Thermal and Dynamic Study of Rapid Cooling in Continuous Galvanizing Lines

Michel Renard

Drever International S.A. Parc Scientifique du Sart-Tilman B-4031 Angleur (Liège), Belgium

Tel. : + 32 43 66 62 62 Fax : + 32 43 67 76 78

E-mail : renard@drever.be

Jean-Baptiste Gouriet, Philippe Planquart, Jeroen van Beeck, Jean-Marie Buchlin von Karman Institute for Fluid Dynamics

Chaussée de Waterloo, 72 B-1640 Rhode-St-Genèse, Belgium

Tel.: + 32 23 59 96 14 Fax: + 32 23 59 96 00

E-mail: buchlin@vki.ac.be

Key words: galvanizing lines, cooling, thermal, vibrations

ABSTRACT

The production of new steel grades requires the improvement of rapid cooling technology. The increase of cooling capabilities can however be limited by the strip vibrations. Experimental and numerical study of the thermal performances of such a gas cooling system is presented. The dynamic behavior of the strip is also studied at both industrial and laboratory levels.

INTRODUCTION

The development of the automotive industry is highly dependent upon the weight reduction and the increase of safety. The UltraLight Steel Auto Body (ULSAB) project has shown that these objectives can be reached by the use of high-strength steels. However the production of these new steel grades necessitates improvements of both process and equipment. In particular, progress is required in cooling technology in order to get better mechanical properties. As in many industrial processes (1), impinging gas jets are widely used to achieve steel strip cooling, in continuous annealing and galvanizing lines (2). The strip is cooled down by strong impingement of cold atmosphere gases on both faces through transversal slot nozzles. Therefore, the optimization of such a gas cooling system requires to identify and to model the design parameters that control the convective heat transfer between the impinging gas and the metal strip. In addition, the impact of gas jets on the steel strip can induce its vibration. So the gas cooling optimization must also include a study of the design parameters for the limitation of strip vibrations.

The methodology adopted in the thermal study combines both experimental tests and numerical simulations. Dedicated laboratory experiments are first conducted on a reduced-scale model of the cooling device. Several design parameters are considered. The experimental approach involves infrared thermography in order to access to a refined description of the thermal exchange between gas jets and steel strip. These experimental data are used to work out engineering correlations. The code FLUENT is used to carry out the numerical analysis. The experimental data also provide support for code validation. Then, numerical simulations are performed to extend the applicability range of the thermal design correlations. The validated code is also used to predict the performance at the scale of industrial cooling devices. The vibration study consists in experimental tests performed on a reduced-scale model. Videos of strip vibrations are recorded in industrial lines, in order to get the vibration characteristics of the steel strip. In the vibration study, we test the same design parameters than in the thermal study. The thermal and vibration studies results are then combined in order to predict the overall performance of the real industrial systems, taking into account the cooling capability as well as the strip dynamic behavior.

THERMAL STUDY

1. Experimental Study The experimental facility uses a model of the cooling device, which has been designed at scale 2/3. Figure 1 shows a schematic of the test set-up while the photograph in Figure 2 displays a general view. The model consists of a plenum (settling chamber) and a set of slot nozzles in the front side, both made in Plexiglas for visualization purpose. The jets impact on a vertical flat plate, which is uniformly heated. The piping system is designed to avoid low frequency contents in the flow and fluidic phenomena that might affect the steadiness of the flow. Several pressure taps have been mounted on the settling chamber in order to control its efficiency. The facility allows the investigation on the cooling rate of the jet Reynolds number Re, based on the hydraulic slot-diameter S, up to 100,000.

Figure 1: Schematic of the multiple slot jet set-up

Figure 2: General view of the thermal study facility

The geometrical arrangements tested include perpendicular (90°) and tilted (60°) nozzles. The slots arrangement can be adjusted to investigate the effect of several design parameters, such as the nozzle spacing W (6 ≤ W/S ≤ 18) and the length of the nozzle E (5 ≤ E/S ≤ 17). More than 25 slot nozzles have been designed to cover the whole range of each parameter. The translation of the multi-jets system allows to change the stand-off distance between the nozzle and the flat plate, Z, in the range of 3 ≤ Z/S ≤ 10. Experiments are conducted with the heating-thin-foil technique. The vertical flat plate is a constant heat-flux surface of 1.7 m long and 0.27 m wide. It consists of adjacent electrical-circuit boards. Each circuit is made of copper foil, coating an epoxy sheet. The copper layer of the plates is machined to produce a Π-shape continuous electrical resistance. The design of the heated flat plate is optimized to provide uniform heat flux, qJ. The Joule heating is monitored by a potentiometer and measured with ammeters and voltmeters. The copper face is exposed to the impinging flow while an infrared camera scans the other side, which is painted black. The IR scanner is an AGEMA Thermovision 900 system with a HgCdTe detector sensitive in the 8-12 µm wavelength range and cooled by liquid nitrogen. The camera is mounted on a displacement carriage to scan the flat plate along the vertical direction. More than 17 images are necessary to reconstruct a complete thermogramme. The thermogrammes are calibrated by measuring the surface temperature at dedicated points with thermocouples flush-mounted on the heated element. The convective heat transfer coefficient is calculated from the plate surface temperature Tw (x,y) by application of the Newton relation:

jrw

cvcv TyxT

yxqyxh

−=

),(),(

),(

In the above expression, Tjr is the temperature of the jet at the nozzle exit. The convective flux qcv is obtained by subtracting the heat losses ql to the Joule heating qJ. From the images recorded by the infrared camera, a complete thermogramme can be reconstructed. Figure 3 shows such a thermogramme corresponding to a typical slots arrangement. It shows the lowest temperatures are observed at the impingement of the jets. The low spanwise distortion of the isotherm contours denotes the two-dimensional character of the impinging flow for this specific configuration.

Figure 3: Typical infra-red thermogramme

The axial distribution of the Nusselt number, Nu, based on the hydraulic slot diameter and corresponding to the jet arrangement and temperature field shown in Figure 3 is plotted in Figure 4; this can also be done for the convective heat transfer coefficient. For this typical jets configuration, the two tilted jets give a lower heat transfer than the perpendicular jets. For engineering purpose, it is more convenient to calculate the average heat transfer coefficient from the local distribution.

-60 -40 -20 0 20 40 60 0

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0.8 Nu/Nu(max)

Z/S = 4.55 ; W/S = 18.2 ; E/S = 11.35

X/S

Figure 4: Axial distribution of Nusselt number

2. Numerical simulations Three-dimensional numerical simulations are performed with the general-purpose commercial code FLUENT (Version 5.5). A typical mesh for a configuration with 4 vertical jets and 2 inclined jets, impinging the steel strip, is shown in Figure 5. The mesh is refined close to the heated plate to capture better the gradients of velocity, temperature and turbulence quantities. Typically 600,000 cells are used for the discretisation of the entire computational domain.

Figure 5: Typical mesh including 2 tilted nozzles

One plane of symmetry is shown in this figure. It was used in order to keep the number of cells relatively small. The computational domain is wider than the width of the slits in order to capture also the flow structure on the side of the jets. The boundary conditions are imposed based on the experimental conditions (temperature and velocity of the impinging jets, temperature of the ambient air, heat flux imposed on the heated plate). The flow velocity field is presented in Figure 6. The numerical simulations allow the determination of the temperature distribution at the level of the impinged plate; then the Nusselt number distribution along a line of the plate is plotted.

Figure 6: Gas flow velocity field

3. Code validation and applications The results obtained in the infrared measurements and by numerical simulations are compared, in order to validate the code. Figure 7 shows such a comparison for 2 perpendicular jets and 1 tilted jet: good agreement is observed for the Nu variation as a function of the reduced axial distance, assessing the correct modeling of the complex flow topology. After this validation, numerical simulations are performed taking into account the real size of industrial cooling systems. Results validate the current engineering correlations used by DREVER International S.A. and provide data for their use in extended range of geometrical and aerodynamical parameters. As examples, use of mixtures of hydrogen and nitrogen and Reynolds number higher than 100,000 can be tested by numerical simulations.

Figure 7: Experimental data - numerical simulations comparison

VIBRATION STUDY Videos of strip vibrations in the rapid cooling section of industrial galvanizing lines are recorded. The tests have been performed in the galvanizing line SIDGAL 3, at Sidmar (Gent, Belgium). Various production conditions are tested, such as different strip widths, strip thicknesses, line speeds, etc. The objective of these tests was to collect information on the strip vibration frequencies in an actual fast cooling system. This information will be used in the design of the dynamic scale-model. The experimental procedure consists to record the vibrations of a moving strip with a video camera. An image processing software processes the video sequences to detect the position of a marker, i.e. the foreground or the background strip border. This experimental procedure provides a correct and fast estimation of the main strip vibration frequencies. Most of the spectral energy is distributed in the low frequency range between 0 and 10 Hz. Figure 8 shows a typical record of the strip position as a function of time and the corresponding power spectral density.

-30 -20 -10 0 100

0.4

0.8

1.2IR dataFLUENT

Z/S = 4.55 W/S = 14.5 E/S = 11.3

Nu/Nu max

X/S

Figure 8: Typical record of strip vibrations and corresponding power spectral density The photography in Figure 9 shows the laboratory facility built for the vibration study. The method, which consists in using a “sectional model”, is a particular way of determining the aero-elastic stability of the strip. It has been developed by the National Physical Laboratory (N.P.L, U.K.) for the simulation in a wind tunnel of the dynamic behavior of a suspension bridge. Strip vibration frequencies (measured in industrial lines) and similarity laws are used to define the elastic connections of the plate simulating the strip. This allows the determination of the spring blades in the facility. In the configuration shown in the Figure 9, the rotation of the plate around vertical axis is blocked: so the translation of the plate is tested.

Figure 9: General view of the vibration study facility

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In the vibration study facility, two reduced-scale plenums with a set of slot nozzles in their front part are installed in a symmetrical way on each side of the sectional model. The plate vibration is recorded using dedicated displacement sensors. The different plenum configurations of the thermal study are used in the vibration facility: then the vibration study is performed as a function of the same parameters than the thermal study. Particular attention is devoted to the performances of pressure pads in the strip dynamic behavior. These pressure pads are formed by two inclined jets linked by a dedicated plate. Figure 10 shows the effect of stand-off distance (between the jet and the strip-simulating plate) on the vibration RMS values, as a function of the jet velocity. The RMS displacement increases with the slot jet velocity. But these displacements are reduced for a low stand-off distance, showing a better efficiency of the pressure pads.

Figure 10: Efficiency of the pressure pads

CONCLUSIONS Convective heat transfer in multiple-jets cooling systems is studied by experimental tests and numerical simulations. The main design parameters are defined and tested in a large range of values. Infrared measurements provide engineering correlations. They also allow a good validation of the CFD code. Therefore, numerical simulations are performed to predict the cooling performances of industrial systems. The same design parameters are considered in the study of strip vibrations. In particular, the efficiency of pressure pads designs in the stabilization of the strip is assessed. Combination of thermal and vibration study results allows overall optimization of rapid cooling devices, providing a high convective heat transfer while preventing strip fluttering.These results are now used in projects and installations of DREVER International S.A. As examples, the dimensioning and the design of cooling devices for Baosteel (China), Vega (Brazil), Hyundai Hysco (Korea) and Severgal (Russia) rely on this study.

ACKNOWLEDGEMENTS

This work was supported by the Walloon Region under Grant n°4125.

REFERENCES 1. H. Martin, Heat and Mass Transfer between Impinging Gas Jets and Solid Surfaces, Adv. Heat

Transfer, Vol. 13, 1977, pp. 1-60. 2. J.M. Buchlin, Lecture Series 2000-03, 2000, von Karman Institute for Fluid Dynamics, Rhode-

St- Genèse, Belgium.

RMS values of the strip vibrations

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