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lecture03 - LECTURE 3 THERMODYNAMICS An Engineering...

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1 LECTURE 3 THERMODYNAMICS An Engineering Approach Mass & Energy Analysis of Control Volume Examples of Steady-State Devices The 2nd Law of Thermodynamics Dr. MinJun Kim Department of Mechanical Engineering & Mechanics Drexel University Conservation of Mass Conservation of mass ( continuity equation ): Stating that if the mass inside the control volume changes with time it is because we add some mass or take some mass out. The present chapter is concerned with development of the control volume forms of the conservation of mass and energy in situations where there are flows of substance present.
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2 Conservation of Mass The flow across the control volume surface can be indicated with an average velocity shown to the left of the valve or with a distributed velocity over the cross section as shown to the right of the valve. The flow rate is So the mass flow rate becomes Where often the average velocity is used. Example Air is flowing in a 0.2 m diameter pipe at a uniform velocity of 0.1 m/s. The temperature is 25 ! C and the pressure 150 kPa. Determine the mass flow rate.
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3 The 1 st Law for a Control Volume The first law of thermodynamics for a control mass, which consists of a fixed quantity of mass, may be written as We have also noted that this may be written as an instantaneous rate equation as The fundamental physical law states that we cannot create or destroy energy such that any rate of change of energy must be caused by rates of energy into or out of the control volume. Rates of heat transfer and work are already included, so we need additional explanation associated with the mass flow rates!!! Flow Work and The Energy of a Flowing Fluid The fluid flowing across the control surfaces enters or leave with an amount of energy per unit mass as relating to the state and position of the fluid. We are able to express the rate of flow work as The flow work per unit mass is then P " , and the total energy associated with the flow of mass is The extension of the first law of thermodynamics becomes The final form of the first law of thermodynamics
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4 Steady-State Process 1. The control volume does not move relative to the coordinate frame. Meaning that all velocities relative to the control surface, and there is no work associated with the acceleration of the control volume. 2. The state of the mass at each point in the control volume does not vary with time which requires: Let us consider a certain set of assumptions that lead to a reasonable model for steady-state process and also Therefore, we conclude that for the steady-state process we can write Energy Analysis of Steady-Flow Systems Many of the applications of the steady-state model are such that there is only one flow stream entering and one leaving the control volume. For this type of process, we can write Continuity equation : !
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This note was uploaded on 02/28/2011 for the course MEM 310 taught by Professor Miller during the Winter '08 term at Drexel.

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lecture03 - LECTURE 3 THERMODYNAMICS An Engineering...

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