Definition of Thermodynamics: Thermodynamics is the Science of Energy and Entropy
- Some definitions. - The zeroth law. - Properties of pure substances. - Ideal gas law. - Entropy and the second law.
Some definitions Intensive properties: independent of the size of the system [T, P] Extensive properties: dependent of the size of the system [V] Specific extensive properties per unit mass [υ] uasi-static process A process in which the system remains infinitesimally close to an equilibrium state at all times. gas
0 th The Zeroth law of thermodynamics Thermal equilibrium: two states are said in thermal equilibrium if, when they are brought into thermal contact with one another, their states do not change. 0 th law of thermodynamics: if A is in thermal equilibrium with B and B is in thermal equilibrium with C the; A is in thermal equilibrium with C. & A B B C A C This obvious law is the basis for the validity of temperature measurement. The 0 th cannot be concluded from the other laws of thermodynamics.
Properties of pure substances. Properties of pure substances. Definition: a pure substance is a substance with fixed and stable chemical composition
The ideal-gas equation of state General formulation: Pressure Specific volume Pv = ZRT Temperature Gas constant Compressibility factor Ideal gas Real gas Z=1 A gas behaves like an ideal gas if: - The pressure is very low. Z<1 Z=1 Z>1 - The temperature is very high (T> 2 Tc), regardless of pressure.
Specific heats Gives you an idea on the energy that you must spend to rise the temperature of 1 kg of a substance by 1 degree. At constant volume: C v = u T v= cte At constant pressure: C p = h T p= cte C p C v =R
First law of Thermodynamics Clausius statement: The variation of energy during a process is equal to the sum work and heat exchanged with the environment during the same process. Variation in internal energy Δ E = W + Work Heat
First law of Thermodynamics ΔE Internal energy variation Kinetic energy variation Potential energy variation mu ( u) 2 2 2 1 2 1 1 ( ) 2 mv V mg( z z ) 2 1 Steady flow process m& i = m& e 2 2 Ve Vi & W& = m& e he + + gze m& i hi + + gzi 2 2 exits inlets
Second law of Thermodynamics
Second law of Thermodynamics The second law states that processes occur in a certain direction. A process is realizable only if the 1 st and 2 nd laws for thermodynamics are fulfilled.
Second law of Thermodynamics Heat engine Heat Work High temperature reservoir Wnet out η th = = 1 H L H H W net out L W net out H L net work output heat supplied to the engine heat rejected by the engine Low temperature reservoir
Second law of Thermodynamics Refrigeration and Heat Pump Work Heat COP COP R HP W L = = netin W H = = netin H L 1 1 1 1 L H
Carnot cycle A-B: Reversible isothermal expansion B-C: Reversible adiabatic expansion C-D: Reversible isothermal compression D-A: Reversible adiabatic compression η th T = T 1 L H Example: T L =430 C T H =1870 C η th = 67.2
Increase in entropy principle: Sgen 0
Entropy relations Tds= du+ Pdv Tds= dh vdp Ideal gases k 1 T 2 v 1 = T v 1 s= cte 2 T 2 P 2 = T P 1 s= cte 1 P 2 v 1 = P v 1 s= cte 2 ( k 1)/ k k
Ideal process for steady flow devices is the isentropic process (adiabatic and reversible), isentropic efficiency is: Turbine Compressors Nozzles η η η T C N Actual turbine work h h = = Isentropic turbine work h h 1 2a 1 2s Isentropic compressor work h h = = Actual compressor work h h 2s 1 2a 1 Actual KE at nozzle exit h h = = Isentropic KE at nozzle exit h h 1 2a 1 2s (a) actual; (s) isentropic