Centrifugal separator for cleaning of crankcase gas

Abstract

A centrifugal separator (100) is disclosed, comprising a barrier device (150) arranged between a separation space and a drainage region of the separator. The barrier device comprises a peripheral edge (156) defining a circumferential gap between the barrier device and an inner wall portion of a housing, the gap being configured to permit a subflow of the gas flow to pass between the separation space and the drainage region. The barrier device further comprises an outlet channel (164) bridging the gap to connect the barrier device to the gas outlet, wherein an outer surface of a cross section of the outlet channel narrows in a direction of the subflow, from a leading edge towards a trailing edge, to reduce a turbulence of at least a portion of the subflow passing the outlet channel.

Description

Technical Field

[0001] The present invention relates to the field of centrifugal separators for cleaning a gas flow comprising contaminants. In particular, the invention concerns a separator for cleaning crankcase gases of an internal combustion engine from contaminants such as oil.Background

[0002] Fluids of different densities can be separated using a centrifugal separator. One common application is the removal of contaminants, such as oil, from gas vented from the crankcase of an internal combustion engine. During engine operation, high-pressure gas from the combustion chambers tends to leak past the piston rings into the crankcase. This gas can increase crankcase pressure and impair engine performance.

[0003] To prevent such issues, the gas is vented and cleaned before being recirculated into the engine's intake system. The cleaning is particularly important in turbocharged engines, where the presence of oil can negatively impact performance. A centrifugal separator mounted near the crankcase may be used for this purpose, directing cleaned gas to the intake system while returning separated oil to the crankcase.

[0004] However, not all contaminants may be effectively removed. Some oil particles and other impurities may become entrapped within the rotating gas flow inside the separator and exit with the cleaned gas.

[0005] Accordingly, there is a need for improved technologies that reduce the risk of contaminants being carried over with the cleaned gas.Summary

[0006] It is an object of the present invention to provide a technology that at least partly addresses the above concerns. The invention is defined by the subject matter of the independent claims. Preferred embodiments are set forth in the dependent claims and described in the following description and drawings.

[0007] According to an aspect, there is provided a centrifugal separator for separating contaminants from a gas flow, comprising a stationary housing enclosing an upper separation space accommodating a rotatable separation unit for separating the contaminants from the gas flow passing through the separation space, and a lower drainage region comprising a drainage outlet for discharging the separated contaminants, and a barrier device arranged between the separation space and the drainage region. The barrier device comprises an upper side facing the separation space and configured to guide the gas flow towards a gas outlet of the housing, a lower side configured to shield the contaminants in the drainage region from the gas outlet, and a peripheral edge defining a circumferential gap between the barrier device and an inner wall portion of the housing, wherein the gap is configured to permit a subflow of the gas flow to pass between the separation space and the drainage region. The separator further comprises an outlet channel bridging the gap to connect the barrier device to the gas outlet, wherein an outer surface of a cross section of the outlet channel narrows in a direction of the subflow, from a leading edge towards a trailing edge, to reduce a turbulence of at least a portion of the subflow passing the outlet channel.

[0008] During operation, the gas to be cleaned may enter the separation space within the housing, where the separation unit induces a whirling motion, generating centrifugal forces that drive heavier contaminants, such as oil droplets and particulate matter, towards the walls of the separation space. These contaminants may then accumulate in the drainage region for discharge. However, the rotational flow pattern of the gas can hinder efficient separation, as swirling gas moving towards the gas outlet may re-entrain liquid contaminants, particularly from the drainage region at the bottom of the housing. Secondary vortices or recirculation zones in this region can prevent proper settling, leading to re-entrainment of contaminants into the gas flow and reducing separation efficiency.

[0009] The barrier device may mitigate this issue by segregating the cleaned gas from the accumulated contaminants. Positioned within the separator, the barrier device may physically obstruct the interaction between the swirling gas flow and the settled liquid contaminants, reducing the risk of re-entrainment. Without this barrier, the swirling gas could disturb the settled contaminants, leading to resuspension and carryover into the gas outlet. The barrier device may thus form a protective partition that shields the liquid contaminants from direct gas flow interaction.

[0010] The barrier device may comprise an annular wall and a cup-shaped profile configured to channel the cleaned gas flow along a controlled path, ensuring that most of the liquid contaminants remain in the drainage region and are properly discharged through the drainage outlet rather than being carried towards the gas outlet.

[0011] The barrier device may be particularly beneficial in applications where oil mist or fine liquid droplets are present, as it reduces the likelihood of contaminants creeping back into the gas stream. By segregating the gas flow from the drainage region, the barrier device may enhance overall separation efficiency, reducing the amount of residual liquid in the cleaned gas.

[0012] In some examples, the barrier device is supported by legs extending downwards into the drainage region, where separated contaminants may accumulate. These legs, being in direct contact with the settled liquid, may facilitate capillary action or liquid creep, causing contaminants to migrate upwardly along the leg surfaces. This effect may be driven by intermolecular forces and may be more pronounced if the leg surfaces exhibit wettability properties that promote liquid adhesion.

[0013] Once the contaminants reach a certain height on the legs, they may be stripped by the swirling gas flow induced by the rotating separation unit. Low-pressure zones and shear forces around the barrier device can detach the creeping liquid, reintroducing it into the gas stream. As a result, instead of being properly drained, the contaminants may become entrained in the exiting gas flow, reducing separation efficiency and increasing carryover of unwanted substances.

[0014] The outlet channel, which may be provided to bridge the gap between the barrier device and the gas outlet, may be designed with a narrowing cross-section in the direction of the subflow swirling around the barrier device. Specifically, the outer surface of the cross-section may taper from a leading edge towards a trailing edge with respect to the flow direction, resembling the profile of an aircraft wing. This geometry may reduce turbulence in at least a portion of the subflow passing across the outlet channel.

[0015] The leading edge of the outlet channel, i.e., the edge facing the swirling subflow, may be broadened or rounded, to allow for a smooth initial interaction between the gas flow and the channel surface. As the flow moves downstream over the outer surface of the outlet channel, the channel's cross-section may gradually narrow, guiding the gas flow along a pathway towards the trailing edge, which may be relatively sharp or truncated to promote flow detachment. This gradual constriction of the flow path may help to regulate the velocity and pressure distribution of the gas, reducing disruptions that could otherwise lead to vortices, recirculation, or unwanted turbulence.

[0016] By adopting a profile similar to an airfoil, the narrowing profile of the outlet channel can help provide a laminarising effect, particularly for subflows that might otherwise interact unpredictably with the surrounding gas.

[0017] The exact contour of the narrowing section may be adapted based on performance requirements. In some examples, the trailing edge may feature an angled taper, directing the flow downwards towards the bottom of the drainage region to further mitigate contaminant entrainment.

[0018] Further, it will be appreciated that the inner cross section of the outlet channel, i.e., the cross section of the actual flow channel, may not necessarily be narrowing in the same way as the outer surface. The cross section of the interior of the outlet channel may, for example, be elongated or elliptic rather than narrowing or tapering in the direction of the subflow.

[0019] The separator housing, or casing, may provide the structural framework for the centrifugal separator, enclosing the separation space and, in some examples, the drainage region. The housing may comprise a top wall, a bottom wall, and an annular wall extending between them, forming a generally enclosed chamber in which the rotating separation unit operates. The bottom wall of the housing may serve as the primary drainage region, where separated liquid contaminants accumulate after being forced outward by centrifugal forces. The drainage outlet may be positioned in the bottom wall, allowing collected contaminants to be discharged from the separator under the influence of gravity or controlled evacuation mechanisms. The drainage outlet may be designed with a sloped or funnel-shaped surface, guiding contaminants towards the outlet.

[0020] The gas outlet may, in contrast, be arranged in the annular wall, or sidewall, which may extend between the top and bottom walls to define the outer boundary of the separator. This arrangement helps separate the cleaned gas from the drainage region, reducing the risk of re-entrainment of contaminants by ensuring that gas exits at a location spatially distinct from where the contaminants accumulate.

[0021] The top wall of the separator may provide structural enclosure and may also serve as a mounting surface for certain components, such as a bearing assembly for the rotating separation unit or an inlet channel for incoming gas flow.

[0022] In some examples, the barrier device comprises additional structural features to enhance gas-liquid separation and prevent re-entrainment of contaminants. These features may include an annual rim protruding from the lower side of the barrier, as well as a funnel-shaped, or cup-shaped, depression on its upper side for guiding the separated gas flow to the gas outlet. Both of these structures may contribute to improved flow control and contaminant retention.

[0023] The annular rim may protrude downwards from the peripheral edge of the barrier device, forming a physical barrier to prevent liquid contaminants from reentering the separation space. The annular rim may shield the collected contaminants in the drainage region from the swirling gas in the separation space, increasing the chances that the contaminants remain within the drainage region.

[0024] The funnel- or cup-shaped depression on the upper side of the barrier device may be provided to guide the cleaned gas towards the opening of the outlet channel. This may ensure that gas exiting the separation space is conveyed in a controlled manner, reducing turbulence and increasing flow efficiency. The funnel-like geometry may provide a gradual transition for the gas flow, reducing pressure drops and velocity fluctuations which could otherwise introduce unwanted disturbances in the separation process. By providing the upper surface with a concave contour that converges towards the opening of the outlet channel, the gas flow may be caused to converge smoothly towards the outlet channel opening, rather than dispersing unpredictably within the separation space.

[0025] The rotatable separation unit, or rotor, may comprise a plurality of separation members arranged in the separation space. The separation members may be surface-enlarging inserts that promote separation of contaminants from the gas. The separation members may be a stack of separation discs. The separation discs of the stack may be frustoconical. A frustoconical disc may have a planar portion extending in a plane that is perpendicular to the axis of rotation, and a frustoconical portion that may extend upwards or downwards with respect to the axis of rotation. The planar portion may be closer to the axis of rotation than the frustoconical portion. Further, the discs of the stack may be radial discs, in which substantially the whole disc extends in a plane that is perpendicular to the axis of rotation.

[0026] It is also to be understood that the separation members, such as separation discs, not necessarily have to be arranged in a stack. The separation space may, for example, comprise axial discs, or plates that extend around the axis of rotation. The axial discs or plates may be planar, i.e., extending in planes that are parallel to the axis of rotation. The axial discs may also have a slightly or significantly curved shape, such as an arcuate or spiral shape, as seen in a radial plane.

[0027] As used herein, the term "axially" may denote a direction which is parallel to the rotational axis (X). Accordingly, relative terms such as "above", "upper", top", "below", "lower", and "bottom" may refer to relative positions along the rotational axis (X). Correspondingly, the term "radially" typically refers to a direction extending radially from the rotational axis (X). A "radially inner position" thus refers to a position closer to the rotational axis compared to a "radially outer position". A radial plane is a plane having its normal parallel to the axis of rotation. An axial plane is a plane having its normal perpendicular to the axis of rotation.Brief Description of Drawings

[0028] The above, as well as additional objects, features, and advantages of the present disclosure, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings, like reference numerals will be used for like elements unless stated otherwise. Figure 1 shows a schematic cross section of a centrifugal separator according to an example. Figure 2 is a perspective view of a barrier device according to an example. Figure 3 is a perspective view of a barrier device according to an example, comprising a flow guide for directing a gas flow downwards. Figure 4 is a zoomed in version of the flow guide in figure 3. Figure 5 is a perspective view of a barrier device according to another example. Figure 6 schematically illustrates a narrowing cross section of an outlet channel connecting the barrier device to the gas outlet in the separator housing. Detailed Description

[0029] Figure 1 shows a cross-section of a centrifugal separator 100 according to an example of the present disclosure. The centrifugal separator 100 comprises a stationary housing 110 enclosing a separation space 120 accommodating a rotatable separation unit 130 for separating contaminants from a gas flow, and a drainage region 140 comprising a drainage outlet 142 through which the separated contaminants can be discharged from the separator 100. The separation space 120 may be arranged above the drainage region 140, with respect to the rotation X of the rotatable separation unit 130.

[0030] The separator further comprises a barrier device 150 arranged between the separation space and the drainage region to segregate cleaned gas in the separation space 130 from the separated contaminants in the drainage region 140, and to prevent the contaminants from being re-entrained by the gas flow on its way out from the separation space 120.

[0031] The separator 100 may be suitable for cleaning crankcase gases ventilated from the crankcase of an internal combustion engine, or gases from other sources, such as the environment of machine tools which may contain liquid contaminants in the form of oil droplets or oil mist. Preferably, the separator 100 may have a size and weight that makes it suitable to be mounted to a combustion engine, such as at a top or a side of the combustion engine, of a vehicle such as a lorry.

[0032] The stationary housing, or casing 110, encloses the separation space 130, through which a gas flow is permitted. The housing 110 may be formed of a surrounding side wall, a top wall, and a bottom wall 116. A major part of the side wall may have a substantially constant radius with respect to the rotation axis X. In different words, the side wall may be an annular wall having a substantially circular, as well as substantially uniform, cross section in a radial plane orthogonal to the rotation axis X.

[0033] A gas inlet 122 may extend through a wall of the housing 110 to allow the gas flow to enter the separation space 120. In the depicted example, the gas inlet 122 is arranged at the upper end wall, coinciding with an axis of rotation X of the separation unit. A gas outlet 124 for evacuating the cleaned gas from the separation space 120 may be arranged at a lower portion of the separation space 120, such as through a lower portion of the side wall of the housing 110.

[0034] The rotatable separation unit, or rotor 130, may be arranged to rotate around the rotation axis X. The stationary housing 110 may be stationary in relation to the rotatable separation unit 130, and preferably in relation to the combustion engine to which the separator 100 may be mounted.

[0035] The rotor 130 may comprise a rotatable shaft 132, or spindle, as well as a stack of separation members 134 attached to the spindle 130. The separation members 134 may form a stack of separation discs, which may be frustoconical and extend outwards and upwards from the spindle 132. The separation discs 134 may thus comprise a flat portion that extends perpendicularly to the rotation axis X and a conical portion that extends outwardly and upwardly from the flat portion. In other examples, the separation discs could extend outwardly and downwardly, or even radially.

[0036] The separation members 134 may be arranged at a distance from each other, forming equidistant interspaces through the gas flow may pass during operation. The axial width of the interspaces may, for instance, be in the order to 0.5-2 mm.

[0037] The separation members 134 may be formed of a plastic material or metal. The number of separation members 134 may, for instance, be 50 to 100, depending on the size of the centrifugal separator.

[0038] During operation, the separation members 134 are brought to rotate around the rotation axis X to induce a whirling motion in the gas flow passing between gas inlet 122 and the gas outlet 124. The rotational speed may be in the range of 7,500-12,000 rpm. This swirling motion generates centrifugal forces that drive heavier contaminants, such as oil droplets and particulate matter, outwards from the rotation axis X towards the inner walls of the separation space 120. These contaminants may then flow downwards under influence of gravity towards the drainage region 140 at the bottom of the housing 110, where they can accumulate for discharge. In the present figure, the flow of the gas is indicated by the arrows in the right half of the separation space 130, whereas the flow of the separated contaminants is indicated by the arrows in the left half of the separation space 130.

[0039] The rotational movement of the rotor 130 and the separation members 134 may be achieved by means of a drive unit 139, which may be driven by a flow of oil from the internal combustion engine of by an electric motor (not shown). In the present example, the separator 100 comprise an oil nozzle 114 arranged to be connected to the engine oil circuit to allow oil to be pumped through the oil nozzle 114 onto a turbine wheel 139 connected to the spindle 132. The rotor 130 may be rotatably supported, or journalled, in the housing 110 by means of an upper bearing 136 and a lower bearing 138, which may be arranged on each axial side of the stack of separation members 134.

[0040] The liquid contaminants, such as oil and or liquid matter, may be drained from the drainage region through a drainage outlet 142 arranged at the bottom of the housing 110. In the depicted example, the drainage outlet 142 extends through the bottom wall of the housing 110, at the lower bearing 138 of the rotor 130. The drainage outlet 142 may lead the contaminant to a lower end portion 112 of the housing 110, which in the present example accommodates the oil-driven drive unit 139 of the rotor 130. This allows the separated oil to join the flow of oil driving the drive unit 139.

[0041] The separator 100 may further comprise a barrier device 150, or stationary insert, which may be arranged in a lower portion of the housing 110, such as axially below the stack of separation members 134. The barrier device 150 may be supported by the bottom wall of the housing 110 via supporting legs (not shown). The barrier device 150 may comprise an upper side 152 facing the separation space 120 and a lower side 154 arranged to face the drainage region 140. This arrangement allows the barrier device to segregate the contaminants in the drainage region 140 from the cleaned gas in the separation space 120.

[0042] The rotational flow pattern of the gas, caused by the rotation of the separation members 134, can hinder efficient separation, as swirling gas moving towards the gas outlet 124 may entrain liquid contaminants from the drainage region 140. Furthermore, vortices or recirculation zones in the drainage region 140 can prevent proper settling of contaminants, leading to re-entrainment of contaminants into the gas flow and reducing separation efficiency. The barrier device 150 may mitigate this issue by segregating the cleaned gas from the accumulated contaminants. The barrier device 150 may physically obstruct the interaction between the swirling gas flow and the settled liquid contaminants, thereby forming a protective partition that shields the liquid contaminants from direct gas flow interaction.

[0043] The barrier device 150 may comprise a peripheral edge 156 running along a major part of the inner wall of the housing 110, thereby forming a circumferential gap between the barrier device 150 and the inner wall of the housing 110. The circumferential gap may be configured to permit at least some of the whirling gas in the separation space 120 to enter the drainage region 140. This portion of the whirling gas flow, passing around the barrier device and between the separation space 120 and the drainage region 140, may be referred to as a subflow of the gas flow.

[0044] The barrier device 150 may comprise an outlet channel 164, bridging the circumferential gap to connect the barrier device 150 to the gas outlet 124 in the sidewall of the housing 110. The outlet channel

[0045] The barrier device 150 may comprise a central frustoconical cup member, or funnel-shaped depression, on the upper side 152 to guide the cleaned gas towards the opening of the outlet channel 164. The outlet channel 164 may hence be arranged with an inlet on the upper side 152 of the barrier device 150 and an outlet connected to the gas outlet 124 in the wall of the housing 110. This allows for the cleaned gas to exit on the separation space-side of the barrier device 150 and the separated contaminants to exit on the drainage region-side of the barrier device 150.

[0046] Figure 2 shows a perspective view of an example barrier device 150, which may be configured similarly to the barrier device 150 shown in figure 1. As discussed in relation to figure 1, cleaned gas may be directed via the upper side 152 of the barrier device 150 into the outlet channel 164, which extends radially across the circumferential gap between the barrier device 150 and the sidewall of the housing to connect the barrier device 150 with the gas outlet 124 (not shown in figure 2). In this example, the outlet channel 164 is formed as a tube or conduit that is formed in a single piece with the barrier device 150. However, the outlet channel 164 may as an alternative be formed as a separate element that is connected to, or attached to, the barrier device 150.

[0047] During operation, a wake zone may be formed at the outlet channel 164, at a downstream or leeward portion of the outlet channel 164 as seen in the direction of the subflow passing in the circumferential gap. In some examples, contaminants may accumulate at the wake zone and risk being re-entrained into the whirling subflow. Therefore, there may be provided a drainage member, or oil bridge structure 157, that connects the wake zone of the outlet channel 164 with the bottom wall 116 of the housing. With such an arrangement, contaminants that accumulate at the wake zone mat be guided from the outer surface of the outlet channel 164 via the oil bridge structure 157 to the bottom wall 116 and further towards the drainage outlet 142.

[0048] The top side 152 of the depicted barrier device 150 is provided with a cup-shape, of funnel-shaped portion, providing a controlled path for cleaned gas into the outlet channel 164. The path for the cleaned gas is indicated by arrows C in the present figure.

[0049] Furthermore, the barrier device 150 comprises a peripheral edge 156 having an annular wall member extending axially downwards from the upper side 152 of the barrier device 150. A circumferential gap, or annular channel, may be formed between the barrier device 150 and the inner surface of the housing 110, forming an annular channel between the barrier device 150 and the inner surface of the housing 110. This channel may provide a protected drainage pathway, guiding separated oil and liquid contaminants downward while at least partly shielding them from swirling gas flow in the separation space, that could otherwise reintroduce them into the gas flow.

[0050] A subflow of the whirling gas may however be permitted to pass the barrier device 150 into the drainage region. Typically, this subflow maintains at least some of the whirling motion induced by the rotor 130. The rotational movement of the subflow is indicated by an arrow Z in figure 2. Hence, when reaching the lower part of the interior of the housing 110, the gas flow may circulate in the drainage region, across the outlet channel 164 that extends radially in the direction from the rotation axis X towards the housing wall. Due to the rotational movement of this subflow, the gas may pass above an outer surface of the outlet channel 164 as well as below the same (with reference to the rotation axis X). Further, a wake zone may be formed downstream, or leeward, of the outlet channel 164.

[0051] The separated liquid contaminants may be transported to the bottom wall 116 of the housing, arranged below the barrier device 150. The surface of the bottom wall 116 may comprise one or more guiding structures 117 in the form of elongated recesses 117 for guiding the separated contaminants towards the drainage outlet 142, which may be arranged at a centre of the bottom wall 116, with respect to the rotation axis X. The recesses 117 may be radially arranged and covering most of the radial distance of the bottom wall 116. The recesses 117 may help guiding a flow of separated oil, indicated by arrow D, from a periphery of the bottom wall 116 to the drainage outlet 142.

[0052] Figure 3 is a perspective view of a barrier device 150 according to some examples. The barrier device 150 may be configured similarly to any of the barrier devices 150 discussed above with reference to figures 1 and 2. Hence, the barrier device 150 may comprise a top side 152 configured to face the separation space and a bottom side 154 configure to face the drainage region. The top side 152 is provided with a concave profile to guide the cleaned gas flow towards the outlet channel 164, which extends radially outwards from the rotation axis X to the gas outlet in the housing wall (not shown). The barrier device 150 further comprises one or more leg members 159, which may be provided to support the bottom side 154 of the barrier device 150 on the bottom wall on the housing.

[0053] During operation, accumulated contaminants, such as oil, may be re-entrained by the whirling gas flow and exit the separator through the gas outlet. Oil may migrate upwards along surfaces of the leg members and be stripped by the swirling gas flow. Further, oil may accumulate in low-pressure zones, such as the wake zone downstream, or behind, the outlet channel 164 (as seen in the direction Z of the subflow).

[0054] The present figure schematically illustrates several measures that may be taken to reduce the risk of oil being re-entrained by the gas flow and reduce the separation efficiency of the separator. It will be appreciated that these measures merely are schematic examples that can be implemented alone or in combination.

[0055] Figure 3 shows an example, where a flow guide 162 is arranged at the peripheral edge 156 to redirect at least a portion of the subflow, passing between the separation space and the drainage space, downwards, i.e., towards the bottom wall of the housing. By directing the subflow downwards, the flow guide 162 may force oil droplets that accumulate on the surface of the barrier device 150, such as in the wake zone at the outlet channel 164, downwards so that they may remain in the drainage region rather than being entrapped in the gas flow and carried towards the gas outlet. Oil migrating upwards along the leg members, as well as oil accumulated in the wake zone, may thus be forced back towards the bottom wall of the housing rather than being entrained by the gas flow and exit through the gas outlet.

[0056] In the present example, the flow guide 162 is arranged in a flow path of the subflow passing an upper side of the outlet channel 164. In different words, the flow guide 162 may be arranged axially above the outlet channel 164 and aligned with the outlet channel 164 in the circumferential direction. The flow guide 162 may be arranged above a trailing edge of the outlet channel 164, i.e., the at the downstream side of the outlet channel 164, with reference to a main flow direction of the subflow, to prevent droplets in the wake zone to be carried upwards, towards the separation space.

[0057] In some examples, the exterior surface of the outlet channel 164 may be provided with one or more flow detachments edges 166, 169 for detaching at least a portion of the subflow from the exterior surface. By detaching the subflow from the exterior surface, undesirable upward flow pattern that could lead to oil re-entrainment may be prevented. These detachments edges 166, 169 may be arranged at a downstream portion of the outlet channel 164, with respect to the subflow's flow direction Z, and configured to interrupt the adhesion of the gas flow to the outlet channel's 164 surface. This configuration may allow the flow to separate cleanly rather than following the curvature of the outlet channel 164 in an upward direction. Without such detachment edge, the gas flow may naturally tend to adhere to the surface contours of the outlet channel 164 due to the Coanda effect. This adherence can result in a portion of the gas flow deflecting upwards, creating a secondary flow that may lift oil droplets from surrounding surfaces and entrain them towards the gas outlet. By incorporating sharp-edged or angled flow detachment edges, having an apex angle of 90° or less, the swirling gas may detach more cleanly from the exterior surface of the outlet channel 164. This, in turn, may reduce the risk of upward secondary flows and instead allows for oil droplets to be redirected into the drainage region.

[0058] The outlet channel 164 may further be provided with an oil bridge structure 157, connecting the wake zone with the bottom wall 116 of the housing to guide accumulated contaminants from the wake zone, as discussed above in connection with figure 2.

[0059] In some examples, the lower side 154 of the barrier device 150 may comprise two or more ring-shaped edges 167 preventing separated oil from migrating towards a centre of the barrier device 150. The edges 167 may function as physical barriers, interrupting the movement oil creeping along the surface of the barrier device due to, e.g., capillary action, surface adhesion, or gas flow patterns in the drainage region. The ring-shaped edges 167 may help preventing this by creating localised containment zones that may disrupt the movement of the oil along the surface of the barrier device 150. A space between the ring-shaped edges may act as a trap zone, where oil accumulates and is directed towards the drainage outlet rather than continuing towards the centre of the barrier device 150. A more detailed example will be discussed below with reference to figure 5.

[0060] The flow guide 162 may be configured to direct the subflow, passing the peripheral edge of the barrier device 150, downwards to prevent re-entrainment of oil droplets. The flow guide 162 may be arranged at the peripheral edge of the barrier device, such that it extends into the circumferential gap in which at least some of the whirling gas flow passes into the drainage region. Beneficially, the flow guide 162 may be arranged to influence the behaviour of the subflow to reduce the risk that separated contaminants are carried towards the gas outlet.

[0061] Figure 4 shows a detailed example of a flow guide 162, which may be configured similarly to the one shown in figure 3. The flow guide 162 is integrally formed in one piece with the barrier device 150 and extends outwardly and downwardly from the barrier device 150, thereby forming a deflecting surface that forces at least a portion of the swirling subflow in a downward direction. The flow guide 162 may be arranged above the outlet channel 164, at a circumferential position coinciding with a downstream edge of the outlet channel 164 and be angled relative to the main flow direction of the subflow to force it downwards, towards the bottom of the drainage region. In some examples, the flow guide 162 may comprise a curved or aerodynamically contoured profile to stabilise the redirected flow and reduce pressure losses.

[0062] The flow guide 162 may extend across the circumferential gap between the barrier device and the separator wall. The flow guide 162 may span the entire width of the gap, or extend into a portion of the gap, thereby forming a deflection path that directs the subflow downwards.

[0063] In some examples, the leading edge of the flow guide 162, i.e., the edge facing the subflow approaching the flow guide 162, may be rounded or tapered to reduce flow resistance and provide a smoother transition for the subflow. In some embodiments, multiple flow guides 162 may be provided, which may be arranged in a stepped or staggered manner, to gradually direct the whirling subflow downwards.

[0064] As illustrated in figure 4, the subflow of cleaned gas, indicated by arrows C, may pass on both sides of the flow guide 162 and be directed downwards, towards the bottom wall of the housing. This downward movement may cause any oil droplets on the wake side of the outlet channel 164, i.e., the side of the outlet channel 164 facing away from the approaching subflow, to be forced downwards towards the bottom wall rather than being carried towards the gas outlet. The flow path of such oil droplets is indicated by arrow D in figure 4.

[0065] Figure 5 shows another example of a barrier device 150, which may be configured similarly to any of the barrier devices discussed above with reference to figures 1-4. Figure 5 is a perspective view of the barrier device 150 from below, showing the lower side 154 facing the drainage region of the separator. The example barrier device 150 comprises three leg members 159 for supporting the barrier device 150 on the bottom wall of the housing. The leg members 159 may, for instance, be attached to the bottom wall by means of rivet joints, formed by means of attachment members or rivets 159' protruding from the bottom part of the leg members 159.

[0066] The barrier device 150 comprises a central passage 158 for a rotor shaft (such as the rotor shaft 132 shown in figure 1), an oil trap structure 167, 168 for preventing oil from migrating towards the central passage 158 and, in some examples, one or more flow detachment edges 166, 169 for reducing the risk of upward secondary flows that might drag oil into the flow of cleaned gas exiting the gas outlet.

[0067] The oil trap structure may comprise a first ring-shaped edge 167 and a second ring-shaped edge 168 similar to the ones described above in connection with figure 3. The first edge 167 may form an outer ring, whereas the second edge 168 may form an inner ring with respect to the central passage 158. The rings 167, 168 may be arranged concentrically around the central passage 158 to form a barrier, or channel, between themselves. Oil that migrates towards the central passage 158 may be trapped in this channel.

[0068] Furthermore, the outer ring 167 may be taller, i.e., protrude more from the surface of the barrier device 150, than the inner ring 168, thereby creating a wake zone that further isolates trapped oil from the influence of the swirling subflow. This configuration helps ensuring that oil remains in the drainage region and prevents re-entrainment into the gas flow.

[0069] The rings 167, 168 may be formed using various manufacturing techniques and structural configurations, balancing barrier efficiency and manufacturability. In some examples, the rings 167, 168 may be integrally formed as part of the barrier device. This may be achieved through, for example, injection moulding, casting, or machining, depending on the material composition of the barrier device. When made from polymeric or composite materials, the rings 167, 168 may be moulded as a single unit with the barrier device. For metallic barrier devices, the rings 167, 168 may be machined or stamped into the surface to form raised structures that serve as oil traps.

[0070] Alternatively, the rings 167, 168 may be separately manufactured components that may be affixed to the lower side 154 of the barrier device. In such cases, they may be attached suing fasteners, adhesives, or welding techniques, depending on the material properties and operational requirements. This module approach allows for customisation of ring dimensions and spacing, enabling adjustments based on the expected oil migration behaviour and fluid dynamics within the separator.

[0071] The rings 167, 168 may be provided with varying heights to increase their trapping efficiency. In some configurations, the outer ring 167 is taller than the inner ring 168, creating a wake zone within the enclosed gap between the rings 167, 168. As mentioned above, this wake zone may isolate trapped oil from the influence of swirling gas flow, reducing the likelihood of droplet re-entrainment. The gap between the rings 167, 168 may also be precisely dimensioned to ensure that oil accumulates in a controlled manner while allowing it to drain efficiently towards the drainage outlet without spreading toward the rotor shaft.

[0072] Surface modifications can further enhance the ability of the rings 167, 168 to contain migrating oil. For instance, the rings 167, 168 may be coated with a material that prevents oil from spreading beyond the intended containment area. Textured or microgrooved surfaces may also be incorporated to promote capillary-driven movement of oil away from critical components and areas of the barrier device.

[0073] One or more flow detachment edges 167, 169 may be provided at a lower (with respect to the rotation axis X), downstream (with respect to the flow direction Z of the subflow) portion of the outlet channel 164. As mentioned above, the gas flow tends to adhere to the surface contours of the outlet channel 164, which may result in a portion of the gas flow to deflect upwards and entrain oil accumulated in the wake zone of the outlet channel 164. This effect may be mitigated by the flow detachment edges 167, 169, which may be arranged to detach the gas flow from the surface of the outlet channel 164 and cause it to continue in a downward direction, towards the bottom of the drainage region.

[0074] Figure 6 is a perspective view of a barrier device 150 according to an example, which may be configured similarly to any of the barrier devices disclosed in figures 1-5. Figure 6 shows a vertical cross section of the outlet channel 164, which is configured to reduce turbulence in the subflow passing around the exterior of the outlet channel 164 while ensuring that contaminants may remain confined to the drainage region. The outlet channel 164 may be arranged to extend between the barrier device 150 and the gas outlet in the housing, intersecting the whirling gas flow in the circumferential gap between the barrier device 150 and the housing wall.

[0075] To improve flow behaviour, the cross-section of the outlet channel 164, and more particularly the outer surface of the outlet channel 164, may narrow in the direction of the subflow, from a leading edge towards a trailing edge, or wake zone, as indicated by arrows C in the figure. This narrowing profile of the outer surface may cause the subflow to converge rather than diverge, reducing unwanted turbulence and the risk of contaminants being dispersed within the separator's interior.

[0076] The leading, or upstream edge of the outlet channel 164, which faces the approaching subflow, may be configured with a rounded profile, similar to the leading edge of an aircraft wing. This aerodynamic shaping may help reduce flow resistance and smoothen the transition as the gas flow moves downstream. As the subflow progresses along the exterior of the outlet channel 164, the cross-section gradually narrows, directing the flow into a controlled path.

[0077] As shown in figure 6, the cross section of the actual flow channel, i.e., the interior surface of the outlet channel 164, may have a shape that differs from the cross section of the outer surface. The interior surface may, for example, have an elliptic or elongated profile, rather than a profile that narrows in the direction of the subflow.

[0078] To promote clean flow detachment, the trailing edge of the outlet channel 164 may be truncated, allowing the subflow to separate from the outer surface of the outlet channel 164. The truncation may form one or more flow detachment edges 169, such as an upper edge and a lower edge serving to further stabilise flow detachment, preventing recirculation zones that could cause oil accumulation, and re-entrainment into the gas stream. By promoting efficient flow separation, these edges 169 may reduce the risk of contaminants collecting in the wake zone and being transported towards the gas outlet. As previously mentioned, contaminants accumulated at the wake zone formed at the trailing edge of the outlet channel 164 may be guided towards the bottom wall of the housing by means of an oil bridge structure 157.

[0079] In addition to reducing turbulence, the profile of the outlet channel 164 may be angled downwards, directing the subflow towards the bottom wall of the separator housing. This downward redirection may be achieved through tapered surfaces on one or both the upper and lower faces of the outlet channel, allowing contaminants in the the subflow to be guided towards the drainage region rather than mixing with the cleaned gas. By maintaining this controlled flow path, the design may prevent contaminants from escaping and enhances the overall efficiency of the separator.

[0080] The features described herein - namely, the narrowing cross-section of the outlet channel 164, the flow guide 162, the oil-trapping rings 167, 168, and the flow detachment edges 166, 169 - may be implemented individually or in any combination to enhance the separation efficiency of the centrifugal separator 100. Each of these features may contribute to the control of gas flow dynamics and the management of liquid contaminants, and their selection and arrangement may be adapted based on the specific operational requirements of the separator 100.

[0081] In some examples, the narrowing cross-section of the outlet channel 164 may be employed alone to reduce turbulence in the subflow, ensuring that gas exits the separator 100 in a more controlled manner while reducing the risk of contaminants dispersing within the housing 110. Alternatively, the flow guide 162 may be implemented independently to redirect the subflow downwards, forcing separated contaminants towards the drainage region 140 and reducing the risk of oil re-entrainment into the gas flow.

[0082] In further examples, the oil-trapping, ring-shaped edges 167, 168 may be used on their own to prevent oil migration along the lower side 154 of the barrier device 150, hindering contaminants from reaching the rotor shaft 132 and instead drain towards the drainage outlet 142. Similarly, the flow detachment edges 166, 169 may be employed separately to promote clean flow separation from the surface of the outlet channel 164, reducing the formation of secondary upward flows that could otherwise entrain oil droplets into the gas flow.

[0083] Furthermore, these features may be combined in various configurations to achieve enhance separation efficiency. A comprehensive configuration may include all four features - the narrowing cross section of the outlet channel 164, flow guide 162, oil-trapping rings 167, 168, and flow detachment edges 166, 169 - working in conjunction to increase flow stability, prevent oil re-entrainment, and ensure that separated contaminants may be directed towards the drainage outlet 142.

Claims

1. A centrifugal separator (100) for separating contaminants from a gas flow, the separator comprising: a stationary housing (110) enclosing an upper separation space (120) accommodating a rotatable separation unit (130) for separating the contaminants from the gas flow passing through the separation space, and a lower drainage region (140) comprising a drainage outlet (142) for discharging the separated contaminants; and a barrier device (150) arranged between the separation space and the drainage region, the barrier device comprising: an upper side (152) facing the separation space and configured to guide the gas flow towards a gas outlet (124) of the housing; a lower side (154) configured to shield the contaminants in the drainage region from the gas outlet; a peripheral edge (156) defining a circumferential gap between the barrier device and an inner wall portion of the housing, the gap being configured to permit a subflow of the gas flow to pass between the separation space and the drainage region; and an outlet channel (164) bridging the gap to connect the barrier device to the gas outlet, wherein an outer surface of a cross section of the outlet channel narrows in a direction of the subflow, from a leading edge towards a trailing edge, to reduce a turbulence of at least a portion of the subflow passing the outlet channel.

2. The separator of claim 1, wherein the leading edge comprises a rounded profile and the trailing edge comprises a truncated profile.

3. The separator of claim 2, wherein the truncated profile forms a flow detachment edge (166) configured to allow at least a portion of the subflow to detach from an exterior surface of the outlet channel.

4. The separator of any of the preceding claims, wherein an upper surface of the outlet channel is configured to direct the subflow in a downwards direction.

5. The separator of claim 1, wherein the barrier device further comprises a flow guide arranged at the peripheral edge and configured to direct at least a portion of the subflow in a downward direction.

6. The separator according to claim 5, wherein the flow guide protrudes from the peripheral edge into the gap.

7. The separator according to claim 5 or 6, wherein the flow guide is elongated and configured to be oriented along the subflow.

8. The separator according to any of the preceding claims, further comprising a rotor shaft configured to drive the separation unit, wherein the barrier device further comprises: a central passage for the rotor shaft; and a first and a second edge protruding from the lower side and arranged to encircle the central passage to trap contaminants flowing on the lower side, towards the central passage.

9. The separator of any of the preceding claims, wherein the housing comprises a top wall, a bottom wall, and an annular wall extending therebetween, and wherein the drainage outlet is arranged in the bottom wall and the gas outlet is arranged in the annular wall.

10. The separator of claim 9, wherein the barrier device further comprises a plurality of leg members configured to support the barrier device on a the bottom wall of the housing.

11. The separator of any of the preceding claims, wherein the peripheral edge of the barrier device comprises an annular rim protruding from the lower side of the barrier device.

12. The separator of any of the preceding claims, wherein the upper side of the barrier device comprises a funnel-shaped depression guiding the gas flow towards an opening of the outlet channel.

13. The separator of any of the preceding claims, wherein the separation unit comprises a stack of separation discs.

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