ME 200 L16: ME 200 L16:Transient & Steady State Processes Read ThermoMentor © Program Launched Spring 2014 MWF AM J. P. Gore Gatewood Wing 3166, Office Hours: MWF TAs: Robert Kapaku Dong Han

2 In this Lecture … Example with significant simplification of the energy equation (note the role of enthalpy difference Δh, kJ/kg) Transient and Steady State Processes in Control Volumes Example of Transient Mass or/and Energy Balance.

3 Example Steam with a specific enthalpy of 3000 kJ/kg and a mass flow rate of 0.5 kg/s enters a horizontal pipe. At the exit, the specific enthalpy is 1700 kJ/kg. If there is no significant change in kinetic energy, determine the rate of heat transfer between the pipe and its surroundings, in kW. Assume steady state. Find –Q = ? in kW Sketch Assumptions –The control volume is at steady state. –ΔW cv = Δke = Δpe = 0 Basic Equations 12 h 2 = 1700 kJ/kgh 1 = 3000 kJ/kg m 1 = 0.5 kg/s steam 3 ;

4 Example Steam with a specific enthalpy of 3000 kJ/kg and a mass flow rate of 0.5 kg/s enters a horizontal pipe. At the exit, the specific enthalpy is 1700 kJ/kg. If there is no significant change in kinetic energy, determine the rate of heat transfer between the pipe and its surroundings, in kW. Assume steady state. Find –Q = ? in kW System (mass flowing through pipe) 12 h 2 = 1700 kJ/kgh 1 = 3000 kJ/kg m 1 = 0.5 kg/s steam 4

5 Example Assumptions W cv = 0 ΔKE = 0 ΔPE = 0 Steady State, Steady Flow Operation Basic Equations Solution 5

Conservation of Mass and Conservation of Energy (CV) Exit Flow work Inlet Flow work

Conservation of Mass (CM) Inlet Flow work=0 =0 Control Mass Mass Note that Volume, Specific Volume and Density of a Control Mass can change, While the “Mass,” remains constant. Example: Piston-Cylinder Device

Examples: Transient Mass Conservation Problem: An initially empty tank is filled with a liquid (density= 1000 kg/m 3, specific heat: 4.2 kJ/kg-K, T=10 o C) at a rate of 2 kg/minute for 20 minutes and then with a rate that decreases linearly with time to 0 kg/minute over 10 minutes Find :(1) The final mass in the tank, (2) Time taken by a 2 kW heater to warm the liquid in the tank from 10 o C to 20 o C, Given: t = 0 40 Kg0 kg 50 kg t = 20 mins.t = 30 mins. Equations Filling time = 30 minutes Heating time = 17.5 minutes HW: Heating begins during filling