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Enthalpy & First Law for CVs

Enthalpy & First Law for CVs. P M V Subbarao Professor Mechanical Engineering Department. Discovery of new Form of Energy Driving the Engineering Systems…. The Family of Steam Engines. A Direct Hardware Creations to the Essential Need …. Piston-Cylinder Systems : Constant Pressure Process.

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Enthalpy & First Law for CVs

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  1. Enthalpy & First Law for CVs P M V Subbarao Professor Mechanical Engineering Department Discovery of new Form of Energy Driving the Engineering Systems…..

  2. The Family of Steam Engines A Direct Hardware Creations to the Essential Need …..

  3. Piston-Cylinder Systems : Constant Pressure Process

  4. The issue of Increasing unit Temperature of A Pure Substance

  5. Microscopic Energy of A Flowing Fluid • Unlike control mass, control volumes involve mass flow across their boundaries. • The substance inside a control volume will be at some pressure, temperature….. • The fluid entering the control volume is pushing itself against the pressure of the control volume. • Some work transfer is involved in pushing this mass into the control volume. • This is an internal work. • This is called flow work or flow energy.

  6. Visualization of Flow Work F p • F= pA will be a driving force responsible for the pushing of the fluid into the CV. • This force will perform a work transfer of F.L to push the fluid. • Therefore, the flow work = F ×L = p × A × L = p × V • This can exists even when there is a fluid pushed out of the CV. L

  7. Flow work • It is interesting to note that unlike other work quantities, the flow work is expressed in terms of state variables. • This is also a state variable, point function and hence a thermodynamic property. • This is also called flow energy, convected energy or transport energy. • The total energy of non flowing fluid: E = m(u + ½V2 + gz) • The total energy of flowing fluid: Q =m(u + pv + ½V2 + gz). • This total energy is also called as Methalpy. • The term u + pv is called as specific Enthalpy, h.

  8. Energy transport by Moving fluid • Amount of energy transport by a moving fluid of mass m: Q= mθ = m ( h + ½V2 + gz ) • Rate of Energy Transport:

  9. Internal Energy & Enthalpy of Wet Mixtures • x is the dryness fraction. • U = (1-x) Uf + x Ug • Specific Internal energy: Internal energy per unit mass ; u • u = (1-x) uf + x ug • Specific enthalpy • h = (1-x) hf+ x hg T ufg uf u ug

  10. Rate Equations for Laws of Nature : Control Mass Conservation of Mass : Conservation of Momentum : First law of thermodynamics :

  11. First Law for A Control Volume • Conservation of mass: • Conservation of momentum: • Conservation of energy:

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