Heat Transfer Questions and Answers – Effect of Variable Conductivity

This set of Heat Transfer Multiple Choice Questions & Answers (MCQs) focuses on “Effect Of Variable Conductivity”.

1. With variable thermal conductivity, Fourier law of heat conduction through a plane wall can be expressed as
a) Q = -k₀ (1 + β t) A d t/d x
b) Q = k₀ (1 + β t) A d t/d x
c) Q = – (1 + β t) A d t/d x
d) Q = (1 + β t) A d t/d x
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2. The inner and outer surfaces of a furnace wall, 25 cm thick, are at 300 degree Celsius and 30 degree Celsius. Here thermal conductivity is given by the relation
K = (1.45 + 0.5 * 10^-5 t²) KJ/m hr deg
Where, t is the temperature in degree centigrade. Calculate the heat loss per square meter of the wall surface area?
a) 1355.3 kJ/m² hr
b) 2345.8 kJ/m² hr
c) 1745.8 kJ/m² hr
d) 7895.9 kJ/m² hr
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3. A plane wall of thickness δ has its surfaces maintained at temperatures T₁ and T₂. The wall is made of a material whose thermal conductivity varies with temperature according to the relation k = k₀ T². Find the expression to work out the steady state heat conduction through the wall?
a) Q = 2A k₀ (T ₁ ³ – T ₂ ³)/3 δ
b) Q = A k₀ (T ₁ ³ – T ₂ ³)/3 δ
c) Q = A k₀ (T ₁ ² – T ₂ ²)/3 δ
d) Q = A k₀ (T ₁ – T ₂)/3 δ
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4. The mean thermal conductivity evaluated at the arithmetic mean temperature is represented by
a) k_m = k₀ [1 + β (t₁ – t₂)/2].
b) k_m = k₀ [1 + (t₁ + t₂)/2].
c) k_m = k₀ [1 + β (t₁ + t₂)/3].
d) k_m = k₀ [1 + β (t₁ + t₂)/2].
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5. With respect to the equation k = k₀ (1 +β t) which is true if we put β = 0?
a) Slope of temperature curve is constant
b) Slope of temperature curve does not change
c) Slope of temperature curve increases
d) Slope of temperature curve is decreases
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6. The accompanying sketch shows the schematic arrangement for measuring the thermal conductivity by the guarded hot plate method. Two similar 1 cm thick specimens receive heat from a 6.5 cm by 6.5 cm guard heater. When the power dissipation by the wattmeter was 15 W, the thermocouples inserted at the hot and cold surfaces indicated temperatures as 325 K and 300 K. What is the thermal conductivity of the test specimen material?

a) 0.81 W/m K
b) 0.71 W/m k
c) 0.61 W/m K
d) 0.51 W/m K
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7. If β is greater than zero, then choose the correct statement with respect to given relation
k = k₀ (1 +β t)
a) k doesn’t depend on temperature
b) k depends on temperature
c) k is directly proportional to t
d) Data is insufficient
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8. The unit of thermal conductivity doesn’t contain which parameter?
a) Watt
b) Pascal
c) Meter
d) Kelvin
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9. The temperatures on the two sides of a plane wall are t₁ and t₂ and thermal conductivity of the wall material is prescribed by the relation
K = k₀ e ^(-x/δ)
Where, k₀ is constant and δ is the wall thickness. Find the relation for temperature distribution in the wall?
a) t ₁ – t _x / t ₁ – t ₂ = x
b) t ₁ – t _x / t ₁ – t ₂ = δ
c) t ₁ – t _x / t ₁ – t ₂ = δ/x
d) t ₁ – t _x / t ₁ – t ₂ = x/δ
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10. “If β is less than zero, then with respect to the relation k = k₀ (1 + β t), conductivity depends on surface area”.
a) True
b) False
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Sanfoundry Global Education & Learning Series – Heat Transfer.

To practice all areas of Heat Transfer, here is complete set of 1000+ Multiple Choice Questions and Answers.

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