蔬菜土豆清洗过程中的特征研究外文文献翻译、中英文翻译
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Biosystems Engineering (2005) 91(4), 441453doi:10.1016/j.biosystemseng.2005.05.009PMPower and MachineryCharacterising the Washing Processes of Vegetables and PotatoesE. Mulugeta; M. GeyerInstitute of Agricultural Engineering Bornim e.V., Max-Eyth-Alle 100, D-14469 Potsdam Bornim, Germany; e-mail of corresponding author:geyeratb-potsdam.de(Received 20 October 2003; accepted in revised form 12 May 2005; published online 7 July 2005)Experiments were conducted to investigate the interdependence between the different influencing factors onthe spray washing process under low-pressure conditions. The washing effect as a function of standoffdistance, spray pressure, and nozzle diameter is derived by considering the spray structure and the spraywashing mechanism. Four measuring systems were used to determine the relevant spray structure parametersof different nozzles and their washing effects. Single droplets formed in the spray were submitted to anenergetic assessment. The nozzles were evaluated with regard to their area washing performance Z as a ratio ofthe effective erosion area to spray area and effectiveness Es,Ve.The agricultural nozzle for plant protection with a flow rate Q lower than 3lmin?1at pressure p of 3bar anda spray angle ah0of 901 was found to be ineffective considering the determined area washing performance(area ratio Z 0?10) as its spray parameters proved to be inappropriate concerning the droplet size spectrum,volume intensity per unit area, and mean impulse distribution. Conversely, the flat-fan nozzle with a flow rateQ of 6?2lmin?1at pressure p of 3bar and a spray angle ah0of 901 produces a spray with a satisfactory areawashing performance (area ratio Z 0?91), but a smaller area washing effectiveness Es,Vefor the sprayconditions used in this experiment.r 2005 Silsoe Research Institute. All rights reservedPublished by Elsevier Ltd1. IntroductionSeveral washing machines with different principles areused for vegetable washing with varying sensitivities(Geyer, 1999). Nozzle washing machines are predomi-nantly used for leafy and bunched vegetables, such aslettuce, leek, bunched carrots and radishes. They cleanthe vegetables hydraulically by means of spray nozzles.Vegetable washing with nozzles must be performedcarefully in a short time without damaging the tissue,using as little fresh water and energy as possible.Although spray cleaning has been used in the dairyand other food industries for many years, there has beenvery little research effort directed towards an analysis ofthe washing performance (Scott et al., 1981). Investiga-tions of the fundamental aspects of spray performancefor cleaning have mostly been focused on chemicalengineering and high-pressure topics, and little of theinformation reported can be applied to vegetablecleaning situations (Mo nicke, 1971; Sandler, 1976; Scottet al., 1981; Schikorr & Louis, 1982; Spillman, 1984;Krautter & Vetter, 1990; Wu & Kim, 1995; Kru ger,1998; Ludewig, 1998; Meng et al., 1998; Louis et al.,1999; Liu, 2000; Sivakumar & Tropea, 2002).Experiments with spray jets for low-pressure cleaninghave been carried out by Kaye et al. (1995), Rose (1997)and Sawamura and Kanazawa (2001). Rose (1997) hasexamined the spray cleaning effect of selected nozzles onvegetable surfaces. However, no conclusions on theapplication and the optimisation of other nozzles can bederived from the paper.Figure 1 summarises the parameters influencing therespective values of the spray structure and the areawashing performance during the vegetable cleaningprocess. The washing effect of the nozzles is primarilybased on the total applied impulse of the spray, which iscomposed of the amount of water used and the waterpressure (Geyer, 1999; Momber, 1993). The main inputsfor the variation of the spray parameters and thusthe spray effects are the operating and the nozzleARTICLE IN PRESS1537-5110/$30.00441r 2005 Silsoe Research Institute. All rights reservedPublished by Elsevier Ltdparameters. The output and thus the direct results of thespray structure are the size and velocity spectra ofdroplets as well as the spray geometry. These physicalspray characteristics are relevant for the emergingdroplet impulses as well as for the spray impact pressureduring washing. The following application-technicaldemands on the spray structure should be fulfilledduring washing (after: Kru ger, 1998; Freudig et al.,2003):(1) optimal microstructure, meaning that the transfor-mation of spray into droplets with optimal size andvelocity spectra leads to an optimal mean dropletimpulse; and(2) optimal macrostructure, meaning that the washingwater is uniformly distributed on the vegetablesurface with an optimal circular spray width andspray effectiveness.From the numerous experimental investigations con-cerning spray coating removal (Adler, 1979; Brunton &Rochester, 1979; Hammitt et al., 1974; Lesser & Field,1983; Obara et al., 1995), the perpendicular componentof droplet impulse generates an extremely high impactpressure as a result of the water-hammer effect. Thedirect deformation and the propagation of mechanicalwaves caused by the impact pressure are responsible forcrack initiation in the erosion process. Lateral outflowspray and hydraulic penetration extend the existingcracks which lead to erosion by the separation ofcoating material from the substrate. The intensity andduration of surface loading is based on the kinematics ofthe droplet impact, and is greater if the impactingsurface is in contact with the compressed zone of theimpacted droplet.The relation between the different factors influencingthe spray structure and the spray washing effect on theimpact surface were analysed in order to show theARTICLE IN PRESSNotationAetotal eroded area, cm2Ae,effeffective eroded area, cm2AI,Tekscanspray impact area on the sensor surface(Tekscan Inc.), cm2Asspray area, cm2apmean area of the detection volume,mm2Cm,specmean specific volume intensity of sprayper unit area, mm3s?1mm?2dddroplet diameter, mm?1d0nozzle diameter, mmd32Sauter mean diameter, mm?1dpendepth of penetration, mmdpen,eff.depth of effective penetration, mmEs,Vearea washing effectiveness, mm3Nm?1Fmax,Tekscanmaximal impact force recorded using asensor (Tekscan Inc.), Nhstandoff distance, cmIdmean impulse of droplet-size group,mgm s?1Id,specspecific impulse rate of effective dro-plets per unit area, gms?2mm?2mdroplet mass, mgPmax,Tekscanmaximal impact pressure using a sensor(Tekscan Inc.), kPapspray pressure, barq3(x)volume density distribution, mm?1Qflow rate, l min?1ndroplet velocity, ms?1yimpact angle of droplet, degZarea ratioah xspray angle at the standoff distance h ofx cm, degOperating parameters spray pressurenozzle distancestandoff distancenozzle numbernozzle arrangementimpact loading time traverse speedNozzle parametersnozzle geometrynozzle typeSpray parametersdroplet sizedroplet velocitynumber of dropletsspray geometrydroplet impulsespray impact pressureProduct characteristicsshape, surface propertiessusceptibilityDirt parametersadhesive forcesof dirt compositionof the adhering soilSprayangleaStand off distanceFig. 1. Influencing factors on the washing process with spraynozzles (after: Geyer & Oberbarnscheidt, 2000)E. MULUGETA; M. GEYER442possibilities of optimising the nozzles and their operat-ing conditions as well as the washing process. Astandard testing method was developed to express thematerial removal as a function of a set of sprayconditions (nozzle and operating parameters) and othersystem parameters based on the spray structure. In thispaper, the relations are described exemplarily for twowashing nozzles.2. Materials and methodsNozzle selection was primarily based on the flow raterepresenting the broad range of nozzle diameters d0asfound in the present nozzle washing machines. In thefollowing article, the results are described for theinvestigation of sprays produced by two 901 flat-jetnozzles out of 11 examined spray nozzles: industrialnozzle 632?726 (I-90) and agricultural nozzle LU 90-04(A-90), both manufactured by Lechler GmbH, Metzin-gen, Germany (Table 1). The nozzle angle ah0of 90o,determined in contrast to the spray angle ahximmedi-ately after spray discharge through the nozzle exit, isrecommended by the nozzle manufacturer.The water pressure used in the experiments was 3, 5,and 8bar. The investigations were conducted for aperpendicular arrangement of the nozzles at fixedstandoff distances h of 10 and 20cm and withouttravelling. The spray structures and their impact effectswere examined. In this context, the mean impulsepotentialIdofindividualdroplet-sizegroupsinmgms?1impacting on a surface is described as:Id m n sin y(1)in which m is the droplet mass in mg, n the verticalvelocity of the droplets in ms?1and y their angle ofimpact on the surface.Thespecificimpulserateofeffectivedroplets(dd4300mm) per unit area Id;specin gms?2mm?2wassimulated by calculating the specific mass flow rate ofspray per unit area and relating that to the correspond-ing measurements of droplet size and velocity spectra.An energetic analysis of the droplets formed in the spraywas obtained. A standardised procedure for the evalua-tion of the spray parameters of the nozzles with regardto their area washing performance and effectiveness wasdeveloped; this was accomplished by records of thedroplet impulse distributions in the spray and theerosion effect along the radial spray dispersion. Withregard to this, the following parameters were deter-mined.(1) Fluid volume distribution and spray geometryThe spatial distribution of the spray was measuredthrough horizontal swivelling of a row of test tubes(internal diameter of 16mm) arranged next to eachother over a time period of 7s (Gebhardt, 1958; Scott etal., 1981). In this way, the spray area Asand theintensityofthespraythroughouttheareaweredetermined. Records made with less than 5mm of thefilling height in the test tubes were ignored. Theinformation was presented in the form of a sheetdiagram. The entire spray area was divided into smallareas of 256mm2, showing the corresponding specificARTICLE IN PRESSTable 1Values of spray geometries obtained at two distances h from the nozzle exit for two nozzle sizes and varied spray pressure pNozzleSpray pressureFlow rateMean values of the spray geometries derived based on measured data(p), bar(Q), l min?1Width, cmAngle, degDepth, cmArea (As), cm2Standoff distance h 10cmA-9031?522?496?44?871?952?125?6104?01?641?082?627?2107?31?643?6I-9037?219?287?61?630?459?921?694?41?633?4812?622?496?41?635?3Standoff distance h 20cmA-9031?541?692?28?0314?952?148?0100?34?8216?682?649?6102?24?8223?8I-9037?235?483?03?294?759?937?686?43?2101?3812?639?288?83?2109?7WASHING PROCESSES OF VEGETABLES AND POTATOES443volume intensity of spray per unit area Cm;specinmm3s?1mm?2. It was computed by assuming thedistribution of the obtained spray volume to be constantover the catchment area of 256mm2.(2) Size and velocity spectra of the dropletsSimultaneous measurements of droplet sizes andvelocities in the spray, which were meant as a basis foranenergeticviewingofindividualdroplets,wereaccomplished by means of a phase-double-particleanalyser (PDA) (Tropea, 1999). These investigationswere carried out in the laboratory of the nozzlemanufacturer Lechler Ltd., Metzingen, Germany.The number of droplets within given upper and lowerlimits of size for contiguous intervals was determined.The raw data were processed using area and volumeweighing factors to compute the droplet-size/numberand velocity distribution for the entire spray, assuming amean size and velocity for each chosen droplet-sizegroup. From the computed distribution the Sauter meandiameter SMD denoted by d32(Damaschke, 1999) wascalculated. The spatial variability of these spray para-meters was considered with regard to the determinationof several spectra within the spray area. Local measure-ments of droplet size and velocity spectra at a designatedinitial cross-section plane of the sprays were achieved byradial moving of the detection volume (mean areaap 0?33mm2) of about 35 or 40mm in each case.Taking into consideration the tolerance range of thepump and the measurement inaccuracy, the variationrange between droplet size measurements under thesame spray conditions ranged from 7 5 up to 10%(Lipthal, 2002).(3) Distribution of the maximum spray impact pressureWith the help of a matrix-based tactile sensor (Type5051, Tekscan Inc., Boston, USA) (Herold et al., 2001)the impact pressure distributions were measured. Thesprays were, thereby, impacted onto the surface of thesensor for an exposure time of 120s (Mulugeta et al.,2002). The entire impact area of the spray was scannedin the respective standoff distance by shifting the sensorin steps of 25mm.The measured values of the impactforce Fmax,Tekscanless than 0?002N have been ignored, asthe operating mode of the sensor makes it impossible toverify the conditions of origin for these smaller loadvalues. The average of two measurements taken for eachset of values of standoff distance, nozzle diameter andspray pressure was used as the representative sprayparameter. To protect the tactile sensor, it was coveredby thin-film polyethylene.(4) Distribution of the erosion depth on a standardisedsand-binder mixture plateAprocedurehasbeendevelopedforobtainingstandardised sand-binder mixture plates following thespray analysis in the high-pressure area (Scott et al.,1981; Krautter & Vetter, 1990; Obara et al., 1995; Menget al., 1998). The sand-binder mixture plates (300mm by250mm by 20mm) were used as a target to observe thewashing mechanisms and process due to the erosionpattern on the plates (see Fig. 6). They were placedperpendicularly to the central axis of the spray nozzles.The exposure time for each sample lay with 300s. Theresulting eroded areas and depths on the plate surfaceswere recorded by a laser scanner. Five measurements foreach set of parameters were taken and the average ofthem was used as the representative value. Recordsmade with less than 0?3mm of scan depth values wereignored. This mean threshold value was obtained fromscan measurements of depth distribution of unloadedplates.The data obtained from the experiments were enteredinto an evaluation program developed at the Institute ofAgricultural Engineering Bornim (Mulugeta et al.,2002). The program permits a coupled analysis of thedifferent spatially related data. Furthermore, one addi-tional descriptive term, the area washing effectivenessEs,Vein mm3Nm?1, has been proposed which rates anozzle in terms of volume removed in mm3on the platesat a defined exposure time per unit of energy in Nmexpended by that nozzle.3. Results and discussionThe following spray characteristics caused by thevariation of nozzle parameter and operating conditionshave been determined:(1) the droplet size distribution;(2) the distribution of volume intensity per unit area;and(3) the mean impulse distribution in the spray which isresponsible for further spray dispersion and thus thespray geometry, and which affects the erosionprocesses on the impact surface.3.1. Effects of spray pressure, nozzle diameter andstandoff distance on spray parameters per unit area andtime3.1.1. Spray structure variables at a standoff distance of10cmCompared to the A-90 the flow rate of nozzle I-90 wasapproximately five-fold higher. The A-90 generatedsprays with a larger width whereas a smaller radialspray dispersion characterised the I-90 (Table 1). Forexample, under a pressure of 3bar, a spray produced bythe A-90 was dispersed over an area with a width ofARTICLE IN PRESSE. MULUGETA; M. GEYER444around8cmlargerthanthesprayoftheI-90.Consequently,thesprayareaAsoftheA-90(?71?9cm2) was larger than the area of the I-90(?30?4cm2).For the set of nozzle diameters and spray pressuresused in this experiment, a mean droplet diameter ddwithin the range of 20900mm and a mean velocitywithin the range of 535ms?1were recorded. Thevolume density distributions q3x in mm?1as a functionof the mean droplet diameters dd; measured at a spraypressure p of 3bar in a standoff distance of 10cm, areshown in Fig. 2. The droplet spectrum of the A-90provided definitely more fine droplets (o300mm) withthe volumetric content of 49% than the one of the I-90with 15% of the total spray volume.There were remarkable differences in Sauter meandiameter SMD between the nozzles (Table 2). This wasdue to the fact that the sprays generated by the I-90contained more large droplets (4700mm), resulting in aclear increase of the SMD. On the other hand, anincrease of the operating pressure only slightly affectedthe formed droplet size spectra. Thus, the droplet sizespectra generally showed a tendency to move towardsthe range of fine droplets and the SMD values slightlydecreased.The mean velocity distribution of droplets in sprayscorresponded to earlier measurements (Ludewig, 1998),and to normal distribution function. Comparing thevalues between the two nozzles at the same operatingpressure, the mean droplet velocities in the sprays of theI-90 increased by about 1433%.The curves of Idcalculated from Eqn (1) versus meandroplet diameter dd, with nozzle diameter d0as aparameter, are presented for the spray pressure p of 3bar in Fig. 3. Comparing these curves, there is a cleardifference in mean impulse between the same droplet-size groups.3.1.2. Energetic view of the sprays at a standoff distanceof 10cmResults of the spray analysis from measurements ofvolume distribution, droplet diameter and velocityspectra are shown in Table 3. Based on the measure-ments of spray volume distribution, a mass-weighedmean specific volume intensity of spray per unit areaCm,specof1?8mm3s?1mm?2hasbeendeterminedfor the entire spray area of the A-90, operating at apressure of 3 bar. Under the same spray situation, thespray area of the I-90 had approximately 18-fold(?32?4mm3s?1mm?2) the Cm,specof the A-90. AsARTICLE IN PRESSTable 2Sauter mean diameters and mean velocities of droplets for two nozzle sizes showing the effect of spray pressure p and standoffdistance hStandoff distance (h), cmSauter diameter (d32),mmDroplet velocity (n), ms?2NozzleSpray pressure (p), barSpray pressure (p), bar35835810A-9027725625113?619?624?3I-9041840739118?123?127?720A-9026623923011?216?216?4I-9037536131917?722?425?8(a)(b)00.0020.00400.0020.0040300600900Mean droplet diameter dd, m0300600900Mean droplet diameter dd, mVolume density distributionq3(x), m1Volume density distributionq3(x), m1Fig. 2. Volume density distribution q3(x) as a function of meandroplet diameter ddfor two nozzle sizes, A-90 (-) and I-90(), at the two standoff distances h: (a) 10cm; (b) 20cm;spray pressure p 3 barWASHING PROCESSES OF VEGETABLES AND POTATOES445shown in Fig. 2, the ratio of droplets larger than 0?3mmto total spray volume significantly increased as thenozzle diameter increased.The specific impulse rate of effective droplets per unitarea Id,specrevealed marked differences between thesprays formed by the different nozzle diameters. Asshown in Fig. 4, at a spray pressure of 3bar, the Id,specfoundinasprayreleasedfromtheI-90is0?48gms?2mm?2. Under the same operating condi-tions, the Id,specfor the spray generated by the A-90 isdetermined to be 0?02gms?2mm?2, about 96% lowerthan the above given value.3.1.3. Effects of a standoff distance elevated to 20cm onspray parametersA number of attempts by the standoff distanceelevated to 20cm are made to find the optimal/criticalstandoff distance for each set of spray pressure andnozzlediameterbyconsideringtheformedspraystructure. From the data presented in Table 1 it can beconcluded that the spray areas for a standoff distance of20cm are about three to four times higher than those ata distance of 10cm.The obtained droplet size spectra generally showed atendency to move towards the fine and middle-sizeARTICLE IN PRESSTable 3Mean values of the spray structure for two nozzle sizes showing the effect of standoff distance h at a spray pressure p of 3 barNozzleStandoff distance(h), cmMean specific volume intensityof spray per unit area(Cm,spec),mm3s?1mm?2Specific impulse rate ofeffective droplets per unit area(Id,spec), gms?2mm?2A-90101?80?02200?40?005I-901032?40?482010?00?160360300600900036Mean impulse Id, mg m s1Mean impulse Id, mg m s10300600900(a)(b)Mean droplet diameter dd, mMean droplet diameter dd, mFig. 3. The mean impulse Idas a function of mean dropletdiameter ddfor two nozzle sizes, A-90 (-) and I-90 (),depending on the standoff distances h:(a) 10cm; (b) 20cm;spray pressure p 3 bar030060090000.40.800.40.80300600900(b)Mean droplet diameter dd, mMean droplet diameter dd, mSpecific impulse rate Id, spec, g m s2 mm2(a)Specific impulse rate Id, spec, g m s2 mm2Fig. 4. The specific impulse rate of effective droplets Id,specas afunction of mean droplet diameter ddfor two nozzle sizes, A-90(-) and I-90 (), depending on the standoff distances:(a)10cm; (b) 20cm; spray pressure p 3 barE. MULUGETA; M. GEYER446droplet classes (see Fig. 2). The number of large dropletsdecreased. While using the I-90, an increasing spraypressure clearly led to smaller droplets and, thus smallerSauter diameters were obtained (see Table 2).Comparingthe droplet velocities of droplet-sizegroups between the two standoff distances, reducedmean velocities within the range of 27% for the I-90and 1733% for the A-90 were determined at a standoffdistance of 20cm, depending on the applied spraypressure p.The difference in impulses when comparing the twostandoff distances as a function of the observed dropletsize seems to be insignificant, with some exceptions(Fig. 3). However, the relations differ depending on theset of applied spray pressure p and nozzle diameter d0:Especially at a spray pressure of 5 and 8 bar, a clear lossof mean impulse Idof droplet-size groups was notice-able for the A-90 at a standoff distance of 20cm. Incontrast to this, the application of the I-90 broughthigher mean impulses of droplets larger than 0?5mmdiameter at the increased spray pressure p of 5 and 8 barthan those at a standoff distance of 10cm (no figuregiven).Enhanced spray areas due to an increased standoffdistance (20cm) caused a reduced level of mean specificvolume intensity of spray per unit area Cm,spec(Table 3).For the A-90, operating at a 3 bar pressure, the Cm,specamounted to 0?4mm3s?1mm?2, corresponding to 22%of the Cm,specat a standoff distance of 10cm. The Id,specwas calculated to be 0?005gms?2mm?2, about 76%lower than the value for the standoff distance of 10cm.Under the same operating conditions, the mean specificvolume intensity found in a mm2spray area of the I-90was 10mm3s?1. The Id,specwas determined to be0?16gs?2mm?2, about 67% lower than the value for10cm of standoff distance.3.2. Impact effect assessment for a range of definedspray characteristicsOf paramount importance for the effect of sprayimpact is the magnitude and time dependence of themaximum pressure-loading generated within the impactsurface (Fig. 5). Sand-binder mixture plates have beenimpacted with sprays to derive a quantitative relation ofthe obtained parameters of spray structure to purelyphysical removal processes (Figs 6 and 7).3.2.1. Spray variables and their effects on erosionat a standoff distance of 10cmThe physical differences in the eroding surface can beseen in Fig. 6. In the case of the I-90, the increasedspecific impulse rate of effective droplets per unit sprayarea Id,specled to a marked increase in the mean depth ofmaterial removal due to the higher spray concentrationand intensity of effective droplet impacts on the surface(Table 4). The eroded surface had a mean depth of1?8mm in the case of the I-90 and 0?9mm in the case ofthe A-90, operating at a pressure of 3 bar. The arearemoved by the I-90 (42?2cm2) is lower than that of theA-90 (52?1cm2). The sprays generated by the A-90eroded a larger width with a shallower depth because thespray was less concentrated with low intensity ofeffective droplet impacts. Taking into considerationthe impact pressure measurements, this spray caused adecreased level of maximum impact pressure Pmax,Tekscancompared to the more concentrated spray of theI-90. For example, the maximal mean impact pressureunder the sprays at an operating spray pressure of3 bar varied from 10?7kPa for the A-90 (impactarea AI,Tekscan 69?5cm2) to 24?3kPa for the I-90(AI,Tekscan 104?6cm2).On the other hand, the increase of the operatingpressure with both nozzles resulted in higher values ofthe maximum mean impact pressure and the enlargeddepth of penetration dpendue to the enhanced impulserate and impact intensity of effective droplets within asmall area.3.2.2. The effect of an increased standoff distanceof 20cm on spray variables and their erosionWhen supplying the I-90 at a pressure of 3 bar, theimpact area AI,Tekscanand the total area eroded Aeincreased with the standoff distance being increased to20cm.During the exposure time of 300s the sampleplates exhibited an erosion trace of 55?2cm2for the I-90.The depth of penetration decreased with the increasingstandoff distance with a mean dpenof 1?4mm (Table 4).The maximum mean impact pressure increased, how-ever, inconsiderably.For the A-90, the total area eroded decreased be-cause of the high content of fine droplets withoutimpact effects near the regime of spray boundary.For example, for the A-90 an erosion pattern of16?3cm2was recorded at a pressure of 3 bar, about68?7% lower than the area obtained at a standoffdistance of 10cm. The area with only slight coatingremoval dominated.For the I-90, it is obvious that with elevating standoffdistance the spray caused only a moderate decrease ofremoval depth on the plates. Thus, the spatial andtemporal decay of the spray parameters due to theelevation of h to 20cm are lower for the larger nozzle I-90. However, the mean value of the maximum impactpressure increased with the rising standoff distance at aspray pressure p of 3 bar.ARTICLE IN PRESSWASHING PROCESSES OF VEGETABLES AND POTATOES4473.2.3. Evaluation of the washing situationsAlthough the sample surface was mounted on asample holder perpendicularly to the central axis of thenozzle, the greater part of the surface was exposed toerosion conditions at a range of spray impact anglesconsiderably less than 901. For impact angles less than901, the direct impact component was reduced. Thecalculated vertical components of mean droplet impulseIdof droplet-size groups indicated a clear decrease withan increasing radial distance from the central axis of thespray for both standoff distances. The pressure on thespray impact surfaces was strongest near the spray coreand decreased to a minimum at the spray boundary.Accordingly, the depth of penetration dpenreachedhigher values near the spray axis and decreased withradial distance from the spray axis.For realistic nozzle/spray evaluation on the basis ofarea washing effectiveness, the eroded areas on theplates must be differentiated, taking into considerationthe obtained results of the penetration depth. To achievea satisfactory removal of coating material adhering to aparticular product surface, a minimum impact pressureARTICLE IN PRESS0.100Maximal impact forceFmax, Tekscan, N015010010050050100150(a)0.100Maximal impact forceFmax, Tekscan, N015050050100150(b)Distance from spray axis, mmFig. 5. Spatial distribution of maximal impact force Fmax,Tekscanat a spray pressure p of 3 bar and a standoff distance h of 10cm fortwo nozzle sizes: (a) A-90; (b) I-90E. MULUGETA; M. GEYER448has to be applied. It has been observed that a meandepth of 2mm on the plates corresponds to the effect ofthis minimum impact pressure. The resulting area isdefined as the effective erosion area Ae.Based on the assumption mentioned above, the areawashing performance and effectiveness, as a function ofnozzle diameter and standoff distance under a spraypressurepof3bar,arepresentedinTable5.The comparisons between effective erosion area Aeand the spray area As, expressed through an arearatio Z (Ae,eff.: As), have shown that the width/area ofeffective material removal is not as wide as the spraywidth/area.The different sizes of the effective erosion area and theerosion depth are related to the intensity of effectivedroplet impacts per unit area, i.e. the impulse/energydensity distribution over a defined impact loading time.With a pressure of 3 bar at a standoff distance of 10cm,the I-90 effectively eroded a larger area Ae,eff.(27?7cm2)than the A-90 (7?2cm2) and eroded more deeply(dpen,eff. 2?8mmversus2?1mm)intotheplates.The effective erosion width accomplished by sprays ofthe nozzle I-90 was measured to be about 11?2cm,corresponding to a spray angle ah 10of 58?31. Themeasurement for A-90 exhibited a marked decrease inwidth (?7?4cm), and consequently in spray angle ah10(?40?41).Thus,theeffectiveerosionareaAe,eff.,created by the less concentrated spray of the A-90,corresponded only to 10% of the liquid spray area As.The area ratio Z was comparably higher (Z ?0?91)as the dense spray of the I-90 impacted on theplates. However, the eroded volume per unit of sprayenergy expended Es,Velay slightly above the valuesdetermined for the A-90 (for example: 0?72 versus0?68mm3Nm?1for h 10cm). Although an enhance-ment in the area ratio was obtained by the increasednozzle diameter, it seemed to be inefficient to materialremoval from the more concentrated spray at thisstandoff distance.At the standoff distance of 20cm, the A-90 didnegligible little effective eroding. The spray situation at apressure of 3 bar resulted in an effective eroded areaAe,eff. of 0?01cm2. This was due to the fact that thecondition of this nozzle is destroyed in its ability toproduce a coherent spray. It may be advantageous toefficient volume removal from the spray only at a lowerstandoff distance. On the other hand, for the I-90 thespray situation at a pressure of 3 bar resulted in a minordecrease of the effective eroded area (Ae,eff.?25?5cm2; Z?0?27) with a mean dpen,eff.of 2?6mm.So, the I-90generatedaspraywithaslightlysmallerEs,Ve(?0?62mm3Nm?1) than the spray in a 10cm standoffdistance. The trend of Es,Veand of increasing Aewithincreasing standoff distance indicates that the optimalstandoff distance with a better area washing effective-ness is located above 10cm.It has been shown that the volume of materialremoval rose with increasing spray pressure for the I-90 and for the A-90, with some restrictions (see Sections3.3.1 and 3.3.2). These observations indicate that theoptimal/critical standoff distance is related to thenozzle/operating parameters such as nozzle diameterand spray pressure. In general, the area washingeffectiveness, expressed as volume eroded per unit sprayenergy expended Es,Ve, decreased as spray pressure andnozzle diameter increased.3.2.4. Discussion of the resultsThe experimental results indicate that there is anoptimum standoff distance for maximal spray perfor-mance in material removal application depending on aset of nozzle diameter and spray pressure at givenmaterialproperties.However,theeffectivenessofremoval is greatly dependent on the spray processparameters including the spray geometry. Previousinvestigations of the spray structure distinguish threeflow regions of a spray as a function of the standoffdistance: the initial, main and final regions (Yanaida &Ohashi, 1980; Zou et al. 1985; Himmelreich, 1993).Previous experiments show that an optimal washingperformance exists only in the main spray region (Menget al., 1998; Wu & Kim, 1995). From the investigation ofYanaida and Ohashi (1980), the length of the initial flowregion hidown to the main flow region, relates to thenozzle diameter d0, as followshi 73.135d0.(2)ARTICLE IN PRESSFig. 6. Photographic sample images showing the variation inerosion trace formed on the surface after an exposure time of300s at a spray pressure p 3 bar using different nozzles: (a)A-90; (b) I-90WASHING PROCESSES OF VEGETABLES AND POTATOES449AccordingtothemeanvaluescalculatedfromEqn (2), the main spray flow region for the A-90begins at a distance of about 7cm from the nozzleexit and at a standoff distance of 17cm for theI-90.Correspondingly,abetterareawashingeffectiveness for the two different nozzle diameterswas observed at different distances from the nozzleexit.ARTICLE IN PRESS105015010050050100150(a)1050Depth of penetrationdpen, mmDepth of penetrationdpen, mm150100500Distance from spray axis, mm50100150(b)Fig. 7. Spatial distribution of depth of penetration dpenat a spray pressure p of 3 bar and a standoff distance h of 10cm for twonozzle sizes: (a) A-90; (b) I-90Table 4Mean values of the impact effect of sprays for two nozzle sizes showing the effect of standoff distance h at a spray pressure p of 3 barNozzleStandoff distance(h),cmImpact area withFmax.Tekscan40?002N(AI,Tekscan),cm2Maximal meanimpact pressure(Pmax,Tekscan), kPaAverage values ofthe erosion patternTotal eroded area(Ae), cm2Depth of penetration(dpen), mmA-901069?510?752?10?9200?71?416?30?6I-9010104?624?342?21?820170?727?555?21?4E. MULUGETA; M. GEYER450The dense spray near the spray core region results instatic loading which achieves an effective materialremoval. In the intermediate and boundary sprayregion, the spray performs a dynamic loading on theimpact surface, which accomplishes only a shallowmaterial removal. According to washing experimentscarried out by Hammitt et al. (1974), damage rate fallsvery rapidly as the angle of impact gets smaller than 901.His results indicate that the curves of volume removedfor a 601 impact angle y drop quickly. Hurley et al.(1983) have found out that at an impact angle y of 301under rain field exposure conditions, the amount ofvisible removal of the given coat material was practicallynil.The agricultural nozzle A-90 for plant protection(flow rate Qo3lmin?1at p 3 bar; ah 0 901) ischaracterised by an increased turbulence which enlargesthe spray angle by increasing the ratio of static-to-dynamic components of the spray impact loading. Inour spray washing analysis, for the A-90 at a standoffdistance h of 10cm, effective material removal on theplates was observed only with a spray impact angle up to701. It was almost caused by vertical droplet impactsnear the spray core regime (area ratio Z 0?10 ath 10cm). While being used at a standoff distance h of20cm, the A-90 was found ineffective with regard to thearea washing performance of the sprays because of aninappropriate spray structure.The use of the industrial nozzle I-90 (flow rateQ47lmin?1at a pressure p 3 bar; ah 0 901)resulted in a marked enhancement of the area washingperformance(arearatioZ 0?91atp 3bar,h 10cm). Under this spray condition it was deter-mined that exposure of the plates up to a spray impactangle y of 601 resulted in an effective material removal.The increased effects of the sprays were caused bychanges in the spray characteristics, i.e. droplet sizespectra and impact intensity of effective droplets perunit area and time. Although an enhancement in thearearatiowasobtainedbytheincreasednozzlediameter, it seemed to be inefficient to material removalfor small standoff distances due to the more concen-trated spray. However, an efficient usage of thesenozzles for coating removal may occur at standoffdistances larger than 10cm, depending on the char-acteristics of coating material. There may be instanceswhere at low standoff distances (hp10cm) it is better touse the less concentrated spray of the A-90 to remove acomparable volume of material, and the advantage maybeduetothebetterareawashingeffectiveness,characterised by bigger Es,Ve, than the more concen-trated spray of the I-90.4. ConclusionsThe developed testing procedure offers a possibility ofevaluating low-pressure nozzles/sprays with reference totheir spray structure and washing performance depend-ing on varying operating parameters. Nozzles shouldgenerate a spray with the greatest effective washed area(maximal area ratio Z), providing a sufficient impactpressure over the entire area (maximum depth ofeffective penetration dpen,eff.).The determination of the interdependence betweenthe different factors influencing the washing processunder low-pressure conditions is of significant impor-tance to optimise the washing performance. As a resultof a better understanding of the relevant hydro-mechanical coherences, application recommendationsconcerning the choice of washing nozzles and theappropriate operating parameters were derived. Thequantitative relations have been verified by spraywashingexperimentsunderlaboratoryconditions.Those experiments provide useful information aboutthesettingofnozzle/operatingparametersinthewashing process. The following conclusions can bedrawn.ARTICLE IN PRESSTable 5Results on spray washing performance and efficiency for two nozzle sizes showing the effect of standoff distance at a spray pressureof 3 barNozzleStandoff distanceAverage values of effective impact areawith a dpen,eff.42mmArea ratio (Z)Area washingefficency (Es,Ve),mm3Nm?1(h), cmAe,eff.Cm2dpen,eff.mmA-90107?22?10?100?6820I-901027?72?80?910?722025?52?60?270?62WASHING PROCESSES OF VEGETABLES AND POTATOES451(1) The effect of the variation of the nozzle diameter onthe droplet size spectrum formed within a spray atstandoff distances up to 20cm is more distinct andmore significant when compared to the variation ofspray pressures up to 8 bar.(2) An effective material removal occurs at an impactangle y of the sprays up to 601 to 701 depending onthe nozzle diameter used under the standard washingconditions.(3) The effects of nozzle diameter on the mean impulseof equal droplet sizes were relatively small.(4) The variation of washing performance of a sta-tionary spray is governed by the standoff distanceand the set of nozzle diameter and spray pressure. Itis caused by the influences of the above nozzle/operating parameters on the structure and impactpressure of generated sprays.(5) Agricultural nozzles for plant protection (flow rateQo3l min?1at pressure p 3 bar; spray angleah 04801) were found ineffective while being usedat a standoff distance d above 10cm.(6) The use of industrial nozzles (flow rate Q47l min?1at pressure p 3 bar; spray angle ah 0 901)resulted in a marked enhancement of the areawashing performance. An efficient usage of thesenozzles may occur at standoff distances larger than10cm.AcknowledgementThe authors wish to thank the German FederalMinistry of Education and Research (project 0339992-TP08) for their financial support.ReferencesAdler, W F (1979). The mechanics of liquid impact. In: Treatiseon Materials Science and Technology (Preece C M, ed), Vol.15, pp. 127183.Brunton J H; Rochester M C (1979). Erosion of solid surfacesby the impact of liquid drops. In: Treatise on MaterialsScience and Technology (Preece C M, ed), Vol. 16, pp.185248.Damaschke N (1999). Grundlage der PDA-meXtechnik. Basics inthe PDA measuring technique. Kurzlehrgang: Stro mungstech-nik in der industriellen Forschung. TU Darmstadt, GermanyFreudig B; Tesch S; Schubert H (2003). Production ofemulsions in high-pressure homogeniserspart II: influenceof cavitation on droplet breakup. Engineering Life Science,3, 266270Gebhardt H (1958). Zersta ubung mit Dralldu sen. Atomisationwith swirl nozzles. Scientific Journal of the TechnicalUniversity Dresden, 7, 249273Geyer M (1999). Gemu sereinigung. Vegetable cleaning.KTBL Publication 384, DarmstadtHammitt F G; Hwang J B; Do L; Huang Y C; Timm E E;Hughes R D (1974). Experimental and theoretical researchon liquid droplet impact. Proceedings of the SecondMeersburg Conference on Rain Erosion and Allied Phe-nomena, pp. 319345Herold B; Geyer M; Studman C J (2001). Fruit contactpressure distributions equipment. Computers and Electro-nics in Agriculture, 32, 167179Himmelreich U (1993). Fluiddynamische Modelluntersuchun-gen an Abrasivstrahlen. Fluid-dynamic model investiga-tions at abrasiv water jets. Dissertation. University ofHannover, GermanyHurley C J; Zahavi J; Do L; Schmitt G F (1983). Rain erosionmechanisms on polyurethane and fluoroelastomer coatedcomposite constructions. Proceedings of the 6th Interna-tional Conference on Erosion by Liquid and Solid Impact,Paper 22, pp. 110Kaye P L; Pickles C S J; Field J E (1995). Investigation oferosion processes as cleaning mechanisms in the removal ofthin deposited soils. Wear, 186(2), 413420Krautter J; Vetter K (1990). Verbesserung der Reinigung-stech
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