An aluminum sphere, initially at a temperature of 25°C, is placed in a 300°C oven. How do the temperatures of the sphere center (black) and the sphere surface (orange) change over time? Assume that the thermal conductivity of the sphere is large and that the rate of convective heat transfer to the sphere is relatively small. Why? A B Tourface Tinside Time Time E D Time Time Time Temperature Temperature Temperature Temperature Temperature

An aluminum sphere, initially at a temperature of 25°C, is placed in a 300°C oven. How do the temperatures of the sphere center (black) and the sphere surface (orange) change over time? Assume that the thermal conductivity of the sphere is large and that the rate of convective heat transfer to the sphere is relatively small. Why? A B Tourface Tinside Time Time E D Time Time Time Temperature Temperature Temperature Temperature Temperature

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter1: Basic Modes Of Heat Transfer

Section: Chapter Questions

Problem 1.53P: Determine the power requirement of a soldering iron in which the tip is maintained at 400C. The tip...

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A horizontal, 3-mm-thick flat-copper plate, 1-m long and 0.5-m wide, is exposed in air at 27C to radiation from the sun. If the total rate of solar radiation absorbed is 300 W and the combined radiation and convection heat transfer coefficients on the upper and lower surfaces are 20 and 15W/m2K, respectively, determine the equilibrium temperature of the plate.
Determine the power requirement of a soldering iron in which the tip is maintained at 400C. The tip is a cylinder 3 mm in diameter and 10 mm long. The surrounding air temperature is 20C, and the average convection heat transfer coefficient over the tip is 20W/m2K. The tip is highly polished initially, giving it a very low emittance.
2. Consider steady heat transfer between two large parallel plates at constant temperatures of T1 = 295 K and T2 = 155 K that are L = 2 cm apart. Assuming the surfaces to be black, determine the rate of heat transfer between the plates per unit surface area assuming the gap between the plates is (a) filled with atmospheric air,(k = 0.01979 W/m- o C) (b) evacuated,(c) filled with superinsulation having an apparent thermal conductivity of 0.00015 W/m · °C.
It is desired to heat the workshop with a stove made of falling cylindrical sheet material with an outer diameter of 400 mm and a height of 1600 mm. The surface temperature of the stove is 217 oC and the radiation emission coefficient is 0.92, the temperature of the workshop surfaces is 12 oC and the ambient temperature of the workshop is 17 oC. Next to the stove, the workshop surface area is very large. From the side surface of the stove to the workshop; a) By transport,b) with radiation Calculate the heat transfer.
Explain the following terms related to the radiation mode of heat transfer: Black body, emissive power, monochromatic emissive power, gray body, white body and opaque body.
You went to Target to buy some masks. As you walk up to the store, you notice the huge red balls in front of the store. Air outside is quiescent. If the sphere surface has a temperature of 17°C and the quiescent air has a temperature of 22.5°C, find the convective heat transfer rate from the sphere. The diameter of sphere is 1 m. -66w
The glass surfaces are polished, having an emissivity of 0.095. The diameters of the inner and outer cylinders are 9 cm and 11 cm respectively and their length is 20 cm. The temperature of the inner and outer cylinders is 92oC and 31oC respectively. Calculate the heat transfer coefficient due to radiation through the cylindrical surface from the inner to the outer cylinder.
Two parallel discs of 1m diameter are situated 2m apart in surroundings at a temperature of 20 oC. The inner side of one disc has an emissivity of 0.5 and is maintained at 500 oC by electric resistance heating and the outer side of the disc is well insulated. The other disc is open to radiation on both sides and reaches an equilibrium temperature. Calculate this equilibrium temperature and the heat flow rate from the first disc, assuming heat transfer is entirely by radiation.
16-51 Consider a person whose exposed surface area is 1.7 m², emissivity is 0.5, and surface temperature is 32°C. Determine the rate of heat loss from that person by radiation in a large room having walls at a temperature of (a) 300 K and (b) 280 K. Answers: (a) 26.7 W, (b) 121 W 16-52 Consider a sealed 20-cm-high electronic box whose base dimensions are 50 cm x 50 cm placed in a vacuum chamber. The emissivity of the outer surface of the box is 0.95. If the electronic components in the box dissipate a total of 120 W of power and the outer surface temperature of the box is not to exceed 55°C, determine the temperature at which the surrounding surfaces must be kept if this box is to be cooled by radiation alone. Assume the heat transfer from the bottom surface of the box to the stand to be negligible. 50 cm 120 W ε = 0.95 T = 55°C 20 cm Stand FIGURE P16-52 50 cm Electronic box
Heat Treatment Steel plates 40 cm per side and 1 cm thick are heat treated in a static oven. Inside the oven there is a shelf where 10 separate plates are placed, in such a way that the heat treatment of one does not affect the treatment of the others. The oven is electric and emits radiation heat towards the plates. In the oven an atmosphere with nitrogen is generated to avoid the oxidation of the material, this gas acts as a convective medium on the plates. Before being placed in the oven, the plates are at room temperature of 25 ° C. The heat treatment of the plates begins with a preheating. At this stage, the combined convection and radiation heat transfer coefficient in the oven is 46 W / m2- ° C. During preheating, the oven controls the internal temperature with a quadratic compensation, therefore, the walls and the nitrogen atmosphere have a temperature Th that varies according to the expression: Th = a + bt2 where a = 750 °C, b = 3 x 10- 5°C/ s2 and t is the time in seconds.…
Question 4:Consider a person standing in a room maintained at20°C at all times. The inner surfaces of the walls, floors, andceiling of the house are observed to be at an average temperature of 12°C in winter and 23°C in summer. Determine therates of radiation heat transfer between this person and the surroundingsurfaces in both summer and winter if the exposedsurface area, emissivity, and the average outer surface temperatureof the person are 1.6 m2, 0.95, and 32°C, respectively.
Consider a 1 m high rectangular closed space with two surfaces separated by an air gap of 0.125 m (at 1 atm pressure). Surface temperatures in the air gap are 34 0C and -9 0C. Find the natural convection heat transfer rates for the horizontal orientation (hot surface down), vertical orientation, and the hot surface inclined at an angle of 80° for this closed gap.