Please, can someone tell me how to calculate the pressure loss in a duct using CFD? Is correct a formula like this:

Press_loss = Total_pressure_inlet - Total_pressure_outlet ?

Thanks for your attention.

TeS

That is correct if you want the total pressure loss across the pipe. If you want the static pressure drop, then you have

Press_loss = Static_pressure_inlet - Static_pressure_outlet

I think for even the simplest case ag needs to consider the inlet-outlet area size difference. and regarding TeS, basically there is no as such conservation of total pressure also. What you take is a momentum, energy and mass conservation at inlet = outlet. but for instance for incompressible flow case conservation of momentum suffices. so for insatance for varying area inlet - i and outlet - o, and neglecting transient effects, and no momentum source (pump, etc) or sink (turbine, etc) case, horizontal pipe case, etc....

momentum balance gives;

(p_i -p_j)A_ij + p_hydraulic loss (darcy friction loss) +(v_i - v_out)m_dot = 0,

and A_ij average area at inlet/outlet, m_dot constant at a branch.

Actually these things are more involved than this simple case when we remove all the assumptions, refer GFSSP program in AIAA publications developed Majumdar paper for further indepth analysis. my msc thesis was on large networked energy systems simulation, but now a cfd student large body simulations are not my interst rather particulate level are more my interest. thx ty

Dear ag and ty, first of all thanks for your answer. My question rises up from the following classical problem:

to find out a correlation from pressure loss obtained from hydraulic formulas

(*) and pressure loss computed with CFD. I'm supposed to compare this results.

(*) (v2^2-v1^2)/2 + g(z2-z1) + (p2-p1)/ro + R = -L

where

v1,2 velocity at inlet(1),outlet(2) z2-z1=H = altitude difference between sections R = R_f + R_c (friction and concentrated loss) R_f = csi*(L/D)*v^2/2 (friction loss) R_c = beta*v^2/2 (concentrated or minor loss) csi = friction factor = f(Reynolds)... beta (shape dependent coefficient)... L = pump work (supposed to be null)

I ask:

>what precisely represents p2 and p1 in that formula?

>what is the best procedure in CFD to obtain pressure loss for stationary

incompressible flow in a generic duct (with friction and bendings)?

>Using an approach in which I assign the mass flow rate at inlet and a 0 static

pressure at outlet can I say that my "conventional" pressure loss (R) is the

difference between total pressure in outlet section and inlet section?

>Can I say that charcteristic curve of the pipe is in terms of P_tot (total

pressure as function of Q_dot (mass flow rate)?

Sorry the verbosity!

TeS

Dear TeS, Let me try to be of help,

>what precisely represents p2 and p1 in that formula?

p2 , p1 are static pressures, in a very vivid term the thermodynamic pressures, for a single component fluid two other thermodynamic values fixes this value, ie for instance for a given temprature T, and and other thermodynamic variable like specific volume, or the others known, and by using equation of state you will arrive to this value, or in experimental set up, the mm of water column you read from the transverse pressure probe in the fluid, the one you read from the flow wise probe is total pressure.

>what is the best procedure in CFD to obtain pressure loss for stationary

incompressible flow in a generic duct (with friction and bendings)?

-according to my msc study you can use finite element method to study this if you consider each duct parts a s a lumped parameters, but the paper I cited last time by majamudar, use finite volume formulation, what he did is he extended the patankar finite volume method to attack large physical system modelling, lum,ped parameter modeling

>Using an approach in which I assign the mass flow rate at inlet and a 0 static

pressure at outlet can I say that my "conventional" pressure loss (R) is the

difference between total pressure in outlet section and inlet section?

-no if there is a an inlet/outlet area difference you have created a static pressure regain, for a practical application for instance in duct design for air condition system refer the static regain design method, in ASHRAE Fundamentals, 1981.

>Can I say that charcteristic curve of the pipe is in terms of P_tot (total

pressure as function of Q_dot (mass flow rate)?

-if what you mean by characterstic curve is the system curve I dont think so, the system curve is a function of the dynamic pressure part which is p_dyn = P_tot - P_staic, i.e. the system curve is a parabolic line which is a fn. of of the the mass flow rate, or function of v^2

-refer the same book of ASHRAE it shows how the hydraulic grade line varies

but generally to summerize what i thought, I dont think you can model a lumped energy system in commercial CFD codes, because what these CFD softwares do is to arrive a pressure drop value by studying a single flow element like duct, you would then use these separate values and that of fittings adding or forming equivalent values of the whole system from the lumped individual values.

Sorry the verbosity, me too!

Ty