1. No category

llniversity of
Al-Qadissiya / A
\
Class; Three
Subjecf; HeatTransfer
Examiner: Dr Khaled Al-Farhany
Time: 2 Hours
College of Engineering
Mechanical Engineering
Depaftmenf
\=-7<, -'/
Darer 08 /04/20,4
SecorTd Semester Exam/Firsf Round 20f3・ 20fイ
Note:
Dr rhru ourtriont onl,
Qr:
(5 marks)
A hot cylinder ingot of 50 mm diameter and 200 mm long is taken out from the furnace at 800oC and
dipped in water (at temperature equal to 30oC) till its temperature fall to 500oC. Then it is directly
exposed to air (at temperature equal to 30oC) till its temperature falls to 100 oC. Find the total time
required for the ingot to reach the temperature from 800 oC to 100 oC.
When ( k=60W/m oC, Cp=260J/m oC, p=800Kg/m3, hwater=200 Wm2oC and hair=20 1ry7m2 oC).
Qz:
(5 marks)
The components of an electronic system are located in a 1.5-mlong horizontal duct whose cross section is 20 cm * 20 cm. The
components in the duct are not allowed to come into direct contact
with cooling air, and thus are cooled by air at 30oC flowing over
uectrrrnic
'"i:fi['t'
the duct with a velocity of 200 m/min.If the surface temperature
200静
ofthe ductis not to exceed 65。 C,determinethetotalpttirrating Air _、
ofthe electronic devices that can be lnounted into the d
―
Please use the data below to answer this question:
Air Properties at(47.5° C):v=1・ 774x10‐ 5m2/s,卜 0.02717W/m.OC,
IIFtlillal.。 w: 再
=0.102*ReD°
For internal flow: NuD
:
0.O23
lil∼
′C
/1二讐
裁
-11
__、 レ
65°
::/
三
ノ
/15話
ツ
￨■ IL城 1111」リ
1/1綱
L「 IT
675 Pr1/3
* nert
●
tooc
Pro'a
Qr:
(5 marks)
Consider a vertical plate of dimension 0.25 m*0.50 m
atT-=20
T, =20
that is at T,=100 oC in a quiescent environment at
the
'C. In the interest of minimizing heat transfer from the
preferred? What
plate,
plate, which orientation, (A) or (B), is preferred?
[----l
| [l
iI
I I I I,
r
I.', II I I
III r,
I]
ll
istheconvectionheattransferfromthefrontsurfaceofllI--J*
the plate when it is in the preferred
Air
Properties
at (60° C):
卜 0。 0285W/m.OC,PFO。 709
Use:両 =0。 707*θ rL1/4 Pr1/2
orientation?.
V=1.89x10 5m2/s,
I
l
I
I
I
I
'
I■・・■ ■■
Q4:
Orientation
A
I
｀U「 lenla1lon
,,`_
ientauon bB ,・
■il,亀
(5″
α″
λリ
C in a
A horizontal pipe 6 m long and 12.5 cm in diameter is lnaintained at a temperature of 150。
large room where the air is 20。 C and l atmo The walls ofthe roonl are at 38。 C.Assume that ε=0。 7
for the pipe.How much heatis lost by the pipe through both convection and radiation?
Usc:Air Properties at(60° C):V=1・ 89x10‐ 5m2/s,卜 0.0285W/m.° C,Pニ ー0。 709
雨
葡
石=0.53*G■ F/4 Pr・ /2
く
ノ
〔
留蒻
/.′
蒻厖ル′
Class; Three
Subiecf; HeatTransfer
Examiner: Dr Khaled Al-FarhanY
ll niversity of Al-QadissiYa
College of Engineering
Time: 2 Hours
Date: 08 l04l 2014
Mechanical Engineering Depaftment
or Second semester Exam/FirsI Ro“ nd 20'3‐ 2014
So′ ufiors
Ql:
“
Bnrt ntrnlhcr.
4=li==(￨)x(指
￨=(1)〔 11:::) =(1)(1))=〔
A、 二 億 1せ S th■ ■ t》 .1,inl● ■γ:l thc■
Tllc lo‖
tt,■ lin卜 ぜ cュ tc● 1:11じ
tl til,1●
in air)nttd addittg ttucil tllat
嵐
1ヽ 1:r輿
ニ
ユ
響号
)(1≧
)=0,04106
:rc雨 ■■,● c Can hC nc8:● ●1● こ■■J tu籠 ■):L● ●ry
1、 ■
1,yc:】
lゞ
￨:機:i:18■
,(tilヽ
:tご
lC Ft11liFCtl in警
t:● 1卜 ●●
t ti:ヽ
=tti、
lc r● lti:=Fd
`1lcri::I,こ
τ==:+て 1.
oFl〔 1=゛ 1)=:=Fι =2π x
‖ca賀だむ
).ll'S “
Xll,1=機 .0314m・
▲Ｔ■υｔ、
ド(■ )==A・ =ι =冗 XO.25燿
｀
x筆 ,1澤 3.91'X il1 4鸞
まrittion w,lh“ ヽ
rtぜ 篠
littttttrt・ 、
:だ ol
1ど
ヽ
rtヾ サ ::tlCく 「
lミ
11、
ill lttttter is tiVむ n
:lC∞ itXd
lo tillle '1｀
wltt・
byIScc
5僣 で
ト
Ftis.4・ 5)
==Xpl一
鸞
τ:]
詩
・
普 ●F●
:、
:′
毎心 蒙
帥
ρ貸
l(lll x:〕
:0314
=fl.099'
`)X3.92'x ll1 4 x 21bf)
・
=・
掛
鱒
:鐸
:
・ ヽ
・
・
時議
5"嬌 C:
:
ril・
ヽやヽ
壁澪
゛集
t.●
`
襟1.0お り
11.1)'99■ 1=0.494
●1.
ヽ=朧 轟
OF.
i“ ■
1■ 1〔 華ゝ
Jlせ 釧
--lr*t,-J0'C
rY'
S
trξ v:轟 震 it■ t Willt r薇 澪 Ft
壼i tittl=智 :lt織 奪●●lett i瀑 ●1●
Fi準
▲︱・一
りヽ
■
`―
Oi
1ヽ
き:ヽ ●■ 卜,lS難 ●
.416,,
F`魔 .4.￨.
礼ハ
30 X O.01:4
pX' - t$tHlx 3.91? x l0 { x ?$tt
`奪
uhr:rc,
100-30
-―
・
―
鸞
岬
ゆ象 む醐
,',
:■ .1.5。
Tt■ titl
ぜ
鏡岬
｀
=辞
螂 ヽ
鸞
(]r
犠鱗
/
ir ilt.J.$,
i- 霊
修.716
繭
轟 ::や 0凛 5
・ ‐
n卜 y
ll:lle tt'i,ぶ lヽ 書
τ=て ,寺
=:=4.'1+1ヽ
ヽ =縦
民).CI事
l‐
組
“
)
撃5.54よ tli 3藤 9籠 量織饉機、ィ(A“ ヽ。
Q2:
Pmperlies The properties of air at I atm and the film temperature
('l, + T -)/2 = (65+30)/2 = 47.5"C are (Table A-15)
of
k=002717W/m° C
υ=1774x105m2/s
Pr=07235
ArlaいiSThe Rcynolds numbcr is
Rc=li22=晰
=3758x104
Using the relation for a square duct from Table 7-1, the Nusselt number is determined to be
!2
Nu =
= o. t oz Reo
k
The heat transfer coefficient is
o=
*
D
lL
=
675
pr
3
ri
=
0. I 02(3.758 x l0
o
5
)o-u, (0.7 23 5)t,
3
=
o'ozzlz-wl* "c (t
n.2) =r5.24 w/m2..c
O2m
Then the rate ofheat transfer fiom the duct becomes
,4 = (a x 0.2 m)(1.5 m) =
Q=
1.2 m2
n4 Q, - T*) = (t s.24 W tm?..C;1
1.2 m
2
116s -
30rC
:
640.0
w
Q::
For minimizing heat transfer,tよ cA
Gr
-
Nu1
sRGJdL'
-
9.797 *
981●
=
亘二二
:ζ?Ill:メ
Grr'ln
生2」こ
=1=92033x107
Pr1/2=58.3
h-kNuL L
0.0285*583
025
-
6.6W/m2K
q_LAAT_ 66*025*05*80=6646レ
Qq:
I=Olilili=423K
Free convectlon
「
ι=6●
1=20° C=293K
ズrad)=α ス(■ 4_.4)=(5 669 xlo 8)π
4(tOtal)=2296+3027=5323W
(。
写 =38° C=311K
125)“ )(4234_3H4)=3027W
/
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lz.2