Lect-36. In this lecture... Tutorial on radial flow turbines. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay

Transcription

1 Lect- 36 1

2 In this lecture... Lect-36 Tutorial on radial flow turbines 2

3 Problem # 1 Lect-36 The rotor of an IFR turbine, which is designed to operate at the nominal condition, is cm in diameter and rotates at 38,140 rpm. At the design point the absolute flow angle at rotor entry is 72 o. The rotor mean exit diameter is one half of the rotor diameter and the relative velocity at rotor exit is twice the relative velocity at rotor inlet. Determine the specific work done. 3

4 Solution: Problem # 1 Volute/Scroll 1 2 Nozzle blades Rotor blades 3 4 Flow Diffuser C α 2 2 β 3 V 2 C a3 V 3 U 2 U 3 90 o IFR turbine arrangement and velocity triangles 4

5 Solution: Problem # 1 5

6 Solution: Problem # 1 We can sum up the three terms and divide by 2 to get the specific work as W=225, 142 m 2 /s 2 The fractional contributions of each of the three terms to the work output is for U 2, for V 2 and for C 2. We can also calculate the specific work by W=U 2 2 = =225, 150 m 2 /s 2 6

7 Problem # 2 Lect-36 A radial inflow turbine develops 60 kw power when running at 60,000 rpm. The pressure ratio (P 01 /P 3 )of the turbine is 2.0. The inlet total temperature is 1200 K. The rotor has an inlet tip diameter of 12 cm and an exit tip diameter of 7.5 cm. The hub-tip ratio at exit is 0.3. The mass flow rate is 0.35 kg/s. The nozzle angle is 70 o and the rotor exit blade angle is 40 o. If the nozzle loss coefficient is 0.07, determine the total-to-static efficiency of the turbine and the rotor loss coefficient. 7

8 Solution: Problem # 2 Volute/Scroll 1 2 Nozzle blades Rotor blades 3 4 Flow Diffuser C α 2 2 β 3 V 2 C a3 V 3 U 2 U 3 90 o IFR turbine arrangement and velocity triangles 8

9 Solution: Problem # 2 The rotor tip rotational speed is U 2 =πd 2 N/60=377 m/s From the velocity triangle at the rotor inlet, β 2 =0, therefore, sin α 2 =U 2 /C 2 C 2 =U 2 cosec α 2 = m/s T 2 =T 02 -(C 22 /2c p )=1130 K To calculate the stagnation temperature drop (isentropic) across the turbine, we shall use the pressure ratio. 9

10 Solution: Problem # 2 10

11 Solution: Problem # 2 11

12 Solution: Problem # 2 12

13 Problem # 3 Lect-36 An IFR turbine with 12 vanes is required to develop 230 kw at an inlet stagnation temperature of 1050 K and a flow rate of 1 kg/s. Using the optimum efficiency design method and assuming a total-to-static efficiency of 0.81, determine (i) the absolute and relative flow angles at rotor inlet; (ii) the overall pressure ratio, P 01 /P 3 ; (iii) the rotor tip speed and the inlet absolute Mach number. 13

14 Solution: Problem # 3 For optimum design, we use the Whitfield s equation, cos 2 α 2 =1/N, where, N is the number of vanes. For 12 vanes, α 2 = o As a consequence of the Whitfield s equation, β 2 =2(90- α 2 )=33.56 o 14

15 Solution: Problem # 3 15

16 Solution: Problem # 3 16

17 Solution: Problem # 3 17

18 Problem # 4 Lect-36 Compare the specific power output of axial and radial turbines in the following casel Axial turbine : α 2 =β 3 =60 o and α 3 =β 2 =0 o Radial turbine: α 2 =60 o and β 3 =α 3 =β 2 =0 o The rotational speed is the same in both the cases. 18

19 Solution: Problem # 4 U 2 =U 3 =U α 2 β 3 C 3 C 2 V 2 V 3 Axial turbine C α 2 2 β V 2 C 3 a3 V 3 U 2 U 3 Radial turbine 19

20 Solution: Problem # 4 Axial flow turbine: Since α 2 =β 3 =60 o and α 3 =β 2 =0 o The specific work is W axial = U(C w2 +C w3 )=U 2 Radial flow turbine: α 2 =60 o and β 3 =α 3 =β 2 =0 o The specific work is W radial = U 2 C w2 -U 3 C w3 )= U 2 U 2 -U 3 x0 =U 2 2 Therefore, the specific work done in both the turbine configurations are the same, given the conditions of operation. 20