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Understanding the First Law of Thermodynamics for Open Systems

This lecture covers the fundamentals of the First Law of Thermodynamics as applied to open systems. It explains the concepts of energy transfer rates into and out of a system and how these relate to the net energy stored. Open systems, also known as control volumes, play a vital role in engineering applications. The lecture expands on the specific energy terms, including kinetic and potential energy, and discusses power transfers across system boundaries. Practical applications such as nozzles, diffusers, heat exchangers, turbines, compressors, and pumps are explored, highlighting their purposes within fluid flow control and energy management.

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Understanding the First Law of Thermodynamics for Open Systems

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  1. Lecture 14 The First Law for Open Systems

  2. The First Law for Open Systems Energy cannot be created or destroyed. Net energy transfer rate into the system Net energy transfer rate out of the system * Net rate of energy stored in the system Therefore, *Open systems are also known as control volumes

  3. The First Law for Open Systems Expanding the specific energy terms, Writing out the kinetic and potential terms and rearranging, All of the power transferred into and out of the system

  4. Open Systems – Power Transfers Breakdown of the power transfers crossing the system boundary All of the power transferred into and out of the system Power required to move the fluid in and out of the system Everything else occurring at the system boundary (boundary displacement, rotating shafts, electrical effects, etc.)

  5. Power Required to Move the Fluid The powerrequired to move the fluid into and out of the system is the force exerted on the boundary multiplied by the velocity of the flow, Flow work

  6. The First Law for Open Systems Substituting these expressions into the First Law, Finally …

  7. The First Law of Thermodynamics Notice that for a closed system, the above equation becomes, Have you seen this equation before???

  8. The First Law of Thermodynamics “I am confused. I don’t understand when to use h and when to use u in the First Law.” Do not be confused! Always start with the First Law in its most general form, Then modify it appropriately for the system that has been identified.

  9. Nozzles and Diffusers (Sect 6.5) Purpose: To increase the velocity of a flow Purpose: To decrease the velocity of a flow

  10. Throttling Devices (Sect 6.6-6.7) Purpose: Fluid flow control

  11. Heat Exchangers (Sect 6.8) Purpose: Heat or cool fluid flow streams Counter Flow Double Pipe Direct Contact Parallel Flow Double Pipe Cross Flow

  12. Heat Exchangers (Sect 6.8)

  13. Turbines (Sect 6.9) Purpose: Power production

  14. Compressors (Sect 6.9) Purpose: Move a flow of vapor or gas

  15. Pumps (Section 6.9) Purpose: Move a flow of liquid

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