Aim: To study the performance characteristics of Impulse turbine at various RPMs and pressure differences. Apparatus: An Impulse turbine setup which is made to run by creating a pressure difference between the inlet and the outlet consists of the following apparatus. An Impulse turbine. External compressor setup for providing high pressure at at the inlet. Tachometer to measure the speed (rpm) of the turbine. Strain gauge to measure the resistance given to the turbine. Thermocouples to measure the temperatures at inlet and exit conditions. Rotameter to measure the air mass flow rate.
Theory: In impulse turbine all fluid energy is converted into kinetic energy by a nozzle and it is is the jet so produced which strikes the runner blades. It has a Zero Degree of Reaction which means that no pressure drop occurs in the rotor.
Figure 1 Impulse Turbine. It shows four nozzles(D) impelling the blades(A) around shaft(c).
Study of Impulse Turbine
Roll No: 163010021 Name: Aravindkumar
By definition, Degree of reaction is the ratio of static pressure drop in the rotor to the static pressure drop in the stage or as the ratio of static enthalpy drop in the rotor to the static enthalpy drop in the stage. In this setup, the flow is initiated by creating a large pressure difference between the inlet and the exit of the turbine. We can change the mass flow rate by changing the inlet pressure. The four nozzles are used to convert most of the flow energy to kinetic energy. With this kinetic energy, the flow impinges on the turbine blades which initiates and maintains the rotation of the turbine. The nozzles are positioned such that they produce couple to rotate the turbine. In his setup, four nozzles are positioned as shown in the figure. They provide two couples to drive he turbine. The nozzle is also provided with valves so that they can be closed or opened to change the inlet flow which facilitates the study of the turbine at different conditions.
A strain gauge is attached to the belt the turbine to measure the force to the resistance created during various operating conditions. This setup measures the strain which is converted to force in Newton by the setup.
Precautions: All the nozzles should not be set to close at the same time. We need to decrease the resistance before reducing the pressure difference. The pressure supply should not be more than 50KPa so that the chances of instabilities due to vibration at high speeds (rpm) are reduced.
Procedure: There are two steps followed in this experiment In the first step, the inlet is provided with different pressures and hence different mass flows. As we vary the pressure, the mass flow will vary and the rpm of the turbine will vary accordingly. The rpm will be controlled by varying the resistance and it will be set at particular constant value for different pressures. The resistance to the load will be measured using a strain gauge while the mass flow rate will be taken from a tachometer. In the second step, the inlet pressure is kept constant and the rpm is varied by varying the resistance load. This gives the value of resistance force at different rpm. The mass flow rate remains constant as the pressure different is maintained constant. Both of the above steps are repeated with two adjacent nozzles shut off and the readings are recorded for calculation of performance parameters.
Roll No: 163010021 Name: Aravindkumar
Study of Impulse Turbine
Observation Tables:
For case 1:
4 nozzle open
Reference RPM=21500 P1
P2
Kpa
Kpa
131.325
101.325
141.325 151.325
Actual RPM
Load
Mass flow rate
T1
T2
N
Kg/sec
K
K
21480
0.66
0.0029
298.7
295.2
101.325
21490
0.96
0.0033
298.7
293.9
101.325
21450
1.16
0.0037
298.8
293
Ref. Pres. = 50KPa RPM
Actual RPM
Load
Mass flow rate
T1
T2
N
Kg/sec
K
K
18500
18400
1.41
0.0037
298.9
294.4
20500
20590
1.23
0.0037
298.9
293
22500
22410
1.14
0.0037
298.9
293.1
Case 2:
2 nozzle open
Reference RPM=19500 P1
P2
Kpa
Kpa
131.325
101.325
141.325 151.325
Actual RPM
Load
Mass flow rate
T1
T2
N
Kg/sec
K
K
19400
0.38
0.0008
299.7
296.5
101.325
19500
0.56
0.001
299.6
296
101.325
19600
0.68
0.0015
299.6
294.5
Ref. Pres. = 50KPa RPM
Actual RPM
Load
Mass flow rate
T1
T2
N
Kg/sec
K
K
16500
16580
0.77
0.0015
299.6
294.4
17500
17490
0.6
0.0015
299.6
294.4
18500
18410
0.74
0.0015
299.6
294.8
Roll No: 163010021 Name: Aravindkumar
Study of Impulse Turbine
Formulae used:
Pideal = mCp ΔT = mCp(T1-Tideal) Pactual=(2.Phi.NT)/60 Here T is torque. Efficiency
η=Pactual/Pideal
Specific Air consumption SAC= m/Pactual
Sample Calculations :
The calculation has been done for the Ref RPM 21500 with inlet pressure of 131.325 KPa. Using isentropic relation to find ideal T2:
Table1. Complete parameter calculated: Four Nozzles open
4 nozzles open RPM Constant P reference =50Kpa Mass flow rate
T1
T2
Kg/sec
K
K
0.0037
298.9
294.4
0.0037
298.9
0.0037
298.9
Rp
T
T2 Ideal
SAC
N-m
W
W
K
%
Kg/KWHr
0.669585
0.020445
39.37434
120.3448
266.5362
32.71793
338.291
293
0.669585
0.017835
38.43597
120.3448
266.5362
31.9382
346.550
293.1
0.669585
0.01653
38.77244
120.3448
266.5362
32.21778
343.5430
Table2. Complete parameter calculated: Four Nozzles open
Roll No: 163010021 Name: Aravindkumar
Study of Impulse Turbine
2 nozzles open RPM Constant:
Reference RPM=19500 Rp
T
T2 Ideal
SAC
N-m
W
W
K
%
Kg/KW-Hr
0.771559109
0.00551
11.18824
17.20905
278.2957
65.0137
257.4131716
0.716964444
0.00812
16.57292
27.30554
272.4303
60.69434
217.2218293
0.66958533
0.00986
20.22746
48.90271
267.1604
41.36266
266.9638029
Table3. Complete parameter calculated: Two Nozzles open
2 nozzles open Inlet Pressure Constant: Reference RPM=19500 Rp
T
T2 Ideal
SAC
N-m
W
W
K
%
Kg/KW-Hr
0.66958533
0.011165
19.37544
48.90271
267.1604
39.62039
278.7033
0.66958533
0.0087
15.92639
48.90271
267.1604
32.56751
339.0598
0.66958533
0.01073
20.67578
48.90271
267.1604
42.27942
261.1752
Roll No: 163010021 Name: Aravindkumar
Study of Impulse Turbine
Graphs: For each case: 2 nozzle open and 4 nozzle open ( × = ) 1. Efficiency – RPM (Inlet pressure constant):
Efficiency vs RPM (Constant Inlet Pressure) 70 60 50 ) % ( 40 y c n e i c 30 i f f E 20
4 nozzle open 2 nozzle open
10 0 15000
16000
17000
18000
19000
20000
21000
22000
23000
RPM
2. Efficiency – Pressure Ratio (Rpm constant)
Efficiency – Pressure Ratio (Rpm constant) 70 60 ) 50 % ( y c 40 n e i 30 c i f f 20 E
4 nozzle open 2 nozzle open
10 0 0.66
0.68
0.7
0.72
0.74
Pressure ratio
0.76
0.78
Roll No: 163010021 Name: Aravindkumar
Study of Impulse Turbine
3. Torque-RPM (Inlet pressure constant):
Torque vs RPM (Inlet pressure constant) 0.023 0.021 0.019 ) 0.017 m N0.015 ( e u 0.013 q r o T 0.011
4 nozzle open 2 nozzle open
0.009 0.007 0.005 16000
18000
20000
22000
24000
RPM
4. Torque-Pressure Ratio (Rpm Constant):
Torque vs Pressure Ratio (Rpm Constant) 0.018 0.016 0.014 ) 0.012 m N 0.01 ( e u 0.008 q r o 0.006 T 0.004
4 nozzle open 2 nozzle open
0.002 0 0.66
0.68
0.7
0.72
0.74
Pressure ratio
0.76
0.78
Roll No: 163010021 Name: Aravindkumar
Study of Impulse Turbine
5. Actual Power – RPM (Inlet pressure constant):
Actual Power vs RPM (Inlet pressure constant) 45 40 ) 35 W ( r e 30 w o P l 25 a u t c 20 A
4 nozzle open 2 nozzle open
15 10 15500
17500
19500
21500
23500
RPM
6. Actual Power – Pressure Ratio (Rpm constant):
Actual Power vs Pressure Ratio (Rpm constant) 40 35 30 ) W ( r 25 e w o 20 P l a u 15 t c A 10
4 nozzle open 2 nozzle open
5 0 0.66
0.68
0.7
0.72 Pressure ratio
0.74
0.76
0.78
Roll No: 163010021 Name: Aravindkumar
Study of Impulse Turbine 7. Specific Air Consumption-Actual Power (Inlet pressure constant):
Specific Air Consumption vs Actual Power (Inlet pressure constant) 350 340 330 )
320
r H - 310 W K 300 / g K (
4 nozzle open
C 290 A S 280
2 nozzle open
270 260 250 0.015
0.02
0.025
0.03
0.035
0.04
0.045
P actual (W)
8. Specific Air Consumption – Actual Power (Rpm constant):
Specific Air Consumption vs Actual Power (Rpm constant) 0.04 0.035 ) 0.03 W ( l a 0.025 u t c a P 0.02
4 nozzle open 2 nozzle open
0.015 0.01 100
150
200
250
300
350
SAC (Kg/KW-Hr )
400
450
500
Study of Impulse Turbine
Roll No: 163010021 Name: Aravindkumar
Conclusion: The efficiency drops for both 4 nozzle open (Case 1) and 2 nozzle open (Case 2) with increase in RPM. This is because of the higher rate of increase of ideal power than the actual power with increase in RPM. So more the RPM, less efficient is the turbine as the thermal losses become more pronounced at higher speeds. In the case with pressure ratio, efficiency increases directly with it as turbine efficiency depends much on its pressure ratio. We get more efficiency for 2 nozzle case as for the same pressure ratio as the RPM of this case is lesser for the same pressure ratio. It is obvious that torque vs. RPM curve shows a negative trend because torque always decreases with RPM. The main reason for this trend is that the performance of the engine in terms of power, torque, efficiency etc. decreases as the turbine is unable to convert the energy efficiently as the losses such as thermal, vibrational and stress related losses begins to increase. The actual power first decreases and then changes the trend with RPM. For both cases (Case 1 and case2) the actual power decreases with pressure ratio because of the losses discussed above. SAC shows a variation which first decreases with power and then mildly increases as at the optimum value of power, the SAC becomes the least and beyond that, SAC increases.
