Venturi Expansion Factor and Discharge Coefficient
Posted: Mon Aug 02, 2010 4:10 pm
I have two questions that I would appreciate if any one help me to find their answers. let's start.
The discharge coefficient used for venturi in the calculator is:
Cd=a+b.(Ln Re)^2+c.(Ln Re)^3
and apparently the expansion factor used for it, is:
Buckingham Eq: Y=1-(0.41-0.35*beta^4).(p1-p2)/(k.p1)
Now first question is this, why we don't use the equation below?
Y= sqrt((r^2/k).(k/k-1).((1-r^(k-1)/k))/(1-r).(1-beta^4/1-beta^4.r^2/k))
Perry's Chemical Handbook, 8th Edition, ch.10, page 10.18, Eq. 10.26, for Venturis and Nozzles
I mean I can't understand why we should use Buckingham equation instead, while in page 10.17 of that Handbook the
Buckingham equation (Eq. 10.23) is introduced as orifice expansion factor.
And the second question is that why the discharge coefficient factor and Reynolds number given by the calculator does not
satisfy the applied formula (Cd= a+b.(Ln Re))^2+c.(Ln Re)^3)
for example consider the given data below:
p1=830 kpa, p2=550 kpa, rho= 9.6998 kg/m3, D1=tube diameter=55 mm, D2=Flowmeter Throat diameter=30 mm, ni=kinematic viscosity=15 mm2/s
the results for this existing data would be:
ReD=270435.38 , Red= 495798.22 , Cd=0.985
but after substituting the parameters a, b, c and ReD in the equation we got: Cd=0.989
Actually I'm working on a personal project to produce a program code for this, but I got some different answers for
Q (Volumetric flow rate). some differences are really unreasonable! for example with this given data I've got
Q=494.9869, but calculator says Q=630.8259 .Please help me figure out this problem.
The discharge coefficient used for venturi in the calculator is:
Cd=a+b.(Ln Re)^2+c.(Ln Re)^3
and apparently the expansion factor used for it, is:
Buckingham Eq: Y=1-(0.41-0.35*beta^4).(p1-p2)/(k.p1)
Now first question is this, why we don't use the equation below?
Y= sqrt((r^2/k).(k/k-1).((1-r^(k-1)/k))/(1-r).(1-beta^4/1-beta^4.r^2/k))
Perry's Chemical Handbook, 8th Edition, ch.10, page 10.18, Eq. 10.26, for Venturis and Nozzles
I mean I can't understand why we should use Buckingham equation instead, while in page 10.17 of that Handbook the
Buckingham equation (Eq. 10.23) is introduced as orifice expansion factor.
And the second question is that why the discharge coefficient factor and Reynolds number given by the calculator does not
satisfy the applied formula (Cd= a+b.(Ln Re))^2+c.(Ln Re)^3)
for example consider the given data below:
p1=830 kpa, p2=550 kpa, rho= 9.6998 kg/m3, D1=tube diameter=55 mm, D2=Flowmeter Throat diameter=30 mm, ni=kinematic viscosity=15 mm2/s
the results for this existing data would be:
ReD=270435.38 , Red= 495798.22 , Cd=0.985
but after substituting the parameters a, b, c and ReD in the equation we got: Cd=0.989
Actually I'm working on a personal project to produce a program code for this, but I got some different answers for
Q (Volumetric flow rate). some differences are really unreasonable! for example with this given data I've got
Q=494.9869, but calculator says Q=630.8259 .Please help me figure out this problem.