The emissivity of iron is approximately 0.25. If an iron cube (with a side length of 0.5 m) is heated to 200• C and is placed into a –10.C chamber then answer the following a) What is the net rate of heat loss due to radiation from the iron cube? b) If the cube is placed onto a 0.1 m thick aluminum sheet, and the ground of the chamber is assumed to be held at –10•C, how long will it take in seconds for the cube to lose 1000 J of heat via conduction to the ground? (Note: the thermal conductivity of aluminum is 240 W/(m K)).

The emissivity of iron is approximately 0.25. If an iron cube (with a side length of 0.5 m) is heated to 200• C and is placed into a –10.C chamber then answer the following a) What is the net rate of heat loss due to radiation from the iron cube? b) If the cube is placed onto a 0.1 m thick aluminum sheet, and the ground of the chamber is assumed to be held at –10•C, how long will it take in seconds for the cube to lose 1000 J of heat via conduction to the ground? (Note: the thermal conductivity of aluminum is 240 W/(m K)).

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 1.30P

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1.26 Repeat Problem 1.25 but assume that the surface of the storage vessel has an absorbance (equal to the emittance) of 0.1. Then determine the rate of evaporation of the liquid oxygen in kilograms per second and pounds per hour, assuming that convection can be neglected. The heat of vaporization of oxygen at –183°C is .
A long wire 0.7 mm in diameter with an emissivity of 0.9 is placed in a large quiescent air space at 270 K. If the wire is at 800 K, calculate the net rate of heat loss. Discuss your assumptions.
1.25 A spherical vessel, 0.3 m in diameter, is located in a large room whose walls are at 27°C (see sketch). If the vessel is used to store liquid oxygen at –183°C and both the surface of the storage vessel and the walls of the room are black, calculate the rate of heat transfer by radiation to the liquid oxygen in watts and in Btu/h.
1.47 A flat roof is modeled as a flat plate insulated on the bottom and placed in the sunlight. If the radiant heat that the roof receives from the sun is , the convection heat transfer coefficient between the roof and the air is 12 W/m2 K, and the air temperature is , determine the roof temperature for the following two cases: (a) Radiative heat loss to space is negligible, (b) The roof is black and radiates to space, which is assumed to be a black- body at 0 K.
1.11 Calculate the heat loss through a glass window 7-mm thick if the inner surface temperature is 20°C and the outer surface temperature is 17°C. Comment on the possible effect of radiation on your answer.
1.54 The soldering iron tip in Problem 1.53 becomes oxidized with age and its gray-body emittance increases to 0.8. Assuming that the surroundings are at 20°C, determine the power requirement for the soldering iron.
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