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. 2020 Nov 13;5(46):30323–30328. doi: 10.1021/acsomega.0c04943

Influence of Foam Characteristics on the Aviation Coolants’ Pollution Degree

Jixin Mao †, Teng Chen †,‡,*, Xin Xu †, Shizhao Yang †, Li Guo †, Jun Ma †, Ting Yao §, Yongliang Xin ∥, Jianqiang Hu †,*
PMCID: PMC7689932  PMID: 33251467

Abstract

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The particulate contamination degree of aviation coolants (ACs) is overestimated commonly because the bubbles in ACs are erroneously recognized as particulate contaminants during the measurement process. In this work, the factors that influence the foam behavior and contamination degree of ACs are investigated. It is evidenced that the foam behavior of ACs is basically unaffected by the ratio of glycol to water of the base solution, which, however, is highly influenced by the organic additive. Also, the more the organic additive arranged at the gas–liquid interface, the lower the surface tension of glycol aqueous (GA) solution and the higher the contamination degree. Furthermore, the foam characteristics and contamination degree of ACs are highly affected by the working conditions, such as airflow, operating temperature, and gas pressure. Besides, the defoaming rate can be accelerated by adding an antifoaming agent or ultrasonic processing; however, the defoaming effect of the natural static method and pressuring positively treatment is disappointing. To further improve the defoaming effect, several efficient synergetic methods of defoaming have been proposed.

1. Introduction

The rapid development of airborne electronic equipment to achieve high frequency, integration, and miniaturization require the cleanliness levels of aviation coolants (ACs) to be higher and higher because excessive particulate contaminants in ACs would bring about the blockage of pipeline, wear and tear of aviation components, and even heat accumulation.1,2 To ensure the stable operation of airborne equipment, the industrial sector had stated that the particulate contamination level of ACs should not exceed class 8 of GJB 420B.2 The particulate contamination degree, however, is overestimated commonly because the bubbles in ACs are erroneously recognized as particulate contaminants during the measurement process. This is mainly because the number of particulate contaminants is measured by an automatic particle counter based on the physically based shading model, which is influenced by both particulate contaminants and bubbles.3

Generally, ACs are composed mainly of base solution and additives,4−6 with glycol aqueous (GA) solution as the main component of the base solution due to its fast heat exchange, high stability, and low toxicity.7−9 In our previous work, we found that the additives, especially organic additives, have a decisive effect on the foam behavior of ACs, including tendency and stability.2 However, the influence of components of ACs on their pollution degrees has not been deeply investigated. Furthermore, the working conditions of ACs, i.e., ventilation rate, pressure, and airflow, which have an effect on the foam behavior of ACs,10−12 is usually unintentionally neglected.

Herein, the influence of the working conditions and composition of ACs on its foam behavior and contamination degree is evaluated. Experimental results show that the foaming tendency of the GA solution is basically unaffected by the ventilation time, temperature, and the ratio of glycol to water, which, however, is highly influenced by the organic additive. Moreover, the working conditions, such as air flow, operating temperature, and gas pressure have a great influence on the foam characteristics and contamination degree of No. 65 AC. The effecting law and action mechanism of the working conditions varies greatly. As for defoaming, the stabilized membrane formed by the surfactant molecules at the gas–liquid interface can be undermined by adding an antifoaming agent, which exhibits a better defoaming effect than other methods, including natural static method and pressuring positively treatment. Based on the effect of such defoaming methods, several efficient synergetic methods have been proposed evidentially.

2. Results and Discussion

2.1. Effect of AC Components

2.1.1. Evaluation of the Composition of Base Solution

Bubbles formed easily in the pure liquid when it is aerated or agitated vigorously; however, the formed bubbles dissipate quickly because the membrane between the bubbles is unstable.13 To investigate the foam behavior and particulate contamination level of the base solution, which is composed mainly of water and glycol, the foaming tendency of a base solution with different contents of glycol was measured. As shown in Figure 1, the foam number (φ ≥ 14 μm) of the GA solutions with different glycol content changes inconspicuously with variations of temperature and ventilation time, with a maximum value of 322. The number of foams with a smaller size, however, is much greater than those with a higher size. For example, after ventilating 20 min at 55°C, the number of foam φ ≥ 6 μm for 95% GA solution is 3712, which is much higher than that of the foam φ ≥ 14 μm. This is mainly because the foam with a smaller size is easier to form in solution. Nonetheless, the foaming tendency of the GA solutions with different ratios of glycol to water is nearly identical. These results suggest that the foaming tendency of the base solution is basically unaffected by the ventilation time, temperature, and ratio of glycol to water. The number of formed foam is too few to affect the result of particle contamination degree.

Figure 1.

Figure 1

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Foam tendency (φ ≥ 14 μm) of (a) 35% GA solution, (b) 65% GA solution, and (c) 95% GA solution at different temperatures and ventilation time.

2.1.2. Evaluation on the Additive

It is widely known that bubbles are stable in lubricating oil due to the presence of additives at the gas–liquid interface, which can reduce the surface tension effectively.14,15 As for No. 65 AC, there are small amounts of corrosion inhibitor, preservative, defoamer, and other additives apart from the GA solution to ensure other properties of the AC. To investigate the effect of additives on the foam behavior and contamination degree, experiments were carried out on 65% GA solution with three kinds of additives commonly used in AC, i.e., sodium molybdate, n-caprylic acid, and T922 (tritolyl phosphate). The percentage addition of additives is 0.2 vol %. As shown in Table 1, the initial contamination degree of GA with sodium molybdate (GA-SM) is 7, which is basically identical to that of GA without an additive. After ventilating for 5 min, the number of foam of different sizes for GA-SM increases slightly with an unchanged contamination degree. With the prolonging of ventilating time to 10 and 20 min, the number of foam of different sizes and contamination degree is still invariable. These results suggest that sodium molybdate, as an inorganic additive, has little effect on the foam behavior and contamination degree of GA.

Table 1. Foam Number and Particulate Contamination Degree of 65% GA Solution with Different Additives for Comparison.
surfactant ventilation time/min ≥4 μm ≥6 μm ≥14 μm ≥21 μm ≥38 μm ≥70 μm degree
sodium molybdate 0 13 111 3320 148 31 1 0 7
5 13 475 3328 198 59 5 0 7
10 13 813 3431 186 51 4 0 7
20 14 097 3580 206 59 3 0 7
n-caprylic acid 0 7252 1614 151 38 7 1 7
5 39 284 40 094 8469 2504 326 4 12
10 62 516 56 606 11 569 4045 915 9 12
20 65 631 58 440 12 060 4642 1253 6 12
T922 (tritolyl phosphate) 0 11 259 2794 134 23 0 0 7
5 43 327 11 369 744 158 0 0 9
10 34 243 12 659 1703 401 7 0 9
20 56 120 20 807 2729 596 9 0 10
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However, the effect of n-caprylic acid on foam behavior should not be overlooked. The number of foam of different sizes for GA with n-caprylic acid (GA-CA) increases dramatically after ventilating for 5 min, and the contamination degree reaches up to 12 from 7, which is well above the prescribed level of 8. For instance, there is a 56-fold increase in the number of foams (φ ≥ 14 μm) for GA-CA relative to the initial value. Furthermore, the number of foam in different sizes increases continually along with the ventilation time extension. This is mainly because the n-caprylic acid, as an organic surfactant, are easily concentrated at the gas-liquid interface regularly in a certain way (Figure 2).16−18 The foamed monolayer additive at the gas–liquid interface reduces the interfacial tension of the GA solution significantly and makes the bubbles thermodynamically stable. The more organic surfactant in the GA solution, the closer the surfactant molecules are arranged at the gas–liquid interface, and the more stable the bubble.19,20

Figure 2.

Figure 2

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Schematic of the effect of n-caprylic acid on the foam behavior of the GA solution.

As for GA with T922 (tritolyl phosphate, abbreviated as GA-TP), after ventilating for 5 min, the foam number increases rapidly with a contamination degree of 9. By comparison, the foam number and contamination degree of GA-TP are less than those of GA-CA. In addition, the growth rate of the foam number of GA-TP is also lower than that of the latter (Table 1). The contamination degree reaches a maximum value of 10 after ventilating for 20 min. Although this value is lower than the corresponding value of GA-CA, which is also well above the prescribed level of 8. It might be because the large molecular volume of tritolyl phosphate increases the intermolecular spacing between themselves at the gas–liquid interface, which reduces the stability of the foam.14,21

All in all, the foam behavior of the GA solution is highly influenced by the organic additive; however, the inorganic additive has little effect on the foam behavior and contamination degree of GA. Moreover, the concentration of organic additive at the gas–liquid interface is affected by the molecular volume and the interaction between the surfactant molecules.2,22 The more organic additive arranged at the gas–liquid interface, the lower the surface tension of the GA solution, and more easily the bubbles are formed.

2.2. Effect of Working Conditions

2.2.1. Air Flow

As shown in Table 2, the number of foam in different sizes increases visibly under the agitation of air and the contamination degree reaches up to 11 from 8 at a flow rate of 600 mL/min. The number of foam with small size increases slowly as the flow rate increases; however, the foam number with a large size (φ ≥ 38 μm) increases quickly after steady growth. These results suggest that the bubbles with small size are easier to be saturated in No. 65 AC than those with large size. Also, with the increase in the flow rate, the foaming ability of No. 65 AC increases rapidly and gradually flattens out.

Table 2. Foam Number and Particulate Contamination Degree of No. 65 AC at Different Air Flows.
air flow/mL/min ≥6 μm ≥14 μm ≥38 μm degree
0 4979 320 2 8
600 48 884 8668 26 11
800 57 564 9070 23 11
1000 59 376 9480 32 11
1200 61 357 10 550 166 12
1400 57 403 9986 332 12
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2.2.2. Operating Temperature

To investigate the effect of temperature on the foam behavior of No. 65 AC, the foam number and particulate contamination degree were measured at 25, 35, 55, 75, and 88 °C, respectively. After ventilating at 25 °C, the foam number of No. 65 AC increases visibly and the contamination degree reaches up to 12 from 8 (Table 3). As the temperature rises continuously to 55 °C, the foam number increases accordingly. If the temperature rises continually to 88 °C, the opposite tendency is observed, and the contamination degree downgrades to 10. The reason for this phenomenon is that in the range of temperature, with the rise in temperature, the kinetic energy of the surfactants increases and more surfactants are assembled at the gas–liquid interface. The enhanced concentration of the surfactant at the gas–liquid interface reduces the surface tension effectively and enhances the stability of the foam accordingly.23

Table 3. Effect of Operating Temperature on the Foam Number and Particulate Contamination Degree of No. 65 AC.
temperature/°C ≥6 μm ≥14 μm ≥38 μm degree
initial 4673 258 1 8
25 44 333 6742 343 12
35 47 947 7523 162 12
55 55 783 11 971 31 12
75 46 080 8682 30 12
88 30 625 4777 30 10
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As the temperature rises to 75 °C or even higher, the intermolecular distance between the surfactants increases, resulting in a weakened interaction between the molecules and decreased stability of the foam.24,25 Furthermore, the interaction between the hydrophilic groups decreases at high temperature, resulting in a decreased surface viscosity, accelerated drainage rate, and reduced foam stability.

2.2.3. Gas Pressure

Apart from airflow and operating temperature, gas pressure is another major factor that greatly affects the foam behavior.26−28 As shown in Table 4, the foam number increases visibly as the gas pressure rises to 0.005 MPa and the contamination degree reaches up to 12 from 8. Moreover, the number of foam in different sizes increase continually with the gas pressure rises, until the pressure rises up to 0.02 MPa. When the gas pressure continues to mount, the foam number increases at a negligible rate, indicating the foam in No. 65 AC is saturated.

Table 4. Effect of Gas Pressure on the Foam Number and Particulate Contamination Degree of No. 65 AC.
pressure/MPa ≥6 μm ≥14 μm ≥38 μm degree
initial 4331 323 1 8
0.005 62 367 47 384 15 194 12
0.01 91 551 73 160 21 236 12
0.015 142 696 117 687 42 604 12
0.02 151 582 122 263 41 858 12
0.025 158 278 121 910 44 196 12
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2.3. Defoaming

To eliminate the effect of bubbles on the contamination degree of AC, several common methods of defoaming were adopted, i.e., addition of antifoaming agent, natural static method, ultrasonic method, positive pressure method, and synergetic method of natural static and ultrasonic/positive pressure treatment.29−32 As shown in Table 5, with the addition of 0.2 μL of an antifoaming agent, the foam number decreases from the maximum value of 3202–2114 and continuously decreases to 657 with its adding amount being 1 μL. This is mainly because the antifoaming agent can replace the surfactant molecule and undermine the stabilized membrane formed by surfactant molecules at the gas–liquid interface, the mechanical balance of the stabilized membrane is destroyed accordingly and the bubble is broken (Figure 3).33−35 Meanwhile, after a period of quiescence, the number of foam also decreases visibly. Also, such a decrement continued with the extension of static time. Compared with the natural static method, the defoaming rate of the ultrasonic method is much faster, indicating the energy brought by ultrasonic wave accelerates the vibration of the surfactant molecules and reduces the concentration thereof at the gas–liquid interface.

Table 5. Defoaming Results (φ ≥ 14 μm) of No. 65 AC with Different Methods.

method number of foam
antifoaming agent/μL initial 0.2 0.5 1
3202 2114   726   657  
natural static method/min initial 5 10 30 60 120 240
120 616 27 803 7324 342 283 211 241
ultrasonic method/min initial 1 2 3 5 10 15
106 462 1118 790 707 667 624 577
positive pressure method/MPa initial 0.01 0.02 0.03 0.04  
82 664 919 911 784 775  
natural static + ultrasonic method/min + min initial 5 + 5 5 + 10 5 +15 5 + 20  
82 095 754 307 268 185  
natural static + positive pressure/min + MPa initial 5 + 0.04 10 + 0.04 15 + 0.04 20 + 0.04
90 299 355 251 225 196
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Figure 3.

Figure 3

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Schematic of the effect of antifoaming agent on defoaming.

Although the drainage effect of bubbles is promoted at a positive pressure, the action time of pressure is too short to reduce the foam number effectively, resulting in an inferior defoaming effect than that of the ultrasonic method. Above all, ultrasonic treatment is an effective way to decrease the foam number compared to the other two methods. However, the number of foam in small size (φ ≤ 6 μm) is invariable after ultrasonic treatment. To further improve the defoaming effect, a synergetic method of natural static and ultrasonic treatment is adopted. As shown in Table 5, the number of foam with different sizes decreases rapidly after static and ultrasonic treatment, and the particulate contamination degree of ACs well meets the operating requirement. A similar defoaming effect is achieved by the synergetic method of natural static and pressuring positively. As shown in Table 5, the foam number decreases from the initial value of 90 299–355 after static treatment for 5 min and pressuring with a value of 0.04 MPa.

3. Conclusions

The effect of additive on the foam behavior and contamination degree is investigated based on the GA solution with different additives. By comparison, the organic surfactant has a measurable effect on the contamination degree. Also, the concentration of organic additive at the gas–liquid interface is affected by the molecular volume and the interaction between the surfactant molecules, which plays a critical role in the foam behavior. Moreover, the contamination degree of No. 65 AC is also affected by the airflow, operating temperature, and pressure. In addition, the effects of several common defoaming methods are investigated. The antifoaming agent can replace the surfactant molecule and undermine the stabilized membrane formed by surfactant molecules at the gas–liquid interface, resulting in unbalanced mechanical equilibrium of the foamed membrane and desirable defoaming effect. Furthermore, compared with natural static and positive pressure methods, the defoaming rate of ultrasonic method is much faster but lower than that of the synergetic method of natural static and ultrasonic/positive pressure treatments.

4. Experimental Section

4.1. Chemicals and Materials

No. 65 AC was produced by Shenyang Teli Co. Ltd., China. Ethylene glycol, sodium molybdate, n-caprylic acid, and T922 defoaming agent were of analytical grade and purchased from Tianjin Chemical Reagent Company, Aladdin Reagent Company and Hongze Zhongpeng Oil Additive Co. Ltd., China, respectively. Distilled water used in this work was home-made in the laboratory.

4.2. Foam Number

One hundred and forty-five milliliters of to-be-detested AC was transferred into a beaker and placed in a thermostatic water bath (25, 55, 75, and 88 °C) for 10 min. And then, at that temperature, the sample was bubbled with compressed air (0.2 MPa) at a rate of 1000 mL/min. Thirty minutes later, the foam number in the AC was detected and recorded by a YSJ automatic particle counter.

4.3. Defoaming

The defoaming of the air-bubbled AC was carried out by means of static placing, ultrasonic processing, raising the pressure and temperature, the addition of antifoaming agent, and synergic treatment of the above two methods. Briefly, 145 mL of AC was placed in a thermostatic water bath for 10 min and bubbled with compressed air (0.2 MPa) for 30 min sequentially. Then, the foam number in the AC was detected after static placing, ultrasonic processing, adding antifoaming agent, or pressuring, in which the ultrasonic frequency was set as 49 kHz in ultrasonic processing. As for synergetic methods, the foam number in the AC was detected after static placing and ultrasonic processing (pressuring) sequentially, in which the pressure on AC was 0.04 MPa.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51575525), the Jiangsu Provincial Natural Science Foundation of China (BK20191155), the Anhui Provincial Natural Science Foundation of China (1908085ME162), and the Air Force Logistics College Fund (KQQNJJ20D003ZD).

The authors declare no competing financial interest.

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