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

Heat and Mass Transfer
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

Three

Two

One

Four

Three

Two

One

Four

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Heat and Mass Transfer
The critical radius is the insulation radius at which the resistance to heat flow is

Minimum

None of these

Maximum

Zero

Minimum

None of these

Maximum

Zero

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Heat and Mass Transfer
Unit of thermal conductivity in S.I. units is

Option (B) and (C) above

J/m² sec

W/m °K

J/m °K sec

Option (B) and (C) above

J/m² sec

W/m °K

J/m °K sec

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Heat and Mass Transfer
Reynolds number (RN) is given by (where h = Film coefficient, l = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, ρ = Density of fluid, cp = Specific heat at constant pressure, and μ = Coefficient of absolute viscosity)

RN = μ cp/k

RN = hl/k

RN = V²/t.cp

RN = ρ V l /μ

RN = μ cp/k

RN = hl/k

RN = V²/t.cp

RN = ρ V l /μ

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Heat and Mass Transfer
Thermal conductivity of solid metals with rise in temperature normally

Remain constant

Increases

Decreases

May increase or decrease depending on temperature

Remain constant

Increases

Decreases

May increase or decrease depending on temperature

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Heat and Mass Transfer
Fourier's law of heat conduction is (where Q = Amount of heat flow through the body in unit time, A = Surface area of heat flow, taken at right angles to the direction of heat flow, dT = Temperature difference on the two faces of the body, dx = Thickness of the body, through which the heat flows, taken along the direction of heat flow, and k = Thermal conductivity of the body)

k. (dx/dT)

k.

(dx/dT)

k.

k. (dT/dx)

(dT/dx)

k. (dx/dT)

k.

(dx/dT)

k.

k. (dT/dx)

(dT/dx)

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Heat and Mass Transfer

Heat and Mass Transfer The critical radius is the insulation radius at which the resistance to heat flow is

Heat and Mass Transfer Unit of thermal conductivity in S.I. units is

Heat and Mass Transfer Reynolds number (RN) is given by (where h = Film coefficient, l = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, ρ = Density of fluid, cp = Specific heat at constant pressure, and μ = Coefficient of absolute viscosity)

Heat and Mass Transfer Thermal conductivity of solid metals with rise in temperature normally

Heat and Mass Transfer
The critical radius is the insulation radius at which the resistance to heat flow is

Heat and Mass Transfer
Unit of thermal conductivity in S.I. units is

Heat and Mass Transfer
Reynolds number (RN) is given by (where h = Film coefficient, l = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, ρ = Density of fluid, cp = Specific heat at constant pressure, and μ = Coefficient of absolute viscosity)

Heat and Mass Transfer
Thermal conductivity of solid metals with rise in temperature normally