NUMERICAL ANALYSIS OF FLOW THROUGH MULTISTAGE ORIFICE PLATE ASSEMBLIES WITH ALTERNATE CONCENTRIC AND ANNULAR ORIFICE PLATES

Transcription

1 NUMERICAL ANALYSIS OF FLOW THROUGH MULTISTAGE ORIFICE PLATE ASSEMBLIES WITH ALTERNATE CONCENTRIC AND ANNULAR ORIFICE PLATES Lakshmisha H R 1, V. Seshadri 2 and Puneeth Kumar S B 3 1 M Tech student Mechanical Engineering, MIT- Mysore 2 Professor (Emeritus) Mechanical Engineering, MIT- Mysore 3 Assistant Professor Mechanical Engineering, MIT- Mysore Abstract Multi stage orifice plate assemblies are used as pressure reduction devices in many industrial applications. These have the advantages of having no moving parts and less noise level. At present the design of these devices is based on empirical correlations. In the present study CFD has been used to generate data needed for the design of such devices. A CFD methodology has been developed by using FLUENT ANSYS software and it is validated by analyzing the flow through concentric orifice plate assembly. The computed values of C d and C L show excellent agreement with standard values. This validated CFD methodology has been used to analyze flow through a number of multistage orifice assemblies. In particular following studies are conducted Flow through single annular orifice plate and its discharge coefficient values are compared with the standard concentric orifice plate (ISO-5167) for different β and Re. Analysis of flow through multistage orifice assembly with concentric and annular orifice assemblies. Computations are made for five, seven and nine stages. The flow is assumed to be incompressible and Newtonian. Data has been generated for C d and C L for various configurations. It has been demonstrated that with alternate concentric and annular orifice plate spacing of X/D=1 gives the best performance Keywords coefficient of discharge, permanent pressure loss coefficient, multistage orifice plate assemblies, annular orifice plate and spacing of the plates I. INTRODUCTION The single annular orifice plate assembly is a variable area orifice created by a circular insert. It can be placed even eccentrically to the tube. Normally multistage orifice plate assemblies are fabricated using concentric and eccentric orifice plates. However in the present study the concentric and annular orifice plates are employed. The concentric and annular orifice plates are placed alternatively. The effect of spacing between the plates (X/D ratio) and Reynolds number are analyzed. Here we tried to find the optimum X/D ratio at which R (efficiency factor) is maximum for the five, seven and nine stage assemblies. The analysis is carried out by using the CFD methodology with ANSYS software. II. LITERATURE SURVEY Mohan Kumar et al [1] studied about the effects of diameter ratio and distance of placement on dual orifice plate assembly. They used Fluent software to analyze the effects of these variables on pressure drop. They validated their methodology by comparing with the ISO-5167 standard values. Sridevi et al [2] studied the comparison of flow analysis through a different geometry of flow meters using ANSYS software. In this study, Fluent was used to define flow characteristics and Gambit software is used to design the 2D model. In their study they employ the k-ε model for analysis. This study gives the values of pressure, velocity, turbulence contours at various sections with water as the media. In this study different types of orifice are used. The numerical results were compared with the experimental data and they were found to be good. DOI: /IJMTER WMF3G 47

2 Francivehar et al [3] carried out study on the multistage orifice plate. The main intension of these works is to minimize the cavitations and to reduce of vibration and pressure pulsations. In these analyses the stages are limited to four stages with eccentric holes. The CFD software ANSYS CFX was used for this design. Here the model employed was SST (shear-stress-transport) turbulence model. For the study more than ten different geometries are analyzed to obtain optimum solution for the problem. The results were verified with the measurements of the model in turbo institute s laboratory and it s confirmed on-site in Nuclear power plant Krsko. Aaron Xavier Fernandez s [4] work is on waste water flow through orifice plates. Here the waste water particles in orifice flow are investigated through numerical simulations. Here the work is carried out to analyze the stress variation in orifice plates. The experimental values are compared with the standard values and results are tabulated. The numerical study confirms that nominal size of orifice cannot explain the particle breakage of single orifice plate assembly compared to that multiorifice plate assembly. However this study of breakages of particles at various radial locations proved to be inconclusive. At last it is concluded that single orifice plate is ideal for strong particles, multi-orifice may be effective in breaking of weak particles. Guido Belforte, Andrea Trivella and Terenziano Raparelli [5] explained about their experimental study on discharge coefficient of gas bearing of external pressure type. Here work is carried on annular orifice (inherent orifice) and simple orifices with feed pocket. Air flow as well as pressure distribution were studied as a function of supply pressure and air gap height. Coefficient of discharge value is approximated by experimental formulae. The validation is carried out by comparing experimental values with numerical values. M.R.Chowdhury.[6] studied loss characteristics for the assembly of three orifices of square-edged type under laminar, transition and turbulent regime. The problems are solved for orifices having β ratios of 0.36, 0.5 and 0.7 with varying aspect ratios. Water is used as fluid with relevant properties. Tube viscometers with three different concentrations are used for obtaining non-newtonian laminar flow loss coefficient data. Flange tapings are employed for the pressure measurements. Values are compared with the numerical values. Gerasimos D. Danilatos.[7] carried out study on annular orifice along with the reverse flow pressure limiting aperture. There are three chambers which are connected through annular orifices which are coaxial with the aperture. The 3 ed chamber is maintained at high pressure as compared with that of other two chambers, to ensure gas flow develops into a supersonic annular jet. The pumping action is created at the core of the jet and any gas leaking through the aperture from 2 ed chamber are entrained and sent to the 1 st chamber, so that pressure difference between 2 ed and 1 st chamber is maintained. Imada et al [8] worked on evaluating discharge coefficient of nozzle and orifice plates. In their study they used ANSYS software to find C d. The diameter ratio used for analysis are 0.5 and R e range of to The model used is k-ε model and the values are in good agreement with the ISO standards. The analyses are carried out with few assumptions such as incompressible, steady and axisymmetric. Sukanta K Dash et al [9] conducted experiments on orifice plate assembly by varying thickness of the horizontal orifice plate assembly with one and two phase flows. For two phase analysis they used water and air as fluids. The thin and thick orifices have been numerically investigated for orifices. The model selected for the CFD analysis is the Eulerian model to calculate pressure drop across the orifice plates. The experiment is carried out by varying velocity and Re to obtain discharge coefficient. The results are validated with ISO standards and data from literature. III. VALIDATION OF CFD METHODOLOGY To validate the methodology of the analysis flow through concentric orifice plate assembly is analyzed. The upstream and downstream straight lengths in the flow domain were 15D and 25D respectively (see Fig 1). Here keeping the boundary conditions such as inlet velocity of 2m/s, Reynolds number 50000, diameter of the pipe 50mm, β(d/d)=0.5, density=1000kg/m 3 and viscosity as 0.002Pa-s the analysis is carried out. The pressure drop obtained across the concentric orifice All rights Reserved 48

3 is used to calculate discharge coefficient (C d ) and permanent pressure drop coefficient (C L ). These values are compared with the ISO 5167 standards. The C d and C L obtained are in good agreement with the standard values (with in 0.1%). So from that we concluded that the CFD methodology adopted is appropriate for the analysis of orifice plate assemblies. During the analysis the assumptions made are the fluid is incompressible, Newtonian, steady and neglecting the effect of heat transfer, vibration and noise. The turbulence model selection study suggested the K- ω model and convergence analysis showed that a minimum of elements are needed to ensure accurate results. Fig1 Flow Domain for the Concentric Orifice Plate Assembly The CFD methodology adopted gives the C d value by using the K- ω model and number of elements of more than a lakh. The value obtained here is exactly same as the ISO 5167 standard value. So from this analysis we concluded that the adopted methodology is perfect. The convergence criterion residual of 10-7 is used for accuracy results. IV. ANALYSIS OF FLOW THROUGH ANNULAR ORIFICE PLATE Annular orifice plate consists of a circular annular opening adjacent to the pipe wall. To keep annular orifice plate in a pipe we need to weld/solder the orifice plate to the pipe. In this study we are neglecting the effect of these tags which are attached to the walls while placing the plate in pipe. Here we are analyzing the effect of the Reynolds number and diameter ratio (β) on annular orifice plate assembly. Here we compare these values with the concentric ISO-5167 standard values. The domain employed here is as shown in Fig2. Thus the annular orifice plate consists of a circular plates of diameter d placed concentrically across the cross section of a circular pipe of diameter D. the open area of orifice will be equal to (π/4)*(d 2 -d 2 ). Fig 2 Flow domain for Annular Orifice Plate Assembly Fig 3 Meshing Around the Annular Orifice Plate Assembly The boundary conditions employed are same as that employed in validation cases. The boundary conditions are inlet velocity 2m/s, Re = 5ᵡ10 4, viscosity of the fluid and density as Pa-s and 1000Kg/m 3 respectively. Gauge pressure at outlet is specified as zero and wall condition is implemented on pipe wall and orifice plate. The number of elements used is more than 1, All rights Reserved 49

4 elements. The specifications of the annular orifice are Diameter of pipe (D) 50mm, upstream length is 750mm, downstream length is 1250mm, Plate thickness (t) is 3mm and Length of pipe (L) is 2509mm. DATA ANALYSIS The discharge coefficient for annular orifice plate in incompressible flow is calculated by using the equation (1) C da = Q act ᵡ / [(A o ᵡ )] (1) Where, Q act = actual flow rate in kg/s A r = area ratio= (D 2 -d 2 )/D 2 A o = orifice opening area (π/4)*(d 2 -d 2 ) in m 2 P= pressure drop across the plate Β 2 = equivalent diameter ratio = A r The permanent pressure loss is obtained by using the equation (2) P perm = (2) where, P perm = permanent pressure drop = drop in pressure between inlet (-15D) and outlet (+25D) of pipe in the presence of orifice plate = drop in pressure between inlet (-15D) and outlet (+25D) of pipe without an orifice plate under same flow conditions. The permanent pressure loss coefficient (C L ) is calculated from using the equation (3) Where, C L = (3) = Pressure drop across the orifice plate. P total = summation of individual pressure drop across the each stage. Comparison between Single Stage Annular and Concentric Orifice Plate Analyses have been made for the flow through annular orifice plate assembly of different equivalent diameter ratios in the range 0.3 to 0.7 at a fixed Reynolds number of Table 1 compares the C d values of annular orifice plate with the concentric orifice plate for same boundary conditions. We can observe that values of C d in case of concentric are lower compared to that of annular orifice plate at higher diameter ratios. As the diameter ratio decreases the annular orifice plates are providing higher pressure drops compared to that of concentric orifice plates. The decrease in the diameter ratio increases the drop in the pressure across both concentric as well as annular orifice plates. Table 1 Comparison of C d values of Annular And Concentric Orifice Plates Diameter ratio (C d ) annular (C d ) conc Table 2 shows the values of the permanent pressure loss across annular orifice plate. Those values are compared with the concentric orifice plate values of ISO 5167 standards of same flow condition. From the table we can observe that recovery of the pressure is better in case of annular orifice plate assembly as compared to concentric orifice All rights Reserved 50

5 Table 2 Comparison Of C L Values of concentric and annular orifice plate Diameter P Pperm ratio (P a ) (P a ) (C L ) conc. (C L ) annular Table 3 shows the variation of C d with Reynolds number at a diameter ratio of 0.5. It is seen that as Reynolds number increases the C d of annular orifice plate increases however for the standard concentric orifice plate C d decreases with increase in Reynolds number. This may be due to the fact that in the standard concentric orifice plate major portion of the flow occurs near axis where as in the case of annular orifice it is concentrated near the wall. Table 3 Comparison of Effect of Reynolds Number on C d (β=0.5) for concentric and annular orifice plate Re P (P a ) (C d ) annular (C d ) conc V. ANALYSIS OF FLOW THROUGH MULTISTAGE ORIFICE PLATE ASSEMBLY In order to study the effect of X/D ratio on the multistage orifice plate assembly we employed five, seven and nine stages. The concentric and annular orifice plates are arranged alternatively. For each stage the analysis is carried out by varying X/D ratio as 1, 2, 3, 5 and 10. The obtained pressure drops across the each stage are used to calculate the C d values. The boundary conditions employed are inlet velocity 2m/s, Reynolds number 50000, β=0.5, diameter of the pipe 50mm, thickness of the plates 3mm, upstream length of 15D and downstream length of 25D. The fine meshing is done around the orifice plates. The numbers of elements employed are more then The wall boundary conditions are employed for orifice plates. The flange tapping are used to calculate the drop in pressure across the orifices. The geometry employed is axisymmetric. A. Five Stage Orifice Plate Assembly Fig 4 represents the geometry used for the analysis. In this geometry we employ the five orifices plate assembly in which three are concentric and two annular orifice plates kept alternatively. The geometric specifications employed for the analysis are as follows, Diameter ratio of concentric orifice plate is 0.5, Diameter ratio of Annular orifice is 0.5(orifice plate diameter 43.3mm) and Area ratio of annular orifice plate is For the analysis we require fine meshing near the orifice plate assembly. The axisymmetric quadratic elements are used. The number of elements used is more than 1ᵡ10 5. The meshing is fine enough to obtain smooth pressure as well as velocity contours along with the pressure drop across the each plate. Fig 5 indicates the discritization done around the multiple orifice stage All rights Reserved 51

6 Fig 4 Flow Domain for Five Stage Orifice Plate Assembly Fig 5 Meshing around the five stages Orifice Plate Assembly B. Analysis of the Results of Five Stages Flange tapings of (+25 and -25mm) are used to find pressure drop across the orifice plates. Figure 6 shows the values of coefficient of discharge calculated from the pressure drop obtained for various spacing X/D in the range 1 to 10. It is observed that C d of intermediate stages is less compared to that of the first and last stages. Thus intermediate stages contribute more to the total pressure drop. This is due to the fact that for these stages the flow is constrained and disturbed on both upstream and downstream stages. Table 4 shows the variation of permanent pressure loss on five stage orifice plate assembly by varying the distance between the plates, we can see from Table 4 that the C L value reaches maximum at X/D=1. So the optimum spacing for obtaining highest pressure drop is selected is 1D. At larger spacing between the plates a partial pressure recovery take place which tends to decrease the overall pressure drop. Fig 6 Variation of C d with X/D for Five Stages Table 5 Effect of Variation of X/D on total pressure drop and C L for five Stage Orifice Plate Assembly X/D Ptotal P* P P Pperm C L All rights Reserved 52

7 C. SEVEN STAGE ORIFICE PLATE ASSEMBLY Here we are analyzing nine stage arrangements to know the effect of X/D ratio. The coefficient of discharge was calculated for different stages individually and are tabled and analyzed for seven stages of orifice plate assembly. Here four concentric and three annulus orifice plate assemblies are kept alternatively. Fig 7 shows the geometry used for the analysis of the seven stage assembly. Fig 7 Geometry of five stage Orifice Plate Assembly In order to analyze the flow we need fine mesh near the concentric as well as annular orifice plate assembly. The number of elements used for the analysis is more than 1 ᵡ 10 5 elements. The types of elements used are quadratic axisymmetric elements. The fine meshing is done near the plate assembly so that uniform and filled contours are obtained. Fig 8 shows the variation of discharge coefficient for seven stages assembly. The discharge coefficients are calculated by using pressure drops. The discharge coefficient is low at 1D it is because of the more interference effect of middle stages of the assembly. From figure we can observe that for a distance X/D=1 the pressure drop is high because of low discharge coefficient. And there after it is decreasing because recovery of pressure due to availability of time for flow to develop. So we are choosing 1D as optimum distance because of high pressure drop. Fig 8 Variation of Cd with X/D for Seven Stages We can see from Table 5 that the total pressure drop is high compared to permanent pressure drop because of partial recovery of pressure after the multistage orifice plate assembly and both the values are maximums at 1D, So optimum value is 1D. D. NINE STAGE ORIFICE PLATE ASSEMBLY X/D Table 5 Effect of Variation of X/D on total pressure drop for Seven Stage Orifice Plate Assembly P total P* All rights Reserved 53 P P perm C L

8 The geometry consists of nine stages in which concentric as well as annular orifice plates are kept alternatively. Here we are analyzing for the pressure drop across each stages. And calculating the coefficient of discharges separately for each stage to know the effect of X/D ratio each stage. The geometry employed for the analysis consists of nine stages. Among this the concentric and annular orifices are placed alternatively to find the optimum X/D ratio. Fig 9 Geometry of Nine stage Orifice Plate Assembly The specifications employed for the flow analyses are, Pipe length = 6027mm, Pipe diameter = 50mm, Concentric orifice plate diameter = 25mm, annular orifice plate diameter = and Diameter ratio (β) of concentric plate = 0.5. Fluid used is a liquid and its properties are Viscosity of fluid 0.002Pa-s, Velocity at inlet 2m/s and Reynolds number 5ᵡ10 4. To create fine meshing around the orifice assembly we are using quadratic axis symmetric elements. The number of elements is more than 1ᵡ10 5 elements. The constant meshing elements are created by varying the size of the elements. Figure 10 shows the variation of discharge coefficient along with X/D for nine stages. The discharge coefficient is less at 1D because of the interference effect of middle stages. So we are choosing 1D as optimum distance. Table 6 shows the comparison of total and permanent pressure drop across the nine stages. The both the values are maximum at 1D and decreases as distance between them is increases because of less interference effect and recovery of pressure. X/D Fig 10 Variation of Cd with X/D for Nine Stages Table 6 Effect of Variation of X/D on nine Stage Orifice Plate Assembly P total P* P P P perm C L All rights Reserved 54

9 VI. COMPARISON OF EFFICIENCY FACTOR FOR DIFFERENT STAGES The efficiency factor R is an indication of interference effect in multistage assembly and it is defined as follows The efficiency factor (R) R= Where, Pi = sum of the pressure drop across the each plate without interference effect P*= the pressure drop between the upstream tap of the first stage and downstream pressure of the last stage Table 7 shows the effects of stages on efficiency factor for various spacing. At smaller spacing (X/D =1 and 2) the values of R increases with increase in number of stages. Thus at nine stages there is almost 81 percent increase in pressure drop as compared to individual stages. However at the large spacing the reverse is true. These are due to the pressure recovery that causes between the stages at that spacing. Table 7 Comparison Table For R Values Of Different Stages X/D 5 Stages 7 Stages 9 Stages R R R Fig 11 Comparison of R for Different Stages Fig 12 indicates the pressure counter obtained for five orifice plate assembly. We can observe from the contours that the drop in pressure is high near the 1st plate and as the plate (stages) increases the pressure is decreasing gradually. We can observe the concentration of the pressure near the annular plates it s because of the straight line movement of the flow which is coming from the concentrated orifice plate Fig 13 is a velocity contour obtained for the five orifice plate assembly. It can be observed from the contour that velocity variation is varying uniformly from stages to stages. Fig 12 Shows the Contours of Static Pressure for Five Orifice Assembly (X/D=1) Fig 13 Indicates the Variation of Velocity Contours Obtained For Five Orifice Assembly All rights Reserved 55

10 From above contours we can observe that velocity increases in a stage between three and four because of more restriction to the flow in that stage and less availability of the time for full development of the flow. And lost stage of the assembly act as a nozzle and inject straight fluid up to full development of flow. Fig 14 shows the velocity vector plot for the five orifice plate assembly. It shows the velocity vector distribution plot of present assembly. From the plot we can observe clearly that drop in pressure is high near the plot so the velocity of flow is high near axis when it passes through the Concentric orifice plate and also when it passes through annular orifice plate the velocity is high near the wall of plate (or gap between the plate and wall). Fig 14 Velocity Vector Contour For Five Orifice Assembly (X/D=1) VII. CONCLUSION The major conclusions drawn from the study are as follows. These are valid for steady, incompressible flow of a Newtonian fluid. Methodology adopted for the analysis gives accurate results as long as standard meshing of required fines is maintained. And K-ω turbulence model will provide accurate results with less computational time. In the case of three orifice of concentric type orifice plate assembly the distances between the plates has a major effect on pressure drop. The different diameter ratio between them gives higher pressure drop as compared to that of same diameter ratio arrangement and the optimum pressure drop is obtained at X/D = 6. In case of multistage arrangement of different orifice types (concentric and annular) highest pressure drop is obtained at a spacing of X/D=1. An intermediate stage gives high pressure drop as compared to end stages due to strong interference effect on small spacing. Effect of Reynolds number is not very significant in a multistage orifice plate assembly. Alternate spacing of concentric and annulus orifice assembly is providing much higher pressure drop. The efficiency of pressure increases with increasing in number of stages. REFERENCES [1] Mohamed A. Siba1, Wan MohdFaizal Wan Mahmood, Mohd Z. Nuawi, RasidiRasani, and Mohamed H. Nassir, Wall Pressure Due to Turbulent Flow Through Orifice Plate International Journal of Mechanical & Mechatronics Engineering, IJMME-IJENS Vol:15, pp 36-41, [2] R. Kis, M. Malcho, M. Janovcova A CFD Analysis of Flow through a High-Pressure Natural Gas Pipeline with an Undeformed and Deformed Orifice Plate.World Academy of Science, Engineering and Technology. International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:8,pp , 2014 [3] Aaron Xavier Fernandes Computational Fluid Dynamics Analysis for Wastewater Floc Breakage in Orifice Flow Chemical Engineering and Applied Chemistry University of Toronto in [4] Andrej Lipej, RokPavlin, AljazSkerlavaj, BogdanJancar, MatjazCernecTurboinstitut,Francivehar, D DRovsnikova, Ljubljana, Slovenia Numerical and experimental design of multi-stage orifice FWRO nd International Conference Nuclear Energy for New Europe, NENE, pp Sep [5] T Sridevi, DhanaSekhar, V Subrahmanyam Comparision of flow analysis through a different geometry of flowmeters using fluent software. International Journal of Research in Engineering and Technology eissn: pissn: , pp , Vol:3 Aug [6] Manish S. Shah, Jyeshtharaj B. Joshi 1, Avtar S. Kalsi, C.S.R. Prasad, Daya S. Shukla. Analysis of flow through an orifice meter: CFD simulation, Chemical Engineering Science, Volume 71, pp , All rights Reserved 56