45 research outputs found
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Dynamic simulation and exergetic optimization of a Concentrating Photovoltaic/ Thermal (CPVT) system
The development of a dynamic, theoretical model suitable for the prediction of the long-term performance of a parabolic-trough Concentrating Photovoltaic/Thermal CPVT system is discussed in the present study. The formulation of the mathematical model and the considered geometrical and operational parameters of the system, such as the characteristics of the employed PV modules and active cooling system are described in detail. The effect of heat capacity is taken into consideration in the thermal balances and thus the model is able to capture the transient behavior of the system. Besides, the model is validated using available experimental data of a manufactured prototype CPVT system. The daily performance of system is predicted for different values of the cooling fluid flow rate and temperature under various environmental conditions. At a second stage, an exergy analysis is conducted in order to point out the effect of the characteristics of the main system sub-components on the exergetic efficiency and exergy output of the CPVT system. It was established that the system exergetic performance is primarily influenced by the optical quality of the parabolic trough and the electrical efficiency of the PV module. Increasing these two factors to achievable values, e.g. ηopt = 0.75 and ηel = 0.25, can yield an increase of the system exergetic efficiency from 12% to 24%
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Experimental Study of Diesel-Fuel Droplet Impact on a Similarly Sized Polished Spherical Heated Solid Particle
The head-to-head impact of diesel-fuel droplets on a polished spherical brass target has been investigated experimentally. High-speed imaging was employed to visualize the impact process for wall surface temperatures and Weber and Reynolds numbers in the ranges of 140–340 °C, 30–850, and 210–1135, respectively. The thermohydrodynamic outcome regimes occurring for the aforementioned ranges of parameters were mapped on a We–T diagram. Seven clearly distinguishable postimpact outcome regimes were identified, which are conventionally called the coating, splash, rebound, breakup–rebound, splash–breakup–coating, breakup–coating, and splash–breakup–rebound regimes. In addition, the effects of the Weber number and surface temperature on the wettability dynamics were examined; the temporal variations of the dynamic contact angle, dimensionless spreading diameter, and liquid film thickness forming on the solid particle were measured and are reported
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Numerical simulation of three-phase flow in an external gear pump using immersed boundary approach
This paper presents a three-phase fully compressible model applied along with an immersed boundary model for predicting cavitation occurring in a two dimensional gear pump in the presence of non-condensable gas (NCG). Combination of these models is capable of overcoming numerical challenges such as modelling the contact between the gears and simulating the effect of NCG in cavitation. The model accounting for the effect of NCG also has broader applicability, since gas dissolved in liquids can come out of the solution when exposed to low pressures; this plays a significant role in the pump performance and cavitation erosion. Here the simulation results are presented for the gear pump at different operating conditions including the contact between gear, gear RPM and % of NCG; their effects on performance and cavitation is demonstrated. The results suggest that modelling the contact between the gears play a role in the cavitation prediction inside the gear pump. An increase in cavitation is observed when the contact is modelled even for the small pressure difference considered between the inlet and outlet. An increase in the RPM of the gears also results in increased cavitation within the pump, whereas an increase in the percentage of NCG content by a small amount can reduce the cavitation to a greater extent. This reduction is due to the expansion of the gas at a lower pressure which recovers the pressure and prevents or delays the phase-change process of the working fluid. The fluctuations in the outflow rate is also found to increase when the gears are in contact and also with increasing gas content
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Comparative evaluation of phase-change mechanisms for the prediction of flashing flows
A numerical study is presented, evaluating in a comparative manner the capability of various mass-transfer rate models to predict the evolution of flashing flow in various geometrical configurations. The examined models comprise phase-change mechanisms based on the kinetic theory of gases (Hertz–Knudsen equation), thermodynamic-equilibrium conditions (HEM), bubble-dynamics considerations using the Zwart-Gerber-Belamri model (ZGB), as well as semi-empirical correlations calibrated specifically for flash boiling (HRM). Benchmark geometrical layouts, i.e a converging-diverging nozzle, an abruptly contracting (throttle) nozzle and a highly-pressurized pipe, for which experimental data are available in the literature have been employed for the validation of the numerical predictions. Consideration on additional aspects associated with phase-change processes, such as the distribution of activated nucleation sites, as well as the deviation from thermodynamic-equilibrium conditions have also been taken into account. The numerical results have demonstrated that the onset of flashing flow in all cases is associated with the occurrence of compressible flow phenomena, such as flow choking at the constriction location and expansion downstream, accompanied by the formation of shockwaves. Phase-change models based on the kinetic theory of gases produced more accurate predictions for all the cases investigated, while the validity of the HRM and ZGB models was found to be situational. Furthermore, it has been established that the inter-dependence between intrinsic physical factors associated with flash boiling, such as the nucleation-site density and the phase-change rate, has a significant, yet not clearly distinguishable influence on the two-phase flow characteristics
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Topology and distinct features of flashing flow in an injector nozzle
The effect of thermodynamic non-equilibrium conditions (liquid superheat) on the two-phase flow field developing inside an axisymmetric, single-orifice nozzle is numerically investigated by means of different variations of a two-phase mixture model. A number of "hybrid" mass-transfer models that take into account both the effect of inertial forces (cavitation) and liquid superheat have been proposed and evaluated against widely used, pure-cavitation models, in order to pinpoint the flow conditions necessary for flash boiling to occur and to elucidate the distinct features of the phase and velocity fields that characterize flashing flows. The effect of the number of nucleation sites, required as an input by the models, on the developing two-phase flow has also been looked into. The numerical results have shown that incorporation of an additional term corresponding to liquid superheat into the mass-transfer rate leads to increased evaporation rate, compared to pure-cavitation models with liquid vaporization taking place within the entire nozzle cross section. The cavitation nucleation sites have been confirmed to act as the necessary flow perturbations required for flash boiling to occur. In addition, the developing velocity field has been found to be in close correlation to the mass-transfer rate imposed. It has been established that increased liquid evaporation leads to choked-flow conditions prevailing in a larger part of the nozzle and accompanied by a more significant expansion of the two-phase mixture downstream of the injector exit that results to increased jet cone angle. Finally, the results demonstrated that liquid cooling due to the increased mass-transfer rate is not significant within the nozzle and thus consider that a constant liquid temperature produces adequately accurate results with a decreased computational cost
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Simulating the effect of in-nozzle cavitation on liquid atomisation using a three-phase model
The aim of this article is to present a fully compressible three-phase (liquid, vapor, air, and mixture) cavitation model and its application to the simulation of in-nozzle cavitation effects on liquid atomization. The model employs a combination of barotropic cavitation model with an implicit sharp interface capturing Volume of Fluid (VoF) approximation. The results from the simulation are compared against the experimental results obtained by (1) for injection of water into the air from a stepped nozzle. Large Eddy Simulation (LES) model is utilized for resolving turbulence. Simulations are performed for a condition where developing cavitation is observed. Model validation is achieved by qualitative comparison against the available images for the cavitation, spray pattern. The model predictions suggest that the experimentally observed void inside the nozzle is not purely vapor, but a mixture of both vapor and back-flowing air. The simulation also identified periodic air entrainment that occurs at developing cavitation condition which further improves primary atomization
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Effect of secondary flows due to buoyancy and contraction on heat transfer in a two-section plate-fin heat sink
The effect of buoyancy forces on laminar heat transfer inside a variable width plate-fin heat sink is numerically analyzed: the configuration under investigation comprises an array of rectangular fins, the number of which is doubled at the streamwise middle length of the plate, leading to a stepwise reduction in the respective channel width and hydraulic diameter. The mixed convection problem is thoroughly examined for Archimedes numbers in the range Ar = 1.32-5.82 and Reynolds numbers, based on the channel hydraulic diameter before the stepwise reduction, in the range Re = 559-667, under the thermal boundary condition of axially constant heat flux. It is illustrated that the secondary flow pattern emanating from the flow contraction and manifested through the presence of a pair of counter-rotating horseshoe vortices and a pair of counter-rotating (fin) sidewall vortices interacts with longitudinal rolls created by buoyancy forces. In fact, the lower horseshoe vortices that are co-rotating with the buoyancy-induced rolls are significantly enhanced in magnitude and cause intense fluid mixing in the vicinity of the channel bottom wall, with a substantial distortion of the temperature field. The numerical results indicate that the joint action of the buoyancy-induced rolls and the combined secondary flow pattern has a beneficial impact on the heat sink thermal performance, a fact quantified through the circumferentially-averaged local Nusselt number distributions. The effect of the top lid thermal conductivity on the heat transfer inside the heat sink is also discussed. Finally, a comparative investigation is conducted between the present variable-channel-width configuration and two configurations of fixed-width heat sink designs. The comparative results reveal that the introduction of stepwise channels leads to superior heat transfer performance, i.e. lower values of the total thermal resistance with mitigated pressure drop penalty and increased temperature uniformity on the cooled surface
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Multi-objective design optimization of a micro heat sink for Concentrating Photovoltaic/Thermal (CPVT) systems using a genetic algorithm
An optimization methodology for a microchannel, plate-fin heat sink suitable for the cooling of a linear parabolic trough Concentrating Photovoltaic/Thermal (CPVT) system is applied in this study. Two different microchannel configurations are considered, Fixed (FWÎĽ) and stepwise Variable-Width (VWÎĽ) microchannels respectively. The performance evaluation criteria comprise the thermal resistance of the heat sink and the cooling medium pressure drop through the heat sink. Initially, the effect of the geometric parameters on the heat sink thermal and hydrodynamic performance is investigated using a thermal resistance model and analytical correlations, in order to save computational time. The results of the 1-D model enable the construction of surrogate functions for the thermal resistance and the pressure drop of the heat sink, which are considered as the objective functions for the multi-objective optimization through a genetic algorithm that leads to the optimal geometric parameters. In a second step, a 3-D numerical model of fluid flow and conjugate heat transfer for the optimized FWÎĽ heat sink is developed in order to investigate in detail the flow and thermal processes. The overall analysis demonstrates that microchannel heat sinks achieve very low values of thermal resistance and that the use of variable-width channels can significantly reduce the pressure drop of the cooling fluid. Furthermore, it is proven that the 1-D model is capable of providing a good estimate of the behavior of the heat sink
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Design and experimental evaluation of a parabolic-trough concentrating photovoltaic/thermal (CPVT) system with high-efficiency cooling
The design and performance evaluation of a novel parabolic-trough concentrating photovoltaic/thermal (CPVT) system are discussed in the present study. Initially, the system design and manufacturing procedures as well as the characteristics of the system sub-components are thoroughly illustrated. At a second stage, the findings in regard to the optical quality of the parabolic trough are presented, as obtained through an experimental procedure that utilizes a custom-made measuring device. The device bears a grid of sensors (photodiodes), so that the irradiation distribution on the receiver surface and the achieved concentration ratio can be determined. Besides, the main factors that have a significant effect on the trough optical quality were identified through ray-tracing simulations. The system electrical and thermal performance was subsequently evaluated in a test rig specially developed for that reason. Three variations of the system receiver incorporating different PV-module and heat-sink designs were evaluated and the prototype CPVT system was found to achieve an overall efficiency approximately equal to 50% (44% thermal and 6% electrical efficiencies, respectively) mainly limited by the trough optical quality, however with a very weak dependency on the operating temperature
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A tabulated data technique for cryogenic two-phase flows
Flashing flows of liquid oxygen (LOX) are prevalent in space applications, where LOX can be used as rocket engine propellant [1]–[3]. Towards this direction, the cryogenic flow in a converging-diverging nozzle has been investigated in the present study by utilising real fluid thermodynamics under the homogeneous equilibrium mixture (HEM) assumption for the LOX. A tabulated data method for the Helmholtz energy equation of state (EoS) has been developed in OpenFOAM (OF) [4] and has been incorporated into an explicit density based solver. Due to the wide variation of the speed of sound and consequently of the Mach number noticed in the liquid, vapour and mixture phases, a Mach consistent numerical flux has been employed suitable for subsonic up to supersonic flow conditions [5]. Since the Helmholtz EoS is computationally inefficient compared to simplified EoS, an ad-hoc thermodynamic table containing all the thermodynamic properties for the LOX has been created and stored prior entering the time loop [6], accompanied by a static linked-list algorithm for reducing the search time. Once the thermodynamic element of the table which satisfies the values of the density and internal energy as predicted from the numerical solution of the Navier-Stokes equations is identified, the unknown thermodynamic properties are approximated by a finite element interpolation [7]. The numerical method has been firstly validated against the Riemann problem at similar cryogenic flow conditions. Then, 2-D axisymmetric simulations of the phase-change process in a converging-diverging nozzle are performed and compared with prediction from other numerical tools as well as experimental data. It is concluded that the results are satisfactory while the applicability of the Helmholtz EoS to LOX simulations is demonstrated. This suggests that the proposed methodology can be utilized for the simulation of flashing flows