Heat Transfer Online Test

1. A polished metal pipe 5 cm outside diameter and 370 K temperature at the outer surface is exposed to ambient conditions at 295 K temperature. The emissivity of the surface is 0.2 and the convection coefficient of heat transfer is 11.35 W/m² degree. Calculate the overall coefficient of heat transfer by the combined mode of convection and radiation

2. What is the thermal conductivity of magnetite in W/m K?

3. Which one is not the unit of thermal conductivity?

4. A steel rod (k = 30 W/m degree) 1 cm in diameter and 5 cm long protrudes from a wall which is maintained at 10 degree Celsius. The rod is insulated at its tip and is exposed to an environment with h = 50 W/m² degree and t _a = 30 degree Celsius. Calculate the rate of heat dissipation

5. For turbulent flow over a flat plate, the average value of Nusselt number is prescribed by the relation
Nu = 0.664 (Re) ^0.5 (Pr) ^0.33
Which of the following is then a false statement?
The average heat transfer coefficient increases as

6. Many types of heat exchangers have been developed to meet the widely varying applications. Based upon their
(i) Operating principle
(ii) Arrangement of flow path
(iii) Design
Identify the correct statements

7. Given a vertical tube exposed to saturated steam at atmospheric pressure, with the outer surface maintained at 80°C by cooling water, calculate the rate at which steam is condensed at the tube surface.

8. What is the value of overall heat transfer coefficient for air to heavy tars and liquid?

9. What is the dimension of energy?

10. Consider two surfaces, one absolutely black and the other non-black. These surfaces are arranged parallel to each other and so close that the radiation of one falls totally on the other. Choose the correct option

11. A thin metal plate of 4 cm diameter is suspended in atmospheric air whose temperature is 290 K. This plate attains a temperature of 295 K when one of its face receives radiant energy from a heat source at the rate of 2 W. If heat transfer coefficient on both surfaces of the plate is stated to be 87.5 W/m ² K, workout the energy lost by reflection

12. “The response of a thermocouple is defined as the time required for the thermocouple to reach the surrounding temperature when it is exposed to it”.

13. A sphere, a cube and a thin circular plate, all made of the same material and having the same mass are initially heated to the temperature of 250 degree Celsius. When left in air at room temperature, what will be their response to cooling?

14. Consider a 1.2 m thick slab of poured concrete (k = 1.148 W/m degree) with both of side surfaces maintained at a temperature of 20 degree Celsius. During its curing, chemical energy is released at the rate of 80 W/m³. Workout the maximum temperature of concrete

15. A two pass surface condenser is required to handle the exhaust from a turbine developing 15 MW with specific steam consumption of 5 kg/k W h. The quality of exhaust steam is 0.9, the condenser vacuum is 66 cm of mercury while the bar meter reads 76 cm of mercury. The condenser tubes are 28 mm inside diameter, 4 mm thick and water flows through tubes with a speed of 3 m/s and inlet temperature 20 degree Celsius. All the steam is condensed, the condensate is saturated water and temperature of cooling water at exit is 5 degree Celsius less than the condensate temperature. Assuming that overall coefficient of heat transfer is 4 k W/m² degree, determine the mass of cooling water circulated

16. Some examples of heat exchanger are
(i) Condensers and evaporators in refrigeration units
(ii) Evaporator of an ice plant and milk chiller of a pasteurizing plant
(iii) Automobile radiators and oil coolers of heat engines
Identify the correct answer

17. Fourier number is given by

18. The total radiant energy A impinging upon a body would be partially or totally absorbed, reflected from its surface or transmitted through it. Identify the correct statement

19. Find the heat transfer rate from a hot surface for 6 fins of 10 cm length? The base temperature of the fin is maintained at 200 degree Celsius and the film is exposed to a convection environment at 15 degree Celsius with convective coefficient 25W/square m K. Each fin has cross-sectional area 2.5 square centimeter and is made of a material having thermal conductivity 250W/m K

20. If I _n denotes the normal intensity and I _α represents the intensity at angle α, then

21. A black body of 0.2 m² area has an effective temperature of 800 K. Calculate the intensity of normal radiations

22. Thermo-physical properties of the fluid are represented by
(i) Density
(ii) Viscosity
(iii) Specific heat
(iv) Thermal conductivity
Identify the correct option

23. The solar radiation that is felt at the earth’s surface includes
(i) Direct radiation that has passed through the atmosphere
(ii) Diffuse radiation from the sky
(iii) Absorbed radiation from the water
Identify the correct statement

24. Two radiating surface A ₁ = 6 m² and A ₂ = 4 m² have shape factor F ₁₂ = 0.1. Then the shape factor F ₂₁ will be

25. Which type of boiling occurs in steam boilers employing natural convection?

26. A motor cycle cylinder consists of ten fins, each 150 mm outside diameter and 75 mm inside diameter. The average fin temperature is 500 degree Celsius and the surrounding air is at 20 degree Celsius temperature. Make calculations for the rate of heat dissipation from the cylinder fins by convection when motor cycle is stationary and convective coefficient is 6 W/m² K

27. A very long copper rod 20 mm in diameter extends horizontally from a plane heated wall maintained at 100 degree Celsius. The surface of the rod is exposed to an air environment at 20 degree Celsius with convective heat transfer coefficient of 8.5 W/m² degree. Workout the heat loss if the thermal conductivity of copper is 400 W/m degree

28. In a parallel flow heat exchanger, engine oil at 150°C is cooled to 80°C by water entering at 25°C and leaving at 60°C. If the fluid flow rates and inlet conditions remain the same, what is the lowest temperature the oil can be cooled to by increasing the length of the exchanger?

29. The following factors need consideration for the optimum design of fins
(i) Cost
(ii) Space considerations
(iii) Weight considerations
Choose the correct option

30. The unit of thermal conductivity doesn’t contain which parameter?

31. A composite wall is made of two layers of thickness δ₁ and δ₂ having thermal conductivities k and 2k and equal surface area normal to the direction of heat flow. The outer surface of composite wall are at 100 degree Celsius and 200 degree Celsius. The minimum surface temperature at the junction is 150 degree Celsius. What will be the ratio of wall thickness?

32. The Prandtl number will be lowest for

33. Interchange factor for infinitely long concentric cylinders is given by

34. A cylindrical cement tube of radii 0.05 cm and 1.0 cm has a wire embedded into it along its axis. To maintain a steady temperature difference of 120 degree Celsius between the inner and outer surfaces, a current of 5 ampere is made to flow in the wire. Find the amount of heat generated per meter length. Take resistance of wire equal to 0.1 ohm per cm of length

35. A steel pipe of 20 mm inner diameter and 2 mm thickness is covered with 20 mm thick of fiber glass insulation (k = 0.05 W/m degree). If the inside and outside convective coefficients are 10 W/m² degree and 5 W/m² degree, calculate the overall heat transfer coefficient based on inside diameter of pipe. In the diagram, the diameter of small circle is 20 mm

36. The velocity profile of the hydrodynamic boundary layer is dependent upon

37. Consider two bodies, one absolutely back and the other non-black and let these be at same temperature. Which one of the following statement is correct?

38. For a cylindrical rod with uniformly distributed heat sources, the thermal gradient at half the radius location will be

39. A steam condenser is transferring 250 k W of thermal energy at a condensing temperature of 65 degree Celsius. The cooling water enters the condenser at 20 degree Celsius with a flow rate of 7500 kg/hr. Calculate the log mean temperature difference

40. For water evaporating at atmospheric pressure, the burnout occurs at temperature excess slightly above

41. Fritz criterion is given by

42. Consider a beam of monochromatic radiation at wavelength λ that enters a layer of absorbing gas. As the beam passes through the gas layer, its intensity

43. Shear stress at the middle of the plate is given by

44. Natural convection heat transfer coefficients over surface of a vertical pipe and a vertical flat plate for same height. What is/are the possible reasons for this?
(i) Same height
(ii) Both vertical
(iii) Same fluid
(iv) Same fluid flow pattern
Select the correct answer

45. The correction length for cylindrical fin is

46. In many flow situations, the duct can be
(i) Circular
(ii) Rectangle
(iii) Trapezoidal
(iv) Annulus
Identify the correct option

47. What percentage of water an average human body can have?

48. Which one is true for an opaque body?

49. Consider the following parameters
(i) Composition
(ii) Density
(iii) Porosity
(iv) Structure
Then, thermal conductivity of glass wool varies from sample to sample because of variation is

50. Two black discs each of diameter 50 cm are placed parallel to each other concentrically at a distance of one meter. The discs are maintained at 1000 K and 500 K. Calculate the heat flow between the discs when the disks are connected by a cylindrical black no-flux surface