# Thermodynamics Questions and Answers – Variable Flow Processes

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This set of Thermodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Variable Flow Processes”.

1. Variable flow processes include
a) filling up a gas cylinder
b) evacuating a gas cylinder
c) all of the mentioned
d) none of the mentioned

Explanation: These are variable flow processes which can be analysed by the control volume technique.

2. The rate at which the mass of fluid within the control volume is accumulated is equal to the net rate of mass flow across the control surface.
a) true
b) false

Explanation: (dm/dt)=w1-w2, where m is the mass of fluid within the control volume at any instant.

3. Rate of energy increase within the control volume is given by
a) rate of energy inflow + rate of energy outflow
b) rate of energy inflow – rate of energy outflow
c) rate of energy inflow = rate of energy outflow
d) none of the mentioned

Explanation: The rate of accumulation of energy within the control volume is equal to the net energy flow across the control surface.
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4. Which of the following is true for steady flow?
a)(dE/dt)=0
b)(dE/dt)>0
c)(dE/dt)<0
d) none of the mentioned

Explanation: Rate of change of energy of fluid with respect to time within the control volume is constant.

5. Variable flow processes can be analysed by
a) system technique
b) constant volume technique
c) both of the mentioned
d) none of the mentioned

Explanation: These two are the main techniques used for analysing variable flow process.

6. Using system technique, energy balance for the process comes out to be
a) m2u2+m1u1+(m2-m1)(((V^2)/2)+(h/2))
b) m2u2-m1u1+(m2-m1)(((V^2)/2)+(h/2))
c) m2u2+m1u1-(m2-m1)(((V^2)/2)+(h/2))
d) m2u2-m1u1-(m2-m1)(((V^2)/2)+(h/2))

Explanation: This comes from the first law by neglecting PE, KE and E is the energy of the gas.

7. Both the techniques for analysing variable flow processes gives same result.
a) true
b) false

Explanation: This is because these techniques have same initial assumptions and hence give same result.

8. In ____ filling a bottle with air at 300K, the gas temperature rises to 420K due to flow work being converted to ____ increase.
c) constant pressure, heat
d) none of the mentioned

Explanation: In the energy equation, m1=0, Q=0, h>>(V^2)/2, we will get we will get that flow work is converted to increase in molecular internal energy.

9. Which of the following is true for a discharging tank?
b) the process is quasi-static
c) dQ=0
d) all of the mentioned

Explanation: Applying first law to the control volume and dW=0, dm=0 and KE and PE of the fluid are assumed to be small.

10. For charging a tank,
a) enthalpy is converted to work done
b) work done is converted to enthalpy
c) enthalpy is converted to internal energy
d) internal energy is converted to work done

Explanation: Tank is initially taken to be empty and ΔU=(m2u2-m1u1)=(mh) at constant state of the fluid in the pipeline.

11. For a variable flow process
a) P.E. terms are neglected
b) K.E. of the fluid is assumed to be small
c) the process is not steady
d) all of the mentioned

Explanation: These are some of the basic assumptions for a variable flow process.

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