tailieunhanh - Lecture Thermodynamics: An engineering approach (8/e): Chapter 17 - Yunus A. Çengel, Michael A. Boles
Chapter 17 - Compressible flow. The objectives of Chapter 17 are to: Develop the general relations for compressible flows encountered when gases flow at high speeds, introduce the concepts of stagnation state, speed of sound, and Mach number for a compressible fluid, develop the relationships between the static and stagnation fluid properties for isentropic flows of ideal gases,. | Chapter 17 Compressible Flow Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach, 8th edition by Yunus A. Çengel and Michael A. Boles Stagnation Properties Consider a fluid flowing into a diffuser at a velocity , temperature T, pressure P, and enthalpy h, etc. Here the ordinary properties T, P, h, etc. are called the static properties; that is, they are measured relative to the flow at the flow velocity. The diffuser is sufficiently long and the exit area is sufficiently large that the fluid is brought to rest (zero velocity) at the diffuser exit while no work or heat transfer is done. The resulting state is called the stagnation state. We apply the first law per unit mass for one entrance, one exit, and neglect the potential energies. Let the inlet state be unsubscripted and the exit or stagnation state have the subscript o. Since the exit velocity, work, and heat transfer are zero, The term ho is called the stagnation . | Chapter 17 Compressible Flow Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach, 8th edition by Yunus A. Çengel and Michael A. Boles Stagnation Properties Consider a fluid flowing into a diffuser at a velocity , temperature T, pressure P, and enthalpy h, etc. Here the ordinary properties T, P, h, etc. are called the static properties; that is, they are measured relative to the flow at the flow velocity. The diffuser is sufficiently long and the exit area is sufficiently large that the fluid is brought to rest (zero velocity) at the diffuser exit while no work or heat transfer is done. The resulting state is called the stagnation state. We apply the first law per unit mass for one entrance, one exit, and neglect the potential energies. Let the inlet state be unsubscripted and the exit or stagnation state have the subscript o. Since the exit velocity, work, and heat transfer are zero, The term ho is called the stagnation enthalpy (some authors call this the total enthalpy). It is the enthalpy the fluid attains when brought to rest adiabatically while no work is done. If, in addition, the process is also reversible, the process is isentropic, and the inlet and exit entropies are equal. The stagnation enthalpy and entropy define the stagnation state and the isentropic stagnation pressure, Po. The actual stagnation pressure for irreversible flows will be somewhat less than the isentropic stagnation pressure as shown below. Example 17-1 Steam at 400oC, MPa, and 300 m/s flows through a pipe. Find the properties of the steam at the stagnation state. At T = 400oC and P = MPa, h = kJ/kg s = kJ/kg K Then and We can find Po by trial and error (or try the EES solution for problem 3-27 in the text). The resulting stagnation properties are Ideal Gas Result Rewrite the equation defining the stagnation enthalpy as For ideal gases with constant specific heats, the enthalpy .
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