By Walter Schwarz
HVAC&R Business Manager
Just as engineers began using finite-element analysis and other analytical tools for structural design, the last several years have seen appliance design engineers turn to computer simulation to optimize fluid flow. New modeling techniques, such as Computational Fluid Dynamics (CFD), let engineers accurately model the performance of design concepts without having to build a prototype. In the past, fluid-flow issues were addressed using trial-and-error procedures, such as hand calculations or experimental methods. However, designing, building, and testing prototypes takes time and can be expensive. Moreover, techniques used to evaluate prototypes provided little information as to why a particular design performed as it did, whether good or bad. Engineers often settled for less than perfect performance because they didn't have the tools for a truly optimized design.
Simulation software, nonetheless, makes it possible to evaluate many designs at lower costs and with shorter lead times, often providing more design data than physical testing.
CFD is said to be the most popular software tool in appliance design because it characterizes fluid velocity, as well as pressure and temperature values throughout the solution domain, including complex geometries and boundary conditions. In fact, a designer may change the system's geometry or boundaries, such as inlet velocity, flow rate, or rotational speed, during analysis to see how it affects fluid flow. Another advantage of CFD is that it provides more information than physical testing, including color-coded graphics that show flow direction and velocity in all locations. This helps engineers see why a design is performing as it is, and allows for quick design improvements.
As this technology gains popularity in the appliance industry, designers may find they are able to improve a range of appliances. Here are a couple examples of CFD in action.
DESIGNING A BETTER LAWN MOWER
Using CFD to develop airflow patterns helps engineers design lawn mowers that pick up more grass from the ground, and distribute the clippings evenly. Flow behavior of mower blades inside a housing has a major impact on cut quality. Typically, mowers are designed to produce smooth and complete swaths with few stragglers, or long pieces of uncut grass. For mulching mowers, the grass is deposited and evenly distributed back onto the lawn. Lawn mowers also bag the trimmings, however to do this, all the trimmings need to be picked up to make the mowed area look its best. Stragglers are normally caused by downward airflow fields inside the housing that push grass down before a mower can properly handle it. Complex flow patterns inside the housing are virtually impossible to measure or visualize using conventional experimental methods. As a result, traditional lawn mowers have been designed using trial-and-error methods.
However, Francisco Saldarriaga, a design engineer for Frigidaire Home Products Inc., Orangeburg, S.C., has made major improvements to this process using a CFD software package from Fluent Inc., Lebanon, N.H. With CFD, he can simulate the flow pattern inside a mower housing with three staggered blades propelling air through its channels. The software provided the capability to import existing CAD files of mower housing and blade designs which were then automatically gridded with an unstructured mesh that captured the complex geometry. Using the multiple rotating reference frame model along with the sliding mesh approach, Saldarriaga predicted the blade's interaction with the housing with reasonably short computational times. Saldarriaga's results show flow patterns illustrating vector velocity and pressure contours at every point within a mower housing.
Since it's best to lift grass just before it's cut, Saldarriaga examined flow fields looking for vectors that showed the grass being pushed to the ground. In most cases, he found that downward vectors formed a section of tangential recirculation zones, and that high-pressure zones in front of the blade tended to create this type of recirculation zone. By pinpointing the zones' location, he found through ongoing analysis, it was possible to alleviate them by changing the geometry of the housing and blade at the precise location where the recirculation zones were formed. Using CFD, Saldarriaga has been able to eliminate recirculation and reverse flow zones from several mower designs, and tests have shown a 30% improvement in cut quality compared to previous designs.
NOW YOU'RE COOKING
Prabhat Tekriwal, Manager of Product Analysis for Maytag Cleveland Cooking Products, used CFD to analyze airflow's effect in home ovens. Outside surface temperature must, of course, be kept below specified levels to avoid a safety hazard. That's why it's necessary to have an insulating compartment between the oven cavity and the outer surface. Air enters at the bottom of the oven and flows mainly through the compartment to keep the outside surface cool to touch. Air typically flows up the oven's sides and passes through, underneath the burner area. Generally, the goal is to provide a relatively even flow throughout the insulating compartment, and in some cases, high flow levels in specific areas to help lessen hot spots. Another important concern in oven design is managing airflow inside the oven cavity. Airflow is important in providing the consistent temperature levels required for even cooking. In some cases, it may also be necessary to minimize flows. For instance, in certain areas of a glass oven door, convection currents between layers of glass are minimized or eliminated to avoid excessive temperatures on the door's outer surface.
CFD's key advantage in oven design, according to Tekriwal, is that it allows him to evaluate many design alternatives in a reasonable period of time. Typically, he and his team investigate the effects of changing the insulation compartment geometry, insulation thickness and type, and size and shape of the entry and exhaust vents. He uses a CFD code that allows for parametric design analysis by letting the engineer enter a range of values for key design variables that the software then evaluates in batch mode. By using CFD to carefully control the convection-driven flow through the insulating compartment, Tekriwal has significantly reduced the gap size required to insulate the cavity from the out-side surface.
Tekriwal points out that CFD doesn't replace experimental methods, but rather provides a method of determining optimum values for critical design parameters before experimental validation.
Edited by Amy Higgins.