Notes_15_Multiple_Flows_Control_Volume_Analysis

Notes_15_Multiple_Flows_Control_Volume_Analysis - Notes...

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1 Notes 14--Sections 5-4 and 5-5
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What we covered Friday Control volume analysis for the following devices: Nozzles Diffusers Throttles Turbines Compressors 2
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Topics for today Control volume analysis for: Heat Exchangers Mixing devices Transient analysis 3
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4 Heat Exchangers and mixing devices Closed heat exchangers are devices which transfer heat between different fluids. The fluids can be at different pressures. Mixing devices (also called open heat exchangers) combine two or more fluids to achieve a desired output, such as fluid temperature or quality. Fluids must be at the same pressure.
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5 Heat Exchangers In a closed heat exchanger, the fluids do not mix. This is a shell-and-tube heat exchanger.
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6 Heat Exchangers Your book has very simple examples of heat exchangers. One is counterflow where the fluids flow in opposite directions in the heat exchanger:
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7 Heat Exchangers Another type is parallel flow, where the fluids flow in the same direction:
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8 Heat Exchangers Yet another type is cross-flow , shown below. These are common in air conditioning and refrigeration systems.
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9 With heat exchangers, we have to deal with multiple inlets and outlets 1 m 2 m 4 m 3 m
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10 Question At steady flow , what is the relationship between ? m and , m , m , m 4 3 2 1 1 m 2 m 4 m 3 m
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11 What assumptions to make? See any devices producing/using shaft work? What about potential energy effects? What about kinetic energy changes? Can we neglect heat transfer?
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12 Apply conservation of mass on both streams. .. 4 2 3 1 0 m m m m dt dm CV - - + = = A 2 1 m m m = = If we have steady flow, then: Further, intuition tell us that: B 4 3 m m m = = 1 m 2 m 4 m 3 m Fluid A Fluid B
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13 Conservation of energy can be a little more complicated. .. 1 m 2 m 4 m 3 m Make the control volume around the whole heat exchanger and say that the heat exchanger is well insulated from the environment . Implications: No heat transfer from the control volume. Fluid A Fluid B
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14 Conservation of energy looks pretty complicated: A 2 1 m m m = = B 4 3 m m m = = + + + + + + - 3 2 3 3 3 1 2 1 1 1 2 2 gz h m gz h m W Q CV V V 0 2 2 4 2 4 4 4 2 2 2 2 2 = + + - + + - gz h m gz h m V V We know from conservation of mass:
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15 Conservation of energy equation for the heat exchanger ( 29 ( 29 A A CV z z g h h m W Q - + - + - + - 2 1 2 2 2 1 2 1 2 2 V V ( 29 ( 29 0 2 2 4 3 2 4 2 3 4 3 = - + - + - + B B z z g h h m V V Apply what we know about the mass flow relationships:
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16 Heat Exchangers Generally, there is no heat transfer from or to the heat exchanger, except for what goes on inside. (There is internal heat transfer).
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Notes_15_Multiple_Flows_Control_Volume_Analysis - Notes...

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