A Suction Mechanism Design
By A. H. Haji
Having a definite flow rate in outlet of a fan, what is the maximum vacuum pressure we can reach using a venturi?
To answer this question we can first use the analytical method i.e. the relations derived in gas dynamics for a converging – diverging nozzle to gain an overall view of the flow in a venturi. In this analysis knowing the input flow rate, input static pressure, output (discharge) stagnation pressure and the inlet diameter of the venturi we should satisfy the maximum vacuum pressure in the throat (which is equivalent to the critical condition in this section) to obtain the throat and outlet section diameters. Assume the known parameters are as follows:
Fig.1: A typical venturi
As we will see in the following, the discharge pressure change will affect only the outlet diameter and does not change the maximum obtainable vacuum pressure. The vacuum pressure i.e. the difference between the throat pressure and the ambient pressure can be computed as follows:
The vacuum pressure attained is desirable:
However some point is remarkable: we have a constraint on the vacuum tube or
suction pipe (Fig. 1) diameter; it should not be less than 5 cm. but here the
throat diameter 
is just 4.2 cm..
So what is the minimum throat diameter in order not to affect the flow in the section where the suction pipe is attached? Considering some similar mechanisms, it seems that the ratio of the throat to the suction pipe is about 2.
We use the computational approach to verify this conjecture. Fig. 2 shows the dimensions of the model we have constructed and analyzed using FLUENT v6.1. The ratio of the middle throat diameter to the suction pipe diameter is 1.71. The inlet conditions are as before:
Fig. 2: the model analyzed with FLUENT. The ratio of the throat diameter to the suction pipe diameter is 1.71
The outlet stagnation pressure is 86 kPa. Fig. 3 shows the contours of the gauge pressure (static pressure minus the ambient pressure) in the longitudinal middle plane of the 3dimensional model.
As we see the pressure in the suction pipe inlet section is 86 + (2.23 + 4.02)/2
= 89.12kPa. Also this pressure is almost uniform on this section from the left
edge to the right where the throat diameter on the left edge is 6.83.
In the other words the flow in the suction pipe expands from a section in
the venturi where the diameter is about 2 times of the suction pipe diameter.
Now it is suitable to compare the pressure calculated by FLUENT v.6.1 with the
answer of analytical method (note that for this model's inlet conditions we have
calculated the inlet stagnation pressure to be
) :
Fig. 3: Contours of the gauge pressure (static pressure minus the ambient pressure) in the longitudinal middle plane of the 3dimensional venturi with dimensions shown in Fig. 2.
This result ( 
) is in excellent agreement with that of FLUENT v.6.1. So
finally we have accessed to powerful design tools: