查看更多>>摘要：Cryogenic air separation plants produce oxygen at 99.5% purity at 150, 230 and 300 bar to fill cylinders. From the production location, liquid oxygen is often transported to a cylinder-filling plant located at a far-off location where it is pumped to high pressure and gasified in ambient vaporizers. Among the methods of cylinder filling, external vaporization is the safest, operationally most flexible, and allows the main heat exchanger to be designed at low pressure. Despite these advantages, external vaporization is not preferred for filling cylinders "in situ" because of high power consumption attributed to the loss of the entire cold exergy of pumped liquid to the environment. An appropriate modification of the plant helps to utilize the thermal energy of pumped liquid oxygen that is externally vaporized to fill cylinders at the plant site. A reduction of specific power consumption by 23% to 29% is possible while the main heat exchanger still operates at low pressure. Cylinders can also be filled at any of these pressures from the same plant with a penalty of 2% to 3% in SPC. The paper shows ways to adjust the operation of the air compressor to obtain oxygen at varying pressures with utilization of the thermal energy of liquid oxygen.
查看更多>>摘要：In this paper, a new approach to calculate the transfer functions (g-functions) for simulating the thermal performance of large diameter, shallow bore helical Ground Heat Exchangers (He-GHE) is proposed using mean fluid temperature rather than borewall temperature. The g-functions are generated using a validated numerical Capacitance Resistance Model-Helical GHE (CaRM-He) for different bore diameters, bore depth and helical pipe pitch. Mathematical formulation is presented which allows calculation of the combined g-function for multiple bores using the g-functions for individual cases. A simplified resistance-based model which enables the calculation of traditional borewall temperature-based g-functions using the mean fluid g-functions for different mass flowrates is also presented. Finally, mean fluid temperature is calculated for an array of eight He-GHEs for Sacramento climate zone using (a) CaRM-He model, (b) mean fluid temperature-based g-function and (c) borewall temperature-based g-functions for maximum (0.128 kg s(-1)), reference (0.063 kg s(-1)) and minimum (0.057 kg s(-1)) mass flowrates. For the reference mass flowrate case, the predicted mean fluid temperature Root Mean Square Deviation (RMSD) between the CaRM-He simulation and the g-function approaches is less than 5% and 6% of the yearly average temperature difference between the inlet and outlet for the mean fluid and borewall based g-functions respectively. For the other mass flowrates, the RMSD in mean fluid temperature varies between 9% and 17% of the yearly average temperature difference between the inlet and outlet temperature. Overall, the proposed g-function approach can be used effectively to estimate the performance of large diameter helical GHEs.
查看更多>>摘要：To solve the instability of conventional single source heat pump and further improve energy efficiency, this paper presented a hybrid solar/air dual source assisted heat pump (S-A-AHP) water heater. Based on the system, a quasi-steady-state mathematical model was constructed to study the mode switching criterion and operation performance. The simulation results showed that it is more suitable to use the mass flow ratio of refrigerating medium (i.e., the ratio of the photovoltaic/thermal collector/evaporator to the whole system) as the criterion of system mode switching than directly using environmental factors, and the critical ratio for switching between the solar-air mode and the solar mode is about 0.75. In addition, compared with the conventional solar assisted heat pump (SAHP) and air source heat pump (ASHP), under the low environmental benefit conditions of ambient temperature of 0 degrees C and solar radiation intensity of 100 W/m(2), the average COP of S-A-AHP is 29.7% and 19.9% higher than that of SAHP and ASHP, respectively. And the S-A-AHP system's heating time is only 36.7% of the SAHP and 83.7% of the ASHP. And the impact of collector area on system performance was also discussed.
Shin, Sung GilCho, Jai OanKo, AreumJung, Hwa-Young...
16页
查看更多>>摘要：The new application of phase change material for passively cooling the containment building of a nuclear power plant provides many benefits such as minimized wall penetration, low maintenance cost, and the possibility of installation in currently operating power plants with a minimal design change. In this study, the applicability of PCM as a passive containment cooling system is verified through design and evaluation of the PCM condenser. In addition, the effective heat capacity method is verified for simulating the melting process of PCM in the condition of a reactor accident. The PCM condenser design is optimized with five design parameters; melting point, volume, heat transfer area, effective thermal conductivity, and total heat absorbed. The performance of the PCM condenser is evaluated by containment safety analysis and is compared to that of an active containment cooling system. Although the PCM condenser had a limitation for absorbing only a fixed amount of thermal energy due to the absence of an external heat sink, the PCM condenser shows better performance in the early stage after the accident initiation compared to the single spray system and finally maintains the integrity of the containment building.
查看更多>>摘要：Loop Heat Pipe (LHP) is a passive phase-change heat transfer device. Among other factors, its performance largely depends on the heat leak from the evaporator to the compensation chamber (CC). For copper wick LHP, a high heat leak creates a problem in successful LHP start-up and can adversely affect its operation. The present study focuses on reducing the heat leak through manipulation of the copper wick properties. The effect of fluid charging, overall pressure drop, and wick oxidation on thermal performance is investigated. A cylindrical copperacetone LHP is tested in favorable orientation for two cases with (1) oxygen-free, or pure copper wick and (2) oxidized copper wick in its evaporator. For Case #1, LHP is filled with acetone at different Charging Ratios (CR) of 50%, 60%, and 70%. For Case #2, the charging ratio is kept constant at 50%. For the pure copper wick, a charging ratio of 50% provides the best thermal performance, and it decreases with an increase in charging ratios. At CR = 50%, LHP can transfer a heat load of 90 W (hEvp = -900 W/m2K) and 180 W (hEvp = -2500 W/ m2K) at the evaporator temperature below 100 degrees C for the pure copper and oxidized wick, respectively. The significant improvement in LHP thermal performance of the latter is attributed to a decrease in heat leak because of the low thermal conductivity of the oxidized porous wick. The heat leak for pure and oxidized wick is found -18% and -7% of input heat loads, respectively. This can be an effective methodology to pave the way for the usage of commercial copper-based LHPs for the thermal management of terrestrial devices.
查看更多>>摘要：The current paper presents the broad-range sensitivity analysis and uncertainty quantification for the performance of a thermoacoustically-driven pulse tube cryocooler to make it robustly perform using a low-grade heat source. Accordingly, the onset and steady-periodic operations of the cryocooler are numerically simulated using Rott's one-dimensional equations. The effects of increasing hot/cold core's number, engine's phase-shifter length, and mean pressure on the cooling features and hot-source temperature are investigated. In this regard, a looped-branched, octa-engine, four-stage cryocooler is proposed that operates with a steady temperature difference of 132 K at a no-load cold temperature of 77 K. In contrast to previous findings, the results show that increasing the number of stages or the charge pressure does not necessarily improve thermodynamic performance. Moreover, the uncertainty in the operation of a quad and octa-engine, four-stage cryocooler caused by the geometric and material characteristics is estimated using the Morris method. It is demonstrated that the overall operation is more sensitive to the hot-core dimensions than to the cold-core dimensions. Besides the hot core dimensions, gas properties and solid thermal conductivity strongly influence the transient and steady periodic features of the cryocooler, respectively. Furthermore, doubling the hot cores in the four-stage system increases the uncertainty of the acoustic power up to five times, leading to more uncertainty in the cooling features. Drawing on the results of this paper, a multi-stage cryocooler can be designed to optimize the overall performance and reliability.
查看更多>>摘要：The use of cooling garments is emerging as an effective, convenient, and energy-efficient way to maintain thermal comfort, which is crucial for conserving physical and psychological health and for avoiding potentially life-threatening situations. Current cooling garments are generally bulky and have short operating periods, uncontrollable cooling capacity, and low effectiveness. Here, a novel man-portable cooling garment based on thermoelectric refrigeration was designed, and the heat generated by body is absorbed by capillary tube and dissipated in semiconductor refrigeration system. The coolant is circulated in the pipeline to recover residual cold energy and improve the thermal stability of the system. Several performance tests were conducted in a simulated hot environment to evaluate and to optimize refrigeration efficiency and cooling effectiveness. The results indicated that the system was capable of providing a water temperature of 15.7 degrees C and cooling power of 340.4 W, with a coefficient of performance of 3.40 at 100 W of electrical power and a climatic chamber at 30 degrees C. Additionally, the operating environment of the system significantly influenced the cooling capacity of the garment; and the cooling power of the system was significantly improved by increasing the heat dissipation capacity. The increase in electrical power was limited by constant heat dissipation capacity restrictions. The wearing trial confirmed the effectiveness of the thermoelectric cooling garment. Furthermore, our study improved the understanding of performance characteristics for the thermoelectric cooling garment, and a road map for the further development of thermoelectric cooling garments was suggested.
查看更多>>摘要：The existence of preferential flow paths, such as fractures and/or fault play an vital role on the thermal breakthrough of geothermal doublet system. The interaction between the preferential flow path and bedrock is often uneven and may have typical fractal characteristic. This study proposes a thermo-hydro-mechanical coupling model considering the deformation of fractal fractures. The fractal fracture is regarded as a thin elastic layer existed in the bedrock, whoes deformation depends to the bedrock and its own mechanical properties. Subsequently, the geothermal doublet system with a three-dimensional fractal fracture is modelled and the parameters affecting thermal breakthrough are investigated numerically. Research results indicate that the thin elastic layer assumption is remarkably robust for modelling fracture opening and closing under coupling conditions. Owing to complex fracture geometry, the fracture permeability evolution presents certain heterogeneity, which is related to fractal dimension, in-situ stress, and geothermal wells layout. The cool water in fracture with larger fractal dimension can interact with the bedrock more fully and further affect the thermal breakthrough. This further suggests that the site selection of geothermal wells should consider the specific geometry of preferential flow paths to avoid premature thermal breakthrough inducing low system efficiency.
查看更多>>摘要：The multi-section absorption heat pump (AHP) is applied in the novel district heating (DH) system of China. This new type of AHP exhibits improved performance in DH systems, but involves a more complicated structural design and the risk of internal flow instability problems such as reversed pressure difference and liquid accu-mulation. The flow circulations of the solution and refrigerant consist of several gravity-driven flow processes that are rarely observed in conventional AHPs, and these processes have not been addressed in previous studies. The present work studies the dynamic response of gravity-driven flow inside a multi-section AHP in a DH system. A dynamic model capable of calculating the liquid level, flow rate, and pressure change of various gravity-driven flow processes is developed and validated. The steady-state conditions calculated by the dynamic model are compared with a validated model in the literature. The gap between the temperature and pressure calculations is less than 0.3 K and 0.05 kPa, respectively. The dynamic simulation results are compared with the start-up process test results of a multi-section AHP. The pressure values and trend change fit the test results well. The standard deviation (SD) and root-mean-square error (RMSE) of the pressure calculation errors are less than 5% and 5.5%, respectively. The model is used to study typical transient processes of multi-section AHPs in a DH system. In the start-up process, the pressure of the generator is 1-2 kPa lower than that of the absorber after the solution circulation begins. This reversed pressure difference leads to solution accumulation at the bottom of the generator. The solution flow rate into the absorber is 20% lower than the designed value. After the heat source is input into the generator, the reversed pressure difference disappears. Solution accumulation expedites the flow of the solution into the absorber. The flow rate is 75% higher than that of the designed value, leading to solution accumulation in the absorber. In the heat source sudden drop process, reversed pressure difference and solution accumulation problems also occur. The reasons for flow instability in each process and possible methods for preventing them are discussed.
Park, Seong HyunHa, Man YeongKim, Kyung ChunYoon, Sang Youl...
12页
查看更多>>摘要：A numerical investigation of a design modification of a two-dimensional supersonic gas ejector was undertaken via an adjoint method, with a focus on enhancing the performance of the ejector. Maximization of the secondary mass flow rate was used as an objective function. The entrance geometry and throat height of the primary nozzle were fixed, but the other geometries were allowed to vary during the design modification. The shape of the divergence of the primary nozzle, the nozzle exit position (NXP), and the entrance region of the mixing chamber were modified simultaneously using the adjoint method. The height of divergent section of primary nozzle (Hne), NXP, and the inlet height of the mixing chamber (Hmi) were enlarged by about 13.33%, 27%, and 49%, respectively, resulting in an entrainment ratio (ER) that was enhanced by about 37.17% compared to that obtained for the baseline geometry. Two simplified geometries from the adjoint modification were suggested, and have the same performance with the adjoint-modified geometry. Further investigation was performed by varying the height of the straight section of the mixing chamber of the simplified geometry. The results show that the ER increased by about 46.77% compared with that obtained using the simplified geometry.
NSTL
Elsevier