Hayder A. Abdulbari, Fiona Ling Wang Ming, Wafaa K. Mahmood

1. Center of Excellence for Advanced Research in Fluid Flow, Universiti Malaysia Pahang, Gambang 26300, Kuantan, Pahang, Malaysia, E-mail:hayder.bari@gmail.com

2. Faculty of Chemical Engineering and Natural Resources, University Malaysia Pahang, Lebuhraya Tun Razak 26300 Kuantan, Pahang, Malaysia

(Received February 15, 2015, Revised June 24, 2015)

Insoluble additives for enhancing a blood-like liquid flow in micro-channels＊

Hayder A. Abdulbari1,2, Fiona Ling Wang Ming2, Wafaa K. Mahmood1

1. Center of Excellence for Advanced Research in Fluid Flow, Universiti Malaysia Pahang, Gambang 26300, Kuantan, Pahang, Malaysia, E-mail:hayder.bari@gmail.com

2. Faculty of Chemical Engineering and Natural Resources, University Malaysia Pahang, Lebuhraya Tun Razak 26300 Kuantan, Pahang, Malaysia

(Received February 15, 2015, Revised June 24, 2015)

This study introduces an approach for flow enhancement in the bloodstream using insoluble additives as non-degradable drag reducing agents that can replace the polymeric soluble additives. An open micro-channel liquid flow system with three different channel sizes was assembled and used to test the drag reduction performances of the solutions investigated. Three different nanopowders (with five different addition concentrations) were investigated and used to form solutions of artificial blood with blood-like rheological properties. The experimental results showed that the optimum drag reduction performance was achieved using bismuth III oxides (65%) for a 200 ppm concentration solution flowing through a 100 μm channel, while titanium IV oxides and fumed silica achieved 57 and 55% drag reduction for a 200 ppm concentration solution flowing in a 50 μm channel, respectively.

Microchannels, pressure drop, drag reduction, nano-powder additives

Introduction

Flow enhancement in pipes and conduits using minute quantities of viscoelastic polymeric additives was first introduced by Toms in the early 1940s. Since then passive[1-4]and active[5-8]drag reduction techniques were introduced and tested by many researchers. Active drag reduction techniques inspired many researchers to introduce different drag reducing agents (DRAs) that can enhance the liquid flow in pipes up to 80% by addition of only a few parts per million of additives. This phenomenon was easily incorporated into many industrial applications, such as pipeline transportation, firefighting, drag reduction in marine vessels, slurry transportation, and heat transfer, among many others.

One of the important medical applications for the drag reduction phenomenon is the enhancement of blood flow in blood streams using long chain polymeric additives[9-11]. Several authors reported a substantialreduction in blood pressure and degree of turbulence by injecting low concentrations of polyethylene oxide (PEO)[12-15]. This polymeric additive was highly capable of laminarizing the turbulent blood flow structures (eddies) and reducing the pressure fluctuation along the blood stream. It was also found that the addition of a long chain polymeric DRA to RBC-phosphate-buffered saline could reduce the thickness of the plasma layer in an artificial macrovesicle system. One of the conditions for any additive to distinguish itself as a drag reducing agent is its solubility in the flow media, a criterion that reduced the availability of economically feasible additives that can be safely used to improve flow in blood streams[16]. Although the tested soluble additives showed excellent drag reduction performance in blood streams, their health effects are not yet completely known due to the artificial nature of these additives and the potential risk of degradation due to the possibilities of reactions with other components of the blood.

The flow of liquids in microchannel was utilized to simulate the flow behavior in real blood streams. It is known that most of the blood flow in the human normal blood vessels is laminar, but turbulent flow will occur when clogged blood vessels are investigated[16]. Superhydrophobic surfaces are considered the

most successfully tested techniques to enhance the laminar blood-flow in vessels[17]. Ou et al.[18]fabricated ultrahydrophobic surfaces from silicon wafers using photolithography. They demonstrated and reported significant drag reduction effect for laminar flow of water in the modified channel (40% pressure drop reduction). The mechanism controlling the drag reduction of superhydrophobic surfaces is completely different when compared with that using additives. Superhydrophobic surfaces are known to show drag reduction behavior due to the slipping effect associated with a thin film of air trapped at the liquid-solid interface which will reduce the turbulent skin friction at different Reynolds numbers. However, the drag reduction mechanism of soluble drag reducing agents depend on the operation conditions, additives types and properties and the flow behavior where drag reduction can only be observed at turbulent flow systems.

Table 1 Bismuth (III) oxide powder physical properties

Fig.1 Particles size distributions of the Bismuth (III) oxide powder using XRD

Many authors showed insoluble additives (fibers) to be effective and environmentally friendly drag reducing agents with high flow enhancement capabilities[19-21]. The insoluble, nonreactive and non-degradable nature of these additives encouraged many researchers to consider them effective additives to replace the commercially available artificial polymers to enhance the flow in pipes. It is well known that when fibers are introduced into the main flow of any closed flow system, their drag reduction capabilities will be controlled by the degree of turbulence inside of the flow media itself[22]. The suspended fibers have a tendency to accumulate and interact even at low concentrations, leading to the formation of small entities that fully interact with the flow structures in a manner different from the interactions between the structures and the individual fiber particles[23]. These entities will act as drag reducing agents with turbulence suppressing properties when interacting with the turbulent flow. Several mechanisms were suggested in the literature, and each mechanism was matched with the properties of the fiber investigated, such as wood pulp[24]and nano-powder[25].

Atherosclerosis (hardening and narrowing of the arteries) is considered to be the number one killing diesis that can cause heart attacks, strokes, and peripheral vascular disease. Such narrow vessels can cause turbulence when the blood flows through, which will produce an increased pressure in the blood vessels and back pressure due to the creation of eddies. In this study, insoluble nano-powders are investigated as drag reducing agents in rheological blood-like streams using microfluidics technology. The goal of this study is to prove the efficiency of insoluble additives as effective drag reducing agents to replace soluble polymeric additives. An artificial rheological blood-like liquid is used to test the effects of adding three different types of nano-powders, with five different concentrations for each powder. The experimental investigation is performed in three microchannels with three different diameters, ranging from 50 μm to 200 μm. Note that the fibers or powders investigated are not intended for use in human blood streams. The purpose of this investigation is to prove the feasibility of using powders with these physical properties as drag reducers.

1. Experimental procedure

1.1Materials

Bismuth (III) oxides, titanium (IV) oxides, and fumed silica were purchased from Sigma Aldrich and used as provided. These nano-powders were selected because of their size range of 100 nm-140 nm. The apparent physical properties of the selected powders are listed in Tables 1, 2 and 3. The particle size distribution was measured using X-Ray Diffraction (XRD) technique and the results are shown in Figs.1 to 3.

Fig.2 Particles size distributions of the fumed silica powder using XRD

The transported liquid was prepared using the method described by Liepsch et al.[26]in which a rheological blood-like liquid was prepared using 5% dextran (70 000 MW), 10 mmol calcium chloride and 12% polystyrene particles (1 μm diameter). It is believedthat the large differences between the polystyrene particle size and the investigated powders are sufficient to show the drag reduction effect. Nanoparticle solutions were prepared at five different concentrations for each oxide, ranging from 100 ppm to 500 ppm. The fluids were homogenized at high speed for 6 h with a stirrerand left overnight on low speed to achieve maximum dispersion. Before each experiment, the solution was stirred for 2 h at 100 rpm to avoid agglomeration.

Fig.3 Particles size distributions of the TiO2powder using XRD

Fig.4 Schematic diagram of the experimental set-up

1.2Experimental set-upthis study. The setup consists of a syringe pump

Figure 4 shows the experimental setup utilized in (model: SN-50F6), a differential pressure transmitte (model: STK336), custom-made microchannels (with three different sizes: 50 μm, 100 μm, and 200 μm fabricated by the Chip-shop (Topas) in Germany, con nections, and a collection beaker. The assembly begins with a syringe pump with two syringes connected with a T-connection, and a micro-tube, respectively. The outlet of this microtube is directed to another T-joint One of the outlets of the T-joint is directed to the differential pressure transmitter, which measures the channel inlet pressure, whereas the other outlet is dire ctly connected to a microchannel through a micro-tube The outlet of this microchannel is connected to anothe T-joint, one of the outlets of the T-joint is connected to the differential pressure transmitter, which measures the outlet pressure, and the other outlet leads to the collection beaker, into which the liquid is discharged.

1.3Experimental procedure

The flow rate of the solution was controlled by setting the required flow rate using the syringe pump. The range of the solution’s flow rate investigated was from 2 500 ml/h to 1 600 ml/h. The pressure drop across the microchannel was observed using a pressure transmitter, and the results were recorded. The drag reduction efficiency (%DR)was calculated using Eq.(1)

where ΔPbis the pressure drop before adding DRA, ΔPais the pressure drop after adding DRA.

Graphs of the%DRversus Reynolds Number are plotted, in which the Reynolds Number is calculated using the following equation

whereρis thedensity of water,vis the velocity,dis the diameter of the microchannel,μis the viscosity of water.

The density of the initial transported liquid was 1.03 kg/m3measured using pycnometer. The velocities for the flow in all the channels investigated is tabulated in Table 4.

Table 4 Measured liquid velocities in micro-channels

Fig.5 Effects of the titanium powder concentrations and Reynolds numbers on the%DR for a solution flowing in channels of length

2. Results and discussion

Figures 5 to 7 show the effects of the powder concentrations (C)and Reynolds Number (NRe)on the drag reduction performance of the investigated additives. It is clear that all of the powders at each concentration showed a good drag reduction performance. Table 4 shows the maximum%DR observed us ing the insoluble additives investigated.Interestingly,anideal %DR-NRerelationship was not observed in which the normal shape should show an incline, peak, and decline behavior (by increasing theNRe), which is considered a typical representation of the additiveturbulence interaction that will lead to the turbulence suppression phenomenon controlling the drag reduction. In this study, a nearly stable drag reduction performance of the insoluble additives was observed. It is believed that this behavior is due to the nature of the additive itself, i.e., the insoluble additives are not degradable, and their properties do not change when they interact with the turbulent structures formed during turbulent flow. The traditional soluble additive may degrade and lose its drag reduction ability when exposed to high shear forces exerted by the pumps or eddies themselves, leading to ineffective performance of drag reduction. Dubief et al.[27]introduced an interesting correlation between the polymer molecules dissolved in the transported solution and the turbulent structures formed in the main core and near the pipe wall. They stated that the polymers are pulled out from the near wall region by the flow vortices and stretched in the vortex structure itself and then it will be re-injected to the near wall region again. Such mechanism will result in a deformation action exerted by the turbulence which will in many cases lead to extensive stretching effect that will result in the degradation of the polymeric molecules. This degradation cannot occur when using insoluble additives.

Fig.6 Effects of the bismuth powder concentrations and Reynolds numbers on the%DR for a solution flowing in channels of length

Fig.7 Effects of the fumed silica powder concentrations and Reynolds numbers on the%DR for a solution flowing in channels of length

Figures 5 to 7 also show the effect of concentration on the drag reduction performances of the investigated additives. Surprisingly, the effect of the additive concentration was not linear and showed an optimum concentration (200 ppm) at which the highest%DR was achieved. Increasing the concentration resulted in lower %DRvalues (although remaining high) compared with 200 ppm. In most of the cases, the lowest %DRwas observed when the additive concentration was 500 ppm. The other surprising phenomenon observed is the effect of the additive type and its relationship to the additive concentration. All of the results indicated that regardless of the additive type, the maximum%DRwas achieved at 200 ppm, which is a direct function of many different parameters, such as the powder density and particle size, as highlighted in Tables 1 to 3. This irregular behavior might be due to th edegree of turbulence inside of the microchannelsand its relationships with particle size and particle density. Increasing the particle concentration increases the number of particles involved in the drag reduction process. Because the additive concentrations were measured on a weight/weight basis, there should be a higher number of particles for the fumed silica than the bismuth (for example), which should lead to an increase in the%DR . However, this increase did not occur in our study. Instead, all of the maximum %DRvalues were achieved at 200 ppm. The range of turbulence investigated did not sufficiently involve all of the suspended particles in the drag reduction system. Thus, 200 ppm is the optimum or maximum concentration that the additives in this concentration range can effectively act as DRAs (within the investigated experimental parameters). Table 5 shows the maximum %DR achieved in the present work.

Table 5 The maximum %DRachieved by DRAs

Fig.8 Effect of powder types on the%DR for solutions (200 ppm) flowing in channels of length

Fig.9 Effect of powder types on the %DRfor solutions (400 ppm) flowing in channels of length

Figures 8 and 9 show the effects of the additive type on the %DRfor selected samples of the experimental data. The results confirm the irregularities highlighted in the previous sections. Figure 8 shows that the bismuth particles showed the highest%DR, followed by the titanium and fumed silica particles. Surprisingly, increasing the additive concentration from 200 ppm (Fig.8) to 400 ppm (Fig.9) changed this behavior, and the fumed silica particles showed the highest%DR, followed by the titanium and bismuth powders (Figs.9(a) and 9(b)). Increasing the channel size resulted in different findings, as shown in Fig.9(c), in which titanium showed the highest%DRcompared with the other particles. One of the goals of selecting the powders in the present study was to investigate the effect of the particle density on the drag reduction performance, but no final or confirmed behavior can be stated in this case because of the interactive effects of all of the parameters investigated. Figure 8 shows the expected behavior in which the particles with the highest density showed the highest%DR values. It is believed that when the suspended nanoparticles interact with the turbulence structures in the main flow stream, the apparent physical properties of eddies change, leading to a large increase in the apparent density of the eddy itself. This process will result in reducing the size and shape of eddies, assuming that the same energy will be extracted from the main stream that formed the eddy in its initial shape, leading to a reduction of the amount of energy dissipated.

Figure 9 suggests a different theory in which the effect of the particle density was not dominant, revealing unexpected results. It is believed that the argument in the previous section is disproved by increasing the additive concentration, in this case, the effect of the additive concentration is dominant, and the effect of the additive type is minor. Because the size of the particles is nearly the same in this case, increasing the concentration led to a large increase in the particle number of the low-density additive (fumed silica) compared with the other two high-density particles.

Figures 10 to 12 show the effect of the microchannel size on the drag reduction performance of selected experimental data. Figures 10 and 11 clearly show that the maximum drag reduction performances were achieved in 100 μm channels with bismuth and titanium solutions, while the maximum %DRwas achieved in 50 μm channels when the fumed silica solutions were tested. It is believed that the interaction between the particle properties (density) and the degree of turbulence(NRe)plays a major role in controlling the drag reduction performance in this case. In small channels (50 μm and 100 μm), the degree of turbulence is high, leading to the increased interaction of the suspended particles with the turbulence media, thus, high-density particles will be more effective for drag reduction than low-density particles. The experimental results show that the optimum turbulence-particle interaction occurs at 100 μm. However, in large channels (200 μm), the degree of turbulence will be lower (at the same flow rate), causing low-density particles to perform better in the drag reduction system, thus, the degree of interaction will be higher in small channels.

Fig.10 Effect of channel size on the %DRin a 400 ppm bismuth solution

Fig.11 Effect of channel size on the%DR in a 400 ppm titanium solution

Fig.12 Effect of channel sizes on the%DRin a 400 ppm fumed silica solution

Generally speaking, the mechanism of drag reduction and how the additives act and affect the flow behavior in pipes, ducts and microchannels was under long and detailed debate and argument since the first observations were made by Toms. The reason behind that is the chaotic nature of the transported media (turbulent flow) where there is no clear or possible mapping that can give a distinguished repeatable image or turbulent-structure that can help in introducing a clear mechanism controlling the flow. In the literatures[28-35], several mechanisms are suggested for thedrag reduction behavior of insoluble additives (suspended solids) in pipelines and ducts. Some of these theories suggest that the flow enhancement occurs due to: a decrease of the liquid viscosity, electrostatic effects, suppression of turbulence, stabilization of transition from laminar to turbulent, and introduce new length and time scales for the turbulence characteristics. All these mechanisms were suggested for the flow in pipes and ducts with dimensions that allow complete formation of turbulent structures inside the pipe or duct itself. It is believed that, the size and shape of the turbulent structures formed during the turbulent flow in conduits are considered as the main pumping power absorbers and suppressing these absorbers will reduce the amount of power dissipated.

In the present work, for the case of micro-channels, we have two theoretically assumed cases which are the flow in straight micro-channels and the flow through clogged (orifice-like) channels. For the first case, laminar sublayer and the transition layers will dominate the scene due to the reduction in the conduit size that eliminates the effect of the turbulent flow core. Introducing nano-powders to the main turbulent flow will result in introducing new properties to the turbulent structures (eddies) formed in the transition layer. Knowing that the degree of turbulence in the transition layer is lower than that in the turbulent core, the main turbulence suppression will occur at that layer and all that will result in laminarizing this layer and increasing the laminar sublayer thickness. The turbulence suppression mechanism is closely related to the properties of the particle itself (density, size and addition concentration) and the suspension stability and its relation to the channel geometry and size. The turbulence suppression will occur when the apparent eddy density is higher (due to the presence of suspended solids) and that will prevent the liquid globe forming this eddy from completing its theoretical shape which will result in reducing the amount of power consumed or dissipated. The experimental data showed that, the effect of the particle concentration on the drag reduction performance is not linear where most of the maximum drag reduction occurred at 200 ppm and any further increase in the concentration resulted in an opposite drag reduction effect. Increasing the particle concentration means increasing the number of particles suspended in the same liquid globe that is forming the eddy and that is highly sensitive to the other controlling factors such as the particle size and density. It is believed that the optimum interaction and suspension properties were achieved at 200 ppm and any further increase in the concentration resulted in overloading the turbulent structures with more suspended particles. And that can form certain kind of solid aggregates due to the interaction forces and these aggregates will be separated from the main suspension stable structure. For the case of clogged (orifice-like) streams, the flow structure is different and the turbulent eddies are expected to be much more clear due to the liquid jetting this orifice-like contraction will create. The eddies created by this contraction in the blood stream is closer (in shape) to the real or normal eddies formed in pipes or conduits due to the direct jetting effect in the core of the main flow. Such flow behavior can result in reducing the thickness of the laminar sublayer. Eddie suppression will also be reached through increasing the density of the initial liquid globe forming eddies and that will prevent the turbulent structures from completing its shape.

3. Conclusion

This study introduced insoluble additives as potential drag reducing agents in blood vessels using rheological blood-like liquid. The nano-powders investigated showed high drag reduction abilities over the investigated range of additive concentrations. The drag reduction performance was not identical to that recorded using pipes, and the effect of the small channels or conduits was clear in changing the hydrodynamic features of the system. The usage of insoluble powders will secure the single dosage of the additives in the blood streams without fear of any degradation that might occur when using polymeric soluble drag reducing agents. The effects of the additive concentration and solution flow rates were highly collaborative with the channel geometry, and no general conclusion can be drawn, while for effects on an individual case basis can be established, depending on how these variables interact. It is believed that medically proven insoluble additives should be examined in future studies to prove the drag reduction effect and to progress toward the introduction of a commercially feasible insoluble additive that can be used in the human body.

Acknowledgment

The authors are grateful to the University Malaysia Pahang for their financial assistance.

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* Biography:Hayder A. Abdulbari (1973-), Male, Ph. D., Professor