KW Micro Power, a small Florida-based manufacturer, designs and manufactures high power density Auxiliary Power Units (APUs) for commercial aviation and military applications. Over the past five years, Enrique Enriquez, 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.

The design team comprised of the engineers at KW Micro Power, nTopology, 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 (AM). 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 AM 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 nTopology’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 nTopology, 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. Efficient cooling leads to increased efficiency and torque, reduced weight, extended machine life, and lower maintenance costs in power generation systems.

In this project, the design team 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 nTopology with CFD simulations in Ansys Fluent.

The results showed the part’s maximum operating 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, the design team 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 the design team at 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 was a good match for the control offered by engineering design software like nTopology.

Because KW Micro Power is a small aerospace engineering and manufacturing company, Enriquez leverages a network of external partners, like nTopology 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.

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 its power generation system keep coming. For example, the design team is 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.

nTopology
www.ntopology.com