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Liquid mass flow equation

  1. Jul 24, 2015 #1

    Can any one answer me how to caluculate the massflow of the liquid.

    I know the inlet mass flow , volume of the pipe , and the inlet pressure and outlet pressure of the pipe.

    Do we have any equation to caluculate the outletmass flow with respect to differential pressure?

    Can any one help me on this.
  2. jcsd
  3. Jul 24, 2015 #2


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    In steady flow and assuming no addition or loss of fluid in transit the mass flow leaving is the same as the mass flow entering .
  4. Jul 25, 2015 #3
    HI ,

    But there is a pressure difference in between inlet and outlet because there is valve in between the flow. So de have any equation to calculate the outlet massflow with respect to pressure difference.
  5. Jul 25, 2015 #4
    I can explain the problem little more elaborative.

    For a pipe of volume V inlet mass flow is M1. Inlet pressure is P1 and the outlet pressure is P2. P1 and P2 are changing over the time based on other actuation . Can we calculate the outlet mass flow M2 with respect to change in pressure.

    Do we have any physical equation? Can any one help here?
  6. Jul 25, 2015 #5


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    No problem with flow equations but I am having difficulty understanding the problem .

    There is no getting past the fact that what goes in must come out .

    Please explain a little more .
  7. Jul 25, 2015 #6


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    A mass balance tells us:
    Input + generation = output + accumulation + consumption
    It's not clear if you are generating, accumulating or consuming mass.
    If none of these then, as Nidium said, it's simply M1 = M2. Clearly you need to provide more information.

    If you want to find mass flow rate for a given P2 and P1 (where M1 isn't fixed as you've implied...) then Bernoulli's equation will work. Account for pressure drop due to friction for better results.
  8. Jul 25, 2015 #7
    Thanks Nidum and billy joule

    With my basic knowledge on physics i can explain the problem little more.

    i am trying to implement a matlab block to calculate fuel mass flow. As i shown in snapshot attached., There is a pump and based on the pump actuation I have the massflow rate (g/s) M1. I know the pressure after the pump P1. In the same fuel path there is a valve placed . Based on the valve actuation and other valves in the same path there is a change in pressure . I understood that if there is a change in pressure then there is a massflow in the path. If the pressure is same then the mass flow rate is zero. In this case i know the pressure after the valve and i want to calculate the mass flow rate after the valve based on the pressure difference with physical equation.

    Please correct if my understanding is not correct. Massflowrate.png
  9. Jul 25, 2015 #8
    How do you know the pump flow value? What kind of pump is it? Does its flow rate change as the discharge pressure changes?
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