Smooth-wall Channel Research Articles

Supersonic turbulent flows over sinusoidal rough walls

Direct numerical simulations were performed to characterize fully developed supersonic turbulent channel flows over isothermal rough walls. The effect of roughness was incorporated using a level-set/volume-of-fluid immersed boundary method. Turbulence statistics of five channel flows are compared, including one reference case with both walls smooth and four cases with smooth top walls and bottom walls with two-dimensional (2-D) and three-dimensional (3-D) sinusoidal roughnesses. Results reveal a strong dependence of the turbulence on the roughness topography and the associated shock patterns. Specifically, the 2-D geometries generate strong oblique shock waves that propagate across the channel and are reflected back to the rough-wall side. These strong shocks are absent in the smooth-wall channel and are significantly weaker in cases with 3-D roughness geometries, replaced by weak shocklets. At the impingement locations of the shocks on the top wall in the 2-D roughness cases, localized augmentations of turbulence shear production are observed. Such regions of augmented production also exist for the 3-D cases, at a much weaker level. The oblique shock waves are thought to be responsible for a more significant entropy generation for cases with 2-D surfaces than those with 3-D ones, leading to a higher irreversible heat generation and consequently higher temperature values in 2-D roughness cases. In the present supersonic channels, the effects of roughness extend beyond the near-wall layer due to the shocks. This suggests that outer layer similarity may not fully apply to a rough-wall supersonic turbulent flow.

Improvement of thermal-hydraulic efficiency of mining power equipment through the application of porous freon steam generators with high heat conductivity

In the article, various areas of technical application of porous freon steam generators in mining power equipment are described, and explanations why freon coolants can provide a positive energy effect in such facilities are given. The paper presents results of calculations of thermal-hydraulic efficiency of the porous once-through tubular steam generators with freon-12 as a model working fluid in the the laminar flow area and with boundary conditions of the first kind. The smooth-wall cylindrical channels with different diameters were used as the reference surfaces to be compared. The following mode and design parameters were taken as a calculation base: the liquid temperature and pressure on the saturation line at the entry into the channel were: Ts0= 110 °С; P0s = 39,9·105 N/m2.; temperature heads, i.e. a difference between the wall temperature and temperature of the liquid at the entry into the channel were: ΔT=Tw-Ts0 = 1 °C; 2 °C; 3 °C; 4 °C; 5 °C; the Reynolds numbers at the entry into the channel were: Reo = 100; 200; 500; 1000; 2000; 2300; the channel porosities were: θ = 0.7; 0.75; 0.8; 0.85; 0.9. The porous material was metal felt with the copper fiber diameter of 200 microns. The channel diameters were: d =3·10-3 m; 4·10-3 m; 5·10-3 m; 6·10-3 m; 7·10-3 m. On the basis of the performed computational studies, it was concluded that for the conditions of the same mass flow rates of the coolant, with laminar flow, and the same channel diameters, it is possible to achieve a significant reduction in the length of the porous once-through steam generator in comparison with the length of the smooth-wall once-through steam generator. Due to the significantly shorter length, differential pressure for pumping the coolant can be several tens of percent less in porous evaporation channels than in the similar smooth-walled channels. This computational study also made it possible to establish main regularities in dynamics of the energy efficiency coefficients and their dependence on the model mode and design parameters. It was shown, that positive dynamics of the efficiency coefficients of porous steam generators occurs with decrease of the channel diameter and temperature head, as well as with increase of the Reynolds number in the investigated region of coolant laminar flow.

Investigation of cold-wire spatial and temporal resolution issues in thermal turbulent boundary layers

In this study, we use simulations and experiments to investigate spatial and temporal attenuation of cold-wire sensors. A direct numerical simulation (DNS) of a heated smooth-wall turbulent channel flow is used to simulate the sensor spatial filtering effect in the spanwise direction, with results confirming that the increasing wire length does not affect the mean temperature, but significantly attenuates the variance of temperature fluctuations, especially in the vicinity of the wall. With increasing viscous-scaled wire length, the peaks of the one- and two-dimensional energy spectrograms of temperature fluctuations are increasingly attenuated, but in a manner that is approximately predictable based on the sensor length. The temporal resolution of the cold-wire is analysed through experimental measurements in a wind-tunnel thermal boundary layer. The wire length-to-diameter aspect ratio (lw/dw) is varied by changing the wire diameter and keeping the wire length constant, such that the viscous-scaled wire length remains constant. The wire aspect ratio has no effect on the mean temperature profile (similar to the observations for viscous-scaled wire length). When the wire aspect ratio lw/dw approaches 1000, we observe no additional temporal attenuation of the temperature fluctuations beyond that attributed to the spatial resolution of the finite sensor size. However, as lw/dw is reduced below this value (<1000), we observe increasing temporal attenuation. The attenuation of temperature fluctuations due to insufficient wire aspect ratios is seen at almost all wall-normal distances and is broadband, affecting all energy scales in the boundary layer. In contrast, the spatial attenuation only influences small-scale energy on the order of the probe scale, which predominates in the near-wall region. For a given wire diameter of the cold-wire sensor, temperature fluctuations are better resolved by maintaining lw/dw≈1000 and sacrificing spatial resolution by increasing the viscous-scaled wire length. Smaller diameter wires must be employed to alleviate this compromise.

Impulse response of turbulent flow in smooth and riblet-walled channels to a sudden velocity increase

ABSTRACT This paper explores the use of a small-span direct numerical simulation for a transient, smooth-wall turbulent channel flow and then applies the small-span simulation to a transient channel flow with riblets. A flow configuration similar to that of S. He and M. Seddighi (J Fluid Mech. 2013;715:60–102) is used to study the impulse response of a half-height channel flow to an abrupt increase in bulk velocity (with a friction Reynolds number increasing from 180 to 418). A minimal domain span sufficient to include the near-wall quasi-streamwise vortices in the ‘healthy turbulence’ region is used. The turbulent flow undergoes reverse transition toward a quasi-laminar state, followed by a retransition phase to the new equilibrium state. On a smooth wall, detailed comparisons with a full-span case show that the small-span test case captures satisfactorily the essential dynamics during the entire transition process, although it yields a slight delay in recovery to the new equilibrium. This difference is attributed to a slower streak transient growth due to an underestimation of near-wall spanwise fluctuations. This underestimation is associated with the missing large attached eddies that are not contained in the small span of the simulation domain. These comparisons justify the use of small-span simulations for identifying the main flow physics in a non-equilibrium accelerating wall turbulence. The application to the riblet flow shows that riblets do not fundamentally affect the flow dynamics, but delay the retransition as a result of significantly milder streak meandering. The streak-stabilisation effect of riblets is still active in a strongly accelerating turbulence and tends to prolong the flow recovery.

Three dimensional flow motions in the viscous sublayer

We employ novel digital Fresnel reflection holography to capture the 3D flows within the viscous sublayer of a smooth-wall turbulent channel flow at Reτ=400. The measurements reveal unsteady and diverse flow patterns in the sublayer including nearly uniform high and low speed flows and strong small-scale (on the order of viscous wall units) spanwise meandering motions. The probability density functions (PDFs) of wall shear stresses show a clear discrepancy in high stress range with those from direct numerical simulation (DNS), which is attributed to the unresolved streamwise and spanwise motions by DNS. Moreover, the PDF of Lagrangian particle accelerations yields a stretched exponential shape like that in homogenous isotropic turbulence, indicating strong intermittency in the sublayer. We find a significant fraction of high accelerations is associated with the small-scale meandering motions. Our study helps explain the effect of sublayer-scale roughness on reducing drag and flow separation reported in the literature.

Particle–fluid–wall interaction of inertial spherical particles in a turbulent boundary layer

Abstract

Experimental study and neural networks prediction on thermal performance assessment of grooved channel air heater

This research presents a study on convective heat transfer and friction factor of turbulent airflow in a channel air heater fitted with isosceles triangular grooves only and isosceles triangular grooves combined with transverse baffles. Experiments were conducted within a rectangular channel with aspect ratio, AR=15 and height, H = 20 mm with three different groove depths (d) of 4, 6 and 8 mm, two different groove inclination angles (α) of 30° and 45° with two different triangular directions of down-stream (DG) and up-steam (UG) of air flow. The airflow rate was presented in terms of Reynolds numbers (Re) based on the inlet hydraulic diameter of the channel and was in the range of 6700 to 17,000. The experimental results showed a significant effect on the heat transfer rate and friction factor in the presence of the isosceles triangular grooves only compared to the smooth-wall channel, by about 1.23–1.91 and 1.12–3.76 times, respectively. While, the case study of isosceles triangular grooves (DG) combined with transverse baffles (TB) provided higher heat transfer rate and friction factor as compared to the smooth-wall channel, by about 1.32–2.00 and 1.25–3.83 times, respectively. The maximum thermal performance enhancement of 1.49 was achieved with the use of 30° down-stream isosceles triangular grooves with the depth of 8 mm combined with transverse baffles at Re ≈6700. Correlations for Nusselt number (Nu) and friction factor (f) for this work were also proposed. The neural networks models were designed for predicting the thermal enhancement factor (η) of a channel air heater with combined turbulators by using the Rapid-Miner Studio 9.5 software. The architecture of the model was 4–20–20–1 with Tanh–Tanh activation function and the use of 5–fold cross validation for the training and testing data segmentation together with the linear_sampling type at a number of the epoch of 1700 provided the best performance for the η prediction which had the coefficient of determination (R2) and mean squared error (MSE) of 0.998864 and 1.2 × 10−5, respectively.

Contributions of very large-scale motions to turbulence statistics in open channel flows

Time-resolved particle image velocimetry measurements were performed in smooth-walled open channels to investigate the contributions of very large-scale motions (VLSMs) to the turbulence characteristics in open channel flows. The focal point is to clarify the free surface effects on the characteristics of VLSMs and the contributions of VLSMs to the unique statistical features in open channel flows (i.e., the turbulent kinetic energy (TKE) redistribution and smaller wake strength of the mean velocity profile). The resulting wavelength of VLSMs in present smooth-walled open channels is approximately $20h$ ( $h$ is water depth), which is comparable to that in pipe and closed channels while smaller than that in rough-walled open channels, and they are shown to make a great contribution to turbulence statistics with over 50 % of streamwise turbulence intensity, Reynolds shear stress and negative net force coming from VLSMs in the outer layer. Compared with other wall-bounded flows, VLSMs maintain higher strength in the outer layer of open channel flows with non-negligible strength even in the near surface region ( $y\sim >0.8h$ ), indicating that the free surface seems to sustain/promote VLSMs. This strength difference of VLSMs closely relates to the TKE redistribution and smaller wake strength of the mean velocity in the outer layer of open channel flows. The higher streamwise turbulence intensity is mainly contributed from the higher strength of VLSMs therein. The decelerating role of VLSMs combining with their higher strength is vital for shaping the mean velocity profile, which therefore is speculated to make a great contribution to the smaller wake strength phenomenon.

Heat Transfer Evaluation for a Two-Pass Smooth Wall Channel: Stationary and Rotating Cases

Abstract This study presents findings on combined effects of Reynolds number and rotational effect for a two-pass channel with a 180-deg turn, numerically and experimentally. To have a better understanding of the flow behavior and to create a baseline for future studies, a smooth wall channel with the square cross section is used in this study. The Reynolds number varies between 6000 and 35,000. Furthermore, by changing the rotational speed, the maximum rotation number of 1.5 is achieved. For the numerical investigation, large eddy simulation (LES) is utilized. Results from the numerical study show a good agreement with the experimental data. From the results, it can be concluded that increasing both Reynolds number and rotational speed is in favor of the heat transfer coefficient enhancement, especially in the turn region.

Parameter extension simulation of turbulent flows

The use of parameter extension simulation (PES) as a mathematical method for simulating turbulent flows is proposed in this study. It is defined as the calculation of a turbulent flow with the help of a reference solution. A typical PES calculation includes three steps, as follows: setting up an asymptotic relationship between the exact solution of the Navier–Stokes equations and the reference solution for the initial parameter values, calculating the reference solution and the necessary asymptotic coefficients, and extending the reference solution to the parameter values to produce the exact solution. The method of controlled eddy simulation (CES) has been developed to calculate the reference solution and the asymptotic coefficients. The CES method is a special case of large eddy simulation (LES), in which a weighting coefficient and an artificial force distribution are used to damp part of the turbulent motions. The distribution of artificial force is modeled with the help of eddy viscosity. The reference-weighting coefficient can be determined empirically or in a convergence study. To demonstrate potential uses, the proposed PES method is used to simulate four types of turbulent flows. The flows are decaying homogeneous and isotropic turbulence, smooth-wall channel flows, rough-wall channel flows, and compressor-blade cascade flows. The numerical results show that the PES solution is more accurate than a Reynolds-average Navier–Stokes simulation solution. Unlike a traditional LES method, which uses the Smagorinsky, k-equation-transport, or WALE subgrid model, the PES requires a lower mesh resolution. These characteristics make it a potential method for simulating the engineering of turbulent flows with complex geometry and a high Reynolds number.

A review of liquid flow and heat transfer in microchannels with emphasis to electronic cooling

Since the realization of microchannel devices more than three and half decades ago with water as the cooling fluid providing heat transfer enhancement, significant progress has been made to improve the cooling performance. Thermal management for electronic devices with their ever-widening user profile remains the major driving force for performance improvement in terms of miniaturisation, long-term reliability, and ease of maintenance. The ever-increasing requirement of meeting higher heat flux density in more compact and powerful electronic systems calls for further innovative solutions. Some recent studies indicate the promise offered by processes with phase change and the use of active devices. But their adoption for electronic cooling still weighs unfavourably against long-term fluid stability and simplicity of device profile with moderate to high heat transfer capability. Applications and reviews of these promising research trends have been briefly visited in this work. The main focus of this review is the flow and heat transfer regime related to electronic cooling in evolving channel forms, whose fabrication are being enabled by the significant advancement in micro-technologies. Use of disruptive wall structures like ribs, cavities, dimples, protrusions, secondary channels and other interrupts along with smooth-walled channels with curved flow passages remain the two chief geometrical innovations envisaged for these applications. These innovations target higher thermal enhancement factor since this implies more heat transfer capability for the same pumping power in comparison with the corresponding straight-axis, smooth-wall channel configuration. The sophistication necessary to deal with the experimental uncertainties associated with the micron-level characteristic length scale of any microchannel device delayed the availability of results that exhibited acceptable matching with numerical investigations. It is indeed encouraging that the experimental results pertaining to simple smooth channels to grooved, ribbed and curved microchannels without unreasonable increase in pumping power have shown good agreement with conventional numerical analyses based on laminar-flow conjugate heat-transfer model with no-slip boundary condition. The flow mechanism with the different disruptive structures like dimple, cavity and rib, fin and interruption, vortex generator, converging-diverging side walls or curved axis are reviewed to augment the heat transfer. While the disruptions cause heat transfer enhancement by interrupting the boundary layer growth and promoting mixing by the shed vortices or secondary channel flow, the flow curvature brings in enhancement by the formation of secondary rolls culminating into chaotic advection at higher Reynolds number. Besides these revelations, the numerical studies helped in identifying the parameter ranges, promoting a particular enhancement mechanism. Also, the use of modern tools like Poincare section and the analysis of flow bifurcation leading to chaotic advection is discussed. Among the different disruptive structures, sidewall cavity with rib on the bottom wall within the cavity plays a significant role in augmenting the thermal performance. Among the different converging-diverging side walls or curved axis, the sinusoidal channel provides the highest mixing by the introduction of secondary vortices or dean vortices to augment the heat transfer with less pressure drop. The optimum geometry in terms of high heat transfer with low pressure plays a major role in the design of heat sink. Directions of some future research are provided at the end.

Assessment of models for velocity distribution in turbulent smooth-wall open channel flows

ABSTRACT This study attempts to provide a more precise and comprehensive model for stream-wise velocities in turbulent smooth-wall open channel flows. Velocity profiles are obtained from the resolution of two ordinary differential equations. The first is derived from the Reynolds-averaged Navier-Stokes (RANS) equation while the second is from the equilibrium between turbulent kinetic energy (TKE) production and dissipation. We assessed different analytical models for eddy viscosity, TKE and mixing length. Computation results for velocity profiles were assessed and compared to experimental data for different flow conditions and the well-known linear, log and log-wake laws. Our results show that the model based on the RANS equation provides more accurate velocity profiles. In the viscous sublayer and buffer layer, the method based on Prandtl’s eddy viscosity model and Van Driest mixing length gives more precise results. For the log layer and outer region, a mixing length equation derived from Von Karman’s similarity hypothesis, provides the best agreement with measured data except near the free surface where we used an additional correction based on a damping function for eddy viscosity. This method allows more accurate velocity profiles with the same value of the damping coefficient that is valid under different flow conditions.

THE RECALCULATING TECHNIQUE THE HEAT-HYDRAULIC CHARACTERISTICS OF DIRECT-FLOW CYLINDRICAL STEAM GENERATORS OPERATING ON A FREON COOLANT, WITH THE BOUNDARY CONDITIONS OF THE SECOND KIND FOR THE BOUNDARY CONDITIONS OF THE FIRST KIND

The article is devoted to the development of a method for recalculating the thermal-hydraulic characteristics of direct-flow cylindrical steam-generating channels operating on a Freon coolant from the boundary conditions of the second kind for the boundary conditions of the first kind. The need for the development of such recalculation procedure is due to the presence in the literature of calculated dependences describing the heat transfer during evaporation of the coolant in porous channel for boundary conditions of the second kind, while the practical plan problems are often conditioned by other boundary conditions, in particular boundary conditions of the first kind. To clarify the essence of the recalculating method, the article briefly summarizes the calculated dependencies for computation of heat transfer and pressure drop in steam generating smooth-walled channels operating on a Freon heat carrier. The proposed method of recalculation is based on the use of the method of successive approximations when calculating the total amount of heat that is necessary for the phase transition process of evaporation. The final goal of the recalculation technique was to create a program for calculating the heat-hydraulic efficiency of direct-flow cylindrical steam generators operating on a Freon coolant. The proposed recalculation technique makes it possible to calculate in such steam generators for boundary conditions of the first kind next thermal-hydraulic characteristics: the channel length necessary for complete evaporation of the heat carrier; power required for pumping the coolant and the total amount of heat transferred to the coolant during the evaporation process.

A coupled DEM-FTSM approach with a new cost-effective searching algorithm of particles for gas-solid flow with heat transfer

The utilization of thermal energy in solid-gas flow is one of matters of general interest, and therefore the phenomena in solid-gas flow play important roles for interactions of particle-gas, particle-particle, and heat transfer. A coupled discrete element method (DEM) - fast transform spectral method (FTSM) approach is developed for non-isothermal gas-solid flow in a smooth-wall vertical channel. The motivation for this research is to use the coupled DEM-FTSM approach to study the influence of mass loading ratio on the heat transfer of gas-solid flow. The mass loading ratio is increased till 4, which is relatively higher under the background of modern direct numerical simulation (DNS) of gas-solid flow with particle diameter of 500 μm. Both the particle-gas and inter-particle interactions are taken into account. A new cost-effective searching algorithm of particles is developed to record the particle-feature point pair. A smoothed Dirac delta function is adopted to link the point particles and the gas flow field for the first time. The feature point is also used to divide the computing domain into a number of blocks for investigating the influence of inter-particle interactions. The comparative results demonstrate that the predictions of the coupled DEM-FTSM approach are in good agreement with the known results. It is shown that the difficulty of the excessive computational cost in the traditional CFD-DEM approach can be overcome by applying the present method. The final simulation results indicate that particles have a great influence on the heat transfer of the gas-solid flow. Moreover, as the mass loading ratio increases, the heat transfer capacity of the gas-solid flow also increases significantly.

Experimental investigation on overall thermal performance of fluid-flow in a rectangular channel with discrete V-pattern baffle

This work presents the results of an experimental study of thermohydraulic performance of rectangular channel having discrete V-pattern baffle attached on the broad wall. Measurements have been carried out for the aspect channel ratio of 10, Reynolds number from 3000 to 21000, relative baffle height value of 0.50, relative baffle pitch value of 1.5, relative gap width value of 1.0, flow attack angle value of 60°, and relative discrete distance values of 0.26 to 0.83. The heat transfer and friction factor data obtained were compared with the data obtained from a smooth wall channel under similar operating conditions. In comparison to the smooth wall channel the discrete V-pattern baffle channel enhanced the Nusselt number and friction factor by 3.89 and 6.08 times, respectively. The overall thermal performance parameter is found superior for the relative discrete distance of 0.67. Discrete V-pattern baffle roughness shape has also been shown to be overall thermal performance higher in comparison to other continuous (without discrete) V-pattern baffle shape rectangular channel.

Flow Interactions Between Streamwise-Aligned Tandem Cylinders in Turbulent Channel Flow

The flow interactions between streamwise-aligned wall-mounted circular cylinders is studied experimentally to explore shear-layer interactions that lead to modifications of the flow around the downstream-most cylinder. Two identical finite-length cylinders are arranged in tandem and fully submerged in the log layer of turbulent channel flow, and particle image velocimetry measurements are conducted to document the flow behavior at various streamwise spacings. Comparisons are made to flow around an isolated cylinder as well as the baseline case of smooth-wall turbulent channel flow with no cylinders present. Strong interactions between the shear layers shed by each individual cylinder are noted, whereby the downstream-most wake exhibits a weaker, more spatially compact recirculation region and significantly reduced reattachment length as compared to flow around an isolated cylinder. First- and second-order turbulence statistics show similar wake modifications, particularly for small cylinder spacings where the shear layer shed from the top of the upstream cylinder directly impinges on the downstream-most cylinder and significantly reduces the turbulence levels in the downstream shear layer. Increasing the spacing of the cylinders relaxes the turbulence statistics in the shear layer of the downstream-most cylinder back toward those of an isolated cylinder, although even a spacing of eight cylinder diameters is not sufficient to fully recover isolated cylinder behavior. Finally, the impact of the cylinder aspect ratio is considered is found to have only a modest impact on the observed trends in reattachment and shear-layer interactions between the cylinders.

Improvements on digital inline holographic PTV for 3D wall-bounded turbulent flow measurements

Three-dimensional (3D) particle image velocimetry (PIV) and particle tracking velocimetry (PTV) provide the most comprehensive flow information for unraveling the physical phenomena in a wide range of fluid problems, from microfluidics to wall-bounded turbulent flows. Compared with other 3D PIV techniques, such as tomographic PIV and defocusing PIV, the digital inline holographic PTV (DIH-PTV) provides 3D flow measurement solution with high spatial resolution, low cost optical setup, and easy alignment and calibration. Despite these advantages, DIH-PTV suffers from major limitations including poor longitudinal resolution, human intervention (i.e. requirement for manually determined tuning parameters during tracer field reconstruction and extraction), limited tracer concentration, small sampling volume and expensive computations, limiting its broad use for 3D flow measurements. In this study, we present our latest developments on minimizing these challenges, which enables high-fidelity DIH-PTV implementation to larger sampling volumes with significantly higher particle seeding densities suitable for wall-bounded turbulent flow measurements. The improvements include: (1) adjustable window thresholding; (2) multi-pass 3D tracking; (3) automatic wall localization; and (4) continuity-based out-of-plane velocity component computation. The accuracy of the proposed DIH-PTV method is validated with conventional 2D PIV and double-view holographic PTV measurements in smooth-wall turbulent channel flow experiments. The capability of the technique in characterization of wall-bounded turbulence is further demonstrated through its application to flow measurements for smooth- and rough-wall turbulent channel flows. In these experiments, 3D velocity fields are measured within sampling volumes of 14.7 × 50.0 × 14.4 mm3 (covering the entire depth of the channel) with a velocity resolution of <1.1 mm/vector. Overall, the presented DIH-PTV method and measurements highlight the applicability of DIH-PTV to obtain 3D characterization of the turbulent structures with velocity spatial resolution and within sampling volume sizes comparable to other the-state-of-the-art 3D whole-field flow measurement techniques.

Nusselt Number as Composite Functions of Aspect Ratio and Wall Inclination in Parallelogram Microchannels with Three Types of Thermal Boundary Conditions

AbstractIn this study, attention is focused on the numerical simulations of laminar fluid flow and heat transfer in straight smooth-walled parallelogram channels with various aspect ratios (α) and inclined angles (θ). The Reynolds number (Re), characterized by the channel hydraulic diameter and the working fluid of water, is fixed at 100. The examinedαandθrange from 1 to 10 and 45° to 90°, respectively. Their effects on the thermal fluid features are explored under three thermal boundary conditions: constant wall temperature (TBC), constant axial heat transfer rate with constant peripheral temperature (H1BC), and constant wall heat flux (H2BC). The SIMPLE algorithm is employed for velocity–pressure coupling with the algebraic multigrid method, while the second-order upwind scheme is utilized for spatial discretization in pressure term; the momentum and energy equations are solved with a QUICK scheme; Least Squares Cell-Based Gradient Evaluation is applied for predicting scalar values at the cell faces and for computing secondary diffusion terms and velocity derivatives. One of the new findings is that there exists a critical value ofθ= 70° below which the Nusselt number under H2BC increases with increasingαwhereas beyond which the trend reverses, a result distinct from those computed with TBC and H1BC. Moreover, TBC is found to be a time-saving alternative to H1BC. Furthermore, both Nusselt numbers under the three thermal boundary conditions and friction factor timesReare successfully and compactly correlated with α andθto offer useful reference for designing micro-cooling channels.

Deflagration-to-detonation transition in narrow channels: hydraulic resistance versus flame folding

This paper is concerned with the identification of the key interactions controlling the deflagration-to-detonation transition in narrow smooth-walled channels. Two agencies contributing to the transition are discussed: hydraulic resistance and flame folding. Depending on the kinetics and parameters of the system, nucleation of detonation may occur near the channel wall and/or in the channel interior. A possibly unexpected outcome of the resistance-folding interplay is the non-monotonicity of the dependence between the predetonation run up time/distance and the channel width.

On the near-wall effects induced by an axial-flow rotor

Laboratory experiments were performed to quantify the near-wall flow characteristics and the bed shear stress in the vicinity of an axial-flow rotor model. It was placed in a smooth-wall open channel flow under subcritical conditions and operated at two heights. A laser Doppler velocimeter was used to collect profiles of the streamwise and vertical velocity at few locations within the rotor symmetry plane as close as one wall unit from the bed. Local estimates of the mean bed shear stress were obtained from the mean velocity profiles in the viscous and logarithmic regions. Results show that the bed shear stress is sensitive to the distance from the rotor and to the relative height of the rotor. Maximum bed shear stress occurred downstream of the rotor with the rotor closer to the bed. Flow statistics shows a reduction of turbulence intensity and turbulent shear stress near the wall and close to the rotor due to flow acceleration and changes in the pressure distribution. Spectral analysis of the streamwise velocity evidences a reduction of the energy content across scales spanning two decades right below the turbine. The increase of bed shear is due to mainly flow accelerating but not to turbulence.