INTRODUCTION….
Advanced Heat Transfer Eswanto., ST.,M.Eng Institut Teknologi Medan
THE FIRST LAW OF THERMODYNAMICS…
The first law of thermodynamics, also known as the conservation of energy principle, states that energy can neither be created nor destroyed; it can only change forms.
Heat Transfer.. •
Perpindahan panas didefinisikan sebagai transfer energi dari suatu sistem ke sistem lainnya akibat dari perbedaan temperatur. Transfer energi akan berlangsung sampai kedua sistem mencapai temperatur yang sama.
Example Heat transfer modes:
Heat Transfer Modes… Kalor ditransfer dengan 3 cara: 1-konduksi 2-Konveksi 3-Radiasi
Conduction through a solid or a stationary fluid
Convection from a surface to a moving fluid
Net radiation heat exchange between two surfaces
1. Perpindahan Panas Konduksi KONDUKSI : Yaitu: Perpindahan panas yang mengalir dari daerah yang bertemperatur tinggi ke daerah yang bertemperatur lebih rendah didalam suatu medium (padat, cair atau gas/uap) atau antara medium yang berlainan tetapi bersinggungan secara langsung (kontak langsung)
The Mechanisms of heat conduction in different phases of a substance.
Konduksi = kontak langsung, dengan media penghantar tetap.
Contoh-contoh konduksi: Konduksi = kontak langsung
Thot
Tcolt
Contoh-contoh konduksi: •
Perpindahan panas konduksi pada bahan dengan ketebalan berbeda, mana yang lebih lama naik suhunya ?
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Perpindahan panas konduksi pada bahan dengan panjang berbeda, mana yang lebihlama panasnya?
PERSAMAAN DASAR PERPINDAHAN PANAS KONDUKSI Hukum Fourier’s pada konduksi pada dinding
Heat Transfer… k ,thermal conductivities...? = (W/m. 0C ) “ adalah kemampuan temperatur untuk merambatkan energi apabila ada perbedaan temperatur (ΔT) “ T ,temperature..? = (Celcius,Kelvin…etc) A ,area..? =(m2) X , distance, thickness or length = (m) Q ,rate of heat…? = (W or kW)
Thermal conductivities (k)….
k, thermal conductivities ….
Iron
Wood
Example 1 : Electrically heated home
Example 2.. The heat flow rate through a wood board L= 2cm thick for a temperature difference of ΔT =25 °C between the two surface is 150 W/m². Calculate the thermal conductivity of the wood.
Thermal Diffusivity (Difusivitas termal, σ ) •
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The product ρCp, which is frequently encountered in heat transfer analysis, is called the heat capacity of a material. Both the specific heat Cp and the heat capacity ρCp represent the heat storage capability of a material. But Cp expresses it per unit mass whereas ρCp expresses it per unit volume, as can be noticed from their units J/kg · °C and J/m³ · °C, respectively. Another material property that appears in the transient heat conduction analysis is the thermal diffusivity, which represents how fast heat diffuses through a material and is defined as.
Note that the thermal conductivity k represents how well a material conducts heat, and the heat capacity ρCp represents how much energy a material stores per unit volume. Therefore, the thermal diffusivity of a material can be viewed as the ratio of the heat conducted through the material to the heat stored per unit volum.
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The thermal diffusivities of some common materials at 20°C are given in Table. Note that the thermal diffusivity ranges from 0.14 X 10¯⁶ m²/s for water to 174 X 10¯⁶ m²/s for silver, which is a difference of more than a thousand times. Also note that the thermal diffusivities of beef and water are the same. This is not surprising, since meat as well as fresh vegetables and fruits are mostly water, and thus they possess the thermal properties of water.
Example : A common way of measuring the thermal conductivity of a material is to
sandwich an electric thermofoil heater between two identical samples of the material, as shown in Figure. The thickness of the resistance heater, including its cover, which is made of thin silicon rubber, is usually less than 0.5 mm. A circulating fluid such as tap water keeps the exposed ends of the samples at constant temperature. The lateral surfaces of the samples are well insulated to ensure that heat transfer through the samples is one-dimensional. Two thermocouples are embedded into each sample some distance L apart, and a
differential thermometer reads the temperature drop T across this distance along each sample. When steady operating conditions are reached, the total rate of heat transfer through both samples becomes equal to the electric power drawn by the heater, which is determined by multiplying the electric current by the voltage. In a certain experiment, cylindrical samples of diameter 5 cm and length 10 cm are used. The two thermocouples in each sample are placed 3 cm apart. After initial transients, the electric heater is observed to draw 0.4 A at 110 V, and both differential thermometers read a temperature difference of 15°C. Determine the thermal conductivity of the sample.
2. Perpindahan Panas Konveksi •
Yaitu : proses perpindahan panas yang terjadi antara permukaan dengan fluida yang bergerak di dekatnya jika terdapat perbedaan temperatur antara keduanya Pergerakan tidak hanya berlangsung secara mikroskopik (molekuler), tetapi terjadi juga pergerakan curah Dengan demikian perpindahan energi panas secara konveksi berlangsung melalui difusi/konduksi (akibat adanya pergerakan molekuler) dan melalui adveksi (akibat adanya pergerakan curah). Pergerakan curah dapat disebabkan oleh adanya peralatan dari luar seperti pompa, kompresor atau kipas (konveksi paksa) atau karena disebabkan oleh adanya perpindahan panas itu sendiri (konveksi bebas/alamiah).
Contoh perpindahan panas konveksi Pada permukaan plat Pada lapis batas
Figure : Heat transfer from a hot surface to air by convection.
Consider the cooling of a hot block by blowing cool air over its top surface (see Figure) energy is first transferred to the air layer adjacent to the block by conduction. This energy is then carried away from the surface by convection, that is, by the combined effects of conduction within the air that is due to rando motion of air molecules and the bulk or macroscopic motion of the air that removes the heated air near the surface and replaces it by the cooler air.
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Despite the complexity of convection, the rate of convection heat transfer is observed to be proportional to the temperature difference, and is conveniently expressed by Newton’s law of cooling as
Dimana : Q = Laju perpindahan panas konveksi h = Koefisien perpindahan panas konveksi (w/m2 0C) As = Luas penampang (m2) Ts = Temperatur permukaan (0C) T∞ = Temperatur fluida (0C)
Contoh.1: Udara pada suhu 20 0C bertiup diatas plat panas 50 x 75 cm. Suhu plat dijaga tetap 250 0C. Koefisien perpindahan kalor konveksi adalah 25 W/m2 0C. Hitunglah perpindahan kalor. Penyelesaian Dari persamaan
:
Q = h As (Ts - T∞) = (25 W/m2 0C) (0,50m x 0,75m) [ 250 0C – 20 0C ] = 21,56 W atau 2,156 kW
= (25 W/m2 0C) (0,50m x 0,75m) [ 250 0C – 20 0C ] = 21,56 W atau 2,156 kW
Contoh.2 .2
3. Perpindahan Panas Radiasi •
Adalah perpindahan panas yang terjadi karena pancaran/sinaran/radiasi gelombang elektro-magnetik, tanpa memerlukan media perantara. Radiasi termal adalah bentuk radiasi yang diemisikan oleh benda karena temperaturnya. Transfer energi radiasi tidak tergantung pada medium
Pancaran Radiasi Permukaan Persamaan Stefan-Boltzmann :
Eb Ts4 = 5,67 x 10 –8 W/m2K4 = 0,1714 x 10 –8 Btu/h ft2 R4
Qbs AEb ATs4 A, Ts Untuk benda pada umumnya:
Qs AE ATs4
= emisivitas permukaan, 0 < < 1
When a surface of emissivity and surface area As at an absolute temperature Ts is completely enclosed by a much larger (or black) surface at absolute temperature Tsurr separated by a gas (such as air) that does not intervene with radiation, the net rate of radiation heat transfer between these two surfaces. 4 4 Rad S S Surr
Q
A T T
.............W
Thus the total heat transfer is determined by adding the contributions of both heat transfer mechanisms. For simplicity and convenience, this is often done by defining a combined heat transfer coefficient h, combined that includes the effects of both convection and radiation. Then the total heat transfer rate to or from a surface by convection and radiation is expressed as
Q total hcombined AS TS4 T .............W
Sifat-sifat radiasi permukaan Emisivitas ,
Radiosity (J)
G, (W/m2)
Reflektivitas ,
G
Absorbtivitas, Transmisivitas,
+ + = 1 G G = Iradiasi (Radiasi yang datang)
E + G = J = Radiosity
G
E=T4
Sifat-sifat permukaan Benda yang tidak transparan (opaque), = 0 Permukaan gray-diffuse, = Hukum Kirchoff :
1< < 1 ; 1< < 1
= pada T dan yang sama
+=+=1
Emisivitas beberapa material pada 300 K Material
Emisivitas
Aluminium foil
0.07
Tembaga dipolish
0.03
Emas dipolish
0.03
Perak dipolish
0.02
Stainless steel dipolish
0.17
Cat hitam
0.98
Cat putih
0.90
Kertas putih
0.92 – 0.97
Jalan aspal
0.85 – 0.93
Bata merah
0.93 – 0.96
Kulit tubuh
0.95
Kayu
0.82 – 0.92
Tanah
0.93 – 0.96
Air
0.96
Tumbuhan
0.92 – 0.96 33
Greenhouse Effect
Handout pjt 2005
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