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Specific speed ''N''''s'', is a dimensionless quantity used to characterize turbomachinery speed.〔 〕 Common commercial and industrial practice uses dimensioned versions which are equally useful. One of the most valued uses of specific speed is its application to pumps to define the suction specific speed -- a quasi non-dimensional number used to classify pump impellers as to their type and proportions. In Imperial units it is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute against one foot of hydraulic head. In metric units flow may be in l/s or m³/s and head in m, and care must be taken to state the units used. Performance is defined as the ratio of the pump or turbine against a reference pump or turbine, which divides the actual performance figure to provide a unitless figure of merit. The resulting figure would more descriptively be called the "ideal-reference-device-specific performance." This resulting unitless ratio may loosely be expressed as a "speed," only because the performance of the reference ideal pump is linearly dependent on its speed, so that the ratio of (to reference-device-performance ) is ''also'' the increased speed the reference device would need to turn, in order to produce the performance, instead of its reference speed of "1 unit." Specific speed is used in engineering design where it is thought of as an index used to predict desired pump or turbine characteristics; e.g., the general shape of a pump's impeller. Often it is used to predict the type of pump or turbine required for a design flow rate and head. Once the desired specific speed is known, basic dimensions of the unit's components can be easily calculated. Several mathematical definitions of specific speed (all of them actually ideal-device-specific) have been created for different devices and applications. == Pump specific speed == Low-specific speed radial flow impellers develop hydraulic head principally through centrifugal force. Pumps of higher specific speeds develop head partly by centrifugal force and partly by axial force. An axial flow or propeller pump with a specific speed of 10,000 or greater generates its head exclusively through axial forces. Radial impellers are generally low flow/high head designs whereas axial flow impellers are high flow/low head designs. Centrifugal pump impellers have specific speed values ranging from 500 to 10,000 (English units), with radial flow pumps at 500-4000, mixed flow at 2000-8000 and axial flow pumps at 7000-20,000. Values of specific speed less than 500 are associated with positive displacement pumps. As the specific speed increases, the ratio of the impeller outlet diameter to the inlet or eye diameter decreases. This ratio becomes 1.0 for a true axial flow impeller. where: : is specific speed (unitless) : is pump rotational speed (radians per second) : is flowrate (m³/s) at the point of best efficiency : is total head (m) per stage at the point of best efficiency : is acceleration due to gravity (m/s²) Note that the units used affect the specific speed value in the above equation and consistent units should be used for comparisons. Pump specific speed can be calculated using British gallons or using Metric units (m3/s or L/s and metres head), changing the values listed above. following equation gives unit less specific speed. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Specific speed」の詳細全文を読む スポンサード リンク
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