查看更多>>摘要：The existence of preferential flow paths, such as fractures and/or fault play an vital role on the thermal breakthrough of geothermal doublet system. The interaction between the preferential flow path and bedrock is often uneven and may have typical fractal characteristic. This study proposes a thermo-hydro-mechanical coupling model considering the deformation of fractal fractures. The fractal fracture is regarded as a thin elastic layer existed in the bedrock, whoes deformation depends to the bedrock and its own mechanical properties. Subsequently, the geothermal doublet system with a three-dimensional fractal fracture is modelled and the parameters affecting thermal breakthrough are investigated numerically. Research results indicate that the thin elastic layer assumption is remarkably robust for modelling fracture opening and closing under coupling conditions. Owing to complex fracture geometry, the fracture permeability evolution presents certain heterogeneity, which is related to fractal dimension, in-situ stress, and geothermal wells layout. The cool water in fracture with larger fractal dimension can interact with the bedrock more fully and further affect the thermal breakthrough. This further suggests that the site selection of geothermal wells should consider the specific geometry of preferential flow paths to avoid premature thermal breakthrough inducing low system efficiency.
查看更多>>摘要：A phenomenon of accidental release from a high-pressure containment into a low-pressure environment is significant in several industrial systems such as chemical reactors and pipeline transportation and storage. The paper aims to investigate the vapor release with condensation, and to develop a vapor release model coupling a blowdown model with a critical flow model. Vapor release tests of the pressurized vessel are performed with micro-cracks, and the fluid pressure and temperature evolutions are recorded. The theory of ideal dropwise condensation is used to describe the interfacial heat transfer, and the wall heat transfer is regarded as the heat conduction in a blowdown model. A six-equation model allowing the vapor leakage simulation with condensation is presented with a bubbly flow regime assumption. The vapor release model is composed of a critical flow model in conjunction with a vessel blowdown model, including the treatment of the vapor condensation, interfacial heat transfer, and interfacial force during the depressurization. The proposed model predictions exhibit strong similarities with the measured evolutions of the fluid pressure, temperature, and the total weight of the released fluid. Finally, the qualitative analysis of the vapor release with a heat transfer/adiabatic process is conducted. The vapor condensation occurs during the release process when the initial pressure is increased, while the condensation is reduced for a heat transfer process. The release time and vapor condensation of the pressurized vessel are significantly affected by the vessel volume and break area.
查看更多>>摘要：The temperatures required by ultra-low temperature applications, ranging from -50 degrees C to -100 degrees C, cannot be reached economically with single stage systems because of the limitation of the compression ratio. Different types of solutions such as cascade or two-stage systems could be implemented to achieve the desired working conditions. However, these systems are usually complex or too expensive. The solution might be found in the use of auto-cascade systems working with zeotropic mixtures. The present article proposes two modifications of the auto-cascade system by including an ejector device to improve the COP. The first modification includes the ejector as an expansion device at the outlet of the phase-separator, while the second includes the ejector as a precompression stage. A mixture of the hydrocarbons iso-butane (R600a) and ethylene (R1150) was used as an alternative to conventional refrigerants, which have a very high GWP. The study performed firstly assessed the sensitivity analysis of the free variables on the operating conditions of each cycle. The evaluated variables were the compressor pressure discharge, the mass fraction of the mixture and the phase separator temperature. In the case of the ejector enhanced cycles, the ejector efficiency and the motive pressure were additionally included. Then, the optimal operating conditions were found by means of an optimization process. The results showed a potential improvement in the COP of 12% for the case of the ejector as an expansion device, with an optimal mass fraction of 0.45 of ethylene. On the other hand, the ejector as a pre-compression stage did not show any improvement with regard to the reference case. The present study concludes that the mixture of ethylene and isobutane is a suitable combination for auto-cascade cycles and that the ejector can be implemented to improve the COP without adding excessive complexity and cost.
Shin, Sung GilCho, Jai OanKo, AreumJung, Hwa-Young...
16页
查看更多>>摘要：The new application of phase change material for passively cooling the containment building of a nuclear power plant provides many benefits such as minimized wall penetration, low maintenance cost, and the possibility of installation in currently operating power plants with a minimal design change. In this study, the applicability of PCM as a passive containment cooling system is verified through design and evaluation of the PCM condenser. In addition, the effective heat capacity method is verified for simulating the melting process of PCM in the condition of a reactor accident. The PCM condenser design is optimized with five design parameters; melting point, volume, heat transfer area, effective thermal conductivity, and total heat absorbed. The performance of the PCM condenser is evaluated by containment safety analysis and is compared to that of an active containment cooling system. Although the PCM condenser had a limitation for absorbing only a fixed amount of thermal energy due to the absence of an external heat sink, the PCM condenser shows better performance in the early stage after the accident initiation compared to the single spray system and finally maintains the integrity of the containment building.
查看更多>>摘要：The supercooling of phase change material (PCM) significantly affects the heat release characteristics of heat storage systems. Therefore, accurate numerical modeling of the solidification process is key for studying heat storage. Few models considering the crystal growth of supercooling PCMs have been established; however, their accuracy under slight supercooling conditions is unsatisfactory. In this study, the implicit finite difference method was used to establish a two-dimensional PCM heat transfer model that considers the crystal growth process in detail. The growth rate of the crystallization front was used to control the crystallization start time at each node, and the solidification speed was used to manage the heating process once the crystallization was triggered. The accuracy of the model was verified by the experimental results obtained by melting, cooling, and triggering crystallization in a stable supercooled state and during cooling. Based on the simulations of a concentric tube PCM heat exchanger, the effects of several parameters on the heat release rate were investigated, such as the supercooling degree, the PCM initial temperature, and the inlet temperature of Heat transfer fluid (HTF). The results show that a large supercooling degree will accelerate the heat release rate of the PCM heat exchanger after the crystallization is triggered. An increase in the initial PCM temperature reduces the sharp increase in HTF outlet temperature caused by crystallization; a decrease in HTF inlet temperature also has the same effect. A comparison of various models demonstrated that the use of the crystallization front to calculate the PCM temperature directly causes up to 3% of the heat to be released earlier or later. Moreover, ignoring the crystal growth process causes up to 9% of the heat to be released in advance, and ignoring the supercooling causes an even greater error.
Bilen, KadirTokgoz, NehirSolmaz, IsmailBalta, Tuba...
14页
查看更多>>摘要：In this study, the influences of swirl generators fixed at the pipe inlet and having different swirl angles (0 degrees, 22.5 degrees, 41 degrees and 50 degrees) on the heat transfer and fluid flow characteristics in a pipe flow were experimentally and numerically studied under a constant heat flux condition in the range of Reynolds (Re) number 2400-23000. The axial type passive swirl generator aims to increase the turbulence effect and gives tangential acceleration while increasing the total flow length in the pipe. This study is novel with the aspects of; (i) the number of helical air passage channels employed in the investigated swirl generators and, (ii) 360 degrees rotation of each helical channel at different swirl angles, i.e., lengths of considered swirl generators are different. Experiments were carried out in a smooth aluminum pipe having an inner diameter of 15 mm and a length of 675 mm with and without swirl generators for turbulent airflows. Empirical correlations for the local and mean Nusselt (Nu) numbers, and friction factor (f) were developed as a function of Re number and swirl angle as well as smooth pipe flow. The obtained experimental results for the heat transfer and friction factor at different swirl angles were compared with the smooth pipe results. The findings showed that the maximum heat transfer rate is achieved at a swirl angle of theta = 50 degrees, and the heat transfer rate increases as the swirl angle and Re number are increased. The decaying swirl flow leads to around 1.1-1.41 times enhancement of Nu and 2.3-6.76 times increase of f compared to the smooth pipe. The maximum value of performance evaluation criteria (PEC), around 0.83, is achieved by the decaying swirl flow when the theta = 0 degrees and Re = 2400. The experimental and numerical Nu and f values for swirl flow are in good agreement within the mean deviation +/- 7.1% and +/- 8.4%.
查看更多>>摘要：A multi-cylinder-type liquid-piston Stirling engine (MCLPSE) is an external combustion engine characterized by a low operation temperature difference below 100 K, a simple structure consisting of only a few parts, and the use of harmless working fluids, namely, air and water at atmospheric pressure. Although the structure is simple, the design method has not been established to date. This study proposes a systematic design guideline for the MCLPSE to attain a target output power under a given temperature condition. The guideline is based on the analysis of the natural mode oscillations of the system, the results of a thermoacoustic theory, and a simple heat transfer model of heat exchangers. The designed MCLPSE was numerically verified using a simulation code (DeltaEC). We introduced a linear alternator based on electromagnetic induction to extract the output power from the designed engine. The results showed that the designed engine achieved the target output power of 102 W under the heat source and sink temperatures of 130 degrees C and 23 degrees C, respectively.
查看更多>>摘要：The properties of geothermal working fluids have great influence on the heat extraction performance of an enhanced geothermal system (EGS). In addition to H2O, N2O and CO2 have also been proposed as geothermal working fluids. In order to analyze and compare the heat extraction performances of the N2O, CO2 and H2O EGS, a two-dimensional thermo-hydraulic-mechanical (THM) coupled EGS model with discrete fractures is established. In addition, the influences of injection-production parameters on the heat extraction effects of EGS with different working fluids are also analyzed. The results indicate that the heat extraction performances of N2O-EGS and CO2-EGS are almost the same under the same conditions. Taking into account the stable production time and the stability of supporting equipment, the performances of N2O-EGS and CO2-EGS are better than H2O-EGS. Furthermore, a lower injection temperature is conducive to the heat extractions of N2O-EGS and CO2-EGS, while the injection mass flow rate and production pressure should be designed reasonably according to the actual situation. As for H2O-EGS, a higher production pressure is conducive to its performance, while its injection temperature and injection mass flow rate should be designed reasonably.
Gomez-de la Cruz, Francisco J.Palomar-Torres, AmaliaPalomar-Carnicero, Jose M.Cruz-Peragon, Fernando...
16页
查看更多>>摘要：Nowadays, the design, control and development of rotary dryers are based on the study of the complete equipment. Rotary dryers are treated as a black box where only inlet and outlet parameters are known. This implies the need to know the main interactions of mass, energy and exergy throughout the trommel to help improve the drying process. In this paper, we carried out an analysis of energy and exergy during drying of olive stone from finites control volumes in an experimental rotary dryer. Mass, energy and exergy balances are applied to each control volume in the drying air. The design of experiments is based on three initial drying air temperatures (210 degrees C, 180 degrees C and 150 degrees C) and three drying air flows (576 kg/h, 425 kg/h and 280 kg/h), with a by-product mass flow of 40 kg/h and a rotational speed of 5.5 rpm. The results indicated that olive stone moisture content was reduced to less than half in the first third of the trommel where a big thermal shock is produced due to the interaction between the by-product and the drying air flow at high moisture contents and high temperatures, respectively. This fact led to the highest exergy destruction values, thereby diminishing the flow exergies for the following control volumes. Heat losses were considerable in all equipment, especially in the first two control volumes. Furthermore, drying process was analyzed from the enersgetic, exergetic and drying efficiencies and the unit energy consumption.
查看更多>>摘要：Low-grade thermal energy can be used for superheating the working fluid in the steam ejector. The purpose of the present study is to investigate the relationship of two-phase heat transfer with the ejector performance in the condensing flow regime under the condition of primary steam superheating. The condensation and evaporation phenomena occur in two-phase ejectors. The heat and mass transfer between the liquid and vapor phases change the flow pattern inside the ejector. The results show that the wet steam model has better fitting with experimental data than the ideal gas model. The primary steam superheating influences on the intensity of shock-wave patterns of supersonic flow, liquid mass fraction and two-phase heat transfer in the ejector. By increasing the superheating level, the two-phase heat transfer and exergy destruction are decreased, entrainment ratio in the wet steam model gets closer to entrainment ratio in ideal gas model, critical and limiting pressure are decreased and on-design and off-design regions become smaller. Also the effects of primary steam superheating in the ejector are investigated on the performance of a renewable refrigeration cycle. Superheating the working fluid reduces the generator energy consumption and increases exergy destruction in a refrigeration cycle. At degree of superheating 100 K, two-phase latent heat, energy consumption, and ejector exergy destruction decrease by 40%, 3.9%, and 11.7%, respectively, and entrainment ratio and total exergy destruction increase by 10% and 50%, respectively.
NSTL
Elsevier