lecture 18 - Thermal and Fluids Engineering I Lecture 18...

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Thermal and Fluids Engineering I Lecture 18 Page 1 Lecture 18 – Steady Flow Energy Equation Consider a flow subject to the following assumptions: - incompressible - steady - one stream in and out The energy equation for this case is [see Eq. (9-26)] 22 11 2 2 2 2 0 cv cv PP qw u g z u g z ρρ ⎛⎞ = + + ++− + ++ ⎜⎟ ⎝⎠ VV Define P w - pump work, always positive T w - turbine work, always positive The control volume work is cv T P ww w =− 2 2 2 2 0 cv P T w u g z u g z =+−+ + + + + Rearranging () 21 12 cv PT uu q Pw P w zz gg g g g −− + = + + Define pump head and turbine head as P P P wW h gm g == & & TT T h g & & Also recall the definition of head loss
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Thermal and Fluids Engineering I Lecture 18 Page 2 () 21 cv L uu q h g −− = 22 11 12 P TL PP zh z h h gg ρρ ++ + = + + VV The mechanical efficiency of a pump is defined as The efficiency of a motor is If a motor is used to drive a pump , P m P motor electric WW ηη = && The mechanical efficiency of a hydroturbine is defined as , P mp in W W η = & & out motor electric W W = & &
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lecture 18 - Thermal and Fluids Engineering I Lecture 18...

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