1,720,993 research outputs found
Buoyancy effect on the flow pattern and the thermal performance of an array of circular cylinders
No full textIn this paper, we found, by means of numerical simulations, a transition in the oscillatory character of the flow field for a particular combination of buoyancy and spacing in an array of six circular cylinders at a Reynolds number of 100 and Prandtl number of 0.7. The cylinders are isothermal and they are aligned with the earth acceleration (g). According to the array orientation, an aiding or an opposing buoyancy is considered. The effect of natural convection with respect to the forced convection is modulated with the Richardson number, Ri, ranging between-1 and 1. Two values of center-to-center spacing (s=3.6d-4d) are considered. The effects of buoyancy and spacing on the flow pattern in the near and far field are described. Several transitions in the flow patterns are found, and a parametric analysis of the dependence of the force coefficients and Nusselt number with respect to the Richardson number is reported. For Ri=-1, the change of spacing ratio from 3.6 to 4 induces a transition in the standard deviation of the force coefficients and heat flux. In fact, the transition occurs due to rearrangement of the nearfield flow in a more ordered wake pattern. Therefore, attention is focused on the influence of geometrical and buoyancy parameters on the heat and momentum exchange and their fluctuations. The available heat exchange models for cylinders array provide a not accurate prediction of the Nusselt number in the cases here studie
Off-design and annual performance analysis of supercritical carbon dioxide cycle with thermal storage for CSP application
Full text linkSupercritical Carbon Dioxide (sCO2) cycles can achieve higher efficiency compared to steam-Rankine or Air-Brayton cycles, therefore they are promising for concentrated solar power applications. Although sCO2 cycles show higher design efficiency, the off-design efficiency is highly sensitive to the ambient conditions, impacting the power block net-power and heat input. In the present work a recompression sCO2 cycle is connected to a central-tower solar field with two-tank thermal storage delivering molten chloride salt at 670 °C. The temperature of the molten-salt exiting from the power block and returning to the cold storage tank increases by 46 °C with respect to the design value when the compressor inlet temperature is raised by 13 °C relative to the design condition of 42 °C, which implies that the capacity of the thermal storage reduces by 25%. The main focus of this work is to investigate the off-design performance of a sCO2 recompression cycle under variable ambient temperature, molten-salt inlet temperature and molten-salt flow rate. Multi-objective optimisation is carried-out in off-design conditions using an in-house code to explore the optimal operational strategies and the Pareto fronts were compared. Since the power cycle can either be operated in maximum power mode or maximum efficiency mode, this study compares these two operational strategies based on their annual performance. Results indicate that the capacity factor of the concentrated solar power can be increased by 10.8% when operating in maximum power mode whilst the number of start-ups is reduced by about 50% when operating in maximum efficiency mode
Impact of Ozone Addition to Gasoline Surrogates Combustion in Spark Ignition Engine
No full textBased on the experimental results, a 3-D Computational Fluid Dynamics investigation is carried out to evaluate the influence of ozone on the combustion process in
spark ignition engine fueled with gasoline/air mixtures. Ozone (O3) is a chemically reactive species capable of improving the laminar flame speed, reducing the ignition delay time,
and stabilizing combustion variability. With the aim of proposing a 3-D numerical model to
simulate combustion of fuel mixtures under ultra-lean conditions, two numerical correlations
are proposed to reproduce the chemical properties of gasoline/air/ozone mixtures in terms
of laminar flame speed. A chemical kinetic mechanism for Toluene Reference Fuel oxidation
(iso-octane, n-heptane, toluene, 63/20/17% by mol.) modified with an ozone sub-mechanism
is used to perform several 1-D numerical simulations. The laminar flame speed correlation
estimates an enhancement of 3.4% at 600 K and 10 bar under ultra-lean condition (φ = 0.6).
For the 3-D numerical simulations, the G-Equation model is used to reproduce the premixed
combustion process in internal combustion engines. The results suggest that the numerical
correlations can predict the combustion properties of gasoline/air mixtures without and with
ozone addition. The presence of ozone traduces in a higher laminar flame speed, leading to an
increase in the in-cylinder pressure peak and the rate of fuel consumption. Furthermore, the
numerical analysis reveals that the greatest improvement is observed for fluid regions within
the cylinder characterized by low turbulent flame speed
Recuperator transient simulation for supercritical carbon dioxide cycle in CSP applications
No full textSupercritical carbon dioxide (sCO2) cycles are considered to provide a faster response to load change owing to their compact footprint. sCO2 cycles are generally highly recuperative, therefore the response time is mainly dictated by the heat exchanger characteristics. This study model the transient behaviour of a recuperator in 10 MWe simple recuperative Brayton cycle. The response for the variation of inlet temperature and mass flow boundary conditions were investigated using two approaches based on temperature and enthalpy. The performance of these two approaches are compared and the numerical schemes were discussed along with the challenges encountered. The simulation results were validated against the experimental data available in the literature with a fair agreement. The characteristic time of the heat exchanger for a step change of the boundary conditions is reported that supports the recuperator design process. Compact recuperator responded in less than 20 seconds for the changes in the temperature boundary condition whilst it can take upto 1.5 minutes for mass flow change. In order to reduce the computational effort, a logarithmic indexed lookup table approach is presented, reducing the simulation time by a factor of 20
A computational model of axial piston swashplate pumps
Full text linkVariable displacement hydraulic machines offer a very promising alternative and energy saving solution for many applications in mobile machines, mobile robots and other applications. In the present paper an axial piston swashplate pump will be theoretically analyzed and explained using the software AMESim in order to estimate the piston friction force and volumetric efficiency loss without hardworking experimental tests. The present paper is aimed at analyzing the forces acting on the swash plate in stationary and non-stationary conditions, in order to optimize the main design parameters of the control actuators of a variable displacement pump. The behavior of the machine is analyzed and presented at different angular velocities and pressure regimes
Pair and multi-particle dispersion in numerical simulations of convective boundary layer turbulence
No full textTracer dispersion within a highly convective planetary boundary layer is studied by means of a large-eddy simulation (LES) model for the continuous phases describing the temperature and velocity fields, and with the Lagrangian tracking of particle trajectories. Particle velocities are decomposed into their resolved and unresolved (or sub-grid) components. The former are evaluated by interpolation from the LES velocity field, the latter are given by a Lagrangian kinematic model that correctly describes the turbulent dispersion of clouds of particles. It is shown that, thanks to the Lagrangian sub-grid model, a clear inertial range is detectable in the time domain. In this range, particle separation grows according to Richardson's law, and nicely compares with previous experimental and numerical measurements. The collective motion of four particles, initially located at the vertices of regular tetrahedra, is also studied. The evolution of tetrad shape and orientation is contrasted with those obtained in homogeneous and isotropic flows. Results show that an agreement is achieved at small time lags. At larger times, the boundary layer reveals its anisotropic structure and the tetrad shape statistics deviate from results obtained in ideal flows
Performance and cost multi objective optimisation of a shell-and-tube LHTES device for mid-temperature applications
No full textIn this contribution the authors investigated by means of an analytical approach the performance of a latent heat thermal energy storage varying the internal HTF tube distribution and other design parameters of the device. The work is based on a simplified prediction model of the discharging phase in order to span several configurations. The effects of the physical parameters of the phase change material and the geometrical dimensions of the device have been studied in order to suggest the optimal configuration. Indeed, the maximisation of the capacity together with the maximisation of the heat transfer rate are often competing and need a multi objective approach in the optimisation procedure. Here, the authors showed several optimal solutions proposing an efficient procedure that can be used in the selection of the most interesting cases to investigate by means of an experimental campaign or detailed numerical simulations. The approach proposed is general and it can be used also to minimise the cost of the device with respect to the maximisation of the performance due to the particular application. Several phase change materials were considered and a preliminary capital cost minimisation was proposed
Analysis of design, off-design and annual performance of supercritical CO2 cycles for csp applications
No full textSupercritical carbon dioxide (sCO2) cycles are studied as the next-generation power cycles in order to reduce the cost of Concentrating Solar Power (CSP) plants. The design performance of numerous cycles has been investigated, nevertheless, the off-design and annual performance of these cycles are seldom studied. This plays a critical role in selecting an optimal cycle for CSP application, as an efficient power cycle influences the solar field size, consequently affecting the Levelised cost of electricity (LCOE). In this study, the design, off-design and annual performance of three sCO2 cycles; simple recuperative, recompression and partial-cooling cycles are studied. Multi-objective optimisation was performed and the off-design Pareto fronts were compared for the changes in the power cycle boundary conditions. Annual performance simulation was carried out, and the performance of the three cycles was compared when the power cycle is operated in maximum efficiency mode, which facilitates selecting the optimal cycle. The LCOE of the simple recuperated cycle was higher by roughly 1.7¢/kWh than recompression cycle when maximising the power cycle efficiency and the partial cooling cycle is higher by 0.2¢/kWh. However, operating the power cycle in the maximum efficiency mode significantly lowers the plant capacity factor (around 10-20%)
A Numerical Investigation of VVA Influence on the Combustion Phase for Premixed Combustion Engine under Partial Load Conditions
No full textNowadays, the vehicle hybridization and the use of non-conventional fuels for heavy-duty applications brings to a new beginning in the use of spark ignition (SI) engines. For a standard intake system, the premixed fuel/ air mixture is controlled by the injection of fuel after the throttle valve. Then, the geometry of the intake system, with the intake duct, the intake valves and the cylinder head shape, influences the characteristics of the flow within the cylinder up to the combustion process. The new technology of fluidpower and electrical actuations gives the opportunity to decouple the intake and exhaust valve actuations with respect to the standard cam shaft distribution. The Variable Valve Actuation (VVA) concept is not new, but its application is now affordable and flexible enough to be applied to partial load conditions. In this work, the intake, compression and combustion processes of an SI engine are studied by means of a three-dimensional numerical approach based on a finite volume approach. In this model, the Unsteady Reynolds-Averaged Navier-Stokes (U-RANS) equations are solved together with a k-ε model for turbulence and an Extended Coherent Flamelet Model (ECFM) for combustion. The 4-valve engine is equipped with two symmetrical intake valves as well as two symmetrical exhaust valves. Two strategies are studied under partial load conditions: a standard valve lift profile for both intake valves, and a single intake valve lift profile, to provide the same overall fresh mass in the cylinder of the 2-valve opening. The valve timing has been kept constant for both strategies, with an Early Intake Valve Closing (EIVC) approach due to the partial load conditions. The intake flow characteristics and their influence on the combustion process are analyzed and a comparison between the two strategies is carried out. The results show flow structures quite different between the single valve opening and the standard 2-valve opening. The asymmetry of the intake flow, induced by the single valve approach, leads to an increase of the swirl ratio with respect to 2-valve opening. The highest swirl ratio of the single valve case is sustained till spark ignition occurs. At spark timing, the Turbulent Kinetic Energy (TKE) is greatly influenced by the valve strategy,
leading to higher values for the single valve lift case with respect to the standard two valves lift. Moreover, the results show that single valve opening provides a faster combustion in lean mixture conditions than the standard lift
Energy and Cost Analysis of Concentrated Solar Thermal Plants Integrated With Latent Heat Thermal Energy Storage for the Decarbonization of Industrial Processes
No full textIndustrial energy demand in the EU is driven by the thermal energy share, which accounts for the most part of the total energy demand. A large part, about one-third, is at medium temperatures, between 100 and 200 °C, and it is still produced by fossil fuels, mainly natural gas. This energy demand could be met by solar thermal energy by using Concentrated Solar Thermal (CST) technologies, suitably combined with thermal energy storage systems.
This paper presents a study of CST plant composed by Parabolic Trough Collectors (PTC) combined with Latent Heat Thermal Energy Storage (LHTES) system in shell-and-tubes configuration for the supply of heat for industrial processes. Firstly, the criteria adopted for the sizing and the quasi-static simulation of the system are described. Significant attention is given to evaluate the variation of the Solar Fraction (SF) with the typical parameters of thermal storage, such as the maximum capacity and exchange surface, given a specific location and the Solar Multiple (SM). The methodology has been applied to a dairy processing facility within the agri-food sector with a thermal load of 500 kWth. Then, an economic analysis has been carried out by evaluating the impact of the component’s costs, such as the linear parabolic collectors, thermal storage materials and landscape preparation. Finally, the Levelized Cost of Heat (LCOH) has been computed for different thermal storage sizes in order to find the best combination that minimizes it. As a result, despite the overall increase of the SF with the thermal storage capacity, the proposed work shows how the minimum value of the LCOH (7.24 c€/kWh) corresponds to low storage capacity (1 MWh)
- …
