ENGR 2213 Thermodynamics F. C. Lai School of Aerospace and Mechanical Engineering University of Oklahoma

First Law of Thermodynamics Energy can be neither created or destroyed; it can only change forms. Conservation of Energy Principle Closed Systems Change in the total energy = of the system Net energy transferred in - by heat Net energy transferred out by work

First Law of Thermodynamics E: total energy includes kinetic energy, potential energy and other forms of energy All other forms of energy are lumped together as the internal energy U. Internal energy U is an extensive property. Specific internal energy u = U/m is an intensive property

Internal Energy ● Sensible Energy It is associated with the kinetic energies of the molecules - Translational kinetic energy - Rotational kinetic energy - Vibrational kinetic energy

Internal Energy ● Chemical (Bond) Energy ● Nuclear Energy It is associated with the atomic bonds of a molecule It is associated with the bonds within the nucleus of an atom. It is associated with the phase change of a system. It is the energy used to overcome the intermolecular forces. ● Latent Energy

Internal Energy For saturated mixtures For compressed liquid based on the given temperature

Example 1 A piston-cylinder assembly contains 8 kg of superheated water vapor at 500 kPa and 300 ºC. Steam is now cooled at constant pressure until 70% of it, by mass, condenses. Find the work associated with this process. A piston-cylinder assembly contains 8 kg of superheated water vapor at 500 kPa and 300 ºC. Steam is now cooled at constant pressure until 70% of it, by mass, condenses. Find the work associated with this process. p v 12 W

Example 2 An insulated rigid tank contains saturated water vapor at 100 ºC. The water is rapidly stirred until the pressure becomes 200 kPa. Find the water temperature at the end of the process and work associated with this process. p v 1 2 An insulated rigid tank contains saturated water vapor at 100 ºC. The water is rapidly stirred until the pressure becomes 200 kPa. Find the water temperature at the end of the process and work associated with this process.

Example 3 p v 2 1 Steam in a radiator (V = 20 L) is at P 1 = 300 kPa and T 1 = 250 ºC. If the valves are closed now, determine the amount of heat that will transfer to the room when the pressure drops to 100 kPa. Steam in a radiator (V = 20 L) is at P 1 = 300 kPa and T 1 = 250 ºC. If the valves are closed now, determine the amount of heat that will transfer to the room when the pressure drops to 100 kPa. Q

Example 4 Water contained in a piston-cylinder assembly undergoes two processes in series. Initially, water is at 1 MPa and 400 ºC. 1 → 2 water is cooled as it is compressed at constant pressure to saturated vapor at p 2 = 1 MPa. 2 → 3 water is cooled at constant volume to 150 ºC. Find heat and work involved in these processes.

Example 4 (continued) p v 12 W 12 3

Example 5 A water mixture with an initial quality of 25% is contained in a piston-cylinder assembly. The mass of the piston is 40 kg, and its diameter is 10 cm. The atmospheric pressure is 100 kPa. As the water is heated, the pressure inside the cylinder remains constant until the piston hits the stops. Heat transfer to the water continues until the pressure becomes 300 kPa. Neglect the friction between the piston and cylinder wall. Determine the total amount of heat transfer.

Example 5 (continued) p v 21 W cm 4 cm

Example 6 Two rigid tanks are connected by a valve. Tank A contains 0.2 m 3 of water at 400 kPa and 80% quality. Tank B contains 0.5 m 3 of water at 200 kPa and 250 ºC. The valve is now opened, and the two tanks eventually come to the same state. Determine the pressure and the amount of heat transfer when the system reaches thermal equilibrium with the surroundings at 25 ºC.

Example 6 (continued) Tank A Tank B p A1 = 400 kPa x A1 = 0.8 p B1 = 200 kPa T B1 = 250 ºC Q