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 cfd novice April 3, 2008 15:13

Calculating mass flow rate

Hi Guys

Can some one tell me please what is the relationship between mass flow rate (Ubar) and friction Reynolds number (Re tau). I need to calculate mass flow rate (Ubar) from Re tau. Thanks in advance

 cfd novice April 3, 2008 18:31

Re: Calculating mass flow rate

Basically I am using periodic/cyclic BC at my inlet & outlet. I dont have U or P given, I have only Re tau. How can I calculte velocity from Re tau and then mass flow rate for my channel flow. Thanks in advance for answer.

cfd novice

 sivaramakrishnaiah April 5, 2008 07:58

Re: Calculating mass flow rate

you know Re number value ok ro(density)*velocity*damiter/dynamic viscosity for Re number. now you can calculate density in that density you can calculate mass flow rate know ok bye

 cfd novice April 5, 2008 15:19

Re: Calculating mass flow rate

Hi sivaramakrishnaiah

How can I calculate mass flow rate from density? Thank You

 alex April 6, 2008 16:12

Re: Calculating mass flow rate

mass flow rate = rho*U*A (Rho=density, U=velocity, A=cross sectional area)

Re = rho*U*l / mu (l=characteristic length, mu=viscosity)

So Mass flow rate = Re*mu*A / l

 cfd novice April 7, 2008 18:45

Re: Calculating mass flow rate

mass flow rate = rho*U*A (1)

Re = rho*U*A/mu (2)

From (2): Re*mu = rho*U*A

So mass flow rate = Re*mu

How did you get above answer?

Thanks

 alex April 8, 2008 13:08

Re: Calculating mass flow rate

Re is based on an length, not the area. Re = rho*U*L / mu

What length you use depends on the problem. L is the characteristic length. You might hear things such as Reynolds number based on pipe diameter for example. Or based on distance from leading to trailing edge of an airfoil.

So... Mass flow = rho*U*A (where A=cross sectional area that the flow is flowing into). --(1)

Re= rho*U*L / mu (where L is the length that you are basing your Reynolds number on) --(2)

Combining (1) & (2) :

Mass flow = Re*mu/L * A

Regards,

Alex

 davoche April 21, 2008 06:38

Re: Calculating mass flow rate

there is a relation (from experimental datas) which can help you to determine the flow rate from re in a turbulent flow. Indeed, according to hinze (1975) : re_tau=1/8.3*(Re)^(7/8)

You could also find more element (in french for the moment sorry) at the following link : http://www.olivierdoche.com/images/T...HESE_DOCHE.pdf

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