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Energy Conservation (Bernoulli’s Equation)

Energy Conservation (Bernoulli’s Equation). Recall Euler’s equation:. Also recall that viscous forces were neglected, i.e. flow is invisicd. If one integrates Euler’s eqn. along a streamline, between two points ,  &. We get :. Which gives us the Bernoulli’s Equation.

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Energy Conservation (Bernoulli’s Equation)

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  1. Energy Conservation (Bernoulli’s Equation) Recall Euler’s equation: Also recall that viscous forces were neglected, i.e. flow is invisicd If one integrates Euler’s eqn. along a streamline, between two points ,  & We get : Which gives us the Bernoulli’s Equation Flow work + kinetic energy + potential energy = constant

  2. Dx p A Bernoulli’s Equation (Continued) Flow Work (p/): It is the work required to move fluid across the control volume boundaries. Consider a fluid element of cross-sectional area A with pressure p acting on the control surface as shown. Due to the fluid pressure, the fluid element moves a distance Dx within time Dt. Hence, the work done per unit time DW/Dt (flow power) is: Flow work or Power Flow work per unit mass 1/mass flow rate Flow work is often also referred to as flow energy

  3. Bernoulli’s Equation (Cont) Very Important: Bernoulli’s equation is only valid for : incompressible fluids,steady flow along a streamline, no energy loss due to friction, no heat transfer. Application of Bernoulli’s equation - Example 1: Determine the velocity and mass flow rate of efflux from the circular hole (0.1 m dia.) at the bottom of the water tank (at this instant). The tank is open to the atmosphere and H=4 m p1 = p2, V1=0 1 H 2

  4. 4 4 3 water height (m) h ( t ) 2 1 2.5e-007 0 0 20 40 60 80 100 0 t 100 time (sec.) Bernoulli’s Eqn/Energy Conservation (cont.) Example 2: If the tank has a cross-sectional area of 1 m2, estimate the time required to drain the tank to level 2. 1 First, choose the control volume as enclosed by the dotted line. Specify h=h(t) as the water level as a function of time. h(t) 2

  5. Energy exchange (conservation) in a thermal system Energy added, hA (ex. pump, compressor) Energy lost, hL (ex. friction, valve, expansion) Energy extracted, hE (ex. turbine, windmill) hL loss through valves heat exchanger hE hA pump turbine hL, friction loss through pipes hL loss through elbows condenser

  6. 2 1 pump zo Z=15 in If energy is added, removed or lost via pumps turbines, friction, etc.then we use Energy conservation(cont.) Extended Bernoulli’s Equation Example: Determine the efficiency of the pump if the power input of the motor is measured to be 1.5 hp. It is known that the pump delivers 300 gal/min of water. No turbine work and frictional losses, hence: hE=hL=0. Also z1=z2 Given: Q=300 gal/min=0.667 ft3/s=AV V1= Q/A1=3.33 ft/s V2=Q/A2=7.54 ft/s 4-in dia.pipe 6-in dia. pipe Looking at the pressure term: Mercury (m=844.9 lb/ft3) water (w=62.4 lb/ft3) 1 hp=550 lb-ft/s

  7. Energy conservation (cont.) Example (cont.)

  8. Frictional losses in piping system P1 R: radius, D: diameter L: pipe length w: wall shear stress P2 Consider a laminar, fully developed circular pipe flow w P+dp p Darcy’s Equation:

  9. Losses in Pipe Flows Major Losses: due to friction, significant head loss is associated with the straight portions of pipe flows. This loss can be calculated using the Moody chart or Colebrook equation. Minor Losses: Additional components (valves, bends, tees, contractions, etc) in pipe flows also contribute to the total head loss of the system. Their contributions are generally termed minor losses. The head losses and pressure drops can be characterized by using the loss coefficient, KL, which is defined as One of the example of minor losses is the entrance flow loss. A typical flow pattern for flow entering a sharp-edged entrance is shown in the following page. A vena contracta region is formed at the inlet because the fluid can not turn a sharp corner. Flow separation and associated viscous effects will tend to decrease the flow energy; the phenomenon is fairly complicated. To simplify the analysis, a head loss and the associated loss coefficient are used in the extended Bernoulli’s equation to take into consideration this effect as described in the next page.

  10. V1 Minor Loss through flow entrance V2 V3 (1/2)V32 (1/2)V22 KL(1/2)V32 pp

  11. Let us now also account for energy transfer via Heat Transfer, e.g. in a heat exchanger The most general form of conservation of energy for a system can be written as: dE = dQ-dW where (Ch. 3, YAC) Energy Conservation (cont.) • dE  Change in Total Energy, E • and E = U(internal energy)+Em(mechanical energy) (Ch. 1 YAC) • E = U + KE (kinetic energy) + PE(potential energy) • dW  Work done by the system where • W = Wext(external work) + Wflow(flow work) • dQ = Heat transfer into the system (via conduction, convection & radiation) • Convention: dQ > 0 net heat transfer into the system (Symbols Q,q..) • dW > 0, positive work done by the system • Q: What is Internal Energy ? mechanical energy

  12. Energy Conservation (cont.) U = mu, u(internal energy per unit mass), KE = (1/2)mV2 and PE = mgz Flow work Wflow= m (p/) It is common practice to combine the total energy with flow work. Thus: The difference between energy in and out is due to heat transfer (into or out) and work done (by or on) the system.

  13. Hence, a system exchanges energy with the environment due to: • Flow in/out 2) Heat Transfer, Q and 3) Work, W • This energy exchange is governed by the First Law of Thermodynamics Energy Conservation (cont.) Heat in, =dQ/dt system Work out dW/dt “Enthalpy”

  14. Example: Superheated water vapor enters a steam turbine at a mass flow rate 1 kg/s and exhausting as saturated steam as shown. Heat loss from the turbine is 10 kW under the following operating condition. Determine the turbine power output. Conservation of Energy – Application From superheated vapor tables: hin=3149.5 kJ/kg P=1.4 Mpa T=350 C V=80 m/s z=10 m 10 kw P=0.5 Mpa 100% saturated steam V=50 m/s z=5 m From saturated steam tables: hout=2748.7 kJ/kg

  15. Q – total heat transfer (J) – rate of total heat transfer (J/s, W) q – heat transfer per unit mass (J/kg) – Heat Flux, heat transfer per unit area (J/m2) Q, q … ?!% Back • Internal Energy ? • Internal energy, U (total) or u (per unit mass) is the sum of all • microscopic forms of energy. • It can be viewed as the sum of the kinetic and potential energies of the molecules • Due to the vibrational, translational and rotational energies of the moelcules. • Proportional to the temperature of the gas. Back

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