cnvergent nzle

International Journal of Dynamics of Fluids ISSN 0973-1784 Volume 6, Number 2 (2010), pp. 161179 Research India Publications http://www.ripublication.com/ijdf.htm

Computational and Heat Transfer Analysis of Convergent Nozzle Used for Gas Atomization of

Liquid Metals

P. Sanjay1*, N.S. Mahesh2 and S. Kishore Kumar3

1Department of Mechanical Engineering, JSS Academy of Technical Education, Bangalore 560060, India

2Dept of Mechanical and Automotive Engineering, MSR School of Advanced Studies, New BEL Road Bangalore 560054, India

3CFD Group, Gas Turbine Research Establishment, Bangalore- 560093, India *Corresponding Author E-mail: [email protected]

Abstract

Gas atomization is a widely used process for manufacturing of fine metal- and alloy- powders. The idea is to transfer kinetic energy from a high velocity jet-gas expanded through a nozzle, to a stream of liquid metal, resulting in fragmentation and break up into metal droplets. Gas atomization process is the powder production technique and nozzles play an important role in the gas atomization process. The gas to metal interaction is determined by geometry of the nozzles. The type of the nozzle and geometry of the flow is the most important parameter for atomization process. The design of an atomizing nozzle determines the degree of contact of the liquid metal with the atomizing gas. The atomizing nozzles have co-axially placed Metal Delivery Tube (MDT), which carry the molten metal to atomizing zone. The flow properties of gas are considerably affected by the presence of MDT. In the present analysis, an attempt has been made to analyze the flow pattern of gas in Convergent nozzle (C - nozzle) using Computational Fluid Dynamics (CFD) techniques. Flow characteristics of gas was determined inside the nozzle and the atomizing zone and different plots (ex: Mach number plot) were obtained using CFD software Fluent. Further, Radial and axial variations of Mach number were determined at the exit of the nozzle and the flow area. CFD results for gas velocity were compared with the analytical results. In addition, variation in pressure, temperature and shock waves were analyzed along axial direction at the nozzle exit and flow area with and without placing the co-axial MDT inside the sonic nozzle.

Keywords: Gas atomization, MDT, CFD, Mach number, sonic nozzles.

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Introduction Gas atomization of molten metals for metal powder production or spray forming (Spray casting) applications is often done by means of atomization nozzles, where the disintegration of the melt is due to impinging high kinetic energy inert gas jets. Atomization is a process wherein a stream of liquid metal is disrupted by high-energy gas jets into fine droplets. These are cooled by gas stream during free flight to generate powders. Rapid solidification effects are naturally produced in this technique. Gas atomization process is one of the widely used powder production techniques. In this method, a high velocity gas stream is made to contact with a relatively low velocity liquid metal stream which is broken up into fine droplets and cooled rapidly by convection. Nozzles are used to achieve the atomization through break-up of molten metal stream into droplets by fast flowing gas. The heart of the gas atomization process is the nozzle. The droplets formed during atomization process can be quenched to form the powders or deposited to form the billets as in Spray Casting process. The type and configuration/parameters of the nozzle determines the gas-metal interaction and hence plays a key role in the gas atomization process. The droplets formed during atomization process can be quenched to form the powders or deposited to form the billets as in Spray Casting process. The type and configuration/parameters of the nozzle determines the gas-metal interaction and hence plays a key role in the gas atomization process. One of the critical parameter in the gas atomization process is the Mach number. The exit velocity of the gas jet in the atomization process controls the extent of the metal breakup. [1, 2, 3, 4, 5, 6] Convergent Nozzle consists of only the convergent portion. The nozzle area decreases from inlet to exit giving a convergent passage. Here the convergent area narrows down from a wide diameter to a smaller diameter in the flow direction. These nozzles can produce subsonic and/or sonic flows. [7] Mach number is one of the critical parameter in the gas atomization process. The extent of the metal breakup is controlled by the exit velocity of the gas jet in the atomization process [8]. A shock wave is a special kind of wave referred to as a steep, finite pressure wave. The change in the flow properties across such a wave are abrupt, in some situations, shocks are undesirable because they interfere with the normal flow behavior. Shock waves will be present in the CD nozzle. But such shockwaves are not present in the C-Nozzle which is advantageous for gas atomization process. [9] Modeling and numerical simulation are introduced as scientific tools for engineering process development as applied to Spray atomization/spray metal forming. Then the individual physical processes that affect spray forming are introduced and implemented into an integral numerical model for spray forming as a whole. Based on state of the art of modeling and simulation tools and techniques, a successful and realistic description of relevant technical and physical process is possible. Numerical simulation has become a fundamental tool for the analysis and optimization of technical process [10]. Computational fluid dynamics is one of the tools which are available to solve fluid-dynamic problems. CFD is the numerical approximation to the solution of mathematical models of fluid flow and heat transfer. Using CFD technique, the

Computational and Heat Transfer Analysis of Convergent Nozzle 163

prediction of fluid flow with the complications of simultaneous flow of heat, mass transfer, phase change, chemical reaction, etc using computers is possible. CFD is one of the branches of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. Fundamental basis of any CFD problem is the Navier-Stokes equations, which define any single-phase fluid flow. These equations can be simplified by removing terms describing viscosity to yield the Euler equations. [11]. Fluent is a state of art computer software for modeling fluid flow and heat transfer in complex geometries. Fluent provides complete mesh flexibilities solving the flow problems with unstructured meshes that can be generated about complex geometries with relative ease. Fluent can be used for modeling fluid flow and heat transfer in complex geometries. Gambit is Fluents geometry and mesh generation module. Gambit has a single interface for geometry creation and meshing that brings together most of Fluent's preprocessing technologies in one environment. [12, 13] In the present study, an attempt has been made to determine the flow characteristics of gas in side CD Nozzle and flow area using CFD technique. The numerical analysis for flow parameters was carried out considering the parameters like Mach number, temperature, velocity and pressure. In addition the pre-heating temperature of atomizing gas was decided based on the exit temperature of the gas from the nozzle. The degree of pre-heating required and the practical limitations associated with experimentation were considered for analysis. The low temperatures at the exit of C nozzle because of the gas expansion will lead to chilling followed by chocking of MDT. This demands preheating of atomizing gas.

Details of the present study Analysis methodology: Fundamental design methodology used for C nozzle to obtain sonic velocity of gas CFD is one of the techniques to obtain the numerical solution to fluid flow and heat transfer problems. A close-coupled C-nozzle in which the co-axially fitted MDT was considered for the present analysis. The nozzle accelerates the gas from subsonic to sonic velocities. The fundamental relationships of isentropic compression and subsequent expansion through a flow passage resulting in a sonic jet have been employed to obtain the areas at the critical sections i.e. inlet and exit. Since the MDT was to be coaxially inserted into the nozzle, the effective area of flow had to be considered in all the design steps. The Area-Mach relation was used to obtain both the inlet and the exit area ratios by suitably substituting the stagnation and exit Mach numbers. [3, 4, 7, 14, 15] CFD trails involved in measurement of velocity and Mach number at the nozzle exit in both in horizontal/axial and radial directions. The objective was to determine the flow characteristics of the gas inside the nozzle and flow area and compute the temperature of gas and molten metal for atomization at the nozzle exit and their pre-heating requirement to prevent the chilling effect along with the heat transfer analysis. Figure 1 below shows the C nozzle geometry parameters that were considered to obtain the Mach number of 1 (sonic). [16]

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Experimental details In the present study, CFD trails have been carried on C nozzle for plotting Mach number, pressure, velocity vector and exit temperature based on the nozzle dimensions. Geometric modeling and meshing of the nozzle were carried out and the results were plotted using Gambit-FLUENT software. The boundary conditions applied were Pressure inlet, Pressure outlet, Wall and the Symmetry. The viscous model was defined as Spalart-Allmaras for the fluid flow, solver as Fluent 5/6 and mesh type as QUAD [17]. Inlet pressure of 0.3 MPa and inlet temperature values of 300K were given as input. The iteration values were set was about 1000 initially and iterated until the solution converges Then the results were plotted for velocity, pressure, temperature and Mach number. The Courant number (CFL) controls the time step used by Fluent during the inner iterations performed during each time step. The default CFL for the coupled implicit solver is 5.0.

C-nozzle dimensions Figure 1 shows the C- Nozzle dimensions without the presence of axially fitted MDT. This C nozzle was modeled and meshed using the Gambit software applying the required boundary conditions for CFD analysis (as shown in figure3) [16]. CFD trails were conducted and results obtained were used for comparison with analytical results.

Figure 1: Convergent nozzle dimensions (without MDT).

Parameters considered for simulation of Mach number for C - nozzle using Gambit/Fluent software Table 1 shows the nozzle and process parameters considered to obtain Mach number 1 at the nozzle exit. [16]


Table 1: Process parameters considered for C-Nozzle.

Sr.No. Nozzle Parameters Values 1 Inlet Diameter 23.35 mm 2 Outlet Diameter 24.12 mm 3 Throat Diameter 16.00 mm 4 Inlet Pressure 0.3 MPa 5 Gauge Pressure 0.1 MPa 6 Exit mach 1 Mach 7 Temperature of the gas at inlet 300 K 8 Boundary condition applied at inlet PRESSUE_INLET 9 Boundary condition applied at outlet PRESSURE_OUTLET 10 Boundary condition applied at axis SYMMETRY 11 Boundary condition applied at the nozzle wall WALL 12 Boundary condition applied for MDT WALL 13 MDT Dimensions OD = 8 mm

ID = 5 mm 14 Atomizing gas used Nitrogen 15 Boundary condition applied to liquid metal Velocity-Inlet 16 Courant number for radial measurement and axial

measurement of mach number 5 CFL

Process parameters considered for simulation of heat transfer Table 2 shows the process parameters considered for simulation. The molten metal considered was aluminum silicon magnesium alloy (A356), atomizing gas was nitrogen, and MDT material is stainless steel (SS 316). The length of the MDT was taken as 14.8 mm [16]

Table 2: Process parameters considered for Heat transfer analysis.

Sl. No

Process Parameters Values

1 Height of the liquid metal head for pour out (h1)

100 mm

2 Inlet velocity of the molten metal [v1 = sqrt (2*g*h1)]

1.4 m/sec

3 Exit velocity of the molten metal [v2 = sqrt (2*g*h2) (h2 = h1 + Length of MDT = 100 + 35 = 135 mm)]

1.68 m/sec

4 Pouring temperature of metal 1125 K 5 Solidus temperate of Metal

(Melting temperature) 925 K

6 Mass of the metal used 3 - 4 Kg

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7 Thermal conductivity h = 800 wt/ (m*m-K) 8 MDT dimension Inner Diameter= 6mm, Outer

Diameter = 8mm, and wall thickness=1 mm

9 Solver used for GAMBIT Segregated 10 Flow used Steady state flow

Heat transfer analysis (Free and Forced convection analysis) The heat transfer analysis was considered for two cases, namely forced and free convection. In free convection phenomenon, the heat transfer from MDT during the time gap between the insertion of the MDT into the nozzle and pouring of the metal [18, 19, 20]. During the spray forming process, the molten metal will be at its pouring temperature. There was a time lag between the placement of the preheated metal delivery tube and the actual pouring. During this time lag heat transfer between the metal delivery tube and the surrounding atmosphere due to heat dissipation and atmospheric cooling. In this case the MDT loses heat by free convection and hence the temperature of the metal delivery tube drops. In Forced convection analysis, the heat transfer between the atomization gas flowing inside the nozzle and the metal delivery tube was considered. The fast flowing gas enters the nozzle and impinges on the hot MDT directly. The MDT loses its heat to the turbulent atomizing inert gas. [16]

C-Nozzle with MDT Figure 2 shows the C- Nozzle with the presence of MDT. The MDT has the dimensions of 5 mm inner diameter and 8 mm outer diameter. [16]

Figure 2: C- Nozzle with MDT.

Figure 3 shows the C-nozzle which was modeled and meshed using Gambit as per the dimensions shown in the above table.


Figure 3: C nozzle geometry (symmetric) with mesh and applied boundary condition (without MDT).

Test Cases considered for the present analysis The different cases that were considered for conducting the experiments on C nozzle to measure the Mach number, Pressure, Velocity Vector and temperature inside the nozzle, at the exit of the nozzle and flow area using CFD trails are: variation of values along radial direction of nozzle with and without MDT, along axial direction with and without MDT.

Results and discussions Radial measurement without MDT (Mach number measurement along the radial direction at the nozzle exit without MDT) Figure 4 shows the Mach number plot for C-Nozzle. Mach number of 1 was obtained which shows that desired Mach number was obtained at the nozzle exit. Mach number varies from sub-sonic to sonic towards the exit. This is as per design and analytical results.

Figure 4: Mach number plot Without MDT, Radial measurement.

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Variation of Pressure, Velocity Vector and Temperature and (Radial measurement of Mach number) Figure 5 shows pressure plot for C-Nozzle for the inlet pressure of 3 Bar. In this case, an exit pressure of 1.5 Bar was obtained at the nozzle exit which is greater than the atmospheric pressure. This pressure prevents chocking of the molten metal at the nozzle exit, which is the desirable pressure for gas atomization at the nozzle exit.

Figure 5: Pressure plot (Without MDT, Radial).

Figure 6 shows the velocity plot for C-nozzle in which velocity of the gas increases from inlet to exit of the nozzle (150 m/sec to 320 m/sec), reaching the sonic velocity towards the nozzle exit.

Figure 6: Velocity plot (Without MDT, Radial).

Figure 7 shows the temperature plot for the C-Nozzle when the gas was passed in to the nozzle at an inlet temperature of 300K. In this case, the exit temperature of the gas found was 250 K which was less then the room temperature. This indicates that gas has to be pre-heated in order to prevent chocking of molten metal inside MDT. From mathematical modeling [24], it was found that at a distance of 15 mm from the


nozzle exit, the temperature of the gas can be around 250K. . This shows that; the temperature results from mathematical modeling calculations and CFD trail values were almost the same

Figure 7: Temperature plot (Inlet temperature of gas at 300K).

Figure 8 shows the Temperature plot when the gas was passed at the inlet temperature of the 350 K. Several iterations were carried out to know at what temperature, the exit temperature of the gas will at room temperature to prevent the chocking of molten metal inside the MDT. The plot shows the exit temperature of the gas was around 300 K which is nearly at room temperature.

Figure 8: Temperature plot (Inlet temperature of the gas at 350K).

CFD experiments were conducted for C Nozzles and found that gas pre-heating of 75-80 deg C was sufficient to achieve the exit temperature of gas around 20o C (293K) which is the room temperature for effective gas atomization to happen. Figure 9 shows the Velocity vector plot for the C-Nozzle. This plot shows that there were no shock waves present inside the nozzle which is advantageous compared to CD nozzles where shock waves are present.

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Figure 9: Velocity Vector plot.

Radial measurement (of Mach number) with MDT Figure 10 shows the C-Nozzle with co-axially fitted MDT which was modeled and meshed using Gambit and applying the boundary conditions applied as per the table 1.

Figure 10: C nozzle geometry with MDT.

Figure 11 shows the Mach number plot when the MDT was co-axially fitted in to the nozzle. This plot shows that there was not much difference between the Mach numbers when the MDT was present and it was almost same when the MDT was not present.

Figure 11: Mach number plot With MDT (Radial).


Similar cases were observed for pressure, velocity and temperature plots when the CFD experiments were conducted for Radial measurements.

Figure 12: Pressure plot With MDT (Radial).

Figure 13: Velocity Plot With MDT (Radial).

Figure 14: Temperature Plot With MDT (Radial).

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Axial measurement of Mach number without MDT: Figure 15 shows the C-Nozzle without the presence of co-axially fitted MDT. For Axial measurement of Mach number, chamber like design was considered at the exit of the nozzle where the exit diameter considered was greater then 2 times the nozzle diameter and the length was 300mm.

Figure 15: C- Nozzle without MDT.

Figure 16 shows the Mach number plot for the C - Nozzle without MDT for axial measurement of Mach number. Here the Mach number increases from subsonic to sonic inside the nozzle. After reaching the maximum value (1 Mach), the Mach number decreases along the axial direction due to the decay of the velocity of the gas (1 mach to 0.75 Mach). The velocity is approximated to an exponential decay of the axial gas velocity with distance as illustrated in equation 1 [15, 21]. This is explained from the equation below:

(1) where, ug (z) is the axial gas velocity, uo the initial gas velocity on exit from the atomizer , z the axial distance from the point of atomization, lamda the exponential gas velocity decay coefficient, given by equation 2.

(2) Where, a is the empirically determined constant relating to kinematic viscosity of gas and Ae the exit area of the atomizer [22, 23]


Figure 16: Mach number plot Without MDT, Axial.

Figure 17 shows the Velocity plot where the velocity decreases from the exit of the nozzle along axial direction in the flow region as explained in the case of the Mach number plot.

Figure 17: Velocity plot Without MDT, Axial.

Figure 18 shows the pressure plot for C - Nozzle without MDT. Here the inlet pressure decreases from 3 Bar to 1.5 bars towards the nozzle exit. Later it attains the value of 1.35 bar at the nozzle exit and remains constant in the chamber region.

Figure 18: Pressure plot: C-nozzle without MDT, Axial.

Figure 19 shows the temperature plot where the temperature decreases in the nozzle region along the axis (from 300K to 240K) and then attains the room

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temperature in the chamber region which is well below the room temperature which explains the need of pre-heating of gas as explained earlier.

Figure 19: Temperature plot: C-nozzle without MDT, Axial.

Figure 20 shows the velocity vector plot where the presences of shockwaves were not found which is more advantageous for gas atomization process.

Figure 20: Velocity vector plot (Axial).

Axial measurement with MDT: Figure 21 shows a C-Nozzle with co-axially fitted MDT which was modeled and meshed using Gambit and applying the boundary conditions applied as per the table 1. This is the similar case as explained from Figure 15.

Figure 21: C-Nozzle With MDT.


Similar results were obtained for Mach number, pressure, velocity and temperature as explained in the case when the MDT was not present.

Figure 22: Mach number plot With MDT (Axial).

Figure 23: Velocity plot With MDT (Axial).

Figure 24: Pressure plot With MDT (Axial).

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Figure 25: Temperature plot With MDT (Axial).

Heat transfer analysis (Forced convection) Figure 26 shows the C - nozzle with the presence of MDT where it was maintained at a constant temperature of 1125 K which the liqidous temperature of molten metal. Gas was preheated at 350K (75 deg) to prevent chocking of molten metal inside in the MDT. The temperature obtained at the exit of the nozzle found was about 290 K (room temperature) and the temperature at the MDT tip obtained was 325K. This is the desirable condition for gas atomization.

Figure 25: Temperature plot (Forced convection C-Nozzle).

Free convection The MDT was modeled and meshed using Gambit as shown in Figure - 2 and the process parameters applied as per Table-2. Passage length is lesser compared to CD nozzle (it is 35 mm in CD nozzle).

Figure 26: MDT with applied boundary conditions.


Case 1: In this case, solid metal is passed in to the MDT at an inlet temperature of 1125K, and outlet temperature was maintained at 300K and MDT was also maintained at room temperature. The temperature plot is shown in below figure 27. Here the atomization is not possible since the outlet temperature obtained was 300K (room temperature)

Figure 27: Temperature plot (Free convection- Case1).

Case 2: In this case, solid metal is passed in to the MDT at an inlet temperature of 1125K, and outlet temperature and MDT was maintained at 1125K. The temperature plot is shown in below figure 28. The outlet temperature obtained was 1120K which is the liquidous temperature of the molten metal which is the desired condition for spray atomization.

Figure 27: Temperature plot (Free convection, Case2).

Conclusions The objective of the present study was to analyze the flow characteristics of the C-Nozzle inside the nozzle and the flow area and to plot the results. Advantages of C-

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Nozzle are that shorter flow passage, absence of shock waves and lesser pre-heating requirement (as compared to CD nozzle of gas jets). In C nozzle, at the nozzle exit, the exit pressure is nearly equal to the atmospheric pressure (0.15 MPa) and gas is not over expanded. Moreover, the flow passage is lesser in C Nozzles (as compared to CD nozzle) which makes delivery of molten metal to the region of atomization at appropriate temperature easy. If the MDT length decreases, lesser degree of superheat is required for the molten metal. In C nozzle, the area converges at the outlet, this should lead to better energy transfer between gas to metal and in turn efficient break up of the molten liquid metal. In the present study, an effort has been made to understand and analyze the temperature and heat transfer pattern for gas and molten metal for C nozzle and MDT using CFD techniques. An attempt was made to find the pre-heating requirement of MDT to prevent chocking. This temperature was found to be 1125K using CFD trials. Pre-heating requirement of gas was also determined in order to prevent chocking of MDT. It was observed that when the inlet temperature of the gas is at room temperature (300K) for C nozzle, the exit temperature of the gas was 250K. In practical conditions, inert gas coming out of the cylinder would have expanded and hence the temperature will be below room temperature at the inlet of the nozzle. Consequently at the exit, the gas jet would be at subzero temperatures leading to chilling followed by chocking of MDT. Moreover, severe quenching of the droplets would occur which is bound to influence the solidification and hence the microstructure. In order to maintain room temperature at exit, pre-heating of the atomizing gas before inlet becomes essential. It was found that gas has to be preheated to 350K. In addition, MDT was to be maintained at a temperature of around 1125 K to avoid chocking.

Acknowledgements The authors thank Management and students of MSRIT, MSRSAS, BIT, Bangalore and JSSATE - Bangalore for the cooperation and encouragement during the present study.

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