Engineering

Pipe with conical insertHeat transfer enhancement is the process of increasing the effectiveness of heat exchangers. This can be achieved when the heat transfer power of a given device is increased or when the pressure losses generated by the device are reduced. A variety of techniques can be applied to this effect, including generating strong secondary flows or increasing boundary layer turbulence.

During the earliest attempts to enhance heat transfer, plain (or smooth) surfaced were used. This surface requires a special surface geometry able to provide higher ha values per unit surface area in comparison with a plain surface. The ratio of heat transfer surface to the plain surface is called Enhancement ratio Eh. Thus,

Eh

 

 

 

The heat transferater  for a two-fluid counter flow heat exchanger is given by Ratio ". Thus,

Q

 

 

In order to better illustrate the benefits of enhancement, the total length 'L' of the tube is multiplied and divided in the equation:

Q2

 

 

Where L/UA is the overall thermal resistance per unit tube length. And it is given by:

L

 

 

 

Internal flow:

There are several available options for enhancing heat transfer. The enhancement can be achieved by increasing the surface area for convection or/and increasing the convection coefficient. For example, the surface roughness can be used to increase h in order to enhance turbulence. This can be achieved through machining or other kinds of insertions like coil-spring wire. The insert provides a helical roughness in contact with the surface. The convection coefficient may also be increased by an insert of a twisted tape that consists in a periodical twist through 360 degrees. Tangential inserts optimize the velocity of the flow near the tube wall, while providing a bigger heat transfer area. While, increased area and convection coefficient can be achieved by applying spiral fin or ribs inserts. Other aspects such pressure drop must be taken into consideration in order to meet the fan or pump power constraints.

 

Classification of Enhancement Techniques:

A list of the various methods or devices under each of these two categories is given in Table 1. 

table

 

 

 

 

 

 

 

 

 

 

                                             

 

 

                                                                  Table: 1

 

The descriptions of passive techniques, as given by Bergles (1998), are as follows:

1. Treated surfaces are heat transfer surfaces that have a fine-scale alteration to their finish or coating. The alteration could be continuous or discontinuous, where the roughness is much smaller than what affects single-phase heat transfer, and they are used primarily for boiling and condensing duties.

2. Rough surfaces are generally surface modifications that promote turbulence in the flow field, primarily in single-phase flows, and do not increase the heat transfer surface area. Their geometric features range from random sand-grain roughness to discrete three-dimensional surface protuberances.

3. Extended surfaces, more commonly referred to as finned surfaces, provide an effective heat transfer surface area enlargement. Plain fins have been used routinely techniques, in many heat exchangers. The newer developments, however, have led to modified finned surfaces that also tend to improve the heat transfer coefficients by disturbing the flow field in addition to increasing the surface area.

4. Displaced enhancement devices are inserts that are used primarily in confined forced convection, and they improve energy transport indirectly at the heat exchange surface by “displacing” the fluid from the heated or cooled surface of the duct with bulk fluid from the core flow.

5. Swirl flow devices produce and superimpose swirl or secondary recirculation on the axial flow in a channel. They include helical strip or cored screw-type tube inserts, twisted ducts, and various forms of altered (tangential to axial direction) flow arrangements, and they can be used for single-phase as well as two-phase flows.

6. Coiled tubes are what the name suggests, and they lead to relatively more compact heat exchangers. The tube curvature due to coiling produces secondary flows or Dean Vortices, which promote higher heat transfer coefficients in single-phase flows as well as in most regions of boiling.

7. Surface tension devices consist of wicking or grooved surfaces, which direct and improve the flow of liquid to boiling surfaces and from condensing surfaces.

8. Additives for liquids include the addition of solid particles, soluble trace additives, and gas bubbles in single-phase flows, and trace additives, which usually depress the surface tension of the liquid, for boiling systems.

9. Additives for gases include liquid droplets or solid particles, which are introduced in single-phase gas flows in either a dilute phase (gas–solid suspensions) or dense phase (fluidized beds).

 

Descriptions for the various active techniques have been given as follows:

1. Mechanical aids are those that stir the fluid by mechanical means or by rotating the surface. The more prominent examples include rotating tube heat exchangers and scraped-surface heat and mass exchangers.

2. Surface vibration has been applied primarily, at either low or high frequency, in single-phase flows to obtain higher convective heat transfer coefficients.

3. Fluid vibration or fluid pulsation, with vibrations ranging from 1.0 Hz to ultrasound (1.0 MHz), used primarily in single-phase flows, is considered to be perhaps the most practical type of vibration enhancement technique.

4. Electrostatic fields, which could be in the form of electric or magnetic fields, or a combination of the two, from dc or ac sources, can be applied in heat exchange systems involving dielectric fluids. Depending on the application, they can promote greater bulk fluid mixing and induce forced convection (corona “wind”) or electromagnetic pumping to enhance heat transfer.

5. Injection, used only in single-phase flow, pertains to the method of injecting the same or a different fluid into the main bulk fluid either through a porous heat transfer interface or upstream of the heat transfer section.

6. Suction involves either vapor removal through a porous heated surface in nucleate or film boiling, or fluid withdrawal through a porous heated surface in single-phase flow.

7. Jet impingement involves the direction of heating or cooling fluid perpendicularly or obliquely to the heat transfer surface. Single or multiple jets (in clusters or staged axially along the flow channel) may be used in both single-phase and boiling applications.

 

Geometry Creation:

Geometry is created in Solid works using Feature workbench. The step. file of Geometry is used for further processing. The three dimensional model of Pipe with insert is shown below in figure1 The total length of the model is 1000 mm and  internal  diameter of pipe is  62 mm and thickness of pipe is 2mm.

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Meshing:

The saved geometry in step format is imported in ICEM CFD. There After name selection has been done i.e. inlet, outlet, Wall, and Insert.

Unstructured Hexa-Core Mesh using Cartesian technique in fluid domain and Tetra mesh using Octree technique generated and prism layer add in the insert part. Total elements is generated 2762058, in fluid domain 2576132 elements and solid 61667 elements. 

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Boundary Condition:

Fluent Setup:

With air used as working fluid, the numerical model for fluid flow and heat transfer in the micro channel is developed under the following conditions.

Velocity Inlet: - 0.1 m/s

Outlet: - Pressure

Wall: Heat Flux: - 759.01 w/m2 

 

Results: 

Temperature Contour at Mid-Section:

R1

 

Velocity Contour at Mid-Section:

R2

 

Temperature Contour at Insert:

R6

 

Temperature Contour at Wall:

Temperature Contour at Wall

 

Velocity Streamline:

R7

 

 

Temperature Plot along the Length of the Pipe:

R8

 

Nusselet Number Plot along the Length of the Pipe:

Nu

 

 

 Nusselet Number VS Reynold's Number Plot:

Nu VS Re

 

 

Future work:

CFD simulation can work with different inserts as well as different Reynolds number. Also can carry out the experimental studies to validate the present results.