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Table 2.1 Dimension of VLD Impeller tip radius, 30.8 Impeller hub radius, 12.5 Impeller exit radius, 62.5 Impeller tip width, 6.3 Tip blade angle, deg 60 Hub blade angle, deg 50 Impeller backsweep angle, deg -42 No. of impeller blade 7(7) Table 2.2 Dimension of VD No. of vanes 13 Vane leading edge, 66.25 Vane leading edge, 76.25 Table 2.3 CFD mesh Impeller 603011 Impeller Inlet 114905 Vaned diffuser 286376 Slot 124740 Table 2.4 CFD Boundary condition Impeller Inlet Diffuser Outlet Slot Outlet Total Pressure mass flow rate Static pressure - 27 -
Table 3.1 CFD recirculation mass flow rate 0.0377 0.1572 0.2249 0.2809 0.5247 Table 3.2 Different 3 cases case 0 Without slot case 1 Without slot case 2 With slot - 28 -
Fig. 2.1 Drawing of the vaneless diffuser Fig. 2.2 Drawing of vaned diffuser - 29 -
Fig. 2.3 Drawing of vaned diffuser with slot Fig. 2.4 Velocity triangle at vane inlet - 30 -
Fig. 2.5 Drawing of vaned diffuser with new slot - 31 -
1.8 1.6 PR 1.4 1.2 1.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f Fig. 3.1 Performance curve for vaneless diffuser 1.0 0.9 0.8 h 0.7 0.6 0.5 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f Fig. 3.2 Efficiency curve for vaneless diffuser - 32 -
90 60 a 2 30 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f Fig. 3.3 Impeller Outlet flow angle for vaneless diffuser - 33 -
1.0 0.8 f = 0.40625 f = 0.6875 f = 0.8125 f = 1.0 0.6 z 0.4 0.2 0.0-0.2-0.1 0.0 0.1 0.2 0.3 0.4 * C x (a) 1.0 0.8 f = 0.40625 f = 0.6875 f = 0.8125 f = 1.0 0.6 z 0.4 0.2 0.0-0.6-0.5-0.4-0.3-0.2-0.1 0.0 0.1 0.2 0.3 0.4 * C t (b) Fig. 3.4 (a) Hub to shroud axial velocity and, (b) Hub to shroud circumferential velocity distributions at impeller inlet - 34 -
Fig. 3.5 Locations of diffuser space - 35 -
1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0-0.2-0.1 0.0 0.1 0.2 0.3 0.4 * C r (a) 1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0-0.2-0.1 0.0 0.1 0.2 0.3 0.4 * C r (b) Fig. 3.6 Hub to shroud radial velocity distribution along diffuser (a) 1.0, (b) 0.6875-36 -
0.40625 0.6875 0.8125 1.0 Fig. 3.7 Meridional radial velocity contour for vaneless diffuser - 37 -
1.8 VLD VD 1.6 PR 1.4 1.2 1.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f Fig. 3.8 Performance curve for vaned diffuser 1.0 0.9 VLD VD 0.8 h 0.7 0.6 0.5 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f Fig. 3.9 Efficiency curve for vaned diffuser - 38 -
0.8 VLD VD 0.6 C p 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f (a) 0.8 VLD VD 0.6 K 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f (b) Fig. 3.10 (a) Pressure coefficient, (b) Pressure loss coefficient - 39 -
Fig. 3.11 Locations of vaned diffuser space - 40 -
1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0-0.2-0.1 0.0 0.1 0.2 0.3 0.4 * C r (a) 1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0-0.2-0.1 0.0 0.1 0.2 0.3 0.4 * C r (b) Fig. 3.12 Hub to shroud radial velocity distribution along diffuser (a) 1.0, (b) 0.6875-41 -
1.0 0.6875 Fig. 3.13 Meridional radial velocity contour for vaned diffuser - 42 -
1.60 1.55 PR 1.50 1.45 1.40 0.00 0.20 0.40 0.60 1.0 f r (a) 0.9 h 0.8 0.7 h max h min 0.6 0.5 0.0 0.2 0.4 0.6 f r (b) Fig. 3.14 (a) Performance curve with different recirculation mass flow rate (b) Efficiency curve - 43 -
1.0 0.8 z 0.6 0.4 case 0 case 1 case 2 0.2 0.0 60 70 80 90 a 2 Fig. 3.15 Impeller Outlet flow angle for different 3 cases - 44 -
Fig. 3.16 Locations of vaned diffuser space with slot - 45 -
1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0-0.2-0.1 0.0 0.1 0.2 0.3 0.4 * C r (a) 1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0-0.2-0.1 0.0 0.1 0.2 0.3 0.4 * C r (b) - 46 -
1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0-0.2-0.1 0.0 0.1 0.2 0.3 0.4 * C r (c) Fig. 3.17 Hub to shroud radial velocity distribution along diffuser (a) case 0, (b) case 1, (c) case 2-47 -
1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 * C t (a) 1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 * C t (b) - 48 -
1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 * C t (c) Fig. 3.18 Hub to shroud tangential velocity distribution along diffuser (a) case 0, (b) case 1, (c) case 2-49 -
(a) (b) (c) Fig. 3.19 Meridional radial velocity contour (a) case 0, (b) case 1, (c) case 2-50 -
(a) (b) - 51 -
(c) Fig. 3.20 Diffuser pressure loss coefficient contour (a) case 0, (b) case 1, (c) case 2-52 -
1.0 0.8 case 0 case 1 case 2 0.6 C p 0.4 0.2 0.0 1.0 1.1 1.2 1.3 1.4 1.5 R * (a) 1.0 0.8 case 0 case 1 case 2 0.6 K 0.4 0.2 0.0 1.0 1.1 1.2 1.3 1.4 1.5 R * (b) Fig. 3.21 (a) Pressure coefficient, (b) Pressure loss coefficient - 53 -
1.8 1.6 VLD VD VD with slot PR 1.4 1.2 1.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 1.0 f (a) 0.8 h 0.6 0.4 VLD VD VD with slot h max VD with slot h min 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f (b) Fig. 3.22 (a) Performance curve for vaned diffuser with slot, (b) Efficiency curve for vaned diffuser with slot - 54 -
1.8 VD with slot VD with new slot 1.6 PR 1.4 1.2 1.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 1.0 f (a) 0.8 h 0.6 0.4 VD with slot h max VD with slot h min VD with new slot h max VD with new slot h min 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f (b) Fig. 3.23 (a) Performance curve for vaned diffuser with new slot, (b) Efficiency curve for vaned diffuser with new slot - 55 -
1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0-0.2-0.1 0.0 0.1 0.2 0.3 0.4 * C r (a) 1.0 0.8 a b c d 0.6 z 0.4 0.2 0.0-0.2-0.1 0.0 0.1 0.2 0.3 0.4 * C r (b) Fig. 3.24 Hub to shroud radial velocity distribution along diffuser (a) vaned diffuser with slot, (b) vaned diffuser with new slot - 56 -
(a) (b) Fig. 3.25 Meridional radial velocity contour (a) vaned diffuser with slot, (b) vaned diffuser with new slot - 57 -
Seoul National University School of Mechanical and Aerospace Engineering Xu Yuxue Abstract Vaned diffuser has higher pressure ratio than vaneless diffuser. Recently requirement for low solidity vaned diffuser is increasing, and many researches focus on improving compressor operating range. Operating range is one of the most important compressor performances. The main reason causing decreasing operating range is stall. Stall can happen at both impeller and diffuser. One of the solutions to solve this problem is to put slot at impeller leading edge. At low mass flow rate, the effect of slot is to make flow stable. After all, it can improve operating range. In this research, it considered that vane in the diffuser space and connected impeller inlet and vane diffuser inlet through slot. The flow which was sucked into slot recirculates into impeller inlet by pressure difference between impeller and diffuser. Than separation flow was also - 58 -
sucked in and the incidence angle at impeller inlet and diffuser inlet could be reduced. It expects that this can control. In this study, it was verified numerically. In numerical simulation, it did not connect impeller and vane diffuser actually. Instead, it used value of vane inlet pressure that was got from CFD and gave proper static pressure condition to slot outlet. In CFD, it could be enough to simulate the real situation. The slot was set into only diffuser, and the result was compared when slot was relocated. As results of CFD, it showed that the slot could control stall. When set slot into vane diffuser inlet, the impeller out let mass flow was increased by recirculation mass flow. Therefore, the impeller outlet angle was reduced which lead to decrease i0ncidence angle. The pressure recovery coefficient got higher and the flow in diffuser space was improved. The loss by separation flow was also reduced. The results showed that the slot has positive effects to both impeller and diffuser flow. Consequently, it improved compressor operating range. Keywords : centrifugal compressor, vaned diffuser, slot, stall, Student Number : 2010-24060 reverse flow, recirculation mass flow - 59 -