has a flow direction shown by arrow 50 either radially inward or radially outward depending on the direction of fan rotation the present invention provides improved noise performance independently of the direction of fan rotation according to the present invention each flow channel 51 maintains a substantially constant cross sectional area measured at each radial position r 1 r 2 r 3 etc by virtue of fan blades 46 and 47 having a predetermined height profile with respect to support plate 45 as shown in fig3 b in general the fan blade is tapered to reduce flow separation of fluid passing through its flow channel this reduction is accomplished over a range of rotational speeds of the fan because of the constant cross sectional area of channel 51 in this example a taper portion 55 extends from a radially inner end 56 of blade 46 to a radially outer end 57 the tapered portion 55 can include substantially all of the radial extent of blade 46 as is shown in fig3 b or can be employed in conjunction with a constant height inner blade portion such as is shown in fig1 and 9 to maintain a high suction pressure for example each cross section of a flow channel taken at radial positions along the flow channel is coincident with an arc portion of a circle having a circumference determined by that radial position since the circumference of a circle increases in direct proportion to the radius 2 u03c0r then the cross sectional area of a channel between a pair of blades is maintained constant by decreasing the blade height in inverse proportion to radius for example in fig3 a and 3b the cross sectional area of the channel at.