Chemical Engineering

Cork is a good insulator because
  • a)
    It is flexible
  • b)
    It can be powered
  • c)
    Low density
  • d)
    It is porous
Correct answer is option 'D'. Can you explain this answer?

Telecom Tuners answered  •  Mar 02, 2021
Even though cork is a solid material, but it’s thermal conductivity is extremely low because of air filled in it’s pores. Typical value of thermal conductivity is in the range of 0.035 to 0.043 W/m-k

Consider the astronaut compartments inside a space shuttle orbiting the moon. Inside these compartments air is maintained at STP conditions for comfortable breathing of astronauts. The cooling system of electronic equipment inside these compartments is based on
  • a)
    Conduction
  • b)
    Forced convection
  • c)
    Free convection
  • d)
    Thermal radiation
Correct answer is option 'B'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
Since the equipment is in air and surface temperature is generally not high, conduction and radiation heat loss is insignificant. Also in space there is no gravity. Thus buoyancy and therefore free convection is not at all possible in the compartment air. Hence the only suitable mode for cooling is forced convection.

The minimum number of view factors that need to be known to solve a 10-surface enclosure completely, is
  • a)
    1
  • b)
    10
  • c)
    22
  • d)
    45
Correct answer is option 'D'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
For n-surface enclosure if nC2 view factors are known directly, entire enclosure can be solved. Thus for 10 surfaces, number of view factors that need to be evaluated directly will be

For the composite slab shown in figure, find out when k represents thermal conductivity and L represents thickness.
  • a)
    110
  • b)
    70
  • c)
    60
  • d)
    90
Correct answer is option 'B'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
The thermal circuit of the system can be represent as follows
By idea of circuits ratio of resistances is equal to ratio of temperature drops across them
150 - Ti = 2Ti - 60
210 = 3Ti
Ti = 70°C

The spectral distribution of surface irradiation is as follows
What is the total irradiation in kW/m2 ?
  • a)
    10
  • b)
    20
  • c)
    30
  • d)
    40
Correct answer is option 'B'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
Total irradiation is given as
i.e. it is equal to area under the curve Gλ Vs λ Hence,
x (25-20) x 1000
∴ G = 20000 W/m2
I.e. G = 20 kW/m2

Solar radiation is incident on a semitransparent body at a rate of . If of this incident radiation is reflected back and is transmitted across the body, the absorptivity of the body is
  • a)
    0
  • b)
    0.25
  • c)
    0.30
  • d)
    0.45
Correct answer is option 'B'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
G = 500 W/m2 Gr = 150 W/m2
Gt = 225 W/m2
By energy conservation principle,
Ga+Ge+Gt = G
Where Ga = absorbed radiation
Gr = reflected radiation
Gt = transmitted radiation
G = incident radiation
= transmissivity
α = 1-0.3 - 0.45 = 0.25

Find out the heat flux (in W/m²) between two fluids separated by a planar wall as shown in the figure.
  • a)
    200
  • b)
    200
Correct answer is between '200,200'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
Consider the thermal circuit for the system
Where nodes H, 1, 2 & C represent hot fluid, left surface of wall, right surface of wall and cold fluid respectively. Therefore,

Solar radiation of falls on a grey opaque surface at steady state. The surface has a temperature of and emissivity of 0.8. Find radiosity from the surface?
Correct answer is 'A'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
J = E+Gr
E = εσT4
E = 0.8 x 5.67 x 10-8 x (50 + 273)4
E = 493.72 W/m2
Given G = 1200 W/m2
But as surface is at steady state
α = ε = 0.8
∴ ρ = 1 -α = 0.2
∴ Gr = ρG
Gr = 0.2 x 1200 = 240 W/m2
Therefore
J = E+Gr
J = 493.72 + 240
J = 733.72 W/m2

Consider H gas, both at temperature of 30°C. The thermal conductivity of H is ________ than that of .
  • a)
    greater
  • b)
    equal
  • c)
    lower
  • d)
    depends upon pressure
Correct answer is option 'A'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
The thermal conductivity of a gaseous medium is dominantly based on diffusion rate, which depends on root mean square velocity.The root mean square velocity for molecules is given by
where M is the molecular mass of the gas & T is the temperature of the gas and ̅ is universal gas constant. Thus diffusion rate and therefore thermal conductivity is lower for gases with higher molecular masses because
Therefore H₂ gas has more thermal conductivity than O₂.

With increase in pressure, the thermal conductivity of an ideal gas __________.
  • a)
    increases
  • b)
    remains constant
  • c)
    decrease
  • d)
    remains constant
Correct answer is option 'B'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
For an ideal gas, thermal conductivity is given as
Where ⊽ is the mean travel velocity of molecules Cv, is the specific heat of gas at constant volume, L is the mean free path of molecules and is the density of the gas.
As pressure increases, density of gas increases but based on experiments it is observed that the mean free path diminishes in the same proportions. Thus the product remains constant.
Also ⊽ depends upon temperature only hence if temperature is constant, change in pressure does not bring any change in ⊽ Also for an ideal gas Cv is independent of pressure.

Consider a composite slab as shown in the figure.
If the left most and right most surfaces are maintained at temperatures of 100°C & 40°C respectively, find out the interface temperature in in °C
Correct answer is 'Range: 66 to 67'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
The slab can be represented by the following thermal circuit.
Considering the width of slab to plane of paper to be equal to W,
R₂ and R₃ are arranged in parallel and can be shown by single equivalent resistance Rₑ
By idea of series circuit, that ratio of temperature drops across resistances is equal to ratio of respective thermal resistances.
I.e.,

Which of the following events represents thermal radiation?
  • a)
    Microwave heating
  • b)
    Nuclear reaction in a sun
  • c)
    Light bulb
  • d)
    None of the above
Correct answer is option 'C'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
In microwave ovens, the radiation emitted is a result of excitation of crystals like magnetrons and klystrons and in microwave wavelength band which is not in the range of thermal radiation. Also during nuclear reactions like those inside sun, -rays are produced which is actually not considered as thermal radiation. Once these -rays get absorbed by the matter in the sun then temperature rises and thermal radiation is emitted. Thus nuclear reaction does not directly emit thermal radiation.
But during lighting of bulb energy is emitted mainly in form of light and IR due to temperature of tungsten filament. Thus this event is termed as thermal radiation.

Which of the following is used to measure the temperature inside a furnace?
  • a)
    Gas thermometer
  • b)
    Optical pyrometer
  • c)
    Alcohol thermometer
  • d)
    Mercury thermometer
Correct answer is option 'B'. Can you explain this answer?

Telecom Tuners answered  •  Mar 01, 2021
In a furnace heat transfer is a result of thermal radiation due to high temperature values and contribution of conduction and convection to the phenomenon is insignificant. Hence optical pyrometers are best suited devices to measure temperature inside the furnace. These devices detect wavelength corresponding to maximum monochromatic emissive power in the infrared radiation and using Wien’s law, i... more

For the radiation between two infinite parallel planes of emissivity respectively, which one of the following is the expression for emissivity factor or equivalent emissivity?
  • a)
    ε1ε2
  • b)
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

Telecom Tuners answered  •  Feb 28, 2021
The equivalent thermal resistance between two gray bodies is given by
Rth = 1-ε1/A1ε1+1/A1F12+1-ε2/A2ε2
Now for infinite parallel panes, F12 =1
A1 = A2 = A
For unit area, A = 1 m2
Rth = 1/ε1+1/ε2-1
Hence equivalent emissivity
Εe = 1/Rth
Fetching relevant content for you
Ask a question