Designing high-performance and aerospace-grade heat exchangers requires detailed characterization of the as-manufactured geometry, including cross-sectional area and surface texture, to reduce uncertainties in performance prediction and issues regarding subsequent system integration. This paper presents experimental testing and analysis of microchannels fabricated using the laser powder directed energy deposition (LP-DED) additive manufacturing (AM) process. Research has shown that as-built surfaces result in differential pressure higher than what is predicted with current correlations and surface enhancements may be required for heat exchangers built using AM to meet the desired pressure drop specifications. Various surface enhancement techniques including abrasive flow machining (AFM), chemical milling (CM), and chemical mechanical polishing (CMP), were applied to the internal surfaces of the channels to tailor flow dynamics and induce variations in pressure drop. Based on experimental flow testing, channels processed with surface enhancements provide a tenfold reduction in differential pressure compared to the as-built channels. After testing, the samples were destructively sectioned to obtain geometric and detailed surface texture information. This characterization helped to inform a new prediction method for determining hydraulic diameter and equivalent sand grain roughness, thus reducing the uncertainty of predicted friction factors. The new correlation allows to estimate friction factor and pressure drop with a deviation from the experimental data that is within 20% of their value. The identification of the mechanisms at the basis of the formation of surface texture allowed to categorize distinct aspects related to friction factor ranges: roughness peaks, peak smoothing/reduction, minimized roughness, and combined waviness and valley reduction.@en

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.@en

This research evaluates Laser Powder Directed Energy Deposition (LP-DED) for producing fine feature internal microchannels. This study is focused on enhancing and characterising the surfaces of microchannels produced using techniques such as abrasive flow machining, chemical milling, chemical mechanical polishing, electrochemical machining, and thermal energy method to modify internal surfaces of microchannels made from NASA HR-1 Fe-Ni-Cr alloy. Flow testing for discharge coefficient measurement is conducted on processed microchannel samples, followed by characterisation through optical microscopy, Scanning Electron Microscopy (SEM), and Computed Tomography. Findings reveal variations in surfaces due to powder adherence, melt pool undulations, and polishing mechanisms. The study emphasises the significance of removing material equivalent to the mean powder diameter to reduce surface roughness and impact the discharge coefficient. The research proposes a ratio for planarising roughness and waviness peak height and density, offering insights for tailored surface adjustments in specific applications requiring reduced flow resistance. Highlights Internal microchannels with thin-walls were fabricated using the laser powder directed energy deposition process. Various surface enhancements and polishing processes were developed to modify the surface texture of the LP-DED channels. Flow testing was conducted to determine the discharge coefficient. Post-test characterisation was completed to obtain cross sectional area, perimeter, surface texture, and general surface condition to analyse results. Ratio of roughness and waviness peak and density (Spk/Spd and Wp/WPc) is proposed as a relevant surface characterisation parameter. Tailored surface modifications for specific end-use applications.

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Metal additive manufacturing (AM) is being used for mission-critical applications in both developmental and production components, driven by economic and technical benefits. Laser powder directed energy deposition (LP-DED) allows manufacturing of thin wall geometric features for various components at diameters larger than 2 m. The characterization of geometric capabilities and limitations is critical for establishing guidelines for end users of the technology. Within this study, several samples of enclosed vertical tracks were fabricated and characterized using LP-DED, with 1 mm-thick walls and varying inclination angles up to 45° using the NASA HR-1 alloy (Fe-Ni-Cr). The wall thickness, melt pool, and surface texture, inclusive of waviness and roughness, were evaluated and results presented. The experimental results indicate that the wall thickness increases exponentially above 30°. The surface texture was shown to be dependent on 1) excess powder adherence, 2) melt pool irregularities causing material droop, and 3) excess material. The experiment revealed that the mean roughness reduces with increasing wall angle for the downskin surface. The upskin roughness reaches a maximum peak at 20° and slowly reduces as powder adheres within the valleys. Both the downskin and upskin surface textures are dominated by irregular waviness generated by the melt pool.@en

Additive Manufacturing (AM) offers new design and manufacturing opportunities of thin-wall microchannel heat exchangers for aerospace and industrial applications. Laser Powder Directed Energy Deposition (LP-DED) is an AM process providing large scale manufacturing of thin-wall microchannel heat exchangers. Successful industrialization of the LP-DED process requires critical quantification and understanding of the metallurgical, geometric, and process limitations. Specifically, understanding the as-built surface texture, inclusive of roughness and waviness, is significant due to its effects on the friction factor and pressure drop within a heat exchanger. This experimental study completed a design of experiments (DOE) to determine the critical build parameters that impact surface texture for enclosed thin-wall samples. This study summarizes the characterization work of the LP-DED process for 1 mm enclosed walls with an Fe–Ni–Cr (NASA HR-1) alloy. The LP-DED parameters including laser power, powder feedrate, travel speed, layer height, and rotary atomized powder feedstock were modified in the experiment. An evaluation of the DOE samples and resulting surface texture is provided along with conclusions from these experiments. Results indicate that 3D areal and 2D profile (directional) surface texture is estimated by 2x the powder diameter that becomes captured or partially melted on the trailing edge of the melt pool. The fine powder showed a higher sensitivity to parameter changes but resulted in a higher density material and 23% reduction in roughness. Surface texture was also shown to vary between closed channel shapes (internal) due to ricochets, recirculation, and higher volume of powder available to bond compared to external (outer) surfaces. The understanding of the LP-DED process as-built surface texture is essential to fluid flow applications such as heat exchanges and can modify performance for enhanced heat transfer or can be a detriment to pressure drop.@en

Heat exchangers for use in propulsion applications are very critical components because they must be efficient, compact and light and often operate with working fluids at extreme temperatures or pressures or both. Various components and systems use heat exchangers such as combustion chambers of gas turbines and internal combustion engines, fuel cells (air supply and thermal management), electric batteries (thermal management), evaporators and recuperators of waste-heat-to-power systems, and rocket engines. Even if the results are more generally applicable, the heat exchangers applications to which this study is more closely related are regeneratively cooled rocket nozzles and chambers, and repressurization systems for the launch vehicles. These components are often thin-walled and contain pressurized fluids, like propellants at cryogenic or elevated temperatures. Given that the environments that these propulsion components must endure are challenging, the manufacturing to meet these specifications often require long lead times due to specialty processes and unique tooling associated with the combined thin-wall integral channel and large-scale structures. Additive manufacturing (AM) offers programmatic advantages for reduction in processing time and cost in addition to various technical advantages, including the possibility to achieve enhanced hardware complexity targeted to superior performance, part consolidation, and the capability of processing of novel alloys. While AM is already being utilized for heat exchanger components in propulsion applications, almost all these AM components are made by means of Laser Powder Bed Fusion (L-PBF). L-PBF allows for fine features but is rather limited with respect to the overall size of the components that can be manufactured. Recent developments are maturing the Laser Powder Directed Energy Deposition (LP-DED) process which may be used, for example, to make integral channel thin-wall regeneratively-cooled rocket nozzles with diameters greater than 1 m. This paper highlights some integral channel heat exchanger demonstrator hardware applications of LP-DED, as well as the characterization of this process in combination with the use of the NASA HR-1 alloy. To properly utilize LP-DED for heat exchanger manufacturing, various aspects are being characterized such as geometry limitations, measurement of surface texture and geometric angled surfaces, surface enhancements for internal channels, and material evaluation. NASA HR-1 (Fe-Ni-Cr) is a high strength hydrogen resistant superalloy developed for use in aerospace applications, such as heat exchangers. Some aspects and considerations about the design of heat exchangers are summarized together with data relevant to LP-DED manufacturing in combination with the NASA HR-1 alloy. Microchannels were successful deposited down to 2.54 mm and 1 mm wall thickness, wall angles of 30°, both with high reproducibility. It was also found that the areal surface roughness is highly dependent on the size of the powder feedstock used for deposition. The characterization of these LP-DED features is critical for fluid flow and heat transfer predictions as it can be exploited to enhance heat transfer at the cost of increased pressure drop. @en

High performance liquid rocket engines require cooling to maintain structural integrity of the combustion chamber which is exposed to high thermal and environmental loads. For many systems, this is achieved by means of regenerative cooling, where a coolant flows through passages around the chamber wall whilst extracting heat from the wall. A novel production technique that is often considered for this is metal additive manufacturing (AM). The use of additive manufacturing opens up new opportunities for engine design, which can result in more competitive designs, from both a technical and economical perspective. This paper provides a detailed literature review on the current state-of-the-art, challenges, and opportunities for designing additively manufactured liquid rocket engines by means of laser powder bed fusion or powder-based and wire-based directed energy deposition (DED) techniques. A detailed, systematic explanation is provided on the steps involving the creation of additively manufactured thrusters including the process considerations, AM techniques and post-processing operations.

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