Heat transfer tells us :
- Bagaimana panas ditransfer
- Pada kecepatan berapa panas ditransfer
- Temperature distribution inside the body
Panas merambat ke segala arah Panas yang diterima (Q output) tergantung luas area dan bahan benda tersebut
Heat flux : the rate of heat transfer per unit area normal to the direction of heat transferConduction
- Needs matter ( padatan)
- Fenomena molekular (proses difusi)
- Tidak ada perpindahan materi hanya energi saja
- Collision ( tumbukan) antara molekul atau atom ( fluida )
- Gelombang lattice disebabkan oleh gerakan atomic dan translasional dari elektron bebas (solid)
the rate of heat conduction through a plane layer is proportional to the temperature difference across the layer and the heat transfer area, but is inversely proportional to the thickness of the layer.
k = conductivity (sifat bahan) W/m 0C
dT = gradien temperatur ( T1- T2) K or 0C
dx = gradien jarak m - makin besar k makin besar q - kalau logam dengan heat yang sama,maka q akan lebih besar dari kayu - q berbanding lurus dengan luas area - q berbanding lurus dengan dT - semakin jauh jaraj maka q makin kecil
Mekanisme Perpindahan Panas Konduksi
- First mechanisme, molecular interaction (eg gas) Greater motion of molecule at higher energy level ( temperature) impacts energy to adjacent molecules at lower energy levels ( molekul dengan temperater lebih tinggi akan mempengaruhi molekul disebelahnya yang temperaturnya lebih rendah ) - Second mechanism, by free electrons ( eg solid) nb Collision : perpindahan panas karena adanya tumbukan antara molekul, dari yang temperaturnya lebih tinggi ke temperatur yang lebih rendah
Difusi : perpindahan panas seperti merambat, materi ga mengalir hanya molekulnya saja Bulk motion : perpindahan panas, materi dan molekul ikut mengalir
Minus mendefinisikan panas bergerak menuju ke temperatur yang lebih rendah
Fourier's Law
Temperatur sebagai fungsi vektor.
nb :
d dibaca de
∂ dibaca du
dibaca do -- > 3 dimensi
Thermal Conductivity
Thermal conductivity of a material can be defined as the rate of heat transfer through a unit thickness of the material per unit area per unit temperature difference Besarnya thermal conductivity menggambarkan besarnya kemampuan material tersebut untuk menghantarkan panas. Semakin besar maka semakin baik menghantarkan panas atau merupakan good heat conductor. Sebaliknya semakin nilai thermal conductivity yang kecil mengindikasi bahwa material tersebut merupakan a poor heat conductor or insulator.
Thermal conductivity : solid > liquid > gas pure metals > alloys > non metallic solids > insulation system Thermal conductivity of liquid increases with decreasing liquid molecular weight Liquid metal > nonmetallic liquid Thermal conductivity of gas increases with decreasing gas molecular weight Thermal conductivity of gas
Cara untuk mengestimasi thermal conductivity of gas : 1. Considering the summation of the energy flux associated with the molecules crossing the control surface 2. The number of molecules involved is related to average random molecular velocity. 3. k: Boltzmann constant, d: molecular diameter, m: mass per molecule.
Thermal conductivity of solids Cara mengesrimasi thermal conductivity of solids :
1. Derived from the Wiedemann, Franz, Lorenz Equation (1872).
2. The free electron mechanism of heat conduction is directly analogous to the mechanism of electric conduction.
3. ke : electrical conductivity [unit = 1/(W-m)], T: absolute temperature (unit = K), L: Lorenz number.
Convection - Heat carried away by bulk motion ( pergerakan materi ) of fluid - needs fluids matter - energy transferred by diffusion + bulk motion of fluid - perpindahan panas secara konveksi melibatkan pertukaran energi antara surface dan adjacent fluids Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion. The faster the fluid motion, the greater the convection heat transfer. In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction. The presence of bulk motion of the fluid enhances the heat transfer between the solid surface and the fluid, but it also complicates the determination of heattransfer rates.
h = heat transfer co-efficient ( W / m2 K ) (not a property ) tergantung pada geometri, sifat aliran, termodinamika properti dll
q = satuan W
q" = satuan W/m2
Free or natural convection contohnya knalpot panas di diamkan akan menjadi dingin karena ada natural convection Forced convection contohnya adanya fin fan di depan bagian mesin motor Phase change contohnya : boiling, condensation
Typical values of h ( W/m2 K)
Tujuan utama dari analisa perpindahan panas konvektif adalah untuk menentukan :
- medan aliran (flow field )
- bidang suhu pada fluida ( temperature field in fluids)
- heat transferred co-efficient (h)
Radiation
- Doesn't need matter ( tidak perlu medium)
- Transmission of energy by electromagnetic waves
- Heat Transfer by electro-magnetic waves or photons( no medium required. ) Emissive power of a surface (energy released per unit area):
E=esTs4 (W/ m2)
e= emissivity of the surface. The property emissivity, whose value is in the range 0 ≤ α ≤1, is a measure of how closely a surface approximates a blackbody for which e= 1 or 0.1714 x 10-8 Btu/h · ft2 R4
Radiation exchange between a large surface and surrounding Q”r a d = es(Ts4-Tsur4) W/ m2
Aplikasi Perpindahan Panas
- Energy production and conversion contoh : steam power plant, solar energy, conversion
- Refrigeration and air-conditioning
- Domestic applications contoh : ovens, stoves, toaster
- Cooling of electronic equipment
- Manufacturing / materials processing contoh : welding, casting, soldering, laser machining
- Automobiles / aircraft design - Nature ( weather, climate )
Summary
