Theory and Practice Review of Heat Transfernsw/chbe446/2020-present/review heat transfer.pdfHeat...
Transcript of Theory and Practice Review of Heat Transfernsw/chbe446/2020-present/review heat transfer.pdfHeat...
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Review of Heat Transfer Theory and Practice
-Manufactured and Distributed by-Kevin Dilley, Kaiwen Jiang, Zachary Landau, Aashish Parekh,
and Natalie Wittick
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What Is Heat Transfer?
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Heat transfer introduction
Basic Rules of Heat Transfer
● Energy, Thermal included, cannot be created nor destroyed
● Thermal energy is naturally transferred down a temperature
gradient○ Always from a warmer to cooler substance
● Heat is either transferred by Conduction, Convection, or
Radiation
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Heat Transfer Mediums
Conduction - Transfer of thermal energy by direct contact or within a substance
● Ex. Electric Stove or Cooling a coke drink with ice
Convection - Transfer of thermal energy by bulk fluid movement
● Ex. Water in bottom of pot transferring energy throughout
Radiation - Transfer of thermal energy by electromagnetic waves (no direct contact!)
● Ex. Roasting marshmallows with a campfire
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Heat Transfer Equations for different Coordinate Systems
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Example: Heat Exchangers
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Heat Exchanger Fundamentals
● General Function: A device whose primary purpose is the transfer of energy between two
fluids
● Three categories of classification:
○ Regenerators
○ open-type exchanger
○ closed-type exchangers or recuperators.
● Heat exchangers are typically classified according to flow arrangement and type of
construction
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Types of Heat Exchangers
● Single Pass (parallel and crossflow)
○ Fluid flows in similar and opposite directions
● Shell and tube
○ Finned/Unfinned tubular HX
● Compact HX
○ Plate and Frame HX
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Single Pass Heat Exchangers
https://www.quora.com/Why-is-a-counter-flow-heat-exchanger-better-than-a-parallel-flow-heat-exchanger
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HEX Theory
Stream Energy Balance:
Total rate of heat transfer:
Heat transfer coefficient:
10
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Heat Transfer Equations for Heat Ex
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Graphical Analysis
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Unsteady State Conduction
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Finite Biot Number
Both surface and internal resistance are measurable
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http://homepages.wmich.edu/~leehs/ME431/ch05_supp_heisler.pdf
Fig. : Centerline Temp as a function of time for a cylinder
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Lumped Parameter Analysis
Used when Biot is very small (≤0.1)
Negligible internal resistance; spatially uniform
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Negligible Surface Resistance
Bi >> 0.1
Surface temperature is constant and essentially equal to the temperature of the surroundings
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Heat Transfer in Multiple Directions
For a rectangular slab:
For a cube:
For a cylinder:
Continue with same process as shown through an iterative method (one direction at a time)
until error is minimized
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Semi-Infinite Wall
Shallow penetration temperature
Short application time
Undefined characteristic length
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Convection
Convective heat transfer--heat transfer between a surface and surrounding fluid
Uses dimensionless numbers and analysis
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Convective Problems
Source: Dr. Goldberg
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Special Circumstances and Exceptions
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Thank for listening
Questions?
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Citations Welty, J. R., Rorrer, G. L., & Foster, D. G. (2019). Fundamentals of momentum, heat, and mass transfer. Hoboken, NJ: Wiley.
Bird, R. Byron, et al. Transport Phenomena. Wiley India Pvt. Ltd., 2014.