Volumetric Liquid Fraction Research Articles

X-ray computed tomography analysis of calcium chloride hexahydrate solidification

Latent Thermal Energy Storages (LTESs) are receiving increasing interest, since they offer many advantages to the field of Thermal Energy Storages (TESs), such as a nearly isothermal solid–liquid phase transition and a high energy storage density. To boost the transition towards a climate-neutral future, a lot of work is surely needed on Phase Change Materials (PCMs) which are used inside LTESs. Salt hydrates, a particular class of PCMs, are very attractive in terms of cost and volumetric latent enthalpy but present drawbacks that still limit their effective deployment in real applications, such as supercooling and phase separation (or segregation). The current work assesses the suitability of X-ray Computed Tomography (XCT) as a novel technique in the field of LTESs to study the solidification of phase-change materials. In this study, calcium chloride hexahydrate (CaCl2·6H2O) is chosen as a representative PCM and the results derived are shown to be repeatable. For the first time, the direct measurement of the volumetric liquid fraction is correlated to temperature measurements allowing for the visualisation of supercooling and the validation of an enthalpy-porosity numerical model, which could be useful in the design of LTES. XCT methodology offers unmatchable features as compared to the other method to measure the volumetric liquid fraction. Given its accuracy and the rigorous protocol, it can be considered as a benchmark for new liquid fraction measurement technologies but it also permits to study how the different PCMs melt and solidify, giving new insights on the mechanisms at the basis of the most crucial challenges, among those: subcooling, segregation, and metastable phase change.

X-ray computed tomography tracking of the calcium chloride hexahydrate crystallisation process

The research conducted on phase change materials (PCMs) for latent thermal energy storages (LTESs) is continuously growing in terms of publications, highlighting the importance of this topic. In fact, PCMs present many advantages that could help the energy transition and reduce CO2 emissions, by enhancing the performance of existing systems and better exploiting renewable energy. Therefore, it is of crucial interest to develop new and reliable methods to control LTES. Differently from sensible thermal energy storages, in LTESs the stored thermal energy is not proportional to the temperature. To really have an insight into the level of charge of these storages, it is important to know the liquid fraction, i.e., the amount of the liquid phase with respect to the whole amount of PCM. X-ray computed tomography (XCT) is a technology that allows to non-intrusively “look inside” the materials. In the current study, it was used to analyse the calcium chloride hexahydrate crystallization. This transient process of calcium chloride hexahydrate was tracked with a sequence of XCT scans, one every 6 minutes, resulting in 3D image stacks that were processed to obtain the volumetric liquid fraction evolution over time. Repeatability tests were run to evaluate the reliability of the XCT technique and the volumetric liquid fraction data was used to validate a numerical model developed within ANSYS Fluent framework. XCT offers great opportunities to study the heat and mass transfer mechanisms underlying the main issues of phase change materials, like, for example, supercooling and salt hydrate segregation.

New liquid fraction measurement methodology for phase change material analysis based on X-ray computed tomography

To tackle climate change, it is of fundamental importance to develop more efficient systems to convert and manage energy. Latent Thermal Energy Storage (LTES) systems, which are based on Phase Change Materials (PCMs), present many interesting features that can lead us to achieve energy sustainability. In fact, these systems can be employed in many applications (e.g., HVAC, buildings thermal management, thermal energy recovery). To use PCMs, it is crucial to understand and monitor their phase-change behaviour. However, the evaluation of two-phase systems is still difficult because of the intrinsic complexity and transient behaviour.In the present study, a new methodology to measure liquid fraction is proposed. X-ray Computed Tomography (XCT) is used to follow the solidification process of eicosane. XCT and the acquisition settings were optimised to ensure maximum contrast between the phases. Then, the solidification process of eicosane was dynamically studied and the volumetric liquid fraction was extracted. Finally, via the Ansys Fluent software, a CFD model, based on the enthalpy-porosity method, was implemented to simulate the solidification of eicosane. The numerical results were compared against the experimental data, showing excellent agreement. This novel experimental methodology opens new possibilities to characterise and understand the solid-liquid phase transition process of PCMs, by directly observing and quantifying the solid/liquid phase evolution in time, the volumetric shrinkage, the presence of voids and the different phases that might occur during the phase-change process.

Modeling Heat Transfer through Permafrost Soil Subjected to Seasonal Freeze-Thaw

The present paper proposes an iterative implicit numerical method for simulating the thaw depth of permafrost soil. For this purpose, the enthalpy-porosity model was used for the phase change process, and the finite difference scheme FTCS (Forward Time Centered Space) was used for discretization. An artificial mushy zone was maintained with the same thickness by keeping the regularization parameter proportional to the temperature gradient. In doing so, we made the scheme more stable and convergence occurred faster. The model accuracy was validated by comparing the numerical results with the analytical Stefan solution and with the results of a derived numerical model, based on an explicit scheme. The model performance was also tested against observation data collected on four different landscapes with different soil profiles and located on a basin underlain by continuous permafrost. It was found that the proposed model matched noticeably well the analytical solution for a volumetric liquid fraction (phi) equal to 0.5 regardless of the grid resolution. Furthermore, compared with the observation data, the model reproduced the annual maximum thaw depth with an absolute error lying between 0.7 and 7.7%. In addition, the designed algorithm allowed the model to converge after a maximum of eight iterations, reducing the computational time by around 75% compared to the explicit model. The results were so encouraging that the model can be included in a hydrological modeling of permafrost watersheds or cold regions in general.

Conjugate Heat Transfer Analysis of PCM Suspensions in a Circular Tube under External Cooling Convection: Wall Conduction Effects

In this paper, a numerical method is used to investigate the conjugate heat transfer of a phase change material (PCM) suspension in a circular tube under external cooling convection. The following parameters and ranges were considered: dimensionless tube wall thickness, t w (0–0.5); wall-to-fluid thermal conductivity ratio, k w f * (0.1–10); volumetric fraction of PCM particles, c v (0.1); Biot number, B i o (1); Stefan number, Ste (0.1); and Peclet number, Pe (1000). The results show that the wall thermal conductivity considerably affects the outer/inner wall temperature of the tube, the average temperature of the working fluid, and the volumetric liquid fraction of PCM particles. Thus, wall conduction effects must be properly accounted for to model heat transfer in a PCM suspension in tube flow.

Influence of aqueous foam syneresis on the shock wave propagation velocity

Numerical simulation of the spherical shock pulse propagation in aqueous foam with volumetric liquid fraction of 0.0083 has been carried out in accordance with the published experimental data on the explosion of HE in aqueous foam. The assumption is used that the foam structure is destroyed by the shock wave, which leads to the transformation of the foam into a monodisperse gas-droplet mixture. The system of equations for the two-phase gas-droplet model of aqueous foam includes the laws of conservation of mass, momentum, energy for each phase and the equation for the dynamics of the volumetric liquid fraction in a single-pressure, two-velocity, two-temperature approximations in a three-dimensional formulation and takes into account the forces of the Schiller-Naumann interfacial drag, the Ranz-Marshall interphase contact heat exchange and the effect of foam syneresis on the initial distribution of its volumetric liquid fraction. Realistic equations of state in the form of Peng-Robinson and Mie-Gruneisen are used to describe the thermodynamic properties of air and water that make up a gas-droplet mixture. Numerical modeling of the processes under consideration was carried out in the open software of computational fluid dynamics OpenFOAM using the finite volume method based on the iterative two-step PIMPLE algorithm. The analysis of the effect of foam syneresis on the dynamics of shock pulse in aqueous foam is given. It was found that the uneven distribution of the liquid fraction in the foam, caused by its sedimentation under the gravity, leads to the increase in the shock pulse velocity in upper layers of the foam. In comparative analysis of numerical solutions and experimental data at sensor locations, the importance of taking into account syneresis phenomena in modeling the dynamics of shock wave in aqueous foam is shown. The reliability of calculations obtained by the proposed model is confirmed by their agreement with experimental data.

Experimental study on the hydrodynamic behavior of gas-liquid air-water two-phase flow near the transition to slug flow in horizontal pipes

The knowledge of the slug flow is essential the design safety of hydrocarbon transportation in the pipeline system of petroleum industries. However, the suitable models to predict the transition condition to slug flow are limited due to insufficient availability of experimental data. In the present study, the hydrodynamic characteristics of the near the transition to slug flow of air-water two-phase flow in horizontal pipes were investigated experimentally for inner pipe diameters of 16 mm, 26 mm, and 50 mm. The superficial velocities of water and air ranged from JL = 0.03 to 0.30 m/s and JG = 0.7 to 10.0 m/s. The hydrodynamic behavior was obtained from the combination of the visualization detected by using a high-speed video camera, the pressure gradient, and instantaneous volumetric liquid fraction measurements.The experimental results indicate that the wave growth and the wave coalescence are involved in the mechanism of the slug flow formation. Both gas and liquid superficial velocities have a significant effect also of those mechanisms. The average volumetric liquid fraction near the transition to slug flow is not affected by superficial gas velocities. Next, under a constant superficial gas velocity, an increase in the inner pipe diameter requires a larger superficial liquid velocity to change the flow patterns from stratified smooth to slug flow, and from pseudo-slug to slug flow.

Effect of geometric design on performance of square cross‐section concentric‐duct and split airlift bioreactors

The performance of internal‐loop airlift bioreactors is affected by their geometry, especially the bottom and gas‐liquid separator designs. Despite efforts to understand the impact of geometry on bioreactor hydrodynamics and oxygen transfer, the contribution of each region to the bioreactor performance as a whole has been evaluated separately. Consequently, it has not been possible to define the best overall geometry. This work discusses the influence of the bottom and gas‐liquid separator geometries on the volumetric oxygen transfer coefficient (kLa) and superficial liquid circulation velocity (UL) of 10‐L square cross‐section concentric‐duct airlift (CDA) and split airlift (SA) reactors. Both airlift reactors were operated with distilled water and Saccharomyces cerevisiae cultivation broth. Based on the results it was possible to indicate the best geometric configurations for biotechnological applications. Gas‐liquid separator design was evaluated by changing the openness angle (α) and the gas‐liquid separator volumetric liquid fraction (FGLS), while the free area for liquid flow between riser and downcomer (Ab) was the geometric parameter modified for the bottom. The results showed that liquid circulation was strongly affected by the bottom geometry, while the gas‐liquid separator significantly affected both variables. Combining the impacts of both regions, promising airlift geometries were identified that exhibited low or high hydrodynamics and oxygen transfer levels. These findings highlight the flexibility of airlift bioreactors and provide information required for their design in order to satisfy specific requirements of different aerobic bioprocesses.

Fluid flow and heat transfer in PCM panels arranged vertically and horizontally for application in heating systems

The rectangular panel is within the most common geometries in phase change materials (PCM). Nevertheless, there is a lack of knowledge regarding how the arrangement (vertical or horizontal) and thickness affect heat flux and phase change duration. Such knowledge would be very helpful both in the design of PCM heat exchangers. This paper studies the behavior of the RT60 paraffin, Pr ≈ 330, phase change temperature between 53 and 61 °C; and the fatty acid Palmitic Acid, Pr ≈ 110, phase change temperature of 65 °C. Parametric studies analyzing the influence of: temperature of the walls (5 × 105 < Ra ≤ 2 × 106), phase change processes (melting and solidification), panel position and PCM thickness (aspect ratio1/20 and 3/20) are performed. The flow behavior is analyzed over time using velocity plots and volumetric liquid fraction contours. The formation of Bénard cells and their evolution is described. Free convection dependence of dimensionless string function and Rayleigh number is discussed. Free convection during melting for horizontal panels becomes very important and the mean heat fluxes increase up to twofold compared to vertical panels. For solidification, however, conduction becomes more relevant. The importance of both mechanisms is highlighted by calculating heat transfer rates.

Effective improvement of defoaming efficiency using foam breaker with synthetic sponge cylinders in foam fractionation

For effectively improving the physical defoaming efficiency in foam fractionation, a new foam breaker with synthetic sponge cylinders was developed and its defoaming mechanism was analyzed. Due to synthetic sponge’s high water absorption ability, it resulted in a large pressure difference inside and outside the liquid film to rapidly collapse the foam. The defoaming efficiency was studied on the basis of the effects of the type and number of synthetic sponge cylinders, SDS concentration, volumetric liquid fraction and volumetric air flowrate. The results show that the synthetic sponge cylinder with 0.23±0.05mm in mean pore diameter and 70.0±0.5% in mean porosity had the highest defaoming efficiency and a larger number of synthetic sponge cylinders resulted in a higher defoaming percentage. Under suitable conditions, defoaming percentage of the new foam breaker with 4 of the optimal synthetic sponge cylinders reached 100.0%. Furthermore, the synthetic sponge cylinders had much higher defoaming efficiency, lower equipment cost, lower energy consumption and simpler preparation than perforated plates and paddle agitator. So the foam breaker with synthetic sponge cylinders is highly promising in industrial defoaming.

CO2 dissolution in water using long serpentine microchannels

The evolution of carbon dioxide bubbles dissolving in water is experimentally examined using long microchannels. We study the coupling between bubble hydrodynamics and dissolution in confined geometries. The gas impregnation process in liquid produces significant flow rearrangements. Depending on the initial volumetric liquid fraction, three operating regimes are identified, namely saturating, coalescing, and dissolving. The morphological and dynamical transition from segmented to dilute bubbly flows is investigated. Tracking individual bubbles along the flow direction is used to calculate the temporal evolution of the liquid volumetric fraction and the average flow velocity near reference bubbles over long distances. This method allows us to empirically establish the functional relationship between bubble size and velocity. Finally, we examine the implication of this relationship during the coalescing flow regime, which limits the efficiency of the dissolution process.

Start‐up transients in a pneumatic foam

AbstractIn the current work, transient features of initiation in a gas‐liquid pneumatic foam are investigated by measuring the evolution of volumetric liquid fraction as a function of height within the column. The addition of wash water to a flotation froth is only effective when the foam liquid fraction has reached a steady state. This makes start‐up transients in pneumatic foam worthy of study. For the conditions adopted in the experiments, an approximately steady state was achieved after typically 500s, but there was significant fluctuation in liquid fraction after this time. In general, three possible regimes in the start‐up transient (induction, growth and evolution) have been identified and a tentative mathematical model has been described for the last two. However, because it has been demonstrated that the method of obtaining bubble size distributions by analysing images taken through the column wall is deficient, no comparison of these models with the data has been attempted. Copyright © 2010 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Intensified Effect of Reduced Pressure on the Foam Fractionation Process of Bovine Serum Albumin

A novel method to intensify the foam fractionation process by operating the column under reduced pressure was reported in this paper. It was found that a poorer foam stability, a bigger mean bubble size and a wider bubble size distribution in the foam layer, a less upward liquid flux and volumetric liquid fraction at the top of the foam layer, and a higher enrichment ratio but a lower mass recovery of the foam fractionation process were obtained under reduced pressures compared to the ones under atmospheric pressure. The most important effect of the reduced pressure on the foam layer during the foam fractionation process is the encouraged bubble coalescence process which was exhibited by those unusual foam properties discussed above. The limitations of this new method were also discussed.

Hydrodynamic theory of rising foam

There is a large body of literature that tries to model flotation based on froth properties with little or no experimental verification of the underlying features. These models adopt the so called channel-dominated theory of foam drainage. There is no experimental evidence to support this foam drainage theory. Instead the new, simple and experimentally validated foam drainage equation of Stevenson (2006a) [Stevenson, P. 2006a. Dimensional analysis of foam drainage. Chem. Eng. Sci. 61, 4503–4510] has been extended to describe the liquid flux and liquid profile in columns of pneumatic froth. The condition for the maximum value of gas rate for froth stability has been described and this shows that there is a maximum volumetric liquid fraction that a foam can exhibit. It is shown that the numerical calculations of liquid profile of Neethling et al. (2003a,b) [Neethling, S.J., Lee, H.T., Cilliers, J.J. 2003a. The recovery of liquid from flowing foams. J. Phys.: Cond. Matter 15, 1563–1576; Neethling, S.J., Lee, H.T., Cilliers, J.J. 2003b. Simple relationships for predicting the recovery of liquid from flowing foams and froths. Miner. Eng. 16, 1123–1130] are incorrect, and this may mean that all of their later simulations of the flotation process are similarly deficient. Instead a simple and accessible method of calculated liquid fraction profiles, both with and without added washwater is shown. In addition, a model for the effect of surface and internal bubble coalescence on the hydrodynamic condition of the froth is presented. It is recognised that the gas–liquid systems considered in the current work are dissimilar to practical mineralised flotation froths and these differences are discussed.

The Wetness of a Rising Foam

A simple graphical method of calculating the liquid volume fraction, or wetness, of a rising pneumatic foam is presented that is deterministic, given system properties (such as the bubble size distribution and the rate at which gas is sparged to the column) and the kinematic viscosity of the interstitial fluid along with some information about the drainage behavior of the stationary foam analogue. Thus, it is shown that data for the wetness of rising columns of SDS foam (Stevenson, P.; Stevanov, C. Ind. Eng. Chem. Res. 2004, 43, 6187) can be well-predicted using an equation (Neethling, S. J.; Lee, H. T.; Cilliers, J. J. J. Phys.: Condensed Matter 2002, 14, 331) for the drainage of liquid through stationary columns of SDS foam. Further, the liquid overflow rate can be well-predicted once the liquid volume fraction is calculated. The effects of washwater added to the top of the column and bubble bursting on the equilibrium volumetric fraction have been considered theoretically. It is shown that the volumetric liquid fraction of the foam will be enhanced and the enhancement can be determined graphically. However, the rate at which liquid will be recovered from the top of the column is seen to be independent of washwater addition and bubble bursting.

Liquid distribution and electrical conductivity in foam

Experiments were conducted with aqueous foam generated by bubbling nitrogen through anionic, cationic and nonionic surfactant solutions. The ratio of the electrical conductivity of the foam to that of the liquid was found to increase monotonically with the volumetric liquid fraction in the foam. The choice of surfactant as well as the degree of inhomogeneity in bubble size were found to be without effect on the relationship. However, for a fixed liquid fraction, it was found that decreasing the mean bubble size can decrease the conductivity ratio somewhat, as well as accelerate the approach toward Lemlich's limit for low density foam. This effect was attributed to increased suction within the Plateau borders.

Electrical conductivity and the distribution of liquid in polyhedral foam

A theoretical model is developed for conduction solely along thin uniform lamellae of polyhedral foam bubbles. For the volumetric liquid fraction divided by the ratio of foam conductivity to liquid conductivity, the model yields a value of 1.4. This complements Lemlich's limit of 3.0 for conduction solely along narrow, uniform, randomly oriented Plateau borders, and is combined with the said limit so as to permit estimation of the relative distribution of liquid between lamellae and Plateau borders in a real polyhedral foam.