Download Steady Open Systems - Thermodynamics - Lecture Slides and more Slides Thermodynamics in PDF only on Docsity! Unit 5 – Steady open systems September 23, 2010 ME 370 - Thermodynamics 1 Unit Five – First Law for Steady, Open Systems Mechanical Engineering 370 Thermodynamics Larry Caretto September 23, 2010 2 Outline • Quiz four results • Unit five – steady open systems – View first law as a rate equation – Have mass crossing system boundaries – Flows across boundaries have several energy forms including internal (u) , kinetic and potential energy plus flow work (Pv) – Internal energy plus flow work is h = u + Pv 3 Future Quizzes • Use equation summary from course web site – Presently in “What’s New’ section – Always available from link to course notes • Only the ten highest quiz grades (of twelve) are counted in final grade • Total quiz grade based on quizzes taken – Students who take N quizzes with N < 10 will receive zeros on 10 – N quizzes 4 Unit Five Goals • Topic is first law for open systems, i.e., systems in which mass flows across the boundary • Will look at general results and focus on steady-state systems. • As a result of studying this unit you should be able to – understand all the terms (and dimensions) in the first law for open systems: 5 Open System Concepts rate of energy change (kW) uW the useful work rate or mechanical power (kW) the mass flow rate (kg/s)m gz the potential energy per unit mass (kJ/kg) the kinetic energy per unit mass (kJ/kg)2 2V systemsystem gz V umE ) 2 ( 2 total energy (kJ) Q heat transfer rate (kW) dt dEsystem Use rate terms mass/time, energy/time = power Engineering units use Btu/hr 6 Unit Five Goals Continued – use the equation relating velocity, mass flow rate, flow area, A, and specific volume – use the mass balance equation outlet i inlet i system mm dt dm v AV m Unit 5 – Steady open systems September 23, 2010 ME 370 - Thermodynamics 2 7 Flow Work • For open systems work is done on (or by) mass entering and leaving the system • Flow work is Pv times mass flow rate • Add this flow work to internal energy (times mass flow rate) • First law for mass flows has h = u + Pv (sum of internal energy plus flow work) 8 Unit Five Goals Continued – use the first law for open systems – use the steady- state assump- tions and equations 0 dt dE dt dm systemsystem outlet o o oou inlet i i ii system gz V hmWQ gz V hm dt dE 2 2 2 2 9 Steady-state equations – Steady- state first law for open systems – Steady-state mass balance for open systems inlet i i ii outlet o o oou gz V hm gz V hmWQ 2 2 2 2 outlet o inlet i mm 10 Unit Five Goals Continued – recognize that kinetic and potential energies are usually negligible • A 1oC temperature change in air (ideal gas with cp = 1.005 kJ/(kg·K) has h = 1005 J/kg • The same kinetic energy change requires a velocity increase from zero to 45 m/s (~100 mph) • The same potential energy change requires an elevation of 102 m (336 ft) 11 Unit Five Goals Concluded – work with ratios q and w – Use the general definition of these ratios and the first law, which are (in simplest case) )hh(mQW inoutu – handle simplest case: steady-state, one inlet, one outlet (one mass flow rate), negligible changes in kinetic and potential energies m Q q m W w uu )( inoutu hhqw 12 Example Calculation • Given: 10 kg/s of H2O at 10 MPa and 700oC enters a steam turbine; the outlet is at 500 kPa and 300oC. There is a heat loss of 400 kW. • Find: Useful work rate (power output) • Assumptions: Steady-state, negligible changes in kinetic and potential energies • Configuration: one inlet and one outlet
