Compressor is a very important part of a gas turbine installation as it absorbs almost two thirds of the power developed by the turbine. The overall efficiency of a gas turbine plant is highly dependent on compressor performance. Introduction Gas turbine development Classification of gas turbines. steam turbine Applications of gas tubes References.
INTRODUCTION
EXERCISES 1
18 GAS TURBINES AND JET AND ROCKET MANUFACTURING 1 '6, Name the two types of compressors used in gas turbines. Explain with a schematic diagram how a simple open cycle constant volume or explosion gas turbine works. What are the advantages of an open cycle gas turbine compared to reciprocating internal combustion engines.
ANALYSIS OF GAS TURBINE CYCLES
On what factors does the ideal standard air efficiency of the constant pressure open gas turbine depend? Derive the expression :rn for the efficiency of the constant pressure open cycle gas turbine taking into account the efficiency of the components. Show that the work output of a simple gas turbine depends on the temperature, temperature, and pressure ratio of the turbine and compressor.
What are the three parameters on which the size of the simple open cycle gas turbine depends. List five m:ii:i thcnu()dynamk: variables that affect the performance of a gas turbine plant. Draw a schematic diagram and a T-diagram of an open-cycle gas turbine with a rrrgrn,;;ra!or.
At what pressure ratio does the regenerator become redundant 'I. GAS TURBINE AND JET .AND VLAUTPOPULl:HON. What are the main effects caused on the performance of an open cycle gas turbine with regeneration by the addition of an intercooler and a reheater. Discuss the advantages and disadvantages of a closed cycle gas turbine compared to an open C) cle gas turbine.
Why can a closed cycle gas turbine be smaller in size than an open cycle gas turbine with the same horsepower?.
BASIC GAS DYNAMICS
EXERCISES 3
118 GAS TURBINE .AND JET AND ROCKET PROPULSION. b) Sl:ow it for an i~entropic flow. is t;,c the value of the specific heat ratio cf. tJds gas. The cross-sectional area of the channel is 1 cm2. a) Balance the stagnation temperature for stre3m. b). Wkt is the maximum percentage area reduction that could be introduced with a reduction in the flow rate of the current.
The photographs of a sphere in flight show that at a great distance from the sphere the total included angle of the wave is 50·3°. The function of a compressor is to increase the pressure of the air or gas introduced. Dynamic compressor, which increases the pressure of the liquid passing through them up to approx. 3 kgf/cm2, referred to as blouses.
Compressors that produce vacuum are called exhaust gases or air pumps, and those that increase the pressure of the liquid already above atmospheric pressure are called boo8ters. The ratio between the mass of air moving towards the compressor and the mass of air which completely fills the swept volume is defined as volumetric efficiency. When the volumetric efficiency is calculated from the conditions at normal prf"SSUFC, and tern peraturc, abbreviated as N .T P.
The mechanical efficiency of a reciprocating compressor is defined as the rado of indicated power to the braking force of the shaft supplied to the compressor.
At the end of delivery the volume instead of being zero, has a value equal to V3 • During inhalation, initially the air in the purge volume expands to pressure P1 and volume V4. process 3-4) and then suction is done. The purge volume only affects the effective suction stroke of the compressor and reduces the volumetric efficiency, but the p-'>m:r required to drive the compressor per kg of air remains the same. The deviation of the actual compression path from the JS·-Jthermal path increases with higher safe advance ratios (see Fig.
It is clear that as the pressure ratio increases, the volumetric efficiency decreases, thereby reducing the air capacity of the compressor. An important feature of the reciprocating cornpreisJr is that the pressure ratio is not affected by the speed and amount of air delivered by the compressor. As the rotors rotate, air is trapped between them and the sides of the housing on the inlet side.
The efficiency of the Roots blower depends on the pressure ratio and decreases as it increases due to increased leakage across the lo:,es. The position of the inlet port is such that air is taken into the compressor cylinder as the distance between the rotor and the cylinder wall increases. Further air compression occurs due to backflow when the inlet opening is uncovered.
About half of the total compression is due to vanes Ztnd this reversible compression is shown by the process b d on this p-v diagram.
INLE.T
The pressure and temperature of the air before compression are 1 kgf /cm2 and 16°0 respectively, and the compression rprocess can be assumed to follow the law pvl·' =constant. If the rate of heat transfer from the cylinder is estimated to be J·6 h.p., calculate the power required/or the compression. If the clearance ratios for the low and high pressure cylinders are 0·0-1 and 0·06 respectively, calculate the sweep and clearance volumes.
Derive the expressions for work done in a single-stage reciprocating compressor if the compression is (a) adiabatic, (b) polytropic, (c) isothermal. What is the effect of the clearance volume in a reciprocating compressor on (a) work done per kg cycle, (b) air delivered, (c) volumetric efficiency. Prove that in a two-stage reciprocating air compressor, when subcooling is complete, the expression for the total work required, W, is given by.
Therefore, prove that in a two-stage compressor, for maximum efficiency, the intermediate pressure p2 is the geometric mean of the initial pressure p1 and the final pre, sure p3. Show that the temperature rise of the gas in both cylinders cf. an ide;,l two-stage compressor is a minimum for the intercooler pressure p=( p1p2)0·5, where Pl and Pz are low pressure -.·y1in suction and high pressure cylinder delivery pressure or Prove that the volumetric efficiency of a single-stage reciprocating compressor with pressure ratio P2IP1 and clearance n.tio C is given by.
Estimate the power required to drive the compressor when (b) all the ports are positioned so that there is no internal compression, (b) the ports are positioned so that there is a 30 percent reduction in volume before back-lbw occurs .
DYJVAMIC COMPRESSORS
The work done in a dynamic compressor can be determined using the steady flow energy equation at the inlet and outlet of the compressor. 5·3] 147 The extra work done in actual practice is due to the increase in volume of air because n is greater than y and is internal. T1d (5·7 b) Thus, the isentropic efficiency is also equal to the ratio between the isentropic temperature rise and the actual temperature rise.
The fact that on a T,s diagram (see Figure 5-5) the vertical distance increases with increasing entropy illustrates well that in a multistage frictionless fusion the isentropi::temi:,;enhure increase is more likely. an elemental stage with a higher entropy than the temperature of another elemental stage with a lower temperature and the temperature increases for all elements. The polytropic efficiency defined in this way is very useful in the case of multi-stage compressors, as it only depends on the. actual temperatures and avoids using ideal ~diabatic tempera•. If n is the same for all stages, the total efficiency is equal to that of each individual stage on a polytropic basis but not on an isentropic definition. The isentropic efficiency of a compressor is the ratio of usable energy output to total energy input.
In the analysis of the centrifugal and axial compressors, the following assumptions are generally made:. l) The flow phenomenon is constant. The air flows through the impeller and is rotated in its passage through an angle of 90° and ideally the outlet is in a radial direction, i.e. the absolute velocity at the outlet 02," is such that its vortex component is equal to the blade tip velocity U2 • However, the large 2. assembly of the air mass flowing through the impeller has a certain inertia and due to the formation of vortices the velocity of the air at the tip i:;i always less than the blade speed. The exit swirl velocity and ttow of the air is turned through an angle less than 90° compared to a 90° turn in ideal condition. the actual power required by the compressor is more than this due to for the following reasons: i) Air has a certain viscosity and the viscous shear forces inhibit the flow near the wall and create a region of low kinetic energy than the main flow.
Thus the absolute exhaust velocity 02 is greater than the absohite inlet velocity 01 • The air is then transferred to the stator blades where further fressure rise takes place, arid in _ adcliticm to.
VORTEX THEORY
The vortex design is an attempt to account for the effect of changing blade radius. One of the important aspects of aerodynamic design is the study of blade profiles and the influence of the presence of other blades on flow phenomena, i.e. from these coefficients. blade angles a11d hr nee efficiency; airfoil can be found.
Both surge and stall impair the performance and limit the operation of the compressors. The mechanism of the instability of flow can be understood as follows: Consider a typical cascade shown in Fig. Fluctuating h als0a:,~Gci 1ted·witll ~epuation of flow or ~talling but the main cause ,}!'· it is the dynamic instability of the system as a whole.
1 and the angle of attack v 1, so there will be a stall in the remaining stages of the compressor. The effect of the flow reduction is cumulative and the pressure drops with the reduced mass flow. This front and back\';stard flow of air rf':result of strong pulses and phenorenma is called a wave. The overall characteristics of the compressor can be divided into stable and unstable regions, separated by the shock line.
Overvoltage Lne shows the limit for stable operation of the compressor - to the right is the stable region, and to the left is the unstable region.
