Virtual Oven to Assist Baking Industry

FRDC researchers selected FIDAP, CFD software from ANSYS of Canonsburg, Pa., as its modeling and analysis tool. FIDAP uses the finite element approach and unstructured grids, which provide considerably greater flexibility in modeling the complex and irregular geometries often associated with food processing. Unstructured grids also make it easy to automate the otherwise tedious process of fitting computational elements to the complex geometries used in food processing equipment, such as mixer blades.

The geometry of the 0.7 m x 0.48 m x 0.56 m oven was created and meshed with Gambit, preprocessor software from ANSYS. Three modes of heating were considered: conduction, wall-to-wall radiation, and convection. The thermal volume expansion of the air was considered as a source of air movement. Boundary and initial conditions were given as follows: initial temperature Ti=21°C; environment temperature Te=21°C; surface heat transfer coefficient h=10 W/m2°C; and air velocity on all the solid surfaces=0.

Simulation Results Match Physical Testing

The results predicted by the CFD model included temperature images and velocity vector plots at times of 100, 300, and 600 seconds. In order to validate the modeling performance, 12 thermocouples were installed at different locations inside the actual oven. The recorded temperatures were used for comparison to those from the CFD simulation. The statistical results of the modeling performance were characterized by a correlation coefficient (R2) of 0.92 and an average relative error (Er) of 7%. These values suggest that the CFD model matched the experimental results well.

The CFD analysis of the initial design for the oven provided the engineering team with a better understanding of the role of certain key design parameters than could have been gained by physical testing alone. The simulation could also be used to perform a sensitivity analysis, and the technique could be extended to more complicated cases.

In the near future, for example, the FRDC plans to extend the simulation to consider not only the heat, mass, and momentum transfer within the oven but also the heat and mass transfer within the product during baking and its effect on baking conditions, with particular focus on the effect of bakery product volume expansion.

Other Food Processing Applications

CFD also has the potential to improve the design of many other types of food processing equipment such as vibro-mixers, disk impellers embedded with dozens of small holes (perforations) that agitate a liquid by moving up and down. Vibro-mixers are commonly used in hermetically sealed vessels where it is impractical to use rotating seals. Common uses include bioprocessing applications such as fermentation.

On a typical vibro-mixer, the tapered ends of the perforations are oriented upward; fluid jets are emitted from the tops of the holes during the downward stroke of the impeller. During the upward stroke, fluid flows in the reverse direction into diverging conical volumes. The upward-directed jets are stronger and give rise to circulation patterns in the bulk fluid. The downward-directed jets are weak by comparison and quickly dissipate.

CFD can be used to help understand the flow patterns and circulation time characteristics of a particular mixer design and a given liquid. If there are solids in the system, the simulation can help determine the processing window of vibration frequency and amplitude to keep the solids in suspension. CFD can also be used to predict the performance of other types of mixers, to assess alterations made to an extrusion die design, and to evaluate design processes that will meet Hazard Analysis and Critical Control Points guidelines in food safety studies.

Computer simulation is an idea whose time has come for the food processing industry. New modeling techniques provide engineers with the ability to evaluate the performance of design concepts with reasonable accuracy without having to build prototypes.