Orifice boundary layer suction method and system
An air-conveying device for use in an HVAC system having an interior surface and an exterior surface. An air-moving device is arranged and disposed to move air through the air-conveying device adjacent to the interior surface of the air-conveying device. The air-conveying device conveys air having passed through an HVAC heat exchanger. The air-conveying device includes one or more openings disposed and arranged to provide a pressure differential sufficient to cause passage of air through the openings from an area adjacent to the interior surface to an area adjacent to the exterior surface in order to decrease aerodynamic drag.
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The present invention is related to devices for conveying air. In particular, the present invention is directed to a diffuser and/or orifice for use in HVAC heat exchanger units.
BACKGROUND OF THE INVENTIONHVAC systems will typically include a heat exchanger unit having a fan arranged to pass air over a heat exchanger. As air is discharged from the unit, the air typically passes through a diffuser/orifice. The diffuser/orifice is a device that permits the passage of air out of a heat exchanger. Diffuser/orifices are typically fabricated with a geometry, which results in less backpressure and an increased airflow. One drawback of the diffuser/orifice is that the flow at or near the diffuser/orifice surface experiences undesirable flow characteristics. In particular, the fluid near the surface of the diffuser/orifice experiences boundary layer formation. Boundary layer separation may also occur further increasing the thickness of the boundary layer region.
Air passing over flat surfaces may flow in a laminar flow profile. Laminar flow profiles experience low drag and results in larger, desirable flow rates. A boundary layer forms as a result of friction between the air and the surface. The boundary layer thickness is defined as the locus of points where the velocity parallel to the flow surface reaches 99% of the mean stream velocity. Therefore, the thicker a boundary layer is, the less area there is available for flow at maximum velocity when passing through the orifice. Separation of the boundary layer from the surface, called boundary layer separation, causes recirculation and/or turbulent flow. Boundary layer separation is a particular problem in applications where the flow of a fluid diverges. In diffusers, boundary layer separation may occur as air passes out of the unit and experiences an air pressure differential. Diverging flow typically occurs when the flow of air out of the heat exchanger unit is diffused through a diffuser/orifice having a flared outlet. The diverging flow provides a reduction in air pressure, which also reduces backpressure against the fan, but may increase the amount of undesirable turbulent flow and susceptibility to boundary layer separation. Air traveling out of a heat exchanger unit through a diffuser/orifice may experience an adverse pressure gradient along the surface of the orifice. The result is that the boundary layer breaks away or separates from the orifice surface forming a broad pulsating wake.
Another type of known diffuser/orifice device is a cylindrical orifice, which conveys air from spaces within heat exchanger units to outside of the heat exchanger units. In diffuser/orifice devices having a substantially cylindrical geometry, the air passing through the cylinder has a relatively large boundary layer. The large boundary layer is due to the friction between the air and the surface of the cylinder. The shape of the entrance to the cylinder plays a large role in determining the eventual thickness of the boundary layer. A smooth and curved orifice entrance will result in less resistance to fluid flow through the orifice. Sharp edges at the orifice entrance result in increased resistance to flow. This resistance is due to the formation of a large boundary layer region that forms just past the orifice entrance. The large boundary layer is susceptible to boundary layer separation and/or turbulent flow, particularly at larger flow rates. In addition, the cylindrical geometry results in a backpressure against the fan that decreases the quantity of air flowing through the diffuser orifice.
As the boundary layer is drawn away from the surface of the diffuser/orifice, the flow loses at least a part of the laminar flow profile and becomes more turbulent. Turbulent flow has increased drag at the surface, has a lower airflow rate and increases backpressure against the fan. The turbulent flow characteristics of the boundary layer are undesirable for heat exchanger unit applications because a significant amount of energy present in the fluid is lost to aerodynamic drag and recirculation of the turbulent flow. The fan blades may extend at least a portion of the way into the orifice. This extension places the tips of the fan blades within this turbulent region rendering this portion of the blade less efficient and may result in increased fan noise. As a result, the fan requires a greater amount of energy to move the air through the diffuser/orifice.
What is needed is a system and method for decreasing the amount of boundary layer separation and/or turbulent flow occurring in orifice/diffusers in order to more efficiently move air out of a heat exchanger unit.
SUMMARY OF THE INVENTIONThe invention includes an air-conveying device for use in an HVAC system having an interior surface and an exterior surface. An air-moving device is arranged and disposed to move air through the air-conveying device adjacent to the interior surface of the air-conveying device. The air-conveying device conveys air having passed through an HVAC heat exchanger. The air-conveying device includes one or more openings disposed and arranged to provide a pressure differential sufficient to cause passage of air through the openings from an area adjacent to the interior surface to an area adjacent to the exterior surface in order to decrease aerodynamic drag.
Another embodiment of the invention includes a method for reducing aerodynamic drag in an air-conveying device. The method includes providing an air-conveying device having an interior and exterior surface. A flow of air is provided with an air-moving device from a heat exchanger through the air-conveying device and along the interior surface. Flow of a portion of air through the air-conveying device is permitted from an area of higher pressure air adjacent to the interior surface to an area of lower pressure air adjacent to the exterior surface through openings in the air-conveying device to reduce aerodynamic drag.
An advantage of the present invention is that the air flowing through the diffuser/orifice experiences reduced boundary layer separation because the boundary layer is drawn closer to the surface of the diffuser/orifice. The reduction in boundary layer permits the air to flow through the diffuser/orifice in a substantially laminar flow profile, reducing the backpressure against the fan and decreasing the power required by the fan to exhaust the air out of the heat exchanger unit.
Another advantage of the present invention is that the fan capacity in a heat exchanger unit may be decreased without decreasing the total amount of airflow through the system.
Another advantage of the present invention is that cylindrical diffuser/orifices may be used with substantially the same fan power requirements as diffuser/orifices having a flared geometry, and without the expensive manufacturing costs associated with flared diverging outlet diffuser/orifices.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
DETAILED DESCRIPTION OF THE INVENTION
Although
Although
While the invention has been described with respect to a diffuser/orifice, any surface that experiences boundary layer separation in an HVAC system may use the system and method of the present invention, such as centrifugal blower housings. In particular, on the exiting side of the centrifugal blower housing, the decrease in boundary layer thickness may provide an increase in the effective flow area.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
Claims
1. An air-conveying device for use in an HVAC system comprising:
- a passageway having an interior surface and an exterior surface opposite the interior surface;
- the passageway being arranged and disposed to permit flow of air through the passageway adjacent to the interior surface upon activation of an air-moving device;
- a plurality of openings in the passageway extending from the interior surface to the exterior surface, the plurality of openings being disposed and arranged to permit flow of air through the plurality of openings from an area adjacent to the interior surface to an area adjacent to the exterior surface in order to decrease aerodynamic drag in the passageway; and
- wherein the flow of air through the plurality of openings occurs in response to a pressure difference between the interior surface and the exterior surface.
2. The air-conveying device of claim 1, wherein the flow of air through the openings reduces an amount of power required by an air-moving device to move air through the passageway.
3. The air-conveying device of claim 1, wherein the plurality of openings each have a substantially rectangular geometry.
4. The air-conveying device of claim 1, wherein the plurality of openings each have a substantially circular geometry.
5. The air-conveying device of claim 1, wherein the passageway is a diffuser for a heat exchanger unit in an HVAC system.
6. The air-conveying device of claim 5, wherein the passageway has a substantially frustoconical geometry.
7. The air-conveying device of claim 1, wherein the passageway is an orifice for a heat exchanger unit in an HVAC system.
8. The air-conveying device of claim 7, wherein the orifice has a substantially cylindrical geometry.
9. A method for reducing aerodynamic drag in an air-conveying device comprising:
- providing an air-conveying device having an interior surface, and an exterior surface and openings extending between the interior surface and the exterior surface;
- flowing air with an air-moving device through the air-conveying device; and
- diverting a portion of airflow through the air-conveying device from an area of higher-pressure air adjacent to the interior surface to an area of lower pressure air adjacent to the exterior surface through the openings in the air-conveying device to increase the laminar flow of the air in the air-conveying device.
10. The method of claim 9, wherein the diverting of air through the openings reduces the amount of power required by the air-moving device to move air through the air-conveying device.
11. The method of claim 9, wherein the step of providing an air-conveying device includes locating the openings around the perimeter of the air-conveying device.
12. The method of claim 9, wherein the step of providing an air-conveying device includes locating the openings adjacent to an exit of the air-conveying device.
13. The method of claim 9, wherein the air-conveying device is a diffuser for a heat exchanger unit in an HVAC system.
14. The method of claim 13, wherein the diffuser has a substantially frustoconical geometry.
15. The method of claim 9, wherein the air-conveying device is an orifice for a heat exchanger unit in an HVAC system.
16. The method of claim 15, wherein the orifice has a substantially cylindrical geometry.
17. A heat exchanger unit for an HVAC system comprising:
- a housing having an interior space;
- a heat exchanger coil disposed in the housing;
- an air-moving device disposed in the housing to move air through the heat exchanger coil and the housing; and
- an air-conveying device being arranged and disposed to permit flow of air from the interior space of the housing upon operation of the air-moving device, the air-conveying device comprising: an interior surface and an exterior surface opposite the interior surface; a plurality of openings extending from the interior surface to the exterior surface, the plurality of openings being disposed and arranged to permit flow of air through the plurality of openings from an area adjacent to the interior surface to an area adjacent to the exterior surface in order to decrease aerodynamic drag in the air-conveying device; and wherein the flow of air through the plurality of openings occurs in response to a pressure difference between the interior surface and the exterior surface.
18. The heat exchanger unit of claim 17, wherein the flow of air through the openings reduces an amount of power required by the air-moving device.
19. The heat exchanger unit of claim 17, wherein the plurality of openings each have a substantially rectangular geometry.
20. The heat exchanger unit of claim 17, wherein the plurality of openings each have a substantially circular geometry.
21. The heat exchanger unit of claim 17, wherein the air-conveying device is a diffuser.
22. The heat exchanger unit of claim 21, wherein the air-conveying device has a substantially frustoconical geometry.
23. The heat exchanger unit of claim 17, wherein the air-conveying device is an orifice.
24. The air-conveying device of claim 23, wherein the orifice has a substantially cylindrical geometry.
Type: Application
Filed: Jul 13, 2005
Publication Date: Jan 18, 2007
Applicant: YORK INTERNATIONAL CORPORATION (York, PA)
Inventors: John Knight (Moore, OK), Anthony Landers (Yukon, OK), Stephen Pickle (Norman, OK)
Application Number: 11/180,763
International Classification: F24F 13/06 (20060101);