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

J. Bae, J. Seol, J. Moon et al.

Abstract High-entropy alloys (HEAs) are a newly emerging class of materials that show attractive mechanical properties for structural applications. Particularly, face-centered cubic (fcc) structured HEAs and medium-entropy alloys (MEAs) such as FeMnCoNiCr and CoNiCr alloys, respectively, which exhibit superior fracture toughness and tensile properties at liquid nitrogen temperature, are the potential HEA materials available for cryogenic applications. Here, we report a ferrous Fe60Co15Ni15Cr10 (at%) MEA exhibiting combination of cryogenic tensile strength of ∼1.5 GPa and ductility of ∼87% due to the multiple-stage strain hardening. Astonishingly, detailed microstructural observations at each stage reveal the sequential operation of deformation-induced phase transformation from parent fcc to newly formed bcc (body-centered cubic) phases. No compositional heterogeneity is observed at phase boundaries, indicating diffusionless phase transformation, as confirmed by atom probe tomography. The transformation to bcc phase occurs predominantly along grain boundaries (GBs) at the early stage of plastic deformation. Simultaneously, numerous deformation-induced shear bands (SBs) having stacking faults associated to the Shockley partial dislocations and thin hcp plates, form within fcc grains. Further deformation leads to the intense nucleation and growth of the bcc phase at the intersections of SBs within fcc grains. These micro-processes consecutively enhance the strain hardening rate, which play a key role in the multiple strain hardening behavior. The in-situ neutron diffraction studies make it clear that the martensite formation and the concurrent load partitioning between the fcc and bcc phases play an important role in the increase in strength. Furthermore, replacing high-cost alloying elements cobalt and nickel with iron, as well as introduction of metastability-engineering at liquid nitrogen temperature, distinguishes the new ferrous MEAs from previously reported equiatomic HEAs. This result underlines insights to provide expanded opportunities for the future development of HEAs for cryogenic applications.

Shelfira Priti Chantika, Dr. Fajri Ashfi Rayhan

Global climate change is closely related to increasing greenhouse gas emissions, including those generated by energy-intensive refrigeration systems on fishing vessels. Conventional onboard cooling systems often operate at relatively low efficiency, resulting in high fuel consumption and environmental impact. This study aims to analyze the coefficient of performance (COP) of a cascade refrigeration system with a flash chamber for fishing vessel applications by evaluating the influence of refrigerant selection and intermediate operating temperature. A steady-state thermodynamic simulation model was developed using Engineering Equation Solver (EES). Three refrigerants, namely R134a, R404A, and R407C, were investigated under intermediate temperature variations of 10–30 °C. The model was validated through comparison with reference data from the literature. Simulation results include compressor power consumption, condenser heat rejection, evaporator heat absorption, and COP values. The results indicate that intermediate temperature significantly affects compressor work distribution between the low-stage and high-stage compressors. Among the evaluated refrigerants, R134a achieved the highest COP under all operating conditions, while R404A showed lower condenser heat rejection and cooling capacity. Overall, the cascade refrigeration system with a flash chamber demonstrates improved energy performance and offers a promising solution to reduce fuel consumption and greenhouse gas emissions in marine refrigeration applications.

Xueli Wang, Yan Jia, D. Zuo

: The development of efficient and clean heating technologies is crucial for reducing carbon emissions in regions with severe cold regions. This research designs a novel two-stage phase change heat storage coupled solar-air source heat pump heating system structure that is specifically designed for such regions. The two-stage heat storage device in this heating system expands the storage temperature range of solar heat. The utilization of the two-stage heat storage device not only makes up for the instability of the solar heating system, but can also directly meet the building heating temperature, and can reduce the influence of low-temperature outdoor environments in severe cold regions on the heating performance of the air source heat pump by using solar energy. Therefore, the two-stage phase change heat storage coupled to the solar energy-air source heat pump heating system effectively improves the utilization rate of solar energy. A numerical model of the system components and their integration was developed using TRNSYS software in this study, and various performance aspects of the system were simulated and analyzed. The simulation results demonstrated that the two-stage heat storage device can effectively store solar energy, enabling its hierarchical utilization. The low-temperature solar energy stored by the two-stage phase change heat storage device enhances the coefficient of performance of the air source heat pump by 11.1% in severe cold conditions. Using the Hooke-Jeeves optimization method, the annual cost and carbon emissions are taken as optimization objectives, with the optimized solar heat supply accounting for 52.5%. This study offers valuable insights into operational strategies and site selection for engineering applications, providing a solid theoretical foundation for the widespread implementation of this system in severe cold regions.

R. Pratama, Ragil Randi Ratmaja Putra

Vocational High School (SMK) graduates are graduates who have the highest Open Unemployment Rate (TPT) among other levels of education. Low work readiness is one of the contributing factors. The purpose of this study was to analyze the influence of industrial internship and self-efficacy on students work readiness. The method used in this study was ex-post facto with a quantitative approach. The data collection technique used a questionnaire with a value range between 1 to 5 likert scale models. Respondents in this study were grade XII students of Heating, Ventilation, Air Conditioning and Refrigeration (HVACR) Engineering Competency at SMKN 8 Bandung. This study consisted of three variables, namely industrial internship (X1), self-efficacy (X2) and students work readiness (Y). The data analysis technique used was multiple regression analysis. The results showed that internship and self-efficacy had a positive and significant effect on students work readiness. Therefore, students work readiness can be through the improvement of industrial internship program and self-efficacy.

Yan Liu, Kaizhi Jia, Hongyang Chen et al.

Matteo Solazzo, M. Monaghan

Endogenous electrically mediated signaling is a key feature of most native tissues, the most notable examples being the nervous and the cardiac systems. Biomedical engineering often aims to harness and drive such activity in vitro, in bioreactors to study cell disease and differentiation, and often in three-dimensional (3D) formats with the help of biomaterials, with most of these approaches adopting scaffold-free self-assembling strategies to create 3D tissues. In essence, this is the casting of gels which self-assemble in response to factors such as temperature or pH and have capacity to harbor cells during this process without imparting toxicity. However, the use of materials that do not self-assemble but can support 3D encapsulation of cells (such as porous scaffolds) warrants consideration given the larger repertoire this would provide in terms of material physicochemical properties and microstructure. In this method and protocol paper, we detail and provide design codes and assembly instructions to cheaply create an electrical pacing bioreactor and a Rig for Stimulation of Sponge-like Scaffolds (R3S). This setup has also been engineered to simultaneously perform live optical imaging of the in vitro models. To showcase a pilot exploration of material physiochemistry (in this aspect material conductivity) and microstructure (isotropy versus anisotropy), we adopt isotropic and anisotropic porous scaffolds composed of collagen or poly(3,4-ethylene dioxythiophene):polystyrenesulfonate (PEDOT:PSS) for their contrasting conductivity properties yet similar in porosity and mechanical integrity. Electric field pacing of mouse C3H10 cells on anisotropic porous scaffolds placed in R3S led to increased metabolic activity and enhanced cell alignment. Furthermore, after 7 days electrical pacing drove C3H10 alignment regardless of material conductivity or anisotropy. This platform and its design, which we have shared, have wide suitability for the study of electrical pacing of cellularized scaffolds in 3D in vitro cultures.

Paulette Cathy Mengue, Michel Mbessa, Ö. Cengiz et al.

ABSTRACT The present study investigated the feasibility of low-energy and low-cost production of ceramic products from widely available alluvial clay in Batchenga, Cameroon, using low firing temperatures. A mixture consisting of 70% kaolin and 30% limestone was prepared and used as an additive in fired alluvial clay bricks. A series of samples was prepared in which the additive made up 5, 10 or 15% of the clay products, and the samples were heated at 700, 750, 800, 850 or 900 °C. The raw materials and ceramic products were characterised using x-ray diffractometry, scanning electron microscopy, bulk density, porosity, thermogravimetry and differential scanning calorimetry analysis. The results revealed that products with 10% additive showed improved mechanical performance and densification after heating at all tested temperatures. A compressive strength of 28 MPa was recorded when the product was heated at 800 °C. A drastic decrease in performance was observed for all samples heated at above 800 °C; this was likely due to the conversion of CaCO3 into CaO, resulting in the appearance of voids within the matrix that weakened the structure. This is in line with the high-water absorption and porosity values recorded. The resultant ceramic products have potential for use in engineering applications.

Gu Guiyu, Sheng Wei, Zheng Haikun et al.

Superhydrophobic surfaces can reduce the attachment of droplets, reduce the increase of thermal resistance caused by the existence of droplets, and thereby improve the efficiency of air conditioning, power generation, and seawater desalination systems. This study experimentally investigates the growth characteristics of condensate droplets on a superhydrophobic surface under different cold surface temperatures (2–8 ℃), relative humidity values (40%–80%), and inclination angles (0°–90°) and analyzes the effects of different working conditions on superhydrophobic-surface condensation. The results show that with a decrease in the cold surface temperature, the average droplet growth radius and surface droplet coverage gradually increase. The lower the cold surface temperature, the faster the droplet growth rate. The droplets on the superhydrophobic surface grow faster under high humidity, while the droplet growth radius under low humidity will exceed that under medium and high humidity after sufficient time. The droplet coverage on the cold surface under low and medium humidity conditions is considerably less than that under high humidity conditions. The critical sweep radius of droplets decreases gradually with an increase in inclination angle, and the droplet coverage on the vertical surface decreases by 42% compared with that on the horizontal surface.

Songhua Li, C. Wei, Yonghua Wang et al.

Zhou Nianyong, Feng Hao, Xu Hongye et al.

In this study, an R134a closed-loop spray cooling system was built to investigate the effects of flow rate, subcooling degree, and refrigerant charge on the steady spray cooling heat transfer performance. The experimental flow rate ranged from 0.20 to 0.25 L/min, the subcooling degree ranged from 5 to 8 ℃, and the refrigerant charge ranged from 0.95 to 1.25 kg. Results show that at a flow rate of 0.184 L/min and refrigerant charge of 0.95 kg, the maximum heat flux and surface heat transfer coefficient were 105.25 W/cm2 and 2.54 W/（cm2?℃）, respectively. At low heat flux (45.93–72.55 W/cm2), with the increase in flow rate, subcooling degree, and refrigerant charge, the surface heat transfer coefficient, overall, increased. Under higher heat flux (84.02–105.25 W/cm2), the surface heat transfer coefficient increases gradually with the increase in flow rate. The surface heat transfer coefficient initially increased and then stabilized as the refrigerant charge increased. In addition, the Jacob number Ja decreases with an increase in charge, which is unfavorable for the improvement of the surface heat transfer coefficient at higher heat flux. There is an optimal refrigerant charge to maximize the heat transfer performance of the closed spray cooling system.

Xie Fulin, Guo Xianmin, Guo Xinwei et al.

In this study, the surface temperature of the frost layer was measured using an infrared thermal imager and calibrated using direct measurements from a micrometer and thermocouple device. The growth characteristics of the frost layer on the surface of the finned-tube heat exchanger were experimentally studied. The effects of fin type and fin pitch on the frost thickness, frost mass, and heat transfer capacity were analyzed. The frost growth characteristics were analyzed by considering the condition at the frost wet air interface as the driving force of heat and mass transfer. The experimental results indicated that the driving force of heat and mass transfer at the interface of the wavy fin and the split fin heat exchangers is higher than that of the flat fin, resulting in higher growth rates of the frost layer on the wavy and split fins than that on the flat fin. In the late frosting period, the growth rate of the frost layer on the split fin was significantly accelerated. Among the three types of fins, the frosting duration on the split fin was the shortest and the longest on the flat fin. The average heat transfer capacity of the wavy fin heat exchanger during the frosting and defrosting periods was the largest, which was 0.61% and 2.67% higher than that of the split and flat fin heat exchangers, respectively. Meanwhile, the influence of the fin pitch on the driving force of heat and mass transfer at the interface is weak, and the larger the fin pitch is, the faster the frost layer thickness increases, and the longer the frosting duration is. Considering the average heat transfer capacity during the frosting defrosting period and the air flow resistance of the heat exchanger, the optimal fin pitch is approximately 2.2 mm under the frosting condition.

Hugo Salmon, M. Rasouli, Nicholas Distasio et al.

Soft thermoplastic elastomers (sTPE) and specifically styrenic block copolymers (SBC) are making rapid progress in the prototyping and mass production of microfluidic chips. However, these new materials lack guidelines and protocols for chips fabrication, curbing their widespread applications compared to polydimethylsiloxane. In this work, the prototyping potential of a commercially available SBC material, Flexdym, for continuous flow applications is explored. This SBC material exhibits both reversible and permanent self‐adhesion depending on the time and bonding temperature, allowing for rapid and adaptive chip fabrication. Replicates are embossed in 2 min, assembled and sealed in 10 min. Under continuous flow, stud interfaces fabricated with this method can withstand 1 bar with reversible bonding and up to 3 bar after permanent bonding. The integration of an acoustic transducer in an SBC chip to induce acoustic streaming enables rapid mixing and local enrichment of polystyrene microparticles up to 8× the injected concentration. The reversible bonding feature of SBC chips allows to culture endothelial cells in open channels and then close and perfuse through channel to stain the cell. Our finding suggests that TPE‐based materials offer numerous possibilities for prototyping microfluidic chips for analytical and biomedical applications when working with continuous flow at high pressure is required.

Yongtao Qin, Le Sun, Liping Tang et al.

In order to fully identify the root cause of cryogenic pneumatic valve failure in the process of liquid rocket engine test, and to analyze the dynamic time change of the process of cryogenic pneumatic valve reliability analysis, this paper first analyzes the failure factors of low temperature pneumatic valve in the test process of liquid rocket engine according to the human-machine environment system engineering. By introducing the dynamic Bayesian network theory, and by means of analyzing the time-change failure failure mode of cryogenic pneumatic valve, constructing the time-change reliability model of cryogenic pneumatic valve was constructed in the test process of liquid rocket engine, and the reliability growth technology of cryogenic pneumatic valve was researched by the optimal failure search strategy. Finally, the validity and accuracy of this method are verified by the example. This method can provide a method guide for quantitative analysis and improvement of the reliability of cryogenic pneumatic valve, and provide support for the accurate and objective evaluation and development of liquid rocket engine performance.

I. Semenova, Julia M. Modina, A. Polyakov et al.

Abstract The dependence of the absorbed energy of ultrafine-grained (UFG) Ti-6Al-4V alloy on the test temperature is studied. Tensile tests and Charpy tests were carried out at room temperature, cryogenic temperature, and temperatures elevated up to 500 °С. A UFG state was produced by equal-channel angular pressing. For the sake of comparison, coarse-grained (CG) alloy was subjected to heat treatment to produce a duplex structure. The UFG alloy demonstrates enhanced strength and reduced ductility-related properties at room temperature. It is shown that at operating temperatures of 200 and 300 °С, the best balance between high strength and absorbed energy is achieved in the UFG alloy. The UFG alloy demonstrates higher strength compared to the CG alloy at the liquid nitrogen temperature, but their absorbed energy is practically the same. The failure mechanisms of CG and UFG alloy and promising engineering applications of UFG alloy are discussed.

Xiaoyun Sun, Mei Zhang, Yang Wang et al.

Abstract Microstructure and mechanical properties of 7Mn steel undertaken different intercritical annealing (IA) with or without deep-cryogenic treatment (DCT) were investigated in this work. It is found that the additional DCT process can lead to the changes in the size and composition of austenite of the steel after IA. With the decrease of the IA temperature, the improvement effect of the DCT on mechanical properties is more remarkable. The product of ultimate tensile strength (UTS) and total elongation (TE) of the pre-DCT specimen IA at 600°C (DCT + IA600) is nearly twice as much as that of the non-DCT specimen. Austenite and ferrite grains exhibit different morphologies, lamellar and equiaxed, and bimodal size distribution at different IA temperatures. With the decrease of IA temperature, the occurring of serration behavior on engineering stress-strain curves is postponed, and the density of serration as well as the mean value of work hardening (WH) rate decrease, both of which are caused by the increasing stability of austenite. The volume fraction, morphology and composition of austenite can be tailored by pre-DCT and IA process to achieve the optimum combination of the retained austenite (RA) volume fraction and stability. The DCT + IA645 specimen exhibits an outstanding combination of 1167 MPa UTS and 35.4% TE.

Maya Schnabel-Lubovsky, Olga Kossover, S. Melino et al.

The present investigation explores the microscopic aspects of cell‐laden hydrogels at high resolutions, using three‐dimensional cell cultures in semi‐synthetic constructs that are of very high water content (>98% water). The study aims to provide an imaging strategy for these constructs, while minimizing artefacts. Constructs of poly(ethylene glycol)‐fibrinogen and fibrin hydrogels containing embedded mesenchymal cells (human dermal fibroblasts) were first imaged by confocal microscopy. Next, high‐resolution scanning electron microscopy (HR‐SEM) was used to provide images of the cells within the hydrogels, at submicron resolutions. Because it was not possible to obtain artefact‐free images of the hydrogels using room‐temperature HR‐SEM, a cryogenic HR‐SEM imaging methodology was employed to visualize the sample while preserving the natural hydrated state of the hydrogel. The ultrastructural details of the constructs were observed at subcellular resolutions, revealing numerous cellular components, the biomaterial in its native configuration, and the uninterrupted cell membrane as it relates with the biomaterial in the hydrated state of the construct. Constructs containing microscopic albumin microbubbles were also imaged using these methodologies to reveal fine details of the interaction between the cells, the microbubbles, and the hydrogel. Taken together with the confocal microscopy, this imaging strategy provides a more complete picture of the hydrated state of the hydrogel network with cells inside. As such, this methodology addresses some of the challenges of obtaining this information in amorphous hydrogel systems containing a very high water content (>98%) with embedded cells. Such insight may lead to better hydrogel‐based strategies for tissue engineering and regeneration.

S. Mohtaram, Wen Chen, Ji Lin

Xing Lin, Yan Jinzhou, Wang Kunhai et al.

A flexible design scheme for a direct expansion solar-assisted heat pump system is presented. Based on the flexible theory for the virtual operating point of the basis design, which was stored in the flexible space, all of the operating points within the flexible space could achieve almost the same heat transfer effect. The weights of the environmental factors for the system’s coefficient of performance (COP) were obtained in a simulation and verified in an experiment, including the irradiation intensity (52.2%), temperature (34.7%), and wind speed (13.1%). A total of 25 sets of systems were simulated under typical weather conditions. The purpose was to verify the operating points that fell in the flexible space based on the weights for the optimal combination of virtual operating points obtained. The values of the irradiation intensity (It), ambient temperature (T0), and wind velocity (uw) were 559.97 W/m2, 21.6 ℃, and 2.89 m/s respectively. The simulation results showed that the variance in the system’s COP was 0.156，which was superior to the traditional static design result.

Chen Zhenhua, Li Gaimin, Wu Jianhua

R290 has been considered for use in air-conditioning systems as a low global warming potential (GWP) refrigerant. This paper describes the tribological characteristic of the vane-piston tribopair in a R290 rotary compressor. For the purpose of simulating the tribopair working conditions, the vanes and rolling pistons of a compressor were directly adopted as test specimens. All of the tribological tests were performed under sealed high-pressure conditions, with test conditions such as the relative sliding speed, loads, and temperatures set based on actual operating conditions of a compressor, and the friction coefficients, scuffing resistances, and wear depths were measured. Furthermore, the morphological changes were investigated using a scanning electron microscope after the tests. As a comparison benchmark, tests were also conducted using R410A/polyolester oil. From these tests, it was evident that the friction coefficients and scuffing load of the vane-piston interface in the presence of R290/mineral oil were respectively 0.05–0.1 and 300 N higher than those of R410A/polyolester. On the other hand, the wear resistance ability with R290/mineral oil was better than that with R410A/polyolester.

E. Mikulin, A. A. Tarasov, M. P. Shkrebyonock