Laminar forced convection flow over a backward facing step using nanofluids

Laminar forced convection flow of nanofluids over a 2D horizontal backward facing step placed in a duct is numerically investigated using a finite volume method. A 5% volume fraction of nanoparticles is dispersed in a base fluid besides using various types of nanoparticles such as Au, Ag, Al₂O₃, Cu, CuO, diamond, SiO₂, and TiO₂. The duct has a step height of 4.8 mm, and an expansion ratio of 2. The Reynolds number was in the range of 50 ≤ Re ≤ 175. A primary recirculation region has been developed after the sudden expansion and it starts to change to become fully developed flow downstream of the reattachment point. The reattachment point is found to move downstream far from the step as Reynolds number increases. Nanofluid of SiO₂ nanoparticles is observed to have the highest velocity among other nanofluids types, while nanofluid of Au nanoparticles has the lowest velocity. The static pressure and wall shear stress increase with Reynolds number and vice versa for skin friction coefficient.     

Control of separated fluid flow and heat transfer characteristics over a backward facing step

The control of laminar fluid flow and heat transfer characteristics over a backward facing step have been studied by varying location and orientation of a thin adiabatic fin mounted on the top wall. The detailed investigation of geometrical parameters of fin (length, location and orientation) for two different Reynolds numbers is performed numerically and the results are compared to the case without fin. It is found that fin location and orientation can be used to control the primary reattachment point and the peak of local Nusselt number effectively and it acts as a passive controller.     

Laminar heat transfer enhancement downstream of a backward facing step by using a pulsating flow

This study is motivated by the need to devise means to enhance heat transfer in configurations, like the back step, that appear in certain types of MEMS that involve fluid flow and that are not very efficient from the thermal transfer point of view. In particular, the work described in this paper studies the effect that a prescribed flow pulsation (defined by two control parameters: velocity pulsation frequency and pressure gradient amplitude at the inlet section) has on the heat transfer rate behind a backward facing step in the unsteady laminar 2-D regime. The working fluid that we have considered is water with temperature dependent viscosity and thermal conductivity. We have found that, for inlet pressure gradients that avoid flow reversal at both the upstream and downstream boundary conditions, the time-averaged Nusselt number behind the step depends on the two above mentioned control parameters and is always larger than in the steady-state case. At Reynolds 100 and pulsating at the resonance frequency, the maximum time-averaged Nusselt number in the horizontal wall region located behind the step whose length is four times the step height is 55% larger than in the steady-case. Away from the resonant pulsation frequency, the time-averaged Nusselt number smoothly decreases and approaches its steady-state value.     

Sidewall effects on heat transfer in narrow backward facing step in transitional regime

In this work, we study numerically with large eddy simulation, the effects induced by the three-dimensional geometry of the channel on the flow topology that exists when the three-dimensional intrinsic instabilities appear in a backward facing step flow with low aspect ratio for Reynolds in the transitional regime (Re = 1,000–1,600), and its impact on the heat flux in the lower wall. Under the transitional regime, the three-dimensional instabilities begin to appear, but they can be masked by the flows due to the presence of the side walls. The study is carried out with two boundary conditions in the sidewalls, slip, and no-slip, to discriminate between the three-dimensionality induced by the geometry and the intrinsic three-dimensional instabilities. The results obtained are compared between the two boundary conditions, establishing what type of flow prevails and its influence on time-averaged mean Nusselt number for all Reynolds.     

Identification of forced convection in pulsating flow at a backward facing step with a stationary cylinder subjected to nanofluid

In the present study, the application of the system identification method for forecasting the thermal performance of forced pulsating flow at a backward facing step with a stationary cylinder subjected to nanofluid is presented. The governing equations are solved with a finite volume based code. The effects of various parameter frequencies (0.25 Hz–8 Hz), Reynolds number (50–200), nanoparticle volume fraction (0.00–0.06) on the fluid flow and heat transfer characteristics are numerically studied. Nonlinear system identification toolbox of Matlab is utilized to obtain nonlinear dynamic models of data sets corresponding to different nanoparticle volume fractions at frequencies of 1, 4 and 8 Hz. It is observed that heat transfer is enhanced with increasing the frequency of the oscillation, nanoparticle volume fraction and Reynolds number. The level of the nonlinearity (distortion from a pure sinusoid) decreases with increasing ϕ and with decreasing Reynolds number. It is also shown that nonlinear dynamic models obtained from system identification toolbox could produce thermal output (length averaged Nusselt number) as close to as output from a high fidelity CFD simulation.     

On characteristics of a non-reacting and a reacting turbulent flow over a backward facing step (BFS)

The turbulent reacting flow over a backward facing step shares some essential characteristics of premixed combustion occurring in a typical gas turbine combustor, while it is a simpler configuration to observe and model. For this reason and to explore the characteristics of the turbulent flow, in this study the combustion and flow dynamics in a backward facing step as a most elementary part of a combustor is studied numerically in atmospheric conditions. Two different configurations representing two laboratory devices are considered. As a first necessary step, the accuracy of predicted results is validated through the detailed comparison of numerical predictions and experimental measurements for a non-reacting flow. First, based on these non-reacting calculations, the turbulent model is selected and then the reacting simulations are done using a standard combustion model (available in CFX). Calculations are well supported with experimental data available from literature. Among the investigated turbulence models (k − ω, SST and SAS–SST), SAS–SST model showed the best agreement with the experimental data. The chosen turbulence model was used for the calculation of well documented case of turbulent flow over a backward facing step with the heated wall, showing satisfactory results compared to experimental data. For modeling of the reacting flow, the BVM combustion model was used. The predicted results using this model showed accurate results with an error about 2% on prediction of reattachment length.     

Heat transfer and fluid flow over microscale backward and forward facing step: A review

Research on convective heat transfer in the microscale backward-facing step (MBFS) and microscale forward-facing step (MFFS) has been extensively conducted in the past decade. This review summarizes numerous researches on the three topics; the first section focuses on studying the effect of the geometry on the fluid flow and heat transfer behavior. The second and the third sections concentrate on the effect of the inclination angle and the flow regime on the fluid flow and heat transfer enhancement. The purpose of this article is to get a clear view and detailed summary of the influence of several parameters such as the geometrical specifications, type of fluids and boundary conditions. The enhancement in the Nusselt number is the main target of such research where correlation equations were developed in numerical and experimental studies are reported.     

Combustion of coal particles in wall-burning swirl combustors

An approximate analysis is made of the performance of the wall-burning coal combustors. The results show that the swirl number plays a more important role than the char gasification chemical kinetics in determining the overall gasification rate. At high pressures, the wall-burning combustors may not be as efficient as the gas-stream combustors. The situation, on the other hand, may be reversed at the lower pressures.     

甲烷/空气混合物在微小型钝体燃烧器中的燃烧特性

为了实现在淬熄距离以下的稳定燃烧,试验设计了两种带钝体的回热型燃烧器,研究预混甲烷在高度小于淬熄距离的燃烧室中的燃烧特性。燃烧器的燃烧室高度为2 mm,长度为20 mm,进口处安装了边长为1mm的等边三角形钝体或相同尺寸的"V"形钝体,并在上下两侧设置了预热通道。利用Fluent6.3软件对甲烷/空气在这两个燃烧器的预混燃烧做了数值模拟,确定了浓度极限和速度极限,并对稳燃机理进行了分析。与三角形钝体燃烧器相比较,"V"形钝体燃烧器的稳燃范围更大。在所给出的稳定燃烧工况下两者的燃烧效率均在99%以上,甲烷/空气的混合气体能够完全燃烧。但当进气速度较小同时当量比较大时会发生回火现象。     

Analysis of turbulent flow and heat transfer over a double forward facing step with obstacles

A numerical study has been conducted to analyze the turbulent forced convection heat transfer for double forward facing step flow with obstacles. Obstacles have rectangular cross-sectional area with different aspect ratio that is located before each step. The numerical solutions of continuity, momentum and energy equations were solved by using a commercial code which uses finite volume techniques. The effect of turbulence was modeled by using a k–ε model. The effects of step height, obstacle aspect ratio and Reynolds number on the flow and heat transfer are investigated. The obtained results show that the rate of heat transfer is enhanced as aspect ratio of obstacle increases and this trend is affected by the step height. Also the results verified that the pressure drop decreases as obstacle aspect ratio increases.     

Experimental study on the unsteady laminar heat transfer downstream of a backwards facing step

The objective of this work is to study experimentally the unsteady heat transfer downstream of a backward-facing step in the 2-D laminar regime when the inlet flow is pulsated. To this aim, an experimental set-up has been prepared with water as the working fluid. The Reynolds number based on the hydraulic diameter of the inlet channel and average inlet velocity is 300. Inlet flow temperature is 30 °C and a region downstream of the step is heated up to 74 °C. Pulsation is achieved using a piston pump and heat transfer is studied up to a maximum pulsation Strouhal number of 1.2. The results obtained confirm previous numerical simulation work in the sense that pulsation could be used to partially recover the heat transfer efficiency that is lost in steady flow conditions downstream of a backward-facing step. It has also been confirmed that the behaviour of the averaged Nusselt number versus pulsation Strouhal number is of the resonant type. That is: the Nusselt number increases from the steady situation up to a certain value of the Strouhal number (0.41 in our case) and, then, it degrades as the frequency of the pulsation is further increased.     

Convective heat transfer of ferrofluid in a lid-driven cavity with a heat-conducting solid backward step under the effect of a variable magnetic field

The effect of variable magnetic field on the mixed convective flow of a ferrofluid within a lid-driven cavity has been analyzed numerically. A heat-conducting solid block is located in the bottom part of the cavity. Governing partial differential equations have been formulated taking into account that the magnetic source is a point source located over the moving lid. Analysis has been performed for a wide range of Hartmann number, nanoparticles volume fraction, and magnetic number. It has been found that the growth of the magnetic number leads to the heat transfer enhancement.     

Control of turbulent channel flow over a backward‐facing step by suction or injection

Experiments have been performed for turbulent channel flow over a backward‐facing step. The backward‐facing step is controlled by equipping a slit at the bottom corner of the step. Low momentum fluids in the recirculation region are sucked or high momentum fluids are injected from the slit. The width of the slit is changed between 2, 3, and 5 mm, and the flow ratio is varied from 0.00 to 0.15. The wall static pressure and local heat transfer coefficient are measured behind the backward‐facing step. The wall shear stress is measured using a micro flow sensor. In addition, the velocity profiles and turbulent intensities are measured by a split hot film probe. It is found that the heat transfer and pressure drop characteristics are controlled by the flow ratio. When the suction flow ratio is 0.06, the highest performance is obtained. Enhancement of the heat transfer is related to the increase of turbulence intensity. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(8): 490–504, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20036     

Influence of nanofluids on mixed convective heat transfer over a horizontal backward‐facing step

Predictions are reported for laminar mixed convection using various types of nanofluids over a horizontal backward‐facing step in a duct, in which the upstream wall and the step are considered adiabatic surfaces, while the downstream wall from the step is heated to a uniform temperature that is higher than the inlet fluid temperature. The straight wall that forms the other side of the duct is maintained at constant temperature equivalent to the inlet fluid temperature. Eight different types of nanoparticles, Au, Ag, Al₂O₃, Cu, CuO, diamond, SiO₂, and TiO₂, with 5% volume fraction are used. The conservation equations along with the boundary conditions are solved using the finite volume method. Results presented in this paper are for a step height of 4.9 mm and an expansion ratio of 1.942, while the total length in the downstream of the step is 0.5 m. The Reynolds number is in the range of 75 ≤ Re ≤ 225. The downstream wall was fixed at a uniform wall temperature in the range of 0 ≤ ΔT ≤ 30 °C which is higher than the inlet flow temperature. Results reveal that there is a primary recirculation region for all nanofluids behind the step. It is noticed that nanofluids without secondary recirculation region have a higher Nusselt number and it increases with Prandtl number decrement. On the other hand, nanofluids with secondary recirculation regions are found to have a lower Nusselt number. Diamond nanofluid has the highest Nusselt number in the primary recirculation region, while SiO₂ nanofluid has the highest Nusselt number downstream of the primary recirculation region. The skin friction coefficient increases as the temperature difference increases and the Reynolds number decreases. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20344     

增压前后柴油机燃烧噪声的对比分析

通过测量稳态和恒转速增扭矩瞬态工况增压前后影响燃烧噪声的参数,对比分析得出增压前后柴油机燃烧噪声的变化规律.结果表明:增压后燃烧噪声降低,增压对稳态工况燃烧噪声的降低比对恒转速增扭矩瞬态工况明显,增压在中速中负荷工况下,对燃烧噪声的控制效果较好.并从压力高频振荡、气体动力载荷和燃烧室壁面温度对试验结果进行了分析,证明了增压前后燃烧室壁面温度的差异影响增压前后柴油机燃烧噪声.     

天生桥一级水电站混凝土面板堆石坝的施工实践与体会

对天生桥一级水电站工程的拦河大坝——混凝土面板堆石坝的主要施工方法进行了介绍,同时对施工中遇到的一些实际问题进行探讨,阐述了我们的实际作法,并结合施工实际提出了建议。     

Periodic mixing and combustion processes in gas fired pulsating combustors

Although gas fired pulse combustors for home and water heating have been in use for a number of years, little research has been carried out, to date, to develop an understanding of the fundamental physical and chemical processes which control the operation of the pulse combustors. Consequently, the past developments of such combustors have been based upon costly trial-and-error approaches. This paper describes some results of an ongoing investigation which studies controlling processes in valved pulse combustors. Specifically, this paper provides experimental data which describe the mixing of the reactants, the time dependence of the combustion process heat release rate, its phase relationship with the combustor pressure, and the spatial characteristics of the combustion process. High speed shadow and Schlieren photography was carried out on a partially transparent pulse combustor. The combustion was observed to take place largely in the mixing chamber. Fuel and air jets enter the mixing chamber prior to the combustor reaching its minimum pressure. The two jets impinge, and mix and the new reactants are ignited by entrained remnants of reacting pockets from the previous cycle. This ignition takes place at the location and time at which the two reactant streams first mix. The resultant reactive stream breaks into two opposing vortices which fill the mixing chamber. Finally, data from CH and CC radiation measurements showed that the reaction is periodic and that it remains nonzero throughout the entire cycle. Furthermore, the reaction rate is in phase with the combustion pressure oscillations, satisfying Rayleigh's criterion for wave driving by heat addition.     

狭长圆柱型燃烧室内的燃烧不稳定性

研究了狭长圆柱型燃烧室内的喷雾火焰燃烧的不稳定性。为了更清楚地了解火焰的构造，首先测量了火焰的温度场，在较大的一次风和二次风变化范围内，测量了压力的振荡特性，得出其均方根值图。结果表明，火焰的稳定是由回流区完成的，在较小的一次风燃料当量比和中等的二次风量时，振荡强度达到100Pa左右。其频率为200—230Hz，与空气/燃料比值关系不大，分析还表明燃烧室中的振荡是轴向驻波振荡。     

Flame speed predictions in planar micro/mesoscale combustors with conjugate heat transfer

An analytical model for flame stabilization in meso-scale channels is developed by solving the two-dimensional partial differential equations associated with heat transport in the gas and structure and species transport in the gas. It improves on previous models by eliminating the need to assume values for the Nusselt numbers in the pre and post-flame regions. The effects of heat loss to the environment, wall thermal conductivity, and wall geometry on the burning velocity and extinction are explored. Extinction limits and fast and slow burning modes are identified but their dependence on structure thermal conductivity and heat losses differ from previous quasi one-dimensional analyses. Heat recirculation from the post-flame to the pre-flame is shown to be the primary mechanism for flame stabilization and burning rate enhancement in micro-channels. Combustor design parameters like the wall thickness ratio, thermal conductivity ratio, and heat loss to the environment each influence the flame speed through their influence on the total heat recirculation. These findings are used to propose a simple methodology for preliminary micro-combustor design.