上游充通大￥ SHANGHAI JIAO TONG UNIVERSITY Engineering Thermodynamics I Lectures 5-6 Spring,2018 Prof.,Dr.Yonghua HUANG 强 MMLLLMMAA http://cc.sjtu.edu.cn/G2S/site/thermo.html AN 1日G
Engineering Thermodynamics I Lectures 5-6 Spring, 2018 Prof., Dr. Yonghua HUANG http://cc.sjtu.edu.cn/G2S/site/thermo.html
Chapter 2 Energy,Energy transfer and Analysis Objective Introduce energy concept. Organize the ideas about energy into forms suitable for engineering analysis. Energy can be stored within systems in various forms. Energy can be converted from one from to another. Energy can be transferred between systems. Total energy is conserved. 上游气通大粤 March 7,2018 2 SHANGHAI JIAO TONG UNIVERSITY
March 7, 2018 2 Chapter 2 Energy, Energy transfer and Analysis Objective • Introduce energy concept. • Organize the ideas about energy into forms suitable for engineering analysis. • Energy can be stored within systems in various forms. • Energy can be converted from one from to another. • Energy can be transferred between systems. • Total energy is conserved
Start with familiar examples Well-sealed and Well-sealed and well-insulated room well-insulated room Fan 洞日 What will happen to the average What if we operated a fan temperature of air in the room? in this room instead? 上游充通大 March 7,2018 3 SHANGHAI JLAO TONG UNIVERSITY
March 7, 2018 3 Start with familiar examples What will happen to the average temperature of air in the room? What if we operated a fan in this room instead?
Energy Exists in several forms:?? Thermal,mechanical,kinetic,potential,electric, magnetic,chemical,nuclear,.. © Total macroscopic energy microscopic energy 上游充通大学 March 7,2018 4 SHANGHAI JIAO TONG UNIVERSITY
March 7, 2018 4 Energy Exists in several forms: ??? • Thermal, mechanical, kinetic, potential, electric, magnetic, chemical, nuclear, … Total = macroscopic energy + microscopic energy
What about Macroscopic Energy? 上游充通大学 March 7,2018 5 SHANGHAI JIAO TONG UNIVERSITY
March 7, 2018 5 What about Macroscopic Energy?
Macroscopic Energy at the system (large)level Potential Energy (PE): -elevation in a gravitational mg field compared to some reference Z level F Reference plane PE =mgz pe g3 V=0 V>0 Kinetic Energy (KE): -motion relative to some m m reference frame V2 V2 KE ke reference m 2 2 Are they properties of a system? 上降文通大学 March 7,2018 6 SHANGHAI JLAO TONG UNIVERSITY
March 7, 2018 6 Macroscopic Energy Potential Energy (PE): –elevation in a gravitational field compared to some reference level Kinetic Energy (KE): –motion relative to some reference frame at the system (large) level Are they properties of a system?
Example:Wind Energy 8.5m/s A site evaluated for a wind farm is observed to have steady winds at a speed of 8.5 m/s.Determine the wind energy (a)per unit mass (b)for a mass of 10 kg,and (c)for a flow rate of 1154 kg/s for air. Solution A site with a specified wind speed is considered.Wind energy per Discussion unit mass,for a specified mass,and for a given mass flow rate of air are to be determined. Assumptions Wind flows steadily at the specified speed. =8.5m/s,12m section, Analysis The only harvestable form of energy of atmospheric air is the Pair=1.2 kg/m3 kinetic energy,which is captured by a wind turbine. (a)Wind energy per unit mass of air is A wind turbine with a V2(8.5m/s)2 1 J/kg e =ke= 2 1m2/s =36.1J/kg wind spanΦl2 m has a 2 power generation (b)Wind energy for an air mass of 10 kg is potential of 41.7 kW. E=me=(10kg)(36.1J/kg)=361J Real wind turbines (c)Wind energy for a mass flow rate of 1154 kg/s is convert about 1/3 of this E=me=(1154kg/s)(36.1J/kg) 1kW potential to electric 41.7kW 1000J/s power. 上海究通大学 March 7,2018 SHANGHAI JIAO TONG UNIVERSITY
March 7, 2018 7 Example: Wind Energy A site evaluated for a wind farm is observed to have steady winds at a speed of 8.5 m/s . Determine the wind energy (a) per unit mass, (b) for a mass of 10 kg, and (c) for a flow rate of 1154 kg/s for air. A wind turbine with a wind span Φ12m has a power generation potential of 41.7 kW. Real wind turbines convert about 1/3 of this potential to electric power. = 8.5m/s, Φ12m section, ρair=1.2 kg/m3 Discussion
What about Microscopic Energy? 上游充通大学 March 7,2018 8 SHANGHAI JLAO TONG UNIVERSITY
March 7, 2018 8 What about Microscopic Energy?
Microscopic Energy at the molecular (small)levels Translational energy:molecules move around Rotational energy:individual atoms rotate about the axis forming the molecular bond Vibrational energy:individual atoms vibrate about the center of mass of the molecular bond Electronic energy:electrons in an atom or molecule are bound in particular configurations combine these to get sensible energy that depends on temperature for ideal gases 上游充通大 March 7,2018 9 SHANGHAI JIAO TONG UNIVERSITY
March 7, 2018 9 – Translational energy: molecules move around – Rotational energy: individual atoms rotate about the axis forming the molecular bond – Vibrational energy: individual atoms vibrate about the center of mass of the molecular bond – Electronic energy: electrons in an atom or molecule are bound in particular configurations Microscopic Energy at the molecular (small) levels combine these to get sensible energy that depends on temperature for ideal gases
Microscopic Energy at the molecular (small)levels Chemical energy:binding energy between electrons and nuclei of atoms - Nuclear energy:binding energy between protons and neutrons (within the nucleus) - Electric/magnetic/surface tension effects usually negligible in most engineering device 上游充通大学 March 7,2018 10 SHANGHAI JLAO TONG UNIVERSITY
March 7, 2018 10 – Chemical energy: binding energy between electrons and nuclei of atoms – Nuclear energy: binding energy between protons and neutrons (within the nucleus) – Electric/magnetic/surface tension effects usually negligible in most engineering device Microscopic Energy at the molecular (small) levels