Cavallini, A.

Doretti, L.

Longo, G. A.

Rossetto, L.

English

CAVALLINI, ALBERTO

DORETTI, LUCA

LONGO, GIOVANNI ANTONIO

ROSSETTO, LUISA

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Flow Patterns during Condensation of Pure Refrigerants on Enhanced Tubes under High Vapour Velocity

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Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering1994Flow Patterns During Condensation of PureRefrigerants on Enhanced Tubes Under HighVapour VelocityA. CavalliniIstituto di Fisica Tecnica dell'Universita di PadovaL. DorettiIstituto di Fisica Tecnica dell'Universita di PadovaG. A. LongoIstituto di Fisica Tecnica dell'Universita di PadovaL. RossettoIstituto di Fisica Tecnica dell'Universita di PadovaFollow this and additional works at: http://docs.lib.purdue.edu/iraccThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlCavallini, A.; Doretti, L.; Longo, G. A.; and Rossetto, L., "Flow Patterns During Condensation of Pure Refrigerants on EnhancedTubes Under High Vapour Velocity" (1994). International Refrigeration and Air Conditioning Conference. Paper 265.http://docs.lib.purdue.edu/iracc/265FLOW PATTERNS DURING CONDENSATION OF PURE REFRIGERANTS ON ENHANCED TUBES UNDER IDGH VAPOUR VELOCITY A.Cavallini, L.Doretti, G.A.Longo and L.Rossetto lstituto di Fisica Tecnica dell'Universita di Padova,Via F.Marzolo 9,1-35131 Padova PD Italy ABSTRACT This work experimentally investigates the effect of vapour velocity during condensation of a refrigerant on the outside of a single horizontal enhanced tube with particular reference to the fluid-dynamic mechanisms induced by vapour shear stress, surface tension and gravity on the condensate flow. More than 70 experimental runs were done in a transparent test section in order to directly observe the condensation of vapour, to take photographs and video-camera film, with two different extended tubes (an integral-fin tube with 1333 fins per meter and a commercial pin-fin type Hitachi Thermoexcel tube) in a wide operative range. These direct observations were implemented with the heat transfer coefficients previously measured by the authors with the same test tubes under the same operative conditions. NOMENCLATURE d =Tube diameter g = Gravitational acceleration hw =Latent heat of vaporization k = Nondimensional group= [(r/2) (glpJ114] 1 a314 Q = Heat flow rate Rer = Film Reynolds number= 2r I IlL Rey =Vapour Reynolds number= (Pv Umaxdo) I Jlv s = Fin spacing u = Vapour velocity 4lr =Flooding angle r = Condensate flow rate per unit length of the tube a = Surface tension p =Density /.. =Taylor wavelength 11 =Dynamic viscosity e = Fin half tip angle Subscripts: L = condensate max =maximum value 0 = outside of finned tube at the fin tip V =vapour INTRODUCTION In film-wise condensation of a pure vapour, the heat transfer coefficients depend on the local thickness and conformation of the liquid film and on the type of condensate flow (laminar or turbulent). This work presents the results of an experimental investigation aimed at observing the condensate flow patterns obtained varying the vapour and condensate flow rates, during condensation of refrigerant 113 on the outside of a single enhanced tube. 311 COOLING WATER INLET COOLING WATER OUTLET INSIDE TUBE 'O'RING PLEXIGLASS WALL TUBE SHEET TEST TUBE Fig.1 Experimental test section with transparent walls. EXPERIMENTAL SETUP AND PROCEDURE The experimental apparatus, fully described in previous papers [1, 2, 3], consists of a refrigerant loop in which dry saturated vapour, supplied by an electric boiler, goes through the test section partially condensing on a single horizontal extended tube cooled by water kept at constant inlet temperature. The test section, sketched in Figure 1, has two transparent walls which allow to directly observe the condensation of the vapour and to take photographs and video-camera films. The refrigerant vapour condenses on the outside enhanced surface of the test tube, while the cooling water flows inside the tube in a bayonet type configuration. Two different extended tubes have been tested: the first one is an integral-fin tube with 1333 constant section fins per meter (fpm) and the second one is the commercial pin-fin type Hitachi Thermoexcel tube. Table 1 gives their main geometrical characteristics. During each experimental run the condensate flow was filmed and photographed and, at the same time, the main experimental parameters such as refrigerant pressure and flow rate, cooling water flow rate, cooling water inlet temperature and temperature gain across the tube were measured, according to the procedures already described in [ 1, 2, 3]. These direct observations were implemented with measures of the heat transfer coefficients previously taken by the authors [ 1, 2, 3] with the same test tubes, instrumented with wall thermocouples, under the same operative conditions in a different test section with the same flow area as the present one, but without the possibility of visualization of the condensate flow pattern. TABLE 1. Geometrical characteristics of the test tubes. Test tube Thermoexcel 1333 fpm Fin type Three-dimensional lnt~gral-fin Tube material Copper Copper Tube length [mm] 150.00 150.00 Outside diameter at the fin tip dn [mm] 18.70 16.40 Outside diameter at the fin root [mm] 14.70 15.00 Fin pitch [mm) 1.25 0.75 Fin height [mm] 2.00 0.70 Effective/Nominal sutface area 2.82 2.40 312 TABLE 2. Operative conditions durin_g experimental runs. Test tube Thermoexcel 1333 fpm Number of experimental runs 30 42 Fluid R113 R113 Vapour inlet pressure [kPa] 115-118 75-119 Maximum vapour velocity [m/s] 2.0-38.7 1.3-28.3 Ref 120-224 37-254 Rev·I0-5 0.29-5.50 0.16-3.56 The two-phase flow considered was characterized by two nondimensional groups: the vapour Reynolds number (1) which refers to the maximum vapour velocity in the test section and to the vapour phase properties, and the film Reynolds number: Rer= 2r I JlL (2) where r= Ql(hLGL) is the condensate flow rate per unit length of the tube and Q is the condensation heat flow rate exchanged in the tube derived from a thermal balance on the cooling water. -Table 2 reports the operative conditions of the experimental runs. RESULTS AND DISCUSSION As shown in [3], vapour velocity effects on condensation heat transfer appear to be different for each geometry tested, both in behaviour and magnitude. Vapour shear effects for the 1333 fpm integral-fin tube appear only at vapour Reynolds number Rev higher than 100,000 with a 50% maximum heat transfer increase at the maximum Rev tested (350,000), while they affect the commercial Hitachi tube in the whole operative range giving a maximum enhancement of 40% at the maximum Rev (550,000). The visual observations in the transparent test section show that the above heat transfer enhancement is due to changes in condensate flow patterns. Figure 2 shows still photographs of the condensate flow patterns for condensation of refrigerant 113 on the 1333 fpm tube at near stationary vapour conditions for different condensate flow rates. The lower portion of the tube is completely flooded by the condensate, and the measured value of the flooding angle 4>r ( i.e. the angle from the top of the tube below which the grooves between the fins are completely filled by the liquid) is 10-15% less than that calculated by the model of Honda et al. [4], that is IJ>r = cos-1 [ ( 4 a cos9) I (Pi$ s d0 ) - 1] (3) At very low condensation rates the liquid condensate leaves the bottom of the tube forming droplets, while when the condensate flow increases, liquid columns, almost equally spaced along the tube length, appear. The distance between two adjacent columns, around 8.5 mm, is very well predicted by the most unstable Taylor wavelength A. for a thin film [4), given by: A= 27t ...J 2a 1 (p~) (4) The transition between droplet and column flow patterns is controlled by the nondimensional group (5) and appears at around 0.1 in accordance with Honda et al. [5]. 313 (Ref= 40 and Rev = 26,000) (Ref= 180 and Rev= 23,000) Fig. 2. Condensate flow patterns for condensation of Rl 13 on a 1333 fpm integral-fin tube for different condensate rates. Figure 3 shows still photographs of the condensate flow patterns for condensation of refrigerant 113 on the 1333 fpm tube at increasing vapour velocities. Starting from stationary conditions, when vapour velocity increases, the value of the flooding angle <Pr seems to slightly decrease due to vapour boundary layer separation, but the border between the flooded and unflooded portion of the tube is not a straight line, as with stagnant vapour, and becomes more and more wave-like. At vapour Reynolds number Rev slightly greater than 100,000 (the value at which vapour velocity effects on the heat transfer rate appear) the liquid columns become less stable, their sites move along the tube and the distance between them is no longer constant: these phenomena produce local reduction of the liquid film thickness in the flooded region. When Rev reaches approximately 200,000 the vapour shear induces waves at the gas-liquid interface in the flooded region, whereas the interaction between vapour and falling liquid columns produces a dispersed flow below the tube. At a higher vapour velocity the entrainment of condensate droplets in the vapour flow appears just below the flooding angle, reducing the thickness and generating turbulence in the liquid film in the bottom part of the tube. At the maximum vapour Reynolds number tested, Rev= 350,000, a fine dispersion of droplets in the vapour flow is entrained mainly at the limit of the flooded region, where the separation of the vapour boundary layer occurs. Fig. 3. Condensate flow patterns for condensation ofR113 on a 1333 fpm integral-fin tube for different vapour velocities. 314 (Ref= 120 and Rev= 29,000) (Ref= 180 and Rev = 32,000) Fig. 4. Condensate flow patterns for condensation ofR113 on the Thermoexcel tube for different condensate rates. Figure 4 and 5 show still photographs of the condensate flow patterns for condensation of refrigerant 113 on the Thermoexcel tube at different condensate rates and different vapour velocities, respectively. At near stationary vapour conditions, the Thermoexcel tube gives a heat transfer rate which is 30% higher than that of the 1333 fpm integral-fin tube, so the condensate rate explored during experimental runs covers only the region of column flow pattern (0.1 < k < 0.2) and the liquid columns do not result equally spaced as happens with the integral-fin tube tested. This three-dimensional pin-finned surface is not affected by condensate retention in the bottom part of the tube and this contributes to explain its higher performance at stationary conditions with respect to integral-fin tubes. The column flow results stable also for high vapour velocities (up to Rev = 160,000), while only at vapour Reynolds numbers higher than 350,000 the transition to dispersed flow is completed. These observations justify the lower effect of vapour velocity on the Thermoexcel hes.t transfer performance and the absence of singularities in the regular enhancement trend. (Rer= 190 and Rev_= 328,000) (Rer= 210 and Rev= 543,000) Fig. 5. Condensate flow patterns for condensation of R 113 on the Thermoexcel tube for different vapour velocities. 315 CONCLUSIONS The effects of vapour shear stress and operative conditions during condensation on two different enhanced tubes were experimentally investigated. The outside heat transfer enhancement with vapour velocity was correlated to characteristic changes in the condensate flow patterns. During almost stagnant vapour condensation on the 1333 fpm tube, a transition between droplet and column flows occurred at increasing condensate flow rates. On the other hand, increasing the vapour velocity, a transition to dispersed flow was observed, starting from Rey:::: 100,000 up to Rey = 200,000. The flooding level was only slightly affected by vapour shear stress. The effect of vapour velocity on the condensate flow for the Thermoexcel tube is less significant, producing a gradual transition from column mode to dispersed flow in the range of vapour Reynolds number from 160,000 to 350,000. The above observations are in good agreement with Honda et al. [6, 7] investigations on condensate flow patterns in a bundle of finned tubes (integral-fin and three-dimensional). This work investigates a wider range of vapour velocities. ACKNOWLEDGMENTS Research carried out with the partial technical support of Ausimont s.p.a.- Milano (Italy) and Hitachi Cable Ltd. (Japan) REFERENCES [1] B.BELLA, A.CAVALLINI, G.A.LONGO and L.ROSSETIO. Pure refrigerant condensation on a single integral finned tube: vapour velocity effects. Proc. 1992 International Refrigeration Conference -Energy Efficiency and New Refrigerants, Purdue University, 1992, Vol.I, p.l77-186. [2] B.BELLA, A.CA VALLINI, G.A.LONGO and ROSSETTO. Pure vapour condensation of refrigerants 1 I and 113 on a horizontal integral finned tube at high vapour velocity. Journal of Enhanced Heat Transfer, 1993, Vol.l, No.1, p.77-86. [3] A.CA VALLINI, L.DORETTI, G.A.LONGO and L.ROSSETIO. Experimental heat transfer coefficients during external condensation of halogenated refrigerants on enhanced tubes. Presented at the 1Oth International Heat Transfer Conference, Brighton, 1994. [4] H.HONDA, S.NOZU, and K.MITSUMORI. Augmentation of condensation on horizontal finned tubes by attaching a porous drainage plate. Proc. of 1st ASME-JSME Thermal Engineering Joint Conference, 1983, Vol.3, p.289-295. [5] H.HONDA, S.NOZU and Y.TAKEDA. Flow characteristics of condensate on a vertical column of horizontal low finned tubes. Proc. of 2nd ASME-JSME Thermal Engineering Joint Conference, 1987, Vol.l, p.517-524. [6] H.HONDA, B.UCHIMA, S.NOZU, H.NAKATA and E.TORIGOE. Film condensation of downward flowing R-113 vapour on in-line bundles of horizontal finned tubes. ASME HTD, 1989, Vol.108, p.ll7-125. [7] H.HONDA, B.UCHIMA, S.NOZU, E.TORIGOE and S.IMAI. Film condensation of downward flowing R-113 vapour on staggered bundles of horizontal finned tubes. ASME/JSME Thermal Engineering Proc., 1991, Vol.2, p.487-494. 316 

Flow Patterns During Condensation of Pure Refrigerants on Enhanced Tubes Under High Vapour Velocity

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Flow Patterns During Condensation of Pure Refrigerants on Enhanced Tubes Under High Vapour Velocity

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