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

One

Three

Two

Four

One

Three

Two

Four

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Heat and Mass Transfer
An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m²). The critical thickness of insulation will be

800 mm

25 mm

40 mm

160 mm

800 mm

25 mm

40 mm

160 mm

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Heat and Mass Transfer
The rate of energy transferred by convection to that by conduction is called

Nusselt number

Stanton number

Biot number

Peclet number

Nusselt number

Stanton number

Biot number

Peclet number

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Heat and Mass Transfer
The time constant of a thermocouple is

Determined by the time taken to reach 100°C from 0°C

The time taken to attain 50% of the value of initial temperature difference

The time taken to attain the final temperature to be measured

The time taken to attain 63.2% of the value of initial temperature difference

Determined by the time taken to reach 100°C from 0°C

The time taken to attain 50% of the value of initial temperature difference

The time taken to attain the final temperature to be measured

The time taken to attain 63.2% of the value of initial temperature difference

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Heat and Mass Transfer
A heat exchanger with heat transfer surface area A and overall heat transfer coefficient U handles two fluids of heat capacities Cmax and Cmin. The number of transfer units (NTU) used in the analysis of heat exchanger is specified as

Cmin/U

U/A.Cmin

U.Cmin

U/Cmin

Cmin/U

U/A.Cmin

U.Cmin

U/Cmin

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Heat and Mass Transfer
Emissivity of a white polished body in comparison to a black body is

Higher

Lower

Same

Depends upon the shape of body

Higher

Lower

Same

Depends upon the shape of body

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Heat and Mass Transfer

Heat and Mass Transfer An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m²). The critical thickness of insulation will be

Heat and Mass Transfer The rate of energy transferred by convection to that by conduction is called

Heat and Mass Transfer The time constant of a thermocouple is

Heat and Mass Transfer A heat exchanger with heat transfer surface area A and overall heat transfer coefficient U handles two fluids of heat capacities Cmax and Cmin. The number of transfer units (NTU) used in the analysis of heat exchanger is specified as

Heat and Mass Transfer Emissivity of a white polished body in comparison to a black body is

Heat and Mass Transfer
An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m²). The critical thickness of insulation will be

Heat and Mass Transfer
The rate of energy transferred by convection to that by conduction is called

Heat and Mass Transfer
The time constant of a thermocouple is

Heat and Mass Transfer
A heat exchanger with heat transfer surface area A and overall heat transfer coefficient U handles two fluids of heat capacities Cmax and Cmin. The number of transfer units (NTU) used in the analysis of heat exchanger is specified as

Heat and Mass Transfer
Emissivity of a white polished body in comparison to a black body is