A sphere, a cube and a disc all of the same material, quality and volume are heated to 900 K and left in air. Which of these have the lowest rate of cooling
Sphere
With equal volume, material and initial temperature, the thermal capacities are equal. Under the same surface heat-transfer coefficient and emissivity, cooling rate scales with exposed area. A sphere has the minimum surface area for a given volume, so it has the lowest cooling rate in this idealized comparison.
Important Questions from Transient Heat Transfer
Consider the following statements about the thermal diffusivity of a substance:
1. It is directly proportional to density.
2. It is inversely proportional to specific heat.
3. It is inversely proportional to thermal conductivity.
4. It is directly proportional to thermal conductivity.
Which of the given statements are INCORRECT?
Lumped heat transfer analysis of a solid object suddenly exposed to a fluid medium at a different temperature is valid when
- The dimensionless numbers used for analyzing the transient heat conduction problems in a plane wall, cylinder, and sphere is ________.
Choose the correct statement.
The curve for unsteady state heating and cooling of bodies with Bi < 0.1 is
A spherical thermocouple junction of diameter 0.706 mm is to be used for measurement of temperature of a gas stream. The convective heat transfer coefficient on bead surface is \(400~\frac{W}{{{m^2} - K}}\). The thermo physical properties of thermocouple material are \(k = 20~\frac{W}{{m - K}},\ c = 400\frac{J}{{kg - K}},\ \rho = 8500~\frac{{kg}}{{{m^3}}}\). If the thermocouple initially at 30°C is placed in a hot stream of 300°C, the time taken by the bead to reach 298°, is
