Received date: January 20, 2017; Accepted date: February 04, 2017; Published date: February 08, 2017
Citation: Kirubadurai B, Rahman FS, Velmurugan P, Kumar S (2017) Effectiveness Analysis of Double Pipe Heat Exchanger with Curls Band in Various Angles. J Appl Mech Eng 6:252. doi: 10.4172/2168-9873.1000252
Copyright: © 2017 Kirubadurai B, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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In the modern learning, effectiveness-NTU studies in a dual tube heat exchanger armed with curly band in view of numerous angles were experimentally deliberate. Additionally, discrepancy of the Dual effectiveness with heated water Reynolds numbers for dissimilar cold water flow rates were obtainable. These turbulators with unlike angles of 45 L, 60 L, 90 L, 120 L and 150 L were made of electrified plates with thickness of 1 mm and were attached in the internal tube of heat exchanger. The experimentations were conceded out at Reynolds numbers of 3000–13,400 at turbulent flow establishment. All over the research, hot and cold water rolled finished the internal pipe and the interplanetary between the pipes (annulus), correspondingly. It was strained to keep the bay hot and cold water temperatures at constant values. Effectiveness-NTU studies were made for the circumstances with and without curly bands including their different angles and associated to each other. Results displayed the substantial decision of turbulators on effectiveness (e) and number of heat transfer units (NTU) of dual tube heat exchanger. In totalling, some experiential associations uttering the domino effect were also industrialized constructed on curve fitting.
Dual tube heat exchanger; Effectiveness; NTU; Turbulator curly bands
A lot of learning has been done on the heat transfer augment and resistance factor of heat exchangers in modern years. In addition, investigative the efficacy and NTU of heat exchangers has been an stimulating topic for academics [1–3]. Generally, methods to increase the heat transfer and effectiveness of heat exchangers are secret into three collections of active, passive and multifaceted methods. In active methods such as using sound waves or attractive fields , exterior power birthplace is used to progress the heat transfer. To increase the heat transfer rate in flaccid attitudes, exterior energy is no longer obligatory. Some of the examples of inactive approaches include using different kind of turbulators. Multiple methods are mixtures of two or more of the active and passive methods such as concurrent using of attractive arenas and Nanofluids . All the revealed methods can be used in dissimilar sectors of industries, for example chilling systems, power plants, food and dairy courses, nuclear and biological reactors. Ibrahim  through some experimentation deliberate the convective heat transfer physiognomies in a flat tube with helical screw component with various twist ratios and insertion length pull-outs. Subsequently dependable associations for both resistance factor and Nusslet number with Re, insertion length and twist ratio were providing. Their results demonstrated that Nusslet number increases with the increase of Renolds number but it changes dramatically with spacer length and twist ratio. Sheikholeslami et al.  deliberate the consequence of using typical and punctured intermittent helical turbulators on heat transfer and friction factor of aquatic to air heat exchanger. Researches were approved out for many open area ratios and pitch ratios. Fallouts show that Nusselt number reduces with the upsurge in open area ratio and pitch ratio while it supplements with the surge in Rey-nolds number. Friction factor is a declining function of Reynolds number, area ratio and pitch ratio. Raineri and Pagliarini  finished some Trials deliberate the convective heat transfer and thermal performances in uneven tubes at different pitch ratios. Achieved results designated that The Nusselt number is almost self-governing of the corrugation shape. Fahed et al.  studied the effects of variations in twisted tape ratio on heat transfer coefficients of dual tube heat exchangers allowing to their results the heat transfer upsurges with the increase of the twist ratio. Naphon and Suchana  obtainable a tentative analysis of heat transfer and pressure drop in a concentric dual tube heat exchanger armed with twisted wires encounter. Three different twisted wire encounter masses (number of the twisted wires per centimeter) were tested during their tests. The heat transfer rate from the basic tube with 300 twisted wires encounter supplements is advanced than those of the plain tube with 200 and 100 twisted wire meeting inserts. Durmus  scrutinized the consequence of cut out tapering turbulators on heat transfer, pressure trickle and dimensionless liveliness loss for the conditions with and without turbulators and associated the results to each other. In their hearings, the outer surface of the internal pipe of the heat exchanger was animated with inundated water vapour, so outside exterior temperature was kept constant. Unceasingly disrupting the hydrodynamic boundary layer is the determination of using this kind of turbulators. Akpinar  deliberate the energy loss in a double tube heat exchanger with helical spring insertions. Also NTU and effectiveness of heat exchanger with this benevolent of turbulators were premeditated. In their learning the increase in friction factor was about 2.74 times equalled to the smooth tube. The highest augmentation was seen to occur in counter flow the exchanger with the helical wire having the pitch of 8 mm and the helical number of 134. The heat transfer rates in this heat exchanger increased up to 2.94 times with the help of this caring of turbulators. In advance endeavours, effectiveness of heat exchangers with swirl initiator was deliberate by Akpinar and Bicer  which led to some weighty results. All of the recent new studies show the noteworthy effect of passive methods on heat transfer and effectiveness of heat exchangers.
By in view of what was debated so far, it can be decided that lots of soundings have been done to study the effect of numerous geometrical alignments of turbulators on the heat transfer and friction factor of heat exchangers. Though, to the author’s best information, this is the first endeavour to examine the effect of mutable curly band angles on NTU and effectiveness of dual tube heat exchanger. Additionally to this, the paper emphases on the investigational study on the e-NTU enquiry of the dual tube heat exchanger with curly crowd seeing numerous angles. In adding, the consequence of cold water rate on the NTU and effectiveness of dual tube heat exchanger were elucidated in this education. The experiential associations of e and NTU are attained based on the arc fitting.
Investigational set-up and route
A representation diagram of the investigational device used in this reconnaissance is shown in Figure 1. The system contains two centrifugal pumps, two flow meters, hot and cold water cisterns, thermocouples and a statistics logger. The internal tube of heat exchanger was finished of copper with inner and outer diameters of di = 26 mm and do = 29 mm, correspondingly, while for the outer tube, electrified pipe with inner diameter of Di = 59 mm was engaged. For the sake of reducing the heat fatalities to environs a great lagging was done for the assessment section. Glass wool and spray are the lagging kind in this investigational setup. Hot water was conceded through the internal pipe, while cold water acting as a cooling intermediate was elegant through the space between the pipes. The cove hot water temperature was familiar in a way to reach the wanted temperature (54 LC) by using two 2 kW fires and consequently hot water flowed into the internal tube of the heat exchanger using a 100 W centrifugal pump. Cold water poured in the annulus by using another indistinguishable pump. Hot and cold water temperatures at inlet and outlet of the heat exchanger were unhurried by the k-type thermocouples. It was tried to keep the cold and hot water inlet temperatures at perpetual values. In order to associate the data at different Reynolds numbers, hot water flow rate was augmented at precise steps of 1 l/min. Hot and cold water flow rates were measured and reasonable by valves and rota meters, with range of 2–11 l/min, positioned after the pumps. All measuring devices were standardized before the assessments. Besides, all data were chronicled at steady state by data attainment system.
Curly bands geometry
The geometrical arrangement of the Curly bands supplements is shown in Figure 2. The Curly bands were prepared of electrified sheet with thickness of 1 mm. they were arranged at five dissimilar angles (h) of 45 L, 60 L, 90 L, 120 L and 150 L and were connected in the inner tube of heat exchanger.
Faults and indecisions cannot be escaped in both the instrument and the procedure of making the quantity. Indecision investigation in each self-governing consideration (e.g., temperatures, flow rates) was taken grounded on the accuracy of the mechanism used to make the quantity. Also the indecisions ascending from scheming a result (WRþ) via some independent variables (X1; X2; . . . ; Xn) are assessed by Moffat  method. The value of insecurity in effectiveness and NTU was 6.8% and 8.2% respectively. This degree of accuracy can be putative by the industry. Where WRþ is the reliant on variables indecision and wn is the independent variables doubt the indecisions that occurred are accessible in Table 1.
|Indecision (Uncertainty in temperature measurement)|
|Hot water inlet temperature||LC||± 0.49|
|Hot water outlet temperature||LC||± 0.49|
|Cold water inlet temperature||LC||± 0.49|
|Cold water outlet temperature||LC||± 0.49|
|Uncertainty in measurement of volume flow rate|
|Hot water flow rate||%||± 3.6|
|Cold water flow rate||%||± 4.9|
|Uncertainty in measurement of pressure drop||%||± 1.8|
|Uncertainties in calculating a result|
|Reynolds number||%||± 3.6|
|Logarithmic mean temperature difference||%||± 1.4|
|Friction factor||%||± 11.2|
|Uncertainty in reading values of table (cp;w , etc.)||%||± 0.11–0.22|
Table 1: Uncertainty limits.
Heat relocated from hot water, and cold water, can be deliberate by following equations, correspondingly:
qh = mhCp1w(Th1i-Th1o).
The change between the heat contributed by the hot fluid into the test tube and the heat engrossed by the cold fluid reaches 2% to 6%. Thus, all the computations are based on average heat transfer rate that can be written as follow:
qave = qc+qh/2 = UAiΔTLMTD
Then investigational overall heat transfer coefficient (U) in a dual pipe heat exchanger can be intended with:
Focused imaginable heat transfer rate is expressed as:
Where Cmin is the minimum thermal capacity and it is defined as below:
Cmin =min [Ch1 Cc]
The effectiveness of a heat exchanger is defined as follow:.
The number of heat transfer units (NTU) is enunciated in terms of thermal capacity,
Using the Curly bands turbulators leads to an improvement in the heat transfer, however, it increases the pressure drop. Therefore, to assess the quality of improvement concept, thermal performance enhancement factor (TEF) can be calculated as follow:
Authentication of plain tube
Nusselt numbers are assessed using old-style ‘‘Wilson plots” as elucidated in Rose  study. Consequently, to authenticate the results, an evaluation has been done between Nusselt number get hold of from the current plain tube and the practical association planned by Dittus and Boelter . In addition, the friction factor gotten from the plain tube was associated to the correspondence unfilled by Petukhov .
Ƒ = (1.82 LoRe-1.64)-2
On the foundation of Figures 3 and 4, it was seen that the outcomes achieved from the try-outs for the plain tube are in a good contract with those anticipated by the typical connections. It is renowned that, in this case the disagreement between the results is less than 9% for Nusselt number and 11% for the friction factor.
The investigations were accompanied in a dual tube heat exchanger tailored with wavy strip turbulators. These turbulators with dissimilar angles of 45 L, 60 L, 90 L, 120 L and 150 L were located along the test tube with the axial way. Firstly, it was strained to keep the coldwater flow rate at constant values, successively the effect of Curly bands turbulators bearing in mind dissimilar angles in the NTU and effectiveness of the heat exchangers were examined. Furthermore, for each turbulator, disparity of the effectiveness with hot water flow rate for different cold water flow rates was appraised .
With the results attained from the investigates, variation of effectiveness with hot water Reynolds number was strained for the contained Curly bands with different angles, as shown in Figure 5. By increasing the Re number, the fluid velocity increases, therefore, the fluid does not have sufficient time for heat extra angles of curly bands. It is understood that the effectiveness ratio prominence was high at inferior Reynolds number and then quickly reduced with the rise of Reynolds number. Outcomes signify a beneficial gain of using the curly bands turbulator over the plain tube, especially at low Reynolds numbers. Using the curly bands turbulators had an effect of 26% to 71% upsurge on effectiveness of heat exchangers (Figure 6).
Figure 7 shows the variation of NTU with Reynolds number for double tube heat exchangers containing curly bands with dissimilar angles. Outcomes show that the NTU reductions with the growth of Reynolds number for all angles of the curly bands. The NTU also increases with the decrease of the curly bands angle. From the data it can be clinched that increasing of NTU causes the effectiveness to be enlarged through a non-linear relation. NTU has a supreme value at the curly bands angle of 45 L (Figure 8).
In tallying, for a given hot water flow rate and curly bands angles, vicissitudes in cold water flow rate upsets the effectiveness of the heat exchanger, The extra the cold water flow rate, the additional the heat transfer, accordingly it causes effectiveness of the heat exchanger to be better. Deviation of the effectiveness with hot water Reynolds numbers for diverse cold water flow rates was shown in Figure 9 for plain tube.
Difference of the effectiveness with hot water Reynolds numbers for various cold water flow rates was shown in Figure 10 for dissimilar angles. It can be unspoken from these figures that the effectiveness at an assumed hot water Reynolds numbers increases with the increase of cold water flow rates transferring. Hence, the temperature inclines between hot water inlet and outlet flow falls, so the effectiveness quenches. The effectiveness also increases with the decrease of the curly bands angle and has a supreme value at the angle of 45 L. This can be clarified by a robust turbulence concentration generated using curly bands turbulator, foremost to quick mixing of the flow largely at lower angles. Figure 6 shows the alteration of effectiveness ratio) e=e0 with the Reynolds number for different curly bands angles. The effectiveness ratio for the curly bands with angle of 45 L was better.
By depleting ‘e’ and NTU become from the tests for altered heat capacity ratios ðCr Þ, link 16 was consequential. Successively as shown in Figure 11, the Difference of the effectiveness with NTU for different Cr, for angle of 60 L was exemplified. It can be seen from these figures that the effectiveness at a given NTU increases with the decrease of Cr .
Ԑ=0.6788Cr-0.03477 NTU-0.8624 RMSD=0.000526
Regression models are significant tools to evaluation the connections between variables, particularly, for which no recognized experimental relation happens. In this grant, the experimental associations of effectiveness and NTU as a occupation of Re and curly bands angle were attained grounded on the arc suitable. Root mean square deviation (RMSD) was used to regulate the quality of the industrialized relation. RMSD is a frequently used measure of the various between values forecast by a model or a regression models and the values really perceived. This limit can be calculated as follow .
Ԑ=11.9Re-0.48(l/L)-0.292 RMSD= 0.0052 4000Re<12500, 0.76<l/L<1.93
Ԑ=0.78g NTU0.932 (l/L) 0.0063 RMSD=0.0018 0.128<NTU0.295
0.76<l/L < 1.93
The consequence of curly bands on the friction factor in the form of ‘f’ is exemplified in Figure 12; also the result was associated with the results of smooth tube. As can be seen in Figure 12, the f somewhat decreases with the rise in Re. The endings designated that the heat transfer was improved, while the pressure drop was increased. To estimate the excellence of improvement formation, thermal performance enhancement factor (TEF) was obtained. Effect of the curly bands on the TEF is accessible in Figure 13. By bearing in mind Figures 8, 12 and 13 into procedure, it is witnessed that the 45 L turbulator has the highest effectiveness and maximum pressure drop. Using Eq. (13), cunning shows that in this case the value of TEF has the minimum amount. Moreover, as the angles are increased the value of TEF increased.
As it is clear the TEF is a non-dimensional inconstant, therefore the results acquired in the extant study can be equalled with those described in previous related revisions. Passive methods may be confidential into two groups including: simple and adapted methods. Researchers instigated some changes to simple methods to improve the heat transfer and pressure drop in heat exchangers (modified methods). Simple twisted cassettes and conical rings are some patterns of simple passive methods and u-cut twisted tapes, v-cut twisted tapes and punctured twisted tape are placed in altered passive methods group. Figure 13 boons the contrast between the simple and altered methods described in other studies and the current study as a newly presented simple method.
Data in this figure would appear to propose that, in judgment with the other simple passive methods, in some cases the attained TEF shows better values and in some other cases the gotten TEF is close to the TEF planned by the preceding methods. However, as a simple method, this TEF is motionless lower than the values of TEF planned by the adapted methods. Moreover, in higher Re numbers, in dissimilarity with the present method, TEF involvements a little drop in other simple methods. It should be directing out that to obtain higher TEFs, one can work on modifying the present simple turbulator via applying some changes such as stamping holes on turbulators or presenting v-cut and u-cut on the turbulators .
The effect of curly bands turbulators with dissimilar angles on the NTU and effectiveness of dual tube heat exchanger was experimentally deliberate. Furthermore, discrepancy of the effectiveness with hot water Reynolds numbers for different cold water flow rates was obtainable. The result specifies that both the effectiveness and NTU are meanings of curly bands angles and Reynolds number. The effectiveness increased with the increase of Reynolds number and increases with the decrease of the curly bands angle. Increasing NTU causes the effectiveness to be increased concluded a non-linear relation. Also, upsurges in cold water flow rate augment the effectiveness. Results showed that in the case of inaugurating curly bands turbulators, value of effectiveness was proceeded 26% to 71%. NTU and effectiveness have a greatest value at the curly bands angle of 45 L. Finally, by making an association between the results of the present study and the results described in writings it was decided that, in comparison with the other simple passive methods, in some cases the obtained TEF shows enhanced values and in some other cases they gotten TEF is close to the TEF projected by the previous methods.