Concept explainers
A microwave oven operates on the principle that application of a high-frequency field causes electrically polarized molecules in food to oscillate. The net effect is a nearly uniform generation of thermal energy within the food. Consider the process of cooking a slab of beef of thickness 2L in a microwave oven and compare it with cooking in a conventional oven, where each side of the slab is heated by radiation. In each case the meat is to be heated from
Want to see the full answer?
Check out a sample textbook solutionChapter 5 Solutions
Fundamentals of Heat and Mass Transfer
- 1.76 Explain a fundamental characteristic that differentiates conduction from convection and radiation.arrow_forward1.10 A heat flux meter at the outer (cold) wall of a concrete building indicates that the heat loss through a wall of 10-cm thickness is . If a thermocouple at the inner surface of the wall indicates a temperature of 22°C while another at the outer surface shows 6°C, calculate the thermal conductivity of the concrete and compare your result with the value in Appendix 2, Table 11.arrow_forwardA furnace must be built with an inner layer of refractory brick (conductivity 1.27 Kcal/h mºC). This layer must be coated with a layer of insulating brick (conductivity 0.134 Kcal/hm ºC) 22 cm thick, in turn covered with a 15 cm layer of building brick (conductivity 0.75 Kcal/ hm°C). The inner surface of the refractory wall will operate at 1150ºC and the outside at 65ºC. How many centimeters of refractory brick would be required for the insulation not to exceed 930ºC (safety limit of the material in question).arrow_forward
- A radiator plate of 0.2-m (wide) x 0.4-m (height) is in a room of 22°C. One side (the hot side facing the room) of the radiator plate is maintained at a temperature of 80°C, the other sides are assumed to be insulated, as shown in Figure 1. If the following assumptions can be made: 1) steady operating conditions exist; 2) the air an ideal gas; 3) the local' atmospheric pressure is 1.00 atm: (a) Briefly and qualitatively analyze the four different locations for the radiators marked A), B), C) and D), as shown in Figure 1, and rank them in terms of heat transfer efficiency by natural convection. (b) Determine the rate of heat transfer from the radiator plate by natural convection if the plate is A) vertical and C) horizontal with the hot surface facing down. Compare the two and provide a brief discussion of the findings. wall A 0.4 mi C 0.4 m 0.4 m B floor ceiling 0.4 m wall Nu Nu=0.59Ra Nu-0.1 Raa 0.387Ral Nu- - {0.825 + 11 + (0.492/Pr)162) (complex but more accurate) Use vertical plate…arrow_forwardHeat is generated in a 1.4 m long and 0.2 cm diameter electrical resistance wire placed in the middle of a room kept at 20 degrees. Under continuous conditions, the surface temperature of the wire was measured as 180 ° C, the voltage drop on the wire as 110 V and the electric current as 3 A. Regardless of radiative heat transfer,Find the heat transfer coefficient for the heat transfer between the surface of the wire shown in the figure and the room air.arrow_forward7.5 m 7.5 m Which of the two configurations above should a company use if it plans on using a fire clay brick refractory with k = 1.54 W/m °C? For the two configurations above, the wall thickness is 400 mm, and the length is 15.75 m. 7.5 marrow_forward
- Correct answer onlyarrow_forwardHeat is generated in a 1.4 m long and 0.2 cm diameter electrical resistance wire placed in the middle of a room kept at 20 ° C. Under continuous conditions, the surface temperature of the wire was measured as 180 ° C, the voltage drop on the wire as 110 V and the electric current as 3 A. Find the heat transfer coefficient for the heat transfer between the surface of the wire shown in the figure and the room air, regardless of radiative heat transfer.arrow_forwardA hollow square box is made from 1ft^2 sheets of a prototype insulating material that is 0.75 inches thick. A 120W electrical heater is placed inside the box. Over time thermocouples attached to the box show that the interior and exterior surfaces of one face have reached the constant temperatures of 150 degrees and 90 degrees. What is the thermal conductivity?arrow_forward
- For one-dimension heat transfer conduction consider a shielding wall for a nuclear reactor. The wall receives a gamma-ray flux such that heat is generated within the wall according to the relation 9x = 90 (2X+3) Where q0 is the heat generation at the inner face of the wall exposed to the gamma- ray Flux. Using this relation for heat generation, derive an expression for the temperature distribution in a wall of thickness L, where the inside and outside temperatures are maintained at Ti and 70, respectively. Also, obtain temperature in the wall at X= 0.1 m. Assume, Ti-100 °C, T0-200 °C, L= 0.2 m, K = 40 w / m. °C, and q0 = 50 W To X- 1 90 Y FLUX Tiarrow_forwardThe section of a vertical wall is made up of fiberglass insulation slabs separated by wooden studs. The thermal conductivity of fiberglass and wood are 0.04 W/m∙K and 0.18 W/m∙K, respectively. The thickness of the wall is 180 mm. The temperatures of the inner and outer surfaces of the wall section are 23.5 °C and 4.7 °C, respectively. The ratio of the insulation area to the total wall area is 0.8. Calculate the total heat flow rate (in W/m2) through the wall per unit area.arrow_forwardAs a researcher in the field of heat transfer, how will you distinguish between radiation by black body and a real body? Explain briefly Point out and briefly explain the various factors affect the thermal conductivity of a material. What is the analogical reason between heat transfer by conduction and flow of electricity through ohmic resistance? Use a composite wall of a building to illustrate the concept. A composite slab with three layers of thermal conductivities k1, k2, k3 and thickness t1, t2, t3 respectively, are placed in a close contact. Derive an expression from the first principle for the heat flow through the composite slab per unit surface area in terms of the overall temperature difference across the slab. Based upon the reradiating properties of absorptivity, reflectivity and transmissivity, how would you distinguish between the following:Black body, white body, transparent body and opaque body. The nature of surfaces affects the absorption and the reflection of…arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning