9_external forced convection
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Perpindahan Panas Konveksi
External Forced Convection
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Pengantar
KONDUKSI
KONVEKSI
RADIASI
PERPINDAHAN PANAS
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Perpindahan Panas Konveksi
Konveksi bebas
(natural convection)
Konveksi paksaan
(forced convection)
External forced convection
Internal forced convection
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Konveksi (Convection)
Transfer energi antara suatu permukaan padatan dengan fluida(gas atau cairan) yang bergerak dan melibatkan efek konduksisekaligus pergerakan fluida. Semakin cepat pergerakan fluida,semakin besar laju perpindahan panas konveksi.
Dengan Ts>T∞
Koefisien perpindahan panas konveksi (h) adalah suatu parameteryang diperoleh melalui eksperimen dan nilainya spesifik untuksetiap sistem tergantung dari beberapa variable seperti geometripermukaan padatan, sifat-sifat fluida, kecepatan alir, dankarakteristik aliran fluida.
Hukum Newton:
TThA sQ
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Perpindahan Panas Konveksi Paksaan (forced convection)Perpindahan panas konveksi paksaan adalahperpindahan panas yang disertai dengan aliranfluida dimana fluida tersebut mengalir karenaadanya gaya dari luar (external force), atau daridalam fluida itu sendiri (internal force) karenadipompa atau ditekan.
Hukum Newton untuk konveksi paksaan:
)T(Tw dengan
) Th.A.(TQ w
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Koefisien Perpindahan Panas Konveksi (h) pada Konveksi Paksaan
Aliran fluida pada perpindahan panas konveksi paksaanterjadi karena adanya energi mekanis yang ditambahkanke fluida sehingga fluida tersebut mengalir. Semakincepat aliran fluida, maka laju perpindahan panassemakin besar.Secara umum, koefisien perpindahan panas konveksipaksaan bernilai lebih besar dibandingkan koefisienperpindahan panas konveksi bebas.
) Th.A.(TQ w
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Dengan adanya pengaruh dari kecepatan aliran, makapenentuan koefisien perpindahan panas konveksimelibatkan suatu bilangan tak berdimensi yang terkaitdengan mekanika fluida, yaitu bilangan Reynolds
denganρ : densitas/rapat massa fluida, kg/m3
v : kecepatan aliran fluida, m/sL : panjang karakteristik (untuk aliran dalam pipa,panjang karakteristik=diameter pipa), mµ : viskositas fluida, kg/m.sν : viskositas kinematik fluida, m2/s
Le
vvLR
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Selain bilangan Reynolds (Re), koefisien perpindahanpanas pada konveksi paksaan juga dipengaruhi olehbilangan Prandtl
Catatan: untuk forced convection, sifat-sifat fisis fluidadapat didekati pada suhu rata-rata aritmatik ataulogaritmik tergantung dari kasusnya.
k
Cp.
heat ofy diffusivitmolecular
momentum ofy diffusivitmolecular Pr
termalsdifusivita.
kinematis viskositas
Cp
k
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Koefisien perpindahan panas konveksi paksaan rata-ratauntuk berbagai situasi dapat didekati dengan persamaanempiris sebagai berikut:
dengank : konduktivitas termal fluidaL : panjang karakteristikRe : bilangan ReynoldsPr : bilangan PrandtlC, m, n : konstanta
Nilai C m, dan n tergantung pada geometri benda dandinamika aliran fluida.
nPrCRek
hLNu m
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External forced convection
External forced convection terjadi ketika fluida (yang memilikibeda suhu dengan permukaan suatu benda) mengalir melewatipermukaan luar dari benda tersebut. Aliran fluida disebabkankarena gaya dari luar (dihembus/dipompa/dengan kecepatanlebih dari 1 m/s) dapat sejajar atau tegak lurus benda.Koefisien perpindahan panas konveksi untuk kasus-kasus yangmelibatkan external forced convection diperoleh melaluipersamaan empiris.
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External forced convection
1. Aliran melalui plat
L
Aliran laminer
Aliran turbulen
Atau bisa juga menggunakan korelasi rata-rata
Pr0,664ReNu 310,5
Lk
hL
Pr0,037ReNu 310,8
Lk
hL
5x10Re 5
75 10 Re5x10
60Pr0,6
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External forced convection
2. Aliran tegak lurus silinder
54
85
41
31
282000
Re1
Pr
0,41
Pr0,62Re3,0Nu
32
0,5
k
hD
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External forced convection
3. Aliran tegak lurus bola
4
1
32
21 4,0PrRe06,0Re4,02Nu
sk
hD
dengan µ∞ adalah viskositas fluida pada suhu fluida,danµs adalah viskositas fluida pada suhu permukaan bola
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External forced convection
4. Aliran tegak lurus melalui jaringan pipa (tube banks)
LS
TS
1,Tv
D
vDS
Sv
T
T
max
LS
TS
1,Tv
D
DS
(a) in-line (b) staggered
v
DS
Sv
D
T
2max
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Arrangement Range of Re Corelation
in-line
0-100
100 - 1000
1000 - 2x105
2x105 - 2x106
staggered
0-100
100 - 1000
1000 - 2x105
2x105 - 2x106
DvmaxRe
25,036,04,0 PrPr/PrRe9,0 sDNu
25,036,05,0 PrPr/PrRe52,0 sDNu
25,036,063,0 PrPr/PrRe27,0 sDNu
25,04,08,0 PrPr/PrRe033,0 sDNu
25,036,04,0 PrPr/PrRe04,1 sDNu
25,036,05,0 PrPr/PrRe71,0 sDNu
25,036,06,02,0
LT PrPr/PrRe/SS35,0 sDNu
dengan
25,036,08,02,0
LT PrPr/PrRe/SS031,0 sDNu
TABLE 7–2Nusselt number correlations for cross flow over tube banks for NL 16 and
0.7 Pr 500 (from Zukauskas, Ref. 15, 1987)*
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Tabel 7-2 hanya berlaku untuk susunan pipa minimal 16 baris.Jika jumlah baris kurang dari 16, maka diperlukan faktorkoreksi dengan persamaan
DND FNuNuL,
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untuk kasus fluida yang mengalir melalui jaringan pipa, sifatfisis fluida ditinjau pada suhu rata-rata aritmatik (Tm) dari suhumasuk (Ti) dan suhu keluar (Te)
dan laju perpindahan panas
dengan ΔTln (logarithmic mean temperature difference):
Ts = suhu permukaan pipa
2
eim
TTT
ielns TTCpm.Δh.AQ T
i
e
ie
is
es
isesln
T
Tln
TT
TT
TTln
TTTTΔT
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Suhu keluar jaringan pipa (Te) dapat diestimasi denganpersamaan:
dengan
Dan
Dengan NT adalah jumlah pipa ke arah tegak lurus aliran
DLNAs
p
s
Cm
hA
expTTTT isse
LSNvm TT
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A long 8-cm-diameter steam pipe whose external
surface temperature is 90°C passes through some
open area that is not protected against the winds.
Determine the rate of heat loss from the pipe per unit
of its length when the air is at 1 atm pressure and 7°C
and the wind is blowing across the pipe at a velocity of
50 km/h.
QUIZ 8
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PR
In an industrial facility, air is to bepreheated before entering a furnace bygeothermal water at 120ºC flowingthrough the tubes of a tube bank locatedin a duct. Air enters the duct at 20ºC and 1atm with a mean velocity of 4.5 m/s, andflows over the tubes in normal direction.The outer diameter of the tubes is 1.5 cm,and the tubes are arranged in-line withlongitudinal and transverse pitches of SL =ST = 5 cm. There are 6 rows in the flowdirection with 10 tubes in each row, asshown in Figure. Determine the rate ofheat transfer per unit length of the tubes.
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