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The jet wiping process is a cost-effective coating technique that uses impinging gas jets to control the thickness of a liquid layer dragged along a moving strip. This process is fundamental in various coating industries (mainly in hot-dip galvanizing) and is characterized by an unstable interaction between the gas jet and the liquid film that results in wavy final coating films. To understand the dynamics of the wave formation, we extend classic laminar boundary layer models for falling films to the jet wiping problem, including the self-similar integral boundary layer and the weighted integral boundary layer models. Moreover, we propose a transition and turbulence model to explore modelling extensions to larger Reynolds numbers and to analyse the impact of the modelling strategy on the liquid film dynamics. The validity of the long-wave formulation was first analysed on a simpler problem, consisting of a liquid film falling over an upward-moving wall, using volume of fluid simulations. This validation proved the robustness of the integral formulation in conditions that are well outside their theoretical limits of validity. Finally, the three models were used to study the response of the liquid coat to harmonic and non-harmonic oscillations and pulsations in the impinging jet. The impact of these disturbances on the average coating thickness and wave amplitude is analysed, and the range of dimensionless frequencies yielding maximum disturbance amplification is presented., SCOPUS: ar.j, info:eu-repo/semantics/published

A natural draft dry cooling tower rejects heat in a power plant. Spray cooling of the inlet air to the cooling tower improves the total efficiency of the power plant. To overcome the scarcity of natural water sources, this research is studying the usage of saline water in spray assisted dry cooling towers. A nozzle is analysed experimentally. It is shown that the CFD model captures the spray well. A full cone spray is simulated in a vertical cylindrical domain representative of cooling tower flows. To investigate the influences of initial and ambient conditions on the spray performance, fourteen different cases are simulated and trends analysed. It is shown that the distances from the nozzle, after which the dry stream starts (wet lengths), are in the range of 4.3–5.25 m depending on the test conditions. A dimensionless study is performed on the wet length and cooling efficiency as the two main parameters. Finally, to predict the wet length and cooling efficiency, two dimensionless correlations are presented and their impact on cooling tower operation is discussed.

In the present study, a cryogenic valve used in launch vehicle liquid propulsion systems is experimentally and numerically characterized. Two independent measurement campaigns are performed with liquid nitrogen and water as working fluids. Two equivalent facilities have been designed and manufactured to perform the cryogenic and the water tests. The characteristic relationship between the volumetric flow rate and pressure drop across the test valve is obtained for both the fluids. As far as cryogenic tests are concerned, temperature measurements at the test valve inlet and outlet are presented as well as visualizations of the flow upstream the test section. The experimental results show that the test valve flow coefficient is independent of the working fluid provided single phase flow conditions. To further validate this result a numerical study is conducted using the commercial code CFD-ACE+. A good agreement between numerical and experimental results is found. Furthermore, test cases in the semi-critical and critical flow conditions are simulated using the so-called full cavitation model. The computed liquid recovery factor is shown to be also independent from the working fluid nature.

The Belgian Antarctic Base Princess Elisabeth is based on an elevated building on top of sloping terrain and connected to an under-snow garage. The integrated design of the base was supported by wind engineering testing that looked into building aerodynamics (pressure taps) and snowdrift management. Wind tunnel modeling using sand erosion technique allowed efficient evaluation of the snow erosion and deposition around different building-block shapes during the conceptual design phase. Parametric testing shows that the positioning of the main building on the ridge has a significant impact on wind loading and snow erosion and deposition. Important reductions in wind loading and snow deposition can be obtained by elevating the building and reducing the windward cantilever. The positioning of the garage roof can further decrease the wind loading by acting as a diffuser in the back of the building. This study shows that, not only for safety and cost reduction but also for the integration of renewable energies, important benefits in the design of a building can be achieved if wind engineering is considered since the conceptual phase of the integrated building design process.

Impinging jets over liquid surfaces are a common practice in the metallurgy and chemical industries. This paper presents a numerical study of the fluid dynamics involved in this kind of processes. URANS simulations are performed using the volume of fluid (VOF) method to deal with the multiphase physics. This unsteady approach with the appropriate computational domain allows resolution of the big eddies responsible for the low frequency phenomena. The solver we used is based on the finite volume method and turbulence is modelled with the realisable k-ϵ model. Two different configurations belonging to the dimpling and splashing modes are under consideration. The results are compared with PIV and LeDaR experimental data previously obtained by the authors. Attention is focused on the surroundings of the impingement, where the interaction between jet and liquid film is much stronger. Finally, frequency analysis is carried out to study the flapping motion of the jet and cavity oscillations.

The current study presents an experimental investigation of a technique for delaying the occurrence of splashing in jet wiping process by means of a side jet. Gas jet wiping is a hydrodynamic method of controlling the film thickness applied on a substrate in coating processes. It consists of a turbulent slot jet impinging on a moving surface coated with a liquid film. The process is limited by splashing; a rather violent film instability which is characterized by the ejection of droplets from the runback flow and results in the detachment of the film from the substrate. In the present study an additional side jet is used close to the main wiping jet in order to stabilize the runback flow and avoid splashing. The mean film thickness after wiping is measured using a light absorption method and the results are compared for the single jet wiping and two jet configuration. It is shown that the use of the side jet allows for stronger wiping, resulting in lower values of the final film thickness which cannot be achieved with a single jet.

This paper presents a study of the gas-jet wiping process, which is used in coating techniques to control the final coating thickness applied on a substrate. Numerical simulations are performed using the FLUENT commercial software, with the Volume of Fluid (VOF) model coupled with Large Eddy Simulation (LES). The comparison with results from an analytical model, (with and without surface tension), and from dedicated experiments shows good agreement. The realizable k-epsilon turbulence model is used to reduce the computation time, but with no satisfactory agreement compared with LES and experiments.

The paper shows that the global rainbow thermometry (GRT) technique is a non-intrusive measurement technique, which provides the entire size distribution and the mean temperature of an ensemble of liquid droplets suspended in a gaseous phase. In this aim a non-isothermal flat-fan water spray is characterized by means of the GRT and the results obtained, both for temperature and for size, are in agreement with predictions already done by other authors. Moreover, it is shown that the main source of uncertainty for the GRT measurements is due to the systematic error committed in the determination of the GRT reference angle.

The mitigation of the consequences of storage-tank fire is a great safety concern in petro-chemical and gas industries. A technique to protect the integrity of neighbouring structures is the water spray curtain. It can be operating downward in front of or oriented to the surface to be shielded. Simple modelling, laboratory experiments and field tests for these two types of thermal shielding are presented. Attenuation factor of 50–75% can be expected with the vertical curtain while 90% can be reached with the impinging curtain if spray overlapping is achieved.

The mitigation of the consequences of accidental releases of dangerous toxic and/or flammable cloud is a serious concern in the petro-chemical and gas industries. Nowadays, the water-curtain is recognized as a useful technique to mitigate a heavy gas cloud. The paper presents a research methodology, which has been established and undertaken to quantify the forced dispersion factor provided by a water-curtain with respect to its configuration. The method involves medium-scale field tests, Wind-Gallery tests and numerical simulations. These different approaches are discussed and exemplified by typical results emphasizing the observed concentration reduction due to the water-curtain.

A physical limit to the galvanizing speed exists over which the liquid film explodes completely and prevents any further wiping. This limit is presently estimated around 180-200 m/min. A dimensionless approach including the Weber and Reynolds numbers is proposed to be able to predict the occurrence of the phenomenon. Practically it comes out that nozzles tilted by 30° down to the pot can pushback the limit by 30 %. A maximal line speed around 220 m/min could thus be reached.

The production of new steel grades - such as dual phase and TRIP steels - requires improvements to both process and equipment of continuous galvanizing lines. In particular, progress has to be obtained in cooling technology in order to get the desired mechanical properties. This paper presents a study of design parameters allowing the optimization of fast gas multi-jet cooling systems. The thermal study involves the application of infrared thermography and three-dimensional numerical simulations. Furthermore, a dynamic study is performed in order to reduce steel strip vibration.

The nozzle design effect on sound production is investigated to improve the understanding of the aeroacoustic coupling that occurs in solid rocket motors with a submerged nozzle. Earlier analytical and experimental work demonstrated that flow-acoustic coupling is observed only for submerged nozzles for which the sound pressure level increases linearly with the nozzle cavity volume. Numerical simulations of the flow-acoustic coupling phenomena and in particular of the effect of the nozzle cavity volume on the pressure pulsations are performed using the code CPS. The numerical and experimental pressure spectra are compared. The frequencies are well simulated by the numerical code even if the pressure levels are overestimated. The nozzle design effect on sound production is also observed with a reduction of pressure level of 55% when the nozzle cavity is removed. Furthermore, the nozzle cavity modifies the flowfield around the nozzle head. With the cavity, the recirculation bubble is shorter, and the flow close to the nozzle head presents high amplitudes of radial mean velocity and fluctuation. That explains why the vortices break up when interacting with the nozzle head. With the cavity, the vortices shed by the inhibitor move along the border of the recirculation bubble and then pass in front of the cavity entrance, where they generate sound by interacting with the velocity fluctuations induced by the cavity volume. This results in a strong interaction between the vortices and the nozzle, which leads to large pressure oscillations.

Aeroacoustic instabilities occur in many applications of technological interest and have undesirable effects on the steady operation of the system. Passive and active means are sought to reduce the level of oscillation and eliminate the instability. In the case of segmented solid rocket motors, observations indicate that low-frequency oscillations are generated by a coupling between vortex shedding in shear regions established in the flow and the acoustic eigenmodes of the system. This process is investigated in this article on a model-scale configuration representing the geometry of the motor. An active control loop is exploited to obtain resonant and non-resonant conditions for the same operating point. Adaptive techniques are used to stabilize the flow and the experiment serves as a testbed for active control. It is shown that an adaptive system may be applied to essentially suppress the pressure oscillations. The instability mechanism is then studied by analyzing the flow field with particle image velocimetry. It is found that control noticeably modifies the mean flow structure. Detailed studies of the vortex pattern in the shedding region indicate that the concentrated vorticity and the corresponding circulation values remain in the same range but that vorticity is shed more randomly when the resonance is eliminated by the controller. This indicates that control is achieved by reducing the level of organization in the vortex pattern. Under resonant conditions the level of pressure fluctuations results from coherent interactions between vortices and the downstream nozzle. This process feeds energy in one of the acoustic modes of the system enhancing the pressure level. It is made less effective by the control loop.

An experimental study of convective heat transfer in a channel implemented with perforated ribs immersed in a turbulent boundary layer is presented. Infrared thermography associated with the steady heated thin foil technique is applied to obtain the mapping of the heat transfer coefficient. Different types of perforations are compared in the case of single turbulator configuration. Conclusions are drawn on the effect of the rib design on the heat transfer enhancement. Next, the study focuses on the turbulator with perforations of chevron type. Then, emphasis is given to the influence of the rib-to-rib spacing, the open-area factor and the channel Reynolds number on the thermal exchange. Compared to solid rib, a local thermal enhancement factor of 3 can be expected just behind the perforated turbulator. Fully developed flow is reached after the 5th to 6th rib. The optimal design combines a rib pitch ratio of 5 with an open area factor of 0.53 for channel Reynolds number ranging from 30000 to 60000.

The ribbing instability hinders the roll coating process, as it produces wavy coating films. The technique studied here prevents ribbing, whereby a string is placed parallel to the rolls, in contact with the unstable meniscus. In this experiment, the fluid is a Newtonian oil. A free-surface visualization technique allows measurement of the interface profile where the string interacts with the forming films. The ribbing can be eliminated under conditions at least 20 times its natural onset. The influence of the string position on the mass transfer is then examined. Measurements show that the streamwise position of the string controls the total flow rate of the process, while its transverse position accurately controls the relative thicknesses applied on each roll. The string proves to be both an efficient concept to eliminate ribbing, and a reliable way of fixing the coating film thickness, without resorting to traditional gap width and roller speeds selection.

In the present paper, experiments are reported for the measurement of the complete meniscus profile in asymmetric forward roll coating. A thin laser sheet technique is used to recover accurately the shape and position of the interface for an extensive database of operating conditions. Knowledge of the interface profile and position is then used to measure the total flow rate and its distribution on each cylinder. The results provide for the first time validation of numerical predictions for the extended shape of the meniscus. Comparison with existing theoretical and numerical predictions validates both these approaches, but reveals the need to model the whole flow region, including gravity, instead of restricting to the region downstream of the nip separating the rotating cylinders.

An experimental study is performed in order to describe the single‐ and two‐phase (air‐water) horizontal flow in presence of a pipe expansion. Two types of singularities are investigated; sudden and progressive enlargement. In the lateral case, the opening angles of the divergence geometry are 5, 8 and 15 degrees. The surface area ratios tested are σ = 0.43 and σ = 0.65. Bubbly flow is the dominant flow regime that is investigated for volumetric quality up to 30%. The pressure recovery for the case of both single and two‐phase was examined versus axial position. It is found that the smallest is the enlargement angle, the largest the recovery pressure for the same flow conditions: the pressure drop caused by the singularity is higher in the case of a sharper enlargement. The comparison of the experimental results to published models leads to proposed corrective coefficient for Jannsen’s (1966) correlation. Flow visualization is also performed; the flow patterns downstream the different singularities are id...

The bubbling phenomena from a downward facing nozzle immersed in a liquid bath are studied experimentally on a water model facility. The pressure fluctuations in the injection lance are correlated to the bubble growth and detachment thanks to simultaneous visualizations and digital processing of the resulting images. The main control parameters for the bubbling frequency are identified within an important experimental database for submerged injection. The effect of the gas volumetric flow rate, the lance submergence depth in the bath, the tip diameter and the gas properties are characterized in particular. An empirical dimensionless correlation is proposed for the frequency prediction.

An experimental investigation of impinging 2d planar air jet on a water surface is performed by means of Particle Image Velocimetry (PIV). The difficulty encountered to measure the two dimensional velocity and turbulence fields close to the interface requires a special treatment of the PIV data obtained in the air and water sides. The resulting technique enables the localisation of the moving interface, the use of the inter-correlation and the calculation of statistics in both phases. The results show that the proposed PIV interface detection technique is in good agreement with the LeDaR detection technique developed at the von Karman Institute. The analysis of the air and water velocity field close to the interface highlights the topological differences between dimpling and incipient splashing configurations. The results obtained in this study will serve to model later turbulence transfer in impinging jet configuration.

An experimental study is performed in order to describe the singleand twophase (air-water) horizontal flow in the presence of pipe expansion and contraction. Three types of singularities are investigated; smooth convergence and sudden and progressive enlargement. The opening angles for progressive singularities are 5, 8, 9 and 15 degrees. The surface area ratios tested are σ = 0.43, 0.64, 0.65 and 1.56. Bubbly flow is the dominant flow regime that is investigated for volumetric quality up to 30%. The pressure distribution for both single and two-phase horizontal flow is examined versus axial position. For expansion geometries, it is found that the smaller the enlargement angle, the larger the recovery pressure for the same flow conditions; the pressure drop caused by the singularity is higher in the case of a sharper expansion. The comparison of the experimental results to published models leads to a proposed corrective coefficient for Jannsen’s correlation. Flow visualization is also performed; the flow patterns downstream from the different singularities are identified in each configuration and plotted in Baker’s map for horizontal flow.

The complex flow in the recirculation region upstream of a rectangular jet issuing normally into a uniform crossflow has been examined using flow visualization techniques. The variation of the primary separation distance with velocity ratio revealed the existence of two different flow regimes. The change from one to the other occurs at a velocity ratio of 5. The recirculation region upstream of the jet is found to be unsteady and periodic. The frequency of this periodic motion appears to be the same as that produced by the vortex shedding in the near wake

The paper describes a methodological study of convective heat transfer in multiple‐jet systems. It consists of a laboratory approach involving the use of infrared thermograpghy and a numerical approach relying on 3D CFD simulation. The resulting dimensionless heat transfer correlations are incorporated in an engineering code, which allows the optimized design of galvanizing lines.

Computational fluid dynamics plays an important role in engineering design. To gain insight into solving problems involving complex industrial flows, such as impinging gas-jet systems (IJS), an evaluation of several eddy viscosity models, applied to these IJS has been made. Good agreement with experimental mean values for the field velocities and Nusselt number was obtained, but velocity fluctuations and local values of Nusselt number along the wall disagree with the experiments in some cases. Experiments show a clear relation between the nozzle-to-plate distance and the Nusselt number at the stagnation point. Those trends were only reproduced by some of the numerical experiments. The conclusions of this study are useful in the field of heat transfer predictions in industrial IJS devices, and therefore for its design.

A numerical analysis of the flow behavior in industrial cooling systems based on arrays of impinging jets has been performed, using several eddy viscosity models to determine their modeling capabilities. For the cooling system studied, and in terms of mean Nusselt number values, the best agreement between experimental results and numerical predictions was obtained with the realizable k-ε model. On the other hand, numerical predictions of the local Nusselt number and its spatial variations along the wall are better adjusted to the experiments when using either the standard k-ε or the standard k-ω models. The results obtained also show that the predicted thermal field depends strongly on the combination of near-wall treatment and selected turbulence model.

This paper deals with the mapping of the convective heat transfer in a multijet anti-icing system by application of the quantitative infrared thermography technique. The experiments are conducted on a full scale mock-up of the leading edge section of a jet aircraft slat. A steady state infrared thermography technique combined with the heating foil method is developed and successfully applied to recover the complete distribution of the thermal exchange coefficient on the concave inner surface of the skin. The heating performance of such a multijet system depends on the jet Reynolds number, the distance of the supply duct and the skin and the spanwise and chordwise jet arrangement. Tridimensional numerical simulations are also performed with the commercial CFD code FLUENT. Despite the complexity of the flow phenomena and the stiff character of the boundary conditions involved, satisfactory agreement is found between the predictions and the IR data.

Impinging fluid jets are widely used in industrial processes where high momentum and/or mass transfer have to be reached. Typical applications are jet wiping, the drying of paper and textile, the cooling of turbine blades, the anti-icing of aircraft, the tempering of glass sheets, the cooling of moving metal strips, the cooling of electronic equipment, ... Numerical simulations of these impinging jets are mainly performed solving the Reynolds Averaged Navier-Stokes equations. However, it has been shown that this approach for the simulation of turbulence overestimates the heat transfer coefficient at the impinging region (overestimates of 100% have been observed). Therefore, the Large Eddy Simulation (LES) approach, which models only the small scales of turbulence and resolves the large scales is used to compute a round impinging jet with a Reynolds number varying from 3.000 to 70.000.

The paper describes a study of convective heat transfer in a multiple-jet systems composed of straight and inclined slot nozzles. The application concerned is the fast cooling of moving strip. The experimental approach involves the application of infrared thermography associated with the steady-state heated foil technique. Three-dimensional numerical simulations performed with the code FLUENT compare agreeably with the IR data. The study aims to determine the effect on the average heat transfer coefficient of the slot Reynolds number up to the value of 100000, the nozzle spacing normalised by the slot hydraulic diameter in the range 6 ≤ W/S ≤ 18, the normalised nozzle emergence length, E/S, from 5 to 17 and the normalised nozzle to strip standoff distance Z/S from 3 to 10. The geometrical arrangements tested include perpendicular (90°) and tilted (60°) nozzles. A thermal entrainment phenomenon is found for cooling system of small width. A corrective factor is derived to account for this effect. The experimental findings are compared with existing correlation; deviations, which are observed at high values of the Reynolds number may reach 25%. The numerical simulation emphasises the benefit to use H2 /N2 gas mixture to enhance significantly the cooling rate.Copyright © 2002 by ASME

Cold gas experiments are used to study the vortex-nozzle interaction, which drives thrust pulsation in solid-rocket motors. The experiments carried out in an axial flow model clearly demonstrate coupling of vortex shedding with acoustical longitudinal resonances of the combustion chamber as observed in actual motors. The amplitudes of the pressure fluctuations correspond to one-thousandth of the static pressure, which is the order of magnitude of the observed pulsations in rocket motors. Experiments show that the cavity formed around the nozzle inlet during combustion is crucial. The pulsation level is proportional to the volume of the cavity. Theory predicts this relationship if we assume vortex-nozzle interaction to be the main source of sound. The proposed analytical model does, however, overestimate the pulsation level by an order of magnitude.

The paper describes a study of convective heat transfer in a multiple-jet systems composed of round nozzles. The application concerned is the fast cooling of moving strip. The experimental approach involves the application of infrared thermography associated with the steady-state heated foil technique. The study aims to determine the effect on the average heat transfer coefficient of the jet Reynolds number in the range 40000 to 85000, the normalised nozzle to strip standoff distance Z/D from 2 to 10 and the normalised nozzle spacing W/D from 2 to 5. The geometrical nozzle arrangements tested include staggered and non-staggered configuration. The experimental findings are compared with an existing correlation; deviations, which are observed at small Z/D-values may reach 30%.

The nozzle design effect on sound production is investigated at VKI to improve the understanding of the aeroacoustic coupling that occurs in the Ariane-5 booster. A 1/30-scale axisymmetric cold flow model of the Ariane-5 EAP is designed on purpose. A flow field investigation demonstrates that the radial flow, injected through porous cylinders, simulates correctly the hot burnt gas flow from the combustion. Acoustic measurements performed by means of PCB piezoelectric transducers allow identification of flow-acoustic coupling. In some conditions, the aft end volume has the effect to increase the pressure fluctuation levels.

Introduction The nozzle design effect on sound production is investigated at VKI to improve the understanding of the aeroacoustic coupling that occurs in the Ariane-5 booster. Acoustic measurements performed by means of PCB piezoelectric transducers are taken in a 1/30scale axisymmetric cold flow model of the Ariane5 SRM with purely axial injected flow. Flowacoustic coupling is observed for the nozzles including cavity and an analytical model of the resonance occurrence is developed. Furthermore, the maximum resonance amplitude is highly dependent on the nozzle design. Without cavity, the fluctuations are damped by at least one order of magnitude. The experimental pressure spectra are compared to numerical simulations performed using the code CPS. The frequencies are well simulated by the numerical code even if the pressure levels are overestimated. The nozzle design effect on sound production is also observed numerically. Furthermore, the nozzle cavity modifies the flow field around the nozzle head. With cavity, the recirculation bubble is shorter and the flow close to the nozzle head presents high amplitudes of radial mean velocity and fluctuation. That explains why the vortices break up when interacting with the nozzle head generating acoustic pressure fluctuations. *Ph.D. Candidate, AIAA Student Member t Professor *Head of CFD Group, AIAA Member Copyright © 2000 by the von Karman Institute for Fluid Dynamics. Published by the American Institute of Aeronautics and Astronautics, with permission. The aeroacoustics of solid propellant booster is currently being investigated at the von Karman Institute (VKI) as a part of the ASSM program (Aerodynamics of Segmented Solid Motors), initiated by the CNES to support the development of the Ariane-5 solid propellant motor (EAP). This accelerator has a segmented combustion chamber consisting of three cylindrical segments and a submerged nozzle (figure 1). Two inhibitor rings ensuring thermal protection separate the three segments. The hot burnt gas flow originates radially from the burning surface of the combustion chamber and then develops longitudinally before reaching the exhaust nozzle. During the combustion, the regression rate of the burning surface is faster than those of the inhibitor rings. Then, the lasts become obstacles into the flow-field and generate a significant risk of hydrodynamic instabilities. Pressure oscillations have been already observed for solid rocket motors, such as the U.S. Space Shuttle, the Titan SRM and the Ariane-5 EAP [1, 2, 3]. Similar results were obtained on sub-scale models of the Ariane-5 booster [4].

An experimental investigation of the forced convection along vertical. slender cylinders aligned in a uniform air flow is performed using quantitative infrared thermography. In the laminar regime, the heat transfer data are correlated in terms of a local dimensionless curvature parameter and validate the thermal boundary layer model for a single slender cylinder. Turbulent heat transfer occurs when the stand­ off distance between two parallel, slender cylinders is sufficiently small to affect the axisymmetry of the boundary layer. An enhancement factor of three can be achieved at the transition region. In the turbulent regime, the curvature parameter alone does not describe the thermal exchange.

The convective heat transfer at the impingement of a vertical turbulent circular air jet on a horizontal flat plate is inferred from temperature measurements performed by quantitative infrared thermography. Steadystate experiments are conducted with the heating-thin-foil method. The effects of the jet Reynolds number and the stand-off distance on the thermal exchange coefficient are emphasized. The influence of jet confinement is determined. The IR results agree with published data and are reproduced by numerical simulations performed with the code FLUENT.

A theoretical investigation of the transient response of a liquid saturated, self heated particle bed is presented. The physical, mathematical and numerical aspects of the problem are examined. The response of the system to fast power transients is studied on the basis of several numerical tests using as input the data corresponding to major in-pile core debris experiments conducted in Europe and USA. In particular the effect of the initial bed conditions on the vapour pressure profile and the possibility of occurrence of bed disturbances is analyzed. Furthermore the formation and growth of a dry zone in the bed interior is simulated. The important role of capillary forces in the boiling bed regime is stressed. Physical interpretations of the results are proposed and discussed.