REVIEW OF HEAT TRANSFER AUGMENTATION WITH TAPE

INSERTS

S.S.JOSHI1*

Student, IV Semester M. Tech (Heat Power Engineering)

V. M. KRIPLANI2

Professor and Head 1*, 2 Mechanical Engineering Department, G. H. Raisoni College of Engineering, Nagpur. 440016 (India)

1* Corresponding author

Abstract:

Heat transfer augmentation techniques refer to different methods used to increase rate of heat transfer with-out affecting much the overall performance of the system. These techniques are used in heat exchangers. Some of the applications of heat exchangers are-in process industries, thermal Power plants, air-conditioning equipments, refrigerators, radiators for space vehicles, automobiles etc. These techniques broadly are of three types viz. passive, active and compound techniques. The present paper is a review of the passive augmentation techniques used in the recent past.

Keywords: Heat transfer augmentation technique, Passive methods, Tape inserts

Introduction:

Generally, heat transfer augmentation methods are classified in three broad categories:

(a) Active method: This method involves some external power input for the enhancement of heat transfer; some examples of active methods include induced pulsation by cams and reciprocating plungers, the use of a magnetic field to disturb the seeded light particles in a flowing stream, etc.

(b) Passive method: These methods generally use surface or geometrical modifications to the flow channel by incorporating inserts or additional devices. For example, use of inserts, use of rough surfaces etc.

(c) Compound method: Combination of above two methods

Passive heat transfer augmentation methods:

Passive heat transfer augmentation methods as stated earlier does not need any external power input. In the convective heat transfer one of the ways to enhance heat transfer rate is to increase the effective surface area and residence time of the heat transfer fluids. The passive methods are based on the same principle. Use of this tech-nique causes the swirl in the bulk of the fluids and disturbs the actual boundary layer so as to increase effective surface area, residence time and consequently heat transfer coefficient in existing system.

Following Methods are used generally used,

1. Inserts

2. Extended surface

3. Surface Modifications

4. Use of Additives.

1. Inserts:

Inserts refer to the additional arrangements made as an obstacle to fluid flow so as to augment heat transfer as explained earlier. Different types of inserts are

1. Twisted tape and wire coils

2. Ribs, Baffles, plates

S.S.Joshi et al. / International Journal of Engineering Science and Technology (IJEST)

ISSN : 0975-5462 Vol. 3 No. 3 March 2011 1949

The present paper Contributes for review of tape inserts.

Twisted tape

Twisted tapes are the metallic strips twisted with some suitable techniques with desired shape and dimension, inserted in the flow. Following are the main categories of twisted tape which are analyzed. i. Full length twisted tape: These tapes have length equal to length of test section. ii. Varying length twisted tape: These are distinguished from first category with regards that they are not having the length equal to length of test section, but half length, ¾ th length, ¼ th length of section etc. iii. Regularly spaced twisted tapes: These are short length tapes of different pitches spaced by connecting to-gether. iv. Tape with attached baffles: Baffles are attached to the twisted tape at some intervals so as to achieve more augmentation. v. Slotted tapes and tapes with holes: Slots and holes of suitable dimensions made in the twisted tape so as to create more turbulence. vi. Tapes with different surface modifications: Some insulating material is provided to tapes so that fin effect can be avoided. In some cases dimpled surfaced material used for tape fabrication. Some of the common tape formats are shown in following figure

Fig 1: short length tape in tube

Fig 2: Full length tape in tube

Fig 3:tape with gradually decreasing pitch

Fig 4: tape with holes

Fig 5:Short tapes connected with rod

Fig 6: tape with attached baffle

Common attributes of tape: i. Width: Small width tapes are preferred to minimize pressure drop. ii. Thickness: Thickness of the tape plays important role in its fabrication and also has contribution in fin effect iii. Pitch: It is the distance between two consecutive twists measured axially. iv Twist ratio: It is the ratio of pitch of tape to tape width. So, if width of the tape considered as a constant (as found generally) twist ratio depends on pitch only. Under this condition if pitch is more it means less number of turns. v. Fin effect: If the tape material is conductive then during the flow some heat will be absorbed by the tape ma-terial itself till its saturation. This is simply the loss of available heat energy.

S.S.Joshi et al. / International Journal of Engineering Science and Technology (IJEST)

ISSN : 0975-5462 Vol. 3 No. 3 March 2011 1950

Review of work carried out: S.K.Saha A.Dutta, [1] experimentally studied the flow of servotherm oil in acrylic circular tube fitted with insu-lated stainless steel twisted tape insert. They studied the effect of varying length and varying pitch twisted tape with different twist ratios on heat transfer rate and friction factor. The important outcomes were - Short length twisted tape reduces pumping losses but also reduces the heat transfer rate, and uniform pitch twisted tape gives maximum heat transfer rate. Zhi-Min Lin, Liang-Bi Wang, [2] in their experimental study of air flow in Plexiglas circular tube used Stain less steel twisted tape insert. Tapes with different twist ratios are used. They concluded that the tape increases friction 3-4 times. With small twist ratio, higher heat transfer is achieved as compared to greater twist ratio. Watcharin Noothong et al. [3] their aim to investigate the efficiency enhancement and to study the heat transfer and friction factor characteristics of heat exchanger. In the experimental study, concentric double tube Plexiglas materialed heat exchanger was used. Cold water as a annulus and hot air as a inner fluid used as a medium. In the inner tube Stainless steel tape with different twist ratios were inserted. They concluded the efficiency and Nusselt number increases with decreasing the twist ratio and friction factor increases with decreasing the twist ratio. The partitioning and blockage of the tube flow cross-section by the tape, resulting in higher flow veloci-ties. Secondary fluid motion is generated by the tape twist, and the resulting twist mixing improves the convec-tion heat transfer. Paisarn Naphon, [4] in his experimental study he used hot and chilled water in horizontal copper double tube heat exchanger fitted with aluminum twisted tape inside. He studied effects of relevant parameters on heat trans-fer and pressure drop. It was concluded that the twisted tape insert has significant effect on enhancing heat trans-fer rate. However, the pressure drop also increases. Correlation for heat transfer coefficient and friction factor based on the experimental data is also presented. Smith Eiamsa-ard et al., [5] their aim was to analyze heat transfer and flow friction characteristics in a copper tube double pipe counter flow heat exchanger, containing the stainless steel helical screw-tape with or without core-rod inside. Hot and chilled water used for experimentation. They concluded that helical screw-tape insert has a significant effect on enhancing heat transfer rate and also considerable increase of friction. The heat trans-fer rate from using the helical screw-tape without core-rod is higher than that from the plain tube at around 340%. The heat transfer rate obtained by using the tape without core-rod is found to be better than that by one with core-rod around 25–60% while the friction is around 50% lower. Ashis K. Mazumder and Sujoy K. Saha, [6] performs the experimental study in a square and rectangular acrylic ducts fitted with full and short length twisted tape. It was concluded that regularly spaced full length twisted tape performs better as compared to short length tape. A Dewan et al. [7] had taken the exhaustive review of some of the methods of augmentation. The important out-comes are given in following table

Sr. no. Details of Tape used Remark

01 Full length twisted tape Heat transfer rate increases but increase in friction factor also observed

02 Short length twisted tape

Low friction factor and low Nusselt number observed As the length of the tape reduces, friction factor reduces and heat transfer co-efficient also reduces.

03 Twisted tape with uniform pitch Performs better than gradually decreasing length tape

04 Twisted tape with gradually decreasing pitch

Poor performance as compared to uniform pitch tape

05 Baffled twisted tape with holes Better Heat transfer rate is observed but as turbulence increases, increase in friction factor is also observed.

06 Tight fit and lose fit tapes Tapes having tight fittings give more frictional loss, whe-reas reduced Width and centrally located loose fit tape gives better result.

S.S.Joshi et al. / International Journal of Engineering Science and Technology (IJEST)

ISSN : 0975-5462 Vol. 3 No. 3 March 2011 1951

References

[1] S.K.Saha A.Dutta “ Thermo hydraulic study of laminar swirl flow through a circular tube fitted with twisted tapes” Trans. ASME Journal of heat transfer June 2001, Vol-123/ pages 417-427.

[2] Zhi-Min Lin, Liang-Bi Wang “Convective heat transfer enhancement in a circular tube using twisted tape” Trans ASME journal of heat transfer Aug 2009,Vol-131/081901-1-12.

[3] Watcharin Noothong, Smith Eiamsa-ard and Pongjet Promvonge” Effect of twisted tape inserts on heat transfer in tube” 2nd joint international conference on “sustainable Energy and Environment 2006” Bangkok, Thiland.

[4] Paisarn Naphon “Heat transfer and pressure drop in the horizontal double pipes with and without twisted tape insert” 2005 Elsevier Ltd.

[5] Smith Eiamsa-ard , Chinaruk Thianpong, Pongjet Promvonge “ Experimental investigation of heat transfer and flow friction in a Cir-cular tube fitted with regularly spaced twisted tape elements” International Communications in Heat and Mass Transfer Vol. 33, Dec 2006.

[6] Ashis K. Mazumder, Sujoy K. Saha “Enhancement of Thermo hydraulic Performance of Turbulent Flow in Rectangular and Square Ribbed Ducts With Twisted-Tape Inserts” Journal of Heat Transfer AUGUST 2008, Vol. 130.

[7] A Dewan1 , P Mahanta1, K Sumithra Raju1 and P Suresh Kumar “Review of passive heat transfer augmentation techniques” Proc. Instn Mech. Engrs Vol. 218 Part A: J. Power and Energy, pages 509-527

S.S.Joshi et al. / International Journal of Engineering Science and Technology (IJEST)

ISSN : 0975-5462 Vol. 3 No. 3 March 2011 1952

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