2-2, Forms of energyTotal energyl 总能 (E, KJ): the sum of numerous forms of energy, e.g.:thermal,mechanical,kinetic,potential,electric,magnetic,chemical,nuclearMagnetic,electric,andsurfacetensioneffects are significantinsomespecializedcasesonlyandareusuallyignored.Intheabsenceof sucheffects,thetotal energyofa systemconsistsof:E=U+mV2/2+mgz(KJ)(KJ/Kg)- Per unit mass: e=u+v2/2+gzForastationarysystem:△E=△U
2-2, Forms of energy • Total energy/总能 (E, KJ): the sum of numerous forms of energy, e.g.: thermal, mechanical, kinetic, potential, electric, magnetic, chemical, nuclear. • Magnetic, electric, and surface tension effects are significant in some specialized cases only and are usually ignored. • In the absence of such effects, the total energy of a system consists of: – E=U+mV2 /2+mgz (KJ) – Per unit mass: e=u+v2 /2+gz (KJ/Kg) – For a stationary system: ∆E=∆U 6
2-2, Forms of energyMass flowrate/质量流率 m:the amount of mass flowing throughacross section perunittime. (kg/s)Volume flow rate/体积流率 V : the volume of a fluid flowing through across section per unit time. (m3/s)m=pV=pA,Vavg一Energyflowrate/能量流率（功率）E：kJ/skWE=meE=me
2-2, Forms of energy – Mass flow rate/质量流率 : the amount of mass flowing through a cross section per unit time. (kg/s) – Volume flow rate/体积流率 : the volume of a fluid flowing through a cross section per unit time. (m3 /s) – Energy flow rate/能量流率（功率） ： kJ/s kW 7
2-2, Forms of energyNuclear energy: changes in the core or nucleus.一fusion（聚变)：twosmallnucleicombineintoalargeroneDifficulttouse.E6neutron5.1x10-13j(b)Fusionof hydrogen-Fission(裂变):split one atom into other elements. E.g.U-235commonlyusedtogenerateelectricityinnuclearpowerplantsnuclearpowersubmarinesandaircraftcarriers,nuclearbombsUranium3.2 × 1011 JU-23514eutronsnneutronRb-93(a)Fissionofuranium
2-2, Forms of energy • Nuclear energy: changes in the core or nucleus. – fusion (聚变)：two small nuclei combine into a larger one. Difficult to use. – Fission(裂变): split one atom into other elements. E.g. U-235. commonly used to generate electricity in nuclear power plants, nuclear power submarines and aircraft carriers, nuclear bombs. 8
2-2, Forms of energyChemical energy(化学能): changes in the structure of theelectrons of the atoms; Mechanical energy(机械能): the form of energy that can beconverted to mechanical work completely and directly by an idealmechanicaldevicesuchasanidealturbine-Kinetic energy: kJ-Potential energy: kJ- Flow energy for flowing fluids: The mechanical energy of a flowing fluid :(量纲一致性)emech=P/p+V2/2+gzkJ/kg Flow energy, P/p: a pressure force acting on a fluid throughadistanceproducesworkV2PTotal energy in rate form:Emech=imnemechm2p
2-2, Forms of energy • Chemical energy(化学能): changes in the structure of the electrons of the atoms; • Mechanical energy(机械能): the form of energy that can be converted to mechanical work completely and directly by an ideal mechanical device such as an ideal turbine. – Kinetic energy: kJ – Potential energy: kJ – Flow energy for flowing fluids • The mechanical energy of a flowing fluid : – emech=P/ρ+ν2 /2+gz kJ/kg (量纲一致性) • Flow energy, P/ρ: a pressure force acting on a fluid through a distance produces work • Total energy in rate form: 9
2-2, Forms of energyThe only two forms of energy interactionsassociatedwith a closed system are:HEAT（传热):drivingforceisatemperaturedifference.一 WORK （作功）： otherwise,itis work10
2-2, Forms of energy • The only two forms of energy interactions associated with a closed system are: – HEAT (传热): driving force is a temperature difference. – WORK （作功）：otherwise, it is work. 10