US20140202444A1

US20140202444A1 - Vortex shedding heat transfer method and apparatus

Info

Publication number: US20140202444A1
Application number: US14/156,707
Authority: US
Inventors: Michael J. Dobie
Original assignee: Standex International Corp
Current assignee: Standex International Corp
Priority date: 2013-01-23
Filing date: 2014-01-16
Publication date: 2014-07-24

Abstract

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. A column is placed perpendicular to the airflow path to provide an obstruction to the airflow. The cross-section of the column can be round, square or another multi-sided design. The obstruction will affect the velocity and intensity at which the shedding occurs thus, causing motion of the jets to deliver a more uniform cook on the product surface.

Description

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).

FIELD OF THE INVENTION

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.

BACKGROUND OF THE INVENTION

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.

SUMMARY OF THE INVENTION

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.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an end view of a typical air impingement oven.

FIG. 2 is a cross-sectional view along section lines A-A as shown in FIG. 1.

FIG. 3 is a detailed cross-sectional view of the vortex shedding heat transfer apparatus in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a typical batch air impingement oven 8 is shown illustrating section line A-A. FIG. 2 shows a cross-sectional side of oven 8 along section A-A.
Referring to FIG. 2 and the detailed view shown in FIG. 3, invention 10 is depicted.
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 FIG. 1, the preferred dimensions are as follows. Plenum 12 is approximately 2.5 inches high, 16 inches wide and 16 inches long with a decreasing cross-sectional area. Jet orifices 22 in this embodiment are approximately ⅝ inch in diameter each with orifices 22 being twenty-three in number. Obstruction 16 is placed horizontally across plenum 12 to cause the creation of vortices 14 downstream. Obstruction 16 in this example is a ½ inch square tube across the full 16 inch plenum 12. These vortices 14 effect the direction of airflow 18 out of jet orifices 22.
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.

Claims

What is claimed is:

1. A heat transfer apparatus for providing uniform airflow to a product, said apparatus comprising:

a motor driven fan for providing a controllable airflow;

a plenum for receiving the airflow, said plenum having a horizontal width, an orifice surface that is essentially oriented parallel to the product;

a plurality of jets within the orifice surface of said plenum, wherein said plurality of jets direct the airflow to the product;

an obstruction perpendicular to the airflow in said plenum, wherein said obstruction has a predetermined cross-section shape, size corresponding to the geometry of said plenum and the output of airflow provided by the motor driven fan, wherein said obstruction extends across the width of said plenum, wherein said obstruction causes the airflow in the plenum to shed into alternating patterns thus causing the airflow exiting said jets to change direction due to vortex shedding taking place in said plenum.

2. The heat transfer apparatus of claim 1 wherein said cross-sectional shape is a shape selected from the group consisting of round, oval, or multi-sided.

3. The heat transfer apparatus of claim 1 wherein said obstruction is rectangular.

4. The heat transfer apparatus of claim 1 wherein said apparatus is part of an air impingement oven.

5. The heat transfer apparatus of claim 1 wherein said apparatus is part of a refrigeration apparatus for quick cooling of products.