查看更多>>摘要：Radiative heat transfer is the dominant mode of heat transfer in glass annealing furnaces. Especially in electrically heated furnaces due to the nonparticipating nature of the furnace atmosphere the radiative heat transfer predominantly occurs between surfaces. In this regard, in container glass manufacturing the layout of the container glasses inside the furnace can considerably affect the radiative heating effectiveness of the furnace. In this numerical study, the effect of the spacing between the bottle rows on the radiant heating effectiveness of the furnace was numerically investigated. An industrial scale continuous annealing furnace model was used and the combined conduction – radiation heat transfer modes in the furnace were solved with the in-house developed transient solver. The convective heat transfer inside the furnace was not taken into account due to its relatively lower contribution in heat transfer. The computational cost of the numerical model is reduced with the use of a computational domain consisting of 7 bottle rows instead of full 29 bottle rows. Reverse Monte Carlo Ray Tracing method is used to solve for the surface-to-surface radiative heat transfer inside the radiant heating zone and integrated into GPU to reduce its computational cost. Further computational speed-up was achieved with the use of a 2nd order accurate radiation boundary condition implementation. It allows a time step increase of 2.5x in comparison with the first order accurate implementation for the same level of accuracy. Finally, a parametric study on the effect of the spacing between the bottle rows on the radiative heating of the bottles have been conducted. In this regard, the spacing between the bottle rows and the conveyor speed have been altered in a way that the throughput of the furnace, the number bottles processed per unit time, is kept constant. The study shows that there is an optimum spacing value between the bottle rows. For the particular bottle geometry considered in this study, the optimum row spacing value is found to be 35% of the bottle diameter. Running the furnace at this optimum value, the length of the heating zone could be reduced by 8.5% compared to the case when the furnace was operated with the bottles almost in touch with each other.
查看更多>>摘要：In a recent article published in this journal, a series of design optimization was executed for a ground source heat pump system. The optimization was conducted based on a COPall index, which considers both the hydraulic loss on the buried pipe network, as well as the soil thermal balance over a one-year period. In that article, it was concluded that a borehole spacing of 4m is the optimal value. In this short communication, the comprehensive COPall index is re-evaluated with the same system setup with both the TRNSYS and OpenGeoSys-TESPy software, but over a 20-year period. The results show that the borehole heat exchanger array with a spacing of 4m will suffer severe heat accumulation over a 20-year operation. This causes the soil temperature to rise by 5.68°C, along with a decrease of COPall from 4.59 to 4.18. Due to the smaller increase of ground-loop temperature and lower electricity consumption from heat pumps, a larger spacing of 6m will bring better COPall value over the long term, and thus should be recommended. The extended numerical study in this work suggests that when evaluating the performance of a ground source heat pump system, different time duration will lead to different results. Therefore, the variation of long-term ground-loop temperature needs to be quantitatively evaluated in advance, and the system optimization is recommended to be conducted over the entire life cycle of the system.
查看更多>>摘要：We present the experimental results for a rotary magnetocaloric prototype that uses the concept of active magnetic regeneration, presenting an alternative to conventional vapor compression cooling systems. Thirteen packed-bed regenerators subjected to a rotating two-pole permanent magnet with a maximum magnetic field of 1.44 T are implemented. It is the first performance assessment of the prototype with gadolinium spheres as the magnetocaloric refrigerant and water mixed with commercial ethylene glycol as the heat transfer fluid. The importance of various operating parameters, such as fluid flow rate, cycle frequency, cold and hot reservoir temperatures, and blow fraction on the system performance is reported. The cycle frequency and utilization factor ranged from 0.5 to 1.7 Hz and 0.25 to 0.50, respectively. Operating near room temperature and employing 3.83 kg of gadolinium, the device produced cooling powers exceeding 800 W at a coefficient of performance of 4 or higher over a temperature span of above 10 K at 1.4 Hz. It was also shown that variations in the flow resistance between the beds could significantly limit the system performance, and a method to correct those is presented. The performance metrics presented here compare well with those of currently existing magnetocaloric devices. Such a prototype could achieve efficiencies as high as conventional vapor compression systems without the use of refrigerants that have high global warming potential.
查看更多>>摘要：The power system of heat pipe reactor is divided into dynamic conversion and static conversion. Segmented thermoelectric generator (STEG) is a typical static conversion, but there is not enough literature to study its application in heat pipe reactor. Therefore, we have to design the STEG used in the heat pipe reactor. Firstly, we successfully simulated the STEG in COMSOL 5.5 and optimized the geometry and performance. The numerical simulation under steady-state conditions is carried out to determine the optimal STEG geometry. Then, the optimal STEG is connected with a heat pipe to form a single-channel model for simulation to explore the performance. The maximum thermoelectric performance can reach 15.75% for a single STEG, and the maximum stress is about 270 MPa. In the single-channel model, thermoelectric conversion efficiency reduces from 15.75% to 15.63%. This work provides a preliminary basis for numerical simulation in the combination between the STEG and heat pipe reactor.
查看更多>>摘要：Liquid desiccant systems have attracted extensive attention due to huge energy-saving and environmental potential. Mass transfer coefficients are key parameters for system design and performance evaluation. Contrast experiments between liquid desiccant dehumidification and regeneration were conducted here to investigate the magnitude of mass transfer coefficients by using LiBr aqueous solutions. Mass transfer coefficients of dehumidification were found to be much higher than regeneration even under the condition that the mass transfer driving force and inlet parameters except solution temperatures were kept the same. To obtain microscopic explanation, free energy curves of water transport were simulated by molecular dynamics based on umbrella sampling. Results show that large energy obstacle should be overcome for water moving into air, which is unnecessary for water transported into aqueous solution. Lower mass transfer coefficients of regeneration should be attributed to the mass transfer direction rather than higher solution temperatures of regeneration than dehumidification. Dimensionless correlations of mass transfer coefficients using water diffusivity in LiBr aqueous solution and correlations using surface tensions show better prediction performance than dimensionless correlations using water diffusivity in air. This study discloses changing mechanism of mass transfer coefficients and helps develop correlations with high accuracy.
查看更多>>摘要：Prediction of snow shedding from overhead structures is crucial to minimize threat to public safety due to snow falling from these structures. Liquid water content (LWC) of snow impacts snow adhesiveness to various surfaces; therefore, its measurement and prediction are crucial in estimating snow shedding from structures. Here, a theoretical heat model was developed to calculate and predict the LWC of snow as a function of temperature, radiation intensity, and wind velocity. To verify the predictions, snow LWC was measured using a surface-mountable sensor in a real-time and non-destructive way, indicating a good agreement between the theoretical results and experimental values. Despite three to four orders of magnitude smaller wind force when compared to snow adhesion force, it was found that wind has a significant effect on snow melting. At ≈3°C average air temperature, a wind with an average velocity of ≈ 2 m/s increased the snow melting rate by at least 90%, when compared to the free convection experiments. In addition, an inclined setup was used to measure snow adhesion on HDPE plates and study effects of snow weight and its LWC on snow shedding from these surfaces. It was observed that the shear adhesion of snow on the HDPE plates decreases from ≈ 59 (Pa) to ≈ 34 (Pa) when LWC changes from ≈ 3% to ≈ 23%. Three snow shedding mechanisms - detachment, melting followed by sliding, and complete melting - were identified. Detachment occurred for snow with LWC of < ≈ 5% when snow shearing weight force dominated snow shear adhesion. Snow melting might reduce its adhesion to a level that its shearing weight force can dominate it, leading to snow sliding. During complete melting experiments shearing weight force never exceeded shear adhesion forces. Finally, snow shedding mechanisms were predicted theoretically and verified experimentally, and a flowchart was presented to predict snow shedding from the HDPE plates.
查看更多>>摘要：Sewage water source heat pump (SWSHP) systems with energy saving and environmental protection advantages have been widely studied, and sewage heat exchangers are an important part of SWSHP systems. Problems such as scale formation, difficulty in cleaning, and rapid decrease of their heat transfer coefficient with time have long been associated with SWSHP systems. With these in mind, in this study, we build a SWSHP system for the recovery of heat from waste using a waste water bath experimental platform, and applied a new, rare earth element (REE) non-metallic immersion sewage heat exchanger, The heat exchanger has heat transfer ability in actual working condition, as well as the operational features of a SWSHP system. From test data, the attenuation of heat transfer and the heat transfer coefficient of the sewage heat exchanger with time were analyzed. An attenuation formula of the heat transfer coefficient was determined using a logistic function. The results show that during 90 days of continuous operation, the surface heat transfer capacity decreased by approximately 7.8%, and the rate of decline over time was much lower than that of traditional heat exchangers. The results show that the system can produce hot water at 40.4–60.6 ℃, and the highest system coefficient of performance (SCOP) of the system is 5.65. Our study shows that varying the depth of immersion of the heat exchanger in the water pool affects the heat transfer ability of the heat exchanger. Results show that when the heat exchanger is closer to the pool surface, water-flushing action is strong. With a long runtime, the heat exchanger's heat transfer ability has a slower rate of decline over time. Based on the test results, the energy consumption of the system was analyzed, and the energy saved by the continuous operation of the system for one year was determined to be equivalent to that of burning 4.29 × 104 kg of standard coal.
查看更多>>摘要：Adsorption desalination is an emerging desalination technology with low energy consumption, which is based on the principle of desalination by using the adsorption and desorption characteristics of porous adsorbents for water vapor. Compared with the traditional distillation and membrane methods, adsorption desalination can use renewable energy sources such as solar energy, geothermal energy, or industrial low-grade waste heat as the driving heat source, and thus has the excellent characteristics of being green and pollution-free. However, as a newly developed desalination technology, there is relatively little theoretical and experimental research on adsorption desalination, and it is still at the stage of small-scale experiments. This paper firstly introduces the adsorption theory and circulation process to clarify the key elements of this desalination method. Further, based on the available results, the related researches are analyzed and summarized according to the critical elements of the adsorption desalination system, i.e., preparation of adsorbents, optimization of system configuration, cyclic operating conditions, and combination systems. Finally, the future directions to be focused on for future research prediction are discussed, which will be useful for related studies in this field.
查看更多>>摘要：Motivated by environmental issues and the needs of communities that live far from the electricity grid, this work presents an innovative methodology for modeling a small-scale concentrated solar power plant based on the organic Rankine cycle. An integrated simulation tool, which consists of accurate computational routines of the energy storage tank, parabolic trough collector, plate heat exchanger, finned air-cooled condenser, and inward-flow radial turbine, is developed. The components are modeled according to distributed models and the mean line method for the turbine in homemade codes to account for local property changes. The transient power plant performance is assessed by employing solar irradiation data of three locations that present mean direct normal irradiation rates of 6306, 4712, and 3220 Wh/m2.day. The following working fluids were considered: a low GWP and ODP refrigerant (R1233zd(E)), a natural one (R600a), and a high GWP fluid that is currently employed for such applications (R245fa). The smallest configuration that led to annual uninterrupted power production is a 565-m2 collector and a 90-m3 storage tank. The R1233zd(E) refrigerant presented the best overall performance by providing about 2% and 30% higher turbine power output, 3% and 29% higher turbine efficiency, and 4% and 36% higher ORC first-law efficiency than R245fa and R600a, respectively. The designed solar power plant generated 48.396 MWh/year of electricity and 80.621 MWh/year of cogeneration energy with R1233zd(E), which is enough to provide electricity and sanitary hot water to about 33 and 64 houses, respectively.
查看更多>>摘要：Capillary-driven thin-film evaporation in the evaporator was modulated using feedback control of the mechanical pump flow rate and capillary pumping head in a mechanical-capillary-driven two-phase loop. The shift from cold-start (forced-convection dominant condition) to post cold-start (boiling dominant condition) was signified by a sudden drop in the evaporator thermal resistance at relatively low heat inputs. In the post cold-start, positive capillary pressure head resulted in another important transition of boiling conditions in the evaporator transitioning from mechanically-driven flooded boiling (flooded mode) to the capillary-driven thin-film evaporation (capillary mode) showing a small but continuous decrease in the thermal resistance. The feedback control significantly expanded the range of the capillary mode by 285% and reduced the mechanical pumping power consumption by 48% compared to a constant flow case. In this study using a monolayer wick in the evaporator, the highest capillary limit was measured to be 227.2 W/cm2 with a thermal resistance of 0.190 K-cm2/W for the highest pump flow rate of 7.5 g/s and the highest flow restriction in the liquid return line from the evaporator.
NSTL
Elsevier