Hasil untuk "Low temperature engineering. Cryogenic engineering. Refrigeration"

Song Yulong, Wang Wencheng, Tong Zhen et al.

Pump-driven two-phase thermosyphon loops (TPTLs) have garnered widespread attention to improve the performance and applicability of TPTLs under various conditions. This study compared the heat-transfer performance and operational states of a TPTL using two parallel downcomer branches to switch between gravity- and pump-driven TPTL operation modes. Significant differences were observed in the normal working load range of the TPTL when the working fluid flowed through downcomer branches 1 and 2 (pump off), or only through branch 2 (pump on). The presence of a liquid pump increased the resistance to the fluid flow within the loop. When the pump was turned off, the heat-transfer limit of the TPTL decreased, and it increased when the pump was on. During the oscillatory operational stage, the reservoir was unable to effectively separate the vapor and liquid. When the working fluid flowed through downcomer branch 2 (pump off), the TPTL exhibited a longer fluctuation cycle and greater amplitude compared to those during the flow through branch 1. In the stable operational stage, the reservoir provided better vapor-liquid separation.

Chu Zhaoping, Han Hua, Gu bo et al.

The optimization of regenerator geometries to enhance the performance of active electrocaloric regenerators （AERs） has attracted significant attention. In this study， corrugated and tapered structures were applied to a parallel-plate AER， and the effects of electrical field parameters on device performance were compared. The results indicated that the tapered structure better balanced the flow resistance and heat transfer efficiency， thus achieving the best refrigeration performance under the same operating conditions， followed by the corrugated AER structure. Short or long device cycle periods resulted in poor refrigeration performance. The electrical field should optimally be switched when the heat transfer reaches 63-66% of the maximum possible heat transfer. For the same cycle period， each device exhibited an optimal polarization duration （0.2 s）， during which the refrigeration capacities of the parallel-plate， corrugated， and tapered AERs were 4.16 W， 4.35 W， and 4.71 W， respectively， with corresponding coefficients of performance of 1.76， 2.04， and 3.17. As the electrical field strength increases， the refrigeration capacity of the device increases exponentially. The greater the field strength， the greater the improvement in the refrigeration capacity of the gradually shrinking AER. When the field intensity increased from 50 MV/m to 225 MV/m， the refrigeration capacity of the tapered AER increased from 0.68 W to 10.06 W， an improvement of approximately 13.79 times.

周汉涛, 张良, 李昊玥 et al.

To address the demands for high-efficiency heat transfer and compact lightweight design in space stations, a high-efficiency condensation dehumidification system utilizing copper foam and driven by a Stirling refrigerator was developed. An experimental study was conducted to investigate its heat and mass transfer characteristics under various conditions. The experimental parameters were set as follows: air inlet temperature ranging from 20 to 30 °C, relative humidity between 50 % and 80 %, cold plate temperature from 8 to 13 °C, and inlet wind speed from 0.4 to 1.4 m/s. The results indicate a positive correlation between the increase in air inlet temperature and the enhancement of both heat transfer coefficient and mass transfer coefficient. Specifically, when the air inlet temperature increased from 20 °C to 30 °C, the heat transfer coefficient rose by 10.5 %, while the mass transfer coefficient exhibited a more substantial increase of 57.1 %. Furthermore, variations in the relative humidity of the air inlet had distinct impacts on the heat and mass transfer coefficients: the heat transfer coefficient decreased with increasing relative humidity, whereas the mass transfer coefficient increased. Notably, although lowering the cold plate temperature can significantly improve heat transfer, it concurrently diminishes the efficiency of heat and mass transfer. Therefore, selecting an appropriate cold plate temperature is crucial. Additionally, the efficiency of heat and mass transfer was markedly enhanced with increasing inlet wind speed; however, a continuous increase in wind speed resulted in higher system energy consumption. Thus, a balance between achieving efficient heat transfer and managing system energy consumption is essential. Based on extensive experimental data, the heat transfer model was refined through regression analysis. The relative average deviation between theoretical and experimental values was found to be 8.97 %, with a relative standard deviation of 8.21 %, demonstrating the model's strong predictive accuracy.

YANG Shuchuan, YIN Yonggao, LI Xiao et al.

In the context of low/zero-carbon heating, heat pump technology has emerged as the inevitable path in the field of medium and low-temperature heating due to their high efficiency in electrical-to-thermal energy conversion. However, due to the lack of stable heat sources in northern winter, it is necessary to store the heat sources within the urban area across seasons to ensure the stability of heating provided by the heat pump system. To address the current limitations of heat sources and the inadequate consideration of daily heat storage, this paper combines the seasonal/daily heat storage project of river water in the Guantao, builds a modeling and simulation platform by using TRNSYS, studies the system's long-term operation performance, and matches and optimizes the buried pipe spacing, river water flow rate, and water tank volume. The results show that after 10 years of operation, the temperature rise of the thermal storage unit reaches 3.2°C, and the heat storage efficiency rises to 98%. For the design of the thermal storage unit, under the large thermal storage unit and larger river water flow, the power consumption to obtain the same heat is smaller. For the tank design, the right tank volume and control strategy can avoid more initial investment and energy consumption. The optimized seasonal/daily thermal storage system can reduce the annual operating cost by about 28.8% compared to the seasonal thermal storage system, which has a good economic benefit.

Jacob Adams, Nathaniel O’Connor, Matthew Jones et al.

Abstract The acoustic expander is an innovative cryogenic component that uses pressure waves for work transfer as part of a continuous flow, recuperative cycle refrigerator. This expander uses passive reed-valves coupled to an acoustic resonator to produce refrigeration. The passive reed-valves are pressure-controlled by the imposed, static pressure difference across the expander and the natural oscillating pressure in the resonator. The resonator is a series of tubes and cones. The practical implications of these simple components are that the acoustic expander does not require controlled valving or close-tolerance sliding seals at low-temperature, unlike existing piston- or turbo-expanders. This work compares two resonator designs, a harmonic resonator and a non-harmonic resonator. The non-harmonic resonator is excited by a single-frequency allowing for operation at an expansion pressure-ratio of 2.4. These expanders are expected to be useful in medium-scale refrigeration applications that are not well served by current small-scale Stirling cryocoolers or large-scale turbo-expander refrigerators.

Zhidong Wen, Tao Bai, Jiahao Wan

Yu-Chen Chen, P. Salter, S. Knauer et al.

A negatively charged nitrogen–vacancy centre — a promising quantum light source — is created in diamond by laser writing (with pulses with a central wavelength of 790 nm and duration of 300 fs) with an accuracy of 200 nm in the transverse plane. Optically active point defects in crystals have gained widespread attention as photonic systems that could be applied in quantum information technologies1,2. However, challenges remain in the placing of individual defects at desired locations, an essential element of device fabrication. Here we report the controlled generation of single negatively charged nitrogen–vacancy (NV−) centres in diamond using laser writing3. Aberration correction in the writing optics allows precise positioning of the vacancies within the diamond crystal, and subsequent annealing produces single NV− centres with a probability of success of up to 45 ± 15%, located within about 200 nm of the desired position in the transverse plane. Selected NV− centres display stable, coherent optical transitions at cryogenic temperatures, a prerequisite for the creation of distributed quantum networks of solid-state qubits. The results illustrate the potential of laser writing as a new tool for defect engineering in quantum technologies, and extend laser processing to the single-defect domain.

Liu Jiaxin, Chen Jianyong, Chen Ying et al.

The use of high-temperature heat pumps to recover industrial waste heat has a high potential for energy conservation. High-temperature heat pumps require high critical temperatures, and the majority of the available working fluids are dry. However, when the dry working fluid is compressed from the saturated vapor phase, the compression process enters the two-phase region, resulting in a risk of liquid slugging that is detrimental to the operation of the compressor and high-temperature heat pump. Two improved vapor injection heat pump cycles (Cycle A and Cycle B) using isohexane, R1336mzz (Z), and R1233zd (E) as working fluids are proposed. The effects of compressor isentropic efficiency, evaporating temperature, and condensing temperature on the minimum superheating degree and heat pump performance are analyzed. The results indicate that, for cycle B, the evaporation temperature increases from 50 ℃ to 80 ℃. For R1336mzz(Z), the maximum COP(coefficient of performance) can be increased by 2.56%, and the maximum volumetric heating capacity (VHC) can be increased by 3.18%. For R1233zd(E), the maximum COP can be increased by 0.44%, and the maximum VHC can be increased by 0.54%. Cycle A has good adaptability to the isentropic efficiency. For cycle B, when the isentropic efficiency is higher than 0.6, isohexane is not suitable as a working fluid. When the isentropic efficiency is higher than 0.95, R1336mzz (Z) is also not suitable.

Lei Xu, F. Gong, Zhi-xiang Liu

Abstract Many underground engineering projects show that rockburst can occur in rocks at great depth and high temperature, and temperature is a critical factor affecting the intensity of rockburst. In general, temperature can affect the energy storage, dissipation, and surplus in rock. To explore the influence of temperature on the energy storage and dissipation characteristics and rockburst proneness, the present study has carried out a range of the uniaxial compression (UC) and single-cyclic loading–unloading uniaxial compression (SCLUC) tests on pre-heated granite specimens at 20 °C to 700 °C. The results demonstrate that the rockburst proneness of pre-heated granite initially increases and subsequently decreases with the increase of temperature. The temperature of 300 °C has been found to be the threshold for rockburst proneness. Meanwhile, it is found that the elastic strain energy density increases linearly with the total input strain energy density for the pre-heated granites, confirming that the linear energy property of granite has not been altered by temperature. According to this inherent property, the peak elastic strain energy of pre-heated granites can be calculated accurately. On this basis, utilising the residual elastic energy index, the rockburst proneness of pre-heated granite can be determined quantitatively. The obtained results from high to low are: 317.9 kJ/m3 (300 °C), 264.1 kJ/m3(100 °C), 260.6 kJ/m3 (20 °C), 235.5 kJ/m3 (500 °C), 158.9 kJ/m3 (700 °C), which are consistent with the intensity of actual rockburst for specimens. In addition, the relationship between temperature and energy storage capacity (ESC) of granite was discussed, revealing that high temperature impairs ESC of rocks, which is essential for reducing the rockburst proneness. This study provides some new insights into the rockburst proneness evaluation in high-temperature rock engineering.

M. Lyutikova, A. Ridel, A. A. Konovalov

Adel Jalaee, E. J. Foster

The engineering thermoplastics industry has largely limited the use of natural fiber reinforcements due to their susceptibility to low-onset thermal degradation and water absorption. Therefore, in order to utilize these economically viable and environmentally friendly materials effectively through common composite fabrication methods such as hot pressing, safeguarding them from thermal degradation becomes essential. This work presents a viable industrially technique called sequential ball milling for processing unbleached softwood kraft pulp fibers (PF) with an engineering thermoplastics polyamide 6 (PA6) with high melting temperatures (>220 °C). An additional eco-friendly modification step that employs ball milling and cellulose nanocrystal (CNC) has been implemented in this study to enhance the mechanical properties of the composites. Special attention is given to fine-tuning key variables, such as milling duration and PF particle size, to produce optimal composites. Leveraging the ability of sequential ball milling to evenly distribute pulp fibers into PA6, a 160% increase in Young’s modulus was achieved with the incorporation of 30 wt % PF. Importantly, the introduction of a 5 wt % CNC modifying agent elevated Young’s modulus to 4.3 GPa, marking a 187% improvement over unmodified PA6. Diverse techniques, including rheological analyses, thermomechanical evaluations, morphological examinations, and assessments of moisture absorption, were utilized to validate the efficiency of the suggested processing approach and the modification phase.

B. Patel, A. Parekh

A major portion of the worldwide emissions arise from mobile air-conditioning systems with hydrofluorocarbon refrigerant as working substance and which is one of major cause for the greenhouse effect. R134a refrigerant having GWP of 1400 has been extensively used in car air conditioning. To reduce greenhouse gas emissions, the current R134a refrigerant must be phase out as per Kigali Amendment. The present study deals with cooling load calculation of car model by heat balance method as per ASHRAE standard using local climate condition. Further, thermodynamic analysis of R1234yf as an alternate refrigerant to R134a has been carried out for automobile air conditioning system. The required properties of refrigerants are extracted from Engineering Equation Software. The thermodynamic analysis is carried out to study the effect of operating parameters viz. condensing temperature, evaporating tempera-ture, degree of superheating and degree of subcooling on COP, EDR, exergy efficiency and entropy generation. The previous literature reports mainly focus on separate study of either cooling load calculation or energy analysis or exergy analysis of R1234yf and R134a for au-tomobile air conditioning system, while this paper presents the comprehensive study of new low GWP R1234yf as an alternate refrigerant to R134a in automobile air conditioning system with cooling load calculation including the concept of energy, entropy and exergy analysis. The percentage difference in COP between R134a and R1234yf system varies from 2.44 % to 4.78 % while percentage difference in EDR varies from 6.79 % to 2.87 % when evaporating temperature varied from -10 °C to 10 °C. With 12 °C of superheating at compressor inlet, the COP of R134a is 3.9 whereas COP of R1234yf is 3.75, which makes 3.85 % lower than that of R134a. The R1234yf has 4.78 % lower value of exergy efficiency as compared to that of R134a at evaporating temperature of -10 °C and it is found that maximum exergy destruction takes place in compressor.

P. Chansoria, S. Asif, K. Polkoff et al.

Gelatin methacryloyl (GelMA) hydrogels have emerged as promising and versatile biomaterial matrices with applications spanning drug delivery, disease modeling, and tissue engineering and regenerative medicine. GelMA exhibits reversible thermal cross-linking at temperatures below 37 °C due to the entanglement of constitutive polymeric chains, and subsequent ultraviolet (UV) photo-cross-linking can covalently bind neighboring chains to create irreversibly cross-linked hydrogels. However, how these cross-linking modalities interact and can be modulated during biofabrication to control the structural and functional characteristics of this versatile biomaterial is not well explored yet. Accordingly, this work characterizes the effects of synergistic thermal and photo-cross-linking as a function of GelMA solution temperature and UV photo-cross-linking duration during biofabrication on the hydrogels' stiffness, microstructure, proteolytic degradation, and responses of NIH 3T3 and human adipose-derived stem cells (hASC). Smaller pore size, lower degradation rate, and increased stiffness are reported in hydrogels processed at lower temperature or prolonged UV exposure. In hydrogels with low stiffness, the cells were found to shear the matrix and cluster into microspheroids, while poor cell attachment was noted in high stiffness hydrogels. In hydrogels with moderate stiffness, ones processed at lower temperature demonstrated better shape fidelity and cell proliferation over time. Analysis of gene expression of hASC encapsulated within the hydrogels showed that, while the GelMA matrix assisted in maintenance of stem cell phenotype (CD44), a higher matrix stiffness resulted in higher pro-inflammatory marker (ICAM1) and markers for cell-matrix interaction (ITGA1 and ITGA10). Analysis of constructs with ultrasonically patterned hASC showed that hydrogels processed at higher temperature possessed lower structural fidelity but resulted in more cell elongation and greater anisotropy over time. These findings demonstrate the significant impact of GelMA material formulation and processing conditions on the structural and functional properties of the hydrogels. The understanding of these material-process-structure-function interactions is critical toward optimizing the functional properties of GelMA hydrogels for different targeted applications.

Every two years the Department of Thermal Systems and Automotive from the “Dunarea de Jos” University of Galati, Romania in cooperation with the Romanian Society of Thermal Engineering (SRT) and the Romanian Society for Refrigeration and Cryogenics Engineers (AFCR) organizes and hosts the National Conference on Thermodynamics with International Participation. This year, the 23rd edition of the conference, NACOT 2023, took place. The Scientific Advisory Board has selected the topic of the conference and its participants. The conference program included invited talks, onsite participants and online participants from Romania, Poland, Belarus, Greece, Portugal, Tunis, Kazakhstan, Bulgaria, Turkey and Republic of Moldova. The conference covers a range of topics related to thermodynamics: Applied thermodynamics; Heat and mass transfer; Fluid dynamics and CFD; Refrigeration and heat pumps; Power plants and CHP; Internal combustion engines; Renewable energy (solar, wind, hydro, biomass, etc.); Energy use (industry, building, transportation, desalination, etc.) and storage; Environmental impact of energy conversion. The participation to this conference represented a good way to inter-personal and inter-institutional communication, discussion and exchange of ideas based on relevant research themes specific to the thermodynamics field, and to identifying potential partners and collaborators. List of NACOT 2023 Organizing Committee are available in this pdf.

K. C. Sekhar, Raviteja Surakasi, Dr. Pallab Roy et al.

Producing items that are of both high quality and long lasting is a difficult task for companies right now. There is a huge need for a wide range of engineering materials in today’s technologically advanced globe. The strength and qualities of the material determine the amount of material that may be used. Due to its excellent mechanical qualities and low density, aluminum-7075 alloy is mostly employed in transportation applications such as aerospace, marine, and vehicle production. This study addresses the fabrication and characterization of Al7075 semisolid metal matrix composite (MMC) reinforced with graphene nanoparticles. Samples are made with and without stirring graphene in aluminum-7075 at various temperatures of 800°, 830°, 860°, 890°, and 920°C. At these temperatures, the material is semisolid, so graphene is introduced and stirred into the molten liquid. The specimens meet the requirements of the American Society for Testing and Material (ASTM). The hardness, tensile strength, impact strength, and compression strength of various materials are evaluated and compared. Temperature lowers tensile strength, hardness, and compression. A scanning electron microscope (SEM) is used to examine the microstructure. The specimen is evaluated using ANSYS. Specimens with stirring have better mechanical characteristics. Graphene has high hardness and strength.

Ziqing Liu, Zhiqiang Dong, Hong Zhu et al.

A new type of large-diameter iron-based shape memory alloy (Fe-SMA) stranded wire was developed, and its stress recovery behaviour was experimentally investigated in this study. The produced Fe-SMA stranded wires had a diameter of 15.2 mm, which were the same as the most commonly used steel-stranded wires. This kind of large-diameter Fe-SMA stranded wire matched the existing prestressed anchorage system to reduce application cost and accelerate the field application of the Fe-SMA in civil engineering. The effects of prestrain levels (4%, 6%, 8%, 10%), activation temperatures (200°C, 300°C, 400°C, 500°C), and secondary activation effect on the recovery stress of Fe-SMA stranded wires were analysed. Test results showed that the recovery stresses of the Fe-SMA stranded wire were 223–357 MPa. The maximum recovery stress was measured for the specimen with a prestrain of 8% and an activation temperature of 500°C. With increasing activation temperature, the recovery stress increased, but the increment decreased gradually. An increased prestrain level resulted in a first increase and a second decrease in the recovery stress, which was highest at the 8% prestrain level. If the shape memory effect (SME) was not fully used during the first activation, then the remaining martensitic phase changed on the second activation. The recovery stress generated by the first activation was essentially the same as those generated by the second activation at the same activation temperature. Taking the Nanshao River concrete box girder bridge in China as an example, the effect of using Fe-SMA stranded wires to compensate the prestressing loss was calculated. The results showed that the Fe-SMA stranded wire developed was able to compensate the vertical prestress loss of the prestressed box girder bridge web well and maintain the main tensile stresses at a low level during the service life of the bridge.

Jong Hoon Kang, Sung-Yong Jung

F. Villa, E. Villa, A. Nespoli et al.