Experimental study of evaporation pressure drop characteristics
of refrigerants R-134a and R-407C in horizontal small tubes
Y.M. Lie, F.Q. Su, R.L. Lai, T.F. Lin
*Department of Mechanical Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC Received 13 October 2006; received in revised form 30 March 2007
Available online 26 June 2007
Abstract
An experiment is carried out in the present study to investigate the characteristics of the frictional pressure drops for the evaporation of refrigerants R-134a and R-407C in horizontal small tubes having the same inside diameter of 0.83 mm or 2.0 mm. In the experiment for the 2.0-mm tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m2s, imposed heat flux q from 5 to 15 kW/m2, inlet vapor
quality xinfrom 0.2 to 0.8, and refrigerant saturation temperature Tsatfrom 5 to 15°C. While for the 0.83-mm tubes, G is varied from 800
to 1500 kg/m2s with the other parameters varied in the same ranges as those for D
i¼ 2:0 mm. In this study, the effects of the inlet
refrig-erant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured frictional pressure drops are examined in detail. Our experimental data clearly show that both the R-134a and R-407C frictional pressure drops increase significantly with the inlet vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the effect of the imposed heat flux on the frictional pressure drop is rather weak. Moreover, a significant decrease in the frictional pressure drop results for a rise in Tsat. Furthermore, both
the R-134a and R-407C frictional pressure drops increase substantially with the refrigerant mass flux. We also note that under the same xin, Tsat, G, q and Di, refrigerant R-407C has a lower frictional pressure drop when compared with that for R-134a. For the same
refrig-erant, a reduction in the duct size from 2.0-mm to 0.83-mm causes a significant increase in DPf. Finally, an empirical correlation for the
friction factor for the R-134a and R-407C evaporation in the 0.83-mm and 2.0-mm small tubes is proposed. Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, there is growing interest in the use of ultra-compact heat exchangers in various thermal systems because of their very high heat transfer density. Thus the heat transfer and pressure drop characteristics in small and capillary tubes have been extensively investigated for various fluids such as air, water and some refrigerants[1]. But the pressure drop characteristics associated with the evaporation and condensation of the HFCs refrigerants in the small tubes are less explored. Data for the two-phase pressure drop in the small tubes are still scarce. In the pres-ent study, we conduct experimpres-ents to measure the evapora-tion pressure drop of the HFC refrigerants R-134a and
R-407C in small tubes. We choose these two refrigerants in this study because they are regarded as the major substi-tutes for refrigerants R-12 and R-22.
In the following, the relevant literature on the evapora-tion pressure drop in small channels is briefly reviewed. Lazarek and Black [2] developed empirical correlations for frictional, spatial acceleration and bend pressure drops for saturated boiling of R-113 in a vertical U-tube with the hydraulic diameter Dh¼ 3 mm. Yan and Lin[3,4]found
that the frictional pressure drop for R-134a evaporation increased with the refrigerant mass flux and imposed wall heat flux in a tube bank with Dh¼ 2 mm for each tube.
Pressure drop of R-123 in a horizontal small tube with inside diameter of 1.12-mm was measured by Fujita et al.
[5]. They indicated that the two-phase friction factor is nearly constant. Warrier et al. [6] reported experimental data and developed pressure drop correlation for saturated flow boiling of FC-84 in a horizontal tube bundle consisting
0017-9310/$ - see front matterÓ 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2007.03.046
*
Corresponding author. Tel.: +886 35 712121x55118; fax: +886 35 726440.
E-mail address:tfl[email protected](T.F. Lin).
www.elsevier.com/locate/ijhmt International Journal of Heat and Mass Transfer 51 (2008) 294–301
of five parallel channels with a hydraulic diameter of 0.75 mm and length to diameter ratio of 409.8 for each channel. Two-phase boiling pressure drop measurements were made by Tran et al.[7]for three refrigerants R-134a, R-12, and R-113 in two round tubes (Di¼ 2:46 and
2:92 mm) and one rectangular channel (Dh¼ 2:4 mm).
Their data were used to develop new correlation for flow boiling frictional pressure drop in small channels. Chang and Ro [8] investigated the pressure drop of refrigerants R-32, R-125, R-134a and their mixtures in capillary tubes with inside diameters ranging from 1.0 to 2.0 mm and derived a model to predict their experimental data. A new correlation which modified the Friedel correlation for two-phase friction pressure drop was developed by Zhang and Webb[9]. The correlation predicts the data for refriger-ants R-22, R-404a and R-134a flowing in a multi-port extruded aluminum tube (Dh¼ 2:13 mm) with a mean
deviation of 11.5%. Recently, Qu and Mudawar[10] inves-tigated the pressure drop in multi-port parallel micro-channels (Dh ¼ 0:348 mm). They identified two types of
two-phase hydrodynamic instability: severe pressure drop oscillation and mild parallel channel instability. They fur-ther noted that the two-phase pressure drop increased appreciably upon commencement of boiling in micro-chan-nels. The pressure drop characteristics of refrigerants R-236ea, R-410A, and R-134a flowing in parallel mini-channels with Dh¼ 1:4 mm were reported by Cavallini
et al.[11]. They compared their data with several models and showed that no one could fit the data of R-410A.
Recently, a comprehensive review of the experimental data and prediction methods reported in the literature for two-phase frictional pressure drop and flow boiling heat transfer in micro-scale channels was conducted by Ribatski et al.
[12]. The data were analyzed and compared against the pre-diction methods.
The above literature review clearly indicates that the experimental data for the evaporation pressure drop of the HFC refrigerants in small tubes are still in urgent need. In this study, we measure the evaporation pressure drop of refrigerants R-134a and R-407C in horizontal small tubes of inside diameter 2.0 and 0.83 mm. The effects of the vapor quality, refrigerant mass flux, imposed heat flux and system pressure on the evaporation frictional pressure drop in the small tubes will be examined in detail.
2. Experimental apparatus and procedures
The experimental system employed in our recent study of evaporation heat transfer in small tubes[13]is also used here to investigate the evaporation pressure drop of the HFC refrigerants in small tubes. It is schematically depicted inFig. 1. The experimental apparatus consists of three main loops, namely, a refrigerant loop, a water–gly-col loop, and a hot-water loop. Refrigerant R-134a or R-407C is circulated in the refrigerant loop. In order to control various test conditions of the refrigerants in the test section, we need to control the temperature and flow rate in the other two loops. The test section along with the entry Nomenclature
a, b, c, d, e, f constants in Eq.(8)
Cc coefficient of contraction
Cr contraction ratio
Dh hydraulic diameter (mm)
Di inside diameter of small tube (mm)
ftp two-phase friction factor (dimensionless)
g gravitational acceleration (m/s2) G mass flux (kg/m2s)
Geq equivalent mass flux (Eq.(10)) ifg enthalpy of vaporization (J/kg)
L length of small tubes (m) Nconf confinement number, Nconf ¼
r gðqlqgÞ
h i0:5
Dh
(dimen-sionless)
q average imposed heat flux (W/m2)
Reeq equivalent Reynolds number, Reeq¼ GeqDi
ll
(dimensionless) T temperature (°C)
Tr;sat saturated temperature of refrigerant (°C) x vapor quality
Greek symbols a void fraction DP pressure drop (Pa)
Dx total quality change in the small tubes l viscosity (Ns/m2) m specific volume (m3/kg) q density (kg/m3) r surface tension (N/m) Subscripts a acceleration exp total f frictional fg phase change g vapor phase
i, in at inlet of the test section l liquid phase
m liquid–vapor mixture
o exit
and exit sections attached to it are schematically shown in
Fig. 2. The test section is a tube bundle forming from 28 parallel side by side contacting small copper tubes having the same inside diameter of 0.83 or 2.0 mm, outside diam-eter of 1.83 or 3.0 mm, and length of 150 mm. Besides, the pressure transducers and thermocouples are placed at the inlet and exit of the test section to measure the pressure drops and refrigerant temperatures. Two copper plates of 5-mm thick are, respectively, soldered onto the upper and lower sides of the tube bundle. The copper plates are
heated directly by an electric-resistance heater with a 500-W DC power supply. The power input to the heater is mea-sured by a power meter with an accuracy of ±0.5%. In order to reduce the heat loss from the heaters, the whole test section is wrapped with a 10-cm thick polyethylene layer. It should be noted that the heated section of the tube bundle is only 100-mm long and there are two unheated sections each having 25-mm in length upstream and down-stream of the heated section. Axial heat conduction in the tube walls can be important in affecting the measured data
TEST SECTION FLOW DIRECTION T T P TTP -50V - 30A DC Power S upp ly + DEGASSING VALVE PUMP THERMOCOUPLE PRESSURE TAP P T DPDIFFERENTIALPRESSURE SIGHT GLASS VALVE RELEASE VALVE WATER-GLYCOL LOOP REFRIGERANT LOOP WATER LOOP WATER FLOWMETER PUM P SUBCOOLER CONDENSER THERMOSTAT WATER-GLYCOL MASS FLOWMETER THERMOSTAT WATER BY-PASS FLOW FILTER/DRYER PREHEATER N2 ACCUM ULATOR BY - PASS FLOW PUMP RECEIVER TOWER COOLING T T T T
Fig. 1. Schematic layout of the experimental system.
FLOW
Units mm
Inlet Section SectionTest Exit Section
in view of the thermal conductivity of the copper being much higher than that of R-134a and R-407C. In the pres-ent study, however, the liquid refrigerant flow in the small tubes is at a very high Peclet number (>5000). Thus the conjugation effects between the convection in the flow and conduction in the tube walls are expected to be small, as evident from our early studies[14,15]. The details of the apparatus, test section, and experimental procedures are already available in our early studies [3,13] and are not repeated here.
3. Data reduction
Note that in the evaporation of R-134a or R-407C refrigerant in the tubes the flow accelerates, causing the pressure drop, as it moves downstream. Besides, the refrig-erant pressure also drops due to the contraction at the test section inlet and rises due to the expansion at the exits of the small tubes. Thus, in the refrigerant flow the two-phase frictional pressure drop DPf associated with the refrigerant
evaporation in the small tubes is calculated by subtracting the pressure drop due to flow acceleration DPa and the
pressure drop at the test section inlet DPi and by adding
the pressure rise at the test section exit DPofrom the
mea-sured total pressure drop DPexp. The frictional pressure
drop is hence given as
DPf ¼ DPexp DPa DPiþ DPo ð1Þ
Note that the acceleration pressure drop is estimated by the homogeneous model for two-phase flow[16]as
DPa¼ G2mfgDx ð2Þ
Moreover, the pressure drop associated with the sudden contraction DPi and the pressure rise associated with the
sudden expansion DPofor two-phase flow moving through
the inlet and exit ports estimated by Collier[16]based on a separated flow model are chosen here. They can be expressed as DPi¼ G Cc 2 ð1 CcÞ ð1 þ CcÞ½x3inm 2 g=a 2þ ð1 x inÞ 3 m2 l=ð1 aÞ 2 2½xinmgþ ð1 xinÞml ( Cc x2 inmg a þ ð1 xinÞ 2 mg ð1 aÞ " #) ð3Þ and DPo¼ G2Crð1 CrÞml ð1 xinÞ 2 ð1 aÞ þ mg ml x2 in a " # ð4Þ where Ccin Eq.(3)is the coefficient of contraction and it is
a function of the contraction ratio Cr. The void fraction a
in the above equations is calculated from the correlation given by Zivi[17]as a¼ 1 1þ 1xin xin q g ql 2=3 ð5Þ
Finally, for the evaporation of R-134a or R-407C in the small tubes the two-phase friction factor is expressed as ftp¼
DPfDi
2G2mmL
ð6Þ where L is the length of the small tubes and mmis the mean
specific volume of the vapor–liquid mixture in the small tubes when they are homogeneously mixed and can be ex-pressed as
mm¼ ½xmmgþ ð1 xmÞml ¼ ðmlþ xmmfgÞ ð7Þ
More detailed description of the data reduction is avail-able from our earlier study [3]. Uncertainties of the mea-sured pressure drops are estimated according to the procedures proposed by Kline and McClintock [18]. The detailed results from this uncertainty analysis are summa-rized in Table 1.
4. Results and discussion
The frictional pressure drops for the R-134a and R-407C evaporation in the small tubes deduced from the measured raw data for DPexpare presented in the following.
The present experiments are performed for refrigerant R-134a or R-407C in the tube bank forming from the 2.0-mm diameter tubes with the refrigerant mass flux G varied from 200 to 400 kg/m2s, imposed heat flux q from 5 to 15 kW/m2, inlet vapor quality xin from 0.2 to 0.8,
and refrigerant saturated temperature Tsat from 5 to
15°C. While for the other tube bank forming from the 0.83-mm diameter tubes, G is varied from 800 to 1500 kg/m2s with the other parameters varied in the same ranges as those for Di¼ 2:0 mm. Note that different ranges
of the refrigerant mass flux are chosen for the different sizes of the tubes. Since at the low mass flow rate the evaporat-ing refrigerant flow in the smaller tubes for Di¼ 0:83 mm is
somewhat unsteady, leading to the unstable intermittent flow in the system. In the following the effects of the refrig-erant vapor quality, imposed heat flux, and refrigrefrig-erant mass flux and saturated temperature on the R-134a and
Table 1
Summary of the uncertainty analysis
Parameter Uncertainty
Small tubes geometry
Length, width and thickness ±0.5%
Area ±1.0%
Parameter measurement
Temperature, T ±0.2°C
Temperature difference, DT 0.28°C
System pressure, P ±2 kPa
Pressure drop, DP ±200 Pa
Mass flux of refrigerant, G ±2%
Evaporation heat transfer in small tubes
Imposed heat flux, q ±4.5%
Inlet vapor quality, xin 9.5%
R-407C evaporation frictional pressure drops are to be examined in detail.
4.1. Frictional pressure drops in 2.0-mm tubes
The variations of the frictional pressure drops with the inlet vapor quality for R-134a evaporation in the 2.0-mm tubes are shown inFig. 3for various refrigerant saturated temperatures, mass fluxes and imposed heat fluxes. The results indicate that for all Tsat, G and q tested here the
fric-tional pressure drop of R-134a in the tubes increases with the inlet vapor quality. The increase is more significant for a lower refrigerant saturated temperature and a higher mass flux. For instance, at Tsat¼ 5C, G = 400 kg/m2s
and q = 5 kW/m2 the data in Fig. 3a show that the fric-tional pressure drop DPf is increased by 48% when xin is
raised from 0.2 to 0.8. This large increase in DPf with xin
is attributed to the noticeable increase in the vapor void
fraction for xin raised from 0.2 to 0.8, as evident from
Eq.(5). At the higher void fraction the liquid film on the tube wall is thinner and more liquid–vapor interfacial area is available for liquid film evaporation. Thus the vapor flow is at a higher speed for a higher xin, resulting in a larger
DPf.Fig. 3a also shows that an increase in the R-134a
sat-urated temperature causes a large reduction in the fric-tional pressure drop. For example, the quality-averaged frictional pressure drop at G = 400 kg/m2s and q = 5 kW/m2 is reduced by 37% for Tsat raised from 5 to
15°C. This mainly reflects the fact that the dynamic viscos-ity of the liquid R-134a is lower and the densviscos-ity of the R-134a vapor is higher at a higher saturated temperature. Thus the speed of the vapor flow in the duct core is lower. Hence the two-phase flow frictions at the liquid–vapor interface and at the wall are reduced at increasing Tsat,
resulting in a significant reduction in DPf. It is also noted
fromFig. 3b that a substantial increase in DPf results for
0 0.2 0.4 0.6 0.8 1 0 10000 20000 30000 40000 Pf (Pa)
Evaporation Frictional Pressure Drop of R-134a (Di =2.0mm) at G=400 kg/m2s, q=5kW/m2 Tsat=5oC Tsat=10oC Tsat=15oC 0 0.2 0.4 0.6 0.8 1 0 5000 10000 15000 20000 Pf (Pa)
Evaporation Frictional Pressure Drop of R-134a (Di =2.0mm) at Tsat=15 o C, q=5kW/m2 G=200 kg/m2s G=300 kg/m2s G=400 kg/m2s 0 0.2 0.4 0.6 0.8 1 xin 0 10000 20000 30000 40000 Pf (Pa)
Evaporation Frictional Pressure Drop of R-134a (Di =2.0mm) at Tsat=5 o C, G=400 kg/m2s q=5kW/m2 q=10kW/m2 q=15kW/m2
a
b
c
Fig. 3. Variations of R-134a evaporation frictional pressure drop with inlet vapor quality in 2.0-mm small tubes: (a) for various Tsat at
G = 400 kg/m2s and q = 5 kW/m2, (b) for various G at Tsat¼ 15C and
q = 5 kW/m2, and (c) for various q at Tsat¼ 5C and G = 400 kg/m2s.
0 0.2 0.4 0.6 0.8 1 0 5000 10000 15000 20000 25000 Pf (Pa)
Evaporation Frictional Pressure Drop of R-407C (Di =2.0mm) at G=400 kg/m2s, q=5kW/m2 Tsat=5oC Tsat=10oC Tsat=15oC 0 0.2 0.4 0.6 0.8 1 0 4000 8000 12000 16000 Pf (Pa)
Evaporation Frictional Pressure Drop of R-407C (Di =2.0mm) at Tsat=15 o C, q=5kW/m2 G=200 kg/m2s G=300 kg/m2s G=400 kg/m2 s 0 0.2 0.4 0.6 0.8 1 xin 0 5000 10000 15000 20000 25000 Pf (Pa)
Evaporation Frictional Pressure Drop of R-407C (Di =2.0mm) at Tsat=5 o C, G=400 kg/m2s q=5kW/m2 q=10kW/m2 q=15kW/m2
a
b
c
Fig. 4. Variations of R-407C evaporation frictional pressure drop with inlet vapor quality in 2.0-mm small tubes: (a) for various Tsat at
G = 400 kg/m2s and q = 5 kW/m2, (b) for various G at Tsat¼ 15C and
an increase of the R-134a mass flux. According toFig. 3b, at Tsat¼ 15C and q = 5 kW/m
2
the quality-averaged DPf
is increased by 83.1% for G raised from 200 to 400 kg/m2s. The higher DPffor a high refrigerant mass flux is attributed
to the fact that both the speeds of the liquid and vapor flows in the tubes are directly proportional to the refriger-ant mass flux. Finally, the frictional pressure drop is not affected to a noticeable degree by the imposed heat flux, as is evident from the results inFig. 3c. In fact, this change of the frictional pressure drop with the imposed heat flux is within the experimental uncertainty. This small change of DPfwith q results from the fact that the increase in the total
rate of liquid film vaporization in the entire tube bundle is very small compared with the vapor flow rate at the inlet since the tubes are short even for q raised from 5 to 15 kW/m2. It is generally <1%.
The DPfdata for the R-407C evaporation in the 2.0-mm
tubes are shown inFig. 4for comparison. The results
man-ifest that the effects of the refrigerant inlet vapor quality, saturated temperature, mass flux and heat flux on the fric-tional pressure drop associated with the R-407C evapora-tion in the 2.0-mm tubes qualitatively resemble those presented above for R-134a. Contrasting the magnitudes of DPf for the corresponding cases with the same Tsat, G
and q shown in Figs. 3 and 4, respectively, for the R-134a and R-407C evaporation in the 2.0-mm tubes reveals that the frictional pressure drop in the R-134a flow is much higher. The higher DPf for the R-134a evaporation
is considered to result from the fact that R-134a has a much higher dynamic viscosity and a slightly lower latent heat of vaporization.
4.2. Frictional pressure drops in the smaller tubes (Di¼ 0:83 mm)
The variations of the frictional pressure drops with xin,
Tsat, G and q for R-134a and R-407C evaporation in the
0 0.2 0.4 0.6 0.8 25000 30000 35000 40000 45000 Pf (Pa)
Evaporation Frictional Pressure Drop of R-134a (Di =0.83mm) at G=800 kg/m2s, q=5kW/m2 Tsat=5oC Tsat=10oC Tsat=15oC 0 0.2 0.4 0.6 0.8 1 25000 30000 35000 40000 Pf (Pa)
Evaporation Frictional Pressure Drop of R-134a (Di =0.83mm) at Tsat=15 o C, q=5kW/m2 G=800 kg/m2 s G=1150 kg/m2 s G=1500 kg/m2 s 0 0.2 0.4 0.6 0.8 1 xin 30000 32000 34000 36000 38000 40000 Pf (Pa)
Evaporation Friction Pressure Drop of R-134a (Di =0.83mm) at Tsat=5 o C, G=800 kg/m2s q=5kW/m2 q=10kW/m2 q=15kW/m2 1
a
b
c
Fig. 5. Variations of R-134a evaporation frictional pressure drop with inlet vapor quality in 0.83-mm small tubes: (a) for various Tsat at
G = 800 kg/m2s and q = 5 kW/m2, (b) for various G at Tsat¼ 15C and
q = 5 kW/m2, and (c) for various q at Tsat¼ 5C and G = 800 kg/m2s.
0 0.2 0.4 0.6 0.8 1 10000 15000 20000 25000 Pf (Pa)
Evaporation Friction Pressure Drop of R-407C (Di =0.83mm) at G=800 kg/m2s, q=5kW/m2 Tsat=5oC Tsat=10oC Tsat=15oC 0 0.2 0.4 0.6 0.8 1 10000 15000 20000 25000 Pf (Pa)
Evaporation Frictional Pressure Drop of R-407C (Di =0.83mm) at Tsat=15 o C, q=5kW/m2 G=800 kg/m2s G=1150 kg/m2s G=1500 kg/m2s 0 0.2 0.4 0.6 0.8 1 xin 10000 15000 20000 25000 30000 Pf (Pa)
Evaporation Frictional Pressure Drop of R-407C (Di =0.83mm) at Tsat=5 o C, G=800 kg/m2s q=5kW/m2 q=10kW/m2 q=15kW/m2
a
b
c
Fig. 6. Variations of R-407C evaporation frictional pressure drop with inlet vapor quality in 0.83-mm small tubes: (a) for various Tsat at
G = 800 kg/m2s and q = 5 kW/m2, (b) for various G at Tsat¼ 15C and
smaller tubes with Di¼ 0:83 mm are presented in Figs. 5
and 6. The results also indicate that the frictional pressure drops for both R-134a and R-407C in the smaller tubes increase significantly with the refrigerant inlet vapor qual-ity, saturated temperature and mass flux. Besides, the fric-tional pressure drop for R-134a is also substantially higher than that for R-407C. Some quantitative data are given here to illustrate the effects of the important parameters on DPf. For the typical cases with Tsat¼ 15C, G =
800 kg/m2s and q = 5 kW/m2 the data in Fig. 5a and
Fig. 6a show that for the inlet vapor quality raised from 0.2 to 0.8, the frictional pressure drop experiences a 10% increase for R-134a and a 14% increase for R-407C.
Fig. 5a and Fig. 6a also show that the quality-averaged frictional pressure drops reduce substantially for R-134a and for R-407C when Tsat is increased from 5 to 15°C
for G = 800 kg/m2s and q = 5 kW/m2. These, respectively, correspond to 18% and 44% reductions in DPf. Moreover,
according toFig. 5b andFig. 6b, the quality-averaged fric-tional pressure drops increase noticeably for R-134a and for R-407C when G is raised from 800 to 1500 kg/m2s for Tsat¼ 15C and q = 5 kW/m2. These, respectively,
cor-respond to 12% and 46% increases in DPf. Furthermore,
the above results clearly manifest that the refrigerant satu-rated temperature and mass flux exhibit more significant effects on the DPf for R-407C. The frictional pressure drop
is not affected to a noticeable degree by the imposed heat flux, as is evident from the results in Fig. 5c andFig. 6c.
Finally, comparing the magnitudes of DPf data inFigs. 3
and 4with that in Figs. 5 and 6reveals that the frictional pressure drop in the smaller tubes for Di¼ 0:83 mm is
much higher than that in the larger tubes for Di¼
2:0 mm for both refrigerants.
4.3. Correlation equation for frictional pressure drops Based on the present data for R-134a and R-407C evap-oration in the 2.0-mm and 0.83-mm tubes, an empirical correlation for the dimensionless frictional pressure drop is proposed in terms of the friction factor. It is expressed as ftp¼ a þ b Receqþ d N e confþ f Re c eq N e conf ð8Þ
where Reeqis the equivalent Reynolds number for the
evap-orating flow and is defined as Reeq¼ Geq Di lf ð9Þ in which Geq¼ G ð1 xinÞ þ xin qf qg !0:5 2 4 3 5 ð10Þ
Here Geqis an equivalent mass flux at the test section inlet
which is a function of the refrigerant mass flux, inlet vapor quality and density at the test condition. The values for the constants in Eq.(8) are determined from a least-square fit
0 0.04 0.08 0.12 ftp(Present data) 0 0.04 0.08 0.12 ftp (Proposed Correlation )
Evaporation Heat Transfer present data R-134a, Di = 2.0 mm R-134a, Di = 0.83 mm R-407C, D = 2.0 mmi R-407C, Di = 0.83 mm Proposed Correlation -35% +35%
Fig. 7. Comparison of the measured data for frictional pressure drops for the evaporation of R-134a and R-407 in 0.83-mm and 2.0-mm small tubes with the proposed correlation.
of the present data and they are a¼ 0:037, b ¼ 147341, c¼ 1:859, d ¼ 0:039, e¼ 0:508 and f ¼ 327726. Besides, xin is the inlet vapor quality of the flow. Fig. 7
shows that the present data for ftp fall within ±35% of
Eq. (8), and the mean deviation between the present data for ftpand the proposed correlation is about 19.4%.
A close inspection of the data given inFig. 7reveals that the friction factor ftpis much smaller for the smaller tubes
with Di¼ 0:83 mm, contrary to the trend presented above
for the frictional pressure drops. These opposite trends in ftpand DPf are simply due to the definition for ftp in Eq.
(6).
5. Concluding remarks
Experimental measurement has been carried out here to investigate how the frictional pressure drops of R-134a and R-407C evaporation in the small tubes are affected by the inlet vapor quality, refrigerant saturated temperature and mass flux, and imposed heat flux. The results show the sig-nificant increase of the frictional pressure drops for R-134a and R-407C evaporation in the small tubes with the inlet vapor quality and the increase is larger for a higher refrig-erant mass flux. But an opposite trend is noted for a rise in the refrigerant saturated temperature. Furthermore, the imposed heat flux exhibits a negligible effect on the fric-tional pressure drops. Besides, we also note that on an average the frictional pressure drop for R-134a is substan-tially higher than that for R-407C in both tubes. Finally, an empirical correlation is proposed to correlate the present data for the frictional pressure drops in terms of the fric-tion factor.
Acknowledgement
The financial support of this study by the engineering division of National Science Council of Taiwan, ROC through the Contract NSC 94-2212-E-009-002 is greatly appreciated.
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