Yam mucilage is a novel environmentally friendly drag reducer. This study investigates drag reduction and degradation characteristics of Chinese yam mucilage, using an in-house rotor device. The effects of temperature, aging, and salts on the drag reduction rate (DR) of yam mucilage were also explored. Furthermore, the synergistic drag reduction properties of Chinese yam-polyethylene oxide (PEO) solution were investigated by blending Chinese yam mucilage with PEO. The rotational speed range of the rotor device was set at 200–700 r/min, corresponding to Reynolds numbers (Re) ranging from 30 396 to 106 385. The results demonstrated that the DR of yam mucilage initially increased, and then decreased at low concentrations, with increasing Re. Conversely, the DR of yam mucilage at high concentrations increased with increasing Re, albeit at a gradually slowing rate as concentration increased; however, the shear stability was gradually enhanced. Degradation testing revealed that yam slime exhibited semi-rigid, or rigid polymer characteristics, with notable shear stability. At a concentration of 2000 ppm and Re = 106 385, the maximum DR reached 44.1%. Prolonged heating and standing resulted in the reduced DR of yam mucilage. However, salt ions exerted dual effects on the DR of yam mucilage: Low concentrations of NaCl improved its effectiveness while Na2SO4 and high concentrations of NaCl diminished its efficacy. The addition of a small quantity of PEO was found to significantly enhance the drag reduction efficacy of yam mucilage, but no significant improvement in the shear stability of yam mucilage was observed. Yam mucilage exhibits promising potential as an environmentally friendly drag reducer with remarkable drag reduction capabilities.

As a method of significantly reducing turbulence resistance, drag reduction (DR) by polymers has been extensively studied since its discovery by Toms.1 After years of exploration and development, it has been successfully applied in applications in various fields, including firefighting, irrigation, biomedical engineering, and pipeline transport. Polymer additives used in pipeline and external flows can be classified into two groups: synthetic polymers-derived from petroleum oil, and natural polymers extracted from resources found in nature.2 It is widely acknowledged that the optimal drag-reducing polymers possess a flexible, linear chain structure with an exceedingly high molecular weight.3 Nevertheless, such polymers are restricted by their vulnerability to flow-induced degradation, which leads to a substantial reduction in drag reduction efficacy. Slow biodegradation rates also raise environmental concerns.4 Natural polymers, on the contrary, are both biodegradable and readily available. Microorganisms and plants produce these materials in the form of polysaccharides. Compared to flexible synthetic polymers, these polymers offer high mechanical stability against degradation; natural polymers of comparable molecular weight are also more cost-effective than their synthetic counterparts.5 Consequently, numerous studies have focused primarily on rigid polymers, despite their pronounced susceptibility to biological degradation.6 

Currently, the focus of research in the field of rigid polymers revolves primarily around guar gum (GG), xanthan gum (XG), and carboxymethyl cellulose (CMC). As early as 1965, Gadd7 compared drag reduction and degradation properties of polyethylene oxide (PEO) and GG. He hypothesized that turbulence mechanically broke up long PEO molecules, causing them to lose their effectiveness. In contrast, little or no mechanical degradation seemed to occur with guar gum solution. The stability of GG and XG over time was found to be superior to that of typical synthetic water-soluble drag reducers (e.g., polyethylene oxide), and the efficiency of DR was closely related to molecular parameters, including stress level, temperature, polymer species, molecular weight, and polymer-solvent interactions.8 The DR of XG can be significantly enhanced under pseudo-homogeneous conditions due to residual polymer entanglements and network structures that do not fully relax or disentangle upon solution dilution.9 A more interesting observation was that the presence of salt also impacted the subsequent drag reduction efficacy of XG by altering its initial polymer conformation. In addition, XG's solution is highly influenced by a pre-shearing, suggesting the existence of polymer aggregates.5 However, the polysaccharide solution with high concentration can also increase the resistance at low flow rates (Re).10 Singh et al.11 synthesized CMC from banana peel by carboxymethylation. The presence of biopolymers in water assists in the performance of the drag reduction agent, and solubility also increases when the percentage of NaOH increases. Salehudin and Ridha12 synthesized the CMC from coconut residue under the alkali-catalyzed reaction. The performance of the synthesized CMC was assessed in a water injection system at different CMC concentrations and flow rates.

Other researchers have conducted broader experiments with the types of natural drag reduction agents. Gasljevic et al.13 found that all the species of marine microalgae tested generated drag reduction effectiveness, allowing the possibility of using them to produce drag-reducing biopolymers for engineering applications. Ogata et al.14 focused on nata de coco (a type of biopolymer that exhibits low mechanical degradation) and found that it reduced drag by up to 25% at a concentration of 50 ppm. They thought the DR effect appeared only when a network of nata de coco fibers was formed in the suspension. Santos et al.15 studied a natural drag reducer, diutan gum (DG), and the maximum DR measured was more than 70%. Moreover, DG has significant thermal stability and shear stability compared with XG and PEO. Recently, Xie et al.16 used two new bio-polysaccharides [locust bean gum (LBG) and tragacanth gum (TG)] to prepare the gelatin-based bio-polysaccharide drag reduction coating, and they obtained a drag reduction effect of more than 20%. However, the drag reduction effect of these rigid polymers was lower than that of typical flexible polymers like PEO.

Relevant studies have employed diverse methodologies to enhance the drag reduction and shear stability of polymers. Sokhal et al.17 observed that GG experienced mechanical degradation under high shear conditions, and the addition of KCl improved shear stability up to 47% (for Re ≈ 45000). However, increasing salt concentration had a negative impact on DR. Dosumu et al.18 found that the DR of GG increased with the pipe diameter and decreased with the oil fraction in oil/water flows. The DR of the solution was enhanced by incorporating a combination of diverse polymer types.19 A composite of rigid (carboxymethyl cellulose, CMC) and flexible (PAM and PEO) polymers had a positive synergistic drag reduction effect, while a composite of two flexible polymers (PAM and PEO) had a negative synergistic effect.20 Polymer and fiber had a synergistic effect causing the fiber to reduce the influence of mechanical molecular degradation on polymer drag reduction.21,22 Sandoval et al.23 demonstrated that polymer-polymer combinations (PAM-XG and PEO-XG) exhibited higher DR and more anti-shearing properties than single polymer solutions with the same concentration. Novelli et al.24 proposed that the incorporation of polymers containing at least one rigid-chain constituent could lead to synergistic drag reduction, with higher levels of synergy observed for polymers possessing similar molecular weights. Maximum drag reduction was achieved when the ratio of flexible polymer to rigid polymer was 4:1. Recently, our group investigated the synergetic drag reduction of PEO and DG, revealing that the mixed solution exhibits a more significant drag reduction and enhanced shear stability.25 The current research findings demonstrated that the combined utilization of rigid and flexible polymers yielded a pronounced synergistic effect on drag reduction.

Furthermore, extensive research has been conducted on the drag reduction performance of natural plant mucilage, revealing a more pronounced drag reduction effect exhibited by the mucilage. The combination of okra fiber and its mucilage was utilized as a novel bio-drag reducer in a turbulent water flowing system. One thousand ppm of okra mucilage resulted in a maximum drag reduction percentage (DR%) of 71%.26,27 Aloe vera mucilage (a natural and environmentally friendly drag reducing agent) was introduced as a flow improver for pipelines operating under turbulent flow conditions. By adding 400 ppm of mucilage to the main flow, a maximum drag reduction percentage of 63% was attained.28 Soares et al.6 subsequently investigated the impact of aging on the drag-reducing capacity of aloe vera using both a rotating apparatus and pipeline systems. It was found that young mucilage samples with higher levels of complex polysaccharides and lower acid contents exhibited superior efficiency as drag reducers. Xie et al.29 recently reported a new type of natural drag reducer—yam mucilage, which demonstrated a remarkable drag reduction effect, exceeding 25% in turbulent flows with injection. However, a comprehensive investigation of the shear stability of yam mucilage is still lacking in the existing body of research.

In this study, the comprehensive drag reduction properties of yam mucilage were investigated on the in-house rotor system, building upon the discovery made by this research group regarding its drag reduction capabilities. It was reported that the Chinese yam mucilage exhibited shear-thinning behavior and possesses viscoelastic properties.30,31 This report resulted in comprehensive tests to assess drag reduction capabilities, degradation characteristics, and influential factors of yam mucilage. A synergistic analysis of yam mucilage and PEO was subsequently conducted with the goal of enhancing the drag reduction performance of yam mucilage.

Fresh yam was purchased from a vegetable market (Fig. 1). For the yam mucilage extraction, the fresh yam was peeled and washed in distilled water; it was cut into pieces which were ground in a blender with an equal amount of water for 3 min30 completely dissolving the gel in the yam, and concurrently reducing the viscosity of the mucilage for convenient extraction. The blender facilitated the separation of yam residue and yam gel post-pressing. The yam mucilage was transferred to a centrifuge and subjected to centrifugation at a rotation speed of 4000 r/min. Following this, the supernatant obtained from centrifugation was decanted into a beaker as the concentrated mucilage in its final form. Steps for preparing the yam mucilage solution are shown in Ref. 29.

FIG. 1.

Chinese yam and the mucilage.

FIG. 1.

Chinese yam and the mucilage.

Close modal

A small portion of the yam concentrated mucilage was weighed and dried in an incubator at 80 °C for 8 hours to determine its concentration based on the resulting dry weight. This calculated concentration was considered as representative of the overall solution substances present in the mother solution of yam. It is important to note that, since the specific drag reducing component within yam mucilage remains unknown, here “concentration” refers to the collective concentration of all the solution constituents.

The concentrated mucilage was subsequently diluted to obtain yam mucilage solutions with concentrations of 100, 150, 300, 500, 1000, and 2000 ppm for experimentation. The diluted mucilage at these specified concentrations was then gently stirred for 10 min and transferred to a suitable container.

The present study investigates the impact of Chinese yam mucilage aging on drag reduction. Specifically, the 500 ppm solutions were placed in a large beaker, sealed with plastic wrap, and stored in a refrigerator at 5 °C. Samples of stored yam mucilage were extracted daily (1, 2, 3, 4, 5, 6, and 7 days) at consistent time intervals for drag reduction testing.

Different concentrations of NaCl and Na2SO4 solution were added to the yam mucilage of 1000 ppm to observe the effect of ions on the drag reduction. The mass of the salt powder was measured using digital scales with a resolution of 0.01 g. The salt powder was gradually introduced into the yam solution while being stirred at a rate of 70 r/min for 10 min to achieve complete dissolution and avoid degradation. Finally, yam salt solutions containing NaCl with 0.005, 0.01, 0.05, 0.1, 0.2, 0.4 mol/l, and Na2SO4 with 0.005 mol/l, 0.2 mol/l, respectively, were prepared.

The active component of yam mucilage is a protein and saccharide-based mucilage, which exhibits temperature sensitivity.30,31 Therefore, the impact of temperature on the drag reduction efficiency of yam mucilage was investigated. First, four equal portions of the yam mucilage mother solution were weighed and heated to temperatures of 40, 60, 80, and 100 °C for a duration of 5 seconds, respectively (prolonged heating could result in precipitation of dissolved substances). Subsequently, the heated mucilage extraction was cooled to room temperature and diluted with de-ionized water to obtain a concentration of 500 ppm. Finally, the solutions were stirred at a rate of 70 r/min for 10 min to achieve complete dissolution and avoid degradation for rotor testing. The drag reduction efficiencies of the heated yam mucilage under different Reynolds numbers (Re) were compared with those without heating at ambient temperature (25 °C).

The flexible polymer utilized in this experiment was PEO (provided by Sigma-Aldrich) with a molecular weight of 8 × 106g/mol. The mass of the polymer powder was measured using digital scales with a resolution of 0.01 g. The polymer powder was gradually introduced into distilled water while being stirred at a rate of 70 r/min for half an hour, to achieve complete dissolution and avoid degradation. During the present work, a total concentration of 150 ppm of Yam-PEO mixtures was prepared. Rheological properties, drag reduction, and degradation performance of the mixed solutions were analyzed. All tests were performed at room temperature of 25 °C.

The rheological experiment was conducted to determine the viscosity and moduli of the solution in different states. At the same time, the rheological rule of yam mucous under different solution ratios was studied. The HAAKE MARS III (manufactured by Thermo Scientific, Germany) was used to measure the rheological properties, and the cone-plate geometry device was applied. The upper plate was the cone plate with an angle α = 1° and diameter D = 35 mm, while the lower plate was flat. The distance between the two plates was a = 0.052 mm.

The drag reduction test was conducted using the in-house rotor apparatus, as shown in Fig. 2. This device (a motor-driven cylinder) consisted of an outer fixed rotor, an inner rotating rotor, a coupling, a torque sensor (HBM T21WN), a motor (MAXON 353297), a motor driver (ESCON 70/10), a data acquisition card (DAQ Card, National Instruments, NI, USB 6210), and a computer system. The motor speed controller determined the rotation speed, and the motor drove the torque sensor, which connected to the cylindrical rotor through the coupling. The torque sensor sensed the physical quantity signal of the friction torque and converted it to an analog quantity electrical signal. An encoder was attached to the torque sensing, which converted the shaft speed of the speed sensor and the rotor speed to a digital signal. These two signals were collected by the data acquisition card and sent to the computer. As a result, the motor with a rated torque of 0.8 N m and a rated speed of 3420 RPM drives the rotor at a rated speed of 1221 RPM. The sample frequency was 1000 Hz. To ensure sufficient experiment time and minimize the impact of polymer degradation, the sample duration was 10 s, that is, an average of 10 000 points. During this 10 s, the shear degradation of the polymer was very weak, so it was ignored.

FIG. 2.

Schematic illustration of the rotating cylinder system.

FIG. 2.

Schematic illustration of the rotating cylinder system.

Close modal
The radii of the inner (R1) and outer rotors (R2) were 108 and 120 mm, respectively, while the heights of the inner rotors were 216 mm. The distance between the outer and inner rotors was Δ = R2-R1 = 12 mm, making the gap ratio η = R1/R2 = 0.9. The Reynolds number (Re) was defined as follows:
Re = ρ ω R 1 Δ η ,
(1)
where ρ is the liquid density, and η is the solvent's viscosity. Re could be adjusted by setting the rotation speed (n) of the device. The set rotating speed and corresponding Re are presented in Table I for this experimental investigation.
TABLE I.

Re vs n.

n(r/min) 200 300 400 500 600 700
Re  30 396  45 594  60 791  75 989  91 187  106 385 
n(r/min) 200 300 400 500 600 700
Re  30 396  45 594  60 791  75 989  91 187  106 385 
The distance between the bottom surface of the inner rotor, the bottom plate of the inner rotor, and the top of the upper cover plate was equal to 2 mm, which meant that the surface height of the liquid in the rotor was L = 220 mm. The rotor was merely fixed to the shaft. Therefore, the friction force received by the rotor in the selection process is only the friction force of the bearing. Because mechanical torque (Tm) resulted from the bearings (which supported the shaft and rotors), a more accurate calculation of DR is obtained by Kim et al.32 
DR = ( T w T m ) ( T p T m ) T w T m × 100 = T w T p T w T m × 100 ,
(2)
where Tw is the torque when the device was filled with water, and Tp is the torque when the device was filled with the polymer solution.
At the same time, the maximum torque when there was no solution in the rotor was far less than the friction of the rotor when it was filled with water. At higher Re, Tm was be much smaller than Tw, so this part of the friction was ignored when calculating the drag reduction rate, which was the same as that in the literature, and DR can be obtained directly from the follow equation:32,33
DR = T W T P T W × 100 .
(3)

The friction of the bearing is very small, and the impact on the drag reduction rate is less than 0.96%. The corresponding percentage uncertainty of drag reduction was 2.28%.

During the test process, the temperature of the liquid in the rotor rose due to high-speed rotation, so a square plexiglass thermostatic bucket was installed outside the cylinder, which is filled with water to maintain the stability of the liquid temperature inside the rotor. It was observed that the constant temperature water box did not consistently maintain a stable temperature of 25 °C. Fluctuations were still present, resulting in an actual temperature range of (25 ± 1) °C.

Applying Rajappan's methodology,34,35 the transition to featureless (Newtonian) turbulence in the particular case of the in-house rotor apparatus was observed to occur at Rec ≈ 2.7 × 104. To achieve featureless turbulence in the rotor, the initial Re was set at 30 396. In addition, the method of Hu et al.36 and Greidanus et al.37 was adopted to determine the contribution of Tb, the torque coming from both the top and bottom cylinder flat surfaces. In this process, the results showed that the measured ratio Tb/Tw ranged from 0.156 to 0.203 for different rotating speeds. It is notable that the final DR was not affected by this proportion.

Prior to the formal experiment, pure water was subjected to repeated tests. The pure water was successively tested from low to high rotation speeds and from high to low rotation speeds before discharging the water. After one round of experiments, the water was discharged, and the same volume of water was added to the rotor to test the torque at each speed. The measurement was repeated 12 times. As shown in Fig. 3, the experimental device exhibited high accuracy and good repeatability (the error bar is partially obscured by the symbol, impeding its visibility and rendering it less discernible), allowing for a single measurement in the subsequent testing process.

FIG. 3.

Relationship between torque and Re of water.

FIG. 3.

Relationship between torque and Re of water.

Close modal

In this study, two experiments were conducted: the main focus was on drag reduction testing, while the other part involved auxiliary analysis based on the rheological testing. The effects of mucilage concentrations, aging, temperature, and salt concentrations on drag reduction capabilities were investigated. Changes in drag reduction rate, relative drag reduction rate, and interaction coefficient of binary composite solutions (PEO and yam mucilage) were also examined.

According to Fig. 4(a), when considering lower concentrations (100 and 150 ppm), the drag reduction rate (DR) initially increased and then decreased with rising Reynolds numbers (Re). The maximum DRs occurred at Re = 60 791 and 91 187 for solutions with concentrations of 100 and 150 ppm, respectively. The primary factor behind this behavior was that more severe shear failure occurred at low concentrations.38 Consequently, as the Re increased, the initial rise in DR was followed by its subsequent decline. With increasing concentration, the shear stability improved and the shear degradation diminished. Regarding the higher concentrations (300 ppm or greater), DR continuously increased with increasing Re. The yam mucilage demonstrated a significant (DR), with DR rapidly increasing as the concentration rose. However, beyond 1000 ppm, further increases in concentration yielded diminishing returns. For example, when increased from 1000 to 2000 ppm, DR increased less than 2%. The maximum achieved DR was recorded at 44.1% for a concentration of 2000 ppm. The DR of the yam mucilage uniform solution was also much higher than that in a water tunnel, with the injection of yam mucilage solutions.29 

FIG. 4.

Effects of yam mucilage concentrations on DR. (a) DR vs Re. Solid symbols represent the process of increasing Re, and hollow symbols represent the process of decreasing Re. (b) ΔDR vs Re.

FIG. 4.

Effects of yam mucilage concentrations on DR. (a) DR vs Re. Solid symbols represent the process of increasing Re, and hollow symbols represent the process of decreasing Re. (b) ΔDR vs Re.

Close modal

In the process of DR testing, the torque generated from low to high Re is recorded first, the torque generated from high to low Re is recorded when the Re reaches the maximum value (Remax = 106 385). Specifically, solid symbols were generated when Re moved from low to high, and hollow symbols were generated by Re going from high to low. The drag reduction difference (ΔDR) is defined as the difference between the torque in the ascending process and the descending process of Re under the same Re, i.e., ΔDR = DRascending - DRdescending, as depicted in Fig. 4(b). The yam mucilage with a concentration of 100 ppm [Fig. 4(b)], exhibited obvious ΔDR, reaching a maximum value of 6.9% at Re = 60 791, and indicating inadequate shear stability of the solution at low concentrations. However, as the concentration increased, the ΔDR diminished noticeably. At a concentration of 500 ppm, the ΔDR fluctuated around zero. Notably, as the concentration surpassed this threshold value, there was minimal decline in DR during decreasing Re, and even a slight increase could occur. When the concentrations were raised to 1000 and 2000 ppm, respectively, ΔDR was negative, i.e., and ultimately a slight increase was observed in DR. The reason being that at low Re, the turbulence intensity diminished, impeding further enhancement of DR by the high concentration of yam mucilage. However, under periodic shear conditions, degradation of long chain molecules occurred, leading to a decrease in viscosity, and an increase in DR.

The temporal changes of DR of yam mucilage were investigated for six different concentrations at r = 500 r/min (Re = 75 989), i.e., the shear stability of yam mucilage, are displayed in Fig. 5. With the increase in concentration, DR decreased slightly then gradually became basically unchanged, indicating a reduced influence of shear on DR. Upon reaching a concentration of 2000 ppm, the DR gradually remained relatively constant at approximately 38%, only a marginal decrease in DR was observed with a δDR% (δDR% represents the difference between the DR at the first test point and the last test point) of merely 0.57%, indicating exceptional stability and remarkable anti-shear properties of yam mucilage. After nearly 50 min of shearing, DR decreased the most at 300 ppm, with a maximum decrease in 5.25%, as listed in Table II, which shows that with the increase in concentration, δDR% first increased then decreased. However, δDR% fails to comprehensively depict the mucilage's shear stability.

FIG. 5.

Effects of different concentrations on drag reduction of yam mucilage as a function of time at Re = 75 989. (a) DR vs time. (b) DR′ vs time. Points obtained by experiment, curves obtained by fitting equation.

FIG. 5.

Effects of different concentrations on drag reduction of yam mucilage as a function of time at Re = 75 989. (a) DR vs time. (b) DR′ vs time. Points obtained by experiment, curves obtained by fitting equation.

Close modal
TABLE II.

Maximum δDR% and δDR′ at different concentrations.

c/ppm 100 150 300 500 1000 2000
δDR%  3.37  4.94  5.25  2.92  2.45  0.57 
δDR′  0.25  0.24  0.18  0.09  0.06  0.02 
c/ppm 100 150 300 500 1000 2000
δDR%  3.37  4.94  5.25  2.92  2.45  0.57 
δDR′  0.25  0.24  0.18  0.09  0.06  0.02 
To better describe the mechanical degradation, the relative drag reduction DR′ = DR(t)/DR(0) regarding the time and concentration is defined as below.39,
DR ( t ) DR ( 0 ) = 1 1 + W ( 1 e h t ) ,
(4)
where DR(t) and DR(0) are the DR at any time t and the initial state t = 0. The parameter W is related to the shear stability and h is related to the degradation rate. A larger h indicates a fast degradation, while a larger W indicates a low shear stability.4,39

As concentration increased, there is a gradual decline of DR. When concentration was 2000 ppm, almost no degradation occurred within nearly 50 min of shearing time, with less than a 2% decrease in DR, i.e., δDR′ < 2% (δDR′ represents the difference between the DR′ at the first test point and the last test point, as listed in Table II). The most significant degradation during this period is observed at concentrations of 100 and 150 ppm where the respective reductions in DR were approximately 25% and 24%, demonstrating their weakest shear stability.

Equation (4) was employed to fit the experimental data, enabling a superior fit, and facilitating comparison of the parameters W and h in the fitting formula at varying concentrations, as depicted in Fig. 5(b). As mucilage concentration increased, parameter W gradually decreased (Table III), indicating an enhanced shear stability and leading to more pronounced shear resistance in solutions. The gradual increase in parameter h suggests accelerated shear degradation of the solution, implying a reduced time required for reaching a stable stage. This phenomenon occurred due to the formation of stronger network structures entangled by long chain molecules in solutions with higher yam concentrations, and resulting in heightened shear stability.38,40,41

TABLE III.

Parameters of Brostow's models for yam mucilage solutions.

c W h
100 ppm  1.030  0.0085 
150 ppm  0.483  0.0229 
300 ppm  0.306  0.0235 
500 ppm  0.121  0.0313 
1000 ppm  0.073  0.0438 
2000 ppm  0.016  0.1447 
c W h
100 ppm  1.030  0.0085 
150 ppm  0.483  0.0229 
300 ppm  0.306  0.0235 
500 ppm  0.121  0.0313 
1000 ppm  0.073  0.0438 
2000 ppm  0.016  0.1447 

According to these results, the yam mucilage exhibited remarkable shear resistance. However, when utilized for drag reduction in engineering applications, its effectiveness could be influenced by various complex environmental factors. To investigate the factors impacting the drag reduction capability of yam mucilage, the effects of heating and aging, and the addition of salt solution on its drag reduction performance were considered. This was performed with the goal of clarifying the drag reduction behavior of yam mucilage and then enhancing its overall efficacy.

The DR of yam mucilage at different heated temperatures with Re is illustrated in Fig. 6. As the Re increased, the DR gradually increased, indicating that the structure of yam mucilage remained intact after temporary heating, and still exhibited the drag reduction effect. The optimal DR of yam mucilage was observed at 25 °C; however, an increase in temperature led to a significant decline in DR across all Re. When heated to 100 °C, a substantial decrease in DR occurred, with a maximum reduction of 13.9% and a minimum reduction of 6.9%. There may be two reasons for this: first, with the increase in temperature, the molecular motion of mucilage became more vigorous, leading to the accumulation of long chain molecules and a reduction in DR.33 Second, high temperatures caused denaturing of macromolecular substances in mucilage, which could generate the aggregation of mucilage molecules.30,32,42 Therefore, altering the properties of mucilage could be achieved by heating the yam mucilage to modify its drag reducing effect.

FIG. 6.

Effects of temperature and bio-degradation on DR at c = 500 ppm. (a) DR vs Re with the effect of temperature. (b) DR vs time.

FIG. 6.

Effects of temperature and bio-degradation on DR at c = 500 ppm. (a) DR vs Re with the effect of temperature. (b) DR vs time.

Close modal

The mucilage of 500 ppm was stored at room temperature and sealed with plastic wrap. On the first, second, third, fourth, fifth, sixth, and seventh days, respectively, the mucilage was taken out for DR testing to investigate the impact of storage time on their properties. As depicted in Fig. 6(b), the DR of yam mucilage exhibited two trends as the storage time increased: First, under identical Re, the DR decreased with prolonged storage time; second, as the storage time increased, the sensitivity of DR to Re diminished. In other words, as Re increased, there was a smaller change in DR of yam mucilage, which performed differently than heated yam mucilage. During the initial experimental phase, applying varying Re to yam, the samples displayed significant differences in DR. However, by the sixth day of the storage period, only a minimal variation was observed across different Re (maximum difference being only 1.63%). By the seventh day however, the DR displayed a near-constant state, although the maximum Re difference was only 0.9%. Static storage primarily affected the yams through microbial activity, known as biological degradation.27 With the increased duration of static storage, microorganisms affected the macromolecular materials, leading to weakened drag reduction performance and inability to further suppress turbulence at higher turbulence levels. According to Soares et al., bio-materials like DG, okra, and aloe vera can undergo biodegradation, but the bio-degradation can be avoided–or at least minimized–using biocides.15,27

These results suggest that heating and aging exerted distinct action mechanisms on yam mucilage. Specifically: heating only diminished the DR of yam mucilage, but it still exhibited an increase with Re, indicating its continued ability to inhibit turbulence. Conversely, aging induced the influence of microorganisms on yam mucilage, leading to decomposition of its effective components, and consequently diminishing its reliance on Re. The likely explanation lies in the biodegradation induced decomposition of macromolecules present in yam mucilage, leading to a reduction in its drag reduction performance—particularly at high Re—which limited the elongation of long chain molecules.

To explore the influence of salt on the drag reduction effect of yam mucilage, different concentrations of NaCl and Na2SO4 were added separately to 1000 ppm mucilage solutions. Subsequently, drag reduction tests were carried out at varying Re [Fig. 7(a)]. The increase in NaCl concentration initially led to an enhancement in DR; however, this trend reversed beyond a threshold concentration (0.01 mol/l). Notably, compared to mucilage solution without salt (c = 0 mol/l), lower NaCl concentrations resulted in increased DR, primarily within low Re ranges, with a maximum improvement potential reaching 3.8% [Fig. 7(b)]. At low NaCl concentrations, the influence of NaCl on DR gradually diminished with the increase in Re. At low NaCl concentrations, the influence of NaCl on DR gradually diminished with increased Re. As NaCl concentration continued to rise above this threshold value (0.01 mol/l), a decline in DR was observed with 0.4 mol/l NaCl exhibiting the most significant decrease at a maximum level of 3.7%. Moreover, with the increase in Re, NaCl reduced DR more and more significantly.

FIG. 7.

Effects of salt on DR at c =1000 ppm: (a) DR vs c and (b) ΔDR vs c.

FIG. 7.

Effects of salt on DR at c =1000 ppm: (a) DR vs c and (b) ΔDR vs c.

Close modal

The DR was inhibited by both 0.005 and 0.2 mol/l Na2SO4, with a greater concentration of Na2SO4 resulting in stronger inhibition effect, reaching a maximum inhibition of 6.3%, as shown in Fig. 7(b). Thus, salt ions exhibited a dual effect on yam mucilage. Based on these findings, adding an appropriate amount of NaCl could enhance the DR of yam mucilage, but excessive NaCl hindered this effect. In comparing the effects of 0.2 mol/l SO42− and 0.4 mol/l Cl on yam mucilage at the same Na+ concentration level, it was observed that the impact of −2 sulfate ions was more pronounced than that of −1 chloride ions. This discrepancy may be attributed to the binding between introduced ions and molecular chain groups in yam mucilage, leading to conformational changes in the molecular chain structure, and ultimately affecting its drag reduction capability.17,19,30 Consequently, controlling drag reducing performance of yam mucilage can be achieved by adding different proportions of salt solution to optimize its comprehensive effectiveness while maximizing efficiency utilization for drag reduction purposes. Due to experimental limitations, further extensive studies are required using various types and concentrations of ions to fully comprehend their influence on drag reduction capabilities of yam mucilage.

Investigations regarding the rheological characteristics of yam mucilage at varying concentrations could be referred to Refs. 29–31 and 43, resulting from extensive research on the rheological behavior of yam mucilage while emphasizing exploration of the rheological behavior of yam mucilage when combined with salt and PEO solution.

The change in viscosity of pure yam mucilage and the addition of salt ions with shear rate are illustrated in Fig. 8. A control sample of 1000 ppm yam mucilage was selected in order for the solution and the ions to adequately interact; it was subsequently supplemented with 0.2 mol/l NaCl, 0.2 mol/l Na2SO4, and 0.4 mol/l NaCl, respectively. The shear thinning behavior of yam mucilage was evident. The addition of 0.2 mol/l Na2SO4 slightly reduced shear viscosity at 102–103(1/s). However, the addition of NaCl led to a slight increase in viscosity at γ ̇>60 s−1. Moreover, the overall impact of these salt solutions on shear viscosity was insignificant, as all four groups exhibited similar viscosities at γ ̇= 2000 s−1. According to Ma et al., the main amino acids in the yam mucilage are aspartic acid and glutamic acid–acidic polar amino acids with a negative charge.31 The ions introduced thus affected the conformation of the molecular chains in the mucilage, and the carbohydrate chains could react and interdigitate, leading to changes in rheological behavior.17,44

FIG. 8.

Variation in shear viscosity of yam-salt solutions.

FIG. 8.

Variation in shear viscosity of yam-salt solutions.

Close modal

Fig. 9 illustrates the moduli of yam mucilage and solution with salt. The moduli of yam mucilage exhibited the following characteristics: At lower frequencies, the modulus remained relatively constant and stable; however, beyond a certain threshold value, it rapidly increased.

FIG. 9.

Storage and loss modulus for yam-salt solutions: (a) storage modulus G′ and (b) loss modulus G″.

FIG. 9.

Storage and loss modulus for yam-salt solutions: (a) storage modulus G′ and (b) loss modulus G″.

Close modal

These same tests were subsequently conducted on yam mucilage with 0.2 mol/l NaCl, 0.2 mol/l Na2SO4, and 0.4 mol NaCl, respectively. There was a significant decrease in the energy storage modulus (G′) of yam mucilage on concentrations and types of ions. However, upon introduction of salt, fluctuations emerged within the low frequency range, as well as a reduction in G′. When high concentrations of the solution were used, an increase in G′ was observed with increasing angular frequency values, which is completely different from the moduli variation of pure yam mucilage. The G″ of yam mucilage increased proportionally with angular frequency increments, while adding 0.2 mol/l NaCl resulted in an elevation of G″ for yam mucilage; conversely, adding 0.4 mol/l NaCl led to a reduction in G″ for mucilage, followed by further reduction upon incorporating 0.2 mol/l Na2SO4 into the system's composition. During this process, the changes observed in G′ and G″ aligned consistently with variations in angular frequency. This process revealed that the inclusion of salt induced a significant alteration in the rheological properties of mucilage, potentially attributed to the synergistic interaction between salt ions and efficacious drag-reducing components.45 According to Ma et al.,30 ions can interact with uronic acids from two or more polysaccharide molecules, thereby contributing to molecular aggregation. This interaction modified the network structure through cross-linking with carboxyl groups and enhanced the strength of gel networks. Additionally, it created junction zones within the mucilage conformation and it induced changes in polymer structures.

The rheological properties of yam mucilage, the typical long chain polymer solution-polyethylene oxide (PEO) solution, and their composite solutions were measured, as depicted in Fig. 10. The fixed hybrid solution had a total concentration of 150 ppm, and the ratio between the two solutions was examined to investigate the rheological behavior of different component solutions. The results revealed that at an equivalent concentration, PEO exhibited a slightly higher shear viscosity than mucilage when γ ̇ > 3 s−1. For the mixed solution, an increase in PEO concentrate resulted in decreased viscosity under high shear rates, while a higher viscosity was observed for the solution containing 50 ppm PEO + 100 ppm yam. At high shear rates, the viscosity tended to remain constant, closely resembling that of pure water. These findings suggest that varying concentrations of PEO and mucus led to distinct interactions and subsequent changes in viscosity.

FIG. 10.

Variation in shear viscosity of yam solution, PEO solution, and their mixtures.

FIG. 10.

Variation in shear viscosity of yam solution, PEO solution, and their mixtures.

Close modal

The dynamic moduli of the mixed solutions were tested (Fig. 11). When the PEO content in the solution was relatively small, the G′ and G″ remained essentially unchanged at low frequencies (w < 1 rad/s) while gradually increasing with further increasing frequency. The pattern of change resembled that of pure yam mucilage in terms of dynamic rheological behavior. As the concentration of PEO equaled or exceeded that of yam, the dynamic moduli continued to increase with increasing angular frequency, indicating that when the concentration of PEO in the mixed solution was relatively high, its dynamic rheological behavior became more like that of a pure PEO solution, rather than pure yam mucilage. It also demonstrated that the dominant component proportion determined the dynamic rheological behavior of the mixed solution. Conversely, it tended to resemble pure yam slime when dominated by yam components. However, for solutions with a large concentration of PEO, there was no significant dependence on concentration observed in terms of modulus values aligning with results obtained from pure PEO solutions.

FIG. 11.

Storage and loss modulus for mixed solutions of yam mucilage and PEO: (a) storage modulus G′ and (b) loss modulus G″.

FIG. 11.

Storage and loss modulus for mixed solutions of yam mucilage and PEO: (a) storage modulus G′ and (b) loss modulus G″.

Close modal

At low concentration, yam mucilage exhibited lower DR than PEO solution. To enhance the overall drag reduction performance of the solution, yam mucilage was combined with PEO under the total concentration of 150 ppm (corresponding DR-Re curves are plotted in Fig. 12). The pure PEO solution demonstrated significantly higher DR than the pure yam mucilage alone, with a maximum DR of 39.2% at the largest Re = 106 385. For the pure yam mucilage solution, the DR initially rose and then decreased slightly for yam mucilage with increasing Re, possessing a maximum DR of 21.1% at Re = 91 187. Notably, incorporating even a small amount (25 ppm) of PEO solution substantially enhanced the DR of the mixed solution. In fact, this addition yielded superior results compared to pure PEO solutions at low Re, and only slightly lower performance at high Re. Also, to a certain extent, both 50 ppm PEO + 100 ppm yam and 75 ppm PEO + 75 ppm yam mixed solutions improved the DR over pure PEO solutions. The greatest improvement occurred with 75 ppm PEO + 75 ppm yam mixed solution at Re = 106 385, with a DR of 40.4%. Furthermore, when compared with pure PEO solutions, the lower Re yielded more pronounced improvement in DR, while convergence in DRs across different solutions occurred as Re gradually increased. Specifically, when Re = 30 395, the mixed solution with 25 ppm PEO + 125 ppm yam caused outstanding increase in DR–up to 7.3%. Furthermore, the mixed solution surpassed pure yam mucus by enhancing its DR by over 20%.

FIG. 12.

Effects of different concentrations rations within mixed solutions on the drag reduction performance, DR, as a function of Re.

FIG. 12.

Effects of different concentrations rations within mixed solutions on the drag reduction performance, DR, as a function of Re.

Close modal

The DR of the mixed solution was investigated over time at a rotational speed of 500 r/min, corresponding to a Re of 75 989. All the DRs of the mixed solution containing PEO decreased significantly within 50 min, as depicted in Fig. 13. In contrast, the DR of the pure yam solution showed less decline with time. Furthermore, increasing PEO concentration in the mixed solution led to a gradual enhancement in DR; conversely, lower concentrations of PEO resulted in greater reductions in DR because of its lower shear resistance properties compared to yam mucilage. After nearly 50 min of shearing, the solution containing PEO still maintained a higher DR than that achieved by pure yam mucilage; however, it was lower than the pure PEO solution. The shear stability of PEO-yam mixed solutions showed no significant improvement over the pure PEO solution, and even decreased slightly. The main reason is that, during the initial stage of shearing, the pure PEO solution exhibited a higher level of shear stability because of its higher concentration of PEO than mixed solutions. Therefore, the mixed solutions experienced rapid degradation under shear, resulting in a significant decline in DR. However, after 40 min of shearing, the mixture containing 75 ppm PEO + 75 ppm yam exhibited the highest DR among all the solutions. Eventually, it approached similar DR values as the other mixed solutions, but remained significantly higher than 150 ppm yam mucilage. For pure PEO solution at low concentration (c < 100 ppm, not examined here), after more than 40 min of high-speed shear, the DR decreased significantly, while for mixed solutions, the DRs approached to equal after 40 min of shear, indicating that synergic DR is generated.

FIG. 13.

Effects of different solutions on drag reduction of yam-PEO mixtures as a function of time at Re = 75 989. (a) DR vs time. (b) DR′ vs time. Points obtained by experiment, curves obtained by fitting equation.

FIG. 13.

Effects of different solutions on drag reduction of yam-PEO mixtures as a function of time at Re = 75 989. (a) DR vs time. (b) DR′ vs time. Points obtained by experiment, curves obtained by fitting equation.

Close modal

Compared with the solution of 25 ppm PEO + 125 ppm yam and 125 ppm PEO + 25 ppm yam, before t = 30 min, the DR of the former was lower than the latter, and between t = 30–45 min, the DR of the two was closer. At t = 50 min, the DR of the two solutions was nearly equal. According to this trend, it could be predicted that when t > 50 min, the DR of the 25 ppm PEO + 125 ppm yam solution would be higher than that of 125 ppm PEO + 25 ppm yam. In the mixed solutions, the main polymer that made significant DR of the solution was PEO (the DR of 150 ppm yam was much lower than that of the mixed solution), while the DR of the mixture containing 25 ppm PEO gradually approached–or even exceeded–that of the mixture containing 125 ppm PEO during the shear process. This proved the existence of PEO and yam synergies, and synergistic drag reduction of 25 ppm PEO + 125 ppm yam was more significant. The synergistic DR of the mixed solution was manifested mainly in two aspects. On one hand, the DR of the mixed solution was much higher than that of the yam mucilage; however, after a long time of shear, the DR of the mixed solution was higher than that of the pure PEO solution.

Based on observations from PEO-yam mixed solutions with a total concentration of 150 ppm, the relative changes in DRs over time were obtained as depicted in Fig. 13(b). Although the DR of pure yam mucilage was lowest, it exhibited the highest resistance to degradation, indicating its superior shear resistance. Initially, the mixture of 25 ppm PEO and 125 ppm yam demonstrated the fastest degradation; as the PEO content in the solution increased, the value of DR′ gradually increased when t < 30 min. However, this pattern changed over time. The primary explanations are as follows: The DR of PEO at 25 ppm in the mixed solution was higher than the yam mucilage, but due to its low concentration, it degraded under continuous shear, significantly decreasing its DR. With the increasing PEO concentration in the mixed solutions, the anti-shear property improved, resulting in a slower descent rate (DR'). Except for the pure yam solution, the combination of 75 ppm PEO and 75 ppm yam exhibited the lowest descent rate of DR' after 40 min, indicating a significant synergistic drag reduction effect and shear stability under this composition. Furthermore, initially there was minimal decrease in DR′ in the mixture containing 125 ppm PEO and 25 ppm yam, but it showed maximum decrease after 45 min—surpassing the mixture of 25 ppm PEO and 125 ppm yam which had the fastest initial decline of DR′. The initial conclusion is that when the content of flexible polymer in the mixed solution was not higher than that of rigid polymer, better synergistic drag reduction effect was observed. This is consistent with the conclusion of Dschagarowa et al., who reported that the positive deviation was stronger when the proportion of less effective drag reducing agent in the mixture (isoprene rubber) was higher. Also, the deviation increased with the flow rate (Re) at a given concentration.46 Malhotra et al. also believed that the synergism was more pronounced with a higher proportion of rigid polymers in the mixed solution.22,47

Parameters W and h [obtained by fitting Eq. (4)], are presented in Table IV. As the concentration of PEO increased for the mixed solutions, the W value gradually rose, indicating weakening shear stability. Meanwhile, the decreasing h values implied a longer time necessary for the solutions to reach the DR platform. Comparing 150 ppm yam to 25 ppm PEO + 125 ppm yam solutions, a small amount of PEO significantly increased both W and h values, resulting in faster shear degradation and lower shear stability. Similarly, comparing 150 ppm PEO to 125 ppm PEO + 25 ppm yam solutions, the addition of a small amount of yam led to a significant increase in the W value and a decrease in h value, indicating slower shear degradation and lower shear stability. These observations could be attributed to two main factors: When there was 25 ppm PEO present in the mixed solution with a total concentration of 150 ppm, an increase in DR was evident for the mixed solution. However, under shearing action, solutions containing low concentration PEO experienced rapid decrease in DR, their plateau region leading to lower shear stability and faster shear degradation. Also, when 25 ppm yam was present in a solution with a total concentration of 150 ppm, the DR for yam mucilage was much lower than that of the PEO solution. Therefore, low concentration yam mucilage had no effect on the anti-shear properties of mixed solutions, but rather reduced overall concentration of PEO, thereby reducing its shear stability and slowing its shear degradation.

TABLE IV.

Parameters of Brostow's models for solutions at total concentration of 150 ppm.

c W h
150 ppm Yam  0.483  0.0229 
25 ppm PEO + 125 ppm Yam  0.714  0.0484 
50 ppm PEO + 100 ppm Yam  0.747  0.0345 
75 ppm PEO + 75 ppm Yam  0.824  0.0236 
100 ppm PEO + 50 ppm Yam  0.937  0.0211 
125ppm PEO + 25 ppm Yam  1.713  0.0106 
150 ppm PEO  1.180  0.0142 
c W h
150 ppm Yam  0.483  0.0229 
25 ppm PEO + 125 ppm Yam  0.714  0.0484 
50 ppm PEO + 100 ppm Yam  0.747  0.0345 
75 ppm PEO + 75 ppm Yam  0.824  0.0236 
100 ppm PEO + 50 ppm Yam  0.937  0.0211 
125ppm PEO + 25 ppm Yam  1.713  0.0106 
150 ppm PEO  1.180  0.0142 
To further quantify the interaction between the binary polymers, the synergistic DR is defined below:48 
DR mix = DR 1 λ 1 + DR 2 λ 2 + I λ 1 λ 2 ,
(5)
where DRmix is the polymeric binary mixture's DR. DR1, DR2, λ1, and λ2 are the DRs and weight factions of the polymeric solutions one and two, respectively. I is the synergistic interaction coefficient. It is noteworthy that DRmix, DR1, DR2, and I are all time-dependent functions as they exhibit variations with increasing shear strength due to polymer degradation. Moreover,
λ 1 + λ 2 = 1 .
(6)

As illustrated in Fig. 14, the synergistic coefficients of all mixed solutions increased with both Re and yam concentration in the mixtures, indicating that a higher proportion of yam led to greater synergy. Reddy and Singh48 observed that the synergism also increased with the Re in most of the mixtures they studied. Notably, I reached its maximum value at 96.98 for the 25 ppm PEO + 125 ppm yam solution. In most cases, a positive synergistic interaction coefficient signified a positive correlation between solution interactions. However, 125 ppm PEO + 25 ppm yams had a negative synergy coefficient (−0.067) under Re = 30 369, indicating that the ratio did not produce the ideal drag reduction effect.

FIG. 14.

Variation law of the synergistic interaction parameter with Re and time of different component (a) I vs Re. (b) I vs time at Re = 75 989.

FIG. 14.

Variation law of the synergistic interaction parameter with Re and time of different component (a) I vs Re. (b) I vs time at Re = 75 989.

Close modal

Furthermore, variations in the synergistic interaction parameter vs time also computed, as plotted in Fig. 14. I decreased gradually over time while exhibiting a slowly reduced rate. Overall, as the concentration of yam in the mixed solution decreased, synergistic coefficients exhibited a gradual decrease. Over time, the synergistic coefficients of the five solutions gradually became negative, with a shorter time for this effect to occur in solutions with a higher PEO content (Table V). This can be attributed to degradation of PEO under increasing shear time. Therefore, the synergistic drag reduction effect became more pronounced when the concentration of PEO did not exceed that of yam mucus in the PEO-yam mixed solution. However, significant variations in concentration between PEO and DG in the solutions (such as 25 ppm PEO + 125 ppm yam and 125 ppm PEO + 25 ppm yam) resulted in substantial contributions from both DR and degradation of PEO within the mixed solution, exerting a profound influence on its overall drag reduction effect. Consequently, the degradation of PEO played a pivotal role in determining its synergistic coefficient decline rate.

TABLE V.

Moment when coefficient, I, of mixed solution turns to negative.

c t/min
25 ppm PEO + 125 ppm Yam  34 
50 ppm PEO + 100 ppm Yam  28 
75 ppm PEO + 75 ppm Yam  22 
100 ppm PEO + 50 ppm Yam  14 
125ppm PEO + 25 ppm Yam 
c t/min
25 ppm PEO + 125 ppm Yam  34 
50 ppm PEO + 100 ppm Yam  28 
75 ppm PEO + 75 ppm Yam  22 
100 ppm PEO + 50 ppm Yam  14 
125ppm PEO + 25 ppm Yam 

The drag reduction effect and degradation characteristics of yam mucilage were investigated as a novel green drag reducer; the impact of temperature, aging, and salt on the drag reduction performance of yam mucilage was also examined. The concentration-dependent DR of yam mucilage varied with Re: it increased at high concentrations but decreased at low concentrations, due to shear degradation. The maximum DR of 44.1% was obtained at the concentration of 2000 ppm and Re = 106 385. Both heating and aging reduced the DR of yam mucilage, although their mechanisms differed. Adding low levels of NaCl enhanced the DR of yam mucilage, while Na2SO4 and high levels of NaCl diminished its effectiveness. There was a significant synergistic effect between yam and PEO in terms of DR. Incorporating a small amount of PEO into the yam mucilage solution significantly improved its DR; moreover, the synergistic coefficient increased with increasing Re but decreased with prolonged shear time exposure. When the concentration ratio of yam and PEO exceeded one, the mixed solution exhibited a more pronounced synergistic effect on DR.

This work is supported by the National Natural Science Foundation of China (Nos. 12102358, and 52201382), the China Postdoctoral Science Foundation (No. 2020M692617), the Fundamental Research Funds for the Central Universities (No. 3102021HHZY030008), the Natural Science Foundation of Chongqing of China (No. cstc2021jcyj-msxmX0393), the Young Talent Fund of Association for Science and Technology in Shaanxi, China (No. 20220512), the Innovation Capability Support Program of Shaanxi (No. 2024RS-CXTD-15), and the Open Fund of Science and Technology on Thermal Energy and Power Laboratory (No. TPL2021B01).

The authors have no conflicts to disclose.

Peng-fei Shi: Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – original draft (equal). Haibao Hu: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Validation (equal). Jun Wen: Formal analysis (equal); Funding acquisition (equal); Project administration (equal); Resources (equal). Tao Zhu: Conceptualization (equal); Funding acquisition (equal); Supervision (equal); Writing – review & editing (equal). Luo Xie: Conceptualization (equal); Funding acquisition (equal); Methodology (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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