Experimental and Computational Analysis of an Extreme Environment Heat Exchanger Co-Designed for Manufacturability and Thermal-Hydraulic Performance
by Zhengda Yao
Supercritical CO2 (sCO2) has recently attracted considerable attention due to its inherent properties, such as high density and volumetric heat capacity, making it an energy-dense and efficient heat transfer medium for various applications, including power generation systems, aerospace and electronics cooling. Heat exchangers operating in extreme conditions must adhere to strict size, weight, and power consumption (SWaP) criteria to ensure efficient thermal systems. Leveraging sCO2 offers the potential to develop high-performance, cost-effective, and compact metal heat exchangers.
Beyond the fluid selection, advanced HX design plays a crucial role in improving thermal performance. In this study, a multi-pass microchannel heat exchanger design (MPMHX) with small fins (0.18 mm width) and microchannels (0.762 mm width) was adopted to achieve high compactness (surface area density = 989 m2/m3). To successfully fabricate this complicated structure, additive manufacturing (AM) was utilized with a development of AM guidance (printing configurations and powder removal process). The multi-pass design concept was applied to fabricate a long microchannel (173 mm) inside a limited printer volume. An elevated relative roughness factor (9.6%) was observed after the manufacturing process and it was incorporated into an AM-based microchannel prediction model to assess its impact on HX thermal-hydraulic performance. The prediction results were validated by experiment, which indicated that the measured roughness increased the pressure loss by 172% while simultaneously enhancing thermal duty by 31%. Compared to other compact HX concepts in the literature, the MPMHX exhibited the highest experimentally demonstrated compactness (Q/V = 45.4 MW/m³, Q/V/dT = 0.34 MW/m³/°C) with a low pumping power of 11.75 W.
To further enhance performance, both genetic algorithm-based parametric optimization and topology optimization were implemented. When applied to a simplified heat sink model, the parametric optimization outperformed topology optimization and was subsequently used to optimize the MPMHX under operating conditions of 147 °C and 800 °C, resulting in thermal performance improvements of 47% and 97%, respectively.
This study presents a highly compact sCO₂ heat exchanger, leveraging additive manufacturing and advanced optimization techniques to enhance HX performance. The findings provide valuable theoretical/experimental insights that can drive the advancement of high-performance HXs for power generation, extreme environments, and high-efficiency applications.
Doctoral Dissertation
http://hdl.handle.net/1903/34258
An Additively Manufactured, Salt Hydrate-Based Latent Heat Thermal Energy Storage for Peak Load Shifting in Building Equipment
by Veeresh Ayyagari
As renewable energy usage increases, imbalances between electricity demand and supply grow, raising the risk of grid failures and blackouts. These imbalances become even more severe during peak load hours. Peak load shifting involves managing demand and supply to reduce the grid load during peak times, helping to match supply levels. A latent heat thermal energy storage (LHTES) device using solid-liquid phase change material (PCM) can be integrated into buildings’ heating and cooling equipment systems, offering energy storage capacity for peak load shifting of thermal loads. An LHTES must be capable of delivering significant power during peak loads, despite the internal thermal resistance caused by the low thermal conductivity of the PCM material. To boost power, conductivity can be enhanced with additives, but this can potentially increase costs and reduce energy storage density (ESD). Alternatively, power density (PD) can be improved by increasing the heat transfer area and reducing heat transfer pathway length without the need for conductivity-enhancing additives. A better thermal design can thus improve PD without increasing the cost, supporting commercialization.
This dissertation examines the thermal performance of a novel cross-media thermal energy storage device (CMTES), a polymer-metal structure made via additive manufacturing, integrated with a low-cost, salt hydrate PCM. The CMTES core structure employs low-cost polymers as a barrier between the heat transfer fluid (HTF) and the PCM with embedded metal wires for high thermal conductance, transferring heat efficiently between the HTF and PCM sides.
For this investigation, an in-house salt-hydrate composite PCM was developed using Glauber’s salt with small doses of economical additives to achieve desired thermal characteristics, enabling its use in Heating Ventilation and Air Conditioning (HVAC) systems. This composition contains 9 wt.% NaCl for melting point reduction, borax to reduce supercooling, and sodium polyacrylate as a thickener. Thermal characterization of the PCM was conducted using a modified T-history method, providing detailed insights into its phase change characteristics. The results indicated a melting range of 18–22 °C, and the latent heat measured at 171 kJ/kg. The PCM also exhibits a high volumetric ESD compared to organic types with similar melting points. The cyclic stability was also investigated, and it was found that the PCM was stable for 300 cycles, equivalent to a year-long TES operation. An experimental investigation was conducted to assess the impact of rehydration time on the PCM’s latent heat of fusion. Findings revealed that the latent heat could change by 70% as the rehydration period increased from three hours to 20 hours.
The CMTES integrated with the composite PCM was experimentally tested. The CMTES demonstrated a volumetric PD of 190-450 kW/m³ during discharging and 238–300 kW/m³ during charging, despite the metal wires occupying only 3% of the total TES volume. A numerical modeling approach was used to compare the CMTES with a competitive low-cost PCM slab design, showing that the developed concept boosts PD, equivalent to a 20-fold increase in thermal conductivity for the same PCM in the slab design. A figure of merit traditionally used for comparing heat exchangers, which combines thermal performance with compactness, was adopted to compare the performance of the CMTES with other enhanced TES designs reported in the literature. Based on the figure of merit introduced in this work, the results indicated that the CMTES performed 1.6–16 times better than conventionally manufactured enhanced TES designs and comparable to state-of-the-art additively manufactured metal fin designs.
The findings of this study provide significant and novel contributions by demonstrating the potential of salt-hydrate composites as a low-cost, high-performance phase change material (PCM) solution for latent heat thermal energy storage (LHTES) applications. By leveraging a cross-media approach, the study shows how these composites effectively combine economic feasibility with enhanced thermal performance. When integrated into the cross-media thermal energy storage system (CMTES), the proposed salt-hydrate composite serves as a cost-effective alternative to traditional microencapsulation techniques, while achieving internal thermal conductance levels comparable to state-of-the-art enhanced TES designs. Additionally, the study addresses key practical challenges associated with salt hydrate PCMs—specifically, the effects of rehydration time on latent heat recovery and the implications for long-term cyclic stability—thereby laying the groundwork for future efforts aimed at improving energy storage density (ESD).
Doctoral Dissertation
Air-Cooled Heat Sink for an Electric-Propulsion Aircraft Application Based on the Manifold-Minichannel Concept
by Murilo Nicoluzzi
The growing demand for efficient thermal management in high-power electric propulsion systems has driven the exploration of innovative cooling solutions for aircraft applications. This research investigates the feasibility of employing a manifold-microchannel heat sink to dissipate waste heat from a 500 kW electric motor designed for aviation use. Given the constraints of airborne systems, the heat sink must achieve a balance between high thermal performance and minimal hydraulic penalties, besides lightweight construction. Air is selected as the working fluid due to its simplicity, reliability, and weight advantages over liquid-cooling alternatives, eliminating the need for pumps and additional infrastructure while making use of the dynamic pressure of the free airstream for cooling.
The study focuses on optimizing the heat sink's geometry and airflow distribution to ensure effective heat dissipation while maintaining the motor’s temperature below the critical threshold of 150°C. A combined numerical-experimental methodology was adopted. Parametric studies and CFD simulations informed the selection of channel and manifold dimensions, which were validated through experimental testing. Thermal-hydraulic characterization established a baseline performance across operating conditions representative of cruise and takeoff. Measured thermal resistance values ranged from 0.002 to 0.0032 K/W, while pressure drops remained below 1,500 Pa, satisfying thermal and hydraulic design constraints. When compared with a conventional straight-fin heat sink under equivalent pumping power conditions, the MMHS achieved up to 50% lower thermal resistance. Conversely, for a fixed thermal resistance, the MMHS reduced pressure drop by as much as a factor of 12.
The study further provided the first experimental evidence of particulate fouling in MMHS configurations. Silica sand ingestion, with particle sizes below channel width, led to only an 8% increase in pressure drop that stabilized within 20 minutes, with negligible effect on thermal resistance. Glass beads with diameters exceeding the channel size produced larger pressure drop increases, in some cases nearing 40%, yet fouling remained localized. These results highlight the intrinsic resilience of MMHS systems, owing to the redundancy provided by thousands of parallel minichannels, in contrast to conventional straight-fin heat sinks that would experience rapid global clogging.
Overall, this work demonstrates that cylindrical air-cooled MMHS systems can deliver high thermal performance, low hydraulic losses, and robust fouling tolerance, positioning them as a viable thermal management solution for next-generation electric propulsion motors. By expanding the MMHS concept beyond electronics to aviation applications, this dissertation contributes new experimental evidence, particularly on fouling behavior, and establishes a foundation for further optimization and integration of lightweight, reliable cooling technologies in electrified aircraft.
Doctoral Dissertation
Development of Modeling and Optimization Methodologies to Facilitate Transition of Heat Pump and Refrigeration Systems to Lower GWP Refrigerants
by Venkata Surya Murali Vijay Preetham Meruva
In light of Kigali Amendment, Fluorinated Gas (F-Gas) regulations, and the United States Environmental Protection Agency (EPA) SNAP program calling for the phase out of hydrofluorocarbon (HFC) refrigerants which have an extremely high global warming potential (GWP), it is necessary to transition to lower GWP alternatives. For example, natural refrigerants such as hydrocarbons (HC, e.g., propane (R290), isobutane (R600a), etc.) and carbon dioxide (CO2) have gained traction as attractive low GWP alternatives for a wide range of residential and commercial heating, ventilation, air-conditioning and refrigeration (HVAC&R) applications which have historically depended on high-GWP refrigerants such as R134a, R410A, R22, and R404A. Moreover, researchers worldwide have also shown immense interest in utilizing refrigerant blends to achieve the required performance whilst minimizing environmental impact. One such area is supermarket refrigeration systems, which often employ multi-stage cascade cycle configurations due to large temperature lifts, making single-stage vapor compression (VC) systems impractical. A key challenge in this field is selecting environmentally-friendly refrigerants, as many common low-temperature refrigerants have high GWP, high ozone depletion potential (ODP), and/or are flammable. The use of HC refrigerants is also highly regulated due to their flammable nature, hence necessitating high-performance heat exchangers (HXs) with significant size, weight, cost, and refrigerant charge reductions compared to current state-of-the-art to comply with flammable refrigerant charge limits.
Decarbonization of buildings is essential to reducing greenhouse gas emissions, as space and water heating account for a substantial share of energy-related emissions. Transition to heat pump technologies is central to this effort, offering a low-emission alternative to fossil fuel and electric resistance heating. They can substantially reduce greenhouse gas emissions from space and water heating when powered by renewable or low-carbon electricity, making them a key technology for building electrification and climate mitigation goals.
This thesis focuses on the development of modeling and optimization methodologies to facilitate the transition of heat pump and refrigeration systems to lower GWP refrigerants and is applied to three different case studies: (i) refrigerant blend optimization and selection for a supermarket refrigeration system, (ii) a water source heat pump system, and (iii) low-charge air-to-refrigerant HXs for a residential air-conditioning system.
In the first part of the thesis, an approach was developed to design optimal refrigerant blends to serve as lower-GWP alternatives to conventional high-GWP refrigerants while also enhancing system efficiency for a two-stage cascade refrigeration system. Significant improvements in COP by up to 49% and decreases in GWP (< 50) were observed when the optimal blends were incorporated into the system. In the second part of the thesis, experimental validation of a water source heat pump system model utilizing R32 as the refrigerant and a brazed plate heat exchanger (BPHX) as the condenser was conducted for a steady-state VC system simulation platform to demonstrate the platform’s capability to accurately model and simulate VC systems with lower-GWP refrigerants and plate heat exchangers.
In the final part, a comprehensive system-level optimization methodology was developed for air-to-refrigerant tube-fin and finless non-round tube HXs which is capable of designing optimal HX pairs (i.e., condenser and evaporator) with minimal refrigerant charge to replace the baseline HXs in an air-to-R410A air-conditioning system to facilitate the transition to lower GWP refrigerants R32 and R290. System-level simulations with the optimal HX pairs suggest that the optimal HX pairs have comparable thermal-hydraulic performance to the baseline system with significant reduction in HX-level charge (> 70%) , thereby supporting the adoption of lower-GWP natural refrigerants such as R290.
Master's Thesis
http://hdl.handle.net/1903/34547
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