ATOMISATION TECHNOLOGY TO MINIMIZE THE EFFECTS OF NOZZLE WEAR ON THE DROPLET SIZE

Transcription

1 ILASS-Europe Darmstadt September ATOMISATION TECHNOLOGY TO MINIMIZE THE EFFECTS OF NOZZLE WEAR ON THE DROPLET SIZE J. Kohlmann*, M. Schmidt*, G. Slowik** and M.Bürgermeister*** * Fachhochschule für Technik und Wirtschaft Berlin University of Applied Sciences Fachbereich II Blankenburger Pflasterweg Berlin ** Universität Halle-Wittenberg Fachbereich Ingenieurwissenschaften Institut für Verfahrenstechnik Geusaer Str. 617 Merseburg *** Hans G. Werner GmbH, Reutlingen Delavan Zerstäubungstechnik Hölzlestr Reutlingen ABSTRACT The effect of the nozzle wear on the droplet size can be calculated by using some experimental data and the presented mathematical model. If it is not possible to minimize the wear by using special materials, like hard metal, the effect of the wear can be reduced to a minimum by using a new technology for the atomisation of the liquids. NOTATION A R area of wall friction α inlet coefficient b 1, b,. width of tangential slots η viscosity d diameter of orifice λ wall friction coefficient d S diameter of swirl chamber µ velocity coefficient D V,.5 volume median diameter θ spray angle F R force of friction ρ density h 1 height of swirl chamber σ surface tension M momentum τ wall shear stress m& mass flow rate n number of tangential slots r radiale coordinate r e average distance between tangential slots Indices Re Reynolds number r S radius of swirl chamber average area, inlet t thickness of liquid layer in the nozzle 1 inlet t s thickness of liquid sheet after leaving nozzle outlet U velocity in axial direction G gas V velocity in tangential direction L liquid V & We volume flow rate Weber number II

2 INTRODUCTION Using nozzles in industrial applications, in fact all nozzles wear out. Corresponding to the specification and the velocity of the fluid, the wear of the nozzle at several stress areas is different. Specially the orifice is a part with a very strong wear. The effects of the wear can be seen here in an increasing of the diameter of the orifice. In figure 1 for a nozzle for industrial use, the increasing of the diameter of the orifice is shown in correlation to the atomised mass of the liquid. Mostly the customer or user is,, 1,8 1,6 trying to reduce the wear by selecting special materials like hard metal (tungsten carbide) or ceramic for the orifice disk and swirl chamber. If everything is done in that way to protect the nozzle against 1,4 1, 1, wear, the user has to supervise the nozzles, to ensure that he is able to exchange the Volume in l nozzle at the right time. This is necessary to keep the droplet size and the capacity in a range that is allowed. Fig. 1: Typical increasing of nozzle diameter The aim of this presentation is, to analyse the wear of a nozzle and to calculate the run time of a nozzle and also to present a new nozzle system what is able to compensate during the use of the nozzles the wear out. With this new nozzle system the user is able to run the industrial application longer with a constant atomisation without exchanging of the wear parts of the nozzles. NEW ATOMISATION SYSTEM The nozzle we have used for the analysis is presented in fig.. The swirl chamber has 4 tangential slots (inlets). The slot sizes are for the slot couples in opposite positions the same. The slot size is decreasing in direction to the swirl chamber. In fig. the details about the pipe system for feeding the slots is not presented. The nozzle presented in fig. is a test nozzle for representing the new nozzle system [1]. The liquid flow is divided into two subflows (see fig.3) and feeds the slots of the Flow Valve Fig. : Cross view of swirl nozzle nozzle by two inlets separately. As presented in fig. the subflow A is connected with the two large slots (b3, b4) and the subflow B is connected with the two small slots (b1, b). Outside the nozzle the subflow A can be varied stepless. Valve Subflow A Fig. 3: Flow sheet Subflow B Nozzle II. 15.

3 MATHEMATICAL MODEL OF THE SWIRL ATOMIZER The mathematical model of the swirl atomizer covers the following steps: 1. Modelling the vortex in the swirl chamber of the nozzle,. Modelling of the thickness of the liquid layer in the orifice and modelling the spray angle, 3. Modelling of the volume median diameter. To calculate the vortex in the swirl chamber respecting the wall friction we can use equation 1 for symmetrical conditions. M d ( V r ) = (1) m& The infinitesimal friction momentum is calculated with the force for the wall friction what is calculated with wall shear stress, as described in equation. dm = r dfr = r τ da R = 4 π τ r dr () The wall shear stress is calculated with the coefficient of the wall friction. λ τ = ρ L V (3) 8 The coefficient of the wall friction depends on the Reynolds number and the velocity itself. That means that equation 1 has to be solved in combination with equation and 3 by using the explicit difference algorithm. The most important velocity component for the vortex is the tangential velocity. The tangential velocity at the radius r = r S =d S / is calculated with equation 4, respecting the contraction effects of the wall jet. 1 re V1 = V (4) α rs V is the average tangential velocity at the inlet, calculated using the volume flow rate and the area size of the tangential slots. V& V = (5) n n h1 b i i= 1 The radius r e is the average distance between the tangential slots and the centre of the swirl chamber. 1 n r e = rs b i (6) n i= 1 The inlet coefficient α is calculated similar to the calculation of this coefficient for cyclones []. The coefficient α depends only on the geometry of the nozzle [3]. The pressure loss is calculated by using the tangential velocity at the outlet of the swirl chamber (r = d/). ρ L P = V (7) The thickness of the liquid layer in the orifice is calculated by using the velocity coefficient µ. d t = 1 1 µ (8) The velocity coefficient µ can be calculated with the velocity component in axial direction U and the pressure loss. For the nozzle, presented in fig., equation 8 can be used..411 ρ L U µ = 1.3 (9) P 4 V& with U =. (1) π d With the restriction, that 1% of the velocity components at the outlet of the orifice is converted, the spray angle can be calculated with equation 11. V θ = arctan µ 1 U (11) II

4 The volume median diameter is calculated with the model WANG & LEFEBRVE [4]. x y D V,.5 = t s C A Re We + B We (1) The thickness of the liquid layer after the exit of the orifice is calculated with equation 13. t s = t cos( θ ) (13) The Reynolds number and the Weber number are defined in equation 14. ρ L V t s ρ G V t s Re = ; We = (14) η L σ The coefficients of [4] were be used for the own calculations and measurements. They were A = 4.5; B =.39; x =.5 and y =.5. The coefficient C is used to describe the correlation between the Sauter mean diameter and the volume mean diameter. For the tested nozzles the coefficient C has a value range of 1,1-1,3. EFFECTS OF NOZZLES WEAR USING SWIRL NOZZLES For the experimental work a spray drying nozzle with the following specification was used. Highness of the swirl chamber: h 1 =,6 mm Quantity of tangential slots: n = 1 Size of the tangential slot: b 1 =,5 mm Orifice diameter d =,89 mm, and d = 1,4 mm The tests were done with water in the lab room. With a Malvern-Sizer the droplet sizes were measured in the spray. To simulate the wear out of the orifice disk, two orifice disks were used with different diameters of the orifice. In fig. 4 the capacity is presented in correlation to the pressure. Increasing the diameter of the orifice from,89 mm up to 1,4 mm has the effect that the used pressure can be reduce from 5 bar down to bar. The influence of the reduction of the pressure is very large. Using a pressure of 5 bar, the measured volume median diameter is 14 µm (see fig. 5). This value is increasing up to µm if the pressure is reduced to bar. In fig. 6 the quality of the calculation of these values is presented. Perssure Difference in bar 5, 4, 3,, 1,, Fig. 4: Exp. d =,89 mm Calc. d =,89 mm Calc. d = 1, mm Calc. d = 1,1 mm Exp. d = 1,4 mm Calc. d = 1,4 mm 4 6 Flow rate in l/h Flow rate Pressure Difference - characteristic 3 As presented in fig. 1, it is possible to calculate the correlation between the wear process and the time. If this correlation is known, then it is possible to get information about the correlation between volume median diameter and time. Than is becomes possible to define the time, when the nozzle has to be exchanged. D V,,5 in µm Exp. d =,89 mm Calc. d =,89 mm Calc. d = 1, mm Calc. d = 1,1 mm Exp. d =1,4 mm Calc. d = 1,4 mm Flow rate in l/h Fig.5: Flow rate - median diameter - characteristic II

5 Flow Rate 4 l/h 6 4 Pressure Difference inbar Pressure Difference Calculation Pressure Difference Experiment 1 DV,.5 Calculation DV,.5 Experiment,85,9,95 1, 1,5 1,1 1,15 1, 1, D V,.5 in µm Fig. 6: Effect of development of nozzle diameter MINIMIZE THE EFFECTS OF NOZZLE WEAR But it is easier and more comfortable to use a technology what is able to compensate or minimize the wear out effects on the atomisation of the nozzles. With a nozzle, constructed as presented in fig. and used as presented in fig. 3, the effect of the new technology is discussed. The pressure can be varied without an influence to the capacity and backwards. The regulation of the capacity can be done in the same way as with pressure atomizers. Additionally the pressure can be changed by varying the ratio between the subflows. The results of the atomisation are presented in fig. 8. The volume median diameter is increasing from 5 µm up to 15 µm because of the wear on a standard nozzle. Pressure Differenze in bar With Swirl Control Whithout Swirl Control,6,7,8,9 1, Fig. 7: Pressure Difference Using the new nozzle technology the volume median diameter is only increasing up to 6 µm when the diameter of the orifice is increasing in the same way. The same effects will be seen on a standard nozzle when the diameter of the orifice will increase from,6 mm up to,65 mm. With the new nozzle technology, it is possible to use the nozzles longer than other nozzles. Activities by choosing hard metal or ceramic to reduce the wear out can be used similar to standard nozzles. The regulation of the pressure needs some additional work to do. II

6 D V,.5 in µm With Swirl Control 11 Whithout Swirl Control ,6,7,8,9 1, Fig. 8: Median diameter 1 3 Pressure Differenze in bar Flow Rate in l/h Both Sub Flows (Exp.) Both Sub Flows (Calc.) Small Sub Flow (Exp.) Small Sub Flow (Calc.) D V,.5 in µm Flow Rate in l/h Both Sub Flows (Exp.) Small Sub Flow (Exp.) Small Sub Flow (Calc.) Both Sub Flows (Calc.) Fig. 9: Pressure difference Fig. 1: Median diameter The size of the droplets can be kept constant, using this technology. To do that, the droplet size is measured in a usable way to regulate the ratio between the subflows at a constant pressure. The minimum and the maximum limit, the nozzle can be used, are presented in fig. 9 and fig. 1. With the presented technology the effect of the wear in combination to the result of the atomisation can be reduce dramatically. REFERENCES [1] Slowik, G., Kohlmann, J., Bürgermeister, M., A new principle for Influencing the droplet size in hollow cone nozzles, ILASS Europe 1999, Toulouse [] Muschelknautz E., D i e B e r e c h n u n g v o n Z y k l o n a b s c h e i d e r n f ü r, Chem.-Ing.-Techn., 44(197)1+, S [3] Schmidt M., Experimentelle Untersuchung der Zerstäubungseigenschaften einer steuerbaren Druck - zerstäuberdüse, Diplomarbeit FHTW Berlin, Berlin [4] Lefebvre A.H., A t o m i z a t i o n a n d S p r a y s, Hemispher Press, NY, 1989 II