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  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME 140 CIRCULAR FINS WITH SLANTED BLADES: UNIFORM HEAT FLUX AND ISOTHERMAL PROCESSES Ali Shakir Al-Jaberi Dept. of Automobiles Eng, Najaf Technical College, Iraq Majid H. M. Foundation of Tech. Education Bassam A. Saheb Dept. of Automobiles Eng, Najaf Technical College, Iraq ABSTRACT This paper investigates experimentally the heat transfer enhancement process over external surface of copper pipe with circular fins attached on theouter surface (heat exchanger) in rectangular channel with air cross-flow. Four types of circular fins were used in the experiments with 32 mm in. I.D., 92 mm. O.D and 1 mm. thick attached on copper pipe. Each type has five (5) circular fins. 1st type has five (5) fins without slanted blades, 2nd type has five (5) slanted blades per one fin, 3rd type has seven (7) slanted blades per one fin and 4th type has nine (9) slanted blades per one fin. Digital thermal camera (FLIR thermal imaging) was used to calculate the average copper pipe/fins surface temperature. Results show that the Nusselt number for the 2nd type is about 11.8 % higher than those for 1st type and with 3rd type is about 20.25 % higher than those for the 1st type and with 4th type is about 27.5 % higher than those for the 1st type. In addition, experimental results show that the 4th type has a good enhancement of heat transfer and fin performance. Moreover, it causes significant reduction in thermal resistance by comparison with 1st type. Keywords: Heat exchanger, Circular fin, Slanted blades, Heat transfer enhancement, Fin performance and thermal resistance. INTRODUCTION Circular fin surfaces are widely used as extended heat exchange surfaces in many fields, including heating, ventilation, air conditioning, and refrigeration (HVACR). Generally, 90% of the INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME: www.iaeme.com/ijmet.asp Journal Impact Factor (2014): 7.5377 (Calculated by GISI) www.jifactor.com IJMET © I A E M E
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME 141 thermal resistance is caused by the air-side thermal resistance. To lower the thermal resistance for improving the performance, many efforts have been concentrated on enhancing the air-side heat transfer by designing new fins. A lot of experimental and numerical studies have been conducted on airside heat transfer performances of fin-and-tube heat exchangers. Stasiulevicius et al. (1988) developed correlations of the convective heat transfer coefficient and resistance of finned-tube bundles in a cross flow including the effects of geometric parameters of fins and tube arrangement within the bundle. Madi et al. (1998) investigated the effects of fin thickness, fin pitch and the number of tube rows on the airside performance of wavy fin-and-tube LuveContardo experimental facilities. Mon et al. (2004) numerically investigated the effect of the ratio of fin-spacing to height on the unsteady flow and heat transfer performance in an annular finned-tube heat exchanger using a numerical renormalization group theory. In finned tubes configuration, the formation of horseshoe vortices (HSV) is observed at each fin-tube. Mon and Gross (2004) investigated the effects of the fin spacing on four-row annular-finned tube bundles in staggered and in-line arrangements by three dimensional numerical study. Cheng et al. (2004) numerical designed slotted fin surface with field synergy principle. Nuntaphan et al. (2005) improved the air-side heat transfer performance of heat exchangers for various fins to increase the total surface area. Also, they studied the air-side of the crimped spiral fin heat exchanger to analyze the effects of the tubes, diameters, fin spacing, transfer tube pitches, and arrangements, and they proposed a correlation between the heat transfer and friction characteristics in the case of low Reynolds numbers under dry and wet surface conditions. The results showed that the heat transfer coefficient for the dry surface was higher than that for the wet surface. Zhou and Tao (2005) studied the strip fin with radial strips. Tao et al. (2007) studied the laminar heat transfer and fluid flow characteristics of wavy fin heat exchangers with elliptic/circular tubes. The results were also analyzed from the view point of field synergy principle. Tang et al. (2008, 2009) experimentally investigated the air-side heat transfer and friction characteristics of five kinds of fin-and-tube heat exchangers crimped spiral fin, plain fin, slit fin, fin with delta wing longitudinal vortex generators, and mixed fin for which the number of tube rows was 12 and the diameter of the tubes was 18 mm throughout both experimental and numerical investigation. Their results indicated that the crimped spiral fin gave a high pressure drop, but it also provided a higher air-side heat transfer performance than the other types. Xie et al. (2009) numerically studied the airside laminar heat transfer and fluid flow characteristics of plain fin-and- tube heat exchangers with large number of large-diameter tube rows. The effects of Reynolds number, the number of tube rows, tube diameter, tube pitch, fin pitch and fin materials were examined heat exchangers. Lee et al. (2010) investigated a spiral-type circular finned-tube heat exchanger in terms of fin pitch, the number of rows, fin alignment, and heat exchanger geometry in comparison with flat- plate finned-tube heat exchangers.Parinya et al. (2012) conducted experiments on the optimized fin pitch for crimped spiral fin-and-tube heat exchangers. The experiments covered a size range of 2.4–6.5 mm, which is the manufacturing limitation for this kind of fin. The water- flow arrangement used in this experiment combined the parallel cross-flow and the counter cross- flow in a two-row configuration. They studied the effects of fin pitches on the heat transfer coefficient and pressure drop characteristics. Dong H. Lee et al. (2012) investigated the air-side heat transfer performance of a heat exchanger with perforated circular finned tubes. SebastienVintrou(2013) designed an experimental set-up involving IR thermography to estimate local heat transfer coefficient distribution with a transient technique. The method consists in time integration of a heat conduction model that takes into account lateral heat conduction into the material and radiation with surrounding. In this paper, experimental study of a circular fins with slanted blades attached on the copper pipe surface is performedto reduce the thermal boundary layer /thermal resistanceby adding slanted blades. The effects of two parameters: number of slanted blades and Reynolds number on the heat
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME 142 transfer characteristics are examined. The fin pitch that was tested covered a size of 1.75 mm which is the manufacturing limitation for this kind of fin. EQUIPMENT AND PROCEDURE The experimental test rig (computer controlled cross flow) was to study the effects of circular fins with slanted blades on the heat transfer enhancement (Fig. 1). The duct channel length (l) is of 700 mmand cross section width (w) of 120 mmand cross section depth (l) 120 mm. test section four models of the circular fins have been tested in the practical side and manufactured in Najaf Engineering Tech. college laboratories. Four types of circular fins were used in the experiments (figure 3) with 32 mm. I.D., 92 mm. O.D and 1 mm. in thick attached on copper pipe. Each type has five (5) circular fins. 1st type has five (5) fins without slanted blades, 2nd type has five (5) slanted blades per one fin, 3rd type has seven (7) slanted blades per one fin and 4th type has nine (9) slanted blades per one fin. All the slanted blades with an angle equal 80 degree from the horizontal line of the copper pipe. Four models (Fig. 2) were made of copper (kcopper= 400 W m-1 K-1 , ρcopper= 8800 kg m-3 ) because of consideration like machinability, and conductivity. Cylindrical heater with control circuit was inserted inside the copper pipe. Heater of approximately the same dimensions as the copper pipe with the power 84 W heated the internal copper pipe for constant heat flux process and for constant base temperature is fixed at 64 °C. The amount of heat supplied by the heater is controlled with autotransformer (Variac) and digital wattmeter (control interface box). Heat loss to the surrounding from backsides of the heater is minimized by insulating the all duct and test section by glass wool. In the experiment, the Reynolds number ranges were8000-31000, which is based on hydraulic channel diameter and inlet velocity. Air is the working fluid in the experiment. The air temperatures for the after and before model were measured by K-thermocouples. Each run of experiments take 35 min even after the steady-state which is between 55-60 min after that, more than seventy readingfor various pin finsurfaceshavebeenmeasuredand recorded to calculate theaveragesurfacetemperature usingthermal imaging infrared camera (FLIR E30). All the above equipments used for various measurements were calibrated. DATA PROCESSING AND ANALYSIS The heat transfer modes in the present work are conduction, convection, and radiation through the air. The magnitude of each mode depends on the temperature of the geometry and the flow rate. Steady state heat transfer from external surface of circular fins attached on copper pipe is: ࡽሶ ࢉ࢕࢔࢜ࢋࢉ࢚࢏࢕࢔ ൌ ࡽሶ ࢚࢕࢚ࢇ࢒െࡽሶ ࢒࢕࢙࢙ (1) Where ࡽሶ ࢉ࢕࢔࢜. is Heat convection through test section and the steady state heat transfer from pin fin array base ࡽሶ ࢚࢕࢚ࢇ࢒. is equal of electrical heat input and calculated from the electrical potential and current supplied to the test section. The radiation heat loss can be neglected. The heat transfer by convection from test section surface including base plate is given by: ࡽሶ ࢉ࢕࢔࢜ࢋࢉ࢚࢏࢕࢔ ൌ ࢎࢇ࢜࡭࢙ ቂࢀ࢙,ࢇ࢜ െ ቀ ࢀ࢕࢛࢚ି ࢀ࢏࢔ ૛ ቁቃ (2)
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME 143 Hence average convective heat transfer coefficient ࢎࢇ࢜, can be find out as: ࢎࢇ࢜ ൌ ࡽሶ ࢉ࢕࢔࢜ࢋࢉ࢚࢏࢕࢔ ࡭࢙ቂࢀ࢙,ࢇ࢜ିቀ ࢀ࢕࢛࢚ష ࢀ࢏࢔ ૛ ቁቃ (3) Where ࢀ࢕࢛࢚ is the outlet airflow temperature that was determined by the averaging of the Three k-type thermocouples measured at the downstream of test section.ࢀ࢏࢔is the inlet airflow temperature that was determined by the averaging of the four k-type thermocouples measured at the upstream of test section. ࢀ࢙,ࢇ࢜is the average test section surface of readingfor varioussurfacesmeasuredand recorded by using thermal imaging infrared camera (FLIR E30). ࡭࢙ is the surface area of test section including the base of test section The dimensionless groups, Nusseltࡺ࢛and Reynolds numberࡾࢋ are calculated as follows: ࡺ࢛ ൌ ࢎࢇ࢜ࢊࢎ ࡷࢇ࢏࢘ (4) ࡾࢋ ൌ ࣋ࢇ࢏࢘ ࢂ ࢊࢎ ࣆࢇ࢏࢘ (5) Where ࢊࢎ, ࡷࢇ࢏࢘, ࣋ࢇ࢏࢘ and ࣆࢇ࢏࢘ are hydraulic diameter, thermal conductivity, density and viscosity for air respectively. The related thermo physical properties of the working fluid are obtained using the bulk mean temperature, which is ࢀ࢓ ൌ ሺࢀ࢏࢔ାࢀ࢕࢛࢚ሻ ૛ (6) For the constant test section base temperature, fin performance ߝ௙ is the ratio of heat transfer from circular fin to heat transfer from fin base without circular fin, as fin effectiveness ,Shaeri (2009), and it is defined as follows; ߝ௙ ൌ Qሶ convection ௛್ ஺್ ሺ்್ି்∞ሻ (7) Where ܶ∞ is the free stream temperature, ݄௕௔௦௘ base circular fins without slanted blades (bare copper pipe) convection heat transfer coefficient, ‫ܣ‬௕௔௦௘ base area circular fins without slanted blades (bare copper pipe) and ܶ௕௔௦௘ is the base temperature of circular fins without slanted blades. The total thermal resistance ܴ௧௛.is primary thermal performance parameter for the circular fins which is considered in this study as: ܴ௧௛. ൌ ሺ்ೞି்∞ሻ ொሶ೎೚೙ೡ೐೎೟೔೚೙ (8)
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME 144 RESULTS AND DISCUSSION Based on the FLIR thermal imaging measurement it was determined after steady state that the maximum, minimum and average temperature of the monitored box area was various as shown in figure 3. The boundary conditions for all images in figure 3 are 6.5 m/s input velocity, 84W input power and 21 °C inlet air temperature. It is clearly shown that the maximum, minimum and average temperature for box area is lowered for 2nd type, 3rd and4th type than that the 1st type. In addition, the temperature distribution bar for four thermal images in figure 3 at the right hand side gives a clear impression for slanted blades effectiveness. In addition, increasing the number of slanted blades leads to destroy the thermal boundary layer (Growth the turbulent eddies) and thus reducing the thermal resistance to heat transfer. Working the slanted blades as a recipient and emitter of heat from the hot surface of the copper pipe at same time, and this is clear from the amount of the light glow in the four images in figure 4 on the rear zones. Fig. 1: schematic diagram of the experimental rig. Item Description 1 Mouth 2 Flow straightener 3 Inlet Pitot tube(Pressure in) 4 Inlet temp. thermocouple(T in) 5 Insulated duct 6 Heat sink insulation 7 Heater (constant heat flux) 8 Heat sink base T av 9 Heat sink insulated base 10 Outlet Pitot tube( Pressure out) 11 thermocouple(T out) 12 differential manometerDigital 13 Control interface box 14 Computer 15 Variable speed blower control 16 Test section 17 Digital wattmeter 18 Variable transformer 19 Mounting Structure 1 Air Inlet Air Ou tle t 2345691211 10 8 71315 14 161718 1 19
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME 145 Table 1: Geometric description of copper pipe /circular fins tested 1st Type 2nd Type 3rd Type 4th Type Fig. 2: Various circular fins with slanted blades configuration. Parameter symbol value Diameter of copper pipe d1 32 mm Circular fin number N 5 Diameter of the circular fin d2 92 mm Height of the copper pipe l 100 mm Circular fin thickness t 1 mm Circular fin pitch p 17.5 mm Duct length L 700 mm Duct cross section width w 120 mm Duct cross section depth D 120 mm Number of slanted blades Ns 0,5,7 and 9 Depth of the notch b 20 mm l d1 d2 p t
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME 146 Rear zone 1st Type 2nd Type 3rd Type 4th Type Maximum temperature point. Minimum temperature point. Figure 3: Temperature distribution field of 1st , 2nd, 3rd and 4th type. Flow Flow Flow Flow Flow Flow Flow
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME 147 Figure 4 shows the variation of Nusselt number as a function of Reynolds number for four models or types considered in the present work. It is clearly that the slanted blades have influence on the rate of average heat transfer. Adding slanted blades to the circler fin increasing the direct contact’s surface area with working fluid. The results of the 4th type (5 circular fins/9 slanted blades per fin) is found good enhancement from other types or models. Results show from figure 5 that the Nusselt number for the 2nd type is about 11.8 % higher than that for 1st type and with 3rd type is about 20.25 % higher than that for the 1st type and with 4th type is about 27.5 % higher than the 1st type. For constant average surface temperature (65 o C), figure 5 shows the variation of fin effectiveness with Reynolds number for four types. It is clearly shown that for the four types of circular fins, fin effectiveness increases as Reynolds number increases up to Reynolds number about 25000 then Nusselt number decreases as Reynolds number increases. In addition, fin effectiveness for 2nd , 3rd and 4th type is higher than for 1st type. Also, for constant average surface temperature, figure 6 shows the variation of total thermal resistance with Reynolds number for four types or models. The results show that the thermal resistance of 1st type (without slanted blades) is higher than the thermal resistance for four types or models. This is because the velocity distribution was more disturbances through the 2nd , 3rd and 4th types. Figure 4: Variation of Nusselt number with Re for copper pipe without fins and for 1st , 2nd , 3rd and 4th type. Figure 5: Variation of fin effectiveness with Re for copper pipe without fins and for 1st, 2nd , 3rd and 4th type.
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME 148 Figure 6: Variation of total thermal resistance with Re for copper pipe without fins and for 1st , 2nd , 3rd and 4th type. CONCLUSION In this study, the overall heat transfer, fin effectiveness and thermal resistance were investigated experimentally. The effects of the working fluid flow and number of slanted blades on the overall heat transfer, fin effectiveness and thermal resistance were determined. The conclusions are summarized as: 1- Average Nusselt number increased with increasing the number of slanted blades. 2- Average Nusselt number for the 2nd type is about 11.8 % higher than that for 1st type and with 3rd type is about 20.25 % higher than that for the 1st type and with 4th type is about 27.5 % higher than the 1st type 3- 4th type (5 circular fins/ 9 slanted blades per fin) has a highly thermal dissipated and fin performance relative to other types. 4- 4th type (5 circular fins/ 9 slanted blades per fin) is achieved in economical. REFERENCES [1] Stasiulevicius, A. Skrinska, A. Zukauskas, Heat transfer of finned tube bundles in crossflow, Hemisphere Publishing, 1988. [2] M. Abu Madi, R.A. Johns, M.R. Heikal, Performance characteristics correlation for round tube and plate finned heat exchangers, Int. J. Refrig. – Rev. Int. Du Froid 21 (7) (1998) 507– 517. [3] M.S. Mon, U. Gross, Numerical study of fin-spacing effects in annular-finned tube heat exchangers, Int. J. Heat Mass Transfer 47 (8–9) (2004) 1953–1964. [4] M.S. Mon, U. Gross, Numerical study of fin-spacing effects in annular-finned tube heat exchangers, Int. J. Heat Mass Transfer 47 (8–9) (2004) 1953–1964. [5] Y.P. Cheng, Z.G. Qu, W.Q. Tao, Y.L. He, Numerical design of efficient slotted fin surface based on the field synergy principle, Numer. Heat Transfer, Part A 45 (6) (2004) 517–538.
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME 149 [6] A.Nuntaphan, T. Kiatsiriroat, C.C. Wang, Air side performance at low Reynolds number of cross-flow heat exchanger using crimped spiral fins, Int. Commun. Heat Mass Transfer 32 (1–2) (2005) 151–165. [7] J.J. Zhou, W.Q. Tao, Three-dimensional numerical simulation and analysis of the airside performance of slotted fin surfaces with radial strips, Eng. Comput. 22 (7/8) (2005) 940–957. [8] Y.B. Tao, Y.L. He, Z.G. Wu, W.Q. Tao, Three-dimensional numerical study and field synergy principle analysis of wavy fin heat exchangers with elliptic tubes, Int. J. Heat Fluid Flow 28 (6) (2007) 1531–1544. [9] L.H. Tang, G.N. Xie, M. Zeng, M. Lin, Q.W. Wang, A comparative study of finand-tube heat exchangers with various fin patterns, ASME Conf. Proc., Heat Transfer Part A and B 4 (2008) 1239–1246. [10] L.H. Tang, M. Zeng, Q.W. Wang, Experimental and numerical investigation on air-side performance of fin-and-tube heat exchangers with various fin patterns, Exp. Therm. Fluid Sci. 33 (5) (2009) 818–827. [11] G. Xie, Q. Wang, B. Sunden, Parametric study and multiple correlations on airside heat transfer and friction characteristics of fin-and-tube heat exchangers with large number of large-diameter tube rows, Appl. Therm. Eng. 29 (1) (2009) 1–16. [12] M. Lee, T. Kang, Y. Kim, Air-side heat transfer characteristics of spiral-type circular fin- tube heat exchangers, Int. J. of Refrig. 33 (2) (2010) 313–320. [13] ParinyaPongsoi, SantiPikulkajorn, SomchaiWongwises, Effect of fin pitches on the optimum heat transfer performance of crimped spiral fin-and-tube heat exchangers, International Journal of Heat and Mass Transfer 55 (2012) 6555–6566 [14] Dong H. Lee, Jin M. Jung, Jong H. Ha, Young I. Cho, Improvement of heat transfer with perforated circular holes in finned tubes of air-cooled heat exchanger,International Communications in Heat and Mass Transfer 39 (2012) 161–166. [15] SebastienVintrou, Daniel Bougeard, Serge Russeil, RabieNacereddine, Jean-Luc Harion, Quantitative infrared investigation of local heat transfer in a circular finned tube heat exchanger assembly, International Journal of Heat and Fluid Flow xxx (2013) xxx–xxx, article in progress. [16] M.R. Shaeri, M. Yaghoubi *, K. Jafarpur, Heat transfer analysis of lateral perforated fin heat sinks, Applied Energy 86 (2009) 2019–2029. [17] Omar Mohammed Ismael, Dr. Ajeet Kumar Rai, Hasanfalah Mahdi and Vivek Sachan, “An Experimental Study of Heat Transfer In A Plate Heat Exchanger” International Journal of Advanced Research in Engineering & Technology (IJARET), Volume 5, Issue 4, 2014, pp. 31 - 37, ISSN Print: 0976-6480, ISSN Online: 0976-6499. [18] Sunil Jamra, Pravin Kumar Singh and Pankaj Dubey, “Experimental Analysis of Heat Transfer Enhancement in Circular Double Tube Heat Exchanger Using Inserts” International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 3, 2012, pp. 306 - 314, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. [19] S. Bhanuteja and D.Azad, “Thermal Performance and Flow Analysis of Nanofluids In A Shell and Tube Heat Exchanger” International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 5, 2013, pp. 164 - 172, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. [20] Prof.Alpesh V Mehta, Nimit M Patel, Dinesh K Tantia and Nilsh M Jha, “Mini Heat Exchanger Using Al2o3-Water Based Nano Fluid” International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 2, 2013, pp. 238 - 244, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 5, May (2014), pp. 140-150 © IAEME 150 SYMBOLS As Heat transfer area b Depth of the notch D Duct cross section depth dh Hydraulic diameter of channel d1 Copper pipe diameter d2 Circular fin diameter h Heat transfer coefficient l Copper pipe length L Duct test length K Thermal conductivity N Circular fins number Ns Number of slanted blades Nu Average Nusselt number Nus Nusselt number for smooth channel p Fin pitch Re Reynolds number R୲୦. Total thermal resistance T Temperature V Average inlet velocity W Duct cross section width Greek ε୤ fin effectiveness ρair Density of air µୟ୧୰ Viscosity of air Subscripts av average base base circular fins without slanted blades in inlet m Mean out outlet s Bare copper pipe ∞ Free stream