Blender Products Inc., Denver, Colorado, has been engineering air-mixing products for close to 40 years, with more than 50,000 installations in the HVAC area alone. In HVAC applications, the company's products mix air coming from the outside with air returning from the building. It is critical that the two air streams mix thoroughly because during the winter months, the temperature of the outside air can be below freezing. If this cold air doesn't mix with the warm indoor air, stratification can result in frozen coils and low-temperature limit controller (freezestat) trips. Blender Products' static air mixers eliminate freeze potential, control inaccuracies created by stratification, and help meet Indoor Air Quality requirements. They range in size from a 6-inch mixer in a small duct to a 208-inch mixer in a large drying system. Instead of moving parts, these devices use a precisely configured series of blades to mix the air. The velocity profile downstream of the mixers has a minimal effect upon components located downstream of the mixer and the pressure drop is a known, predictable amount.
Adding a low-cost model
An essential part of the design process at Blender Products involves evaluating how effectively a new product blends hot and cold air streams. For the AIR BLENDER® mixer, this was done by testing physical prototypes. A prototype was installed on one of the test stands in the company's test facility, where cold air was pumped into the bottom half and warm air was pumped into the top. The two air streams were indicated with different colors of smoke so it was possible to visualize the amount of blending as the two air streams passed through the mixer. The test was performed multiple times with different percentages of hot and cold air as well as various air temperatures. In all, about 30 tests were performed. Usually, the test process revealed some things about the design that needed to be changed, so additional prototypes were built and tested until the design was finalized. For the AIR BLENDER, the testing process took approximately one year. In addition to delaying the introduction of the product, this approach had the additional drawback of requiring the company to maintain a fully equipped test facility. "The cost of keeping all the instruments running and calibrated was high, plus we had to devote a lot of space to this activity," says Keith Robinson, engineering manager, HVAC Products, at Blender Products.
Several years ago, Blender Products' market research revealed the need to add a lower-end mixer to the product line. "Until then, all of our mixers had been very high-performance models but in HVAC work, cost is always a big concern," explains Robinson. "We wanted to come up with a mixer that was equal in performance to our older models yet provided a cost advantage over the AIR BLENDER." The way they intended to accomplish this was by staying with same basic design used in the AIR BLENDER mixer, but reducing the part count. "The idea was to take out some of the mixing blades, while leaving the most important ones. That would take us from 32 parts down to 14 and reduce the cost of manufacturing," says Robinson. "But we needed to be sure that this new configuration had acceptable mixing efficiency."
To eliminate the time and expense of testing prototypes and get this new product to market quickly, Blender Products decided to add CFD analysis to its design process. CFD analysis involves the solution of the governing equations for fluid flow, heat transfer, and chemistry at many thousand discrete points on a computational grid representing the flow domain. The use of CFD enables the company to obtain solutions for problems with complex geometries and boundary conditions. A CFD analysis yields values for fluid velocity, fluid temperature, and fluid pressure throughout the solution domain. Based on the analysis, a designer or an engineer can optimize fluid flow patterns by adjusting either the geometry of the system or the boundary conditions such as inlet velocity or temperature. Blender Products chose CFX-5 software from AEA Technology because this program's modeling tools were easier to use than other CFD programs they evaluated. "The fact that CFX-5 has very easy ways to generate geometry was the biggest thing that drew us to the program," says Robinson. "The preprocessor is so intuitive that you can build models with virtually no training."
Two types of benefits
Prior to using CFD analysis on the design of the low-end mixer, Robinson wanted to model an existing mixer, the AIR BLENDER, and run some analyses to see how CFD results would compare with the test results for the product. He created the geometry of the mixer and test stand using the software's pre-processor. "The geometry generation tools were very intuitive so this part went quickly," Robinson says. In CFX-5, he modeled a straight duct divided into two passages. Hot air entered the top half and cold air entered the bottom half. This arrangement was chosen to minimize the amount of natural mixing due to buoyancy. Next he modeled the geometry of the AIR BLENDER mixer. Other boundary conditions and fluid properties, such as with the velocity and temperature of each air stream, were entered by selecting options from menus. After the geometry of the mixer had been defined, the geometry was meshed automatically in CFX-5.
The analysis results showed temperatures downstream from the mixer at 1,000 data points per cross-section. This was much more information than the 24 to 32 data points per cross-section captured from the test stand. Although the initial CFD results were very close to the prior test results, Robinson went through several iterations until it yielded a 99 percent correlation with the testing. This very close correlation gave the company confidence that CFD could replace much of the testing during the design of the new low-end mixer. "Once we had validated the analysis model, we no longer needed to physically test every design iteration. We could use CFX-5 to check all the possibilities," Robinson says.
Creating the geometry of the low-end mixer went especially quickly because when Robinson had modeled the AIR BLENDER, he had created separate representations of the various test stand and mixer components. He was able to pull many of these into the new analysis model. Robinson analyzed three different design iterations before he reached an acceptable design. From the analysis results, he determined which mixing blades from the AIR BLENDER design could be removed and which had to stay to give an acceptable level of air mixing. The three iterations took only four weeks. Although the company still tested a few prototypes, the CFD approach made it possible to develop this product in a fraction of the time it took for previous models. Also, since adopting CFD, Blender Products has reduced the size of its test facility substantially. They got rid of their very large test stands and kept only a few two-feet by two-feet, 500 CFM models. In the near future, much of the space previously used to house lab equipment will be converted to manufacturing.
Blender Products also uses CFD analysis to determine how its product will work in actual systems. As Robinson explains, "We have defined the performance of the basic mixer configurations very well. Unfortunately, in many cases, the actual installation varies considerably from the lab setup. Often HVAC systems are installed without a mixer and then experience lots of problems once they are operating. As a result, the owners try to squeeze a mixer into a system. These retrofit applications have very little space, so mixers are installed at angles and all sorts of creative ways. The question is almost always asked, 'How well will it work?' Before CFD, we would make a scale model and test it but this was always time and cost-intensive process. Usually it took two to three months at a minimum. Now with CFD, we can turn that around in a week." He notes that the ability to show the customer, through CFD analysis, how the existing system performs without a mixer and then with a mixer often helps make the sale.
The new low-end mixer that Blender Products developed with CFD will be on the market soon. The company expects to offer it for 25 to 33 percent less than their other models. Blender Products credits CFX analysis for helping them address this market niche soon after identifying it. And they have come to rely on CFX analysis for boosting sales of all products. "This technology is valuable in both new product development and for on-going sales support," Robinson concludes.