A small metal bead (radius 0.5 mm), initially at 100°C, when placed in a stream of fluid at 20°C, attains a temperature of 28°C in 4.35 seconds. The density and specific heat of the metal are 8500 kg/m3 and 400 J/kgK respectively. If the bead is considered as a lumped system, the convective heat transfer coefficient (in W/m2.K) between the metal bead and the fluid stream is
300.0
For a sphere, V/A = r/3. The lumped relation gives \(h=-\frac{\rho cr}{3t}\ln\frac{28-20}{100-20}=\frac{8500(400)(0.0005)}{3(4.35)}\ln10=299.9536\ \mathrm{W/(m^2K)}\). To one decimal place: 300.0 W/(m² K).
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
