임펄스 터빈, 리액션 터빈 Impulse turbine and Reaction turbine

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임펄스 터빈, 리액션 터빈 Impulse turbine and Reaction turbine

임펄스 터빈(impulse turbine)은 고정된 노즐(fixed nozzle)과 블레이드(blade)로 구성되어 있다.

리액션 터빈(reaction turbine)은 회전하는 노즐(rotating nozzle)로 구성되어 있다. 

In impulse turbine, there are nozzle and moving blades are in series while there are fixed blades and moving blades are present in Reaction turbine (No nozzle is present in reaction turbine).

 

In impulse turbine pressure falls in nozzle while in reaction turbine in fixed blade boiler pressure falls.

 

In impulse turbine velocity (or kinetic energy) of steam increases in nozzle while this work is to be done by fixed blades in the reaction turbine.

 

Compounding is to be done for impulse turbines to increase their efficiency while no compounding is necessary in reaction turbine.

 

In impulse turbine pressure drop per stage is more than reaction turbine.

 

The number of stages is required less in impulse turbine while required more in reaction turbine.

 

Not much power can be developed in impulse turbine than reaction turbine.

 

Efficiency of impulse turbine is lower than reaction turbine.

 

Impulse turbine requires less space than reaction turbine.

 

Blade manufacturing of impulse turbine is not difficult as in reaction turbine it is difficult.

스팀터빈의 작동

Working of Steam Turbine 

스팀터빈은 작동유체가 가지고 있는 열에너지를 기계적 에너지로 전환시켜주는 장치로 발전 사이클의 중요 구성요소이다. 

고온고압의 작동유체가 로터(rotor)에 붙어 있는 블레이드(Blade)를 통과하면, 블레이드 주변에 압력 차가 발생하여 로터가 회전하게 된다. 이로서 열에너지가 기계적 에너지로 변환되는 것이다.  

Fig.1 Rotating blades of turbine helps in transforming thermal in fluid to mechanical energy

하나의 로터에서 작동유체가 지닌 열에너지를 기계적 에너지로 변환시키지 못하기 때문에, 여러 개의 로터가 필요하게 된다. 로터 한 개를 살펴보면 아래 그림과 같다. 

Fig.2 A typical steam turbine rotor

블레이드를 자세히 살펴보면, 로터 바닥에서 시작해 끝단까지 에어포일(airfoil) 형태인 것을 확인할 수 있다. 

즉 블레이드 윗면과 아랫면에서 압력 차이가 발생하고, 이 압력 차이에 의해 로터가 회전하게 된다. 

Fig.3 Fluid flow around airfoil cross sectioned blade induces a high pressure (P) and low pressure(Ps) on blade surfaces

Energy Associated with a Fluid

유체는 3가지 형태의 에너지 요소를 가지고 있다. 

Kinetic energy - Virtue of its velocity

Pressure Energy - Virtue of its pressure

Internal Energy - Virtue of its temperature

여기서, 내부에너지(Internal energy)와 압력에너지(Pressure energy)는 엔탈피(H, enthalpy) 개념으로 대체할 수 있다. 

즉, 유체가 가지고 있는 에너지는 운동에너지(Kinetic energy)와 엔탈피로 구성된다. 

Energy Transfer to Rotors

When fluid passes through rotor blades it loses some amount of energy to the rotor blades. Due to this both kinetic and enthalpy energy of fluid come down for a typical rotor. As kinetic energy comes down velocity of flow decreases. If we directly pass this stream to next stage of rotor blades it will not transfer much energy because of low velocity of flow stream. So before passing the stream to next rotor stage we have to increase the velocity first. This is achieved with use of a set of stationary nozzle blades, also known as stator. When fluid passes through stator blades velocity of fluid increase due to its special shape thus one part of enthalpy energy will get converted into kinetic energy. Thus enthalpy of stream reduces and kinetic energy of stream increase. It is to be noted that here there is no energy addition or removal from flow, what happens here is conversion of enthaply energy into kinetic energy. Now this steam of fluid can be passed to next rotor blades and process can be repeated. Velocity and enthalpy variation of flow is shown in following figure.

Fig.4 Velocity and enthalpy variations across rotor and stator stages of a typical steam turbine

Degree of Energy Transfer

Total energy transfer to the rotor blade is sum of decrease in kinetic energy and decrease in enthalpy. Degree of contribution of each term is an important parameter in axial flow machines. This is represented by a term called of degree of reaction, which is defined as D.O.R 

0 % Reaction - Impulse Turbines

When D.O.R = 0 there will not be any enthalpy change across the rotor, such a turbine is known as impulse turbine. Blades of such a turbine would like as shown below.

Fig.5 A typical impulse turbine rotor cross section and flow pattern

Here incoming flow stream hits the blade and produces and impulse force on it. Since enthalpy across the blade does not change temperature will also remain same. There will be minor pressure drop across the rotor, but this is almost negligible. Here energy transfer to the blade is purely due to decrease in kinetic energy of fluid.

100 % Reaction Turbines

When D.O.R = 1 kinetic energy change across the rotor will be zero, energy transfer will be purely due to decrease in enthalpy. Since kinetic energy is same across the rotor absolute value of velocities remain same. This is shown in figure below.

Fig.6 A typical reaction turbine rotor cross section and flow pattern

Usually people use compromise of above two discussed cases,that is 50% D.O.R . Such turbines are known as Parson turbines, where both kinetic and enthalpy energy transfer contribute equally to power transfer to rotor.

출처 - http://www.learnengineering.org/2013/02/working-of-steam-turbine.html

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2016. 01. 16 작성