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- 1. BIRLA VISHVAKARMA MAHAVIDYALAYA vallabh vidyanagar BIRLA VISHVAKARMA MAHAVIDYALAYA vallabh vidyanagar Div – 8 Nichay Agrawal 140080125001 group no : - 6 Ullas Bhalani 140080125002 Jaydeep Chaudhary 140080125003 Dobariya Yash 140080125004 Div – 8 Nichay Agrawal 140080125001 group no : - 6 Ullas Bhalani 140080125002 Jaydeep Chaudhary 140080125003 Dobariya Yash 140080125004
- 3. 3 Heat:Heat: The form of energy that can be transferred from one system to another as a result of temperature difference. ThermodynamicsThermodynamics is concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another. Heat TransferHeat Transfer deals with the determination of the rates of such energy transfers as well as variation of temperature. The transfer of energy as heat is always from the higher-temperature medium to the lower-temperature one. Heat transfer stops when the two mediums reach the same temperature. Heat can be transferred in three different modes: conduction, convection,conduction, convection, radiationradiation
- 4. HHeateat as the form of energy that can be transferred from one system to another as a result of temperature difference. A thermodynamic analysis is concerned with the amountamount of heat transfer as a system undergoes a process from one equilibrium state to another. The science that deals with the determination of the ratesrates of such energy transfers is the heatheat transfertransfer.. The transfer of energy as heat is always from the higher-temperature medium to the lower-temperature one, and heat transfer stops when the two mediums reach the same temperature. Heat can be transferred in three basic modes: conductionconduction convectionconvection radiationradiation All modes of heat transfer require the existence of a temperature difference.
- 5. 5 ConductionConduction: The transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. In gases and liquidsIn gases and liquids,, conduction is due to the collisionscollisions and diffusion of the molecules during their random motion. In solidsIn solids, it is due to the combination of vibrationsvibrations of the molecules in a lattice and the energy transport by free electrons. 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. ConductionConduction: The transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. In gases and liquidsIn gases and liquids,, conduction is due to the collisionscollisions and diffusion of the molecules during their random motion. In solidsIn solids, it is due to the combination of vibrationsvibrations of the molecules in a lattice and the energy transport by free electrons. 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.
- 6. 6 Fourier’s law of heat conduction Fourier’s law of heat conduction Thermal conductivity, k: A measure of the ability of a material to conduct heat. Temperature gradient dT/dx: The slope of the temperature curve on a T- x diagram. Heat is conducted in the direction of decreasing temperature, and the temperature gradient becomes negative when temperature decreases with increasing x. The negative sign in the equation ensures that heat transfer in the positive x direction is a positive quantity. Thermal conductivity, k: A measure of the ability of a material to conduct heat. Temperature gradient dT/dx: The slope of the temperature curve on a T- x diagram. Heat is conducted in the direction of decreasing temperature, and the temperature gradient becomes negative when temperature decreases with increasing x. The negative sign in the equation ensures that heat transfer in the positive x direction is a positive quantity. When x 0→When x 0→
- 7. Thermal conductivity: The rate of heat transfer through a unit thickness of the material per unit area per unit temperature difference. The thermal conductivity of a material is a measure of the ability of the material to conduct heat. A high value for thermal conductivity indicates that the material is a good heat conductor, and a low value indicates that the material is a poor heat conductor or insulator. Thermal conductivity: The rate of heat transfer through a unit thickness of the material per unit area per unit temperature difference. The thermal conductivity of a material is a measure of the ability of the material to conduct heat. A high value for thermal conductivity indicates that the material is a good heat conductor, and a low value indicates that the material is a poor heat conductor or insulator. A simple experimental setup to determine the thermal conductivity of a material.
- 8. 8 ConvectionConvection: 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. Heat transfer from a hot surface to air by convection.
- 9. 9 •Forced convection: If the fluid is forced to flow over the surface by external means such as a fan, pump, or the wind. •Natural (or free) convection: If the fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid. •Forced convection: If the fluid is forced to flow over the surface by external means such as a fan, pump, or the wind. •Natural (or free) convection: If the fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid. The cooling of a boiled egg by forced and natural convection. Heat transfer processes that involve change of phase of a fluid are also considered to be convection because of the fluid motion induced during the process, such as the rise of the vapor bubbles during boiling or the fall of the liquid droplets during condensation.
- 10. 10 h convection heat transfer coefficient, W/m2 · °C As the surface area through which convection heat transfer takes place Ts the surface temperature T∞ the temperature of the fluid sufficiently far from the surface. •The convection heat transfer coefficient h is not a property of the fluid. •It is an experimentally determined parameter whose value depends on all the variables influencing convection such as - the surface geometry - the nature of fluid motion - the properties of the fluid - the bulk fluid velocity •The convection heat transfer coefficient h is not a property of the fluid. •It is an experimentally determined parameter whose value depends on all the variables influencing convection such as - the surface geometry - the nature of fluid motion - the properties of the fluid - the bulk fluid velocity
- 11. •Energy transferred in the form of rays or waves or particles. •We will concentrate on the type of radiation that travels as electromagnetic waves.
- 12. Heat transfer through the wall of a house can be modeled as steady and one-dimensional. The temperature of the wall in this case depends on one direction only (say the x-direction) and can be expressed as T(x). Heat transfer through the wall of a house can be modeled as steady and one-dimensional. The temperature of the wall in this case depends on one direction only (say the x-direction) and can be expressed as T(x). forfor steady operationsteady operation Fourier’s law of heat conductionFourier’s law of heat conduction In steady operation, the rate of heat transfer through the wallIn steady operation, the rate of heat transfer through the wall is constant.is constant. dEdEwall / dt = 0
- 13. •The rate of heat conduction through a plane wall is proportional to the average thermal conductivity, the wall area, and the temperature difference, but is inversely proportional to the wall thickness. •Once the rate of heat conduction is available, the temperature T(x) at any location x can be determined by replacing T2 by T, and L by x. •The rate of heat conduction through a plane wall is proportional to the average thermal conductivity, the wall area, and the temperature difference, but is inversely proportional to the wall thickness. •Once the rate of heat conduction is available, the temperature T(x) at any location x can be determined by replacing T2 by T, and L by x.Under steady conditions, the temperature distribution in a plane wall is a straight line: dT/dx = const.
- 14. Analogy between thermal andAnalogy between thermal and electrical resistance concepts.electrical resistance concepts. rate of heat transfer -- electric currentrate of heat transfer -- electric current thermal resistance -- electrical resistancethermal resistance -- electrical resistance temperature difference -- voltagetemperature difference -- voltage differencedifference rate of heat transfer -- electric currentrate of heat transfer -- electric current thermal resistance -- electrical resistancethermal resistance -- electrical resistance temperature difference -- voltagetemperature difference -- voltage differencedifference Conduction resistance of the wall: Thermal resistance of the wall against heat conduction. Thermal resistance of a medium depends on the geometry and the thermal properties of the medium. Conduction resistance of the wall: Thermal resistance of the wall against heat conduction. Thermal resistance of a medium depends on the geometry and the thermal properties of the medium.
- 15. •When the convection heat transfer coefficient is very large (h → ), the convection resistance becomes zero and Ts T. •That is, the surface offers no resistance to convection, and thus it does not slow down the heat transfer process. •This situation is approached in practice at surfaces where boiling and condensation occur. •When the convection heat transfer coefficient is very large (h → ), the convection resistance becomes zero and Ts T. •That is, the surface offers no resistance to convection, and thus it does not slow down the heat transfer process. •This situation is approached in practice at surfaces where boiling and condensation occur. •Schematic for convectionSchematic for convection resistance at a surface.resistance at a surface.
- 18. The thermal resistance network for heat transfer through a two-layer plane wall subjected to convection on both sides. The thermal resistance network for heat transfer through a two-layer plane wall subjected to convection on both sides.
- 19. Dr. Şaziye Balku 19 gapcontact QQQ ••• += erfacec TAhQ int∆= • erface c T AQ h int / ∆ = • (W/m2 0 C) (m2 0 C/ W) AQ T h R erface c c / 1 int • ∆ == hC: thermal contact conductance
- 20. Dr. Şaziye Balku 20 Thermal contact resistance is inverse of thermal contact conduction, Depends on Surface roughness, Material properties, Temperature and pressure at interface, Type of fluid trapped at interface
- 21. Dr. Şaziye Balku 21 Effect of metallic coatings on thermal contact conductance For soft metals with smoot surfaces at high pressures Thermal contact conductance Thermal contact resistance
- 22. Dr. Şaziye Balku 22 ) 11 )(( 21 21 2 21 1 21 21 RR TT R TT R TT QQQ +−= − + − =+= ••• totalR TT Q 21 − = • 21 111 RRRtotal += 21 21 RR RR Rtotal + = Resistances are parallel
- 23. Dr. Şaziye Balku 23 totalR TT Q ∞ • − = 1 convconvtotal RR RR RR RRRR ++ + =++= 3 21 21 312 11 1 1 Ak L R = 22 2 2 Ak L R = 33 3 3 Ak L R = 3 1 hA Rconv =
- 24. Dr. Şaziye Balku 24 Steady-state heat conduction Heat is lost from a hot-water pipe to the air outside in the radial direction. Heat transfer from a long pipe is one dimensional
- 25. Dr. Şaziye Balku 25 dr dT kAQ cylcond −= • , Fourier’s law of conduction = • cylcondQ , constant ∫∫ == • −= 2 1 2 1 , T TT r rr cylcond kdTdr A Q rLA π2= )/ln( 2 12 21 , rr TT LkQ cylcond − = • π cyl cylcond R TT Q 21 , − = • Lk rr Rcyl π2 )/ln( 12 =
- 26. Dr. Şaziye Balku 26 2 4 rA π= krr rr Rsph 21 12 4π − = sph sphcond R TT Q 21 , − = • including convection 2 2 221 12 4 1 4 hrkrr rr Rtotal ππ + − = totalR TT Q ∞ • − = 1
- 27. Dr. Şaziye Balku 27 )2( 1 2 )/ln( 2 12 11 LrhLk rr TT RR TT Q convins ππ + − = + − = ∞∞ • 0/ 2 = • drQd h k r cylindercr =, Thermal conductivity External convection heat transfer coefficient show CYLINDER
- 28. Dr. Şaziye Balku 28 cr cr cr rr rr rr > = < 2 2 2 max Before insulation check for critical radius h k r spherecr 2 , =
- 29. Dr. Şaziye Balku 29 Two ways of increasing - increase h - increase As By adding fins (Car radiators) • Q )( ∞ • −= TThAQ SSconv
- 30. Dr. Şaziye Balku 30 rate of heat conduction into the element at x rate of heat conduction from the element at x+Δx rate of heat convection from the element += ))(( ∞ • −∆= TTxphQ Sconvconvxxcondxcond QQQ • ∆+ •• += ,, 0)( ,, =−+ ∆ − ∞ • ∆+ • TThp x QQ xcondxxcond 0→∆x 0)( =−+ ∞ • TThp dx dQcond
- 31. Dr. Şaziye Balku 31 dx dT kAQ ccond −= • 0)( =−− ∞TThp dx dT kA dx d c 02 2 2 =− θ θ a dx d axax eCeCx − += 21)(θ At constant AC and k Solution is; CkA hp a =2 ∞−= TTbbθ ∞−= TTθ (fin) Boundary condition x = 0
- 32. Thank you…