Patent Publication Number: US-7896618-B2

Title: Centrifugal compressing apparatus

Description:
This is a divisional application of U.S. application Ser. No. 11/240,527, filed Oct. 3, 2005 now U.S. Pat. No. 7,476,081. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a centrifugal compressing apparatus. 
     2. Description of the Related Art 
     As shown in  FIG. 7 , a centrifugal compressing apparatus  10  has an impeller  1  that is driven to rotate by a motor (not shown), etc., and a casing  2  that houses the impeller  1 . The impeller  1  has a hub (rotor)  4  that is formed into a substantially conical shape and blades  3  that are mounted radially onto the hub  4 . The casing  2  is formed to a substantially conical-cylindrical shape so as to house the impeller  1  across a predetermined clearance CL. The clearance CL is made substantially fixed in value from a front edge side  5  to a rear edge side  6  of the impeller  1 . 
     Reference symbol H denotes the height of the blade  3 , and the height H of the blade  3  is made to decrease gradually from the front edge  5  side to the rear edge  6  side of the impeller  1 . The height H of the blade  3  is the amount of protrusion of the blade from the hub surface in a direction orthogonal to the main air flow inside the impeller. In the following description, the value obtained by dividing the amount of change of the blade height with respect to the meridional distance along the hub surface by the meridional distance shall be defined as the blade height changing rate. 
     At the impeller  1  of the centrifugal compressing apparatus  10 , there exists a clearance flow that flows in from the clearance CL between a top edge  7  of the blade  3  and the shroud casing  2 . The clearance flow (CLF) refers to a phenomenon, wherein, as shown in  FIG. 10 , a portion of the air at a pressure surface  3   a  of the blade  3  of the impeller  1  flows past the clearance CL between the blade  3  and the casing  2  and into the negative pressure surface  3   b  side of the blade  3 . 
     A modeled flow inside the impeller for an ideal case where the clearance flow CLF does not exist is illustrated in  FIG. 8  and  FIG. 9 .  FIG. 8  is a diagram corresponding to a view taken on line A-A of  FIG. 7 . If, as shown in  FIG. 8 , it is assumed that the clearance CL does not exist between the blade  3  and the casing  2 , the flow velocity distribution (inter-blade flow velocity distribution) of the flow (main flow) flowing in the depth direction orthogonal to the paper surface along the section taken along line B-B of  FIG. 8  will, as shown in  FIG. 9 , be such that the flow velocity decreases gradually from the negative pressure surface  3   b  side to the pressure surface  3   a  side of the blade  3 . 
     Meanwhile, a modeled flow for the case where the clearance flow CLF exists is shown in  FIG. 10  and  FIG. 11 . Since the clearance flow CLF flows substantially perpendicular to the main flow direction as shown in  FIG. 10 , the flow velocity near the negative pressure surface  3   b  is a mixture of the ideal flow velocity and the substantially zero flow velocity of the clearance flow CLF and thus drops, as shown in  FIG. 11 , to half the ideal flow velocity shown in  FIG. 9 . The decrease in flow velocity in the main flow direction resulting from this mixture is a pressure loss. 
     As shown in  FIG. 7 , in the impeller  1  of the centrifugal compressing apparatus  10 , the height H of the blade  3  decreases from an entrance to an exit in the flow direction.  FIG. 12A  is a diagram for illustrating a case where the height H of the blade  3  is relatively high, and  FIG. 12B  is a diagram for illustrating a case where the height H of the blade  3  is relatively low. Since the clearance CL between the blade  3  and the casing  2  is substantially fixed from the front edge  5  side to the rear edge  6  side of the blade  3  as mentioned above, when the height H of the blade  3  decreases, the ratio of the width Δb of the clearance CL to the height H of the blade  3  (Δb/H) becomes relatively large, and thus the ratio of the area occupied by the clearance flow CLF to the area occupied by the main flow becomes large as shown in  FIG. 12A  and  FIG. 12B  and thus the pressure loss increases. The pressure loss due to the clearance flow CLF is greater the lower the height H of the blade  3 , and is greater at the rear edge  6  side than at the front edge  5  side of the blade  3 . 
     Japanese Published Unexamined Patent Application No. 2000-64998 discloses a centrifugal compressing apparatus, wherein an abradable layer that is abraded by an impeller is provided on an inner surface of a casing that houses the impeller, and with this centrifugal compressing apparatus, when the length from a front edge to a rear edge of the impeller along the inner surface of the casing is M and a length from the front edge of the impeller to an arbitrary position is m, the abradable layer is disposed in the range of M-m, with 0.2≦m/M≦0.4 being satisfied. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a centrifugal compressing apparatus having low pressure loss and that can restrain the loss of efficiency. 
     According to an aspect of the present invention, in a centrifugal compressing apparatus, a height of a blade of an impeller is made to decrease gradually from a front edge thereof to a rear edge thereof, and an absolute value of a rate of change of the height of the blade is relatively large near the rear edge. 
     According to another aspect of the present invention, in a centrifugal compressing apparatus, at a top edge of a blade of an impeller, a shroud line of a shroud surface that opposes a casing that houses the impeller is made convex in a direction of increasing a height of the blade beyond a tangent drawn to the shroud line from a point of an exit width from a hub surface along a rear edge of the blade and towards an interior of the blade, at a rear edge of the blade with respect to an intersection of the tangent and the shroud line. Here, the point of the exit width from the hub surface on the rear edge of the blade may be a point that is separated from the hub surface on the rear edge of the blade by just the exit width. 
     According to still another aspect of the present invention, in a centrifugal compressing apparatus, at a base end of a blade of an impeller, a hub line that is a boundary with respect to a hub onto which the blade is mounted is made concave in a direction of increasing a height of the blade beyond a radial line drawn in a radial direction of the impeller from an intersection of a rear edge of the blade and the hub line. 
     According to still another aspect of the present invention, in a centrifugal compressing apparatus, a height of a blade of an impeller is made to decrease gradually from a front edge to a rear edge, and a rate of change of the height of the blade has at least one inflection point. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional side view of an impeller of a centrifugal compressing apparatus of a first embodiment of the present invention; 
         FIG. 2  is an enlarged view of  FIG. 1 ; 
         FIG. 3  is a diagram of a velocity triangle of a conventional impeller; 
         FIG. 4  is a diagram of a velocity triangle of the impeller of the first embodiment; 
         FIG. 5  is a sectional side view of an impeller of a centrifugal compressing apparatus of a second embodiment of the present invention; 
         FIG. 6  is a sectional side view of an impeller of a centrifugal compressing apparatus of a third embodiment of the present invention; 
         FIG. 7  is a sectional side view of an impeller of a conventional centrifugal compressing apparatus; 
         FIG. 8  is a view taken on A-A of  FIG. 7  and is a diagram of a modeled flow for an ideal case where a clearance flow does not exist; 
         FIG. 9  is a diagram of an inter-blade flow velocity distribution along line B-B of  FIG. 8 ; 
         FIG. 10  is a view taken on A-A of  FIG. 7  and is a diagram of a modeled flow for a case where a clearance flow exists; 
         FIG. 11  is a diagram of an inter-blade flow velocity distribution along line C-C of  FIG. 10 ; 
         FIG. 12A  is a diagram of the ratio of the area occupied by a clearance flow and the area occupied by a main flow when the blade height is high; and 
         FIG. 12B  is a diagram of the ratio of the area occupied by a clearance flow and the area occupied by a main flow when the blade height is low. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of a centrifugal compressing apparatus according to the present invention shall now be described in detail with reference to the drawings. 
     In the following embodiments, portions in common to those of the conventional art described above shall be provided with reference symbols in common and detailed description thereof shall be omitted. 
     As described with reference to  FIG. 12A  and  FIG. 12B , an object of these embodiments is to reduce pressure loss at a rear edge  6  side of a blade  3  at which the pressure loss is relatively large and thereby effectively restrain the lowering of the efficiency of the centrifugal compressing apparatus. 
     First Embodiment 
     A first embodiment shall now be described with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  is a side view of an impeller  1  of a centrifugal compressing apparatus  20  according to a first embodiment, and  FIG. 2  is an enlarged view of the principal portions. 
     As shown in  FIG. 1 , a line (shroud line)  12  of a shroud surface  11  of the blade  3  that opposes a casing (not shown) at a top edge  7  side of the blade  3  is formed so as to bulge in the direction of expanding the height H of the blade  3  in comparison to a shroud line  13  of the conventional centrifugal compressing apparatus  10  of  FIG. 7 . With the blade  3 , the bulged portion (convex portion) is indicated by reference symbol  14 . By the blade  3  having the convex portion  14 , the height H of the blade  3  is made higher than in the conventional arrangement. 
     In  FIG. 2 , reference symbol TA 1  denotes, in the blade  3  of the centrifugal compressing apparatus  20 , a tangent that is drawn starting from a point P, located at a distance of an exit width L to the shroud side from a hub line at the rear edge of the blade  3 , to the shroud line  12  in the upstream direction in the interior of the blade  3 . The point P is the intersection of the shroud line  12  and the rear edge of the blade  3 . The blade  3  of the centrifugal compressing apparatus  20  has the convex portion  14 , which bulges in the direction of enlarging the height H of the blade  3  beyond the tangent TA 1 , at the rear edge  6  side of the intersection of the shroud line  12  and the tangent TA 1 . In the conventional centrifugal compressing apparatus  10 , the height H of the blade  3  is relatively low at the rear edge  6  side of the blade  3  so that the pressure loss due to the clearance flow CLF becomes a problem. However, in the centrifugal compressing apparatus  20  of the first embodiment, the convex portion  14  is provided at the rear edge  6  side of the blade  3  so that the pressure loss due to the clearance flow CLF is reduced effectively. 
     Reference symbol TA 2  denotes a tangent drawn from point P to the shroud line  13  of the conventional centrifugal compressing apparatus  10  of  FIG. 7 . Because the blade  3  of the centrifugal compressing apparatus  10  does not have a convex portion that bulges in the direction of enlarging the height H of the blade  3  beyond the tangent TA 2 , the blade  3  is low in height H in comparison to the blade  3  of the centrifugal compressing apparatus  20  and is large in pressure loss due to the clearance flow CLF. 
     In regard to the meridional shape of the exit portion of the impeller  1  of the centrifugal compressing apparatus  20  of the first embodiment, whereas the conventional shroud line  13  is concave in the height H direction of the blade  3  from the hub  4  along the shroud direction, the shroud line  12  of the first embodiment is convex. By making the shroud line  12  convex with respect to the conventional impeller  1  with the same exit width L ( FIG. 2  and  FIG. 7 ) as the impeller  1  of the centrifugal compressing apparatus  20 , the height H of the blade  3  can be made high at an intermediate portion between the entrance and the exit of the impeller  1  (with the first embodiment, the portion at the exit side at which the pressure loss is especially high). 
     Thus, in the centrifugal compressing apparatus  20  of the first embodiment, the ratio of the width Δb of the clearance CL to the height H of the blade  3  (Δb/H) is relatively small in comparison to that of the conventional centrifugal compressing apparatus  10 . As a result, the ratio of the flow path area occupied by the clearance flow CLF to the flow path area occupied by the main flow is increased, and since the pressure loss is thus made small, the lowering of the efficiency can be prevented. 
     As with the conventional centrifugal compressing apparatus  10  shown in  FIG. 7 , the clearance CL between the casing and the impeller  1  is set to a substantially fixed value from the front edge  5  side to the rear edge  6  side of the blade  3  in the centrifugal compressing apparatus  20  of the first embodiment as well. With the first embodiment, the shape of the casing of the centrifugal compressing apparatus  20  is formed (though not illustrated) so that the clearance CL will be of a substantially fixed value from the front edge  5  side to the rear edge  6  side of the blade  3  according to the blade  3  having the convex portion  14  and the blade  3  having a shape such that the height H of the blade  3  is higher than that of the conventional arrangement (the blade  3  of  FIG. 7 ). 
     Though the respective embodiments of the present invention that shall be described below also share the feature that the height H of the blade  3  of the impeller  1  changes so as to decrease gradually from the front edge  5  side to the rear edge  6  side of the blade  3  as in the conventional arrangement, these embodiments have the characteristic of being arranged in such a manner that while the blade  3  is provided with an adequate blade height even at the rear edge side so that the proportion occupied by the clearance flow CLF will be small, an inflexion point is provided in the blade height changing rate in order to smoothly guide air to the impeller exit that is made relatively narrow. That is, when an ordinate is set to the blade height and an abscissa is set to the meridional distance from the front edge of a blade along the hub surface, whereas the conventional blade shape will be a monotonously decreasing curve that is convex towards the lower side, with the respective embodiments of the present invention, the curve will be convex towards the lower side at the front edge side, be convex towards the upper side at the rear edge side (and more convex towards the lower side near the rear edge), and have an inflection point in between. 
     The above embodiment may be summarized as follows. 
     Basic Principle 
     A basic principle of the first embodiment is that by making the proportion of the clearance CL with respect to the height H of the blade  3  small, the leakage loss is decreased and the efficiency is improved. Since priorly, the absolute value of the clearance CL was decreased to 1) decrease the absolute value of the leakage amount and 2) make the ratio of the clearance CL to the height H of the blade  3  small. Meanwhile, with the first embodiment, since the absolute value of the clearance CL can be made small by the conventional means, measures are taken to make the height H of the blade  3  high and thereby make small the ratio of the clearance CL to the height H of the blade  3  to reduce the leakage loss. 
     Additional Effect 
     The following additional effect is provided by the first embodiment. 
       FIG. 3  is a diagram of the blade  3  of the conventional centrifugal compressing apparatus  10 . In  FIG. 3 , the reference symbol U denotes the rotation direction velocity of the impeller  1 , the reference symbol W denotes the relative flow velocity, and the reference symbol C denotes the absolute flow velocity. By these, the velocity triangle shown in  FIG. 3  is formed.  FIG. 4  is a diagram of the velocity triangle of the impeller  1  of the centrifugal compressing apparatus  20  of the embodiment, and in  FIG. 4 , the velocity triangle of the impeller  1  of the conventional centrifugal compressing apparatus  10  is depicted by the broken lines. 
     As shown in  FIG. 3  and  FIG. 4 , with the impeller  1  of the centrifugal compressing apparatus  20 , by increasing the height H of the blade  3  by just the amount of convex portion  14 , the Cm within the velocity triangle decreases to Cm′, given that the flow rate is the same. In order to keep the work fixed (keep the pressure fixed), Cu′=Cu must be satisfied, and for this, modification is made to make the blade angle βk′&lt;βk so that the flow angle β′&lt;β (see  FIG. 3 ). 
     Consequently with the centrifugal compressing apparatus  20  of the first embodiment, the absolute flow velocity C′ also decreases in comparison to the conventional centrifugal compressing apparatus  10 . Since this absolute flow velocity C′ generates frictional loss with the casing, the loss of the impeller  1  is reduced by this reduction of the absolute flow velocity C′. 
     Thus as an additional effect of the first embodiment, by decreasing of the blade angle βk, the frictional loss can be reduced to restrain the reduction of the efficiency of the centrifugal compressing apparatus  20 . 
     Second Embodiment 
     A second embodiment shall now be described with reference to  FIG. 5 . 
     In the second embodiment, description of portions in common to the first embodiment shall be omitted and only the characteristic portions of the second embodiment shall be described. 
     As shown in  FIG. 5 , with a centrifugal compressing apparatus  30  of the second embodiment, a hub line  17 , at a base end  16  side that is the side of the blade  3  that is mounted to the hub  4 , is formed so as to be depressed in the direction of increasing the height H of the blade  3  in comparison to a hub line  15  of the conventional centrifugal compressing apparatus  10  of  FIG. 7 . The portion of difference (concave portion) of the blade  3  is denoted by the reference symbol  18 . The blade  3  has the convex portion  18  and the height H of the blade  3  is thereby made greater than that in the conventional arrangement. The hub line  17  is the boundary between the base end  16  of the blade  3  of the impeller  1  and the hub  4  onto which the base end  16  of the blade  3  is mounted. 
     In  FIG. 5 , the conventional hub line denoted by the reference symbol  15  is, at the same time, a segment (radial line) in the radial direction of the hub  4  that passes through a point Q at the hub  4  side of the exit width L of the rear edge  6  of the blade  3 . The point Q is the intersection of the hub line  17  and the rear edge  6  of the blade  3 . The blade  3  of the centrifugal compressing apparatus  30  has the convex portion  18  that bulges in the direction of enlarging the height H of the blade  3  beyond the radial line  15  that passes through the point Q. Whereas the height H of the blade  3  is relatively low at the rear edge  6  side of the blade  3  of the conventional centrifugal compressing apparatus  10  and the pressure loss due to the clearance flow CLF becomes a problem in particular, with the centrifugal compressing apparatus  30 , the convex portion  18  is provided at the rear edge  6  side of the blade  3 . The pressure loss due to the clearance flow CLF is thereby reduced effectively. Since the conventional centrifugal compressing apparatus  10  does not have a convex portion that bulges in the direction of enlarging the height H of the blade  3  beyond the hub line  15 , the height H of the blade  3  is low in comparison to the blade  3  of the centrifugal compressing apparatus  30  and the pressure loss due to the clearance flow CLF is large. 
     In regard to the meridional shape of the impeller  1  of the conventional centrifugal compressing apparatus  10 , with respect to an axial length Z 1  of the hub line  15  from the front edge  5  to the rear edge  6  of the impeller  1 , an axial length Z 2  from the front edge  5  of the impeller  1  at an intermediate portion between the front edge  5  and the rear edge  6  of the impeller  1  is such that Z 1 ≧Z 2 . Meanwhile, with the second embodiment, with respect to the axial length Z 1  from the front edge  5  to the rear edge  6  of the impeller  1 , a maximum value Z 2 max of the axial length Z 2  from the front edge  5  of the impeller  1  at an intermediate portion is such that Z 1 &lt;Z 2 max. 
     By making the maximum value of the length in the axial direction of the impeller  1  at an intermediate portion between the front edge  5  and the rear edge  6  of the impeller  1  satisfy Z 1 &lt;Z 2 max, the height H of the blade  3  can be made high at an intermediate portion between the front edge  5  and the rear edge  6  of the impeller  1 . The ratio (Δb/H) of the width Δb of the clearance CL and the height H of the blade  3  is thereby made relatively large. As a result, the ratio of the flow path area occupied by the clearance flow CLF to the flow path area occupied by the main flow is reduced and since the pressure loss is thus made small, the lowering of the efficiency can be prevented. The above-described additional effect obtained in the first embodiment is also obtained in the second embodiment. 
     As shown in  FIG. 1  and  FIG. 5 , in both the first embodiment and the second embodiment, though the height H of the blade  3  of the impeller  1  is made to change in a gradually decreasing manner from the front edge  5  side to the rear edge  6  side of the blade  3  as in the conventional arrangement, the embodiments are characterized in being arranged so that the changing rate of the height H of the blade  3  becomes relatively large near the rear edge  6  of the blade  3 . That is, both the first embodiment and the second embodiment have the arrangement where the height H of the blade  3  is secured to be as high as possible until immediately before the exit of the impeller  1  and the flow path is constricted sharply near the exit than at other portions. As a result, the height H of the blade  3  can be made large at the rear edge  6  side under the design restriction of setting the exit width of the rear edge  6  of the blade  3  to the predetermined design value L. 
     Third Embodiment 
     A third embodiment shall now be described with reference to  FIG. 6 . 
     In the third embodiment, description of portions in common to the above-described embodiments shall be omitted and only the characteristic portions of the third embodiment shall be described. 
     With a centrifugal compressing apparatus  40  of the third embodiment, the blade  3  has both the convex portion  14  of the first embodiment and the convex portion  18  of the second embodiment. The third embodiment can therefore exhibit the actions and produce the effects of both the first embodiment and the second embodiment. 
     As described above, in each of the first to third embodiments, by changing the exit shape of the impeller  1  and thereby making the height H of the blade  3  high at an intermediate portion, the ratio (Δb/H) of the width of the clearance CL and the height H of the blade  3  is made relatively small. As a result, the ratio of the flow path area occupied by the clearance flow CLF to the flow path area occupied by the main flow is reduced and since the pressure loss is thus made small, the lowering of the efficiency of the centrifugal compressing apparatus can be prevented.