AI HOC QUOC GIA Tp. HCM - Trng H BACH KHOA

Khoa C KH

BM CONG NGHE NHIET LANH

GV ra e

THI HC K I NH 2011-2012

Mon Thi: Truyn Nhit

Thi gian : 90 phut

NgayThi : 05/01/2012

TS. H Anh Tng

----- -----

Ghi chu: SV c s dung tai lieu

Bi 1: ( 4 im)

Xt mt on dy in di 3m treo ng trong khng gian phng. Li dy in c

ng knh 12 mm c bc bn ngoi mt lp cch in c h s dn nhit = 0,1

W/(m.oC). Nu bit nhit trn b mt li dy in l tw = 90

oC, nhit mi trng

khng kh xung quanh l tf = 30oC, hi:

a) Trong trng hp khng bc lp cch in, h s ta nhit i lu t b mt li

dy in ra xung quanh l bao nhiu?

b) Phi bc li dy in vi lp cch in c b dy bao nhiu nhit lng truyn

ra ngoi t gi tr cc tr. Khi tnh xem nh c gi tr khng i ging cu a.

Bi 2: ( 3 im)

Mt vch phng din tch 6 m2 t thng ng c lm bng hai tm kim loi mng

cch nhau 25 mm. Bit nhit v en mt bn trong ca hai tm kim loi ln lt l

tw1 = 140oC, 1 = 0,65 v tw2 = 40

oC, 2 = 0,4. Hy xc nh nhit lng trao i qua vch

khi gia vch l khng kh.

Bi 3: ( 3 im)

Kho st mt thit b trao i nhit dng ng lng ng, lu ng ngc chiu. Nc

lm mt chy trong ng c nhit vo t2 = 20oC, nhit ra t2 = 80

oC, tc 2 = 2,5

m/s. Mi cht nng l du i bn ngoi ng c nhit vo t1 = 180oC, nhit dung ring

cp1 = 2,6 kJ/(kg.), lu lng G1 = 2,5 kg/s. Bit h s truyn nhit ca thit b k = 600

W/(m2.) v ng c ng knh trong dtr = 30 mm, hy xc nh din tch truyn nhit

ca thit b bng 2 phng php:

a) Phng php chnh nhit trung bnh logarit t .

b) Phng php hiu sut.

--- Ht ---

P N Bi 1: ( 4 im)

a) KTX: l = h = 3 m

NX: Cttt owfm 602

1 tra bng

11

26

3

2

3

10*3249,13090*10*97,18

3*81,9*003,0

t

glGr

Gr.Pr = 1,3249*1011*0,696=9,2213*1010 chy ri: C = 0,135; n = 1/3

Nu = C (Gr. Pr)n = 0,135*(9,2213*10

10)

1/3 = 609,9

9,53

029,0*9,609*

l

Nu W/(m

2.) (2 im)

b) Gi b dy lp cch in l 2 dD

Dd

D

ttq

fw

L1

ln2

1

cc tr khi

Dd

DDf

1ln

2

1

cc tr

mdDDf 011,0012,09,5

1,0*2

2

12

2

120

Nhit lng truyn ra ngoi cc tr khi bc dy vi lp cch in dy 11 mm

(2 im)

Bi 2: (3 im)

Trao i nhit bc x gia 2 vch:

444

2

4

1

21

100

27340

100

273140*

14,0

1

65,0

1

67,5*6

100100*

111

* TTCFQ oBX

WQBX 2183 (1 im)

Trao i nhit i lu trong khng gian hp:

+ KTX: l = = 0,025 m

+ NX: tf = 0,5*(tw1 + tw2) = 90oC tra bng

8630540140*10*1,22

025,0*81,9*00275,026

3

2

3

tgl

Gr

= 2,9*10-2 W/(m.)

= 18,97*10-6 m2/s Pr = 0,696

003,0

27360

1

-1

= 3,13*10-2 W/(m.)

= 22,1*10-6 m2/s Pr = 0,69

00275,0

27390

1

-1

Gr*Pr = 86305*0,69=59550,5 > 103 812,25,59550*18,0Pr**18,0 25,025,0 Grtd

088,0812,2*0313,0* tdtd W/(m.)

211240140*025,0

088,0*6*

*21 ww

tdDL tt

FQ

W

Tng nhit lng trao i qua hai vch: Q = QBX + QL = 2183 + 2112 = 4295 W

(2 im)

Bi 3: (3 im)

* PP chnh nhit trung bnh

3

)(

/1,988

./174,4502

8020

mkg

okgkJcCt

nuoc

nuocp

o

tb

Nhit lng trao i:

kW

tcd

tcGQ ptr

p

3,437

2080*174,4*1,988*4

03,0**5,2***

4**

2

222

2

2222

Mt khc:

CcG

QtttcGQ o

p

p 1136,2*5,2

3,437180

11

'

1

"

1111

Ct

Ct

o

o

9320113

10080180

min

max

C

t

t

ttt o5,96

93

100ln

93100

lnmin

max

minmax

23

55,75,96*600

10*3,437m

tk

QFtkFQ

(1,5 im)

* PP hiu sut 5,66,2*5,21 C kW/

288,7174,4*1,988*4

03,0**5,2

2

2 C kW/

kWttCQ 104020180*5,6* 21minmax

42,01040

3,437

max

Q

Q

89,0288,7

5,6

max

min C

C 7,0NTU 2

3

min 58,7600

10*5,6*7,0*m

k

CNTUF

(1,5 im)