Compact Heat Exchangers -- A New Approach

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Compact Heat Exchangers -- A New Approach
P.M.V Subbarao Associate Professor Mechanical
Engineering Department Indian Institute of
Technology, Delhi
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Introductory Remarks

• Power, Process, Refrigeration and A/c and
  Aerospace industries require small size and light
  weight heat exchanger devices.
• The size of heat exchanger is very large in those
  applications where gas is a medium of heat
  exchange.
• Continuous research is focused on development of
  Compact Heat Exchangers --- High rates of heat
  transfer per unit volume.
• The rate of heat exchange is proportional to
• The value of Overall heat transfer coefficient.
• The surface area of heat transfer available.
• The mean temperature difference.

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Overall Heat Transfer Coefficient and Thermal
Resistance

• In general the heat transfer coefficient of the
  gas may be 10 to 50 times smaller than that of
  the liquid.
• In phase-change heat exchangers, the air side
  heat transfer often limits the thermal
  performance of heat exchangers.
• The air side can comprise 75 of the thermal
  resistance in an evaporator and 95 in a
  condenser used in typical refrigeration
  applications.
• In applications where the thermal resistance of
  one fluid dominates, significant cost reductions
  and energy savings can be achieved by using heat
  transfer augmentation devices or methods.
• For this reason development of high-performance
  surfaces for air side heat transfer augmentation
  is an important area of interest.

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Large surface area Heat Exchangers

• The use of extended surfaces will reduce the gas
  side thermal resistance.
• To reduce size and weight of heat exchangers,
  many compact heat exchangers with various fin
  patterns were developed to reduce the air side
  thermal resistance.
• Fins on the outside the tube may be categorized
  as
• 1) flat or continuos (plain, wavy or interrupted)
  external fins on arrays of tubes,
• 2) Normal fins on individual tubes,
• 3) Longitudinal fins on individual tubes.
• Kays and London presented pressure drop and
  heat transfer characteristics of a wide variety
  of configurations of compact heat exchanger
  matrices.

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Fin and Tube Heat Exchanger A primitive Compact
Heat Exchanger
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Anatomy of Fin Tube Heat Exchanger
Gas Flow
Plate
Tube
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Two Dimensional Conduction In Cylindrical
Coordinates System
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Conduction Equation
Local Heat Flux Vector
GDE for Steady State Temperature Distribution in
Fin
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Variation of Heat Transfer Coefficient
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Comments on Conventional Compact Heat exchangers

• Conventional compact heat exchangers were
  developed by increasing heat transfer surface
  area per unit volume of heat exchanger.
• In spite of much larger heat transfer area the
  major part of thermal heat transfer resistance is
  due to gas side.
• The potential for increasing the fin area is
  limited by the fact that the increasing fin area
  leads to drop in fin efficiency.
• Major part of the fin area is wetted by low heat
  transfer coefficient gas flow.
• large number of fins makes the system costlier,
  heavier, and spacious.
• An attempt for having a broader vision on the
  enhancement of heat transfer coefficient is yet
  to be achieved.

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Enhancement of Heat Transfer Coefficient

• The magnitude of heat transfer coefficient is
  proportional to Reynolds number.
• Huge increase in heat transfer coefficient is
  seen in turbulent flows when compared to laminar
  flows.
• Turbulent flow is due to a combination of set of
  vortices (Eddies).
• Recent research findings have shown that the
  vortices play an important role in enhancing the
  heat transfer.
• At a given Reynolds number, the flow can be made
  more turbulent by introducing artificial vortices
  into the flow.
• Artificial introduction of vortices on the gas
  side can be another potential alternative for
  augmenting heat transfer in compact heat
  exchangers.

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Compact Heat Exchangers -- A New Concept

• Creation of concept.
• Testing and understanding of concept.
• Explanation of the concept.
• Discovery of various performance parameters of
  the concept.
• Development of compact heat exchangers using the
  new concept.
• Testing and performance evaluation.
• Generation of Design data.

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VORTEX GENERATORS

• Wing type turbulators (a,b)
• Winglet type turbulators (c,d)

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Mechanical Department, IIT Delhi
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WINGLET VORTEX GENERATORS
h Height of trailing edge L Base length ß
Angle of attack X and Y position coordinates
with respect to center of tube t Thickness of
winglet AR 2h/L, Aspect ratio
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Mechanical Department, IIT Delhi
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VORTICAL DESCRIPTION OF FLOW PAST A DELTA WINGLET
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HEAT TRASNFER ENHANCEMENT BY WINGLET
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COMMON FLOW DOWN AND COMMON FLOW UP
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Fin-Tube Heat Exchangers with winglets
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A Model Heat exchanger
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Theory of circular fins
Local heat Transfer Coefficient
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Measurement of Isotherms

• Fifty thermocouples are embedded in central plate
• The cylinder is electrically heated.
• A special wind tunnel is designed developed and
  tested for this purpose.
• Steady state temperature distribution is
  transferred to computer.

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Anatomy of Fin-Tube Heat Exchangers
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Wind Tunnel
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Heat Transfer in Circular Fins
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Positioning of Winglets around Tube

• Objective is to raise to promote heat transfer
  rate in downstream of a tube.
• High temperature gradients and heat transfer
  coefficients are to be created.
• A placement of winglet also leads to raise in
  pressure drop.
• An optimum location will produce a maximum heat
  transfer enhancement with low raise in pressure
  drop.

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Heat Transfer in Circular Fins with winglets
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Effect of Winglet Geometry
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Effect of winglet Position
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Main Observations

• Mechanism of Heat Transfer Enhancement by Winglet
• The winglet entrains more fluid into the
  downstream (wake) region and enhances fluid
  circulation.
• This leads to increase in the value of local heat
  transfer coefficient and hence enhancement in the
  heat transfer rates in wake region.
• Best winglet and best positions are those, which
  entrain more fluid into wake region.

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A Prototype Compact Heat Exchanger
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Performance of Prototype
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The Compact Heat Exchanger
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Concluding Remarks

• The winglet with ? 1.33, located at X 0.5D and
  Y0.5D is the the option
• The Result on prototype are encouraging.
• The enhancement in heat transfer coefficient
  shows a great promise in reducing gas side
  thermal resistance.
• Another effective alternate concept for
  construction of Compact Heat Exchangers.
• Testing of real equipment and generation of final
  design procedure is our final goal.