Mechanical Engineering - Hydraulics and Fluid Mechanics - Discussion

According to NPTEL, pressure increases as velocity reduces in the Divergent section So, the answer should be Option B.

The answer is definitely A!

Actually, in case of compressible fluid combining the BERNOULLI equation and CONTINUITY equation we get the resulting equation dA/A=-dV/V (1-Ma*2). In the divergent section of the venturi meter, the fluid velocity reaches beyond sonic velocity and attains a supersonic velocity and in such case with the increase in area pressure decreases and velocity increases. Since we generally consider liquid to be incompressible so we do not take into account this Ma factor in liquid flow but in case we consider liquid to be compressible we had to understand this peculiar behavior. SO THE GIVEN ANSWER C IS CORRECT.

Answer should be B.

Option B is CORRECT.

The velocity reaches its maximum value and pressure reaches its minimum value at the throat. Subsequently, a decrease in the velocity and an increase in the pressure takes place in course of flow through the divergent part. This typical variation of fluid velocity and pressure by allowing it to flow through such a constricted convergent-divergent passage was first demonstrated by an Italian scientist Giovanni Battista Venturi in 1797.

Venturimeter is an instrument used to measure the discharge of liquid flowing in a pipe. It consists of three parts, i.e the converging cone, the throat and the diverging cone. The length of the divergent cone is made about three to four times convergent cone in order to avoid the tendency of breaking away the stream of liquid and to minimise frictional losses. It may be noted that

(a) The velocity of liquid at the throat is higher than that of the inlet.
(b) The pressure of the liquid at the throat is lower than that of the inlet.
(c) The velocity and pressure of the liquid flowing through the divergent portion decrease.

The discharge through a Venturimeter is given by;

Q = Cda1a2 √2gh/(a1^2 - a2^2).
where
Cd = Coefficient of discharge,
a1 = Area at inlet,
a2 = Area at the throat, and
h = Venturi-head.

In divergent portion, velocity is supersonic so velocity is increase as increase in area of nozzle.

As per Bernoulli's equation, velocity increase will decrease pressure.

Option B is absolutely correct.

Av = constant implies as area increases velocity decreases.

From applications of Bernoulli equation we know, pressure α 1/velocity (pressure is inversely proportional to velocity, as velocity decreases pressure increases. As simple as that don't complicate it guys.

Why draft tube is used in turbine?

To convert part of kinetic energy into pressure head, right?

In the same way. In the divergent part, pressure increases.

The answer should be B. If it decreases it doesn't obey the Bernoulli's equation which is on principle of conservation of energy.