Circular Channel Research Articles

Undular bores in a large circular channel

Abstract An experimental device previously developed for studying rotating baroclinic flows has been used to investigate undular bores formation, propagation and collision. Up to our knowledge this is the first experimental study of undular bores in a circular channel. For a setup without barriers, this geometry accomplishes in a natural way the periodic lateral boundary conditions, very often used in numerical simulations. An excellent agreement between the experiment and simulation has been achieved. The spatio-temporal structure of bores is well reproduced for the first few reflections or collisions.

Hydraulic Jump in Circular Open Channels with Mild Slope

A hydraulic jump is a well-known transitionalphenomenon from supercritical to subcritical flows withundulations of water-surface. In this transition, the water flowhas a high-velocity, water surface rises abruptly, surface rollersare formed, intense mixing occurs, the air is entrained, and alarge amount of energy is dissipated. In the present studycharacteristics of the hydraulic jumps in circular open channelswith a mild sloped have been discussed under a wide range ofexperimental conditions. The study aims to determine the effectof the channel’s mild slope on the characteristics of thehydraulic jumps with circular channels. A theoretical study hasbeen done and led to obtaining equations that can be used to getspecific force for the hydraulic jump that occurred in circularopen channels. A theoretical study is based on usingmomentum, and Froude’s number equations. An experimentalstudy has been investigated using Laboratory 40 experimentalruns in circular section flume for eight different dischargesranged from 3.92 to 12.07 l/sec were given with mild slopeequals 0.000833. In total, the experiments were conducted overa range of Froude numbers from 1.78 to 8.87. The experimentalworks concluded some dimensionless curves and new usefulformulas to get the length of the jump, energy dissipated andefficiency. The conjugated depth ratio is determined and plottedversus the upstream Froude number, downstream Froudenumber, head loss, and efficiency for different values of criticaldepths. The resulting graphs and equations of the present studyare readily applicable for a design for the hydraulic jumps inthe circular open channels with mild slope.

Experimental Investigation of Magnetic Particle Transport in a Circular Minichannel in a Constant Magnetic Field

Experimental investigation of the deposition of nanoparticles in a circular channel in a constant magnetic field has been carried out. The dynamics of growth of nanoparticles deposits on the channel walls depending on the Reynolds number and magnetic field strength has been studied in the experiments. The influence of magnetic field on pressure drop at different Reynolds numbers has been investigated. Characteristic profiles of the zones of nanoparticles depositions have been obtained for the first time; the sizes of the deposition areas and the rates of their formation depending on the Reynolds numbers, magnetic field strength and time have been determined.

MODELS FOR DETERMINING MAXIMUM DEGREE FILLING CHANNELS OF CIRCULAR SECTION SHAPE

Objectives The task was to obtain a model for determining the maximum possible degree of filling the circular section channels in the earthen channel based on the condition of ensuring the stability of slopes and the minimum volume of excavation during their construction, as well as finding the optimum degree of filling fortified channels of a closed profile corresponding to maximum throughput.Method In work analytical methods of differential calculus and the solution of implicit equations are used.Result The experience of domestic and foreign researchers was taken into account to solve the set tasks, select research methods and criteria for optimizing channel parameters. Two cases of circular-shaped channels are considered: 1) in the earth channel, 2) reinforced with a closed transverse profile. For the case of a hydraulically most advantageous circular channel in the earthchannel channel, equating the first derivative of the equation of a circle with the reciprocal of the allowable embedding coefficient of the slopes obtained an analytical solution for determining the maximum degree of filling from the slope stability condition, which was not dependent on hydraulic flow elements. In the case of non-cohesive soils that form the channel, the circular channel can be filled to a depth not exceeding 20 percent of the channel radius. At the same time, the average flow rate should remain in the range from non-venting to non-blurring. In order to be able to determine the flow rate, analytical expressions are given for finding hydraulic flow elements in a circular channel. When determining the volume of excavation for the construction of the channel, the excess of the channel edge above the maximum water level in the channel was taken into account. For the case of a fortified channel of a closed transverse profile, by taking the derivatives from the Chezy formula, we obtained the optimal values of flow rate and average velocity.Conclusion A fortified closed circular profile channel has a maximum capacity with a relative degree of filling of 0.938, and the maximum average velocity of a fluid in a pressureless channel is achieved with a degree of filling of 0.815. To determine the maximum permissible relative degree of filling in the case of a circular section channel in the earthen channel, analytical dependences were obtained, before using which, according to the reference literature, it is necessary to take the value of the slope coefficient for this type of channel bed soil.

Experimental and Computational Methodology for the Determination of Hydrodynamic Coefficients Based on Free Decay Test: Application to Conception and Control of Underwater Robots.

Hydrodynamic coefficients are essential for the development of underwater robots; in particular, for their design and navigation control. To obtain these coefficients, several techniques exist. These methods are usually experimental, but, more recently, some have been designed by a combination of experiments with computational methods based on Computational Fluid Dynamics (CFD). One method for obtaining the hydrodynamic coefficients of an ROV (Remote Operated Vehicle) is by using an experimental PMM (Planar Motion Mechanism) or CWC (Circular Water Channel); however, the use of these experimental infrastructures is costly. Therefore, it is of interest to obtain these coefficients in other ways, for example, by the use of simple experiments. The Free Decay Test is an ideal type of experiment, as it has a low cost and is simple to implement. In this paper, two different free decay tests were carried out, to which three different methods for obtaining coefficients were applied. They were compared with results obtained by CFD simulation to conduct a statistical analysis in order to determine their behaviours. It was possible to obtain values of the drag and added mass coefficients for the models analysed, where the values were obtained for an Underwater Drone Robot (UDrobot).

Investigation of heat transfer and fluid flow behaviors of CuO/(60:40)% ethylene glycol and water nanofluid through a serpentine milichannel heat exchanger

PurposeThis paper aims to investigate the three-dimensional (3D) numerical simulations, based on the Navier–Stokes equations and the energy equation. Forced convection of a mixture of (60:40) percent ethylene glycol and water, was used as the base fluid and CuO nanoparticles, through a serpentine minichannel.Design/methodology/approachIn this simulation, a serpentine mini-channel heat exchanger was simulated. The fluid studied in this simulation was composed of a mixture of (60:40) per cent ethylene glycol and water, was used as the base fluid and CuO nanoparticles. Four slabs and three serpentines were used in this study. The serpentine section is connected to the slab. Three equidistant circular channels (1 mm in diameter) were implemented inside the slab.FindingsResults show that nanoparticles increase the fluid pressure drop and by changing volume fraction of nanoparticles from 0 to 1 per cent, the pressure drop of nanofluids increases between 42and 47 per cent, for Reynolds numbers from 100 to 500. The existence of serpentine bend in the minichannel heat exchanger causes the heat transfer rate to increase. Increase the volume fraction of nanoparticles reduces the fluid temperature at the outlet of the heat exchanger. The numerical results show that in Re = 500, at the beginning of the last slab in middle channel by changing volume fraction of nanoparticles from 0 to 2 per cent, local Nusselt number 57.40 per cent increase. The existence of the serpentine bend causes the heat transfer rate to increase.Originality/valueForced convection of a mixture of (60:40) per cent ethylene glycol and water by using of 3D numerical simulations, based on the Navier–Stokes equations.

Diameter Effect on the Wall Temperature Behaviors During Supercritical Water Heat Transfer Deterioration in Circular Tubes and Annular Channels

Diameter effect on heat transfer deterioration (HTD) was numerically studied for supercritical water flowing upward in circular tubes and annular channels at high heat flux and low mass flux based on validated turbulence model. When the same boundary conditions were applied, i.e. heat flux, mass flux, and inlet temperature, it was found that in circular tubes the first wall temperature peak moves upstream and the magnitude of the peak increases first and then decreases with the increase of tube diameter, the second peak moves downstream with the increase of tube diameter. Whereas in annular channels with a fixed inner diameter, HTD is suppressed when the outer diameter is small and HTD occurs gradually with the increase of outer diameter. These phenomena are consistent with previous experimental results. The mechanism was analyzed based on fully developed turbulent velocity profile at the inlet of the heated sections. Increasing inner diameter for circular tubes or outer diameter for annular channels will result in the decrease of maximum velocity and velocity gradient in the near wall region, which makes velocity profile in this region much easier to be flattened by the buoyancy. Then an M-shaped velocity profile is formed, which will significantly decrease the Reynolds shear stress and turbulent kinetic energy and hence impair the heat transfer and cause HTD. For the same flow conditions, HTD is much easier to occur in circular tubes than in annular channels with the same hydraulic diameters.

Two-Phase Flow Analysis and Design of Geothermal Energy Turbine

A two-phase geothermal turbine generates mechanical energy by means of a total flow system. Literature references are available for experimental investigation of such flow machines but there is no validated numerical model for prediction of flow field and flashing for optimal design of the turbine’s flow channels. This paper presents a test reaction-turbine design study with two-phase flow using CFD. An Eulerian-Eulerian multiphase model with the Thermal Phase-Change criteria for two-phase heat, mass and momentum transfer has been used. Phase change validation has been conducted on published experimental data on a converging-diverging nozzle flow at various operating conditions. The stationary nozzle model was then extended to curved nozzle turbine and the results of flow and power has been validated with the data available in literature. Liquid water and vapour distribution due to flash evaporation was predicted by the analysis. Two-phase turbine model so established has been used further to re-design and optimise turbine nozzle by evaluating its cross section, area ratio, throat design, specific torque variation and curvature. Two new channel geometries, square and blended, were compared with the base circular channel and it was found that a blended channel offers on an average 6% higher specific power at 4623rpm. For the same rotor diameters and feed water flow rate, the power output could thus be improved by 4.5%. Isentropic efficiency in the range of 10 to 19% was estimated. Final design will be prototyped and tested in the laboratory for validation of design and deployment in eastern Africa.

Link between Photoelectron Circular Dichroism and Fragmentation Channel in Strong Field Ionization.

The investigation of the photoelectron circular dichroism (PECD) in the strong field regime (800 nm, 6.9 × 1013 W/cm2) on methyloxirane (MOX) reveals a flip of the sign of PECD between different fragmentation channels. This finding is of great importance for future experiments and applications in chemistry or pharmacy using PECD in the strong field regime as analysis method. We suggest that the observed sign change of PECD is not caused by ionization from different orbitals but by effectively selecting differently oriented nonisotropic subsamples of molecules via the fragmentation channel.

An analytical method to predict and compensate for residual stress-induced deformation in overhanging regions of internal channels fabricated using powder bed fusion

Powder bed fusion (PBF) is ideally suited to build complex and near-net-shaped metallic structures such as conformal cooling channel networks in injection molds. However, warpage occurring due to the residual stresses inherent to this process can lead to shape deviation in the internal channels and needs to be minimized. In this research, a novel analytical model based on the Euler-Bernoulli beam bending theory was developed to estimate the residual stress-induced deformation of internal channels printed horizontally using PBF. The model was used to predict the shape deviation for three different shapes of channel cross sections (circular, elliptical, and diamond-shaped), and showed very good agreement with the experimentally determined shapes of nine different internal channels (three cases per cross-sectional shape). Further, the model predictions were used to compensate for the shape deviation in the design stage, resulting in a reduction in root mean square (RMS) deviation of the circular channel by a factor of 2. The proposed approach is thus expected to be a useful tool to generate design-for-AM guidelines for the additive manufacturing of overhangs and internal channels.

The uncertainty of the Shannon entropy model for shear stress distribution in circular channels

The shear stress distribution at alluvial stream beds and banks is one of the essential parameters in channel stability analysis. In the current paper, a novel uncertainty analysis method based on the framework of a Bayesian Forecasting System (BFS) is presented to evaluate the Shannon entropy model for prediction of the shear stress distribution in both circular rigid-bed and alluvial-bed channels. The Johnson and Box-Cox transformation functions were applied to select the optimum sample size (SS) and corresponding transformation factor for determining a 95% confidence bound (CB) for the Shannon entropy model. The Shapiro-Wilk (SW) test is applied according to the SS used to evaluate the power of transformation functions in the data normalization. The results show that the error distribution between predicted and experimental shear stress values generated using the Box-Cox transformation is closer to a Gaussian distribution than the generated using the Johnson transformation. The indexes of the percentage of the experimental values within the CB (Nin) and Forecast Range Error Estimate (FREE) are applied for the uncertainty analyses. The lower values of FREE equal to 1.724 in the circular rigid-bed channel represent the low uncertainty of Shannon entropy in the prediction of shear stress values compared to the uncertainty for the circular alluvial-bed channel with a FREE value equal to 7.647.

Numerical study on heat transfer performance using Al2O3/water nanofluids in six circular channel heat sink for electronic chip

Abstract In this numerical study, the heat transfer rate, surface temperature, Nusselt number, thermal resistance, power consumption and reliability of electronic chip in the six circular channel heat sink are investigated with water and the Al2O3/water nanofluids as coolants. The performances of electronic chip are studied with the ANSYS (v12) fluent software. It is observed that the Al2O3/water nanofluids decreases the surface temperature, the power consumption, and thermal resistance of electronic chip than water. It is also studied that the Nusselt number increases and the reliability of electronic chip using nanofluids is 70% higher than water as coolant.

Comparison of interfacial heat transfer correlations for high-pressure subcooled boiling flows via CFD two-fluid model

Heat transfer processes using two-phase boiling flows are often found in the industry, due to the high-efficiency heat removal, with a minimum difference of temperature between the heated surface and fluid. Moreover, the use of computational fluid dynamics to solve safety issues related to nuclear reactors has increased rapidly, but a complete validation is still being carried out. Therefore, this study aims the assessment of different sub-models of the interfacial heat transfer coefficient which is used as a closure relation in the two-fluid multiphase model. As a manner to validate the numerical results, the set of experimental conditions of Bartolomei and Chanturiya (Therm Eng 14:123–128, 1967) were applied to an upwards flow in a circular channel, under high water saturation pressures. The interfacial sub-models were implemented into an axisymmetric simulation domain, using the Eulerian two-fluid approach. Three different saturation pressures and two uniform heat fluxes were considered in the simulation runs. Fixed mass flux and subcooled degree of 900 kg/m2 and 60 K were applied, respectively. A satisfactory agreement was achieved for the estimation of the heated wall temperature, the liquid bulk temperature and the onset of saturated boiling. Different heat transfer closure relations promoted different vapor volume fraction along the channel, indicating the importance of an adequate interfacial heat transfer correlation to predict the flow boiling phenomena.

Modeling of supersonic gas flows in radial nozzles

In the present paper, we report on the results of a study of supersonic gas flows in a radial nozzle. The pressurized gas is supplied into a circular channel, from which it then flows into the gap between two parallel disks (radial nozzle). It is shown that the structure of the jet ejected into ambient space substantially depends on the friction force acting on the gas flow from the side of the disks. The force of friction leads to a considerable decrease of the velocity of the supersonic jet, which acquires a fan-shaped form instead of being barrel-shaped. The numerical results proved to be in good agreement with experimental data.

Parametric analysis of axial wall conduction in a microtube subjected to two classical thermal boundary conditions

Heat transfer in laminar flow microtube is numerically explored with an objective of discriminating conjugate heat transfer process experienced in a microtube under two different thermal conditions. Two classical thermal conditions – constant heat flux and constant wall temperature – are imposed separately on the outer surface of a microtube. Wide parametric variations are considered in this study, for the two thermal conditions, albeit the problem under consideration being very classical from both geometry and thermal condition point of view. The parametric variations considered in this work include wall thickness, wall conductivity and coolant flow rate. An expression for Nusselt number in terms of radial (or transverse) and axial conduction number is presented and validated against existing theoretical correlation as well as reported experimental data for both circular and non-circular channels. Dominance of axial conduction over radial (or transverse) conduction is explored and it is found that the effect of wall material on conjugate heat transfer plays an important role. Additionally, it is also observed that with the increase in coolant flow rate, the ratio of radial to axial conduction number increases for both thermal boundary conditions.

Nonuniform heat transfer of supercritical pressure carbon dioxide under turbulent cooling condition in circular tubes at various inclination angles

In current work, we conducted a numerical simulation about the turbulent mixed convection and heat exchange of supercritical pressure carbon dioxide (SCO2) cooled by constant heat flux in inclined circular channel with equal cross-section. Result shows that the inclination angle of the channel has considerable effect on the arrangement of temperature, velocity, turbulent kinetic energy (TKE) as well as wall shear stress. The nonuniformity of local heat transfer coefficient (LHTC) is remarkably influenced by inclination angle, heat flux density, and gravity force level. Parameter Gr/Re2 is chosen to describe the buoyancy effect induced by density difference, and the buoyancy effect results in heat transfer enhancement. The maximal heat transfer coefficient (HTC) is seriously decreased with the deviation of the operating pressure from the critical pressure.

3D Printed Liquid Cooling Interface for a Deep-UV-LED-Based Flow-Through Absorbance Detector.

Ultraviolet (UV)-light-emitting diodes (LEDs) are now widely used in analytical absorbance-based detectors; as compared to conventional UV lamps, they offer lower cost, faster response time, and higher photon conversion efficiency. However, current generation deep-UV-LEDs produce excess heat when operated at normal operating currents, which affects output stability and reduces their overall performance and lifespan. Herein a 3D printed liquid cooling interface has been developed for a deep-UV-LED-based optical detector, for capillary format flow-through detection. The interface consists of a circular channel that is tightly wrapped around the LED to provide active liquid cooling. The design also facilitates easy plug-and-play assembly of the various essential components of the detector: specifically, a 255 nm UV-LED, a capillary Z-cell, and a broadband UV photodiode (PD). The unique liquid cooling interface improved the performance of the detector by reducing the LED temperature up to 22 °C, increasing the spectral output up to 34%, decreasing the required stabilization time by up to 6-fold, and reducing the baseline noise and limits of detection (LODs) by a factor of 2. The detector was successfully used within a capillary HPLC system and could offer a miniaturized, rapidly stabilized, highly sensitive, and low-cost alternative to conventional UV detectors.

Attenuation and shape effects on IR transceiver irradiation behaviour during air–hydrogen peroxide two-phase flow

Hydrogen peroxide is a green propellant used in nanosatellites for micro-propulsion. The flow pattern and concentration of hydrogen peroxide decide the thrust produced in such micro-thrusters in which the thrust is of the order of milli-Newton. A mini/micro-fluidic cross-sectional geometry can dramatically vary the fluid flow characteristics, particularly for a capillary-driven flow in such devices. In the present work, experiments are carried out in a mini-square channel of 50 × 2 × 2 mm size and thickness of 0.5 mm with air–hydrogen peroxide as two-phase flow fluids. Bubble and slug flow regimes formed during the flow are recorded using a PROMON high-speed camera. An infrared transceiver circuit is used for identifying the flow regimes. COMSOL Multiphysics package is used to develop a numerical model. The developed numerical model is validated with the experiments by comparing the signal behaviour. The shape effects of the square, circular, and triangular channel on IR transceiver irradiation during two-phase flow are detailed. The effects of dimensional ratio of the test section on IR transceiver irradiation are discussed. The attenuation behaviour of the hydrogen peroxide fluid (6, 30, and 50% concentration) on IR transceiver irradiation during two-phase flow is presented.

An experimental study on flow boiling in microchannels under heating pulses and a methodology for predicting the wall temperature fluctuations

Flow boiling in microchannels is a particularly interesting cooling method to maintain great temperature uniformity in applications exhibiting transient and/or non-uniform heat dissipation. The two-phase flow heat transfer coefficient is directly proportional to the local heat flux and higher heat loads are counterbalanced by lower thermal resistance. This study provides a fundamental understanding of how the temperature fluctuations caused by transient heat flux are affected by the mass velocity, local vapor quality, average heat flux and the heating pulses waveform, amplitude and its frequency. Transient wall superheat temperature data were obtained from flow boiling experiments for R134a at a saturation temperature of 31 °C and 0.5 and 1.1 mm diameter stainless steel circular channels. The results revealed that the average wall superheat temperature during heating pulses behaves similarly to its respective time-averaged conditions. Also, the amplitude of temperature fluctuations decreased for increasing average wall superheat temperature. A methodology for predicting the temperature fluctuations based on first-order transfer functions is proposed. In this approach, methods developed for steady heating conditions are extrapolated to estimate the heat transfer coefficient corresponding to the average and the fluctuating component of the wall superheat temperature. The accuracy of the methodology was evaluated for seven well-known methods from literature for estimating the flow boiling heat transfer coefficient in small diameter channels.

Rotating detonation engines with two fuel orifice schemes

Hollow combustors without centerbodies are becoming popular for rotating detonation engines. The present experimental study compares detonation and pressure gain performances of hollow combustors with two fuel orifice schemes to choose a better one. Hydrogen and air sources at room temperatures of 280–285 K are used as propellants. The combustors are optically accessible by embedding a piece of quartz glass in the outerbody. The hollow combustor channel here has an outer diameter of 100 mm. Fuel is injected into the combustor from 150 cylindrical orifices in a diameter of 0.8 mm or 90 orifices in a diameter of 2 mm, and air is injected through a circular channel with a throat width of 1 mm. Results show that the hollow combustor with 150 cylindrical orifices in a diameter of 0.8 mm has a better detonation performance and a higher pressure gain performance than the other one. High-speed images show the wall surface roughness affects the RDW strength greatly.