ApplianceMagazine.com | Designing Better Ovens—Faster

Figure 1. Simulated ducts (above) and oven cavity (below).

Correct oven cavity design is at the heart of all good self-cleaning ovens. In fact, this design element is often the starting point for new oven projects. Some of the key challenges facing engineers include complying with cost budgets (especially those concerning materials and investments) and performance standards, as well as many other nontrivial constraints such as reliability, certification, noise, and manufacturing processes.

This paper describes design methodologies applicable both for self-cleaning and non-self-cleaning domestic ovens based on the following three-step design approach.

Virtual simulation of door and air-cooling systems/devices (external portion of the oven cavity such as the electronics bay) so as to establish, as a minimum, the external thermal conditions of the door (the largest heat dispersion source for ovens) and areas where important cooling phenomena occur.
The thermal-fluidynamic response of the cavity in loaded and unloaded conditions and using real cooking modes (e.g., convection roast, thermal bake, etc.).
The thermal-mechanical response of the cavity structure coupled with the fluidynamic simulation. In this way, engineers can appreciate the real response of the system with and without thermal load (i.e., real food).

This approach is not dependent upon the firm’s product development process (e.g., technology-push, market-pull, concurrent design, etc.) since, once in place, it can be iterated on demand. Interestingly, the results not only provide better knowledge of the product, but also can strengthen intellectual property and verify or quantify tacit knowledge.

Initial Design Process

In general, domestic ovens are designed from the outside-in: First, aesthetics are established and then product characteristics are fixed. Once this is complete, product specs are issued, and the design engineers start work. A typical draft spec will usually include the following factors.

Aesthetics. For example, this could include flat brushed stainless-steel trim with black glass for oven door and control panel; black speckled enamel and silver door gasket; and large rounded handle with brushed stainless finish.

Controls. Possible control specs might include a single multifunctional display that provides clear, clutter-free feedback for the end-user. Modern oven controls nowadays are electronic with oven settings established using one of the following three methods: touch control panel with a full keypad and no knobs, i.e., nonmechanical selectors or encoders; semitouch control panel, i.e., a reduced keypad and a series of selector switches and/or encoders with knobs; and no touch control with only selector switches and/or encoders with knobs.

Overall Measures and Weights. This might include cabinet cut-out size; cavity volume and dimensions; service requirements such as installed power, energy consumption; and oven weight.

Performance. Possible specifications could be warm-up time, self-clean cycle duration, self-clean cycle temperature, energy class, cooking performance, and noise, etc.

Accessories. This might include enameled or stainless-steel racks, cookie trays, meat probe, telescopic cooking racks, and additional halogen lighting.

Market specifications. For example, this could cover launch and rollout dates, quantities (volumes for each market), final product cost, selling price, and margins, etc.

Product Development Phase

Once those details are finalized, it is customary to start the product development phase. In the suggested approach, this implies using one of the following three methods.

Coupled, with feedback and in sequence. In simulating the door and key externalities (e.g., electronics, rear external wall, etc.), the authors conducted a standard fluidynamic simulation (CFD) of the cooling system, with special attention to the door, the electronics bay, the catalyzer duct and cooling of oven fan(s), and control panel. This work was conducted with FLUENT software, but was also supported via a specific program developed in MathCAD. All mechanical drawings, including the cooling ducts and parts, were drawn with Pro-Engineer Wildfire 3.0.

In the next phase, the thermal-fluidynamic conditions were simulated with FLUENT using a finite volume methodology. This was conducted in empty cavity conditions (unloaded cavity) and in loaded conditions (i.e., a simulated standard energy consumption “refractory brick”). With the thermal-fluidynamic response in hand, it was possible to move to the third and final step, which consisted of simulating the thermal-mechanical response of the cavity structure (using NASTRAN) under the conditions established in the previous step (FLUENT). In the authors’ opinion, this approach is the most complete and beneficial, but it requires dedicated resources (outsourced or in-house) for a specific allocated time, which may not always be possible for all projects.

Coupled, without feedback and in sequence. This approach is almost identical to the previous method. The difference is that there is no feedback, and the MathCAD worksheet can be avoided, provided that essential duct and door operation data are known or set (e.g., temperature contours, air velocities, pressure drop, etc.).

Uncoupled and stand-alone (results can be grouped or pooled at the end). The third approach splits the design steps into at least two parts (i.e., steps 1 and 2, or steps 2 and 3, or step 1 only). Because each of the three steps can be conducted separately, this approach affords two main advantages. First of all, it can significantly reduce development time and costs (since each step can be outsourced separately), although it still implies that the results should be pooled together. In essence, this is typical of the “work-package” approach. Secondly, it can be applied for singular design purposes (e.g., determining mechanical stress on cavity walls, improving catalyzer duct performance, predicting energy consumption, warm-up time, etc.). This is convenient when the oven is already far down the development process.

There are, however, two main drawbacks. The first is that the knowledge is pooled at the end rather than shared and coupled during the development process. In addition, tacit knowledge tends to remain within each department involved. Both of these aspects may add cost and extend development time for future projects.

Figure 3. Detail of typical mesh of oven cavity.

Examples of Design Input and Output

The MathCAD computation worksheet was set up to deliver three types of information based on a monodimensional conceptualized system of ducts:

Estimate pressure drop and velocities in each section of the cooling duct(s). Using the coupled with feedback and in-sequence approach, data were verified to test the complete simulated model of the cooling system (door included) simulated with FLUENT.
Estimate temperature domains in each part of the cooling system and ducts.
Estimate heat transfer in each part of the cooling system(s) and ducts.

Among the data included (and modifiable), there was the characterization of cooling fans, duct geometry, etc. The geometric model was directly obtained from the original Pro-E model of the complete oven. Figure 1 shows the simulated collection of ducts and cooling system (left) and half of the oven cavity (right).

A typical part of the MathCAD worksheet is the section concerning input and output data. The input data array shown in Table I refers to one of the ducts. The data array shown in Table II represents a typical output for pressure drop calculations.

Output for the cooling system thermal-fluidynamic simulation might look similar to Figure 2. This kind of output can be conducted in steady-state conditions or, more rarely, during transitory. Operating conditions can be adjusted to take into account products working at altitude or in very hot conditions.

While only briefly summarized, these three examples of computed information are typical of step 1 of the suggested approach. Clearly much more information can be obtained and provided as input into the design process (i.e., the characterization of moving cooling fans).

Figure 4. Simulated transitory vertical plane.

Taking It a Step Further

In designing domestic ovens, this is usually where product manufacturers stop and continue development using an experimental approach. However, if the next two steps are followed, the design engineer has the opportunity to discover much more about the full operation of the oven.

Figure 3 shows the meshed domain (finite volume) of a section of the oven cavity. (The outline of the grilling or broil element is visible.) The meshing was applied directly to the exported drawing file from Pro-Engineer and, together with the data obtained during both experimentation and the previous step, was used to set the boundary and initial conditions of the thermal-fluidynamic simulation.

It should be noted that it is also possible to include cooking racks and thermal loads in the mesh, although for empty cavity simulations, nothing was actually included. In load conditions, it is customary to emulate the cooking rack as a plane, but as stated, it is quite possible to include the rack(s) as well. This is worth noting because rack mass is not negligible for certain thermal simulation conditions since ovens often work at temperatures in excess of 325°F (163°C) and typical mass is at least 1.5 kg for U.S. ovens.

A typical requirement for design engineers is to predict warm-up time or guarantee warm-up performance. Ideally, this implies predicting performance for different cooking modes or for the self-clean cycle. In order to achieve this, it is necessary to conduct simulations where the relative transitory are assessed. This provides the design engineer with not only a measure of the temperature rise gradient, but also how the heat is dispersed and circulated within and across the system. In this way, thermal currents are quantified and qualified. Figure 4 depicts a typical vertical plane showing thermal currents within the oven cavity.

As previously mentioned, when this type of analysis includes transitory conditions, it is possible to predict and compare oven warm-up performance, as shown in Figure 5 (experimental versus predicted). Similarly, this analysis can be conducted at any plane in the oven cavity, as shown in Figure 6.

Another possible output concerns the structural response of the oven, especially the oven cavity. As suggested, it is possible to carry out a complete coupled simulation in which the thermal-fluidynamic response is combined with a complete structural simulation both for steady-state and transitory conditions. Such circumstances can also include different structural conditions (e.g., hyperstatic or isostatic, simple supported, fixed-end, etc.), as well as linear and nonlinear materials response.

Figure 7 shows a typical output in two consecutive time frames for temperature; hence, this is a typical transitory. Similarly, we can have the stress contours and deformation contours, as shown in Figures 8 and 9. Since temperature, stress, and deformation are obtained in steady-state and transitory conditions, it is possible to plot, in time, these results and, therefore, conditions for peak stress, yield conditions, etc. Figure 10 shows a typical example of maximum cavity stress plotted versus time for two design cases.

In a concluding note, it is also possible to simulate the presence of thermal loads, such as the previously mentioned “refractory brick” used for energy measurements in Europe. Figure 11 shows a typical output for this type of simulation.

Conclusion

The three approaches discussed all provide reliable and fast results, thus saving product development time and costs. They also offer the benefit of more knowledge and understanding of the product.

These approaches are applicable to companies of all sizes. It is not necessary to have in-house, expert engineers or a dedicated department that specializes in such work. However, in order for the organization to learn, it is necessary that the knowledge acquired is shared and that any work outsourced uses appropriate project coordination and hand-in-hand cooperation.

This article was submitted for publication in January 2009.

Bibliography

Incropera, FP and De Witt DP. Fundamentals of Heat and Mass Transfer. 3rd ed. New York: Wiley, 1990.

Ward, D. “Thermal Modelling of Oven Cooking and the ‘Brick’ Test.” APPLIANCE Magazine—European Edition, January 2003, 16–18.

Young, WC. Roark’s Formulas for Stress and Strain. 6th ed. New York: McGraw-Hill, 1989.

Figure 5. Comparison of experimental and simulated transitory

Figure 6. Thermal-fluidynamic simulation of cooking planes.

Figure 7. Coupled thermal-fluidynamic and structural simulations (transitory).

Figure 8. Stress contours during thermal transitory.

Figure 9. Deformation contours during thermal transitory.

Figure 10. Stress versus time for two different cases.

Figure 11. Thermal response of refractory brick.

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