Method of manufacturing a fin structure for heat exchanger

The present disclosure discloses a method of manufacturing a structure with a wall, comprising manufacturing a structure with a wall by using additive manufacturing technology, and dissolving a surface of the wall for reducing thickness of the wall.

BACKGROUND

This disclosure relates generally to manufacture process, and more particularly to a method of manufacturing a fin structure.

For manufacturing a fin structure, thin rolled sheets are usually used. Heat exchangers may utilize such structures and are key components in aircraft engine for managing temperatures of various fluids like lubrication oil, fuel, generator cooling fluid, air, and the like. Conventional heat exchangers are manufactured through a fabrication process where ultra-thin rolled sheets are used to make and stack the fin structures. Conventional manufacturing process by using rolled sheets allows 4-5 mils thick fins. These fin structures are then attached to the main heat exchanger fluid channels through brazing process.

Due to reliability issues with braze joints, manufacture of the traditional heat exchangers is transitioning to using additive manufacturing technology. Current additive manufacturing technologies have limitations on feature sizes that can be produced, for example, a limit on wall or fin structure size. This limit is a thickness of 15 mils. As fins contribute to major portion of heat exchanger weight, any increase or excessive fin thickness can severely impact the overall heat exchanger mass. Also, higher fin thickness results in higher pressure loss and reduced heat exchanger performance.

BRIEF DESCRIPTION

In one embodiment, the present disclosure provides a method of manufacturing a structure with a wall, comprising manufacturing a structure with a wall by using additive manufacturing technology, and dissolving a surface of the wall for reducing thickness of the wall.

In another embodiment, the present disclosure provides a method of manufacturing a fin structure with a wall, comprising manufacturing a fin structure with a wall by using additive manufacturing technology, and dissolving a surface of the wall of the fin structure for reducing thickness of the wall.

In another embodiment, the present disclosure provides a method of manufacturing a heat exchanger comprising a fin structure with a wall, comprising manufacturing the heat exchanger by using additive manufacturing technology, and dissolving a surface of the wall of the fin structure for reducing thickness of the wall.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the disclosure in unnecessary detail.

In the present disclosure, additive manufactured heat exchangers are used to replace conventional heat exchangers. However, the limitation of current additive manufacturing process is the minimum thickness of the fin structure. The minimum thickness that can be printed with current technology is more than 9 mils which is double the conventional fin thickness. As fins are major elements of heat exchanger this thickness limitation will substantially increase the overall heat exchanger weight. The present disclosure a method of manufacturing a fin structure or a heat exchanger, which can address the limitation of the thickness problem in additive manufacturing process.

Referring toFIG. 1andFIG. 2of the drawings, a fin structure10with walls12having upright walls121and a bottom wall122is shown. The fin structure10is made with additive manufacturing technology, and the walls are thicker than the desired thickness. Referring to the drawings ofFIG. 3andFIG. 4, a heat exchanger20with a fin structure10is shown. The heat exchanger comprises a core22having passages21thereinside with surrounding walls23for allowing the fluid passing through and a fin structure10for dissipating heat of the fluid passing through the core22. The heat exchanger20with core22is made with additive manufacturing technology. The additive manufacturing technology may be Electron beam melting (EBM), Selective Laser Melting (SLM), Direct metal laser melting, (DMLM), binder jet, Infrared melting technology or other any other suitable additive manufacturing technology.

Referring toFIG. 5of the drawings, the present disclosure discloses a method of manufacturing a structure with a wall, comprising101) manufacturing a structure with a wall by using additive manufacturing technology,102) dissolving a surface of the wall12for reducing thickness of the wall. In one embodiment, as shown inFIG. 1andFIG. 2, the structure may be a fin structure10for a heat exchanger20. In this embodiment, dissolving a surface of the wall comprises dissolving a surface of the wall of the fin structure. In another embodiment, as shown inFIG. 3andFIG. 4, the structure may be the heat exchanger20with a core22having passages21with surrounding walls23. In this embodiment, dissolving a surface of the wall comprises dissolving a surface of a surrounding wall of a passage in a core of the heat exchanger.

In one embodiment, the step of102) dissolving a surface of the wall for reducing the overall thickness of the wall comprises dissolving by103) conducting an etching process applied to the wall or the structure. Before the etching process, the part that does not need to be etched can be covered with a protective layer, such as the bottom wall122of the fin structure inFIG. 1andFIG. 2.

During the etching process, acidic and alkaline solutions are used to dissolve the surface of the wall or the structure made of metal and form appropriate salt. Then, the salt is removed by using running water or other solvents. Therefore, a wall with reduced uniform thickness is produced. For example, aluminum is the material of the wall or the structure, and aluminum can be easily dissolved by NaOH or KOH solutions. Implementing NaOH or KOH solutions for etching the aluminum wall or structure to reduce the thickness of the wall is cost effective and can be scaled to any requirements. In this embodiment, dry etching process or wet etching process may be used.

Hydrogen embrittlement may occur, when the hydrogen gas is evolved during the etching process. If hydrogen is not going to penetrate, the step of102) dissolving a surface of the wall for reducing the thickness of the wall further comprises104) baking the wall or the structure after step103) of conducting an etching process applied to the wall or the structure, so that the hydrogen gas can be removed that would prevent hydrogen embrittlement. The baking temperature may be from 200˜500° C., and the duration is 0-4 hours.

If hydrogen embrittlement occurs, the step of102) dissolving a surface of the wall for reducing the thickness of the wall further comprises105) neutralizing the surface of the wall or the structure with vinegar, acetic acid or other acids that would cause aluminum hydroxide formation by using alkaline material, and cleaning the products of neutralization by water or other solvents.

In another embodiment, the step of102) dissolving a surface of the wall for reducing the thickness of the wall comprises106) conducting a corrosion process to the wall or the structure.

During the corrosion process, acidic and alkaline solutions are used to dissolve the surface of the wall or the structure made of metal and form appropriate salt. For example, aluminum is the material of the wall or the structure, and NaOH or KOH solutions are used to corrode aluminum wall or structure to uniformly reduce the thickness of the wall, which is cost effective and can be scaled to any requirements.

During a corrosion process, the wall, on the order of nano-meters, gets corroded and forms an oxide layers on the surface of the metal wall. These oxide layers are then dissolved or washed away with some suitable solvents that would not form hydrogen gas and prevent embrittlement problem. Further, by baking process the hydrogen embrittlement can be prevented as discussed above.

In another embodiment, the step of102) dissolving a surface of the wall for reducing the overall thickness of the wall comprises107) conducting an electropolishing process to the wall or the structure.

Electropolishing with electrolyte solution can remove the surface of the wall or structure made of metal at a reasonable rate and result in formation of oxide layer that prevent hydrogen embrittlement. Salts formed by the electropolishing processes are washed by fluid circulation mechanisms. Continuous flow of etching solution washes away the formed salts from the critical location thus reducing the probability of forming thick salt deposits.

In one example, the electrolyte solution for electropolishing process may be a solution containing phosphoric acid 65-75 ml/L, sulfuric acid 5-10 ml/L, glycerin 1-15 ml/L, melamine 1-10 wt %, and hydrofluoric acid 1-5 ml/L. The parameters used in the electropolishing process are shown below. The temperature is 85° C., the voltage is 20V, the duration time is 5 minutes, the anode material is stainless steel, and the anode current is 25 A/dm2.

In the experiments, commercially available aluminum alloy is used for a fin structure with three upright walls121and one bottom wall122as shown inFIG. 1. The thickness of the upright walls121is 29 mils and the thickness of the bottom wall is 32 mils. Commercially available NaOH is used for the etching process.

Commercially available NaOH solution is used for the etching process. The etching process is uniform throughout the length of the walls. After 13 hours, the thickness of upright walls is from 6.5 to 7.5 mils.

The NaOH solution is diluted by half from the experiment 1. The etching process is uniform throughout the length of the walls. After 17 hours, the thickness of upright walls is from 2.5 to 3.5 mils.

The NaOH solution is diluted by half from the experiment 1. The etching process is uniform throughout the length of the walls. After 16 hours, the thickness of upright walls is from 9.5 to 11 mils.

Referring toFIGS. 6aand 6bof the drawings,FIG. 6ashows an additively printed straight fins with 0.010 inches thickness,FIG. 6bshowed the chemically etched straight fins with reduced fin thickness of 0.004 inches.

Referring toFIGS. 7aand 7bof the drawings,FIG. 7ashows an additively printed inclined fins with 0.015 inches thickness,FIG. 7bshowed the chemically etched inclined fins with reduced fin thickness of 0.004 inches.

Referring toFIGS. 8aand 8bof the drawings,FIG. 8ashows an additively printed circular core heat exchanger with wall thickness between the passages of 0.03 inches and the diameter of the circular passage21is 0.08 inches,FIG. 8bshowed the chemically etched circular core heat exchanger with reduced wall thickness of 0.02 inches, and the diameter of the circular passage21is 0.0085 inches after the etching process of etching the surrounding walls23of the passages21of the heat exchanger core22.