Conventional Compression Research Articles

CFD supported thermodynamic analysis of a CO2 pressure exchanger based refrigeration system for supermarkets

This manuscript presents a method to thermodynamically model a pressure exchanger device. CFD-based model of a rotary energy recovery device used in desalination process is developed and validated with published experimental data with a maximum deviation of 5.19%. The dimensionality of the validated model was reduced to investigate the potential of the pressure exchanger device operating with CO2 as the working fluid. Based on the data obtained from several numerical experiments, two correlations were developed, by means of non-linear regression analysis, for estimating the pressure lift generated by the device as a function of inlet velocity of high pressure fluid (0.5 – 3 m/s), rotor speed (500 – 3000 rpm), receiver pressure (25 – 40 bar) and gas-cooler pressure (90 – 120 bar). The correlations were found to be within acceptable error margin. The pressure lift generated by the pressure exchanger was found to be directly proportional to the inlet velocity of the high pressure fluid and inverse proportionality to the rotor speed. The developed correlations are used to evaluate the thermodynamic performance of a multi evaporator supermarket refrigeration system integrated with pressure exchanger to a conventional parallel compression system and a published multi-ejector system in the ambient temperature range of 30 to 40 °C. The pressure exchanger based system was found to outperform the parallel compression as well as the multi-ejector system and demonstrate significant energy savings.

Experimental investigation into rock burst proneness of rock materials considering strain rate and size effect

In deep rock engineering, evaluating the likelihood of rock burst is imperative to ensure safety. This study proposes a new metric, the post-peak dissipated energy index, which accounts for strain rate and size effects in assessment of the rock burst proneness of a rock mass. To investigate rock burst proneness, conventional compression tests were conducted on limestone and slate samples with different length to diameter (L/D) ratios (ranging from 0.3 to 1.5) at four different strain rates (0.005, 0.01, 0.5, and 1.0 s−1). Based on the testing observations, the actual rock burst proneness was classified into three categories (no risk, low risk, and high risk). A new criterion was also established using the post-peak dissipated energy index, which is the ratio of elastic energy to total dissipated energy. The impact of the strain rate and L/D ratio on rock burst proneness was analyzed. The results indicated that increased strain rates cause a strong hardening effect, leading to staged growth of rock burst proneness. However, the rock burst proneness decreases non-linearly with the increasing L/D ratio. The accuracy of the proposed criterion was validated by comparison with existing criteria, demonstrating that the energy-based index ensures a reliable evaluation of the rock burst proneness of a rock mass. The proposed method has excellent potential for practical application in deep rock engineering.

Influence of Fabrication Technique on Adhesion and Biofilm Formation of Candida albicans to Conventional, Milled, and 3D-Printed Denture Base Resin Materials: A Comparative In Vitro Study.

The aim of this study was to evaluate the adhesion and biofilm formation of Candida albicans (C. albicans) on conventionally fabricated, milled, and 3D-printed denture base resin materials in order to determine the susceptibility of denture contamination during clinical use. Specimens were incubated with C. albicans (ATCC 10231) for 1 and 24 h. Adhesion and biofilm formation of C. albicans were assessed using the field emission scanning electron microscopy (FESEM). The XTT (2,3-(2-methoxy-4-nitro-5-sulphophenyl)-5-[(phenylamino) carbonyl]-2H-tetrazolium hydroxide) assay was used for the quantification of fungal adhesion and biofilm formation. The data were analyzed using GraphPad Prism 8.02 for windows. One-way ANOVA with Tukey's post hoc testing were performed with a statistical significance level set at α = 0.05. The quantitative XTT biofilm assay revealed significant differences in the biofilm formation of C. albicans between the three groups in the 24 h incubation period. The highest proportion of biofilm formation was observed in the 3D-printed group, followed by the conventional group, while the lowest candida biofilm formation was observed in the milled group. The difference in biofilm formation among the three tested dentures was statistically significant (p < 0.001). The manufacturing technique has an influence on the surface topography and microbiological properties of the fabricated denture base resin material. Additive 3D-printing technology results in increased candida adhesion and the roughest surface topography of maxillary resin denture base as compared to conventional flask compression and CAD/CAM milling techniques. In a clinical setting, patients wearing additively manufactured maxillary complete dentures are thus more susceptible to the development of candida-associated denture stomatitis and accordingly, strict oral hygiene measures and maintenance programs should be emphasized to patients.

Thermodynamic optimisation of a booster-ejector vapour compression refrigeration system using solar energy and R152a/Cu nano-refrigerant

Conventional vapour compression refrigeration systems require high electrical energy for low temperature applications. For the first time in the literature to reduce this required energy, comprehensive thermodynamic analysis and optimization of booster-assisted ejector expansion vapour compression refrigeration system with R152a/Cu nano-refrigerant are presented for low-temperature applications. Solar panels are used to cover the total compressor work, using the climate data of Izmir, Turkey. According to the thermodynamic optimization findings, the use of nanoparticles in the refrigeration system led to several improvements when compared to the R152a refrigeration system. These enhancements include a reduction of 6.52-11.71% in the main compressor discharge temperature, a decrease of 18.46-34.48% in the total exergy destruction rate, an increase of 3.08-4.04% in the entrainment ratio, a reduction of 0.089-3.25% in the ejector area ratio, a decrease of 8.34-14.27% in the panel area, and an increase of 8.09-14.05% in both the coefficient of performance and exergy efficiency. Another thing worth mentioning is that coefficient of performance and exergy efficiency improved by 2.06% and 1.96%, respectively, while total exergy destruction reduced by 4.83% compared to our previous study. The results obtained from the thermodynamic optimisation provide a useful reference for the design of an experimental system, as they provide evidence of model validity and can guide the implementation of the system.

A new micromechanical damage model for quasi-brittle geomaterials with non-associated and state-dependent friction law

In this paper, we shall present a new micromechanical damage model for capturing the mechanical behaviors of quasi-brittle geomaterials under a wide range of compressive stress. Two essential coupled non-linear mechanisms are taken into account: plastic deformation related to frictional sliding along the microcrack surfaces, and microscopic damage induced by the initiation and propagation of microcracks. Within the framework of thermodynamics and Mori–Tanaka homogenization scheme, free energy and thermodynamic forces, including the local stress applied on the microcracks and damage driving force are deduced. The main novelty of this work lies in presenting a new physically based non-associated and state-dependent friction law to capture the plastic distortion, in which the friction coefficient is no longer a constant but a state-dependent variable varying with the surface asperities of microcracks. The relationship between the friction coefficient and mean local stress, as well as microscopic damage is subtly defined. Intriguingly, a macroscopic strength criterion is derived as an inherent part of the proposed model without rotation of the principal stress. Moreover, semi-analytical stress–strain–damage relationships of the proposed model can be derived under conventional triaxial compression. These up-scaling analyses provide a powerful tool for parameter calibration. For application, the semi-implicit return mapping algorithm with plasticity-damage decoupling correction procedure (SRM-PDDC) is adopted for numerical implementation. To demonstrate the performance of the micromechanical model, we compare the model predictions with experimental data of quasi-brittle rocks in literature as well as the results obtained by Zhu et al. (2016). Finally, the applicable condition of the SRM-PDDC algorithm is discussed in detail.

Dual‐sensor wavefield separation in a compressed domain using parabolic dictionary learning

AbstractIn the marine seismic industry, the size of the recorded and processed seismic data is continuously increasing and tends to become very large. Hence, applying compression algorithms specifically designed for seismic data at an early stage of the seismic processing sequence helps to save cost on storage and data transfer. Dictionary learning methods have been shown to provide state‐of‐the‐art results for seismic data compression. These methods capture similar events from the seismic data and store them in a dictionary of atoms that can be used to represent the data in a sparse manner. However, as with conventional compression algorithms, these methods still require the data to be decompressed before a processing or imaging step is carried out. Parabolic dictionary learning is a dictionary learning method where the learned atoms follow a parabolic travel time move out and are characterized by kinematic parameters such as the slope and the curvature. In this paper, we present a novel method where such kinematic parameters are used to allow the dual‐sensor (or two‐components) wavefield separation processing step directly in the dictionary learning compressed domain for 2D seismic data. Based on a synthetic seismic data set, we demonstrate that our method achieves similar results as an industry‐standard FK‐based method for wavefield separation, with the advantage of being robust to spatial aliasing without the need for data preconditioning such as interpolation and reaching a compression rate around 13. Using a field data set of marine seismic acquisition, we observe insignificant differences on a 2D stacked seismic section between the two methods, whereas reaching a compression ratio higher than 15 when our method is used. Such a method could allow full bandwidth data transfer from vessels to onshore processing centres, where the compressed data could be used to reconstruct not only the recorded data sets, but also the up‐ and down‐going parts of the wavefield.

An Experimental Study on the Emission Characteristics in CI Engine using Nano Particle Coated Catalytic Converter

A proven viable technology to reduce vehicular exhaust gas emission is the catalytic converter. Although catalyst converters are an effective and efficient method to react to certain pollutants, they are expensive due to the use of noble metals like palladium, platinum and rhodium. Various research has been carried out to find a viable alternative to the noble metal catalyst that can react over vehicular pollutants such as oxides of nitrogen (NOx), carbon monoxide (CO), unburnt hydrocarbon (UBHC) and particulate matter (PM) emitted by the combustion of hydrocarbon burnt internal combustion engine. This paper investigates the emission characteristics of a nano-particle-coated catalytic converter in a conventional fossil-fuelled compression ignition engine. The catalyst used in the research is a nano-particle of metal oxides such as aluminium oxide (nano-sized Al2O3) particle and titanium oxide nano-sized (TiO2) particle. This catalyst is less expensive compared to a noble catalyst. Experiments were made on a single cylinder four strokes water-cooled diesel-fuelled compression ignition engine with a catalytic converter coated with alternative nano-particles. The exhaust gas was allowed to pass through the nano-sized catalyst deposited over the honeycomb structure of the substrate of the catalytic converter and the emission was measured using a five-gas analyser. The result showed a decreased level of oxides of nitrogen.

Effects of Self-heating Process on the Mechanical Properties and Energy Consumption in Molding of Carbon Fiber/Epoxy Composites

In order to increase the molding efficiency of carbon fiber composites and reduce their energy consumption in molding and manufacturing costs, this study explores the self-heating molding process of carbon fiber composites and investigates the uniformity of temperature distribution of carbon fiber/epoxy (CF/EP) composites. Besides, the mechanical properties, molding efficiency and energy consumption of CF/EP composite laminates are studied, and the comparison with the conventional hot compression molding process is conducted. The results show that the fixing mode of the electrodes greatly affects the heating rate and temperature uniformity-specifically, the use of bolted pressure plates leads to a faster heating rate and a more uniform temperature distribution than the use of conductive adhesive tape. During the self-heating process, the temperature difference inside the prepreg stack of CF/EP composite laminates was found less than 5°C. Compared with the CF/EP composite laminates molded by hot compression, those molded by self-heating exhibited an increase in tensile strength by 14.9%, compressive strength by 14.2%, interlaminar shear strength by 2.5%, tensile modulus by 11.6%, and flexural modulus by 11.1%, except for bending strength that showed no significant improvement. The self-heating molding process showed a heating rate of 5-10 times higher than the hot compression molding process, consumed only about 6.8% of the energy that the latter required, and increased the molding efficiency by 32%.

Feasibility and clinical benefits of the double-ProGlide technique for hemostasis after cryoballoon atrial fibrillation ablation with uninterrupted oral anticoagulants.

To access the efficacy and safety of the double-ProGlide technique for the femoral vein access-site closure in cryoballoon ablation with uninterrupted oral anticoagulants (OAC), and its impact on the electrophysiology laboratory time as well as hospital stay after the procedure in this observational study. Patients with atrial fibrillation undergoing cryoballoon ablation with uninterrupted OAC at Department of Cardiology, Beijing Anzhen Hospital, Capital Medical University, Beijing, China from May 2019 to May 2021 were enrolled in this study. From October 2020, double-ProGlide technique was consistently used for hemostasis (ProGlide group), and before that conventional manual compression was utilized (manual compression group). The occurrence of vascular and groin complications was accessed during the hospital stay and until the three-month follow-up. A total of 140 participants (69.30% of male, mean age: 59.21 ± 10.29 years) were evaluated, 70 participants being in each group. Immediate hemostasis was achieved in all the patients with ProGlide closure. No major vascular complications were found in the ProGlide group while two major vascular complications were occurred in the manual compression group. The incidence of any groin complication was obviously higher in subjects with manual compression than patients with ProGlide devices (15.71% vs. 2.86%, P = 0.009). In addition, compared with the manual compression group, the ProGlide group was associated with significantly shorter total time in the electrophysiology laboratory [112.0 (93.3-128.8) min vs. 123.5 (107.3-158.3) min, P = 0.006], time from sheath removal until venous site hemostasis [3.8 (3.4-4.2) min vs. 8.0 (7.6-8.5) min, P < 0.001], bed rest time [8.0 (7.6-8.0) h vs. 14.1 (12.0-17.6) h, P < 0.001] and hospital stay after the procedure [13.8 (12.5-17.8) h vs. 38.0 (21.5-41.0) h, P < 0.001]. Utilization of the double-ProGlide technique for hemostasis after cryoballoon ablation with uninterrupted OAC is feasible and safe, which has the clinical benefit in reducing the total electrophysiology laboratory time and the hospital stay length after the procedure.

Molecularly-controlled high swirl combustion system for ethanol/1-octanol dual fuel combustion

To achieve future targets for reducing greenhouse-gas and pollutant emissions from internal combustion engines, alternative fuels as well as alternative combustion modes offer great potential. The simultaneous use of two different fuels offers additional degrees of freedom to realize highly efficient and low-emission combustion, but also poses further challenges. To this end, this paper presents the Molecularly-Controlled high swirl Combustion system, MCC, which involves dual direct injection of a low- and a high-reactivity fuel in combination with lean relative air-fuel-ratios, high compression ratio, and compression ignition. The principle centers on the generation of a strong “molecular spark” – a spacially confined area in the combustion chamber where the reactivity of the fuel mixture is sufficient to form a robust, controllable ignition source. Subsequently, combustion spreads to the rest of the combustion chamber. Ethanol was used as the low-reactivity fuel and 1-octanol or conventional diesel fuel as the high-reactivity fuel to investigate the combustion characteristics, efficiency, and raw emission potential under various operating conditions with a light-duty single-cylinder research engine. The results are compared with compression ignition operation of conventional diesel fuel and neat 1-octanol. The MCC system achieves a large reduction in soot-emissions, even at very low NOx-levels below 0.1 g/kWh and a considerable increase in indicated efficiency over conventional compression ignition combustion.

Study on the ejector-expansion refrigeration system for low-temperature freezer application: Experimental and exergetic assessments

In this study, the authors propose applying the ejector-expansion refrigeration system (ERS) to develop a -50 °C low-temperature freezer. The main performance of the ERS based prototype freezers were investigated by experimental and exegetic methods. The influences of nozzle throat diameter and compressor displacement on ejector performance were discussed in detail. The higher efficiency and transiting exergy efficiency of ejector could be obtained with a larger nozzle throat diameter and compressor displacement. When the nozzle throat diameter rises from 0.4 to 0.7, ejector efficiency and transiting exergy efficiency increase from 16.1% to 20.1% and 26.1% to 30.2%, respectively. Moreover, these results demonstrated that the ERS-based freezer could operate stably with appropriate nozzle throat diameter and compressor displacement. Compared with the conventional single-stage vapor compression refrigeration system (CVRS), the compression ratio of ERS was reduced by 15.8%. In addition, the system performance decreased with the increase of capillary tube length. When capillary tube length varied from 40 mm∼ 80 mm, the COP of the freezer decreased from 0.59 to 0.52, and the overall exergy efficiency of freezer decreased from 22.1% to 17.1%. This research confirms that the ERS could provide a novel way for developing a -50 °C low-temperature freezer.

Performance assessment of a two-stage evaporation grade compression heat pump system for double temperature level drinking water production

The heat pump is widely used for space and water heating, but it has problems of low heating efficiency and inadequate heating at a large temperature span. According to the concept of energy cascade utilization and production, a two-stage evaporation grade compression heat pump (TE-GCHP) system is proposed in this paper for double temperature level boiled drinking water production. In this novel system, the application of grade compression matching multistage evaporation process realizes cascade utilization of the low grade thermal energy and cascade production of the hot drinking water, i.e. the refrigerant vapor from the intermediate-temperature water-cooling evaporator is sucked into the intermediate pressure suction port of the compressor to undergo the compression process of low compression ratio, while the refrigerant vapor from the low-temperature air-cooling evaporator is sucked into the low pressure suction port of the compressor to undergo the compression process of high compression ratio, so as to improve heating performance of the TE-GCHP system at large temperature spans. The energetic and exergetic analysis methods are used to evaluate heating performance of the proposed system and comparisons with the conventional waste heat-recovery single-stage compression heat pump (WH-SCHP) system are also discussed. The results indicate that there are the optimum water inlet temperature of medium-temperature water chamber and the optimum first-stage compression ratio to obtain the highest COP and exergy efficiency as well as the lowest power consumption, and energy-saving and economic performances of TE-GCHP system are markedly superior to those of WH-SCHP system and electric boiler. At evaporating temperature in the range of −10 °C to 30 °C, COP of TE-GCHP system is increased by1.06～1.29, and its exergy efficiency is improved by 2.17 %～10.62 %, while exergy loss of TE-GCHP system is reduced by 27.49 % on average, as compared to WH-SCHP system. In addition, as condensing temperature declines from 130 °C to 100 °C, power consumption of TE-GCHP system is decreased by 20.13–35.07 % compared with WH-SCHP system, whereas exergy efficiency of TE-GCHP system is on average 7.99 % higher than that of WH-SCHP system. In addition, the annual costs of the proposed system are 53.20 % and 69.68 % lower than those of the WH-SCHP system and electric boiler respectively.

Preliminary Study for the Commercialization of a Electrochemical Hydrogen Compressor

A global energy shift to a carbon-neutral society requires clean energy. Hydrogen can accelerate the process of expanding clean and renewable energy sources. However, conventional hydrogen compression and storage technology still suffers from inefficiencies, high costs, and safety concerns. An electrochemical hydrogen compressor (EHC) is a device similar in structure to a water electrolyzer. Its most significant advantage is that it can accomplish hydrogen separation and compression at the same time. With no mechanical motion and low energy consumption, the EHC is the key to future hydrogen compression and purification technology breakthroughs. In this study, the compression performance, efficiency, and other related parameters of EHC are investigated through experiments and simulation calculations. The experimental results show that under the same experimental conditions, increasing the supply voltage and the pressure in the anode chamber can improve the reaction rate of EHC and balance the pressure difference between the cathode and anode. The presence of residual air in the anode can impede the interaction between hydrogen and the catalyst, as well as the proton exchange membrane (PEM), resulting in a decrease in performance. In addition, it was found that a single EHC has a better compression ratio and reaction rate than a double EHC. The experimental results were compatible with the theoretical calculations within less than a 7% deviation. Finally, the conditions required to reach commercialization were evaluated using the theoretical model.

A numerical study of elastocaloric regenerators of tubular structures

Solid-state elastocaloric cooling is increasingly recognized as preferred cooling technology to conventional vapor compressions without using coolants that are volatile atmospheric pollutants or greenhouse gases with global warming potential. However, the specific cooling power (SCP) of current prototypes has yet to be improved in meeting practical requirements. In this work, tubular nickel-titanium (NiTi) elastocaloric regenerators with enhanced heat transfer structures were proposed in order to increase cooling performance in compression-loaded regenerative systems. A numerical model was developed to evaluate this cooling performance. Using a nondimensional analysis of the governing equations, the model simplifies the investigation of the design and operation parameters and greatly reduces the computation complexity of the regenerative elastocaloric cooling systems. As a demonstration, the design principles have been applied to yield an elastocaloric regenerator of spiral-shaped tubular structures, which improves the maximum SCP and maximum temperature span by 186% and 146% respectively, compared to a plain tube design, proving that the numerical model provides a promising pathway for developing energy-efficient elastocaloric cooling systems.

A preliminary study of diffusion tensor imaging in root entry zone of primary trigeminal neuralgia.

Primary Trigeminal Neuralgia (PTN) is a common and refractory neurological disease. Conventional vascular compression theory could not completely explain the etiology and pathogenesis of PTN. This study used diffusion tensor imaging (DTI) to demonstrate the microstructural changes of root entry zone (REZ) region in PTN patients. DTI sequences was performed on PTN patients and healthy controls (HCs). Clinical data included affected side, disease course and visual analogue scale (VAS) were collected. Quantitative DTI variables such as FA, MD, AD and RD of the root entry/Exit zone (REZ) were measured and compared in PTN/HCs, affected/unaffected side, and pre/post operation groups. The PCoA was established to conduct overall differences between PTN group and the HCs. A total of 17 patients with PTN (mean age 59.29 ± 8.53; 5 men) and 34 HCs (mean age 57.70 ± 6.37; 10 men) were included. Lower FA value of the affected side of PTN group was observed compared to the unaffected side and the HCs (p = 0.001), whereas the values of MD, AD and RD were significantly increased (p < 0.001). Moreover, the decrease of FA value was recovered post operation. PCoA results of the comprehensive indexes can significantly distinguish PTN group from HCs (r = 0.500, p < 0.001). Quantitative variables derived from DTI in REZ had significantly different profiles between PTN patients and HCs, which were associated with VAS situation and the disease course of PTN. The comprehensive index established on DTI variables were of great potential to reveal the microstructure changes in PTN patients and predict the therapeutic effect.

Performance analysis of a double-efficient CO2 cooling and heating system using an ejector

This paper presents a double-efficient CO2 cooling and heating system using an ejector, which can operate as modified dual-evaporators ejector expansion cycle (MDEEC) in cooling mode or as conventional heat pump cycle (CHPC) in heating mode through the reasonable arrangement of valves, and countercurrent heat transfer of CO2 with cooled or heated air is achieved in both modes. An ejector is utilized to recover the expansion work and two evaporators provide independent air dry-cooling and dehumidification in cooling mode. Two thermodynamic models are developed to investigate the effects of various parameters on the cycle performance. Comparison of proposed cycles with ejector expansion refrigeration cycle (EERC) and conventional dual-evaporator vapor compression cycle (CDVCC) are conducted based on energy analysis. The results show that coefficient of performance (COP) of this system is improved by more than 25% compared to EERC, and 33.62–56.27% compared to CDVCC. Besides, performance comparison of CHPC using CO2 and R134a is conducted, which reveals that COP of CO2 cycle is consistently higher, maintained above 2.7, and the maximum value is 5.36. The COP improvement is up to 29.74%, and the minimum value is 5.37%. This study provides a solution for efficient cooling and heating applying CO2 as refrigerant.

Experimental Study on Creep Characteristics and Long–Term Strength of Anthracite

Coal pillars from old mines undergo creep, a type of time−dependent deformation. Research on underground engineering stability of coal mines has increasingly focused on long−term strength as an important mechanical index of the creep characteristics of coal pillars. This study performed conventional triaxial compression and triaxial creep tests of anthracite under different confining pressures. The creep law of anthracite and the long−term strength of anthracite was studied according to the test results. The test results demonstrated the following: (1) The conventional elastic modulus and peak strain of anthracite increased exponentially with the confining pressure. (2) Under low stress levels, anthracite exhibited only instantaneous deformation and attenuation creep. In contrast, anthracite exhibited instantaneous deformation, attenuation creep, and steady creep under high stress levels; accelerated creep occurred until failure when the stress reached a particular value. (3) By fitting the steady−state creep rate of anthracite under high stress levels, the functional relationship between axial stress and steady−state creep rate was established, and the threshold of the steady−state creep rate in high−stress−level areas was suggested as the optimum long−term strength of anthracite. (4) The ratio of the long−term strength to instantaneous strength under various confining pressure grades ranged from 70% to 91%.

A Simple and Practical Tool for Indirect Determination of the Unconfined Compressive Strength of Most Common Construction Materials

Determination of the unconfined compressive strength (UCS) of construction materials in the laboratory is tedious and time-consuming. There have been many attempts to indirectly predict UCS using simpler tools and techniques. One of them is the nail gun. The scope of this investigation is to design a nailer which can be applied all construction materials whose UCS range from 1-100 MPa. In the research, rocks, bricks, and concretes prepared in different cement/sand ratios with different strength ranges were used as materials. The unconfined compressive strength of the materials used in the experiments was first determined by conventional compression tests. The nail penetration depths were determined by conducting experiments on the same materials using a nailer with two different energy levels. An empirical relationship was developed by using nail penetration depths, driving energies, and nail diameters as the independent variables and the UCS determined by the conventional method as the dependent variable. According to the empirical relationship determined by multiple regression analysis, the UCS of building materials can be estimated with significance level of 99% by the nail penetration method. The research also revealed that the UCS of rocks might have a coefficient of variation as high as 30%.

Creep behavior and long-term strength characteristics of pre-peak damaged sandstone under conventional triaxial compression

A series of creep tests were carried out on sandstone specimens with different pre-peak instantaneous damage characteristics under different confining pressures. The results revealed that the creep stress was the key factor affecting the occurrence of the three stages of creep, and the steady-state creep rate increased exponentially with increasing creep stress. Under the same confining pressure, the larger the instantaneous damage of the rock specimen was, the more quickly creep failure occurred and the lower the creep failure stress was. For the pre-peak damaged rock specimens, the strain threshold for accelerating creep was the same for a given confining pressure. The strain threshold increased with increasing confining pressure. In addition, the long-term strength was determined using the isochronous stress–strain curve and the variation in the creep contribution factor. The results revealed that the long-term strength decreased gradually with increasing pre-peak instantaneous damage under lower confining pressures. However, the instantaneous damage had little effect on the long-term strength under higher confining pressures. Finally, the macro–micro-failure modes of the sandstone were analyzed according to the fracture morphology observed via scanning electron microscopy. It was found that the macroscale creep failure patterns of the sandstone specimens could be divided into a shear-dominated failure mode under high confining pressures and a mixed shear-tensile failure mode under low confining pressures. At the microscale, as the confining pressure increased, the micro-fracture mode of the sandstone changed gradually from a single brittle fracture to a mixed brittle and ductile fracture mode.

Experimental and numerical study on a novel fanless air-to-air solar thermoelectric refrigerator equipped with boosted heat exchanger

Peltier cooling systems are generally small, portable and simple in operating principle compared to conventional vapor compression cooling systems. For these reasons, Peltier cooling systems are proposed widely to use in the field. In studies on Peltier coolers presented in the literature, equipment such as pumps and fans is used and this causes noise and vibration. In order to resolve these problems and negative effects, a cooling system was designed and manufactured using completely motionless elements. Additionally, the electricity energy need of this thermoelectric cooling chamber is fully met by solar energy. This study was investigated both experimentally and numerically by using computational fluid dynamics (CFD) methods. By designing a special heat exchanger in the produced air-to-air cooler system, it has been proven that, the Peltier system can work effectively without the need for any elements such as fans. Then the effects of the voltage values applied to the system and the number of Peltier modules have been investigated. The experiments were carried out at 4 different voltage values (1.5, 2.0, 2.5 and 3.0 V) and in 3 different Peltier numbers (1, 3 and 5) in consecutive connections. According to the obtained results, the most suitable situation for reducing the temperature of the cooling chamber were obtained as the voltage was at a maximum of 3 V and the Peltier number was maximum (5 Peltiers mode), where the internal temperature of the cooling chamber decreased to 11.28 °C. It was observed that, the coefficient of performance (COP) values decreased over time in all experiments. Considering the data taken at the beginning of the experiments, the maximum COP value was obtained as 0.04 when 1 Peltier and 1.5 V voltage were applied.