2-3， Energy transfer by heat （热Heat(热):the form of energy that is transferred between two systems(orasystemanditssurroundings)byvirtueofatemperaturedifference.Heat Unit, Q: kJHeat transfer perunit mass, : q=Q/m (kJ/kg)Heat transfer per unit time, Q: kJ/s=kW2k3themalenergyHeattransferto a system:positive QinHEATSURROUNDINGAIRHeattransferfromasystem:negativeQout21healBAKEDPOTATOSystem2themalboundaryAdiabaticprocess:aprocesshasnoheattransferenergyWellinsulatedSystemand surroundings are atthe sametemperatureInsulationAdiabaticsystem
2-3，Energy transfer by heat（热） • Heat(热): the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference. – Heat Unit, Q: kJ – Heat transfer per unit mass, q: q=Q/m (kJ/kg) – Heat transfer per unit time, Q: kJ/s=kW – Heat transfer to a system: positive Qin – Heat transfer from a system: negative Qout • Adiabatic process: a process has no heat transfer – Well insulated – System and surroundings are at the same temperature 11 Adiabatic system Insulation
Mechanisms of Heat TransferConduction(导热):interactions betweenAIRHealparticles.（依靠分子、原子级自由电子等微观粒子的COLAWall of热运动而产生的热能传递）Convection(对流)：energytransferbetween surfaceand adjacentfluid(指流ForcedNaturalconvectionconvection体的宏观运动引起的流体各部分之间发生相对位移，冷AIR热流体相互掺混所导致的热量传递叫对流。）AIR，因流体中分子进行着不规则热运动，对流必伴随导热过程，所以8Ot流体流过物体表面时流体与物体表面间的热量创地过程叫对流传热Radiation(辐射):energy emitted byPerson30°Cmatterinformofelectromagnetic waves(Air物体通过电磁波来传递能量的方式）Fin900°℃
Mechanisms of Heat Transfer • Conduction(导热): interactions between particles. (依靠分子、原子级自由电子等微观粒子的 热运动而产生的热能传递) • Convection(对流): energy transfer between surface and adjacent fluid (指流 体的宏观运动引起的流体各部分之间发生相对位移，冷 热流体相互 掺混所导致的热量传递叫对流。) – 因流体中分子进行着不规则热运动，对流必伴随导热过程，所以 流体流过物体表面时流体与物体表面间的热量创地过程叫对流传热 • Radiation(辐射): energy emitted by matter in form of electromagnetic waves( 物体通过电磁波来传递能量的方式) 12
2-4， Energy transfer by work(功)Work（功）:theenergytransferassociated withaforceactingthroughadistanceWorkunit,W:kJWorkdoneperunitmass,w:w=W/m (kJ/kg)Work done per unit time (Power),P:kJ/s=kWWork done byasystem:positive WoutWorkdoneonasystem:negativeWinSurroundingsSystem
2-4，Energy transfer by work(功) • Work（功）: the energy transfer associated with a force acting through a distance. – Work unit， W: kJ – Work done per unit mass,w: w=W/m (kJ/kg) – Work done per unit time (Power)， P: kJ/s=kW – Work done by a system: positive Wout – Work done on a system: negative Win 13 + - - +