Heat Transfer

1. Air at -10ºC flowing over the isothermal plate at 100ºC (w=3.63 m/s). The length of the plate is 1 m, width is 0.5 m. Kinematic viscosity of air at 45ºC: 0.1772 cm2/s, Prandtl number: 0.72; thermal conductivity: 0.02699 W/(m∙K). Total heat transfer from the plate to the air: 300 W. 350 W. 400 W. 450 W.

2. An example of a one-dimensional steady-state heat conduction problem given below has: one solution. two solutions. infinite number of solutions. no solution.

3. For the one-dimensional steady-state heat conduction problem defined in the schematic picture below, determine the temperature in the middle, inside the wall, if thermal conductivity of wall material k=10 W/(m·K) and thickness of the wall L=0.5 m.s The calculated temperature for x=0.25 m is: 47.5ºC. 50ºC. 52.5ºC. 55ºC.

4. Photon is a quantum of: vibrational mechanical energy. light energy. kinetic energy. potential energy.

5. Stefan-Boltzmann constant unit is: W/K. W/(m2K). W/(m2K4). W/(m4K2).

6. Emmitance in radiation heat transfer: depends only on the type of a surface material. depends only on the type and color of surface material. depends on the type, color and temperature of surface material. is a parameter dependent on the characteristics of both radiating surfaces.

7. If Nusselt number Nu=2, it means that: conduction heat transfer is two times more efficient than convection heat transfer. convection heat transfer is two times more efficient than conduction heat transfer. heat transfer is by pure convection. heat transfer is by pure conduction.

8. Thermal radiation is of the same nature as: x-rays. radio waves. visible light. all types of radiation mentioned above.

9. The correct formula for the Reynolds number for the flow inside the pipe is: Re=w·d/ν. Re=w·ρ·d/μ. some 𝑅𝑒=4 𝑚̇/(𝜋∙𝑑∙μ). all formulas above are correct.

10. Prandtl number is: ratio of thermal conductivity to viscosity. ratio of viscosity to thermal conductivity. ratio of thermal diffusivity to viscosity. ratio of viscosity to thermal diffusivity.

11. The unit of thermal expansion coefficient β is: 1/m^3. 1/K. m^3/K. thermal expansion coefficient is nondimensional.

12. Choose a true statement about critical Reynolds number for the flow over a flat plate: for curved surface the transition to turbulence occurs for higher Reynolds numbers. critical Reynolds number depends on the width of the plate. critical Reynolds number depends on the length of the plate. critical Reynolds number is higher in the presence of structural/acoustic vibrations.

13. For the fully developed laminar flow in pipe (both velocity and temperature profile developed): Nusselt number is dependent on Reynolds number. Nusselt number is dependent on Reynolds and Prandtl numbers. Nusselt number is dependent on Reynolds, Prandtl and Graetz numbers. Nusselt number value is constant.

14. Nusselt number for natural convection is usually dependent on: only Prandtl Number. Prandtl and Reynolds numbers. Prandtl and Grashof numbers. Prandtl, Grashof and Graetz numbers.

15. The higher the Prandtl number the intensity of convective heat transfer in fully-developed turbulent flow inside a pipe is: lower. higher. no difference. it depends on the pipe’s diameter.

16. Thin layer of fluid very near to the wall (tens to hundreds micrometer thick) is called: inertial sublayer. buffer sublayer. viscous sublayer. laminar sublayer.