The First Law of Thermodynamics Cengel & Boles, Chapter 4 ME 152

The First Law of Thermodynamics The First Law of Thermodynamics is also known as Conservation of energy principle Energy balance equation The “First Law” In basic terms, it states: ME 152

Energy Balance for a Closed System and ME 152

Energy Balance for a Closed System, cont. The energy balance statement becomes Rate form: ME 152

Closed System Energy Balance: Special Cases Stationary system (KE, PE = 0) Adiabatic system (Q = 0) No work interactions (W = 0) Cycle (initial state = final state) ME 152

Energy Balance for Control Volumes Balance of energy transfer by work, heat, and mass flow across system boundary plus stored energy within control volume: ME 152

Energy Transport by Mass Recall from Chapter 3: ME 152

Energy Balance for Steady-Flow Systems At steady-state, dEsystem/dt = 0, thus: ME 152

Energy Balance for Steady-Flow Systems, cont. For single-stream control volumes, Dividing by mass flow rate gives the energy balance on a unit-mass basis: ME 152

Steady-Flow Engineering Devices Nozzles and diffusers Turbines, compressors, and pumps Throttling valves Mixing chambers Heat exchangers Pipe and duct flow See section 4-4 in text and PDF file ME152SSdevices.pdf on website ME 152

Energy Balance for Unsteady-Flow Processes Unsteady = transient Examples: Filling and discharging of tanks Start-up and shut-down of turbines, compressors, hxers, etc. Mathematically, need to solve 1st order differential equation ME 152