# ME 423 Chapter 8 PREDICTION OF PERFORMANCE OF SIMPLE GAS TURBINES

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ME 423 Chapter 8 PREDICTION OF PERFORMANCE OF SIMPLE GAS TURBINES
Prof. Dr. O. Cahit ERALP

Prediction of Performance of Simple Gas Turbine
From cycle calculations it is possible to determine the PRESSURE RATIO ( Rc ) which will give the best overall efficiency for a given Tmax. MASS FLOW RATE to give the most suitable desired power output.    After such preliminary calculations, the most suitable design data for a particular application can be chosen. Then, it is possible to design individual components to give the required operation at the design point. That is running at the design speed N*, mass flow rate m* and pressure ratio R*. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Prediction of Terformance of Simple Gas Turbine
Then the off-design performance has to be determined which is the divergence from the design point over the complete operating range of speed and power output. The performance ¢ of the individual components may be estimated on the basis of the previous experience or actual experiments. When they are combined in an engine their operating range is considerably reduced. The problem is to find the Operating point (OP) on each component ¢ when the engine is running at a steady speed (EQUILIBRIUM).  The plot of these OP's form the EQUILIBRIUM RUNNING LINE (ERL). Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Prediction of Performance of Simple Gas Turbine
For the whole range of operating speeds, it will generate the EQUILIBRIUM RUNNING DIAGRAM. Determining the OP; the power output, thrust and the SFC can be obtained. The Equilibrium Running Diagram indicates the margin of operation from the surge line (SL) . This margin indicates a Margin of stability; indicates if there is enough margin to operate with adequate compressor efficiency. If the surge line is crossed some action has to be taken to recover, not to give rise to a failure. Ideally the engine should be operated within the region of maximum possible efficiencies. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Prediction of Performance of Simple Gas Turbine
Variation of SFC with reduction in power  PART LOAD PERFORMANCE. This is important while running the GT at low power settings. Poor sfc at part load is the biggest disadvantage of a GT, especially a vehicular one. The effect of ambient conditions on maximum output is also important, i.e. high & low Ta and Pa. Peak load energy generation:  Europe: cold days in winter,  America: hot days in Summer  for airplanes: Runway length (safety) and pay load (economics) are affected. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Off-Design Performance of Simple GT
Here we will try to analyse a :  a) Single shaft unit delivering shaft power.  b) Free turbine engine - power turbine drives the load. c) Simple jet engine, where the useful output is from the propelling nozzle. More complex arrangements - two spool engines, Turbofan & transient performance Chapter 9 Flow characteristics of a free turbine and propelling nozzle are similar and impose the same restrictions on the Gas Generator.  As a result of this several jet engines have been converted to Free Turbine Power engine for peak load electric generation, and marine applications. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Component Characteristics ¢
Axial compressor ¢ constant speed lines become vertical so ηc , Rc vs is plotted. FIG.1 Compressor Characteristics Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Component Characteristics ¢
Turbine ¢  do not show a significant variation in ND speed. Their operating range is usually severely restricted by another component downstream. FIG.2 Turbine Characteristics Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Off-Design Operation of The Single - Shaft GT
Since inlet and exhaust pressure losses are ignored; pressure ratio across the turbine is determined by the compressor pressure ratio and the pressure loss in the combustion chamber; ΔP034 = P012 - P032 The mass flow through the turbine = mass flow through the compressor - Bleeds + fuel flow; Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
a) Select a constant speed line on the C¢ and choose an OP on this line thus N/ T01 are selected. b) The corresponding point on the T¢ is obtained by the Compatibility of Speed and Flow. COMPATIBILITY OF ROTATIONAL SPEED Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
COMPATIBILITY OF FLOW Here combustion chamber pressure loss P03/P02 = 1 - Pb/P02   assume Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
and are fixed by the chosen OP on the is assumed to be constant. Neglecting inlet and exhaust pressure losses Pa = P01 = P04 is a function of Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
Now in the flow compatibility the only unknown is The rest can be obtained from C¢ and T¢. Thus, Thus, knowing T01, T03 can be calculated. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
Having determined T03 , the SPEED COMPATIBILITY : The compressor & turbine temperature changes can be determined. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
And the NET POWER corresponding to selected OP is : m could be calculated knowing P01 , T01 c) Having matched the C¢ & T¢ it is necessary to ascertain whether the work output corresponding to the OP is compatible with that required by the driven load.   For this; variation of power with speed "P(N)" should be known. This will indicate whether the OP selected represents a valid solution (Equilibrium).  Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
Examples: If the engine were run on a test bed, Coupled to an electric/or hydraulic dynamometer, the load could be set independent of speed. Then, it is possible to operate at any point on C¢ within safety limits (T03 , N). With a Propeller load - Power absorbed varies with as N3 of propeller. Knowing ṁ and gear ratio, the load characteristics in terms of Pout turbine vs Nturbine can be plotted which corresponds to a single Poutput per constant speed curve i.e single point on a fixed C¢. Only at this point the required output is given. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
FIG.3 Load Characteristics Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
Then the single point on each constant speed line of the C¢ has to be found. This is done by trial error, taking several OP on the C¢ and establishing the power output for each OP.  If the power output by turbine is not equal to power required by propeller then the engine will not be in equilibrium but accelerate or decelerate.   Finding the equilibrium points on a series of constant speed lines, and joining them the equilibrium running line is obtained.  The most common type of load used with a single shaft GT is the ELECTRIC GENERATOR which runs at constant N with the electrical load varying . Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
Then the single point on each constant speed line of the C¢ has to be found. This is done by trial error, taking several OP on the C¢ and establishing the power output for each OP.  If the power output by turbine is not equal to power required by propeller then the engine will not be in equilibrium but accelerate or decelerate.   Finding the equilibrium points on a series of constant speed lines, and joining them the equilibrium running line is obtained.  The most common type of load used with a single shaft GT is the ELECTRIC GENERATOR which runs at constant N with the electrical load varying . Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
FIG.4 Equiblirium Running Lines Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
The equilibrium running line for a generator set would correspond to a particular line of constant Each point on the line would represent a different value of T03 and Pout. At each speed it is possible to find by trial error the compressor OP corresponding to zero net output and connecting the No-Load Running Line for a Generator Set is obtained. Looking at the C¢ and propeller equilibrium line, the operation is generally at a high ηc. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Procedure of Obtaining an Equilibrium Running Point
Generator load results in a rapid drop in ηc as the load is reduced.  The location of equilibrium running line w.r.t. surge line indicates whether it could be brought to full power without any complications. If ERL and SL intersects a blow-off valve around the compressor rear is employed. No such problem for bringing up an electric generator (No load condition).  With the above findings T032 and hence from Combustion curves, f could be determined for an assumed b  then, sfc can be calculated. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on single shaft gas turbine
The following data refer to a SSGT operating at design speed: Ambient conditions:Pa=1.013 bar, Ta=288 K, m=98% (Neglect all pressure losses!) Calculate: T03 for Power=3800 kW Establish the T03 for each point given on the CC Establish (T02 - T01), (T03 - T04) and find Pout Plot T03 vs Pout to find the T03 for Pout =3800 kW Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on single shaft gas turbine
1) Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Equilibrium Running of a Gas Generator
The GG performs the same function for both the jet engine and free turbine engine. It generates continuous flow of gas at high pressure and temperature, to be expanded to lower pressure to produce either shaft work or a high velocity propulsive jet.   The compatibility of speed and flow are the same as the single shaft engine. Thus; Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Equilibrium Running of a Gas Generator
However, the pressure ratio of the turbine is not known. It must be determined by equating the turbine work to the compressor work.   The work requirement; These equations are linked by (T03/T01) and a trial-and-error procedure is necessary to determine T03 for any arbitrary point on C¢ Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Equilibrium Running of a Gas Generator
a) Select a comp. OP b) Calculate c) Guess a value of P03/P01 & calculate d) Find T03 / T01 from FLOW compatibility e) Using T03 / T01 calculate from SPEED COMPATIBILITY Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Equilibrium Running of a Gas Generator
f) With and P03 / P04 find c from T¢ g)Calculate h) Calculate (T03/T01 ) using (T034/T03) and POWER COMPATIBILITY i) Check T03/T01 with the "one" from flow compatibility (Step d) j) If different modify P03/P04 and repeat the steps c to i until obtaining the correct T03/T01 Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Equilibrium Running of a Gas Generator
k) The agreement of T03/T01 indicates that the turbine OP is compatible with the compressor OP for the temperature increase in CC satisfying T03/T01. It is not necessary to calculate this for a series of points because the downstream components impose limits on the operating zone of the C¢.   This could be repeated for a series of points and points of constant T03/T01 could be joined up, but unnecessary since the flow compatibility with the downstream components (power turbine/or/ propelling nozzle restricts the operating zone on the C¢.  Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Equilibrium Running of a Gas Generator
The matching procedure outlined here has been developed on the assumption that turbine ¢ do not exhibit a variation of with   This is true if the flow correspond to choked mass flows.  If not choked; before guessing P03 / P04  *Guess T03/T01 calculate N / T03 from speed compatibility   *calculate from flow compatibility *Then P03 / P04 and ηt can be obtained from T¢ *T034/T03 can be calculated and the GG work compatibility T03/T01 *Compare T03/T01 with the initial guess. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Off-design Operation of Free Turbine Engine
The matching is done by select a point on C¢. i) Flow compatibility i.e mass flow of GG = mass flow FT where ii) The pressure ratio available is fixed by the compressor and GGT press ratios.    Inlet and exit duct losses ignored. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Off-design Operation of Free Turbine Engine
iii) Having found the pressure ratio across the power Turbine, the value of can be found from the FT¢. iv) If from (i) and (iii) do not match; a new point on the constant speed C¢ has to be selected and this procedure has to be repeated until the flow compatibility between 2 turbines is satisfied. For each line on the C¢ there will be only one point which will satisfy both the requirement of the GG and the flow compatibility of the FT. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Off-design Operation of Free Turbine Engine
Equilibrium running line can be produced for different on C¢. The running line for the FT engine is independent of the load and determined by the swallowing capacity (ṁ) of the PT. FT engine has quite a different load performance than the single shaft GT. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Off-design Operation of Free Turbine Engine
FIG.5 Equilirium Running Line for Free Turbine Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Matching of 2 TURBINES IN SERIES
The iterative procedure of a FT/GG matching can be simplified if the 2-Turbines in series are considered. The variation of t at any pressure ratio is not large, particularly in the restricted range of operation. As a result the change in t does not affect so has a little effect on Therefore, a mean value of ηt is taken at any given pressure ratio. Then, Now the GG turbine exit conditions can be mapped on the GGT¢. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Off-design Operation of Free Turbine Engine
FIG.6 Operation of Turbines in Series Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Off-design Operation of Free Turbine Engine
The flow compatibility between the 2 turbines places a major restriction on the OP of GGT. As long as the PT is choked, the GGT will operate at a fixed ND point at all choked OP. With the PT unchoked the GG will operate at a fixed pressure ratio for each PT pressure ratio (i.e. fixed OP) Thus the maximum pressure ratio across the GGT is controlled by choking PT. (i.e the SWALLOWING capacity the GT). The turbine pressure ratios can be expressed in terms of the Rc as: Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Off-design Operation of Free Turbine Engine
FIG. 7 Compressor Pressure Ratio vs GGT Pressure Ratio For any value of the compressor pressure ratio, GGT pressure ratio can be obtained. Thus and T034 /T03 are fixed for GG flow compatibility & GG power compatibility. Thus for the GG, pressure ratio iteration is not necessary to find the correct equilibrium point. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Variation of Power Output & Sfc with Output Speed of a Free Turbine Engine
Power output of a FT engine = ṁ Cpg  T045 where FIG.8 Variation of Power Output with Output Speed Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Variation Of Power Output & sfc with Output Speed of a Free Turbine Engine
power output for each equilibrium running point (one for each compressor speed); i) P04 /Pa will be known ii) T04 can be calculated from T04 = T03 - T034 knowing Pa, Ta ; m can be found from Free turbines are used to drive a variety of loads each of which are different (pump, propeller, electric generator), each with different vs Npt ¢ .These curves are quite flat in the higher Npt region where pt is fairly constant. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Variation Of Power Output & sfc with Output Speed of a Free Turbine Engine
FIG.9 Variation of sfc With Power Output sfc increases as power is reduced, since as fuel flow decreases; Nc decreases, T03 decreases; but as T03 decreases cycle decreases. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Variation of Power Output & sfc with Output Speed of a Free Turbine Engine
Fuel consumption can be calculated similar to the single shaft units since the fuel consumption depends only on GG parameters. There will be one value for each . sfc however, is a function of both Nc and Npt as Pout The off-design performance can be expressed by plotting sfc vs Pout for different Npt. This shows the performance of the unit when coupled to different types of loads. Although for convenience Ncomp is chosen as the independent variable; in practice the fuel flow is the independent variable. A chosen value of fuel flow and (T03) determines Ncomp and therefore Pout. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Torque Characteristics
In case of a GT delivering shaft power, the variation of torque with output speed at a given power determines its suitability for different applications (e.g. high starting torque for traction). a) For the single shaft engine the compressor is constrained to turn at some multiple of load speed. Load speed decrease = Compressor speed decrease unsuitable for traction (since m decrease out decrease) b) Normal curve of Internal Combustion Engine is flat. c) Free power turbine has a favourable torque ¢ over a wide load-speed range for a fixed Nc. This is because the compressor can supply an essentially constant flow at a given compressor speed irrespective of the FT speed. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Torque Characteristics
Therefore, at constant Pout as Npt decrease  t increase. The torque might stall at high t or very low Npt.   With a reduction in Npt quite a large increase in t can be obtained efficiently. But at least a speed gear box have to be used for traction (usually 5-6 speed automatic transmission is used in heavy load vehicles) Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Torque Characteristics
FIG.10 Torque Characteristics Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on gas turbine with Free Power Turbine
Given: Calculate: Power developed and the turbine ND flows If the engine is running at same mechanical speed at ambient temp. of 268 K, calculate T03, P03 / P04 and Pout assuming the following: a)Combustion pressure loss remains constant. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on Gas Turbine with Free Power Turbine
b)Both turbines are choking with values of and as calculated above. No change in c)At 268 K and the same N, the line on the C¢ is a vertical line with ND flow 5% greater than the design value. d)Variation of compressor efficiency with pressure ratio at the relevant value of is: P02 / P01 6.0 6.2 6.4 6.6 0.873 0.843 0.845 0.840 Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on gas turbine with Free Power Turbine
Solution: Design Point Calculation: OP on CC: Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on Gas Turbine with Free Power Turbine
GG Power Turbine: Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on Gas Turbine with Free Power Turbine
Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on gas turbine with Free Power Turbine
OFF DESIGN At Ta= T01 =268K If the PT remains choked, the GGT will be constrained to operate at a fixed ND point and thus the value of as for the design condition The Work Compatibility: Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on gas turbine with Free Power Turbine
hence Flow Compatibility Now the problem is to find the OP that satisfies A and B simultaneously for With the variation in efficiency; Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on gas turbine with Free Power Turbine
With the constant value of CC loss Iterations Tabulated : Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on gas turbine with Free Power Turbine
With the constant value of CC loss Iterations Tabulated: Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on gas turbine with Free Power Turbine
Solving Graphically the required pressure ratio : P02 / P01 = with T03 / T01 = 4.34 Therefore; To3 = 4.34*268=1163K P developed can be calculated; since the GGT still operates at the same ND point Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Example on gas turbine with Free Power Turbine
Power output P = 32.7kg/s* 1.147J/kg.K*179,6K*0,99= 6680kW Thus on a cold day a decrease of Tamb to To1=268K results in a decrease of max. cycle temperature from 1200K to 1163K. T03/T01 increases from 4.17 to 4.34 due to the increased Power increases from 5910 to 6680 kW. This is due to increase of mass flow rate and compressor pressure ratio. The beneficial effect of low Ta on GT is evident also the adverse effect of increase of Ta. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Off Design Operation Of The Jet Engine Propelling Nozzle Characteristics
The propelling nozzle area is determined from the design point calculations. A fixed nozzle area has a major influence on the off-design operation. The nozzle ¢ in terms of ND variables is given in terms of and P04/Pa Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Propelling Nozzle Characteristics
For a nozzle of given area and j These are valid up to the critical point. The CRITICAL point of the nozzle is when Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Propelling Nozzle Characteristics
FIG.11 Propelling Nozzle Characteristics Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Propelling Nozzle Characteristics
for P04/Pa > P04/Pc , the nozzle is choked P5 = Pc > Pa and = const (not a function of P04/Pa ). The similarity between this and the turbine ¢ is evident. When the nozzle is choked Generally Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Matching of GG with Nozzle
The Nozzle will exert the same restriction on the operation of the GG as the FT, at STATIC conditions, The equilibrium running line can be determined as for FT. Here the effect of forward speed (Va) on the equilibrium running line has to be considered. FORWARD SPEED  RAM PRESSURE RATIO = f(Ma, ηi) RAM  P02 increase P04 increase  P04/P5 increase when choked maximum and independent of P04/P45 thus Va . Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Matching of GG with Nozzle
Then the Turbine OP will be unchanged because of the compatibility of flow between turbine & nozzle. That is; As long as the nozzle is choked, the equilibrium running line is uniquely determined by the fixed Turbine OP and independent of flight speed. Practically ALL JET ENGINES during take off, climb and cruise operate with Choked Nozzle. The nozzle may be unchoked when preparing to land or taxying. Since the running line is close to surge line at low ,the effect of Va on ERL has to be considered. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Matching of GG with Nozzle
Nozzle pressure ratio and Ram pressure ratio can be related as: The ram pressure ratio is: Therefore, for a given intake efficiency i ; P04/Pa = f (GG parameters and flight Mach Number). *The same procedure as for the FPT can be followed to obtain the equilibrium running point. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Matching of GG with Nozzle
FIG. 12 Jet Engine Running Lines Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Matching of GG with Nozzle
For each compressor speed the calculation is repeated for several Ma to cover the desired range of flight speed. The result  A fan of Equilibrium RL of constant Ma. These merge to a single RL at higher , where the nozzle is choked.  Increasing Ma pushes the equilibrium RL away from SL at low compressor speeds.   Therefore, the Ram pressure rise allows the Rc decrease for the required flow. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Variation of Thrust With Rotational-speed; Forward-speed; Altitude
The Net Thrust of the jet is; F = m (V5-Va) + (P5-Pa) A5 Fnet over the complete range of inlet conditions (Va, ) is determined by ND quantities as: Since: Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Variation of Thrust With Rotational-speed; Forward-speed; Altitude
When the Nozzle is UNCHOKED; with P5 = Pa and the pressure thrust is 0 since P5 / Pa = 1 When the Nozzle is CHOKED; and where the critical pressureratio, Pc / P04: Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Variation of Thrust With Rotational-speed; Forward-speed; Altitude
FIG.13 Variation of Thrust (F/ Pa ) with engine speed (N/T01) and flight speed (Ma) Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Variation of Thrust With Rotational-speed; Forward-speed; Altitude
There; The thrust for a given N/T01= f (Ma) ; although for choked flow there is a UNIQUE ERL. Increasing flight speed Va, m*Va= momentum drag increases P02 increases (i.e RAM increases ) At low N/T01, momentum drag increase predominates thus Ma increases  Fn decreases . At high N/T01, Ram pressure rise predominates Thus Ma increases  Fn increases Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

ENGINE SPEED Although performance is expressed in terms of ND speed, N/T01 , the actual mechanical speed N imposes a limit due to turbine stresses, and controlled. If the speed is kept well below this limit, the take-off thrust is substantially reduced. If N exceeds the correct limit: i) The centrifugal stresses increase with the square of speed N2 ii) A rapid increase in Turbine Inlet Temperature T03 (2% in N may cause 50K in T03) Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

ENGINE SPEED Since the blade life is determined by CREEP, the time which the high speeds are permitted must be controlled. Take-off rating t < 5 min 100% Nmax   Climb rating - reduction in fuel flow t < 30 min at 98 % Nmax   Cruise rating - further reduction in fuel and rotor speed at 95 % Nmax Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Effect of Ambient Conditions on the take off rating
Ta: With Engine running at max speed, Ta  ,N/  Ta  hence N/T01  , along the equilibrium running line ; Therefore, Ta   Fn  ( loss of thrust) T03  T03 = ( T03 / T01 )*T01 On a hot day T03 > T03max  N  is required, thus Fn  Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Effect of Ambient Conditions on the take off rating
Pa: Fn and Pa in direct proportion (since (F / Pa) ...) Altitude  , Pa  and Ta  ( up to m) since as Pa  Fn  but as Ta   Fn  Then Fn    Therefore ; thrust decreases with increase in altitude. Airports at high altitudes, especially around tropical zones are critical (Mexico-City, Nairobi ) suffer from this problem. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Variation in fuel consumption & sfc with rotational Speed, forward speed & altitude
Fuel consumption and fuel capacity of the aircraft determine the range sfc(fuel flow /per unit thrust) indicates economy Both are functions of N/T01 and Ma. With combustion efficiency b assumed, fuel consumption can be determined from : m, f/a curves, with ΔT032 . Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Variation in fuel consumption & sfc with rotational Speed, forward speed & altitude
Dependence of fuel flow on Ambient conditions can be eliminated by ND fuel flow The fuel parameter slightly depends on Ma when based on T01 and P01. They merge to a single line, for the choked nozzle conditions. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Variation in fuel consumption & sfc with rotational Speed, forward speed & altitude
FIG. 14 S.f.c. Curves Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Methods of Displacing the Equilibrium Running Line
If the Equilibrium Running Line (ERL) intersects the Surge Line (SL), it is not possible to bring the engine up to full power directly. The compressor may surge when the engine accelerates even ERL is not cutting the SL. Many high performance compressors have a kink in the SL. A running line intersecting SL at low N/T01 and at the kink is shown in Figure 15. To overcome ERL is lowered down in dangerous regions. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Methods of Displacing the equilibrium running line
BLOW-OFF is a method to achieve this. Air is bled from some intermediate compressor stage.  Some turbine work is wasted. blow-off valve only operates when it is essential. Variable Area Propelling Nozzle; an alternative method to blow-off. Either method will produce a reduction in P02/P01 at a given N/T01, hence lower the ERL. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Methods of Displacing the equilibrium running line
FIG. 15 Effect of Blow-off and Increased Nozzle Area Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Methods of Displacing the equilibrium running line
FIG. 16 Effect of Variable Area Propelling Nozzle Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

Methods of Displacing the equilibrium running line
In a variable area nozzle as nozzle area increases, A5 increases (P03/P04 ↑ ,so ΔP034/T03 ↑). If N/T01 is held constant P02/P01 ↓. Therefore, RL will be moved away from SL. To keep N/T01 constant fuel flow to be reduced. for N/T01 held constant and A5↑ ; ERL will be removed away from SL. Me 423 Spring 2006 Prof. Dr. O. Cahit ERALP

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