Unit-1 Question Bank1. Define thermal conductivity.2. Define the convection heat-transfer coefficient.3. Discuss the mechanism of thermal conduction in gases and solids.4. Discuss the mechanism of heat convection.5. What is the order of magnitude for the convection heat-transfer coefficient in free convection? Forced convection? Boiling?6. When may one expect radiation heat transfer to be important?7. Name some good conductors of heat; some poor conductors.8. What is the order of magnitude of thermal conductivity for (a) metals, (b) solid insulating materials, (c) liquids, (d) gases?9. Suppose a person stated that heat cannot be transferred in a vacuum. How do you respond?10. Review any standard text on thermodynamics and define: (a) heat, (b) internal energy, (c) work, (d) enthalpy.11. What are the physical mechanisms associated with heat transfer by conduction, convection, and radiation?12. What is the driving potential for heat transfer? What are analogs to this potential and to heat transfer itself for the transport of electric charge?13. What is a temperature gradient? What are its units? What is the relationship of heat flow to a temperature gradient?14. What is the thermal conductivity? What are its units? What role does it play in heat transfer?15. What is Fouriers law? Can you write the equation from memory?16. If heat transfer by conduction through a medium occurs under steady-state conditions, will the temperature at a particular instant vary with location in the medium? Will the temperature at a particular location vary with time?17. What is Newtons law of cooling ? Can you write the equation from memory?18. What role is played by the convection heat transfer coefficien in Newtons law of cooling? What are its units?19. What effect does convection heat transfer from or to a surface have on the solid bounded by the surface?20. What is predicted by the StefanBoltzmann law, and what unit of temperature must be used with the law? 21. What is the emissivity, and what role does it play in characterizing radiation transfer at a surface?22. What two outcomes characterize the response of an opaque surface to incident radiation?23. Which outcome affects the thermal energy of the medium bounded by the surface and how? What property characterizes this outcome?24. What conditions are associated with use of the radiation heat transfer coefficient?25. Consider the surface of a solid that is at an elevated temperature and exposed to cooler surroundings. By what mode(s) is heat transferred from the surface if (1) it is in intimate (perfect) contact with another solid, (2) it is exposed to the flow of a liquid, (3) it is exposed to the flow of a gas, and (4) it is in an evacuated chamber?26. What is the inherent difference between the application of conservation of energy over a time interval and at an instant of time?27. Heat is conducted through a material with a temperature gradient of 9000 C/m. The conductivity of the material is 25W/mK. If this heat is convected to surroundings at 30C with a convection coefficient of 345W/m2K, determine the surface temperature. If the heat is radiated to the surroundings at 30C determine the surface temperature.28. A wall is exposed on one side to a heat flux of 1.5 kW/m2which is conducted through the wall. For the following combinations, determine the temp drop through the wall (a) thickness 0.16m and k = 1.4, 15, 25, 45, 210 and 340 W/mK. (ii) Thickness 0.25m and k as above. Plot the temperature drop against the radio (L/k) and also (k/L).29. The heat flux through a layer of material 40 mm thick conducting heat under steady state with a temperature drop of 40C, was measured as 106 W. Determine the thermal conductivity of the material.30. A glass pane is 8mm thick and the inside surface temperature was 25C and outside surface temperature was 33C. If k = 1.4 W/mK determine the heat flow through an area of 0.8m 1m size pane.31. The surface temperature of a plate over which air flows was measured as 80C. The air temperature was 40C. In order to maintain the surface temperature over an area of 0.1m2, the heater rating required was found to be 1.5 kW. Determine the value of convection coefficient.32. A strip heater of area 0.2m2 and rating of 1200W is fixed on a vertical wall and mostly convects the heat into the room air at 20C. Determine the value of convective heat transfer coefficient if the surface temperature of the heater is not to exceed 60C. Indicate whether such a value can be achieved by natural convection.33. A strip heater with an area of 0.05m2 has to radiate at 600C to surroundings at 30C. Determine the rating assuming that convection is negligible.34. The filament of an incandescent lamp of 60 W rating has a total surface area of 40mm2. If the surrounding is at 30C and if 90% of the power is converted to heat and radiated, determine the temperature of the filament.35. A sphere of 0.5m diameter containing hot fluid has an insulation of 0.1m thickness. Before adding the insulation, the surface temperature was 260C and heat loss was mainly due to radiation. After adding the insulation, the surface temperature is reduced to 160C and again the heat loss is mainly due to radiation, the surroundings in both cases being at 25C. Determine the change in heat transfer rate.36. A surface maintained at 80C dissipates heat to surroundings at 25C both by radiation and convection. If convection accounts for 60% of heat loss, determine the value of convection coefficient and also the total heat loss.37. A pipe of internal diameter 0.2m and wall thickness 0.008m carries steam at 220C, the thermal conductivity of the material is 12.5 W/mK. Heat is lost from the outside surface by convection to surroundings at 30C. Determine the outside surface temperature if the convection coefficient has a value of 38.5 W/m2. Also find the heat loss /m length.38. Considering the problem 12, if the heat loss from the surface is only by radiation, determine the surface temperature. the loss from the surface is both by radiation and convection, determine the surface temperature.39. A flat plate solar collector absorbs 80% of radiation of 820 W/m2 received. The top loss coefficient is 12 W/m2K. Determine the temperature of heat collection if the efficiency of collection is 0.6.40. Heat is generated in a solid having a total surface area of 0.56m2 and a volume of 0.26m3 at a rate of 1MW/m3. Determine the surface temperature if the heat is convected as well as radiated to the surroundings at 27C. The convective heat transfer coefficient has a value of 245 W/m2K.41. A thermocouple junction of 2mm dia spherical shape receives heat from a flowing fluid at 800C with a convection coefficient of 250 W/m2K and radiates to the surroundings at 450C. Determine the temperature of the surface at equilibrium.42. One side of an insulation layer of 25mm thickness is maintained at 300C, while the other side is exposed to convection at 30C with a convective heat transfer coefficient of 400 W/m2K. Determine the surface temperature neglecting radiation. Also find the heat flow. Conductivity of the material is 40 W/mK.43. A steel billet of 0.2m 0.2m 0.4m lies on its 02m 0.4m face on the floor of a furnace and is exposed to radiation on all the other faces from a source at 1500C. The density of the material is 7830 kg/m3 and the specific heat is 960 J/kgK. Determine the rate of rise in the average temperature of the billet when its surface temperature is 500C. Will the heating rate increase or decrease with rise in temperature of the surface?44. For an air conditioned space, a double glass window pane with a thin air gap between the plates the preferred when compared to a single glass pane. Discuss the reasons for the above.45. Heat is convected and also radiated from the surface of a solid at 160C to the surroundings at 30C. The convection coefficient is 45 W/m2K. Assuming the solid to be isotropic and to have constant properties and the heat is conducted through the solid at steady state, determine the temperature gradient in the solid for k = 15, k = 45, k = 210 and k = 340 W/mK.46. Heat is conducted through a solid layer of 0.3m and is then convected at the surface. The temperature drop in the solid and the temperature drop in the convective layer are equal. The conductivity of the material is 12.5 W/mK. Determine the value of convection coefficient. Also evaluate the conduction and convection resistances.47. The temperature gradient in a solid of thermal conductivity k = 12.5 W/mK, conducting heat under steady conditions is 6C/cm. If the heat is radiated at the surface, determine the temperature at the surface. The surroundings are at 30C.48. A spherical container filled with a fluid of good thermal conductivity which is constantly being stirred is to be insulated. The inner diameter of the container is 0.4 m. The heat capacity of the fluid is 70 kJ/C. The wall thickness is 15 mm and the conductivity of the material is 15 W/mK. The fluid is at 160C. The convection coefficient on the inside is 45 W/m2K. The outside is exposed to air at 30C with a convection coefficient of 25 W/m2K. Determine the overall heat transfer coefficient based on outside area, heat loss and the instantaneous rate of cooling of the fluid.49. A hollow spherical insulation has internal diameter of 16 cm and a thickness of 8 cm. The inner surface is at 10C while the outer surface is at 35C. Determine the radius at which the temperature is 0C.50. A spherical vessel 10 cm OD at 24C is exposed to air at 20C with convection coefficient of 10W/m2K. Determine the heat gain rate. If the sphere is insulated with 5 cm thick insulation of thermal conductivity of 0.5 W/mK, determine the heat gain rate. Also investigate thicknesses of 2.5 cm and 7.5 cm of insulation.51. A spherical vessel containing hot fluid at 160C (in a chemical process) is of 0.4 m OD and is made of Titanium of 25 mm thickness. The thermal conductivity is 20 W/mK. The vessel is insulated with two layers of 5 cm thick insulations of thermal conductivities 0.06 and 0.12 W/mK. There is a contact resistance of 6 104 and 5 104 m2 C/W between the metal and first insulation and between the insulating layers. The outside is exposed to surrounding at 30C with a convection coefficient of 15 W/m2K. Determine the rate of heat loss, the interface temperatures and the overall heat transfer coefficient based on the metal surface area.52. An insulating wall of a furnace has the following relationship for its thermal conductivity k = 0.7(1 + 15 104 T) W/mK where T is in C. The wall is 0.25 m thick. The inside surface is at 760Cand the outside surface is at 60C. Determine the heat loss per unit area and also the temperature at the mid section.