# Power Loss or gain from a fluid flow

1. Jul 21, 2015

### 99percent

Hi guys,

I have quick question about calculating the power lost in fluid flow.

So imagine there is a simple setup as in the attached image, where you know the mass flow in and out, pressures at inlet and outlet and the temperature at the inlet.

Is the equation to find the power lost or gained by the flow just (volume flow)*(pressure difference)?

I know that you can use this to calculate the power required for a pump using liquids, does this also work for gases?

Thanks!

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2. Jul 21, 2015

### Nidum

Simple formula only works with incompressible fluids and low velocities .

Even if doing only simple tests or calculations then at least two properties of fluid really need to be evaluated at inlet and outlet to get energy change .

As flow regime gets more complex and compressiblity and temperature changes become more relevant then more complex calculations are nescessary .

Some fluids like vapours which might go through phase transition are even more complicated to test or analyse .

All covered in Thermodynamics and Fluid Flow theory but you'll have to study that .

3. Jul 21, 2015

### 99percent

Edit: What equation would be best to use? Would I also need to know the temperature at both the inlet and the outlet to get energy change for compressible flow?

Last edited: Jul 21, 2015
4. Jul 21, 2015

### Nidum

Depends on what type of flow you are actually trying to monitor . Give me a specific example and I'll get back to you .

5. Jul 22, 2015

### 99percent

I am basically looking at a flow before and after a venturi like device, however the inlet and outlet diameters are different.

Could I just use the mass flow* (enthalpy change)to get the power? where the enthalpy is cp*T?

If I dont have the temperatures, is there a way of just working it out using the pressures?