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The heat transfer by conduction through a thick cylinder (Q) is given by (where T₁ = Higher temperature, T₂ = Lower temperature, r₁ = Inside radius, r₂ = Outside radius, l = Length of cylinder, and k = Thermal conductivity)
A. Q = [2πlk (T₁ - T₂)]/2.3 log (r₂/r₁)
B. Q = 2.3 log (r₂/r₁)/[2πlk (T₁ - T₂)]
C. Q = [2π (T₁ - T₂)]/2.3 lk log (r₂/r₁)
D. Q = = 2πlk/2.3 (T₁ - T₂) log (r₂/r₁)
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Unit of thermal diffusivity is
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When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by
D. Conduction and convection
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The concept of overall coefficient of heat transfer is used in case of heat transfer by
D. Conduction and convection
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Heat is closely related with
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Depending on the radiating properties, a body will be black when
A. P = 0, x = 0 and a = 1
C. P = 0, x = 1 and a = 0
D. X = 0, a + p = 0 Where a = absorptivity, p = reflectivity, X = transmissivity.
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Joule sec is the unit of
A. Universal gas constant
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Which of the following is expected to have highest thermal conductivity?
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The emissive power of a body depends upon its
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Reynolds number is the ratio of
A. Energy transferred by convection to that by conduction
B. Kinematic viscosity to thermal diffusivity
C. Inertia force to viscous force
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According to Dalton's law of partial pressures, (where pb = Barometric pressure, pa = Partial pressure of dry air, and pv = Partial pressure of water vapour)
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The thermal diffusivities for gases are generally
A. More than those for liquids
B. Less than those for liquids
C. More than those for solids
D. Dependent on the viscosity
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The heat of sun reaches to us according to
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A composite slab has two layers of different materials with thermal conductivities k₁ and k₂. If each layer has the same thickness, then the equivalent thermal conductivity of the slab will be
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The amount of radiation mainly depends on
C. Type of surface of body
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The heat transfer from a hot body to a cold body is directly proportional to the surface area and difference of temperatures between the two bodies. This statement is called
A. First law of thermodynamics
B. Newton's law of cooling
C. Newton's law of heating
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Two balls of same material and finish have their diameters in the ratio of 2: 1 and both are heated to same temperature and allowed to cool by radiation. Rate of cooling by big ball as compared to smaller one will be in the ratio of
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The ratio of the thickness of thermal boundary layer to the thickness of hydrodynamic boundary layer is equal to (Prandtl number) n, where n is equal to
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The most commonly used method for the design of duct size is the
A. Velocity reduction method
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A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h₁, h₂ and h₃ respectively. The average heat transfer coefficient for the cube is
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In heat transfer, conductance equals conductivity (kcal/hr/sq.m/ °C/cm) divided by
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Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be
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The heat is transferred by conduction, convection and radiation in
C. Condensation of steam in condenser
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Wiens law states that the wave length corresponding to ________ is proportional to the absolute temperature.
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Heat transfer in liquid and gases takes place by
D. Conduction and convection
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Emissivity of a white polished body in comparison to a black body is
D. Depends upon the shape of body
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According to Prevost theory of heat exchange
A. It is impossible to transfer heat from low temperature source to t high temperature source
B. Heat transfer by radiation requires no medium
C. All bodies above absolute zero emit radiation
D. Heat transfer in most of the cases takes place by combination of conduction, convection and radiation
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Which of the following statement is correct?
A. A grey body is one which absorbs all radiations incident on it.
B. At thermal equilibrium, the emissivity and absorptivity are same.
C. The energy absorbed by a body to the total energy falling on it, is called emissivity.
D. A perfect body is one which is black in colour.
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In free convection heat transfer, Nusselt number is function of
A. Grashoff number and Reynold number
B. Grashoff number and Prandtl number
C. Prandtl number and Reynold number
D. Grashoff number, Prandtl number and Reynold number
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The heat transfer by conduction through a thick sphere is given by
A. Q = 2πkr1 r2 (T1 - T2)/ (r2 - r1)
B. Q = 4πkr1 r2 (T1 - T2)/ (r2 - r1)
C. Q = 6πkr1 r2 (T1 - T2)/ (r2 - r1)
D. Q = 8πkr1 r2 (T1 - T2)/ (r2 - r1)