查看更多>>摘要：To fully utilize the solar energy captured by the photovoltaic module (PVM) and improve its performance, a coupled system consisting of a PVM, the solar selective absorber (SSA) and the two-stage thermoelectric generator (TTEG) is investigated, where the TTEG considers the Peltier effect, Seebeck effect and Thomson effect. Taking account of the various irreversible losses within the coupled system, the mathematical expressions of energy conversion efficiency, power output, exergy efficiency and exergy destruction rate of the coupled system are established. The numerical results show that the maximum energy efficiency and maximum output power of the coupled system are, respectively, increased by 9.89% and 9.87% compared with a standalone PVM, and the maximum exergy efficiency is 10.4% higher than a standalone PVM. Finally, through sensitivity analysis, it can be seen that the coupled system is affected by several key parameters, including the diode ideality factor, thermoelectric elements, solar irradiance, and PVM operating temperature, so the coupled system can be improved by adjusting these parameters. The results of this paper may provide a new insight into the design and optimization of an actual coupled system.
查看更多>>摘要：Cooling garments are gradually becoming a simple and energy-saving way to relieve the human body from thermal stress. In this article, we built a set of cooling garment prototypes based on thermoelectric cooling. A series of performance tests were carried out in a simulated high-temperature environment to select the appropriate refrigerant and on this basis to determine the best inlet temperature that can meet the refrigeration requirements of personnel. The experimental results show that water and 40% glycerin have own advantages as refrigerants. The energy utilization efficiency of the cooling garment is higher when water is used as the refrigerant and the average cooling effect is better when 40% glycerol is used as the refrigerant. The higher the energy utilization efficiency, the relatively small battery capacity required by the liquid cooling garment, so water is selected as the refrigerant in this article. The effective cooling power can reach 210 W when the ambient temperature is 35 degrees C, and the maximum coefficient of performance at this time is 3.3. Through the wear experiments, we determined that the optimal water inlet temperature when the ambient temperature is 35 degrees C is 22 degrees C, which reduces the average temperature in the microclimate area to about 28.5 degrees C, and the relative humidity in the microclimate area is reduced from 90% to 58%, which proves the effectiveness of wearing this cooling garment to improve the thermal comfort of the human body. In addition, it is found that the inlet temperature of the cooling garment has a critical value at 22-24 degrees C when the ambient temperature is 35 degrees C. The water inlet temperature near this critical value has a more significant change in the cooling effect.
查看更多>>摘要：The rotating detonation engine (RDE) is an innovative and practical pressure-gain device for rocket and ramjet propulsion, and has been widely investigated. The present work aims to explore at length the characteristics of two-phase rotating detonation waves in a typical RDE, particularly characterizing the effects of the injection area and equivalence ratios on the propagation modes. Series of experimental tests were carried out in the specifically designed rotating detonation combustor propelled by liquid kerosene and oxygen-rich air, where the high-speed imaging technique was adopted for visually capturing the traveling trajectories of the detonation waves. The experimental results demonstrated that the rotating detonation wave always propagated in a hybrid mode of single-wave and dual-wave collision. In the single-wave mode, traces of local combustion in the combustor were clearly observed. While for the dual-wave collision mode, the luminosity denoting the reaction intensity was much weaker, and positions of the collision point shifted frequently. The spontaneous mode transitions in the combustor were presented through the synchronous detection of the high-speed camera and pressure sensors, which are the typical characteristics of the hybrid wave mode. The results also implied that a reduction in the injection area increased the pressure of the RDC and the velocity of the detonation waves, and was likely to suppress the deflagration mode. Such a decrease in the injection area ratio finally increased the occurrence time of the dual-wave collision mode during the operation of RDE. The findings could serve as a reference for the design of the propellant injection structure of liquid-fueled RDE.
查看更多>>摘要：Supercritical CO2 heat exchangers are used in many industries. In some applications, the channel may not be straight, but rather serpentine. The effects of mass flux (G = 100-200 kg/m2s) and heat flux (q(w) = 30-90 kW/m(2)) on the heat transfer of a vertical serpentine micro-tube (d = 1 mm) and the different effects of buoyancy and centrifugal force on the flow and heat transfer of a vertical serpentine micro-tube in the liquid-like region and the gas-like region are studied by numerical simulation. Under the same operating conditions, the heat transfer coefficient decreases with the increase of heat flux and increases with the increase of mass flux. The liquid-like region is greatly affected by buoyancy, resulting in the heat transfer coefficient of upward flow is significantly lower than that of downward flow, and the heat transfer coefficient fluctuates violently. The gas-like region is less affected by buoyancy, resulting in small difference between upward flow and downward flow heat transfer coefficients, and the heat transfer coefficient fluctuates less. At the turning point of the serpentine micro-tube, the centrifugal force mutation results in the weakened heat transfer of the local tube wall, and the weakened layer in the liquid-like region is larger than that in the gas-like region. These findings are important for understanding the effects of buoyancy and centrifugal force on flow and heat transfer in the liquid-like region and the gas-like region.
查看更多>>摘要：The thermal management of a thermoelectric cooler (TEC) has a significant impact on its cooling performance. The heat generated in the hot side of the TEC system reduces the performance and lifespan. Therefore, an efficient thermal management technique is vital for ensuring the TEC to operate at optimum temperature. In this study the performance of TEC is investigated by varying input voltage, air velocity at hot side, heat sink configuration and fill volume of PCM in the heat sink. A 54 W Bismuth Telluride based TEC was chosen for this study and OM35 was used as the phase change material (PCM). The three configurations of aluminium heat sink used in this study are square pocket, rectangle pocket and circular pocket. The input voltage to TEC is varied from 2.5 V to 12.5 V. The results show that, both the cold and hot side temperature of TEC increases when the input voltage is increased. The changes in air velocity at the hot side of the TEC from 1 m/s to 3 m/s reduced the temperature on both the cold and hot side of TEC. Among the three configurations of heat sink, square pocket heat sink with fully filled PCM showed better performance compared to other configurations. Moreover, the heat sink with a square pocket filled with PCM provided cold and hot side temperatures of 27 degrees C and 58.7 degrees C respectively when compared to 34 degrees C and 68.3 degrees C for the similar operating conditions without PCM. The results indicate that the performance of TEC can be significantly improved by the use of heat sinks filled with PCM.
查看更多>>摘要：Convective drying of textiles is a multi-physical problem coupled with fluid flow, heat, and mass transfer, and is directly affected by the form of air supply. In this study, a coupled heat and mass transfer model for convective drying of textiles was established by focusing on the effects of a porous relative impinging jet on the coupled heat and mass transfer of textiles under different influencing factors. The results show that temperature and wind velocity, as the two most important influencing parameters, change the variation of steam concentration and convective heat transfer coefficient, respectively, thus affecting the drying process. Reducing the relative humidity of the air in the oven can accelerate the drying process. The accuracy of the proposed model was validated against the experimental results, and the error was within 10%. The optimal operating conditions (v = 10 m/s, T = 180 degrees C, RH = 10%) were determined via numerical simulation. The air supply mode of the orifice impinging jet accelerated the drying rate and simultaneously improved the uniformity of the change in textile moisture. The model enhances the understanding of the impinging jet drying mechanism for textiles and provides theoretical guidance for fabric drying equipment such as heat-setting machines.
查看更多>>摘要：Hybrid-wetting surfaces are commonly used in enhancing dropwise condensation (DWC) because of provide high droplet mobility and small droplet size. The shape of the patterns plays a significant role in determining droplet departure size, frequency, and thus, heat transfer rates, which are not introduced yet. Therefore, different pattern shapes such as circle, ellipse, and diamond share the same area, gap distance, and arrangement have been developed to conduct condensation experiments on horizontal copper tubes at atmospheric pressure. Results show that condensation heat flux and heat transfer coefficient on the diamond-shaped pattern are 40% and 60% are higher than the complete DWC and outperform the other two shapes, i.e., ellipse-and circle-shaped patterns with a gap distance of 1 mm. Unfavorable bridged droplets were observed between two or more patterns on all hybrid surfaces. The bridged droplets would result in high thermal resistance and deteriorate condensation rate. We observed that bridged droplets have been effectively reduced on the diamond-shaped patterns, which should be a primary mechanism for outperforming the circular and elliptic patterns. In addition, we observed that the gap between the patterns also plays a significant role in determining droplet dynamic and heat transfer rate on all hybrid surfaces.
查看更多>>摘要：Multi-channel LED light sources possess complicated nonlinear photo-electro-thermal (PET) dynamics. Modeling such PET dynamics is challenging, but important to ensure the performance of these light sources. Traditional first-principle modeling methods that have been reported in the literature yield either singlechannel LED dynamic models, or multi-channel static models. On the other hand, system identification (SID) methods have also been applied, and lead to both single-channel nonlinear dynamic models and multi-channel linear time invariant (LTI) models. However, there are still few attempts given to build multiple-input-multipleoutput (MIMO) nonlinear dynamic models of multi-channel LED systems. In this work, we try to model the PET dynamics of a dual-spectral ultraviolet (UV) light source, with a structured nonlinear system identification technique, i.e., linear parameter varying (LPV) SID technique. The experimental prototype contains a 280 nm UVC channel and a 365 nm UVA channel, respectively driven by two separate constant current sources, and is hence a MIMO system. Its nonlinear electro-thermal model is first identified from experimental data by the LPVSID method, which is then integrated with the estimated photo-electro model to form a complete PET dynamic model. The experimental results have verified that the model can accurately predict the temperature variations at two representative points on the LED board. Moreover, the identified model has been applied in designing a feedforward-feedback controller to precisely track the reference radiation levels of the two UV channels, despite the strong nonlinearity caused by both the electro-thermal dynamics and the temperature-dependent lumen efficacy
查看更多>>摘要：Pulsating heat pipe (PHP), as an efficient heat transfer component, can be used for waste heat recovery and electronic cooling. The influences of typical ocean motions, such as inclination and swing, on thermal performances of PHPs are inevitable for practical offshore applications. In this study, surfactant-free functionalized multi-walled carbon nanotubes (f-MWCNTs) nanofluids with long-term stability were prepared. Thermal performances of PHP with f-MWCNTs nanofluids and mixed fluids under static and swing conditions were comparatively investigated. Results indicate that 0.25 mg/ml f-MWCNTs nanofluids (base fluid, 1:1 ethanol-DI water) shows the lowest thermal resistance among all working fluids. Thermal performances of flat-plate PHP with ethanol-DI water mixed fluids are better than that of ethanol-based f-MWCNTs nanofluids, and HFE-7100-DI water mixed fluids are more suitable for performance enhancements at the lower heat loads. As for effects of swing motion, the increased swing amplitudes and decreased swing periods will reduce thermal performances. In the worst case, swing motions can reduce thermal performances by 5.85%. Moreover, frequencies of temperature difference fluctuations increase with the decrease of swing periods. Although the change of swing amplitudes has a certain effect on frequencies and amplitudes of temperature difference fluctuations, but the influence seems negligible as well. These results can be used to guide practical offshore applications of PHPs.
Sawadogo, MohamedBenmahiddine, FerhatHamami, Ameur El AmineBelarbi, Rafik...
14页
查看更多>>摘要：In latent heat storage, energy is stored through the change of state of a material and then released when the material returns to its original phase. Latent heat thermal energy storage systems incorporate phase change materials (PCMs) as storage materials. The objective of this study is the fabrication and characterization of a biosourced PCM hemp concrete. Hemp shives are vacuum impregnated with CA with an incorporation rate of 53%. The resulting shape-stabilized hemp shives/CA composite is used to fabricate PCM hemp concrete. The morphology and thermophysical properties of the materials at all stages (from pure CA to hemp concrete PCMs) are characterized using differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and compared to the reference state (hemp concrete without PCMs). Results of the DSC measurement indicated melting and solidification temperatures of hemp shives/CA composite of 28.9 degrees C and 23 degrees C, respectively and enthalpy of melting and solidification calculated as 78.7 and 76.4 J.g(-1), respectively. The TGA/dTG results showed that the form stable composite exhibits good thermal stability under 170 degrees C, making it suitable for building application. The thermal performances of PCM hemp concrete are evaluated in a climatic chamber and compared to those of a reference hemp concrete. Results showed a good thermoregulation capacity of the PCM hemp concrete with a maximum time shift of 30 min and a temperature difference between reference and PCM hemp concrete of about 4.6 degrees C. Featured Application: Passive latent heat storage for building applications, thermal comfort.
NSTL
Elsevier