29 April 2024, Volume 33 Issue 3
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Special Column for the 9th Asian Joint Workshop on Thermophysics and Fluid Science 2022 (AJWTF 2022)Journal of Thermal Science. 2024, 33(3): 793. https://doi.org/10.1007/s11630-024-1988-y
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Journal of Thermal Science. 2024, 33(3): 794-806. https://doi.org/10.1007/s11630-024-1968-2The oscillatory response of multiple shock waves to downstream perturbations in a supersonic flow is studied numerically in a rectangular duct. Multiple shock waves are formed inside the duct at a shock Mach number of 1.75. The duct has an exit height of H, and the effect of duct resonance on multiple shock oscillations is investigated by attaching exit ducts of lengths 0H, 50H, and 150H. The downstream disturbance frequency varied from 10 Hz to 200 Hz to explore the oscillation characteristics of the multiple shock waves. The oscillatory response of shock waves under self-excited and forced oscillation conditions are analyzed in terms of wall static pressure, shock train leading-edge location, shock train length, and the size of the separation bubble. The extent of the initial shock location increases with an increase in exit duct length for the self-excited oscillation condition. The analysis of the shock train leading edge and the spectral analysis of wall static pressure variations are conducted. The variation in the shock train length is analyzed using the pressure ratio method for self-excited as well as forced oscillations. The RMS amplitude of the normalized shock train length (ζST) increases with an increase in the exit duct length for the self-excited oscillation condition. When the downstream perturbation frequency is increased, ζST is decreased for exit duct configurations. For all exit duct designs and downstream forcing frequencies, the size of the separation bubble grows and shrinks during the shock oscillations, demonstrating the dependence on duct resonance and forced oscillations.
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Journal of Thermal Science. 2024, 33(3): 807-814. https://doi.org/10.1007/s11630-024-1969-1A pair of unidirectional turbines (UT) can operate in oscillatory airflow without additional units. However, this arrangement suffers from poor flow rectification. A fluidic diode (FD) offers variable hydrodynamic resistance based on the flow direction, and this can be coupled with UT to improve flow rectification. In this work, a numerical investigation on the effect of FD with UT is presented using the commercial fluid dynamics software ANSYS Fluent 16.1 with k-ω SST turbulence closure model. Periodic domains of UT and FD are numerically validated individually with experimental results. Later, both are coupled to obtain the combined effect, and these results are compared with the analytical approach. It was observed that coupling FD with UT improved the unit’s performance at the lower flow coefficient (<1), but its performance decreased as the flow coefficient increased. Due to the diode’s presence, fluid leaving the turbine experiences higher resistance at a higher flow coefficient, which decreases the overall performance of the combined unit.
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Journal of Thermal Science. 2024, 33(3): 815-832. https://doi.org/10.1007/s11630-024-1980-6Flow around a pair of flat plates is a basic hydrodynamics problem. In this paper, the flow and heat transfer characteristics of two parallel plates with different edge shapes are numerically calculated. Under different inclined angles, the influence of chamfered and rounded structures with different sizes at the end-edge on unsteady flow and heat transfer characteristics of two parallel plates are analyzed. It is found that the instability and unsteadiness of flow decrease with the increase of end-edge size, and the non-uniformity of wake velocity of both rounded and chamfered plates decreases gradually. The non-uniformity of wake temperature increases firstly and then decreases at a small inclined angle, and the amplitude becomes the largest when Srou(Scha)=3, while it basically keeps monotonically increasing at a large inclined angle. Moreover, the global heat transfer performance of the flat plate is obviously affected by the end-edge modification, especially the chamfered structure. With the increase of chamfered size, the global Nusselt number basically shows the decreasing trend. This study provides a theoretical basis for the application of plate-shape structure in engineering fields.
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Journal of Thermal Science. 2024, 33(3): 833-846. https://doi.org/10.1007/s11630-024-1986-0Hot gas ingestion refers to the phenomenon of mainstream hot gas flowing into the space cavity of a turbine wheel. Previous studies have found that mainstream annulus pressure distribution plays an important role in hot gas ingestion, but due to its complexity, the mechanism of the interaction between mainstream flow and hot gas ingestion remains unclear. This paper adopts the URANS method, and three sealing flow rates are considered, named Cw=0, Cw=500, and Cw=5000. The time-averaged annulus pressure distribution shows that an increase in the sealing flow decreases the pressure value, and the effects of the sealing flow on the pressure distribution of the leading edge of the blade are much more influential than that of the trailing edge of the vane. The unsteady pressure time-space distribution in the annulus indicates that a time-space tilted distribution of pressure at the rim exits when the sealing flow exists. This phenomenon is mainly due to the strong feedback mechanism of the sealing flow to the annulus pressure field. A comparison of the pressure and mean radial velocity distribution of the mainstream shows that the ingestion mainly occurs on the blade side, where the pressure is lower than on the vane side. The flow characteristics at the wheel rim are analyzed with a sealing flow rate Cw=5000, and under these conditions, both pressure-induced ingestion and ingestion caused by a passage vortex can be inferred. The three-dimensional and inertial effects of the mainstream at the wheel rim lead to the generation of separation vortices on the blade side, and the presence of separation vortices leads to ingestion along the blade side. At the same time, pressure on the blade side will cause the fluid to have a radial inward flow tendency, which will promote the formation of separation vortices, leading to more serious ingestion in the high-pressure region on the blade side. The blade pressure field can be more significant than the vane trailing pressure field in the rim seal ingestion, and it contributes some explanations to the open question: the effect of blade on ingestion.
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Journal of Thermal Science. 2024, 33(3): 847-855. https://doi.org/10.1007/s11630-024-1940-1A dispersion system fluid can convect even if the dispersoid is a solid phase. Therefore, heat exchange performance can be improved while maintaining fluidity using a material with high thermal conductivity as the dispersoid. This study presents the melting performance evaluation results of a latent heat storage material with a carbon nanotube (CNT) dispersion system with high thermal conductivity, which enhances the thermal conductivity of the latent heat storage material and does not limit free convection. Increasing the thermal conductivity and enhancing the melting convection of the heat storage material result in increased latent heat storage speed. In this study, the thermal conductivity of the latent heat storage material was successfully increased by dispersing CNTs in the material. When 0.1% (in mass) of multi-wall CNT (MWCNT) was dispersed in a paraffin-based latent heat storage material, the shear stress increased by 1.5 times at a shear rate of 500 s–1, while taking into account the potential effects of convective inhibition. Therefore, a latent heat storage experiment was conducted in a rectangular heat storage tank using the CNT dispersion composition ratio as a parameter. A rectangular vessel with a heated vertical surface was used for the latent heat storage experiment. The melting speed was determined by comparing the amount of latent heat stored in a CNT-dispersed latent heat storage material and a single-phase latent heat storage material sample. The experimental results show that the time required for the latent heat storage material to completely melt in the heat storage tank was the shortest for the single-phase latent heat storage material sample. However, the fastest melting progress was observed for the sample with 0.02% (in mass) MWCNT content in the melting rate range of up to approximately 40% in the tank. The results indicate that this phenomenon is caused by the difference in the melting rates in the upper part of the tank. The generated data are useful for determining the shape and heat transfer surface arrangement of the latent heat storage tank.
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Journal of Thermal Science. 2024, 33(3): 856-871. https://doi.org/10.1007/s11630-024-1961-9Based on a small perturbation stability model for periodic flow, the effects of inlet total temperature ramp distortion on the axial compressor are investigated and the compressor stability is quantitatively evaluated. In the beginning, a small perturbation stability model for the periodic flow in compressors is proposed, referring to the governing equations of the Harmonic Balance Method. This stability model is validated on a single-stage low-speed compressor TA36 with uniform inlet flow. Then, the unsteady flow of TA36 with different inlet total temperature ramps and constant back pressure is simulated based on the Harmonic Balance Method. Based on these simulations, the compressor stability is analyzed using the proposed small perturbation model.Further, the Dynamic Mode Decomposition method is employed to accurately extract pressure oscillations. The two parameters of the temperature ramp, ramp rate and Strouhal number, are discussed in this paper. The results indicate the occurrence and extension of hysteresis loops in the rows, and a decrease in compressor stability with increasing ramp rate. Compressor performance is divided into two phases, stable and limit, based on the ramp rate. Furthermore, the model predictions suggest that a decrease in period length and an increase in Strouhal number lead to improved compressor stability. The DMD results imply that for compressors with inlet temperature ramp distortion, the increase of high-order modes and oscillations at the rotor tip is always the signal of decreasing stability.
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Journal of Thermal Science. 2024, 33(3): 872-887. https://doi.org/10.1007/s11630-024-1899-yAs a kind of large-scale physical energy storage, compressed air energy storage (CAES) plays an important role in the construction of more efficient energy system based on renewable energy in the future. Compared with traditional industrial compressors, the compressor of CAES has higher off-design performance requirements. From the perspective of design, it needs to pay attention not only to the performance of the design point, but also to the performance of all the stable working range. However, from the previous literature, no diagonal compressor was used in CAES which can meet the requirements, which also reflects the design program can be further improved. Therefore, this paper studies the design strategy of high efficient diagonal compressor for large-scale CAES, and gives the complete strategy algorithms used for different program modules. The pressure ratio, isentropic efficiency and stable working range are comprehensively considered. In the design process, the criteria for the key parameters of the diagonal flow angle of the diagonal compressor are given for the first time. The results show that the isentropic efficiency at the design point is 92.7%, the total pressure ratio is 1.97, and the stable working range exceeds 20%, which meets the design requirements of the compressor for CAES and exceeds the overall performance of the previous compressors in the entire working range.
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Journal of Thermal Science. 2024, 33(3): 888-898. https://doi.org/10.1007/s11630-024-1944-xThe low power consumption of the near-critical compressor is the key factor for the high efficiency of supercritical CO2 Brayton cycle. In the numerical simulation of the compressor, the rapid changes in the thermophysical properties of the CO2 near the critical point make it difficult to capture the condensation phenomenon. This paper investigates the influence of fluid physical properties on the condensation phenomenon. Firstly, the differences in the physical properties of CO2 in the SRK EOS (equation of state), PR EOS, and SW EOS are compared. Then, the simulation of nozzles and compressors were carried out and discussed. Results show that the condensation positions predicted by the three EOSs are basically the same. Compared with SW EOS, the disparities between the maximum condensation mass fraction predicted by the PR and SRK EOSs is 5.7% and 11.5%, and that of total pressure ratio is 0.3% and 3.8%, respectively. The results show that PR EOS can be considered for numerical simulation in engineering practice. Since its physical property calculation results are closer to the actual physical properties while the physical properties change more gently, it has considerable accuracy and numerical stability.
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Journal of Thermal Science. 2024, 33(3): 899-913. https://doi.org/10.1007/s11630-024-1951-yThe rotating stall mechanism is of high importance for the stability of centrifugal compressors and thermal power cycles. The majority of research concerning this topic has concentrated on the initial stall phase. However, the evolution of stall cells in wide-long diffusers has not been comprehensively studied. In this paper, the causes of rotating stall in the wide-long diffuser and the three-dimensional evolution mechanism of stall cells during the stall process were thoroughly analyzed. During the stall induction phase, an annulus vortex structure was found in the reverse-flow zone near the hub side of the diffuser outlet, which was the initial form of stall cells. The whole evolution process of stall cells was divided into three phases as the flow rate decreased. During the initial stall phase, the dynamic equilibrium was built under effects of the impeller wake and the adverse pressure gradient. As a result, the number of stall cells was kept at seven and the size of stall cells remained constant. During the transition phase, the flow in the diffuser became unstable. Stall cells extended to the impeller outlet, and the effect of the wake flow was strengthened significantly. Stall cells started integrating and separating regularly. As a result, the number and propagation speed of stall cells varied periodically at a constant mass flow rate. During the deep stall phase, the size of stall cells remained unchanged, and the number of stall cells kept at one. This study has important practical guidance and engineering value for the high-efficiency design and safe operation of centrifugal compressors.
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Journal of Thermal Science. 2024, 33(3): 914-930. https://doi.org/10.1007/s11630-024-1949-5The constrained multi-objective multi-variable optimization of fans usually needs a great deal of computational fluid dynamics (CFD) calculations and is time-consuming. In this study, a new multi-model ensemble optimization algorithm is proposed to tackle such an expensive optimization problem. The multi-variable and multi-objective optimization are conducted with a new flexible multi-objective infill criterion. In addition, the search direction is determined by the multi-model ensemble assisted evolutionary algorithm and the feature extraction by the principal component analysis is used to reduce the dimension of optimization variables. First, the proposed algorithm and other two optimization algorithms which prevail in fan optimizations were compared by using test functions. With the same number of objective function evaluations, the proposed algorithm shows a fast convergency rate on finding the optimal objective function values. Then, this algorithm was used to optimize the rotor and stator blades of a large axial fan, with the efficiencies as the objectives at three flow rates, the high, the design and the low flow rate. Forty-two variables were included in the optimization process. The results show that compared with the prototype fan, the total pressure efficiencies of the optimized fan at the high, the design and the low flow rate were increased by 3.35%, 3.07% and 2.89%, respectively, after CFD simulations for 500 fan candidates with the constraint for the design pressure. The optimization results validate the effectiveness and feasibility of the proposed algorithm.
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Journal of Thermal Science. 2024, 33(3): 931-950. https://doi.org/10.1007/s11630-024-1953-9The complementary of biomass and solar energy in combined cooling, heating and power (CCHP) system provides an efficient solution to address the energy crisis and environmental pollutants. This work aims to propose a multi-objective optimization model based on the life cycle assessment (LCA) method for the optimal design of hybrid solar and biomass system. The life-cycle process of the poly-generation system is divided into six phases to analyze energy consumption and greenhouse gas emissions. The comprehensive performances of the hybrid system are optimized by incorporating the evaluation criteria, including environmental impact in the whole life cycle, renewable energy contribution and economic benefit. The non-dominated sorting genetic algorithm II (NSGA-II) with the technique for order preference by similarity to ideal solution (TOPSIS) method is employed to search the Pareto frontier result and thereby achieve optimal performance. The developed optimization methodology is used for a case study in an industrial park. The results indicate that the best performance from the optimized hybrid system is reached with the environmental impact load reduction rate (EILRR) of 46.03%, renewable energy contribution proportion (RECP) of 92.73% and annual total cost saving rate (ATCSR) of 35.75%, respectively. By comparing pollutant-eq emissions of different stages, the operation phase emits the largest pollutant followed by the phase of raw material acquisition. Overall, this study reveals that the proposed multi-objective optimization model integrated with LCA method delivers an alternative path for the design and optimization of more sustainable CCHP system.
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Journal of Thermal Science. 2024, 33(3): 951-969. https://doi.org/10.1007/s11630-024-1842-2Humid air turbine cycle (HAT) has potential of electrical efficiencies comparable to combined cycle, with lower investment cost and NOx emission. The typical heat exchanger network of HAT consists of intercooler (if there is), aftercooler, recuperator, economizer and humidifier, which brings higher efficiency but makes the system more complex. To simplify HAT layout, a novel humidifier concept is proposed by integrating the aftercooler into traditional counter-current humidifier. Based on this concept, a one-dimensional model including pressure drop and exergy calculation is established to distinguish the thermodynamic and hydrodynamic characteristics, and then the structural parameters, such as the number of rows and columns, tube diameter, pitch and type for a micro HAT are identified. The results show that the aftercool-humidifier plays the same role as original aftercooler and humidifier, and can match the in-tube air, out-tube air and water stream well with lower volume. In the case of micro HAT cycle, the volume of heat and mass transfer area can be reduced by 47% compared with traditional design. The major thermal resistance occurred in the convection heat transfer process inside the tube; however, using enhanced tube cannot effectively improve the compactness of device.
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Journal of Thermal Science. 2024, 33(3): 970-984. https://doi.org/10.1007/s11630-024-1906-3For the efficient use of solar and fuels and to improve the supply-demand matching performance in combined heat and power (CHP) systems, this paper proposes a hybrid solar/methanol energy system integrating solar/exhaust thermochemical and thermal energy storage. The proposed system includes parabolic trough solar collectors (PTSC), a thermochemical reactor, an internal combustion engine (ICE), and hybrid storage of thermal and chemical energy, which uses solar energy and methanol fuel as input and outputs power and heat. With methanol thermochemical decomposition reaction, mid-and-low temperature solar heat and exhaust heat are upgraded to chemical energy for efficient power generation. The thermal energy storage (TES) stores surplus thermal energy, acting as a backup source to produce heat without emitting CO2. Due to the energy storage, time-varying solar energy can be used steadily and efficiently; considerable supply-demand mismatches can be avoided, and the operational flexibility is improved. Under the design condition, the overall energy efficiency, exergy efficiency, and net solar-to-electric efficiency achieve 72.09%, 37.65%, and 24.63%, respectively. The fuel saving rate (FSR) and the CO2 emission reduction (ERCO2) achieve 32.97% and 25.33%, respectively. The research findings provide a promising approach for the efficient and flexible use of solar energy and fuels for combined heat and power.
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Application of Response Surface Methodology for Analysing and Optimizing the Finned Solar Air HeaterJournal of Thermal Science. 2024, 33(3): 985-1009. https://doi.org/10.1007/s11630-024-1934-zIn this research paper, a solar air heater with triangular fins has been experimentally analysed and optimized. Initially, an experimental set-up of a solar air heater having triangular fins has been developed at the location of 28.10°N, 78.23°E. The heat transfer rate through fins and fins efficiency has been determined by the Finite Difference Method model equations. The experimental data and modeled data of response parameters have been optimized in MINITAB-17 software by the Response Surface Methodology tool. For creating the response surface design, three input parameters have been selected namely solar intensity, Reynolds number, and fin base-to-height ratio. The range of solar intensity, Reynolds number, and fin base-to-height ratio is 600 to 1000 W/m2, 4000 to 6000, and 0.4 to 0.8 respectively. The response surface design has been analyzed by calculating the outlet temperature, friction factor, Nusselt number, fin efficiency, thermal performance factor, and exergy efficiency. The optimum settings of input parameters: solar intensity is 1000 W/m2; Reynolds number is 4969.7, and the fin base to height ratio is 0.6060, on which these response: namely outlet temperature of 92.531°C, friction factor of 0.2350, Nusselt number of 127.761, thermal efficiency of 50.836%, thermal performance factor of 1.4947, and exergy efficiency of 8.762%.
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Journal of Thermal Science. 2024, 33(3): 1010-1025. https://doi.org/10.1007/s11630-024-1937-9Recently, natural draft dry cooling system with the main-auxiliary integrated air-cooled heat exchangers in the up and lower layers, has drawn attention to the electric power industry. This research firstly develops two physical models for the integrated cooling system, namely Case A and Case B. In Case A, the main air-cooled heat exchanger is arranged in the upper layer and the auxiliary air-cooled heat exchanger arranged in the lower layer, while in Case B, the two heat exchanger systems are arranged in the opposite way. And then, directing at the engineering TMCR and TRL 1 working conditions, the unit-local-overall thermo-flow characteristics of Case A and Case B are obtained and compared by numerical simulation. The findings show that, for the auxiliary air-cooled exchanger, Case A has obviously higher cooling performances than Case B, with the difference varying from 5.46% to 7.55%. Whereas, for the main air-cooled exchanger, Case B shows the recovered cooling performances, with the difference changing from 1.15% to 2.99%. Case A is preferably recommended to the engineering application in consideration of more strict cooling demand of the auxiliary cooling system. Conclusively, this research will provide some theoretical guidelines for the design and construction of the main-auxiliary integrated natural draft dry cooling system.
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Journal of Thermal Science. 2024, 33(3): 1026-1036. https://doi.org/10.1007/s11630-024-1946-8For combined sensible-latent heat storage system (CSLHS) (termed as the hybrid configuration), macro encapsulation can effectively solve the leakage problem of PCMs. However, due to the poor thermal conductivity of PCMs, the charging performance of the hybrid configuration slightly increases compared to the solid structure (with only sensible materials). Meanwhile, the natural convection in the PCMs zone could improve the charging performance. So, how to improve natural convection intensity is a key issue for the CSLHS by macro encapsulating. It is found that adding fins can significantly enhance natural convection and accelerate the melting of PCM. In this paper, we proposed the hybrid configuration with fins built-in by macro encapsulation, and analyzed its charging performance with different fin structural parameters in the PCM zone by CFD simulation. In the case, the sensible heat storage material is high-temperature concrete and the PCM is a low-melting-point mixed molten salt. We analyzed the effects of fin number, fin length and fin thickness on the charging performance of the hybrid configuration respectively. From the result, the charging performance increases with the fin number, but the increase rate gradually decreases. When the fin number is 6, the charging performance increases by 20.18% compared to the situation without fin. The charging performance increases gradually with the fin length. Compared with the hybrid configuration without fin, for each 10 mm increase in fin length, its charging performances increase by 4.09%, 5.26%, 7.02%, 8.77%, 11.70%, and 15.79%, respectively. Different from number and length of fins, the effect of thickness on the charging performance is very small. When the fin thickness increased from 1 mm to 4 mm, the charging performance only increased by 2.3%. It indicates that the main reason for the improving the charging performance is to increase the natural convection intensity by dividing the PCM zone through fins. These results show that the charging performance of the CSLHS with macro encapsulation can be improved by optimizing fin structural parameters.
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Journal of Thermal Science. 2024, 33(3): 1037-1054. https://doi.org/10.1007/s11630-023-1933-5In Iran, the intensity of energy consumption in the building sector is almost 3 times the world average, and due to the consumption of fossil fuels as the main source of energy in this sector, as well as the lack of optimal design of buildings, it has led to excessive release of toxic gases into the environment. This research develops an efficient approach for the simulation-oriented Pareto optimization (SOPO) of building energy efficiency to assist engineers in optimal building design in early design phases. To this end, EnergyPlus, as one of the most powerful and well-known whole-building simulation programs, is combined with the Multi-objective Ant Colony Optimization (MOACO) algorithm through the JAVA programming language. As a result, the capabilities of JAVA programming are added to EnergyPlus without the use of other plugins and third parties. To evaluate the effectiveness of the developed method, it was performed on a residential building located in the hot and semi-arid region of Iran. To obtain the optimum configuration of the building under investigation, the building rotation, window-to-wall ratio, tilt angle of shading device, depth of shading device, color of the external walls, area of solar collector, tilt angle of solar collector, rotation of solar collector, cooling and heating setpoints of heating, ventilation, and air conditioning (HVAC) system are chosen as decision variables. Further, the building energy consumption (BEC), solar collector efficiency (SCE), and predicted percentage of dissatisfied (PPD) index as a measure of the occupants’ thermal comfort level are chosen as the objective functions. The single-objective optimization (SO) and Pareto optimization (PO) are performed. The obtained results are compared to the initial values of the basic model. The optimization results depict that the PO provides optimal solutions more reliable than those obtained by the SOs, owing to the lower value of the deviation index. Moreover, the optimal solutions extracted through the PO are depicted in the form of Pareto fronts. Eventually, the Linear Programming Technique for Multidimensional Analysis of Preference (LINMAP) technique as one of the well-known multi-criteria decision-making (MCDM) methods is utilized to adopt the optimum building configuration from the set of Pareto optimal solutions. Further, the results of PO show that although BEC increases from 136 GJ to 140 GJ, PPD significantly decreases from 26% to 8% and SCE significantly increases from 16% to 25%. The introduced SOPO method suggests an effective and practical approach to obtain optimal solutions during the building design phase and provides an opportunity for building engineers to have a better picture of the range of options for decision-making. In addition, the method presented in this study can be applied to different types of buildings in different climates.
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Journal of Thermal Science. 2024, 33(3): 1055-1064. https://doi.org/10.1007/s11630-024-1945-9In recent years, Enhanced Geothermal System (EGS) technologies have been applied to the geothermal resources production in the Hot Dry Rock (HDR). The core of EGS technologies is to adopt hydraulic fracturing in the reservoir to create a connected network of discrete fractures with the consideration of water as a working fluid for hydraulic fracturing and heat production. This paper investigates the characteristics of water flow behaviors through a single rough fracture under different temperature and pressure conditions. A single fracture model with rough fracture surfaces is constructed and then characterized, and influences of the anisotropic factor on the average tortuosity and frictional resistance coefficient of water flow through a single fracture with rough surfaces have been compared and analyzed. With consideration of other impacting factors (temperature, pressure, fracture roughness), the impact of mass flow rate has also been presented. Numerical simulation results present that changes of average tortuosity for water flow through a single rough facture are mainly affected by temperature. It can be observed that higher temperature leads to larger average tortuosity but the frictional resistance coefficient shows an opposite trend. As for pressure conditions, it is found that effects of pressure on average tortuosity and frictional resistance coefficient is very small, which can be neglected under high pressure conditions. Furthermore, the average tortuosity shows a progressively linear relationship with the mass flow rate. On the contrary, the frictional resistance coefficient has a negative relationship with the mass flow rate. It is revealed that when the mass flow rate reaches a critical point, the influences of temperature on the frictional resistance coefficient will be negligible. Comparisons of single rough fractures with different anisotropic factors show that values of average tortuosity and frictional resistance coefficient have positive relationships with the increase of anisotropic factors.
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Journal of Thermal Science. 2024, 33(3): 1065-1081. https://doi.org/10.1007/s11630-024-1927-yThis paper aims to conduct a comprehensive exergoeconomic analysis of a novel zero-carbon-emission multi-generation system and propose a fast optimization method combined with machine learning. The detailed exergoeconomic analysis of a novel combined power, freshwater and cooling multi-generation system is performed in this study. The exergoeconomic analysis model is established by exergy flow theory. A comprehensive exergy, exergoeconomic and environmental analysis is carried out. Five critical decision variables are researched to bring out effects on the multi-generation system exergoeconomic performance. A novel fast optimization method combining genetic algorithm and Bagging neural network is proposed. The advanced nature comparison is made between the proposed system and four similar cases. Results display that increasing the turbine inlet temperature can improve exergy efficiency and decrease the total product unit cost. The multi-generation system exergy destruction directly determines exergy efficiency and total exergy destruction cost rate. The total product unit cost in the cost optimal design case is reduced by 7.7% and 25%, respectively, compared with exergy efficiency optimal design case and basic design case. Compared with four similar cases, the proposed multi-generation system has great advantages in thermodynamic performance and exergoeconomic performance. This paper can provide research methods and ideas for performance analysis and fast optimization of multi-generation system.
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Journal of Thermal Science. 2024, 33(3): 1082-1093. https://doi.org/10.1007/s11630-024-1952-xWaste-to-Energy treatment is a promising path to environment and energy management in the future. This work detailed a binary molten salt thermal treatment methodology for the detoxification of spent cathode carbon block (SCCB) waste and the recycling of carbonaceous materials. The thermal behavior of SCCB and SCCB blended with molten salts was investigated. It was found that the NaCl-Na2CO3 binary molten salts significantly contributed to reducing pyrolysis onset temperature by 334.3 K compared to that of SCCB itself (i.e., activation energy of pyrolysis reaction was reduced from 4.24×105 to 2.30×105 J/mol), thus helping to lower thermal treatment energy consumption. With the addition of binary molten salts, the residue after thermal treatment in a horizontal tube furnace experiment was separated into two layers. The bottom-layer residue was mainly composed of molten salts. The fluorine content in the form of NaF and CaF2 of top-layer residue was reduced significantly while the carbon content remained unchanged. Specifically, the leaching concentration of fluoride ion was decreased from 4620 mg/L to 856 mg/L. It is noted that the NaF and CaF2 can be removed through water-leaching and hydrothermal acid-leaching methods and thus the carbonaceous materials with a calorific value of 17.5 MJ/kg were obtained.
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Journal of Thermal Science. 2024, 33(3): 1094-1108. https://doi.org/10.1007/s11630-024-1978-0The recovery of heat and water from low-grade flue gas is of considerable importance for energy conservation and environmental preservation. While the full-open absorption heat pump shows promise as a means of achieving heat and water recovery, the lack of research on heat and mass transfer performance of open-type solution evaporation regeneration represents a significant impediment to its design and operation. This paper experimentally investigates the regeneration performance of an open-type spaying tower equipped with ceramic structured packings. Two different regeneration modes are proposed, namely ambient air receiver mode and flue gas receiver mode, to utilize air or low-grade flue gas as a driving source. The impact of different input parameters on the regeneration characteristics, including heat transfer capacity, water removal rate, thermal efficiency, and humidity effectiveness, are demonstrated. The findings indicate that the enhancement of regeneration can be achieved through the increase of solution flow rate, solution temperature, and flue gas flow rate in both regeneration modes. However, high solution concentration and flue gas humidity ratio can weaken water removal rates and reduce thermal efficiency. For the regeneration of CaCl2-H2O with a concentration of 55%, flue gas around 200°C with a humidity ratio below 44 g/kg can successfully drive the solution regeneration process. When the solution concentration or flue gas humidity ratio continues to rise, additional energy is necessary for regeneration. Furthermore, the coupled heat and mass transfer coefficients are fitted, which can contribute to the design and optimization of the open-type regenerator.
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Journal of Thermal Science. 2024, 33(3): 1109-1118. https://doi.org/10.1007/s11630-024-1918-zLithium-ion batteries are used in a wide range of applications. However, their cycle life suffers from the problem of capacity fade, which includes calendar and cycle aging. The effects of storage time, temperature and partial charge-discharge cycling on the capacity fade of Li-ion batteries are investigated in this study. The calendar aging and cycle aging are presented based on the storage and cycling experiment on LiCoO2/graphite cells under different storage temperature and different ranges of state of charge (SOC). Based on the measurement data, a one-component and a double-component aging model are presented to respectively describe the capacity fade caused by calendar and cycle aging. The calendar aging of LiCoO2/graphite batteries is mainly affected by temperature and SOC during the storage. Mean SOC and change in SOC (∆SOC) are the main factors affecting battery degradation during cycling operation.
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Journal of Thermal Science. 2024, 33(3): 1119-1131. https://doi.org/10.1007/s11630-023-1858-zThe external surface heat transfer coefficient of building envelope is one of the important parameters necessary for building energy saving design, but the basic data in high-altitude area are scarce. Therefore, the authors propose a modified measurement method based on the heat balance of a model building, and use the same model building to measure its external surface heat transfer coefficient under outdoor conditions in Chengdu city, China at an altitude of 520 m and Daocheng city at an altitude of 3750 m respectively. The results show that the total heat transfer coefficient (ht) of building surface in high-altitude area is reduced by 34.48%. The influence of outdoor wind speed on the convective heat transfer coefficient (hc) in high-altitude area is not as significant as that in low-altitude area. The fitting relation between convection heat transfer coefficient and outdoor wind speed is also obtained. Under the same heating power, the average temperature rise of indoor and outdoor air at high- altitude is 41.9% higher than that at low altitude, and the average temperature rise of inner wall is 25.8% higher than that at low altitude. It shows that high-altitude area can create a more comfortable indoor thermal environment than low-altitude area under the same energy consumption condition. It is not appropriate to use the heat transfer characteristics of the exterior surface of buildings in low-altitude area for building energy saving design and related heating equipment selection and system terminal matching design in high-altitude area.
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Journal of Thermal Science. 2024, 33(3): 1132-1147. https://doi.org/10.1007/s11630-024-1942-zPrinted circuit heat exchanger (PCHE) has been widely used in supercritical carbon dioxide (S-CO2) power systems because of its high heat transfer efficiency and good compactness. However, due to the large variety of PCHE configurations, channel selection in practical applications lacks a basis. Therefore, this paper discussed the heat transfer and friction characteristics and the synergy of three fields in the channel under the guidance of the field synergy principle for four typical PCHE channels. Additionally, the comprehensive performance of four channels was compared. Finally, the heat transfer and friction factor correlations for S-CO2 in four channels were established. The findings demonstrate that the synergy of velocity and pressure fields of the straight channel PCHE is better (β≈180°), so its resistance loss is relatively small. The zigzag and sinusoidal wavy channels and the airfoil fins can reduce the angle α between the temperature gradient and velocity, thus enhancing the heat transfer. The sinusoidal wavy channel can reduce flow resistance compared to the zigzag channel due to the rounded corners. The streamlined airfoil structure can guide the flow and reduce backflow, thus reducing resistance losses. In the range of Re studied in this paper, the maximum error of the proposed heat transfer and friction factor correlations of PCHE is 7.0%, which shows good fitting accuracy. The research in this paper can provide a reference for the selection and design of PCHE with different channel configurations.
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Journal of Thermal Science. 2024, 33(3): 1148-1160. https://doi.org/10.1007/s11630-024-1912-5This paper proposed a new structure of highly-dense micro-jet arrays for hybrid jet-impingement/microchannel heat sinks (~120 jet per cm2 with jet width of 0.35 mm). Parametric study is performed to investigate the influence of structure and flow parameters on the convective heat transfer of water jet cooling in confined space. The simulation results show that the optimal jet width, jet spacing and impingement distance are around 0.2 mm, 0.2 mm and 0.3 mm, respectively, which can achieve a low thermal resistance as well as a relatively low pressure drop. The analysis of the heat transfer pathways shows that the micro-fin jet structures can extend the impinging heat transfer area and conduct a considerable proportion of heat, which can reach up to 38.9% with the Reynolds number ranging from 797.1 to 5602.2. The heat transfer characteristics in the heat sink will shift from impingement dominated heat transfer to the channel-convection dominated heat transfer as the jet impingement distance increases. A correlation is proposed to predict the average Nusselt number on the stagnation area for the heat sink with different structure parameters, and the deviations of predictions from the numerical results are less than 10%.
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Journal of Thermal Science. 2024, 33(3): 1161-1173. https://doi.org/10.1007/s11630-024-1956-6A suitable mixing rule is important for vapor liquid equilibrium (VLE) investigations for mixed refrigerants. In this work, a new excess free energy mixing rule (MRv) was proposed at zero pressure based on the linear relationship between dimensionless parameter 1/(u–1) and α. MRv mixing rule was explicit adopted variable liquid molar volume. The applicable temperature range of MRv could be extended by means of an empirical method to estimate the liquid molar volume for components at high temperatures. Three mixing rules modified Huron-Vidal mixing rule (MHV1), Wong-Sandler mixing rule (WS), and MRv at two reference pressures were used to compare the VLE data in the calculation of 37 mixed refrigerants. Results demonstrated that MRv had a relatively similar accuracy to MHV1 and WS for component and pressure calculation. Moreover, the average excess Gibbs free energy using the MRv mixing rule for the 37 selected mixed refrigerants (0.0013) was much lower than those using the MHV1 (0.0078) and WS (0.0809) mixing rules, which was very valuable for the design and optimization of thermodynamic systems using mixed refrigerants.
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Journal of Thermal Science. 2024, 33(3): 1174-1188. https://doi.org/10.1007/s11630-024-1959-3Microchannel flow boiling heat transfer has the advantages of strong heat dissipation capacity, good temperature uniformity, and compact structure. It is an excellent way to thermally manage electronic devices, but when the heat flux exceeds CHF (Critical Heat Flux), the heat transfer performance deteriorates as the working fluid dries out. Non-azeotropic mixtures have the potential to effectively delay or avoid dry-out during the boiling process due to their temperature slide characteristics which causes the mass transfer resistance. To understand the influence of non-azeotropic mixtures on microchannel flow boiling, using the phase-change microchannel heat sink as the research object, the experiments on the flow boiling heat transfer performance of R245fa/R134a mixtures under different working conditions were carried out, and the characteristics of flow boiling heat transfer were obtained under the different working conditions, and comparison was developed with those of pure substance R245fa. The results demonstrated that a small amount of low-boiling-point components in the high-boiling-point working fluid inhibited boiling heat transfer to some extent, and lowered the average heat transfer coefficient under the non-dryout condition slightly lower than that of the pure substance; however, it also effectively delayed the onset of local dry-out and prevented significant deterioration in thermal transfer performance under the lower mass flow rate and higher heat flux, which could enhance the heat sink’s stability.
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Journal of Thermal Science. 2024, 33(3): 1189-1199. https://doi.org/10.1007/s11630-024-1844-0This paper focuses on the laminar flame instability of three high molecular weight n-alkanes, namely n-hexane, n-octane, and n-decane. The experiment was carried out in a constant volume combustion bomb to get the flame images. The critical radius under different conditions was extracted using the image processing program. Combined with the existing critical Peclet number theory, the dominant factors of flame instability under current conditions for three n-alkanes can be figured out. Moreover, the average cell size (equivalent cell radius, Rcell) was extracted to provide quantitative analysis of the flame cellular structure, based on the method developed in this work. The theoretical Rcell were also calculated and compared with the experimental results to validate the proposed method.
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Journal of Thermal Science. 2024, 33(3): 1200-1215. https://doi.org/10.1007/s11630-024-1883-6In this study, the multi-objective intelligent optimization of the crevice structure in a rapid compression machine (RCM) is carried out based on the RCM simulation model modified with the KIVA-3V program. A multi-objective optimization simulation model of the crevice structure based on the large eddy simulation model coupled with the genetic algorithm NSGA-III is established. Six optimization parameters and seven optimization objectives are selected in the optimization process. The results show that the genetic algorithm can quickly find the values of the optimized parameters. The crevice volume ratio shows a trade-off relationship with the dimensionless temperature ratio Tmax/Taver and the tumble ratio. A larger crevice volume can reduce the flow of boundary layer cryogenic gas into the combustion chamber, thus improving the temperature uniformity. In addition, the crevice entrance width and the connecting channel length should be smaller, while the volume of the crevice main chamber should be larger, so as to sufficiently introduce the low-temperature gas of the boundary layer into the crevice and reduce their influence on the temperature field of the combustion chamber. When the crevice volume accounts for 10% of the total volume, the temperature uniformity of the combustor is significantly enhanced, and when the crevice volume accounts for 30.4%, there is almost no low-temperature vortex in the combustion chamber.
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Journal of Thermal Science. 2024, 33(3): 1216-1230. https://doi.org/10.1007/s11630-024-1935-yTo address the pressing need for intelligent and efficient control of circulating fluidized bed (CFB) units, it is crucial to develop a dynamic model for the key operating parameters of supercritical circulating fluidized bed (SCFB) units. Therefore, data-knowledge-driven dynamic model of bed temperature, load, and main steam pressure of the SCFB unit has been proposed. Firstly, a knowledge-driven method is employed to develop a dynamic model for key operating parameters of SCFB units. The model parameters are determined based on the operating data of the unit and continuously optimized in real time. Then, Bidirectional Long Short-Term Memory combined with Convolutional Neural Network and Attention Mechanism is utilized to build the dynamic model of bed temperature, load, and main steam pressure. Finally, a collaboration and integration method based on the critic weight method and the variation coefficient method is proposed to establish data-knowledge-driven model of key operating parameters for SCFB units. The model displays great accuracy and fitting ability compared with other methods and effectively captures the dynamic characteristics, which can provide a research basis for the design of intelligent flexible control mode of SCFB unit.
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Journal of Thermal Science. 2024, 33(3): 1231-1241. https://doi.org/10.1007/s11630-024-1939-7Natural gas is a promising alternative fuel for the internal combustion engine, and natural gas engine has become an efficient and feasible measure to deal with the energy shortage and climate change. Since the laminar flame characteristics are the foundation of the turbulent flame, the laminar flame characteristics of natural gas have a significant impact on the combustion status and efficiency of the engine. A visual constant volume bomb was used to study the influence of the gas components, different excess air coefficient (λ), and initial conditions on the laminar combustion characteristics of natural gas. The experimental results showed that when the initial pressure and temperature were 0.1 MPa and 300 K respectively, compared to propane, ethane had a remarkable influence on the equivalent-combustion laminar-combustion-speed, with an average increase of approximately 5.1% for every 2.5% increase in the ethane proportion. The laminar combustion velocity of the natural gas under different excess air coefficients had a maximum value at about λ=1.0, and the Markstein length of the flame decreased with the increase of the λ. The increase in the initial pressure of the mixture resulted in a decrease in the equivalent-combustion laminar-combustion-speed of the flame, a significant decrease in the Markstein length. The increase of the initial temperature of the mixture led to a rapid increase of the equivalent-combustion laminar-combustion-speed, but the effect on the flame Markstein length was not dominant.
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Journal of Thermal Science. 2024, 33(3): 1242-1256. https://doi.org/10.1007/s11630-024-1987-zThe co-combustion of low-rank coals through fluidized bed boiler (CFB) is an effective approach to enhance the level of resource utilization. To date, there has been a lack of investigation concerning the co-combustion kinetics and self-desulfurization characteristics of coal slime, coal gangue, and raw coal. In this study, we adopted multiple model-free and model-fitting methods to comparatively analyze co-combustion kinetics of blended coals on the basis of thermogravimetric data. Then, the sulfur balance and self-desulfurization of blended coals in the co-combustion were intensively investigated using a tube furnace set-up. The results reveal that in the presence of coal gangue in blended coals, the average activation energy (Ea) falls within the range of 65.7 kJ/mol to 100.4 kJ/mol, as determined by four model-free methods. Conversely, in the absence of coal gangue, only the Flynn-Wall-Ozawa (FWO) and Friedman (FM) methods are deemed appropriate for calculating the average Ea, yielding a value of 77.3 kJ/mol. The first order reaction model is confirmed to be reliable for analyzing the co-combustion kinetics of low-rank blended coals. Irrespective of the specific composition of the blended coal, a significant linear correlation exists between the Ea and the natural logarithm of the pre-exponential factor (lnA) within an extensive range of parameters. Moreover, the addition of coal gangue to the blended coal substantially enhances the self-desulfurization level, resulting in an increase from 25.7% to 60.7% at 1073 K. The self-desulfurization efficiency exhibits a good linear relationship with both the mass ratio of coal gangue to coal slime and the molar ratio of calcium to sulfur. In a practical implementation, the optimal addition ratio of coal gangue is a trade-off between the self-desulfurization efficiency and the ignition capacity.
