查看更多>>摘要：This paper presents a series of three-dimensional numerical simulations on the flow and heat transfer characteristics of the RP-3 aviation kerosene in a horizontal tube heated by different heat fluxes under altered gravity levels at the supercritical pressure. The RP-3 aviation kerosene with the initial temperature of 373 K flows in the horizontal tube heated by heat fluxes of 400 or 480 kW/m2 with the mass velocity of 982.4 kg/(m2 s). The results indicate that the evolution of the secondary flow pattern strongly depends on the gravity level. The critical gravity levels for the incipience of two kinds of the secondary flow pattern evolution along the main flow direction are 0.1g and 0.3g. The secondary flow enhanced by the gravity level promotes the local heat transfer on the inner wall near the bottom, while causes the heat transfer deterioration near the top of the horizontal tube. Moreover, the increase of the gravity level enhances the average heat transfer on the inner wall. The secondary flow pattern and tendencies of the turbulent kinetic energy are almost independent of the heat flux. However, higher heat flux accelerates the transition of the secondary flow patterns along the main flow direction and enlarges the local heat transfer difference between the top and the bottom. In addition, the heat transfer weakens with the increase of the heat flux.
查看更多>>摘要：In this study, critical heat flux (CHF) of Zirconium Silicide (ZrSi2) ATF coating for the zircaloy-4 (Zry-4) substrate has been assessed in flow boiling and enhanced by homogeneous porous surfaces fabricated by electrophoretic deposition (EPD). Three different porous structures were fabricated on physical vapor deposition (PVD) ZrSi2 coating using EPD. The particle size distribution, thickness, and porosity were adjusted by the changing process parameters of EPD to produce different porosity levels. Mean particle size and porosity of the EPD porous layer were increased by 35% and 21% respectively when the applied voltage was increased from 100 to 400 V. The CHF of the bare Zry-4, ZrSi-Zry, and ZrSi-Zrys modified by EPD (EPD ZrSi-Zrys) was evaluated in flow boiling at a mass flux of 300 kg/m2s. The optimized ZrSi2 coating modified by EPD enhanced the CHF of ZrSi-Zrys up to 146% when deposited at 400 V. Flow boiling enhancements of CHF of EPD ZrSi-Zrys were analyzed by evaluating liquid inflow rates of the surfaces.
查看更多>>摘要：Micro-fins (<0.5 mm tall) are an engineered roughness with the advantage of reducing thermal resistance and the disadvantage of increased pressure drop when applied inside a tube in heat-exchange applications. The competing effects highlight the need for careful optimization that identifies micro-fin surfaces with the potential to match heat exchanger design needs. Hence, the objectives of this study are chosen to enable efficient optimization in future studies. The main goals are: (1) study the effects of micro-fin design variables on heat transfer and friction factors; and (2) evaluate the potential of a data-mining model as a surrogate of computational fluid dynamic (CFD) models in 2 dimensional (D) and 3D micro-fin tubes. This study applied conductive and convective heat transfer and turbulent fluid flow simulation to evaluate the performance of different 2D and 3D micro-fin tubes. Different configurations were generated by varying micro-fin height (e), helix angle (α), number of starts (Nf), and discontinuity features. Coupled solid and periodic fluid domains were applied in ANSYS Fluent 19.1. Performance was mapped for 210 different simulations (including a smooth tube) using a realizable k-ε turbulence model at Reynolds number (Re) of 48,928. Two different least squares-support vector regression (LS-SVR) models were employed to estimate the Colburn j factor as a function of geometric variables and the Fanning friction factor (f) as a function of geometric variable and j factor. Results of the parametric study showed that the best 2D micro-fin tube can enhance efficiency index (η) up to 1.18. Results of the LS-SVR model showed that the percentage of average absolute error (AAE) between simulated and estimated j and f factors are 2.05% and 2.93% for 3D micro-fin tubes, respectively.
查看更多>>摘要：Accurately predicting the performance loss caused by thermal interaction between boreholes is essential for estimating the initial investment cost of ground source heat pump systems. In this study, based on the finite volume method and considering groundwater seepage, a multiple Borehole Heat Exchangers (BHEs) heat transfer model in layered soil is established and validated. By introducing the thermal interaction coefficient (δ) as the evaluation index of thermal interaction between boreholes, an empirical method is proposed for quickly obtaining the total Heat Transfer Rate (HTR) of a BHE field in stratified soil. For different types of multiple-layer soil, various δ values are compared and the effects of soil thermal conductivity, inlet temperature, inlet velocity, and seepage velocity on δ between boreholes are investigated. Results indicate that in soils with thermal conductivities of 1 W?m?1?K?1 to 4 W?m?1?K?1 and seepage velocities of 5 × 10-7 m·s?1, where the thickness of each layer does not vary significantly, the δ value for multiple BHEs is largest in the first layer of soil. Besides, the effects of inlet temperature and inlet velocity on δ could be ignored. Greater seepage velocities lead to lower δ values in the soil layer, as well as faster stabilisation times. This results and proposed empirical approach can be used to accurately predict the total HTR of a BHE field in stratified soil and provide directions for optimising the layout of a BHE field.
查看更多>>摘要：In a novel combustion mode, intelligent charge compression ignition (ICCI), the effects of fuel injection strategies were studied on a modified single-cylinder engine with the given engine load and speed. Commercial gasoline and diesel were used to represent low-reactivity fuel and high-reactivity fuel, respectively. The in-cylinder stratification, fuel–air mixture formation, heat release rate profile and combustion phasing were investigated in detail. Engine thermal efficiency, fuel consumption, gaseous and solid emissions were further studied. In the single-injection strategy and the multiple-injection strategy, different diesel injection timings were configured to find the influence of their changing trends on engine performance, combustion and emissions. In the single injection strategy, the diesel injection of 60 °CA BTDC can lead to the highest indicated thermal efficiency. In contrast, only a specific injection strategy influences the heat release in small-scale split injection. Using multiple injections of high-reactivity fuel and adjusting the combustion phasing decrease the excessive in-cylinder pressure and maximum pressure rise rate in the single injection. Meanwhile, alternate fuel injection to build a stratified environment in the cylinder can maintain the indicated thermal efficiency at a high level. The indicated thermal efficiency is close to 50% when the diesel first injection timing is 60 °CA BTDC, the second injection timing is 38 °CA BTDC or 100 °CA BTDC. The single diesel injection strategy of 60 °CA BTDC reaches the best emissions, in which the NOx emissions are lower than 0.5 g/kWh. The multiple split injection strategies can further reduce carbon monoxide and total hydrocarbon emissions.
查看更多>>摘要：Strengthening the heat transfer technology of the furnace tube in the radiant section of the cracking furnace is very important in the field of energy-saving and environmental protection. This research proposes a new type of open-tooth spiral to enhance heat transfer and increase ethylene productivity. The mathematical computation has been conducted to evaluate the thermal-hydraulic performance with an open-tooth spiral. The classical Ordinary tube was also investigated for comparison. At the same time, the vortex identification method based on the Q criterion reveals the enhanced heat transfer mechanism of the open-toothed spiral tube. Then use the field synergy principle to qualitatively judge the heat transfer performance, combined with the comprehensive heat transfer evaluation factor (PEC) to quantitatively evaluate the comprehensive heat transfer performance of the open-tooth spiral tube. The results show that although the Nusselt number of the open-tooth spiral is 8%-10% lower than that of the ordinary screw, the friction resistance coefficient of the spiral tube with an opening angle of 15° can be reduced by 33%～43%. Its PEC value is between 1.15 and 1.25, with excellent comprehensive heat transfer performance. It can effectively improve key products, such as ethylene, propylene, etc.
查看更多>>摘要：The combination of flow-induced vibration and pulsating flow has a greater potential to enhance heat transfer. This study aims to investigate the effect of flow-induced vibration using pulsating flow generated by a vortex generator on the heat transfer enhancement of planar elastic tube bundle (PETB) in three kinds of flow fields. Water was utilized as working fluid to study the vibration characteristic and heat transfer of PETB comprehensively in a laminar flow using the experimental platform of flow-induced vibration and two-way fluid structure interaction (FSI) calculation. Vibration results found that the main vibration frequency of PETB in three flow fields was mainly concentrated in 24–25 Hz and the vibration amplitude was most obviously affected by the pulsating flow, leading to an amplitude reduction of 18.5% and 51.75% in coupled flow, 62.2% and 59% in steady flow. The difference in constrain support caused the local vibration displacement on No.3 tube showing a trend of first decreasing and then increasing and the local vibration displacement on No. 4 tube presenting a gradually increasing trend. Findings indicated that there was the same variation trend between the vibrational Reynolds number and heat transfer enhancement. The pulsating flow enhanced the vibration intensity of PETB, resulting in a increase in the turbulence kinetic energy and variation in the vorticity patterns. As a result, the passive heat transfer enhancement using the flow-induced by pulsating flow was obtained with a heat transfer coefficient increase of 28%, 25% and 19.5% in pulsating flow, coupled flow and steady flow, respectively. The maximum difference in local heat transfer was obtained at the position near the big connecter, where the local heat transfer coefficients in pulsating flow and coupled flow were 11.4% and 2.8% higher than that in steady flow, respectively.
查看更多>>摘要：Optimizations of the heat exchanger of the heat pump drying system were usually carried out individually, linkage effect of the whole system in practical operating was usually not considered. In addition, for a heat pump sludge drying system, optimizations mainly concentrated on the operation parameters, few theoretical analyses focused on the effect of structural parameters to the system performance. In the present study, based on the laws of energy conservation and mass conservation, a steady-state model for the heat pump sludge drying system, which interconnects the refrigerant circulation and the moist air circulation, was established. Three variables which are coefficient of performance (COP), dehumidification rate (DR), and specific moisture extraction rate (SMER) are simultaneously introduced to evaluate the system's operating and drying performances. Effects of the number of horizontal tube rows, the number of longitudinal tube rows, and the fin spacing to the above three variables are analyzed. The results show that the COP decreases with increasing the number of horizontal tube rows while increases with increasing the number of longitudinal tube rows and fin spacing. The DR increases with increasing the number of horizontal tube rows and fin spacing and decreases with increasing the number of longitudinal tube rows. The SMER has a similar variation trend to that of the DR due to their directly proportional relationship. Finally, based on the sensitivity analysis, the heat pump sludge drying system achieves better operating and drying performances when the fin spacing is 2.85 mm, the numbers of horizontal tube rows of the evaporator and condenser are 18 and 14, and the numbers of longitudinal tube rows are respectively 12 and 9.
查看更多>>摘要：Solar energy is the cleanest and most abundant renewable energy source that can be utilized to displace fossil fuels and provide an effective solution for future energy shortage and climate change. Aiming at enhancing the capture efficiency of solar photothermal energy, we developed a novel type of phase-change composites based on polyimide (PI)/phosphorene (PR) hybrid aerogel and polyethylene glycol (PEG). The composites were prepared by fabricating a series of PI/PR hybrid aerogels with different loadings of PR nanosheets using freeze-drying and thermal imidization techniques, followed by vacuum impregnation of PEG as a phase change material into the aerogel framework. The obtained hybrid aerogels exhibited a lightweight nature with a density of 21.9 mg·cm?3 when 16 wt% PR nanosheets were incorporated, resulting in an extremely high PEG loading of 4067% in the aerogel system. The combination of PI and PR nanosheets leads to a significant enhancement in solar light absorption and photothermal energy conversion for the hybrid aerogel/PEG composites. The loading amount of PEG was also improved remarkably due to an increase in the volume capacity of the hybrid aerogels resulting from the introduction of PR nanosheets. The resultant hybrid aerogel/PEG composites not only exhibit a high latent-heat capacity of over 170 J·g?1 and a high photothermal conversion efficiency of 82.5%, but also show good thermal impact resistance to keep their original shape and form well after heating at 80 °C for 20 min and maintain a high thermal cycle stability after thermally cycled for 500 times. These superior performances make the hybrid aerogel/PEG composites capable of dealing with a wide range of applications in solar photothermal energy capture and storage. This study provides a promising strategy for the design and development of high-performance and lightweight composite PCMs to meet the requirement of applications for efficient utilization of solar energy.
查看更多>>摘要：The number of vapor compression cycles in microgravity is still small, especially relative to the increasing rate of developments supporting space exploration. Ground-based inclination testing helps to understand the effect of gravity on two-phase cycles and can increase confidence into their operability in space. An investigation was conducted by operating a vapor compression cycle at different orientations, always for a long enough time to achieve steady-state operation. Experiments were conducted with R134a and R1234ze(E) across three different mass fluxes. In general, both refrigerants reacted similarly to inclination changes. Significant mass flow rate oscillations were observed in the suction line due to inclination changes in a transient study. These had larger amplitudes at lower flow rates. The steady-state conditions plotted as a function of the inclination angle for one set of control parameters resulted in sinusoidal behaviors with varing “amplitudes”. A semi-mechanistic heat exchanger model was leveraged to track the hydrostatic pressure drop of all coil-segments as the test rig was rotated. Based on comparing experimental and model results, it is hypothesized that changes in orientation led to an accumulation of refrigerant in the evaporator causing a higher pressure drop not captured by the model.
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Elsevier