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    • 专利摘要:
    Centrifugal compressor comprising a casing (1), an impeller (2), a bladed diffuser (3), in which said diffuser is arranged around the impeller so as to collect the flow exiting from the impeller itself and rotate coaxially in the opposite direction to said impeller .
    公开号:CH712976A2
    申请号:CH01291/16
    申请日:2016-09-29
    公开日:2018-03-29
    发明作者:Bedetti Gianfranco
    申请人:Bedetti Gianfranco；
    IPC主号:
    专利说明:

    Description Field of application [0001] The invention concerns the sector of centrifugal compressors.
    Prior art [0002] The centrifugal compressor is a well-known machine which essentially comprises an impeller and a diffuser.
    [0003] The impeller receives the fluid to be compressed and throws it to the diffuser at increased pressure and very high speed which in some cases may be slightly supersonic.
    [0004] Fig. 1 shows the typical design of a centrifugal compressor. The diffuser has the task of converting the kinetic energy of the gas leaving the impeller into pressure energy. For this purpose the diffuser typically comprises a spiral duct adapted to reduce the speed of the gas so as to increase the pressure of the gas itself. In the case of medium and high prevalences (pressure energy) the first zone of the diffuser is equipped with vanes that have the purpose of improving the conversion of kinetic energy into pressure energy. It should be noted that most of this conversion takes place in the spiral, not in the blading.
    [0005] One of the main disadvantages of centrifugal compressors is given by the low efficiency of the diffuser. The impeller substantially converts all the mechanical work provided to its axis into energy transferred to the fluid (pressure energy, kinetic energy and internal energy); the diffuser does not perform mechanical work and limits itself to converting the kinetic energy of the fluid discharged from the impeller into pressure energy. In the diffuser the gas undergoes a considerable slowing down, for example from 300 m / s to 20 or 30 m / s and this requires a subsonic slow-down zone with an increasing section which greatly increases the length and the size of the diffuser.
    [0006] The impeller has a very high efficiency (over 0.9) in large and medium-sized machines, and still maintains good efficiency in small and very small machines; the diffuser, on the other hand, can achieve high efficiency only if it is made in very large dimensions and with the best expedients of gas dynamics as for example in the known Oerlikon blower illustrated in fig. 2 which, however, is cumbersome and not always usable. For these reasons, in many applications, for example in aeronautical engines, the axial compressor has been preferred but it is expensive and cumbersome because it requires many compression stages, since the compression ratio obtainable from a single axial stage is very low.
    [0007] It should be noted that the interest in centrifugal compressors is very high. Centrifugal compressors are universally used in the turbo-compressors of internal combustion engines, especially for automotive applications; the centrifugal compressor is also interesting for light aeronautical engines (eg helicopters and small airplanes), to achieve turbo-reactors of lower cost and size compared to axial turbo-jets, for the small-power gas turbine sector (so-called gas microturbines).
    Summary of the invention [0008] The invention aims to improve the technique of centrifugal compressors by obtaining: high efficiency; high specific power in relation to size; stable operation and high efficiency even in transients.
    [0009] The objects are achieved with a centrifugal compressor according to claim 1.
    [0010] The centrifugal compressor according to the invention comprises a case and an impeller, and is characterized in that it comprises a bladed diffuser which rotates coaxially opposite to said impeller and is arranged around the impeller, so as to collect the flow outgoing from itself; the diffuser blades facing backwards with respect to the direction of rotation of the diffuser itself.
    [0011] The diffuser is a movable body and keyed on the same axis as the impeller so as to rotate in the opposite direction of the impeller; the diffuser also receives mechanical work on its shaft, which forces it to rotate in the opposite direction of the impeller. In essence, the diffuser can be considered as a second counter-rotating impeller with backward-facing blades.
    [0012] Further preferred aspects of the invention are in accordance with the dependent claims.
    [0013] Advantageously, the rotating diffuser operates in a clearly supersonic regime. In this way the rotating diffuser uses shock waves to compress the gas. This compression, as will be shown below, has a higher efficiency than the subsonic polytropic transformation that occurs in conventional compressor diffusers.
    [0014] According to the above, advantageously, the blades of the rotating diffuser define channels within which there is a supersonic flow during the operation of the compressor. Preferably the flow in the diffuser is clearly supersonic (not transonic); more preferably it has a number greater than 1.1, more preferably between 1.1 and 2.5 and even more preferably between 1.3 and 2.
    [0015] Preferably the shock waves are generated at the entrance of the channels defined by the vanes of the rotating diffuser. In a preferred embodiment the diffuser blades have an leading edge having two inclined planes which define an attachment edge for the formation of said impact waves. Said attachment edge preferably has an angle of 30 ° or about 30 °.
    [0016] The advantage of the invention is that the flow exiting from the impeller "meets" the rotating bladed diffuser which rotates in the opposite direction and, consequently, the relative speed of the flow entering the diffuser is very high. Said speed can be of many hundreds of m / s and therefore clearly supersonic. Within the rotating diffuser (ie in the channels defined by the diffuser blade) the flow slowing is modest; however at the outlet of the diffuser an absolute low flow speed is obtained due to the composition with the drag speed.
    [0017] Consider the following two simplified examples, in which: both the radius of the impeller and the inner radius of the diffuser (neglecting the clearance). R2 is the outer radius of the mobile speaker.
    [0018] First simplified example (subsonic regime).
    [0019] The following data are considered: peripheral speed of the impeller: 150 m / s on the radius Fh, diffuser rotating in the opposite direction to the peripheral speed in Ft, of 150 m / s, desired output speed (outgoing from the case) of 50 m / s.
    [0020] In these conditions the diffuser is fed at a relative speed of 150 + 150 = 300 m / s and to obtain an absolute output speed of 50 m / s it will have to slow down the air inside it from 300 to 200 m / s only; in fact the gas leaving the rotating diffuser has a relative speed (with respect to the diffuser) of 200 m / s. Said speed, decreased by the drag speed of 150 m / s (assuming R2 approximately equal to Ri), is equal to the absolute output speed of 50 m / s. A ratio between the input / output speed in the speaker is 300/200 = 1.5. With a conventional compressor equipped with a static (non-rotating) diffuser, under the same conditions, the diffuser should have a ratio between the speeds of 150/50 = 3. This high ratio, in conventional compressors, requires a large diffuser. Thanks to the invention, and in particular thanks to the rotation of the diffuser opposite to the rotation of the impeller, the said ratio between the speeds is drastically reduced.
    [0021] It should be noted that in the example just illustrated the difference between the radii Ri and R2 has been neglected, considering R2 slightly different from R · ,. If R2 is significantly greater than R, the result is even more advantageous. In the previous example, setting R2 = 1.2 R-, the exit speed is 180 m / s; therefore the mobile diffuser of the invention slows down the gas from 300 to 230 m / s only. The ratio of speeds drops to 1.30 against 3 of the conventional speaker.
    [0022] Second simplified example (supersonic regime).
    [0023] A peripheral speed of the impeller and of the diffuser is assumed to be 300 m / s over the radius R ,. The diffuser is powered by a supersonic speed of 600 m / s, relative to an observer attached to it. If the air is to be released at the counter at a speed of 50 m / s it will have to slow down inside the internal mobile diffuser from 600 to 350 m / s. In fact the relative gas speed of 350 m / s minus the drag speed of 300 m / s equals the absolute speed of 50 m / s. The result is a ratio between the absolute speeds = 600/350 = 1.71 against 300/50 = 6 for a conventional compressor, for R2 approximately equal to Ri. As in the previous case, with R2 = 1.2 Ri the drag speed at the exit is 1.2 x 300 = 360 m / s; the diffuser slows down the gas from 600 to 410 m / s and the ratio 0, / ¾ further decreases to 600/410 = 1.46.
    [0024] An aspect of the invention consists in a slowing down of the air in a supersonic regime inside the mobile diffuser. The gas is compressed almost instantaneously in an extremely small path of the order of 10 "5 cm and with a very high yield.
    [0025] These examples, although very simplified, show the advantages of the invention which include, among other things: high performance, very high compression ratio even with a single stage and therefore a lighter and more compact machine. These advantages are achieved thanks to the configuration with two impellers (impeller and mobile diffuser). The following detailed description will give more complete and rigorous examples to show the advantages of the invention.
    [0026] The invention can be applied to all types of centrifugal compressor. Among the preferred applications, by way of non-limiting example, there are turbochargers for internal combustion engines and gas turbine compressors or turbojets of limited power.
    Description of the figures [0027]
    Fig. 1 shows a known centrifugal compressor.
    Fig. 2 shows a compressor with a known type Oerlikon blower.
    Fig. 3 shows a compressor according to a way of carrying out the invention, comprising an impeller and a mobile diffuser.
    Fig. 4 shows an example of velocity triangles of a compressor known as that of fig. 1.
    Fig. 5 shows an example of triangles of the speeds of a compressor according to the invention, respectively on a first input radius of the impeller and on a second output radius of the impeller and mobile diffuser inlet.
    Fig. 6 shows another example of triangles of the speeds of a compressor according to the invention, respectively on a first ray of entry of the mobile diffuser and on a second ray of exit from said diffuser.
    Fig. 7 shows an example drawing of the movable diffuser, according to a preferred way of making the compressor of Fig. 3.
    Fig. 8 shows an example of embodiment of the vanes of the mobile diffuser, with particular reference to the leading edge.
    Detailed description [0028] Fig. 1 shows a centrifugal compressor of the known art with impeller and spiral diffuser.
    [0029] Fig. 2 illustrates the known Oerlikon blower and gives an idea of the disadvantageous encumbrance of conventional diffusers to improve performance.
    [0030] Fig. 3 shows schematically (front section and along the axis) a centrifugal compressor according to a way of realizing the invention, comprising a case 1, an impeller 2, a rotary and bladed diffuser 3.
    [0031] The impeller 2 comprises a series of blades 4 and rotates with speed coG. The diffuser 3 comprises vanes 5 and rotates with a speed cod in the opposite direction to the impeller.
    [0032] The impeller 2 and the diffuser 3 are keyed on the same axis. For example in fig. 3 it can be seen that the impeller 2 is keyed on a shaft 6 and the diffuser 3 is keyed to a hollow shaft 7 coaxial to the shaft 6.
    [0033] The diffuser 3 rotates coaxially in the opposite direction to the said impeller 2 and is also arranged around the impeller 2 so as to collect the flow exiting the impeller itself as can be seen in fig. 3.
    [0034] The case 1 advantageously has a section increasing towards the outlet mouth 8 in the direction of rotation of the diffuser.
    [0035] Fig. 3 also shows the location of sealing labyrinths 9.
    [0036] The blades 5 of the diffuser 3 face backwards with respect to the direction of rotation (oD of the diffuser 3 itself.
    [0037] Fig. 4 and 5 show velocity triangles with these symbols, which reflect the symbols commonly used in the study of turbomachines. u peripheral speed of the impellers (drag speed), w relative gas speed, c absolute gas velocity, at an angle between the vector of the absolute velocity c and the vector of the drag velocity u, ß angle between the vector of the relative velocity we the drag speed vector u, R0 impeller inlet radius,
    Impeller exit radius and diffuser inlet, diffuser outlet R2.
    [0038] The subscripts 0,1 and 2 denote the speeds on the radii R0, Ri and R2 respectively. The radius R corresponds to the external radius of the impeller and (neglecting the clearance) to the inner radius of the diffuser; therefore with reference to said radius R ·, we will use an apex to denote the sizes of the diffuser. Thus for example w1 denotes the relative speed with respect to the impeller on the radius Ri whereas w ·, denotes the relative velocity in the diffuser. The subscript r indicates the radial component of velocities, so for example it is the radial component of w · ,.
    [0039] Fig. 4 relates to the known art and shows the typical situation at the entrance and exit of the impeller. The gas enters the impeller with a radial speed c0, but due to the peripheral driving speed u0, the impeller "sees" the fluid entering with relative speed w0. At the exit of the impeller blading, the flow has relative speed w-ι corresponding to an absolute speed c-ι. It can be seen that the absolute speed c-ι is quite high in absolute value, and this corresponds to the fact that most of the compression takes place in the diffuser, where the kinetic energy is converted into pressure energy, but with a not very efficient performance satisfactory.
    [0040] Fig. 5 shows an example of speed triangles in a way of putting the invention into practice. The absolute speed Ci in output from the impeller 2 is composed with the speed Ui of the mobile diffuser 3, which has a sense of rotation opposite to the impeller. Consequently the relative speed wr at the speaker input is very high and preferably it is clearly supersonic, making it possible to use compression shock waves.
    [0041] Fig. 6 shows an example of speed triangles at the entrance (subscript 1) and respectively at the exit (subscript 2) of the mobile diffuser 3. In fig. 6 it can be seen that the gas in the mobile diffuser 3 slows down relatively little passing from the speed Wr to the speed w2. However the composition of said speed w2 with the dragging speed u2 provides an absolute output speed c2 of absolute value (modulus) contained. Said speed c2 for example is a few tens of m / s, typically it is between 30 and 60 m / s, for example about 50 m / s.
    [0042] Fig. 7 shows schematically the blading of the movable diffuser 3. The point P1 indicates the leading edge (leading edge) and the point P2 indicates the trailing edge. The figure also indicates one of the channels 10 defined by the vanes 5 of the movable diffuser 3.
    [0043] Fig. 8 indicates a way of realizing a sharp-edged leading edge, for a supersonic flow, of one of the vanes 5. Said sharp edge is formed by two surfaces 11,12 on the extrados and on the intrados of the blade 5. Said surfaces are inclined with angles 0e, Θ, with respect to a median plane 13 of the blade 5. Said angles 0e, 0, in some embodiments are equal and simply indicated with Θ; more preferably said angles are about 30 °. Fig. 8 shows a preferred embodiment in which Θ = 30 °; the figure also shows the characteristic dimensions s and I of said leading edge. When the angles are the same, as in fig. 8, the relation s / l = 2 tg 0 applies.
    [0044] In the following a systematic treatment is given which illustrates the operation of the present invention and the theoretical bases.
    Symbols used [0045] The main symbols used are the following: T Temperature p pressure p density V specific volume (v = 1 / p) cv specific heat at constant volume m polypropic transformation index k ratio between specific heats at constant pressure and volume constant T | compression efficiency β compression ratio; angle between w and u M Mach number
    Internal work
    Lwg Work lost by the impeller
    Lwd Work lost by the speaker [0046] We will also use the symbols already introduced for the study of speed triangles. Other symbols will be introduced from time to time with reference to the specific formulas. The angles are expressed in degrees.
    Air slowing in a supersonic regime [0047] The problem of slow gas slowing and its consequent compression has been studied with the birth of supersonic aerodynamic tunnels. When a supersonic current of gas encounters an obstacle, shock waves arise. Shock waves represent a phenomenon known and widely studied in the art. The basis of the study of shock waves consists in the study of the spontaneous formation of a plane shock wave (normal shock wavé) which is formed, orthogonal to the direction of velocity, in a supersonic uniform gaseous stream with a non-isentropic compression , that is, with energy loss. Compression takes place according to an equation known as the Hugoniot or Rankine-Hugoniot equation.
    [0048] It is interesting to show here that the efficiency of said compression is better than that of a subsonic polytropic compression when the compression ratios are of the order of those provided by centrifugal compressors, even if they are very loaded. Reference 1 indicates the speaker input and reference 2 indicates the speaker output. The compression ratio is therefore p2 / pi- [0049] The comparison between Hugoniot compression and polytropic compression was done for the air by deriving the exponent m from the known polytropic transformation formula: P2 / P1 = (Vi / v2) m (1) [0050] The ratio v -, / v2 is calculated from the following Hugoniot equation which gives the values of p2 / pi as a function of vi / v2 (v = 1 / p indicates the specific volume of the gas). p2 / pi = [A (V1 / V2) -1] / [A - (V1 / V2)] (2) where A = (k + 1) / (k-1). For air, k = 1.4 and consequently A = 6.
    [0051] The yield can be calculated with the formula:
    Had = [(β2Λ (k-1) / k) -1] / [(β2Λ (m-1) / m) -1] (3) where β2 indicates the compression ratio p2 / pi [0052] the following values: p2 / pi = 1.5 2 3 4 5 6 vi / v2 = 1.3333 1.625 2.111 2.500 2.818 3.083 m = 1.4094 1.428 1.470 1.513 1.553 1.591 pad = 0.9826 0.948 0.876 0.810 0.755 0.707
    Table 1 [0053] The results of table 1 show that the compression given by the shock wave is almost isentropic (yield very close to 1) when the compression ratio is less than 2. Note that the value of m is slightly higher than 1.4 and it is 1.6 when the compression ratio is equal to 6, a ratio notoriously unattainable by a single-stage centrifugal compressor.
    [0054] The compression efficiency is therefore excellent up to a ratio p2 / pi = 3, and remains very good or good even for higher values of said ratio, which are very high for a compressor. It is interesting to note that the compression of Hugoniot has a power to slow down the gas such as to be able to avoid the subsonic slowing zone, always long in the subsonic regime, or to drastically reduce it.
    Efficiency [0055] Consider that the kinetic energy supplied to the diffuser is typically about 50% of the total energy received by the compressor and that the efficiency of the compression with the shock waves in the diffuser according to the invention takes place with an efficiency of approximately equal ( in theory even better) to the performance calculated in the previous paragraph. A yield higher than 0.90 can therefore be calculated up to β = 2, greater than 0.85 at β = 4, and greater than 0.80 at β = 6.
    Impeller operation [0056] For simplicity, we refer to an impeller with radial blades.
    [0057] The compression ratio through the impeller is defined as β! = ρ -, / ρο. As above, β2 = p2 / pi is the compression ratio through the diffuser.
    [0058] Other symbols used: φ = wri / ui (which in the case of the radial impeller equals cos α1). The CP and CC symbols indicate the two fractions of the net work (Li-Lwg) made by the impeller, CP of the pressure energy, CC of the kinetic energy.
    [0059] Table 2 below shows the calculation results in three different examples, with different values of φ, CP and CC.
    ψ = 0.1 CP = 0.495 CC = 0.505 ul = 200 m / s ßl = 1.249 cl = 202 m / s TI = 315 K (al = 5.70) ul = 250 ßl = 1.408 cl = 252.5 TI = 321 ul = 300 ßl = 1.620 cl = 303 TI = 326 φ = 0.3 CP = 0.455 CC = 0.545 ul = 200 ßl = 1.228 cl = 218 TI = 319 (al = 16.70) ul = 250 ßl = 1.371 cl = 272.5 TI = 319 ul = 300 ßl = 1.562 cl = 327 TI = 325 φ = 0.5 CP = 0.375 CC = 0.625 ul = 200 ßl = 1.185 cl = 250 TI = 312 (al = 26.60) ul = 250 ßl = 1.300 cl = 312.5 TI = 317 ul = 300 ßl = 1,450 cl = 375 TI = 322
    Table 2
    Diffuser operation [0060] In table 3 below: [0061] the values of the first five lines are taken from Appendix A.2 of the Modem Compressible Flow volume by John D. Anderson, McGraw-Hill Sériés in Aeronautics! and Aerospace Engineering, Third Edition; [0062] in the sixth line there are the calculated values of the ratio w27w1 'of the speeds in the diffuser downstream and upstream of the shock wave obtained by the Mach number (M) downstream and by the temperature ratio; [0063] the last three lines give the values of the exponent m of the polytropic drawn (approximately) from the previous table; the Lcom work required for the compression operated by the shock wave, calculated with the exponent m; and finally the same work LCom decreased by the kinetic energy Ci2 / 2 that collaborates with the mechanical work Li 'supplied to the axis of the mobile Diffuser; [0064] the values of the last row represent this net work to be supplied to the Diffuser axis. M upstream = 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 p2 / pi = 1.2451.513 1.805 2.120 2.458 2.820 3.205 3.613 p2 / pi = 1.1691.342 1.516 1.690 1.862 2.032 2.198 2.359 T2 / T1 = = 1.0651.128 1.191 1.255 1.320 1.388 1.458 1.532 M downstream = 0.9118 0.8422 0.7860 0.7397 0.7011 0.6684 0.6405 0.6165 w2 '/ wl' = 0.855 0.745 0.660 0.592 0.537 0.492 0.455 0.424 m = 1.42 1.42 1.42 1.45 1.45 1.45 1.49 1.49 L IO 3 = 20.80 40.45 59.26 81.55 99.96 117.85 144.9 163.2 (L com - Ci2 / 2) 10 3 = -20,15-0.50 18.30 40.6 59.0 76.9 103.95 122.25
    Table 3 [0065] Table 3 shows that the area of the diffuser working in the subsonic regime downstream of the shock wave is eliminated or drastically reduced. The table also shows that it is possible to double the compressor power.
    [0066] In the following the flow sections of the generic diffuser conduit are estimated. We refer as always to the second example. Specifically, we see the theoretical ratio between the input S and S2 sections of one of the diffuser ducts, in a supersonic regime.
    [0067] Thanks to table 3, using the values for M = 1.4, the ratio between the sections can be calculated as follows: [0068] S1 / S2 = (p2 / p1) (w27w1 ') = 0.592 x 1.690 = approximately 1 .
    [0069] This remarkable result is also due to the fact that the pressure is quite high: the impeller gives a compression ratio of 1.562, the diffuser provides a ratio of 2.12; the compression ratio is ps / Po = 1.562 x 2.120 = 3.31. Note that the example is not optimized and an even higher ratio can be obtained.
    [0070] Assume the values in output from the impeller for φ = 0.3. The speed w1 at the input of the mobile diffuser, which has a value of about 355 + 300 = 635 m / s in normal temperature and pressure conditions, is equal to about 500 m / s. The sound speed is about 360 m / s, and the Mach number upstream = 500/360 = 1.40.
    [0071] At this value from the table is, downstream of the shock wave, w2 = 0.59 χ 500 = about 295 m / s only, a value even lower than the 410 m / s previously selected to release the gas at 50 m / s in the direction expected then (ie that in the direction of rotation of the impeller, not of the diffuser). Instead the distributor slows the gas down to 295 m / s and releases it at a speed of 295 - 360 = -65 m / s in the same direction as the movement of the mobile diffuser with the advantage of eliminating also the losses that would occur if the flow of compressed gas it moved in the opposite direction to that of the diffuser.
    [0072] It should be noted that this example is not optimized and can be further improved; for example you can load it to double the compressor power: in the example you need 40.6 χ 1000 = 40 600 J / kg net against 90000 J / kg of gross power, having chosen 300 m / s for the speaker. The «charge» can be made, as the efficiency of the diffuser is much greater than 40 600/90 000 = 0.51. For example choosing φ = 0.2 or 0.1 to increase the air to the diffuser and consequently the Mach number. This way you get more pressure from the impeller and more air to the diffuser to increase your work.
    Description of the diffuser (example) [0073] The variables that define the diffuser are shown in fig. 7 where the two radii Rr and R2 of the mobile Diffuser are noticed (neglecting the play between R-i and R-r) and the diagrams of the absolute speeds c and relative w, including their symbols; a and β are the angles of the velocity vectors c and w with the tangents to the circumferences of radius R. The apex indicates the magnitudes on the diffuser in R1 even if wr1 · = wr- |. Note that the angles ßv and ß2 in the diagrams are the angles of the palettes in Rr and R2.
    [0074] The following reports apply. IMPELLER: u = ul ßl = 90 °
    DIFFUSER IN ul '= -ul
    al '= al cl' = cl (4) DIFFUSER OUT:
    [0075] A mass balance on the mobile diffuser allows to calculate the ratio R2 / R1 between the external radius and the internal radius. In the following the iti symbol indicates the mass flow of the gas. Said flow rate is expressed as a function of the relevant variables in the case of radial impeller (ß1 = 90 °), by the following equation: rii = pi wrl '2π RI LI' = p2 wr2 2n R2 L2 (5) [0076] From the equation (5 ) the following is obtained: pi (wr1 '/ ul) = p2 (wr2 / u2) (u2 / ul) (R2 / R1) (L2 / L1') (6) [0077] Equation (6) can be simplified taking into account the relation: u2 / ul = (ω2 R2 / ü) l RI) (7) and taking into account the following relations (8) deducible from the triangles of velocities and from the fact that e = c2 cos cs2 is very small (see equations (4)). wrl / ul = tg al and wr2 / u2 = tg ß2 (8) [0078] The balance equation (6), taking into account the relations (7) and (8), becomes: pi tg al = p2 tg β2 ( ω2 / ωΐ) (R2 / R1) 2 L2 / L1 (9) [0079] In the supersonic regime it is preferable that the ratio of the widths L2 / Li is unitary, then it is assumed L2 / Li = 1 and is maintained as in the example the hypothesis of equal rotation speed (¢ 02/0) 1) = 1.
    [0080] The relationship follows:
    (10) [0081] From table 3 we take p2 / pi = 1.69 and we assume tg ai = 0.5 to which corresponds c * i = 26.6 °.
    [0082] We obtain from the relation (10) the following values: ß2 = 20 15 14 13 12 11 10 9 8 R2 / Ri = (0.90) (1.05) 1.09 1.13 1.18 1.23 1.30 1.37 1.45 [0083] The mass balance and the small tables show that in order not to have too low angles 62 the value of the ratio R2 / R1 is advantageously kept rather low, less than 1.30 in the case considered.
    [0084] The following relation is also valid: β1 '= tg-1 (1/2 tg al) = tg "1 (0.5 / 2) = tg * 1 (0.25) = 14 °. (11) [0085] Fig 7 represents the new diffuser and was constructed in the following way: R2 / R! = 1.25 was assumed to which corresponds β2 = 10.7 ° and fixed the number of vanes (12) and the angles ß1 '= 14 ° and ß2 = 10.7 °; the tangents to the circumferences of radius Ri and R2 were traced on two neighboring points Pi and P2 and the straight lines inclined on them with the corresponding angles ßr on Ri and ß2 on R2. arc of a circle tangent to its inclined line with radius Re of such value as to be tangent to the line passing through Pv. The result represents the (middle) profile of the first blade and of all the others constructed in an obvious way after having traced the circle of radius Reo place of the arcs centers of the palette profile.
    [0086] Fig. 7 represents only one example of a mobile diffuser made in accordance with the present invention and other embodiments may be performed by a person skilled in the art.
    Shock wave theory [0087] This section explores the topic of shock wave formation to define the diffuser palettes.
    [0088] The supersonic compression described so far and analyzed with the Hugoniot equation, consisting in the formation of a plane shock wave (normal shock wave), occurs when the supersonic current is uniform and suddenly finds itself in the condition of being compressed. The real physical model to consider is this: a supersonic current "is turned into itself by a sharp deviation in a point caused by the obstacle called Concave Corner 0 compression corner (formed by two rectilinear segments that form the concave angle Θ at that point ).
    [0089] A flat impact wave is formed inclined by an angle ß on the direction of the incoming speed of the current. The value of ß depends on Θ and on the initial Mach number M1. For example, if M1 = 1.4 and Θ = 2 °, 4 °, 6 °, so-called «weak» waves are formed 0 Weak Shock Waves with ß = 48.2 °, 52.8 °, 54.7 ° (all on about fifty degrees) , if the pressure downstream of these plane waves is not affected by overpressure due to extraneous causes.
    [0090] If on the contrary it suffers, so-called "strong" waves are formed 0 Strong Shock Waves with ß = 87.2, 83.3, 80.5 ° (all almost normal waves). All the waves of this family, the weak and the strong, are reflected on the opposite walls, remaining straight. Normally weak waves are formed downstream of which the current remains supersonic; downstream from the Strong, the current is subsonic. These flat shock waves are called Oblique Shock Waves.
    [0091] The waves defined as "curved and detached shock waves" belong to another family similar to the one now considered; said waves form when the angle Θ of the deviation (already defined) exceeds the value of the fundamental variable 9max, function only of the Mach number. For example if M1 = 1.4 and 2, Omax = 9.5 ° and 23 °. These waves start before the obstacle and are curved above all in the first section and adapt to the type of obstacle; this peculiarity is very useful for the mobile diffuser of the invention.
    [0092] Another very important family includes the shock waves called "detached shock waves in front of a blunt bodf of fundamental importance for supersonic aeronautics. These shock waves have the ability to adapt to any type of obstacle. Their compression capacity varies depending on their local curvature which is very variable: it varies, in general, from zero (where it behaves like normal shock wave) to infinity (where it behaves like an evanescent shock wave. Consider this case of importance for the new mobile diffuser: in a rectangular section duct a «oblique wave» starts due to the impact on a compression corner, but this straight wave is not able to reflect on the opposite wall keeping itself straight after the impact, because the "angle Θ of the reflected wave to the new N, of Mack would exceed 9max. The wave does not reach the opposite wall because from this last part a strong normal to the wall that curves with a variable curvature joining the rectilinear wave: from the point of union between the two waves part towards the valley a weak curved wave, which tends to zero in a short path, in order to "fix" the downstream motion field as far as it was not possible to do at first shot This phenomenon is called Mach Reflection.
    [0093] Said phenomenon of adaptation to the real situation is very frequent and occurs even in the most complex cases; for example when two Strong flat waves, starting from two opposite walls collide: before the junction point they curve and form a single surface that occupies the whole section of the supersonic current downstream of which the regime is subsonic everywhere; it is evident that this is a phenomenon similar to the Mach Reflection operated not on the wall but on the conjunction of the two curved waves; in fact from the conjunction two evanescent waves start towards the valley that are exhausted along the path to the purpose previously indicated.
    [0094] If, on the other hand, two Weak waves had left, they would have been reflected at the meeting point but, given that in this case the downstream field is still weakly supersonic, two straight Weak waves curve from the line (very thin) of junction. both and normally adhere to the two walls; it is the same phenomenon of reflection formed by the two reflected waves.
    [0095] If a wave is Weak and the other Strong the phenomenon is analogous with the difference that reflect a curved and light side Strong side that runs out along the path (because it has done all the compression), and the other reflection stronger than it normally reaches the second wall.
    Diffuser palette [0096] In order to obtain a total or partial compression in a very short distance, we can choose the shock waves to be formed in the ducts of the supersonic mobile diffuser.
    [0097] It is preferable to choose the family of waves called "curved and detached shock waves" and previously described. This choice is preferred because both the waves that form at the entrance of the generic duct always remain in a position behind the impeller which therefore does not suffer disturbances.
    [0098] These waves are always formed on the edge of the inlet profile of the diffuser (fig. 8) formed by two small inclined planes with angle of inclination Θ both on the extrados side and from that of the intrados. The preferred angle is 30 °, a value suitable for forming this type of wave up to the value of Mach No. M1 = 2.5. The value of Θ can be decreased but must be greater than the angle Omax which is a function of M1.
    [0099] When the angle of incidence is zero, two perfectly symmetrical curved waves start from one edge and are welded together in a central area in the manner already explained. In general, when the variations of the angle of incidence are predictable, the two planes on the edge are inclined with different angles θί, 0 and respectively for intrados and extrados, [0100] The compression efficiency on shock waves can theoretically increase slightly compared to the considered case of the normal shock wave because the waves are not normal in all their points, neither on the curved waves nor on the oblique waves up to the line of their welding.
    [0101] The path of compression is always very short as deduced from aerodynamics.
    Compressor control [0102] The control shafts are preferably two and the control can be independent or constrained. An independent control is preferred and also allows to control the group of the two impellers (impeller and mobile diffuser) according to the chosen objectives; for example, if the efficiency should be maximum in different stationary speeds (not too far away), when the peripheral speed u1 varies, the impeller must correspond to a variation u2 of the diffuser, calculated based on the diagrams of the speed of the diffuser and, in practice, at manometric characteristics at different speeds.
    Implant control [0103] Preferably, the mobile diffuser operates in a supersonic regime. If the diffuser is sized to operate exclusively in supersonic mode, the Mach No. at the input must not fall below the value of about 1.10 to which corresponds the value of 0max = 1.5.
    Transients [0104] So far we have considered the regular supersonic current in the sense that the speeds are directed towards the vanes of the mobile diffuser with zero angle of incidence.
    [0105] When the angle of incidence to the input profile of the generic blade varies, the phenomena differ considerably if the gas flow is in a subsonic or supersonic regime. In the following we refer to the supersonic regime and to simplify the argument we consider 0i = 0e = Θ.
    [0106] Yes it considers that the angle of incidence of the current assumes a positive value, placing itself (ideally) in front of the entrance of the generic conduit.
    [0107] On the edge of the lower blade the deviation of the current Θ decreases. Until Θ remains above 0max (M1) the type of wave does not change (the waves remain curved and slightly detached from the edge); if the angle of incidence exceeds the value of 30 ° (almost impossible) the straight waves would form, which in any case would not give problems because they would slip inside the duct.
    [0108] Now we consider the hypothesis of angle of incidence that assumes a negative value. This time the deviation of the current 0 decreases on the edge of the upper blade and everything is fine until the deviation resets the angle of incidence to zero; if this happens, straight waves with an angle of 60-70 ° start from the band of the intrados which do not slip into our duct and disturb the impeller.
    [0109] The clearance between the impeller and the mobile diffuser in the supersonic regime is preferably such as not to disturb the formation and shape of the shock waves which start from the edge of the diffuser blades.
    [0110] The disturbance would consist in causing torsional vibrations on the impeller shaft and bending on its blades. The game is of the order of a fraction of the minimum distance between an edge of the edge of the blade and the nearest blade, of which the value, with the same dimensions, also depends on the number of the blades. The value of this fraction, which can be determined theoretically, is preferably less than 1/2.
    [0111] Note that the presence of this game has the only effect of slightly increasing the diameter of the compressor (apart from that of changing the previous calculations slightly).
    [0112] With regard to the construction technology, the present invention can be realized with the technique known to those skilled in the art: with respect to conventional centrifugal compressors, it has a peripheral impeller (movable diffuser) and two more labyrinths. Faced with this modest constructive complication, the invention substantially allows to double the volume power and to obtain a higher yield than that of the axial compressors.
    Energy balance [0113] This section reports the equations that dominate the phenomena of the centrifugal compressor according to the invention.
    [0114] The first equation (12) refers to the impeller, the second equation (13) refers to the mobile diffuser. The third equation (14) is obtained by summing the (12) and (13) and refers to the complete compressor. Li and Li 'are the works provided on the axis of the impeller and the mobile diffuser, respectively. The other symbols belong to the university literature.
    Li + cv T0 + po / po + c02 / 2 = cv Ti + pi / pi + ci2 / 2 + Lwg (12) cv (Ti - To) + Lwg is the loss on the Impeller
    Li '+ cv Ti + pi / pi + Ci2 / 2 = cv T2 + p2 / p2 + c22 / 2 + Lwd (13) cv (T2-Ti) + Lwd is the leak on the Speaker
    Li + Li '+ cv To + po / po + co2 / 2 = cv T2 + p2 / p2 + c22 / 2 + Lwg + Lwd (14) cv (T2-To) + Lwg + Lwd is the loss on the compressor [0115 ] If the compressor is correctly designed, co = c2, and the compressor equation becomes:
    Li + Li '= + (p2 - pi) / p2 + [cv (T2 - To) Lwg + Lwd] [0116] In a conventional compressor the mechanical work is obviously lacking because the distributor is fixed and formed by the Cassa. Furthermore, it is customary to consider (even in university texts) a single polytropic compression (p2-p0) to simplify the compressor sizing calculation. In the written equations the mechanical works Li and Li 'have the following expressions:
    Li = ul cl cos al if the impeller is designed so as to place the vector co in the radial direction,
    权利要求:
    Claims (10)
    [1]
    Li '= ul' cl cos al if the impeller is designed to arrange the vector c2 in the radial direction, otherwise: Li '= ul' [cl cos al - c2 cos al (R2 / R1)] where ui and ur ' are the two peripheral speeds of the impeller and the diffuser in R1. It should be noted that the peripheral speed ur can be different from the peripheral speed u-i of the Impeller. The last three equations derive from the well-known theorem of momentum momentum variation. claims
    1. Centrifugal compressor comprising a case (1) and an impeller (2), characterized in that it comprises a palletized diffuser (3) which: coaxially rotates in the opposite direction to said impeller; it is arranged around the impeller so as to collect the flow exiting the impeller itself; includes vanes (5) facing backwards with respect to the direction of rotation of the diffuser.
    [2]
    2. Compressor according to claim 1, wherein the impeller (2) and the diffuser (3) are keyed onto respective coaxial shafts (6, 7).
    [3]
    3. Compressor according to claim 1 or 2 in which the blades (5) of the rotating diffuser (3) define channels (10) within which there is a supersonic flow.
    [4]
    4. Compressor according to claim 3, wherein the flow inside said channels (10) of the diffuser has a Mach number greater than 1.1, preferably between 1.1 and 2.5 and more preferably between 1.3 and 2.
    [5]
    5. Compressor according to Claim 3 or 4, in which the rotary diffuser converts kinetic energy of the gas leaving the impeller into pressure energy through shock waves.
    [6]
    6. Compressor according to claim 5, wherein said impact waves are generated at the entrance of the channels (10) defined by the vanes (5) of the rotating diffuser.
    [7]
    7. Compressor according to claim 6 in which each pallet (5) of the diffuser has an leading edge having two inclined surfaces (11, 12), respectively on the intrados and extrados of the blade, which define an attachment edge for the formation of said shock waves.
    [8]
    8. Compressor according to claim 7 wherein said two inclined surfaces have the same angle of inclination.
    [9]
    9. Compressor according to claim 7 or 8, wherein the inclination angle of said surfaces is about 30 °.
    [10]
    10. A compressor according to any one of Claims 5-9, in which the shock waves comprise waves of the "curved and detached" type.
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    同族专利:
    公开号 | 公开日
    CH712976B1|2020-06-30|
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    CH01291/16A|CH712976B1|2016-09-29|2016-09-29|Centrifugal compressor.|CH01291/16A| CH712976B1|2016-09-29|2016-09-29|Centrifugal compressor.|
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