Bed-to-wall heat transfer behavior in a pressurized circulating fluidized bed

An experimental investigation has been made to study the effect of pressure and other relevant operating parameters on bed hydrodynamics and bed-to-wall heat transfer in a pressurized circulating fluidized bed (PCFB) riser column of 37.5 mm internal diameter and 1940 mm height. The experiments have been conducted with and without bed material for the consideration of frictional pressure drop due to gas density at elevated pressures. The pressure drop measured without sand particles is assumed as the pressure drop due to gas density for the calculation of bed voidage and suspension density profiles. The specially designed heat transfer probe is used to measure the bed-to-wall heat transfer coefficient. The experimental results have been compared with the published literature and good agreement has been observed. The axial bed voidage is less in the bottom zone of the riser column and is increasing along the height of the bed. With the increase in system pressure, the bed voidage is found to be increasing in the bottom zone and decreasing in the top zone. The heat transfer coefficient increases with the increase in system pressure as well as with the gas superficial velocity. The heat transfer coefficient is also observed to be increasing with the increase in average suspension density.     

Numerical study of different gas–solid flow regimes effects on hydrodynamics and heat transfer performance of a fluidized bed reactor

In this study, a two‐?uid Eulerian–Eulerian model has been carried out applying the kinetic theory of granular flow (KTGF) to study the hydrodynamics and heat transfer behavior of a fluidized bed reactor simultaneously. The effects of different gas–solid flow regimes on the operating conditions and heat transfer rate between the hot air and two types of low and high‐density inert particles are investigated in a fluidized bed dryer. Different gas–solid flow regimes for wood and glass particles of groups A, B, and D of Geldart's classification are simulated to introduce the most optimal flow regime in terms of heat transfer rate and operating costs. The compromise between the heating rate, the height required for the reactor, and the ratio of the final mass to the initial mass of solid particles, which specifies the need for a cyclone separator showed that the bubbling regime of Geldart B powder for low‐density particles and the turbulent regime of Geldart D powder or bubbling regime of Geldart B powder for high‐density particles are the optimal operating conditions and flow regimes. Furthermore, it was concluded that the convective heat transfer is the dominant mechanism, which increases with increasing the air velocity and decreasing the particle diameter in each group.     

Modeling of polymer electrolyte membrane fuel cell with circular and elliptical cross‐section gas channels: A novel procedure

The main objective of this paper is to develop an analytical solution based on the perturbation method to solve the continuity and momentum equations governing the flow in gas channels of a PEMFC having circular and elliptical cross sections. The equations are solved in both the anode and cathode gas channels with appropriately defined perturbation parameters to obtain the velocity profile in these channels. It was observed that by changing the circular cross section to an elliptical one (ie, increasing the value of perturbation parameter), the axial velocity increases. As a result, the penetration of species into the reaction areas decreases. Then, the effect of species penetration speed on the performance of PEMFC is discussed. Increasing the penetration speed (ie, radial velocity) of the reactant gases causes the maximum value of the gas velocity in the channel to decrease. This would imply that the diffusion rate of the reactant species to the reaction areas, and thereby the cell performance would be optimized. Apart from the analytical solution, 3‐D numerical solution of the governing equations using collocated finite volume method along with the SIMPLE algorithm is also performed. The results are validated against the available published data. The numerical results confirm that by converting the circular cross section to the elliptical one, while other conditions are fixed, the PEMFC produces less current density.     

Numerical modeling of axial bed-to-wall heat transfer in a circulating fluidized bed combustor

The water-wall surfaces located above the secondary air inlet within the circulating fluidized bed (CFB) combustor are exposed to the axial bed-to-wall heat transfer process. In the current work, the axial bed-to-wall heat transfer coefficients are estimated for three different axial voidage profiles (covering three widely occurring average particle concentrations) in order to investigate the effect of voidage, time, initial and fixed temperature of the bed and annulus, and gas gap between wall and solid particles; on the axial heat transfer process. A 2D thermal energy balance model is developed to estimate the axial heat transfer values for the gas–solid suspension along the height of the riser column with horizontally changing mass distribution. The gas–solid mass distribution is fixed with time thus providing a spectrum of changes in axial bed-to-wall heat transfer profile with time. The current work provides an opportunity to understand the axial heat transfer relationship with particle concentration and instantaneous behaviour. The results from the work show that: (i) first few seconds of the suspension temperature near the wall has maximum energy thus providing a small time frame to transfer more heat to the surface (CFB wall); (ii) both axial and horizontal particle concentrations (influenced by the operating conditions) affect the axial heat transfer locally; (iii) initial temperature of the bed between average and maximum values provide end limits for the axial heat transfer; (iv) annulus region has higher thermal energy than the core due to increased particle presence; and (v) a particle-free zone near the wall (gas gap) having a maximum thickness of 1 mm, tends to reduce up to 25% of axial heat transfer value. The model trends have close agreement with experimental trends from published literature; but the model values differ when correlating with real values due to inconsistencies in riser diameter and nature of variation in parameters.     

二维流化床内射流深度的试验研究

研究二维流化床内的水平射流和垂直向上射流。利用图象处理技术得到了水平射流的水平深度和垂直向上深度以及垂直向上射流的射流深度。通过多元线性最小二乘回归,得到了射流深度的关联式。研究了流化数、射流速度、颗粒平均粒径和静态床层高度对射流深度的影响。试验结果表明,流化数和射流速度的增加,射流深度将增加;颗粒粒径增加,射流深度将减小;静态床层高度变化时,射流深度基本不变。发现射流深度决定于射流与乳化相气体和颗粒的动量交换。对水平射流的垂直向上深度和垂直向上射流的射流深度进行了比较。     

Effect of convex platform structure on hydrogen and oxygen combustion characteristics in micro combustor

The combustion characteristics of the micro combustor with a convex platform were simulated and the effects of the height of the convex platform and the inlet velocity on the combustion process were analyzed. The results show that the setting of convex platform can significantly increase the maximum velocity and reduce the outlet velocity. When the height of the boss continues to increase, the maximum velocity is more significant, but has little effect on the outlet velocity. At the same time, the increasing height of the convex platform increases, the turbulent kinetic energy and reduces the intensity of combustion on the axis. However further increase in the height does not reduce the effect significantly. The fuel conversion rate increases significantly, but the velocity decreases. In the micro combustor with a convex platform, increasing the inlet velocity increases the axial temperature, the fuel conversion rate decreases.     

Gas–solid turbulent flow and heat transfer with collision effect in a vertical pipe

A turbulent gas–solid suspension upward flow in a vertical pipe is simulated numerically using Eulerian–Lagrangian approach. Particle–particle and particle–wall collisions are simulated based on deterministic approach. The influence of particle collisions on the particle concentration, mean temperature and fluctuating velocities are investigated. Numerical results are presented for different values of loading ratios. The profiles of particle concentration, mean velocity and temperature are shown to be flatter by considering inter-particle collisions, while this effect on the gas mean velocity and temperature is not significant. It is demonstrated that the effect of inter-particle collisions have a dramatic influence on the particle fluctuation velocity. It is shown that the profiles of particle concentration and particle velocity are flattened due to inter-particle collisions and this effect becomes more pronounced with increasing loading ratio. Also, the attenuation of turbulence by inter-particle collisions in the core region of the pipe is increased by increasing loading ratio.     

循环流化床冷态流动特性的试验研究

研究了循环流化床气固两相流动在轴向的宏观流体动力特性,采用计算机数据采集系统测量不同工况下沿主床高度上的压力分布,进而计算出空隙率沿循环流化床主床高度方向上的分布情况,研究不同操作条件(流化风速、存料量)对空隙率分布、物料循环量和压力平衡的影响。     

温度和压力对旋风分离器内气相流场的综合影响

借助Fluent6.1软件,采用改进的RSM模型对不同温度(293~1373 K)和不同压力(0.1~6.5 MPa)下旋风分离器的气相流场进行了数值模拟.结果表明:切向速度随温度的升高而降低,随压力的升高而升高;但当温度超过1000 K、压力超过1.0 MPa以后,切向速度的变化幅度趋于减少.据此,建立了包含温度与压力综合影响在内的切向速度计算公式.温度升高使内旋流中心区域的轴向速度增大,而压力升高则使其减小.     

Simulation of gas species and temperature separation in the counter-flow Ranque–Hilsch vortex tube using the large eddy simulation technique

A computational fluid dynamic model is used to predict the species and temperature separation within a counter flow Ranque–Hilsch vortex tube. The large eddy simulation (LES) technique was employed for predicting the gas flow and temperature fields and the species mass fractions (nitrogen and helium) in the vortex tube. A vortex tube with a circumferential inlet stream of nitrogen–helium mixture and an axial (cold) outlet stream and a circumferential (hot) outlet stream was considered. The temporal evolutions of the axial, radial and azimuthal components of the velocity along with the temperature, pressure and mass density and species concentration fields within the vortex tube are simulated. Even though a large temperature separation was observed, only a very minimal gas separation occurred due to diffusion effects. Correlations between the fluctuating components of velocity, temperature and species mass fraction were calculated to understand the separation mechanism. The inner core flow was found to have large values of eddy heat flux and Reynold’s stresses. Simulations were carried out for varying amounts of cold outlet mass flow rates. Performance curves (temperature separation/gas separation versus cold outlet mass fraction) were obtained for a specific vortex tube with a given inlet mass flow rate.     

Thermal stochastic collision model in turbulent gas–solid pipe flows

New thermal stochastic particle collision model in gas–solid flow in a riser is developed. The simulation is based on four-way coupling of phases considering inter-particle collision and heat transfer. It is shown that the limitation of excessive computational time in Eulerian–Lagrangian simulation of gas–solid flows for the high loading ratios is eliminated by using the stochastic particle collision model. The simulation results demonstrate that the predictions of the developed thermal stochastic particle collision modem are in good agreement with those obtained by the direct particle collision model and the available experimental data. The new stochastic modeling is used and nearly dense gas–solid flow is simulated for high loading ratios up to eight and the results are presented and discussed.     

Mixing enhancement mechanism of combined H2–Water jets in supersonic crossflows in a combustor with an expanded section

To obtain the mixing enhancement mechanism of H₂–Water combined jets in supersonic crossflows in a combustor with expanded section for rotating detonation ramjet, the flow field shape and spray structure were studied by experimental and numerical methods. The Eulerian–Lagrangian method was used to investigate the diffusion mechanism and H₂–Water interaction law of combined jets with different sequences. At the same time, high-speed photography and the schlieren technique were used to capture the flow field. The effects of jet pressure drop, orifice diameter, orifice spacing, incoming Mach number, and other parameters on the penetration depth of water jets were studied. The results of experiment and simulation show that using H₂–Water combined jets, the penetration depth of the jet spray can be greatly increased and the jet mixing effect can be significantly improved, which will contribute to the engine's ignition and stable combustion. In the case of pre-water/post-H₂, the penetration depth of the hydrogen jet is greater. In the case of pre-H₂/post-water, the hydrogen jet raises the water spray mainly by protecting the integrity of the water column.     

湍流射流火焰抬举高度的实验研究

湍流射流燃烧作为工业燃烧室中普遍存在的燃烧方式,研究湍流射流火焰不仅能促进实际燃烧室的设计改造,更能增强对湍流燃烧理论的理解。在轴对称伴流射流燃烧器实验平台上,研究了湍流自由射流火焰抬举高度随射流速度的变化及氮气稀释和伴流速度对火焰抬举高度的影响。实验结果表明湍流自由射流燃烧火焰抬举高度随射流速度呈线性增长;随氮气稀释摩尔分数的增加其抬举高度的线性斜率增大,射流火焰吹出喷嘴的雷诺数降低,火焰更易发生抬举;同时,氮气稀释摩尔分数的增加也导致射流火焰发生吹熄时雷诺数减小,射流火焰在射流速度完全进入湍流之前发生吹熄;伴流速度小于0.3 m/s时对火焰抬举高度的影响不大,当伴流速度大于0.3 m/s时抬举高度随伴流速度的增加呈线性增长,当射流速度大于20 m/s时,伴流速度的影响降低;对比伴流与稀释对抬举高度的影响可知射流速度大于30 m/s时对伴流的敏感性大于稀释,而在射流速度小于30 m/s时对稀释更敏感。     

The behaviour of lifted oxy-fuel flames in burners with separated jets

Oxy-fuel combustion in separated-jet burners has been proven to increase thermal efficiency and to have a potential for NOx emission reduction. This paper presents an investigation into confined, turbulent, oxy-flames generated by a burner consisting of a central natural gas jet surrounded by two oxygen jets. The study is focused on the identifying the influence of burner parameters on the flame characteristics and topology, namely stability, lift-off height and flame length. The effects of the natural gas and oxygen jet exit velocities, the distance separating the jets and the deflection of oxygen jets towards the natural gas jet are examined. The OH chemiluminescence. Results show that the lift-off heights increase when jet exit velocities and the distance separating the jets are increased. The deflection of oxygen jets decreases the lift-off height and increases the volume of flame in the transversal plane. The flame length increases principally with the oxygen exit velocity and the separation distance, and decreases considerably when the angle of oxygen jets is increased.     

Effectiveness measurements for a cooling film disrupted by a row of jets

The influence of cold air jet injection upon filmcooling of combustion chamber walls has been investigated by simulation. The experiments, in which a row of cold air jets is injected perpendicularly to the film cooled test chamber wall, show considerable decreases of the film cooling effectiveness downstream of the jet exit. The same behavior occurs in real combustion chambers of aircraft gas turbine engines. The tests are selected to represent combinations of geometries and flow variables (except the temperature ratios) which are pertinent to gas turbine combustor design. The present investigations cover effectiveness measurements for various combinations of the parameters: a) velocity ratio coolant film to mainstream b) velocity ratio jet to mainstream c) dimensionless distance between coolant film and jet exit d) spacing ratio e) ratio jet diameter to slot height f) ratio lip thickness to slot height.     

Experimental investigation on the microscopic characteristics of underexpanded transient hydrogen jets

The microscopic characteristics of hydrogen jet affect the concentration distribution and gas-air mixing of high-pressure gas jets for a hydrogen engine. In this study, the schlieren method was used to record the hydrogen jet for an outward-opening injector. The results showed that the jet asymmetricity was large in the initial stage of the jet development, then decreased rapidly as the jet developed, and finally entered a stable range of 13%–38% when the valve fully opened. The increase of the ambient pressure was beneficial to reduce the fluctuation range of the asymmetricity. The steady S of an outward-opening injector (S = 0.7 ± 0.03) is larger than that of a single-hole injector (S = 0.25 ± 0.05). The increase in injection pressure and the decrease in ambient pressure caused S to decrease to 1 faster. The steady Γ of an outward-opening injector was experimentally determined, and the value (Γ = 0.98 ± 0.1) was smaller than that of a single-hole injector. According to the transient Γ, the injection process can be divided into the developing phase and the self-similar phase. Γ increased rapidly with time and then stabilized after the injector valve reached its maximum lift position. The gas concentration distribution was uneven along the axial direction. The mole fraction decreased with the increase in axial penetration, and entered a stable range of 0.27–0.37 when axial penetration was greater than 20 mm. The average equivalent ratio was large at the beginning of injection under different pressure ratios, and then decreased and finally stabilized in the range of 1.4–5.     

Investigation on mechanism of critical cavitating flow in liquid jet pumps under operating limits

The critical cavitating flow in liquid jet pumps under operating limits is investigated in this paper. Measurements on the axial pressure distribution along the wall of jet pumps indicate that two-phase critical flow occurs in the throat pipe under operating limits. The entrained flow rate and the distribution of the wall pressure upstream lowest pressure section does not change when the outlet pressure is lower than a critical value. A liquid–vapor mixing shockwave is also observed under operating limits. The wave front moves back and forth in low frequency around the position of the lowest pressure. With the measured axial wall pressures, the Mach number of the two-phase cavitating flow is calculated. It’s found that the maximum Mach number is very close to 1 under operating limits. Further analysis infers a cross-section where Mach number approaches to 1 near the wave front. Thus, the liquid–vapor mixture velocity should reach the local sound velocity and resulting in the occurrence of operating limits.     

Dynamical Study of Pulsed Impinging Jet with Time Varying Heat Flux Boundary Condition

Pulsed jets in different configuration are potentially considered for enhancing transport phenomenon generally. Flow and temperature field in a pulsed impinging jet are simulated numerically by solving the governing equations using the control volume method. Ensemble Averaging Method as well as Phase Averaging has been employed for reporting the results in this study. In order to simulate a pulsating jet, inlet velocity profile was exerted as a time dependent sinusoidal and step signals. The results of this simulation showed an oscillatory jet could lead to an increase in jet development and its cross section with the wall and also a more uniform Nusselt profile would be obtained compared to the steady jet. For parametric investigations and extracting flow and thermal characteristics of a pulsed impinging jet, the effects of various parameters including flow frequency and amplitude and heat flux frequency were considered. It has been seen that Nusselt number varies by the changes in frequency, amplitude and the type of the excitation. It has been shown that the oscillating impinging jet has a better performance rather than the steady case when the excitation amplitude and frequency increase. Finally, it is also observed how a thermal field is going to respond with two pulsating inputs.     

木屑在循环流化床中的流动特性研究

研究了冷态实验条件下木屑在内径0.28m、高10m的循环流化床中的流体力学特性。着重研究了表观气速U_g=1.81～2.26m/s、循环流率G_s=0.42～0.76kg/(m~2·s)的工况下木屑在循环流化床上升管中颗粒速度与床层空隙率的轴向及径向分布特点。实验发现:循环流化床的下降管中保持足够料高是木屑实现先定循环的必要条件;颗粒在上升管中的流动为典型的环-核结构;面积平均空隙率沿床高呈现先增大再减小的现象。通过将截面按径向位置r/R=0～0.71,0.71～0.93,0.93～1.00分为3个区域,分析了环区、核区和环核过渡区各自的床层空隙率以及颗粒速度沿床高变化的特点。     

Effect of fluidic obstacles on flame acceleration and DDT process in a hydrogen-air mixture

Using solid obstacles to accelerate the deflagration to detonation transition (DDT) process induces additional thrust loss, and fluidic obstacles can alleviate this problem to a certain extent. A detailed simulation is conducted to investigate the effects of multiple groups of fluidic obstacles on the flame acceleration and DDT process under different initial velocities and gas types. The results show that, initially, the propagation of reflected shock wave formed by jet impingement is opposite to the flame acceleration direction, thus increasing the initial jet velocity will hinder the flame acceleration. Later, the vortex structure and enhanced turbulence can promote flame acceleration. As the flame accelerates, the virtual blockage ratio of the fluidic obstacles decreases, and increasing initial jet velocity or using reactive jet gases both affect the virtual blockage ratio. Further, increasing initial jet velocity or using reactive jet gases can shorten the detonation initiation time and distance. Compared with solid obstacles, it is concluded that fluidic obstacles can achieve faster detonation initiation with a smaller blockage ratio. Overall, the detonation phenomena in this study are all triggered by hot spots formed by the interaction between reflected waves and distorted flame, but the formation of reflected waves varies.