Experimental Study on Ice Accretion of Aviation Jet Fuel Tube

Round 1

Reviewer 1 Report

Paragraph in lines 92-96 is not very clear, I suggest to re-write, for example the first sentence seems to miss a verb or that should not end with a period and link it to the next one.

Line 142 is missing the dot after the 3, and the section is called “Results and Analysis”, same as section 4, see suggestion below.

Figure 7 is missing the final period of the text explaining the figure.

Redaction in lines 252 and 253 is not clear, It seems to be missing a verb.

Line 256 is missing the dot after the 4, and the section is called “Results and Analysis” same as section 3; my suggestion is that section 4 be called “Conclusions”.

Author Response

Response to Referee #1:

Dear Editors and Reviewers:

Thank you very much for your review and comments concerning our manuscript entitled “Experimental study on ice accretion of aviation jet fuel tube (ID: aerospace-2077665)”. Especially, we sincerely appreciate you for giving us a chance to improve our manuscript, as well as the important guiding to our work. Your comments are all valuable and very helpful for improving our paper. We have revised our manuscript carefully in accordance with your comments. The changes are marked in red in revised version. Below we would like to address your comments step by step:

 

Comment 1: Paragraph in lines 92-96 is not very clear, I suggest to re-write, for example the first sentence seems to miss a verb or that should not end with a period and link it to the next one.

Response 1: We are sorry for the incomplete statement of the required water flow for mixing with fuel in the manuscript. The corresponding statement has been re-written to our marked manuscript in lines 99-103: ‘The atomizer was composed of a water storage tank and a piezoelectricity sheet. The water was pushed by an external micro syringe pump. The water droplet dripped on the piezoelectricity sheet was rapidly atomized. The required flow volume of water was regulated  according to the fuel flow rate and power of the piezoelectricity sheet:’

 

Comment 2: Line 142 is missing the dot after the 3, and the section is called “Results and Analysis”, same as section 4, see suggestion below.

Response 2: We are sorry for the mistake. The corresponding statement has been re-written to our marked manuscript in line 145: ‘3. Results and Analysis’.

 

Comment 3: Figure 7 is missing the final period of the text explaining the figure.

Response 3: We are sorry for the mistake. The corresponding statement has been revised to our marked manuscript: ‘Ice accretion in the test sections at different fuel temperatures at t = 2 h,  = 200 ppm, V = 600 L/h. (a-d), (e-h) are the ice accretion at upstream and downstream test section, respectively.’

 

Comment 4: Redaction in lines 252 and 253 is not clear, It seems to be missing a verb.

Response 4: We are sorry for the mistake. The corresponding statement has been revised to our marked manuscript in lines 279 -282: ‘The amount of ice accretion increased with the water concentration, as shown in Figure 12. This was because the frozen fraction of droplets increased with the increase in water concentration. More free water in fuel appeared nucleation, icing, and adhering to the inner wall.’

 

Comment 5: Line 256 is missing the dot after the 4, and the section is called “Results and Analysis” same as section 3; my suggestion is that section 4 be called “Conclusions”.

Response 4: We are sorry for the mistake. The corresponding statement has been re-written to our marked manuscript in lines 283: ‘4. Conclusions’.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors presented a study of several experimental parameters governing internal icing of fuel lines, specifically for aviation jet fuel. Their setup improved on others reported by introducing an enhanced control of water injection into the fuel. Through the measurement of mass of ice, fuel water content, and fuel temperature, the authors observed variations in icing type and severity as a function of flow duration, temperature, and water content. Interestingly, they observed a peak in icing severity at 120 minutes of flow time, followed by a de-icing event. Similarly, the most severe icing occurred at -12°C, then decreased in severity for warmer or colder conditions. Finally, they observed an increase in icing severity with increasing water content, with a higher rate of increase at lower water content than at higher.

 

The article is generally very well-written. The motivation was effectively communicated, the experimental section was clear, as was the results section, which included an expertly analyzed dataset, and the conclusion which was presented in an easy-to-read format. It is unclear to me, however, how this study complements those from literature. In particular, this work is closely related to that of (Schmitz and Schmitz, 2018), except with a more sophisticated water injection apparatus and some data on the influence of water content. It appears that the authors were able to draw parallels between their observations and those of other works, but did not clearly identify where theirs stands out from other works. In other words, what is the scientific novelty? What should the reader learn from this article that they cannot learn from the cited ones? I would therefore recommend publication of this article following some improvement.

General Comments:

Statistical significance and measurement uncertainty:

1-      How many test runs were performed at each condition?  The authors present variations in ice accretion mass within a range of 2.0 g, but what is the uncertainty on each of the data points? Due to the randomness of ice accretion in general, especially in dynamic conditions, it would be surprising that the uncertainty for repeated test runs is less than 1%.

 

From Fig. 6, we see that that there is a measurable random uncertainty in fuel water content between test runs (comparing the difference values at the same test-period time on different curves). That suggests that the amount of ice accreted and the water content for all other tests likely vary noticeably for repeated tests. Providing the data of multiple test runs of the same condition, and representing it by uncertainty bars on your plots would increase the significance of your results.

 

2-      What is the average and standard deviation of the mass of the test section without ice? By my estimation, according to the images in the manuscript and the description of the test section, the mass may be around 3 kg. Assuming a high-precision balance was used with a maximum reading above 3000 g, the size of the scale plate was likely smaller than the 600-mm test section. Therefore, any misalignment during the placement of the test section on the balance may cause some uncertainty in mass measurement (i.e. if the center of gravity is off-center, that could cause a force-moment on the scale). Normally, it may not be significant, but the measurement of interest is that of ice, which you reported to be 3.0 – 4.0  g (roughly 0.1% of the total mass being measured by the balance). How does the uncertainty in mass measurement compare to the magnitude of the measurement of interest?  

Specific comments:

3-      In Fig. 5 and Fig. 8, the authors plotted a polynomial fit on the data. Given the sparsity of data points and their potential uncertainty, it is not convincing that such a fit is appropriate. For example, in Fig. 5, one could argue that a flat line could be drawn through the water concentration data. Similarly, a straight (sloped) line could be drawn through the ice accretion data with the 120-minute point as an outlier. If the authors so choose, I would suggest replacing the fitted curves with straight lines connecting the data points to guide the eye as in Fig. 6 and Fig. 9.

 

4-      Line 196. Could the authors comment on or estimate the magnitude of the shear stress experienced by the ice due to the fuel flow?

 

 

5-      Line 221 “…the ice adhesion strength increased with a decrease in temperature.” Can this statement be made definitively based on the data you obtained? I would suggest rewording the statement to reflect that it is only speculation based on your observations. Alternatively, I would suggest you search in ice adhesion measurement literature to support this claim.

 

I find the second explanation more plausible than the first, however, it is also speculative, and therefore I suggest modifying the wording slightly to not appear misleading. i.e. Without direct observation of heterogeneous nucleation, nor of any quantitative impurity measurement, one cannot say with certainty that “most of the water in fuel happened to heterogeneous nucleation…”  nor “The frozen fraction of droplets increased with the decrease in temperature…” Suggested phrasing, for example, “The fraction of frozen droplets has been reported to increase with a decrease in temperature [13]…”  

In the conclusion:

 

6-      Line 257. The first line of the conclusion is almost identical to the first line of the conclusion found in (Schmitz and Schmitz, 2018). Comparing that article and this manuscript, it is clear that this work was inspired by theirs, which is great, but perhaps it would be worth considering rewording slightly to avoid any tendency toward plagiarism.

7-      Line 261, Statement 1 is vague. What exactly is meant by “correctness of the experimental method”? Do the authors mean that their observations are in agreement with those reported in literature?

8-      Lines 273-278, Statements 5 and 6. Could the authors also comment on what they observed at temperatures until -20 °C? That would highlight the peculiarities observed near -12°C and -15°C.

9-      Line 279. Could the authors relate their conclusions to motivations they discussed in the introduction? Based on your results is there a specific range of conditions for which fuel-line icing is most likely to occur, and therefore should be avoided? Are certain parameters more critical than others?

 

Author Response

Response to Referee #2:

Dear Editors and Reviewers:

Thank you very much for your review and comments concerning our manuscript entitled “Experimental study on ice accretion of aviation jet fuel tube (ID: aerospace-2077665)”. Especially, we sincerely appreciate you for giving us a chance to improve our manuscript, as well as the important guiding to our work. Your comments are all valuable and very helpful for improving our paper. We have revised our manuscript carefully in accordance with your comments. The changes are marked in red in revised version. Below we would like to address your comments step by step:

 

The article is generally very well-written. The motivation was effectively communicated, the experimental section was clear, as was the results section, which included an expertly analyzed dataset, and the conclusion which was presented in an easy-to-read format. It is unclear to me, however, how this study complements those from literature. In particular, this work is closely related to that of (Schmitz and Schmitz, 2018), except with a more sophisticated water injection apparatus and some data on the influence of water content. It appears that the authors were able to draw parallels between their observations and those of other works, but did not clearly identify where theirs stands out from other works. In other words, what is the scientific novelty? What should the reader learn from this article that they cannot learn from the cited ones?

Response: Thank you very much for your suggestion. We are sorry for the unclear statement of the innovations.

In this paper, the experiments were carried out to research the pipe icing problem of super-saturated low-temperature fuel. Thus, the key parameters, such as test duration, fuel temperature, and water concentration under the condition of super-saturated fuel were explored.

• Two types of accreted ice were observed. The soft ice was mentioned in the literature [1]. However, the hard ice exhibited a ‘pebbly’ appearance, which was not mentioned in other literature. The phenomenon was related to the water injection method and characteristics of super-saturated fuel, which entrained more supercooled water droplets and ice particles.
• The non-uniformity of accreted ice at the cross-sectional area with the distance of flow was observed, which was rarely reported in the literature. The phenomenon was related to the impingement and coalescence of water droplets/ice particles in the flowing process.
• The water solubility of RP-3 jet fuel range from -30 to +40   was tested systematically, which complemented the content about RP-3 fuel in the Handbook of Aviation Fuel Properties [2].
• The variation law of accreted ice mass in the fuel pipe with test duration, fuel temperature, and water concentration was discussed, which might serve as the basis for aircraft fuel system design and airworthiness certification.

The above discussion has been emphasized in our marked manuscript in the Abstract, Introduction, and Conclusion:

In Abstract, ‘The different kinds of accreted ice, ‘fluffy’ and ‘pebbly’, were observed. As the distance of flow increased, a non-uniform distribution of ice on the cross-sectional area was noted. The amount of ice accretion increased with a decrease in the temperature from -2℃ and -12℃, and with an increase in entrained water concentration. Besides, the amount of ice accretion showed an increasing trend as time went on, and became stable after 2 hours.’

In Introduction, line 74-76, ‘Although the accreted ice in fuel pipes has studied, there was difference for different fuels due to chemical composition [12]. Thus, the test data was insufficient, especially for su-per-saturated low-temperature fuel.’

In conclusion, ‘A method for preparing quantitative water concentration of flowing fuel in a loop was presented, and the key parameters, such as test duration, fuel temperature, and water concentration, were analyzed. There were two different depositions on the inner wall of the pipe: one was in the form of soft ice, and the other was hard and exhibited a ‘pebbly’ appearance. The water solubility decreases exponentially with decreased fuel temperature, which promotes soft ice accumulation. The accreted ice exhibited a non-uniformity at the cross-sectional area with the distance of flow. With respect to the test duration, the amount of ice accretion showed an increasing trend and became stable after 2 hours. Besides, the amount of accreted ice increased with a decrease in the fuel temperature between -2℃ and -12℃. Part of the soft ice is shed off at fuel temperature -15 . The amount of accreted ice increased with the increase in water concentration in the fuel.’

 

General Comments:

Statistical significance and measurement uncertainty:

Comment 1: How many test runs were performed at each condition? The authors present variations in ice accretion mass within a range of 2.0 g, but what is the uncertainty on each of the data points? Due to the randomness of ice accretion in general, especially in dynamic conditions, it would be surprising that the uncertainty for repeated test runs is less than 1%.

From Fig. 6, we see that that there is a measurable random uncertainty in fuel water content between test runs (comparing the difference values at the same test-period time on different curves). That suggests that the amount of ice accreted and the water content for all other tests likely vary noticeably for repeated tests. Providing the data of multiple test runs of the same condition, and representing it by uncertainty bars on your plots would increase the significance of your results.

Response 1: Thank you very much for your suggestion. We are sorry for the incomplete statement of the uncertainty in the manuscript.

The accreted ice in fuel pipes, especially for flowing low-temperature fuel, was difficult to measure. On the one hand, the amount of accreted ice was small. On the other hand, the icing position on the inner wall was random and it was difficult to observe directly. In this study, the critical parameters, including the initial water concentration and fuel temperature, were controlled. The weight difference of test section before and after tests indicated the accreted ice mass.

• Figure 6 in the marked manuscript showed the water concentration at the inlet and outlet. The result showed the water concentrate was relatively steady before entering the test sections. After a long test loop, the low-temperature fuel entrained supercooled water droplets, ice particles, and even snow shower. The sampled fuel from the outlet was re-stored to room temperature and was dealt with by the Karl Fischer method, leading to a large error bar. Thus, only accreted ice mass in the upstream test section was used for comparison.

Figure 6. Water concentration of the fuel sample at the inlet and outlet of the test loop.

• For various icing conditions, three times repeated tests were carried out and the average value was obtained to eliminate the uncertainty. Table 1 showed the accreted ice mass under typical working conditions.

Table 1. Mass of accreted ice in the upstream test section for repeated tests.

	Test period ()	Fuel Temperature ()	Average Water Concentration ()	Fuel Flow Volume Rate ()	Accreted Ice Mass ()
Test-01	120	-12	197.15	600	4.81
Test-02	120	-12	190.86	600	4.71
Test-03	120	-12	206.37	600	4.88
Average Accreted ice mass					4.80

The above discussion has been added to our marked manuscript in lines 187-195: ‘For various icing conditions, three times repeated tests were carried out and the average value was obtained to eliminate the uncertainty.’; in lines 207-214: ‘In order to more clearly clarify the relationship between the amount of icing and the test durations, the water concentration of the fuel samples at the inlet and outlet of the test loop were compared, as shown in Figure 6. The results suggested that the water concentration at the inlet is relatively steady, which demonstrated the initial water concentration was repeated and controlled. However, the water concentration at the outlet changed drastically. This was because the sampled fuel at the outlet was mixed with ice. The water concentrate was analyzed by Karl Fischer method after sampled fuel was restored to room temperature, leading to a large error bar.’

 

Comment 2: What is the average and standard deviation of the mass of the test section without ice? By my estimation, according to the images in the manuscript and the description of the test section, the mass may be around 3  Assuming a high-precision balance was used with a maximum reading above 3000 , the size of the scale plate was likely smaller than the 600- test section. Therefore, any misalignment during the placement of the test section on the balance may cause some uncertainty in mass measurement (i.e. if the center of gravity is off-center, that could cause a force-moment on the scale). Normally, it may not be significant, but the measurement of interest is that of ice, which you reported to be 3.0 – 4.0  (roughly 0.1% of the total mass being measured by the balance). How does the uncertainty in mass measurement compare to the magnitude of the measurement of interest?

Response 2: Thank you very much for your suggestion. The average mass of the test section without accreted ice was 495.20 . In order to remain the test pipe in the center as much as possible and reduce rolling, a gentle foam was used as a support in the rear, as shown in the following figure. The measuring accuracy was 0.01 . At the end of test, the measurement of accreted ice mass and photography were completed in the chilling chamber.

The mass of accreted ice was weighed three times to eliminate the uncertainty. Table 2 showed the accreted ice mass under the same test.

Table 2. Mass of accreted ice in the upstream test section for the same test.

	Test period ()	Fuel Temperature ()	Average Water Concentration ()	Fuel Flow Volume Rate ()	Accreted Ice Mass ()
Mass01	120	-12	197.15	600	4.80
Mass02	120	-12	197.15	600	4.84
Mass03	120	-12	197.15	600	4.79
Average Accreted ice mass					4.81

The data acquisition details of accreted ice mass have been added to our marked manuscript in lines 135-140: ‘After the fuel inside the test section was drained, the weight of the test section was measured and used to quantify the ice accretion. The average value was got to eliminate the uncertainty in mass measurement. The test section was photographed to record the accreted ice. The whole procedures, including weight measurement and photograph, was completed in the chilling chamber to prevent the influence from ambient. The ice suspended in the fuel was drained with fuel together, which was not counted in the accreted ice mass in the fuel pipe.’

Figure 1. The mass of the test section without accreted ice.

 

Comment 3: In Fig. 5 and Fig. 8, the authors plotted a polynomial fit on the data. Given the sparsity of data points and their potential uncertainty, it is not convincing that such a fit is appropriate. For example, in Fig. 5, one could argue that a flat line could be drawn through the water concentration data. Similarly, a straight (sloped) line could be drawn through the ice accretion data with the 120-minute point as an outlier. If the authors so choose, I would suggest replacing the fitted curves with straight lines connecting the data points to guide the eye as in Fig. 6 and Fig. 9.

Response 3: Thank you very much for your suggestion. Figure 5, Figure 8, and Figure 12 showed the influence of test period, fuel temperature, and water concentration, respectively. The straight lines connecting the data points were used to show the variation of accreted ice. Besides, the water concentration was represented by scatter, and the error bar indicated the root means error. The corresponding figures have been replaced with our marked manuscript.

Figure 5. Mass of accreted ice in the upstream test section versus the test durations.

 

Figure 8. Mass of accreted ice in the upstream test section versus the fuel temperature.

 

Figure 12. Mass of accreted ice in the upstream test section versus the water concentration.

 

 

Comment 4: Line 196. Could the authors comment on or estimate the magnitude of the shear stress experienced by the ice due to the fuel flow?

Response 4: Thank you very much for your suggestion.

• The flow in the pipe was conducive to the deposition of accreted ice. The low-temperature fuel containing undissolved water flowed in the pipe. Under the action of velocity profile, the slip speed near the wall was relatively slow, which increased the residence time of water droplet/ice particles, leading to ice accumulation and subsequent adsorption. A higher flow velocity caused a larger shear force, which might carry away part of accreted ice. Therefore, a dimensionless parameter Reynolds number was used to describe the fuel flow process in the pipeline:

where  was the spatially averaged flow velocity (),  was the inner diameter of fuel pipe (), and  was the fuel kinematic viscosity ().

• Laminar internal flow. The average Reynold number in this study (-22 , 600) was near 937, which could be considered as a laminar flow. It assumed that the fuel flow was fully developed in the pipe. The accreted ice on the wall could be regarded as a porous medium. However, the interaction between the solid wall and porous media was complex and would be the focus of future work. Thus, the wall shear stress of a simple pipe flow without accreted ice was estimated.

where  was the spatially averaged flow velocity,  was the density of fuel pipe. The wall shear stress was 0.083 . The relative literature [3] showed that the slip velocity at the interface of porous fluid can be much larger than the spatially average velocity in porous media. Thus, the actual shear stress was greater. When the accreted ice increased, the shear strength increased and overcame the ice adhesion strength. Once the equilibrium state was reached, the accreted ice no longer increase significantly.  

• Adhesion strength of accreted ice in fuel. Lam et al. [3] measured the adhesion strength of accreted ice immersed on the aluminum block surface in the low-temperature fuel. The adhesion strength at horizontal and vertical direction were 0.36 and 2.19 , respectively. The interface shear strength of the accreted ice on subcooled surfaces submerged in fuel was several orders of magnitude smaller than those tabulated on the cold surface.

The above discussion has been added to our marked manuscript in lines 196-206: ‘In this study, the undissolved water was entrained by low-temperature fuel and impacted on cold wall under the action of velocity profile, leading to ice accumulation and subsequent adsorption. Without considering the complex interface of porous media, the wall shear strength increased with the average spatial flow velocity. With the test duration prolonging, the water droplets/ice particles continuously adhered to deposition within the test loop. Meanwhile, the initial thin layer of soft ice was gradually filled with tiny water droplets and ice crystals. As the amount of accreted ice increased, the average spatially flowing velocity increased. The shear strength increased and gradually overcame the ice adhesion strength. After the equilibrium state was reached, the accreted ice no longer increased significantly. Lin et al. [17] believed that most accreted ice accumulated over a short period of time (generally less than 3 hours).’

 

Comment 5: Line 221 “…the ice adhesion strength increased with a decrease in temperature.” Can this statement be made definitively based on the data you obtained? I would suggest rewording the statement to reflect that it is only speculation based on your observations. Alternatively, I would suggest you search in ice adhesion measurement literature to support this claim.

I find the second explanation more plausible than the first, however, it is also speculative, and therefore I suggest modifying the wording slightly to not appear misleading. i.e. Without direct observation of heterogeneous nucleation, nor of any quantitative impurity measurement, one cannot say with certainty that “most of the water in fuel happened to heterogeneous nucleation…”  nor “The frozen fraction of droplets increased with the decrease in temperature…” Suggested phrasing, for example, “The fraction of frozen droplets has been reported to increase with a decrease in temperature [13]…”

Response 5: Thank you very much for your suggestion.

(1) The influence of supercooled degree of water droplets. Gibbs free energy must be overcome for ice to be formed from water [4]:

where  was the radius of ice nucleus,  was the average entropy of fusion per unit molar volume,  was the liquid surface tension, and  was the temperature difference between the melting point and droplet temperature. The difference of Gibbs energy decreased with the increase in .

The presence of supercooled water droplets was the precondition of nucleation and ice formation. The supercooled water remained liquid state as long as there was no contact with any particulates until it reached the homogenous freezing point of about -36  [5]. In fact, the dust particles, microbiological mats, and other impurities, even rough surface or surface defects, might cause nucleation of supercooled water droplets dispersed in the fuel [6]. Most of the water in fuel happened to heterogeneous nucleation due to higher fuel temperature [7]. However, whether homogeneous or heterogeneous nucleation, the supercooled degree of water droplets increased with the decrease in fuel temperature, leading to less difference in Gibbs energy. Therefore, the frozen fraction of droplets increased with the decrease in temperature.

（2）The sensitive adhesion strength of accreted ice. There was general agreement in the literature that the adhesion strength of ice to arbitrary substrates increased with a decrease in temperature [8]. Dong et al. [9] explained that by the thin liquid-like layer, which existed between ice and structural solids. With increasing temperature, the ice adhesion was weakened due to the growing liquid-like layer. However, the accreted ice in fuel had a ‘sticky’ temperature region between -5  and -20  [7]. Schmitz et al.[1] observed that the ice thickness gradually leveled off at test temperatures below −15  and a large fraction of the initially accreted ice was subsequently shed off in case of −20.8 . Similarly, Maloney et al. [10] observed the accreted ice in fuel pipe at -7 , while no ice accumulation was observed when the fuel temperature dropped to -19 . Therefore, in the range of ‘sticky’ ice, the adhesion of ice increased with a decrease in fuel temperature. When fuel temperatures near the edges of the “sticky” ice region, the accreted ice was decreased significantly.

The above discussion has been added to our marked manuscript in lines 244-257: ‘The increase in accreted ice could be owned to two aspects: nucleation rate and ice adhesion strength. Firstly, the presence of supercooled water droplets was the precondition of nucleation and ice formation. Gibbs free energy [22] must be overcome for ice to be formed from water. The fuel temperature could be assumed as the supercooled water temperature. Whether homogeneous or heterogeneous nucleation, the difference in Gibbs energy decreased with increases in the supercooled degree of water droplets. Thus, the frozen fraction of droplets entrained in the fuel increased with the decrease in fuel temperature.

Besides, the adhesion strength of accreted was sensitive to temperature [9]. It was agreed that the adhesion strength of ice to arbitrary substrates increases with the decrease in temperature [23]. The pipe wall temperature in the test section was cooler than the fuel temperature. Thus, the adhesion strength of ‘pebbly’ ice increased with lower wall temperature, leading to more ice particle deposition. However, when the fuel temperature is near the edges ‘sticky’ temperature region, the ice accumulation is less [9].’

 

Comment 6: Line 257. The first line of the conclusion is almost identical to the first line of the conclusion found in (Schmitz and Schmitz, 2018). Comparing that article and this manuscript, it is clear that this work was inspired by theirs, which is great, but perhaps it would be worth considering rewording slightly to avoid any tendency toward plagiarism.

Response 6: Thank you very much for your suggestion. The corresponding statement has been re-written to our marked manuscript in lines 283-287: ‘When the fuel system encounters cold environment, it is inevitable from ice accumulation in the absence of additives. The super-saturated fuel entrains water drop-lets/nucleation particles and flows in pipes in emergency conditions, leading to accreted ice. The experiments were carried out to study accreted ice of super-saturated low-temperature fuel in the pipe.’

 

Comment 7: Line 261, Statement 1 is vague. What exactly is meant by “correctness of the experimental method”? Do the authors mean that their observations are in agreement with those reported in literature?

Response 7: Thank you for your suggestion. And we are sorry for the misunderstanding. In fact, the different experimental methods, including the test process and the method of water injected into fuel, presented different outcomes. Therefore, ‘the verification of experimental method’ was inappropriate. Statement 1, ‘By comparing the experimental results of appearance and tendency of icing with these in literature, the correctness of the experimental method is verified’ has been deleted.

 

Comment 8: Lines 273-278, Statements 5 and 6. Could the authors also comment on what they observed at temperatures until -20 °C? That would highlight the peculiarities observed near -12°C and -15°C.

Response 8: Thank you for your suggestion. Accreted ice was very sensitive to fuel temperature. And, the characteristics of accreted ice were related to test methods.

• The different test methods. In general, the icing tests were divided into two scenarios: saturated and super-saturated fuel. The saturated fuel at room temperature flowed through an insulated test pipe in a chilling chamber, and the temperature difference between the fuel and the inner wall was helpful for ice accumulation [10,11]. Schmitz et al. [1] observed the accreted ice on the wedge-shaped test body, which was cooling with the flowing fuel. No Temperature difference between the test body and the fuel. No additional water was injected before or during the experiments.

Another scenario was the accreted ice of super-saturated fuel. According to Standard Airbus Method, Lam et al. [12] maintained the fuel temperature at 5 ℃, and circulated fuel to facilitate fuel and water mixing. When the requisite water concentration was attained, the fuel was transported to a cold test pipe. The pipe wall was exposed to the free water and its temperature was cooler than fuel temperature. No additional water was introduced once the cooling phase commenced. However, in this paper, the test process was inspired by the Injection Method. At the cooling stage, the circling fuel kept saturated and quantitative water mist was injected after reaching the target temperature.

• The characteristics of accreted ice. When saturated fuel at room temperature flowed in the cold test section [10,11], two types of ice accumulation were formed. The dissolved water was precipitated into free water because of a decrease in fuel temperature. However, the fuel temperature was higher than the pipe wall, leading to the adhesion of free water and ‘hard’ ice formation. With the decrease in fuel temperature, some entrained water froze (heterogeneous nucleation) and became the ‘soft’ ice, which adsorbed on the surface of the ‘hard’ ice. When circling saturated fuel and solid surface were cooling simultaneously [1], the dissolved water precipitated. The water droplets/ice particles started to adhere to each other and form ‘soft/fluffy’ ice on the solid surface after reaching the target fuel temperature. Based on Standard Airbus Method [12], the ice accumulation in fuel pipes was mostly ‘soft’ ice with large porosity.

In this paper, two kinds of ice accumulation characteristics could be observed, ‘soft’ and ‘pebbly’ ice. On the one hand, during the cooling stage, the chilling chamber cooled the pipes and circulating fuel simultaneously. The fuel temperature decreased and water was precipitated.  The entrained water appeared heterogeneous nucleation and adhered to the inner wall to form ‘soft’ ice at cooling stage. On the other hand, the water mist injected into the cold fuel underwent nucleation and adhered to the inner wall to form ‘pebbly’ ice. Besides, the super-saturated water concentration in this study is relatively large, leading to a large average volume of water droplets. Thus, large ice beads were formed on the wall.

• The influence of temperature on adhesion strength of ‘soft’ ice. Schmitz et al.[1] observed that the ice thickness gradually leveled off when fuel temperature was below -15 and a large fraction of the initially accreted ice was subsequently shed off in the case of -20.8 . They believed the change seems to be a consequence of ice properties and related to the ice adhesion strength. Similarly, Maloney et al. [10,11] observed ice accumulation at -7 , especially at the weld position. However, the accreted ice is almost invisible at -19 .

In this study, the fuel temperature ranged from -2  to -15 , which belonged to the ‘sticky’ region. However, the part of accreted ice, especially the top soft ice, is shed off at -15. Thus, the accreted ice decreased when the fuel temperature was close to the edge of the ‘sticky’ region.

• The corresponding statement has been added to our marked manuscript in lines 59-76: ‘In order to simulate the real fuel system, a series of tests were designed to mimic the fuel flow and ice accumulation. The accreted ice found in the AAIB tests was found to be soft and mobile [2]. The release of the accreted ice in the fuel system could clog the FOHE, thereby threatening flight safety. Maloney et al. [8,9] tested accreted ice of the saturated fuel flowing through the cold pipe and observed two types of ice accumulation due to temperature differences between the fuel and pipes. Schmitz et al. [10] observed that the soft accreted ice upon the aluminum wedge block, but a large fraction of them shed off at fuel temperature below -20℃, which was considered to be a consequence of the changing ice properties and adhesion strength. Lam et al. [11] obtained the accreted ice with a large porosity of accreted ice for super-saturated fuel, which was mixed with water at 5℃. Besides, the adhesion of accreted ice, especially soft ice, was sensitive to fuel temperature. Lam et al. [7] measured the adhesion of accreted ice on horizontal and vertical sub-cooled surfaces immersed in jet fuel. Maloney et al. [9] observed the ice accumulation at -7 ℃, especially at the weld position. However, the accreted ice was almost invisible at -19 ℃. AAIB observed critical icing temperature, called the ‘sticky’ range, was between -5 ℃ and -20 ℃ [2]. Although the accreted ice in fuel pipes has studied, there was difference for different fuels due to chemical composition [12]. Thus, the test data was insufficient, especially for super-saturated low-temperature fuel.’
• The corresponding statement has been added to our marked manuscript in conclusions in lines 291-293: ‘There were two different depositions on the inner wall of the pipe: one was in the form of soft ice, and the other was hard and exhibited a ‘pebbly’ appearance. The water solubility decreases exponentially with decreased fuel temperature, which promotes soft ice accumulation.’ Lines 295-297: ‘Besides, the amount of accreted ice increased with a decrease in the fuel temperature between -2℃ and -12℃. Part of the soft ice was shed off at fuel temperature -15 ℃.’’

 

Comment 9: Line 279. Could the authors relate their conclusions to motivations they discussed in the introduction? Based on your results is there a specific range of conditions for which fuel-line icing is most likely to occur, and therefore should be avoided? Are certain parameters more critical than others?

Response 9: Thank you for your suggestion. when saturated fuel was cooled, dissolved water precipitated and formed soft ice. When super-saturated fuel was cooled, free water formed hard ice. When the fuel system encounters cold environment, it is inevitable from ice accumulation in the absence of additives. Thus, it is meaningful to understand the complex icing process in fuel system. Besides, the experiments in this study provided actual accreted ice mass, visual observation, and water solution, which enrich the related test data. In addition, the method of mixing water with fuel is presented and validated to regulation of water concentration. Based on the experiment rig, the key parameters, including test duration, fuel temperature, and water concentration, were systemtically studied.

The corresponding statement has been re-written to our marked manuscript in lines 283-298: ‘When the fuel system encounters cold environment, it is inevitable from ice accumulation in the absence of additives. The super-saturated fuel entrains water droplets/nucleation particles and flows in pipes in emergency conditions, leading to accreted ice. The experiments were carried out to study accreted ice of super-saturated low-temperature fuel in the pipe. A method for preparing quantitative water concentration of flowing fuel in a loop was presented, and the key parameters, such as test duration, fuel temperature, and water concentration, were analyzed. 

There were two different depositions on the inner wall of the pipe: one was in the form of soft ice, and the other was hard and exhibited a ‘pebbly’ appearance. The water solubility decreases exponentially with decreased fuel temperature, which promotes soft ice accumulation. The accreted ice exhibited a non-uniformity at the cross-sectional area with the distance of flow. With respect to the test duration, the amount of ice accretion showed an increasing trend and became stable after 2 hours. Besides, the amount of accreted ice increased with a decrease in the fuel temperature between -2℃ and -12℃. Part of the soft ice was shed off at fuel temperature -15 ℃. The amount of accreted ice increased with the increase in water concentration in the fuel.’

 

[1] Schmitz M.; Schmitz G. Experimental study on the accretion and release of ice in aviation jet fuel. Aerospace Science and Technology 2018, 82-83, 294-303.

[2] CRC. Handbook of Aviation Fuel Properties. Technical Report, CRC Report No. 635, 3rd Edition 2004.

[3] Lam J.; Lao L.; Hammond D. et al. Character and Interface Shear Strength of Accreted Ice on Subcooled Surfaces Submerged in Fuel. The Aeronautical Journal 2015,119, 1377-1396.

[4] Dang Q, Song M, Dang C, et al. Experimental study on solidification characteristics of sessile urine droplets on a horizontal cold plate surface under natural convection. Langmuir 2022, 38(25): 7846-7857.

[5] Johnson J U, Carpenter M, Williams C, et al. Complexities associated with nucleation of water and ice from jet fuel in air-craft fuel systems: A critical review. Fuel 2022, 310: 122329.

[6] Baena-Zambrana S, Repetto S L, Lawson C P, et al. Behaviour of water in jet fuel—A literature review. Progress in Aero-space Sciences 2013, 60: 35-44.

[7] AAIB. Report on the accident to Boeing 777-236ER, G-YMMM, at London Heathrow Airport on 17 January 2008. 2014.

[8] Work A.; Lian Y. A critical review of the measurement of ice adhesion to solid substrates. Progress in Aerospace Sciences 2018, 98, 1-26.

[9] Dong W, Ding J, Zhou Z X. Experimental study on the ice freezing adhesive characteristics of metal surfaces. Journal of Aircraft 2014, 51(3): 719-726.

[10] Maloney T C, Diez F J, Rossmann T. Ice accretion measurements of Jet A-1 in aircraft fuel lines. Fuel 2019, 254: 115616.

[11] Maloney T C. The collection of ice in Jet A-1 fuel pipes. Rutgers The State University of New Jersey-New Brunswick, 2012.

[12] Lam J K W, Woods R D. Ice accretion and release in fuel systems: Large-scale rig investigations. The Aeronautical Journal 2018, 122(1253): 1051-1082.

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have certainly improved the manuscript significantly, and have addressed the reviewers’ comments with great detail and attention. They have convincingly shown by quantitative measurement, how certain conditions can influence the formation of varying types and quantities of ice in fuel lines. I would therefore recommend accepting this article for publication in Aerospace.