1 Thermal conduction Mechanism The achievement of heat transfer must have the aid of the motion of the microscopic particles the microscopic particles have the kinetic motion or impact of molecules as in the fluids at rest the drift of electrons as in the case of metals the vibration of the lattice as in the case of solids Basic heat conduction law fourier law The heat conduction regularity is result of Biot's experimenta observation and Fourier's analysis theory of heat
1 Thermal Conduction Mechanism The achievement of heat transfer must have the aid of the motion of the microscopic particles. the microscopic particles have: the kinetic motion or impact of molecules, as in the fluids at rest; the drift of electrons, as in the case of metals;the vibration of the lattice, as in the case of solids. Basic heat conduction law—Fourier law The heat conduction regularity is result of Biot’s experimental observation and Fourier’s analysis theory of heat
1 Thermal conduction a The law states that i heat flow rate by conduction in a given direction is proportional to the area normal to the direction to the difference of temperature in the direction, and is inversely proportional to the distance in the direction For example, large plate(slab) at K → K where, x proportionality constant is called thermal conductivity of materials
The law states that: heat flow rate by conduction in a given direction is proportional to the area normal to the direction, to the difference of temperature in the direction, and is inversely proportional to the distance in the direction. For example,large plate(slab) 1 Thermal Conduction t1 t2 0 x Q A x t A q A t t q ∂∂ ⇒ = − − = κ δ κ where,κ proportionality constant is called thermal conductivity of materials
1 Thermal conduction a Thermal conductivity There is a wide difference in conductivity of various materials metals alloys >nonmetal solids > nonmetal fluids> insulating materials >gases (at atmospheric pressure The conductivity of materials is a physical property quantity which is related to the state parameters of system
Thermal conductivity There is a wide difference in conductivity of various materials. metals > alloys > nonmetal solids > nonmetal fluids > insulating materials >gases (at atmospheric pressure) The conductivity of materials is a physical property quantity,which is related to the state parameters of system. 1 Thermal Conduction
2 Thermal convection The achievement of heat transfer has the aid of the macroscopic displacement of the fluids, and is accompanied with the conduction. Convective heat transfer-the fluid flows over a solid body or inside channel, while temperature of fluid and solid surface are different the heat transfer between the fluid and the solid surface takes place as a consequence of the motion of fluid relative to the surface Forced convection fluid motion is artificially induced Free(natural) convection fluid motion is set up by buoyancy effect, resulting from density diff. caused by temp diff. The convective heat transfer is important to the engineering application
2 Thermal convection The achievement of heat transfer has the aid of the macroscopic displacement of the fluids, and is accompanied with the conduction. Convective heat transfer —the fluid flows over a solid body or inside channel, while temperature of fluid and solid surface are different,the heat transfer between the fluid and the solid surface takes place as a consequence of the motion of fluid relative to the surface. Forced convection fluid motion is artificially induced. Free(natural) convection fluid motion is set up by buoyancy effect, resulting from density diff., caused by temp. diff.. The convective heat transfer is important to the engineering application
2 Thermal convection In order to simplicity the heat transfer calculation, a heat transfer coefficient h is defined as q= hA (t-to) The h can be computed analytically for laminar flow over body having simple geometries, but an experimental approach is only means to determine it for flow over body having complex configuration. Typical values of heat transfer coefficient various factors as velocity geometric size flow type, fluid sort fluid temp flow state, change in phase
The h can be computed analytically for laminar flow over body having simple geometries,but an experimental approach is only means to determine it for flow over body having complex configuration. Typical values of heat transfer coefficient various factors as velocity, geometric size,flow type, fluid sort, fluid temp., flow state, change in phase. 2 Thermal convection ( ) = T − T ∞ q hA w In order to simplicity the heat transfer calculation,a heat transfer coefficient h is defined as