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Heat Transfer

Heat Transfer

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1. 1. Heat Transfer DM23815 Chapter 1. Introduction Eunseop Yeom esyeom@pusan.ac.kr School of Mechanical Engineering, Pusan National University
2. 2. 2 1.1 What is heat transfer? The form of energy that can be transferred from one system to another as a result of temperature difference.  Thermodynamics  Heat It deals with the amount of energy as a system undergoes a process from one state to another, and gives no indication about how long the process will take. (equilibrium)  Heat transfer It deals with the rate of heat transfer to or from a system, and thus determine the rates of heat transfer and the times of cooling or heating, as well as the variation of the temperature. (non-equilibrium) “Heat transfer is energy in transit due to temperature difference.” E=[J] q=[W]=[J/s], q'=q/L=[W/m], q''=q/A=[W/m2] Time State 1 Temp State 2
3. 3. 3 Examples of heat transfer mechanisms Conduction (Heat diffusion) Convection Radiation
4. 4. 4 1.2.1 Conduction Heat transfer from the more energetic to the adjacent less energetic particles of a substance due to interactions between the particles. Fourier’s law of heat conduction k : thermal conductivity (열 전도율) [W/m·K] Two mechanisms 1. The atoms and molecules having energy will pass those energy with their adjacent atoms or molecules by means of lattice vibrations. 2. Through the translational motion of free electrons, heat energy can be transferred in a conductor like metals having a plenty of free electrons. Conductive heat flux Under steady-state conditions and temperature distribution is linear L T - T dx dT 1 2   L T k L T T k q 2 1 x       x q 
5. 5. 5 1.2.1 Conduction Thermal conductivity (k) is a measure of material’s ability to conduct heat. Material k (W/m·K) Water (liquid) 0.607 Air(gas) 0.026 Human artery 0.476 ± 0.041 Human blood (43%Ht) 0.530 Human plasma 0.572 Human bone 0.373 - 0.496 Human fat 0.23 - 0.27 Human kidney 0.513 - 0.564 Human liver 0.467 - 0.527 Human lung 0.302 - 0.550 Human muscle 0.449 - 0.546 Human skin 0.385 - 3.393  Thermal conductivities Duck, Physical properties of tissues: a comprehensive reference book. (Academic press, 2013). - If k is high, the material is a good conductor. - If k is low, the material is a poor conductor or an insulator. - Thermal conductivity varies with temperature.
6. 6. 6 1.2.1 Convection Heat transfer due to a superposition of energy transport by the random motion of the molecules (diffusion), and by the bulk motion of the fluid (advection). Newton's law of cooling h : Convection heat transfer coefficient [W/m2·K]. (The term convection refers to heat transfer that will occur between a solid surface and the adjacent fluid when they are at different temperatures.) Convective heat flux Ts and T∞ : Temperatures at surface and fluid [K]. conv q 
7. 7. 7 1.2.1 Convection  Forced convection  Natural convection Process h (W/m2·K) Free convection Gases 2 - 25 Liquids 50 - 1,000 Forced convection Gases 25 - 250 Liquids 100 - 20,000 Convection with phase change Boiling and condensation 2,500 - 100,000 Fluid motion is set up by buoyancy effects resulting from density difference caused by temperature difference in the field. Fluid motion is forced by external means, such as a fan, a pump, etc. h depends on conditions in the boundary layer, which are influenced by ① Surface geometry ② The nature of the fluid motion ③ An assortment of fluid thermodynamic properties  Convection with phase change A latent heat exchange is associated with phase change between liquid and vapor states of the liquid. Two special cases are boiling and condensation.  Convection heat transfer coefficient Forced convection Free convection conv q  conv q  Boiling Condensation
8. 8. 8 1.2.3 Radiation This mode of heat transfer didn’t require any medium to occur. Every matter having a temperature above absolute zero will emit energy in the form of electromagnetic waves (or alternatively, photons) and called radiation. Radiation transfer occurs most efficiently in a vacuum. Stefan-Boltzmann’s law 4 s b T E   σ : Stefan-Boltzmann’s constant (5.67×10-8) [W/m2·K4]. T : Absolute temperature of surface [K] (For black body; ideal radiator) 4 s T E   (For real body) ε : Emissivity(방사율). (0 ≤ ε ≤ 1) A measure of how efficiently a surface emits energy relative to a blackbody. It depends strongly on the surface material and finish. rad q 