This set of Food Processing Unit Operations Multiple Choice Questions & Answers (MCQs) focuses on “Heat Convection”.
1. Which of the following example explain convection?
a) The heat from a hot liquid makes the cup itself hot
b) Walking on the beach on a hot summer day will warm the feet
c) A metal spoon becomes hot from the boiling water inside the pot
d) The formation of sea and land breeze
View Answer
Explanation: The formation of sea and land breeze is an example of convection. During a day, the sun the sun warms the ground and sea water. Since the ground has lower specific heat compared to water and so its temperature increases more than the temperature of the water. The air above ground is heated thus forming a rise in the convection currents and the cool air from over sea flows towards shore. This is called as sea breeze. Similarly in the evening, ground cools faster than water and therefore breeze flows from shore to sea.
2. Which law governs the thermal convection?
a) Fourier’s law
b) Pascal law
c) Newton’s law
d) Stefan-Boltzmann Law
View Answer
Explanation: Newton’s law governs the thermal convection especially in convection cooling. It states that rate of heat loss of a body is directly proportional to the temperature difference between the body and its surroundings.
3. ___________ is the transfer of thermal energy through the movement of a liquid or gas.
a) Convection
b) Conduction
c) Radiation
d) Evaporation
View Answer
Explanation: Convection is the transfer of thermal energy through the movement of a liquid or gas.Convection can be defined as the heat transfer by the movement of a fluid between areas of temperature difference.
4. Is conduction faster than convection?
a) True
b) False
View Answer
Explanation: Convection is faster than conduction. In convection the medium in which heat transfer takes place is in motion whereas the heat transfer through conduction takes place by collision between the atoms.
5. Are these statements about the thermal convection true?
Statement 1: Thermal convection depends on temperature gradient.
Statement 2: It does not depend on relative motion of the fluid.
a) True, False
b) True, True
c) False, True
d) False, False
View Answer
Explanation: The properties of heat transfer by the convection between a body and its surrounding are 1. The convection takes place when there is temperature difference between two medium. 2. It depends on the relative motion of the fluid as convection takes place only when the medium has movement.
6. _________ is the currents produced only by temperature difference.
a) Natural convection
b) Forced convection
c) Conduction
d) Radiation
View Answer
Explanation: Natural convection is a type of convection where the heat transfer takes place only due to density difference that leads to the temperature difference in the fluid. The gravitational force is the main driving force in the natural convection.
7. In __________, fluid is forced to flow over a surface or in a tube by external means.
a) natural convection
b) forced convection
c) conduction
d) radiation
View Answer
Explanation: Forced convection is a mechanism of heat transfer in which fluid motion is produced by an external source such as ceiling fan, a pump and suction device. It creates uniform and comfortable temperature throughout the medium.
8. Convective heat transfer coefficients for air under natural convection is __________
a) 0.5 – 1000 (W/(m2K))
b) 50 – 3000 (W/(m2K))
c) 10 – 1000 (W/(m2K))
d) 50 – 10000 (W/(m2K))
View Answer
Explanation: Convective heat transfer coefficients for air under natural convection is 0.5 – 1000 (W/(m2K)). For liquids under natural convection is 50 – 3000 (W/(m2K)). For air under forced convection it is 10 – 1000 (W/(m2K)). For liquids under forced convection it is 50 – 10000 (W/(m2K)).
9. Convective heat transfer coefficients for condensing water vapor is __________
a) 0.5 – 1000 (W/(m2K))
b) 5 – 100 (W/(m2K))
c) 10 – 1000 (W/(m2K))
d) 50 – 10000 (W/(m2K))
View Answer
Explanation:A convective heat transfer coefficient for condensing water vapor is 5 – 100 (W/ (m2K)). It is proportionality constant between heat flux and temperature difference that plays an important factor as driving force for the flow of heat.
10. Convective heat transfer coefficients for liquid under forced convection is __________
a) 0.5 – 1000 (W/(m2K))
b) 50 – 3000 (W/(m2K))
c) 10 – 1000 (W/(m2K))
d) 50 – 10000 (W/(m2K))
View Answer
Explanation: Explanation: Convective heat transfer coefficients for air under natural convection is 0.5 – 1000 (W/(m2K)). For liquids under natural convection is 50 – 3000 (W/(m2K)). For air under forced convection it is 10 – 1000 (W/(m2K)). For liquids under forced convection it is 50 – 10000 (W/(m2K)).
Sanfoundry Global Education & Learning Series – Food Processing Unit Operations.
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