Wall Of Flow Channel Research Articles

Distribution parameter and drift velocity for upward gas-liquid metal two-phase flow

In view of the needs in the developments of the fast-neutron nuclear reactor, the accelerator driven nuclear reactor system, the fusion reactor and so on, this study has conducted an extensive literature survey on the past experimental and theoretical studies of the upwardly-moving gas-liquid metal two-phase flows in the vertical flow channels with various channel geometries. The experimental data of the upward gas-liquid metal two-phase flows taken in the N2-Hg (nitrogen and mercury), N2-Pb/Bi (nitrogen and lead/bismuth eutectic alloy), N2-Ga (nitrogen and gallium) and N2-Na/K(nitrogen and sodium/potassium eutectic alloy) mixtures in the vertical circular, annular and rectangular flow channels and seven available drift-flux type correlations used for the void fraction predictions in the gas-liquid metal two-phase flows have been collected. The performances of these seven drift-flux type correlations have been checked against the collected experimental data, and no available drift-flux type correlation is able to predict well the void fraction of the gas-liquid metal two-phase flows with low gas-to-liquid density ratios. So, this study has analyzed the effects of the two-phase density ratio, the flow channel size and shape, the local two-phase flow conditions and the wettability between the liquid metal and the flow channel wall surface on the two-phase flow behaviors and has proposed general mathematical expressions for the distribution parameter and the drift velocity. By fitting the experimental data, a new constitutive correlation for the distribution parameter and the drift velocity consisting of two flow-regime-independent correlation sets, which are, respectively, for the gas-liquid metal two-phase flows with the low and high wettability between the liquid metal and the flow channel wall surface, has been developed. The newly-developed drift-flux type correlation has been evaluated with the collected experimental data of the gas-liquid metal two-phase flows, and their mean relative error is 0.109. The predicted mean relative errors of the low and high wettability drift-flux type correlation sets are, respectively, 0.132 and 0.0782 for the gas-liquid metal two-phase flows with the low and high wettability between the liquid metal and the flow channel wall surface.

Flow modeling and experimental verification of flow resistors used in microfluidic chips driven by capillary force

A microfluidic chip driven by capillary force has the advantage of low cost and ease of manufacturing in batches, and its flow behavior is dominated by the geometry and surface characteristics of the microchannel. The design of mirochannel structures is very important for the microfluidic chips. This study presents a semi analytical method for the design of capillary microchannel. In this method, the quasi-steady state approximate solution method of the Young–Laplace equation is used to derive the capillary surface tension, and the parallel flow assumption based on the Reynolds equation is used to derive the resistance of the channel wall. A computational fluid dynamics simulation is used to provide the inlet effect coefficient and channel shape effect coefficient of this model. The availability of the semi analytical model is verified by the experiment. This model realizes the flow analysis of two-dimensional capillary flow channel with a continuous shape of the flow channel wall, providing a fast and accurate method for the structural design of the microfluidic chip driven by capillary force.

Cold wall effects of control rod guide tubes and experimental flow channel walls

In rod bundle experiments, coarse flow housing walls are used to provide flow boundaries for the working fluid. In the PWR (Pressurized Water Reactor) reactor core, the guide tubes are needed to provide channels for control rods. The non-prototypical flow channel walls and guide tubes have similar cold wall effects on the thermal and hydraulic conditions of the surrounding flow channels. They might influence the local temperature, quality and mass flux distributions, which could have potential impacts on local mass and heat transfer characteristics. For example, when the flow resistance in the boiling hot channel increases due to steam generation, the adjacent cold channels around the guide tube can bypass the coolant and lead to the escalation of flow starvation in the steaming hot channel, which might ultimately lead to the early occurrence of a boiling crisis. In the rod bundle experiment, in order to ensure the representation of the CHF experiments, it is important to minimize the effects of the non-prototypical flow channel walls.In this paper, the wall effects of flow channel walls in a typical 5 × 5 rod bundle CHF test configuration have been minimized by optimizing the rod-to-wall gap (the gap between the outer rod wall and the channel inner wall design) through analytical analysis, CFD modeling and subchannel code calculation. This study concluded that the proper rod-to-wall gaps in the thermal field are often different from that in the hydraulic field. Therefore reasonable weight factors have been proposed to incorporate different impacts from these two fields. In order to determine proper design of rod-to-wall gap, the average temperature criterion, average mass flux criterion and maximum wall temperature criterion have been examined. Similarly, the cold wall effects of guide tube have been investigated using the theoretical model developed for the thermal field.

A numerical model for estimation of water droplet size in the anode channel of a proton exchange membrane fuel cell

Abstract The effects of working conditions of the proton exchange membrane fuel cell (PEMFC) on the amount of produced water on the walls of the anode gas flow channel (GFC) were numerically investigated. For this purpose, a single-region, steady, two-dimensional, along-the-channel and non-isothermal model was used. Results showed quantitatively that by increasing the working temperature (Tcell), decreasing the relative humidity at the anode GFC inlet (RHa), increasing the current density (i) and increasing the hydrogen stoichiometry ratio (SH2), the amount of produced water is reduced and consequently, water flooding is prevented. The numerical results also demonstrated that how the size of water droplet increases along the channel. In fact the increase in the size of the droplets with various working conditions were investigated and it was shown that RHa, Tcell and SH2 are the most effective in increasing the droplet size while the current density is the least responsible for this matter. Finally, a relationship for the size of the produced water droplets in the anode GFC as a function of the working parameters of a PEMFC was proposed which is applicable for the water management in the anode GFC.

Comparisons of Experimental and Simulated Velocity Fields in Membrane Module Spacers

Spacers are used in spiral wound and plate and frame membrane modules to create flow channels between adjacent membrane layers and mix fluid within the flow channel. Flow through the spacer has a significant beneficial impact on mixing and resulting mass transfer rates but is accompanied by an undesirable increase in pressure drop. Computational Fluid Dynamics (CFD) is a common tool used to evaluate the effect of spacer design on fluid flow. While numerous theoretical studies are reported in the literature, confirmation of simulation results through experimental velocity field measurements is limited. Comparisons of CFD simulations with experimental velocity measurements using Particle Image Velocimetry (PIV) for traditional symmetric diamond and asymmetric spacer designs and a novel static mixing spacer design are presented. The results include comparisons of the two velocity components in planes parallel to the flow channel walls for the diamond and asymmetric spacer as well as the first reported comparisons of all three velocity components for the static mixing spacer. The results indicate good agreement between theory and experiment and help validate the use of CFD for spacer design.

Experimental and theoretical investigations of two-phase flow in low pressure steam–water injector

Unlike the typical steam–water injector, the device investigated here operates with relatively low pressure motive steam which is expanded in convergent nozzle to critical (sonic) velocity only. Experimental test were conducted with motive steam pressure in the range of 62–130 kPa (abs), superheated up to 30 K. Distributions of pressure and temperature along the injector’s mixing chamber and diffuser were recorded for selected inlet steam and water flow rates and outlet backpressures. The pressure was measured not only along the flow channel walls but also at the centerline near the steam nozzle outlet plane. Maximum backpressure for which the injector works stably was also determined for various inlet flow parameters. In theoretical part of the paper, control volume model was applied to predict the flow parameters in key cross-sections of the injector working with maximum discharge pressure. The proposed model results were compared with the measurements and good prediction of the outlet pressure and temperature was achieved.

Quantitative Evaluation of Disorders of the Swirled Blood Flow Structure in the Aorta with Pathological Alteration of Its Channel Geometry Using Numerical Simulation of the Aorta

It is now an established fact that blood flow in the heart and the major vessels has normally a helical pattern. The structure of this flow is described by the exact solution of the nonstationary hydrodynamics equations for a class of self-organized tornado-like jets of viscous fluids. Our previous studies have shown that the geometry of the aortic channel corresponds to streamlines of this class throughout the cardiac cycle, which is supported by a special distribution of viscoelastic properties of the flow channel wall along the aorta. As a result of age-related pathological processes and/or their surgical correction, normal viscoelastic characteristics of vessel walls inevitably become impaired. A diversity of impairments in the structure of the cardiovascular system causes negative alterations in the normal geometrical configuration of biological flow channels. Thus, any pathological alterations in vessel channels are accompanied by deteriorations in the structural parameters of blood flow. However, a direct experimental study of all the diversity of vessel lesions is associated with considerable complications. Thus, no studies have been conducted so far to identify the regularities of alterations in the structural parameters of a swirling blood flow in different pathological conditions of the vessel channel. In this study, we have created a physical model of the flow channel of the aorta, which was based on the nonlinear distribution of transverse elasticity in the walls of the aorta along its length. Further, we created a computer simulation of the constructed model flow channel dynamics throughout the cardiac cycle in correspondence with the standard curve for blood pressure changes. The simulation results were interpreted within the semi-empirical theory of self-organized tornado-like jets of viscous fluids. This interpretation allowed us to formulate the time-dependent quantitative characteristics establishing functional relations between the structure of blood flow and the geometry of the flow channel of the aorta throughout the cardiac cycle.

Prediction of critical heat flux on surface under boiling in case of IVR-ERVC by using CFD simulation

The efficacy of in-vessel through external reactor vessel cooling (IVR-ERVC) is strongly correlated with the critical heat flux (CHF) on the lower head of reactor pressure vessel (RPV). The high limit of CHF prevents the RPV from failure by extracting much heat out of the vessel through the circulation of working fluid in external flow channel. In present study, the deionized water is allowed to flow through curved flow channel having radius of curvature of 368mm and hydraulic diameter of 40mm with square cross-section. The heat flux of 1500kWm−2 is applied perpendicular to the convex wall of the channel. The CHF on the wall of flow channel has been predicted by incorporating CFD based two-phase (liquid-vapor) boiling model. The investigations on spatial variation of pressure, temperature, and velocity and heat flux acquired as a result of numerical simulations have been discussed in detail. The CHF and average HTC is predicted as 1.798e+04kWm−2 and 38.981kWm−2K−1 respectively.

Direct determination of anaerobe contributions to the energy metabolism of Trypanosoma cruzi by chip calorimetry.

We describe a chip calorimetric technique that allows the investigation of biological material under anoxic conditions in a micro-scale and in real time. Due to the fast oxygen exchange through the sample flow channel wall, the oxygen concentration inside the samples could be switched between atmospheric oxygen partial pressure to an oxygen concentration of 0.5% within less than 2h. Using this technique, anaerobic processes in the energy metabolism of Trypanosoma cruzi could be studied directly. The comparison of the calorimetric and respirometric response of T. cruzi cells to the treatment with the mitochondrial inhibitors oligomycin and antimycin A and the uncoupler FCCP revealed that the respiration-related heat rate is superimposed by strong anaerobic contributions. Calorimetric measurements under anoxic conditions and with glycolytic inhibitors showed that anaerobic metabolic processes contribute from 30 to 40% to the overall heat production rate. Similar basal and antimycin A heat rates with cells under anoxic conditions indicated that the glycolytic rates are independent of the oxygen concentration which confirms the absence of the "Pasteur effect" in Trypanosomes. Graphical abstract.

Experimental Investigation of Natural Circulation During Air-Ingress Scenario in a Very High Temperature Reactor

In order to investigate air-ingress phenomena in a gas-cooled very high temperature reactor (VHTR), natural circulation experiments have been conducted in a helium flow loop after the injection of nitrogen into the lower plenum. A pair of helium analyzers were used to measure the nitrogen and helium concentrations in the lower plenum and upper plenum. The changes in the nitrogen concentration in the upper plenum were used to calculate the time required for the transport of nitrogen from the lower plenum to upper plenum through a riser flow channel made of graphite. The effect of system temperature and pressure on the rate of nitrogen transport has been studied extensively. Furthermore, a close examination of the graphite flow channel wall temperatures at different elevations showed small but sudden drops indicating the arrival of nitrogen at each elevation. From these data, the upward transport of nitrogen injected into the lower plenum under natural circulation conditions could be quantitatively investigated. The experimental findings indicate that the driving mechanisms for air transport through the reactor core of VHTR would result from both molecular diffusion and natural circulation. At low graphite temperatures in the riser, molecular diffusion is the dominating mechanism; however, as the riser temperature increases, natural circulation becomes dominant and the rate of nitrogen transport increases. Further, the time constants for these mechanisms have been calculated using a simplified species transport equation.

Self-Immobilizing Biocatalysts Maximize Space–Time Yields in Flow Reactors

Maximizing space–time yields (STY) of biocatalytic flow processes is essential for the establishment of a circular biobased economy. We present a comparative study in which different biocatalytic flow reactor concepts were tested with the same enzyme, the (R)-selective alcohol dehydrogenase from Lactobacillus brevis (LbADH), that was used for stereoselective reduction of 5-nitrononane-2,8-dione. The LbADH contained a genetically encoded streptavidin (STV)-binding peptide to enable self-immobilization on STV-coated surfaces. The purified enzyme was immobilized by physisorption or chemisorption as monolayers on the flow channel walls, on magnetic microbeads in a packed-bed format, or as self-assembled all-enzyme hydrogels. Moreover, a multilayer biofilm with cytosolic-expressed LbADH served as a whole-cell biocatalyst. To enable cross-platform comparison, STY values were determined for the various reactor modules. While mono- and multilayer coatings of the reactor surface led to STY < 10, higher productivity was achieved with packed-bed reactors (STY ≈ 100) and the densely packed hydrogels (STY > 450). The latter modules could be operated for prolonged times (>6 days). Given that our approach should be transferable to other enzymes, we anticipate that compartmentalized microfluidic reaction modules equipped with self-immobilizing biocatalysts would be of great utility for numerous biocatalytic and even chemo-enzymatic cascade reactions under continuous flow conditions.

Systematic study of packaging designs on the performance of CMOS thermoresistive micro calorimetric flow sensors

We systematically study the effect of two packaging configurations for the CMOS thermoresistive micro calorimetric flow (TMCF) sensors: S-type with the sensor chip protrusion-mounted on the flow channel wall and E-type with the sensor chip flush-mounted on the flow channel wall. Although the experimental results indicated that the sensitivity of the S-type was increased by more than 30%; the corresponding flow range as compared to the E-type was dramatically reduced by 60% from 0–11 m s−1 to 0–4.5 m s−1. Comprehensive 2D CFD simulation and in-house developed 3D numerical simulations based on the gas-kinetic scheme were applied to study the flow separation of these two packaging designs with the major parameters. Indeed, the S-type design with the large protrusion would change the local convective heat transfer of the TMCF sensor and dramatically decrease the sensors’ performance. In addition, parametric CFD simulations of the packaging designs provide inspiration to propose a novel general flow regime map (FRM), i.e. normalized protrusion d* versus reduced chip Reynolds number Re*, where the critical boundary curve for the flow separation of TMCF sensors was determined at different channel aspect ratios. The proposed FRM can be a useful guideline for the packaging design and manufacturing of different micro thermal flow sensors.

Cylindrical porous inserts for enhancing the thermal and hydraulic performance of water-cooled cold plates

In the present study, a new class of porous inserts with the Rhombi-Octet unit cell design was proposed for enhancing the thermal and hydraulic performance of water-cooled cold plates with a cylindrical flow channel. Selective laser melting (SLM) was employed to fabricate the cold plates where the inserts were integrated to the flow channel walls in a single built piece. In all, three cold plates (P1, P2 and P3) with their flow channels fully filled with the porous structures of different unit cell sizes and three partially-filled cold plates (AP1, AP2 and AP3) with porous structures of the same unit cell size but with different fill ratios pp+s were investigated. Their results were compared against the cold plates with empty flow channel and flow channel with helical inserts of different twist ratios (Y/D). The geometrical characteristics of the porous structures such as porosity (ε), strut length (l), width (w) and average pore diameter (dave) were determined and their hydraulic performances were characterized using the Forchheimer-extended Darcy equation where the permeability (K) and inertia coefficient (CE) of the respective porous structures were obtained. The cold plates with porous inserts show significant enhancements in the local (hx) and length-averaged (have) heat transfer coefficients as compared to the cold plate with empty channel and helical inserts. In all, P1 exhibits hx values as high as 18322W/m2·K and up to 383% enhancement in have as compared to the empty channel cold plate. On the other hand, the heat transfer coefficients also increase with increasing fill ratio, with AP3 exhibiting the highest h‾ave amongst the cold plates with partially-filled porous inserts and surpassing those of the empty channel, helical insert and a fully-filled porous insert (P3) cold plate. At constant pumping power, P1, P2, P3 and AP3 also show consistently lower average thermal resistances (Rave) than all the other cold plates tested.

Numerical simulation of three dimensional two-phase flow and prediction of oil retention in an evaporator of the automotive air conditioning system

Abstract This paper presents a three-dimensional model of the refrigerant-oil two phase flow developed with the aim of predicting the oil retention in evaporators of air-conditioning systems. The developed model is based on the two-fluid model approach. The governing mass, momentum and energy balance equations are written for each phase. The gas phase is the refrigerant vapor, while the liquid phase is the mixture of liquid refrigerant and oil. The balance equation for the oil mass fraction in the mixture with liquid refrigerant is included. Transfer processes at the vapor-liquid interfaces and on the flow channel walls are predicted with closure laws. The model is solved by the in-house computer code based on the SIMPLE type numerical procedure. The model is validated by comparing numerically predicted refrigerant mass and oil retention data in a brazed plate and fin evaporator, typically used in automotive applications, against measured values. Two sets of experiments performed with refrigerants R134a and R1234yf in the mixture with PAG oil are simulated. The numerical results provide a complete picture of the two-phase flow structure in the evaporator. Model predicts that oil is mainly retained in the bottom header and in smaller amounts in the top header and in parallel evaporating channels with upward refrigerant flow. The developed three-dimensional modeling and numerical approach has the advantage of being more reliable for the prediction of oil retention than existing one-dimensional models.

A statistical model to predict streamwise turbulent dispersion from the wall at small times

Data from simulations are used to develop a statistical model that can provide the streamwise dispersion distribution of passive particles released from the wall of a turbulent flow channel. It is found that a three-point gamma probability density function is the statistical distribution that can describe the dispersion of particles with Schmidt numbers ranging from 6 to 2400 at relatively short times after the release of the particles. Scaling arguments are used to physically justify and predict the parameters of the gamma three-point distribution. The model is used to predict particle separation that can occur in turbulent flow under special conditions. Close to the channel wall, turbulent convection is not the dominant transport mechanism, but molecular diffusion can dominate transport depending on the Schmidt number of the particles. This leads to turbulence-induced separation rather than mixing, and the currently proposed model can be used to predict the level of separation. Practically, these results can be applied for separating very small particles or even macromolecules in dilute suspensions.

Multidimensional Heat Transfer and Flow Boiling Curve Measurements in Single-Side-Heated Flow Channels

In order to accommodate high thermal loading of single-side-heated (SSH) components, robust thermal management and high-heat-flux-removal approaches are essential to prevent thermal instability, thermal runaway, or a thermal spiral toward component failure. This paper presents multidimensional steady-state heat transfer measurements for a high-strength-copper SSH monoblock (heat sink) coolant flow channel with a helical wire insert (HI) and thermally developing internal laminar and turbulent water (coolant) flow. In the present case, the term “monoblock” refers to a solid parallelepiped with a central coolant flow channel along the axial centerline. In addition to producing local two-dimensional (axial and circumferential) flow boiling curves, multidimensional monoblock wall temperature distribution comparisons were made between flow channels with and without a HI. Further, flow boiling curves were measured up to ∼4.0 MW/m2 at the inside flow channel wall. For the same inside flow channel temperature, the HI enhanced (1) the incident heat flux by >70% when compared with the flow channel without the insert and (2) the inside flow channel wall heat flux by up to a factor of 5 near the monoblock heated side and at all axial locations. These results can be used for validation of computational fluid dynamics codes.

Design and development of guide vane cascade for a low speed number Francis turbine

Guide vane cascade of a low speed number Francis turbine is developed for the experimental investigations. The test setup is able to produce similar velocity distributions at the runner inlet as that of a reference prototype turbine. Standard analytical methods are used to design the reference turbine. Periodic walls of flow channel between guide vanes are identified as the starting profile for the boundary of the cascade. Two alternative designs with three guide vanes and two guide vanes, without runner, are studied. A new approach, for the hydraulic design and optimization of the cascade test setup layout, is proposed and investigated in details. CFD based optimization methods are used to define the final layout of the test setup. The optimum design is developed as a test setup and experimental validation is done with PIV methods. The optimized design of cascade with one guide vane between two flow channels is found to produce similar flow conditions to that in the runner inlet of a low speed number Francis turbine.

Performance analysis on solid-liquid mixed flow in a centrifugal pump

In order to study the solid-liquid mixed flow hydraulic characteristics of centrifugal pump, the Pro/E software was used for three-dimensional modeling of centrifugal pump chamber. By using the computational fluid dynamics software CFX, the numerical simulation calculation of solid-liquid two-phase flow within whole flow passage of centrifugal pump was conducted. Aim at different particle diameters, the Reynolds-averaged N-S equations with the RNG k-Ɛ turbulence model and SIMPLEC algorithm were used to simulate the two-phase flow respectively on the condition of different volume fraction. The influence of internal flow characteristic on pump performance was analyzed. On the conditions of different particle diameter and different volume fraction, the turbulence kinetic energy and particle concentration are analyzed. It can be found that the erosion velocity ratio on the flow channel wall increases along with the increasing of the volume fraction

Curved surface turbulence precision machining method for artificial joint complex of titanium alloy

Titanium alloy is widely used in the design of artificial joints because of its high strength and low modulus. But, titanium alloy has its defects such as poor machinability, low thermal conductivity. The surface of artificial joints is usually complex curved surface, which makes that high surface quality cannot be easily obtained by using conventional processing methods. Aiming at the processing problems for the complex surface of titanium alloy artificial joints, a partial coverage constraint style complex surface turbulence precision machining method is proposed for titanium alloy artificial joint. The partial covering surface binding component is established by profiling of the artificial joint surface, so the controlled flow channel of abrasive flow is built. The processing surface is a part of the flow channel wall, which making abrasive flow into the turbulent fully developed state. The micro-cutting of the processing surface is realised by the disorderly movement of the abrasive to reach the mirror-level surface roughness. The experiment results show the effectiveness and feasibility of this method.

Impact of micromixing on performance of a membrane-based absorber

In this study, microstructures are employed to manipulate thermohydraulic characteristics of the lithium bromide (LiBr) solution flow in a membrane-based absorber in order to enhance the absorption rate. In a membrane-based absorber, the liquid absorbent is constrained between a solid wall and a highly permeable membrane, thus facilitating manipulation of the flow properties. Recent numerical studies have shown that transport mode in a laminar flow can be changed from diffusive to advective via the implementation of surface microstructures on the flow channel walls. Here, we experimentally evaluate the enhancement in absorption rate caused by the introduction of microstructures on the solution flow channel wall of a membrane-based absorber. The experiments are conducted in a fully instrumented membrane-based absorption refrigeration system. The geometry and dimensions of the microstructures are based on the optimal values determined in our previous numerical studies. Absorption rates as high as that of a 100-μm-thick solution film (in the absence of wall features) is achieved but at two orders of magnitude less pressure drop. The achievement of a high absorption rate at a relatively low solution pressure drop in the proposed approach enhances the prospect of developing large-scale membrane-based absorbers.