Effect of change of minor flow rate in a Aerosol

In a real impactor , the nozzle flow impacts on a flat plate placed at right angle to the nozzle flow and flows

radially outwards on the plate. Hence, the flow field is constant in real impactor provided the flow rate Q₁

through the nozzle is constant, whereas in a virtual impactor this is not the case. Here keeping the total flow

rate Q₁through nozzle constant, the minor flow rate Q_m through collection prove can be changed by

changing the major flow rate Q₂through the vacuum pump.

Hence, in virtual impactor , the effective velocity responsible for pushing the particles into the minor flow is a

difference of velocities (V₁-V₂) , Where V₁ and V₂ are the velocities corresponding to the nozzle flow

rate and major flow rate.

The experimental analysis has been done in set II in accordance with the methodology described below.

Stokes number for a virtual impactir is given by:

Stk= Ʈ (V₁-V₂) / D/2

Where Ʈ is the relaxation time of the particle. Putting Ʈ = ῤ_pd_p²/ 18 µ in the above equation gives

9µD(Stk)₅₀= ῤ_p (d p₅₀)² Q₁/A₁- ῤ_p (d _p50)²/A₂ (Q₁- Q_m) …………….(4)

Where (St)₅₀ and d_5oare Stokes number and cut of diameter respectively at 50 % collection efficiency, A₁

and A₃ are areas of major and minor flows respectively and Q₂= (Q₁- Q_m).

Keeping Q₁ to be constant and separating the constant terms, the above equation, for a particular virtual impactor, yields:

(d _p50)²= C/Q_m

Where C is constant.

Effect of change of total flow rate:

The size of the narrow distributed aerosols can be changed by changing by total flow rate Q₁.

Above equation (4) dividing both sides by Q₁ and taking Q_m/Q₁as constant, we have , for particular

virtual impactor ,

(d _p50)²= C₁/Q₁

Where C₁is a constant.

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