Influence of interfacial tension, temperature and recirculating time on the 3D properties of ice particles in jet A-1 fuel

https://doi.org/10.1016/j.ces.2021.116737Get rights and content

Highlights

  • We propose a new approach of sampling, with better separation of ice particles.

  • 3D images are obtained by X-ray tomography using a specific cryogenic cell.

  • 3D geometrical characterization of ice gives access to size and shape parameters.

  • Process parameters influence the size and shape of the ice particles.

Abstract

Icing in aircraft fuel systems remains a serious issue for aviation safety and security. It is linked to the geometrical properties of the ice particles formed in situ, which are themselves highly sensitive to the operating parameters of the fuel system. In this work, an experimental campaign was conducted with a fuel-icing bench to explore the impact of the interfacial tension (γ), the sampling temperature (θ) as well as the recirculation time of the particles in the injection loop (τ). For this purpose, a new sampling method has been defined, tested and validated, using paraffin oil (PO) as an impregnation product for ice particles. The specimens were prepared, set up in a cryogenic cell and then scanned using X-ray Micro-Computed Tomography (μCT). Image processing and analyses allowed computation of the 3D properties at 5 μm resolution. The impregnation with PO revealed satisfying results in terms of physical separation of ice particles (more than 66%), especially for the smallest ones (200 μm), thus limiting the sintering phenomenon over time. The decrease in γ values showed a slight tendency to form smaller particle size distributions widths (PSDw). The observed influence of θ was minor due to its interference with τ, which proved to have the strongest effect on particle size and morphology. Thus, the increase in τ promoted the formation of smaller PSDw with modes ~ 100 μm. The elongation (Ee) and flatness (Ef) form factors showed significant tendencies (40%) towards rounded and spherical shapes for all ice specimens. The mean curvature distributions showed a trend towards convexities either by decreasing γ or increasing τ values. This study illustrates the complex impact of process parameters on the geometric properties of ice particles in jet fuel. The obtained results may be useful for further testing of filter clogging phenomena or numerical simulation and modeling studies.

Introduction

Jet fuels always contain a relatively small amount of water that can cause serious problems in aviation fuel systems. Since aircrafts are subject to low temperature conditions during the flight, this amount of water, which exists in different forms, may be transformed into ice crystals mainly by heterogeneous nucleation (Baena et al., 2012, AAIB). As the temperature decreases over time, the solubility of dissolved-form water becomes lower. This solubility decrease leads to the formation of micron-sized droplets of water (Lam et al., 2014, Zherebtsov and Peganova, 2012, Carpenter et al., 2011, Lao et al., 2011, Murray et al., 2011), which turn into ice particles in the fuel, thus promoting ice accretion on their surroundings.

The ice-layer buildup is potentially dangerous and has been a concern for aviation safety. The accumulated ice can consequently be detached due to sudden changes in flow rates. Therefore, such a release of large amounts of ice is often referred as a “snow shower”, which may stick in critical parts of the fuel system e.g. heat exchanger inlet screens and filters, and could in severe cases cause aircraft engine failure. Such an accident occurred in 2008, causing the crash of a Boeing 777-GYMMM, for which, the Fuel–Oil Heat Exchangers were believed to be blocked (AAIB). This accident drew attention to the lack of understanding of this clogging phenomenon, thus prompting the need to better understand the behavior of water at low temperature conditions in aviation fuels. Consequently, several researches have been conducted on the formation of ice in aircraft fuels in order to better understand the involved physical mechanisms (Murray et al., 2011, Lao et al., 2011, Lam et al., 2013, Lam et al., 2015, Lam and Woods, 2018).

This phenomenon of ice formation showed to be quite complex as it depends on different interrelated processes such as nucleation (either heterogeneous or homogeneous (Murray et al., 2011, Murray et al., 2011)) and morphological evolution (e.g. the sintering process (Szabo and Schneebeli, 2007, Kuroiwa, 1961, Ramseier and Keeler, 1966, Ramseier and Sander, 1966, Kaempfer and Schneebeli, 2007)), which are themselves very sensitive to the process parameters such as temperature, fuel additives, water concentration and types of surrounding materials (Maloney et al., 2019, SAE-International AIR790-C, Considerations on ice formation in aircraft fuel systems, 2006, SAE-International ARP1401B, Aircraft fuel system and component icing test, 2006, Schmitz and Schmitz, 2018, EASA, 2010).

Based on previous results obtained on aircraft fuel system mock-ups, a sticky range has been identified between −20 and −5 °C, in which the ice tends to adhere preferentially to the surrounding metal surfaces and to quickly agglomerate (AAIB). Such a sticky behavior has also been found between −18 and −12 °C (SAE-International AIR790-C, Considerations on ice formation in aircraft fuel systems, 2006, SAE-International ARP1401B, Aircraft fuel system and component icing test, 2006), and between −20 and −6 °C (Schmitz and Schmitz, 2018). The accreted ice was described as being “soft” by the Air Accidents Investigation Branch (AAIB) (AAIB) and appeared to have a fluffy behavior with high porosity similar to fresh snow (Lam et al., 2015, Lam and Woods, 2018). Moreover, during experiments conducted with a test rig containing a glass-windowed pipe, Schmitz and Schmitz (2018) showed the formation of fine water droplets having diameters between 2–10 μm as the temperature drops below  +10 °C. When reaching −18 °C, they turned into snowflake-like ice particles, confirming thus the fluffy appearance with sizes reaching up to 250 μm. Recently, Maloney et al. (2019) conducted experiments with a recirculating fuel system using jet A-1 fuel. They showed a significant formation of the so-called “soft” ice accreted on the inner surface of the fuel lines. According to their observations, ice accretion was most effective at −10.5 °C and less particles were generated at both −19.7 and −7 °C.

Following this ongoing need to further investigate these phenomena, a fuel-icing bench was developed by the IFTS (2013) to generate ice particles mainly at −18 °C and to perform clogging tests for aircraft filters.

Thus, to create a reproducible method, several tests were carried out to prevent the sticky behavior of the ice particles on their environment (e.g. fuel tank, pipes and heat exchanger), and to ensure a better accuracy of the water content targeted to the filter clogging test. For this purpose, a surfactant, dioctyl sulfosuccinate sodium salt (DOSS or AOT: CAS Number: 577–11-7), was used (Flynn and Wand, 2001). This chemical product is applied to modulate the interfacial tension (γ) of various fuel types in many filter test applications. It is widely known and used as an anionic surfactant in the preparation of reverse micelles as well as a wetting agent. In fact, modulating the γ parameter has proven to be a key factor in various applications and for the proper functioning of some chemical processes (Yu et al., 2017, Yu et al., 2018, Zhang and Hassanizadeh, 2017, Pal et al., 2018, Liu et al., 2020, Pal and Mandal, 2020), thus allowing to control particle’s geometrical properties (e.g. minerals, water droplets, etc.), surface wettability, emulsion and foam formation (Tai and Chen, 2008, Chen and Tai, 2010, Peng et al., 2011, Shlegel et al., 2020).

The variation in γ values is usually inversely proportional to the affinity of the two immiscible phases (e.g. water-fuel). Thereby, by decreasing γ values, a better dispersion from one phase to the other is achieved (Abel, 2007), thus enhancing breaking and limiting coalescence (Hu et al., 2000, Peng et al., 2011). From a practical point of view, the use of DOSS provides homogeneous mixtures of ice particles in the jet fuel at −18 °C, minimizing as much as possible the accumulation of ice on its metallic environment.

The development of such fuel-icing bench and its application to measure filters performances brought then the need for an accurate particle characterization method, a need that is mostly shared by the entire fuel icing community. In this context, we recently developed an experimental procedure to sample and observe the size and morphology of such particles using X-ray absorption tomography (Haffar et al., 2021).

This non-destructive characterization technique is widely used in several applications, opening access to the 3D properties of various types of materials (Baker, 2019, Lin and Miller, 2005, Erdoğan et al., 2007, Farber et al., 2003, Wallenstein et al., 2015, McGuire et al., 2007, Funk et al., 2018). The first experiments were focused essentially on the feasibility to characterize ice particles in jet fuel at low temperature. However, we observed a rapid sintering of the ice particles when they came into contact. This sintering phenomenon (Kaempfer and Schneebeli, 2007, Kuroiwa, 1961, Ramseier and Keeler, 1966, Szabo and Schneebeli, 2007, Ramseier and Sander, 1966) makes the particle separation and their subsequent analysis more complicated. To overcome this problem, we applied numerical algorithms (Holzer et al., 2006, Münch et al., 2006) to separate the ice particles and get access to both size and shape distributions. These distributions have been computed for two values of the concentration of injected water (C) and the recirculating time (τ) by which the particles recirculated inside the injection loop.

The particle separation method can be optimized by doing a preliminary impregnation step (see e.g. Feng et al., 2020) during sample preparation, in order to limit the sintering phenomenon, to homogenize the distribution of particles and to stabilize the microstructure. Hence, this study aims to: (i) optimize and define a reliable experimental protocol for the sampling of ice particles in jet fuel using Paraffin Oil (PO) as an impregnation product and (ii) study the effect of the interfacial tension between water-fuel (γ), the sampling temperature (θ) and the recirculation time (τ) occuring inside the injection loop on the 3D properties of the produced ice particles. The obtained results expand the knowledge of process parameters that may potentially impact the ice microstructure in jet fuels and can thus feed simulations and modelings of icing phenomena (see e.g. (Marechal et al., 2020, Maréchal et al., 2016, Marechal and Perrin, 2017, Marechal, 2016, Kuruneru et al., 2019)).

Section snippets

IFTS fuel-icing bench

Fig. 1 shows the fuel-icing bench developed by the IFTS for the characterization of aeronautical filters. It is composed of two linked fuel units, which, by means of a cooling generator and a cold chamber containing the major part of the equipments, can achieve very low temperatures down to −45 °C. This double-linked system is composed of:

  • -

    An injection loop, which consists of a 45-liter fuel tank, a centrifigual pump and a tubular heat exchanger. Water is injected into this loop. After initial

Comparison of sampling methods

In this section, we present a comparison of ice microstructure and particle size distributions (PSD) between the two sampling methods. The ice particles were sampled under the same operating conditions: C = 3%, γ = 28 mN.m−1, θ =-18 °C and τ = 0.25 h, and were scanned (i) in jet A-1 fuel and (ii) impregnated with PO.

Conclusions

A new protocol for the characterization of ice particles produced in jet A-1 fuel using X-ray tomography has been proposed. It consists in the impregnation of the particles with paraffin oil, which significantly reduces the sintering phenomenon of the particles and the formation of agglomerates. With this new protocol, more than 66 % of the particles were physically separated, which limited the contribution of numerical separation tools. This new protocol has also improved the recognition of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to express their gratitude to the IFTS/CEEF and 3SR teams for their assistance and support during the experiments. Special thanks are granted to the CEN members: A. Dufour, J. Roulle, P. Lapalus, N. Calonne and L. Pézard for their help during the tomography sessions. The authors are also grateful to B. Münch for his contribution to the image processing method by implementing an improved version of the ImageJ plugin “Xlib”.

This work was supported by the ANRT (Association

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  • Cited by (2)

    • Characterization of ice particles in jet fuel at low temperature: 3D X-ray tomography vs. 2D high-speed imaging

      2022, Powder Technology
      Citation Excerpt :

      In this study, we proposed two experimental methods for the characterization of ice particles produced in Jet A-1 fuel with the IFTS fuel-icing bench: (i) using the X-ray tomography technique, referred to as the 3D ex-situ method, and (ii) using the high-speed imaging technique, which we designated as the 2D in-situ method. Concerning the ex-situ method, we relied on our previous studies [26,27] to optimize the sampling protocol, using a mixture of paraffin oil (70% v/v) with Jet A-1 fuel (30% v/v), allowing to have a gel-like mixing at θ = -20 °C and a good impregnation of the particles. After tomography and 3D image processing, this mixture showed the improvement of the physical separation of ice particles (mostly > 70% in number count), as well as the homogenization of the particles within the samples, limiting the contact, thus the sintering phenomenon between particles.

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