查看更多>>摘要：Supercritical CO2 Brayton cycle is widely used in industry because of its small compression work and considerable cycle efficiency. In this work, dynamic simulation numerical models of Supercritical CO2 Brayton cycle with three typical layouts (recompression, reheating, intercooling) and a newly proposed layout are developed using thermodynamic equations. After verifying the simulated steady-state values with the experimental ones, key parameters in the recompression and the new layout, as well as their responses under temperature perturbations, are calculated for different extraction ratios. It has been found that larger extraction ratios correspond to lower efficiency but higher stability, and similarly, the new proposed layout is 4.1% less efficient but with a 34% smaller fluctuation amplitude compared to the recompression layout. Then system parameters are calculated for different interstage pressure ratio assignments for the turbine in the reheating model and for the compressor in the intercooling model. The results show the 1st-stage with a pressure ratio of 1.2 has higher power generation and cycle efficiency, as well as more stable generated power. For the newly proposed layout, the pre-compressor power, as well as the fluctuation amplitude of the (RC + IC + PC) model, is much larger than the other compression powers, and the fluctuation amplitude from largest to smallest are IC, (RC + IC + PC), RC, RH. The effects of extraction ratio on efficiency and generated power are much greater than the distribution of interstage pressure ratio, and the maximum efficiency is obtained at the small extraction ratio and the equal pressure ratio of the two stages.
Anand, R. S.Jawahar, C. P.Solomon, A. BruslyDavid, Shibin...
10页
查看更多>>摘要：Thermosyphon operating with refrigerants are usual choices for low temperature heat transfer applications. The present study focuses on the performance evaluation of thermosyphons employing refrigerant R134a and Al2O3/ R134 as working fluids, with novel designs to enhance the heat transfer. Two designs are proposed in the study in which eight internal axial fins are incorporated in one design termed as finned thermosyphon 1, and in addition to the internal fins, a uniform radial cut or gap is provided in the evaporator section in another design termed as finned thermosyphon 2. The performance of the thermosyphon shows that the thermal resistance of finned thermosyphon 1 and finned thermosyphon 2 decreases by 26.8% and 44.8%, respectively than conventional thermosyphon. Experimental investigations were also carried out using nanoparticles Al2O3 mixed with refrigerant R134a at a concentration of 1.0% concentration by weight on the modified design was employed in finned thermosyphon 2, as its performance was better when R134a was utilized. The thermal resistance of Al2O3/R134 based finned thermosyphon 2 decreases by about 55.3%, and 38.9% when compared to the conventional thermosyphon and finned thermosyphon 1, respectively. At the optimum condition, the heat transfer coefficient and surface area are determined for the conventional, finned type thermosyphons 1 and 2. The experimental analysis results are validated with a machine learning algorithm to estimate the deviation between the experimental and predicted results. The predicted and experimental results were found to be in good agreement with each other.
查看更多>>摘要：Pyrolysis process through circulating fluidized bed (CFB) is a promising technology to produce synthetic fuel and other products from biomass feedstocks. Computational fluid dynamics (CFD) computing means provide valuable insights to better understand gas-solid flow hydrodynamics, troubleshoot performance issues and optimize reactor operations. In this study, gas-solid flow hydrodynamics and heat transfer characteristics of a CFB riser for fast pyrolysis are investigated using a three-dimensional (3D) Eulerian-Eulerian CFD model. The main observations are discussed to provide insights on the factors affecting CFB riser performance. The model parameters, specularity and particle-particle restitution coefficients, were considered and tuned to accurately predict of gas-solid flow hydrodynamics and heat distribution with respect to different gas velocities and solid circulation rates. The results have shown that the CFD model predicted well the flow hydrodynamics and both specularity and particle-particle restitution coefficients are critical parameters as they affect particle behavior and temperature distribution fields. The lower specularity coefficient (phi -0.00001) was fairly able to predict the axial solid holdup profile into CFB riser. However, the lower value for particle-particle restitution coefficient (e(ss)- 0.8) significantly overpredict the bottom dense region. The results shows that the increase of operating velocity promotes the mixing behaviors and heat transfer performance. In this work the suitable gas and solid circulation flow rates are U-g = 4.5 m/s and G(s) = 81.23 kg/m(2)s.
查看更多>>摘要：Ejectors in Joule-Thomson (JT) cooling cycles can reduce the evaporator pressure and the compressor power consumption, achieving lower temperatures and higher efficiencies. The ejector, being a major component of a JT cooling cycle with an ejector, is critical to overall cooling performance. In this study, the effects of inlet pressures, outlet pressures, inlet temperatures, nozzle, and mixing geometries on the performance of nitrogen ejectors are investigated experimentally and numerically. The critical back pressure of the ejector increases as the primary and secondary inlet pressures increase. Depending on the working mode, the entrainment ratio varies with the ejector inlet temperature. The clogging due to the deposition of water molecules as impurities occurs when the ejector inlet temperature is below 222 K, resulting in the rapid reduction of mass flow rates. For nitrogen ejector designs, the following ratios are recommended: constant-area mixing chamber diameter to nozzle outlet diameter, constant-area mixing chamber diameter to nozzle throat diameter, nozzle exit position to constant-area mixing chamber diameter, constant-area mixing chamber length to constant-area mixing chamber diameter. The results of this study will be beneficial in promoting the use of ejectors in JT cooling cycles.
查看更多>>摘要：A helically coiled microtube was firstly introduced to microfluid technology owing to its large specific surface area. In this study, an experimental investigation into the effects of internal diameter (di), helical pitch (P), coil diameter (Dc), and mass flow rate (m) on the flow and heat transfer characteristics when n-decane flows through a helically coiled microtube was executed. Microtubes with various di (267.4, 404.4 and 465.3 mu m) were adopted; the Reynolds number (Re) ranged from 829 to 3395, and the amended Dean number (De') ranged from 262 to 1058. The pressure drops (Delta p) both in the helically coiled microtube and straight microtube were compared. The friction factor (f) suddenly increased when the di decreased to 267.4 mu m, but it decreased with ascending Dc and P. Moreover, the Nusselt number (Nu) decreased with increasing di and Dc. In addition, the amended coil diameter (D'c) was used to comprehensively evaluate the effects of P on the flow behavior. The capacity of comprehensive heat transfer enhancement was quantified by Performance evaluation criteria (PEC). PEC increases with the increasing De, but decreases with the increasing di and Dc. The biggest PEC is 1.554, which occurs in the helically coiled microtube with di = 267.4 mu m. Finally, Empirical correlations of f and Nu for the helically coiled microtube were developed, considering di and D'c. The maximum relative errors for f and Nu were +/- 10% and +/- 8.3%, respectively.
查看更多>>摘要：More than 30% of the world's energy consumption depends on building sector which causes 27% of total greenhouse gas emissions. More than 50% of this load is associated to space heating and cooling in buildings. The ground source heat pumps are used in building sector especially in harsh and warm climates, as a valid alternative to electric air-to-air/water heat pumps traditionally used to supply heating, cooling and domestic hot water loads. This study proposes an analysis on experimental basis of a ground source heat pump with a combithermal storage by means of real components and a Hardware in Loop test bed installed in a laboratory in Germany. The study will be conducted by using Second Law analysis of Hardware in Loop system allow to investigate different operating conditions and to identify the devices of the thermal plant that exhibit the largest exergy losses, efficiency defect and the greatest room for improvement. By considering an emulated constant thermal load equal to 5 kW for building during heating period, the ground source heat pump results the highest dissipative component of whole plant by destroying about 36% of exergy input in experimental test conditions which are a ground temperature of 6 degrees C, evaporator inlet temperature of about 0 degrees C and condenser water supply temperature of 46.8 degrees C. In this condition the heat storage exhibits an efficiency defect of 19.1% demonstrating that it is a strategic component for plant. A sensitivity analysis has been defined by varying on experimental basis the brine input to evaporator and water supply temperatures. The variation in the first parameter corresponds to a range on ground temperature from 5 to 25 degrees C while the second one allows to evaluate a range of supply temperature to storage of 35 - 60 degrees C. In addition, an exergoeconomic analysis has been conducted in order to evaluate the unit exergoeconomic product cost by showing that the exergoeconomic unit price of product for end-users depends on balancing of both electricity cost and investment cost that highly depend on ground and heat pump supply temperature condition.
查看更多>>摘要：The mantled solar heat storage (MSHS) tank can separate water utilization and heat collection, and its structure is not only conducive to the formation of temperature stratification, but also easy to be combined with the building, which has a wide range of application value. However, when it is in the simultaneous charging/discharging mode (CD-Mode), the mixing in the inner tank is affected by the heat transfer of the mantle exchanger, and the mixing mechanism and the development law need to be further explored. In this paper, the mixing mechanism of negative buoyancy jet at the inlet and the variation trend of thermocline in the tank under different operating modes were explored numerically and experimentally. As well as its influence on the overall thermal performance of the tank, a simple and feasible way to optimize the diffuser is discussed as well. It is found that in the CD-Mode, the mixing inside the water tank becomes more severe in the early stage of water use, and the thermocline thickness shows a trend of thickening first and then thinning over time. However, the mixing state is relatively stable during the whole water use period, and the change rate of thermocline thickness is smaller. But it is more affected by the inlet negative buoyancy jet than that in the discharging mode (D-mode), especially when the Rei number is large. Under the two operation modes, when the dimensionless time t* = 0.4-0.8, it enters the stable water use process, and the thermocline is in the stable development stage. The thickness of thermocline changes less in the CD-mode, which makes it easier for users to obtain longer hot water service time, reflecting the advantages of mantled heat exchange water tank in maintaining thermal stratification. For the working conditions studied, after optimized the diffuser, the hot water output rate can be 109.9% higher than the straight tube with the same tube length, and the hot water service time can be prolonged by 90.2%, which has practical application value.
查看更多>>摘要：As traditional fossil energy sources are continuously diminishing, the demand for optimising output from renewable energy sources is gaining particular importance. Among these, solar energy is certainly one of the most prominent technology and it is widely used in a variety of applications, either concerning electricity and heat production. Nonetheless, the global efficiency of solar systems still has to be largely improved, reducing at the same time generation costs, in order to make solar an even more relevant source of clean energy. In modern photovoltaic, concentrated photovoltaic as well as concentrated solar power plants, the net output can be increased through solar tracking solutions aiming at the optimal positioning of the solar panels/mirrors on a daily and seasonal basis. This typically requires electromechanical motors, which are designed to align the incident solar radiation with the optical axis, thus enhancing the overall energy conversion efficiency but draining at the same time up to 1-2% of the theoretically achievable net power output. Furthermore, to increase energy dispatchability concentrated solar power plants usually incorporates thermal energy storage units, which can be of the sensible-heat or latent-heat storage type. The latter imply phase transition of the storage material which, in turn, can generate up to 20% volumetric expansion for a solid-to-liquid transition. Although generally assumed as an undesired side effect, such expansion can represent an opportunity to extract mechanical work and thus increase the overall efficiency of the solar system. The main objective of this study is to provide an initial quantitative assessment of the passive tracking potential related to the phase-change induced expansion of thermal storage media in concentrated solar power plants. To this aim, a solar-integrated waste-to-heat steam power plant, rated at 15 MWe, has been taken as a reference and a coupled finite-difference/finite-volume numerical model of the latent-heat thermal energy storage unit of the plant has been developed. The model takes input data from the power plant operating conditions and is able to retrieve time-resolved temperature and volumetric density changes of the thermal storage media. Results from the numerical model shows that passive solar tracking is achievable for a fraction of the heliostat field that ranges from 10% to 100%, depending on the season and operating pressure of the tracking system. In terms of electrical power savings, this is up to 2% of the net power output of the reference plant, thus representing a promising basis for further investigations on the applicability of the proposed novel integrated passive solar tracking concept.
查看更多>>摘要：With the popularity of electric vehicles, lithium-ion batteries are widely used. As a temperature-sensitive component, the performance, life span, and safety of lithium-ion batteries are affected by their working temperature. Therefore, the monitoring of lithium-ion battery temperature is of great significance for electric vehicles. Currently, the temperature monitoring methods are mainly based on the measurement of battery surface temperature. It is difficult to obtain the more important internal temperature that reflects the actual electrochemical reaction status inside the battery. Herein, a prediction model for cylindrical 18,650 lithium-ion batteries is established to reveal the internal temperature under various boundary conditions. Firstly, T-type thermocouples are inserted into the battery to obtain the internal and surface temperature. The characteristics of the battery temperature variation at different discharge rates under different cooling conditions are analyzed in detail. Then, the calculation model of the battery internal temperature under different cooling modes is established by using the thermal network method. Finally, the accuracy of the prediction model is verified by the experimental data. The results show that the internal temperature at the positive terminal is higher than that in other parts. From the perspective of safety, this temperature can be used as a target parameter for the battery thermal management system and is also suitable as a warning parameter to monitor the battery thermal runaway. The battery internal temperature prediction model can achieve a high calculation precision based on the thermal network method. The absolute and mean square deviations can be controlled within 1.5 degrees C and 0.8 degrees C under the discharge rate of up to 1.5C and 1C pulse discharge. It provides high precision and reliable calculation method for the internal temperature prediction of power batteries in electric vehicles under daily driving with low discharge rates.
查看更多>>摘要：Regenerative cooling is a promising method to meet the increasing heat management requirement of hypersonic vehicles. In contrast to the traditional co-directional flow (CDF) pattern, here we propose an innovative bidirectional flow (BDF) pattern inside rectangular parallel channels to improve the active regenerative cooling performance. Based on a reliable CFD model, the flow and heat transfer performance of n-decane under two types of thermal boundaries are analyzed. Three types of channels at different locations considering buoyancy effect are also investigated. By observing the contour of specific heat, mass fraction of n-decane, temperature distribution, velocity field and calculating the heat sink and surface heat transfer coefficient, the cooling performance of BDF pattern is discussed and compared with that of CDF pattern. Results reveal that compared to the CDF pattern a more homogeneous temperature distribution and a higher conversion of n-decane (around 14% increment under thermal boundary: 1.9 MW/m2 and 1400 K) are obtained by BDF pattern. However, an increased pressure drop within a reasonable range and a higher surface temperature are observed for the BDF pattern. With overall consideration, a hybrid flow pattern is proposed which could maximize the cooling effect within the surface temperature constraint yet keeping its simple inner structure.
NSTL
Elsevier