Week 2. Energy, Energy Transfer, And General Energy Analysis

Week 2. Energy, Energy Transfer, And General Energy Analysis

Objectives 1. Introduce the concept of energy and define its various forms 2. Discuss the nature of internal energy 3. Define the concept of heat and the terminology associated with energy transfer by heat 4. Discuss the three mechanisms of heat transfer: conduction, convection, and radiation 5. Define the concept of work, including electrical work and several forms of mechanical work 6. Introduce the first law of thermodynamics, energy balances, and mechanisms of energy transfer to or from a system 7. Determine that a fluid flowing across a control surface of a control volume carries energy across the control surface in addition to any energy transfer across the control surface that may be in the form of heat and/or work 8. Define energy conversion efficiencies 9. Discuss the implications of energy conversion on the environment

Forms of Energy Macroscopic Forms: those that related to motion and the influence of some external effects (e.g. Kinetic and Potential energy) Microscopic Forms: those that related to the molecular structure of system and the degree of the molecular activity (e.g. Internal energy) Energy - Kinetic Energy : result of its motion V V KE = m (kj); ke = 2 2 2 2 (kj/kg) - Potential Energy : result of its elevation in a gravitational field PE = mgz (kj); pe = gz (kj/kg) Total Energy V E = U + KE + PE = U + m 2 2 V e = u + ke + pe = u + + gz 2 2 + mgz (kj/kg) (kj)

Total Energy

Summary PE KE TE = + + PE KE IE KE PE PE KE

Forms of Energy II Stationary system: a closed systems whose velocity and elevation of the center of gravity remain constant during a process D E = DU Flow rate: the amount of properties flowing through a cross section per unit time ex) mass flow rate, volume flow rate, energy flow rate m& = r Av (kg/s) V& = Av 3 (m /s) E& = me & (kj/s or kw)

Summary PE KE

Some Physical Insight to Internal Energy Sensible Energy : Energy associated with the kinetic energies of the molecules proportional to the temperature Latent Energy : Energy associated with the phase of a system Chemical Energy : Energy associated with the atomic bonds Nuclear Energy : Energy associated with the strong bonds within the nucleus Two forms of energy interactions: Heat transfer and Work

Energy Transfer by Heat Heat: the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference Heat simply means heat transfer in thermodynamics Adiabatic Process : A process during which there is no heat transfer - Well insulated boundary - Isothermal process (Caution! Adiabatic Process ß Isothermal process)

Heat Transfer Convection Radiation Conduction Radiation Convection

Energy Transfer by Work Work: energy transfer associated with a force acting through a distance Heat and Work are directional quantities requires magnitude & direction Heat transfer to a system & work done by a system : + Heat transfer from a system & Wonk done on a system : Heat and Work are boundary phenomena Systems possess energy, but not heat or work Both are associated with a process (path functions) Gain + Lose - Spend - Produce +

Mechanical Forms of Work Work: done by a constant force F on a body displaced a distance s in the direction of the force d W = Fdx = PAdx = PdV W V 2 = ò PdV (kj) V 1 P Final state Process Path 2 Initial state Expansion and compression work 1 V 2 V 1 V The P-V diagram of a compression process Shaft work, Spring work, work done on elastic solid bars, work associated with the stretching of a liquid film

Summary? Energy: How can I transfer into the system? System: What is Energy thinking about? Energy: Yes, Heat transfer and Work transfer

Summary

Summary - Work + Work

Denotation Common denotation about heat and work - Q, W : the amount of heat transferred and work done during the process between two states (kj) - q, w : heat and work transfer and work done per unit mass of a system Q&, W& Q W q =, w = ( kj kg) m m - : the heat transfer per unit time (the rate of heat transfer) and the work done per unit time (power) ò t & t ; & (kj) Q 2 2 = Qdt W = Wdt t1 t1 ò ( kj s or kw)

The First Law of Thermodynamics The conservation of energy principle The energy can be neither created nor destroyed during a process; it can only change forms Energy balance ætotal energy ö ætotal energy ö æchange in the total ö ç entering the system - ç leaving the system = ç energy of the system è ø è ø è ø E - E = DE (kj) in out system E& - E& = de dt (kw) rate form in out system D E = D U + D KE + DPE system D E = D W + D Q + DE system mass Energy change of a system Energy transfer Energy Balance Equation D W + D Q + D Emass = D U + D KE + DPE Adiabatic process: No heat transfer -The change in the total energy during an adiabatic process must be equal to the net work done for a control mass

Flow Work And The Energy of A Flowing Fluid Flow work (or flow energy): some work required to push the mass into or out of the control volume W = FL = PAL = PV (kj) flow wflow = Pv (kj/kg)

Summary

The First Law of Thermodynamics II Assumptions 1. Adiabatic process: ΔQ =0 2. Closed system: ΔE mass =0 3. For a closed system undergoing a cycle: ΔU =0 4. Stationary system: ΔKE+ ΔPE=0

Summary

Summary For a closed system undergoing a cycle, the initial and final states are identical

Examples of the First Law (No work) D E = D U + D KE + DPE system D E = D W + DQ + DE system mass D U + D KE + DPE = D W + D Q + DE mass

Examples of the First Law (No heat Transfer)

Examples of the First Law (work& heat)