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Heat Transfer Enhancement In A TubeUsing Rectangular-Cut

Twisted-Tape Insert

• Active method: involves some external power input for the enhancement of heat transfer

• Passive method: These methods generally use surface or geometrical modifications to the flow channel by incorporating inserts or additional devices

• Compound method: Combination of above two methods

Heat transfer enhancement methods

What is a twisted tape tube insert?

Twisted Tapes

• Twisted tapes are metallic strips twisted with some suitable techniques with desired shape and dimension

Inserted in the flow of pipe

Major Classifications• Full length twisted tape: These tapes have length equal to length of test

section • 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.

• Regularly spaced twisted tapes: These are short length tapes of different pitches spaced by connecting together

• Tape with attached baffles: Baffles are attached to the twisted tape at some intervals so as to achieve more augmentation

• Slotted tapes and tapes with holes: Slots and holes of suitable dimensions made in the twisted tape so as to create more turbulence

Major Types

Objective

• experimental investigation was carried for measuring tube-side heat transfer coefficient, friction factor, heat transfer enhancement efficiency of water for turbulent flow in a circular tube fitted with rectangular-cut twisted tape insert

Experimental Setup

Twisted Tape Used

Equations Involved

Results• Heat transfer and friction data for the smooth tube were collected first. These data were taken to check

the validity of the set up and measurement techniques over the range of Reynolds number 10000 to 19070.

• Data fall within -24.7% and -5.6% of the Gnielinski, 1976 values with r.m.s. value of error 20.3%. • Nusselt numbers for the smooth tube and the tube with twisted tape insert are shown in next fig. It is

seen that, Nusselt numbers increased with the increase of Reynolds number and twisted tape insert gave higher values of Nusselt number than those for smooth tube.

• Swirl flow generated by twisted tape was responsible for thinning the thermal boundary layer and increasing the mixing between core and tube wall flows

• Rectangular-cut in the twisted tape was responsible for additional disturbances which increased the tangential contact between secondary flow and the wall surface of the tube

Fig. 3

Results contd.

• The variation of heat flux with Reynolds number for smooth tube and tube with rectangular-cut twisted tape insert is shown in Fig. 4

• It is found that with the increase of Re heat fluxes increased and rectangular-cut twisted tape insert gave higher heat fluxes than those for smooth tube

• Higher values of heat transfer coefficient was responsible for this enhancement, although temperature difference between wall and bulk fluid, (Twi - Tb),e significantly decreased for tube with insert, Fig. 5

Fig. 4

Fig. 5

• Figure 6 shows the variations of friction factor with Reynolds number. Friction factors for both smooth tube, fs and tube with insert, fe decreased with the increase of Re. And fe were found to be 39% to 80% higher than fs. These higher values of fe than fs are due to the high viscous loss near the wall regions caused by swirl flow

Fig. 6

Results contd.

• Heat transfer enhancement efficiency, , calculated using Eqn. is shown in Fig. 7. It is quite obvious that with the increase of Reynolds number heat transfer enhancement efficiencies increased continuously. The value of increased from 1.8 to 2.2 with the increase of Re from 10116 to 19070.

Fig. 7

Conclusion

References[1]Salam,B.,Biswas,S.,Saha,S. and Bhuiyya,M.M.K. (2013)Heat transfer enhancement in a tube using rectangular-cut twisted tape insert,Procedia Engineering ,Vol 56,pp . 96-103[2]Prajapati,C.,Patel,P.,Patel,J. and Patel,U. (2012)A review of heat transfer enhancement using twisted tape International Journal of Advanced Engineering Research and Studies Vol.2(1),pp. 162-164

[3]”www.princeton.edu”

[4]”www.wikipedia.com”

[5]”www.google.com”