bài 2 truyỀn nhiỆt Ống lỒng Ống.doc

7/28/2019 bi 2 TRUYN NHIT NG LNG NG.doc

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TRUYN NHIT NG LNG NG

I. MC CH

- Lm quen vi thit b truyn nhit ng lng ng- Xc nh h s truyn nhit trong qu trnh truyn nhit gia hai dnglnh, nng ngn cch bi vch ngn kim loi ch chy khc nhau- Thit lp cn bng nhit lng.

II. C S L THUYT

Qu trnh truyn nhit trong thit b dng ng lng ng l mt v d cas truyn nhit phc tp. y din ra s trao i nhit gia hai lu chtc ngn cch bi vch ngn kim loi, bao gm truyn nhit i lu t

dng nng n vch, dn nhit qua thnh ng kim loi v i lu nhitgia dng lnh vi ng.

1. Phng trnh cn bng nhit lng cho hai dng lu cht

Q = G1C1(tV1-tR1) = G2C2(tR2-tV2), W (1)

Trong :

G1, G2: Lu lng dng nng v lnh, kg/s.C1, C2: Nhit dung ring trung bnh ca dng nng v dng lnh, J/kg.KtV1, tR1: Nhit vo v ra ca dng nng, CtV2, tR2: Nhit vo v ra ca dng lnh, C

2. Phng trnh biu din qu trnh truyn nhit

Q = K1.tlog.L, W (2)

L: Chiu di ng, m

K1: H s truyn nhit di,W/m. Ctlog: Chnh lch nhit trung bnh logarit, C

3. chnh lch nhit trung bnh logarit


2/17

tlog =nho

lon

nholon

t

t

tt

ln(3)

Trong :

tlon: H s nhit gia dng nng v dng lnh u vo hoc ra cgi tr lntnho: H s nhit gia dng nng v dng lnh u vo hoc ra cgi tr b.

4. H s truyn nhit di l thuyt, K1* tnh trn 1m chiu di ng

K1* =ca

cau

ngtr

ng

tr d

r

dd

d

d+++

21

1ln

2

11

Trong :

dng, dtr : ng knh ngoi v trong ca ng truyn nhit, m. : h s dn nhit ca ng, W/m. C

1, 2 : h s cp nhit, W/m. Crcau : nhit tr ca lp cu, m2.s0.C/Jdcau : ng knh lp cu, m

5. H s cp nhit 1, 2 gia vch ngn v dng lu cht

Nu =A.Rem Prn Rt

1

25,0

Pr

Pr

(5)

Cc h s A, m, n, 1, R l cc h s thc nghim ph thuc vo cc yut sau:

Cc ch chy ca dng lu cht S tng quan giu dng chy v b mt truyn nhit c trng b mt truyn nhit ( nhm, hnh dng, )


3/17

III. THIT B TH NGHIM

H thng thit b th nghim c hai kiu kt cu b mt truyn nhit nh

sau: Kiu A: Loi ng lng ng m lu cht chy ngang mt ngoi ca ngtrong. Hai dng chy c phng vung gc vi nhau.

Kiu B (B1, B2): Loi ng lng ng n gin, lu cht chy dc bmt ngoi ca ng trong. Hai dng chy c phng song song vinhau.

Kch thc ng:

Kiu ng ng knh (mm) Chiu di (mm)ng trong ng ngoi

A 17/21 30/24 1020B 17/21 30/24 1020

S h thng thit b (xem hnh):

Ch thch:

: K hiu dng nng vo v ra.> : K hiu dng lnh vo v ra.A : B trao i nhit dng dng chy vung gc.B1 : B trao i nhit dng dng chy song song ngc chiu.B2 : B trao i nhit dng dng chy song song cng chiu.1A, 1B, 1C : Ln lt l van cp nc tng, van cp nc cho dng lnh,van cp nc cho bn gia nhit.2, 3 : Van chnh lu lng dng nng.4, 5, 6 : Van ng m dng nng vo cc b trao i nhit.7 : Van chnh lu lng dng lnh.8, 9, 10 : Van ng m dng lnh vo cc b trao i nhit.11 : Van x y bn.


4/17

IV. PHNG PHP TH NGHIM Sau khi chun b v lm quen vi thit b ta o cc i lng: lu

lng dng nng v dng lnh, nhit vo v ra ca cc dng2211 ,,, RVRV tttt .

Trnh t th nghim:Lu : Cc van 1A, 1B khi m ch m 30 - 45. Khi m hoc ngbt k mt van no sinh vin u phi xoay tht t t, tuyt i khng cng m t ngt. Trc khi th nghim tt c cc van u v tr ng.

o M van 1A tht t t (ch xoay mt gc khong 30).o ng van 11. T t m van 1C cho nc vo y bn cha, sau

kha van v y np bn cha li.o M cu dao ngun, bt cng tc GUN trn hp iu khin ln.o Bt cng tc GIA NHIT ln. n hot ng mu sang.

Cm gia nhit hot ng.o Sau khi nc trong bn gia nhit si ( khong t 30-60 pht),

cng tc gia nhit t ng ngt, lc bt u lm th nghim.o M van 3, bm nt chy ca BM (mu xanh), n hot ng

ca bm (mu ) sng. Mm ly tm hot ng v bm nc tun hon.o Ch cho h thng n nh, m mt trong cc cp van tng

ng: 4/8, 5/9, 6/10. iu chnh cp van 2, 3 iu chnh lu lng dngnng v chnh van 7 iu chnh lu lng dng lnh theo yu cu. Ch slu lng c c trn lu lng k.o Ch khong 5-10 pht, khi qu trnh n nh, n v gi mt

nt (mu vng) cn thit trong bn nt trn hp iu khin c nhit tiv tr tng ng.o Sau khi o xong tt c cc thng s cn thit, tt cng tc GIA

NHIT, tip theo tt BM, tt cng tc ngun CB. M np bn gia nhitcho nc ngui bt. Kha tt c cc van li. i khi nc ngui ri mvan 11 x ht nc trong bn cha gia nhit.


5/17

V. Kt qu th nghim:

B trao i nhit A:

Ln Lulng Nng Lnhtv tr tv tr1 4 62.8 60.2 29.7 36

6 69.7 62.2 29.8 35.58 76.8 64.9 29.9 35.5

2 4 83.2 73.5 30.3 39.36 83.7 75 30.3 38.38 83.5 75.8 30.3 37.4

3 4 83 75 30.2 37.26 83.1 74 30.3 37.58 83.2 74.7 30.1 37.5

Ta c bng gi tr trung bnh i vi b trao i nhit A:

G1 G2 Nng Lnhtv tr tv tr

4 4 76.33 69,56 30.06 37.56 6 78.83 70.4 30.13 378 8 81.166 71.8 30.1 36.8

B trao i nhit B1: ngc chiuLn Lu

lngNng Lnh

tv tr tv tr1 4 86.7 72.5 30.1 38.9

6 82.2 72.9 30.3 38.18 83.9 75.5 30.2 37.6

2 4 83.1 74.5 30.3 39.7

6 83.2 74.5 30.3 35.58 83.1 75.1 30.4 383 4 83.1 75 30.3 37.7

6 83 74.7 30.1 35.48 83 75.5 30.2 37.5


6/17

Ta c bng gi tr trung bnh 3 ln o i vi b trao i nhit B1:

G1 G2 Nng Lnhtv tr tv tr4 4 84.3 74 30.23 38.766 6 82.8 74.03 30.23 36.338 8 83.33 75.36 30.266 37.7

B trao i nhit B2: cng chiuLn Lu

lngNng Lnh

tv tr tv tr1 4 83 74.6 30.2 38.9

6 83.7 75.5 30.3 388 83.7 76.1 30.3 37.6

2 4 82.7 74 30.4 38.66 83 75 30.4 38.48 83.1 75.4 30.4 37.7

3 4 83 74.1 30.3 30.86 83 75 30.5 37.58 83.2 75.5 30.1 38

Ta c bng gi tr trung bnh 3 ln o ca b trao i nhit B2:

G1 G2 Nng Lnhtv tr tv tr

4 4 82.9 74.23 30.3 36.16 6 82.23 75.16 30.4 37.968 8 83.33 75.66 30.26 37.76

VI. Tnh ton:1. Nhit lng Q

Q = G1C1(tV1-tR1) = G2C2(tR2-tV2), W


7/17

i vi dng nng ca 3 thit b A, B1v B2:

Thit b G1 tv1 tv2 Cp(j/kg.o)

Q1 (w)

A4 76.33 69,56 4203.4 1138286 78.83 70.4 4203.5 2126138 81.166 71.8 4203.5 103103

B14 84.3 74 4203.5 1731846 82.8 74.03 4203.5 2211888 83.33 75.36 4203.5 268015

B24 82.9 74.23 4203.5 1457776 82.23 75.16 4203.5 1783128 83.33 75.66 4203.5 257926

i vi dng lnh ca 3 thit b A, B1v B2:

Thit b G2 tr1 tr2 Cp(j/kg.o)

Q2 (w)

A

4 30.06 37.5 4178.4 124349

6 30.13 37 4178.4 1679718 30.1 36.8 4178.4 223962

B14 30.23 38.76 4178.4 1425676 30.23 36.33 4178.4 1529298 30.266 37.7 4178.4 248497

B24 30.3 36.1 4178.4 969396 30.4 37.96 4178.4 1895328 30.26 37.76 4178.4 250704

Ta c tn tht nhit:

Thit bQ1 (w) Q2 (w) Q ( W)

| Q2 - Q1 |

A113828 124349 10521212613 167971 44642103103 223962 120859173184 142567 30617


8/17

B1 221188 152929 68259268015 248497 19518

B2145777 96939 48838178312 189532 11220

257926 250704 72222. Tnh Dtlog, K

Thit b tv1 tr1 tv2 tr2 Dtlog K1

A76.33 69,56 30.06 37.5 41.597 1825.0878.83 70.4 30.13 37 41.045 3423.03

81.166 71.8 30.1 36.8 43.019 1597.79B1

84.3 74 30.23 38.76 43.985 2624.8982.8 74.03 30.23 36.33 44,723 3297.1583.33 75.36 30.266 37.7 44.922 3977.48

B282.9 74.23 30.3 36.1 45.349 2143.0382.23 75.16 30.4 37.96 44.514 2670.583.33 75.66 30.26 37.76 33.597 5118.03

tlog =nho

lon

nholon

t

t

tt

ln

Q = K1.tlog.L, W t y => K vi L= 1020 mm

3. Tnh h s cp nhit 1 v 2a. Tnh ch chy ca lu cht bng chun s Re:

=

l.Re

Ta c bng s liu:

Thitb

t1tb(oC)

t2tb(oC)

D1.103(Ns/m2)

D2.103(Ns/m2)

D1(w/mK)

D2(w/mK)

A76.33 30.06 0.3574 0.7089 0.672 0.61378.83 30.13 0.3543 0.7016 0.675 0.628


9/17

81.166

30.1 0.36090.6947 0.677 0.629

B184.3 30.23 0.3609 0.6685 0.677 0.63282.8 30.23 0.3436 0.7085 0.676 0.627

83.33 30.266 0.3416 0.7016 0.677 0.628

B282.9 30.3 0.3436 0.7085 0.676 0.62782.23 30.4 0.3521 0.6814 0.675 0.63183.33 30.26 0.3416 0.7016 0.677 0.628

Bng Re

Thitb

D1.103(Ns/m2)

d1(m) w1 Re1 D2.103

(Ns/m2)d2(m) w2 Re2

A0.3574 0.017 299.5 56.98 0.7089 0.017 299.5 28.730.3543 0.017 449.4 86.19 0.7016 0.017 449.4 43.530.3609 0.017 599.1 112.8

8 0.69470.017 599.1 58.64

B10.3609 0.017 299.5 56.43 0.6685 0.017 299.5 30.460.3436 0.017 449.4 88.88 0.7085 0.017 449.4 43.10

0.34160.017 599.1 119.2

6 0.70160.017 599.1 58.06

B20.3436 0.017 299.5 59.27 0.7085 0.017 299.5 28.74

0.3521 0.017 449.4 86.73 0.6814 0.017 449.4 44.82

0.34160.017 599.1 119.2

6 0.70160.017 599.1 58.06

Vi l = dtd=4d=0.068 mb. Xc nh chun s Nu cho phng chy ngang kiu A:

Do : 5 < Re < 103

Nn :0,25

0,5 0,38 Pr

0, 5. Re . Pr . Prt

Nu

=

Prv

a

=

Vi : : h s nht ng hc ca lu cht, m2/s


10/17

: h s dn nhit ca lu cht, m2/s.

Thit b tv1 tr1 tv2 tr2 1 2

A 76.33 69,56 30.06 37.5 57.82 53.178.83 70.4 30.13 37 57.89 53.181.166 71.8 30.1 36.8 58.11 53.1

B184.3 74 30.23 38.76 58.25 53.182.8 74.03 30.23 36.33 58.17 53.183.33 75.36 30.266 37.7 58.21 53.1

B282.9 74.23 30.3 36.1 58.20 53.182.23 75.16 30.4 37.96 58.20 53.183.33 75.66 30.26 37.76 58.21 53.1

Ta c bng gi tr:

Thitb

Re1 Pr 1 Prt1 Nu Re2 Pr2 Prt2 Nu

A56.98 1.88 5.400 3.68 28.73 4.050 5.400 4.2486.19 1.88 5.400 4.53 43.53 4.450 5.400 5.54112.88 1.89

5.400 5.20 58.644.700

5.400 6.66

c. Xc nh chun s Nu cho phng thc chy dc theothan ng kiu B

Thitb

Re1 Pr 1 Prt1 Nu Re2 Pr2 Prt2 Nu

B156.43 1.92 5.400 3.72 30.46 4.930 5.400 4.9488.88 1.9 5.400 4.63 43.10 4.400 5.400 5.47

119.26 1.9 5.400 5.36 58.06 4.500 5.400 6.45

B259.27 1.91 5.400 3.79 28.74 4.700 5.400 4.6686.73 1.93 5.400 4.62 44.82 4.850 5.400 5.94119.26 1.9

5.400 5.36 58.064.400

5.400 5.47


11/17

Thit b t

v1 t

r1 t

v2 t

r2 t2

A76.33 69,56 30.06 37.5 38.83 39.578.83 70.4 30.13 37 41.83 40.2781.166 71.8 30.1 36.8 44.366 41.69

B184.3 74 30.23 38.76 45,54 43.7782.8 74.03 30.23 36.33 46.47 43.883.33 75.36 30.266 37.7 45.63 45.094

B282.9 74.23 30.3 36.1 52.6 38.1382.23 75.16 30.4 37.96 51.83 37.283.33 75.66 30.26 37.76 53.07 37.9

Tnh sai s:

Thitb

t2* Pr1 Prt1 1 2 Nu1 Nu2

A 15.77 29.62 2.53 2.6 5.397 6.1010.0108 0.0150

6.72 35.66 2.28 2.35 5.333 6.903

0.010

7 0.0170

10 33.98 2.38 2.44 7.401 3.4560.0148 0.0084

B1 19.73 22.02 2.73 2.79 7.278 5.2320.0145 0.0129

7.85 34.62 2.17 2.26 7.157 6.1960.0143 0.0152

10.9 32.59 2.32 2.48 7.064 6.9710.0141 0.0172

B2 8.07 30.88 2.46 2.52 8.339 3.479

0.016

6 0.0085

10.23 30.77 2.59 2.66 7.797 5.2850.0155 0.0129

12.26 30.21 2.1 2.2 8.032 6.2130.0160 0.0156

4. H s truyn di l thuyt :


12/17

K1* =ca

cau

ngtr

ng

tr d

r

dd

d

d+++

21

1ln

2

11

5. Lp bng tnh K1* v K1 theo Re

Thitb

dn dtr 1 2 K l*

A34 21 57.82 53.1 3.65334 21 57.89 53.1 3.63534 21 58.11 53.1 3.662

B134 21 58.25 53.1 3.667

34 21 58.17 53.1 3.66934 21 58.21 53.1 3.666

B234 21 58.20 53.1 3.66534 21 58.20 53.1 3.66534 21 58.21 53.1 3.666


13/17

Thit b Re Kl* K1

A

56.98 3653 1825.08

86.19 3635 3423.03112.88 3662 1597.79

B156.43 3667 2624.8988.88 3669 3297.15119.26 3666 3977.48

B259.27 3665 2143.0386.73 3665 2670.5119.26 3666 5118.03

6. V th K1* v K1 theo Re

Theo h thng A:

i vi h thng B1:


14/17

i vi h thng B2:

VII. Bn lun:1. Tn tht nhit c ng k khng ti sao:


15/17

Tn tht nhit l ng k th hin vic nhit lng mt i cadng nng ln hn nhiu so vi nhit nhn vo ca dng lnh. Nguynnhn:

Nhit lng truyn cho ng lm ng nng ln.

Ti nhng v tr dng nng chy khng c dng lnh bao quanhc tn tht do qu trnh truyn nhit t dng nng qua ng n mitrng xung quanh do khng c bc lp cch nhit.

Trn ng ng do lu ngy c ng cn bn cng gp phnlm tn hao nhit lng.

Tn tht nhit qua cc van do van b r r sau mt thi gian sdng.

Sai s khi c nhit trong qu trnh th nghim lm kt qutnh ton khng chnh xc.

S mt mt nhit dc chiu di ng. ng i cng di lngnhit tn tht cng nhiu. ng ng lm bng ng khng bc lpcch nhit nn s tn hao nhit ra mi trng xung quanh cng nhiuhn so vi nhng vt liu khc.

y, s mt mt nhit dng lnh khng ng k. V tuy cs truyn nhit nhng nhit dng lnh tng khng ng k nnchnh lch nhit vi ng cng khng ng k c th b qua.

2. Mc sai s, nguyn nhn gy ra sai s trong lc lm thnghim? bin php khc phc:

a. Nguyn nhn gy ra sai s trong qu trnh th nghim: Sai s do xc nh lu lng dng nng v lnh: lu lng

dng nng v dng lnh d c iu chnh cng khng t ctrng thi n nh tuyt i, c ln nhiu bt kh.

Sai s do qu trnh c nhit : c nhit khng cng thiim (cha c s cn bng nhit lng), nhit khng n nh.

Bm lu cht hot ng khng n nh (do in khng nnh).

Sai s do s r r lu lng cht lng trong h thng ng: tinhng v tr ng ni, van... khng kn chy nc ra ngoi lm gim lulng.

Sai s do cc ng khng c bc lp cch nhit gy tht thotnhit do s trao i nhit gia ng vi mi trng bn ngoi.

Sai s khi chnh van II thay i chiu dng chy o lulng: chnh van khng hp vi ng gc 450, dng chy khng c


16/17

i hng hon ton, mt phn dng nng s ha trn vi dng lnhgay tn tht nhit.

Sai s do nc th nghim khng l nc tinh khit nhng ccthng s c dng trong tnh ton l ca nc tinh khit.

Sai s h thng do dng c o khng chnh xc. B qua nh hng ca lp bn trn thnh ng.

b. nh hng ca sai s n s khc bit gia kt qu thcnghim v l thuyt:

Dn n sai s dy chuyn trong qu trnh tnh ton. T nhit c ta suy ra tlog. Do c sai s dn n Kl, Q sai

do:

Kl = LtQ

.log

Q = G.C.

t Nhit c sai --> tra h s sai --> tnh sai --> sai h struyn nhit di l thuyt Kl*.

ng thi khi tra khi lng ring khng chnh xc dn n sais khi tnh Re lm mt tnh chnh xc ca gin .

c. Bin php khc phc: Thao tc th nghim phi chnh xc, chnh van II phi ng v

lc chnh phi n vo nc khng r r ra ngoi. C th tin hnhth nhiu ln cho thun thc trc khi th nghim chnh thc.

ng h o nhit , lu lng k n nh mi c, cchnh xc s liu bng cch c gi tr trung bnh trn ng h o. Nc dng trong qu trnh th nghim cng sch cng tt. Kim tra v tr cc van trc khi lm th nghim. Thng xuyn v sinh ng ng. Tin hnh th nghim lp li nhiu ln tm ra gi tr chnh

xc nht vi sai s nh nht.3. So snh h s truyn nhit di thc nghim Kl vi h s truyn

nhit di l thuyt Kl*:

y Kl v Kl*

khng ging nhau. Nguyn nhn:

b

b

ngtr

ng

tr

l

d

r

dd

d

d

K

+++

=

21

*

1ln.

2

11


17/17

B qua gi trb

b

d

r

: khng xc nh c b dy ca lp cu

(bn) v nhng nh hng ca n i vi qu trnh truyn nhit (b

quab

b

d

r

K s ln).

Xc nh h s dn nhit cho ng bng ng nguyn chtnhng thc t nguyn liu lm ng dn khng nguyn cht.

Cc gi tr xc nh c lun mc phi sai s do tnh tonnhit vch, nhit lu cht xc nh cc chun s Nu, Pr...

Nhit lng tn tht vn cha b ni sai s trong qu trnh lmth nghim.

y qu trnh truyn nhit l phc tp nhng khi tnh ton chk n nhng nh hng chnh chng hn b qua s i lu t nhintrong dng chy mng ng B dn n sai s cao...

bài 2 truyỀn nhiỆt Ống lỒng Ống.doc

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