Fluid Mechanics MCQ Questions & Answers | Chemical Engineering

Magnetic flowmeters work on the principle of electromagnetic induction.
This means they rely on the liquid being measured to have some electrical conductivity.

Here's a simplified explanation:
Imagine the liquid flowing through a pipe surrounded by a magnetic field.
If the liquid is electrically conductive, its moving through this field generates a small voltage.
The flowmeter measures this voltage and relates it to the flow rate.

Liquid hydrocarbon fuels (like gasoline or kerosene) are typically poor conductors of electricity.
In other words, their electrical conductivity is very low.

Therefore, if you try to use a magnetic flowmeter on a liquid hydrocarbon fuel, the signal generated will be too weak or nonexistent, making an accurate measurement impossible.

Let's look at why the other options are less likely to be correct:

Option A: Thermal conductivity: How well a substance conducts heat, while important in some applications, doesn't directly impact the operation of a magnetic flowmeter.
Option C: Specific gravity: This is the ratio of the density of a substance to the density of water. While it relates to the fluid properties, it's not the primary factor affecting measurement in this type of flowmeter.
Option D: Electrical resistivity: This is the opposite of electrical conductivity (how much a substance resists electrical current). While related, the more direct factor influencing the flowmeter's operation is the conductivity itself - whether it's high enough to generate a measurable signal.

The capillary rise of mercury in a small diameter tube is proportional to 1/d, where d is the diameter of the tube, and σ is the surface tension of mercury. Capillary rise occurs due to the balance between cohesive forces within the liquid and adhesive forces between the liquid and the tube wall. In a small diameter tube, the capillary rise is inversely proportional to the diameter of the tube.

Mathematically, the capillary rise h is given by the capillary rise equation:

h = (2σ cosθ) / (ρgr)

Where:

σ is the surface tension of the liquid (mercury).
θ is the contact angle between the liquid and the tube wall (usually close to 0° for mercury in glass).
ρ is the density of the liquid (mercury).
g is the acceleration due to gravity.
r is the radius of the tube (half of the diameter).

Since the radius r is directly proportional to 1/d, and the capillary rise is proportional to the radius r, the capillary rise is inversely proportional to the diameter d, which is represented by option B.

The head developed by a centrifugal compressor is dependent on the kinetic energy imparted to the gas. The kinetic energy is directly related to the velocity of the gas molecules. With an increase in molecular weight of the gas, the average velocity of the gas molecules decreases due to higher mass. This results in a decrease in kinetic energy and consequently, a decrease in the head developed by the centrifugal compressor.
Hence, the correct answer is Option A: Decrease.

The correct answer is Option B: Develop effective pressure rise by the pump.
Here's why:
Priming a centrifugal pump means filling the pump casing and suction pipe completely with the liquid to be pumped *before* starting the pump.

Why is this important?
If the pump casing is filled with air or gas, the impeller (the rotating part) will mostly just churn the air.
Centrifugal pumps need to be filled with liquid to work correctly.

Let's look at the other options:
* Option A: Increase the mass flow rate of fluid: Priming doesn't directly increase the flow rate once the pump is running, but is a prerequisite for pumping to commence.
* Option C: Avoid chances of separation inside the impeller: While cavitation (separation) *can* be worsened by improper priming, the *primary* reason for priming is to create an initial pressure head.
* Option D: None of these: This is incorrect since priming does have a specific purpose.

Therefore, the main reason for priming is to remove air and create a liquid-filled environment so the impeller can effectively develop pressure and begin pumping.

The terminal velocity of a small sphere settling in a viscous fluid is given by Stokes' law for low Reynolds numbers, which is expressed as:

v = (2/9) * (r² * g * (ρ_s - ρ_f)) / μ

Here: v = terminal velocity r = radius of the sphere g = acceleration due to gravity ρ_s = density of the sphere ρ_f = density of the fluid μ = dynamic viscosity of the fluid

From the equation, it is evident that the terminal velocity is inversely proportional to the viscosity of the fluid (μ). A higher viscosity results in lower terminal velocity, while a lower viscosity increases the terminal velocity.

Thus, the terminal velocity varies as the inverse of the fluid viscosity.