Effect of turbulent fluctuation on the ignition of millimeter particle:Experimental studies and numerical modelling with a new correlation of nusselt number Author links open overlay panel

Understanding the influencing mechanism of turbulent fluctuation on the ignition characteristics of millimeter coal particles is essential.In this work,to study the effect of turbulent fluctuation on ignition time,millimeter coal particles are subjected to a specific flow field,generated in a furnace with symmetric fans.A one-dimensional model with the new proposed correlation and the Ranz-Marshall(R-M)correlation for Nu(Nusselt number)is established to simulate the coal ignition process.In addition,the effects of fan speed,temperature,particle diameter,particle distance and coal type on the ignition time are investigated.It is found that an increase in fan speed from 0 to 3000 rpm leads to a particle Reynolds number Re_(p)increase from 0 to 22.5,and a turbulent particle Reynolds number Re_(t)*increase from 0 to 71.5.With a consideration of the fluctuation effect,the new correlation of Nu gives a better prediction of ignition time compared to the R-M correlation.Moreover,the ignition time is revealed to decrease with an increasing fan speed and an elevating temperature.While the ignition time shows merely an initial boost with enlarging particle distance,it exhibits a linearity with the term of particle diameter dp1.3-1.7 and Reynolds numbers(Nu*/Nu)-0.6(Nu*is turbulent Nusselt number).Based on this relationship,the difference of predicted ignition time is calculated at different Re_(p)and Re_(t)*.It is shown that at low Re_(p)or high Re_(t)*values,the new correlation should substitute for the R-M correlation.

Numerical analysis of turbulent fluctuations around an axisymmetric body of revolution based on wall-modeled large eddy simulations

Wall-modeled large eddy simulation(WMLES)is used to investigate turbulent fluctuations around an axisymmetric body of revolution.This study focuses on evaluating the ability of WMLES to predict the fluctuating flow over the axisymmetric hull and analyzing the evolution of turbulent fluctuations around the body.The geometry is the DARPA SUBOFF bare model and the Reynolds number is 1.2×10^(7),based on the free-stream velocity and the length of the body.Near-wall flow structures and complex turbulent fluctuation fields are successfully captured.Time-averaged flow quantities,such as time-averaged pressure and skin-friction coefficients,and time-averaged velocity profiles on the stern,achieved great agreements between WMLES results and experimental data.Self-similarity of time-averaged velocity defects within a self-similar coordinate up to twelve diameters from the tail.A comprehensive analysis of second-order statistics in the mid-body,stern,and wake regions is condutced.Numerical results agree well with experimental data and previous wall-resolved large eddy simulation(WRLES)results about root mean square(rms)of radial and axial fluctuating velocities at the stern.Turbulent fluctuations including turbulent kinetic energy(TKE)and second-order velocity statistics are identified as dual peak behavior and non-self-similar over the wake length,consistent with previous findings in the literature.This assessment enhances the understanding of WMLES capabilities in capturing complex fluctuating flow around axisymmetric geometries.

Effect of inertial particles with different specific heat capacities on heat transfer in particle-laden turbulent flow

The effect of inertial particles with different specific heat on heat transfer in particle-laden turbulent channel flows is studied using the direct numerical simulation（DNS） and the Lagrangian particle tracking method. The simulation uses a two-way coupling model to consider the momentum and thermal interactions between the particles and turbulence. The study shows that the temperature fields display differences between the particle-laden flow with different specific heat particles and the particle-free flow,indicating that the particle specific heat is an important factor that affects the heat transfer process in a particle-laden flow. It is found that the heat transfer capacity of the particle-laden flow gradually increases with the increase of the particle specific heat. This is due to the positive contribution of the particle increase to the heat transfer. In addition,the Nusselt number of a particle-laden flow is compared with that of a particle-free flow.It is found that particles with a large specific heat strengthen heat transfer of turbulent flow, while those with small specific heat weaken heat transfer of turbulent flow.

Broadening of cloud droplet size distributions and warm rain initiation associated with turbulence: an overview

In the study of warm clouds,there are many outstanding questions.Cloud droplet size distributions are much wider,and warm rain is initiated in a shorter time and with a shallower cloud depth than theoretical expectations.This review summarizes the studies related to the effects of turbulent fluctuations and turbulent entrainment-mixing on the broadening of droplet size distributions and warm rain initiation,including observational,laboratorial,numerical,and theoretical achievements.Particular attention is paid to studies by Chinese scientists since the 1950s,since most results have been published in Chinese.The review reveals that high-resolution observations and simulations,and laboratory experiments,are needed because knowledge of the detailed physical processes involved in the effects of turbulence and entrainment-mixing on cloud microphysics still remains elusive.The effects of turbulent fluctuations and entrainment-mixing processes have been unrealistically separated in most theoretical studies.They could be unified by further advancement of a systems theory into a predictive theory.Developing parameterizations for the effects of fluctuations and entrainment-mixing processes is still in its infancy,and more studies are warranted.

Experimental study for frequency spectrum of turbulent boundary layer pressure fluctuation

An experimental study on TBL (turbulent boundary layer) pressure fluctuation frequeny spectrum of a revolution body is presented. With the measured results, a relation of convective frequency fo of the models is obtained. Relations of turbuleat wall-pressure fluctuation spectrum in transition region and development region to frequency and speed are obtained also

Noise radiation of an elastic plate excited by the turbulent boundary layer pressure fluctuations

A unified theory for calculating the noise radiation of an infinite elastic plate excited by the turbulent boundary layer pressure fluctuations is presented . Using the wave number frequency transfer function to desribe the whole system , consisting of the plate and the liquid loading, a general expression of the cross spectrum was derived. It is an integral in the complex wave number plane and leads to a sum of the residues at the poles of two types. One pole introduced by the convective ridge of the pressure fluctuations yields a direct transfer component , which is an evanescent wave in liquid because the pole lies in the high wave number region . The other poles introduced by the transfer function of structure produce the radiation field components with the resonance modes of a liquid loaded plate. The pole positions and their residues can be computed approximately by use of the Resonance Scattering Theory . For the case of hydrodynamic noise, where the range of the frequency-thickness product of interest is relatively low , the symmetric zero-order mode dominates the noise radiation.

A new method of LES verification and validation for attached turbulent cavitating flow

The large eddy simulation(LES)is used to resolve the flow structure in the cavitating turbulent flow around the Clark-Y hydrofoil coupled with a homogeneous cavitation model.A new method is proposed in this paper to calculate the LES error of the time-averaged streamwise velocity for the LES verification and validation(V&V).From the instantaneous cavity patterns,it is demonstrated that the predicted results agree fairly well with the experimental data.With this new proposed method,the LES errors can be easily and effectively calculated with a limited mesh number,and the method might be used in the other applications of the LES V&V.Results of the LES errors obtained by the new method show that the relatively steady flow can be simulated with small errors,while the complex flow structures at the cavity shedding region might lead to an increase of errors in the LES modeling.In addition,the distributions of the resolved Reynolds stresses are used to estimate the influences of the cavitation on the turbulent fluctuations.Results indicate that the turbulent fluctuations for the cavitating flow are much larger in magnitude as compared to the cases without cavitation.

EXPERIMENTAL STUDY ON TURBULENT BOUNDARY LAYER CHARACTERISTICS OVER STREAMWISE RIBLETS

Measurements of characteristics by means of a two-component Laser DopplerVelocimeter (LDV) were carried out in turbulent boundary layers over both a symmetric V-shapedribbed plate and a smooth one in a low speed wind tunnel. The present results clearly indicate thatthe logarithmic velocity profile over the riblets surface is shifted upward with a 30. 9% increasein the thickness of the viscous sublayer. Also a change in the log-law region is found. And themaximum value of streamwise velocity fluctuations is reduced by approximately 17%. The skewness andflatness factors do not show any change besides those in the region of y^+ 【 0. 6 . It is evidentthat the Reynolds shear stress over the riblets is reduced. Further more, in log-law region, theReynolds shear stress has a larger reduction of up to 18%.

DIRECT NUMERICAL SIMULATIONS OF TURBULENT CHANNEL FLOWS WITH CONSIDERATION OF THE BUOYANCY EFFECT OF THE BUBBLE PHASE

Turbulent channel flows with consideration of the buoyancy effect of the bubble phase is investigated by means of the Direct Numerical Simulation （DNS）. This two-phase system is solved by a two-way coupling Lagrangian-Eulerian approach. The Reynolds number based on the friction velocity and the half-width of the channel is 194, and the gravitational acceleration varies from -0.5 to 0.5, ranging from the upflow to the downflow cases. This study aims to reveal the influence of buoyancy on the turbulence behavior and the bubble motion. Some typical statistical quantities, including the averaged velocities and velocity fluctuations for the fluid and bubble phases, as well as the flow structures of the turbulence fluctuations, are analyzed.

An overview of flow field computational methods for hydrodynamic noise prediction

Recently,the hydrodynamic noise is becoming a research hotspot because it not only affects the concealment and comfort of ships,but also affects the living condition of underwater mammals.Accurate prediction of hydrodynamic noise requires that the detailed flow field has been simulated temporally and spatially with high fidelity method.In this paper,we introduce the current issues and challenges for the prediction of hydrodynamic noise,and provide an overview to several detailed flow field simulation methods which aim to resolve these issues.The overview could point the future directions for hydrodynamic noise prediction.

The inception cavitating flows over an axisymmetric body with a blunt headform

The inception cavitating flows around a blunt body are studied based on flow visualizations and velocity field measureme- nts. The main purpose of the present work is to study the incipient cavity evolution and the interplay between the inception cavitation and the local turbulent flows. A high-speed video camera is used to visualize the cavitating flow structures, and the particle image velocimetry （PIV） technique is used to measure the velocity field, the vorticity, and the Reynolds stresses under non-cavitating and inception cavitating flow conditions. It is found that the appearance of visible cavities is preceded by the formation of a cluster of micro-bubbles not attached to the body surface and in a hairpin-shaped vortex structure. During its evolution, the cavity moves downstream with a lower speed. The effect of the incipient cavity is significant on the local vortical structures but slight on the time- averaged velocity distribution. The mean Reynolds stress distributions in the turbulent shear flow can be substantially altered by the incipient cavities. The presence of the incipient cavities can lead to the production of turbulent fluctuations.

Marine-Atmospheric Boundary Layer Characteristics over the South China Sea During the Passage of Strong Typhoon Hagupit

The structures and characteristics of the marine-atmospheric boundary layer over the South China Sea during the passage of strong Typhoon Hagupit are analyzed in detail in this paper. The typhoon was generated in the western Pacific Ocean, and it passed across the South China Sea, finally landfalling in the west of Guangdong Province. The shortest distance between the typhoon center and the observation station on Zhizi Island （10 m in height） is 8.5 km. The observation data capture the whole of processes that occurred in the regions of the typhoon eye, two squall regions of the eye wall, and weak wind regions, before and after the typhoon’s passage. The results show that： （a） during the strong wind （average velocityˉu≧10 m s？1） period, in the atmospheric boundary layer below 110 m, ˉu is almost independent of height, and vertical velocity ˉw is greater than 0, increasing with ˉu and reaching 2–4 m s？1 in the squall regions;（b） the turbulent fl uctuations （frequency＆gt;1/60 Hz） and gusty disturbances （frequency between 1/600 and 1/60 Hz） are both strong and anisotropic, but the anisotropy of the turbulent fl uctuations is less strong;（c） ˉu can be used as the basic parameter to parameterize all the characteristics of fl uctuations;and （d） the vertical fl ux of horizontal momentum contributed by the average fl ow （ˉu＆#183; ˉw） is one order of magnitude larger than those contributed by fl uctuation fl uxes （u＆#39;w＆#39; and v＆#39;w＆#39;）, implying that strong wind may have seriously disturbed the sea surface through drag force and downward transport of eddy momentum and generated large breaking waves, leading to formation of a strongly coupled marine-atmospheric boundary layer. This results in ˉw ＆gt; 0 in the atmosphere, and some portion of the momentum in the sea may be fed back again to the atmosphere due to ˉu ＆#183; ˉw＆gt;0.

Performances and Acoustic Noise of Micro Multi-blade Fan

In this paper, the performances and the acoustic noise of the traditional type micro multi-blade fan were investi- gated experimentally and numerically, to optimize the specifications of the fan for the resident circumstances. The acoustic noise level decreases but the efficiency deteriorates slightly with the increase of the blade number of the impeller. Besides, the acoustic noise decreases with the increase of the distance between the impeller outlet and the volute tongue, in accompanying with the increase of the input and the deterioration of the fan efficiency.

Effect of the Uneven Circumferential Blade Space on the Performance of Small Axial Flow Fan

Effects of the uneven circumferential blade space on static characteristics and aerodynamic noise of a small axial flow fan are studied in this work.The blade angle modulation is adopted to design a series of unequally spaced fans,which have different maximum of modulation angular displacement.The steady flow is simulated by the calculations of Navier-Stokes equations coupled with RNG k-epsilon turbulence model,while the unsteady flow is computed with large eddy simulation.According to theoretical analysis,a fan with a maximum of modulation angular displacement of 6° is regarded as the optimal unequally spaced fan.The experiment of static characteristic is carried out in a standard wind tunnel and the aerodynamic noise of both fans is tested in a semi-anechoic room.Then,performances of the optimal unequally spaced fan are compared with those of the prototype fan.The results show that there is reasonable agreement between the simulation results and the experimental data.It is found that the discrete noise of the optimal unequally spaced fan is lower than that of the prototype fan at the near field monitoring point.This can be explained that the total pressure fluctuation of the optimal unequally spaced fan is much more regular than that of the prototype fan.