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# GATE 2020: Heat Transfer Quiz 2 (App update required to attempt this test)

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Question 1

A thermocouple of diameter 1 mm is used to measure the temperature of a gas stream for which convective heat transfer coefficient is 500 W/m2k. The time period after which an acceptable reading of temperature can be recorded is (take specific heat = 400 J/kgK, mass density = 8000 kg/m3)

Assume thermocouple to be cylindrical.

Question 2

A spherical body of diameter 750mm of initial temperature 25 is immersed in the fluid at temperature 400. Thermo physical properties of the body are k=22W/m-K, c=400 J/kg and ρ=9000kg/m3 (Regular notations used). Determine the time (in seconds) when the temperature will become 390, given the convective heat transfer coefficient between body and fluid is 300W/m2-k.

Question 3

Find the time period for a lumped system to reach the average temperature of initial and surrounding temperature. The properties of the lumped system spherical in shape having diameter of 1.2mm are h= 85W/ K, 𝞺= 8500kg/ , Cp=320J/kgK.

Question 4

A steel ball l of radius 30 mm is initially in thermal equilibrium at 1000 ° C in furnace. It is then suddenly removed and cooled in ambient air at 30 ° C with convection heat transfer coefficient h = 20 W/m2K. find the time required in seconds to cool the steel ball in air from 1000 ° C to 450 ° C, (Take thermo physical properties of steel: density ρ = 7600 kg/m3, Conductivity k = 40 W/mK, specific heat Cp = 500 J/kg K.)

Question 5

A sphere of mass 5.5 kg and initially at a temperature of 290 K is suddenly emerged in a liquid of 15 K and convective heat transfer coefficient is 58 W/m2K. Estimate the time(in minutes) required to cool the sphere to 95K and the given properties are
ρ = 2700kg/m3, C= 900J/kg and k=205W/m K r=4.5 mm

Question 6

A steel ball of diameter 60 mm is initially in thermal equilibrium at 1030 °C in a furnace. It is suddenly removed from the furnace and cooled in ambient air at 30 °C, with convective heat transfer coefficient h = 20 W/m2K. The thermo-physical properties of steel are: Density ρ = 7800 kg/m3. Conductivity k = 40 W/mK and specific heat c = 600 J/kgK. The time required in seconds to cool the steel ball in air from 1030 °C to 430 °C is
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