Experimental Investigation of Elliptical Tube Bank Cross Flow Heat Exchanger with Inline Arrangement & 45 Angle of Attack #1 Digvijay A. Shelar, #2 N.S. Gohel, #3 R. S. Jha #1 Mechanical Engineering Department, SPPU #2 Professor, Mechanical Engineering Department, SPPU #3 Thermax India Ltd. Abstract In the present study thermal and hydraulic analysis of elliptical tube bank cross flow heat exchanger with inline arrangement is carried out. Cold atmospheric air is passed over the elliptical tube bank and hot water is circulated through these tubes. The elliptical tubes are arranged at 45 angle of attack. The thermal and hydraulic analysis is carried out at different Reynolds numbers on air side and constant mass flow rate of water. Experimental investigation is carried out to study the heat transfer augmentation and results are compared with Zukauskas correlation for circular tube bank. Keywords: cross flow heat exchanger, elliptical tubes, angle of attack, heat transfer augmentation I. INTRODUCTION Heat exchangers are devices that provide the flow of thermal energy between two or more fluids at different temperatures. These include power production, process, chemical and food industries, electronics, environmental engineering, waste heat recovery, manufacturing industry, and air conditioning, refrigeration and space applications. In these applications, thermal hydraulics and energy usage play dominant roles. Energy, space and materials saving considerations as well as the present day global economics have led to the expansion of efforts to produce more efficient heat exchange equipment for minimizing cost, which is to reduce the physical size of heat exchanger equipments for a given heat duty. Therefore the main thermal- hydraulic objectives are to reduce the size of a heat exchanger required for a specific heat duty, to upgrade the capacity of an available heat exchanger and its operation with smaller approach temperature difference, or to reduce the pumping power. There are various techniques used for heat transfer enhancement, which are segregated in two groups: (1)Active techniques (2) Passive techniques. The active techniques require external power to the surface (surface vibration, acoustic or electric fields). Passive techniques use specific surface geometries with surface augmentation. Talaat A Ibrahim et al. [4] carried out experimental and numerical investigations of the turbulent flow through bundle of elliptic tubes heat exchanger. They investigated the effect of design parameters such as minor to major axis ratio and flow angle of attack. The experiment was conducted on five bundle of elliptic tube heat exchanger with different axis ration. From experimental date, it was found that increasing angle of attack clockwise until 90 enhances the convective heat transfer coefficient considerably. The maximum thermal performance (heat transfer per unit pumping power) occurred at 0 where as the minimum thermal performance of the elliptic occurred at 90 angle of attack. The elliptic tube bundle produced a considerable frontal area reduction of heat exchanger at 0 angle of attack with minor to major axis ratio 0.25 to 1. Lei Sun et al. [2] compared the overall thermal performance of refrigerant to air finned tube condensers with elliptical and circular tubes. The results show that the air pressure drop and the heat transfer rate of elliptical tube condensers are 20% -27.3% lower and 8.3% to 30.9% higher than those of circular tube condenser respectively. Compared to circular tube condensers, the system capacity improvement using elliptical tube condensers was found to be as high as 21.3% -27.5% where as the COP improvement ranges from 3.6% to 6.7%. Arash Mirabdolah Lavasani et al. [1] experimentally investigated flow around cam shaped tube bank with inline arrangement. It was found that friction factor of cam shaped tube bank is about 95 and 93 percent lower than circular tube bank. By increasing longitudinal pitch ratios from 1.5 to 2, heat transfer increases about 7 to 14 percent. Mesbah G. Khan et al. [3] experimentally investigated Flows of hot air at 41.5 ± 1.5 C across an array of elliptical tubes carrying cold water. The results showed that the heat transfer rate (q) increased with the increase of both water and air flows. The Nu Re correlation obtained from the experiment was found to be in the form Nu a =0.26Re a 0.66 The objectives of experimental investigation are to study the thermal performances of elliptical shape tubes in cross flow heat exchanger & to study the effect of different mass flow rate of air on Nu, Re, Pr, Overall heat transfer coefficient. 2015, IERJ All Rights Reserved Page 1
Nomenclature - Heat transfer area (m 2 ) C p - Specific heat ( J/kg.K) - Hydraulic diameter (m) h - Convective heat transfer coefficient(w/m 2 K) ṁ - Mass flow rate (kg/s) Nu th - Nusselts number for circular tubes. Nu exp - Nusselts number for elliptical tubes calculated from experimental data ΔP - Pressure drop across heat exchanger (Pa) Q - Heat transfer due to convection (Watt) Re - Reynolds number (dimensionless) T - Temperature ( C) U - Overall heat transfer coefficient (W/m 2 K) Greek symbol α - Angle of attack θ - Log mean temperature difference ( C) - Density (kg/m 3 ) Subscripts a - Air i - Inlet o - Outlet w - Water II. EXPERIMENTAL APPARATUS AND PROCEDURE Fig.1Experimetnal setup The heat exchanger consists of a duct of 1 m length. The air enters the heat exchanger from inlet duct which is attached to the blower. The stainless steel elliptical tubes are used in the experiment. The tube array consists of 30 tubes; arranged in inline manner. The tubes are oriented at 45 angle of attack. The water is heated in tank with electric heaters. The hot water is circulated through tubes using centrifugal pump. To avoid heat loss, heat exchanger is fully insulated with nylon foam. The hot water was supplied to the heat exchanger at constant temperature. The inlet and the outlet temperature of the waterside and airside were measured using PT100 sensors. A 6-channeled digital temperature indicator is used to view the temperature readings of the sensors at different locations. Vane type anemometer was used to measure the velocity of air at the outlet duct. Velocity was measured at 9 grid points and average was taken. The outlet of pump is attached to rotameter with bypass valve in between. Bypass valve is used to control the flow rate of water. The pressure drop across the heat exchanger is measured using U tube manometer. The water flow rate was kept constant whereas air flow rate is varied using the valve. 2015, IERJ All Rights Reserved Page 2
Fig.2. Tube layout The tube layout is shown in fig.2. The lateral & longitudinal pitches are 52 mm. The major tube axis makes 45 angle with the horizontal. III. DATA REDUCTION The heat transfer rate in the heat exchanger is calculated from Eqn. (1). The overall heat transfer coefficient U (W/m 2 k) is calculated from Eqn. (2) & Eqn. (3) Q = ṁ a C pa ( T ao -T ai ) (1)...(2) The waterside Nu is calculated from Dittus-Boelter equation, Eqn. (4). The water side h can be obtained from Eqn. (5) (3) Nu w = 0.023 Re 0.8 Pr 0.3 (4) h w = (5) Initially h a is assumed and U is calculated from Eqn.(6) & is further compared with U calculated from Eqn. (3) The conductive resistance of tubes is neglected, as it is very small compared to convective resistance. The iterations are performed using solver in excel and air side heat transfer coefficient (h a ) is found out. (6) Nu a = For the comparison air side Nu for circular tubes is also found out using Zuakauskas correlation, Eqn. (8) (7) Nu = 0.27 Re 0.63 Pr 0.36 (8) 2015, IERJ All Rights Reserved Page 3
Re air IV. RESULTS AND DISCUSSION Table 1 Experimental readings Nu Nu (Circular) (elliptical) Ratio 7778 65.23 67.29 1.031 10706 79.66 91.66 1.150 12104 86.13 101.74 1.181 13296 91.40 113.51 1.241 15077 98.97 124.36 1.256 20000 118.057 156.11 1.322 Fig.3 Nusselt number vs Reynolds number The Nusselt number versus Reynolds number plot for circular and elliptical tubes is illustrated in Fig.2.. The highest Nusselt number is obtained at Re = 20000. Table 1 shows heat transfer enhancement ratio. 32% heat transfer enhancement is observed at Reynolds number = 20000, which implies that elliptical tubes with 45 angle of attack shows better thermal performance compared to circular tube bank. With increase in Reynolds number, pressure drop also increases. As the air flows over the inclined tubes with 45 angle of attack, the free flow area varies continuously, which results into flow mixing. The air flow coming out from first row of tubes as jet, strikes on the next row tubes and the jet stream is redirected to the inter-passages of next row. V. CONCLUSIONS Thermal performance of cross flow heat exchanger using elliptical inline arrangement is studied. The experiment was conducted by varying the air flow in Reynolds number range 5000 to 20000. Compared to circular tubes considerable heat transfer enhancement is found in case of elliptical tubes. With increasing Reynolds number, pumping power required to pump the air needs to be increased. REFERENCES [1] Arash Mirabdolah Lavasani, Hamidreza Bayat, Taher Maarefdoost, Experimental study of convective heat transfer from inline cam shaped tube bank in cross flow, Applied Thermal Engineering 65 (2014) 85-93. [2] Lei Sun, Liang Yang, Liang Shao, Overall thermal performance orieinted numerical comparison between elliptical and circular finned tube condensers, International Journal of Thermal Science 89 (2015) 234-244. [3] Mesbah G. Khan, Amir Fartaj, David S.-K. Ting, An experimental characterization of cross-flow cooling of air via an in-line elliptical tube array, International Journal of Heat and Fluid Flow 25 (2004) 636 648. 2015, IERJ All Rights Reserved Page 4
[4] Talaat A Ibrahim, Abdall Gomma, Thermal performance criteria of elliptic tube bundle in crossflow, International Journal of Thermal Science 48 (2009) 2148-2158. [5] Heat Transfer Enhancement of Heat Exchangers, S Kakac [6] Heat and Mass Transfer, Y Cengel. 2015, IERJ All Rights Reserved Page 5