Several sources have estimated 20 to 25% of all electrical energy produced in the world is used to power fluid moving equipment. An opportunity to reduce global energy consumption lies in improving the efficiency of such equipment. Computational fluid dynamics (CFD) and finite element analysis (FEA) play essential roles in optimizing these complex machines to improve efficiency and reliability, and reduce product weight and cost.
Relative velocity vectors of flow moving through a vertical turbine pump stage.
Fluid moving equipment has been designed using hand calculations based on assumptions and simplifications, relying on the designer’s “feel” to produce efficient designs. Any departure from previous designs was a leap of faith, or a trial and error research and development exercise. CFD and FEA allow us to predict the internal interaction of fluids and components, giving us insight into energy-robbing phenomena within any new machine design.
The three Rs in the green movement are reduce, reuse, and recycle. As designers, we can make our products from recyclable materials and make them robust enough to be reused. But generally the factor over which we have the most control is reduction. We can increase efficiency, thereby reducing energy requirements, and increase effective use of material, thereby reducing weight and material costs. Additional benefits are in reduced shipping costs and decreased fuel in transport. We don’t need to reuse or recycle as much if we reduce in the first place.
Drive key stress in a double suction pump impeller.
Finding Efficiencies with CFD
For pump design engineers, most of the “low-hanging fruit” or “easy fixes” have been worked out over the years through testing, trial, and error. Now, we are left to make hard fought incremental gains. Though gains may not be of an order of magnitude, most turbo machinery has potential for significant improvement. Pumps are said to be the second-most-common machine in the world, second only to the electric motor. Think of all of the pumps you use every day in your car, dishwasher, washing machine, waste water services, gas pumps, supplying your water, etc. Efficiency improvement of just a few percent in these and millions of pumps could mean annual power savings of billions of dollars to the nation’s economy.
CFD offers an opportunity to look inside the blade passages of the spinning impeller in a pump or fan and see all the areas where efficiency is stolen by recirculation, stall, choking, or other factors. Traditionally, the internal shape of impeller blades has been treated as a black box. We control the inlet and outlet parameters, but design of the “in-between” was more art than science. Leading pump manufacturers had “fluid dynamic savants” with a feel for good design who could generally, after years of experience and lots of trial and error, produce quality designs. But significant variation from previous designs often produced unfortunate surprises. With CFD, “non-savants” can consistently produce credible high-quality designs meeting the performance requirements with minimal prototyping. This not only reduces development costs but also compresses the development cycle. This is a competitive advantage, and leads to the only kind of green the accounting department will acknowledge.
There are many more opportunities for CFD to improve our designs. I did a redesign of a complicated side inlet on an existing pump for a client, and by properly orienting the flow into the impeller we increased the design point efficiency by 12%. We were able to reduce the operating noise level and cavitation problems that had been reported. Without the flow visualization that CFD provided, I would not have been able to find the areas of high loss and make such an improvement. For another client I redesigned the volute of the pump, changing the angle of the cutwater, which boosted the pump head and efficiency.
Mapping CFD to FEA
In addition to fluid dynamic uses, CFD also provides the designer with a method to predict and/or troubleshoot the interaction between the fluid and the pump’s structural components. This can predict bearing loads, blade tip stresses, and dynamic loading for shafts, keyways, etc. Often these outputs can be fed directly into FEA programs, so the fluid imposed loads can be mapped to the structural surfaces providing convenient setup of the FEA simulation.
Mechanical and fluid dynamically imposed stress in a compressor impeller. Courtesy of Concepts NREC
FEA is the structural equivalent of CFD. The designer can take a complex shape that is beyond the limits of hand computation and dissect it into a mesh of thousands or millions of small elements from which the computer can iteratively solve for stresses to within a pre-specified error. Many CFD programs provide pressure, temperature, force, etc. in formats that can be read by FEA programs so the designer can quickly transfer that data and develop an accurate model of the stresses imposed by the fluid.
Create Competitive Advantages
This brings us back to the three Rs. With accurate input data for the FEA program, we can predict the actual stresses and strains experienced by the mechanical components during operation.
In the pump industry, like most industries, the old adage “when in doubt use more metal” was/is the motto. But metal costs money. For commodity products, to control costs, manufacturers have to find ways to reduce the amount of metal used while maintaining the structural strength and durability of the product. This has led to extensive use of FEA to optimize component strength-to-weight ratios, strategically placing ribs and bosses to provide strength and rigidity while reducing the weight, often by as much as 30%. In the past, without proper FEA of components, shaving off metal often resulted in massive product warranty costs as areas of unexpected high stress failed in the field, costing some brands their reputation and market share. Done carefully, CFD analysis and FEA simulations can give manufacturers a competitive advantage, while simultaneously allowing them to lower material consumption and reduce their ecological footprint.
The ability of CFD software to produce equations describing pressure, forces, velocity, and temperature of the fluid with respect to time allows those who work in the field of vibration and rotor dynamics to predict structural response of the machine before installation. This avoids retrofitting expenses after the equipment is installed.
Such upfront analysis has been invaluable to many large pump stations. Foundations and structural supports are being reduced, but this diminishes their ability to damp out vibrations. At the same time, variable speed technology is being used to reduce pump power requirements during off-peak demand. Variable speeds of the motors increases the likelihood of structural resonance problems. Approximations and simplifications that have been standard for years no longer accurately encompass all of the variables in the complex operating scenarios found in modern pumping systems.
In the green engineer’s toolbox, CFD and FEA can and should play a role in developing products that are more efficient, use fewer resources, and increase performance, permitting companies that use them to go green for our planet and their balance sheets.
Greg Case is president of PD3, Pump, Design Development & Diagnostics, LLC, Contact him via email@example.com or pdcubed.net.