The turbulent wake of a monopile foundation

An experimental programme is presented, examining the turbulent wake of a monopile foundation in a current. Velocity was recorded across an extensive domain downstream of a model monopile in a 0.5 m deep basin, using an acoustic Doppler velocimeter array. The distribution of turbulent kinetic energy (TKE) is examined across the entire domain. Tests were undertaken using several combinations of pile diameter (D = 0.1 and 0.2 m) and mean flow velocity ($\overline{u_0}$ = 0.08–0.24 m/s), representing typical prototype conditions at a scale of 1:50. It is shown that turbulence can be predicted using the distance downstream (x) and off axis (y), the pile diameter, and the mean flow velocity. Two new parameters are introduced to simplify assessment of proposed structures. Relative Excess Turbulence (RET) is the extra turbulence generated by the pile, normalised by the ambient turbulence. Turbulence Recovery Lengthscale (TRL) is the distance downstream (normalised by D) required for RET to fall below a given threshold. Results show that RET decays exponentially with distance downstream. Across the wake, RET fitted a Gaussian function with peak values at the wake centreline. TRL is estimated at 40 for an RET threshold of 1.0 and 400 for an RET threshold of 0.1.     

Analysis of turbulent combustion in inert porous media

The objective of this paper is to present an extension of a simplified reaction kinetics model that, combined with a thermo-mechanical closure, entails a full-generalized turbulent combustion model for flow in porous media. In this model, one explicitly considers the intra-pore levels of turbulent kinetic energy. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. The rate of fuel consumption is described by an Arrhenius expression involving the product of the fuel and oxidant mass fractions. These mass fractions are double decomposed in time and space and, after applying simultaneous time-and-volume integration operations to them, distinct terms arise, which are here associated with the mechanisms of dispersion and turbulence. Modeling of these extra terms remains an open question and the derivations herein might motivate further development of models for turbulent combustion in porous media.     

Thermal performance evaluation of heat exchangers fitted with twisted-ring turbulators

Heat transfer, friction factor and thermal performance characteristics in a tube equipped with twisted-rings (TRs) are experimentally investigated. The experiments were conducted using TRs with three different width ratios (W/D = 0.05, 0.1 and 0.15) and three pitch ratios of (p/D = 1, 1.5 and 2) for Reynolds numbers ranging from 6000 to 20,000 using air as a test fluid. The typical circular rings (CRs) were also tested for an assessment. The experimental results reveal that most TRs yield lower Nusselt numbers and friction factor than CRs, except at the largest width ratio (W/D = 0.15) and the smallest pitch ratio (p/D = 1.0). In addition, Nusselt number and friction factor increase as width ratio increases and pitch ratio decreases. However, a maximum thermal performance factor is associated by TRs with the smallest width ratio and pitch ratio. The empirical correlations of the heat transfer (Nu) and friction factor (f) are also included in this paper.     

Acceleration of hydrogen/air flames in a cylindrical envelope

Flame front behavior during hydrogen/air deflagration in initially quiescent mixtures in cylindrical envelopes was experimentally studied using ionization gauges and infrared photography. Hydrogen/air mixtures with hydrogen content from 12% to 30% were filled in the polyethylene envelope of 4.5 m³ and ignited with exploding wire of 5 J energy. The dependences of the flame front position on time were obtained. Dynamics of the flame front was analyzed on harmonic instabilities development. The mechanisms for the creation of turbulence are discussed. Flame front acceleration is analyzed using Kolmogorov law.     

COMPUTATION OF THE FLOW FIELD IN A RECIRCULATING TURBULENT GAS JET

As part of an overall investigation of flash-smelting processes, numerical computations were performed for the turbulent recirculating flow field of a gas jet in a confined cylindrical system. The two-equation (k - e) model was used to describe turbulence. The two-dimensional elliptic partial differential equations cast into finite difference forms were solved by the TEACH code (Gosman and Pun, 1973), and the pressure field was computed by the SIMPLER method (Patankar, 1980). Various correlations for the dissipation rate of turbulent kinetic energy at the inlet were tested, and the relation yielding the best results was obtained as E= C_μK^1.5/(0.015d_e where d_e = 4 x hydraulic radius. Extensive comparisons have been made between the computed results using the proposed form of e at the inlet and experimental data taken from the literature. Much better predictions than previously possible have been obtained     

Multiple mapping conditioning of velocity in turbulent jet flames

Multiple mapping conditioning (MMC) has emerged as a new approach to model turbulent reacting flows. This study revises the standard MMC closure for velocity in turbulent jet flows from linearity in the reference space to linearity in the composition space. This modeling amendment ensures that the standard velocity model in conditional moment closure studies can now be used for MMC computation as well. A simplified model for the velocity-dependence of MMC drift coefficients is derived without loss of generality and is implemented for the revised velocity closure. Modeling results have been corroborated against the Direct Numerical Simulation database of a spatially evolving, planar turbulent jet flame. The revised model shows marked improvement over standard MMC closure in predicting velocity statistics close to the nozzle.     

Prediction of swirling confined diffusion flame with a Monte Carlo and a presumed-PDF-model

This work aims to compare numerical results obtained by using the Monte Carlo composition-PDF method and a presumed-β-PDF in order to reveal their effects on the prediction of flow and scalar fields in swirling confined methane diffusion flame. Using the intrinsic low dimensional manifolds method for modelling the chemistry and a second moment closure for the turbulence, it is shown that both PDF-methods provide a similar accuracy level of the prediction of mean quantities. While the presumed-β-PDF performs using reasonable computational efforts, the Monte Carlo-PDF allows to capture well the turbulence-chemistry interaction and strong finite-chemistry effects such as local extinction.     

湍流值对风电功率预测的影响与分析

风电功率预测是缓解弃风现象的有效手段。文中针对风电波动性,提出了一种在模糊C均值聚类算法(Fuzzy C-Means algorithm,FCM)中引入湍流值IT的风电功率预测方法。在FCM算法中引入湍流值IT对训练样本进行聚类,可以进一步增强训练样本与预测样本间的相似性,避免因训练样本减少,导致风电功率波动性影响能力增大的情况。以山西某风电场实测数据为依据,在MATLAB平台上通过支持向量机(Support Vector Machine,SVM)对FCM的聚类结果进行训练和预测,仿真结果表明,FCM-IT-SVM能有效增强风电功率的相似性,减小预测误差。     

增强燃烧室内空气紊流降低柴油机油耗,噪声及有害排放物的研究

柴油机燃烧室中空气紊流运动对促进燃烧有重要作用。为改善Ｘ４１０５Ｂ柴油机性能，作采用了高紊流的“Ｑｕａｄｒａｍ”型燃烧室。通过合理选择进气涡流强度、供油速率和供油提前角，取得了良好的效果。不仅改善了燃油经济性，减小了烟度，而且降低了其排放物和噪声。