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 Paul January 9, 2006 00:10

Pressdrop across in nozzle: Fluent

Hi,

I want to know what to measure, to find pressure drop across nozzle in fluent(3D geometry). Should i take difference of "total pressure" at inlet and outlet or Should i take diffence of "static" Pressure at inlet and outlet.

Thank you Paul

 S K Prasad January 9, 2006 01:41

Re: Pressdrop across in nozzle: Fluent

Pressure drop should be taken as difference of static pressure. It is this drop that is responsible for flow through the nozzle. Total pressure differnece accounts for the losses in the nozzle.

 breathless January 9, 2006 12:37

Re: Pressdrop across in nozzle: Fluent

if that is true, then is static pressure difference also responsible for flow thru a diverging nozzle?

 carno January 10, 2006 04:21

Re: Pressdrop across in nozzle: Fluent

It is not the static pressure who drives the flow. The total head (Bernoulli's head) is responsible for it.

 Paul January 12, 2006 00:36

Re: Pressdrop across in nozzle: Fluent

Suppose if i take nozzle and diffuser of same dimensions, with wide angle, around 70 degrees, for a particular flow rate Q, where should i apply more pressure, at nozzle inlet or diffuser inlet. what pressure shold i measure at both inlet and outlet, that is responsible for the flow Q.

Regards Paul

 carno January 12, 2006 05:47

Re: Pressdrop across in nozzle: Fluent

paul,

To achieve same flow rate from a nozzle and same diffuser (say same nozzle with reversed flow direction). Nozzle will have more staic pressure at inlet than diffuser, since diffuser rises static pressure. But the total pressure at inlet of both should be same to get same mass flow rate. mass flow rate is a function of total pressure at inlet. (although values of static and dynamic components of total pr are different)

Apply total pressure at inlet and static pressure at outlet. The drop responsible for it is difference of total pressure at inlet and outlet.

 paul January 13, 2006 13:41

Re: Pressdrop across in nozzle: Fluent

Dear carno,

If i apply velocity inlet boundary condition and zero pressure outlet boundary condition for both nozzle and diffuser, what is that should i measure at both inlet and outlet to to find out the pressure drop, is it "total pressure " or "static pressure".

And one more question is should i measure "mass weighted average" quantities or "area weighted quantities". Fluid is incompressible.

Thank you Paul

 Ahmed January 15, 2006 03:18

Re: Pressdrop across in nozzle: Fluent

First: I would like to invite all the participants in this forum to STOP using the term "Static pressure" from any discussion related to Fluid Dynamics, Static pressure is developed in non moving fluids. For the exact meaning of this ugly term, all are invited to consult the reference book written by George Batchelor "An introduction to Fluid Dynamics" and to save your time, go directly to page 14 Equation 1.3.11 and the following comment. The correct terminology is Equilibrium Pressure or Thermodynamic Pressure whichever you prefer.

Second: Grab a pencil, draw a 2D duct of the shape you like (Just for simplicity), draw an integral element(from wall to wall), take a force balance and after a simple mathematical manipulation you will find that the pressure drop along that element is a function of the shear stress at the boundaries Now how the wall shear stress is calculated, all CFD programmes that I am familiar with use the wall functions to do this calculation, in plain language, if your Y+ values are out of bounds, then your calculated pressure drop is not the correct one

 Ahmed January 15, 2006 03:24

Re: Pressdrop across in nozzle: Fluent

One last comment, differences in the total pressure account for the Energy dissipation (Entropy Generation).

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