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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.

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.

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 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.

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.

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.

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.

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.

Coating techniques are frequently used in industrial processes such as paper manufacturing, wire sleeving, and in the iron and steel industry. Depending on the application considered, the thickness of the resulting substrate is controlled by mechanical (scraper), electromagnetic (if the entrained fluid is appropriated), or hydrodynamic (gas-jet wiping) operations. This paper deals with the latter process, referred to as gas-jet wiping, in which a turbulent slot jet is used to wipe the coating film dragged by a moving substrate. This mechanism relies on the gas-jet–liquid film interaction taking place on the moving surface. The aim of this study is to compare the results obtained by a lubrication one–dimensional model, numerical volume of fluid–large eddy simulation (VOF-LES) modeling and an experimental approach. The investigation emphasizes the effect of the controlling wiping parameters, i.e., the pressure gradient and shear stress distributions induced by the jet, on the shape of the liquid film. Those...

This paper presents an analysis of the jet wiping process in hot-dip galvanization. This technique consists in reducing the liquid film thickness on a moving substrate by applying gas slot jets. A theoretical development allows the computation of the film thickness distribution in the wiping zone. It is further simplified to an engineering model which predicts the final coating thickness, in good agreement with wiping experiments, as well as numerical simulations. The limit of applicability of jet wiping is due to the occurence of a violent film instability, called splashing, which takes the form of a liquid droplet emission just below the nozzle. An experimental investigation of this phenomenon is conducted on a water model facility. Two nozzle designs are tested. The effect of the process parameters such as the strip speed, the nozzle pressure, the standoff distance and the tilt angle of the nozzle on splashing is emphasized. A dimensionless correlation is established to allow the prediction of the operating conditions leading to splashing occurence. It is confronted to observations made on galvanization lines.Copyright © 2006 by ASME