6.1. Steel balls 12 mm in diameter are annealed by heating to 1150 K and then slowly cooling to400 K in an air environment for which T∞=325 K and h = 20 W/m² • K. Assuming the propertiesof the steel to be k = 40 W/m• K, p = 7800 kg/m³, and c = 600 J/kg • K, estimate the timerequired for the cooling process.

Step 1: Given: Diameter of the steel ball, d=12mm=0.012m Initial temperature, T0=1150K Final…

Answered: 6.1. Steel balls 12 mm in diameter are…

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.

Chapter3: Transient Heat Conduction

Section: Chapter Questions

Problem 3.36P

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A 0.6-cm diameter mild steel rod at 38C is suddenly immersed in a liquid at 93C with hc=110W/m2K. Determine the time required for the rod to warm to 88C.
5.6 Steel balls 12 mm in diameter are annealed by heating to 1150 K and then slowly cooling to 400 K in an air envi- ronment for which T325 K and h = 20 W/m².K. Assuming the properties of the steel to be k = 40 W/mK, p= 7800 kg/m³, and c = 600 J/kg K, estimate the time required for the cooling process.
A polished stainless steel sphere (r= 5cm) is at a temperature of 540 C. The ambier air is at 20 C. What is the instantaneous heat transfer rate?
6. a. The heat flux applied to the walls of the biomass combustion furnace is 20 W/m2. The furnace walls have a thickness of 10 mm and a thermal conductivity of 12 W/m.K. If the wall surface temperature is measured to be 50oC on the left and 30oC on the right, prove that conduction heat transfer occurs at a steady state!b. Heating the iron cylinder on the bottom side is done by placing the iron on the hotplate. This iron has a length of 20 cm. The surface temperature of the hotplate is set at 300oC while the top side of the iron is in contact with the still outside air. To reach the desired hotplate temperature, it takes 5 minutes. Then it takes 15 minutes to measure the temperature of the upper side of the iron cylinder at 300oC. Show 3 proofs that heat transfer occurs transiently
(Q4) A 4m x 6m wall consists of 4 glass windows of 2m x 1.5m dimensions. The wall has thickness of 0.13m and a thermal conductivity of 0.5 W/m.K, while the glass windows are 6 mm thick with a thermal conductivity of 1.228 W/m.K. The values of intemal and external surface conductance for the wall (including glass) are 7.8 W/m? K and 34.4 W/m².K, respectively. The intemal and extemal temperatures are 22° C and 42°C, respectively. Calculate the total heat transfer rate through the wall. What percentage of this heat transfer is through the windows?
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5.52 Steel is sequentially heated and cooled (annealed) to relieve stresses and to make it less brittle. Consider a 100-mm-thick plate (k = 45 W/m.K, p = 7800 kg/m³, Cp c, = 500 J/kg .K) that is initially at a uniform tempera- ture of 300°C and is heated (on both sides) in a gas-fired furnace for which T = 700°C and h = 500 W/m² K. How long will it take for a minimum temperature of 550°C to be reached in the plate?
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Suppose that the 10 cm thick concrete wall of a commercial building measures 30 mby 4 m. The energy impinging on the surface is 700 Wm-2. Under these conditionsthe external air temperature is 25ºC, and the internal and external surfacetemperatures of the concrete wall are 17ºC and 40ºC, respectively.(a) State and briefly describe the three heat transfer mechanisms.[30%](b) Calculate the net amount of thermal energy radiated by the wall.[40%](c) State your assumptions and briefly discuss their effect on the relativesizes of the heat transfer mechanisms.[30%]Data for Question A3.Heat transfer coefficient of air at 25ºC = 5 Wm-2K-1Thermal conductivity of concrete = 1.2 Wm-1K-1Emissivity of concrete = 0.85Stefan’s constant = 5.7 x 10-8 Wm-2K-4
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