Mid-Term Review

Classical Thermodynamics The science of the conversion of energy from one form to another. The science of energy and entropy.

Topics of Study Control volumes Properties of pure substances Work and heat 1 st Law 2 nd Law Entropy Power and refrigeration cycles Thermodynamic relations

Experimental observations have evolved into a set of laws that form the basis of the science of Thermodynamics: 0 th law (temperature) 1 st law (energy) 2 nd law (entropy) 3 rd law (absolute entropy)

Applications of these laws requires the use of mathematical models which, in turn, contain variables that describe the “state” of the system. We call these “state variables” the properties of the thermodynamic system: temperature pressure mass density, or specific volume enthalpy entropy

Temperature A sense of hotness or coldness at the touch. Not very satisfying! Equality of temperature. Zeroth law of thermodynamics

Temperature Scales Celcius triple point of water steam point Kelvin (absolute scale) Fahrenheit Rankine

Chapter 3 PROPERTIES OF A PURE SUBSTANCE

Independent Properties of Pure Substances The state of a simple, compressible pure substance can be defined by two independent properties Not any two properties; e.g., pressure and temperature are not always independent

Low and Moderate Density Gases (high specific volume) Implies very low intermolecular potential energy; i.e., ideal gas behavior PV = nR * T, PV = (m/M)R * T, PV = mRT, Pv = RT Where is P α 1/v on the P-v-T surface?

Chapter 4 WORK AND HEAT

04-05

Polytropic Processes PV n = constant

Modes of Heat Transfer Conduction Convection Radiation

Heat and Work Comparisons Both are transient; systems possess neither; both can cross system boundary when the system undergoes a change of state Both are boundary phenomena representing energy crossing a boundary Both are path functions; inexact differentials

Chapter 5 THE FIRST LAW OF THERMODYNAMICS

E = U + KE + PE dE = dU + d(KE) + d(PE) = δQ – δW and integrating between states 1 and 2, U 2 – U 1 + ½m(V 2 2 – V 2 1 ) + mg(Z 2 – Z 1 ) = Q 1-2 – W 1-2

Internal Energy as a Thermodynamic Property

U = U liq + U vap mu = m liq u f + m vap u g u = (1-x)u f + xu g u = u f + xu fg

Specific Heats δQ = dU + δW = dU + PdV

Internal Energy, Enthalpy, and Specific Heat of Ideal Gases P, kPa T, ° C Internal Energy for Superheated Steam

The First Law as a Rate Equation We’ve already seen the first law in differential form in equation 5.7: dE = dU +d(KE) + d(PE) = δQ – δW Dividing by δt and taking the limit, we can also write the first law as a rate equation:

05-13

Chapter 6 FIRST-LAW ANALYSIS FOR A CONTROL VOLUME

06-01

06-02

06-03

06-04

The Steady-State Process The control volume is stationary The state of the mass at each point in the control volume does not vary with time For mass flowing across the boundary, the mass flux and the state of mass at each area of flow on the control surface do not vary with time. The rates at which heat and work cross the control surface are constant.

Steady-State Devices: Heat Exchangers Heat transferred to/from fluids flowing through pipes Usually constant pressure No work gets done ΔKE and ΔPE usually small Little heat transfer with surroundings if C.V. includes both fluids

06-06 Steady-State Device: A Heat Exchanger

Steady-State Devices: Nozzles A device for creating high-velocity fluid streams Smooth transition to higher velocity produces lower pressures (Bernoulli’s equation, which is just another statement of the 1 st Law) No work done Little or no change in potential energy Little or no heat transfer Inlet KE usually negligible

06-07 Steady-State Device: A Nozzle

Steady-State Devices: Diffusers Anti-nozzle: A device for decelerating fluid flow to produce an increase in pressure As with nozzles, only inlet and exit enthalpies and inlet KE contribute to the 1 st Law

06-16

Steady-State Devices: Throttles Sudden restrictions in flow passage that produces a drop in pressure Not smooth like a nozzle; not much change in KE No change in PE No work done No heat transfer Net result: pressure drop at constant enthalpy Can involve a change in phase; e.g., an expansion valve in a refrigerator

06-08 Steady-State Device: A Throttle

Steady-State Devices: Turbines Rotary machines that produce shaft work at the expense of working fluid pressure Steam or gas Inlet pressure controlled by previous pumping or compression process Exit pressure determined by environment Two internal processes: – Nozzles to increase velocity and reduce pressure – High velocity fluid directed at rotating blades that turn the shaft and generate work; low-pressure, low-velocity fluid exits the turbine Negligible change in PE Negligible inlet KE Normally taken to be an adiabatic process Normally, work output is change in enthalpy from inlet to outlet

06-09 Steady-State Device: A Turbine

Steady-State Devices: Compressors and Pumps Devices that use shaft work to increase pressure in the working fluid Two types: – Rotary; an anti-turbine – Piston/cylinder Usually taken to be adiabatic Negligible change in PE Negligible inlet KE Heat transfer negligible for rotary compressors; can be significant for piston/cylinder type

06-11 Steady-State Device: A Pump

06-12 Steady-State System: A Power Plant

06-13 Steady-State System: A Refrigerator