Channels Of Circular Cross Section Research Articles

Experimental investigation of condensation flows in circular cross-section channel

In this work, condensation flows of pure ethanol in a horizontal water-cooled circular cross-section heat exchanger were investigated for a mass velocity ranging between 30 and 100 kg m−2 s−1. The inner channel of condensation has inner and outer diameters of 4 mm and 10 mm, respectively, with a heat exchange length of 1020 mm. An adiabatic section consisted of a transparent circular cross-section channel, of 4 mm inner diameter and of 130 mm visible length, was placed downstream of the condenser in order to visualise the condensation flows by shadowgraph technique. Two main flow regimes were identified: the wavy-stratified regime and the wavy-annular regime. A flow regime map was developed and compared with flow regime maps available in the literature. Besides, the local vapour quality and internal heat transfer coefficient were determined and compared with prediction models of the literature. Finally, the total pressure drop in the condenser was determined.

Thermal expansion effect on the propagation of premixed flames in narrow channels of circular cross-section: Multiplicity of solutions, axisymmetry and non-axisymmetry

The present study examines, in presence of thermal expansion effects, the existence of the multiplicity of solutions previously reported within the context of diffusive-thermal modeling in [15], for lean premixed flames with low Lewis number (Le < 1) propagating in narrow circular adiabatic channels subject to a Poiseuille flow. For this, direct numerical simulations have been carried out within the framework of variable-density Navier–Stokes equations and zero-Mach-number approximation. The simulations, conducted for both axisymmetric and three-dimensional cylindrical geometries, confirm the coexistence of multiple steady flame structures for a given flow rate. They show that axisymmetric flames concave towards the upstream are more unstable to three-dimensional perturbations than convex (toward the upstream) flames. This result evinces earlier findings obtained from stability analysis. The non-axisymmetry property of the flame is also found to push back the critical flashback limits at larger flow rate when compared to those predicted under the assumption of flame axisymmetry.

Experimental investigation of in-tube condensation in microgravity

This work is aimed at studying the effect of gravity when convective condensation occurs inside a small diameter channel. An experimental apparatus has been specifically designed to perform microgravity experiments during the 62nd ESA Parabolic Flight Campaign. Convective condensation heat transfer tests have been carried out at mass velocity between 70 kg m−2 s−1 and 170 kg m−2 s−1 using HydroFluoroEther HFE-7000 (1-methoxyheptafluoropropane) as the working fluid in a circular cross section channel with an internal diameter equal to 3.4 mm. The data of heat transfer coefficient and the flow pattern visualizations show that, when gravity acts perpendicular to the channel flow, it has a beneficial effect on the heat transfer coefficient by acting on the liquid distribution along the channel perimeter. In microgravity conditions this mechanism leads to a penalization which is proportional to the mass velocity.

New portable microchannel molding system based on micro-wire molding, droplet formation studies in circular cross-section microchannel

This work describes a new appliance for design and fabrication of various 3D microchannels by a quick process known as micro-wire molding. The method provides several possibilities including simultaneous curing and molding without any displacement or a curing oven. Both metallic and Nylon micro-wires can be used in this method. The method is designed so that the molding container needs minimal polydimethylsiloxane providing possibility to produce channels with different cross-sections, shapes and geometries. Inside a circular cross-section channel by 800 μm diameter, a T-junction is used to generate micro-droplets of water in oil with two different examined liquids as the continuous phases. The effect of relative flow rate and viscosity of the phases have been studied and demonstrated that these parameters affect the shape, size, velocity and separation of the droplets inside the running phase within the channel.

On the propagation of particulate gravity currents in circular and semi-circular channels partially filled with homogeneous or stratified ambient fluid

We present a combined theoretical-experimental investigation of particle-driven gravity currents advancing in circular cross section channels in the high-Reynolds number Boussinesq regime; the ambient fluid is either homogeneous or linearly stratified. The predictions of the theoretical model are compared with experiments performed in lock–release configuration; experiments were performed with conditions of both full-depth and partial-depth locks. Two different particles were used for the turbidity current, and the full range 0≤S≤1 of the stratification parameter was explored (S = 0 corresponds to the homogeneous case and S = 1 when the density of the ambient fluid and of the current are equal at the bottom). In addition, a few saline gravity currents were tested for comparison. The results show good agreement for the full-depth configuration, with the initial depth of the current in the lock being equal to the depth of the ambient fluid. The agreement is less good for the partial-depth cases and is improved by the introduction of a simple adjustment coefficient for the Froude number at the front of the current and accounting for dissipation. The general parameter dependencies and behaviour of the current, although influenced by many factors (e.g., mixing and internal waves), are well predicted by the relatively simple model.

Cracks with interfacial bonds in perforated heat-releasing nuclear fuel

We consider a fracture mechanics problem for heat-releasing isotropic material weakened by a doubly periodic system of identical cylindrical channels of circular cross section. It is considered that as the intensity of heat release increases in the material, its elastic properties become temperature dependent and failure of the material occurs. It is accepted that rectilinear cracks with bonds between the faces at the end zones originate from the surface of holes. The condition of limit equilibrium of a crack with end zone is formulated with regard to criterion of ultimate stretching of material’s bonds.

Nuclear coupled thermal hydraulic analysis of fast transient depressurization in supercritical channels

PurposeThe objective of the present work is to simulate the nuclear coupled thermal–hydraulic fast transient case studies for a vertically up-flowing supercritical pressure water channel of circular cross section. The emphasis is on analyzing the phenomenon of the deterioration in heat transfer (DHT) inside the channel subjected to sharp pressure variations.Design/methodology/approachThe thermal–hydraulic model, THRUST, is integrated with the neutron point kinetic (NPK) solver to account for the non-linear interactions between the thermal–hydraulic and neutronic temperature and density reactivity feedback effects. The model implemented and studied accounts for the time-dependent reactor power and is used to analyze various steady-state and flow-induced transient case studies (time-dependent and step change in exit pressure).FindingsThere is good agreement in the predicted behavior of the supercritical water pressure system with that of the available experimental data for the steady-state case. The event of DHT in the second transient case (step decrease in exit pressure) is found to be more severe than that of exponential pressure decrease.Originality/valueThis study evaluated a novel implementation of the thermal–hydraulic model, THRUST, integrated with NPKs applied to supercritical pressure water systems for predicting DHT.

Numerical Simulation of Multidimensional Modes of Gaseous Detonation

ABSTRACTProcesses observed in experiments with the initiation and propagation of divergent detonation in a narrow gap between plates was simulated. The detonation wave is formed in a round tube adjoined to one of the plates at a right angle. Complicated cellular structures of the 2D detonation have been obtained. The process of formation of 3D cellular detonation in channels of square, rectangular, circular, and elliptical cross sections was studied. The process of spontaneous formation of spin detonation in channels of circular cross section was considered. Calculations of the transition of spin detonation to the channel of larger or smaller diameter were conducted and conditions of self-sustained spin detonation were formulated. Detonation is initiated in a quiescent combustible mixture by an instantaneous homogeneous electric discharge in a finite-width zone near the flat closed end of the channel. All computations of 3D detonation were performed in a correct formulation for the entire channel of large length (1 m) rather than only in a certain zone moving together with the wave.

Computational and experimental research of thermal processes in solid heat storage materials with high thermal storage properties

A comprehensive calculated and experimental research of thermal processes in the solid heat storage elements of the proposed construction of two channels of circular cross section of chamotte. Adapted mathematical model of heat exchange processes to modes of operation electrothermal storage (ETS). Has been solved the direct conjugate heat conduction problem by finite element method. Reveals the factors which significantly influence the distribution of temperature in the heat storage element during the charge period and the heat output of ETS: natural convection in the air channel of heat storage element in charge of the period, the change of the heated air temperature along the channel height and heat transfer by radiation and convection from the outside ETS surface of the housing ETS. Experimental studies consist in measuring the temperatures at certain points of the heat storage elements and of the ETS using thermocouples chromel-alumel. The purpose of the experiment is to obtain data on the change of temperatures in the heat storage material, heat insulation, heated air in the channels and on the surface of the housing ETS during the period of the charge and the heat output. Obtained values of temperatures allows to judge about the nature and dynamics of heating and cooling the heat storage elements and also allow to check the validity of the obtained results of numerical modeling of heat transfer processes in heat storage elements ETS by comparing them.

Gravity currents in a linearly stratified ambient fluid created by lock release and influx in semi-circular and rectangular channels

We present an experimental investigation, supported by a theoretical model, of the motion of lock-release, constant inflow, and time varying inflow gravity currents (GCs) into a linearly stratified ambient fluid at large Reynolds number. The aim is the experimental validation of a simple model able to predict the slumping phase front speed and the asymptotic self-similar front speed for rectangular and circular cross section channels. The first investigated system is of Boussinesq type with the dense current (salt water dyed with aniline) released in a circular channel of 19 cm diameter and 400 cm long (605 cm in the inflow experiments), half-filled of linearly stratified ambient fluid (salt water with varying salt concentration). The second system has the same components but with a channel of rectangular cross section of 14 cm width, 11 cm ambient fluid depth, and 504 cm length. The density stratification of the ambient fluid was obtained with a computer controlled set of pumps and of mixing tanks. For the experiments with inflow, a multi-pipes drainage system was set at the opposite end with respect to the inflow section, computer controlled to avoid the selective withdrawal. The numerous experiments (28 for circular cross section, lock release; 26 for circular and 14 for rectangular cross section, constant inflow (fluid volume ∝tα, with α = 1); 6 for circular cross section, linearly increasing inflow (α = 2)), with several combination of the stratification parameter (0 &amp;lt; S &amp;lt; 1) confirm the theory within ≈30% (≈40% for a single series of experiments), which is considered a good result in view of the various underlying simplifications and approximations. The results on the front speed of the GCs are discussed in the presence of the internal waves, which have a celerity given by a theoretical and experimentally tested model for the rectangular but not for the circular cross section. The theoretical analysis of internal waves in circular cross sections has been extended and experimentally validated.

A Numerical Thermal-Hydraulic Model to Simulate the Fast Transients in a Supercritical Water Channel Subjected to Sharp Pressure Variations

AbstractThe present work demonstrates the extension of a thermal-hydraulic model, THRUST, with an objective to simulate the fast transient flow dynamics in a supercritical water channel of circular cross section. THRUST is a 1-D model which solves the nonlinearly coupled mass, axial momentum and energy conservation equations in time domain based on a characteristics-dependent fully implicit finite difference scheme using an Eulerian approach. The model developed accounts for the compressibility of the supercritical flow by considering the finite value of acoustic speed in the solution algorithm and treats the boundary conditions naturally. A supercritical water channel of circular cross section, for which the experimental data is available at steady state operating conditions, is chosen for the transient simulations to start with. Two different case studies are undertaken with a purpose to assess the capability of the model to analyze the fast transient processes caused by the large reduction in system pressure. The first transient case study is where the initial exit pressure is reduced by 1MPaexponentially in a time span of 5s. In the second case study, the transient is initiated with a sudden step decrease in the exit pressure by the same amount. Results obtained for both the case studies show the desired performance from the model developed.

Mathematical model of mass transfer processes of pervaporation water purification

Object. Theoretical studies of pervaporation separation of water -organic mixture by mathematical modeling of process.Design. Methodology . Approach. The current state of the problem, namely the existing mathematical model of membrane processes had been analyzed. After analysis of features of pervaporation purification process the mathematical model was formulated. The mathematical model describes the pervaporation of organic liquid component through the non-porous membrane in a circular cross section channel. Initial and boundary conditions for this mathematical model had been recorded. Without these conditions the solution is impossible Conclusions. Solution of the equations of mathematical model with the initial and boundary conditions allows to determine the concentration distribution of liquid flow which passes the membrane, and also though the membrane thickness depending on the size of the membrane, the modes of motion of initial and gas-vapor mixture, the concentration of organic impurities in initial and gas-vapor mixture. A mathematical model allows to finding a mode of operating of unit at which completely realized possibilities of membranes and minimized the cost of liquid heating and condensate cooling. In further the research of experimental studies of pervaporation process are planned to continue, to confirm the results obtained by mathematical modeling.

Structure and stability of premixed flames propagating in narrow channels of circular cross-section: Non-axisymmetric, pulsating and rotating flames

Analysis of lean premixed freely propagating flames in narrow adiabatic channels of circular cross-section subject to a Poiseuille flow is carried out systematically within the context of a diffusive-thermal model. It is found that for flames with sufficiently low Lewis number multiplicity of solutions exists, in particular it is shown that for a given flow rate up to three different axisymmetric flames can coexist.The rest of the study is focused on the influence of the Lewis number and the channel radius on the flame stability, linking first the results to the stability properties of corresponding planar flame fronts. The global stability analysis, giving accurate threshold values of instability, shows that for flames with Lewis number lower than one non-axisymmetric monotonous perturbations can grow, while for flames with Lewis number larger than one unstable axisymmetric and non-axisymmetric oscillatory modes can appear. An increase in the flow rate leads to a loss of stability of axisymmetric flames for Le < 1 while for flames with Le > 1 the Poiseuille flow produces a stabilization effect. This analysis seems to have received no attention in the literature for flames in circular channels.Finally, the stability results are compared successfully with three-dimensional unsteady numerical simulations. They show that for Le < 1 non-axisymmetric flames propagating with a constant velocity appear for positive values of the flow rate. For flames with Le > 1, depending on the channel width, pulsating axisymmetric and rotating non-axisymmetric flames arise. It is demonstrated that when the non-axisymmetric oscillatory perturbations are the most unstable, the flame adopts a multi-headed non-axisymmetric shape and can propagate in a rotating manner while the central part of the flame advances at a constant velocity.

Gravity currents produced by constant and time varying inflow in a circular cross-section channel: Experiments and theory

We investigate high-Reynolds number gravity currents (GC) in a horizontal channel of circular cross-section. We focus on GC sustained by constant or time varying inflow (volume of injected fluid ∝ tα, with α=1 and α > 1). The novelty of our work is in the type of the gravity currents: produced by influx/outflux boundary conditions, and propagation in circular (or semi-circular) channel. The objective is to elucidate the main propagation features and correlate them to the governing dimensionless parameters; to this end, we use experimental observations guided by shallow-water (SW) theoretical models. The system is of Boussinesq type with the denser fluid (salt water) injected into the ambient fluid (tap water) at one end section of a circular tube of 19 cm diameter and 605 cm long. The ambient fluid fills the channel of radius r* up to a given height H*=βr* (0 < β < 2) where it is open to the atmosphere. This fluid is displaced by the intruding current and outflows either at the same or at the opposite end-side of the channel. The two different configurations (with return and no-return flow) allow to analyze the impact of the motion of the ambient fluid on the front speed of the intruding current. For Q larger than some threshold value, the current is expected theoretically to undergo a choking process which limits the speed/thickness of propagation. Two series of experiments were conducted with constant and time varying inflow. The choking effect was observed, qualitatively, in both series. The theory correctly predicts the qualitative behavior, but systematically overestimates the front speed of the current (consistent with previously-published data concerning rectangular and non-rectangular cross-sections), with larger discrepancies for the no-return flow case. These discrepancies are mainly due to: (i) the variations of the free-surface of the ambient fluid with respect to its nominal value (the theoretical model assumes a fixed free-slip top of the ambient fluid), and (ii) mixing/entrainment effects, as shown by specific measurements of the open interface level and velocity profiles.

Formation of spin detonation in channels of circular cross section

Formation of spin detonation in channels of circular cross section

The propagation of gravity currents in a circular cross-section channel: experiments and theory

AbstractHigh-Reynolds number gravity currents (GC) in a horizontal channel with circular/semicircular side walls are investigated by comparing experimental data and shallow-water (SW) theoretical results. We focus attention on a Boussinesq system (salt water in fresh water): the denser fluid, occupying part of the depth or the full depth of the ambient fluid which fills the remaining part of the channel, is initially at rest in a lock separated by a gate from the downstream channel. Upon the rapid removal of the gate (‘dam break’), the denser ‘current’ begins propagating into the downstream channel, while a significant adjustment motion propagates upstream in the lock as a bore or rarefaction wave. Using an experimental channel provided by a tube of 19 cm diameter and up to 615 cm length, which could be filled to various levels, we investigated both full-depth and part-depth releases, considered the various stages of inertial-buoyancy propagation (in particular, the initial ‘slumping’ with constant speed, and the transition to the late self-similar propagation with time to the power $3/4$), and detected the transition to the viscous-buoyancy regime. A first series of tests is focused on the motion in the lock while a second series of tests is focused on the evolution of the downstream current. The speed of propagation of the current in the slumping stage is overpredicted by the theory, by about the same amount (typically 15 %) as observed in the classical flat bottom case. The length of transition to viscous regime turns out to be ${\sim}[\mathit{Re}_{0}(h_{0}/x_{0})]^{{\it\alpha}}$ ($\mathit{Re}_{0}=(g^{\prime }h_{0})^{1/2}h_{0}/{\it\nu}_{c}$ is the initial Reynolds number, $g^{\prime }$ is the reduced gravity, ${\it\nu}_{c}$ is the kinematic viscosity of the denser fluid, $h_{0}$ and $x_{0}$ are the height of the denser current and the length of the lock, respectively), with the theoretical ${\it\alpha}=3/8$ and experimental ${\it\alpha}\approx 0.27$.

Initiation and propagation of multidimensional detonation waves

Three-dimensional detonation in channels of different cross sections and spinning detonation in channels of circular cross sections are numerically studied. A possibility of galloping detonation in a supersonic flow of a mixture with a variable fuel concentration over a plane channel width is discussed. Conditions of detonation formation under the action of moving boundaries of the flow region due to rotation of an elliptical cylinder and in a variable-size square chamber are obtained. Formation of three-dimensional detonation in a supersonic flow in a helical channel of elliptical cross section and in a variable-size square channel is calculated.

Condensation in a Square Minichannel: Application of the VOF Method

A number of steady-state numerical simulations of condensation of R134a at mass fluxes of 400 kg m−2 s−1 and 800 kg m−2 s−1 inside a 1-mm square cross section minichannel are proposed here and compared against simulations in a circular cross section channel with the same hydraulic diameter. The volume of fluid (VOF) method is used to track the vapor–liquid interface, and the effects of interfacial shear stress, surface tension, and gravity are taken into account. A uniform wall temperature is fixed as a boundary condition. At both mass velocities the liquid film and the vapor core are treated as turbulent; a low-Re form of the SST k-ω model has been used for the modeling of turbulence through both the liquid and vapor phases. Numerical simulations are validated against experimental data. The influence of the surface tension on the shape of the vapor–liquid interface may provide some heat transfer enhancement in a square cross section minichannel, but this depends on the mass flux and it may be not significant at high mass velocity, as confirmed by experimental data and by the present numerical work. The gravity force is shown to be responsible for the liquid film thickness increase at the bottom of the channel in the circular cross section, but the gravity force has a minor effect in the square minichannel; at these mass velocities, the heat transfer mechanism is dominated by shear stress and surface tension.

사출금형의 냉각채널 성능 평가

Design of the cooling channels of a plastic injection mold affects the quality and the productivity of the injection processes. In the injection process, the melted resin with high temperature enters the mold cavity, and just after the cavity is filled the heat should be dissipated through the cooling channels simultaneously. The purpose of this study is to analyse the heat transfer phenomenon and to estimate the temperature distribution in the mold to evaluate the cooling effect of the channels. The injection mold is assumed to have cooling channels of circular cross section and each channel has the same coolant flow rate. and The cavity has a rectangular shape. The results show that as the cooling channels get closer to the cavity surface, the cooling efficiency increases as might easily be guessed. However, due to the final hot resin flow from the gate an intensive cooling is required in that region.

Mold temperature control using high-frequency proximity effect induced heating

A rapid heating in an injection molding cycle has the advantage of improving product quality without significant increase in cycle time. In this study, high-frequency proximity effect induced heating (HFPEIH) was developed and combined with water cooling to achieve dynamic mold surface temperature control. By applying the HFPEIH system on a pair of mold plates separated with a small gap, the relevant influence of HFPEIH design was evaluated under various parameters including different mold plate material, inductor designs, and inductor channel depths beneath mold surface as well as mold separations. Simulation was also conducted and verified with experiments. Results show that all the heating rates range within 2°C/s to 4°C/s for the mold plate size of 100mm by 100mm. For the inductor design with three channels of circular cross section, the heating rate is fastest whereas one inductor design of rectangular shape exhibits the best the uniformity of temperature distribution. When the channel depth is reduced from 12mm to 4mm, the heating rate is increased significantly. The heating rate is also sensitive to mold plate surface area. When stainless steel N700 was used as the plate materials in a smaller plate of 60mm by 60mm, the heating rate can reach 7.6°C/s using one channel inductor design. The mold separation exhibits that it is less sensitive to the heating rate within 1mm to 5mm range and when it is greater than 5mm, the heating rate starts to decrease slightly. All the simulated results show good coincidence with experimental measurements.