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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online),...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online),...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online),...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online),...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online),...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online),...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online),...
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THERMO HYDRAULICS PERFORMANCE OF TURBULENT FLOW HEAT TRANSFER THROUGH SQUARE DUCTS WITH INSERTS

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This paper describes the experimental study of square ducts with inserts. Experiments are conducted for air with uniform heat flux condition. The top wall surface is made rough with metal ribs of square section. The roughened wall is uniformly heated and other walls are insulated. The heat transfer coefficient enhances square channel at injection of different inserts. The performance of the geometry under investigation has been evaluated .The heat transfer coefficient of air is increase by 46% than plane square ducts with inserts. The heat transfer and pressure drop measurements have been taken in separate sections .

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THERMO HYDRAULICS PERFORMANCE OF TURBULENT FLOW HEAT TRANSFER THROUGH SQUARE DUCTS WITH INSERTS

  1. 1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME 59 THERMO HYDRAULICS PERFORMANCE OF TURBULENT FLOW HEAT TRANSFER THROUGH SQUARE DUCTS WITH INSERTS M. Udaya Kumar Associate Professor, Indur Institute of Engineering and Technology, Siddipet, MEDAK (District), Telangana - 502 277, India M. Manzoor Hussian Professor, JNTU Hyderabad, Kukatpally, Hyderabad, Telangana - 500085 Md. Yousuf Ali Nawab Shah Alamkhan College of Engineering and Technology, No.16-4-1, New Malakpet, Near Railway Station, Malakpet, Hyderabad, Telangana 500024 ABSTRACT This paper describes the experimental study of square ducts with inserts. Experiments are conducted for air with uniform heat flux condition. The top wall surface is made rough with metal ribs of square section. The roughened wall is uniformly heated and other walls are insulated. The heat transfer coefficient enhances square channel at injection of different inserts. The performance of the geometry under investigation has been evaluated .The heat transfer coefficient of air is increase by 46% than plane square ducts with inserts. The heat transfer and pressure drop measurements have been taken in separate sections .The flow friction and thermal characteristics are governed by duct aspect ratio, twist ratio, length, Reynolds number and prandtl number. The heat transfer and pressure drop characteristics of turbulent flow of air (10.000<Re<100,000) through square ducts with inserts on all surfaces of the ducts have been studied experimentally. INTRODUCTION The enhancement of single phase heat transfer inside a duct is often achieved by forming some swirling or secondary flows. It is usually accompanied with high turbulence intensity, which INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME: www.iaeme.com/IJMET.asp Journal Impact Factor (2014): 7.5377 (Calculated by GISI) www.jifactor.com IJMET © I A E M E
  2. 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME 60 promotes the mixing of different parts of fluids, hence enhances the heat transfer. The swirling and secondary flow can be initiated by several ways, among which the insert of twisted tape or wire coil, the injecting or imparting a tangential flow, the use of integral tube with helical fins, spiral fluted duct and the twisted duct are often encountered. Most of these methods have been intensively studied. For example, the heat transfer and fluid flow behavior of the twisted tape have been studied and compared comprehensively by Manglik and Bergles [1],[2]. Yampolsky [3] and Obot et al. [4] tested the fluid flow and heat transfer characteristics of the spirally fluted tubes. The imparting of a tangential flow was investigated by Gau and Chen[ 5]. Relatively speaking the study on the twisted duct is quite limited. The purpose of this paper is to investigate the turbulent heat transfer and fluid flow in twisted uniform cross section, converging and diverging square ducts. As a technique for enhancing heat transfer, twisted duct has a unique feature in that it may induce swirling secondary flow and increase fluid contacted surface in some extent within a given axial length while still keeping smooth surfaces. Apart from the enhancement consideration, the present study was also motivated by the need in understanding the heat transfer mechanism of gas turbine blade cooling process. The cooling channel of a gas turbine is often modeled by a duct with a square cross section [6]. Periodic ribs are frequently employed to enhance the heat transfer process in various cooling passages such as turbine blades, guide vanes and combustor walls. Heat transfer augmentation inside cooling channels is achieved by using repeated ribs as turbulence promoters. The periodic ribs break the laminar sub-layer and create local wall turbulence due to flow separation and reattachment between them ribs, greatly enhancing the heat transfer. Several researchers have studied the heat transfer characteristics in straight channels with various shaped ribs using air as coolant flow. Chandra et al. [7], Han et al. [8], Lanjewar et al. [9], Srinath et al. [10], Salameh and Sunden [11], Tanda [12] and Wang and Sunden [13] studied experimentally the effect of ribs configuration and angled ribs on heat transfer and friction. While by numerical predictions of the flow and heat transfer in rib-roughened passages have been conducted previously by several investigators: Kashmiri et al. [8] investigated the rib pitch effect on heat transfer. Insertion of twisted tapes in circular tubes provides a simple passive technique for enhancing the convective heat transfer coefficient on the tube side of a heat exchanger. Twisted tape inserts are used to achieve compact heat exchangers, as well as, to prevent hot spots in high heat flux transfer situations encountered with gas flows. A duct of square cross-section provides higher surface to volume ratio than a circular tube. Further, if a square duct is inserted with a twisted tape, whose width equals the side of the duct, the flow becomes periodically fully developed with the distance of periodicity equals to 900 rotation of the tape. Similarly, the heat transfer also attains periodically fully developed state. Thus, both the flow and heat transfer are under continuous state of periodic development. Therefore, compared to a circular tube with a twisted tape insert, a higher thermal hydraulic performance can be expected from a square duct with a twisted tape insert. Mano et al. [14] and Bhadsavle [15] have presented experimentally determined correlations of friction factors for flow in a square duct with twisted tape insert. On comparison, however, it is observed that the correlations are at considerable variance with each other. In general, turbulent heat transfer in the duct is dominated by the transport of heat by turbulence. Therefore, in order to understand the mechanism of turbulent heat transfer in the complex through a square duct, the characteristics of the temperature flow field, such as temperature fluctuation intensity and turbulent heat fluxes, should be examined detail. Square ducts are widely used in heat transfer devices. For instance in compact heat exchangers, gas turbine cooling systems, cooling channels in combustion chambers and nuclear reactors. The forced turbulent heat convection in a square duct is one of the fundamental problems in the thermal science and fluid mechanics. The problem of turbulent heat and fluid flows in a straight square duct is fundamental in thermal science and fluid mechanics. The turbulence in the straight square duct has a remarkable change in Flow structure due to existence of the so called Prandtls second kind secondary flows. Enhancement leads to reduce size and cost of heat exchanger. An increase in heat transfer coefficient generally leads to
  3. 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME 61 additional advantage of reducing temperature driving force. Whenever inserts are used for the heat transfer enhancement, along with the increase in the heat transfer rate, the pressure drop also increases. This increase in pressure drop results in rise in pumping cost. Therefore, any augmentation device should optimize between the benefits due to the increased heat transfer coefficient and the higher cost involved because of the increased frictional losses. EXPERIMENTAL SET UP Schematic diagram of the experimental set-up is shown in Fig1. Experiments were carried out in a double-pipe heat exchanger. It has square duct (Aluminum duct, D = 25 mm, L = 4000 mm, AR =1.0, thickness (δ) = 4 mm) .It consists of calming section, test section, U Tube Manometers, air was flowing through the inner square duct at very high flow rate in counter-flow, through the annular channel formed between inner square duct and outer surface to attain nearly uniform wall temperature conditions. The temperature of hot air is maintained at desired set point with the deviation of plus or minus 10 C by controlled heating. All the RTD PT 100 type (Teflon coated) temperature sensors used to measure inlet, outlet temperatures of hot air. The wall temperature is obtained by taking average of all thermocouples installed in axial location along the wall of the duct. A groove of 1 mm depth is made on outer wall of the duct, temperature measuring probe inserted in to the groove and adhesive was applied around the groove to fix up the thermocouples to the wall. This is particularly essential to get inner wall temperature. The test section and connected piping were wound with asbestos insulation of approximately 30 mm thickness to minimize heat loss to the surrounding area. The flanges are used to connect calming section to test section and mixing section at the other end. Mixing section (stainless steel, square duct, 500 mm L and 25.2 mm De), whose 100mm length filled with wire mesh, Teflon gaskets of 8 mm thickness placed in between flanges to prevent heat conduction loss to calming section and mixing section. The pressure drop across the test section was measured by using vertical inclined U-tube manometer containing water. Heat Transfer calculations Ts = (T2+T3+T4+T5+T6+T7+T8+T9)/8 Tb= (T1+T10)/2 Mean temperature, Te = ( Ts+Tb ) / 2 Reynolds Number, Re = De vρ/µ De = 4A /p Nusselt Number: Nu = hiDe /k Experimental Nusselt Number (Nu): Heat transfer coefficient, h = q/(Ts-Tb) Heat flux, q =Q/A (2)Theoretical Nusselt Number (Nuth): 0.024 Re 0.8 Pr 0.4 f =[(∆P /L)De )]/[(ρU2 /2)]
  4. 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME 62 SCHEMATIC DIAGRAM Nomenclature A convective heat transfer area (πDL), (m²) Qd air discharge through test section (m³/sec) Dh hydraulic diameter (4A/P), (m) F friction factor fi friction factor obtained using inserts h experimental convective heat transfer coefficient, (W/m²K)
  5. 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME 63 hair equivalent height of air column, (m) k thermal conductivity, (W/mK) L length of test section, (m) m mass flow rate of air, (Kg/sec) Nui Nusselt number (experimental) with inserts, (hDh/k) Nu Nusselt number (experimental) for plain square duct Nuthe Nusselt number for plain square duct(theoretical) Pr Prandtl number P wetted perimeter, (m) ∆P pressure drop across the test section, (Pa) Re Reynolds number, T1, T10 - air temperature at inlet and outlet, (°C) T2, T3 T4, T5 - duct wall temperatures, (°C) T6, T7 T8, T9 T s average Surface temperature of the working fluid, (°C) T bulk temperature, (°C) Te means temperature, (°C) V velocity of flow (m/sec) U air velocity through test section, (m/sec) Greek symbols ν Kinematic viscosity of air, (m²/sec) µ dynamic viscosity, (kg/m s) η Over all enhancement ratio ρa density of air (Kg/m³) v velocity (m/sec) Thermo hydraulic performance For a particular Reynolds number, the thermo hydraulic performance of an insert is said to be good if the heat transfer coefficient increases significantly with a minimum increase in friction factor. Thermo hydraulic performance estimation is generally used to compare the performance of different inserts such as twisted tape, wire coil, etc., under a particular fluid flow condition. Overall enhancement ratio The overall enhancement ratio is defined as the ratio of the heat transfer enhancement ratio to the friction factor ratio .This parameter is also used to compare different passive techniques and enables a comparison of two different methods for the same pressure drop. The friction factor is a measure of head loss or pumping power. Nusselt number The Nusselt number is a measure of the convective heat transfer occurring at the surface and is defined as hd/k, where h is the convective heat transfer coefficient, d is the diameter of the tube and k is the thermal conductivity.
  6. 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME 64 Prandtl number The Prandtl number is defined as the ratio of the molecular diffusivity of momentum to the molecular diffusivity of heat. Aspect ratio It is ratio of duct width to duct height. This factor also plays very crucial role in investigating thermo hydraulic performance. Pitch Pitch is defined as the distance between two points that are on the same plane, measured parallel to the axis of a twisted tape. CONCLUSIONS 1. The friction factor and Nusselt number in square duct is higher than that for circular tube under a same operating conditions. This happen due to fact that a duct of square cross section provides higher surface area to volume ratio than that of circular tube when both were fitted with different inserts. 2. Experimental results of plain square duct were compared and found in good agreement with well known correlation of friction factor. 3. The friction factor decreases with increasing Reynolds number and increases with decreasing twist ratio. 4. The increase in friction factor and heat transfer can be explained by the generation of swirling flow as result of secondary flows of fluid. 5. In the present study experimental set up was validated for friction factor and heat transfer results. Isothermal friction factor and Nusselt number variation with Reynolds number for a square duct fitted with different inserts are also investigated. REFERENCES [1] Manglik, R. M., and Bergles, A. E., 1993, ‘‘Heat Transfer and Pressure Drop Correction for Twisted-Tape Inserts in Isothermal Tubes: Part I—Laminar Flow,’’ ASME J. Heat Transfer, 115, pp. 881–889. [2] Manglik, R. M., and Bergles, A. E., 1993, ‘‘Heat Transfer and Pressure Drop Correction for Twisted-Tape Inserts in Isothermal Tubes: Part II—Turbulent Flow,’’ ASME J. Heat Transfer, 115, pp. 890–896. [3] Yampolsky, J. S., 1983, ‘‘Spirally Fluted Tubing for Enhanced Heat Transfer,’’ Heat Exchangers—Theory and Practice, J. Taborek, G. F. Hewitte, G., Afgan, eds. Hemisphere, Washington, D. C., pp. 945–952. [4] Obot, N. T., Esen, E. B., Snell, K. H., Rabas, T. J., 1991, ‘‘Pressure Drop and Heat Transfer for Spirally Fluted Tubes Including Validation of the Roles of Transition,’’ in ASME HTD- 164, T. J. Rabas, J. M. Chenoweth, eds., New York, pp. 85–92. [5] Gau, C., and Chen, H. R., 1998, ‘‘Enhancement of Heat Transfer With Swirling Flows Issued Into a Divergent Pipe,’’ J. Thermophys. Heat Transfer, 12, pp. 87–93. [6] Han, J. C., Zhang, Y. M., Lee, C. P., 1991, ‘‘Augmented Heat Transfer in Square Channels With Parallel, Crosses, and V-Shaped Angled Ribs,’’ ASMEJ. Heat Transfer, 113, pp. 590–596. [7] P. R. Chandra, C. R. Alexander and J. C. Han, “Heat transfer and friction Behaviors in Rectangular Channels with Varying Number of Ribbed Walls,” Int. J. Heat Mass Transfer, Vol. 46, pp. 481-495. 2003.
  7. 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 11, November (2014), pp. 59-65 © IAEME 65 [8] J. C. Han and J. S. Park “Developing Heat Transfer in Rectangular Channels with Rib Turbulators,” Int. J. Heat Mass Transfer, Vol. 31, pp.183-195, 1988. [9] A. Lanjewar, J. L. Bhagoria and R. m. Sarviya “Heat Transfer and Friction in Solar Air Heater Duct with W- Shaped Rib Roughness on Absorber Plate,” Energy, Vol. 36, pp.4531-4541, 2011. [10] V. Srinath, Ekkad and J. C. Han “Detailed Heat Transfer Distributions in Two- pass Square Channels with Rib Turbulators,” Int. J. Heat Mass Transfer, Vol. 40, pp. 2525-2537, 1997. [11] T. Salameh and B. Sunden “An Experimental Study of Heat Transfer and Pressure Drop on the Bend Surface of a U- Duct,” ASME Paper GT2010-22139, 2010. [12] G. Tanda “Heat Transfer in Rectangular Channels with Transverse and V- Shape Broken Ribs,” Int. J. Heat Mass Transfer, Vol. 47, pp. 229-243, 2004. [13] L. Wang and B. Sunden “Experimental Investigation of Local Heat transfer in a Square Duct with Various Shaped Ribs,” Heat Mass Transfer., Vol. 43, pp. 759-766, 2007. [14] A. Kashmiri, M. A. Cotton and Y. Addad “Numerical Simulations of Flow and heat transfer over Rib-Roughened Surfaces,” 17th Annual Conference of CFD Society of Canada, Ottawa, May 3-5, 2009. [15] Y. Mano, N. Funahashi, S. Kobayashi, M. Kobayashi, Y.Mori, K. Ohhori, I. Nikai, M. Akane, A. Fuji, T. Kaneta, S. Urabe, Development of high temperature plate-fin heat exchanger for phosphoric acid fuel cell power system, in: High Temperature Heat, Exchangers, Hemisphere Publishing Corporation, 1986, pp. 421–432. [16] K. Obual Reddy, M. Srikesh, M. Kranthi Kumar and V. Santhosh Kumar, “CFD Analysis of Economizer to Optimize Heat Transfer”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 5, Issue 3, 2014, pp. 66 - 76, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. [17] Jawdat A. Yakoob and Ehsan F. Abbas, “Experimental Investigation of Convection Heat Transfer for Laminar Flow in an Inclined Annulus”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 5, Issue 4, 2014, pp. 160 - 168, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. [18] 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.

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