Fluidised bed for stripping sand casting process. Guido Belforte, Massimiliana Carello, Vladimir Viktorov

Transcription

1 Fluidised bed for stripping sand casting process Guido Belforte, Massimiliana Carello, Vladimir Viktorov Dipartimento di Meccanica - Politecnico di Torino C.so Duca degli Abruzzi, Torino Italy Abstract An innovative application of a non-traditional casting system, in particular a stripping fluidised bed, whose working principle is based on air sand interaction phenomenon, is presented. A bed s prototype used to analyse the fluidisation phenomena is described. The purpose of the experimental tests is to measure: pressure, air flowrate and velocity. Different measurement points inside the prototype have been taken into account. Stripping pieces with different shapes and dimensions have been considered. Moreover visualisation of air bubbling in the sand is possible and its behaviour is compared to a computational fluid flow simulation. Key words fluidised bed, casting, air-sand interaction, and fluidisation effect. 108/1

2 Introduction In the last years the foundry processes have had significant development, connected to the product evolution. This evolution involved aero and earth transports, where faster and larger systems require the realization of structural elements with big dimensions and small manufacturing tolerance. Then the stripping of big pieces becomes very important. In this case it is very difficult to use the traditional techniques based on vibration and shake that require vibration and noise isolation for the safeguard of ambient. An alternative system for stripping is the fluidised bed with hot air that allows the stripping of piece without stress and damage. In this paper an experimental bed prototype is described. Experimental tests have been performed to investigate the fluidisation phenomena. The main parameters in terms of air flow-rate and pressure have been identified; also it has been possible to take into account the presence of stripping pieces. The visualisation of bubbling phenomena is in good agreement with the first results obtained by means of computational fluid flow analysis. Experimental test bench When solid particles of sand interact with air flow their behaviour becomes similar to a fluid; if the sand particles are contained in a chamber a relative movement of the particles is possible. [1 8] A fluidised bed furnace has been designed and built (scaled down version of a real stripping bed, in particular 1:5) to investigate the fluidisation phenomena. The prototype is made up of three important parts: a rectangular chamber (riser or bed or furnace); a cover; an air distribution system positioned on the lower part of the chamber. Some glass windows allow the visualisation of the air-sand interaction phenomena during the experimental tests. The dimensions of the chamber are: 800 mm x 470 mm x 370 mm. A frontal view of the prototype is shown in figure 1. The air distribution system is made by means of 760 nozzles, whose diameter equal is 0.8 mm; the downward realisation of the nozzles on proper distribution pipes avoids their block up. The supply of the nozzles occurs through two flexible pipes positioned in opposed points on the rectangular chamber. Figure 2 shows a lower view of the nozzles. Figure 3 shows an internal view of the chamber, where it is possible to see the upper part of the air distribution system and the metal crate used to contain the casting piece (internal dimensions 720 mm x 270 mm x 320 mm). The tests were carried out filling up with foundry siliceous sand the chamber of the bed prototype; in particular sand with particle medium diameter of 0.3 mm (A.F.A. 55 type) was used. Figure 4 shows the scheme of the experimental test bench, whose the most important components are: the intersection valve 1; the filter 2; the pressure reducer 3; the flowmeter 4 to measure the inlet air flow; the manometer 5 to measure the flow meter upstream pressure, the air heater 108/2

3 6; the manometer 7 to measure the air pressure at the entrance of the furnace; the fluidised bed furnace prototype 8; the differential water manometer 9 to measure the pressure inside the bed; one or more thermocouples 10 to measure the sand temperature. Two orthogonal slides positioned between the rectangular chamber and the cover allow to position the pressure or the temperature measurement instruments; in this way it is possible to have different measurement points inside the fluidised bed. The experimental tests were carried out to measure the fluidisation curves in term of pressure vs. air flow-rate for different sand levels H (10; 15, 20 cm). Lower sand levels did not produced interesting results. The experimental analysis presented in this paper refers to ambient temperature. For this reason the cover of the prototype was not used. The pressure was measured by means of the manometer 9 of figure 4. Different measurement points, varying the longitudinal, the transversal and the height position inside the chamber, were used. For air flow-rate before fluidisation Q 1 (Q 1 = 760 dm 3 /min (ANR)) and for the minimum air flow fluidisation Q 2 (Q 2 = 840 dm 3 /min (ANR)) the piezometric curves in term of pressure vs. distance from the bottom of the chamber are used. The purpose of other tests presented in this paper is to establish the influence of the presence/absence of a stripping piece on the fluidisation phenomena; in particular rectangular pieces have been taken into account with equal thickness (10 mm) and different dimensions (600 mm x 200 mm; 300 mm x 150 mm; 150 mm x 150 mm). The pieces were fixed in the centre of the furnace chamber, at the height equal to 10 cm from the base of the metal crate. Experimental results The outline of the fluidisation curves obtained with the experimental tests reproduces the behaviour shown by other authors [1 8], in particular varying the flow-rate the pressure increases until it reaches a maximum constant level. Near the edge of the furnace chamber a marginal bubbling phenomenon may be observed, while in the main internal zone it is very important. In correspondence it is possible to measure the minimum fluidisation flowrate. Figure 5 shows a top view of the chamber where it is possible to note the bubbling effect, but also the metal crate and the two orthogonal slides for the pressure system positioning. The chamber of the furnace was filled with sand until level H = 20 cm was reached. The pressure was measured at the centre of the riser at different distance from the bottom of the chamber h (h = 1, 6, 11, 16, 18 cm). The fluidisation curves in terms of pressure vs. flow-rate are shown in figure 6. Increasing h the pressure becomes very low, in fact for h = H the pressure is equal to the atmospheric pressure; the fluidisation effect is important for lower level of h, for witch a minimum flow-rate equal to 800 dm 3 /min (ANR) is required. 108/3

4 The variation of the sand level H influences the maximum pressure reached inside the furnace, but the minimum flow-rate fluidisation has a negligible variation ( dm 3 /min (ANR)). Similar behaviour were obtained in other measurement points inside the metal crate. The piezometric curves have been obtained for three different levels of sand H (H = 10, 15, 20 cm) and in each case using two flow-rate Q 1 (760 dm 3 /min (ANR)) and Q 2 (850 dm 3 /min (ANR)). Figure 7 shows the results in term of pressure vs. distance from the bottom of the chamber h. It is possible to note a linear trend of the characteristics, because the behaviour of the air-sand mixture becomes similar to that of a liquid. The influence of the stripping piece on the pressure and flow fluidisation has been investigated. Figure 8 shows the pressure level curve on the surface of the bigger piece considered (600 mm x 200 mm) positioned in the center of the chamber with sand level H = 20 cm and flow-rate Q 2. In this way the passage area between the piece and the crate are constant. The fluidisation of the sand corresponds to the development of big bubbles. Il is possible to note that on the piece s edge the pressure is bigger than in the central part. It is interesting to consider that in a real application the stripping would be possible in all part of the piece. Then probably, a small pressure corresponds to low or not efficient stripping. The piece dimensions influence the fluidisation and the bubbling phenomena then it is important to evaluate the better position of the piece to have efficient stripping. Simulation results A CFD program was used to model and simulate the behaviour of a fluidised bed using Eulerian - Eulerian model. Figure 9 shows the countour lines of sand fraction volume with air velocity equal to 0.25 m/s, corresponding to the fluidisation. It is possible to note different sand fraction volume in different zone of the chamber that indicates the bubbling phenomena. Fluidisation analysis The study of fluidisation phenomena started with Ergun [9], witch obtained an empirical formula to calculate the pressure drop in a fixed bed, when a gas stream goes through solid particles. Starting from Ergun formula some authors [10 13] proposed other methods to evaluate the gas pressure drop and the minimum fluidisation velocity or flow-rate. The minimum fluidisation velocity is reached when the air flow forces are equal to the total mass force of the sand; in this condition the system is balanced and it is possible to consider the equilibrium equation along the vertical direction. In this paper two different formulations from the literature have been taken into account to calculate the gas mass flow per unit of bed cross section G mf and the theoretical results have been compared to the experimental data G exp. 108/4

5 The first formulation, developed by Levenspiel allows calculating G mf1 : a 4b G mf 1 = g ρ ( 1 ε mf ) ( ρ p ρ ) 2 b 2 a The second formulation, developed by Delebarre [12-13] allows calculating G mf2. G mf a 2 = b 4b H mf 1 + g ρ p 1 ε mf ρ H mf ρ H p 1 ε mf a ρ H mf Where the symbols significance for the two formulations are: a: first coefficient of Ergun s equation (kg/s m 3 ); b: second coefficient of Ergun s equation (m -1 ). g: gravity acceleration (m/s 2 ); ρ: gas density (kg/m 3 ); ρ p : solid apparent density (kg/m 3 ); ε mf : bed voidage at minimum fluidization (equal to 0.44); H mf : bed height at minimum fluidisation (equal to 0.22 m); H 0 : atmospheric pressure in gas height at bed surface conditions (m); In our case has been calculated ε mf equal to Knowing the bed cross-section it is possible to calculate the mass flowrate and the standard flow volume rate. In particular in our case the following fluidisation flow-rate have been obtained: G mf1 = 857 dm 3 /min; G m2f1 = 850 dm 3 /min; G mf1 = dm 3 /min. It is important to note a good agreement between the two analytical formulation and the experimental results. Conclusions The fluidised bed furnace could be an alternative system for stripping pieces without stress and damage. In this paper an experimental bed prototype has been presented. The experimental tests have been carried out to investigate the fluidisation phenomena. The main parameters in terms of air flow-rate and pressure have been identified and measured, showing the influence of the presence of stripping pieces. The glass windows of the prototype allows the visualisation of the air-sand interaction phenomena, in particular the bubbling phenomena, that it is in good agreement with the first results obtained with a computational fluid flow analysis. A good agreement has been obtained comparing the experimental minimum air-flow fluidisation and the values obtained by analytical formulation of other authors. Other experimental tests will be carried out to establish the influence of the temperature on the fluidisation curves. 108/5

6 References 1. Arastoopour H, Numerical simulation and experimental analysis of gas/solid flow system, Powder Technology, 119, 2001, pp Benyahia S, Arastoopour H, Knowlton T M, Massah H, Simulation of particles and gas flow behavior in the riser section of a circulating fluidized bed using the kinetic theory approach for the particulate phase, Powder Technology, 112, 2000, pp Detamore M S, Swanson M A, Freder K R, Hrenya C M, A kinetictheory analysis of the scale-up of circulating fluidized bed, Powder Technology, 116, 2002, pp Gidaspow D, Bezburuah R, Ding J, Hydrodynamics of Circulating Fluidized Beds, Kinetic Theory Approach, 7th Engineering Foundation Conference on Fluidization, 1992, pp Guenther C, Syamlal M, The effect of numerical diffusion on simulation of isolated bubbles in a gas-solid fluidized bed, Powder Technology, 116, 2001, pp Polashenski W, Chen J C, Normal solid stress fluidized beds, Powder Technology, 90, 1997, pp Zhang S J, VanderHeyden W B, High resolution three-dimensional numerical simulation of a circulating fluidized bed, Powder Technology, 116, 2001, pp Zhang S J, Yu A B, Computational investigation of slugging behavior in gas-fluidized beds, Powder Technology, 123, 2002, pp Ergun S, Fluid flow through packed columns, Chemical Engineering Prog., 48, 1952, Marthur K B, Epstein N, Spouted beds, Academic Press, New york, Sutherland J P, The measurement of pressure droop across a gas fluidized bed, Chemical Engineering Sci., 19, 1964, Delebarre A, Does the minimum fluidization exist?, Journal of Fluids Engineering, 124, September 2002, pp Delebarre A, Morales J M, Ramos L, Influence of the bed mass on its fluidization characteristics, Chemical Engineering Journal, 98, 2004, pp Acknowledgements The authors would tanks you ing. M. Iannuzzi and ing. G. Tagliarini for their cooperation in experimental tests. The research is part of the Eureka Project Sand Cast, and it has been carried out in cooperation with IMF (Luino, Italy) and SFU (Ussel, France). 108/6

7 Figures Figure 1 Frontal view of the fluidized bed Figure 2 Air distribution nozzles Figure 3 Inside view of the fluidised bed 108/7

8 Figure 4 Scheme of the experimental test bench Figure 5 Example of bubbling phenomenon 108/8

9 3,0 2,5 2,0 h = 1 cm h = 6 cm h = 11 cm h = 16 cm h = 18 cm Pressure [kpa] 1,5 1,0 0,5 0, Flow-rate [dm 3 /min (ANR)] Figure 6- Fluidisation curves pressure vs. flow-rate (sand level H= 20 cm) 3 Pressure [kpa] 2,5 2 1,5 1 H=20 cm; Q1 H=20 cm; Q2 H=15 cm; Q1 H=15 cm; Q2 H=10 cm; Q1 H=10 cm; Q2 0, Distance z [cm] Figure 7 - Pressure vs. distance h (different sand levels H and flow-rate Q 1 =760 dm 3 /min (ANR) and Q 2 =850 dm 3 /min (ANR)) 108/9

10 Pressure [10 kpa] Figure 8 Pressure level curve on the surface of the stripping piece (sand level H= 20 cm, flow-rate Q 2 = 850 dm 3 /min (ANR)) Figure 9 Contour lines of sand fraction volume in fluidisation conditions 108/10