REVIEW ON HEAT TRANSFER ENHANCEMENT WITH TWISTED TAPE By Mr. Shekhar S. Babar Prof. K. D. DevadeMECHANICAL ENGINEERING(Indira College of Engineering & Management,Pune)

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OVERVIEW INTRODUCTION CLASSIFICATION & APPLICATIONS DESIGN CONSIDERATIONS MECHANISM OF HEAT TRANSFER ENHANCEMENT NEED FOR HEAT TRANSFER ENHANCEMENT ENHANCEMENT TECHNIQUIES TWISTED TAPE : A PASSIVE METHOD THERMAL CHARACTERISTICS OF HEAT EXCHANGER WITH TWISTED TAPE CONCLUSION

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INTRODUCTION HEAT EXCHANGER A heat exchanger is a device that transfers thermal energy from a high-temperature fluid to a low-temperature fluid with both fluids moving through the device. A heat exchanger is a device that is used to transfer thermal energy (enthalpy) between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid, at different temperatures and in thermal contact. In heat exchangers, there are usually no external heat and work interactions.

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CLASSIFICATION OF HEAT EXCHANGERS According to Transfer process

Indirect contact type Direct contact type

According to Construction Tabular Type -Double Pipe, Shell& tube, Spiral tube, Pipe coil Plate Type Extended surface type Regenerative

According to Flow arrangement According to Heat transfer mechanism

Single phase convection on both side Two phase convection on both side Single phase convection on one side & two phase on other side

Process function Condenser Evaporator & Heaters, Cooler, Chillers

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APPLICATIONS OF HEAT EXCHANGERS

Applications involve heating or cooling of a fluid stream of concern and evaporation or condensation of single- or multi component fluid streams. In other applications, the objective may be to recover or reject heat, or sterilize, pasteurize, fractionate, distill, concentrate, crystallize, or control a process fluid. Heat exchanger for an ocean thermal energy conversion (OTEC), Heat exchangers for process industries, Sugar factories, diary plants, Chemical industries Heating and cooling in evaporators, Heat exchangers thermal power plants, air-conditioning equipment, refrigerators, radiators for space vehicles, automobiles.

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DESIGN CONSIDERATIONS

Resistance to heat transfer should be minimized Contingencies should be anticipated via safety margins; for example, allowance for fouling during operation. The equipment should be sturdy. Cost and material requirements should be low. Corrosion should be avoided. Pumping cost should be kept low. Space required should be kept low. Required weight should be kept low.

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NEED FOR HEAT TRANSFER ENHANCEMENT To make the equipment compact To achieve a high heat transfer rate using minimum pumping power Minimize the cost of energy and material A need for miniaturization of a heat exchanger in specific applications Space, OTEC Working fluids of low thermal conductivity (gases and oils) and desalination plants Increase efficiency of process & system. Design optimum heat exchanger size Transfer required amount of heat with high effectiveness Reduce the volume & weight For given temperature difference improved HT Effective utilization of energy-Minimum operating cost

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MECHANISMS OF AUGMENTATION OF HEAT TRANSFER Use of a secondary heat transfer surface. Disruption of the unenhanced fluid velocity. Disruption of the laminar sub layer in the turbulent boundary layer. Introducing secondary flows. Promoting boundary-layer separation. . Enhancing effective thermal conductivity of the fluid under static conditions Enhancing effective thermal conductivity of the fluid under dynamic Delaying the boundary layer development. Thermal dispersion Increasing the order of the fluid molecules. Redistribution of the flow. Modification of radiative property of the convective medium. Increasing the difference between the surface and fluid temperatures. Increasing fluid flow rate passively. Increasing the thermal conductivity of the solid phase using special Nanotechnology fabrications

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HEAT TRANSFER ENHANCEMENT TECHNIQUES Active method: external power input for the enhancement of heat transfer pulsation by cams and reciprocating plungers magnetic field to disturb seeded light particles in a flowing stream, fluid vibration, jet impingement ,Suction Passive method: surface or geometrical modifications to the flow channel by incorporating inserts or additional devices Inserts, Rough surfaces, Coiled / Twisted tape Baffles, Extended surface additives, Compound method: When any two or more techniques employed simultaneously

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ACTIVE METHODS

Mechanical Aids: Surface vibration: Fluid vibration: Electrostatic fields: Injection Suction: Jet impingement:

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PASSIVE METHODS Treated Surfaces:. Rough surfaces: Extended surfaces: Displaced enhancement devices:. Swirl flow devices: Coiled tubes:. Surface tension devices: Additives for liquids: Additives for gases:

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WHY PASSIVE TECHNIQUES?

These techniques generally use simple surface or geometrical modifications to the flow channel by incorporating inserts or additional devices It does not need any external power input. Insert manufacturing process is simple and these techniques can be easily employed in an existing heat exchanger. Passive insert configuration can be selected According to the heat exchanger working condition It can be used in design of compact heat exchangers. It is not only applicable in heat exchanger but also in solar air heater and cooling of electronic components (heat sink).

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TWISTED TAPE TWISTED TAPE :A PASSIVE METHOD Inserts refer to the additional arrangements made as an obstacle to fluid flow so as to augment heat transfer. Twisted tapes are the metallic strips twisted with some suitable techniques with desired shape and dimension, inserted in the flow.

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TYPES OF TWISTED TAPES Plain Twisted Tape Full length twisted tape Varying length twisted tape Short length TT Regularly spaced twisted tapes Tape with attached baffles Slotted tapes and tapes with holes Tapes with dimpled surface modifications Serrated twisted tape Edge fold twisted tape Oblique teeth Twisted tape

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TYPES OF TWISTED TAPES

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ATTRIBUTES OF TWISTED TAPE Width (W) Thickness (δ) Pitch (y) Twist ratio (Y)= y/W Clearance ratio(CR)=D/W Depth ratio(DR)=d/W Width ratio(WR)=w/W Wing depth ratio Perforated diameter ratio

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THERMAL CHARACTERISTICS OF HEAT EXCHANGER WITH TWISTED TAPE TT induce swirl into bulk flow-disrupting boundary layer at tube surface due to repeated changes in surface geometries, TT induce turbulence & superimpose vortex motion results thinner boundary layer Effect of TT on Thermal Characteristics of Heat exchangers like Thermo hydraulic performance, Thermal performance factor, Nu, Friction factor Effect of Twist ratio-TT with Twist ratio-5,7 increases the Heat transfer rate about 188%-159% than plain tube Smaller the TR larger is the Heat transfer but penalty of greater friction factor & Pressure drop Small TR give rise to more Nu, Friction Factor & Enhancement efficiency at high Re

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THERMAL CHARACTERISTICS OF HEAT EXCHANGER WITH TWISTED TAPEEffect of parallel rectangular wingTT are cut at the edge of tape in straight line at every pitch length & each cut bend in 450 It is observed that increase in wing depth ratio increases heat transfer rate & Friction factor as well as maximum thermal performance For Wing depth ratio 0.1,0.2,0.3 HTR increases 78%,91%,100% than plain tube Thermal performance Factor is about 1.36 for Wing depth ratio 0.3

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THERMAL CHARACTERISTICS OF HEAT EXCHANGER WITH TWISTED TAPEEffect of Varying width Twisted Tape Enhancement is due to the centrifugal forces resulting the spiral motion of fluid Increasing the width of TT results in increase in Nu hence the heat transfer rate as well as Friction factor For Three TT with different width 26,22,18 mm enhancement of HT is 48%,40% & 35% than plain tube & Friction factor rise of 18%,17%15% than plain tube Overall enhancement ratio is 1.62,1.39,1.22 22mm gives better results with 60% material saving

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THERMAL CHARACTERISTICS OF HEAT EXCHANGER WITH TWISTED TAPEEffect of single, Twin & Triple TT When Number of TT increases HTR increases also increases Friction factor Thermal performance factor better for the Twin& Triple TT than Single TT HTR increases 2.5,3,3.5 times more than plain tubeEffect of Half length TT HTR increases 40% than plain tube On the basis of equal mass flow rate HT performance is better than Plain tube but on unit pressure drop basis plain tube is better than half length TT

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THERMAL CHARACTERISTICS OF HEAT EXCHANGER WITH TWISTED TAPEEffect of Perforated TT with parallel wing Wing induce extra turbulence near tube wall & efficiently disrupt thermal boundary layer Hole existing along a core tube diminish pressure loss within the tube, stream behaves in between axial & swirl flow. Wing depth ratio (w/W) increases higher turbulence intensity & better mixing fluid near tube wall hence higher HTR & greater Friction factor, Higher Thermal performance factor Perforated diameter ratio(d/W) larger the flow become axial flow lose swirl intensity between tape & surface wall HTR , Friction factor, Thermal performance factor increases with decreases in Perforated diameter ratio

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THERMAL CHARACTERISTICS OF HEAT EXCHANGER WITH TWISTED TAPEEffect of Peripherally cut Twisted tape TT modified by Peripheral cutting in order to generate additional turbulence in vicinity of tube wall HT, Nu, Friction factor as well as Thermal performance factor associated by PCTT found increased with increasing Tape depth ratio(d/W) & decreasing width ratio (w/W)Effect of dimpled tube with TT Increase of Nu due to both Turbulence (dimpled tube) & Swirl flow (TT) higher reduction of boundary layer thickness & increase of resultant velocity HTR & Friction factor increases with decreasing both Pitch ratio & Twist ratio

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CONCLUSIONTwisted tape is commonly used heat transfer enhancement tool in heat exchanger because its manufacturing process is simple and these can be easily employed in an existing heat exchanger. Study of influence different types Twisted Tapes with different attributes e.g. Twist Ratio, Pitch, Depth ratio etc & different configuration on Thermal characteristics of heat exchangers like Heat transfer rate, friction factor, Thermal performance ratio etc., reveals that heat exchanger employed with Twisted tape always gives the enhanced heat transfer with penalty of appreciable rise in friction & Pressure drop hence selection of the optimum Twisted Tape for the required working conditions gives scope for the future work. Future work may be extended to Change the tape material from Aluminium to Copper & employing Compound enhancement techniques.

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