This invention relates to apparatus that have heat transfer requirements for air impingement ovens, in particular, impingement ovens wherein the product remains stationary relative to the oven.
Typically, batch impingement ovens provide air directly from above and below the product that is to be cooked or heated. Batch impingement ovens direct columns of air directly at the product from above and below. In practice, most items, heated or cooked, are supported by a pan or some other means of support such as packaging material or cooking trays. The columns or jets of air that strike the cooking support from below impart heat to the support, which, in turn, delivers heat to the product via conduction through the support.
Good batch oven design includes mechanisms for moving the top air jets in relation to the product. This motion is necessary to prevent localized overheating and browning of the product surface. The motion evens out the heat transfer, resulting in even cooking and browning of the products. Motion mechanism may move the jets themselves, while the product remains stationary, or as in alternative technologies, the product, itself, moves via a moving product support.
Conveyor ovens solve this problem by moving the product under the jets of air. Other ovens solve the problem by moving the jets or changing the direction of the airflow out of the jet orifices. Ovens that change the jet direction have used an air vane that is mounted on a motor driven shaft.
All of the above solutions are mechanisms that have the complication of moving parts within a heated environment. These mechanisms are subject to maintenance issues due to the heat and can lead to mechanical breakdowns and failure of electrical components due to the hostile environment.
Vortex shedding is a well-known fluid dynamics principle that occurs when fluid flows past a blunt object, resulting in vortices that alternately shed and detach from the blunt object. The fluid then flows in alternating wave-like patterns due to the low-pressure vortices behind the blunt object.
The inventor has discovered new and unexpected uses for this vortex shedding oscillating flow principle for use in impingement oven designs. Vortex shedding creates a new, superior method of imparting motion to heated air jets resulting in improved evenness of cooking and heating, with minimal mechanisms having to be located in hostile environments.
While the primary use of the vortex shedding principles are directed toward batch air impingement ovens, the invention can also be used with other ovens and for quick cooling devices such as beverage coolers, ice makers, and the like.
There is not found in the prior art the use of vortex shedding principles for batch impingement ovens or quick cooling apparatus.
It is an aspect of the invention to provide a heating transfer method and apparatus that has a fixed physical obstruction placed in the air path causing the airflow to cause the airflow to shed into alternating patterns.
It is another aspect of the invention to provide a heating transfer method and apparatus that has a column running perpendicular to the airflow path to provide an obstruction to the airflow.
Another aspect of the invention is to provide a heating transfer method and apparatus wherein the cross-section of the column can be round, square or another multi-sided design.
It is still another aspect of the invention to provide a heating transfer method and apparatus where the obstruction will affect the velocity and intensity at which the shedding occurs.
Finally, it is an aspect of the invention to provide a heating transfer method and apparatus such that once the cross-sectional shape and area of the obstruction are optimized for the oven air duct geometry, air velocity and fluid density, an alternating wave in the duct will cause motion of the jets to deliver a more uniform cook on the product surface.
Referring now to
Referring to
As shown, obstruction 16 is placed within plenum 12 causing airflow 18 to shed into alternating patterns 14 as shown. Obstruction 16 preferably has a rectangular cross-sectional shape. However, other cross-sectional shapes could also be used such as round, oval, or even multi-sided. Obstruction 16 is a column that extends across the entire plenum 12 perpendicular to airflow 18. The cross-sectional shape and size of obstruction 16 will affect the velocity and intensity at which the shedding of airflow 18 occurs. When the cross-sectional shape and area of obstruction 16 are optimized for a particular oven plenum 12 geometry, an alternating wave 14 in plenum 12 will cause the airflow exiting jets 22 to deliver a more uniform heating to product 26 supported by pan 24.
Airflow 18 is provided by motor driven fan 20. Airflow from jets 22 is of an intensity that could cause an increased heat transfer coefficient directly below each jet 22. This localized higher transfer rate is usually undesirable if this localized spot remains in the same place. Invention 10 using vortex shedding principles solves this problem. The direction of the air exiting jets 22 changes due to vortex shedding taking place in the air plenum delivery duct 12.
When optimized, jets 22 deliver essentially a perpendicular airflow to the surface of product 26, which is supported by pan 24. This a major improvement as this method of vortex shedding eliminates the problem of a localized heat transfer coefficient without the need for mechanically changing the position of jets 22 or changing the direction of airflow 18 as it exits jets 22 by using air vanes or other methods. Invention 10 accomplishes this task without the need for moving parts.
For a typical oven 8 as shown in
Fan 20 is delivering approximately 120 cfm into plenum 12; airflow 18 crosses obstruction 16 travelling at approximately 500 fpm to create the vortices 14. The air vortices 14 then exits orifices 22 at approximately 3000 fpm. The location of obstruction 16 was determined experimentally by observing the direction of the airstreams from jets 22 while moving obstruction 16 in plenum 12. The location selected for obstruction 16 is where the maximum movement of the airstreams was observed.
Although the present invention has been described with reference to certain preferred embodiments thereof, other versions are readily apparent to those of ordinary skill in the preferred embodiments contained herein.
This application claims benefit of U.S. Provisional Application Ser. No. 61/755,573 filed on Jan. 23, 2013, pursuant to 35 USC §119 (e).
Number | Date | Country | |
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61755573 | Jan 2013 | US |