Introduction to Heat Transfer *Energy transfer Pre work : + how does passive cooling work in fanless laptop? + how does wireless charging work to charge the smartphone? + how does microwave work? Home appliances use heat transfer process Fanless laptop https://picklaptop.com/best-fanless-laptops/ March 8, 2021, 9.01 am Cooling Fan laptop https://surfacetip.com/is-surface-go-fanless/ March 8, 2021, 9.01 am https://www.ifixit.com/Guide/HP+Notebook +15-ba018ds+Fan+Replacement/115650 March 8, 2021, 9.01 am Cooling Fan PC Electronic devices use heat transfer process Automotive devices use heat transfer process Laboratory equipments use heat transfer process Mechanism of Heat Transfer *atomic/molecular point of view • Dynamics of atomic structure • Atom movement • vibration v=0 • rotation electron neutron v=0 • translation electron proton Electromagnetic wave vīš0 • Temperature is a physical quantity which describes how quickly atoms are moving inside a material. • Temperature also represents the momentum of atoms. Conduction (atoms do not have translation movement, collides between atoms due to vibration or rotation = momentum transfer) before collision after collision Convection (atoms have translation movement, collides between atoms due to translation = momentum transfer) v before collision v’ v”after collision Mechanism of Heat Transfer *atomic/molecular point of view Radiation (atoms that have dynamics atomic structure produce electromagnetic wave, the propagation of electromagnetic wave that is called electromagnetic radiation) influences the other atomic structure. Electric field electron neutron Magnetic field proton Electromagnetic radiation Atom 1 proton neutron Atom 2 electron Modelling of Heat Transfer (Conduction) *microscopic point of view đ đ´ Heat flow đ đ´ Heat flow T1 > T2 đ đ´ Heat flow īT đ đ đđ = −đ đ´ đđĨ Fourier’s formula Group 1 of atoms Group 2 of atoms atomic/molecular point of view īx īx microscopic point of view Physical phenomena Mathematical model Application of Heat Transfer Study *example of one dimensional steady state heat conduction The inside space of a refrigerator is maintained to be constant at 15 oC, where the surrounding temperature is about 32 oC all the day. The wall of refrigerator is isolated to prevent heat transfer from surrounding except the glass door. The thickness of glass door is 4 mm. If the thermal conductivity of glass is about 0.96 W/(m.oC) and the area of glass door is about 1.5 m2, determine the heat that flows from surrounding into the freezer to estimate the minimum power of refrigerator. Assumptions : the problem becomes simple but still physically realistic. 1. The outside surface temperature of glass door is homogeny and same with the temperature of surrounding. 2. The inside surface temperature of glass door is homogeny and same with inside temperature of refrigerator. 3. The situation is steady state. Heat flow T1 Temperature of surrounding (T1) = 32 oC Inside temperature of refrigerator (T2) = 15 oC īT Boundary condition: x1 â¯→ T1 x2 â¯→ T2 T2 đ đđ = −đ đ´ đđĨ đ đđĨ = −đđđ đ´ đ đ x1 īx đ2 đ đĨ2 āļą đđĨ = −đ āļą đđ đ´ đĨ1 đ1 x2 đ = −đđ´ đ đĨ − đĨ1 = −đ đ2 − đ1 đ´ 2 đ2 − đ1 đĨ2 − đĨ1 đ 15 − 32 đ đļ 2 đ = −0.96 đ 1.5đ đ. đļ 4. 10−3 đ đ = 2520đ Important factors of Heat Transfer Study Applications • • • • • • Steady state or Transient Heat Transfer Multidimensional Heat Transfer Multilayer Plane Walls Heat Generation Variable conductivity Boundary and initial conditions • Specified Temperature Boundary Condition • Specified Heat Flux Boundary Condition • Insulated Boundary • Thermal Symmetry • Convection Boundary Condition • Radiation Boundary Condition • Interface Boundary Conditions • Generalized Boundary Conditions • Solutions of Differential Equations *Conduction Modelling of Heat Transfer (Convection) *microscopic point of view Heat flow and đ Fluid flow đ´ Heat flow and đ Fluid flow đ´ T TīĨ > v â v v v v đ = â đ − đ∞ đ´ Newton’s law of cooling Group 1 of atoms Group 2 of atoms atomic/molecular point of view microscopic point of view Physical phenomena Mathematical model Important factors of Heat Transfer Study Applications • Classification of fluid flows • Laminar flow • Turbulent flow • Boundary layer • Velocity boundary layer • Thermal boundary layer • External forced convection • Internal forced convection • Natural convection *Convection