- Email: [email protected]
Development of a method to determine normal and shear stress necessary to remove a swollen soil from a surface
Food and Bioproducts Processing 1 1 3 ( 2 0 1 9 ) 86–92
Contents lists available at ScienceDirect
Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Development of a method to determine normal and shear stress necessary to remove a swollen soil from a surface Roman Murcek a,∗ , Enrico Schöhl a , Hannes Köhler b , André Boye a , Sabine Gold a a
Fraunhofer IVV, Branch Lab for Processing Machinery and Packaging Technology Dresden, Heidelberger Str. 20, 01189 Dresden, Germany b Institute of Natural Materials Technology, Technische Universität Dresden, Bergstr. 120, 01069 Dresden, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
To simulate cleaning processes is of constantly growing importance with regard to opti-
Received 13 June 2018
mising the cleaning efficiency in food production lines. This requires suitable parameters
Received in revised form 20
to describe the cleaning behaviour of different soils. Therefore, a method was developed
November 2018
to determine the normal and shear stress, which are necessary to remove a swollen soil
Accepted 20 November 2018
layer from a surface, by an impinging jet. The method consists of a cleaning procedure in
Available online 27 November 2018
combination with normal force measurements. For the cleaning tests, a stainless steel plate (AISI 304 with a 2B finish) with a defined soil layer was soaked reproducibly for a defined
Keywords:
time. Subsequently, a very short impulse of a liquid jet impinged on the surface in order to
Cleaning
determine the gauge pressure limit state, which is necessary to remove the soil from the
Simulation
surface in the impact area. At this determined limit state, the measurement was repeated
Cleaning behavior
with the same parameters, but with a piezo-based force sensor instead of the soiled surface.
Limit stress
Out of this measured forces, it was possible to calculate the target values: the normal and
Test method
the shear stress.
Spray cleaning
Measurements were performed with petroleum jelly and egg yolk as a food based model soil at different jet angles and for different swelling times. As a result, it was possible to draw the calculated stress as a function of the jet angle. By that, it is possible to quantify and compare the cleaning behaviour of different soils and at different swelling states. © 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Contents 1. 2.
3.
4.
∗
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87 Experimental. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87 2.1. Soiling method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87 2.2. Cleaning method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 2.3. Force measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 3.1. Determination of the limit gauge pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 3.2. Calculation of the normal and shear limit stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Corresponding author. E-mail address: [email protected] (R. Murcek). https://doi.org/10.1016/j.fbp.2018.11.009 0960-3085/© 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Food and Bioproducts Processing 1 1 3 ( 2 0 1 9 ) 86–92
1.
Introduction
With regard to the increasing flexibility and the decreasing batch sizes in food production processes the cleaning efficiency is becoming more and more important in order to save time and resources during the increasing number of cleaning steps. One potential way to avoid the oversizing of cleaning processes while also guaranteeing the product safety is the simulation of cleaning procedures. In the recent past research has been carried out in order to lay the basics of such process simulations, since they require knowledge about the different influencing factors on the cleaning process. The shape structure of liquid jets and the thereby resulting flow mechanical processes within a free surface jet have been examined e.g. by Yanaida and Ohashi (1978) and Gauntner et al. (1970). They showed the structural change of the liquid jet with increasing length due to turbulences within the jet and frictional forces due to the surrounding air. Investigations on the mechanical processes during the impact of droplets and liquid jets on a solid surface and the thereby resulting effects on cleaning have been performed e.g. by Faßmann (2015), Meng (1996), Leu et al. (1998) and Köhler et al. (2016). They mentioned that the centre of the jet stream is most relevant for the cleaning process whereas the periphery area where atomization is stronger does not have a relevant effect. On this basis, e.g. Springer (1976) and Adler (1974) provided mathematical approaches to quantify the impact pressure in correlation with the fluid hammer. Tong, (2003a) and Joppa et al. (2015) e.g. examined the compression within the impact area of a liquid jet. They could show that the compression can be described with a bell shaped function around the centre of impingement, which is sharply decreasing towards the edge. For a ratio of diameter around the centre D and jet radius r of D/r = 1 the pressure is already nearly null. For oblique impact angles the distribution of the compression shifts out of the centre (Tong, 2003b). Regarding the shear stress within and around the impingement area e.g. Lienhard and John (2006) and Stevens and Webb (1993) could show that it is null in the impact centre and sharply increases towards the edge. At D/r = 0.7 the maximum is reached. A practical approach regarding cleaning simulation was provided by Boye and Gumhold (2012) within the research project “Simulation system for designing spray cleaning processes”. There, a software tool was developed which is able to simulate the quantitative wetting behaviour of spray cleaning nozzles on complex surfaces in a CAD environment based on the nozzles’ impact. With this tool it is possible to design cleaning systems for entire production plants in order to make sure that all areas are reached by the cleaning fluid and that the nozzles are placed most efficiently. What is not taken into account here is the cleaning behaviour of the soil. Therefore, an approach has been provided by the work of Helbig et al. (2018) and Föste et al. (2014). They described the cleaning behaviour of different soil types with the help of dimensionless numbers, which can be determined due to the combination of different experimental set-ups. The current research project “Virtual Nozzle Development” at the Fraunhofer IVV Dresden and the LSTM Erlangen is dedicated to the development of spray cleaning nozzles with the help of CFD simulations in order to optimise impact distribution. On the one hand, the spray parameters are simulated out of the nozzle’s geometry (Abubaker et al., 2017). On the other hand, the influence of those spray parameters on the cleaning effect is examined. Since all kinds of soils have also
87
different cleaning behaviours, a method was developed within this project in order to determine the normal and shear stress, which are necessary to remove a soil from a surface. This method is described within this paper. It consists of a sensitive cleaning test with soiled test plates in combination with force measurements of the liquid jet. In addition, the jet angle and the swelling state of the soil are taken into account for those measurements. The measured forces enable to calculate the normal and shear stress and to draw them as a function of the jet angle (Hansen and Lundgren, 2013). This form of visualisation makes it possible to compare the required cleaning effort for different soils and for different swelling states quantitatively and to implement this data into a cleaning simulation system. The test method was developed with petroleum jelly as test soil and then verified with egg yolk powder dissolved in water as a food based test model soil.
2.
Experimental
2.1.
Soiling method
In order to keep a constant initial state for the cleaning tests, it is crucial that the soil layer on the test plates can be applied reproducibly and evenly. Therefore, a soiling test rig was used to generate consistent soil layers on even test plates of stainless steel (AISI 304 with a 2B finish) using a doctor blade. The soil layer thickness can be adjusted by using thin spacing templates between the doctor blade and the test plate. At first, the soil is applied extensively on the test plate. The excess soil amount is then removed by the doctor blade, which is moved automatically and with a constant speed (50 mm/s) over the test plate. The soiled plates are then dried for 24 h under standard atmosphere conditions (23 ◦ C, 50% air humidity) before they are used for the cleaning tests. For petroleum ® jelly (Roth Vaseline, CAS No.: 8009-03-8, mixed with 0.5 g RC ® Tritec Storelite , CAS No.: 68611-70-1, as a fluorescent tracer on 100 g petroleum jelly) a soil layer thickness of 550 m was applied. For the egg yolk (600 g OVOBEST egg yolk powder EMULTHERM mixed with 1 L deionized water at 1400 rpm) spacing templates with a thickness of 270 m were used. The test plates were weighed before soiling and after drying to determine the actual soil mass. Thereby, a variation coefficient of 10% could be determined, which shows the reproducibility of the method.
2.2.
Cleaning method
For the cleaning tests one short impulse with a solid water jet is applied on a soiled test plate. This procedure is repeated at different gauge pressures for each set of parameters (soil type, swelling state, jet angle). The aim of those tests is to determine the fluid pressure, which is necessary to remove the soil from the surface. This pressure is then considered as the limit gauge pressure for the current parameter set, which is then used for the force measurements. To implement this measuring procedure, a test rig was built up which is shown in Fig. 1. To realise the short impulse with the water jet, a perforated plate is moved by a linear axis with a constant speed beneath a continuously spraying jet nozzle. Thereby, only a short part of the water jet can pass the plate and impinges on the soiled test plate. The used nozzle has an outlet diameter of 0.84 mm (Lechler 544.360, flow rate: 0.45 L/min @ 1 bar, 1.00 L/min @ 5 bar), the length of the
88
Food and Bioproducts Processing 1 1 3 ( 2 0 1 9 ) 86–92
Fig. 1 – Cleaning test rig for the determination of the limit cleaning pressure.
Fig. 2 – Images from the high speed camera filming the segmented water jet impinging the force sensor plate at 0.5 bar. straight section before the nozzle is 185 mm (length/diameter ratio = 20:1). The gauge pressure is measured directly before the straight section. The perforated plate is moved with a speed of 0.8 m/s while the orifice where the water jet passes the plate has a width of 5 mm. The distance between the plate and the target is 90 mm. The parameters were chosen in order to create a segment of the water jet, which is as short and stable as possible. The water jet was filmed with a high-speed ® camera (MotionBLITZ Cube 4, frame rate: 4 kHz) to monitor it as seen in Fig. 2. By that, it was possible to assess the jet quality and to adjust the parameters for a suitable result. The tests showed that a too small orifice width (<5 mm) can lead to damages at the jet segment. The operating gauge pressure can be varied in increments of 0.1 bar so that the limit gauge pressure can be determined precisely as shown in Fig. 3. To investigate different jet angles, the nozzle can be inclined around the test plate in steps of 10◦ . In addition, the soiled plates can be soaked for a defined time before the segmented water jet impinges the surface. Therefore, the soiled test plates are positioned in a basin, which can be filled and drained automatically with water so that the plates are submerged reproducibly. After the water has drained from the basin, the cleaning procedure starts immediately. After the cleaning step, the test plates are examined with an optical soil detection system consisting of a camera (Matrix Vision mvBlueCOUGAR-X) and a UV light source. With this system, the amount of residual soil in the impact area was determined due to the fluorescence of the soil. By analysing the local intensity in the image, it can be determined if the water jet was able to remove the soil completely from the sur-
face or if there are still soil layers remaining. The threshold for the determination of a cleaned area was determined by taking images of clean test plates. It is also possible to determine the size of the cleaned area where the soil was removed completely. If the diameter of the cleaned area was at least as big as the outlet diameter of the jet nozzle, the cleaning step was considered as successful.
2.3.
Force measurements
After the limit gauge pressure, which is needed to remove the soil from the surface, has been determined, it is necessary to calculate also the stresses, which are induced by the water jet on the soiled surface at the determined pressure. There® fore, a piezo-based force sensor (Kistler Piezo Force Sensor ® Type 9215A, 0–10 V, used with Kistler Charge Amplifier Typ 5165A) is used to measure the normal force, which is applied by the water jet. Subsequently, together with the jet angle and the cross sectional area of the liquid jet in the impact area, it is possible to calculate the normal and shear stress. For the measurements, the force sensor is placed at the same position as the soiled test plates were during the cleaning tests. The impulse with the water jet is applied in the same way and with the same parameters as they were determined at the limit cleaning state. The sensor works with a sampling frequency of 62.5 kHz and within a range of 2 N. Fig. 4 shows exemplary the force progression during the short impact of the water jet at 2 bar. The impact duration in this case is ca. 12 ms. During that time 750 measured values are recorded. After a short moment at the beginning where the first impact on the dry plate is visible, the curve shifts into
Food and Bioproducts Processing 1 1 3 ( 2 0 1 9 ) 86–92
89
Fig. 3 – Example for different cleaning results at different fluid pressures for petroleum jelly.
Fig. 4 – Measured normal force progression during the impact of the water jet at 2 bar and 90◦ (mean value: 0.15 N, variation coefficient: 18.7%). a stationary progression around a constant value. Since the measured peak value from the first impact was too error-prone due to its short duration, the average value from the stationary section where the reproducibility of the measurement was significantly higher was chosen as target value. Subsequently, this value was used for the calculations of the normal and shear stress.
3.
Results and discussion
3.1.
Determination of the limit gauge pressure
Petroleum jelly was used in first tests to adjust and optimise the parameters of the cleaning procedure since it does not require swelling to make it cleanable and therefore allows a high number of tests within short time. The operating gauge pressure was varied iteratively for each set of parameters while checking if the cleaning step was successful or not at this pressure. The tests showed that due to general fault effects (e.g. varying local soil mass, local granular ingredients) and within a range around the searched limit pressure the cleaning step can sometimes be successful and sometimes not for the same operating pressure. Therefore, the percentile number of successful cleaning steps at the same operating gauge pressure is used as target value. At first, the abovementioned range around the limit pressure is determined in approximation tests with only 3 repetitions and by increasing the operating gauge pressure in steps of 0.3 bar. Afterwards, detailed tests were performed within and closely around this range with 9 repetitions and by an operating gauge pressure increase of 0.1 bar. Fig. 5 shows the results of this procedure for the cleaning tests with petroleum jelly at a jet angle of 45◦ . A sigmoid regression function was fit onto the measured results: P=
1 1 + exp (−ˇ0 + p ∗ ˇ)
(1)
In this function P is the percentile number of successful cleaning steps, p is the operating gauge pressure and ˇ and ˇ0 are regression coefficients which fit the curve to the measured
Fig. 5 – Determination of the limit gauge pressure for petroleum jelly at a jet angle of 45◦ .
Fig. 6 – Limit gauge pressure needed to remove petroleum jelly from the surface depending on the jet angle. values. Those regression coefficients are determined by maximum likelihood estimation (Andreß et al., 1997; Stegemann, 1995). The searched limit state is defined as the operating gauge pressure where 50% of the cleaning steps are successful. This value was chosen because it was assumed that for the limit state it is characteristic that it describes the border between getting the surface clean or not. Since the cleaning process is very prone to discontinuities regarding process parameters like e.g. the soil mass, a tolerance range of ±30% (value chosen arbitrarily) was implemented to determine the deviation of the limit gauge pressure. This means, that the pressure values where 20% and 80% of the cleaning steps are successful are taken as the lower and upper deviation limit, respectively. This means that the steeper the regression curve’s slope is the smaller is the deviation of the limit gauge pressure. Fig. 6 shows the determined limit gauge pressures for petroleum jelly as a function of the jet angle. The error bars there are resulting from the intersection of the tolerance limits and the regression curves.
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Food and Bioproducts Processing 1 1 3 ( 2 0 1 9 ) 86–92
Table 1 – Limit gauge pressure and associated measured normal force for petroleum jelly depending on the jet angle. Soil: petroleum jelly Jet angle in degree
Limit gauge pressure in bar
Tolerance range of the limit gauge pressure in bar
Normal force at limit gauge pressure in N
Standard deviation of the normal force measurements in N
90 80 70 60 45
1.65 1.71 1.87 2.29 2.72
0.10 0.05 0.05 0.21 0.18
0.154 0.156 0.161 0.181 0.183
0.0041 0.0039 0.0056 0.0040 0.0028
Since the cross sectional area of the water jet in the impact area is differing depending on the jet angle, those force values cannot be compared directly. It is necessary to calculate the normal and shear stress, which are induced by the water jet on the soil according to the mechanisms shown in Fig. 8. This is a simplified two-dimensional approach, which considers the lateral components of the stress tensor negligible. The normal stress can be calculated as the quotient of the measured normal force FN and the size of the cross sectional area:
(˛) = Fig. 7 – Limit gauge pressure needed to remove egg yolk from the surface depending on the jet angle and the swelling time. After the cleaning procedure was set up with petroleum jelly, the tests were also performed with an egg yolk powder solution. Since it was not possible to remove this kind of soil from the surface without swelling, the soiled plates were soaked in water before cleaning as described above. Three different swelling times were examined: 150 s, 300 s and 450 s. Fig. 7 shows the results that were determined for egg yolk at those swelling times depending on the jet angle. The graph already shows a good differentiation between the swelling states of the soil. The longer the soil is soaked, the less operating pressure is necessary to remove it from the surface.
3.2.
Calculation of the normal and shear limit stress
After the limit gauge pressures have been determined for the different soils at the different jet angles and swelling states, the normal force induced by the segmented water jet on the soiled plate has been measured at those limit gauge pressures as described above. Table 1 shows the results of those measurements for petroleum jelly.
FN A2 (˛, L)
(2)
In this equation A2 is the cross sectional area of the water jet which is depending on the jet angle ˛ and the distance from the nozzle outlet to the soiled surface L. With extending distance to the nozzle outlet, the diameter of the water jet is spreading. Since the distance from the nozzle to the surface could not be kept constantly due to geometrical limitations of the used test rig while varying the jet angle, the extent of this spread is also differing for the different jet angles. Therefore, the diameter of the water jet has been measured at different jet angles with the high speed camera. Out of this measured diameter and the diameter of the nozzle outlet D the spread coefficient k could be calculated which describes the relative increase of the water jet’s cross sectional diameter at the measuring site. The determined spread coefficients at the limit gauge pressures for petroleum jelly are shown in Table 2. With the determined spread coefficients k and the diameter of the nozzle outlet D, it is possible to calculate the cross sectional area depending on the jet angle and the nozzle distance to the surface:
A2 (˛, L) =
2 (D ∗ k (˛, L)) ∗ sin ˛ ∗ 4
Fig. 8 – (a) Approach for normal and shear stress from the solid mechanics, (b) approach for normal and shear stress regarding cleaning.
(3)
Food and Bioproducts Processing 1 1 3 ( 2 0 1 9 ) 86–92
91
Table 2 – Spread coefficient of the water jet’s cross sectional area depending on the jet angle, the operating pressure and the distance from the nozzle to the surface. Soil: petroleum jelly Jet angle in degree
Distance to the measuring site L in mm
Spread coefficient k of the water jet’s cross sectional diameter
90 80 70 60 45
200 201 211 228 256
1.65 1.66 1.77 1.95 2.12
Fig. 10 – Normal and shear stress needed to remove egg yolk from the surface depending on the jet angle and the swelling state.
certain tolerance range, this method can also be used to estimate the needed normal and shear stress for other jet angles. In order to improve the validity of the measured and calculated data the test rig used within this thesis can still be improved. Due to the geometrical circumstances the different jet angles could not be measured at the same distances between the nozzle and the test plate what increases the susceptibility to errors. Especially for the lower jet angles with a higher nozzle distance this can lead to break ups within the liquid jet. Therefore, a smaller and constant distance of the jet nozzle is preferable. Fig. 9 – Normal and shear stress needed to remove petroleum jelly from the surface depending on the jet angle. The calculated cross sectional area A2 is now used with Eq. (2) to determine the normal stress : (˛) =
4 ∗ FN 2 (D ∗ k (˛, L)) ∗ sin ˛ ∗
(4)
The total stress S and the shear stress can be calculated as follows by using trigonometrical functions (as described in Fig. 8b): S = (˛) ∗ sin ˛ (˛) =
S cos ˛
(5) (6)
It is possible to draw the calculated normal and shear stress values as a function of the jet angle, the soil and the swelling state what is shown for petroleum jelly (no swelling) in Fig. 9. The stress values are showing an elliptic characteristic. It is assumed that this shows the different influence of the normal and shear component on the cleaning effect. Therefore, the equation of an ellipse is calculated out of the stress values for 90◦ and 45◦ and the ellipse was drawn into the graph. It shows that the values for the other jet angles are lying very closely to this curve and all values are lying within the tolerance range (dashed lines, based on the previously shown error bars). According to this method, the stress values were also calculated for the limit gauge pressure values determined for egg yolk at the different swelling states. The resulting values and the calculated elliptic curves are shown in Fig. 10. The graph shows that this display format is very suitable to compare the cleaning effort needed at the different swelling states. The same applies for the comparison of different soils. Since all values fit very tight to the calculated ellipse within a
4.
Conclusions
The results of this paper show that it is possible with the developed method to describe the normal and shear stress required to remove different soils from a surface. By displaying the values in a stress diagram, it is possible to compare different soils and swelling states qualitatively and quantitatively. Within this work the form of the ellipses was calculated out of the measurements from two jet angles. The measurements at the other jet angles were used to validate the determined curve. Prospectively, the effort can be reduced by measuring only at two jet angles. Cleaning simulation systems can use this data to read out the normal and shear stress required for cleaning at any relevant jet angle. The method therefore is helping to close a crucial gap on the way to the simulation of cleaning processes. In order to validate the transferability to other cleaning mechanisms, like e.g. spray cleaning, it is recommended to extend the presented test method by additional cleaning tests.
References Abubaker, H., Sivaram, A., Münsch, M., Murcek, R., Boye, A., Delgado, A., 2017. Optimization of design parameters of CIP spray cleaning nozzle using genetic algorithm. In: 4th International Conference on Computational Engineering, Darmstadt. Adler, W.F., 1974. The mechanics of liquid impact. In: Treatise on Materials Science and Technology. Andreß, H.J., Hagenaars, J.A., Kühnel, S., 1997. Analyse von Tabellen und kategorialen Daten: log-lineare Modelle, latente Klassenanalysen, logistische Regression und GSK-Ansatz. Springer, Berlin. Boye, A., Gumhold, S., 2012. Erarbeitung eines entwicklungsbegleitenden Simulationswerkzeugs zur konstruktiven Auslegung von düsengebundenen Sprühreinigungssystemen in
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Lebensmittelverarbeitungsmaschinen. Project report, IGF 16438 BR. Fraunhofer IVV Dresden, Dresden. Föste, H., Helbig, M., Schulte, T., Rudolph, M., Majschak, J.-P., Augustin, W., Scholl, S., 2014. A unified approach to classify removal processes of different soils using dimensionless numbers. In: Fouling and Cleaning in Food Processing, Cambridge. Faßmann, B.W., 2015. Zeitabhängige Charakterisierung der Sekundärtropfen aus dem Hochgeschwingdigkeitsaufprall einzelner Flüssigkeitstropfen. Dissertation. TU Braunschweig. Gauntner, J.W., Linvingood, J.N.B., Hrycak, P., 1970. Survey of Literature on Flow Characteristics of a Single Turbulent Jet Impinging on a Flat Plate. National Aeronautics and Space Administration, Lewis Research Center, Washington D.C. Hansen, J.B., Lundgren, H., 2013. Hauptprobleme der Bodenmechanik. Springer, Heidelberg, Berlin. Helbig, M., Zahn, S., Böttcher, K., Rohm, H., Majschak, J.P., 2018. Laboratory methods to predict the cleaning behaviour in a flow channel exemplified by egg yolk layer. In: Fouling and Cleaning in Food Processing, Lund. Joppa, M., Köhler, H., Rüdiger, F., Majschak, J.P., Fröhlich, J., 2015. Vereinfachte Berechnung der Strömung bei der Strahlreinigung. In: VVD 2015 Verarbeitungsmaschinen und Verpackungstechnik — 8. wissenschaftliche Fachtagung, 12th/13th March 2015, Dresden. Köhler, H., Stoye, H., Weyrauch, T., Boye, A., Mauermann, M., Majschak, J.P., 2016. Study on the application of cleaning models with high speed water jets to CIP-processes. Tech. Gaz. 23 (April (2)), 349–355, http://dx.doi.org/10.17559/ TV-20140213152946.
Leu, M.C., Meng, P., Geskin, E.S., Tismeneskiy, L., 1998. Mathematical modeling and experimental verification of stationary waterjet cleaning process. J. Manuf. Sci. Eng. 120 (3), 571–579. Lienhard, V., John, H., 2006. Heat transfer by impingement of circular free-surface liquid jets. In: 18th National & 7th ISHMT-ASME Heat and Mass Transfer Conference, Guwahati (India). Meng, P., 1996. Experimental and Analytical Investigation of Waterjet Cleaning Process. Dissertation. New Jersey Institute of Technology. Springer, G.S., 1976. Erosion by Liquid Impact. Scripta Pub Co. distributed by Halsted Press Division, Wiley, Washington, New York. Stegemann, S., 1995. Logistische Regression und Diskrimination bei zufällig variierenden Parametern. Dissertation. Universität Dortmund. Stevens, J., Webb, B.W., 1993. Measurements of flow structure in the stagnation zone of impinging free-surface liquid jets. Int. J. Heat Mass Transfer 36 (17), 4283–4286. Tong, A.Y., 2003a. A numerical study on the hydrodynamics and heat transfer of circular liquid jet impinging onto a substrate. Numer. Heat Transfer A 44, 1–19. Tong, A.Y., 2003b. On the impingement heat transfer of an oblique free surface plane jet. Int. J. Heat Mass Transfer 46, 2077–2085. Yanaida, K., Ohashi, A., 1978. Flow characteristics of water jets in air. In: Proceeding — 4th Int Symposium on Jet Cutting Technology, Canterbury.
Contents lists available at ScienceDirect
Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Development of a method to determine normal and shear stress necessary to remove a swollen soil from a surface Roman Murcek a,∗ , Enrico Schöhl a , Hannes Köhler b , André Boye a , Sabine Gold a a
Fraunhofer IVV, Branch Lab for Processing Machinery and Packaging Technology Dresden, Heidelberger Str. 20, 01189 Dresden, Germany b Institute of Natural Materials Technology, Technische Universität Dresden, Bergstr. 120, 01069 Dresden, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
To simulate cleaning processes is of constantly growing importance with regard to opti-
Received 13 June 2018
mising the cleaning efficiency in food production lines. This requires suitable parameters
Received in revised form 20
to describe the cleaning behaviour of different soils. Therefore, a method was developed
November 2018
to determine the normal and shear stress, which are necessary to remove a swollen soil
Accepted 20 November 2018
layer from a surface, by an impinging jet. The method consists of a cleaning procedure in
Available online 27 November 2018
combination with normal force measurements. For the cleaning tests, a stainless steel plate (AISI 304 with a 2B finish) with a defined soil layer was soaked reproducibly for a defined
Keywords:
time. Subsequently, a very short impulse of a liquid jet impinged on the surface in order to
Cleaning
determine the gauge pressure limit state, which is necessary to remove the soil from the
Simulation
surface in the impact area. At this determined limit state, the measurement was repeated
Cleaning behavior
with the same parameters, but with a piezo-based force sensor instead of the soiled surface.
Limit stress
Out of this measured forces, it was possible to calculate the target values: the normal and
Test method
the shear stress.
Spray cleaning
Measurements were performed with petroleum jelly and egg yolk as a food based model soil at different jet angles and for different swelling times. As a result, it was possible to draw the calculated stress as a function of the jet angle. By that, it is possible to quantify and compare the cleaning behaviour of different soils and at different swelling states. © 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Contents 1. 2.
3.
4.
∗
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87 Experimental. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87 2.1. Soiling method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87 2.2. Cleaning method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 2.3. Force measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 3.1. Determination of the limit gauge pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 3.2. Calculation of the normal and shear limit stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Corresponding author. E-mail address: [email protected] (R. Murcek). https://doi.org/10.1016/j.fbp.2018.11.009 0960-3085/© 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Food and Bioproducts Processing 1 1 3 ( 2 0 1 9 ) 86–92
1.
Introduction
With regard to the increasing flexibility and the decreasing batch sizes in food production processes the cleaning efficiency is becoming more and more important in order to save time and resources during the increasing number of cleaning steps. One potential way to avoid the oversizing of cleaning processes while also guaranteeing the product safety is the simulation of cleaning procedures. In the recent past research has been carried out in order to lay the basics of such process simulations, since they require knowledge about the different influencing factors on the cleaning process. The shape structure of liquid jets and the thereby resulting flow mechanical processes within a free surface jet have been examined e.g. by Yanaida and Ohashi (1978) and Gauntner et al. (1970). They showed the structural change of the liquid jet with increasing length due to turbulences within the jet and frictional forces due to the surrounding air. Investigations on the mechanical processes during the impact of droplets and liquid jets on a solid surface and the thereby resulting effects on cleaning have been performed e.g. by Faßmann (2015), Meng (1996), Leu et al. (1998) and Köhler et al. (2016). They mentioned that the centre of the jet stream is most relevant for the cleaning process whereas the periphery area where atomization is stronger does not have a relevant effect. On this basis, e.g. Springer (1976) and Adler (1974) provided mathematical approaches to quantify the impact pressure in correlation with the fluid hammer. Tong, (2003a) and Joppa et al. (2015) e.g. examined the compression within the impact area of a liquid jet. They could show that the compression can be described with a bell shaped function around the centre of impingement, which is sharply decreasing towards the edge. For a ratio of diameter around the centre D and jet radius r of D/r = 1 the pressure is already nearly null. For oblique impact angles the distribution of the compression shifts out of the centre (Tong, 2003b). Regarding the shear stress within and around the impingement area e.g. Lienhard and John (2006) and Stevens and Webb (1993) could show that it is null in the impact centre and sharply increases towards the edge. At D/r = 0.7 the maximum is reached. A practical approach regarding cleaning simulation was provided by Boye and Gumhold (2012) within the research project “Simulation system for designing spray cleaning processes”. There, a software tool was developed which is able to simulate the quantitative wetting behaviour of spray cleaning nozzles on complex surfaces in a CAD environment based on the nozzles’ impact. With this tool it is possible to design cleaning systems for entire production plants in order to make sure that all areas are reached by the cleaning fluid and that the nozzles are placed most efficiently. What is not taken into account here is the cleaning behaviour of the soil. Therefore, an approach has been provided by the work of Helbig et al. (2018) and Föste et al. (2014). They described the cleaning behaviour of different soil types with the help of dimensionless numbers, which can be determined due to the combination of different experimental set-ups. The current research project “Virtual Nozzle Development” at the Fraunhofer IVV Dresden and the LSTM Erlangen is dedicated to the development of spray cleaning nozzles with the help of CFD simulations in order to optimise impact distribution. On the one hand, the spray parameters are simulated out of the nozzle’s geometry (Abubaker et al., 2017). On the other hand, the influence of those spray parameters on the cleaning effect is examined. Since all kinds of soils have also
87
different cleaning behaviours, a method was developed within this project in order to determine the normal and shear stress, which are necessary to remove a soil from a surface. This method is described within this paper. It consists of a sensitive cleaning test with soiled test plates in combination with force measurements of the liquid jet. In addition, the jet angle and the swelling state of the soil are taken into account for those measurements. The measured forces enable to calculate the normal and shear stress and to draw them as a function of the jet angle (Hansen and Lundgren, 2013). This form of visualisation makes it possible to compare the required cleaning effort for different soils and for different swelling states quantitatively and to implement this data into a cleaning simulation system. The test method was developed with petroleum jelly as test soil and then verified with egg yolk powder dissolved in water as a food based test model soil.
2.
Experimental
2.1.
Soiling method
In order to keep a constant initial state for the cleaning tests, it is crucial that the soil layer on the test plates can be applied reproducibly and evenly. Therefore, a soiling test rig was used to generate consistent soil layers on even test plates of stainless steel (AISI 304 with a 2B finish) using a doctor blade. The soil layer thickness can be adjusted by using thin spacing templates between the doctor blade and the test plate. At first, the soil is applied extensively on the test plate. The excess soil amount is then removed by the doctor blade, which is moved automatically and with a constant speed (50 mm/s) over the test plate. The soiled plates are then dried for 24 h under standard atmosphere conditions (23 ◦ C, 50% air humidity) before they are used for the cleaning tests. For petroleum ® jelly (Roth Vaseline, CAS No.: 8009-03-8, mixed with 0.5 g RC ® Tritec Storelite , CAS No.: 68611-70-1, as a fluorescent tracer on 100 g petroleum jelly) a soil layer thickness of 550 m was applied. For the egg yolk (600 g OVOBEST egg yolk powder EMULTHERM mixed with 1 L deionized water at 1400 rpm) spacing templates with a thickness of 270 m were used. The test plates were weighed before soiling and after drying to determine the actual soil mass. Thereby, a variation coefficient of 10% could be determined, which shows the reproducibility of the method.
2.2.
Cleaning method
For the cleaning tests one short impulse with a solid water jet is applied on a soiled test plate. This procedure is repeated at different gauge pressures for each set of parameters (soil type, swelling state, jet angle). The aim of those tests is to determine the fluid pressure, which is necessary to remove the soil from the surface. This pressure is then considered as the limit gauge pressure for the current parameter set, which is then used for the force measurements. To implement this measuring procedure, a test rig was built up which is shown in Fig. 1. To realise the short impulse with the water jet, a perforated plate is moved by a linear axis with a constant speed beneath a continuously spraying jet nozzle. Thereby, only a short part of the water jet can pass the plate and impinges on the soiled test plate. The used nozzle has an outlet diameter of 0.84 mm (Lechler 544.360, flow rate: 0.45 L/min @ 1 bar, 1.00 L/min @ 5 bar), the length of the
88
Food and Bioproducts Processing 1 1 3 ( 2 0 1 9 ) 86–92
Fig. 1 – Cleaning test rig for the determination of the limit cleaning pressure.
Fig. 2 – Images from the high speed camera filming the segmented water jet impinging the force sensor plate at 0.5 bar. straight section before the nozzle is 185 mm (length/diameter ratio = 20:1). The gauge pressure is measured directly before the straight section. The perforated plate is moved with a speed of 0.8 m/s while the orifice where the water jet passes the plate has a width of 5 mm. The distance between the plate and the target is 90 mm. The parameters were chosen in order to create a segment of the water jet, which is as short and stable as possible. The water jet was filmed with a high-speed ® camera (MotionBLITZ Cube 4, frame rate: 4 kHz) to monitor it as seen in Fig. 2. By that, it was possible to assess the jet quality and to adjust the parameters for a suitable result. The tests showed that a too small orifice width (<5 mm) can lead to damages at the jet segment. The operating gauge pressure can be varied in increments of 0.1 bar so that the limit gauge pressure can be determined precisely as shown in Fig. 3. To investigate different jet angles, the nozzle can be inclined around the test plate in steps of 10◦ . In addition, the soiled plates can be soaked for a defined time before the segmented water jet impinges the surface. Therefore, the soiled test plates are positioned in a basin, which can be filled and drained automatically with water so that the plates are submerged reproducibly. After the water has drained from the basin, the cleaning procedure starts immediately. After the cleaning step, the test plates are examined with an optical soil detection system consisting of a camera (Matrix Vision mvBlueCOUGAR-X) and a UV light source. With this system, the amount of residual soil in the impact area was determined due to the fluorescence of the soil. By analysing the local intensity in the image, it can be determined if the water jet was able to remove the soil completely from the sur-
face or if there are still soil layers remaining. The threshold for the determination of a cleaned area was determined by taking images of clean test plates. It is also possible to determine the size of the cleaned area where the soil was removed completely. If the diameter of the cleaned area was at least as big as the outlet diameter of the jet nozzle, the cleaning step was considered as successful.
2.3.
Force measurements
After the limit gauge pressure, which is needed to remove the soil from the surface, has been determined, it is necessary to calculate also the stresses, which are induced by the water jet on the soiled surface at the determined pressure. There® fore, a piezo-based force sensor (Kistler Piezo Force Sensor ® Type 9215A, 0–10 V, used with Kistler Charge Amplifier Typ 5165A) is used to measure the normal force, which is applied by the water jet. Subsequently, together with the jet angle and the cross sectional area of the liquid jet in the impact area, it is possible to calculate the normal and shear stress. For the measurements, the force sensor is placed at the same position as the soiled test plates were during the cleaning tests. The impulse with the water jet is applied in the same way and with the same parameters as they were determined at the limit cleaning state. The sensor works with a sampling frequency of 62.5 kHz and within a range of 2 N. Fig. 4 shows exemplary the force progression during the short impact of the water jet at 2 bar. The impact duration in this case is ca. 12 ms. During that time 750 measured values are recorded. After a short moment at the beginning where the first impact on the dry plate is visible, the curve shifts into
Food and Bioproducts Processing 1 1 3 ( 2 0 1 9 ) 86–92
89
Fig. 3 – Example for different cleaning results at different fluid pressures for petroleum jelly.
Fig. 4 – Measured normal force progression during the impact of the water jet at 2 bar and 90◦ (mean value: 0.15 N, variation coefficient: 18.7%). a stationary progression around a constant value. Since the measured peak value from the first impact was too error-prone due to its short duration, the average value from the stationary section where the reproducibility of the measurement was significantly higher was chosen as target value. Subsequently, this value was used for the calculations of the normal and shear stress.
3.
Results and discussion
3.1.
Determination of the limit gauge pressure
Petroleum jelly was used in first tests to adjust and optimise the parameters of the cleaning procedure since it does not require swelling to make it cleanable and therefore allows a high number of tests within short time. The operating gauge pressure was varied iteratively for each set of parameters while checking if the cleaning step was successful or not at this pressure. The tests showed that due to general fault effects (e.g. varying local soil mass, local granular ingredients) and within a range around the searched limit pressure the cleaning step can sometimes be successful and sometimes not for the same operating pressure. Therefore, the percentile number of successful cleaning steps at the same operating gauge pressure is used as target value. At first, the abovementioned range around the limit pressure is determined in approximation tests with only 3 repetitions and by increasing the operating gauge pressure in steps of 0.3 bar. Afterwards, detailed tests were performed within and closely around this range with 9 repetitions and by an operating gauge pressure increase of 0.1 bar. Fig. 5 shows the results of this procedure for the cleaning tests with petroleum jelly at a jet angle of 45◦ . A sigmoid regression function was fit onto the measured results: P=
1 1 + exp (−ˇ0 + p ∗ ˇ)
(1)
In this function P is the percentile number of successful cleaning steps, p is the operating gauge pressure and ˇ and ˇ0 are regression coefficients which fit the curve to the measured
Fig. 5 – Determination of the limit gauge pressure for petroleum jelly at a jet angle of 45◦ .
Fig. 6 – Limit gauge pressure needed to remove petroleum jelly from the surface depending on the jet angle. values. Those regression coefficients are determined by maximum likelihood estimation (Andreß et al., 1997; Stegemann, 1995). The searched limit state is defined as the operating gauge pressure where 50% of the cleaning steps are successful. This value was chosen because it was assumed that for the limit state it is characteristic that it describes the border between getting the surface clean or not. Since the cleaning process is very prone to discontinuities regarding process parameters like e.g. the soil mass, a tolerance range of ±30% (value chosen arbitrarily) was implemented to determine the deviation of the limit gauge pressure. This means, that the pressure values where 20% and 80% of the cleaning steps are successful are taken as the lower and upper deviation limit, respectively. This means that the steeper the regression curve’s slope is the smaller is the deviation of the limit gauge pressure. Fig. 6 shows the determined limit gauge pressures for petroleum jelly as a function of the jet angle. The error bars there are resulting from the intersection of the tolerance limits and the regression curves.
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Table 1 – Limit gauge pressure and associated measured normal force for petroleum jelly depending on the jet angle. Soil: petroleum jelly Jet angle in degree
Limit gauge pressure in bar
Tolerance range of the limit gauge pressure in bar
Normal force at limit gauge pressure in N
Standard deviation of the normal force measurements in N
90 80 70 60 45
1.65 1.71 1.87 2.29 2.72
0.10 0.05 0.05 0.21 0.18
0.154 0.156 0.161 0.181 0.183
0.0041 0.0039 0.0056 0.0040 0.0028
Since the cross sectional area of the water jet in the impact area is differing depending on the jet angle, those force values cannot be compared directly. It is necessary to calculate the normal and shear stress, which are induced by the water jet on the soil according to the mechanisms shown in Fig. 8. This is a simplified two-dimensional approach, which considers the lateral components of the stress tensor negligible. The normal stress can be calculated as the quotient of the measured normal force FN and the size of the cross sectional area:
(˛) = Fig. 7 – Limit gauge pressure needed to remove egg yolk from the surface depending on the jet angle and the swelling time. After the cleaning procedure was set up with petroleum jelly, the tests were also performed with an egg yolk powder solution. Since it was not possible to remove this kind of soil from the surface without swelling, the soiled plates were soaked in water before cleaning as described above. Three different swelling times were examined: 150 s, 300 s and 450 s. Fig. 7 shows the results that were determined for egg yolk at those swelling times depending on the jet angle. The graph already shows a good differentiation between the swelling states of the soil. The longer the soil is soaked, the less operating pressure is necessary to remove it from the surface.
3.2.
Calculation of the normal and shear limit stress
After the limit gauge pressures have been determined for the different soils at the different jet angles and swelling states, the normal force induced by the segmented water jet on the soiled plate has been measured at those limit gauge pressures as described above. Table 1 shows the results of those measurements for petroleum jelly.
FN A2 (˛, L)
(2)
In this equation A2 is the cross sectional area of the water jet which is depending on the jet angle ˛ and the distance from the nozzle outlet to the soiled surface L. With extending distance to the nozzle outlet, the diameter of the water jet is spreading. Since the distance from the nozzle to the surface could not be kept constantly due to geometrical limitations of the used test rig while varying the jet angle, the extent of this spread is also differing for the different jet angles. Therefore, the diameter of the water jet has been measured at different jet angles with the high speed camera. Out of this measured diameter and the diameter of the nozzle outlet D the spread coefficient k could be calculated which describes the relative increase of the water jet’s cross sectional diameter at the measuring site. The determined spread coefficients at the limit gauge pressures for petroleum jelly are shown in Table 2. With the determined spread coefficients k and the diameter of the nozzle outlet D, it is possible to calculate the cross sectional area depending on the jet angle and the nozzle distance to the surface:
A2 (˛, L) =
2 (D ∗ k (˛, L)) ∗ sin ˛ ∗ 4
Fig. 8 – (a) Approach for normal and shear stress from the solid mechanics, (b) approach for normal and shear stress regarding cleaning.
(3)
Food and Bioproducts Processing 1 1 3 ( 2 0 1 9 ) 86–92
91
Table 2 – Spread coefficient of the water jet’s cross sectional area depending on the jet angle, the operating pressure and the distance from the nozzle to the surface. Soil: petroleum jelly Jet angle in degree
Distance to the measuring site L in mm
Spread coefficient k of the water jet’s cross sectional diameter
90 80 70 60 45
200 201 211 228 256
1.65 1.66 1.77 1.95 2.12
Fig. 10 – Normal and shear stress needed to remove egg yolk from the surface depending on the jet angle and the swelling state.
certain tolerance range, this method can also be used to estimate the needed normal and shear stress for other jet angles. In order to improve the validity of the measured and calculated data the test rig used within this thesis can still be improved. Due to the geometrical circumstances the different jet angles could not be measured at the same distances between the nozzle and the test plate what increases the susceptibility to errors. Especially for the lower jet angles with a higher nozzle distance this can lead to break ups within the liquid jet. Therefore, a smaller and constant distance of the jet nozzle is preferable. Fig. 9 – Normal and shear stress needed to remove petroleum jelly from the surface depending on the jet angle. The calculated cross sectional area A2 is now used with Eq. (2) to determine the normal stress : (˛) =
4 ∗ FN 2 (D ∗ k (˛, L)) ∗ sin ˛ ∗
(4)
The total stress S and the shear stress can be calculated as follows by using trigonometrical functions (as described in Fig. 8b): S = (˛) ∗ sin ˛ (˛) =
S cos ˛
(5) (6)
It is possible to draw the calculated normal and shear stress values as a function of the jet angle, the soil and the swelling state what is shown for petroleum jelly (no swelling) in Fig. 9. The stress values are showing an elliptic characteristic. It is assumed that this shows the different influence of the normal and shear component on the cleaning effect. Therefore, the equation of an ellipse is calculated out of the stress values for 90◦ and 45◦ and the ellipse was drawn into the graph. It shows that the values for the other jet angles are lying very closely to this curve and all values are lying within the tolerance range (dashed lines, based on the previously shown error bars). According to this method, the stress values were also calculated for the limit gauge pressure values determined for egg yolk at the different swelling states. The resulting values and the calculated elliptic curves are shown in Fig. 10. The graph shows that this display format is very suitable to compare the cleaning effort needed at the different swelling states. The same applies for the comparison of different soils. Since all values fit very tight to the calculated ellipse within a
4.
Conclusions
The results of this paper show that it is possible with the developed method to describe the normal and shear stress required to remove different soils from a surface. By displaying the values in a stress diagram, it is possible to compare different soils and swelling states qualitatively and quantitatively. Within this work the form of the ellipses was calculated out of the measurements from two jet angles. The measurements at the other jet angles were used to validate the determined curve. Prospectively, the effort can be reduced by measuring only at two jet angles. Cleaning simulation systems can use this data to read out the normal and shear stress required for cleaning at any relevant jet angle. The method therefore is helping to close a crucial gap on the way to the simulation of cleaning processes. In order to validate the transferability to other cleaning mechanisms, like e.g. spray cleaning, it is recommended to extend the presented test method by additional cleaning tests.
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