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Research Papers: Multiphase Flows

Separation of Entrained Air Bubbles From Oil in the Intake Socket of a Pump Using Oleophilic and Oleophobic Woven and Nonwoven Fabrics

[+] Author and Article Information
David Bach

Plant Biomechanics Group Freiburg,
Botanic Garden,
Faculty of Biology,
University of Freiburg,
Schänzlestr. 1,
Freiburg 79104, Germany;
Freiburg Materials Research Center (FMF),
Stefan-Meier-Straße 21,
Freiburg 79104, Germany
e-mail: david.bach@fmf.uni-freiburg.de

Jan Hauke Harmening

Department of Engineering,
Westphalian University of Applied Sciences,
Gelsenkirchen 45877, Germany
e-mail: jh.harmening@gmx.net

Matthias Höfer

Department of Engineering,
Westphalian University of Applied Sciences,
Gelsenkirchen 45877, Germany
e-mail: matthias.hoefer@web.de

Tom Masselter

Plant Biomechanics Group Freiburg,
Botanic Garden,
Faculty of Biology,
University of Freiburg,
Schänzlestr. 1, Freiburg 79104, Germany;
Competence Networks Biomimetic and
BIOKON e.V.,
Schänzlestr. 1,
Freiburg 79104, Germany
e-mail: tom.masselter@biologie.uni-freiburg.de

Thomas Speck

Plant Biomechanics Group Freiburg,
Botanic Garden,
Faculty of Biology, University of Freiburg,
Schänzlestr. 1,
Freiburg 79104, Germany;
Freiburg Materials Research Center (FMF),
Stefan-Meier-Straße 21,
Freiburg 79104, Germany;
Competence Networks Biomimetic and
BIOKON e.V.,
Schänzlestr. 1,
Freiburg 79104, Germany
e-mail: thomas.speck@biologie.uni-freiburg.de

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received August 24, 2016; final manuscript received September 6, 2017; published online October 19, 2017. Assoc. Editor: Mhamed Boutaous.

J. Fluids Eng 140(3), 031301 (Oct 19, 2017) (16 pages) Paper No: FE-16-1546; doi: 10.1115/1.4037944 History: Received August 24, 2016; Revised September 06, 2017

Entrained air in oil can cause malfunctions and damages within hydraulic systems. In this paper, we extend existing approaches to reduce the amount of entrained air by separating air bubbles from oil using filter elements. The aim of this study was to investigate the ability of different untreated and surface modified woven and nonwoven fabrics (NWF) to separate air bubbles from oil when directly integrated into an intake socket of an oil pump. An experimental setup was constructed to generate entrained air in oil and to characterize changes in oil aeration and pressure drop induced by the filters. Measurements were conducted at volume flow rates of 2.2 and 5.4 l/min with an inflow angle normal to the filter elements. The developed setup and aeration measurement method proved to be suitable to generate entrained air in oil in a reproducible manner and to accurately characterize aerated oil up to air contents of about 5%. Significant influences on the aeration characteristics were found only for the NWF. Whereas the number of air bubbles decreased by up to 33% relative to the values in the oil reservoir for a flow rate of 2.2 l/min, a significant reduction of the volumetric air ratio could not be achieved as resulting bubble distributions comprised a higher number of large bubbles. We suggest that the lack of effective bubble separation was a result of the flow-induced pressure drop by the filters, which increased with the flow rate.

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Figures

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Fig. 1

Schematic drawings of the experimental setup. (a) Overview of the four different oil circuits used for oil aeration and sample analysis. (b), (c) Detailed computer-aided design images of the air injection unit (b) and the intake socket with the sample holder (c). Components: impeller pump with electric motor (1), hose (2), adjustable ball valve to regulate system back pressure, manometer with restrictor valve to take readings of the current back pressure (3), 12 liter oil tank (4), intake socket (5) with sample holder (6), white oil (7), circulating pump used for oil conditioning (8), venturi nozzle with sinter filter (sf) (aeration unit) (9), flow sensor with regulator valve and shut-off valve (10), three-way ball valve to switch between the oil aeration circuit and the sample testing circuit (11), oil sampling tube (ST) (12), Festo pneumatics hose (13), shut-off valves to switch between the two different oil sampling routes, 1: sampling from the oil reservoir via a sampling tube (14), 2: sampling from the intake socket (IS) (15), oil sample analysis unit (16), electric gear pump (17). Shaded part in (a) indicates the oil-filled part of the reservoir and the oil aeration circuit. Important operating parameters are recorded with a number of sensors (pressure sensor indicated by P/U, temperature sensor indicated by T/U).

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Fig. 2

Performed image processing steps with ImageJ to determine the rate of oil aeration for each oil sample. (a) Original image after cropping and scaling (image size: 1600 × 1166 pixels/approximately 2.0 × 1.5 mm). (b) Image after conversion to black and white using auto thresholding. Incomplete and truncated air bubbles at the image edges are colored gray. (c) Final processed image after elimination of incomplete and truncated bubbles (gray areas in (b)). The smallest complete air bubble in (c) has a diameter of 22 μm and the largest of 244 μm.

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Fig. 3

Overview of the texture (a)–(d) and wetting behavior (e)–(l) of the tested woven and nonwoven polyester filter fabrics. (a) and (b) Top view of woven filter fabrics with a mesh size of 22 μm (twill weave) (a) and 51 μm (plain or linen weave) (b). (c) and (d) Top view of nonwoven filter fabrics with an average pore size of 33 μm (c) and 60 μm (d). Wetting behavior of untreated (oleophilic) ((e), (g), (i), and (k)) and surface treated (oleophobic) ((f), (h), (j), and (l)) fabrics with a drop of white oil (15 μl). Woven filter fabrics with a mean mesh size of 22 μm ((e) and (f)) and 51 μm ((g) and (h)). Nonwoven filter fabrics with an average pore size of about 33 μm (i), (j), and about 60 μm (k), (l). (a)–(d) SEM micrographs.

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Fig. 4

Influence of woven and NWF on bubble count ((a) and (b)), relative bubble area ((c) and (d)) and relative bubble volume ((e) and (f)) for Q = 2.2 l/min ((a), (c), and (e)) and Q = 5.4 l/min ((b), (d), and (f)). Diamonds represent mean values. Woven and non-woven samples: oleophilic surfaces (left boxes and data points), oleophobic surfaces (right boxes and data points). Significant statistical differences between oil samples from oil reservoir and intake socket indicated by *(*0.05 > p ≥ 0.01, **0.01 > p ≥ 0.001, *** p < 0.001) (SM: support mesh, WC: coarse woven fabrics, WF: fine woven fabrics, NWC: coarse nonwoven fabrics, NWF: fine nonwoven fabrics).

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Fig. 5

Influence of woven and NWF on the bubble diameter distribution for Q = 2.2 l/min (a) and Q = 5.4 l/min (b) (n > 20,000). 15 μm threshold of minimum bubble diameter set for image analysis. Woven and nonwoven samples: oleophilic surfaces (left boxes and data points), oleophobic surfaces (right boxes and data points). Significant statistical differences between oil samples from oil reservoir and intake socket indicated by * (* 0.05 > p ≥ 0.01, ** 0.01 > p ≥ 0.001, *** p < 0.001), (SM: support mesh, WC: coarse woven fabrics, WF: fine woven fabrics, NWC: coarse nonwoven fabrics, NWF: fine nonwoven fabrics).

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Fig. 6

Induced mean pressure drop in the intake socket of the pump in dependence of the tested samples and volume flow rate (n = 6…7, CI of mean 95%, SM: support mesh, WC: coarse woven fabrics, WF: fine woven fabrics, NWC: coarse nonwoven fabrics, NWF: fine nonwoven fabrics)

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Fig. 7

Influence of pump back pressure and air supply on the oil aeration. (a) Measured relative bubble area in dependence of the measured back pressure of the circulation pump. (b) Calculated relative bubble volume plotted over measured back pressure. Relative bubble area and bubble volume determined from USB microscope images with a size of: 2000 × 2000 pixels/approximately 8.5 × 8.5 mm.

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Fig. 8

Aeration and deaeration process for a pump back pressure of 2 bar, an oil flow rate of approximately 6.5 l/min and an air supply of 100 ml/min. Development of mean bubble count (a), mean relative bubble area (b), and mean relative bubble volume over time (c) (n = 5, confidence interval (CI) of mean 95%). The shaded regions indicate the deaeration process with the circulation pump shut off; the dotted lines mark the mean value calculated from the time interval 5–10 min. Development of air bubble diameter distribution over time during the aeration process (d).

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Fig. 9

Influence of the sequence of sample taking for the control group measurement (only support mesh installed) (n = 6). Sampling location: oil reservoir (left boxes and data points), intake socket (right boxes and data points).

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