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Integral Relations for a Control Volume T Arayyes Introduction • in Analysing Fluid Motion, We Might Take One of Two Paths: 1

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Integral Relations for a Control Volume T Arayyes Introduction • in Analysing Fluid Motion, We Might Take One of Two Paths: 1 Integral Relations for a Control Volume T Arayyes Introduction • In analysing fluid motion, we might take one of two paths: 1. Seeking to describe the detailed flow pattern at every point (x, y, z) in the field OR 2. working with a finite region, making a balance of flow in versus flow out, and determining gross flow effects such as the force or torque on a body or the total energy exchange • The second is the “control-volume” method and is the subject of this chapter. • The first is the “differential” approach and is developed in Chap. 4.is • Control volume is a very quick analysis that give a quantitative results. • In Fluids: control volume used for conservation of mass, linear momentum, angular momentum, and energy • In thermodynamics it is mainly conservation of mass and energy. What is a control volume? • control volume approach: is to consider a fixed interior volume of a system • System approach: We follow the fluid as it moves and deforms.—no mass crosses the boundary, and the total mass of the system remains fixed. • an arbitrary quantity of mass of fixed identity. Everything external to this system is denoted by the term surroundings, • • Control volumes can be Fixed, moving, and deformable a) Fixed control volume for nozzle-stress analysis; b) Control volume moving at ship speed for drag-force analysis; c) Control volume deforming within cylinder for transient pressure- variation analysis. Review, Basic laws for a fixed mass or system approach • Conservation of mass: 푑푚 = 0 푑푡 Mass of the system can be defined as: 푀 = 푑푚 = 휌푑푉 푉(푠푦푠푡푒푚) • Conservation of Linear momentum, p: 푝 = 푚푣 푑푝 F = = 푚 푣 = 푚푎 (time rate of change of momentum) 푑푡 Linear momentum: 푝 = (푠푦푠) 푣 푑푚= 푉(푠푦푠) 푣 휌푑푉 • Conservation of Energy 훿푄 + 훿푊 = 푑퐸 Where Q is hear and W is work 푑퐸 푄 + 푊 = 푑푡 푠푦푠푡푒푚 퐸푠푦푠푡푒푚 = 푒. 푑푚 = 푒. 휌푑푉 푠푦푠푡푒푚 푠푦푠푡푒푚 푣2 Where 푒 = 푢 + + 푔푧. 2 • The main aim of the following sections is translate from fixed mass state where these equations were developed for into control volume where we have the mass crossing the boundaries. Review, intensive and extensive properties • Extensive properties= B (total mass, Linear momentum and energy) • Intensive properties=b=B/m Extensive property Intensive property Conservation of mass m 1 Conservation of momentum p=mv v Conservation of Energy E e Review. Classification of fluid Fluid flow may be classified as: • Uniform Flow conditions(velocity, pressure, cross section or depth) are the same at every point in the fluid. • Non-uniform Flow: Flow conditions are not the same at every point. • Steady Flow: Flow conditions could differ from point to point but it doesn’t change with time. • Unsteady Flow: If at any point in the fluid, the conditions change with time, the flow is described as unsteady. THE REYNOLDS TRANSPORT THEOREM • The previous equations in the system approach are all expressed in time rate of 푑퐵 change for . 푑푡 system • We need now to relate the change in extensive property B for a system to a control volume • The relationship between the time rates of change of an extensive property for a System, B and for a control volume is expressed by the Reynolds transport theorem (RTT). The Reynolds transport theorem (RTT) provides a link between the system approach and the control volume approach. The time rate of change of the extensive property B of the system is EQUAL *to the time rate of change of extensive B of the control volume PLUS Net rate of flux of extensive property B through the control surface For a Uniform flow: Where & RTT Special Cases • For steady flow, the time derivative drops out, 0 RTT Special Cases For a uniform velocity control volumes with well-defined inlets and outlets RTT Special Cases, For moving and/or deforming control volumes • For a control volume moving at constant velocity • The absolute velocity V in the second term is replaced by the relative velocity Reynolds transport theorem applied to a control volume moving at constant velocity. Conservation of Mass Principle The conservation of mass equation is obtained by replacing B in the Reynolds transport theorem by mass m, and b by 1 (m per unit mass =m/m =1). Steady—Flow Processes For steady flow, the total amount of mass contained in CV is constant. Total amount of mass entering must be equal to total amount of mass leaving mm in out For incompressible flows, VAVAn n n n in out conservation of Linear momentum • In linear momentum the variable is B =mV and intensive property b= dB/dm =V, and application of the Reynolds transport theorem gives the linear-momentum relation for a deformable control volume The term ∑퐹 is the vector sum of all forces acting on the control-volume material considered as a free body; i.e., it includes: • surface forces Fs on all fluids: due to shear force and pressure. • Body force Fb gravity and electromagnetic) acting on the masses within the control volume. The velocity is is the fluid velocity relative to an inertial (nonaccelerating) coordinate system; Linear momentum special case • In general in RTT equation , a uniform velocity control volumes with well-defined inlets and outlets will be as follows • Applying this to Linear momentum Net pressure force on a close control volume inwards Normal vector outwards is negative • Pressure Inwards is positive • Pressure outwards is negative Momentum-Flux Correction Factor • For flow in a duct, the axial velocity is usually nonuniform, • For this case the simple momentum-flux calculation 푢휌 푽. 풏 푑퐴 = 푚 푣 = 휌퐴푉2 is somewhat in error and should be corrected to 훽휌퐴푉2. Where 훽 is the dimensionless momentum-flux correction factor, 훽>1. Noninertial Reference Frame • All previous derivations and examples in this section have assumed that the coordinate system is inertial, i.e., at rest or moving at constant velocity. • In many cases it is convenient to use a noninertial, or accelerating, coordinate system. An example would be coordinates fixed to a rocket during takeoff. • Suppose that the fluid flow has velocity V relative to a noninertial xyz coordinate system, as shown in. Then dV/dt will represent a noninertial acceleration which must be added vectorially to a relative acceleration arel to give the absolute acceleration ai relative to some inertial coordinate system XYZ, a • The RTT equation will be modified to account for the non-inertial reference 푑 퐹 − 푎 푑푚 = 푣휌푑푉 + 푣(푣. 푛) 푑퐴 푟푒푙 푑푡 퐶푉 퐶푉 퐶푆 • Assume the rocket start from rest and ignore drage • Exhaust flow rate: 푚 푒 =5kg/s • Exhaust Velocity ve=3500m/s • m0=400 kg • Find a) Initial acceleration b) U(t) or velocity as a function of time Questions to answer • 3.13,3.14,3.15,3.16,3.20,3.22,3.39,3.42,3.43,3.45,3.49,3.54,3.55,3.60 ,3.64,3.68. • Assuming friction has a very little effect In a typical engineering problem, the control volume may contain many inlets and outlets; energy flows in at each inlet, and energy flows out at each outlet. Energy also enters the control volume through net heat transfer and net shaft work. 37 ENERGY ANALYSIS OF STEADY FLOWS The net rate of energy transfer to a control volume by heat transfer and work during steady flow is equal to the difference between the rates of outgoing and incoming energy flows by mass flow. single-stream devices A control volume with only one inlet and one outlet and energy interactions. 38 Ideal flow (no mechanical energy loss): The lost mechanical energy in a fluid flow Real flow (with mechanical system results in an energy loss): increase in the internal energy of the fluid and thus in a rise of fluid temperature. A typical power plant has numerous pipes, elbows, valves, pumps, and turbines, all of which have irreversible losses. Energy equation in terms of heads Note, hL is refer to as hf in the Frank white book. The main reason for mechanical loss is friction loss that Mechanical energy flow chart for a fluid flow system that involves a pump and a turbine. Vertical dimensions show each energy term expressed as an equivalent column height of fluid, i.e., head. 66 THE BERNOULLI EQUATION Bernoulli equation: An approximate relation between pressure, velocity, and elevation, and is valid in regions of steady, incompressible flow where net frictional forces are negligible. Despite its simplicity, it has proven to be a very powerful tool in fluid mechanics. The Bernoulli approximation is typically useful in flow regions outside of boundary layers and wakes, where the fluid motion is governed by the combined effects of pressure and gravity forces. The Bernoulli equation is an approximate equation that is valid only in inviscid regions of flow where net viscous forces are negligibly small compared to inertial, gravitational, or pressure forces. Such regions occur outside of boundary layers and wakes. 32 Conditions for Bernoulli equations 1. Steady flow The Bernoulli equation is applicable to steady flow. 2. Frictionless flow Every flow involves some friction, no matter how small, and frictional effects may or may not be negligible. 3. No shaft work The Bernoulli equation is not applicable in a flow section that involves a pump, turbine, fan, or any other machine or impeller since such devices destroy the streamlines and carry out energy interactions with the fluid particles. When these devices exist, the energy equation should be used instead. 4. Incompressible flow Density is taken constant in the derivation of the Bernoulli equation. The flow is incompressible for liquids and also by gases at Mach numbers less than about 0.3.
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