Heat transfer in a rectangular chamber with differentially heated horizontal walls: Effects of a vibrating sidewall

Heat transfer in a gas-filled closed enclosure with differentially heated horizontal walls is investigated numerically. One of the sidewalls vibrates with specified frequency and amplitude to induce forced convective flows in the enclosure. The vibrating and the stationary sidewalls are considered to be thermally insulated while the two horizontal walls are differentially heated. To simulate the flow field, the full compressible form of the Navier–Stokes equations is considered and solved by a highly accurate flux-corrected transport algorithm. In the numerical model, temperature dependant heat conductivity and viscosity are taken into account. The presence of acoustic streaming is found to have significant effect on the heat transfer. Also the presence of temperature gradients in the enclosure is found to affect the formation of acoustically induced streaming flows.     

Experimental study on the effects of acoustic streaming of high frequency ultrasonic waves on convective heat transfer: Effects of transducer position and wave interference

In the present study, an experimental study was carried out in order to find the effect of high frequency ultrasound wave transducer position on heat transfer rate. For this purpose, a thin platinum wire, which was used as a heater and sensor, was submerged in deionized water. The high frequency ultrasound transducers were installed at various positions including; two on lateral wall and three at the bottom of a cylindrical container. The interference effects of multiple transducers were investigated and it was found that a single transducer installed at the lateral wall works more efficiently than other layouts from heat transfer enhancement point of view.     

On the quasi-instantaneous aerodynamic load and pressure field measurements on turbines by non-intrusive PIV

An experimental technique is presented to non-intrusively measure the quasi-instantaneous aerodynamic loads and surrounding pressure field for a turbine by using particle image velocimetry (PIV). The PIV measurements provide the velocity flow field needed to calculate the pressure field around the turbine using three different methods. In the first method, the quasi-instantaneous and mean pressure fields are obtained by solving the Poisson equation and by calculating the boundary conditions from the Navier–Stokes equations. In the second method, the pressure at the boundaries is determined by spatial integration of the pressure gradient. In the third method, the pressure is calculated using the Bernoulli equation. The experimental results are compared to aerodynamic load theoretical predictions from the Blade Element Momentum theory (BEM). An analysis of the experimental results showed the importance of the local acceleration, convective and pressure terms when calculating the forces and the pressure field in a stationary reference frame. Only the Poisson method includes all these terms, and had a small standard deviation between the calculated instantaneous forces. Furthermore, the Poisson method results are independent of the control volume size investigated while the other two experimental methods are affected. This experimental technique could be used to simultaneously replace instrumentation such as force balance and pressure taps while providing for the first time quasi-instantaneous information about the surrounding flow in any turbine immersed in an incompressible flow. In addition, it could be applied to evaluate unsteady wind loads and aerodynamic stall and also provide much needed information for validating computational studies.     

The wake structure in a 2D grid installation of the horizontal axis micro wind turbines

In order to develop applications for micro-wind turbines, an experimental analysis of the flow field around integrated micro-wind turbines was performed. The wake flows of a single turbine and 5×5 array unit were measured by using hot-wire and ultrasonic anemometers and particle image velocimetry (PIV). The present array of turbines follows a fundamental lattice layout; however, it has the flexibility to optimize its layout according to the environmental conditions. hot-wire and ultrasonic anemometers and PIV measurements were used for stand-alone turbines and their integrated systems. Comparisons between the mean velocity field and turbulent intensity were described for stand-alone full-scale and 1/10-scale wind turbine models. Thereafter, a typical array of the 1/10-scale model was assumed and its wake flow was investigated in a wind tunnel. The velocity profile and turbulence behind the array were measured and studied at different streamwise locations. The scale effect and model similarities were discussed. The experimental results show that a zone exists with constant and linear wake deficit ratios in the downstream regions.     

Effects of aspect ratio on mixed convection in a horizontal rectangular duct with heated and cooled side walls

This paper describes the effect of aspect ratio on mixed convection in a horizontal rectangular duct with heated and cooled side walls numerically and experimentally. In the numerical analysis, fluid flow and temperature distributions for Ri=1.61, Pr=6.99, Re=100 and aspect ratio, Ar=0.2–10, were obtained by solving dimensionless governing equations using the SIMPLE procedure. The QUICK scheme was applied to the convective term of these equations. In the experimental analysis, the flow behavior for A_r=0.5–2 was visualized by the dye‐injection method. Numerical results showed that the swirl flow was generated along the flow direction, and its pitch length was influenced by A_r. The pitch length was the shortest when A_r=0.5–1, and this tendency was the same in numerical results and experimental results. The heat transfer behavior was also discussed corresponding to the flow, and the heat transfer ratio was highest at A_r=1 in 0.2 ≤ A_r ≤ 10. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20391     

Effect of aspect ratio on entropy generation in a rectangular cavity with differentially heated vertical walls

In the present study, entropy generation in rectangular cavities with the same area but different aspect ratios is numerically investigated. The vertical walls of the cavities are at different constant temperatures while the horizontal walls are adiabatic. Heat transfer between vertical walls occurs by laminar natural convection. Based on the obtained dimensionless velocity and temperature values, the distributions of local entropy generation due to heat transfer and fluid friction, the local Bejan number and local entropy generation number are determined and related maps are plotted. The variation of the total entropy generation and average Bejan number for the whole cavity volume at different aspect ratios for different values of the Rayleigh number and irreversibility distribution ratio are also evaluated. It is found that for a cavity with high value of Rayleigh number (i.e., Ra = 10⁵), the total entropy generation due to fluid friction and total entropy generation number increase with increasing aspect ratio, attain a maximum and then decrease. The present results are compared with reported solutions and excellent agreement is observed. The study is performed for 10² < Ra < 10⁵, 10^− 4 < ? < 10^− 2, and Pr = 0.7.     

Influence of sleeve tube on the flow and heat transfer behavior at a T-junction

The flow structure of a sleeved jet into a main crossflow was experimentally investigated employing particle imaging velocimetry technology and numerically simulated using a CFD code. The jet-to-crossflow velocity ratio, VR, was ranged from 0.5 to 8. Three basic flow patterns were marked, namely attaching jet, lift-off jet and impinging jet as VR gradually increased. The flow in the main duct was characterized by a stream of discharge from the annular space at the rear part of the sleeve near the jet exit, which primarily came from the upstream crossflow. This annulus discharge isolated the leeward wall from the jet fluid and also caused weak local heat transfer in the large momentum deficiency region, and hence could supply an effective protection of the leeward wall from the thermal shock caused by a very cold jet injection.     

Influence of convective cooling on the temperature in a frictionally heated strip and foundation

The analytical solution of a boundary-value heat conduction problem of friction for the tribosystem consisting of a semi-infinite foundation and a plane-parallel strip sliding over its surface is obtained. The evolution of temperature and its distribution in depth from a contact surface for materials of frictional couple, such as aluminum-steel, is studied.     

Effects of Prandtl number on three‐dimensional mixed convection in a horizontal square duct with heated and cooled side walls

We examined the effects of Prandtl number on three‐dimensional mixed convection in a horizontal square duct with heated and cooled side walls numerically. Non‐dimensional governing equations were solved for Re = 100, Pr = 0.1–10, and Ri = 36.44 by the SIMPLE method. The numerical results show that the swirl flow was generated along the flow direction, and its pitch lengthened with the increase of Pr. We also examined the strength of swirl flow using the swirl number, S, and we discuss heat transfer behavior as it corresponded to the flow. Heat transfer was promoted by the swirl flow with all Pr, and the optimum value existed within these Pr. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20319     

Numerical and experimental investigations on effect of fan height on the performance of piezoelectric fan in microelectronic cooling

A piezoelectric fan is an attractive device to remove heat from microelectronic systems due to its low power consumption, minimal noise and compactness. In the present study, a piezoelectric fan is investigated to analyze the cooling capability for possible use in electronic devices. Both numerical and experimental analyses are carried out on the piezoelectric fan which was oriented horizontally. The FLUENT 6.3 software is used in the 2D simulation to predict the heat transfer coefficient and the flow fields using a dynamic mesh option to observe the fan swinging phenomena. Two heat sources in in-line arrangement are used in the experiment. The flow measurements are carried out at different piezoelectric fan heights by using a particle image velocimetry (PIV) system. The result shows that the piezofan height of h_p/l_p = 0.23 can reduce the temperature of the heat source surface as much as 68.9 °C. The numerical and experimental values of heat transfer coefficients are plotted and found in good agreement.     

Heat and fluid flow resulting from the chimney effect in a symmetrically heated vertical channel with adiabatic extensions

An experimental study on a channel-chimney system was carried out in order to elucidate the behavior of heat transfer and fluid flow. The results are presented in terms of local air temperature measurements inside the symmetrically heated channel and between the adiabatic extensions. Different fluid motion regions are observed inside the chimney. Inflows of air are detected in the lower extension ratio, particularly for large values of the ratio of the width of chimney to that of the heated channel. Some typical configurations show the presence of a vortex structure for an expansion ratio greater than one close to the corner regions in the chimney. Some monomial correlation equations between the local Nusselt number, the channel Rayleigh number and the geometric parameters are proposed. The dimensionless parameters are in the following ranges: 10²Ra^*(B/b)10⁶; 1.5L/L_h4.0; 1.0B/b4.0, in which L is the total height of the system, L_h is the height of the heated channel, B is the width of the chimney and b is the width of the heated channel. A good agreement between the correlation and the experimental data is observed.     

Influence of surface radiation on the transition to unsteadiness for a natural convection flow in a differentially heated cavity

Abstract

The influence of surface radiation on the transition to the unsteady state in natural convection is studied numerically. The configuration of the differentially heated square cavity with adiabatic horizontal walls is chosen to generate an internal natural convection flow. It is known that radiative transfers reduce the temperature difference between the adiabatic walls, which consequently reduces the thermal stratification of the central zone and increases the velocity flow. Many studies have focused on the stationary regime, but few of them have investigated the transition to unsteady flow. For this purpose, the effect of the wall emissivity on the critical Rayleigh number and the associated critical frequency was studied for a given cavity length. The cavity length and mean temperature of isothermal walls are set for the whole study. The results show that all these values are between the values obtained without radiation and those obtained for perfectly conducting horizontal walls. The critical Rayleigh number decreases with emissivity while the associated frequency increases. Moreover, the symmetry of fluctuating properties of the flow is changed when the radiation is taken into account.     

Influence of vertical vibrations on the separation of a binary mixture in a horizontal porous layer heated from below

We study the influence of vertical high-frequency and small-amplitude vibrations on the separation of a binary mixture saturating a shallow horizontal porous layer heated from below. The monocellular flow obtained for a separation ratio ψ > ψ_mono > 0 leads to a migration of the species towards the two vertical boundaries of the cell. The 2D direct numerical simulations and the linear stability analysis of the averaged governing equations show that the vertical vibrations delay the transition from monocellular flow to multicellular flow. The vibrations also decrease the value of ψ_mono, which allows species separation for a wide variety of binary mixtures.     

Influence of ramped pressure gradient on transient flow formation in a horizontal porous channel

The transient flow formation in a horizontal porous channel assuming a ramped pressure gradient is presented. The equation governing the flow is modeled into a partial differential equation (PDE) which is solved by employing the Laplace transformation technique to transform the PDE to an ordinary differential equation (ODE). The obtained ODE is solved by employing the method of undetermined coefficients to obtain the velocity profile in the Laplace domain. The Riemann sum approximation technique is then adopted to change the obtained solution from the Laplace domain into the time domain. For accuracy checks, the numerical results of the obtained equation are reckoned with previously published work, and an excellent agreement is found. For a clearer understanding of the impact of various flow parameters entering the solutions obtained, graphical and tabular representations are offered using MATLAB software. We noticed that the velocity is slower with ramped pressure gradient compared to a constant pressure gradient. This is because the motion of the fluid occurs gradually with ramped pressure gradient.     

Effects of ridged walls on the heat transfer in a heated square duct

Turbulent flows in rectangular cooling ducts of rocket engine thrust chambers are characterized by secondary motions of Prandtl’s first and second kinds. These secondary currents play a prominent part in heat transfer between the thrust chamber and the cooling gas conveyed in the duct. Previous numerical and experimental works reveal that attaching ridges on the walls of the duct causes the formation of new secondary flows of Prandtl’s second kind. These new structures are likely to increase the heat transfer. The present study has investigated numerically, through large eddy simulations, the effect of different forms of ridges on heat transfer in straight square duct flows.     

Flow characterization in the near‐wake region of a horizontal axis wind turbine

An experimental study is conducted to investigate the flow dynamics within the near‐wake region of a horizontal axis wind turbine using particle image velocimetry (PIV). Measurements were performed in the horizontal plane in a row of four radially distributed measurement windows (tiles), which are then patched together to obtain larger measurement field. The mean and turbulent components of the flow field were measured at various blade phase angles. The mean velocity and turbulence characteristics show high dependency on the blade phase angle in the near‐wake region closer to the blade tip and become phase independent further downstream at a distance of about one rotor diameter. In the near‐wake region, both the mean and turbulent characteristics show a systemic variation with the phase angle in the blade tip region, where the highest levels of turbulence are observed. The streamlines of the instantaneous velocity field at a given phase allowed to track a tip vortex which showed wandering trend. The tip vortices are mostly formed at r/R > 1, which indicates the wake expansion. Results also show the gradual movement of the vortex region in the axial direction, which can be attributed to the dynamics of the helical tip vortices which after being generated from the tip, rotate with respect to the blade and move in the axial direction because of the axial momentum of the flow. The axial velocity deficit was compared with other laboratory and field measurements. The comparison shows qualitative similarity. Copyright © 2015 John Wiley & Sons, Ltd.     

燃煤流化床声发射频谱域特征分析

对燃煤流化床内声发射进行实验和理论分析 ,将床内声源分为单极子声源、双极子声源和四极子声源 ,从声射频谱域特征上确定了气固相对运动 (双极子声源 )对流化床声发射的作用以及噪声信号的影响 ,并对声发射信息与气固运动信息间的关系进行了理论探讨。     

Temperature oscillations in a differentially heated cavity with and without a fin on the sidewall

Temperature measurements of the thermal flows in a differentially heated cavity with and without a fin on the sidewall were performed. The oscillatory behaviours in the transition from an initially isothermal state to an eventually stratified interior fluid ambient are characterized. It is revealed that, for the case without a fin, the amplitude of the oscillations in the thermal boundary layer increases as the stratification of the interior fluid increases. For the case with a fin, the temperature measurements show that the oscillations are induced by an unstable fluid layer above the fin. The spectral analysis reveals that the frequency properties of the oscillations are different between the cases with and without a fin.     

Experimental study of industrial gas turbine flames including quantification of pressure influence on flow field,fuel/air premixing and flame shape

A commercial swirl burner for industrial gas turbine combustors was equipped with an optically accessible combustion chamber and installed in a high-pressure test-rig. Several premixed natural gas/air flames at pressures between 3 and 6 bar and thermal powers of up to 1 MW were studied by using a variety of measurement techniques. These include particle image velocimetry (PIV) for the investigation of the flow field, one-dimensional laser Raman scattering for the determination of the joint probability density functions of major species concentrations, mixture fraction and temperature, planar laser induced fluorescence (PLIF) of OH for the visualization of the flame front, chemiluminescence measurements of OH^* for determining the lift-off height and size of the flame and acoustic recordings. The results give insights into important flame properties like the flow field structure, the premixing quality and the turbulence–flame interaction as well as their dependency on operating parameters like pressure, inflow velocity and equivalence ratio. The 1D Raman measurements yielded information about the gradients and variation of the mixture fraction and the quality of the fuel/air mixing, as well as the reaction progress. The OH PLIF images showed that the flame was located between the inflow of fresh gas and the recirculated combustion products. The flame front structures varied significantly with Reynolds number from wrinkled flame fronts to fragmented and strongly corrugated flame fronts. All results are combined in one database that can be used for the validation of numerical simulations.     

On the ignition and flame development in a spray-guided direct-injection spark-ignition engine

High-speed fuel, flow, and flame imaging are combined with spark discharge measurements to investigate the causes of rare misfires and partial burns in a spray-guided spark-ignited direct-injection (SG-SIDI) engine over a range of nitrogen dilution levels (0–26% by volume). Planar laser induced fluorescence (PLIF) of biacetyl is combined with planar particle image velocimetry (PIV) to provide quantitative measurements of equivalence ratio and flow velocity within the tumble plane of an optical engine. Mie scattering images used for PIV are also used to identify the enflamed region to resolve the flame development. Engine parameters were selected to mimic low-load idle operating conditions with stratified fuel injection, which provided stable engine performance with the occurrence of rare misfire and partial burn cycles. Nitrogen dilution was introduced into the intake air, thereby displacing the oxygen, which destabilized combustion and increased the occurrence of poor burning cycles. Spark measurements revealed that all cycles exhibited sufficient spark energy and duration for successful ignition. High-speed PLIF, PIV, and Mie scattering images were utilized to analyze the spatial and temporal evolution of the fuel distribution and flow velocity on flame kernel development to better understand the nature of poor burning cycles at each dilution level. The images revealed that all cycles exhibited a flammable mixture near the spark plug at spark timing and a flame kernel was present for all cycles, but the flame failed to develop for misfire and partial burn cycles. Improper flame development was caused by slow flame propagation which prevented the flame from consuming the bulk of the fuel mixture within the piston bowl, which was a crucial step to achieve further combustion. The mechanisms identified in this work that caused slower flame development are: (1) lean mixtures, (2) external dilution, and (3) convection velocities that impede transport of the flame into the fuel mixture.