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### Transcript of Heat Transfer

HEAT TRANSFER MECHANISM1. CONDUCTION

T Q =k.A. x

(W)

x Rcond = k . A

( C/W)

o

HEAT TRANSFER MECHANISM1. CONVECTION

Q =h.A(Ts - T)Rconv =1 h. A

(W)

( C/W)

o

Q =AsT

4 s

(W)

CONDUCTION

CONDUCTION

CONDUCTION (SOLUTION)The roof of an electrically heated home is 6 m long, 8 m wide, and 0.25 m thick, and is made of a flat layer of concrete whose thermal conductivity is k = 0.8 W/m.C. The temperatures of the inner and the outer surfaces of the roof one night are measured to be 15C and 4C, respectively. Determine the rate of heat loss

CONVECTION, Q = h A (Ts T )

CONVECTION (EXAMPLE)A 2-m-long, 0.3-cm-diameter electrical wire extends across a room at 15C, as shown in Fig. 133. Heat is generated in the wire as a result of resistance heating, and the surface temperature of the wire is measured to be 152C in steady operation. Also, the voltage drop and electric current through the wire are measured to be 60 V and 1.5 A, respectively. Disregarding any heat transfer by radiation, determine the convection heat transfer coefficient for heat transfer between the outer surface of the wire and the air in the room.

CONVECTION (SOLUTION) P = VI = (60 V)(1.5 A) = 90 W As = DL = (0.003 m)(2 m) = 0.01885 Q = h A (Ts T)

Q = . . As .T 4 4 Q = (Ts Tsurr )4 s

RADIATION (EXAMPLE)Consider a person standing in a room maintained at 22C at all times. The inner surfaces of the walls, floors, and the ceiling of the house are at an average temperature of 10C in winter and 25C in summer. Determine the rate of radiation heat transfer between this person and the surrounding surfaces if the exposed surface area and the average outer surface temperature of the

SIMULTANS MODE

Konveksi = Q1, Q5, Q8 Konduksi = Q2, Q6 Radiasi = Q3, Q4, Q7

COMPOSITE WALL Q = Ttot/Rth,tot Rth,tot = Rth,A + Rth,B + Rth,C

COMPOSITE WALLQ = Ttot/Rth,tot Rth,tot = Jabarkan (tugas)

COMPOSITE WALL + CONVEKSI

COMPOSITE WALL + CONVEKSI

SILINDER dan BOLA

SILINDER dan BOLASilinder :

Bola :

SILINDER dan BOLA Komposit

SILINDER Komposit

= (T1 T2)/Rtotal

SOAL LATIHAN Sebuah lapisan serat kaca tebalnya 13 cm. Beda suhu antara kedua permukaan adalah 85 oC. Konduktivitas termal serat kaca adalah 0.035 W/m.oC. Hitunglah kalor yang dipindahkan melalui bahan itu per jam per satuan luas.

JAWAB Diketahui serat kaca X = 13 cm; 1.4 W/m.oC.C; delta T 85 o Konduktivitas termal 0.035 W/m.oC. Q/A = 22.8 W/m2 = 22.8 W.h/m2

SOAL LATIHAN Sebuah dinding dilapisi isolasi yang memiliki konduktivitas termal 1.4 W/m.oC setebal 2,5 cm. Suhu bagian dalam isolasi adalah 315 oC. Suhu udara luar 38 oC. Dinding melepaskan kalor ke lingkungan secara konveksi. Hitunglah nilai koefisien perpindahan panas konveksi (h) agar suhu permukaan luar lapisan isolasi menjadi 41 oC.

JAWAB Q = Q konduksi = Q konveksi Q konduksi = k.delta T/ x = 15344 W/m2 h = (Q/A)/T = 5115 W/m2 OC.

SOAL LATIHAN Salah satu sisi dinding mempunyai suhu 100 oC, sedang sisi lainnya berada dalam lingkungan konveksi dengan T = 10 oC dan h = 10 W/m2.oC. Dinding memiliki nilai k = 1.6 W/m.oC dan tebalnya 40 cm. Hitunglah laju perpindahan kalor melalui dinding.

JAWAB R1 (konduksi) = 0.4/1.6 = 0.25 R2 (koveksi) = 1/10 = 0.1 R tot = R1 + R2 = 0.35 Q = delta T/ R tot = 90/0.35 = 257 W

KONDUKSI DG SUMBER KALOR BIDANG DATAR SILINDER BOLA

KONDUKSI DG SUMBER KALOR

KONDUKSI DG SUMBER KALOR

SIRIP (NEXT)

CONTOH SIRIP LIHAT TABEL 3-4

SIRIP SANGAT PANJANG

PANJANG SIRIP TERKOREKSI

EFISIENSI SIRIP

EFISIENSI SIRIP

EFISIENSI SIRIP

EFISIENSI SIRIP

EFECTIVITAS SIRIP

EFISIENSI VS EFEKTIVITAS

KONDUKSI DUA DIMENSI FAKTOR BENTUK KONDUKSI (S) : LIHAT TABEL 3-5 Q = k.S.T

KONDUKSI TRANSIEN Steady (Tunak): variabel tidak berubah sebagai fungsi waktu. Transient (Fana): variabel berubah sebagai fungsi waktu. Konduksi transien: konduksi yang berubah menurut waktu.

PENDEKATAN SOLUSI Menggunakan metode sistem tergabung (lumped system), dimana temperature of such bodies can be taken to be a function of time only, T(t) Parameter: Angka Biot (Bi) Kriteria Bi 0,1

PRINSIP

T sebagai fungsi t

Prosedur Hitung panjang karakteristik, Lc Hitung Angka Biot, Bi dan

CONTOH: Bola Tembaga

A 7.5 cm diameter orange is subjected to a cold environment. Assuming that the orange has properties similar to those of water at 20 C and that h = 11 W/m2 oC, determine the suitability of a lumped analysis for predicting the temperature of the orange during cooling.

Solution: From Table B-3 (SI), the thermal conductivity of water at 20 oC is 0.597 W/m-K. Also, for a sphere