© 2021 Elsevier LtdScramjet (supersonic combustion ramjet) engines are often seen as a promising alternative to place payloads in the Earth's orbit. Such air-breathing engines have a simple structure and few moving parts. On the other hand, high heat fluxes and pressure loads on the walls, shock-wave-boundary-layer interactions, and the risk of choked flow inside the isolator channel are a few examples of obstacles that need to be addressed during scramjet design. Therefore, this study aims to evaluate of different air freestream conditions on the flow field in a scramjet inlet (compression ramps and isolator) through detailed three-dimensional computational fluid dynamics (CFD) simulations. The Mach number, flight altitude, and angle of attack were the evaluated conditions. Moreover, this study also focuses on the behavior of the boundary layer separation located at the compression ramp corners, isolator entrance, and scramjet sidewalls. Regarding the Mach number variation, the results showed that high Mach numbers yielded high-pressure levels throughout the engine. Furthermore, a higher altitude promoted a lower total pressure field. Considering the angle of attack changes, it is evident that a higher angle of attack results in a decrease in airflow pressure, while an increase in the total pressure along the walls is observed. By investigating these freestream parameters, this work can contribute to the early phase of engine design, avoiding critical failures in the scramjet structure due to aerodynamic load and thermal stress.

© 2021, The Brazilian Society of Mechanical Sciences and Engineering.The present study proposes the assessment of closure relations related to bubble parameters found in the wall heat flux partitioning model, as well as the assessment of interfacial heat transfer correlations, via CFD (computational fluid dynamics) simulation. The selection of these closure relations, most of the time, does not consider its applicability and scope, neglecting the effects that might affect the results obtained in two-phase flow simulations. In order to assess which relations can properly predict the boiling two-phase flow characteristics found in PWRs, an upward subcooled flow boiling in an annulus, with R-134a as the working fluid, was simulated based on the Eulerian two-fluid model. The void fraction radial profile of two-phase flow was attained, analyzed and compared to a benchmark study at different elevations. It can be concluded that the increase in pressure has a distinguishable impact on the predicting performance of the correlations. A trend of void fraction overprediction was observed at the inlet channel, whereas an underpredicting performance was found at the outlet channel. A crossed effect between bubble departure diameter, nucleate site density, and interfacial heat transfer coefficient on the void fraction profile was evidenced. Insightful considerations regarding the best models to be chosen for the simulation of subcooled boiling flows can be extracted from this investigation.

© 2021 Elsevier LtdCompact 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.

© 2020 Elsevier LtdNuclear power conversion in space has been approached by various means since the first space missions, with the advent of concepts such as thermoelectric, thermionic and thermodynamic conversion. Nowadays, thermal cycles are under greater focus for being capable of providing higher conversion efficiencies. In this context, one of the main concerns of engineers is the trade-off between power and mass. Therefore, this work aims the optimization of a recuperator used in a regenerative closed Brayton cycle applied for power conversion in the project of a small-scale nuclear reactor. The recuperator consists of a cross-flow, shell-and-tube heat exchanger with a matrix of tubes distributed in a staggered configuration. In this work, the number of tubes and the mass flow rate are varied. The number of tubes distributed axially is fixed as 4, whereas the quantity around the axis can be 5, 7, 9, 12 and 16 tubes. The working fluid considered in this study is a mixture of noble gases He-Xe with a molecular weight of 40 g/mol, whereas Inconel alloy 617 is applied as the recuperator material. The optimization procedure was based on the entropy generation minimization and the heat exchanger effectiveness, using the Computational Fluid Dynamics (CFD) technique to obtain the flow field. Optimum mass flow rates are obtained for all the geometries at the points of minimum entropy generation number, around which lie the ranges of tested mass flow rates. The ratio between the entropy generation number and effectiveness associated with the optimum mass flow rate is considered a performance evaluation criterion, and the dependence of this parameter with exchanger mass is assessed in order to select the most suitable geometry for the studied application. This analysis leads to the optimum design point at the geometry of 9 tubes around the recuperator axis, yielding a lost available work of 929.76 W for an ambient temperature of 298 K.

© 2020, The Brazilian Society of Mechanical Sciences and Engineering.Air-breathing hypersonic vehicles can be powered by scramjet (supersonic combustion ramjet) engines, whose inlet requires blunt leading edges in order to overcome the high heat fluxes that are inherent of hypersonic flights. Thus, this work has the purpose of evaluating, through detailed two-dimensional computational fluid dynamics analysis, the influence of the leading-edge bluntness and isolator height on the airflow of a scramjet intake. Three radii have been analyzed for the forebody and cowl leading edges: sharp, 0.5 and 1.0 mm. Moreover, two different isolator heights were varied: 15 and 20 mm. Fixed temperatures of 300 K and 1300 K were considered along the hypersonic vehicle wall. The static temperature, pressure, and Mach number contour are part of the analysis, as well as the Stanton number and the adiabatic kinetic efficiency of the scramjet. It was evidenced that both leading-edge bluntness and isolator height performed great influence on the intake airflow structure. Results showed that the isolator height had a major effect on the flow separation at the isolator entrance yielding a recirculation zone that increases with the isolator height. Furthermore, higher Stanton numbers and temperature along the isolator were found for the shorter isolator height due to stronger shock waves and higher frequency of shock-wave train. The leading-edge bluntness, in this analysis, was taken as a parameter to reduce the heat flux at the stagnation point of leading edges. However, a decrease in the adiabatic kinetic efficiency was observed when the leading-edge radius is increased. Moreover, blunt leading edges promoted a decrease in Stanton number along the scramjet intake walls and also a decrease in average temperatures throughout the isolator.

© 2020 Elsevier LtdRegarding space exploration, an adequate design of an energy conversion system for power generation is essential and plays an important role in the mission feasibility, having low total mass and size as key features that differ space power plants from grounded power stations. The waste heat system of space power plants consists of heat pipes attached to a radiator panel and, as they are responsible for the highest size proportion of the total energy conversion system, special attention must be directed to these components during the design phase. Considering these aspects, this short communication aims the optimization of the cold-side temperature of a recuperated closed Brayton cycle for space power production. For this purpose, an endoreversible thermodynamic modeling is proposed, based on overall thermal conductances of heat exchangers and heat pipes, whereas a set of algebraic equations characterized the compressor and turbine performance. The ratio of the cycle power output per radiator area was considered as the objective function which is maximized. For a fixed heat input of 157 kW and heat source and sink temperatures of 1150 K and 200 K respectively, it was evidenced an optimum operating temperature ranging from 450 to 500 K for the cold heat pipe. Based on this range of operation, heat pipes made of titanium-water with rectangular grooves as the wick structure were chosen as reference for design modeling and assembly of the heat rejection system.

© ECOS 2020.All right reserved.Focusing on the great challenges of space exploration for the coming decades, there is a need for more efficient and compact energy conversion systems for space applications. To this end, a thermodynamic model was applied to analyze the energy and exergy efficiency of space nuclear power plant that operates with freepiston Stirling cycle for deep space exploration purposes. The model considered conduction losses (thermal bridge), heat pipe efficiency, finite regeneration time, and radiation losses. Thermodynamic irreversibilities of the reactor, regenerator, and radiator were computed in order to find which component is accountable for the highest exergy destruction. Results have shown that most of the exergy is destroyed in the regenerator and the radiator. With the increasing radiator area, the irreversibility of the radiator tends to decrease, but the mass per power output ratio also decreases. The optimum power output per radiator area and mass per radiator area pointed for 2.697m2of the radiator area for this energy conversion system. Furthermore, it was evidenced that the regenerator effectiveness considerably influences the overall cycle efficiency, as well as reference temperature (environment) and cold source temperature. These results can provide relevant guidelines for future design of a space power plant where the Stirling cycle is the chosen dynamic energy conversion system.

© 2019 Elsevier Ltd and IIRThe use of hydrocarbons as refrigerants has increased substantially in the past decades due to more severe regulations on direct and indirect carbon dioxide emissions. However, safety standards related to flammable fluids limit the amount of propane (R-290) used in air-conditioning (AC) systems. In this study, an experimentally validated numerical model for a split-type AC system is employed and, for the first time, an air-side peripheral fin geometry is used in the condenser in an attempt to reduce the refrigerant charge for a given system cooling capacity. To establish a fair basis for comparison, plain, wavy and louvered fins were also investigated in the model. A more realistic assessment of the system performance was achieved by considering the POE ISO 22 oil circulation ratio (OCR) in the computation of the physical properties of the mixture. The results have shown that, for the simulated range, heat exchangers with peripheral fins provided higher values of thermal conductance (UA) and cooling capacity than other types of fins. A substantial decrease in the R-290 mass (as high as 12.13%) was observed when peripheral fin condensers were employed.

© 2019 Elsevier LtdThe research and technological development towards compact energy conversion systems for space applications allow the emergence of new mission possibilities, especially those directed for deep space explorations. Besides the final efficiency, the most crucial factor of an energy conversion system for nuclear propulsion purposes is the total mass and size of the system. Considering the Closed Brayton Cycle (CBC) as the energy conversion system for a nuclear power system, a numerical analysis was carried out in order to predict the thermal performance of cold side of the system (i.e., heat pipes and radiator) for initial design purposes. The complete space heat pipe-radiator array was discretized in control volumes where a variation of geometrical parameters was included, resulting in a stepped trapezoidal-shaped radiator (RAD) as output. The heat capacity was limited by the geometry of each panel section, being its heat pipe modeled to fit the given geometry and verified against operational limits. The proposed design-based model considered a physical and thermal coupling with temperature drops along the heat pipe (HP) axial direction, the radiator panel surface, and the cold heat exchanger duct, providing reasonable global parameters to aid the design considering mass and size optimization of a heat pipe-radiator assembly. The number of heat pipes and the total heat pipe-radiator assembly mass and length were evaluated for different heat pipe spacing, heat transfer rate, cold heat exchanger (CHE) inlet temperatures, and radiation shield shadow angles. It was observed a point of minimum HP-RAD mass and length when the heat pipe spacing and CHE inlet temperature are varied, for a given heat transfer rate and shadow angle.

© 2019, The Brazilian Society of Mechanical Sciences and Engineering.Heat transfer processes using two-phase boiling flows are often found in the industry, due to the high-efficiency heat removal, with a minimum difference of temperature between the heated surface and fluid. Moreover, the use of computational fluid dynamics to solve safety issues related to nuclear reactors has increased rapidly, but a complete validation is still being carried out. Therefore, this study aims the assessment of different sub-models of the interfacial heat transfer coefficient which is used as a closure relation in the two-fluid multiphase model. As a manner to validate the numerical results, the set of experimental conditions of Bartolomei and Chanturiya (Therm Eng 14:123–128, 1967) were applied to an upwards flow in a circular channel, under high water saturation pressures. The interfacial sub-models were implemented into an axisymmetric simulation domain, using the Eulerian two-fluid approach. Three different saturation pressures and two uniform heat fluxes were considered in the simulation runs. Fixed mass flux and subcooled degree of 900 kg/m2 and 60 K were applied, respectively. A satisfactory agreement was achieved for the estimation of the heated wall temperature, the liquid bulk temperature and the onset of saturated boiling. Different heat transfer closure relations promoted different vapor volume fraction along the channel, indicating the importance of an adequate interfacial heat transfer correlation to predict the flow boiling phenomena.

© 2018 Elsevier Ltd and IIRThe number of air-conditioning (AC) applications where natural refrigerants are used has grown in the past few years, mainly due to their favorable thermodynamic characteristics. Hence, a numerical investigation based on the second law of thermodynamics and Seasonal Energy Efficiency Ratio (SEER) for R-290/POE ISO 22 condensers is performed, where the impact of the transversal number of tubes, fin density and length of the finned tube is evaluated for three types of fins (plain, wavy and louver). Thermodynamic properties were generated via the departure function approach using the Peng–Robinson equation of state. The influence of the POE ISO 22 concentration is also accounted for in the heat transfer and pressure drop calculations. Depending on the available heat transfer area, plain or louver fins are more indicated to enhance the AC performance enhancement. For a fixed cooling capacity, points of maximum second law efficiency are achieved when the transversal number of tubes and tube length are varied.

© 2017 Elsevier LtdThe use of natural convection as a passive heat removal mechanism has been widely explored as a safety system of nuclear power plants. Characteristics like low complexity and economy makes Natural Circulation Loop (NCL) an encouraging technology which is being investigated for the Generation IV nuclear energy systems. As a manner to enhance the performance prediction of NCL, this study provides an evaluation of the adaptive time-stepping scheme that the RELAP5/3D code uses to calculate the time-step size applied during a transient simulation. Additionally, the Fast Fourier Transform (FFT) and the Root-Mean-Square Deviation (RMSD) procedures were conducted in order to investigate the agreement between the numerical and experimental data during the time frame when two-phase flow instabilities appear. Temperature measurements extracted from an experimental facility were used as the baseline data. For this evaluation, four different maximum time-step sizes were considered and their effect on the prediction of NCL temperatures was analyzed.

© 2017The heat transfer mechanism of natural convection has been extensively studied as a passive heat removal system of new nuclear power plants. Considering this aspect, the main objective of this study is to present an assessment of RELAP5 linear-equation solver under a transient two-fluid model for a two-phase natural circulation loop (NCL). For this assessment, three different approaches of linear-equation solvers for the hydrodynamic model are presented: the sparse matrix solver based on the Lower-Upper (LU) decomposition, the Border-Profile Lower Upper (BPLU) solver and the iterative method named Generalized Minimal Residual Method (GMRES). For comparison purposes, an experimental natural circulation loop made of glass tubes and using water as working fluid is analyzed. The onset of nucleate boiling observed during the experiment was predicted by all RELAP5 solvers as well as the representation of flow oscillations along the loop. Furthermore, it was noticed that the choice of the solver algorithm has a strong influence on the prediction of the two-phase natural circulation phenomena, since different wavelengths and amplitudes of flow instabilities were obtained for each approach.

© 2016 Elsevier B.V.The present study concerns a detailed analysis of flow boiling phenomena under high pressure systems using a two-fluid Eulerian approach provided by a Computational Fluid Dynamics (CFD) solver. For this purpose, a vertical heated pipe made of stainless steel with an internal diameter of 15.4 mm was considered as the modeled domain. Two different uniform heat fluxes and three saturation pressures were applied to the channel wall, whereas water mass flux of 900 kg/m2 s was considered for all simulation cases. The model was validated against a set of experimental data and results have indicated a promising use of the CFD technique for estimation of the wall temperature, the liquid bulk temperature and the location of the departure of nucleate boiling. Changes in factors applied in the modeling of the interfacial heat transfer coefficient and bubble departure frequency were suggested, allowing a better prediction of the void fraction along the heated channel. The commercial CFD solver FLUENT 14.5 was used for the model implementation.

© 2016 Elsevier Ltd and IIRRefrigeration equipment as the compressor has had its size reduced and cooling capacity increased in the last years, which leads to a continuous rise of the level of heat flux dissipated through the compressor shell and may cause reliability issues related to the compressor's moving parts. Furthermore, extreme conditions as high ambient temperatures augment the importance of keeping the compressor shell temperature under safe limits. To overcome such difficulties, a novel vapor-compression refrigerating loop was created especially for the compressor thermal management. A cooling unit was assembled using the proposed loop and later compared experimentally with two thermoelectric coolers under ambient temperatures of 25, 35 and 55 °C. The results have shown that the novel refrigerating loop enables the vapor-compression unit to work at a high ambient temperature (55 °C) and nearly a two times higher coefficient of performance (COP) was achieved, when compared to thermoelectric solutions.

© 2016 Elsevier LtdAir-conditioning applications using propane (R-290) have several environmental and thermodynamic advantages over more commonly used refrigerants, such as R-410A and R-22. This paper presents the development of a mathematical model for variable capacity air conditioning systems that use R-290/POE ISO 22 as refrigerant/lubricant. The thermodynamic performance of the refrigeration system is evaluated in terms of the SEER (Seasonal Energy Efficiency Ratio). The thermodynamic properties of the refrigerant/lubricant mixture were obtained from a departure-function approach using the Peng-Robinson equation of state. The effect of the oil on the condenser and evaporator heat transfer coefficients and pressure drops was also taken into account. Sub-models were developed for each component of the air conditioning system, including the connecting lines and the scroll compressor. Furthermore, an air conditioner experimental calorimeter was constructed and tested in order to validate the proposed model.

© 2016 The Author.The analysis based on the second law of thermodynamics of a lightweight vapor compression refrigeration system is presented. A small-scale linear compressor was applied in a DC-powered portable cooler for vehicles and for the medical field, using finned-tube heat exchangers and R600-a (Isobutane) as the working fluid. The cooler was tested in an environmental chamber (with controlled temperature and humidity) under three different ambient temperatures (21, 25 and 32 °C) in order to measure key parameters of the system, such as cooling capacity, power consumption and internal air temperature. For the comparison of thermodynamic irreversibilities, a conventional vapor compression refrigerating system was also tested under the same ambient conditions. Results indicated that the system with the proposed lightweight system kept the lowest internal air temperature with higher coefficients of performance, showing how a lightweight cooling unit can enlarge the use of refrigerating systems due to its attributes.

© 2015 Elsevier Ltd.Abstract Nuclear power systems turned to space electric propulsion differ strongly from usual ground-based power systems regarding the importance of overall size and mass. For propulsion power systems, size and mass are essential drivers that should be minimized during conception processes. Considering this aspect, this paper aims the development of a design-based model of a Closed Regenerative Brayton Cycle that applies the thermal conductance of the main components in order to predict the energy conversion performance, allowing its use as a preliminary tool for heat exchanger and radiator panel sizing. The centrifugal-flow turbine and compressor characterizations were achieved using algebraic equations from literature data. A binary mixture of Helium-Xenon with molecular weight of 40 g/mole is applied and the impact of the components sizing in the energy efficiency is evaluated in this paper, including the radiator panel area. Moreover, an optimization analysis based on the final mass of heat the exchangers is performed.

Nuclear power systems turned to space electric propulsion differs strongly from usual ground-based power systems regarding the importance of overall size and weight. For propulsion power systems, size/weight are essential drivers that should be minimized during conception processes. Considering that, this paper aims the development of a design-based model of a closed Brayton cycle that applies the thermal conductance of the main components in order to predict the energy conversion performance, allowing its use as preliminary tool for the heat exchangers and the radiator panel sizing. The centrifugal-flow turbine and compressor characterization were achieved using algebraic equations from literature data. The binary mixture of He-Xe with molecular weight of 40 g/mole is applied and the impact of the components sizing in the energy efficiency is evaluated in this paper, including the radiator panel area. Moreover, an optimization analysis based on the final area/size of these components is performed.

Experimental work was conducted to compare the thermal-hydraulic performances of cross-flow microchannel condensers using louvered fins and metal foams as extended surfaces. Three copper foam surfaces with pore densities of 10 and 20 pores per inch (PPI) and porosities of 89.3 and 94.7%, and three aluminum louvered fins with lengths of 27 and 32 mm (in the flow direction) and heights of 5 and 7.5 mm were evaluated. The experiments were carried out in a closed loop wind-tunnel calorimeter equipped with a R-600a refrigeration loop. A condensing temperature of 45 °C was used in all tests, with face velocities ranging from 2.1 to 7.7 m/s. A comparison based on the thermal conductance and air-side pumping power showed that the surfaces enhanced with louvered fins performed better than the metal foams under all conditions investigated. © 2012 Elsevier Ltd. All rights reserved.

For electronics cooling, the matching between size and efficiency is a desirable aspect of the design of cooling units. Hence, the compact cooling units are suitable for this area of refrigeration due to high capacity by size ratio and lightweight. For this purpose, Embraco developed a new refrigerating system for small enclosures and command panels with the use of a new miniaturized linear compressor called microcompressor. This unit consists in an outdoor mounted air-conditioning with finned-tube heat exchangers whereas the microcompressor is a direct current (DC) oil-free compressor that uses R-134a as refrigerant fluid. The paper presents a detailed description about the cooling unit and its performance tests. In order to run all experimental tests, a cooling unit calorimeter was assembled. In the experimental facility, the ambient temperature was varied for evaluation purposes whereas the bulk temperature inside the enclosure was kept at 35 °C via an electric heater. A commercially available thermoelectric system used for electronics cooling was also tested and compared with the vapor compression system. The results have shown that the microcompressor unit presented a coefficient of performance (COP) approximately two times larger than thermoelectric solution and higher cooling capacities were achieved under low ambient temperatures. © 2012 IEEE.

The thermal-hydraulic performance of microchannel condensers with open-cell metal foams to enhance the air-side heat transfer is investigated in this paper. Three different copper metal foam structures with distinct pore densities (10 and 20 PPI) and porosities (0.893 and 0.947) were tested. A conventional condenser surface, with copper plain fins, was also tested for performance comparison purposes. The experimental apparatus consisted of a closed-loop wind tunnel calorimeter and a refrigerant loop, which allowed the specification of the mass flow rate and thermodynamic state of R-600a at the condenser inlet. The experiments were performed at a condensing temperature of 45 °C. The air-side flow rate ranged from 1.4 × 10 -3 to 3.3 × 10 -3 m 3/s (giving face velocities in the range of 2.1-4.9 m/s). The heat transfer rate, the overall thermal conductance, the Colburn j-factor, the friction factor and the pumping power were calculated as part of the analysis. © 2011 Published by Elsevier Ltd. All rights reserved.

We present a critical review of the literature on the fundamentals, design, and application aspects of compact and miniature mechanical vapor compression refrigeration systems. Examples of such systems are those envisaged for electronics and personal cooling. In comparison to other refrigeration technologies (e.g., solid-state), vapor compression enables the attainment of low evaporating temperatures while maintaining a large cooling capacity per unit power input to the system. Over the past decade, there have been a significant number of studies devoted to the miniaturization of system components, with the most critical being the compressor. When compared with competing cooling technologies, such as flow boiling in microchannels, jet impingement, and spray cooling, refrigeration is the only one capable of lowering the junction temperature to values below the ambient temperature. The combination of vapor compression refrigeration with the aforementioned technologies is also possible, necessary, and beneficial, since it increases greatly the potential for reducing the system size. For each main application, this paper sheds some light on the thermodynamic and thermal aspects of the cooling cycle and on recent developments regarding its components (compressor, heat exchangers, and expansion device). Whenever appropriate, issues and challenges associated with the different cycle designs are addressed. An overview of the ongoing efforts in competing technologies is also presented. © 2012 Copyright Taylor and Francis Group, LLC.

We investigate a novel evaporator design for a small-scale refrigeration system whose function is to assist the existing heat pipe technology currently used in chip cooling of portable computers. A heat transfer model for the evaporator/heat pipe assembly was devised specifically for sizing the evaporator in order to keep the chip surface temperature below a certain value. A prototype was tested with R-600a at saturation temperatures of 45 and 55 °C, mass flow rates between 0.5 and 1.5 kg h-1 and heat transfer rates between 30 and 60 W. The experimental results demonstrated that the average refrigerant-side heat transfer coefficient is more sensitive to a change in the refrigerant mass flux than to changes in the saturation temperature and heat transfer rate. The agreement between the calculated heat transfer coefficient and the data was within ±10% for the conditions evaluated. © 2010 Elsevier Ltd and IIR. All rights reserved.