Dimensional Analysis of Specific Speed: Explained

In summary, specific speed is a dimensionless number used to compare the performance of pumps and turbines. It is calculated using the rotational speed, flow rate, and head of the pump or turbine. A high specific speed indicates that the equipment is designed for high-flow, low-head applications, making it more efficient for tasks such as irrigation. Specific speed is an important factor to consider when selecting a pump or turbine, as it can help determine the best type for a specific application and indicate its efficiency. However, it has limitations, such as not taking into account fluid viscosity and only being applicable to centrifugal pumps and turbines. Other factors, such as cavitation and NPSH, should also be considered when choosing equipment.
  • #1
Mikealvarado100
55
0
Hi
Specific Speed is defined as:
Ns=N(Q^0.5)/(H^.75)
How Dimensional Analysis of it can be done? Unit of N is 1/sec and Ns is unitless. Then Dimension of Q^0.5 must be equal to dimension of H^0.75. It seems no to be equal.
Please explain it.
 
Last edited:
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  • #2
What is your question ?
 
  • #3
Please analyse the formula dimensionally.
 
  • #4
Your formula is only pseudo dimensionless . The calculated Ns number is only valid for a specific system of units .

The proper dimensionless formula has a g in the denominator . Explained reasonably well in Wikipedia .
 

Related to Dimensional Analysis of Specific Speed: Explained

1. What is specific speed in the context of dimensional analysis?

Specific speed is a dimensionless number that is used to characterize the performance of a pump or turbine. It is calculated using the pump or turbine's rotational speed, flow rate, and head, and is used to compare different types of pumps or turbines.

2. How is specific speed calculated?

The specific speed is calculated using the following formula: Ns = N * Q^0.5 / H^0.75, where N is the rotational speed in revolutions per minute (rpm), Q is the flow rate in gallons per minute (gpm), and H is the total head in feet. The result is a dimensionless number that is used for comparison.

3. What does a high specific speed indicate?

A high specific speed indicates that the pump or turbine is designed for high-flow, low-head applications. This means that it is more efficient at pumping or generating large volumes of fluid at lower pressures, such as in irrigation systems.

4. How does specific speed affect pump or turbine selection?

Specific speed is an important factor to consider when selecting a pump or turbine for a specific application. It can help determine which type of pump or turbine is best suited for a particular flow rate and head, and can also indicate the efficiency of the pump or turbine.

5. Are there any limitations to using specific speed in dimensional analysis?

Specific speed is a useful tool for comparing pumps and turbines, but it does have limitations. It does not take into account the viscosity of the fluid being pumped or generated, and it is only applicable to centrifugal pumps and turbines. Other factors, such as cavitation and net positive suction head (NPSH), should also be considered when selecting a pump or turbine.

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