International Journal of Heat and Mass Transfer 95 (2016) 996–1007

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International Journal of Heat and Mass Transfer

journal homepage: www.elsevier.com/locate/ijhmt

Enhancement of forced convection in wide cylindrical annular channel using rotating inner pipe with interrupted helical fins ⇑ Hosny Z. Abou-Ziyan a,b, , Abdel Hamid B. Helali a, Mohamed Y.E. Selim a,c a Mech. Power Eng. Dept., Faculty of Engineering, University of Helwan, Cairo, Egypt b Mech. Power Eng. Dept., Faculty of Technological Studies, PAAET, Kuwait c Mech. Eng. Dept., UAE University, 15551 Al-Ain, United Arab Emirates article info abstract

Article history: This paper presents the results of heat transfer and pressure drop in concentric annular wide channel Received 8 October 2015 with inner plain or finned pipe under stationary and rotating conditions in Taylor–Couette–Poiseuille Received in revised form 24 December 2015 flow. The experiments are conducted for one plain pipe and three finned pipes with helical fin spacing Accepted 30 December 2015 of 75, 110 and 150 mm at rotating speeds of 0, 200, 250, 300, 350 and 400 rpm. The experiments cover Available online 15 January 2016 axial Reynolds number of 8.07 104–1.82 105, rotational Reynolds number of 0 and from 1428 to 3008, corresponds to Taylor number of 0 and from 1.22 106 to 8.32 106. The reported results in the form of Keywords: Nusselt numbers and friction factors are correlated in terms of Re, Ta, Pr and fin geometrical parameters. Annular channel The results proved that at Re = 1.5 105, the wide annular channel with inner pipe of helical fin spacing Rotating inner pipe Finned inner pipe 75 mm that rotates at 400 rpm enhances Nu by a factor of 7.5 and also boosts the ratio of heat exchange Interrupted helical fins to pumping power by a factor of 7.6, compared to the case for plain stationary pipe. Heat transfer enhancement Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Passive heat transfer augmentation techniques do not require any external power input [5]. The passive technique, for single Many mechanical systems comprise a concentric cylindrical phase fluid in annular channel, includes use of either: a. swirl flow annulus where either the inner or outer cylinder is stationary or devices such as twisted tape, wire coils, springs, ribs, baffles, plates, rotating including annular heat exchangers, rotating pipe heat etc. [6–10]; b. extended surface such as helical, longitudinal, annu- exchangers, gas-cooled nuclear reactors, packed beds, gas turbines, lar and pin fins [10–13]; c. surface modifications such as treated, jet engines, electric motors, mechanical and chemical mixers and coated or rough surfaces [4,14]; d. fluid additives including phase drilling operations in the oil and gas industry. Thus, the study of change materials; and e. modification of physical properties of forced convection heat transfer in annular passages is of interest fluid such as nanofluids [15]. Most passive techniques cause swirl for many process and aeronautical industries. Owing to that, in the bulk of the fluids and disturbs the actual boundary layer so numerous experimental and theoretical investigations that were as to increase effective surface area and residence time and conse- initiated decades ago are reported in literature. These have been quently enhance the heat transfer coefficient in the system. reviewed across the years by Childs and Long in 1996 [1], Fenot Active heat transfer augmentation techniques are those involv- et al., in 2011 [2] and Togun et al., in 2014 [3]. Recently, environ- ing the supply of external power to drive devices such as mechan- mental and economical considerations have drawn the attention ical aids, surface vibrators, fluid vibrators, electrostatic suppliers, of researchers to seek maximum energy with less volume, mass suction and jet impingements [4]. The common active techniques, and cost by enhancing thermal performance of the systems. This for single phase fluid in annular channel, include rotating either encourages the development of many passive, active and com- inner or outer pipe [15–21] or vibrating the heat transfer surfaces pound heat transfer augmentation techniques as discussed by or fluids [22] or using jet/jet-array impingement [23]. Léal et al., [4]. These augmentation techniques are based on the fact It should be stated that both active and passive techniques that heat transfers between a fluid and a wall are influenced by the applied in annular channels depend mainly on creating curved, thermal and hydrodynamics boundary layers. rotating or vortex/decaying swirling flows [6]. Curved flows tend to be continuous swirling flows and are created by coil inserts,

⇑ Corresponding author at: Mech. Power Eng. Dept., Faculty of Technological twisted tapes, helical vanes, etc. Rotating flows also sustain contin- Studies, Shuwaikh, Kuwait. Tel.: +965 99381336; fax: +965 24811753. uous swirling flows and are created by either rotating the inner or E-mail address: [email protected] (H.Z. Abou-Ziyan). the outer section of the annular channel. Vortex flows or swirling http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.12.066 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved. H.Z. Abou-Ziyan et al. / International Journal of Heat and Mass Transfer 95 (2016) 996–1007 997

Nomenclature

2 Aax Cross sectional area of the annulus channel, m T Temperature, °C 2 Af Surface area of the helical fins, m Ta1, Ta2 Average inlet and outlet air temperature across the test 2 As Surface area of the inner rotating pipe, m section, °C b Annular gap width = (D d)/2 = 0.05 m for plain Tcm Mean temperature of the cold air in the annular chan- pipe = (D df)/2 = 0.033 m for finned pipe nel, °C cp Specific heat of air, J/kg K Tf Average fin temperature, °C d Outer diameter of the inner rotating pipe = 0.05 m Ts Average surface temperature of inner pipe, °C df Effective fin diameter of the rotating pipe = 0.084 m Va Axial air velocity in the annular channel, m/s D Inner diameter of the outer pipe = 0.150 m D Difference

Dh Hydraulic diameter of the annular channel, C Length ratio, L/b m=D d = 0.100 m for plain pipe = D df = 0.0657 m g Radial cylindrical gap aspect ratio, R1/R2 for finned pipe l Dynamic viscosity of air, Pa.s h Convection heat transfer coefficient, W/m2K m Kinematic viscosity of air, m2/s H Fin height, m q Density of air, kg/m3 k Thermal conductivity of air, W/mK x Angular velocity of rotating inner pipe, rad./s L Effective length of test section, m ma Mass flow rate of air, kg/s Dimensionless groups n Rotational speed of inner pipe, rpm 2 f Friction factor, Dp.Dh/(0.5 q Va L) P Pressure, Pa Nu Nusselt number, hDh/k Q Rate of heat transfer, W Pr Prandtl number of air, cp l/k R Ratio of heat transfer to pumping power Re Reynolds number, Va.Dh/m R Outer radius of inner pipe, m 1 Rer Rotational Reynolds number, x db/2m 2 2 3 2 R2 Inner radius of outer pipe, m Ta Taylor number, x d b /t (D + d) S Fin spacing, pitch, m t Fin thickness, m

decaying flows are usually created by means of tangential inlet, et al., [28] considers a small annulus gap to simulate the case for radial or axial canes, etc. The swirling decaying flows are created electric motor and concluded that the transport of momentum at the inlet of the annular duct and then let to decay out of the sys- and heat is raised by a factor of 1.2 in the case of cavities embedded tem. Studies on different vortex configurations by Prandtl, Von in the inner cylinder and by a factor of l.1 in the case of cavities Karman, and Taylor among others help in understanding the insta- embedded in the outer cylinder. Instead of that, Bouafia et al., bility of vortices which leads to a turbulence built-up. [26] investigated heat transfer in a narrow annular gap flow for The compound heat transfer augmentation techniques denote rotating inner cylinder and stationary outer cylinder with axial the simultaneous use of two or more techniques in the annular grooves. Their results in turbulent flow show that the situation of channel to enhance the heat transfer greater than that produced a smooth air gap is more favorable for heat transfer at the rotor. by either of them when used individually. The use of compound The heat transfer in annuli with rotation of the inner cylinder technique in annular channel is dated back to 1991 and still and axial flow (Taylor–Couette–Poiseuille flow) is dependent on promising of more development. Various compound techniques several parameters, including the radius ratio, flow regime, ratio such as fins and twisted-tape inserts [10], nanofluids and rotation of tangential to axial flow and hydraulic diameter [1]. The review [15] and Jet-array impingement with rotating inner cylinder [23] of the work done on annular channel indicated that limited work are used to augment heat transfer in annular channel. However, has been conducted using compound heat transfer enhancement the additional of fins, swirl devices or cavities along rotating pipes techniques. Most of the work treated annuli with small gaps that [17,24–28] would greatly increase heat transfer in the annulus and mimic the conditions of rotor/stator in electric machines, [2] or this will be on focus for the present work as discussed below. in gas turbines. With few exceptions, the other work cover limited Jeng et al., [17] investigated the heat transfer enhancement of Reynolds number in the range of 0 6 Re 6 5 104 [2]. In particular, Taylor–Couette–Poiseuille flow in an annulus of radii ratio of knowledge of the compound effects of helical fins and rotating 0.90 by mounting four longitudinal ribs on the rotating inner cylin- inner pipe on convection heat transfer in annular channel with der. The authors investigated the effects of the axial Reynolds num- wide gap still lacking. Annular channel of wide gap has wide range bers from 30 to 1200, rotational Reynolds number from 0 to 2922 of applications including mechanical and chemical mixers and agi- and the test mode on the convective heat transfer. Fenot et al., [24] tators and double pipe heat exchangers. In addition, Chen et al., in used slotted rotating inner cylinder to enhance the convective heat 2015 [6] reviewed the topic of heat transfer in annular ducts and transfer in the entry region of an annular channel. The authors con- stated that despite the extensive investigation throughout the ducted experiments for axial Reynolds numbers from 2140 to 6425 years; much still remain unknown for this unique geometrical and rotational Reynolds numbers from 1750 to 35,000 (corre- structure. Also, Togun et al., [3] in their recent review paper stated sponding to Taylor numbers from 104 to 4 106) to imitate the that diverse results were reported in literature, including on the conditions of the air gap of an open four-pole synchronous motor. effect of radii ratio between inner and outer pipes, type of fluid, Hamakawa et al., [25] and Hayase et al., [28] created cavities and fluid velocity in an annular passage on the heat transfer pro- between two concentric rotating cylinders to augment the heat cesses. In addition, Fenot et al., [2] after reviewing the subject con- flow in the annular channel. Hamakawa et al., [25] tested two cluded that notwithstanding the sizable quantity of studies annular gaps of 2.5 and 5.5 mm in Reynolds number from 290 to focused on the subject of flows in rotating annular channel, the 1700 and Taylor number from 6300 to 143,000. Also, Hayase similarly large number of influence and impact factors still leaves Download English Version: https://daneshyari.com/en/article/656696

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