Fluid Inlet Velocity Research Articles

Electrical and thermal performance of silicon concentrator solar cells immersed in dielectric liquids

Direct liquid-immersion cooling of concentrator solar cells was proposed as a solution for receiver thermal management of concentrating photovoltaic (CPV) and hybrid concentrating photovoltaic thermal (CPV-T) systems. De-ionized (DI) water, isopropyl alcohol (IPA), ethyl acetate, and dimethyl silicon oil were selected as potential immersion liquids based on optical transmittance measurement results. Improvements to the electrical performance of silicon CPV cells were observed under a range of concentrations in the candidate dielectric liquids, arising from improved light collection and reduced cell surface recombination losses from surface adsorption of polar molecules. Three-dimensional numerical simulations with the four candidate liquids as the working fluids, exploring the thermal performance of a silicon CPV cell array in a liquid immersion prototype receiver, have been performed. Simulation results show that the direct-immersion cooling approach can maintain low and uniform cell temperature in the designed liquid immersion receiver. The fluid inlet velocity and flow mode, along with the fluid thermal properties, all have a significant influence on the cell array temperature.

Numerical simulation and optimization of single-phase turbulent flow in chevron-type plate heat exchanger with sinusoidal corrugations

The thermal-hydrodynamic characteristics of a single-phase turbulent flow in chevron-type plate heat exchangers with sinusoidal-shaped corrugations have been numerically investigated in this article. The computational domain contains a corrugation channel, and the simulations adopted the shear-stress transport κ-ω model as the turbulence model. The numerical simulation results in terms of Nusselt number and friction factor were compared with limited experimental data and existing correlations in order to verify the accuracy of the numerical model. The validation was performed using single-phase water as the working fluid. A bi-objective optimization was carried out using an approximation-assisted optimization technique, which provided considerable computational savings to ensure the feasibility of optimization. The corrugation depth, corrugation angle, corrugation pitch, and fluid inlet velocity were identified as design variables, and 200 samples were selected using the maximum entropy design method to build the metamodel for obtaining the heat transfer coefficient as well as the pressure drop per unit length. A multi-objective genetic algorithm was utilized as the optimizer. The optimization results were presented in the form of Pareto solution set, which clearly showed its dominance over the entire design space and the tradeoff between the two optimization objectives: maximizing heat transfer coefficient and minimizing drop per unit length. Also, the Pareto optimal designs were validated against the values directly obtained from numerical simulations. The approximation-assisted optimization shows that all optimal designs have largest enlargement factor values inside the design space, and the optimal corrugation angle increases with the increase of maximum heat transfer coefficient.

Fabrication of Microfluidic Rapid Micromixer

A microfluidic multi-cylindric rapid micromixer is fabricated in the present paper. The key features in the presented MEMS-based microchannel design are (1) micro pump (2) Y-junction type channel (3) cylindric obstacle (4) notch with the edge of sharp teeth. Two different fluids (DI water and red ink) were pumped and injected into Y-type channel, and the fluids were broken-up by a cylindric obstacle in the center of tapered microchannel. The chaotic convection occurs in the mixing channel behind the cylindric obstacle. The mixing index is defined to qualify the mixing efficiency, which demonstrates the outlet notch with sharp teeth along the sidewall plays an important role for mixing effects. The developed micromixer can enhance mixing using the mechanisms of diffusion and convection for wide range of Reynolds number (0.01<Re<100). Parametric studies for volumetric flow rate include the number of cylindric obstacles, the number of notches with sharp-teeth and the width of microchannel. Preliminary results demonstrate that the mixing index reaches the desired effect (<0.1) within 0.08 second when the inlet fluid velocity is 0.49992m/s, i.e. volumetric flow rate is 1200μl /min. The presented device is faster than most of reported micromixers.

Effect of micro-channel geometry on fluid flow and mixing

Understanding the flow fields at the micro-scale is key to developing methods of successfully mixing fluids for micro-scale applications. This paper investigates flow characteristics and mixing efficiency of three different geometries in micro-channels. The geometries of these channels were rectangular with a dimension of; 300 μm wide, 100 μm deep and 50 mm long. In first channel there was no obstacle and in the second channel there were rectangular blocks of dimension 300 μm long and 150 μm wide are placed in the flow fields with every 300 μm distance attaching along the channel wall. In the third geometry, there were 100 μm wide fins with 150° angle which were placed at a distance of 500 μm apart from each other attached with the wall along the 50 mm channel. Fluent software of Computational Fluid Dynamics (CFD) was used to investigate the flow characteristics within these microfluidic model for three different geometries. A species 2D model was created for three geometries and simulations were run in order to investigate the mixing behaviour of two different fluid with viscosity of water (1 mPa s). Models were only built to investigate the effect of geometry, therefore only one fluid with similar viscosity was used in these models. Velocity vector plots were used in the CFD analysis to visualise the fluid flow path. Mass fractions of fluid were used to analyse the mixing efficiency. Two different colours for water were used to simulate the effect of two different fluids. The results showed that the mixing behaviour strongly depended on the channel geometry when other parameters such as fluid inlet velocity, viscosity and pressure of fluids were kept constant. In two geometries lateral pressure and swirling vortexes were developed which provided better mixing results. Creation of swirling vortexes increased diffusion gradients which enhanced diffusive mixing.

Fluid–structure interaction analysis of flow and heat transfer characteristics around a flexible microcantilever in a fluidic cell

This study analyzes the effect of the flow conditions and the geometric variation of the microcantilever’s bluff body on the microcantilever detection capabilities within a fluidic using a finite element fluid–structure interaction (FSI) model. Periodic steady-state results of the current investigation show that the magnitude of the inlet fluid velocity, elasticity of the microcantilever, random noise, and the height of the bluff body has respective profound effect on deflection of the microcantilever. Low inlet fluid velocity condition exhibits no vortices around the microcantilever. However, the introduction of a random noise in the fluidic cell may cause the microcantilever to oscillate in a harmonic mode at low velocity. The results of this study show that microcantilevers excite earlier for large height compared with smaller heights of the bluff body at high inlet fluid velocity. This work paves the road for researchers in the area microcantilever to design efficient microcantilevers that display least errors in the measurements.

Research on Porous Micro Heat Sink for Thermal Management of High Power LED

In view of the characteristics of high power light-emitting diode(LED),such as high heat fluxes and requirement of strict temperature control,a porous micro heat sink system is presented for thermal management of high power LED,which has high heat transport capability.The working principle and heat transfer characteristics of porous micro heat sink are analyzed.Numerical model for the micro heat sink is proposed to describe liquid flow and heat transfer based on the local thermal equilibrium of porous medium,and it is solved with SIMPLE algorithm.The numerical results show that the heated surface temperature of porous micro heat sink is low at high heat fluxes and the maximal temperature of heated surface is only 55.2 ℃ at the heat flux of 200 W/cm2.Increasing inlet fluid velocity can decrease the temperature in the micro heat sink and the heated surface temperature.The micro heat sink has good performance for the cooling of high power LED,and it can improve the reliability and life of LED.The numerical results obtained are useful for performance optimization and design of porous micro heat sink.

A prediction of cell differentiation and proliferation within a collagen–glycosaminoglycan scaffold subjected to mechanical strain and perfusive fluid flow

Mesenchymal stem cell (MSC) differentiation can be influenced by biophysical stimuli imparted by the host scaffold. Yet, causal relationships linking scaffold strain magnitudes and inlet fluid velocities to specific cell responses are thus far underdeveloped. This investigation attempted to simulate cell responses in a collagen–glycosaminoglycan (CG) scaffold within a bioreactor. CG scaffold deformation was simulated using μ-computed tomography (CT) and an in-house finite element solver (FEEBE/linear). Similarly, the internal fluid velocities were simulated using the afore-mentioned μCT dataset with a computational fluid dynamics solver (ANSYS/CFX). From the ensuing cell-level mechanics, albeit octahedral shear strain or fluid velocity, the proliferation and differentiation of the representative cells were predicted from deterministic functions. Cell proliferation patterns concurred with previous experiments. MSC differentiation was dependent on the level of CG scaffold strain and the inlet fluid velocity. Furthermore, MSC differentiation patterns indicated that specific combinations of scaffold strains and inlet fluid flows cause phenotype assemblies dominated by single cell types. Further to typical laboratory procedures, this predictive methodology demonstrated loading-specific differentiation lineages and proliferation patterns. It is hoped these results will enhance in-vitro tissue engineering procedures by providing a platform from which the scaffold loading applications can be tailored to suit the desired tissue.

A finite element study of mechanical stimuli in scaffolds for bone tissue engineering

Mechanical stimuli are one of the factors that affect cell proliferation and differentiation in the process of bone tissue regeneration. Knowledge on the specific deformation sensed by cells at a microscopic level when mechanical loads are applied is still missing in the development of biomaterials for bone tissue engineering. The objective of this study was to analyze the behavior of the mechanical stimuli within some calcium phosphate-based scaffolds in terms of stress and strain distributions in the solid material phase and fluid velocity, fluid pressure and fluid shear stress distributions in the pores filled of fluid, by means of micro computed tomographed (CT)-based finite element (FE) models. Two samples of porous materials, one of calcium phosphate-based cement and another of biodegradable glass, were used. Compressive loads equivalent to 0.5% of compression applied to the solid material phase and interstitial fluid flows with inlet velocities of 1, 10 and 100 μm/s applied to the interconnected pores were simulated, changing also the inlet side and the viscosity of the medium. Similar strain distributions for both materials were found, with compressive and tensile strain maximal values of 1.6% and 0.6%, respectively. Mean values were consistent with the applied deformation. When 10 μm/s of inlet fluid velocity and 1.45 Pa s viscosity, maximal values of fluid velocity were 12.76 mm/s for CaP cement and 14.87 mm/s for glass. Mean values were consistent with the inlet ones applied, and mean values of shear stress were around 5×10 −5 Pa. Variations on inlet fluid velocity and fluid viscosity produce proportional and independent changes in fluid velocity, fluid shear stress and fluid pressure. This study has shown how mechanical loads and fluid flow applied on the scaffolds cause different levels of mechanical stimuli within the samples according to the morphology of the materials.

Multiphase flow model to study channel flow dynamics of PEM fuel cells: deformation and detachment of water droplets

Deformation and detachment conditions for a water droplet adhering to the surfaces of the cathode in PEM fuel cells are investigated. This problem is numerically modelled by considering the dynamic behaviour of a droplet located between two parallel plates. The numerical multiphase flow model is based on solving Navier–Stokes equations for Newtonian fluids in the channel. The study includes the effect of interfacial forces with constant surface tension. The volume-of-fluid method is used to numerically determine the deformation of free surfaces. Water droplet and channel fluid properties, inlet velocity and droplet size determine whether a droplet can separate from the surface or just deforms and remains stationary. The critical conditions for the droplet break-up are determined based on the Reynolds and Capillary numbers. Effects of droplet size relative to the channel width, density and viscosity ratios between the water droplet and oxidant fluid are addressed.

Modeling evaluation of the fluid‐dynamic microenvironment in tissue‐engineered constructs: A micro‐CT based model

Natural cartilage remodels both in vivo and in vitro in response to mechanical stresses, hence mechanical stimulation is believed to be a potential tool to modulate extra-cellular matrix synthesis in tissue-engineered cartilage. Fluid-induced shear is known to enhance chondrogenesis in engineered cartilage constructs. The quantification of the hydrodynamic environment is a condition required to study the biochemical response to shear of 3D engineered cell systems. We developed a computational model of culture medium flow through the microstructure of a porous scaffold, during direct- perfused culture. The 3D solid model of the scaffold micro-geometry was reconstructed from 250 micro-computed tomography (micro-CT) images. The results of the fluid dynamic simulations were analyzed at the central portions of the fluid domain, to avoid boundary effects. The average, median and mode shear stress values calculated at the scaffold walls were 3.48, 2.90, and 2.45 mPa respectively, at a flow rate of 0.5 cm(3)/min, perfused through a 15 mm diameter scaffold, at an inlet fluid velocity of 53 microm/s. These results were compared to results estimated using a simplified micro-scale model and to results estimated using an analytical macro-scale porous model. The predictions given by the CT-based model are being used in conjunction with an experimental bioreactor model, in order to quantify the effects of fluid-dynamic shear on the growth modulation of tissue-engineered cartilage constructs, to potentially enhance tissue growth in vitro.

Transient temperature and thermally induced stress distributions in a partly-circumferentially heated cylindrical workpiece

This paper presents the numerical solutions of the transient temperature and thermally induced stress distributions in a partly-circumferentially heated cylindrical hollow workpiece (steel) with conjugate heat transfer. Outer surface of the workpiece is heated partly-circumferentially heat flux as its remainder outer surface is circumferentially cooled with fluid (water). Three phenomena have been considered as; (1) conduction inside the cylinder, (2) convection from the cylinder surface to the surrounding fluid, and (3) thermal stress produced by high temperature gradient inside the cylinder. The governing flow and energy equations have been solved numerically by using a control volume approach. The PHOENICS 3.2 and HEATING7 computer codes have been used for the numerical evaluation. The transient calculations have been performed individually for four fluid inlet velocities, ui = 0.005, 0.01, 0.015 and 0.020 m/s, until the system attains steady-state. The results of this study clearly demonstrate that the temperature contours in the low inlet velocity cases are more near to a symmetric case with respect to the y = 0 plane than that in the high inlet velocity cases, and the increment of the inlet velocity exponentially reduces the temperatures and thermally induced stresses in the workpiece. The effective thermal stress differences occurring in the workpiece can be significantly reduced by the high fluid inlet velocity.

Mathematical Analysis of the Conditions Necessary for the Appearance of the “Overshoot” Phenomenon in Countercurrent Packed Columns

This paper analyzes the model describing the transient behavior of a countercurrent packed-bed column exposed to a change of the inlet fluid flow rate. Functional mathematical analysis of the model equations is employed to establish the conditions that would lead to the overshoot phenomenon of liquid holdup in the column undergoing a sudden change of the inlet fluid velocity. It has been proven that a mere nonlinear relationship between the liquid holdup and liquid velocity is not a sufficient condition for the overshoot. The liquid holdup overshoot can appear if the liquid holdup is also a function of the gas velocity, if the implemented step change of either the gas or liquid velocity is to a higher velocity, and if the higher value of the velocity is maintained for a sufficiently long time.

Modeling a turbulent fibre suspension flowing in a planar contraction: The one-dimensional headbox

An Eulerian model of a turbulent fibre suspension flowing through a planar contraction is proposed to predict the fibre orientation distribution. The model specifically applies to the flow of pulp suspensions through a papermachine headbox. The model accounts for the convection of orientation distribution of the mean fluid velocity and the dispersion caused by the turbulent velocity fluctuations and describes each by a dimensionless dispersion coefficient and a dimensionless velocity gradient characterized by the contraction ratio (ratio of duct inlet height to outlet height) of the duct. A numerical solution of the model equations was shown to compare well to two different experimental studies available in the literature. From comparison with experiment the rotational dispersion coefficient was estimated to be 2 s −1 for both studies. Using the experimental value of the rotational dispersion coefficient and assuming it remains constant, the fibre orientation distributions was calculated for a range of dimensionless dispersion coefficients and contraction ratios. It was demonstrated that increasing the contraction ratio, over the industrial range of 5–50, significantly increases the alignment of fibres exiting the headbox. It was also shown that varying the inlet fluid velocity, over a practical range, provides only small changes in the fibre orientation distribution. Both predicted effects agree with experimental observation. Furthermore, this study suggests that fibre orientation is approximately dependent on a single dimensionless Peclet number that is a function of the fluid contraction ratio, inlet velocity, contraction length and rotational dispersion coefficient and that this parameter can be used to guide the design and operation of commercial headboxes.

Boiling two-phase flow under microgravity

To study the effects of reduced gravity on the flow regime and the heat transfer characteristics of a boiling two-phase horizontal flow, parabolic flight experiments were performed by using an aircraft. The gravity level during the parabolic flight and the duration time were about −0.01 g e ∼ +0.01 g e and 20 s, respectively. Under earth gravity, many small bubbles are detached very frequently from the heater rod surface, flowing into the upper stream due to the buoyancy and resulting in a stratified flow in the cases of lower inlet fluid velocity and higher heat flux. Under microgravity conditions, bubbles are hardly detached from the heater rod, growing and coalescing to become much larger along the heater rod, surrounding the heater rod in the downstream. This tendency was more noticeable in the cases of lower inlet fluid velocity, higher heat flux and lower inlet fluid subcooling. The local heat transfer coefficients at the bottom of the heater rod tend to decrease slightly under microgravity compared with those under earth gravity because of the reduction of the heat removal due to natural convection. On the other hand, the local heat transfer coefficients at the top of the heater rod tend to increase slightly under microgravity. However, the differences of the local heat transfer coefficients are very small in spite of large differences of the flow regimes under earth gravity and microgravity.

Thermocline Degradation in a Packed Bed Thermal Storage Tank

A time-dependent mathematical analysis of the fluid and solid temperature profiles of the bed during holding periods when no new fluid enters the tank was developed for the special case where the inlet fluid velocity is zero. The analysis incorporates the additional effects of thermal conduction and finite tank length.