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Convective boiling performance of refrigerant R-134a in herringbone and microfin copper tubes
International Journal of Refrigeration 29 (2006) 81–91 www.elsevier.com/locate/ijrefrig
Convective boiling performance of refrigerant R-134a in herringbone and microfin copper tubes Enio P. Bandarra Filhoa,1, Jose´ M. Saiz Jabardob,* Faculdade de Engenharia Mecaˆnica, Universidade Federal de Uberlaˆndia, Av. Joa˜o Naves de A´vila, 2160 Bloco 1M, Santa Moˆnica, Uberlaˆndia, MG, Brasil b Escuela Polite´cnica Superior, Universidad de la Corun˜a, Calle Mendiza´bal s/n, 15403 Ferrol, Espan˜a, Spain
a
Received 5 April 2004; received in revised form 19 January 2005; accepted 25 May 2005 Available online 19 August 2005
Abstract This paper reports an experimental investigation of convective boiling heat transfer and pressure drop of refrigerant R-134a in smooth, standard microfin and herringbone copper tubes of 9.52 mm external diameter. Tests have been conducted under the following conditions: inlet saturation temperature of 5 8C, qualities from 5 to 90%, mass velocity from 100 to 500 kg sK1 mK2, and a heat flux of 5 kW mK2. Experimental results indicate that the herringbone tube has a distinct heat transfer performance over the mass velocity range considered in the present study. Thermal performance of the herringbone tube has been found better than that of the standard microfin in the high range of mass velocities, and worst for the smallest mass velocity (GZ 100 kg sK1 mK2) at qualities higher than 50%. The herringbone tube pressure drop is higher than that of the standard microfin tube over the whole range of mass velocities and qualities. The enhancement parameter is higher than one for both tubes for mass velocities lower than 200 kg sK1 mK2. Values lower than one have been obtained for both tubes in the mass velocity upper range as a result of a significant pressure drop increment not followed by a correspondent increment in the heat transfer coefficient. q 2005 Elsevier Ltd and IIR. All rights reserved. Keywords: Experiment; Evaporation; R-134a; Microfin tube; Wavy tube; Heat transfer; Pressure drop
R-134a: performance en ebullition convective a` l’inte´rieur de tubes en cuivre a` chevrons et a` microailettes Mots cle´s : Expe´rimentation ; E´vaporation ; R-134a ; Tube microailete´ ; Tube ondule´ ; Transfert de chaleur ; Chute de pression
1. Introduction
* Corresponding author. Tel.: C34 981 337 400x3275. E-mail addresses: [email protected] (E.P. Bandarra Filho), [email protected] (J.M. Saiz Jabardo). 1 Tel.: C55 34 3239 4192x235.
0140-7007/$35.00 q 2005 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2005.05.011
During the last 20 years efforts have been made to upgrade the performance of cooling and condensing coils in refrigeration applications. As should be expected, most of the effort has been focused in the air side due to its higher thermal resistance. Manufacturers have tried to upgrade the
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Nomenclature AR D e E G h i L m n Q t T x
Area ratio Diameter (mm) Microfin height (mm) Enhancement parameter Mass velocity (kg sK1 mK2) Heat transfer coefficient (W mK2 KK1) Enthalpy (J kgK1) Length (m) Mass flow rate (kg sK1) Number of microfins Electrical power (W) Wall thickness (mm) Temperature (K) Quality
Greek symbols b Helix angle (8)
internal heat transfer coefficient, especially in dry expansion evaporator coils, by inserting devices to promote better contact of the liquid with the tube wall. These devices are in fact efficient in upgrading the heat transfer coefficient but at the cost of significant pressure drop. More recently, thin wall copper tubes have been developed having the inner surface covered by fins of reduced size, known as ‘microfin tubes’. Several of these fins have been developed, and are presently used in the refrigeration field, mostly in air cooled condensers, but also in evaporators. These fins promote significant increments in heat transfer though not affecting in the same proportion the pressure drop. These characteristics have promoted their widespread application in the refrigeration industry. In the last 15 years microfin tubes have been submitted to an intense scrutiny regarding their thermo-hydraulic performance especially under condensing conditions. Despite the progress made in understanding convective boiling and condensation of refrigerants in these tubes, some aspects of the heat and momentum exchange mechanisms are not clearly understood and trends remain to be adequately justified. Microfin tubes were introduced into the market in the
DP f q
Pressure drop (kPa) Heat flux (W mK2) Apex angle (8)
Subscripts e external f fluid fg liquid/vapor i internal or root diameter in inlet o outlet PH pre heater r refrigerant sat saturation TS test section w wall
late seventies by Hitachi Cable Ltd. A schematic drawing and a microphotograph of a microfin tube by one international manufacturer is shown in Fig. 1. As a general rule, the fins run helically along the tube. The helix angle, b, varies in the range between 16 and 308, depending on the manufacturer, and the number of fins typically varies from 60 to 70. The height of the fins varies from 0.15 to 0.25 mm, and the actual thickness of the copper tubes is rather small, varying in the range between 0.3 and 0.5 mm. The reduced thickness makes the manufacturing process rather simple and economically sound to the coil manufacturer. Though the investigation of convective boiling in tubes with internal heat transfer enhancement devices is not new, dating back to the 1950s and 1960s, Lavin [1] and Lavin and Young [2], studies involving microfin tubes are more recent. According to Webb [3], one of the first known papers dealing with microfins is the one by Fujie [4], published in the late seventies, and reporting a research performed for Hitachi Cable Ltd. As a general rule, all the published papers report that the microfin tubes under convective boiling conditions present a better heat transfer performance though the pressure drop is significantly increased with respect to their smooth counterpart. Reported increments in
Fig. 1. A cut-way view and a microphotography of the standard microfin tube of the present investigation.
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the heat transfer coefficient with respect to the smooth tubes range between 1 and 3 times whereas the pressure drop increases between 20 and 80%, depending on the mass flow rate. Comprehensive studies of the performance of microfin tubes have been published in the past, especially under condensing conditions. One of such reviews is the one recently published by Cavallini et al. [5], who also included results from a new geometry introduced in the late nineties and rightfully designated by the name ‘herringbone’. A microphotograph of such microfin geometry is shown in Fig. 2. Noteworthy in this geometry is that the microfins run in opposite directions in the lower and upper halves of the tube. Miyara et al. [6] performed heat transfer and pressure drop condensation experiments involving three 7 mm external diameter tubes. One of them was smooth whereas one of the other two was a 50 fins standard microfin tube and the other a herringbone tube with 60 fins. Refrigerants R-410A and R-22 have been tested at a saturation temperature of 40 8C and mass velocities of 100 and 300 kg sK1 mK2. Since a refrigerating compressor was used, lubrication oil was mixed with the refrigerant, its mass concentrations being limited to a maximum of 0.1%. Heat transfer results from this study presented two different trends depending on the mass velocity. For the higher mass velocity (GZ300 kg sK1 mK2), the herringbone geometry performance was better than the standard microfin over the whole range of qualities. For the lower mass velocity, herringbone and standard microfin results were very close, though, for lower qualities (x!0.5), the standard microfin tube presented a performance slightly better than the herringbone, the opposite being observed for higher qualities. Miyara et al. [7] carried out an experimental study of the thermal performance under convective boiling conditions of refrigerant R-410A at 10 8C of saturation temperature in five 7.00 mm external diameter herringbone tubes and a standard microfin tube of the same diameter. The effects of herringbone tube geometric parameters such as the helix and apex angles, fin height and number of fins over the thermal performance were investigated. Results from the investigation can be summarized as follows: (1) for a mass
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velocity of 100 kg sK1 mK2, the heat transfer coefficient of herringbone tubes with intermediate apex angles (14 and 168) increased up to a quality of the order of 40%, decreasing for higher qualities, pointing to a possible surface dry out; (2) for a mass velocity of the order of 300 kg sK1 mK2, the highest heat transfer coefficient was obtained in the herringbone tube with the highest helix angle (288); (3) as should be expected, the pressure drop increased with the helix angle and the fin height. Recently, Miyara and Otsubo [8] confirmed previous results by performing experiments with three herringbone tubes and two additional tubes, one a standard microfin and the other smooth. It has also been determined that pressure drop was higher for two configurations of herringbone tubes with higher helix angle. The pressure drop in the one with small helix angle (68) was lower than that in the standard microfin tube. In a study involving the herringbone and standard regular microfin geometries, Ebisu and Torikoshi [9] performed convective boiling and condensation experiments with refrigerant R-407C in 7.0 mm external diameter tubes, at the saturation temperatures of 5 and 50 8C, respectively. Experiments were performed at mass velocities ranging from 150 to 400 kg sK1 mK2 and a constant heat flux of 7.5 kW mK2. Contrary to Miyara and co-workers condensation results, those from the Ebisu and Torikoshi [9] study revealed a better thermal performance of the herringbone geometry over the whole range of mass velocities. The variation of the heat transfer coefficient with mass velocity for a quality of 60% revealed that the herringbone tube heat transfer performance was better than its standard microfin counterpart over the whole range of mass velocities of the study. The increment factor in heat transfer observed in the herringbone tube with respect to the standard microfin one varied in the range from 3 to 4 for condensing and in the range from 1 to 2 for convective boiling conditions. Present paper reports results from an experimental investigation of convective boiling of refrigerant R-134a in smooth, herringbone and standard microfin copper tubes of the same diameter. Comparison of the obtained results for these microfin geometries are performed in terms of the so called ‘enhancement parameter’ which takes into account the combined effects of heat transfer and pressure drop.
Fig. 2. Microphotography and a cut-way view of the herringbone tube of the present investigation.
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2. Experimental set-up and procedures A schematic circuit diagram of the experimental bench used in the present investigation is shown in Fig. 3(a). The refrigerant is pumped from the condenser through a filter dryer and a sight glass (SG) to the mass flow meter and pre heater before reaching the entrance of the test section (TS). Upon leaving the pre heater, the refrigerant flows through a straight section of 1.2 m length made out of the same tube as the test section in order to allow for flow development. The results reported herein have been obtained in copper tubes of 1.5 m length and 9.52 mm nominal external diameter. The tubes investigated in the present study had the geometric characteristics shown in Table 1. The vapor/liquid mixture of refrigerant leaving the TS is directed to a shell and tube condenser where it is condensed by exchanging heat with a 60% ethylene glycol aqueous solution cooled by a chiller specially built for operation in the experimental set up. The pre heater and the test section are electrically heated with tape resistors, uniformly wrapped on the external surface of the tube to guarantee a uniform heat flux. The heaters are covered by successive layers of fiber glass and foam thermal insulation. Voltage converters manually operated adjust the electrical output in the pre-heater and
test section. The quality of the refrigerant at the TS entrance is determined by performing an energy balance in the pre heater and set by adequately adjusting the electrical output. The heat exchanged with the ambient air through the thermal insulation in the pre heater and test section was estimated in preliminary tests carried out to check the correct operation of the instruments. These tests used to be performed with the refrigerant flowing as subcooled liquid to prevent any possibility of phase change either in the preheater or in the test section. It has been determined that ‘heat leakage’ in both components is less than 5% of the total heat input. The refrigerant bulk temperature was measured at the TS entrance and exit through 30 AWG type T sheathed thermocouples. The refrigerant pressure at the entrance and exit of the TS was measured by pressure transducers whereas the TS pressure drop was directly measured by a differential pressure transducer. Since thermodynamic equilibrium prevailed in all the tests, the refrigerant temperature (saturation temperature) could also be determined from the measured pressure at the entrance and exit of the test section. The surface temperature was measured at four cross sections along the test section tube, as shown in Fig. 3(b). In
Fig. 3. (a) Schematic diagram of the experimental set-up; (b) surface thermocouples location along the test section.
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Table 1 Geometric characteristics of the tubes used in the present investigation Tube
De
Di
t
e
n
b (8)
q (8)
ARa
Smooth Standard microfin Herringbone
9.52 9.52 9.52
8.76 8.92 8.92
0.38 0.30 0.30
0.2 0.2
82 70
18 18
33 33
1.00 1.91 1.78
a
Total finned area over root smooth area.
each cross section 30 AWG type T thermocouples were adequately attached to the external surface of the tube at the bottom, mid plane, and top of the tube. The surface temperature at each cross section was evaluated as the average of the three readings from the surface thermocouples. Heat conduction effects in the tube wall, though small, have been taken into account in the inner surface temperature evaluation. The mass flow rate was measured through a Coriolis type meter and the electrical power (pre heater and TS) with power transducers. The precision of the power transducers given by the manufacturer was checked by single phase experiments conducted as a preliminary procedure previous to the effective experiments with each tube. Each thermocouple and the associated data acquisition system channel were calibrated in a thermostatic bath using both NIST traceable precision glass thermometers and the digital temperature reading of the bath with a precision of G 0.02 8C. Pressure transducers were calibrated using either a reference transducer previously calibrated with a dead weight tester or a column manometer. The accuracy of the calculated parameters was determined according to the procedure suggested by Albernethy and Thompson [10] with an interval of confidence of 95%. The accuracy of measured and calculated parameters is shown in Table 2. It can be noted that heat transfer coefficient accuracy varies from G8.96 to G34.0%. The upper limit corresponds to the highest heat transfer coefficient which occurs under conditions where the difference between the average surface and the refrigerant temperatures is the smallest. The average heat transfer coefficient in each of the four measuring sections in the test section can be determined Table 2 Accuracy of measured and calculated parameters with confidence interval of 95% Parameter
Accuracy
Temperature Pressure Pressure drop Mass flow rate Electrical power Quality Heat transfer coefficient
G0.15 K G0.3% G0.25% G0.15% G5.0% G5.1% G8.96 to G34.0%a
a
Higher accuracy corresponds to higher surface/refrigerant temperature difference.
from Newton’s Cooling Law: hZ
f Tw K Tf
(1)
where Tw is the average wall temperature corresponding to the readings of the thermocouples attached in the particular cross section and Tf is the saturation temperature in the cross section. This temperature is the saturation temperature corresponding to the local pressure of the refrigerant obtained by linearly interpolating the readings of the test section inlet and outlet pressure transducers. The test section average heat transfer coefficient is the one corresponding to the average of the four measuring stations, which in turn is related to the average quality in the test section. The quality at the test section entrance and exit is evaluated from energy balances at both the pre-heater and test section according to the following equations: QPH Z mr ½ðio Þ K ðiin ÞPH
(2)
where io and iin are the enthalpies of the refrigerant at the pre-heater exit and entrance, determined from the pressure and temperature measured at these locations. The liquid entering is sub cooled though an equilibrium saturated liquid/vapor mixture must leave the pre-heater. Since the total electric power output is measured, and the inlet enthalpy is determined from the pressure and temperature readings, the exit enthalpy can thus be determined from Eq. (2), and the quality from the expression for the enthalpy of a liquid/vapor mixture in thermodynamic equilibrium, i Z if C xifg
(3)
The quality at the test section inlet is equal to that at the pre-heater exit since the flow development tube length can be assumed adiabatic. The exit quality can be determined from an energy balance in the test section: QTS Z pDi Lf Z mr ½ðio Þ K ðiin ÞTS
(4)
where (i)o is given by an equation similar to Eq. (3) from which one can obtain the value of the exit quality. QTS is the electric power output at the test section. The quality considered in the present analysis is the average quality in the test section, corresponding to the average heat transfer coefficient.
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3. Analysis of results The range of mass velocities covered in this study varied from 100 to 500 kg sK1 mK2, whereas the qualities varied form 10 to 95%. It has been observed that, for smooth tubes, the annular flow pattern is dominant in the high quality and mass velocities ranges. In the low quality range, the intermittent flow pattern has been observed whereas for low mass velocities (lower than 150 kg sK1 mK2) stratified flow occurs over the whole range of qualities. Uniform and relatively low surface temperature over the tube circumference is the main heat transfer characteristic of the annular flow pattern. The heat transfer coefficient is generally high and increases as one proceeds downstream as the thickness of liquid film diminishes by evaporation at the liquid-vapor interface. Since the upper region remains dry in stratified flow, the superficial temperature increases significantly in the upward direction with the circumferential average heat transfer coefficient being generally small as compared to the one in the annular flow. The intermittent flow is generally a transition flow pattern to the annular one for the higher mass velocities. The quality range it extends over is small, though for transitional mass velocities, from 150 to 200 kg sK1 mK2, it can encompass a higher quality range, Saiz Jabardo et al. [11]. Flow patterns are affected by secondary flows induced by the grooves formed by successive fins in microfin tubes. Two different patterns have been observed in the range of operating conditions considered in the present investigation, both for the standard microfin and the herringbone geometries:
Fig. 4. Heat transfer coefficient variation with the quality. TsatZ 5 8C; fZ5 kW mK2; GZ100 kg sK1 mK2.
(1) internal tube surface covered by liquid (2) mist formation
the smooth tube. It has been argued in the past, Bandarra Filho et al. [12] that this trend is related to the spreading of a liquid film over the surface of the tube promoted by the grooves formed by the microfins. This film flows in what appears to be a secondary flow attached to the tube wall. The thermal resistance associated to the film diminishes with its thickness, a trend which is clearly displayed in Fig. 4 for the standard microfin geometry. This heat transfer pattern is similar to that of the annular flow in smooth tubes for higher mass velocities. According to Fig. 4, the herringbone tube heat transfer coefficient presents a rather peculiar pattern. In the lower quality range, up to 50%, the variation of the heat
Herringbone tube results can be divided into three different ranges regarding the effect of the mass flow rate: the low range, 100 kg sK1 mK2, the intermediate range, 200 and 300 kg sK1 mK2, and the high range, 500 kg sK1 mK2. These ranges are clearly discernible in a first glance in Figs. 4–7, where the average heat transfer coefficient for the three tubes of the present investigation is plotted against the average quality, each figure corresponding to one of the cited mass velocities. Only two ranges are apparent for the standard microfin tube, with the mass velocity of 300 kg sK1 mK2 being the boundary between the two. The variation of the heat transfer coefficient for the three tube surface geometries with the quality for a mass velocity of 100 kg sK1 mK2 is shown in Fig. 4. It can clearly be observed that the smooth tube heat transfer coefficient is the lowest over the whole range of qualities. In addition, it remains essentially constant over the whole range of qualities, a typical heat transfer pattern of stratified flow. The standard microfin surface heat transfer coefficient increases with quality in most of the quality range. In addition, its value is more than four times higher than that of
Fig. 5. Heat transfer coefficient variation with the quality. TsatZ 5 8C; fZ5 kW mK2; GZ200 kg sK1 mK2.
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tubes, the heat transfer coefficient undergoes a significant trend shift with respect to that of Fig. 4. In the lower quality range, up to 40%, the heat transfer coefficient becomes larger than that of the standard microfin tube, the difference increasing up to a factor of the order of two. The rate of increment of the heat transfer coefficient diminishes for higher qualities (higher than 40%), and, as a result, differences between herringbone and standard microfin results tend to come closer to each other. Two conclusions can be drawn from Fig. 5:
Fig. 6. Heat transfer coefficient variation with the quality. TsatZ 5 8C; fZ5 kW mK2; GZ300 kg sK1 mK2.
transfer coefficient with quality follows closely that of the standard microfin geometry. However, for higher qualities, the heat transfer coefficient experiences a significant reduction with quality, approaching the value of the smooth tube at higher qualities. This reduction is certainly related to the depletion of liquid refrigerant in the upper surface of the tube caused by the microfins arrangement. Mass velocity increment to 200 kg sK1 mK2 promotes significant flow pattern changes in smooth and herringbone microfin tubes as can be noted in Fig. 5. Intermittent pattern occurs up to qualities of the order of 20% in the smooth tube. Then annular flow sets in, as the upward trend of the heat transfer coefficient suggests. In the case of the herringbone
(1) Standard microfin and smooth tubes results present similar trends, but the heat transfer coefficient of the former is still higher. It could be argued that the higher heat transfer coefficient in the microfinned surfaced could be related to the increment in the heat transfer area with respect to the smooth surface. However, the area ratio between the microfinned and the smooth surfaces for the tubes of the present investigation is of the order of two whereas the heat transfer coefficient ratio is something higher. This additional enhancement might be related to the aforementioned secondary flow promoted by the grooves in the microfinned surface. (2) The heat transfer coefficient trend shift when the mass velocity varies from 100 to 200 kg sK1 mK2 in the herringbone tube certainly is related to the wetting of the upper region of the tube surface as a result of some sort of dragging mechanism. The increment in the vapor core velocity might be sufficiently high to drag liquid to the mid section of the tube, at which point the disposition of the upper grooves might allow liquid to be dragged efficiently to the top of the tube. At this point it is difficult to speculate about the mechanism responsible for the observed differences between the results of the herringbone and the standard microfin surfaces. The increment of the mass velocity from 200 to 300 kg sK1 mK2 does not cause major changes, as the plot in Fig. 6 clearly shows. However, two points are worth mentioning:
Fig. 7. Heat transfer coefficient variation with the quality. TsatZ 5 8C; fZ5 kW mK2; GZ500 kg sK1 mK2.
– Heat transfer coefficients for standard microfin and smooth tubes are closer than in the previous case, the difference diminishing with the quality. – The herringbone tube heat transfer coefficient is still significantly higher than that of its counterparts in the range of low and medium qualities. A trend shift at the quality of the order of 40% is also observed in this case. The heat transfer coefficient levels off for higher qualities, diminishing slightly with quality. The heat transfer coefficient for high qualities is closer to those of the smooth and standard microfin tubes. The herringbone tube results reveal a clear ‘transition point’ at a quality
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Fig. 8. Photographs of the flow at the test section exit. (a) Annular flow pattern, smooth surface; (b) misty flow, standard microfin surface. TsatZ 5 8C; fZ5 kW mK2; GZ500 kg sK1 mK2.
of the order of 40%, independently of the mass velocity. The herringbone tube heat transfer coefficient for qualities lower than 40% and mass velocity of 500 kg sK1 mK2 is higher than that of the other two geometries, as can be seen in Fig. 7. At the transition point, 40% quality, the heat transfer undergoes a clear trend shift, diminishing further continuously with quality. It is interesting to note that the transition occurs at the same ‘quality transition point’ as before, and the heat transfer coefficient for both microfin tubes of this investigation tend to each other at higher qualities (higher that 60%). On the other hand, smooth tube results keep their upward trend in the high quality range, typical of the annular flow pattern, with the heat transfer coefficient being even higher than those of the microfin tubes. The peculiar behavior observed in the results of Fig. 7 for both microfin tubes has been related to the premature misty flow formation through visual observations made at the sight glass window located at the exit of the test section, see Fig. 3(b). The annular flow pattern with roughened liquid–vapor interface of Fig. 8(a), for the smooth tube, contrasts with the misty flow of the microfin tube of Fig. 8(b) for similar operating conditions. The physical mechanism behind the mist formation in the standard microfin and herringbone tubes seems to be related to the reduction in the liquid film thickness to the extent of exposing the tip of the microfins to the vapor core. This seems to be responsible for the mechanism of liquid removal from the grooves, and its dispersion into the vapor core. The variation of the test section pressure gradient with the quality for several mass velocities and tube surfaces considered in the present study is shown in Fig. 9. It must be stressed at this point that mass velocities in microfin tubes have been determined based on the cross sectional area corresponding to the microfins root diameter of the tube. This is a slightly larger area than the actual one. It can be noted that absolute differences in pressure drop are relatively small in the low and intermediate ranges of mass velocity. Pressure drop in the microfin tubes (standard microfin and herringbone), though being higher, present
similar trend as the smooth one. As a general rule, pressure drop in the herringbone tube is higher than in the standard microfin tube. The pressure gradient in the test section tends to increase with quality, the increment being higher in the low quality range. For higher qualities, the gradient tends to level off due to the thinning of the liquid film on the tube surface. At the highest mass velocity, 500 kg sK1 mK2, differences tend to widen. From Fig. 9 it is also apparent that the pressure gradient trends at this mass velocity for the standard microfin and the herringbone tubes are different. The herringbone tube pressure gradient levels off in the range of high qualities whereas for the standard microfin the pressure gradient presents a continuous rising trend. The observed trend for the standard microfin tube might be related to the mechanism of liquid dispersion previously referred to. This is not the case for the herringbone tube despite the occurrence of the dispersion mechanism. The interplay of momentum and heat transfer mechanisms
Fig. 9. Test section pressure gradient versus quality. TsatZ5 8C. Key to symbles: blank-smooth tube; blackened-standard microfin tube; gray-herringbone tube.
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involved in the dispersion mechanism at elevated mass velocities should be addressed in future studies. A qualitative comparison with results for refrigerant R-407C in 7.0 mm diameter herringbone tubes obtained by Ebisu and Torikoshi [9] has revealed different trends under similar operating conditions. It must be emphasized that the tube diameter and the refrigerant in the Ebisu and Torikoshi [9] study are different from those of the present investigation. According to Ebisu and Torikoshi [9], for a mass velocity of 300 kg sK1 mK2 and heat flux of 7.5 kW mK2, the heat transfer coefficient increases displaying a trend similar to that of the annular flow in smooth tubes. The present investigation results, shown in Fig. 6, display different trends for herringbone and standard microfin tubes The former tend to level off at the quality of the order of 40%, presenting a downward trend for higher qualities, whereas the standard microfin pattern is similar to that of Ebisu and Torikoshi [9]. In addition, whereas the Ebisu and Torikoshi heat transfer coefficient increases with the mass velocity, for both microfin tubes, present results display a maximum at the mass velocity of 200 kg sK1 mK2, as can be noted in Fig. 10. It is difficult at this point to speculate about the possible causes for these discrepancies since the variation of the heat transfer coefficient with the mass velocity is affected by the quality and the refrigerant. It must be added that effects such as the herringbone tube orientation, as noted by Miyara and co-workers, could also affect the results.
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to their smooth counterpart: (1) higher heat transfer coefficient; and (2) higher pressure drop. In the design of a heat transfer surface one must take into account not only the thermal performance but also the momentum transfer between the fluid and the surface, in other words, the pressure drop experienced by the fluid. The latter is related to the mechanical work needed to pump the fluid through the heat exchanger. This is particularly important in refrigerating evaporators, where the refrigerant flows due to the action of the system compressor, whose capacity and driving power vary with the suction pressure. Several parameters or criteria have been proposed in the past to evaluate the heat transfer surface performance, see, for example, Webb [3]. The objective of the investigation reported herein is to evaluate and compare the thermo-hydraulic performance of refrigerant R-134a under convective boiling conditions in microfin tubes. The so called evaluation has been done in the preceding section. In the present section, the comparison will be made in terms of an enhancement parameter, E, defined as the ratio between the relative heat transfer and pressure drop of the microfin with respect to the smooth tube, that is, EZ
h hsmooth Dp ðDpÞsmooth
(5)
The results discussed in the preceding section have shown two important, though apparent, general trends regarding the performance of microfin tubes with respect
Values of E higher than 1 would indicate that the particular microfin geometry would be an interesting substitute for the smooth one since the heat transfer coefficient increment with respect to the smooth tube would be higher than the pressure drop increment. The variation of the enhancement parameter with quality for the standard microfin and herringbone tubes is presented in Figs. 11 and 12, respectively. As a general rule, the
Fig. 10. The effect of the mass velocity over the heat transfer coefficient for a quality of 60%: a comparison between Ebisu and Torikoshi [9] and present study results.
Fig. 11. Enhancement parameter versus quality for the standard microfin tube.
3.1. Enhancement parameter
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Fig. 12. Enhancement parameter versus quality for the herringbone microfin tube.
enhancement parameter diminishes with the mass velocity. For standard microfin tubes, E diminishes from the order of 4 to the order of 1 when the mass velocity increases from 100 to 300 kg sK1 mK2. It is interesting to note that for the mass velocity of 500 kg sK1 mK2, E is mostly less than 1, diminishing with the quality, since the rate of pressure drop increment with the quality is higher. At this mass velocity, the heat transfer coefficient diminishes from qualities of the order of 50% on, contributing to the enhancement parameter reduction noted in Fig. 11. Similar results can be noted in Fig. 12 for the herringbone geometry. It must be noted that the heat transfer coefficient is generally higher in this geometry than in the standard microfin tube. The pressure drop is equally higher so that the enhancement parameters for both geometries are similar, as can be noted in Figs. 11 and 12.
4. Conclusions The general conclusions drawn from the present investigation can be summarized as follows. (1) Microfin tube heat transfer coefficients are generally higher than those for the smooth tube. The pressure drop is equally higher. (2) Heat transfer and pressure drop in herringbone tube are higher than in the standard microfin tube for mass velocities higher than 100 kg sK1 mK2. Thermal performance is comparable for the smallest mass velocity of the investigation (100 kg sK1 mK2) up to a quality of the order of 50%. From this quality on, it has been observed a progressive deterioration of the herringbone heat transfer coefficient, possibly due to a premature dry out of the upper tube surface region.
(3) It has been found that the heat transfer and pressure drop for both microfin tubes considered in the present study at the highest mass flow rate deteriorate at qualities higher than 40–50%. This deterioration has been related to a substantial liquid removal from the grooves and its consequent dispersion into the vapor stream. (4) An enhancement parameter, E, that includes the relative effect of heat and momentum transfer, has been introduced as a performance comparison tool between both microfin geometries. Similar enhancement parameter trends have been found for both microfin geometries over the whole range of mass velocities. (5) The enhancement parameter, E, diminishes with the mass velocity, varying from values of the order of 4 at 100 kg sK1 mK2 to values even lower than one at 500 kg sK1 mK2. (6) Given the preceding analysis, it can be concluded that the microfin tubes are adequate substitutes for the smooth ones for low mass velocities, specially the standard microfin geometry, since, in this case, the grooves operate as liquid conveyors to the upper regions of the tube, enhancing the heat transfer coefficient with a pressure drop comparable to that of the smooth tube.
Acknowledgements The authors gratefully acknowledge the support given to this investigation by Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo, FAPESP, Brazil, and Termomecanica Sa˜o Paulo S/A.
References [1] J.G. Lavin. Heat transfer to refrigerants boiling inside plain tubes and tubes with internal turbulators. PhD. Thesis, University of Michigan; 1963. [2] J.G. Lavin, E.H. Young, Heat transfer to evaporating refrigerants in two phase flow, Am Inst Chem Eng J 11 (1965) 1124–1132. [3] R.L. Webb, Principles of enhanced heat transfer, Wiley, New York, 1994. [4] Y. Fujie, H. Ohta, T. Innami, H. Kimura, N. Nakayama, T. Yanagidi. Heat transfer pipe. US Patents 4,044;797, assigned to Hitachi, Ltd; 1977. [5] A. Cavallini, D. Del Col, L. Doretti, G.A. Longo, L. Rosseto, Heat transfer and pressure drop during condensation of refrigerants inside horizontal enhanced tubes, Int J Refrigeration 23 (2000) 4–25. [6] A. Miyara, K. Nonaka, M. Taniguchi, Condensation heat transfer and fluid flow pattern inside a herringbone-type micro-fin tube, Int J Refrigeration 23 (2000) 141–152. [7] A. Miyara, Y. Otsubo, S. Ohtsuka, Evaporation heat transfer of
E.P. Bandarra Filho, J.M. Saiz Jabardo / International Journal of Refrigeration 29 (2006) 81–91 R410A in herringbone micro fin tubes, thermophysical properties and transfer processes of new refrigerats, CDROM, IIR, France, 2002. p. 314–19. [8] A. Miyara, Y. Otsubo, Condensation heat transfer of herringbone micro fin tubes, Int J Therm Sci 41 (2002) 639–645. [9] T. Ebisu, K. Torikoshi, Experimental study on evaporation and condensation heat transfer enhancement for R-407C using herringbone heat transfer tube, ASHRAE Trans 106 (1998) 1044–1052.
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[10] R.B. Albernethy, Uncertainties in gas turbine measurements, ISA I-483-3 1980. [11] J.M. Saiz Jabardo, E.P. Bandarra Filho, Convective boiling of halocarbon refrigerants flowing in a horizontal copper tube— an experimental study, Exp Therm Fluid Sci 23 (2000) 93–104. [12] E.P. Bandarra Filho, J.M. Saiz Jabardo, P.E.L. Barbieri, Convective boiling pressure drop of refrigerant R-134a in horizontal smooth and microfin tubes, Int J Refrigeration 27 (2004) 895–912.
Convective boiling performance of refrigerant R-134a in herringbone and microfin copper tubes Enio P. Bandarra Filhoa,1, Jose´ M. Saiz Jabardob,* Faculdade de Engenharia Mecaˆnica, Universidade Federal de Uberlaˆndia, Av. Joa˜o Naves de A´vila, 2160 Bloco 1M, Santa Moˆnica, Uberlaˆndia, MG, Brasil b Escuela Polite´cnica Superior, Universidad de la Corun˜a, Calle Mendiza´bal s/n, 15403 Ferrol, Espan˜a, Spain
a
Received 5 April 2004; received in revised form 19 January 2005; accepted 25 May 2005 Available online 19 August 2005
Abstract This paper reports an experimental investigation of convective boiling heat transfer and pressure drop of refrigerant R-134a in smooth, standard microfin and herringbone copper tubes of 9.52 mm external diameter. Tests have been conducted under the following conditions: inlet saturation temperature of 5 8C, qualities from 5 to 90%, mass velocity from 100 to 500 kg sK1 mK2, and a heat flux of 5 kW mK2. Experimental results indicate that the herringbone tube has a distinct heat transfer performance over the mass velocity range considered in the present study. Thermal performance of the herringbone tube has been found better than that of the standard microfin in the high range of mass velocities, and worst for the smallest mass velocity (GZ 100 kg sK1 mK2) at qualities higher than 50%. The herringbone tube pressure drop is higher than that of the standard microfin tube over the whole range of mass velocities and qualities. The enhancement parameter is higher than one for both tubes for mass velocities lower than 200 kg sK1 mK2. Values lower than one have been obtained for both tubes in the mass velocity upper range as a result of a significant pressure drop increment not followed by a correspondent increment in the heat transfer coefficient. q 2005 Elsevier Ltd and IIR. All rights reserved. Keywords: Experiment; Evaporation; R-134a; Microfin tube; Wavy tube; Heat transfer; Pressure drop
R-134a: performance en ebullition convective a` l’inte´rieur de tubes en cuivre a` chevrons et a` microailettes Mots cle´s : Expe´rimentation ; E´vaporation ; R-134a ; Tube microailete´ ; Tube ondule´ ; Transfert de chaleur ; Chute de pression
1. Introduction
* Corresponding author. Tel.: C34 981 337 400x3275. E-mail addresses: [email protected] (E.P. Bandarra Filho), [email protected] (J.M. Saiz Jabardo). 1 Tel.: C55 34 3239 4192x235.
0140-7007/$35.00 q 2005 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2005.05.011
During the last 20 years efforts have been made to upgrade the performance of cooling and condensing coils in refrigeration applications. As should be expected, most of the effort has been focused in the air side due to its higher thermal resistance. Manufacturers have tried to upgrade the
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Nomenclature AR D e E G h i L m n Q t T x
Area ratio Diameter (mm) Microfin height (mm) Enhancement parameter Mass velocity (kg sK1 mK2) Heat transfer coefficient (W mK2 KK1) Enthalpy (J kgK1) Length (m) Mass flow rate (kg sK1) Number of microfins Electrical power (W) Wall thickness (mm) Temperature (K) Quality
Greek symbols b Helix angle (8)
internal heat transfer coefficient, especially in dry expansion evaporator coils, by inserting devices to promote better contact of the liquid with the tube wall. These devices are in fact efficient in upgrading the heat transfer coefficient but at the cost of significant pressure drop. More recently, thin wall copper tubes have been developed having the inner surface covered by fins of reduced size, known as ‘microfin tubes’. Several of these fins have been developed, and are presently used in the refrigeration field, mostly in air cooled condensers, but also in evaporators. These fins promote significant increments in heat transfer though not affecting in the same proportion the pressure drop. These characteristics have promoted their widespread application in the refrigeration industry. In the last 15 years microfin tubes have been submitted to an intense scrutiny regarding their thermo-hydraulic performance especially under condensing conditions. Despite the progress made in understanding convective boiling and condensation of refrigerants in these tubes, some aspects of the heat and momentum exchange mechanisms are not clearly understood and trends remain to be adequately justified. Microfin tubes were introduced into the market in the
DP f q
Pressure drop (kPa) Heat flux (W mK2) Apex angle (8)
Subscripts e external f fluid fg liquid/vapor i internal or root diameter in inlet o outlet PH pre heater r refrigerant sat saturation TS test section w wall
late seventies by Hitachi Cable Ltd. A schematic drawing and a microphotograph of a microfin tube by one international manufacturer is shown in Fig. 1. As a general rule, the fins run helically along the tube. The helix angle, b, varies in the range between 16 and 308, depending on the manufacturer, and the number of fins typically varies from 60 to 70. The height of the fins varies from 0.15 to 0.25 mm, and the actual thickness of the copper tubes is rather small, varying in the range between 0.3 and 0.5 mm. The reduced thickness makes the manufacturing process rather simple and economically sound to the coil manufacturer. Though the investigation of convective boiling in tubes with internal heat transfer enhancement devices is not new, dating back to the 1950s and 1960s, Lavin [1] and Lavin and Young [2], studies involving microfin tubes are more recent. According to Webb [3], one of the first known papers dealing with microfins is the one by Fujie [4], published in the late seventies, and reporting a research performed for Hitachi Cable Ltd. As a general rule, all the published papers report that the microfin tubes under convective boiling conditions present a better heat transfer performance though the pressure drop is significantly increased with respect to their smooth counterpart. Reported increments in
Fig. 1. A cut-way view and a microphotography of the standard microfin tube of the present investigation.
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the heat transfer coefficient with respect to the smooth tubes range between 1 and 3 times whereas the pressure drop increases between 20 and 80%, depending on the mass flow rate. Comprehensive studies of the performance of microfin tubes have been published in the past, especially under condensing conditions. One of such reviews is the one recently published by Cavallini et al. [5], who also included results from a new geometry introduced in the late nineties and rightfully designated by the name ‘herringbone’. A microphotograph of such microfin geometry is shown in Fig. 2. Noteworthy in this geometry is that the microfins run in opposite directions in the lower and upper halves of the tube. Miyara et al. [6] performed heat transfer and pressure drop condensation experiments involving three 7 mm external diameter tubes. One of them was smooth whereas one of the other two was a 50 fins standard microfin tube and the other a herringbone tube with 60 fins. Refrigerants R-410A and R-22 have been tested at a saturation temperature of 40 8C and mass velocities of 100 and 300 kg sK1 mK2. Since a refrigerating compressor was used, lubrication oil was mixed with the refrigerant, its mass concentrations being limited to a maximum of 0.1%. Heat transfer results from this study presented two different trends depending on the mass velocity. For the higher mass velocity (GZ300 kg sK1 mK2), the herringbone geometry performance was better than the standard microfin over the whole range of qualities. For the lower mass velocity, herringbone and standard microfin results were very close, though, for lower qualities (x!0.5), the standard microfin tube presented a performance slightly better than the herringbone, the opposite being observed for higher qualities. Miyara et al. [7] carried out an experimental study of the thermal performance under convective boiling conditions of refrigerant R-410A at 10 8C of saturation temperature in five 7.00 mm external diameter herringbone tubes and a standard microfin tube of the same diameter. The effects of herringbone tube geometric parameters such as the helix and apex angles, fin height and number of fins over the thermal performance were investigated. Results from the investigation can be summarized as follows: (1) for a mass
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velocity of 100 kg sK1 mK2, the heat transfer coefficient of herringbone tubes with intermediate apex angles (14 and 168) increased up to a quality of the order of 40%, decreasing for higher qualities, pointing to a possible surface dry out; (2) for a mass velocity of the order of 300 kg sK1 mK2, the highest heat transfer coefficient was obtained in the herringbone tube with the highest helix angle (288); (3) as should be expected, the pressure drop increased with the helix angle and the fin height. Recently, Miyara and Otsubo [8] confirmed previous results by performing experiments with three herringbone tubes and two additional tubes, one a standard microfin and the other smooth. It has also been determined that pressure drop was higher for two configurations of herringbone tubes with higher helix angle. The pressure drop in the one with small helix angle (68) was lower than that in the standard microfin tube. In a study involving the herringbone and standard regular microfin geometries, Ebisu and Torikoshi [9] performed convective boiling and condensation experiments with refrigerant R-407C in 7.0 mm external diameter tubes, at the saturation temperatures of 5 and 50 8C, respectively. Experiments were performed at mass velocities ranging from 150 to 400 kg sK1 mK2 and a constant heat flux of 7.5 kW mK2. Contrary to Miyara and co-workers condensation results, those from the Ebisu and Torikoshi [9] study revealed a better thermal performance of the herringbone geometry over the whole range of mass velocities. The variation of the heat transfer coefficient with mass velocity for a quality of 60% revealed that the herringbone tube heat transfer performance was better than its standard microfin counterpart over the whole range of mass velocities of the study. The increment factor in heat transfer observed in the herringbone tube with respect to the standard microfin one varied in the range from 3 to 4 for condensing and in the range from 1 to 2 for convective boiling conditions. Present paper reports results from an experimental investigation of convective boiling of refrigerant R-134a in smooth, herringbone and standard microfin copper tubes of the same diameter. Comparison of the obtained results for these microfin geometries are performed in terms of the so called ‘enhancement parameter’ which takes into account the combined effects of heat transfer and pressure drop.
Fig. 2. Microphotography and a cut-way view of the herringbone tube of the present investigation.
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2. Experimental set-up and procedures A schematic circuit diagram of the experimental bench used in the present investigation is shown in Fig. 3(a). The refrigerant is pumped from the condenser through a filter dryer and a sight glass (SG) to the mass flow meter and pre heater before reaching the entrance of the test section (TS). Upon leaving the pre heater, the refrigerant flows through a straight section of 1.2 m length made out of the same tube as the test section in order to allow for flow development. The results reported herein have been obtained in copper tubes of 1.5 m length and 9.52 mm nominal external diameter. The tubes investigated in the present study had the geometric characteristics shown in Table 1. The vapor/liquid mixture of refrigerant leaving the TS is directed to a shell and tube condenser where it is condensed by exchanging heat with a 60% ethylene glycol aqueous solution cooled by a chiller specially built for operation in the experimental set up. The pre heater and the test section are electrically heated with tape resistors, uniformly wrapped on the external surface of the tube to guarantee a uniform heat flux. The heaters are covered by successive layers of fiber glass and foam thermal insulation. Voltage converters manually operated adjust the electrical output in the pre-heater and
test section. The quality of the refrigerant at the TS entrance is determined by performing an energy balance in the pre heater and set by adequately adjusting the electrical output. The heat exchanged with the ambient air through the thermal insulation in the pre heater and test section was estimated in preliminary tests carried out to check the correct operation of the instruments. These tests used to be performed with the refrigerant flowing as subcooled liquid to prevent any possibility of phase change either in the preheater or in the test section. It has been determined that ‘heat leakage’ in both components is less than 5% of the total heat input. The refrigerant bulk temperature was measured at the TS entrance and exit through 30 AWG type T sheathed thermocouples. The refrigerant pressure at the entrance and exit of the TS was measured by pressure transducers whereas the TS pressure drop was directly measured by a differential pressure transducer. Since thermodynamic equilibrium prevailed in all the tests, the refrigerant temperature (saturation temperature) could also be determined from the measured pressure at the entrance and exit of the test section. The surface temperature was measured at four cross sections along the test section tube, as shown in Fig. 3(b). In
Fig. 3. (a) Schematic diagram of the experimental set-up; (b) surface thermocouples location along the test section.
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Table 1 Geometric characteristics of the tubes used in the present investigation Tube
De
Di
t
e
n
b (8)
q (8)
ARa
Smooth Standard microfin Herringbone
9.52 9.52 9.52
8.76 8.92 8.92
0.38 0.30 0.30
0.2 0.2
82 70
18 18
33 33
1.00 1.91 1.78
a
Total finned area over root smooth area.
each cross section 30 AWG type T thermocouples were adequately attached to the external surface of the tube at the bottom, mid plane, and top of the tube. The surface temperature at each cross section was evaluated as the average of the three readings from the surface thermocouples. Heat conduction effects in the tube wall, though small, have been taken into account in the inner surface temperature evaluation. The mass flow rate was measured through a Coriolis type meter and the electrical power (pre heater and TS) with power transducers. The precision of the power transducers given by the manufacturer was checked by single phase experiments conducted as a preliminary procedure previous to the effective experiments with each tube. Each thermocouple and the associated data acquisition system channel were calibrated in a thermostatic bath using both NIST traceable precision glass thermometers and the digital temperature reading of the bath with a precision of G 0.02 8C. Pressure transducers were calibrated using either a reference transducer previously calibrated with a dead weight tester or a column manometer. The accuracy of the calculated parameters was determined according to the procedure suggested by Albernethy and Thompson [10] with an interval of confidence of 95%. The accuracy of measured and calculated parameters is shown in Table 2. It can be noted that heat transfer coefficient accuracy varies from G8.96 to G34.0%. The upper limit corresponds to the highest heat transfer coefficient which occurs under conditions where the difference between the average surface and the refrigerant temperatures is the smallest. The average heat transfer coefficient in each of the four measuring sections in the test section can be determined Table 2 Accuracy of measured and calculated parameters with confidence interval of 95% Parameter
Accuracy
Temperature Pressure Pressure drop Mass flow rate Electrical power Quality Heat transfer coefficient
G0.15 K G0.3% G0.25% G0.15% G5.0% G5.1% G8.96 to G34.0%a
a
Higher accuracy corresponds to higher surface/refrigerant temperature difference.
from Newton’s Cooling Law: hZ
f Tw K Tf
(1)
where Tw is the average wall temperature corresponding to the readings of the thermocouples attached in the particular cross section and Tf is the saturation temperature in the cross section. This temperature is the saturation temperature corresponding to the local pressure of the refrigerant obtained by linearly interpolating the readings of the test section inlet and outlet pressure transducers. The test section average heat transfer coefficient is the one corresponding to the average of the four measuring stations, which in turn is related to the average quality in the test section. The quality at the test section entrance and exit is evaluated from energy balances at both the pre-heater and test section according to the following equations: QPH Z mr ½ðio Þ K ðiin ÞPH
(2)
where io and iin are the enthalpies of the refrigerant at the pre-heater exit and entrance, determined from the pressure and temperature measured at these locations. The liquid entering is sub cooled though an equilibrium saturated liquid/vapor mixture must leave the pre-heater. Since the total electric power output is measured, and the inlet enthalpy is determined from the pressure and temperature readings, the exit enthalpy can thus be determined from Eq. (2), and the quality from the expression for the enthalpy of a liquid/vapor mixture in thermodynamic equilibrium, i Z if C xifg
(3)
The quality at the test section inlet is equal to that at the pre-heater exit since the flow development tube length can be assumed adiabatic. The exit quality can be determined from an energy balance in the test section: QTS Z pDi Lf Z mr ½ðio Þ K ðiin ÞTS
(4)
where (i)o is given by an equation similar to Eq. (3) from which one can obtain the value of the exit quality. QTS is the electric power output at the test section. The quality considered in the present analysis is the average quality in the test section, corresponding to the average heat transfer coefficient.
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3. Analysis of results The range of mass velocities covered in this study varied from 100 to 500 kg sK1 mK2, whereas the qualities varied form 10 to 95%. It has been observed that, for smooth tubes, the annular flow pattern is dominant in the high quality and mass velocities ranges. In the low quality range, the intermittent flow pattern has been observed whereas for low mass velocities (lower than 150 kg sK1 mK2) stratified flow occurs over the whole range of qualities. Uniform and relatively low surface temperature over the tube circumference is the main heat transfer characteristic of the annular flow pattern. The heat transfer coefficient is generally high and increases as one proceeds downstream as the thickness of liquid film diminishes by evaporation at the liquid-vapor interface. Since the upper region remains dry in stratified flow, the superficial temperature increases significantly in the upward direction with the circumferential average heat transfer coefficient being generally small as compared to the one in the annular flow. The intermittent flow is generally a transition flow pattern to the annular one for the higher mass velocities. The quality range it extends over is small, though for transitional mass velocities, from 150 to 200 kg sK1 mK2, it can encompass a higher quality range, Saiz Jabardo et al. [11]. Flow patterns are affected by secondary flows induced by the grooves formed by successive fins in microfin tubes. Two different patterns have been observed in the range of operating conditions considered in the present investigation, both for the standard microfin and the herringbone geometries:
Fig. 4. Heat transfer coefficient variation with the quality. TsatZ 5 8C; fZ5 kW mK2; GZ100 kg sK1 mK2.
(1) internal tube surface covered by liquid (2) mist formation
the smooth tube. It has been argued in the past, Bandarra Filho et al. [12] that this trend is related to the spreading of a liquid film over the surface of the tube promoted by the grooves formed by the microfins. This film flows in what appears to be a secondary flow attached to the tube wall. The thermal resistance associated to the film diminishes with its thickness, a trend which is clearly displayed in Fig. 4 for the standard microfin geometry. This heat transfer pattern is similar to that of the annular flow in smooth tubes for higher mass velocities. According to Fig. 4, the herringbone tube heat transfer coefficient presents a rather peculiar pattern. In the lower quality range, up to 50%, the variation of the heat
Herringbone tube results can be divided into three different ranges regarding the effect of the mass flow rate: the low range, 100 kg sK1 mK2, the intermediate range, 200 and 300 kg sK1 mK2, and the high range, 500 kg sK1 mK2. These ranges are clearly discernible in a first glance in Figs. 4–7, where the average heat transfer coefficient for the three tubes of the present investigation is plotted against the average quality, each figure corresponding to one of the cited mass velocities. Only two ranges are apparent for the standard microfin tube, with the mass velocity of 300 kg sK1 mK2 being the boundary between the two. The variation of the heat transfer coefficient for the three tube surface geometries with the quality for a mass velocity of 100 kg sK1 mK2 is shown in Fig. 4. It can clearly be observed that the smooth tube heat transfer coefficient is the lowest over the whole range of qualities. In addition, it remains essentially constant over the whole range of qualities, a typical heat transfer pattern of stratified flow. The standard microfin surface heat transfer coefficient increases with quality in most of the quality range. In addition, its value is more than four times higher than that of
Fig. 5. Heat transfer coefficient variation with the quality. TsatZ 5 8C; fZ5 kW mK2; GZ200 kg sK1 mK2.
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tubes, the heat transfer coefficient undergoes a significant trend shift with respect to that of Fig. 4. In the lower quality range, up to 40%, the heat transfer coefficient becomes larger than that of the standard microfin tube, the difference increasing up to a factor of the order of two. The rate of increment of the heat transfer coefficient diminishes for higher qualities (higher than 40%), and, as a result, differences between herringbone and standard microfin results tend to come closer to each other. Two conclusions can be drawn from Fig. 5:
Fig. 6. Heat transfer coefficient variation with the quality. TsatZ 5 8C; fZ5 kW mK2; GZ300 kg sK1 mK2.
transfer coefficient with quality follows closely that of the standard microfin geometry. However, for higher qualities, the heat transfer coefficient experiences a significant reduction with quality, approaching the value of the smooth tube at higher qualities. This reduction is certainly related to the depletion of liquid refrigerant in the upper surface of the tube caused by the microfins arrangement. Mass velocity increment to 200 kg sK1 mK2 promotes significant flow pattern changes in smooth and herringbone microfin tubes as can be noted in Fig. 5. Intermittent pattern occurs up to qualities of the order of 20% in the smooth tube. Then annular flow sets in, as the upward trend of the heat transfer coefficient suggests. In the case of the herringbone
(1) Standard microfin and smooth tubes results present similar trends, but the heat transfer coefficient of the former is still higher. It could be argued that the higher heat transfer coefficient in the microfinned surfaced could be related to the increment in the heat transfer area with respect to the smooth surface. However, the area ratio between the microfinned and the smooth surfaces for the tubes of the present investigation is of the order of two whereas the heat transfer coefficient ratio is something higher. This additional enhancement might be related to the aforementioned secondary flow promoted by the grooves in the microfinned surface. (2) The heat transfer coefficient trend shift when the mass velocity varies from 100 to 200 kg sK1 mK2 in the herringbone tube certainly is related to the wetting of the upper region of the tube surface as a result of some sort of dragging mechanism. The increment in the vapor core velocity might be sufficiently high to drag liquid to the mid section of the tube, at which point the disposition of the upper grooves might allow liquid to be dragged efficiently to the top of the tube. At this point it is difficult to speculate about the mechanism responsible for the observed differences between the results of the herringbone and the standard microfin surfaces. The increment of the mass velocity from 200 to 300 kg sK1 mK2 does not cause major changes, as the plot in Fig. 6 clearly shows. However, two points are worth mentioning:
Fig. 7. Heat transfer coefficient variation with the quality. TsatZ 5 8C; fZ5 kW mK2; GZ500 kg sK1 mK2.
– Heat transfer coefficients for standard microfin and smooth tubes are closer than in the previous case, the difference diminishing with the quality. – The herringbone tube heat transfer coefficient is still significantly higher than that of its counterparts in the range of low and medium qualities. A trend shift at the quality of the order of 40% is also observed in this case. The heat transfer coefficient levels off for higher qualities, diminishing slightly with quality. The heat transfer coefficient for high qualities is closer to those of the smooth and standard microfin tubes. The herringbone tube results reveal a clear ‘transition point’ at a quality
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Fig. 8. Photographs of the flow at the test section exit. (a) Annular flow pattern, smooth surface; (b) misty flow, standard microfin surface. TsatZ 5 8C; fZ5 kW mK2; GZ500 kg sK1 mK2.
of the order of 40%, independently of the mass velocity. The herringbone tube heat transfer coefficient for qualities lower than 40% and mass velocity of 500 kg sK1 mK2 is higher than that of the other two geometries, as can be seen in Fig. 7. At the transition point, 40% quality, the heat transfer undergoes a clear trend shift, diminishing further continuously with quality. It is interesting to note that the transition occurs at the same ‘quality transition point’ as before, and the heat transfer coefficient for both microfin tubes of this investigation tend to each other at higher qualities (higher that 60%). On the other hand, smooth tube results keep their upward trend in the high quality range, typical of the annular flow pattern, with the heat transfer coefficient being even higher than those of the microfin tubes. The peculiar behavior observed in the results of Fig. 7 for both microfin tubes has been related to the premature misty flow formation through visual observations made at the sight glass window located at the exit of the test section, see Fig. 3(b). The annular flow pattern with roughened liquid–vapor interface of Fig. 8(a), for the smooth tube, contrasts with the misty flow of the microfin tube of Fig. 8(b) for similar operating conditions. The physical mechanism behind the mist formation in the standard microfin and herringbone tubes seems to be related to the reduction in the liquid film thickness to the extent of exposing the tip of the microfins to the vapor core. This seems to be responsible for the mechanism of liquid removal from the grooves, and its dispersion into the vapor core. The variation of the test section pressure gradient with the quality for several mass velocities and tube surfaces considered in the present study is shown in Fig. 9. It must be stressed at this point that mass velocities in microfin tubes have been determined based on the cross sectional area corresponding to the microfins root diameter of the tube. This is a slightly larger area than the actual one. It can be noted that absolute differences in pressure drop are relatively small in the low and intermediate ranges of mass velocity. Pressure drop in the microfin tubes (standard microfin and herringbone), though being higher, present
similar trend as the smooth one. As a general rule, pressure drop in the herringbone tube is higher than in the standard microfin tube. The pressure gradient in the test section tends to increase with quality, the increment being higher in the low quality range. For higher qualities, the gradient tends to level off due to the thinning of the liquid film on the tube surface. At the highest mass velocity, 500 kg sK1 mK2, differences tend to widen. From Fig. 9 it is also apparent that the pressure gradient trends at this mass velocity for the standard microfin and the herringbone tubes are different. The herringbone tube pressure gradient levels off in the range of high qualities whereas for the standard microfin the pressure gradient presents a continuous rising trend. The observed trend for the standard microfin tube might be related to the mechanism of liquid dispersion previously referred to. This is not the case for the herringbone tube despite the occurrence of the dispersion mechanism. The interplay of momentum and heat transfer mechanisms
Fig. 9. Test section pressure gradient versus quality. TsatZ5 8C. Key to symbles: blank-smooth tube; blackened-standard microfin tube; gray-herringbone tube.
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involved in the dispersion mechanism at elevated mass velocities should be addressed in future studies. A qualitative comparison with results for refrigerant R-407C in 7.0 mm diameter herringbone tubes obtained by Ebisu and Torikoshi [9] has revealed different trends under similar operating conditions. It must be emphasized that the tube diameter and the refrigerant in the Ebisu and Torikoshi [9] study are different from those of the present investigation. According to Ebisu and Torikoshi [9], for a mass velocity of 300 kg sK1 mK2 and heat flux of 7.5 kW mK2, the heat transfer coefficient increases displaying a trend similar to that of the annular flow in smooth tubes. The present investigation results, shown in Fig. 6, display different trends for herringbone and standard microfin tubes The former tend to level off at the quality of the order of 40%, presenting a downward trend for higher qualities, whereas the standard microfin pattern is similar to that of Ebisu and Torikoshi [9]. In addition, whereas the Ebisu and Torikoshi heat transfer coefficient increases with the mass velocity, for both microfin tubes, present results display a maximum at the mass velocity of 200 kg sK1 mK2, as can be noted in Fig. 10. It is difficult at this point to speculate about the possible causes for these discrepancies since the variation of the heat transfer coefficient with the mass velocity is affected by the quality and the refrigerant. It must be added that effects such as the herringbone tube orientation, as noted by Miyara and co-workers, could also affect the results.
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to their smooth counterpart: (1) higher heat transfer coefficient; and (2) higher pressure drop. In the design of a heat transfer surface one must take into account not only the thermal performance but also the momentum transfer between the fluid and the surface, in other words, the pressure drop experienced by the fluid. The latter is related to the mechanical work needed to pump the fluid through the heat exchanger. This is particularly important in refrigerating evaporators, where the refrigerant flows due to the action of the system compressor, whose capacity and driving power vary with the suction pressure. Several parameters or criteria have been proposed in the past to evaluate the heat transfer surface performance, see, for example, Webb [3]. The objective of the investigation reported herein is to evaluate and compare the thermo-hydraulic performance of refrigerant R-134a under convective boiling conditions in microfin tubes. The so called evaluation has been done in the preceding section. In the present section, the comparison will be made in terms of an enhancement parameter, E, defined as the ratio between the relative heat transfer and pressure drop of the microfin with respect to the smooth tube, that is, EZ
h hsmooth Dp ðDpÞsmooth
(5)
The results discussed in the preceding section have shown two important, though apparent, general trends regarding the performance of microfin tubes with respect
Values of E higher than 1 would indicate that the particular microfin geometry would be an interesting substitute for the smooth one since the heat transfer coefficient increment with respect to the smooth tube would be higher than the pressure drop increment. The variation of the enhancement parameter with quality for the standard microfin and herringbone tubes is presented in Figs. 11 and 12, respectively. As a general rule, the
Fig. 10. The effect of the mass velocity over the heat transfer coefficient for a quality of 60%: a comparison between Ebisu and Torikoshi [9] and present study results.
Fig. 11. Enhancement parameter versus quality for the standard microfin tube.
3.1. Enhancement parameter
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E.P. Bandarra Filho, J.M. Saiz Jabardo / International Journal of Refrigeration 29 (2006) 81–91
Fig. 12. Enhancement parameter versus quality for the herringbone microfin tube.
enhancement parameter diminishes with the mass velocity. For standard microfin tubes, E diminishes from the order of 4 to the order of 1 when the mass velocity increases from 100 to 300 kg sK1 mK2. It is interesting to note that for the mass velocity of 500 kg sK1 mK2, E is mostly less than 1, diminishing with the quality, since the rate of pressure drop increment with the quality is higher. At this mass velocity, the heat transfer coefficient diminishes from qualities of the order of 50% on, contributing to the enhancement parameter reduction noted in Fig. 11. Similar results can be noted in Fig. 12 for the herringbone geometry. It must be noted that the heat transfer coefficient is generally higher in this geometry than in the standard microfin tube. The pressure drop is equally higher so that the enhancement parameters for both geometries are similar, as can be noted in Figs. 11 and 12.
4. Conclusions The general conclusions drawn from the present investigation can be summarized as follows. (1) Microfin tube heat transfer coefficients are generally higher than those for the smooth tube. The pressure drop is equally higher. (2) Heat transfer and pressure drop in herringbone tube are higher than in the standard microfin tube for mass velocities higher than 100 kg sK1 mK2. Thermal performance is comparable for the smallest mass velocity of the investigation (100 kg sK1 mK2) up to a quality of the order of 50%. From this quality on, it has been observed a progressive deterioration of the herringbone heat transfer coefficient, possibly due to a premature dry out of the upper tube surface region.
(3) It has been found that the heat transfer and pressure drop for both microfin tubes considered in the present study at the highest mass flow rate deteriorate at qualities higher than 40–50%. This deterioration has been related to a substantial liquid removal from the grooves and its consequent dispersion into the vapor stream. (4) An enhancement parameter, E, that includes the relative effect of heat and momentum transfer, has been introduced as a performance comparison tool between both microfin geometries. Similar enhancement parameter trends have been found for both microfin geometries over the whole range of mass velocities. (5) The enhancement parameter, E, diminishes with the mass velocity, varying from values of the order of 4 at 100 kg sK1 mK2 to values even lower than one at 500 kg sK1 mK2. (6) Given the preceding analysis, it can be concluded that the microfin tubes are adequate substitutes for the smooth ones for low mass velocities, specially the standard microfin geometry, since, in this case, the grooves operate as liquid conveyors to the upper regions of the tube, enhancing the heat transfer coefficient with a pressure drop comparable to that of the smooth tube.
Acknowledgements The authors gratefully acknowledge the support given to this investigation by Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo, FAPESP, Brazil, and Termomecanica Sa˜o Paulo S/A.
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