Mean Bubble Research Articles

Steady and transient response of a laminar separation bubble to controlled disturbances

The steady and transient response of a laminar separation bubble to flow disturbances is examined experimentally. Wind tunnel experiments are performed on a NACA 0012 aerofoil at a chord Reynolds number of 130 000 and angle of attack of $2^{\circ }$. Under the investigated conditions, a laminar separation bubble forms on the suction side of the aerofoil in the unperturbed flow. Periodic disturbances are introduced into the boundary layer just upstream of separation by means of a surface-mounted dielectric barrier discharge plasma actuator. Two-component, time-resolved particle image velocimetry measurements are performed to characterise both quasi-steady and transient response of the flow to periodic disturbances. The results show that the dynamics of the laminar separation bubble is dominated by the periodic shedding of shear layer vortices, forming upstream of the mean reattachment location due to the amplification of unstable flow disturbances. Introducing the controlled perturbations leads to significant changes in separation bubble topology and the characteristics of the dominant coherent structures, with the effect dependent on both amplitude and frequency of disturbances. Linear stability analysis demonstrates that the induced changes to the mean bubble topology affect the stability characteristics, reducing the maximum growth rate and the frequency of the most amplified disturbances by 35 % and 20 %, respectively, when the bubble length is reduced by up to 40 %. The observed changes in stability characteristics are shown to correlate with the attendant variations in the shape factor. The transient response of the bubble is associated with significant changes in the separation bubble dynamics, with significant differences observed between the relative duration (${\approx}45\,\%$) of the transients flow response associated with the introduction and removal of the controlled disturbances. A detailed analysis of the results offers new insight into the response of laminar separation bubbles to changes in the disturbance environment.

Possibilities and Limitations of CFD Simulation for Flashing Flow Scenarios in Nuclear Applications

The flashing phenomenon is relevant to nuclear safety analysis, for example by a loss of coolant accident and safety release scenarios. It has been studied intensively by means of experiments and simulations with system codes, but computational fluid dynamics (CFD) simulation is still at the embryonic stage. Rapid increasing computer speed makes it possible to apply the CFD technology in such complex flow situations. Nevertheless, a thorough evaluation on the limitations and restrictions is still missing, which is however indispensable for reliable application, as well as further development. In the present work, the commonly-used two-fluid model with different mono-disperse assumptions is used to simulate various flashing scenarios. With the help of available experimental data, the results are evaluated, and the limitations are discussed. A poly-disperse method is found necessary for a reliable prediction of mean bubble size and phase distribution. The first attempts to trace the evolution of the bubble size distribution by means of poly-disperse simulations are made.

Gas holdup, bubble behaviour, and mass transfer characteristics in a two‐stage internal loop airlift reactor with different screens

AbstractScreen internals were used in a two‐dimensional two‐stage internal loop airlift reactor (ILALR). The influence of screens on the hydrodynamics and mass transfer characteristics in the two‐stage ILALR, including gas holdup, mean bubble diameter, bubble rise velocity, and volumetric mass transfer coefficient (KLa), have been experimentally investigated. It is found that screens can efficiently break bubbles up. Radial bubble velocity distribution is more uniform when screens are installed. Mean bubble diameter with smaller screen pores seems smaller and slightly increases with the increasing of superficial gas velocity Ug. A larger gas holdup and KLa can be obtained when a suitable screen is mounted. The variation trend of overall gas holdup and interfacial area a with Ug is similar. A nearly constant liquid‐side mass transfer coefficient (KL) is found. The empirical correlations for the overall gas holdup and KLa have been proposed, which fit the experimental data well.

Simulating Spiraling Bubble Movement in the EL Approach

Simulating the detailed movement of a rising bubble can be challenging, especially when it comes to bubble path instabilities. A solution based on the Euler Lagrange (EL) approach is presented, where the bubbles show oscillating shape and/or instable paths while computational cost are at a far lower level than in DNS. The model calculates direction, shape and rotation of the bubbles. A lateral force based on rotation and direction is modeled to finally create typical instable path lines. This is embedded in an EL simulation, which can resolve bubble size distribution, mass transfer and chemical reactions. A parameter study was used to choose appropriate model constants for a mean bubble size of 3 mm. To ensure realistic solution, validation against experimental data of single rising bubbles and bubble swarms are presented. References with 2D and also 3D analysis are taken into account to compare simulative data in terms of typical geometrical parameters and average field values.

Experimental investigation into the drag volume fraction correction term for gas-liquid bubbly flows

Estimates have been made using data from an instrumented pilot-scale bubble column of the volume fraction correction term over a range of local volume fractions (0.03–0.38) and mean bubble diameters (4–10mm) for Bubble Size Distributions (BSDs) of varying ‘broadness’. The value of this correction term was found to depend on both the mean bubble diameter and the dispersity of the BSD, not simply on the local volume fraction. Using the available experimental data, no hindered bubble rise was observed. The results from this study should assist in the development of more accurate predictive CFD models of bubble columns and other gas-liquid systems operated at high local volume fractions.

Evaluation of structural characteristics determining surface and foaming properties of β-lactoglobulin aggregates

Due to its structural versatility and remarkable surface activity, β-lactoglobulin (β-Lg) is a prominent food structuring agent in both its native state and in the form of heat-induced aggregates. In this study, we considered the properties of soluble aggregates of pure β-Lg having a median diameter of 50nm. The behavior of the aggregates at the air/water interface has been probed as a function of pH, with the aim of evaluating the controversial role of surface charge in connection with the functionality of particle-stabilized foams. Based on these new experimental results, it would appear that charge shielding around the isoelectric point (pI) leads to significantly increased foam stability as compared to samples having a significant positive or negative zeta potential. Against expectations, however, maximum foam stability was not linked to minimum foam drainage. High foamability and foam stability have been found to correlate with maxima in the rates of initial surface pressure increase and with interfacial dilatational properties. Evidence for efficient stabilization of the air/water interface was also reflected in a small average bubble size and a low rate of mean bubble area increase for aggregates with pH∼pI. Information from additional comparative experiments on systems with native β-Lg enabled an evaluation of the significance of surface hydrophobicity and aggregate particle size in relation to the foaming properties.

Gas Hydrate Growth Kinetics: A Parametric Study

Gas hydrate growth kinetics was studied at a pressure of 90 bars to investigate the effect of temperature, initial water content, stirring rate, and reactor size in stirred semi-batch autoclave reactors. The mixing energy during hydrate growth was estimated by logging the power consumed. The theoretical model by Garcia-Ochoa and Gomez for estimation of the mass transfer parameters in stirred tanks has been used to evaluate the dispersion parameters of the system. The mean bubble size, impeller power input per unit volume, and impeller Reynold’s number/tip velocity were used for analyzing observed trends from the gas hydrate growth data. The growth behavior was analyzed based on the gas consumption and the growth rate per unit initial water content. The results showed that the growth rate strongly depended on the flow pattern in the cell, the gas-liquid mass transfer characteristics, and the mixing efficiency from stirring. Scale-up effects indicate that maintaining the growth rate per unit volume of reactants upon scale-up with geometric similarity does not depend only on gas dispersion in the liquid phase but may rather be a function of the specific thermal conductance, and heat and mass transfer limitations created by the limit to the degree of the liquid phase dispersion is batched and semi-batched stirred tank reactors.

Oscillatory behavior of the bed bulk and the bubbles in a vertically vibrated pseudo-2D bed in bubbling regime

The effect of the bed vessel vibration on the oscillatory behavior of the bed bulk and the bubbles is experimentally studied in the present work by means of Digital Image Analysis (DIA) in a pseudo-2D bed. The bed material was three different powders of Geldart A, B and A/B classifications and was operated in bubbling regime for different superficial gas velocities and vibration amplitudes and frequencies. A tracking methodology was developed in order to follow the oscillatory motion of the bed bulk and each individual bubble in the system. This allowed the analysis of the interaction of the dense phase of the bed with the oscillations of the bubble diameter, position and velocity. The results indicate that both the center of mass of the bed and the bubble characteristics follow the oscillation of the bed vessel with a similar frequency but with a phase delay. The amplitude and phase delay of the oscillation of the center of mass of the bed are more sensitive to variations of the vibration frequency than to variations of the vibration amplitude of the bed vessel. Both the amplitude and the frequency of the bed vessel vibration have a stronger impact on the bubble behavior of beds filled with small particles. The existence of a phase delay between the oscillations of bubble characteristics in the lower and upper sections of the bed indicates the existence of compression-expansion waves in the dense phase that modify the bubble behavior along the bed despite bubbles are interacting with each other. The presence of compression-expansion waves may shed light onto the different behaviors encountered for the mean bubble behavior in vibrated fluidized beds.

TEM study and modeling of bubble formation in dual-beam He+/Fe3+ ion irradiated EUROFER97

The Reduced Activation Ferritic/Martensitic (RAFM) steels are promising structural materials for the first wall and blanket components of future fusion reactors. To obtain further insight into the temperature dependence of helium effects induced by 14 MeV energy neutrons under fusion like conditions, EUROFER97 was exposed to He+/Fe3+ dual-beam ion irradiation at the JANNUS laboratory at Saclay. The implantation was carried out at temperatures of 330 °C, 400 °C and 500 °C and induced a damage and helium concentration up to 26 dpa and 450 appm He, respectively. TEM microstructure analysis indicates a spatially homogeneous distribution of helium bubbles at 330 °C and 400 °C whereas a coexistence of homogeneous and heterogeneous nucleation of bubbles is found at 500 °C. An increasing mean bubble diameter and decreasing concentration of bubbles with rising irradiation temperature, as predicted by numerical results of a kinetic rate model for diffusion governed homogeneous nucleation of helium bubbles, are mostly confirmed by the irradiation experiment. Furthermore, within the rate model two approaches for the determination of the thermodynamic properties of helium filled voids in α-iron are applied. With respect to the final bubble size distribution, the commonly used surface energy of a void in the iron matrix is compared to the “variable gap model” of [1], J. Nucl. Mater. 418 (2011), which includes additionally the interaction between the helium atoms themselves, the energy at the helium-iron interface and the elastic deformation of the iron matrix.

Ab Interno Collagen Stent implantation as a treatment option for open angle glaucoma

PurposeTo establish the efficacy and safety of an Ab Interno Collagen Stent (AICS) in combination with subconjunctival Mitomycin C (MMC) in patients with open angle glaucoma (OAG), in the reduction of intraocular pressure (IOP), hypotensive treatment (HT) and progression on structural retinal nerve fiber layer (RNFL) exams.MethodsProspective, non‐randomized study involving 10 eyes with OAG, not controlled by HT, submitted to AICS implantation (Xen Gel Stent, Aquesys, Calif., USA) following subconjuntival MMC, with 6 month of follow‐up. Patients were reevaluated on the 1st day, 1st week and at 1st, 3rd and 6th months of follow‐up. IOP and bubble dimensions were registered and the need for re‐interventions or HT was analyzed. Structural RNFL evaluation was performed at 6 months (Spectralis OCT, Heidelberg Engineering, Heidelb., Germany), and compared to preoperative values. Correlation analysis was made between IOP reduction, bubble dimensions and age.ResultsMean IOP values were 21.8 ± 5.87 mmHg, 11.4 ± 4.5 mmHg, 13.2 ± 3.93 mmHg; 18.1 ± 6.9 mmHg, 14.2 ± 3.19 mmHg and 13.7 ± 3.10 mmHg and mean bubble dimensions were 3.1; 2.5; 2.5; 2.5 and 2.6 at 1st day, 1st week and 1st, 3rd and 6th months, respectively. Re‐interventions were necessary in 4 cases. IOP reductions compared to preoperative values were statistically significant at all follow‐up visits (p < 0.05), except for the 1st post‐operative month. There was a statistically significant correlation between bubble dimensions and IOP reduction (p < 0.05; r = −0.788). There was no significant progression of RNFL loss. Hypotensive medication was reduced by 87.5%. No complications were reported.ConclusionsThe insertion of an Ab Interno Collagen Stent with adjunctive MMC significantly reduced IOP over a 6 months follow‐up period without significant complications, notwithstanding a considerable need for re‐interventions.

Enhanced hydrodynamics in a novel external-loop airlift reactor with self-agitated impellers

In the present work, self-agitated impellers, as a novel type of internals, have been proposed for improving hydrodynamic and mass transfer characteristics of external-loop airlift reactors. The influence of inserted self-agitated impellers, in the riser section, in various liquid phases and with different sparger types, on main hydrodynamic parameters, was studied. The results show that the insertion of impellers led to significant bubble breakage and decrease in mean bubble size, particularly in pseudoplastic liquid. Obtained riser gas holdup values were up to 47% higher, in comparison to the configuration without impellers. Higher improvements were obtained with single orifice, given that this is the least effective gas distributor. Even though impellers represent an obstacle to the flow, relatively low reduction (about 10%) in downcomer liquid velocity was observed, for all investigated cases. Having this in mind, the benefit of inserting self-agitated impellers for improving the performance of external-loop airlift reactors was apparent.

Estimation of bubble column hydrodynamics: Image-based measurement method

Bubble column reactors (BCRs) have proven their worth in the chemicals industry by achieving high heat and mass transfer rates at relatively low cost. Limiting the design and scale-up, however, are the complex hydrodynamics that they exhibit, whose influence on transport characteristics are not fully understood to date. In addition, most studies in the literature focus on single phase system, whereas typical industry systems are multiphase.In this paper, a better understanding of the BCR hydrodynamics is obtained through investigating the coupling of Electrical Resistance Tomography (ERT) and Dynamic Gas Disengagement (DGD) technique. The increase in a Population Balance Model (PBM) simulation accuracy is achieved by allowing for the inference of bubble population property distributions such as bubble size and axial position. However, using a PBM to simulate bubble swarm phenomena under the influence of bubble coalescence and breakage requires accurately known boundary conditions. These are gas void fractions, the bubble size profile and the bubble number density distribution (BNDD).Electrical Resistance Tomography (ERT) is a non-invasive imaging technique suitable for creating images of changing gas void fractions in BCRs. In this work, a steady-state two-phase air-water system was set-up in a BCR. The ERT data for analysis of the local disengaging gas volume were captured over four rings of electrodes on a column of height 1.545m and diameter 0.29m. To facilitate measurements of bubble population distributions, a Dynamic Gas Disengagement (DGD) approach was used to induce transient gas holdup fractions which were captured by the ERT apparatus.The time profiles for the disengagement of gas void fractions locally were calculated for superficial gas velocities (ugs) in the range 5.0×10−3 to 1.0×10−2ms−1 (bubbly flow regime). From the known time profiles and height of the column sections, the bubble population properties were calculated. At ug of 1.0×10−2ms−1; the local average bubble rise velocities were found to range from 2.0×10−1 to 6.5×10−1ms−1; the local Sauter Mean Bubble Diameters (SMBD) were found to range from 2.3×10−3to 2.5×10−2m; and the axially averaged BNDD of bubble sizes were found to range from 3.6×10−3 to 7.6×10−6m−1.The smallest bubble size classes were found to be 2.3×10−3m at ug in the range 7.0×10−3 to 1.0×10−2ms−1 with relative error of 4% compared to the theoretical prediction of 2.2×10−3m . At ug in the range of 5.0×10−3 to 6.0×10−3ms−1, the smallest bubble size classes were determined to be 1.65×10−3m yielding the relative error of 37% compared to the theoretical prediction. The determined BNDDs were log-normal distributions and are consistent with both theoretical predictions and experimental findings. The low error values of the obtained results indicates the method is suitable for the development of PBM boundary conditions as well as the BCRs design and scale-up.

Influence of gas flow rate and sodium carboxymethylcellulose on foam properties of fatty alcohol sodium polyoxyethylene ether sulfate solution

ABSTRACTFoamability, foam initial liquid volume, and bubble size of fatty alcohol sodium polyoxyethylene ether sulfate (AES) surfactant solution were studied with and without the addition of sodium carboxymethylcellulose (CMC) at different gas flow rates, using a sparging method. The generation time decreased with increasing gas flow rate. At low gas flow rates, the added CMC greatly enhanced the foamability by preventing bubble collapse. The initial liquid volume of the foam first increased rapidly, and then gradually decreased. Increasing the CMC concentration increased the initial liquid volume of the foam. The mean bubble diameter first clearly decreased, then increased slowly with increasing gas flow rate. CMC showed different effects on bubble size at high and low gas flow rates. Adsorption of CMC on AES molecules forms a network structure and improves bubble film stability, which can explain the above results. These findings provide guidelines for generating foam with excellent properties suitable for coal mine dust control by adjusting the gas flow rate and the concentration of the added water-soluble polymer.

Applications of computational fluid dynamics to modeling hydrodynamics in tilapia rearing tank of Recirculating Biofloc Technology system

Biofloc Technology (BFT) has been widely used in tilapia rearing. However, the Total Suspended Solids (TSS) at the bottom of rearing tank will increase to high concentration due to biofloc sedimentation and accumulation. Therefore, it will be have negative effects on the feeding enthusiasm, gill and even survival of cultured species, and especially in the initial stages of larvae culture. The Recirculating Biofloc Technology (RBFT) would be one of the potential solution for the above concerns. To achieve an accurate TSS regulation, the biofloc distribution should be as homogenous as possible. The aim of this work is to specify the optimal bubble size and Hydraulic Retention Time (HRT) to the uniformity of biofloc distribution in a RBFT system through Computational Fluid Dynamics (CFD) method. For this, a three-dimensional and three-phase unsteady transient model was developed to simulate the hydrodynamics in a gas-liquid-solid tilapia rearing tank based on the commercial software Workbenching 15.0. Firstly, to achieve a more reliable prediction, the influence of the grid types and the computation turbulence model (Standard, Renormalization-Group (RNG), Realizable) to the simulation result were discussed by contrasting the simulations and experiments results of solid holdup at dimensionless radial position. Secondly, appropriate mesh size (181395 elements) and mixture standard k-e model were implemented to study the effect of bubble size and HRT on solid distribution uniformity. Simulations were performed by using three different mean bubble sizes (diameter=1, 2, 3mm) and three different HRTs (0.56, 1.13, and 2.25h) to study the sensitivity of the results to the uniformity of the biofloc distribution, respectively. The results showed that to get a better biofloc distribution, the bubble size should be range in 1mm and 2mm, and as the HRT decreasing, the distribution of biofloc distribution will be more homogeneous (0.45≤HRT≤0.56h). This paper provides an essential data set for determining the bubble size and HRT in production, as well as evaluating the accuracy of various CFD models for capturing the complex flow field in a BFT rearing tank.

Correlations of Bubble Diameter and Frequency for Air–Water System Based on Orifice Diameter and Flow Rate

Prediction correlations of air bubble diameter and frequency in stagnant clean water were established. Eleven different orifice diameters were tested under flow rate of 0.05–0.15 SLPM. The resulted bubble size and frequency were traced using high-speed camera. It was found that the mean Sauter diameter and bubble frequency are in the range of 3.7–6.9 mm and 6.4–47.2 bubbles per second, respectively. Nonlinear regression was performed to design the new correlations of estimating diameter and frequency with a correlation factor of 0.93 and 0.94, respectively. Flow rate and orifice size had the highest impact on the studied parameters.

Numerical modelling of air–nanofluid bubbly flows in a vertical tube using the MUltiple-SIze-Group (MUSIG) model

The MUltiple-SIze-Group (MUSIG) model was used in this study to simulate bubbly flows of air–water and air–nanofluid in a vertical tube. Flow parameters including the void fraction, gas velocity, interfacial area concentration and Sauter mean bubble diameter were predicted and compared against the experimental data available in the literature. The model agreed well with the experimental data of air–water bubbly flow, but exhibited notable discrepancies from the data of air–nanofluid bubbly flow. With the aim to improve the MUSIG model for an effective modelling of air–nanofluid bubbly flows, some latest experimental and theoretical research outcomes were summarised and analysed. It was proposed that the key job when modelling bubbly flows of nanofluids using the MUSIG model is to address the spontaneous assembly of nanoparticles at bubble surfaces and its effects on the interfacial forces and bubble coalescence process.

Effect of vertical vibration and particle size on the solids hold-up and mean bubble behavior in a pseudo-2D fluidized bed

The solids hold-up and mean bubble behavior in a vertically-vibrated fluidized bed are experimentally studied in the present work by means of Digital Image Analysis (DIA) for four different powders with Geldart classifications A, B and A/B. The bed has a small thickness (i.e. pseudo-2D bed) and operates in bubbling regime subject to a wide range of gas superficial velocities, vibration frequencies and vibration amplitudes. Mean parameters of the bed and the bubbles, such as solids hold-up, bubble fraction, bubble number density and bubble diameter and velocity, are characterized here by averaging the results over time and space. The results reveal that vibration of the bed promotes a confinement of the bubble path to the central section of the bed. This bubble confinement is more intense for the smallest particles tested and for high vibration strengths and creates two different bubble regimes in the bed. In particular, close to the distributor, the bubble velocity decreases when increasing the vibration amplitude of the bed vessel because bubbles are smaller and less confined, and they behave like isolated bubbles. The behavior of bubbles changes when they are far from the distributor, where the interaction between bubbles becomes greater due to their bigger size and the confinement of bubbles induced by vibration. This confinement promotes coalescence of bubbles. It is shown that consideration of these two different regimes of bubble dynamics allows to shed light on understanding the apparently contradictory results encountered in the literature regarding bubble behavior in bubbling vibrated fluidized beds.

Experimental investigation on the influence of ethanol on bubble column hydrodynamics

We experimentally investigate the effect of ethanol (0.05% in mass) on the hydrodynamics of a large-diameter and large-scale bubble column. The bubble column is 5.3m in height, has an inner diameter of 0.24m, and we consider gas superficial velocities in the range of 0.004–0.20m/s. The experimental investigation consists of gas holdup measurements and image analysis. The gas holdup measurements are used to investigate the flow regime transition and the global bubble column hydrodynamics. The image analysis is used to investigate the bubble shapes and size distributions near the sparger and in the developed region of the column. The presence of ethanol increases the gas holdup, stabilizes the homogeneous regime and modifies the bubble shapes and size distributions. In particular, the addition of ethanol increases the mean bubble aspect ratio and decreases the bubble diameters. The results suggest that the addition of ethanol changes the bubble properties, which modifies the bubble size distribution and shapes, thus, stabilizing the homogeneous flow regime and, finally, increasing the gas holdup.

Correlation between bulk characteristics of aggregated β-lactoglobulin and its surface and foaming properties

Bovine β-lactoglobulin features a pronounced surface activity, and therefore, is widely applied in order to provide stability to food-related aerated systems. Due to its distinct tertiary and quaternary structure, it can be used both in native state as well as in the form of thermally aggregated particles. In this context, heat treatment (80 °C/90 min) of highly purified solutions of β-lactoglobulin (c = 10.0 g L−1) under variation of solution pH (6.8 or 8.0) and NaCl concentration (0–130 mM) resulted in the formation of soluble aggregates, whose median particle diameters ranged from about 2.5 nm to 1.1 μm. These differences in particle size in combination with differences in particle characteristics (e.g. surface hydrophobicity, zeta potential) had a significant impact on surface properties, i.e. surface tension, dynamics of protein adsorption and interfacial dilatational properties. Thereby, diffusion rates decreased with increasing median particle diameter. Initial protein adsorption was majorly influenced by surface hydrophobicity. However, reverse observations emerged in terms of foam characteristics. Foam stability clearly increased with increasing aggregate size, showing maxima for the largest particles examined. The increasing foam stabilization ability was also reflected in time-resolved bubble properties, e.g. lower mean bubble areas as well as higher bubble counts for larger aggregates. This observation was traced back to the increasing surface hydrophobicity as well as more negative zeta potential of the aggregates with increasing particle diameter. As a result, sterically stabilized and more impermeable surface films in combination with electrostatic repulsive forces led to a reduction in coalescence rate as indicated by a lower coarsening exponent, and thus, a decrease of foam decay.

Porous structure of fibre networks formed by a foaming process: a comparative study of different characterization techniques.

Recent developments in making fibre materials using the foam-forming technology have raised a need to characterize the porous structure at low material density. In order to find an effective choice among all structure-characterization methods, both two-dimensional and three-dimensional techniques were used to explore the porous structure of foam-formed samples made with two different types of cellulose fibre. These techniques included X-ray microtomography, scanning electron microscopy, light microscopy, direct surface imaging using a CCD camera and mercury intrusion porosimetry. The mean pore radius for a varying type of fibre and for varying foam properties was described similarly by all imaging methods. X-ray microtomography provided the most extensive information about the sheet structure, and showed more pronounced effects of varying foam properties than the two-dimensional imaging techniques. The two-dimensional methods slightly underestimated the mean pore size of samples containing stiff CTMP fibres with void radii exceeding 100 μm, and overestimated the pore size for the samples containing flexible kraft fibres with all void radii below 100 μm. The direct rapid surface imaging with a CCD camera showed surprisingly strong agreement with the other imaging techniques. Mercury intrusion porosimetry was able to characterize pore sizes also in the submicron region and led to an increased relative volume of the pores in the range of the mean bubble size of the foam. This may be related to the penetration channels created by the foam-fibre interaction.