this process the wet steam at point 3 is compressed till the steam regains its original state of temperature T 1 and pressure P 1.

27 EFFICIENCY OF CARNOT CYCLE: Efficiency of carnot cycle= (Net work done)/ (Heat supplied) Net work done = Heat supplied-heat rejected η=1-sink temperature/source temperature η=1-t 2 /T 1

28 WORK RATIO: It is defined as the ratio of net work output to the turbine output.

29 LIMITATIONS OF CARNOT CYCLE: 1. Isothermal addition of heat at constant temperature (4-1) may be readily achieved. Extension of this Isothermal process into the superheated region requires simultaneous reduction pressure which is most difficult. 2. In order to return saturated water to the boiler the condensing process (2-3) must be terminated at state 3 where the working fluid is a mixture of liquid and vapour. 3. It is difficult to compress a wet vapour isentropically to the saturated state as required by the process The efficiency of the carnot cycle is greatly affected by temperature T 1 at which heat is transferred to the working fluid. Since the critical temperature of steam is only 374 C Therefore the cycle is to be operated in the wet region and the maximum possible temperature is limited.

30 RANKINE CYCLE: The thermodynamic cycle for steam power plant is Rankine cycle.many limitations of Carnot cycle are eliminated in Rankine cycle by superheating the steam in turbine and condensing it completely in the condenser.in an ideal Rankine cycle there is no pressure drop during heating and condensation.also in the absence of irreversibilities and heat interaction with the surroundings the expansion and compression in the turbine and pump would be isentropic.rankine cycle is also called vapour cycle.it is similar to the Brayton cycle or Joule cycle.but in Rankine cycle the working substance changes its phase from vapour to liquid and vice versa.in Brayton cycle the working substance will be in gas phase throughout the process.rankine cycle consists of 2 reversible adiabatic process(isentropic process) and 2 reversible constant pressure process.it is shown in the above chart.

31 The sequence of operation of Rankine cycle is: 1. Isentropic expansion (1-2) Turbine 2. Constant pressure heat rejection (2-3) Condenser 3. Isentropic compression process (3-4) Pump 4. Constant pressure heating process (4-1) Boiler

32 Reheat Rankine cycle Sequence of processes: 1-2: Isentropic expansion in high pressure turbine 2-3: Reheating of steam at constant pressure 3-4: Isentropic expansion in low pressure turbine 4-5: Constant pressure heat rejection in condenser 5-6: Isentropic compression process

33 REGENERATION RANKINE CYCLE: The mean temperature of heat addition can be increased by regeneration cycle. In this cycle, steam is extracted from the turbine at several locations and supplied to the feed heaters (regenerative heaters) to heat the feed water. Then the amount of heat supplied in the boilers increases. Extracting or draining the steam from the turbine for the purpose of heating feed water is called bleeding. So the cycle is also called bleeding cycle. The steam drained or extracted from the turbine is called bled steam.

34 STEAM NOZZLE By Mr. K.Venugopal Assistant professor MLRI T

35 Steam Nozzle Steam nozzle is an insulated passage of varying cross-sectional area through which heat energy (Enthalpy), pressure of steam is converted into kinetic energy.

36 Functions of Nozzle :- 1) The main function of the steam nozzle is to convert heat energy to kinetic energy. 2) To direct the steam at high velocity into blades of turbine at required angle. Applications :- 1) Steam & gas turbines are used to produces a high velocity jet. 2) Jet engines and rockets to produce thrust (propulsive force)

37 Types of nozzles :- 1) Convergent nozzle 2) divergent nozzle 3) convergent - divergent nozzle

38 1) Convergent nozzle :- It is a nozzle with large entrance and tapers gradually to a smallest section at exit. It has nodiverging portion. 2) Divergent nozzle :- It is a nozzle with small entrance and tapers gradually to a large section at exit. It has noconverging portion at entry. 3) convergent - divergent nozzle :- convergent - divergent nozzle is widely used in steam turbines. The nozzle converges first to the smallest section and then diverges up to exit. The smallest section of the nozzle is called throat. The divergent portion of nozzle allows higher expansion ratio i.e., increases pressure drop. The taper of diverging sides of the nozzle ranges from 6 0 to If the taper is above 15 0 turbulent is increased. However if it is less than 6 0, the length of the nozzle will increases.

39 Flow of steam through a nozzle :- The steam enters the nozzle at a high pressure with a relatively small velocity. As the steam flows (expands)the velocity will increase and pressure drops. The time spent by steam in the nozzle is very small and the heat exchange across the nozzle wall may be neglected. Therefore the steam flow through a nozzle may be regarded as adiabatic expansion. The enthalpy drop during the expansion is utilised to increase kinetic energy. In practice there is a friction between steam and wall of the nozzle, as a result the process is irreversible. It is important to note that the expansion of steam through a nozzle is not a free expansion nor throttling expansion, but it is an adiabatic with or without friction. Due to rapid expansion, steam does not get time to condense, i.e., condensation is delayed. This phenomena is known as supersaturated flow.

40 Reaction steam turbine By Mr. K.Venugopal Assistant professor MLRI T

41 Reaction steam turbine In reaction turbine, there is no nozzle to convert steam energy to mechanical energy. Moving blades work due to differential pressure of steam between front and at behind of moving blades In general, reaction turbine is not stand alone, but works at behind impulse turbinewhether constructed in one rotor or at separated rotor, but still connected by coupling. The purpose of impulse turbine is to control speed and reduce steam enthalpy to specified level. Reaction turbine is just receiving steam condition from impulse blades. Typical pairs of reaction and impulse turbines are;

42 Curtis (Stand alone or Single Stage) a. Compact. b. Power is relative small ( up to 2000 kw). c. Speed is relative low ( up to 6000 rpm, except for special design up to rpm). d. Enthalpy drop is high.

43 Rateau (Multi rows) a. Efficiency is higher than Curtis b. Power is high ( up to 30,000 kw) c. Generally, speed is higher than Curtis (up to15000 rpm) d. Enthalpy drop for each row lower than Curtis but still high, higher than Reaction

44

45

46

47

48 Reaction (Multi row reaction + 1 row impulse for control stage) a. More efficient b. Power is high c. Speed is high (up to15000 rpm) d. Enthalpy drop each row is low e. For low steam pressure

49 STEAM PROCESS IN STEAM TURBINE

50 GAS TURBINES By Mr. K.Venugopal Assistant professor MLRI T

51 IDEAL GAS TURBINE CYCLE If the losses are not considered then the cycle is called ideal gas turbine cycle. The gas turbine works on brayton cycle or joule cycle. The brayton cycle consists of two constant pressure processes and two isentropic processes

52 Sequence of Operations : 1) Isentropic compression processes (1-2) 2) constant pressure heating processes (2-3) 3) Isentropic expansion processes (3-4) 4) Constant pressure cooling processes (4-1)

53 The major difference in the performance of actual cycle is due to irreversibility caused by the friction. the pressure drop due to friction is less significant, and usually ignored. however the effect of friction on the performance of turbine and compressor is more significant. The work developed by the turbine decreases and work input to the compressor increases.

54 The following methods are used to improve the performance of gas turbine cycle. a) Multi compression with inter cooling b) Reheating c) Regeneration

55 A compressor in a gas turbine plant utilizes major portion of work developed by the turbine. The work required by the compressor is reduced by compressing the air in two or more stages and providing an inter cooler between the compressor. The inter cooler is a heat exchanger which decreases temperature of air at constant pressure.