HIGH EFFICIENCY CASCADE-STYLE HEAT EXCHANGER

Abstract

A heat exchanger used for wind tunnel temperature control applications. The heat exchanger is a finned tube design with each tube aligned perpendicular to the wind tunnel flow direction but with the tube bundle aligned at an oblique angle to flow direction for increased surface area. Tube fins may be aligned with the bulk flow direction. The heat exchanger is built in modules with horizontal splitter plates separating each tube bundle. The benefit of this heat exchanger design for wind tunnel applications is the combination of low pressure loss and favorable heat transfer performance in a compact design, while maintaining flow quality consistent with stringent test requirements.

Description

This application claims benefit of and priority to U.S. Provisional Application No. 61/392,980, filed Oct. 14, 2010, by Eugene A. Tennyson, et al., and is entitled to that filing date for priority. The specification, figures and complete disclosure of U.S. Provisional Application No. 61/392,980 are incorporated herein by specific reference for all purposes.

FIELD OF INVENTION

This invention relates to a heat exchanger used for wind tunnel temperature control.

BACKGROUND OF THE INVENTION

Wind tunnels help engineers simulate the forces acting on an object moving through air. To obtain useful results, the conditions in the wind tunnel should closely match the conditions the object will encounter in actual service.

A fan generally drives the air flow in a wind tunnel to create the wind tunnel flow stream. All of the mechanical energy supplied to the fan propeller in a wind tunnel is converted into an increase in heat energy in the wind tunnel flow stream. Low powered wind tunnels may balance this heat gain through various heat losses such as surface cooling and ambient air exchange. High-powered wind tunnels must be cooled in order to maintain functional testing conditions.

Since the inception of heat exchangers in wind tunnels, “fin-tube” style or spiral wound, fluid-cooled radiator type heat exchangers have been used. Such heat exchangers transfer the heat energy from the flow stream to a coolant. The heated coolant is then pumped out of the heat exchanger, cooled by external means such as a cooling tower, and then recirculated to the heat exchanger. This “fin-tube” type of heat exchanger consists of coolant-carrying tubes that cross back and forth across the air flow passage. These tubes include attached fins that provide increased surface area for improved heat transfer between the flow stream and coolant.

Several problems exist with this type of heat exchanger for wind tunnel applications. For example a “fin-tube” type of heat exchanger presents a large resistance to the flow stream. The resistance increases the power needed to operate the wind tunnel at a specified wind velocity, which in turn increases the temperature of the flow even more. To reduce this resistance, many wind tunnels increase the size of the heat exchanger in the cross-flow stream direction. This, in turn, requires the expansion of the wind tunnel duct cross-section to house the larger heat exchanger. The transition from a smaller duct upstream of the heat exchanger to the larger duct required to house the heat exchanger may require the use of a wide angle diffuser, which significantly increases the risk of flow separation, turbulence, and angularity problems.

Still further, the flow around the cross-stream tubes in a “fin-tube” heat exchanger produces unsteady turbulent flow characteristics. This causes dynamic forces on the tube. These forces may induce tube vibration due to the low natural frequency of the slender, long span coolant tube. The cross-stream tubes also cause flow unsteadiness and increased turbulence in the flow steam. Unless the turbulence and flow unsteadiness is allowed to decay sufficiently, these effects may degrade the quality of the experimental results. Still further, because the fins in the heat exchanger are press fit onto the tubes, the unsteady flow and resulting vibrations over time can cause the fins to separate or lose their grip on the tube. This results in a degradation of the heat transfer effectiveness of the heat exchanger.

Some early closed circuit wind tunnels were originally built without a heat exchanger or other method to control air temperature. Retrofits to the wind tunnel often include heat exchanger cooling systems in order to meet the more stringent modern test conditions. In order to maintain the top speed requirements of these wind tunnels, the flow resistance of any installed heat exchanger needs to be minimized. Installation location and overall footprint are also limited in retrofits.

Accordingly, there is a need for a wind tunnel heat exchanger structure which minimizes flow resistance (or pressure loss) in the wind tunnel and provides adequate heat exchange in a compact area, while maintaining the flow quality at a level suitable for wind tunnel testing.

SUMMARY OF INVENTION

In various embodiments, the present invention comprises a high-efficiency, cascade-style heat exchanger. The heat exchanger provides for a more compact and efficient heat exchange capability, minimizes the flow stream blockage or resistance resulting from the heat exchanger, and provides for high aerodynamic quality of the airstream exiting the heat exchanger.

In one exemplary embodiment, the heat exchanger has finned tubes grouped in tube bundles, each bundle comprising a plurality of tubes. Each tube is aligned perpendicularly (i.e., across the wind tunnel) to the wind tunnel flow direction but with the tube bundle aligned at an oblique angle to the flow direction for increased surface area. In the embodiment shown, tube fins are aligned with the bulk flow direction for, minimum airside pressure loss.

In one embodiment, the heat exchanger may be built in modules with horizontal splitter plates separating each tube bundle. The splitter plates serve as additional flow conditioning control to reduce both flow angularity and flow non-uniformity within the wind tunnel airstream, consistent with the high level of aerodynamic flow quality required for wind tunnel testing. The tube bundles are angled relative to the bulk flow stream direction for improved heat transfer with greater surface area within a compact location. The angle of the tubes to the bulk flow stream, the number of modules, the height, width and length of the heat exchanger, and the number of tubes in a bundle may vary. In one embodiment, the tube bundles are set at a 30-degree angle to the air flow, with six total modules.

The tubes are hollow and capable of transporting a coolant or fluid. In one embodiment, the tubes are arranged in a cross-flow configuration (i.e., perpendicular) to the air flow. A fluid inlet may be located at one end or both ends of a tube bundle, with corresponding fluid outlets or drains also located at one end or both ends of the bundle.

Tubes and tube bundles are captured in a framework to create modules. The face area of these modules are angled with regard to the direction of air flow such that the face area of the overall exposed tube surface is larger than it would be for a planer face area. As a cold heat transfer fluid travels inside the tubes, fins attached to the tubes are cooled by conduction.

Multiple fins are attached to the tubes by mechanical extrusion (or separately formed and attached) and can be arranged with the surface area aligned with or perpendicular to the air flow. The heat transfer from the tubes to the fins cools the fins, which in turn provides convective cooling to the airflow passing over the fins.

The heat exchanger modules are installed within a structural support frame. This frame maintains the plane of the tube face area in the angled position thus increasing the overall face area of the heat exchanger, with respect to the flow direction of the air stream. Structural horizontal splitter plates are aligned parallel to the airflow between the heat exchanger tube bundles. The splitter plates create an aerodynamic boundary, thereby controlling the air flow path into the sloped heat exchanger face.

The air flow will follow the path of least resistance and assume a pitch angle (i.e. non-horizontal flow direction) as it travels through the tube module. The pitch angle could be upward or downward depending on (a) whether the leading edge of the modules is angled so as to be above or below the direction of the incoming air flow and (b) the geometric configuration of the tubes within each module. As the airflow exits the heat exchanger tubes, the aft end of the horizontal splitter plates located above and below each module encourages the uniform redistribution of the airflow and a return to the incoming horizontal air flow direction. The wakes of the thin splitter plates decay naturally downstream, returning the airflow to a uniform distribution across the duct of the wind tunnel near the exit of the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the fin-tube heat exchanger with six modules of tube bundles, with flow direction as shown.

FIG. 2 is an end elevation view of the heat exchanger assembly, showing slight upward flow angularity on the downstream side as the flow passes through the heat exchanger.

FIG. 3 is an enlarged end view of a tube bundle module with tubes ending before reaching the lower splitter plate.

FIG. 4 is an isometric view of a single module with support brackets, and tube bundles set at a 30-degree angle to the incoming flow.

FIG. 5 is an isometric view of a heat exchanger module showing a cut-away view of tubes with intermediate supports.

FIG. 6 is an isometric view of the fin-tube heat exchanger with six modules of tube bundles installed within the wind tunnel walls, with flow direction as shown.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention comprises a high-efficiency, cascade-style heat exchanger. The heat exchanger provides for a more compact and efficient heat exchange capability, minimizes the flow stream blockage or resistance resulting from the heat exchanger, and provides for high aerodynamic quality of the airstream exiting the heat exchanger.

In one exemplary embodiment, as shown in FIGS. 1-3, a heat exchanger 4 has tubes 14 grouped in tube bundles 12, each bundle comprising a plurality of tubes. Each tube is aligned perpendicularly (i.e., across the wind tunnel) to the wind tunnel flow direction 2 but with the tube bundle 12 aligned at an oblique angle to the flow direction for increased surface area (in the embodiment shown, a forward cant angle). Tubes have or are attached to fins 16. In the embodiment shown, tube fins 16 are aligned with the bulk flow direction 2, consistent with minimum airside pressure losses.

In one embodiment, the heat exchanger may be built in modules 20 with horizontal splitter plates 22 separating each tube bundle. The splitter plates 22 serve as additional flow conditioning control to reduce flow angularity. In the embodiment shown in FIGS. 1 and 2, six modules 20 are stacked in a frame to form the heat exchanger.

The tube bundles 12 are angled relative to the bulk flow stream direction 2 for improved heat transfer with greater surface area within a compact location. The angle of the tubes to the bulk flow stream, the number of modules, the height, width and length of the heat exchanger, and the number of tubes in a bundle may vary. In the embodiment shown in FIGS. 1-3, the tube bundles are set at a 30-degree angle to the air flow, with six total modules.

The tubes 14 are hollow and capable of transporting a coolant or fluid. In one embodiment, the tubes are arranged in a cross-flow configuration (i.e., perpendicular) to the air flow. A fluid inlet 24 may be located at one end or both ends of a tube bundle, with corresponding fluid outlets or drains 26 also located at one end or both ends of the bundle.

Tubes and tube bundles are captured in a framework to create modules. The face area of these modules are angled with regard to the direction of air flow such that the face area of the overall exposed tube surface is larger than it would be for a planer face area. As a cold heat transfer fluid travels inside the tubes, fins 16 attached to the tubes are cooled by conduction.

Multiple fins are attached to the tubes by mechanical extrusion (or separately formed and attached) and can be arranged with the surface area aligned with or perpendicular to the air flow. Alignment with air flow provides less impedance to air flow. The heat transfer from the tubes to the fins cools the fins, which in turn provides convective cooling to the airflow passing over the fins.

The heat exchanger modules are installed within a structural support frame 30. This frame maintains the plane of the tube face area in the angled position thus increasing the overall face area of the heat exchanger, with respect to the flow direction of the air stream. Structural horizontal splitter plates 22 are aligned parallel to the airflow between the heat exchanger tube bundles. The splitter plates create an aerodynamic boundary, thereby controlling the air flow path into the sloped heat exchanger face.

The air flow will follow the path of least resistance and assume a pitch angle (i.e., non-horizontal flow direction) as it travels through the tube module. The pitch angle could be upward or downward depending on (a) whether the leading edge of the modules is angled so as to be above or below the direction of the incoming air flow and (b) the geometric configuration of the tubes within each module. As the airflow exits the heat exchanger tubes, the aft end of the horizontal splitter plates located above and below each module encourages the uniform redistribution of the airflow and a return to the incoming horizontal air flow direction. The wakes of the thin splitter plates decay naturally downstream, returning the airflow to a uniform distribution across the duct of the wind tunnel near the exit of the heat exchanger.

Thus, it should be understood that the embodiments and examples described herein have been chosen and described in order to best illustrate the principles of the invention and its practical applications to thereby enable one of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as are suited for particular uses contemplated. Even though specific embodiments of this invention have been described, they are not to be taken as exhaustive. There are several variations that will be apparent to those skilled in the art.

Claims

1. A heat exchanger for use in a windtunnel with a direction of bulk air flow, comprising:

a frame for mounting one or more tube modules;

one or more tube modules, each tube module comprising a tube bundle with a front face and one or more fins, each tube bundle comprising a plurality of parallel hollow tubes aligned across the direction of bulk air flow, wherein a coolant or fluid flows through the hollow tubes, and further wherein said fins are attached to the tubes;

wherein the front face of each tube module is angled with respect to the direction of bulk air flow.

2. The heat exchanger of claim 1, wherein the front face of each tube module is angled at an oblique angle with respect to the direction of bulk air flow.

3. The heat exchanger of claim 2, wherein the oblique angle is 30 degrees.

4. The heat exchanger of claim 1, wherein multiple modules are stacked vertically in the frame.

5. The heat exchanger of claim 1, wherein six tube modules are stacked vertically in the frame.

6. The heat exchanger of claim 4, further comprising a splitter plate located between adjacent tube modules, each said splitter plate aligned parallel to the direction of bulk air flow.

7. The heat exchanger of claim 1, each tube module further comprising one or more fluid inlets and one or more fluid outlets.

8. The heat exchanger of claim 1, wherein heat transfer from said tubes to said fins cools the fins, which provides convective cooling to the air flow passing over the fins.

9. The heat exchanger of claim 1, wherein the fins are aligned with the direction of bulk air flow.