Q. An egg with mean diameter of 4 cm and initially at 25 degree Celsius is placed in an open boiling water container for 4 minutes and found to be boiled at a particular level. For how long should a similar egg boil at the same level, when refrigerator temperature is 5 degree Celsius? Use lumped parameter theory and assume following properties of egg

k = 12 W/m K

h = 125 W/m² K

c = 2 k J/kg K

p = 1250 kg/m³

View solution

Q. A person is found dead at 5 pm in a room whose temperature is 20 degree Celsius. The temperature of body is measured to be 25 degree Celsius, when found and heat transfer coefficient is estimated to be 8 W/m² K. Modeling the body as a 30 cm diameter, 1.7 cm long cylinder. Estimate the time of death of person.

View solution

Q. The following data pertains to the junction of a thermocouple wire used to measure the temperature of the gas stream.

Density = 8500 kg/m³, specific heat = 325 J/kg K, thermal conductivity = 40 W/m K and the heat transfer coefficient between the junction and gas = 215 W/m² K.

If thermocouple junction can be approximated as 1 mm diameter sphere, determine how long it will take for the thermocouple to read 99% of the initial temperature difference.

View solution

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

View solution

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

View solution

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

View solution

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

View solution

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

View solution

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

View solution

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

View solution

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

View solution

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

View solution

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

View solution

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

View solution

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

View solution

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

View solution

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

View solution

Q. The perturbation time varies as

View solution

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

View solution

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

View solution