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issue: November 2006 APPLIANCE Magazine European Edition

Convection Ovens
Design Procedure for Air Distribution in Convection Ovens


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by Levent Akdag, R&D group leader, Haluk Karatas, product development engineer, and Talip Çaglar, product development engineer, Arçelik AS

This paper summarizes a study for the design of an air distribution system for a forced-air convection oven. Advantages of using advanced design tools in an iterative design process are discussed.

Energy consumption and cooking quality are two important performance measures for electrical domestic ovens. Throughout the last few years, forced convection ovens have gained popularity in the market because of their cooking performance. Employing forced air as the heat-carrying medium, convection ovens can achieve shorter cooking times, which yields to less energy consumption compared to conventional-type ovens. Since the major heat transfer mechanism is forced convection, air distribution inside the oven cavity becomes the driving force of cooking
and strongly affects the performance, especially when cooking several items at once.
The air distribution system in a forced convection oven usually comprises an impeller (fan) and a cover plate (fan cover) to keep the rotating parts out of reach of the users. A fan cover may also include air suction and blowout ports; therefore, it also acts as an air guide. The heater element is usually located in the back of the fan cover, around the fan. Typically, air is sucked from the cooking cavity through the sucking ports by the impeller, and the air is directed towards the heater, where the air temperature is increased. From there, the heated air is distributed into the cooking cavity through blowout ports. In this configuration, the cover plate is responsible for most of the air management, as it has sucking and blowout ports.
This paper discusses a study of an iterative design procedure for the cover plate and describes a sample application. 

Figure 1. Iterative Design Procedure Flow Chart

Design Procedure

As shown in Figure 1, the design process has a multi-step structure, which includes conceptual design, detailed design, and prototyping and testing. Iteration is the main characteristic of a typical design study. To reach design targets, many modifications and redesign activities should be followed.
In this study, an iterative design procedure with the following steps was proposed and followed in order to obtain even air distribution, which can be helpful for even cooking performance.

Conceptual & Detailed Design
The first step in the design study was forming alternative concepts. The concepts were defined by considering the needs and constructional restraints of the product. Reviewing and stating pros and cons of the present design were also helpful. 

Figure 2. Desired Airflow Inside Cavity

The different concepts were then compared and ranked. The one at the top of the ranking was determined to be the preferred concept that would be used in the design steps. By the end of the first step, the design team reached a conceptual design with the desired flow distribution scheme. Figure 2 shows a sample design flow distribution scheme.
Next, the detailed design of the parts was studied. This step included reviewing both system and constructional design. In system design, starting with the primitive shapes, the team focused on the functions or the utilities of the part. A concept design was then formed for a fan cover plate in a forced convection oven. The design team defined three design zones for the cover plate—suction zone, blow-out zone and air-handling zone. A sample design comprising three functional zones is illustrated in Figure 3.

Figure 3. Sample Cover Plate Concept Design

Numerical Analysis
The next step was virtually testing the built-up design geometry. Producing prototypes is an expensive and time-consuming process; therefore, a numerical modeling and analysis study was conducted to investigate the airflow inside the oven cavity. A commercial computational fluid dynamics (CFD) analysis software was used in this application. In order to make a CFD analysis, the cooking cavity, the impeller, the cover plate, the food trays, and the heater were all embedded in the model (see Figure 4).

Figure 4. Model Geometry

During CFD analysis, the researchers used geometrical models of the problems being investigated. In most of the methods used in CFD, the “physical domain” is transferred/mapped to the “computational domain” by dividing the model into a finite number of subdomains (also called “elements,” “volumes” or “cells”). This dividing process is called “meshing,” and the obtained result where the computation is processed is called the “mesh” or “grid.” It should be noted that the quality of the mesh and the number of the elements are critical for CFD analysis. A low-quality mesh may cause the analysis to diverge with no solution, while a high number of elements require large computation power and long solution time.

Figure 5. Air Velocity Distribution on Tray Surfaces

The meshed geometry was then transferred to the CFD software and necessary boundary conditions were applied just before the solution process. Both the flow and the energy behavior of the system were modeled. When the analysis converged to a solution, detailed post-processing and investigation of airflow characteristics were conducted. Air velocity distribution on the food tray surfaces was plotted for each tray for both upper and lower surfaces. As shown in Figure 5, some challenges were discovered, but were fixed by modifications.
The airflow path in the backside of the cover plate was also plotted to determine whether the design was functioning properly. Using these results, necessary modifications were made on the cover plate design (see Figure 6).

Figure 6. Flow in the Backside of Fan Cover

Rapid Prototyping & Flow Measurements
When the design study reached a mature state, a cover plate prototype was built to validate the CFD results and obtain other information. Employing rapid prototyping techniques, such as stereolithography (SLA), plastic prototype parts were produced (see Figure 7).

Figure 7. Sample Plastic Prototype

Flow measurements were conducted using laser doppler anemometry (LDA) and particle image velocimetry (PIV), as these methods give quick and accurate results. Both methods are based on introducing small seeding particles into the flow field and visualizing them with lasers.

Figure 8. LDA Experimental Setup

Figure 8 shows the experimental setup for the LDA measurements. The LDA method utilizes two laser beams to measure the velocity at a single point, while in PIV method, a camera is used in order to take consequent pictures of flow field. The pictures were processed to obtain the velocity distribution of the measured plane.
As revealed in Figure 9, the results of the CFD analysis and the flow measurement are congruent.

Figure 9. Comparison of CFD and Flow Measurement results

Metal Prototyping & Experiments

After validating the flow characteristics of the design, a metal cover plate prototype and a test oven were built to perform high-temperature tests. Three types of experiments were performed:
• Tests for obtaining the temperature distribution in the cavity.
• Energy consumption tests to obtain the energy level according to related conditions.
• Cooking tests to measure the actual performance in terms of cooking quality and time.

Figure 10. Temperature Distribution with Old (Upper) and New (Lower) Cover Plates

Experimental setup for obtaining temperature distribution involved a grid with several thermocouples. Energy consumption tests were performed according to EN 50304 standard conditions, which consist of electric oven energy consumption measurement procedures. A specific brick (hipor) was used in the energy tests with standard dimensions and density.
After all analysis and experimental studies, homogeneous air distribution via the new plate was validated through cooking tests. Two kinds of baking foods were used in cooking tests—pastry and a small cake.
The pastry item contained six phyllo (filo) layers. Each phyllo is spread on a cooking tray from end to end and covered with a sauce containing yogurt, egg and cheese. The baking of  multiple-layered, golden brown pastry is a crucial test to show whether or not the design’s homogenous air distribution was improved. Therefore, simultaneous tests were performed with two similar ovens, one with the old cover plate and one with the new cover plate. Each oven contained three pastry trays during cooking tests. All three trays were placed at the same level within the oven cavity.

Table 1.

The second test food was a small cake. Small cake tests demonstrated cooking performance of the new cover plate on discrete foods such as cookies. In the small cake tests, 20 cakes with the same properties were positioned on one cooking tray, and the trays were placed at the same levels in each oven cavity. The tests were again conducted simultaneously.

Experimental Results

The temperature distribution inside the oven cavity is plotted for old and new cover plates, as shown in Figure 10. A more homogenous distribution is achieved by the new design. The oven energy consumption test values with old and new cover plates are given in Table 1.
As Table 1 shows, the new cover plate reduced both the time and the energy consumption values. Considering the values obtained, the usage of the new cover plate improved the energy level of the oven.
The results of cooking tests are given in Figure 11. The cooking time for the new cover plate is 5 minutes shorter than the old cover plate (45 minutes compared to
50 minutes). As shown in Figure 11, the pastries baked with the new cover plate had more golden brown zones, showing a more even cooking on all three trays. Pastries baked with the old cover plate have both dark brown zones and light-brown zones.
The results of the small cake tests are given in Figure 12. In this test, a shorter cooking time was obtained with the new cover plate (22 minutes compared to 25 minutes). The appearance of each cake was compared using color analysis software. In this study, cake colors were scaled from 0 (i.e., black, dark brown) to 255 (i.e., white, uncooked). The mean value of cakes with the old plate design was 150, while the mean value of the cakes with the new plate was 123, which shows a better cooking level.

Figure 11. Pastry Appearances

Conclusion

A new fan cover plate design has been obtained using a conceptual design procedure. The detailed design method involved virtual testing by using numerical analysis tools. Using the results, proper modifications were made on the design geometry. Proper modifications were also made in parallel with the iterative characteristic of the design process at each design step. Flow and temperature measurements were performed for further testing of the design, followed by energy and cooking performance tests. The energy consumption and cooking test results agreed with numerical analysis studies and design targets were reached. 

Figure 12. Results of Cake Color Comparisons

The cover plate obtained with the iterative design procedure provided a homogenous and even air distribution system for a forced convection oven. The design process, using advanced design tools, reduced necessary time and expenses to reach finalized fan cover design.
Using the concepts and design information of this study, a new fan cover design is being developed and applied to a new double-cavity oven. This oven consists of two cavities with different cooking methods—microwave and forced convection (“turbo”) cooking. The new cover plate design will be applied to the “turbo” cooking zone. There are also pending patent applications related to this study.

About the Authors

Levent Akdag , Ph.D., is a mechanical engineer and works as Group Leader in the R&D Center of Arçelik AS in Istanbul, Turkey. Talip Çaglar and Haluk Karatas are working as product development engineers in the Arçelik AS Cooking Appliance Plant in Bolu, Turkey. If you wish to contact the authors, e-mail: editor@appliance.com

 

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