The diameter and surface emissivity of an electricallyheated plate are D = 300 mm and ∈ = 0.80 , respectively. (a) Estimate the power needed to maintain a surfacetemperature of 200°C in a room [or which the airand the walls are at 25°C. The coefficient characterizing heat transfer by natural convection depends on the surface temperature and, in units of W/m 2 ⋅ K , may be approximated by an expression ofthe form h = 0.80 ( T s − T ∞ ) 1 / 3 . (b) Assess the effect of surface temperature on thepower requirement, as well as on the relative contributions of convection and radiation to heat transfer from the surface.
The diameter and surface emissivity of an electricallyheated plate are D = 300 mm and ∈ = 0.80 , respectively. (a) Estimate the power needed to maintain a surfacetemperature of 200°C in a room [or which the airand the walls are at 25°C. The coefficient characterizing heat transfer by natural convection depends on the surface temperature and, in units of W/m 2 ⋅ K , may be approximated by an expression ofthe form h = 0.80 ( T s − T ∞ ) 1 / 3 . (b) Assess the effect of surface temperature on thepower requirement, as well as on the relative contributions of convection and radiation to heat transfer from the surface.
Solution Summary: The author explains the power needed to maintain the surface temperature of heated plate at 200°C in a room.
The diameter and surface emissivity of an electricallyheated plate are
D
=
300
mm
and
∈
=
0.80
,
respectively. (a) Estimate the power needed to maintain a surfacetemperature of 200°C in a room [or which the airand the walls are at 25°C. The coefficient characterizing heat transfer by natural convection depends on the surface temperature and, in units of
W/m
2
⋅
K
, may be approximated by an expression ofthe form
h
=
0.80
(
T
s
−
T
∞
)
1
/
3
. (b) Assess the effect of surface temperature on thepower requirement, as well as on the relative contributions of convection and radiation to heat transfer from the surface.
A food product with 80% water content in a 10 cm diameter can be frozen. Mass of the product type is 1000 kg / m³, thermal conductivity is 1.0 w / (m k), and the frozen early temperature is -1.75 ° C.After 15 hours in the freezing medium -25 ° C, the temperature of the product becomes -10 ° C. Estimating the coefficient of moving the heat of the medium freezing convection. Assume cans as an unlimited cylinder. H = Answerw / (m² k).
Consider steady heat transfer between two large
parallel plates at constant temperatures of T1 =
500 K and T2 = 300 K that are L = 2.5 cm apart.
The emissivity of the surfaces, ɛ = 0.95. Calculate
the total rate of heat transfer and the percentage
by thermal radiation between the plates per unit
surface area assuming the gap between the
plates is (a) filled with atmospheric air (k = 0.02
W/m.°C), (b) evacuated.
%3D
%3D
%3D
O = 5.67 x 10-8 W/m².K4
A mectal cylinder of 5 cm in diameter and initially at a temperature about 150 °C. the eylinder
is suddenly immersed in liquid with temperature at 50 °C and heat transfer coeflicient about 2000
W/m.°C. Under typical operation conditions the center temperature and surface temperature are
measured to be 110 °C and 90 °C after 1 minute, respectivielly. Calculate the thermal
conductivity for this material.
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