Increase Productivity Using CFD Analysis CFD of Process Engineering Plants for Performance Estimation and Redesign Vinod Taneja Vidhitech Solutions Abhishek Jain abhishek@zeusnumerix.com +91 9819009836 Defense Nuclear Power Aerospace Infrastructure Industry
Overview Introduction to CFD CFD Methodology Difference between CFD and TFD Advantages of CFD Case Studies Flow uniformity Temperature and pressure gradients Conclusion 7-Nov-17 Performance Enhancement using CFD 2
Fluid Dynamics The study of liquids and gases in motion Governed by laws of motion and thermodynamics Classified as Compressible vs. Incompressible Laminar vs. Turbulent Steady vs. Unsteady Uniform vs. Nonuniform Defined by Navier-Stokes equations (complex coupled partial differential equations) 7-Nov-17 Performance Enhancement using CFD 3
Fluid Mechanics Branches Analysis Tool Year 1600 1700 1800 1900 2000 Experimental Theoretical Computational Experimental methods Theoretical Fluid Dynamics (TFD) write eqns. for flow Computational Fluid Dynamics (CFD) 7-Nov-17 Performance Enhancement using CFD 4
Equations: Incompressible Flow Incompressible flow is governed by: Conservation of mass (continuity equation) u/ x + v/ y + w/ z = 0 (1) Conservation of momentum (Euler equation) ( u/ t + u u/ x + v u/ y + w u/ z) + p/ x = 0 (2) ( v/ t + u v/ x + v v/ y + w v/ z )+ p/ y = 0 (3) ( w/ t + u w/ x + v w/ y + w w/ z) + p/ z = 0 (4) Density is constant. Temperature doesn t take part in motion of flow Heat energy of element (e) or temperature (T) (if Cp constant) is convected as if T is independent attribute fluid not related to motion More equations for combustion (specie transport) 7-Nov-17 Performance Enhancement using CFD 5
The Difference TFD TFD can give equations but cannot easily solve them To solve, assumptions are to be made like Flow is ideal, uniform, continuous e.g. Bernoulli s eqn It gives average values and not exact Example Flow rate of 1000 Kg/s water from 1sqm area pipe will mean 1m/s average flow velocity in TFD It usually cannot give velocity at each point in that 1 sqm TFD is based on gross values Inexpensive and solves in small time 7-Nov-17 Performance Enhancement using CFD 6
The Difference CFD Can take any complex equation and apply numerical methods Assumptions made in CFD are Law of Conservation of Bulk and Species Mass Newton s 2nd Law of Motion 1st Law of Thermodynamics Allows friction, heating, non-uniformity, discontinuity etc. Gives exact values of flow variables at each location 7-Nov-17 Performance Enhancement using CFD 7
CFD in Six Steps 2 Build Computational Domain 3 Create suitable Mesh Boundary Conditions & Initial conditions Interpret solution Plot flow Field Solution of discrete equations 7-Nov-17 Performance Enhancement using CFD 8
CFD Brief Steps Identify the computational domain area of study Generate the correct type of mesh discrete points Set up Simulation Assign boundary conditions, initial conditions, etc Execute the solver Choose accuracy, Viscous, Turbulent, Incompressible, Reactive etc Requires large computer to solve Post-process data Organize data and understand results Understand the fluid dynamics Do the results make any sense? Is the design correct? Note: At every step, good understanding of theoretical fluid dynamics is essential! 7-Nov-17 Performance Enhancement using CFD 9
The Difference Illustration by simple example TFD CFD Final average velocity based on mass flow principle. May assume blausius profile at outlet Each red dot is one velocity value Millions of such discrete values possible. Leads to interesting insight in the flow, even flow paths Similarly for temperature and other variable 7-Nov-17 Performance Enhancement using CFD 10
Steps for Design Improvement System Definition Prediction & Data Visualization Identification of problem areas Analyse Identification of root cause of problem and feasibility of solution possibility Gap between desired and predicted, Incremental design changes and trade-offs Change the system parameters, reevaluate to get desired result Validation of design change using CFD 7-Nov-17 Performance Enhancement using CFD 11
Case Studies Case studies from industrial examples 7-Nov-17 Performance Enhancement using CFD 12
ESP Bank Study 1 ESP Bank having non-uniform flow 17% difference in mass flow rate in four branches Pressure drop 9.3 mmwc original design (case 1) 15 Kg/s difference in mass flow rate 7-Nov-17 Performance Enhancement using CFD 13
Redesign Case 2 Diverter plates installed at six places of flow bend Pressure drop decreased to 8.26 mmwc stagnation zone formed by diverter plate 1 Flow is not attached to the wall at turnings 4 Kg/s difference in mass flow rate 7-Nov-17 Performance Enhancement using CFD 14
Redesign Case 3 Change in diverter plate design Pressure drop reduced to 7.54 mmwc Flow non-uniformity increased max flow in duct three 11 Kg/s difference in mass flow rate 7-Nov-17 Performance Enhancement using CFD 15
Redesign Case 4 Change again in diverter plates Flow is not uniform at the exit of duct no. 2 & 4 Pressure drop across duct is 7mmWc 10 Kg/s difference in mass flow rate 7-Nov-17 Performance Enhancement using CFD 16
Redesign Case 5 Design change to bring more uniformity Pressure loss across duct is 7.9mmWC Uniformity achieved in two branches and its subbranches this design chosen as final Energy saved approx. 9.5 KW 4 Kg/s difference in mass flow rate 7-Nov-17 Performance Enhancement using CFD 17
Pulse Jet Bag Filters Study 2 Flow uniformity and pressure drop reduction Design failing in operation 558 bags each in 6 bag nests (large simulation) Optimum pressure drop across bag filter, Uniform distribution of gas to each bag nest Absence of critical velocity (> 1.5 m/s) Optimum deposition of dust particles on bags & inlet plenum 7-Nov-17 Performance Enhancement using CFD 18
X component of velocity at a vertical plane at the mid span of the porous zone/bag filter Results for Bag Filter CFD showed twice flow impinging on outer compartment than middle and inner Non-uniformity is a root cause for failure 7-Nov-17 Performance Enhancement using CFD 19
Solution Guide vanes to redirect flow, change in size of the damper plates to make flow uniform to all compartments Performance improved to desired level and gave life of 3 years 7-Nov-17 Performance Enhancement using CFD 20
Exhaust and Plume Trajectory Study 3 Problems to be solved Flue gases to pass through ducting and reach the chimney with each flow branch loaded equally Unequal loading causes some fans to be overloaded Leads to uneconomical choice of fans Chimney plume to clear nearby living areas May transport contaminants and cause soot inhalation Mass flow rate uniformity achieved within 0.2% Pressure loss in chimney reduced by 15% 7-Nov-17 Performance Enhancement using CFD 21
Results 7-Nov-17 Performance Enhancement using CFD 22
Evaporation and Combustion Study 4 Estimation of evaporation of feedstock and its combustion characteristics Evaporation due to high temperature Time delay in liquid converting to vapor Combustion after activation temperature reached Infinite rate chemistry vs. finite rate of combustion Advantage of CFD Possible to control the evaporation rates Possible to control the high temperature zones as desired 7-Nov-17 Performance Enhancement using CFD 23
Results Combustion Hotter core Normalized droplet radius vs time 7-Nov-17 Performance Enhancement using CFD 24
Carbon Black Industry: Uses Typical CB combustor operates at 1850 to 1900 C CFD analysis shows on a typical reactor temperature profile along a cross section varies from 150 to 300 C implying some parts of refractory at 1700 C and some at 2000+C By getting a more uniform temperature and a profile hotter in the core it should be possible to operate the reactor at 100 125 C higher This would give 1 to 1.5% higher yield and 2-3% higher productivity In fact using high temperature refractories requires CFD studies must be done to normalize the temperature profile to get the best advantage 7-Nov-17 Performance Enhancement using CFD 25
Carbon Black Industry: Uses Air-preheater (APH) Problem of flow non-uniformity leads to non-uniform flow velocities in different tubes The velocity in different tubes varies from 35 to 55 m/s Tubes of low velocity flow tend to have fouling and low heat transfer reducing the overall efficiency of APH Properly designed flow profile can give 30 to 35 deg C average higher air temperature that would increase the yield and productivity of the reactor 7-Nov-17 Performance Enhancement using CFD 26
Carbon Black Industry: Uses Bag filter Proper design with help of CFD can provide uniform flow to the entire bag filter Pulsing flow uniformity to all bags This will provide bag filters with higher air to cloth ratio that would mean lower capex and opex cost CFD is very useful tool for improving the bag filter system 7-Nov-17 Performance Enhancement using CFD 27
Conclusions CFD can simulate most aspects of process Simulations mimic actual physics qualitatively and quantitatively Correct simulations help in finding the way to improvement Visualization possible in experimentally inaccessible areas Multiple redesigns can be economically tested before actual installation Domain knowledge essential apart from CFD In short, CFD is a very important tool for process improvement 7-Nov-17 Performance Enhancement using CFD 28
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