Enhancement in heat transfer characteristics by using truncated conical turbulators in a double pipe heat exchanger under turbulent conditions

Figures & data

Table 1. Influence of turbulators on the solar air heater performance

Authors	Geometry modification	Outcomes	Remarks
Gill et al., (2017)	Broken arc rib type of turbulator	The maximum increase in Nu and f were 3.06 and 2.5 times that of a plain absorber plate	A relative staggered rib ratio of 4 gave maximum thermo hydraulic efficiency.4
Prasad & Saini, (1988).	Small diameter wire type protrusions on the absorber plate	They observed a higher Stanton number and friction factor for the modified absorber plate compared to that of normal.	They concluded that both pitch and height of the roughness turbulators significantly influence the thermal performance
Gilani et al., (2017).	Conical-shaped fin protrusions on the absorber plate	Thermal efficiency was increased by 26% for the fin pitch of 16 mm compared to that of a smooth one.	Fin height of 4 mm pitch 16 mm with staggered arrangement gave optimum results
Promthaisong & Eiamsa-Ard, (2019).	Wavy triangular ribs	Nu and friction factor increased in the range of 5.8–304% and 13.8–171% respectively	Increase in Re, blockage ratio and decreasing the longitudinal pitch enhanced the thermal performance
Manjunath et al., (2017).	Spherical shaped turbulators	A spherical diameter of 3mm with P/d ratio 3, at Re 23,600 showed the highest thermal performance about 23.4% higher than the base model	Thermal efficiency increased by increasing the spherical turbulator diameter and decreasing the relative roughness pitch
Ravi & Saini, (2018).	Multi V-shaped staggered ribs	A maximum increase in Nu by 4.52 times and friction factor of 3.13 times that of a smooth duct was noticed for the modified absorber duct	Optimum performance was witnessed for rib size of 3.5, roughness width ratio of 7, and pitch ratio of 0.6
Zaboli et al., (2021).	Inner helical axial fins	For pitch =2500 and 1000 mm 14% and 21.5% improvement in performance respectively	Overall 21% improvement of thermal performance obeserved
Nouri Kadijani et al., (2022)	Rectangular channel solar air heater with v-shaped ribs	Increasing RH from 0 to 50% led to an increase in heat transfer by 50%	For a Re of 4000 and 12,000 performance increased by 7.3% and 31.4 %
Olfian et al., (2020)	Rectangular and V-shaped baffles	For 90 angled baffle, Nu increased by 148% and ΔP increased by 316% as compared to without baffle case	For 90 ° and 30 ° V-shaped baffles, thermal performance is 27% and 13% higher than no baffled case.

Table 2. Details of the inserts used in a tubular heat exchanger

Authors	Geometry modification	Outcomes	remarks
Kumar & Prasad, (2000).	A twisted tape insert	Increase of Nu by 18–70% whereas ΔP increased by 87–132% as compared to the plain tube.	In water heater applications twisted tape collectors performed better in the lower Re ie up to 12,000 beyond which the performance deteriorated.
Eiamsa-Ard & Promvonge, (2010).	Alternate clock-wise and anticlockwise twisted tape	Alternate clockwise-anticlockwise twisted tapes showed an increased Nu in the range of 12.8–41.8 and 27.3–90.5 compared to that of a single twisted tape insert and a plain tube.	Twist angle of 90°, twist ratio 3 gave a maximum increase in thermal performance.
Bhuiya et al., (2013).	Perforated twisted tape	Twisted tape with porosities 1.6,4.5, 8.9, and 14.7 was used. They revealed Nu, f, and thermal performance was enhanced by 110–340, 110–360, and 28–59% as compared to that of a normal plain tube.	Empirical correlations were developed and found that Nu, f, and thermal performance were within the deviation of 4%,4%, and 2% respectively for predicted and experimental results.
Murugesan et al., (2011).	V-cut twisted tape insert	V-cut twisted tape with a depth-to-width ratio of 0.34,0.43 is used. Enhancement in the thermal performance was noticed in decreasing twist ratio, width ratio, and increasing depth ratio.	Developed Correlations showed a good agreement between empirical and experimental results with an average deviation of 6% for Nu and 10% for friction factor.
Akyürek et al., (2018).	Wire coiled insert	Coiled wire type inserts with different pitch values were used for heat transfer augmentation in a concentric heat exchanger with Al2O3 nanofluid. For pitch 25 mm wire coil, Nu increased by 51.6% and heat transfer coefficient by 12.6%	An increase in volume concentration of nanoparticles enhanced the thermal performance, pitch 25 mm, volume concentration of 1.6% at Re of 20,000 showed maximum thermal performance.
Kumar et al., (2019).	Helical tape insert	Helically coiled tape inserts with varying width ratios and twist ratios are used. A combination of width ratio 3 and twist ratio 1.25 maximum thermos hydraulic performance was noticed.	Both tape width and tape pitch significantly contribute to the heat transfer enhancement in a double pipe heat exchanger.
Wongcharee & Eiamsa-Ard, (2011).	Twisted tape with triangular, rectangular, and trapezoidal wings.	Experiments were conducted using triangular, rectangular, and trapezoidal winglets with chord length ratios of 0.1,02, and 0.3 in each type. Trapezoidal wings outperformed the other types with better thermal performance	A maximum performance factor of 1.42 was noticed for trapezoidal wings with a chord ratio of 0.3. Nu and f increased by 2.84 and 8.02 times that of a bare pipe.
Kazemi Moghadam et al., (2020)	Corrugated tube with a coiled wire as a tubulator	Maximum increase in performance noted for Re =5000 with Nu =11.16% for p/d =3 and 17.4% for p/d =5	Roughness height of 3 exhibited best performance
Zaboli et al., (2022)	Corrugate tube with different lobe shaped c/s with spiral twisted tape	Five lobe c/c showed 9.1% and 3.7% higher Nu and ΔP than 3 lobed c/s	Providing corrugated shape with swirl generator effectively increased the thermal performance.
Mousavi Ajarostaghi et al., (2022)	Curved conical turbulator	Re varied from 10,000–35000 As the angle of curvature is increased Nu also increased.	Curved swirl generator showed higher than the uncurved structure.
Noorbakhsh et al., (2020).	Twisted tapes on both sides of the double pipe heat exchanger	Increasing the number of twisted tape from one-fin to four-fin increased Nu by 3.1%, ΔP by 64%, and coefficient of performance by 63.9%.	Creating hollows with aspect ratio =1 on the twisted tape yielded higher coefficient of performance.

Figure 1. Comparison between the CFD predicted and analytical values for Nu.

Figure 2. Schematic representation of the heat exchanger with the conical turbulator.

Figure 3a. Meshed model of the truncated conical turbulated heat exchanger.

Figure 3b. Grid independence study for the truncated conical turbulated heat exchanger.

Table 3. Dimensions and Specifications of the heat exchanger

Specifications	Dimensions
Inner tube Diameter	15mm
Outer tube Diameter	40mm
Tube Thickness	1mm
Length of the heat exchanger	500 mm
Base diameter of the truncated conical turbulator	12 mm
Height of the turbulator	12 mm
Pitch values in mm	25, 50,75 and 100
Material of heat exchanger and turbulator	Copper
Inner pipe fluid	Water
outer pipe fluid	Air
Water Reynolds number	3000
Reynolds number of air	3000, 6000, 9000,12000 and 15,000

Figure 4. Contour plot showing the velocity variation (a) along the length (b) exit velocity for Re 3000 and 15,000.

Figure 5. Contour plot showing the temperature variation along the length for Re 3000 and 15,000.

Figure 6. Contour plot showing the turbulent intensity variation along the length for Re 3000 and 15,000.

Figure 7. Contour plot showing the turbulent kinetic energy variation along the length for Re 3000 and 15,000.

Figure 8. (A) variation of velocity contours (b) variation of turbulent intensity (c) exit temperature variation for different pitch values.

Figure 9. Variation of temperature along the length of the heat exchanger for different pitch values.

Figure 10. Variation of Nu and annulus HTC with pitch values for different Re.

Figure 11. Variation of ΔP and friction factor with pitch values for different Re.

Figure 12. Variation of THPI with pitch values for different Re.

Table 4. Comparison of the present results with Published literature

Authors	Geometry	Operating conditions	Results obtained
Eiamsa-Ard & Promvonge, (2010).	Alternate clockwise and anticlockwise twisted tape inserts	Re varied from 3000–27000	Nu is 41.9% higher than the plain tube Heat transfer rate increased by 90% than the plain tube
Kumar et al., (2020).	Hemispherical Turbulators in the annulus body	Re varied from 2500 – 10000	Maximum Nu and THPI occurred for pitch ratio of 0.58. Nu is 2.71 times the base value and the corresponding THPI value 1.41
Kazemi Moghadam et al., (2020).	Corrugated tube with coiled wire	Re varied from 5000–20000	For P/d = 5 max increase in Nu observed for Re =5000
Zhang et al., (2012).	Helical fins with vortex generators	Re varied from 6000–14000	Nu for rectangular winglet pair was 46% higher than with only helical fins. For the same pitch
Present study	Truncated conical turbulators	Re varied from 3000–15000	Highest Nu and THPI of 78 and 1.13 obtained for pitch 50 mm.

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