查看更多>>摘要：Bubble columns are recently used for the humidification of air in water treatment systems and fuel cells. They are well applicable due to their excellent heat and mass transfer and their low technical complexity. To design and operate such devices with high efficiency, the humidification process and the impact of the operating parameters need to be understood to a sufficient degree. To extend this knowledge, we use a refined and novel method to determine the volumetric air-liquid heat and mass transfer coefficients and the humidifier efficiency for various parametric settings. The volumetric transfer coefficients increase with both of the superficial air velocity and the liquid temperature. It is further shown that the decrease of vapor pressure with an increase of the salinity results in a corresponding decrease in the outlet humidity ratio. In contrast to previous studies, liquid heights smaller than 0.1 m are investigated and significant changes in the humidifier efficiency are seen in this range. We present the expected humidifier efficiency with respect to the superficial air velocity and the liquid height in an efficiency chart, such that optimal operating conditions can be determined. Based on this efficiency chart, recommendations for industrial applications as well as future scientific challenges are drawn.
查看更多>>摘要：In this paper, a numerical study has been conducted on the effect of the offset of jet holes on the flow structure and heat transfer of swirling flow in a concave target chamber with various dimple structures and effusion holes at the turbine blade leading edge. The ratio of jet holes off the centerline distance to jet hole diameter (e/d) varies from 0 to 2.0, and the ratio of jet-to-target plate spacing to jet diameter (H/d) is 4. The effusion hole diameters of 0.5d are arranged in a staggered pattern relative to the jet holes. Four types of dimple structure with a large depth ratio (delta/D) of 0.25, including spherical dimples (SDs) and oval-trench dimples (OTDs) in an in-line and staggered arrangement, are considered. The heat transfer characteristic and pressure loss of the different leading-edge are evaluated and compared at a Reynolds number of 30,000 based on the jet hole diameter. The results show that the offset of the jet holes provides 15%-higher overall heat transfer performance and more uniform heat transfer of the target surface, while having little impact on the friction factor within the e/d range of 0-2.0. The introduction of the dimple structures on the target surface decreases the overall averaged Nusselt number but enhances the total heat transfer rate from the target chamber due to the clear increase of heat transfer areas, for which a maximum of a 24% improvement of the thermal-hydraulic performance is obtained for e/d = 2.0 with OTDs in a staggered arrangement. Under the same e/d, the OTD structure, especially with the staggered arrangement, is superior to the SD structure.
查看更多>>摘要：The use of an expander is a promising technique to improve the performance of vapor compression refrigeration systems, but its use in a system with HFC refrigerants has not been extensively explored. This paper studies the performance of a four-intersecting-vane expander prototype and the effect of different expander location ar-rangements to the overall system performance. A customized R134a vapor compression test rig was constructed for the experiments. The rig's coefficient of performance was determined when using either a throttling valve or an expander for the same operating conditions. The relationships between expander placement arrangements, expander rotational speed, evaporating load and condensing load on the performance of the expander and the thermodynamic behaviors of the system were also investigated. Transient behaviours of the system are discussed too. The experimental results show that the expander could improve the performance of the system by 6.4% in comparison with a system with a throttling valve. The maximum expansion power efficiency was 34.9% at 500 rpm. Of the four expander arrangements, that in which the expander is located right after the condenser exhibited the highest performance. The results show the potential benefits of using a four-intersecting-vane rotary expander for energy recovery of a vapor compression refrigeration system, and the importance of the correct placement of the expander in the system.
查看更多>>摘要：High-power Stirling-type pulse tube cryocoolers (SPTC) are expected to be an ideal candidate for cooling high temperature superconductivity (HTS) for its advantages of compact structure, low maintenance and long service life. However, the regenerator temperature non-uniformity is still a key problem needs to be solved since it significantly deteriorates the cooling performance. As a main heat exchanger, the insufficient and asymmetric heat exchange in the aftercooler may be one of the factors affecting the temperature inhomogeneity inside the regenerator. However, it has been seldom investigated. In this study, the relationship between the performance of the aftercooler and the temperature non-uniformity of the regenerator was investigated based on a high-power SPTC working at ~ 80 K. With the help of Sage simulation, it is found that the inlet temperature of the regenerator has a large effect on its inside temperature distribution. Even an inapparent temperature inhomogeneity at the inlet of the regenerator will induce a large temperature difference in the middle of the regenerator, due to an obvious DC flow generated in the regenerator. By changing the water inlet and outlet directions of the aftercooler, the temperature non-uniformity of the regenerator will consequently change, which means the uneven heat transfer of aftercooler can be one of the main inducements to the temperature nonuniformity inside the regenerator. Further, methods of using copper wire mesh filling at the hot end of the regenerator, and optimizing heat exchange tubes inside the aftercooler were studied to reduce the temperature non-uniformity.
查看更多>>摘要：Ejector is meaningful and valuable to improve the transcritical CO2 refrigeration system. To study in-depth on the exergy destruction is significant for the performance enhancement of CO2 ejector. This paper presents a comprehensive investigation on the exergetic performance of CO2 two-phase ejector using conventional and advanced exergy analysis method to explore the interactions among ejector components and their improvement potential. The effects of primary flow pressure, suction flow temperature and area ratio (motive nozzle throat area to mixing chamber area) on the ejector exergy destruction are discussed. The results indicate the largest exergy destruction in the ejector is caused by the mixing chamber, followed by the motive nozzle, diffuser and suction chamber. Based on the avoidable exergy destruction rate and exergy destruction proportion, the order of optimization potential for ejector components is motive nozzle, diffuser, mixing chamber and suction chamber. However, the sum of avoidable exergy destruction of mixing chamber is the largest, which has the highest priority for improvement of overall ejector performance. The exogenous exergy destructions of the mixing chamber and diffuser are largely caused by the motive nozzle, and the exergy destructions would be increased with the improvement of motive nozzle performance. There exists an optimal primary flow pressure 9.5 MPa to minimize the exergy destruction of the each component of the ejector. To reduce the suction flow temperature and the ejector area ratio can decrease the avoidable exergy destruction of the components. Moreover, the variation of operating parameter has no effect on the optimization sequence of the overall ejector and its components. This work would provide a helpful way to guide the optimization on the ejector efficiency.
查看更多>>摘要：Rotating heat pipe (RHP) has wide application prospects in the field of heat dissipation of rotating machinery. But researches on the dynamic characteristics of RHP are very few at present, which limits its application on real engineering. In this paper, the dynamic characteristics of RHP are investigated experimentally, including: startup characteristic, temperature fluctuation, and failure phenomenon. Results show that the rotating motion can obviously improve the performance of RHP during its startup process. Temperature fluctuation is obvious at 0 RPM, the fluctuation frequency increases and the fluctuation amplitude decreases with the increase of heating power. Rotation of RHP will obviously inhibit the temperature fluctuation. RHP will encounter failure phe-nomenon. Failure phenomenon is prone to occur under low filling ratio, low cooling temperature, and high heating power. Low rotating speed is easy to induce failure phenomenon and high rotating speed can restrain failure phenomenon. Increasing filling ratio can effectively inhibit failure. There are two inertial effects on failure phenomenon: the irreversibility of failure phenomenon caused by change of rotating speed and the hysteresis of the influence of cooling temperature variation on failure phenomenon. The dynamic performance of RHP is influenced by the inertial effects, the intrinsic mechanism is non-linear.
查看更多>>摘要：The thermo-hydraulic performance of solar thermal collector can be significantly improved by the utilization of roughness geometries in the fluid flow channel. However, the experimental investigation of turbulence pro motors is a tedious and costly process. Thereby, the hybrid Taguchi-TOPSIS technique has been utilized in this work for optimization of potential parameters of discrete V down baffle in the fluid flow duct of the collector. The potential parameters of the discrete V down baffle have been selected, the relative angle of attack (alpha/90), relative discrete width (g(w)/H-b) and relative discrete position (D-d/L-v). The experimental alternatives are decided by Taguchi's L-16 orthogonal array. The optimization of potential parameters and ranking of alternatives based on the closeness index has been conducted. The optimum values of potential parameters have been found to be alpha/90 = 0.666, g(w)/H-b = 1 and D-d/L-v = 0.67. The quantification of contribution percentage for each potential parameter on the performance of solar air heater has been conducted and found to be 33.29%, 37.25% and 29.46% for alpha/90, g(w)/H-b and D-d/L-v respectively. The CFD analysis of solar air channel (SAC) with discrete V down baffle has been carried out for understanding the fluid flow and heat transfer mechanism, the performance of the optimum set of potential parameters and the difference in the contribution of potential parameters. This study will be beneficial for researchers and designers for optimization and understanding the effect of potential parameters on the performance of solar thermal collector utilizing air as working fluid.
查看更多>>摘要：Latent heat thermal energy storage (LHTES) has two primary LHTES packing methods, encapsulation and bulk PCM storage. Since there is still a lack of experimental comparison between these two methods, two types of LHTES units based on separated methods were built for direct comparison. Moreover, the impact of heat transfer fluid (HTF) flowrate on LHTES performance was evaluated. Unit one is a 0.38 m(3) tank containing slab-shaped macro-encapsulated phase change material (PCM); unit two is a 0.54 m(3) tank containing submerged spiral coil heat exchanger (SCHE) in PCM. PCM with a melting temperature of 58-60 degrees C was charged/discharged between 46 and 72 degrees C. Parametric studies on constant and time-varying HTF flowrates were conducted to test the impact on the thermal storage performance. The time-varying flowrate control enables the system to supply the needed power at different discharging stages. Moreover, partial charging/discharging demonstrates higher mean thermal power than full charge/discharge rendering this control strategy adequate under specific operating conditions. Finally, the comparison between the slab-encapsulated PCM storage unit and the SCHE based unit shows that the former requires a shorter completion time while the latter has a higher energy storage density.
查看更多>>摘要：Condensation in the presence of noncondensable gas is of interest to the passive containment cooling system in the nuclear power plant. Most researches focus on the heat transfer characteristics of condensation in an individual vertical tube or a parallel tube bundle. However, the influence of the condenser structure on condensation heat transfer is rarely considered in previous works. Thus, a numerical simulation was conducted for condensation in the presence of NCG in a condenser composed of two headers and 158 condenser tubes. A steady threedimensional model was developed to predict the NCG distribution and the heat transfer performance of the condenser when the PCCS loop was stably running. The fluid inside the condenser was regarded as a single-phase mixture composed of steam and air, and the liquid film was evaluated by the Eulerian Wall Film model. The results indicated that the air tends to accumulate in the middle of the condenser tubes rather than the headers. The fluid flow in most of the tubes was blocked, so the heat transfer coefficients of pure steam and the steam-air mixture were lower than expected. An orifice plate was further designed to optimize the pressure and flow field in the PCCS condenser. The effect of the optimization was quantitatively evaluated. The results showed that the heat transfer performance of the condenser was improved by up to 87.8% with the orifice plate.
查看更多>>摘要：Heat pump drying is an effective method to reduce the energy consumption and environmental pollution especially for industry and agriculture. Drying process analysis is the basis of optimizing operating condition, structure design and improving product quality. In this paper, a mathematical model is developed to simulate the flow field and drying process of forage seed in a double-layer drying chamber, and the simulated results are verified by the experiment. The moisture content of seeds in the tray is approximate normal distribution in the total drying process except the lowest tray (tray-1). The lowest and highest moisture contents of seeds are 5.95% (tray-1) and 8.77% (tray-8) respectively while the maximum and minimum non-uniformities are 13.7% (tray-9) and 3.7% (tray-5) after the drying time of 3 h. All the moisture content of seeds and the surrounding air increase rapidly in the first half hour and decrease rapidly in the following half hour, and then they change slowly until the end of drying time. The moisture content of seeds is still uneven after 3 h and the range of 5.95%-8.77% is shown for different trays. The final moisture content of seeds decreases 3.00%, 4.23%, 1.67% respectively when the air temperature increases from 45 & DEG;C to 60 & DEG;C, air velocity increases from 1.5 m.s(-1) to 2.5 m.s(-1), and the thickness of seeds decreases from 6 cm to 4 cm. Although double-tray structure improves the air flow though the central seeds, it has little effect on the average drying degree.
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Elsevier