Q. A thermocouple junction in the form of 4 mm radius sphere is to be used to measure the temperature of a gas stream. The junction is initially at 35 degree Celsius and is placed in a gas stream which is at 300 degree Celsius. The thermocouple is removed from the hot gas stream after 10 seconds and kept in still air at 25 degree Celsius with convective coefficient 10 W/m² K. Find out the time constant of the thermocouple. Assume the thermos-physical properties as given below:

h = 37.5 W/m² K

p = 7500 kg/m³

c = 400 J/kg K

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Q. What percentage of water an average human body can have?

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Q. According to lumped system analysis, solid possesses thermal conductivity that is

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Q. The temperature and rate of heat conduction are undoubtedly dependent on

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Q. Glass spheres of 2 mm radius and at 500 degree Celsius are to be cooled by exposing them to an air stream at 25 degree Celsius. Find the minimum time required for cooling to a temperature of 60 degree Celsius. Assume the following property values

Density = 2250 kg/m³

Specific heat = 850 J/kg K

Conductivity = 1.5 W/m K

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Q. Which is true regarding lumped system analysis?
(i) Conductive resistance = 0

(ii) Convective resistance = 0

(iii) Thermal conductivity = 0

(iv) Thermal conductivity = infinity

Identify the correct statements

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Q. Which of the following is an example of lump system analysis?

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Q. What is the criterion for the applicability of lump system analysis?

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Q. What is the value of characteristics length for cylinder?

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Q. During heat treatment, cylindrical pieces of 25 mm diameter, 30 mm height and at 30 degree Celsius are placed in a furnace at 750 degree Celsius with convective coefficient 80 W/m² degree. Find the value of biot number if thermal conductivity is 40 W/m degree

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Q. What is the value of characteristics length for sphere?

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Q. What is the value of characteristics length for cube?

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Q. “An infinite solid is one which extends itself infinitely in all directions of space”. Identify the correct option

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Q. The boundary conditions in case of transient heat conduction in infinite thick solids are

(i) t (x = 0) = t i
(ii) t (0, T) = t a for T greater than zero
(iii) t (infinity, T) = t i for T greater than zero
Identify the correct statements

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Q. The perturbation time varies as

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Q. The temperature perturbation at all the surface has penetrated to the depth (Where, T is time constant and α is thermal diffusivity.)

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Q. At the penetration depth d, there will be 1% perturbation at a time given by

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Q. A large steel ingot, which has been uniformly heated to 750 degree Celsius, is hardened by quenching it in an oil bath that is maintained at 25 degree Celsius. What length of time is required for the temperature to reach 600 degree Celsius at a depth of 1 cm? Thermal diffusivity for the steel ingot is 1.21 * 10¯⁵ m²/s. The ingot may be approximated as a flat plate

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Q. A mild steel plate 5 cm thick and initially at 40 degree Celsius temperature is suddenly exposed on one side to a fluid which causes the surface temperature to increase to and remain at 90 degree Celsius. Calculate maximum time that the slab be treated as a semi-infinitely body
For steel, thermal diffusivity = 1.25 * 10¯⁵ m²/s

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Q. Water pipes are to be buried underground in a wet soil (thermal diffusivity = 2.78 * 10¯³ m²/s) which is initially at 4.5 degree Celsius. The soil surface temperature suddenly drops to -5 degree Celsius and remains at this value for₁0 hours. Calculate the minimum depth at which the pipe be laid if the surrounding soil temperature is to remain above 0 degree Celsius. The soil may be considered as semi-infinite solid

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