Computational Fluid Dynamics By:- Thombare A.S.

What is CFD? Computational Fluid Dynamics (CFD) is the science of predicting fluid flow, heat transfer, mass transfer, chemical reactions, and related phenomena by solving the mathematical equations which govern these processes using a numerical process. The result of CFD analyses is relevant engineering data used in: conceptual studies of new designs detailed product development troubleshooting redesign CFD analysis complements testing and experimentation. Reduces the total effort required in the laboratory.

Base of the CFD The fundamental basis of almost all CFD problems are the Navier-Stokes Equations ,which defines any single phase fluid flow. By some simplifications equation becomes :-

Where is CFD used? Aerospace Automotive Biomedical Chemical Processing HVAC Hydraulics Marine Oil & Gas Power Generation Sports Aerospace Biomedical F18 Store Separation Automotive Temperature and natural convection currents in the eye following laser heating.

Where is CFD used? Aerospacee Automotive Biomedical Chemical Processing Hydraulics Marine Oil & Gas Power Generation Sports Chemical Processing Polymerization reactor vessel - prediction of flow separation and residence time effects. Hydraulics

Flow of lubricating mud over drill bit Flow around cooling towers Where is CFD used? Aerospace Automotive Biomedical Chemical Processing HVAC Hydraulics Marine Oil & Gas Power Generation Sports Sports Marine (movie) Oil & Gas Power Generation Flow of lubricating mud over drill bit Flow around cooling towers

Domain for bottle filling problem. CFD - How It Works Filling Nozzle Analysis begins with a mathematical model of a physical problem. Conservation of matter, momentum, and energy must be satisfied throughout the region of interest. Fluid properties are modeled empirically. Simplifying assumptions are made in order to make the problem tractable (e.g., steady-state, incompressible, inviscid, two-dimensional). Provide appropriate initial and/or boundary conditions for the problem. Bottle Domain for bottle filling problem.

Mesh for bottle filling problem. CFD applies numerical methods (called discretization) to develop approximations of the governing equations of fluid mechanics and the fluid region to be studied. Governing differential equations  algebraic The collection of cells is called the grid or mesh. The set of approximating equations are solved numerically (on a computer) for the flow field variables at each node or cell. System of equations are solved simultaneously to provide solution. The solution is post-processed to extract quantities of interest (e.g. lift, drag, heat transfer, separation points, pressure loss, etc.). Mesh for bottle filling problem.

An Example: Water flow over a tube bank Goal compute average pressure drop, heat transfer per tube row Assumptions flow is two-dimensional, laminar, incompressible flow approaching tube bank is steady with a known velocity body forces due to gravity are negligible flow is translationally periodic (i.e. geometry repeats itself) Physical System can be modeled with repeating geometry.

Mesh Generation Geometry created or imported into preprocessor for meshing. Mesh is generated for the fluid region (and/or solid region for conduction). A fine structured mesh is placed around cylinders to help resolve boundary layer flow. Unstructured mesh is used for remaining fluid areas. Identify interfaces to which boundary conditions will be applied. cylindrical walls inlet and outlets symmetry and periodic faces Section of mesh for tube bank problem

Advantages of CFD Low Cost Speed Ability to Simulate Real Conditions Using physical experiments and tests to get essential engineering data for design can be expensive. Computational simulations are relatively inexpensive, and costs are likely to decrease as computers become more powerful. Speed CFD simulations can be executed in a short period of time. Quick turnaround means engineering data can be introduced early in the design process Ability to Simulate Real Conditions CFD provides the ability to theoretically simulate any physical condition

Ability to Simulate Ideal Conditions CFD allows great control over the physical process, and provides the ability to isolate specific phenomena for study. Example: a heat transfer process can be idealized with adiabatic, constant heat flux, or constant temperature boundaries. Comprehensive Information Experiments only permit data to be extracted at a limited number of locations in the system (e.g. pressure and temperature probes, heat flux gauges, LDV, etc.) CFD allows the analyst to examine a large number of locations in the region of interest, and yields a comprehensive set of flow parameters for examination.

Limitations of CFD Physical Models Numerical Errors CFD solutions rely upon physical models of real world processes (e.g. turbulence, compressibility, chemistry, multiphase flow, etc.). The solutions that are obtained through CFD can only be as accurate as the physical models on which they are based. Numerical Errors Solving equations on a computer invariably introduces numerical errors Round-off error - errors due to finite word size available on the computer Truncation error - error due to approximations in the numerical models Round-off errors will always exist (though they should be small in most cases) Truncation errors will go to zero as the grid is refined - so mesh refinement is one way to deal with truncation error.

Fully Developed Inlet Profile Boundary Conditions As with physical models, the accuracy of the CFD solution is only as good as the initial/boundary conditions provided to the numerical model. Example: Flow in a duct with sudden expansion If flow is supplied to domain by a pipe, you should use a fully-developed profile for velocity rather than assume uniform conditions. Computational Domain Fully Developed Inlet Profile Computational Domain Uniform Inlet Profile poor better

2. CFD for Dryers:- CFD models are applied to determine optimum equipment configuration and process settings. Drying equipment is usually large and expensive. As a result, efficiency is an important factor that influences production and operation cost. CFD is applied to examine configuration changes and thus minimize risk and avoid unnecessary downtime during testing.

Spray Dryer

Summary Computational Fluid Dynamics is a powerful way of modeling fluid flow, heat transfer, and related processes for a wide range of important scientific and engineering problems. The cost of doing CFD has decreased dramatically in recent years, and will continue to do so as computers become more and more powerful.

References Websites :- www.leads.ac.uk/cfd www.wikipedia.com www.osun.org Journal :- Chemical Engg. Education (CEE)

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