Compressor

Abstract

The invention relates to a compressor having a rotatably mounted compressor wheel (10), the compressor wheel (10) having a radially inner inflow region (11) and a radially outer outflow region (14), the inflow region (11) being connected to the outflow region (14) via a flow channel (15) which extends from the inflow region (11) to the outflow region (14), the flow channels (15) each have a flow cross-sectional area measured in the circumferential direction of the compressor wheel (10), the flow channels (15) having a channel length extending from the inflow region (11) to the outflow region (14), and the flow channels (15) having a channel volume which is derived from the flow cross-sectional area and the channel length. In order to simultaneously achieve a high pressure ratio with a very low diameter-specific productivity by means of such a compressor, according to the invention the volume occupied by the compressor wheel (10) is at least three times greater than the sum of the channel volumes of the flow channels (15), and / or the angular distance, measured in the circumferential direction of the compressor wheel (10), of two adjacent flow channels (15) at the flow inlet (15.1) thereof located in the inflow region (11) and / or at the flow outlet (15.2) thereof located in the outflow region is at least 18 DEG.

Description

Technical field

[0001] The invention relates to a compressor having a compresssor wheel rotatably supported therein, wherein the compressor wheel has a radially inner inflow region and a radially outer outflow region, wherein the inflow region and the outflow region are connected via a flow channel extending from the inflow region to the outflow region, wherein the flow channel forms a flow outlet in the outflow region, the flow outlet being arranged at a distance from the axis of rotation of the compressor wheel, wherein the flow channel has a flow cross-section extending in the circumferential direction of the compressor wheel, wherein the flow channel has a channel length extending from the inflow region to the outflow region, and wherein the flow channel has a minimum flow channel cross-sectional area along the channel length. Background art

[0002] Compressors in various embodiments are known from the prior art. EP 3 421 825 A1 shows an example of a turbo compressor. A compressor wheel is used in the turbo compressor and is held on a drive shaft. In order to generate high pressure and high-quality flow, these compressors are implemented as radial compressors or centrifugal compressors. The compressor wheel inhales the air to be compressed at its inflow region centered in the axial direction. The inhaled air then enters the flow channel via a flow inlet. The air is guided in the flow channel and is conveyed radially outwards by the acting centrifugal force when the compressor wheel rotates. The compressed air thus leaves the flow channel at the flow outlet.

[0003] For certain applications, it may then be necessary to provide a compressor that is suitable for generating high pressure but at the same time only provides a small mass flow.

[0004] For example, in an application according to the invention, it may be necessary to provide a compressor that conveys a fluid with a very small mass flow (for example in the range between 1 - 10 g / s) while having a high pressure ratio (the pressure difference between the pressure in the outflow region and the pressure in the inflow region being in the range greater than 2 bar). These very specific requirements can result from the application of the compressor according to the invention, in which ambient air is compressed in order to supply it to a cleaning system for a radar sensor operating with compressed air. Such a cleaning system according to the invention can be used, for example, in an autonomous vehicle having a radar sensor.

[0005] Associated with a turbo compressor, the required pressure ratio can be achieved by means of the respective circumferential speed at the flow outlet of the compressor wheel. Since the rotational speed of the drive is limited to approximately 100,000 U / min for driving the compressor wheel (e.g., by means of an electric motor), achieving a high pressure ratio is a challenge. Therefore, the circumferential speed required for this purpose can only be achieved by a large compressor wheel outlet diameter. This results in a design with a very low diameter-specific productivity. The diameter-specific productivity of a turbo compressor is mainly determined by the flow cross-section between the blades, the rotor sleeve, and the housing profile, and can only be changed to a limited extent by design. When designing a very low diameter-specific productivity, the size of the channel cross-section must be correspondingly small. This leads to an unfavorable ratio between the blade height and the clearance height between the compressor impeller and the housing in extreme designs.

[0006] Ultimately, the channel height cannot be lower than the clearance height. If, in the extreme case, the channel height corresponds to the clearance height, there will no longer be any space for the rotor blades and the compressor will no longer be able to perform its function. If the diameter-specific productivity in this hypothetical case is still always greater than the requirement, it is technically impossible to design the corresponding turbo compressor. Even before the described extreme case, a very unfavorable surface area to volume ratio is achieved by reducing the blade height. This results in almost no free flow cross-section due to the boundary layer, and thus a technically reasonable design cannot be achieved. Summary of the Invention

[0007] The object of the present invention is to provide a compressor that has a very small diameter-specific productivity, but still achieves a high pressure ratio and has stable operating performance.

[0008] This object is achieved in that the radius for calculating the circular area is formed by the distance furthest from the axis of rotation of the compressor wheel at the flow outlet, and the sum of the minimum flow channel cross-sectional areas of all the flow channels of the compressor wheel is less than 0.01, preferably less than 0.008, relative to this circular area.

[0009] In other words,

[0010] Preferably <0.008.

[0011] Compared with known turbo compressors, in the solution according to the present invention, the flow through the compressor wheel is restricted by implementing the flow channels. Only a small mass flow is conveyed here, but at the same time, the compressor principle used by the radial compressor remains unchanged, thus achieving the desired high pressure ratio.

[0012] The compression ratio can be adjusted by spacing the flow outlet from the axis of rotation. The productivity is adjusted by adjusting the flow channels (in terms of their number and their cross-sectional areas). Thereby, variability of the compressor in two important dimensions (productivity and pressure ratio) is achieved. Here, the compressor can operate reliably and stably. The stable operation of the compressor is characterized in that the operating point lies within two operating limits (surge and choke limits).

[0013] For example, an implementation according to the invention can provide a compressor with which a dimensionless rotational speed characteristic value σ can be achieved in the range between 0.01 and 0.05, preferably between 0.01 and 0.03, where σ is calculated according to the following formula:

[0014]

[0015] The following provisions apply here:

[0016] Δp = pressure difference between the pressure in the outflow region and the pressure in the inflow region

[0017] ρ = density of the conveyed fluid

[0018] V = volume flow rate of the conveyed fluid.

[0019] Additionally or alternatively, for example, an implementation according to the invention can provide a compressor with which a dimensionless diameter characteristic value δ can be achieved in the range between 20 and 100, preferably between 20 and 70, where δ is calculated according to the following formula:

[0020]

[0021] The following provisions apply here:

[0022] Δp = pressure difference between the pressure in the outflow region and the pressure in the inflow region

[0023] ρ = density of the conveyed fluid

[0024] V = volume flow rate of the conveyed fluid.

[0025] Such dimensionless rotational speed and / or diameter characteristic values can result in completely new working machines unknown in the prior art.

[0026] According to a possible variant of the invention, it can be provided that the volume occupied by the compressor wheel is at least three times as large as the sum of the channel volumes of the flow channels, and / or that the maximum opening dimension of the flow outlet of the compressor wheel or at least a part of the flow outlet extends in the circumferential direction over a maximum central angle of 10°.

[0027] Here, the volume of the compressor wheel is determined by the outer contour of the compressor wheel, where internal cavities within the compressor wheel, such as recesses or cavities, are not taken into account.

[0028] The volume of the impeller is many times larger than the sum of the volumes of the flow channels of the compressor wheel. The compression ratio can be adjusted by adjusting the outer diameter of the compressor wheel. The productivity is adjusted by adjusting the flow channels (in terms of their number and their cross-sectional area). Thus, variability of the compressor in two important dimensions (productivity and pressure ratio) is achieved.

[0029] Preferably, the volume occupied by the compressor wheel can be at least 6 times, preferably at least 8 times, the sum of the channel volumes of the flow channels, and / or the maximum opening dimension of the flow outlet or at least a part of the flow outlet of the compressor wheel extends in the circumferential direction at a maximum central angle of 0.5° to 10°, preferably at a maximum central angle of 1° to 7°, and / or the maximum opening dimension of the flow inlet or at least a part of the flow inlet of the flow channels of the compressor wheel extends in the circumferential direction at a maximum central angle of 3° to 20°, preferably at a maximum central angle of 6° to 14°. These designs are particularly suitable for the effective cleaning of the sensor surface.

[0030] According to a preferred variant of the invention, it can be provided that at least a part of the flow channels are at least locally configured as circumferentially closed cavities. In contrast to known turbo compressors, no blades are used in this implementation. Thus, there is no limiting case between the possible blade height and the necessary clearance in this implementation. Instead, the cross-sectional area determining the productivity can be adjusted by adjusting the cross-sectional area of the cavity and the number of cavities.

[0031] A preferred variant of the invention can be designed such that the dimensions of the flow cross-section of at least a part of the flow channels do not change at least locally in the direction of the channel length. The design of such a compressor wheel can be simply accomplished. Preferably, it can be provided here that at least a part of the flow channels have the shape of a cylindrical hole in the region between the flow inlet and the flow outlet. For example, holes can be simply drilled in the compressor wheel. The holes can extend radially here, or can also extend inclinedly in the direction of rotation of the compressor wheel or opposite to the direction of rotation.

[0032] In the present invention, it can also be contemplated that the dimensions of the flow cross-section of at least a part of the flow channels change at least partly in the direction of the channel length. Thereby, the flow rate through the flow channels can be further influenced, in particular the pressure ratio generated at a given rotational speed.

[0033] Preferably, it can be provided that the flow cross-section in the region between the flow inlet and the flow outlet in at least a part of the flow channels narrows outward in the radial direction. Thereby, the increase in the density of the fluid conveyed in the flow channels can be effectively compensated.

[0034] The compressor according to the invention can be designed such that the flow outlets of at least a part of the flow channels are arranged offset in the circumferential direction and / or in the axial direction with respect to the flow inlets. Thereby, the incorrect oncoming flow at the flow inlet of the compressor wheel and the outflow angle at the flow outlet can be adjusted, thereby affecting the compressor, for example, in terms of operating stability and pressure magnitude. This can be improved additionally or alternatively by extending at least a part of the flow channels curved in the circumferential direction in the region between the flow inlet and the flow outlet.

[0035] Offset in the axial direction enables an optimized design of the structural space of the compressor wheel.

[0036] If at least a part of the flow channels are formed in the region between two mutually connected components, a complex flow channel course can also be simply realized at this time. Particularly preferably, it is provided here that one of the components has a groove extending in the direction of the channel length, the groove defining the flow channel and the other component at least partially completing or covering the groove. The groove can be simply machined, for example milled, into the component in a desired shape.

[0037] According to a possible variant of the invention, it can be provided that the cross-sectional area of at least one flow channel at the flow inlet and / or at the flow outlet is at least 0.5 mm 2 and at most 15 mm 2 , preferably at least 1 mm 2 and at most 5 mm 2 , in order to achieve a desired conveying effect.

[0038] The object of the invention is also achieved by a component, in particular for a land vehicle or a ship or an aircraft, having an optical sensor, in particular a radar sensor, and a compressor according to any one of claims 1 to 12, wherein a channel connection is arranged between the compressor wheel and the sensor, so as to guide the fluid compressed by the compressor wheel, in particular air, to the surface of the sensor for cleaning. Description of the Drawings

[0039] The invention is explained in detail below on the basis of the embodiments shown in the drawings. Shown therein are:

[0040] Figure 1 A partial perspective view of the compressor is shown;

[0041] Figure 2 Shown according to Figure 1 An overall section of the compressor;

[0042] Figure 3 An overall section of the compressor wheel of the compressor and a sectional view along the Figure 4 section curve marked III-III in

[0043] Figure 4 shows a cross-sectional view of a compressor wheel along the Figure 3 section curve marked IV-IV in Figure 3 ; and

[0044] Figures 5a - 5f shows different design variants of the compressor wheel. DETAILED DESCRIPTION

[0045] Figure 1 shows a compressor which can be used, for example, in a cleaning system, in which compressed air can be used for cleaning. Here, the cleaning system can be used, for example, to clean the optical surfaces of sensors, in particular radar sensors.

[0046] Figure 2 shows details and the entire cross-section of the compressor. As shown in the schematic diagram, the compressor has a drive unit which can be configured, for example, as an electric motor 41. The electric motor 41 has a motor rotor 44, and a motor stator 45 is assigned to the motor rotor. The motor rotor 44 is connected to a drive shaft 42. The drive shaft 42 and the motor rotor 41 can form an integral component. The drive shaft 42 is non-rotatably connected to the compressor wheel 10.

[0047] The motor stator 45 of the electric motor 41 is installed in a housing having a housing part 40. Here, the housing part 40 can be closed by means of a bearing part 30. The bearing part 30 has a bearing receiving part 32 in which a bearing 33 for supporting the drive shaft 42 is received. The bearing part 30 is sealed relative to the housing part 40 (seal 34).

[0048] The compressor wheel 10 can preferably be made in one piece. However, it is also conceivable that the compressor wheel 10 is implemented as a multi-piece, in particular two-piece, component.

[0049] The compressor wheel 10 has a sleeve 12, and the compressor wheel 10 is non-rotatably connected to the drive shaft 42 by means of the sleeve. For this purpose, it can be provided that the compressor wheel 10 is connected to the drive shaft 42, in particular clamped, by means of a nut 43 which is screwed onto the thread of the drive shaft 42. In the present embodiment, a spacer sleeve 18 is arranged between the nut 43 and the support surface of the sleeve 12.

[0050] A seal sleeve 35 can be arranged between the compressor wheel 10 and the bearing 33, and the seal sleeve seals the bearing 33 relative to the flow channel. In addition, a support disk 31 can be provided, and the support disk fixes the bearing 33, for example fixes a ball bearing on the outer ring of the ball bearing.

[0051] The compressor wheel 10 has a central inflow region 11, which can be machined into the compressor wheel 10 in the form of a recess in the region of the inflow side 16. Starting from the inflow region 11, a flow channel 15 is machined into the compressor wheel 10, and the flow channel is configured in the form of a cavity. Here, the cavity is surrounded by the geometry of the compressor wheel 10 on all sides in its circumferential direction.

[0052] The inflow region 11 can be designed such that the inflow region is at least partially bounded by a surrounding inner wall 13. The inner wall 13 has an opening, which is configured in the form of a flow inlet 15.1 of the flow channel 15.

[0053] The compressor wheel 10 has an outflow region 14 in the region of its outer circumference. The flow channels 15 communicate with flow outlets 15.2 respectively here.

[0054] According to the invention, the flow channel 15 has a flow cross-sectional area that varies or remains constant in the direction of the channel length. Therefore, the volume of the flow channel 15 is calculated from the channel length and the flow cross-sectional area. The flow channel 15 machined into the compressor wheel 10 in the form of a cavity has a total channel volume. The compressor wheel has a compressor wheel volume. According to the invention, the compressor wheel volume is several integer multiples larger than the channel volume. As described above, the compressor wheel volume is determined by the outer edge of the compressor wheel 10.

[0055] The flow channels 15 are arranged spaced apart from each other in the circumferential direction. Here, the angular spacing in the region of the flow outlet 15.2 and / or in the region of the flow inlet 15.1 is at least 18°.

[0056] Figure 2 It is also shown that the housing can be provided with at least one housing part 20, and the housing part has a spiral channel 22 or forms a spiral channel. The spiral channel 22 opens into the outflow region 14 on the circumferential side of the compressor wheel 10. Therefore, the outflow channel 15 opens into the spiral channel 22 at its flow outlet 15.2.

[0057] A compressed air interface 24 is provided downstream of the spiral channel 22 in the flow direction, and a compressed air pipeline can be connected to the compressed air interface. The compressed air interface 24 is preferably connected integrally with the housing or the housing part 20. In addition, the housing or the housing part 20 can have a connecting pipe 23, and the connecting pipe forms a suction channel 23.1. The suction channel 23.1 is guided to the inflow region 11. An intake pipeline can be connected to the connecting pipe 23, whereby the compressor wheel 10 can suck air to the flow inlet 15.1 of the flow channel 15 via the suction channel 23.1.

[0058] Figure 2 It is shown that the housing part 20 can have a centering receiving part 21, and the housing part 20 is oriented relative to the housing part 40 or relative to the bearing part 30 at the centering receiving part.

[0059] Finally, the housing part 20 can form a wheel receiving part 25 in which the compressor wheel 10 is at least partially received. In the present embodiment, the wheel receiving part 25 is defined by the housing part 20 and the housing of the electric motor 41, and is currently defined by the bearing part 30 of the electric motor 41.

[0060] According to a preferred variant, the compressor wheel 10 is received in the wheel receiving part 25, and the wheel receiving part is sealed relative to the environment except for the air inlet (suction channel 23.1) and the air outlet (spiral channel 22). In the present embodiment, a seal 34 is provided between the bearing part 30 and the housing part 40 for this purpose.

[0061] Figure 3 A possible design variant of the compressor wheel is shown, which can be applied, for example, in a compressor according to Figure 1 and Figure 2 . As shown in the schematic diagram, the compressor wheel 10 has a flow channel 15, which extends in the radial direction and is machined into the material of the compressor wheel 10 in the form of a tapered hole. Here, the flow channel 15 narrows continuously or discontinuously in the radial direction outwards. It is also conceivable to use a cylindrical hole for the flow channel 15.

[0062] Furthermore, it is also conceivable to use a flow channel 15, the flow cross-section of which widens continuously or discontinuously along the channel length and in the flow direction at least in some regions.

[0063] Figure 3 and Figure 4 show that the compressor wheel 10 can also have a sleeve 12, which has an inflow region 11 in the region of the inflow side 16 and can have a drive side 17 opposite to the inflow side 16 in order to be connected to a drive shaft 42 of, for example, an electric motor 41. Figure 3 It is shown that all the flow outlets of the flow channel 15 are spaced apart from the axis of rotation of the compressor wheel 10 by a dimension R to a uniform extent, where R is preferably the radius of the outer circumference of the compressor wheel 10. The circular area calculated by using the radius R is A mm 2 . The flow channel 15 has a minimum flow channel cross-sectional area in the region of the flow outlet. The sum of the minimum flow channel cross-sectional areas of all the flow channels 15 of the compressor wheel 10 is B mm 2 . According to the present invention:

[0064] B / A < 0.005.

[0065] The outer contour of the compressor wheel 10 occupies a space including a volume. Here, this volume is determined without considering the cavities or recesses in the compressor wheel 10. In other words, the volume of the compressor wheel 10 results from the volume that constructs the compressor wheel 10. This volume is at least three times as large as the sum of the volumes of the flow channels 15.

[0066] Figure 4 It is shown that the maximum opening dimension of the flow outlet of the flow channel 15 of the compressor wheel 10 extends along the circumferential direction over a maximum central angle of 10°.

[0067] Figures 5A to 5F show different design variants of the compressor wheel 10. The structure of the compressor wheel 10 corresponds in principle to the compressor wheel 10 described in the reference Figures 1 to 4 For the sake of avoiding repetition, reference may thus be made to the foregoing embodiments. Only the differences of the compressor wheel will be explained below.

[0068] According to Figures 5a to 5f the design variants differ in the arrangement and implementation of their flow channels 15.

[0069] Figure 5a It is shown that the compressor wheel 10 has a radially extending flow channel 15, and the flow channel is implemented in the form of a cylindrical hole.

[0070] Figure 5b It is shown that the compressor wheel 10 has a radially extending flow channel 15. The flow cross-section of the flow channel 15 continuously narrows outward along the radial direction along the channel length.

[0071] Figure 5c and Figure 5d It is shown that the compressor wheel 10 has a flow channel 15 that extends in a curved manner along the channel length. Here, according to Figure 5c a flow channel 15 is provided, and the flow channel bends against the direction of rotation ω. Figure 5d It is shown that the flow channel 15 extends in a curved manner in the direction of the rotation direction ω.

[0072] It is also possible to make the flow channel 15 have regions that extend in a curved manner in the rotation direction and in a direction opposite to the rotation direction ω It is also conceivable that the flow channel 15 has a constant flow cross-section or a variable flow cross-section along its channel length.

[0073] Figure 5e and Figure 5f It is shown that in the present invention, the flow channel 15 can be implemented such that the flow inlet 15.1 is arranged offset relative to the flow outlet 15.2 in the rotation direction ω. This feature can be achieved in all compressor wheels 10 according to the present invention, especially in the compressor wheels according to Figures 5a to 5d the compressor wheels.

[0074] The fluid inlet 15.1 can be retracted (5e) relative to the fluid outlet 15.2 in the rotational direction ω, or the fluid outlet 15.2 can be retracted relative to the fluid inlet 15.1 in the rotational direction ω ( Figure 5f ).

Claims

1. Compressor, which has a rotatably supported compressor wheel (10), wherein, The compressor wheel (10) has a radially inner inflow region (11) and a radially outer outflow region (14), wherein the inflow region (11) is connected to the outflow region (14) via a flow channel (15) that extends from the inflow region (11) to the outflow region (14), wherein the flow channel (15) forms a flow outlet in the outflow region (14), and the flow outlet is arranged at a distance from the rotational axis of the compressor wheel, wherein the flow channel (15) has a flow cross-section that extends in the circumferential direction of the compressor wheel (10), wherein the flow channel (15) has a channel length extending from the inflow region (11) to the outflow region (14), and wherein the flow channel (15) has a minimum flow channel cross-sectional area along the channel length. It is characterized in that the maximum distance of the flow outlet from the rotational axis of the compressor wheel (10) forms a radius for calculating a circular area, such that the sum of the minimum flow channel cross-sectional areas of all the flow channels (15) of the compressor wheel (10) is less than 0.01, preferably less than 0.008, relative to this circular area.

2. The compressor according to claim 1, wherein, The compressor wheel volume occupied by the compressor wheel (10) is at least three times as large as the sum of the channel volumes of the flow channels (15), and / or the maximum opening dimension of the flow outlet or at least a part of the flow outlet of the compressor wheel (10) extends in the circumferential direction over a maximum central angle of 10°.

3. The compressor according to claim 1 or 2, characterized in that, At least a part of the flow channels (15) are at least locally configured as circumferentially closed cavities.

4. The compressor according to any one of claims 1 to 3, characterized in that, The dimensions of the flow cross-section of at least a part of the flow channels (15) do not change at least locally in the channel length direction, wherein preferably it is provided that at least a part of the flow channels (15) have the shape of a cylindrical hole in the region between the flow inlet (15.1) and the flow outlet (15.2).

5. The compressor according to any one of claims 1 to 4, characterized in that, The dimensions of the flow cross-section of at least a part of the flow channels (15) change at least locally in the channel length direction, wherein preferably it is provided that the flow cross-section in the region between the flow inlet (15.1) and the flow outlet (15.2) in at least a part of the flow channels (15) narrows radially outwards.

6. The compressor according to any one of claims 1 to 5, characterized in that, The flow outlet (15.2) of at least a part of the flow channels (15) is arranged offset in the circumferential direction of the compressor wheel (10) and / or in the axial direction of the compressor wheel (10) relative to the flow inlet (15.1).

7. The compressor according to any one of claims 1 to 6, characterized in that, At least a part of the flow channels (15) extend curvilinearly in the circumferential direction in the region between the flow inlet (15.1) and the flow outlet (15.2).

8. The compressor according to any one of claims 1 to 7, characterized in that, At least a part of the flow channels (15) are formed in the region between two mutually connected components. Wherein preferably it is provided that one of the components has a groove extending in the direction of the channel length, which groove delimits the flow channel, and the other component at least locally completes or covers the groove.

9. The compressor according to any one of claims 1 to 8, characterized in that, The compressor wheel (10) has a centering notch or centering recess in the region of its axis of rotation, and the notch or the centering recess is fluidly connected to the flow inlet of the flow channel (15) in the inflow region (11).

10. The compressor according to any one of claims 1 to 9, characterized in that, The volume occupied by the compressor wheel (10) is at least 6 times, preferably at least 8 times, the sum of the channel volumes of the flow channels (15). and / or the maximum opening dimension of the flow outlet or at least a part of the flow outlet of the compressor wheel (10) extends in the circumferential direction over a maximum central angle of 0.5° to 10°, preferably over a maximum central angle of 1° to 7°.

11. The compressor according to any one of claims 1 to 10, characterized in that, The cross-sectional area of at least one flow channel (15) at the flow inlet (15.1) and / or at the flow outlet has a size of at least 0.5 mm 2 and at most 15 mm 2 , and / or the cross-sectional area of at least one flow channel (15) at the flow inlet (15.1) and / or at the flow outlet (15.2) has a size of at least 1 mm 2 and at most 5 mm 2 .

12. The compressor according to any one of claims 1 to 11, characterized in that, In the region of the outer circumference of the compressor wheel (10), a housing including a spiral channel is provided, and the flow channel (15) opens into the spiral channel.

13. The compressor according to any one of claims 1 to 12, characterized in that, The compressor wheel is coupled to a drive, such as an electric motor, via a drive shaft.

14. A component, in particular for a vehicle or a ship or an aircraft, the component having an optical sensor, in particular a radar sensor, and a compressor according to any one of claims 1 to 12, wherein, A channel connection is arranged between the compressor wheel (10) and the sensor to guide the compressed air generated by the compressor wheel (10) to the surface of the sensor for cleaning.

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