KW Micro Power creates lightweight multifunctional microturbine housing with embedded cooling channels

KW Micro Power redesigned the housing of their aerospace-grade, high-power-density, compact turbogenerator for metal Additive Manufacturing. Using nTop, they reduced its weight by 44% for a total savings of 4.5 kg.

They achieved this by applying a shell with variable thickness to their generator’s housing, removing material from areas not bearing significant loads. This operation was performed in nTop in less than 100 milliseconds.

Redesigning the part for Additive Manufacturing opened new opportunities for KW Micro Power. The team converted the empty shell into a conformal cooling channel to improve the thermal management of their high power density generator.

The result was a multifunctional, lightweight part with conformal channels that reduced the maximum operating temperature by 33%, improved the system’s efficiency and significantly extended its machine life.

Technical takeaways

• Weight reduction: Leverage variable shelling to remove material from non-load-bearing areas of the part for significant lightweighting gains.
• Conformal cooling: Smoothen conformal cooling channels to maximize flow and create self-supporting internal features.
• Multi-objective optimization: Use fuel as the heat transfer medium to further improve efficiency and take advantage of available resources.

Business value

• Innovation for SMBs: Develop new ways to design new products to gain a competitive advantage and stay at the forefront of your industry.
• Open new markets: Modify your existing product line to serve the needs of different industries or use cases.
• Highest-performing products: Take advantage of cutting-edge engineering design tools to provide more value to your customers.

Key statistics

• Lightweighting: 44% weight reduction
• Maximum temperature: 33% temperature reduction
• Time to DfAM redesign: Less than 1 day
• Function consolidation: 3 functions in a single part
• Manufacturing process: Metal LPBF by VELO3D
• Material: Aluminum F357

Introduction

KW Micro Power designs and manufactures high power density Auxiliary Power Units (APUs) for commercial aviation and military applications. They are a small Florida-based manufacturer looking for new, cutting-edge solutions. Over the past five years, Enrique Enriquez, the president of KW Micro Power, has worked tirelessly to create a microturbine generator roughly the size of a microwave oven that can crank out more power than systems ten times as large.

Enriquez is no stranger to aerospace design and manufacturing. Over his long and successful career, he has led engineering teams in Rolls-Royce, worked with DARPA on the first Micro Air Vehicle (MAV) VTOL drone microturbine propulsion systems, and bought the second 3D printing system ever manufactured by Stratasys. But he is astonished by the capabilities of contemporary design software and additive manufacturing systems. “I think this is like the renaissance of engineering,” he mentions.

In this case study, we document the design of a critical component of KW Micro Power’s airborne microturbine: the generator housing. The engineers of KW Micro Power, nTop, and VELO3D created a housing that is not only much lighter than the original design and manufacturable as one piece with minimal support structures but also features internal conformal channels for cooling the engine and preheating the fuel.

Lightweighting APUs for aerospace

KW Micro Power offers a range of micro generator products, each optimized for a different use-case. For landbound applications, weight is not a big concern. Yet, for APUs on board an aircraft or drone, lightweighting is a number one priority — and every gram counts.

The engineering team managed to reduce the generator’s housing weight by 44% — from approximately 10.4 kg down to 5.9 kg. This significant weight reduction greatly surpassed the expectations of the KW Micro Power team. “I would be more than happy with just 20-25% weight reduction!” Enriquez mentioned.

As a bonus, the originally CNC machined housing can now be manufactured in a single piece with metal Additive Manufacturing. When the engine spins at 90,000 rpm, everything needs to be precisely aligned. Having multiple parts in an assembly increases the chances of misalignment. This way, part consolidation is an essential technique for improving machine reliability.

Redesigning the part for Additive Manufacturing was a straightforward process. To achieve these results, Enriquez’s team followed a Field-Driven Design approach:

• They first confirmed that the loads on the housing were relatively small using nTop's integrated static and modal analysis simulation tools.
• Then they removed unnecessary material to create a hollow shell with a variable wall thickness.
• Finally, they smoothed the internal geometry to ensure that it required no support structures during manufacturing with VELO3D’s metal AM process.

The entire process required only a few simple design blocks in nTop, was performed almost instantaneously without errors and took less than a day’s work before the part was ready to manufacture. It also opened up opportunities to add additional functionality: conformal cooling channels for heat management.

Cooling electric machines using conformal channels

Cooling systems are a crucial component of high-power energy generation systems. In fact, thermal management is one of the main size constraints of electric machines. Simply put, better cooling means more power.

Efficient cooling minimizes generator hot spots, enables higher current densities, reduces ohmic losses, and lessens heat stress on machine components — especially the windings and the magnets. This leads to increased efficiency and torque, reduced weight, extended machine life, and lower maintenance costs in power generation systems.

In this project, KW Micro Power seized the opportunity to incorporate multiple functions into the same component. The hollow structure that was initially conceived to reduce the motor casing’s weight could also function as a cooling channel.

The team evaluated the effect of the conformal cooling channels on the performance of the system through simulations. They combined results from thermal FE analysis carried out in nTop with CFD simulations in Ansys Fluent.

The results showed a dramatic reduction in the part's operating temperature. The maximum temperature was reduced by 33%, while the external temperature of the generator dropped by 86% and down to 27°C, making it safe to the touch.

This temperature drop allowed the use of aluminum as the material for the generator housing. Without the conformal cooling channels, KW Micro Power would potentially be forced to use a material with higher thermal resistance — increasing costs — or run their generator at lower speeds — throttling its power output. 

Creating conformal cooling channels was not a new concept for KW Micro Power. Enriquez’s team has experimented in the past on test parts with internal spirals with great success. This was the first time, though, that they applied the concept to a component of their microturbine.

The team experimented with different cooling mediums. A very appealing option was engine fuel. Using fuel as the heat transfer medium cools the engine with a liquid that is already onboard the aircraft and preheats the fuel itself — from room temperature to 55°C, based on simulation results — increasing the efficiency of the combustion process.

From design to manufacturing

The new design of the generator housing was manufactured by VELO3D on the company’s Sapphire metal 3D printing system in Aluminum F357. This foundry-grade aluminum alloy can be anodized and is certified for mission-critical applications.

VELO3D’s LPBF additive manufacturing system’s capabilities were taken into consideration during the design phase to create a manufacturable part with minimal support structures and post-processing requirements. 

VELO3D’s SupportFree technology and tight control of processing parameters is an excellent match for the unparalleled level of control offered by advanced engineering design software like nTop. Using this combination of advanced tools, KW Micro Power took advantage of cutting-edge technology to remove barriers in product development — from design to manufacturing.

“This component is a testament to what can be achieved by aggressive engineering companies that leverage next-generation design and manufacturing tools. KW Micro Power was able to use the unique capabilities of nTop to design a part that is specifically optimized for the application, and using VELO3D’s manufacturing technology meant that there were no compromises for manufacturability.”

— Zachary Murphree, VP of Sales at VELO3D

Engineering collaboration for advanced product development

KW Micro Power is a small aerospace engineering and manufacturing company. As such, Enriquez leverages a network of external partners, like nTop and VELO3D, to push the boundaries of his field. “I like to work with people that really know what they are doing and can push the limits of engineering,” he says.

As for his experience with using nTop, Enriquez notes:

“Nothing is fast enough for me, but everything is instantaneous in nTop. I can do the things that I want to do that are impossible with other design tools. I think this is like the renaissance of engineering. In the past, we were always working from the outside. Now, we can control every aspect of the geometry of our designs and their microstructure. If you want to make jet engines and APUs lighter and more efficient, this is the way!”

— Enrique Enriquez, President of KW Micro Power

The team also took advantage of an extensive simulation package to test the candidate solutions for fatigue strength, grip force, pull-out breaking force, and behavior during take-off and landing. These simulations enabled them to limit the experimental validation steps to a minimum, accelerating their development process.

The next steps

KW Micro Power is planning to launch its lightweight aerospace-grade microturbine in 2021. However, new ideas to increase the functionality of each component of their power generation system keep coming. For example, they are now examining how to use lattice structures to further reduce the weight of their airborne model and embed electronic sensors and filters to monitor their generator’s performance in real-time.