A 10-mm-thick workpiece, initially at 25°C, is to be annealed at a temperature above 725°C for a period of at least 5 minutes and then cooled. The workpiece is opaque and diffuse, and the spectral distribution of its emissivity is shown schematically. Heating is effected in a large furnace with walls and circulating air at 750°C and a convection coefficient of 120 W/m².K. The thermophysical properties of the workpiece are p = 2700 kg/m³, c = 885 J/kg-K, and k = 165 W/m.K. Assume that ₁ = 0.3 and ₂ = 0.8. α = 82 i &₁ (a) Calculate the emissivity and the absorptivity of the workpiece when it is placed in the furnace at its initial temperature of 25°C. 8 = 2.5 λ (um) (b) Determine the net heat flux into the workpiece for this initial condition, in kW/m². What is the corresponding rate of change in
A 10-mm-thick workpiece, initially at 25°C, is to be annealed at a temperature above 725°C for a period of at least 5 minutes and then cooled. The workpiece is opaque and diffuse, and the spectral distribution of its emissivity is shown schematically. Heating is effected in a large furnace with walls and circulating air at 750°C and a convection coefficient of 120 W/m².K. The thermophysical properties of the workpiece are p = 2700 kg/m³, c = 885 J/kg-K, and k = 165 W/m.K. Assume that ₁ = 0.3 and ₂ = 0.8. α = 82 i &₁ (a) Calculate the emissivity and the absorptivity of the workpiece when it is placed in the furnace at its initial temperature of 25°C. 8 = 2.5 λ (um) (b) Determine the net heat flux into the workpiece for this initial condition, in kW/m². What is the corresponding rate of change in
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.
Chapter11: Heat Transfer By Radiation
Section: Chapter Questions
Problem 11.27P
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Keep in mind that both the bottom and top surfaces of the workpiece are exposed to radiation and convection. The workpiece is suspended in air in the furnace.
Transcribed Image Text:A 10-mm-thick workpiece, initially at 25°C, is to be annealed at a temperature above 725°C for a period of at least 5 minutes and then
cooled. The workpiece is opaque and diffuse, and the spectral distribution of its emissivity is shown schematically. Heating is effected
in a large furnace with walls and circulating air at 750°C and a convection coefficient of 120 W/m².K. The thermophysical properties of
the workpiece are p = 2700 kg/m³, c = 885 J/kg-K, and k = 165 W/m.K.
Assume that ₁ = 0.3 and ₂ = 0.8.
ε =
α =
i
dT/dt =
(a) Calculate the emissivity and the absorptivity of the workpiece when it is placed in the furnace at its initial temperature of 25°C.
Ya
i
82
i
&1
(b) Determine the net heat flux into the workpiece for this initial condition, in kW/m². What is the corresponding rate of change in
temperature, dT / dt, for the workpiece, in K/s?
qnet.in
2.5
kW/m²
λ (um)
K/s
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