CN104334857B - Turbine windscreen - Google Patents

Abstract

The invention discloses the windscreen (36) of a kind of turbine rotor component for gas-turbine unit (24).Windscreen includes width dimensions (12), height dimension (14) and length dimension (10) and header board (76) and back plate (78).Along width dimensions and height dimension, back plate is more than header board, and is included on height dimension the top (128) extended, and top has asymmetric configuration.Windscreen is additionally included on length dimension vertical structure (80) that is that extend and that connect front and rear panels.

Description

Turbine windscreen

Technical field

Present disclose relates generally to a kind of turbine windscreen, flow through turbine more particularly, to for regulation A kind of turbine windscreen of rotor assembly air-flow.

Background technology

Known a kind of gas-turbine unit (" GTE ") includes turbine assembly, and this turbine assembly has There are the one or more turbine rotor component installed on the driving shaft.Each turbine rotor component includes edge Extend radially outwardly and around turbine rotor multiple turbo blades spaced circumferentially from one another.GTE lights The mixture of air and fuel flows through the high temperature compressed air-flow of turbo blade to produce, and this makes turbine leaf Sheet revolving wormgear rotor assembly.The rotational energy of each turbine rotor component can be passed to drive shaft with Drive load, such as electromotor, compressor or pump.

Turbo blade generally includes root structure and the aerofoil of the opposite side extension from turbine blade platform. Turbine rotor includes the groove for accommodating each turbo blade root structure.The shape of each groove can be similar to Shape in each turbo blade root structure.When multiple turbo blades are assemblied on turbine rotor, Between the turbine stage of adjacent turbine blades with under can form platform underside cavity.

The known parts being positioned in platform underside cavity, these parts are used for regulating around turbine rotor component Compressed air stream.Authorizing United States Patent (USP) No.7097429 (" ' 429 patent ") of Athans et al. In describe an embodiment of this base part.' 429 patent discloses and includes multiple turbo blade Rotor disk.Each turbo blade includes aerofoil, platform and shank.Shank can extend downwardly into many lobes Dovetail is to be installed to rotor disk by turbo blade.Seal body is between shank and phase adjacent vortex Below the platform of impeller blade.Seal body includes the increase sealing plate being arranged on seal body front end. Increase sealing plate overlapping with the front surface portion of adjacent turbine blades shank to provide sealing.Sealing member master Body also includes the rear end with substantially rectangular head, and this rear end is arranged on the top of a pair axial lobe. The area of rear end head is less than the sealing plate at front end.

Summary of the invention

The disclosure provides the windscreen of the turbine rotor component for gas-turbine unit.Windscreen includes width Degree size, height dimension and length dimension and front and rear panels.Along width dimensions and height dimension, Back plate is more than header board and is included on height dimension the top extended, and top has asymmetric configuration. Windscreen is additionally included on length direction the vertical structure extending and connecting front and rear panels.

The disclosure additionally provides the windscreen of the turbine rotor component for gas-turbine unit.Windscreen bag Include width dimensions, height dimension and length dimension, and header board.Windscreen also includes back plate, this back plate Area on width and height dimension is more than the area of header board；Top, this top have relative to The upper end of the back plate axis biasing extended on height dimension；And on length dimension from back plate backward The rectangle baffle plate extended.Windscreen is additionally included on length dimension the longitudinal direction extending and connecting front and rear panels Structure.

The disclosure additionally provides a kind of gas-turbine unit with turbine rotor component.Turbine rotor Assembly includes the turbine rotor with multiple turbo blade groove, and has aerofoil, platform and root knot Multiple turbo blades of structure, the root structure of each turbo blade is configured to be accommodated in turbine rotor In respective turbine blade groove.Turbine rotor component be additionally included near the platform of adjacent turbine blades and under The platform underside gap of square one-tenth, and tie at radially-outer surface and the adjacent turbine blades root of rotor Between structure and at adjacent turbine blades platform platform underside cavity formed below.Turbine rotor component is also Including the turbine windscreen being positioned at least one of which platform underside cavity, turbine windscreen includes broad-ruler Very little, height dimension and length dimension.Windscreen also includes being sized at platform underside cavity and platform Underside gap provides the header board in forward flow gap, and is sized to cover platform underside cavity A part and the back plate of a platform underside gap part.

Accompanying drawing explanation

Fig. 1 is the schematic diagram of the turbine rotor component of part, and this turbine rotor component includes exemplary whirlpool Wheel windscreen；

Fig. 2 is the schematic diagram of exemplary turbine windscreen from Fig. 1 of front end and lateral view, should Turbine windscreen is independent of turbine rotor component；

Fig. 3 is exemplary turbine windscreen from Fig. 2 of rear end and lateral view；

Fig. 4 shows the side view of the turbine windscreen of Fig. 2；

Fig. 5 shows the front end view of the exemplary turbine windscreen of Fig. 2；

Fig. 6 shows the rear end view of the exemplary turbine windscreen of Fig. 2；

Fig. 7 is to observe from the front surface of turbine rotor component, has Fig. 1 turbine of extra turbine blade The schematic diagram of rotor assembly；And

Fig. 8 is to observe from the rear surface of turbine rotor component, has Fig. 1 turbine of extra turbine blade The schematic diagram of rotor assembly.

Detailed description of the invention

With reference to Fig. 1, a kind of gas-turbine unit (GTE) can include turbine assembly, this turbine group One or more turbine rotor component that part includes being arranged on drive shaft (not shown) (or the turbine disk Assembly) 24.Such as, turbine rotor component 24 can include turbine rotor or the turbine disk 30, turbine leaf Sheet 32 and turbine windscreen 36.For the ease of describing, refer to relative with reference to " inner side " with " outside " The radially inner side position of rotation axis and position in turbine rotor 30.Equally, term is " front Side " refer to flow through the upstream position of GTE fluid, and " rear side " refers to downstream position.Multiple whirlpools Wheel rotor assembly 24 can be axially aligned to form multiple stage of turbines of GTE on the driving shaft.Fig. 1 shows Go out and observed turbo blade 32 He turbine rotor 30 from substantially front side at a certain angle to rear direction The relative position of windscreen 36.Although fig 1 illustrate that have single turbo blade 32 and single windscreen 36 Turbine rotor component 24, it is understood that, each turbine rotor component 24 includes around whirlpool Take turns the circumferentially positioned multiple turbo blades 32 of rotor 30 and multiple windscreen 36 being associated.

As it is shown in figure 1, turbo blade 32 can include from the upwardly extending aerofoil of platform 50 48.The wing Face 48 can include the spill aerofoil surfaces 65 on side and the convex aerofoil surfaces on opposite side 67 (figure 8).Additionally, each turbo blade 32 may also include the root structure 52 downwardly extended from platform 50. Root structure 52 has front surface 54 and rear surface 56 (Fig. 8).Front surface 54 and spill aerofoil table Face 65 can be generally facing the equidirectional corresponding to turbine rotor component 24 front part or upstream portion. Rear surface 56 and convex aerofoil surfaces 67 can turn corresponding to turbine generally facing the back side of front surface 54 Sub-component 24 rear section or downstream part.Root structure 52 also includes shank 53 and bottom 55. The bottom 55 of root structure 52 can have fir-tree type shape, thus provides a series of radial direction to be spaced Lobe.

Turbine rotor 30 is configured to be contained in respective grooves 58 the multiple turbo blades being radially spaced 32.Turbine rotor 30 includes front surface 38, rear surface 40 (Fig. 8) and circumferential outer edge 42. Groove 58 axially extends to rear surface 40 from front surface 38.Groove 58 is configured to coordinate and fixed turbine The corresponding root structure 52 of blade 32.

When a pair turbo blade 32 is arranged in the adjacent grooves 58 of turbine rotor 30, at contiguous roots Between the shank 53 of structure 52, below adjacent platforms 50 and outside the circumference of turbine rotor 30 Platform underside cavity 60 is formed above edge 42.Platform underside cavity 60 can include neighbouring turbine rotor Behind the front end 61 of the front surface 38 of 30, and the rear surface 40 (Fig. 8) of neighbouring turbine rotor 30 End 63.As described below, windscreen 36 can be located at turbine rotor 30 and two adjacent turbine blades 32 it Between platform underside cavity 60 in.

Fig. 2 and Fig. 3 respectively illustrates windscreen 36 view observed at a certain angle from front-end and back-end. Windscreen 36 includes length dimension 10, width dimensions 12 and height dimension 14.Windscreen 36 includes passing through Header board 76 that vertical structure 80 is connected to each other and back plate 78.Back plate 78 can include lower extension 124 With upside extension 128.Rectangle baffle plate 120 can extend on rear direction from back plate 78.

With reference to Fig. 2, header board 76 can have profile 84, and the area that this profile 84 limits is more than longitudinally knot The cross-sectional area of structure 80, but less than the area occupied by back plate 78.From Fig. 5 it can be clearly seen that, The whole width of header board 76 and be highly smaller than whole width and the height of back plate 78.The wheel of header board 76 Wide 84 define have conical upper 77 and substantially straight side and the shape of bottom (79,81) Shape.With reference to Fig. 3, the rear surface 75 of header board 76 can include lateral recess 89 and along header board 76 times The bias antelabium 90 that border width extends.The front surface of header board 76 can include substantially smooth surface. Front support face 94 can extend from the top 77 of header board 76 in rear direction.Front support face 94 shapes For wedge shape to coordinate geometry on the downside of the platform 50 of turbo blade 32.

As it has been described above, back plate 78 can include upside extension 128 and lower extension 124.Back plate 78 (that is, having bigger surface area, lower extension 124 extends can be more than platform underside cavity 60 Outside the rear end 63 of platform underside cavity 60).Rear side bearing-surface 98 in front side upwards from back plate The upside extension 128 of 78 extends.Rear side bearing-surface 98 is shaped to wedge shape, and this wedge shape is converged in base Originally it is perpendicular on the vertical line of back plate 78.Rear side bearing-surface 98 also has the length of significantly greater than back plate 78 Size.

The upside extension 128 of back plate 78 can include the outward flange of extension 128 profile on the upside of restriction 86, and lower extension 124 can include limit lower extension 124 profile outward flange 87.As Shown in Fig. 5 and Fig. 6, on height dimension 14 and width dimensions 12, outward flange 86,87 compares header board The external periphery outline 84 of 76 prolongs and projects farther.The overall size of upside extension 128 just can be set to Extend to well the lower section of platform 50.

From Fig. 6 it can be clearly seen that, the upside extension 128 of back plate 78 can include about back plate The asymmetric profile in axis 101 extended along height dimension of 78.Specifically, upside extension 128 First side 132 asymmetric with second side 134, wherein, first side 132 and can be included Dual side-edge 134 is separated by axis 101.First side 132 can include the first straight profile part 136, And second side 134 can include the second straight profile part 138.First straight profile part 136 Extend along the direction intersected in diverse location and axis 101 with the second straight profile part 138.? Fig. 6 illustrate with dashed lines these intersection points.

The extension, upside 128 of back plate 78 also includes subtracting on width dimensions 12 along height dimension 14 The little profile to upper end 130, upper end 130 can somewhat bias between the platform underside to cover similar inclination Gap 74 (Fig. 1), platform underside gap 74 is between adjacent turbine blades 32 and is in adjacent The platform 50 of turbo blade 32 is near and below.This upper end includes the straight profile 140 of top surface, This top surface is basically perpendicular to the axis 101 of back plate 78.Additionally, upper end 130 includes approximate right angle Profile 142, this right angle profile is included in the top surface of first side, top 128 132.Upper end 130 Including obtuse angle profile 144, this obtuse angle profile 144 includes the top table on second side, top 128 134 Face.In detailed description and terminology in claims " generally ", " approx ", " reality On border " or " substantially " be intended to allow correlation values or the minor variations of condition.This minor variations It is understood in the range of ± 3%.

Referring again to Fig. 3 and Fig. 4, substantially rectangular baffle plate 120 can be located at upside extension 128 He Between lower extension 124.Baffle plate 120 can extend from the side of back plate 78 on width dimensions 12 To the opposite side of back plate 78, and extend to form fin like structures in rear direction.Baffle plate 120 Width can be more than the width of upside extension 128.Should be appreciated that baffle plate 120 can be other shapes Shape, and can omit.

Lower extension 124 can include substantially rectangular part 126, the width of this substantially rectangular part 126 Degree is approximately equal to the width of baffle plate 120.Additionally, lower extension can include inferior horn 146 He of rounding Substantially straight profile part 148, profile part 148 is basically perpendicular to the axis of back plate 78 Line 101 and be roughly parallel to the width dimensions of baffle plate 120.Therefore, lower extension divides 124 Width dimensions is more than the width dimensions of upside extension 128.

Referring again to Fig. 2 to Fig. 4, the vertical structure 80 of windscreen 36 can include center wall 104 and extremely A few reinforcement structure element.Such as, vertical structure 80 can include outer structural element 106 and inner structure Element 108 is to strengthen the rigidity of structure of windscreen 36.In the exemplary embodiment, vertical structure 80 Cross section can be the most I-shaped.Outer structural element 106 and inner structural element 108 are at them Length direction on width can substantial constant.Such as, vertical structure 80 may also include and extends into header board The rear surface 75 of 76 and through inner structural element 108 and the rounding recess 110 of center wall 104.Configuration This rounding recess 110 is to assist the bias characteristic of header board 76.It is contemplated that vertical structure 80 can be wrapped Include one or more foot extended internally to be placed in the circumference of turbine rotor 30 during assembling On outward flange 42.Such as, vertical structure 80 can include forefoot 114 and rear feet 116 (Fig. 4).

Fig. 7 and Fig. 8 side-looking field (Fig. 7) and rear side visual field (Fig. 8) in the past shows and includes windscreen The overall structure of the turbine rotor component 24 of 36.Vertical structure 80 is positioned in platform underside cavity 60 The surface of circumferential outer edge 42 of rotor 30, and adjacent by forefoot 114 and rear feet 116 Switch through the circumferential outer edge (Fig. 4) of son 42.

As it is shown in fig. 7, windscreen 36 is between a pair turbo blade 32A and 32B and rotor 30. The size arranging header board 76 makes it be slightly less than the front end of platform underside cavity 60 61, thus front Leaving gap 82 between the root structure 52 of plate 76 and adjacent turbine blades 32A and 32B.With Sample, as mentioned above, the profile of outward flange 84 includes conical upper 77, makes header board 76 have wedge Shape feature, when header board 76 is close to the downside of platform 50, wedge shaped features follows the angle of root structure 52 Degree.Fig. 7 also show flat side and bottom (79,81) of header board 76, and header board 76 terminates at Below the circumferential outer edge of turbine rotor 42, but the first of root structure 52 fir configuration the protrusion Above lobe.

Fig. 8 shows the windscreen 36 between turbo blade 32A and 32B and rotor 30.After Plate 78 is formed at the groove 58 of root structure 52 and rotor 30 together with lower extension 124, covering The portion gap of interface.

Baffle plate 120 substantially extends on width and length direction.Baffle plate 120 may extend into back plate The outward flange of 78, so makes baffle plate outward flange 121 nearly contact relevant to adjacent back plate adjacent The second baffle outward flange of baffle plate.As mentioned above, each turbine rotor component 24 can include around Multiple turbo blades 32 that turbine rotor 30 is circumferentially positioned and multiple relevant windscreen 36.Due to multiple This size of baffle plate 120 and position, multiple baffle plates 120 form ring around rotor 30 together.Baffle plate 120 the most substantially extend (clearly illustrating as in Fig. 4) in rear direction.Fig. 8 also show baffle plate Upside extension 128 above in the of 120, the somewhat offset side 130 of upside extension 128 can cover Cover the platform underside gap 74 with the similar inclination of lower section between adjacent turbine platform 50.Upside is prolonged The radial height of extending portion 128 is slightly below the bottom of platform 50.

Industrial applicibility

Disclosed turbine rotor component 24 is applicable to any rotary power system, such as, combustion gas whirlpool Turbine.Now to the assembling process of turbine rotor component 24 and by turbine rotor component 24 The regulation process of air-flow 44,46 be described in detail.

During turbine rotor component 24 is assembled, such as, each windscreen 36 can pass through interference fit It is attached to turbine rotor 30.In order to position windscreen 36 on turbine rotor 30, can be along away from back plate The direction of 78 temporarily urges the bias antelabium 90 of header board 76, thinks header board 76 and the back plate of windscreen 36 78 provide enough spaces to coordinate in the circumferential outer edge 42 of turbine rotor 30.Once windscreen 36 When being appropriately positioned on the turbine rotor 30 being among one of them groove 58, applying can be removed Power on header board 76, so that windscreen 36 is clamped in the circumferential outer edge 42 of turbine rotor 30.

Such as, turbo blade 32 is slidably mounted in turbine rotor 30 along vertical direction In groove 58.As shown in the figure 7, the first turbo blade 32A can be at the first groove 58A of turbine rotor 30 In be slideably mounted into the side of one of them windscreen 36.Second turbo blade 32B pacifies slidably It is contained in the second groove 58B.The header board 76 of windscreen 36 can provide enough spaces thus allow the first turbine Blade 32A and the second turbo blade 32B slips into the first groove 58A and the second groove 58B through windscreen 36. It is also contemplated that replace installing all windscreens 36 before mounting turbine blades 32, phase can be installed Between adjacent first turbo blade 32A and the second turbo blade 32B, windscreen 36 is arranged on turbine and turns On son 30.May be repeated and turbo blade 32 and windscreen 36 are arranged on turbine rotor 30 with shape Become the process of turbine rotor component 24, until all grooves 58 on turbine rotor 30 are by turbo blade 32 occupy.

Prepare to run once assembling turbine rotor component 24 and GTE completely, turbine rotor component 24 can help to regulate the thermal current 44 shown in Fig. 1 and cold airflow 46.In the running of GTE, Before at least some of compressed air entrance combustor section forms thermal current 44 with burning, compressor Air can be passed through in air inlet duct suction GTE and compressed air by part.At least one The residual compression air (referred to as cold airflow 46) divided can be used for non-burning purpose and (such as cools down GTE One or more parts) and can with compressed air for combustion part separate turn up the soil through GTE.Can send into rotate one or more turbine rotor component by turbine portion by thermal current 44 24.The term about air-flow " hot " and " cold " is used to be only intended to mark: compared to " cold air Stream ", " thermal current " is generally of different temperatures or pressure.

As it is shown in figure 1, thermal current 44 and cold airflow 46 can flow upwardly through turbine vertical side Rotor assembly 24.Thermal current 44 generally can be separated with cold airflow by a wall (not shown).

At least one of thermal current 44 rotates one or more turbine rotor component 24.But, logical Cross gap 74 (Fig. 7) enter platform underside cavity 60 thermal current 44 can cause turbo blade due to Overheated and premature fatigue.In order to help prevent the generation of this situation, make at least of cold airflow 46 Divide and turn to inside the platform underside cavity 60 and/or groove 58 of turbine rotor component 24, provide pressurization Fluid.Part cold airflow 46 also can cool down one or more parts of turbine rotor component 24.

In order to contribute to below turbine blade platform 50 and be in the front table of turbine rotor component 24 Region between face and rear surface keeps normal pressure, it is envisioned that compared in turbine rotor component The sealing member that the rear surface of 24 is formed, the gap 82 at the front end 61 of platform underside cavity 60 can have Have less restricted.Cold airflow 46 can flow through the front surface 54 of root structure 52, flows through gap 82 And entering the front end 61 of platform underside cavity 60, gap 82 is formed at the whole of header board 76 or portion Exceptionally between the front surface 54 of edge 84 and contiguous roots structure 52.It is allowed to enter platform underside empty The cold airflow 46 in chamber 60 often can make the pressure in platform underside cavity 60 and groove 58 increase to above Pressure outside platform underside cavity 60 or outside groove 58.This is the front surface 88 due to back plate 78 Cover the root structure 52 of part and the interface of the groove 58 of rotor 30, thus limit cold airflow 46 Leave the rear end 63 of platform underside cavity 60.It is, can be in the rear end 63 of platform underside cavity 60 Limiting cold airflow 46 and leave rear end and the rear end of groove 58 of platform 50, this restriction is more than turning at turbine Restriction at the front end of sub-component 24.Owing to air-flow trends towards moving to low-pressure area from higher-pressure region, turbine High pressure cold stream 46 below platform 50 can trend towards suppression and flow radially inwardly into platform underside cavity 60 Thermal current 44.

With reference to Fig. 8, the profile of lower extension 124 can limit a kind of shape, and this shape is along part Root structure 52 and groove 58 provide seal.Equally, upper end 130 can have and prolongs essentially outward The shape stretched is with the gap between additional seal rear surface 56.More specifically, upside extension 128 The parts of two adjacent rear surfaces can covering rotor immediately below platform 50, upper end 130 close to complete Envelope.

Fig. 8 be also show windscreen 36 and can be limited the most at least in part by baffle plate 120 Heat air-flow 44 to flow downward.Because baffle plate 120 substantially extends on width dimensions and length dimension, The air stream between hot-fluid and cold flow is suppressed to mix in the posterior region of turbine rotor component 24 further Close.It is to say, baffle plate 120 inhibits substantially introversive radial air flow, reason is baffle plate 120 Rear side extension component be used as separates walls.Due at baffle plate 120 its outer edges 121 and adjacent to baffle plate 120 Alignment and almost in contact with, thus form the ring around rotor assembly, baffle plate 120 is by angle Define the most subcontinuous separates walls on direction and suppress air-flow radially further.

Although describing in the illustrative embodiments of Fig. 1-8 and showing windscreen 36, however, it is possible to Expect implementing other configuration of windscreen 36.Such as, the header board 76 of windscreen 36 can include one Or multiple passage (not shown) is with regulation cold airflow 46 further in platform underside cavity 60.This Outward, windscreen 36 can include that less or more extension is to realize additional seal and/or turbine rotor component Parts between maintaining part.

It is obvious for the man skilled in the art that, can turbo blade of this disclosure Assembly makes various modifications and variations without departing from the scope of the present disclosure.By considering system disclosed herein The explanation of system and practice, other embodiments of turbine blade assemblies for those skilled in the art will be Obviously.Should be appreciated that what description and embodiments was merely exemplary, the disclosure real Scope is shown by claims below and equivalent thereof.

Claims (9)

1. the windscreen (36) for the turbine rotor component (24) of gas-turbine unit, bag Include:

Width dimensions (12), height dimension (14) and length dimension (10)；

Header board (76)；

Back plate (78), described back plate (78) along described width dimensions and height dimension more than before described Plate, and there is the top (128) extended on described height dimension, described top has asymmetric Configuration；

Wherein, described top includes that upper end, described upper end include the uppermost top surface on described top, Described top surface forms the straight profile being basically perpendicular to axis, and described axis is at the height of back plate The upper and lower of back plate is extended through in size；

Wherein, the described top including upper end has the length of substantial constant on length dimension；

Wherein, described top includes the first side with the first straight profile part, and has second The second side of straight profile part, first side and second side are by the height gauge in back plate of back plate The described axis of very little upper extension separately, first straight profile part line along described upper end and Extend along the direction intersected in diverse location and axis with the second straight profile part,

And

Vertical structure (80), it extends and connects described header board and described on described length dimension Back plate.

Windscreen the most according to claim 1, wherein, the width on described top is along described height gauge Very little reduction.

Windscreen the most according to claim 1, also includes: on described length dimension after described The generally rectangular baffle plate (120) that plate extends back.

Windscreen the most according to claim 3, wherein, described baffle plate prolongs on described width dimensions Stretch more farther than described top.

Windscreen the most according to claim 3, wherein, described baffle plate prolongs from the side of described back plate Reach the opposite side of described back plate.

Windscreen the most according to claim 1, wherein, the described bottom (124) of described back plate Generally rectangular.

Windscreen the most according to claim 6, wherein, described bottom includes substantially straight profile Partly (148), described outline portion (148) be approximately perpendicular to described back plate at described height gauge The axis of very little upper extension, and be roughly parallel to prolong backward from described back plate on described length dimension The width dimensions of the baffle plate stretched.

Windscreen the most according to claim 7, wherein, the width of described bottom is more than described top Width.

9. a gas-turbine unit, including:

Turbine rotor component (24), described turbine rotor component includes:

Turbine rotor (30), it has multiple turbo blade groove (58),

Multiple turbo blades (32), described turbo blade (32) has aerofoil (48), platform (50) With root structure (52), the described root structure of each turbo blade is configured to be accommodated in described whirlpool In the respective turbine blade groove of wheel rotor,

Platform underside gap (74), is formed at the described platform of adjacent turbine blades near and below, And

Platform underside cavity (60), is formed at outer radial face and the adjacent turbine blades of described rotor Between root structure, and it is positioned at below adjacent turbine blades platform；And

Turbine windscreen (36), it is positioned at least one described platform underside cavity, described turbine wind Gear includes:

Width dimensions (12), height dimension (14) and length dimension (10)；

Header board (76), is sized between described platform underside cavity and described platform underside Gap provides forward flow gap；

Back plate (78), is sized to cover a part for described platform underside cavity and described The part in platform underside gap；

Wherein, the top of described back plate has asymmetric configuration and includes upper end, and described upper end includes The uppermost top surface on described top, described top surface forms the straight wheel being basically perpendicular to axis Exterior feature, described axis extends through the upper and lower of back plate on the height dimension of back plate, including on The described top of end has the length of substantial constant on length dimension, and described upper end is configured to cover The part in platform underside gap, described top includes the first side with the first straight profile part, With there is the second side of the second straight profile part, first side and second side by back plate rear The described axis extended on the height dimension of plate separately, the first straight profile part along described upper end it Under line and along intersect in diverse location and axis with the second straight profile part direction extension.