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Applied Thermal Engineering 68 (2014) 125e132

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Applied Thermal Engineering

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

Analysis on performance characteristics of ejector with variablearea-ratio for multi-evaporator refrigeration system based onexperimental data

Cui Li a,b, Yanzhong Li a,*, Wenjian Cai b, Yu Hu b, Haoran Chen b, Jia Yan b

a School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, Chinab School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore

h i g h l i g h t s

� Experiment and analysis are performed on a variable area-ratio ejector used in MERS.� ER increases linearly with pS, but decreases monotonously with pP in a way of y ¼ axb.� PRR is a quadratic function of pS, but a piecewise-linear function of pP.� Critical AR is proposed to indicate the pressure recovery and energy saving status.� Critical AR can be predicted by a linear function of pP or a power function of pS.

a r t i c l e i n f o

Article history:Received 17 December 2013Accepted 13 April 2014Available online 24 April 2014

Keywords:Variable ejectorMulti-evaporator refrigeration systemPressure recoveryEntrainment ratioCritical area ratioExperiment

* Corresponding author. Tel.: þ86 29 82668738; faE-mail addresses: yzli-epe@mail.xjtu.edu.cn, yzli-e

http://dx.doi.org/10.1016/j.applthermaleng.2014.04.031359-4311/� 2014 Elsevier Ltd. All rights reserved.

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a b s t r a c t

This paper presents a study on experiment and analysis of variable area-ratio ejector used in a multi-evaporator refrigeration system (MERS). The experimental rig and method are described, and theentrainment and pressure recovery performances of the variable ejector are measured at various oper-ating and geometric conditions. The critical area ratio is proposed as an indicator of pressure recoverystatus; area ratios smaller than the critical ones are required to make sure that the system operates atenergy saving mode. The investigation results indicate that the entrainment ratio, pressure recovery ratioand critical area ratio are strongly affected by the primary pressure and secondary pressure. Greaterentrainment ratio can be obtained by increasing the secondary pressure or decreasing primary pressure.The opposite trends are found for pressure recovery ratio. Moreover, the critical area ratio varies greatlywith the operation condition, and can be predicted by a linear fitting function of primary pressure or apower function in terms of secondary pressure.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

As a device without moving parts, ejector has the advantage ofbeing quiet, reliable, low-cost, easy to maintain and operate, etc.[1]. It can be powered by low grade heat energy or renewable en-ergy [2e4], and has found many applications in engineering, suchas refrigeration, aerospace, chemical and biochemical process in-dustries [5,6]. Depending on its area of application, ejector could bedesigned with the following intentions: (a) To get large entrain-ment of the secondary fluid, (b) To produce intensemixing betweenthe primary and secondary fluids, or (c) To pump fluids from aregion of low pressure to a region of high pressure [7].

x: þ86 29 82668725.pe@163.com (Y. Li).

1

ica.ir

The application of ejector in refrigeration has a long-establishedhistory. The first ejector refrigeration system was introduced byMaurice Leblanc in 1910 [8], and this system experienced a wave ofpopularity during the early 1930s for air conditioning of largebuildings [9]. Although investigation on ejector refrigeration hasbeen almost at standstill after 1950s as most effort has beenconcentrated on vapor-compression refrigeration systems, therehas been a strong resurgence recently in research and developmentof ejector due to the growing concern towards environment, energyutilization and sustainable development [10e20].

In ejector refrigeration systems, ejector plays an important roleas the incentive for its application is either replacing the mechan-ical compressor or optimizing the refrigeration cycle (for example,the combined ejector-absorption refrigeration system). And theejector performance of primary interest is large entrainment of

C. Li et al. / Applied Thermal Engineering 68 (2014) 125e132126

secondary fluid. Since the entrainment ratio of ejector using wateris very low, various refrigerants have been used as working fluid forejector refrigeration. The earliest reported research on ejectorrefrigeration using a refrigerant other thanwater was performed inthe 1950s [21,22]. After that, many pure fluids as well as azeotropicor non-azeotropic mixtures were used as candidate fluids forejector refrigerators. For example, halocarbon compound re-frigerants such as R11, R12, R13, R113, R114, R123, R133a, R134a,R141b, R142b, R152a, R21, R22, R245ca, R245fa, and RC318 were allused and tested. Among these refrigerants, CFC R12, HCFC R142b,HFC R134a and R152a are reported to give high entrainment ratiosand comparative ejector performance [21]. Configuration optimi-zation is another effective way to enhance the entrainment per-formance of ejector. In fact, a considerable portion of the publishedliterature on ejectors was directed toward the design and optimi-zation of ejector [7,19,20,23e33]. And CFD technique is consideredas a reliable tool to perform such research if sufficiently validated.Up to present, the effects of such geometry parameters as area ratio,mixing tube length and primary nozzle position on ejectorentrainment performance have been all studied. The results indi-cate that the optimal value of these geometry parameters thatprovides maximum entrainment rate varies greatly with theoperation conditions, and it is difficult to find a universal value thatmeets all the conditions [33].

Apart from getting large entrainment of the low-pressure sec-ondary fluid, ejector used in refrigeration systems is expected tocontribute to reducing power consumption by its pressure recoveryeffect. Typical applications include multi-evaporator refrigerationsystem [34,35], household refrigerators [36,37], CO2 heat pump[38e42], etc.

The multi-evaporator refrigeration system (MERS) consists ofmore than one evaporator that operates at different pressure andtemperature levels, but one condenser and a single compressor[34,43]. It is widely used in circumstances that involve two or threeevaporating temperatures. For example, in supermarket or otherfood processing, transportation and storage applications, threeevaporating temperatures, namely þ7 �C, e5 �C and e30 �C, areusually required for space cooling, storage of perishable or tem-perature sensitive food, and freezing, etc [44e46].

The application of ejector for two-evaporating temperaturerefrigeration system dates back to early 1990’s for domestic re-frigerators [37,47e49]. And an energy-efficient three-evaporatorrefrigeration system with two ejectors was also proposed recentlyby Kairouani L. et al. [34]. In these systems, ejector is used tomaintain the required pressure differences between the high-temperature and low-temperature evaporators and more impor-tantly, to lower the power consumption by its pressure recoveryeffect. The exergy analysis results show that the system COP can beimproved by about 10% and 20% for two-temperature ejector-basedMERS and three-temperature ejector-based MERS, respectively, ascompared to the conventional MERSs. The main drawback ofejector-based multi-evaporator refrigeration system is that it canonly work well at the on-design condition which is, however,difficult to guarantee in actual operation due to ambient environ-mental variations. Therefore, a variable ejector is highly needed tomeet the variable cooling load conditions. To the best of the au-thors’ knowledge, information on variable ejector as a pressurerecovery device in a refrigeration cycle is still limited. Preliminaryresults of an adjustable ejector applied in multi-evaporator refrig-eration system can be found in the CFD study of Lin et al. Theyreported that pressure recovery ratio is sensitive to the varying ofcooling loads [50].

However, little or no information is available with respect to theoptimal ejector geometry for energy saving running of multi-evaporator refrigeration system. In the present work, the

entrainment and pressure recovery performances of the variablearea-ratio ejector applied in MERS are first studied in detail. Theconcept of critical area ratio is then proposed as an indicator ofpressure recovery status, and the main concern of this study is toobtain the relationship between the critical area ratio and theoperating pressures. These works are expected to contribute to theenergy savings potential of MERS in supermarket or other situa-tions where MERSs are required, and can be easily extended toother ejector refrigeration systems.

2. Variable ejector and the relevant experimental setup

The present experimental study is performed on the basis of anejector with variable area ratios. As presented in Fig. 1, this variableejector consists of five parts: primary nozzle, suction chamber,mixing chamber, diffuser, and a spindle. The primary nozzle, whichis illustrated in detail, has an inlet diameter (d1) of 15 mm, a throatdiameter (dt) of 4mm and an exit diameter (d2) of 6mm. It is placedupstream of the constant-area section of mixing chamber, whichhas a length (Lm) of 40 mm and a diameter (Dm) of 10 mm, and thedistance between the nozzle exit and the start of constant-areamixing section (NXP) is 15 mm. Other geometric data are given inTable 1. The spindle is driven by a motor. With its movement, theeffective flow area of primary nozzle changes accordingly. By thismeans, the primary flow rate of ejector can be adjusted, which inturn leads to the variation of secondary flow rate. It should be notedthat the ability of spindle to adjust the primary flow rate hasalready been proved by the analysis of Varga S. et al. performed on avariable area ratio steam ejector [51].

The application of variable ejector in a multi-evaporatorrefrigeration system is expected to (a) achieve the required allo-cation of cooling capacity between the evaporators by adjusting theentrainment of secondary fluid, and (b) reduce power consumptionby its pressure recovery effect. The former is represented by theentrainment ratio of ejector, which is defined as

ER ¼ mS

mP(1)

where mP is the primary mass flow rate, and mS is the secondarymass flow rate. And the latter is evaluated by the pressure recoveryratio (PRR), which is defined as:

PRR ¼ pb � pSpS

(2)

where pb is the outlet pressure of ejector and pS is the secondarypressure. For the compressor, higher PRR implies a reduction in itscompression ratio and an increase in its efficiency. Therefore, thehigher PRR the ejector provides, the less power the systemconsumes.

The ejector based multi-evaporator refrigeration system isschematically shown in Fig. 2. Besides the variable ejector, the mainelements of this test facility include an inverter compressor, an air-cooled condenser, two evaporators (Evaporator 1 and Evaporator2), and two electronic expansion valves (indicated by EEV1 andEEV2, respectively). The working cycle of the present multi-evaporator refrigeration system consists of two parts: the refrig-eration process and the subsequent pressure recovery process. Theformer is accomplished by continuously compressing (State 1e2),condensing (State 2e3), throttling (State 3e4, 3e6), and evapo-rating (State 4e5, 6e7) the refrigerant, while the latter ishappening within the variable ejector (State 5,7e9). Since CFCs andHCFCs are no longer usable or will be phased out soon due to theirozone depletion potential (ODP) or high global warming potential

Primary Flow

Secondary Flow

Spindle

Suction Chamber

Constant-area MixingChamber

Diffuser

Primary Nozzle

Mixed Flow

Motor

Constant-pressure MixingChamber

N2

N1

S1

S2

d

Fig. 1. Schematic diagram of the variable ejector and primary nozzle.

C. Li et al. / Applied Thermal Engineering 68 (2014) 125e132 127

(GWP) [52], this experimental study is carried out with R134a asthe working refrigerant. Apart from its good entrainment perfor-mance and environmentally-friendly properties, R134a has areasonable cost and is safe for normal handling due to its non-toxic,non-flammable and non-corrosive properties.

Evaporator 1 and Evaporator 2 operate at different pressure andtemperature levels, and use ethylene glycol solution with differentconcentrations as heat transfer liquids. The ethylene glycol solutionin Evaporator 1 (the high-temperature evaporator) has a concen-tration of 20% and a freezing temperature of �8 �C. It can meet thetemperature requirements for both air conditioning and freshstorage. Evaporator 2 (the low-temperature evaporator) is aimed atstorage of dairy and frozen products, the temperature of whichgenerally starts at 0 �C to as low as �30 �C. Therefore the ethyleneglycol solution used in this evaporator has a concentration of 50%and a freezing temperature of �37 �C. It should be noted that theallocation of cooling capacity between the two evaporators isdirectly linked to the entrainment ratio of ejector. That is, thehigher the entrainment ratio, the bigger cooling capacity the low-temperature evaporator provides.

The BITZER 4cc-9.2 semi-hermetic type of inverter compressorhas a rated output power of 6.6 kW, while the condenser has amaximum condensing load of 10 kW. The electronic expansionvalves are driven by PID controlled step motor. The area ratio ofvariable ejector is regulated by motor-driven spindle.

All the pressures are measured using pressure transducers withthe accuracies of 0.5% of full scales. The temperatures are measuredby PT1000 platinum resistance with an error of�0.3 �C. The motiveand the entrained flow rates are measured by two metal tube ro-tameters mounted upstream of the expansion valves, each with anaccuracy of 1.6%. The output signals from the measurement devicesare transferred to a PC through a data acquisition board, and thenmonitored and controlled by a system developed in Labview 2010of the National Instruments.

Table 1Configurations and dimensions of the variable area-ratio ejector.

Geometry parameters Value

Inlet diameter of primary nozzle, d1 15 mmThroat diameter of primary nozzle, dt 4 mmExit diameter of primary nozzle, d2 6 mmConverging angle of constant-pressure mixing chamber, qm 25�

Diameter of constant-area mixing chamber, Dm 10 mmLength of constant-area mixing chamber, Lm 40 mmLength of diffuser, Ld 80 mmDiverging angle of diffuser, qd 3.5�

Primary nozzle position, NXP 15 mmArea ratio, AR 6.25e8.33

For a view of the real experimental rig, see Fig. 3.

3. Entrainment performance and pressure recovery ofvariable area-ratio ejector

In consideration of its role in the MERS, this study is first con-cerned with the performances of variable ejector, that is, the effectsof ejector area ratio on the entrainment ratio and the pressure re-covery ratio. In this section, the low-temperature evaporator(Evaporator 2) of the system is applied as a freezer and thus thecorresponding secondary pressure of ejector is 110 kPa. The casethat the high-temperature evaporator (Evaporator 1) used for airconditioning is compared with that for refrigerator. And the pri-mary pressure conditions covered by this comparison are 380 kPaand 240 kPa, respectively. The results are summarized in Table 2.

The variation of entrainment ratio ER with area ratio AR is givenin Fig. 4(a). As mentioned above, the area ratio of ejector can beadjusted by moving the spindle. When the spindle moves forwardin the primary nozzle under the driving of motor, the effective flowarea of primary nozzle decreases, which leads to the rapid increaseof area ratio. It is found that the entrainment ratio ER increaseslinearly with the area ratio AR for fixed conditions. Take the airconditioning case (the corresponding primary pressure is 380 kPa)for example. As AR increases from 6.25 to 8.33, the entrainmentratio ER increases from 0.6 to 1.56. Moreover, the entrainment ratioER measured for pP ¼ 380 kPa is lower when compared with that ofpP ¼ 240 kPa for all the area ratios investigated.

The pressure recovery performance of variable ejector is alsodependent upon the area ratio AR. As shown in Fig. 4(b) and (c), thepressure recovery ratio PRR decreases gradually with the area ratioAR, so does the power consumed by the whole system. And theoptimum value of area ratio is 6.25 in the present study from theviewpoint of high pressure recovery performance. The relationshipbetween PRR and AR can be described by quadratic function. Still,larger primary pressure gives higher values of PRR for constant arearatios. As AR increases from 6.25 to 8.33, the pressure recovery ratiounder pP¼ 380 kPa condition decreases from 31% to 15% (the powerconsumption decreases from 2.63 kW to 2.40 kW), while for thecondition of pP ¼ 240 kPa, it decreases from 11% to about 2%.

4. Effects of operating pressures on the variable ejectorperformances

The primary pressure pP and secondary pressure pS of the var-iable ejector are determined by the evaporating pressures of MERS,and their effects on the entrainment and pressure recovery per-formances have been studied in detail in this section. According to

TestingChamber 1

T9

TestingChamber 2

T10

Evaporator 1 Evaporator 2 Compressor

Condensor

P3 T3

Accumulator

ReceiverF Flowmeter 2F Flowmeter 1

T5 T7

P

T6P6

T4P4

P7P5

2

3

6

7

4

5

Ejector

T8P8

T2P2

T1P1

8

1

9

Liquid indicator

Fig. 2. Schematic diagram of experimental apparatus.

Fig. 3. Photograph of the experimental rig.

C. Li et al. / Applied Thermal Engineering 68 (2014) 125e132128

the characteristics of the experimental results, performance cor-relations are obtained and can be used to estimate ejector perfor-mance at other working conditions.

The variation of entrainment ratio ER with the secondarypressure pS under different area ratios is shown in Fig. 5(a). Thesecondary pressure varies in the range of 90e240 kPa while theprimary pressure pP keeps constant at 380 kPa. It is obvious that theentrainment ratio is very sensitive to the variation of secondarypressure. For all the area ratios studied, the entrainment ratio ERincreases linearly with the secondary pressure pS, and this rapidgrowth of ER depends to a large extent on the area ratio. Thisconclusion can be demonstrated by the linearly correlating resultsof entrainment ratio data with secondary pressure. The slope oflinear fitting equation is 0.73 for AR ¼ 6.25 and 1.0 for AR ¼ 7.03. Inother words, larger area ratio gives higher values of ER for fixedsecondary pressures. To be specific, in the case of AR ¼ 6.25, theentrainment ratio increases from 0.2 to 1.1 when the secondarypressure pS rises from 120 kPa to 250 kPa, while for AR ¼ 7.03, thesame increase in ER requires an increase of secondary pressure pSfrom 90 kPa to 180 kPa.

Table 2Entrainment and pressure recovery performances of the variable area-ratio ejector.

pP ¼ 240 kPa, pS ¼ 110 kPa pP ¼ 380 kPa, pS ¼ 110 kPa

AR ER PRR (%) AR ER PRR (%)

6.250 0.608 11.15 6.250 0.301 31.096.294 0.659 10.93 6.294 0.312 29.396.429 0.725 9.35 6.429 0.353 26.586.667 0.814 7.04 6.667 0.410 22.457.031 0.982 6.06 7.031 0.485 19.747.563 1.244 4.39 7.563 0.687 18.898.333 1.559 2.47 8.333 0.911 14.97

5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.00.0

0.5

1.0

1.5

2.0

Primary pressure: 240 kPa Primary pressure: 380 kPa

Entra

inm

ent r

atio

Area ratio, AR(a)

5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.00

10

20

30

40

Primary pressure 240 kPa Primary pressure 380 kPa

Pres

sure

reco

very

ratio

, PR

R (%

)

Arear ratio, AR(b)

(c)

5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.01.9

2.0

2.1

2.4

2.5

2.6

2.7

Primary pressure 240 kPa Primary pressure 380 kPa

Pow

er (k

w)

Arear ratio, AR

Fig. 4. Effect of area ratio on: (a) entrainment ratio (b) pressure recovery ratio and (c)power consumption.

C. Li et al. / Applied Thermal Engineering 68 (2014) 125e132 129

The variation of pressure recovery ratio with secondary pressureis quite different from that of entrainment ratio. Fig. 5(b) shows theplot of pressure recovery ratio PRR versus secondary pressure pSunder different area ratios. It can be seen that PRR is stronglyinfluenced by pS. As the secondary pressure pS increases, thepressure recovery ratio PRR goes down gradually until it hits zeroand keeps unchanged. This monotonic decreasing property of PRRversus pS can be described by quadratic functions, the coefficientsof which are also presented in Fig. 5(b). For convenience, the arearatio corresponding to the first zero PRR is referred to in the presentstudy as the critical area ratio. The decreasing rate of PRR forAR ¼ 6.25 is close to that for AR ¼ 7.03. The difference betweenthese two curves is that under same secondary pressures, the largerarea ratio gives the lower pressure recovery ratio. In other words,larger area ratio is required to achieve the same pressure recoveryeffect for smaller pS. This suggests that the critical area ratio isdifferent when the secondary pressure varies, for example, thecritical area ratio is 7.03 for pS ¼ 200 kPa and 6.25 for pS ¼ 250 kPa.Since the energy saving effect of MERS is negligible when the PRR

80 100 120 140 160 180 200 220 240 260-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Area ratio: 6.25Area ratio: 7.03E

ntra

inm

ent r

atio

, ER

Secondary pressure (kPa)

Equation y = a + b*xa (Intercept) b (Slope)

Area ratio : 6.25 --0.70768 0.00729Area ratio : 7.03 -0.66644 0.01005

(a)

80 100 120 140 160 180 200 220 240 260-5

0

5

10

15

20

25

30

35

40

45

Area ratio: 6.25Area ratio: 7.03

Pre

ssur

e re

cove

ry ra

tio, P

RR

(%)

Secondary pressure (kPa)

Equation y = Intercept + B1*x^1 + B2*x^2

Coefficient Value

Area ratio: 6.25

Intercept 0.9647B1 -0.00738B2 1.43197E-5

Area ratio: 7.03

Intercept 0.94001B1 -0.00902B2 2.16658E-5

(b)

Fig. 5. Effect of secondary pressure on: (a) entrainment ratio and (b) pressure recoveryratio.

C. Li et al. / Applied Thermal Engineering 68 (2014) 125e132130

drops to 0, this critical area ratio is of great importance in ejector-based MERS and awaits further analysis to gain a betterunderstanding.

Fig. 6 shows the effect of primary pressure on the performancesof variable ejector, with (a) for entrainment ratio and (b) for pres-sure recovery ratio. The primary pressure varies between 150 kPaand 400 kPa while the secondary pressure has a constant value of150 kPa. From Fig. 6(a), it is evident that an increase in the primarypressure pP leads to a decrease in the entrainment ratio ER ofejector. The decreasing ER can be estimated by a power function ofpP in the form of y ¼ axb. As the area ratio AR increases from 6.25 to7.03, an obvious rise in ER can be observed.

The variation of pressure recovery ratio with primary pressuredisplays a quite different characteristic. As the primary pressure pPincreases from 150 kPa to 400 kPa, the pressure recovery ratio PRRfirst keeps constant at 0 and then increases linearly when pP

150 200 250 300 350 4000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Area ratio: 6.25Area ratio: 7.03

Ent

rain

men

t rat

io, E

R

Primary pressure (kPa)

Equation y = a*x^bCoefficient Value

Area ratio: 6.25a 29180.19804b -1.87085

Area ratio: 7.03a 15280.89477b -1.66363

(a)

150 200 250 300 350 400-5

0

5

10

15

20

Area ratio: 6.25Area ratio: 7.03

Pre

ssur

e re

cove

ry ra

tio, P

RR

(%)

Primary pressure (kPa)

Equation y = a + b*x

Coefficient Value

Area ratio: 6.25a -0.2532b 0.0011

Area ratio: 7.03a -0.23438b 8.06377E-4

(b)

Fig. 6. Effect of primary pressure on: (a) entrainment ratio and (b) pressure recoveryratio.

becomes higher than a certain value. In other words, the variationof pressure recovery ratio with pP can be described by a piecewise-linear function. From the viewpoint of energy saving, ejector withfixed geometry parameters should avoid operating within theconstant zero-PRR region. Fig. 6(b) also shows that for the sameprimary pressures, an increase in area ratio decreases the pressurerecovery ratio. This is considered mainly due to two facts. One isthat the linear variation region starts at different pP. In the case ofAR ¼ 6.25, the transition of PRR from constant to linear variationoccurs at a pP of 225 kPa, while in the case of AR¼ 7.03, the pressurerecovery ratio PRR does not enter the linear variation region untilthe primary pressure pP increases to 300 kPa. And the other is thatthe growth rate of AR ¼ 6.25 is a bit faster when compared withthat of AR ¼ 7.03.

As explained earlier, the cooling capacity provided by the low-temperature evaporator in MERS increases with an increase inthe entrainment rate. Therefore, the low-temperature evaporatorwill make a greater contribution to the total cooling capacity withan increase in secondary pressure pS or a decrease in primarypressure pP. This is, however, unfavorable to the energy saving ofthe whole system because in this process the pressure recoveryratio drops to a lower value.

5. Analysis and discussion

From the results mentioned above, it can be concluded that thepressure recovery effect generated by the variable ejector dependsnot only on the operating pressures but on its area ratio. For a givenoperation condition, there is a critical area ratio that corresponds tothe first hit of zero pressure recovery ratio. This critical area ratio isthe maximal value of area ratio that can provide pressure recovery.In other words, pressure recovery effect can only be achieved byejector with an area ratio smaller than this critical value. In view ofmeasurement and accuracy, the criterion used in the present studyto determine the critical value is PRR ¼ 1%, which assumes that thepressure recovery effect vanisheswhen the ratio PRR falls to 1%. Thecritical area ratio is found to differ widely when the primary andsecondary pressures vary. It is therefore necessary to investigatethe relationship between the critical area ratio and the operatingpressures.

The relationship between the critical area ratio and the primarypressure is shown in Fig. 7(a). It is clear that the critical area ratioincreases linearly with the primary pressure, and the increase of pPfrom 180 kPa to 530 kPa (as shown in Table 3) doubles the criticalAR (from 6.2 to 12.25). The red (in the web version) line in Fig. 7(a)is obtained by correlating the critical values of AR as a linearfunction of primary pressure, which is expressed as

y ¼ 0:01687xþ 3:139 (3)

where y is the critical value of AR, and x is the primary pressure pP.Area ratios those below this curve are capable of providing pressurerecovery effect and should be selected to decrease the powerconsumption of this refrigeration system.

The variation of critical area ratio with the secondary pressure isshown in Fig. 7(b). It appears that the critical area ratio decreaseswith the increasing secondary pressure pS, and the increase of pSfrom 125 kPa to 250 kPa causes a 40 percent decrease in critical AR(from 10.4 to 6.2). It should be noted that the decreasing rate ofcritical AR is not a constant; instead, the critical AR continues todecrease but at a slower pace. The critical AR can be predicted by apower function in terms of the secondary pressure, which is givenas

y ¼ 495x�0:8 (4)

150 200 250 300 350 400 450 500 5505

6

7

8

9

10

11

12

13

PRR=0

Are

a ra

tio, A

R

Primary Pressure (kPa)

Equation y = a + b*x

CoefficientIntercept 3.13877Slope 0.0168

PRR>1%

critical area ratios

(a)

100 120 140 160 180 200 220 240 2605

6

7

8

9

10

11

critical area ratios

PRR=0

PRR>1%Are

a ra

tio, A

R

Secondary Pressure (kPa)

Equation y = a*x^b

Coefficienta 494.98398b -0.79842

(b)

Fig. 7. Relationship between the critical area ratio and: (a) the primary pressure and(b) the secondary pressure.

Table 3Critical area ratio for different working conditions.

Primary pressure (kPa) Secondary pressure (kPa) Critical area ratio

183.5 124.2 6.28200.8 125.2 6.43214.7 124.7 6.73240.3 125.1 7.033262.4 125.7 7.443280.8 124.9 7.993299.8 124.6 8.3333317.5 125.5 8.733373.1 125.2 9.473397.5 126.2 9.77439.9 125.9 10.44496.7 125.6 11.25533.2 124.9 12.25379.0 125.4 10.44380.4 149.6 9.21380.2 175.1 7.99380.1 199.7 7.03379.3 225.5 6.43379.5 249.5 6.25

C. Li et al. / Applied Thermal Engineering 68 (2014) 125e132 131

where y is the critical value of AR, while x is the secondary pressurepS. Still, the fitting curve is a representative that the pressure re-covery effect begins to disappear. For a given pS, an area ratiosmaller than the critical one is required to make sure that thesystem operates at the energy saving mode.

Since the data used for prediction of critical area ratio indicatewide variations in the primary and secondary pressures, thesecorrelations can serve as a guideline for design and operation ofvariable ejector.

6. Conclusion

In this paper, the performance characteristics of a variable area-ratio ejector have been investigated based on analysis of experi-mental data measured in a multi-evaporator refrigeration system(MERS). The area ratio of variable ejector can be adjusted bymovinga spindle axially within the primary nozzle. The experimental

results demonstrate the effects of operating pressures on theentrainment and pressure recovery performances, and haveconfirmed the advantages of ejector with variable geometry. Theconcept of critical area ratio is then proposed for better under-standing of pressure recovery effect, and the relationships betweenthe critical area ratios and the primary and secondary pressures arediscussed. The main results can be summarized as follows:

� For given operation conditions, larger area ratio gives higher ERand lower PRR. And the relationship between ER and AR can bedescribed by linear functions, while PRR is represented byquadratic functions in term of AR.

� For all the area ratios studied, the entrainment ratio ER increaseslinearly with the secondary pressure pS, but decreases monot-onously with the primary pressure pP in a way of y ¼ axb.

� The pressure recovery ratio PRR decreases quadratically withthe secondary pressure pS. When it comes to the increasingprimary pressure pP, the pressure recovery ratio has a piecewise-linear character, that is, the PRR first keeps constant at 0 andthen increases linearly when pP becomes higher than a certainvalue.

� The critical area ratio is considered as an indicator of pressurerecovery status. It increases with the primary pressure and de-creases with the secondary pressure. For a given pP or pS, an arearatio smaller than the critical one is required to make sure thatthe system operates at the energy saving mode.

� The critical AR can be predicted by a linear fitting function ofprimary pressure pP or a power function in terms of the sec-ondary pressure pS.

Acknowledgements

This work was supported by the A*STAR-MND Green BuildingJoint Grant of Singapore (1121760027), China Postdoctoral ScienceFoundation funded project (2013M532041) and the FundamentalResearch Funds for the Central Universities.

Nomenclatures

d1 inlet diameter of primary nozzle, mmd2 exit diameter of primary nozzle, mmdN1S1 the relative spindle position, mmdt throat diameter of primary nozzle, mm

C. Li et al. / Applied Thermal Engineering 68 (2014) 125e132132

Dm diameter of constant-area mixing chamber, mmLd length of diffuser, mmLm length of constant-area mixing chamber, mmmP mass flow rate of primary flow, kg s�1

mS mass flow rate of secondary flow, kg s�1

pP primary flow pressure of ejector, kPapS secondary flow pressure of ejector, kPapb outlet pressure of ejector, kPaqd diverging angle of diffuser, �

qm converging angle of constant-pressure mixing chamber, �

AbbreviationAR area ratioER entrainment ratio, mS/mPNXP primary nozzle position, mmPRR pressure recovery ratio, (pb � pS)/pSMERS multi-evaporator refrigeration system

IndexN1 inlet cross section of primary nozzleN2 outlet cross section of primary nozzleS1 cross section of spindleS2 spindle tip

References

[1] S. He, Y. Li, R.Z. Wang, Progress of mathematical modeling on ejectors, Renew.Sustain. Energy Rev. 13 (2009) 1760e1780.

[2] M. Sokolov, D. Hershgal, Enhanced ejector refrigeration cycles powered bylow grade heat. Part 3. Experimental results, Int. J. Refrig. 14 (1991) 24e31.

[3] B.J. Huang, J.H. Wu, H.Y. Hsu, J.H. Wang, Development of hybrid solar-assistedcooling/heating system, Energy Convers. Manag. 51 (2010) 1643e1650.

[4] B.M. Diaconu, S. Varga, A.C. Oliveira, Numerical simulation of a solar-assistedejector air conditioning system with cold storage, Energy 36 (2011) 1280e1291.

[5] D.W. Sun, I.W. Eames, Recent developments in the design theories and ap-plications of ejectors e a review, J. Inst. Energy 68 (1995) 65e79.

[6] C. Li, Y.Z. Li, Investigation of entrainment behavior and characteristics of gaseliquid ejectors based on CFD simulation, Chem. Eng. Sci. 66 (2011) 405e416.

[7] R.L. Yadav, A.W. Patwardhan, Design aspects of ejectors: effects of suctionchamber geometry, Chem. Eng. Sci. 63 (2008) 3886e3897.

[8] W.B. Gosney, Principle of Refrigeration, Cambridge University Press, Cam-bridge, 1982.

[9] W.F. Stoecker, Steam-jet refrigeration, in: Refrigeration and Air Conditioning,McGraw-Hill, New York, 1958, pp. 194e205.

[10] Z. Aidoun, M. Ouzzane, The effect of operating conditions on the performanceof a supersonic ejector for refrigeration, Int. J. Refrig. 27 (2004) 974e984.

[11] S. Aphornratana, I.W. Eames, A small capacity steam-ejector refrigerator:experimental investigation of a system using ejector with movable primarynozzle, Int. J. Refrig. Revue Int. Du Froid 20 (1997) 352e358.

[12] Y. Bartosiewicz, Z. Aidoun, Y. Mercadier, Numerical assessment of ejectoroperation for refrigeration applications based on CFD, Appl. Therm. Eng. 26(2006) 604e612.

[13] K. Chunnanond, Ejectors: applications in refrigeration technology, Renew.Sustain. Energy Rev. 8 (2004) 129e155.

[14] I.W. Eames, S. Aphornratana, H. Haider, A theoretical and experimental-studyof a small-scale steam jet refrigerator, Int. J. Refrig. Revue Int. Du Froid 18(1995) 378e386.

[15] B.J. Huang, C.B. Jiang, F.L. Hu, Ejector performance-characteristics and designanalysis of jet refrigeration system, J. Eng. Gas Turbines Power Trans. ASME107 (1985) 792e802.

[16] J.T. Munday, D.F. Bagster, New ejector theory applied to steam jet refrigera-tion, Ind. Eng. Chem. Process Des. Dev. 16 (1977) 442e449.

[17] S.B. Riffat, S.A. Omer, CFD modelling and experimental investigation of anejector refrigeration system using methanol as the working fluid, Int. J. EnergyRes. 25 (2001) 115e128.

[18] E. Rusly, L. Aye, W. Charters, A. Ooi, CFD analysis of ejector in a combinedejector cooling system, Int. J. Refrig. 28 (2005) 1092e1101.

[19] D.W. Sun, Variable geometry ejectors and their applications in ejectorrefrigeration systems, Energy 21 (1996) 919e929.

[20] R. Yapici, H. Ersoy, A. Aktoprakoglu, H. Halkaci, O. Yigit, Experimental deter-mination of the optimum performance of ejector refrigeration systemdepending on ejector area ratio, Int. J. Refrig. 31 (2008) 1183e1189.

[21] D.W. Sun, Comparative study of the performance of an ejector refrigerationcycle operating with various refrigerants, Energy Convers. Manag. 40 (1999)873e884.

[22] J. Mizrahi, M. Solomiansky, T. Zisner, W. Resnick, Ejector refrigeration fromlow temperature energy sources, Bull. Res. Counc. Israel 6C (1957) 1e8.

[23] A.E. Kroll, The design of jet pumps, Chem. Eng. Prog. 1 (1947).[24] P. Havelka, V. Linek, J. Sinkule, J. Zahradnik, M. Fialova, Effect of the ejector

configuration on the gas suction rate and gas hold up in ejector loop reactors,Chem. Eng. Sci. 52 (1997) 1701e1713.

[25] P.H.M.R. Cramers, A.A.C.M. Beenackers, Influence of the ejector configuration,scale and the gas density on the mass transfer characteristics of gaseliquidejectors, Chem. Eng. J. 82 (2001) 131e141.

[26] K. Cizungu, M. Groll, Z. Ling, Modelling and optimization of two-phase ejec-tors for cooling systems, Appl. Therm. Eng. 25 (2005) 1979e1994.

[27] S. Watanawanavet, Optimization of a High-efficiency Jet Ejector by Compu-tational Fluid Dynamics Software (Master of Science), Texas A&M University,2005, p. 238.

[28] P. Chaiwongsa, S. Wongwises, Effect of throat diameters of the ejector on theperformance of the refrigeration cycle using a two-phase ejector as anexpansion device, Int. J. Refrig. 30 (2007) 601e608.

[29] S. Balamurugan, V. Gaikar, A. Patwardhan, Effect of ejector configuration onhydrodynamic characteristics of gaseliquid ejectors, Chem. Eng. Sci. 63 (2008)721e731.

[30] N.I.I. Hewedy, M.H. Hamed, F.S. Abou-Taleb, T.A. Ghonim, Optimal perfor-mance and geometry of supersonic ejector, J. Fluids Eng. 130 (2008) 041204.

[31] S. Varga, A.C. Oliveira, B. Diaconu, Influence of geometrical factors on steamejector performance e a numerical assessment, Int. J. Refrig. 32 (2009) 1694e1701.

[32] Y. Zhu, W. Cai, C. Wen, Y. Li, Numerical investigation of geometry parametersfor design of high performance ejectors, Appl. Therm. Eng. 29 (2009) 898e905.

[33] C. Li, Y. Li, L. Wang, Configuration dependence and optimization of theentrainment performance for gasegas and gaseliquid ejectors, Appl. Therm.Eng. 48 (2012) 237e248.

[34] L. Kairouani, M. Elakhdar, E. Nehdi, N. Bouaziz, Use of ejectors in a multi-evaporator refrigeration system for performance enhancement, Int. J. Refrig.32 (2009) 1173e1185.

[35] C. Lucas, J. Koehler, Experimental investigation of the COP improvement of arefrigeration cycle by use of an ejector, Int. J. Refrig. 35 (2012) 1595e1603.

[36] S. Disawas, S. Wongwises, Experimental investigation on the performance ofthe refrigeration cycle using a two-phase ejector as an expansion device, Int. J.Refrig. 27 (2004) 587e594.

[37] Y. Liu, T. Xin, L. Cao, C. Wan, M. Zhang, Compressioneinjection hybridrefrigeration cycles in household refrigerators, Appl. Therm. Eng. 30 (2010)2442e2447.

[38] J.-Q. Deng, P.-X. Jiang, T. Lu, W. Lu, Particular characteristics of transcriticalCO2 refrigeration cycle with an ejector, Appl. Therm. Eng. 27 (2007) 381e388.

[39] J. Sarkar, Optimization of ejector-expansion transcritical CO2 heat pump cycle,Energy 33 (2008) 1399e1406.

[40] M. Yari, Performance analysis and optimization of a new two-stage ejector-expansion transcritical CO2 refrigeration cycle, Int. J. Therm. Sci. 48 (2009)1997e2005.

[41] M. Nakagawa, A.R. Marasigan, T. Matsukawa, A. Kurashina, Experimentalinvestigation on the effect of mixing length on the performance of two-phaseejector for CO2 refrigeration cycle with and without heat exchanger, Int. J.Refrig. 34 (2011) 1604e1613.

[42] M. Yari, S.M.S. Mahmoudi, Thermodynamic analysis and optimization of novelejector-expansion TRCC (transcritical CO2) cascade refrigeration cycles (noveltranscritical CO2 cycle), Energy (2011).

[43] W. Chen, X. Zhou, S. Deng, Development of control method and dynamicmodel for multi-evaporator air conditioners (MEAC), Energy Convers. Manag.46 (2005) 451e465.

[44] D. Datta, S.A. Tassou, Artificial neural network based electrical load predictionfor food retail stores, Appl. Therm. Eng. 18 (1998) 1121e1128.

[45] S.A. Tassou, J.S. Lewis, Y.T. Ge, H.I. Chaer, A review of emerging technologiesfor food refrigeration applications, Appl. Therm. Eng. 30 (2010) 263e276.

[46] P.G. Jolly, C.P. Tso, Y.W. Wong, S.M. Ng, Simulation and measurement on thefull-load performance of a refrigeration system in a shipping container, Int. J.Refrig. 23 (2000) 112e126.

[47] M. Sokolov, D. Hershgal, Enhanced ejector refrigeration cycles powered bylow grade heat. Part 1. Systems characterization, Int. J. Refrig. 13 (1990) 351e356.

[48] M. Sokolov, D. Hershgal, Enhanced ejector refrigeration cycles powered bylow grade heat. Part 2. Design procedures, Int. J. Refrig. 13 (1990) 357e363.

[49] M. Sokolov, D. Hershgal, Enhanced ejector refrigeration cycles powered bylow grade heat. Part 3. Experimental results, Int. J. Refrig. 14 (1991) 24e31.

[50] C. Lin, W. Cai, Y. Li, J. Yan, Y. Hu, The characteristics of pressure recovery in anadjustable ejector multi-evaporator refrigeration system, Energy 46 (2012)148e155.

[51] S. Varga, A.C. Oliveira, X. Ma, S.A. Omer, W. Zhang, S.B. Riffat, Experimentaland numerical analysis of a variable area ratio steam ejector, Int. J. Refrig. 34(2011) 1668e1675.

[52] J.M. Abdulateef, K. Sopian, M.A. Alghoul, M.Y. Sulaiman, Review on solar-driven ejector refrigeration technologies, Renew. Sustain. Energy Rev. 13(2009) 1338e1349.