查看更多>>摘要：Compressed Expanded Natural Graphite (CENG) impregnated with Phase Change Material (PCM) is an interesting material for latent heat storage application requiring a high heat transfer rate. This composite material, that has orthotropic properties, undergoes expansion while the PCM changes phase that might affect its thermal properties and its mechanical integrity. In order to assess this possible evolution, the CENG-PCM thermal conductivities in the two principal directions have been measured before and after a 1500 thermal cycling test. The thermal characterisation was performed using two methods: a classic hot guarded plate method and a more originally inverse method. The results show no significant change of the thermal conductivity values after ageing cycles in spite of the appearance of cracks in the planar direction of the composite. We can conclude that the delamination of the CENG was not sufficient to affect the thermal conductivities. In addition, a DSC analysis shows that the latent heat is also almost unchanged after the thermal cycling.
查看更多>>摘要：A mathematical model of gravity-assisted flat loop heat pipe has been established, based on mass conservation, energy balance, combining pressure-temperature and heat & mass transfer relations. The effects of gravity, heat sink and ambient temperature on steady-state performance of the loop heat pipe were comprehensively investigated. The modeling results show that there exist two driving modes: gravity-driven mode and capillary-gravity co-driven mode when plate loop heat pipes operate under gravity-assisted condition. Besides, a critical heat flux has been found between the gravity-driven mode and the co-driven mode. The condensate capacity of heat sink and ambient temperature to the heat regenerative ability of the liquid line affect the variable heat-conductance zone and constant heat-conductance zone. These findings are beneficial to the comprehensive understanding of the operating characteristics, which can guide the design and optimization of the flat loop heat pipes.
Tarish, Ali Lateef J.Khalifa, Abdul Hadi N.Hamad, Ahmed
15页
查看更多>>摘要：The energy demand in HVAC utilization has been steadily increasing, specifically in warm climate countries such as Iraq. Therefore, searching for advanced air conditioning systems driven by waste energy sources such as adsorption air conditioning systems operated by solar energy becomes necessary. The objective of the present study is to investigate the performance of the adsorption thermophysical battery (ATB) employed with the silica gel-water under various operating conditions. In addition, examine and analyses the influence of outlet chilled temperature and pressure during the charge and discharge processes. The performance parameters of ATB including, cooling capacity (CC), coefficient of performance (COP), specific cooling power (SCP) under different effective temperatures are evaluated. The lumped parameter method (LPM) and modified Freundlich adsorption equations are used to develop a theoretical approach. The results revealed that, relatively low temperature range (50-70 degrees C) can be used during the charging process based on a common waste heat source. A significant reduction in the temperature and pressure of ECU during the charging-condensation process can be achieved by decreasing the inlet coolant temperature to the chilled temperature. This relatively low temperature is close to the chilled water temperature which can be used for air conditioning purposes. Investigating three inlet chilled temperatures 15 degrees C, 20 degrees C, and 30 degrees C showed that the increase in the chilled temperatures enhanced the evaporation process and increased COP, SCP, CC and the adsorption uptake. The reduction in the inlet coolant temperature from 40 to 20 degrees C, resulted in 11% enhancement in CC. The determined performance parameters of the ATB system under specified operating conditions are 2 kW, 0.3 and 100 Wkg(-1) for CC, COP, and SCP respectively.
查看更多>>摘要：The design of passive micromixers is a trade-off among the mixing efficiency, pressure drop, length/time scale and the complexity (or cost) of the mixing channel. The advent of chaotic advection in micromixers has encouraged researchers to explore the untapped potential of disturbance in the flow. Over the past years, the performance of T micromixers has increased significantly, however, the mixing efficiency is still far behind the conventional threshold. In order to enhance the mixing further, non-aligned inlets with rigid diffuser plates (or vortex generators) and cylindrical obstacles are proposed with the present study. The vortex formed at the T junction is intensified at the leading edge of the diffuser plate and each diffuser plate creates two vortices in opposite directions. These vortices are energized across each pair of the diffuser plate and gets propelled along the mixing chamber. The common flow-up approach with the staggered arrangement is suggested for enhancing the mixing efficiency. For the Reynolds number around 30; mixing efficiency as high as 85% is achieved within the pressure loss of 2300 Pa. The proposed design is simple and similar approach can be used for desired mixing efficiency at specific Reynolds number. Configurations such as in-line & staggered arrangement, flow-up & flow down arrangement, presence of obstacles etc. are compared and their implementation is proposed accordingly.
查看更多>>摘要：Cannabis sativa L. is an herbaceous, dioicous, annual medicinal plant of the Cannabaceae family. Two of the most important cannabinoids with pharmaceutical properties produced are the Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and the cannabidiol (CBD). The present work aims to investigate the convective drying characteristics of hemp inflorescences, evaluate the effects of constant and time-varied temperature schemes on the CBD and Delta(9)-THC concentrations and estimate the energy requirements of each drying regime. Constant temperature drying of inflorescences was performed for 40, 50 and 60 degrees C, while the rates of temperature increase evaluated for the non-isothermal drying experiments have been 1.5, 2.5 and 4 degrees C.h(-1) for the aforementioned temperature range limits (40-60 degrees C), under a constant airflow of 1 m.s(-1). Considering all experiments, the highest temperature level and temperature increase rate resulted in reduced processing time and energy consumption, while the total content of CBD and Delta(9)-THC of inflorescences ranged between 0.653-1.335% w/w and 0.036-0.062% w/w, respectively. Concentration of samples dried under variable temperature regimes resulted to higher CBD mean levels (+46.7% for inflorescences and +65.3% for leaves), while the Delta(9)-THC levels did not seem to be affected significantly at any drying treatment. Different constant temperature levels of isothermal drying or rates of temperature increase of non-isothermal drying, were not found to have a significant effect on the total content of the two investigated cannabinoids. Thus, non-isothermal drying at the maximum rate of temperature increase (4 degrees C.h(-1)) is proposed since reduced drying duration and energy consumption is achieved by retaining high CBD levels.
查看更多>>摘要：The ventilation power and the temperature increase experienced by steam turbine last stages operating at low load are strongly influenced by some design parameters such as radial clearances, degree of reaction and pitch to chord ratio. This paper presents a detailed numerical analysis about the role of these different design parameters. A series of temperature and flow field measurements performed on a steam turbine last stage blades are used for the validation of the numerical results. For the considered test cases, it seems that the size of the radial clearances of the rotating blades does not a play a significant role on the ventilation phenomenon. The impact of the pitch to chord ratio on ventilation power can be divided into two components such as number of moving blades and chord size. The increase on the number of blades is more significant than the increase in the chord. The degree of reaction at design influences the ventilation power since it drives the inception of flow separation, affecting the reverse flow height downstream the last stage moving blades (LSMB).
Mohseni-Gharyehsafa, BehnamLyulin, Yuriy V. A.Evlashin, Stanislav A.Kabov, Oleg...
15页
查看更多>>摘要：We report our experimental study of the performance of a 3D-printed closed two-phase thermosyphon. Using laser powder-bed fusion (L-PBF) technique, the thermosyphon has been additively manufactured with stainless steel (316L), which has excellent resistance and compatibility with chemicals and coolants. The specific feature of the thermosyphon and the novelty of the study is that the device is printed as a single unit, including an extruded helical heat exchanger into the condenser body and integrated pillar arrays in the evaporation section. The evaporation, adiabatic, and condensation section lengths are 20 mm, 30 mm, and 50 mm, respectively. The micropillars size is 2.5 mm. We considered the effects of the filling ratio of acetone as a working fluid and heat flux on the thermal performance of the thermosyphon. The filling ratios used are 0%, 1.7%, 4.9%, 6.3%, 10.4%, 17.1%, 27.9%, 40.5%, 50.4%, 64.2%, 76.6%, and 84.1%. We found that the minimum total thermal resistance is 0.48?.W-1, which is achieved at the minimal value of the filling ratio of 1.7% and thermal power of 13.7 W. The filling ratio of 4.9% shows the best stable thermal performance of the thermosyphon.
查看更多>>摘要：This work deals with experiments on the thermal performance of phase change materials (PCMs) used as thermal interface materials (TIMs) between a heat pipe cooling system and an electronic component. A test rig is conceived and realized so that experiments can be carried out under various heat input powers, clamping pressures, and cooling temperatures. Three commercial PCMs (Hi-flow 105, Hi-flow 200, and Hi-flow 625), having different thicknesses, thermal conductivities, and phase change temperatures, were tested. A heat transfer enhancement factor is introduced to compare the contact thermal resistance obtained without PCMs to that obtained with PCMs. The experimental results show that the thermal performances obtained by Hi-flow 105 and Hi-flow 625 are affected greatly by the heat input power and they are less sensitive to the clamping pressure. The maximum heat transfer enhancements obtained by these two PCMs, which increase with the heat input power, are 75 % and 55 % for Hi-flow 105 and Hi-flow 625, respectively for a heat input power of 115 W. However, with Hi-flow 200, the thermal performances are sensitive to heat input power, and the clamping pressure for heat input powers lower than 100 W. The heat transfer enhancements obtained by Hi-flow 200, which decrease with the heat input and the clamping pressure, are lower than those obtained with Hi-flow 105 and Hi-flow 625 PCMs whatever the heat input power and the clamping pressure, and they do not exceed 25 %. Thermomechanical simulations were carried out to analyze the combined effects of the heat input load, the temperature level, and the clamping pressure. The comparison between the results issued from the thermomechanical model and those obtained from the experiments which are relative to axial temperature distribution along the heating and cooling blocks shows a good agreement that is demonstrated by statistical indicators. An analytical model is developed to calculate the contact thermal resistance between two rough surfaces including or not PCMs. The comparison between the analytical results and those issued from the literature and the present study shows a good agreement.
查看更多>>摘要：This paper addresses a theoretical investigation about the effect of pressure drop along the heat exchangers on the coefficient of performance, heat transfer area and compressor capacity of a single-stage vapor compression refrigeration system. This analysis is performed based on a model of the complete system with one-dimensional heat exchangers, compressor and expansion device modelled as concentrated parameters. The fluid thermodynamic state along the heat exchangers is evaluated based on the energy and momentum balance considering predictive methods available in the open literature for heat transfer coefficient, pressure drop and void fraction. The compressor is modelled based on single isentropic efficiencies and the expansion device is assumed as adiabatic. Uniform wall temperatures are imposed in both heat exchangers to mimic heat removal from a refrigerated space and rejection to external ambient. Commercially available tube diameters were considered for the channel dimensions. Based on the analysis, it can be concluded that the pressure drop presents significant impact at the efficiency and sizing of the main components in a refrigeration system depending on the operational conditions. COP reductions of more than 15% for R600a and R134a were observed, as well as up to 29.2% increase of the heat exchanger area for the condenser. Correlations were proposed to prompt estimative of the impact of pressure drop on the COP and heat exchangers area.
查看更多>>摘要：In comparison with the conventional turbine-based power unit, liquid metal magnetohydrodynamic (LMMHD) power units have simple structure since they have no moving parts, and hence they are highly reliable systems with low maintenance cost. However, a LMMHD power unit needs a high-temperature prime mover which is conventionally provided by fossil fuels. To cover up this shortcoming, the present study aims at proposing a new LMMHD cogeneration plant for power and fresh water supply, using a concentrated solar power (CSP) system. The devised integrated system was designed and evaluated from the first- and second-laws of thermodynamics perspectives. The results indicated that the proposed cogeneration plant can produce total power of 38.3 kW and fresh water of 881.6 L/h, in which in this case the energy utilization factor (EUF) and total exergy efficiency were computed 99.69% and 5.57%, respectively. Considering the LMMHD power unit as the reference system, it is found that when the idea of cogeneration was employed, the energy and exergy efficiencies of the LMMHD power system can be improved by 150.2% and 4.3%, respectively. Among all constituents, the receiver has the highest exergy destruction of 256.9 kW, followed by the heliostat with exergy destruction of 228.2 kW. The results of parametric study indicated that the energy and exergy efficiencies of the proposed cogeneration system can be improved by increasing the humidifier/dehumidifier effectiveness or decreasing the mass flow rate of the second MHD loop.
NSTL
Elsevier