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Biosystems Engineering (2005) 91(4), 441453 doi:10.1016/j.biosystemseng.2005.05.009 PMPower and Machinery Characterising the Washing Processes of Vegetables and Potatoes E. Mulugeta; M. Geyer Institute 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 infl uencing factors on the spray washing process under low-pressure conditions. The washing effect as a function of standoff distance, spray pressure, and nozzle diameter is derived by considering the spray structure and the spray washing mechanism. Four measuring systems were used to determine the relevant spray structure parameters of different nozzles and their washing effects. Single droplets formed in the spray were submitted to an energetic assessment. The nozzles were evaluated with regard to their area washing performance Z as a ratio of the effective erosion area to spray area and effectiveness Es,Ve. The agricultural nozzle for plant protection with a fl ow rate Q lower than 3lmin?1at pressure p of 3bar and a 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 fl at-fan nozzle with a fl ow rate Q of 6?2lmin?1at pressure p of 3bar and a spray angle ah0of 901 produces a spray with a satisfactory area washing performance (area ratio Z 0?91), but a smaller area washing effectiveness Es,Vefor the spray conditions used in this experiment. r 2005 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd 1. Introduction Several washing machines with different principles are used for vegetable washing with varying sensitivities (Geyer, 1999). Nozzle washing machines are predomi- nantly used for leafy and bunched vegetables, such as lettuce, leek, bunched carrots and radishes. They clean the vegetables hydraulically by means of spray nozzles. Vegetable washing with nozzles must be performed carefully 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 dairy and other food industries for many years, there has been very little research effort directed towards an analysis of the washing performance (Scott et al., 1981). Investiga- tions of the fundamental aspects of spray performance for cleaning have mostly been focused on chemical engineering and high-pressure topics, and little of the information reported can be applied to vegetable cleaning situations (Mo nicke, 1971; Sandler, 1976; Scott et al., 1981; Schikorr Spillman, 1984; Krautter Wu Kru ger, 1998; Ludewig, 1998; Meng et al., 1998; Louis et al., 1999; Liu, 2000; Sivakumar Momber, 1993). The main inputs for the variation of the spray parameters and thus the spray effects are the operating and the nozzle ARTICLE IN PRESS 1537-5110/$30.00441r 2005 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd parameters. The output and thus the direct results of the spray structure are the size and velocity spectra of droplets as well as the spray geometry. These physical spray characteristics are relevant for the emerging droplet impulses as well as for the spray impact pressure during washing. The following application-technical demands on the spray structure should be fulfi lled during washing (after: Kru ger, 1998; Freudig et al., 2003): (1) optimal microstructure, meaning that the transfor- mation of spray into droplets with optimal size and velocity spectra leads to an optimal mean droplet impulse; and (2) optimal macrostructure, meaning that the washing water is uniformly distributed on the vegetable surface with an optimal circular spray width and spray effectiveness. From the numerous experimental investigations con- cerning spray coating removal (Adler, 1979; Brunton Hammitt et al., 1974; Lesser Obara et al., 1995), the perpendicular component of droplet impulse generates an extremely high impact pressure as a result of the water-hammer effect. The direct deformation and the propagation of mechanical waves caused by the impact pressure are responsible for crack initiation in the erosion process. Lateral outfl ow spray and hydraulic penetration extend the existing cracks which lead to erosion by the separation of coating material from the substrate. The intensity and duration of surface loading is based on the kinematics of the droplet impact, and is greater if the impacting surface is in contact with the compressed zone of the impacted droplet. The relation between the different factors infl uencing the spray structure and the spray washing effect on the impact surface were analysed in order to show the ARTICLE IN PRESS Notation Aetotal eroded area, cm2 Ae,effeffective eroded area, cm2 AI,Tekscanspray impact area on the sensor surface (Tekscan Inc.), cm2 Asspray area, cm2 apmean area of the detection volume, mm2 Cm,spec mean specifi c volume intensity of spray per unit area, mm3s?1mm?2 dddroplet diameter, mm?1 d0nozzle diameter, mm d32Sauter mean diameter, mm?1 dpendepth of penetration, mm dpen,eff.depth of effective penetration, mm Es,Vearea washing effectiveness, mm3Nm?1 Fmax,Tekscanmaximal impact force recorded using a sensor (Tekscan Inc.), N hstandoff distance, cm Idmean impulse of droplet-size group, mgm s?1 Id,spec specifi c impulse rate of effective dro- plets per unit area, gms?2mm?2 mdroplet mass, mg Pmax,Tekscanmaximal impact pressure using a sensor (Tekscan Inc.), kPa pspray pressure, bar q3(x)volume density distribution, mm?1 Q fl ow rate, l min?1 ndroplet velocity, ms?1 yimpact angle of droplet, deg Zarea ratio ah xspray angle at the standoff distance h of x cm, deg Operating parameters spray pressure nozzle distance standoff distance nozzle number nozzle arrangement impact loading time traverse speed Nozzle parameters nozzle geometry nozzle type Spray parameters droplet size droplet velocity number of droplets spray geometry droplet impulse spray impact pressure Product characteristics shape, surface properties susceptibility Dirt parameters adhesive forces of dirt composition of the adhering soil Spray angle a Stand off distance Fig. 1. Infl uencing factors on the washing process with spray nozzles (after: Geyer M. GEYER442 possibilities of optimising the nozzles and their operat- ing conditions as well as the washing process. A standard testing method was developed to express the material removal as a function of a set of spray conditions (nozzle and operating parameters) and other system parameters based on the spray structure. In this paper, the relations are described exemplarily for two washing nozzles. 2. Materials and methods Nozzle selection was primarily based on the fl ow rate representing the broad range of nozzle diameters d0as found in the present nozzle washing machines. In the following article, the results are described for the investigation of sprays produced by two 901 fl at-jet nozzles out of 11 examined spray nozzles: industrial nozzle 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, is recommended by the nozzle manufacturer. The water pressure used in the experiments was 3, 5, and 8bar. The investigations were conducted for a perpendicular arrangement of the nozzles at fi xed standoff distances h of 10 and 20cm and without travelling. The spray structures and their impact effects were examined. In this context, the mean impulse potentialIdofindividualdroplet-sizegroupsin mgms?1impacting on a surface is described as: Id m n sin y(1) in which m is the droplet mass in mg, n the vertical velocity of the droplets in ms?1and y their angle of impact on the surface. The specifi cimpulserateofeffectivedroplets (dd4300mm) per unit area Id;specin gms?2mm?2was simulated by calculating the specifi c mass fl ow rate of spray 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 spray was obtained. A standardised procedure for the evalua- tion of the spray parameters of the nozzles with regard to their area washing performance and effectiveness was developed; this was accomplished by records of the droplet impulse distributions in the spray and the erosion effect along the radial spray dispersion. With regard to this, the following parameters were deter- mined. (1) Fluid volume distribution and spray geometry The spatial distribution of the spray was measured through horizontal swivelling of a row of test tubes (internal diameter of 16mm) arranged next to each other over a time period of 7s (Gebhardt, 1958; Scott et al., 1981). In this way, the spray area Asand the intensityofthespraythroughouttheareawere determined. Records made with less than 5mm of the fi lling height in the test tubes were ignored. The information was presented in the form of a sheet diagram. The entire spray area was divided into small areas of 256mm2 , showing the corresponding specifi c ARTICLE IN PRESS Table 1 Values of spray geometries obtained at two distances h from the nozzle exit for two nozzle sizes and varied spray pressure p NozzleSpray pressureFlow rateMean values of the spray geometries derived based on measured data (p), bar(Q), l min?1Width, cmAngle, degDepth, cmArea (As), cm2 Standoff distance h 10cm A-9031?522?496?44?871?9 52?125?6104?01?641?0 82?627?2107?31?643?6 I-9037?219?287?61?630?4 59?921?694?41?633?4 812?622?496?41?635?3 Standoff distance h 20cm A-9031?541?692?28?0314?9 52?148?0100?34?8216?6 82?649?6102?24?8223?8 I-9037?235?483?03?294?7 59?937?686?43?2101?3 812?639?288?83?2109?7 WASHING PROCESSES OF VEGETABLES AND POTATOES443 volume intensity of spray per unit area Cm;specin mm3s?1mm?2. It was computed by assuming the distribution of the obtained spray volume to be constant over the catchment area of 256mm2. (2) Size and velocity spectra of the droplets Simultaneous measurements of droplet sizes and velocities in the spray, which were meant as a basis for anenergeticviewingofindividualdroplets,were accomplished by means of a phase-double-particle analyser (PDA) (Tropea, 1999). These investigations were carried out in the laboratory of the nozzle manufacturer Lechler Ltd., Metzingen, Germany. The number of droplets within given upper and lower limits of size for contiguous intervals was determined. The raw data were processed using area and volume weighing factors to compute the droplet-size/number and velocity distribution for the entire spray, assuming a mean size and velocity for each chosen droplet-size group. From the computed distribution the Sauter mean diameter SMD denoted by d32(Damaschke, 1999) was calculated. The spatial variability of these spray para- meters was considered with regard to the determination of several spectra within the spray area. Local measure- ments of droplet size and velocity spectra at a designated initial cross-section plane of the sprays were achieved by radial moving of the detection volume (mean area ap 0?33mm2) of about 35 or 40mm in each case. Taking into consideration the tolerance range of the pump and the measurement inaccuracy, the variation range between droplet size measurements under the same spray conditions ranged from 7 5 up to 10% (Lipthal, 2002). (3) Distribution of the maximum spray impact pressure With the help of a matrix-based tactile sensor (Type 5051, Tekscan Inc., Boston, USA) (Herold et al., 2001) the impact pressure distributions were measured. The sprays were, thereby, impacted onto the surface of the sensor for an exposure time of 120s (Mulugeta et al., 2002). The entire impact area of the spray was scanned in the respective standoff distance by shifting the sensor in steps of 25mm.The measured values of the impact force Fmax,Tekscanless than 0?002N have been ignored, as the operating mode of the sensor makes it impossible to verify the conditions of origin for these smaller load values. The average of two measurements taken for each set of values of standoff distance, nozzle diameter and spray pressure was used as the representative spray parameter. To protect the tactile sensor, it was covered by thin-fi lm polyethylene. (4) Distribution of the erosion depth on a standardised sand-binder mixture plate Aprocedurehasbeendevelopedforobtaining standardised sand-binder mixture plates following the spray analysis in the high-pressure area (Scott et al., 1981; Krautter Obara et al., 1995; Meng et al., 1998). The sand-binder mixture plates (300mm by 250mm by 20mm) were used as a target to observe the washing mechanisms and process due to the erosion pattern on the plates (see Fig. 6). They were placed perpendicularly to the central axis of the spray nozzles. The exposure time for each sample lay with 300s. The resulting eroded areas and depths on the plate surfaces were recorded by a laser scanner. Five measurements for each set of parameters were taken and the average of them was used as the representative value. Records made with less than 0?3mm of scan depth values were ignored. This mean threshold value was obtained from scan measurements of depth distribution of unloaded plates. The data obtained from the experiments were entered into an evaluation program developed at the Institute of Agricultural Engineering Bornim (Mulugeta et al., 2002). The program permits a coupled analysis of the different spatially related data. Furthermore, one addi- tional descriptive term, the area washing effectiveness Es,Vein mm3Nm?1, has been proposed which rates a nozzle in terms of volume removed in mm3on the plates at a defi ned exposure time per unit of energy in Nm expended by that nozzle. 3. Results and discussion The following spray characteristics caused by the variation of nozzle parameter and operating conditions have 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 is responsible for further spray dispersion and thus the spray geometry, and which affects the erosion processes on the impact surface. 3.1. Effects of spray pressure, nozzle diameter and standoff distance on spray parameters per unit area and time 3.1.1. Spray structure variables at a standoff distance of 10cm Compared to the A-90 the fl ow rate of nozzle I-90 was approximately fi ve-fold higher. The A-90 generated sprays with a larger width whereas a smaller radial spray dispersion characterised the I-90 (Table 1). For example, under a pressure of 3bar, a spray produced by the A-90 was dispersed over an area with a width of ARTICLE IN PRESS E. MULUGETA; M. GEYER444 around8cmlargerthanthesprayoftheI-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 pressures used in this experiment, a mean droplet diameter dd within the range of 20900mm and a mean velocity within the range of 535ms?1were recorded. The volume density distributions q3x in mm?1as a function of the mean droplet diameters dd; measured at a spray pressure p of 3bar in a standoff distance of 10cm, are shown in Fig. 2. The droplet spectrum of the A-90 provided defi nitely more fi ne droplets (o300mm) with the volumetric content of 49% than the one of the I-90 with 15% of the total spray volume. There were remarkable differences in Sauter mean diameter SMD between the nozzles (Table 2). This was due to the fact that the sprays generated by the I-90 contained more large droplets (4700mm), resulting in a clear increase of the SMD. On the other hand, an increase of the operating pressure only slightly affected the formed droplet size spectra. Thus, the droplet size spectra generally showed a tendency to move towards the range of fi ne droplets and the SMD values slightly decreased. The mean velocity distribution of droplets in sprays corresponded to earlier measurements (Ludewig, 1998), and to normal distribution function. Comparing the values between the two nozzles at the same operating pressure, the mean droplet velocities in the sprays of the I-90 increased by about 1433%. The curves of Idcalculated from Eqn (1) versus mean droplet diameter dd, with nozzle diameter d0as a parameter, are presented for the spray pressure p of 3 bar in Fig. 3. Comparing these curves, there is a clear difference in mean impulse between the same droplet- size groups. 3.1.2. Energetic view of the sprays at a standoff distance of 10cm Results of the spray analysis from measurements of volume distribution, droplet diameter and velocity spectra are shown in Table 3. Based on the measure- ments of spray volume distribution, a mass-weighed mean specifi c volume intensity of spray per unit area Cm,specof1?8mm3s?1mm?2hasbeendetermined for the entire spray area of the A-90, operating at a pressure of 3 bar. Under the same spray situation, the spray area of the I-90 had approximately 18-fold (?32?4mm3s?1mm?2) the Cm,specof the A-90. As ARTICLE IN PRESS Table 2 Sauter mean diameters and mean velocities of droplets for two nozzle sizes showing the effect of spray pressure p and standoff distance h Standoff distance (h), cmSauter diameter (d32),mmDroplet velocity (n), ms?2 NozzleSpray pressure (p), barSpray pressure (p), bar 358358 10A-9027725625113?619?624?3 I-9041840739118?123?127?7 20A-9026623923011?216?216?4 I-9037536131917?722?425?8 (a) (b) 0 0.002 0.004 0 0.002 0.004 0300600900 Mean droplet diameter dd, m 0