查看更多>>摘要：It is well known that non-adsorbable gas, e.g. residual air, can dramatically slow down the adsorption stage of adsorption heat transformation cycles. So far, this effect has been studied for the cycles initiated by temperature change (temperature-initiated cycles). This work addresses the dynamic effect of residual air for another way of adsorption initiation, namely, by changing vapour pressure over adsorbent (pressure-initiated cycle). Comparison of the pressure- and temperature-initiated cycles is also made. The effects are studied for the working pairs "AQSOA FAM-Z02 - water" and "LiCl/(silica gel) - methanol" promising for adsorption heat transformation. The residual partial air pressure Delta Pair was varied from 0 to 5 mbar. The main finding of this study is that the pressure-initiated adsorption is less sensitive to the presence of residual air than the temperature-initiated adsorption. This is especially true at a low partial air pressure Delta Pair. For instance, at Delta Pair <= 0.5 mbar, residual air has little or no effect on the pressure-initiated adsorption dynamics compared to the temperature-initiated one. A qualitative explanation of this finding is proposed. Thereby, closed adsorption heat transformation cycles based on the pressure-initiated process are more robust and resistant against the presence of residual air that could be a significant practical advantage.
查看更多>>摘要：This study proposed a novel micro-scale falling film absorber with its experimental data on a complete absorption refrigeration system. Micro-scale liquid-distributor has been carefully designed and manufactured by special techniques. An experimental system for absorption refrigeration has been established to test the novel absorber. This unique absorber applies ammonia/lithium nitrate as working pair which is considered to own application potential for small-scale absorption refrigeration units. Experimental analysis has been done under various of working conditions for making contrasts. Relationships between significant working parameters and absorption parameters have been discussed. Under series of working conditions, absorption rates ranging from 1.2 to 2.6 x 10(-3) kg/(m(2).s) have been obtained. Approach to equilibrium factor which characterize the perfection of absorption process ranges from 0.5 to 0.98 while absorption ratio ranges from 0.02 to 0.12. With the increase of Reynolds number from 3 to 52, the mass transfer coefficient rises from 12 to 98 x 10(-6)m/s and Sherwood number ranges from about 30 to 60 respectively. According to the analysis, absorption pressure potential and cooling water temperature are two major factors which enhance the absorption intensity. Absorption pressure potential and inlet solution subcooling degree owns nearly linear relationship with parameters such as absorption rate, absorption ratio and approach to equilibrium factor. However, normalized absorption rate and approach to equilibrium factor gain negative correlations with absorption pressure potential and Reynolds number respectively. Effects of inlet solution flowing features on absorption performance are distinguished under different working condition series. In general, solution flowing intensity inside absorber is a positive factor for absorption process. All the experimental results reveal that, this micro-scale falling film absorber has its priority on absorption perfection and absorption rate for per unit of mass transfer area. Compact structure makes it more valuable for further research and application on small-scale absorption refrigeration unit with the working pair of ammonia/lithium nitrate.
查看更多>>摘要：ABSTR A C T Miller cycle has been gradually applied on marine low-speed engines to reduce NOx emission, due to its convenient implement with lower cost. However, current studies about relevant effects, influence mechanism and turbocharger re-match all have neglected the coupling relationship between engine and turbocharger, which will have non-negligible impact on final results. Therefore, based on this relationship, the influence of Miller cycle is studied through a numerical simulation. Then, a thermodynamic analysis is conducted to summarize the relevant impact mechanism and provide guidance for turbocharger re-match. Numerical results show that the in-cylinder charger temperature is actually reduced, but the combustion duration is prolonged. Hence, the exhaust gas temperature is increased, causing compressor operating points moving to the direction with higher pressure ratio and mass flow rate. Furthermore, from the thermodynamic analysis, the fundamental reason of this phe-nomenon is the uniflow scavenging adopted by marine low-speed engine: for this scheme, the more the Miller cycle is applied, the more the mass will flow across the cylinder. For compensating the losses caused by Miller cycle, achieving the original cylinder compression pressure is an effective method, hence a higher boost pressure is needed and the turbocharger should be re-matched. Finally, based on the thermodynamic analysis, the demand of Miller cycle for turbocharger is summarized and the relevant guidance for turbocharger re-match is proposed: the relationship between boost pressure and Miller cycle should follow the original poly-tropic process in cyl-inder; the high efficiency area of compressor should include the operating point with maximum Miller cycle, where the pressure ratio and the mass flow rate are decided by the maximum Miller cycle and the index of original poly-tropic process.
查看更多>>摘要：Adsorption-based atmospheric water harvesting has received significant interest owing to its promise of decentralized water supply and a wide applicability. In this study, the adsorbent bed is specially designed and optimized for efficient moisture capture. A transient three-dimensional non-equilibrium model has been developed that takes both internal and external mass transfer resistances into account. It is found that the simulation results agree better with the experimental results, which indicates the reliability of the model. Then this model was applied to investigate the influence of the adsorbent bed structure, and the operating parameters on the transient distributions of the adsorbent temperature, adsorption kinetics, dynamic up-taken moisture capture, mass transfer resistance and relative capture efficiency of the adsorbent bed. ASLi30 (Activated carbon fiber + Silica gel + 30 wt% LiCl)-vapor is selected as the working pair. It has been found that generally, the effects of the air channel aspect ratio on the investigated indexes are negligibly small. A thinner layer thickness and a smaller air channel length characterize an excellent moisture capture, small vapor transport resistance and faster adsorption kinetics. By considering different parametric variations for performance optimization and the effect of operating conditions, these results provide important insights and design guidelines of adsorbent bed for efficient moisture capture.
查看更多>>摘要：The main purpose of this work is to conduct detailed comparisons of spray behavior among four important components of gasoline and to investigate the effects of fuel properties and aerodynamic breakup on spray behavior under flash boiling conditions. Isooctane, hexane, pentane, and ethanol were used as test fuels. Pressure ratio (Rp) was used as an indicator for the superheated degree and varied from 0.14 to 1.1 by adjusting the ambient pressure. Both Diffused Backlight Imaging (DBI) and Phase Doppler Anemometry (PDA) measurements were used to obtain macroscopic and microscopic characteristics of sprays. Four dimensionless numbers were used to evaluate the aerodynamic breakup processes. The results showed that as the reduction of Rp, the flash boiling intensity increased, leading to dramatic spray morphology changes and smaller droplet size, regardless of fuel type. Spray plumes merged into the spray center when lowering Rp from 1.0. Spray collapsed at an Rp of 0.28 for isooctane, hexane, and ethanol, and at an Rp of 0.18 for pentane. Fuel properties also had significant effects on spray behaviors. Spray with pentane had the smallest penetration length, and a dramatic far-field angle reduction was observed at Rp between 0.2 and 0.4, due to its higher vapor pressure and lower latent heat of vaporization. Under the same Rp, ethanol sprays had relatively larger near-field angles, far-field angles, and Sauter mean diameter (SMD), due to its high latent heat of vaporization. All the dimensionless numbers showed that the spray with pentane had the largest aerodynamic breakup intensity, followed by hexane and isooctane, while ethanol had the lowest aerodynamic breakup intensity under the tested conditions. Aerodynamic breakup still played an important role under low flash boiling intensity conditions (Rp over 0.8). Microscopic results showed that SMD of isooctane, hexane, and pentane under an Rp of 0.28 had a similar value of 10 mu m.
查看更多>>摘要：In this paper, the 2D model of the elliptical tube is established to simulate the film hydrodynamics and heat transfer performance. The volume of fluid (VOF) is used and the model is verified by the experimental data in the previous literature. The effects of the heating condition, fluid medium, inlet temperature (Tin) on the film thickness and heat transfer coefficients are explored. The studied 11 refrigerants include ethane, propane, R123, R1234yf, R1234ze(E), R125, R134a, R143a, R152a, R227ea, R245fa at Tin = -30, -10 and 10 degrees C. The results show at Re = 2000, the local heat transfer coefficients of the propane at Tin = 10 degrees C under fixed heat flux are 19% greater than those under fixed wall temperature. Both the local film thickness and local heat transfer coefficients increase with Reynolds number (Re). The overall external heat transfer coefficients increase with Re, but the growth rate slows down at Re >= 1500. Fluids have different results of film thickness and heat transfer coefficients even they have a similar Kapitza number (Ka). Different from the water and seawater, the heat transfer coefficients reduce as the inlet temperature increases for the refrigerants. A correlation is proposed to predict local film thickness on both circular and elliptical tubes. A correlation is acquired by the fitting method to predict the overall external heat transfer coefficients under the heating condition of fixed wall temperature, which can capture 89% of 132 data within the deviations of +/- 10%.
查看更多>>摘要：Direct-contact total heat exchange with liquid desiccant produces enormous gains in both sensible and latent heat recovery of low-grade flue gas. Previous studies related to that has tended to focus on the system performance rather than the heat and mass transfer characteristics, which is equally essential to their design, simulation and operation. This paper establishes an experimental system to investigate the heat and mass transfer characteristics in direct-contact total heat exchanger for flue gas total heat recovery. Ceramic structured packing is adopted considering its corrosion and high-temperature resistances; calcium chloride aqueous solution is employed as the coolant and absorbent for its low cost and high latent heat recovery capability. Based on those, the effects of flue gas and solution inlet parameters on the performance indices, including sensible and latent heat recovery capacities, latent heat rate and total heat recovery rate, are tested and analyzed. Coupled heat and mass transfer models between flue gas and solution in this process are developed, which feature the enthalpy of dilution. By combining the models and experimental data, heat and mass transfer coefficients are calculated and fitted. In addition, the effect of packing height on total heat recovery capacity and ratio is investigated through simulations. The results show that the total heat recovery rate of 74.7% can be achieved at packing height to 1.5 m when liquid-gas ratio is 5.172.
查看更多>>摘要：The properties of geothermal working fluids have great influence on the heat extraction performance of an enhanced geothermal system (EGS). In addition to H2O, N2O and CO2 have also been proposed as geothermal working fluids. In order to analyze and compare the heat extraction performances of the N2O, CO2 and H2O EGS, a two-dimensional thermo-hydraulic-mechanical (THM) coupled EGS model with discrete fractures is established. In addition, the influences of injection-production parameters on the heat extraction effects of EGS with different working fluids are also analyzed. The results indicate that the heat extraction performances of N2O-EGS and CO2-EGS are almost the same under the same conditions. Taking into account the stable production time and the stability of supporting equipment, the performances of N2O-EGS and CO2-EGS are better than H2O-EGS. Furthermore, a lower injection temperature is conducive to the heat extractions of N2O-EGS and CO2-EGS, while the injection mass flow rate and production pressure should be designed reasonably according to the actual situation. As for H2O-EGS, a higher production pressure is conducive to its performance, while its injection temperature and injection mass flow rate should be designed reasonably.
查看更多>>摘要：In this study, a new heat dissipation device with a composite heat sink of a loop heat pipe (LHP) system was evaluated for efficient cooling of electronic components. To improve the heat dissipation ability of the high heat flux chip and its local hot spot area, LHP, which had a gas-liquid phase change as the main mechanism of heat dissipation, was selected to remove heat. The experimental results showed that the average temperature performance was improved after the graphene copper foil heat sink was fitted, and the maximum temperature under the heat loads of 80 W, 90 W, 100 W, and 120 W was reduced by 3.8 degrees C, 5.7 degrees C, 6.5 degrees C, and 7.7 degrees C, respectively, compared with without a heat sink. The hot spot temperature increased linearly with an increase in the inlet water temperature. Increasing the ambient temperature within a certain range had little effect on the heat-dissipation effect of the hot spots. To interpret the experimental results, the hot spot heat transfer performance of the evaporation section of the LHP with a composite heat sink was set up by the equivalent thermal resistance model. The CFD simulated results were in good agreement with the experimental values. This indicated that the heat sink significantly reduced the hot spot temperature and improved the cooling uniformity largely. When the transverse thermal conductivity of the heat sink is sufficiently high, the increase in longitudinal thermal conductivity is more effective in reducing the hot spot temperature than the transverse thermal conductivity. Moreover, it has been suggested that the hot spots and heating power play a major role in the actual electronic structure and heat dissipation design. In summary, the study proposes a valuable method for improving the uniform cooling of chips, which was conducive to the heat management of the system.
Mahmoudi, FarhadNajafabadi, Tooraj AbbasianMoarefianpour, Ali
17页
查看更多>>摘要：Stress control of boilers is a major concern in the aged power plants. The studied forced circulation boiler suffers from a malfunction of the combustion control system. In order to reduce cyclic stress on boiler equipment and prevent undesirable shutdowns of power unit during fuel shortage events, an operating instruction is proposed. In order to predict dynamic behavior of the boiler, a nonlinear state space model is developed using mathe-matical modelling approach. A gray box identification procedure is introduced using only steady state mea-surements of plant. The dynamic model is verified compared to actual plant data and accuracy of model parameters is evaluated by sigma-normalized sensitivity analysis. Considering aim of steam pressure regulation, a three-step optimization approach is proposed based on golden section search, parabolic interpolation and interior-point methods to optimize arbitrary parameters of the proposed manual control scheme. Simulation results for different loads and various fuel limitation scenarios show that proposed instruction improves the cyclic stress in superheater tubes at least 84.5%. In addition, Monte Carlo experiments illustrate despite a 10% operators' error in implement of the instruction, the stress is still reduced by an average of 78.1% for a given case.
NSTL
Elsevier