Thermodynamics 1, Chapter 1 & 2 week 1
Thermodynamics 1, Chapter 1 & 2 week 1 ME 236
University of Hartford
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This 7 page Class Notes was uploaded by Jeffrey Severino on Friday May 20, 2016. The Class Notes belongs to ME 236 at University of Hartford taught by Dr. Ribarov in Summer 2016. Since its upload, it has received 20 views. For similar materials see Thermodynamics I in Engineering and Tech at University of Hartford.
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Date Created: 05/20/16
CHAPTER 1 CONCEPT OF ENERGY Macroscopic: Things we can see - (Classical Thermodynamics) Microscopic: Statistical Thermodynamics – (Energy being quantized) A Microscopic amount of mass can present energy in the following forms: - Internal – internal structure - Kinetic Energy-Related to motion - Potential Energy-External forces acting on this mass - Rotational Energy-Rotational force TotalEnergy=I+KE+PE+ℜ=U+KE+PE+ℜ Dividingbymassgivesus:e=E /m=u+ke+pe+ℜ=u+1/2v +gh+1/2I ꙍ 2 ¿ Internal Energy (macroscale)-similar set of energies (associated with macroscale motion of individual molecules .U=U external molectranslatio∫molecule Where: U externalmoleculeermolecular forces (PE sum) U translationKE of molecule) U molecule Internal/atomic structure ∫ But there is a difference between intermolecular forces High Density (ρ) =close spacing = high Low Density (ρ) =loose spacing = weak molecular interaction molecular interaction U ≈0 In the limit of a very low ρ (rarified gas) => ext - No external forces, molecules are too far apart, so the forces are little between them - Ex: leaving the earth’s atmosphere Translated Energy depends only on mass & velocity of center of mass of the molecule Internal Energy depends on the molecules internal structure We rewrite the energy eqn: .U=U molecule rotationvibrationatoms ∫ There are three principal vibration modes of the H2O molecule While we evaluate the energy of the molecule, we refer to these energy modes as Degrees of Freedom [(d.o.f) -> modes of energy] Atomic/Molecular Structure Element or Compound Degrees of Freedom Monatomic molecule He 3 (x,y,z) Diatomic molecule O2 6 (x,y,z) rotation (x,z) vibration (y)-> causes ‘stretch’ Triatomic molecule H2O 9 (x,y,z) translation: 3 rotation: 3 vibration: 3 Most complex polyatomic molecules are 3-D structures that have multiple vibration modes, each of which contributes to the energy storage capabilities of the molecules Phases: solid, liquid, vapor (gas) State: specific condition, expressed by a unique set of properties such as Pressure, Temperature and Density - Two independent properties can define a state - Intensive properties independent of mass (P,T,ρ) - Extensive properties dependent on mass (m,V,E) Process: a change of substance beginning at state 1 to final state 2 through a continuous variation of state. 1) Process path: specific succession of states that which this happens - Device behaves as a process equation/device equation ex: heating a cup without container; exposes cup to atmospheric pressure-> constant pressure process => ”isobaric” process 2) Heating Air in constant volume container => “isochoric” process 3) Air compressed in a piston, cylinder walls are at constant temperature => “isothermal” process 4) Air compressed in an insulated piston (no cooling/heating) => “adiabatic” process Cycle: a process path, that ends in the initial state, it can be a complex process or a series of simple processes. - Cycles do not change the working substance - However, outside of the cycle; the surrounding environment may change while the working substance is at different conditions during this cycle=>Net effect is energy conversion process ex)engine needs surrounding! CHAPTER 2 PROPERTIES OF A PURE SUBSTANCE Pure Substance- has homogeneous & invariable chemical composition; may exist in one or more phases A mixture of gases is considered a pure substance *as long as there is no phase change Phase Boundaries: 1) Saturation Temperature – temperature at whiche vaporization takes place at a given pressure (“boiling temperature”) 2) Fusion Line- border between solid and liquid phases 3) Sublimation Line- border between solid & vapor phases 4) Triple Point- only P,T combinations where all three phases exist @ the same time (T=0.001 degrees Celcius, P=0.6113 kPa)=> water *(P of atm = 101.3 kPa) bellow Triple Point is no liquid 5) Vaporization Line- border between liquid and vapor phases Critical Point for H2O is at P=22.09 MPa, this is where vaporization stops. CHAPTER 2 PROPERTIES OF A PURE SUBSTANCE The P-V-T surface: CHAPTER 2 PROPERTIES OF A PURE SUBSTANCE X=0 -> Saturated Liquid X=1-> Saturated Vapor Sup. Vapor->Superheated Vapor (point where no liquid is left, only vapor as a result from the liquid being “superheated”) Steam Tables give the difference between the given temperature and the saturated temperature for the same pressure Superheated Vapor Region: Table B.1.3 Compressed Liquid Region: Table B.1.4 Saturated liquid/Saturated Vapor-> as f(T) -> table B.1.1 as f(P) -> table B.1.2 Saturated solid/Saturated Vapor-> as f(T) -> table B.1.5 Two Phase States If: Saturated liquid = “f” Saturated steam = “g” Then: TotalVolume=V=V +V liq vapm ∗liqm fV vap g V m liq m vap Dividingby massgivesus:v= = V f V g m m m m vap Quality,x= = (−x V )xfV g m CHAPTER 2 PROPERTIES OF A PURE SUBSTANCE VfgV −g =¿f=V +x∗V f fg Example: Find overall specific volume (v) if a saturated mixture of water at 200 degrees C, x=70% 70%=0.7 m 3 m 3 B1.1→V =f.01156 kg,V g0.12736 kg 3 3 3 v=(1−x ) +x∗V = 1−0(7 0.0)156 m +0.7∗ 0.12736 m = 0.0895m f g ( kg) ( kg ) kg
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