Unlock document.
This document is partially blurred.
Unlock all pages and 1 million more documents.
Get Access
9-138
9-156 A solar collector consists of a horizontal copper tube enclosed in a concentric thin glass tube. Water is heated in the
tube, and the annular space between the copper and glass tube is filled with air. The rate of heat loss from the collector by
natural convection is to be determined.
1-
25
K 003135.0
K)27346(
11
7238.0Pr
/sm 10760.1
C W/m.02706.0
=
+
==
=
=
=
−
f
T
k
2
2=
and
)m 02.0)(K 3260)(K 003135.0)(m/s 81.9(
)(
3-12
3
−
−
coi LTTg
Do =9 cm
Glass cover
To = 32C
9-139
9-157 A solar collector consists of a horizontal tube enclosed in a concentric thin glass tube is considered. The pump
circulating the water fails. The temperature of the aluminum tube when equilibrium is established is to be determined.
Assumptions 1 Steady operating conditions exist. 2 Air is an
ideal gas with constant properties. 3 The local atmospheric
1-
25
K 003284.0
K)2735.31(
11
7278.0Pr
/sm 10622.1
C W/m.02599.0
=
+
==
=
=
=
−
f
T
k
Analysis This problem involves heat transfer from the aluminum tube to the glass cover, and from the outer surface of the
glass cover to the surrounding ambient air. When steady operation is reached, these two heat transfers will be equal to the rate
Do =7 cm
Di = 5 cm,
= 1
Air space
9-140
1-
25
K 003203.0
K)27317.39(
11
7257.0Pr
/sm 10694.1
C W/m.02656.0
=
+
==
=
=
=
−
f
T
k
9-142
Fundamentals of Engineering (FE) Exam Problems
9-159 Consider a hot boiled egg in a spacecraft that is filled with air at atmospheric pressure and temperature at all times.
Disregarding any radiation effect, will the egg cool faster or slower when the spacecraft is in space instead of on the ground?
9-160 A hot object suspended by a string is to be cooled by natural convection in fluids whose volume changes differently
with temperature at constant pressure. In which fluid will the rate of cooling be lowest?
9-161 A spherical block of dry ice at –79°C is exposed to atmospheric air at 30°C. The general direction in which the air
moves in this situation is
9-162 The primary driving force for natural convection is
9-144
9-164 A 4-m-long section of a 5-cm-diameter horizontal pipe in which a refrigerant flows passes through a room at 20ºC. The
pipe is not well insulated and the outer surface temperature of the pipe is observed to be -10ºC. The emissivity of the pipe
surface is 0.85 and the surrounding surfaces are at 15ºC. The fraction of heat transferred to the pipe by radiation is
D=0.05 [m]
T_infinity=20 [C]
nu=1.382E-5 [m^2/s]
Pr=0.7350
g=9.81 [m/s^2]
sigma=5.67E-8 [W/m^2-K^4]
Ra=(g*beta*(T_infinity-T_s)*D^3)/nu^2*Pr
