Directed Energy Deposition: an Additive Manufacturing Technology for Large High-Temperature Compact Heat Exchangers: Process characterization and fluid dynamic performance

Heat exchangers are essential for temperature regulation across various industries, from everyday applications to space exploration. Over time, these devices have evolved from simple clay vessels to complex structures made from advanced metal alloys and ceramics. In modern engineering, additive manufacturing (AM) has revolutionized heat exchanger production, enabling the creation of thin-walled structures with complex internal channels designed for efficient fluid flow. This is particularly critical in industries such as aerospace, power generation, and manufacturing, where components must be compact, lightweight, and capable of withstanding extreme temperatures and pressures. However, traditional AM methods like laser powder bed fusion (L-PBF) are constrained by size limitations, necessitating new manufacturing techniques to meet industry demands for compact large scale heat exchangers.

This research addresses the challenges of developing the Laser Powder Directed Energy Deposition (LP-DED) process for extreme-environment heat exchangers. Process and flow test experiments were conducted, along with comprehensive characterization of LP-DED-fabricated microchannels, which are thin-walled (1 mm) and capable of containing cryogenic or high-temperature pressurized fluids. The results from the research establishes LP-DED as a viable technology for heat exchanger fabrication by addressing challenges related to geometry, wall thickness, surface texture, and the fluid dynamics of these unique AM surfaces. Three key research questions guide the study:

1. How is the surface texture of heat exchanger microchannels affected by the LP-DED fabrication process?
2. What are the geometrical relationships and sensitivities affecting fluid flow performance if heat exchanger channels are manufactured with the LP-DED process?
3. What improvements can be made to control the surface texture of thin-wall LP-DED internal microchannels?

A detailed literature review identifies significant gaps in current thin-wall LP-DED manufacturing and internal surface enhancement techniques. An experimental study examines LP-DED process mechanics and build parameters, focusing on their influence of the thin-wall surface texture and the effect of build angles on both open and closed structures. This research establishes guidelines for Design for Additive Manufacturing (DfAM), addressing process limitations, surface texture, and wall thickness metrics for the LP-DED process.
This research introduces microchannels fabricated using LP-DED in various sizes, with their internal and external surface textures, wall thicknesses, and repeatability characterized. These microchannels are then processed internally using various surface enhancement techniques to provide variations of the surface finish. Two experimental studies were conducted, with comprehensive characterization performed of the internal channel surfaces to evaluate the variations in surface texture resulting from the enhancement processes and their relationship to flow resistance and friction factors.

Key innovations include the characterization of a new hydrogen-resistant alloy (NASA HR-1), which provides foundational data for heat exchanger design. The study also identifies surface texture mechanisms that affect fluid friction factors, resulting from distinct enhancement techniques such as peak smoothing, roughness minimization, waviness, and valley reduction. Additionally, friction factors and differential pressure in LP-DED-fabricated microchannels, both in as-built and surface-enhanced conditions, were investigated. Surface treatments such as abrasive flow machining, chemical milling, and chemical mechanical polishing were evaluated. The experimental results and comprehensive surface texture characterization led to the development of new correlations for calculating the hydraulic diameter of square channels and predicting sand grain roughness and friction factors. These correlations resulted in pressure drop predictions with deviations of less than 20% from experimental data, offering a 50% improvement over previous models.