Heat Rejection System Research Articles

Conceptual design and optimization of heat rejection system for nuclear reactor coupled with supercritical carbon dioxide Brayton cycle used on Mars

During the conceptual design of the Mars surface nuclear reactor coupled with supercritical carbon dioxide Brayton cycle, the heat rejection system occupies most of the weight, and reducing the weight of heat rejection system can effectively cut down the transportation cost from Earth to Mars. This paper explored the feasibility of adopting a combination of heat pipe cooling device and convective heat exchanger to design the heat rejection system for the Mars surface nuclear power station, established flow and heat transfer model and weight model, developed a thermal-hydraulic design program for the heat rejection system, and verified its accuracy; then, the NSGA-II multi-objective optimization method is used to optimize the weight of heat rejection system. Pareto front shows that with higher heat rejection proportion of convective heat exchanger, corresponding inlet velocity of Mars' atmosphere should also be increased. As the optimization result of heat rejection system, the ratio of weight to heat rejection is between 0.94 kg/kW and 2.92 kg/kW, and the maximum power consumption of convective heat exchanger accounts for 3.29% of the total power generation.

Numerical study of anisotropic scattering and solar radiation impact on liquid droplet radiator performance

The Liquid Droplet Radiator (LDR), a lightweight frameless radiant heat rejection system, is widely regarded as the optimal heat rejection solution for megawatt-scale space nuclear power systems. The LDR dissipates heat to space through its droplet layer, which plays a crucial role in determining its radiative rejection capacity. Radiation beams undergo a series of emission, scattering, and absorption processes within the layer before being emitted into space. Prior studies typically assume isotropic scattering in this process. However, this study develops a three-dimensional model for the LDR’s droplet layer based on Mie theory and Monte-Carlo methods to investigate the impact of anisotropic scattering and solar radiation. Our calculations reveal that the working fluid exhibits strong forward anisotropic scattering characteristics. Under the anisotropic assumption, the heat rejection capability of the LDR is 27–33 % higher compared to the isotropic assumption. Regarding the temperature distribution within the LDR, under the anisotropic assumption, the layer’s temperature is 6 K lower at the XY boundary surface and 3 K higher at the layer median, as opposed to the isotropic assumption. We introduce the concept of the probability of a light beam escape to elucidate the distinct beam transmission mechanisms and temperature distribution. When the distance between the LDR and the sun is in the vicinity of 1 AU, the impact of solar radiation on the LDR’s ability to operate is on the scale of 1E-5, which is almost negligible. This study lays down a theoretical groundwork for calculating the heat rejection capacity during the LDR design phase.

Modelling and optimization of a forced convection heat exchanger for Mars waste heat rejection for power cycle and cryocooling applications

A forced convection heat exchanger for waste heat rejection to the Martian atmosphere is modelled for both a power generation and cryogenic refrigeration application. The modeling is based on existing as well as recent, experimentally validated heat transfer and pressure drop correlations for low-Reynolds-number, moderate-Knudsen number finned tube arrays and represents the first detailed examination of optimal heat exchanger design and performance in this regime. These correlations were developed from heat exchanger performance data taken in Mars-like conditions and are described in a separate, companion work [1]. A heat exchanger model based on the effectiveness-NTU method is developed using these correlations and the heat exchanger geometry is optimized using an open-source solver to minimize the overall system mass for a range of candidate heat exchanger materials and heat transfer rates. The optimal heat exchangers generally consisted of shallow, staggered banks of 1–2 mm diameter tubes and 2–4 mm fin spacing, although in some conditions bare tube arrays were optimal. These geometries tended to be preferred due to their low relative pressure drop, a necessity given the power requirement of producing high pressure head in the thin atmosphere. Curve fits that capture the heat exchanger performance are generated based on the input conditions in order to develop a reduced order model that can be used to integrate the heat exchanger model with an existing recuperated Brayton cycle high-temperature gas cooled reactor (HTGR) power cycle model. This power cycle model is used to determine the impact of using convective heat rejection on optimal system mass and compare this approach to radiative heat rejection. For a 40 kWe power output, the optimal overall cycle mass is found to decrease by 78 % when using convective heat rejection compared to a radiator; this is due to the decreased mass of the heat rejection system as well as the lower optimal rejection temperature which results in a lower recuperator mass. The heat exchanger itself has a mass of only 37 kg, a frontal area of 12.7 m2, and requires 2134 W of fan power to operate. The system is shown to perform well over a wide range of Mars ambient conditions. A high-power liquid Oxygen (LOx) reverse Brayton cryocooler cycle is also modeled to determine the benefit of using convective heat rejection for in-situ resource utilization (ISRU) applications. Across a range of power source options and ambient temperatures, using a convective heat rejection system reduced total system mass by up to 43 % and frontal area by up to 95 % compared to a radiator and also reduced the mass-optimal cycle power draw by increasing the optimal cycle efficiency. However, the magnitude of the mass reduction depends strongly on the specific power of the electrical source; the forced convection system provides only a 7 % improvement over a radiator when coupled with a photovoltaic power source due to the increased power demand of the fan. Overall, this technology offers significant performance advantages over radiative heat rejection systems for waste heat rejection applications on Mars and therefore may significantly benefit future crewed Mars missions.

Experimental investigation of an optimized indirect free cooling system including a dry cooler equipped with evaporative cooling pads for data center

Different heat rejection systems could be used to discharge the heat of liquid cooled data center to the ambience. The dry cooler as an indirect free cooling solution is one of the most used, where outside air is applied as a cooling fluid for its finned heat exchangers (FHEXs). In ambiences where dry coolers ambient temperatures are higher than supplied FHEX’s water temperature, evaporative cooling system can be used as pre-coolers. This paper presents an experimental investigation conducted on an indirect free cooling system including a dry cooler equipped with evaporative cooling pads (IFC+EC) under 20 K data center temperature difference for high outdoor conditions, variable fan speed, reduction in cooling pad’s surface and increasing in water supply temperature. The results show that the dry cooler is successfully operated under 20 K temperature difference. Furthermore, by increasing the air speed from 2 m/s to 3 m/s, the pads’ outlet air temperature increases by 0.6 K, the relative humidity tends to decrease about 9% and its efficiency decreases from 97% to 88%. Surface clogging had no significant impacts. A slight rise of 1.24 K in air outlet temperature and a decrease of about 3.3% and 2.5% in relative humidity and cooling pad thermal efficiency respectively were detected while increasing water supply temperature from 23 °C to 44 °C. This allowed authors to propose an optimized IFC+EC system with precooling DC supply water before the dry cooler to reduce its cooling demand by at least 31%.

An Innovative Heat Rejection System for High Altitude Unmanned Aerial Vehicles

An Innovative Heat Rejection System for High Altitude Unmanned Aerial Vehicles

Decarbonization and enhancement of LNG cascade cycle by optimizing the heat rejection system, hourly evaluation

This study is motivated by global directions towards reducing carbon emissions. LNG is a very energy intensive industry and improving its efficiency is an essential area of study. LNG process is sensitive to cooling media, so conventional air cooling is replaced by proposed water cooling with a cooling tower. Aspen Hysys V11 is used for simulating the base and proposed systems in this study. The proposed water-cooling system is simulated and compared with the conventional air-cooling system in Alexandria, Egypt as the base location of the study, at a single simulation point. For more data refinement, hourly temperature records are used for simulation and total annual performance is calculated based on 8784 simulation points, which achieved a total year improvement of 3.98% reduction in SPC, fuel gas and CO2 emissions and 7.21% reduction in annual propane consumption. To assess the suitability of the system in different climates, four different locations are assessed: Nigeria, Qatar, Egypt, and Russia. Performance is measured throughout the year on hourly calculations. Results revealed that power reduction up to 4.26% can be achieved based on total year performance. The power reduction achieved is 1.24%, 1.6%, 3.98%, and 4.26% for Russia, Nigeria, Egypt, and Qatar, respectively.

Thermal hydraulic design and mass optimization of a 100-kWe S-CO2 cooled Mars-surface fission reactor system

Fission reactor with electrical power of 100 kW to megawatt is a promising power option for upcoming deep space exploration. To reduce the number of space launches and the costs, it is necessary to design an energy supply system coupling fission reactor with energy conversion system of high efficiency and compactness. The combination of gas-cooled fast reactor and supercritical carbon dioxide (S-CO2) Brayton cycle has the advantages of simple system layout, compact structure and high thermal efficiency.This paper aims to carry out researches on the thermal hydraulic design and mass optimization of 100kWe reactor system for Mars-surface application.An analysis code which includes thermodynamic models for different components in the system, as well as mass evaluation models for radiator, recuperator and reactor, is developed and validated to provide an analysis tool. An S-CO2 Brayton cycle directly cooled reactor system is proposed, which applies Tube-In-Duct fuel assembly to make up the core and heat pipe radiator as heat rejection system. The thermal hydraulic evaluation of the reactor core, thermodynamic performance evaluation as well as overall mass evaluation of the whole system are carried out to achieve an optimized 100kWe fission reactor system for Mars surface with minimum mass. The optimal design has an operating pressure of 16 MPa, core outlet temperature of 600 °C, radiator panel area of 120 m2 and an overall mass of 1862.98 kg considering the reactor and key components of Brayton cycle. S-CO2 Brayton cycle directly cooled reactor system has a comparable specific mass as He-Xe cooled reactor with lower operating temperature, which is beneficial for material selection and provides another option for space reactor development.

Design of a Low-Enrichment Uranium Reactor to Power a Future Martian Colony

The Megawatt Implementation of a NuclEar ReActor using Low-enrichment uranium (MINERAL) is designed to deliver 2 MW(electric) of steady-state electricity to a colony established on the surface of Mars with a minimum lifetime of 10 years. The main challenge associated with a low-enrichment uranium fission surface power system is reducing the total mass, which will be higher than that of an equivalent high-enrichment uranium system. Optimizing the mass of the system is crucial to limit the amount of Earth-Mars cargo needed to deploy a MINERAL unit. The use of yttrium hydride as a moderator has shown promise in reducing the overall mass of the reactor. An in-house Python framework evaluates the neutronic, thermal-hydraulic, and heat rejection performance throughout the design process. The final design iteration uses a CO2 Brayton cycle cooled by a passive heat rejection system consisting of six panels with a total surface area of 4752 m2. The cylindrical core is fueled with low-enrichment uranium monocarbide with 0.83 wt% of pure 157Gd moderated with yttrium hydride and surrounded by a beryllium oxide reflector. The reactivity is controlled by ten control drums and a central control rod, which provide enough margin to operate the reactor and ensure its subcriticality in case of a submersion accident. The mass of the core with the reflector, reactivity control system, and shield is 7.2 tonnes.

Performance analysis of new liquid cooling topology and its impact on data centres

A large amount of the energy consumed by data centres is related to the cooling demand. The use of optimised liquid cooling technology enables one to increase the water inlet temperature and the water temperature difference of a data centre. Accordingly, the energy consumption of the data centre infrastructure can be reduced by appropriately utilising free cooling available on-site. This paper presents an experimental investigation and performance analysis conducted on a novel liquid-cooled topology deployed within OVHcloud data centres. A rack cooling system based on a combination of close-coupled cooling and direct-to-chip cooling is presented. The experimental setup used comprised five information-technology racks with operational servers under different temperature profiles (15 K and 20 K) for two thermal conditions (27 °C and 35 °C). The results indicated a repartitioning of thermal load absorption between close coupled cooling and direct-to-chip cooling to approximately 56% and 44%, respectively. Thereby, air was still necessary as a coolant for the data centre. A temperature difference was validated for all the information technology racks, whereas the 20 K temperature-difference profile was validated with the proposed new rear-door heat exchanger (RDHX) configuration. In addition, a performance analysis was performed on a data centre of 600 kW cooled with this topology for three heat rejection systems (mechanical cooling system using chillers, indirect free cooling with evaporative cooling, and hybrid chillers with intelligent dry coolers) under four climatic conditions. Indirect free cooling combined with an evaporative cooling system showed optimal energy savings. A reduction in annual power consumption by 89% was achieved compared with the mechanical cooling system at four locations where data centres were located. The cooling partial power usage effectiveness of data centre was reduced by at least 16% with respect to mechanical cooling. An increase in the temperature difference of the information technology racks from 15 K to 20 K caused a reduction in the water usage effectiveness of the data centre by at least 40%. An increase in the temperature of the cold water supplied to the data centre by 5 K caused a reduction in water usage effectiveness and annual water consumption by at least 30% and 47%, respectively.

Nonnuclear Experimental Capabilities to Support Design, Development, and Demonstration of Microreactors

This work provides a summary of selected experimental capabilities being developed to support nonnuclear testing and demonstration of technology in support of microreactors under the U.S. Department of Energy’s (DOE’s) Microreactor Program. Major capabilities include the Single Primary Heat Extraction and Removal Emulator (SPHERE) and the Microreactor Agile Non-nuclear Experimental Test Bed (MAGNET). The SPHERE facility allows for controlled testing of the steady-state and transient heat rejection capabilities of a single heat pipe using electrical heaters that simulate nuclear heating. The facility is capable of monitoring axial temperature profiles along the heat pipe and surrounding test articles during startup, steady-state operation, and transients. Instrumentation includes noncontact infrared thermal imaging, surface thermocouples, spatially distributed fiber optic temperature and strain sensors, electrical power meters, and a water-cooled, gas-gap calorimeter for quantifying heat rejection from the heat pipe. The facility can be operated under both vacuum and inert-gas conditions. The MAGNET facility is a large-scale, 250-kW electrically heated microreactor test bed to enable nonnuclear experimental evaluation of a variety of microreactor concepts. It can be supplied to electrically heat a scaled section of a microreactor and further test the capabilities of heat rejection systems. The initial MAGNET experiments will support technology maturation and reduce uncertainty and risk associated with the design, operation, and deployment of monolithic heat pipe–based reactors. However, this test bed can broadly be applied to multiple microreactor concepts to evaluate a wide range of thermal-hydraulic and structural phenomena such as interface coupling with power conversion units and other collocated systems. MAGNET can evaluate integral thermomechanical effects during electrical heating of an array of heat pipes in a larger test article. Examples of initial testing will include thermal stresses in the monolith and the impact of debonding of a heat pipe from the core block and how that failure could impact surrounding heat pipes, i.e., understanding the potential for cascading failure. This work also discusses some modeling capabilities that can support experiment design, analysis, and interpretation, including the heat pipe code Sockeye and a comparison of thermal-structural simulations performed using ABAQUS and STAR-CCM+.

An Enhanced Calculation Method of the Heat Rejection System of a Free-Piston Stirling Engine (FPSE) Operating on the Moon

In this paper, an enhanced calculation method of a heat rejection system operating on the moon is presented. This was taken into consideration in the developed calculation method and in the propagation of heat fluxes with the radiation of the removed heat. The developed method made it possible to effectively evaluate the capabilities of various refrigerants and choose the radiator parameters and the refrigerant flow regime in a less time-consuming process and with minimal deviations (<5%) compared to the previously developed two-dimensional radiator model by the authors. A comparative analysis was carried out for two refrigerants: helium and liquid ammonia. It has been established that when using liquid ammonia, there are more possibilities for varying the geometric parameters of the radiator. The use of liquid ammonia as a refrigerant made it possible to reduce the power spent on pumping the refrigerant through the radiator. Using helium, the power for pumping the refrigerant was NR= 5.1 W during a turbulent flow Re = 4500. On the other hand, the power for pumping liquid ammonia was NR= 0.27 W. In addition, using liquid ammonia increased the heat flux radiated by the radiator pipe by 3.9 times, which made it to possible to increase the fin width and reduce the length of the radiator pipe.

Forced Convection Heat Rejection System for Mars Surface Applications

A forced convection heat exchanger for waste heat rejection to the Martian atmosphere is designed based on existing heat transfer and pressure drop correlations for low-Reynolds-number finned tube arrays. The design is optimized to determine the mass-optimal heat exchanger geometry for a range of candidate heat exchanger materials and power loads. For a 100 kW thermal load, the optimal heat exchanger mass is found to be 27.0 kg and a frontal area of 3.94 m2. This is 95% less mass and area than a comparable radiator and only requires 638 W of fan power (or 0.6% of the output power) to operate. Optimal geometries are also found for heat rejection loads of 1 kW to 250kW across a range of coolant and atmosphere temperatures, indicating wide applicability of this technology for Martian heat rejection applications such as cryofuel refrigeration or in-situ resource utilization (ISRU) plant cooling. An experimental facility has been designed and partially constructed to experimentally validate the predicted heat exchanger performance in a Mars-like environment. A subscale heat exchanger has been built based on the predicted mass-optimal geometry. Heat transfer and pressure drop performance data of this heat exchanger will be collected and will refine the modelling predictions.

Optimization of a heat pipe-radiator assembly coupled to a recuperated closed Brayton cycle for compact space power plants

• Analysis of recuperated closed Brayton cycle conditions is carried out. • Brayton cycle and heat pipe-radiator models are numerically coupled. • Heat source and cold heat exchanger inlet temperatures are optimized. • The specific mass minimization is used as the objective variable. • Optimization provides cycles with specific masses ranging from 40 to 50 kg/kW. Compact and efficient energy conversion systems for space applications enable the appearance of new mission opportunities and technological discoveries resulted from space exploration. Besides energy availability, another crucial factor of any energy conversion system for space purposes is its total mass and size. Focusing on a recuperated closed Brayton cycle (CBC), thermodynamic modeling of a CBC is proposed. Moreover, a thermal model is carried out to predict the overall properties of the cold side of the system (i.e., heat pipes and radiator) for different CBC conditions. Both models are coupled and their conjunct solution provides operational data for the design of the heat rejection system, such as the number of heat pipes (HP), total assembly mass, length, and second-law efficiency. Furthermore, by means of this coupling, the heat source temperature and the cold heat exchanger (CHE) inlet temperature are defined, using an optimization procedure where the specific mass (i.e., radiator mass to cycle power ratio) is minimized. Based on this objective variable, the optimized heat source temperature of 1200 K is achieved, while the CHE inlet temperature of 513.2 K is obtained. Such temperature conditions ensure the future design of a space energy conversion system that aligns good efficiency and compactness.

Thermal Performance Evaluation of Space Radiator for Single-Phase Mechanically Pumped Fluid Loop

This paper presents the experimental and analytical results of a heat rejection system consisting of a single-phase mechanically pumped loop and a space radiator. It has been developed for future crewed exploration missions to provide a large amount of heat dissipation capability. The radiator consists of a honeycomb radiator, eight aluminum pipes inserted through the honeycomb core, and two aluminum manifolds joined at both ends of the eight pipes. A graphite sheet is attached to the back of the aluminum skin, facing deep space to improve the thermal diffusivity in the in-plane direction. Silverized Teflon® tapes are put on the surface of the skin on the deep-space side, and the surface of the opposite skin is covered with multilayer insulation. A gap between the radiator surface and the flow channel protects against micrometeoroids and orbital debris. A thermal vacuum test was conducted to evaluate the thermal performance of the radiator. The coolant was HFE7200, and its flow rate was varied from about 50 to 200 kg/h. More than 50 temperatures across the radiator surface were measured for each case. Thermal Desktop® and SINDA/FLUINT software were used to develop the thermal mathematical model, which was correlated with the test results and evaluated through thermal analysis. The fin efficiency of the radiator increased with the mass flow rate of the coolant, reaching 0.9 or higher.

A Review on Thermal Design of Liquid Droplet Radiator System

Liquid Droplet Radiator (LDR) system is regarded as a quite promising waste heat rejection system for aerospace engineering. A comprehensive review on the state-of-the-art of LDR system was carried out. The thermal design considerations of crucial components such as working fluid, droplet generator and collector, intermediate heat exchanger, circulating pump and return pipe were reviewed. The state-of-the-art of existing mathematical models of radiation and evaporation characteristics of droplet layer from literatures were summarized. Furthermore, thermal designs of three LDR systems were completed. The weight and required planform area between the rectangular and triangular LDR systems were respectively compared and the evaporation models for calculating the mass loss were evaluated. Based on the review, some prospective studies of LDR system were put forward in this paper.

Fabrication and Characterization of Heat Pipe with Composite Structure for the Adiabatic Section

Background:Innovative cooling technology is required in every field of life, ranging from satellite to terrestrial applications. A novel heat rejection system is of great concern for space applications. Futuristic applications in heat pipe will involve composite structures in various ways as they offer flexibility in design with their inherent advantage of being lightweight. However, it remains a challenge to join a composite structure with metals. This study investigates the effect of a composite section in the adiabatic region of the heat pipe and also proposes a novel approach to join metals with a composite structure.Methods:A flexible composite tube, such as carbon fiber, reinforced thermoplastics to make the adiabatic section. This section is adhesively bonded with the metal tubes, i.e., evaporator and condenser section. The inherent roughness of the metal tube makes the first layer for mechanical interlocking, followed by adhesive bonding.Results:The effect of adiabatic, condenser, and evaporator length, for a normal vs. composite heat pipe, on specific thermal conductivity is evaluated.Conclusion:The numerical studies confirm that the use of composite material for the adiabatic section improves the performance of the heat pipe. It is proposed to use reinforced thermoplastic as the material for the adiabatic section.

Design and Costing of an ION Clean Energy CO2 Capture Plant Retrofitted to a 700 MW Coal-fired Power Station

ION Clean Energy’s (ION) advanced solvent is one of the leading solvent systems currently under development for post-combustion carbon dioxide (CO2) capture. ION has partnered with Nebraska Power Public District (NPPD), Sargent & Lundy (S&L), Koch Modular Process Systems (KMPS), and Siemens to design a commercial-scale (700 MW) capture system utilizing ION’s advanced solvent. The resulting CO2 capture plant design utilizes ION’s ICE 21 solvent and was designed to take full advantage of the solvent benefits, which include an efficient physical plant layout, reduced energy requirements, less solvent degradation, lower emissions, and lower capital costs relative to systems built with (U.S. Department of Energy) DOE Bituminous Baselines Study (BBS) case benchmark solvents. This Front-End Engineering and Design (FEED) is the continuation of a successful pre-FEED study on a 300 MW capture system at NPPD’s Gerald Gentleman Station (GGS) in Nebraska, USA, where cost of capture was determined to be approximately $32 per tonne of CO2. The overall objective of this project is to provide a detailed design resulting in an accurate cost estimate for a commercial-scale CO2 capture facility retrofitted onto a 700 MW existing coal-fired power station. The engineering design and costing will be provided at an Association for the Advancement of Cost Engineering (AACE) Class 2 estimate, which will result in cost accuracy in the range of 15 to +20%, supported by completion of up to 70% of the engineering effort. This paper reviews the engineering work completed to support the cost estimate, which includes process flow diagrams, utility flow diagrams, and general arrangement drawings. Balance of plant (BOP) engineering design was also completed to support the costing effort and includes the necessary tie-ins to the power station for steam, electricity, cooling water, makeup water, and flue gas. The culmination of this work is a 3D model of the carbon capture system as integrated at GGS. The 3D model incorporates the capture island and all the BOP modifications and necessary subsystems for the inclusion of the CO2 capture plant and ensure that the necessary level of detailed engineering has been achieved to support the cost estimate. A preliminary cost evaluation is discussed projecting the expected capital cost, operation and maintenance (O&M) cost, and cost of capture outcome based on the above engineering work. The CO2 capture rate efficiency was not confined or restricted for the FEED project but when optimized for this design basis remained at approximately 90% for full load operation. During the periods of turndown for the power plant, the CO2 capture efficiency increases to over 95% from the flue gas to fully utilize the installed equipment. This paper will also discuss the design and engineering challenges encountered while striving to integrate a CO2 capture system onto an existing power station. These challenges are not unique to this installation and include: • Sizing the system to best match the power station demands in a flexible power market • Designing a steam supply system that covers operations at all load conditions • Designing a heat rejection system with limited availability of cooling water resources and other factors that are site specific The project team has integrated several independent studies into this FEED project to address these specific challenges and this paper will review these results.

A calculation method of a heat rejection system in a lunar power plant consisting of a free-piston Stirling engine (FPSE)

The colonization of the Moon and building permanent lunar outposts in the near future has become a main objective for most space agencies around the world. Future lunar outpost with a permanent human presence would have high power requirements to support such activities as scientific experimentation, in situ mining and processing, astronomical observation, and surface exploration. Several challenges are associated with the habitation of the lunar surface, such as, choosing an appropriate power generation unit. Stirling engines are one of the attractive power generation options which were implemented in space missions before, and are most likely to be used in future space exploration missions. In this paper, a detailed calculation methodology of a radiator design in a lunar power plant is developed. A brief research on some of the challenges that are associated with the habitation of the Moon and an overview of a lunar power plant consisting of a free piston Stirling engine (FPSE) are presented. A program code based on the developed calculation methodology consisting of two algorithms (one- and two-dimensional cases) was developed, and the results of both algorithms were compared and presented. Moreover, three different radiator design options having helium as a refrigerant circulating in them were considered and their results were analyzed. Finally, the conclusions and future directions of this work are provided.

Experimental Evaluation of Performance Characteristics of a Horizontal Copper Mesh Wick-Based Miniature Loop Heat Pipe

The ever-increasing computational power of the electronic devices accompanies a greater power dissipation. Understanding and optimization of the heat rejection systems are thus vital to ensure effective thermal management of electronic devices in an often highly confined space. This paper reports the design, development, and results of an experimental investigation of a fast responding and passive heat-rejecting copper–water-based miniature loop heat pipe (mLHP) system. The designed mLHP has a square flat-faced evaporator of 20 mm length with opposite replenishment, four extended surfaces with vapor space, a liquid reservoir or compensation chamber, fin- and tube-type condenser, and different diameter transport lines. Capillary pumping action was ensured by a composite of 100PPI copper mesh with 37% porosity and absorbent wool of 20 µm average pore size. Wick hydration was safeguarded by a 67% filling ratio of mLHP at a cold state. A decrease in thermal resistance and an increase in heat transfer coefficient for evaporation were observed. The thermal resistance of the evaporator and mLHP was 0.109–0.396 °C/W and 0.132–0.644 °C/W with an uncertainty of 3.749% and 3.744%, respectively. The heat transfer coefficient for evaporation achieved during experimentation was 6.313–23.00 kW/m2K with 0.345% uncertainty. No reverse flow of vapors and evaporator dry-out was detected. The maximum temperature sustained was 108 °C against the 205 W heat load with a 4.5-min start-up duration. These results reveal the suitability of the designed miniature loop heat pipe for the heat dissipation of electronics.

Performance evaluation of an air conditioning system based on quasi isothermal dehumidifcation

This paper presents a first study of novel air conditioning system based on quasi isothermal dehumidification combined with dew-point evaporative cooling through the Maisotsenko cycle (M-Cycle). The dehumidification process is realized in desiccant coated heat exchanger with heat rejection system. The key idea behind the solution is to maximize the cooling effectiveness by creating favourable conditions for dehumidification and evaporative cooling process. This is achieved by rejecting the heat of sorption in the desiccant system and providing cold and dry air for M-Cycle air cooler, which allows the system to be powered by using low-grade heat energy at 50 °C. The analysis is performed on the basis of experimentally validated numerical models of each component combined in a unified model of the system. The goal of the study is to determine the performance of the solution under different operational conditions. In addition two types of M-Cycle air coolers (i.e. cross-flow and regenerative unit) are compared to establish which unit is more suitable as the source of cooling energy for the system. It was established that proposed system is able to provide comfortable room parameters (supply air temperature equal 17 °C and humidity ratio equal 0.010 kg/kg-da) for outdoor air temperatures ranging from 28 to 36 °C and outdoor air humidity ranging from 0.010 to 0.018 kg/kg-da. This means that the proposed solution is able to provide comfortable conditions in temperate and humid climates. Results also indicate that the system consisting of the regenerative M-Cycle HMX allows better cooling load management, offers higher thermal Coefficient of Performance (COPth) and uses less water compared to the system with the cross-flow HMX. However, the system with cross-flow heat and mass exchanger (HMX) has a relatively greater electrical Coefficient of Performance (COPel) in most cases.