Application of water loop variable refrigerant flow air-conditioning system in large-scale buildings in northern China

A water loop variable refrigerant flow（WLVRF）air-conditioning system is designed to be applied in large-scale buildings in northern China.The system is energy saving and it is an integrated system consisting of a variable refrigerant flow（VRF）air-conditioning unit,a water loop and an air source heat pump.The water loop transports energy among different regions in the buildings instead of refrigerant pipes,decreasing the scale of the VRF air-conditioning unit and improving the performance.Previous models for refrigerants and building loads are cited in this investigation.Mathematical models of major equipment and other elements of the system are established using the lumped parameter method based on the DATAFIT software and the MATLAB software.The performance of the WLVRF system is simulated.The initial investments and the running costs are calculated based on the results of market research.Finally,a contrast is carried out between the WLVRF system and the traditional VRF system.The results show that the WLVRF system has a better working condition and lower running costs than the traditional VRF system.

Analysis of periodic pulsating nanofluid flow and heat transfer through a parallel-plate channel in the presence of magnetic field

In this paper,we focus on the two-dimensional pulsating nanofluid flow through a parallel-plate channel in the presence of a magnetic field.The pulsating flow is produced by an applied pressure gradient that fluctuates with a small amplitude.A kind of proper transformation is used so that the governing equations describing the momentum and thermal energy are reduced to a set of non-dimensional equations.The analytical expressions of the pulsating velocity,temperature,and Nusselt number of nanofluids are obtained by the perturbation technique.In the present study,the effects of the Cu-H2O and Al_(2)O_(3)-H2O nanofluids on the flow and heat transfer in pulsating flow are compared and analyzed.The results show that the convective heat transfer effect of Cu-H2O nanofluids is better than that of Al_(2)O_(3)-H2O nanofluids.Also,the effects of the Hartmann number and pulsation amplitude on the velocity,temperature,and Nusselt number are examined and discussed in detail.The present work indicates that increasing the Hartmann number and pulsation amplitude can enhance the heat transfer of the pulsating flow.In addition,selecting an optimal pulsation frequency can maximize the convective heat transfer of the pulsating flow.Therefore,improved understanding of these fundamental mechanisms is conducive to the optimal design of thermal systems.

Advances and challenges of electrolyzers for large-scale CO_(2) electroreduction

CO_(2) electroreduction(CO_(2) ER)to high value-added chemicals is considered as a promising technology to achieve sustainable carbon neutralization.By virtue of the progressive research in recent years aiming at design and understanding of catalytic materials and electrolyte systems,the CO_(2) ER performance(such as current density,selectivity,stability,CO_(2) conversion,etc.)has been continually increased.Unfortunately,there has been relatively little attention paid to the large-scale CO 2 electrolyzers,which stand just as one obstacle,alongside series-parallel integration,challenging the practical application of this infant technology.In this review,the latest progress on the structures of low-temperature CO_(2) electrolyzers and scale-up studies was systematically overviewed.The influence of the CO_(2) electrolyzer configurations,such as the flow channel design,gas diffusion electrode(GDE)and ion exchange membrane(IEM),on the CO_(2) ER performance was further discussed.The review could provide inspiration for the design of large-scale CO_(2) electrolyzers so as to accelerate the industrial application of CO_(2) ER technology.

Prediction of Pulsation Frequency of Pulsing Flow in Trickle Beds Based on Artificial Neural Network

An extensive database (946 measurements) for the frequency of pulsing flow in trickle beds was established by collecting the experimental results published over past 30 years. A new correlation based on artificial neural network (ANN) to predict the pulsation frequency was developed. Seven dimensionless numbers (groups) employed in the proposed correlation were liquid and gas Reynolds, liquid Weber, liquid Eotvos, gas Froude, and gas Stokes numbers and a bed correction factor. The comparisons of performance reported in the of literature and present correlations show that ANN correlation is a significant improvement in predicting pulsation frequency with an average absolute relative error (AARE) of 10% and a standard deviation less than 18%.

A Fluid Experiment of Large-Scale Topography Effect on Baroclinic Wave Flows

The effects of topography on baroclinic wave flows are studied experimentally in a thermally driven rotating annulus of fluid.Fourier analysis and complex principal component (CPC) analysis of the experimental data show that, due to topographic forcing, the flow is bimodal rather than a single mode. Under suitable imposed experimental parameters, near thermal Rossby number ROT = 0.1 and Taylor number Ta = 2.2 × 107, the large-scale topography produces low-frequency oscillation in the flow and rather long-lived flow pattern resembling blocking in the atmospheric circulation. The 'blocking' phenomenon is caused by the resonance of travelling waves and the quasi-stationary waves forced by topography.The large-scale topography transforms wavenumber-homogeneous flows into wavenumber-dispersed flows, and the dispersed flows possess lower wavenumbers.

Effect of bionic mantis shrimp groove volute on vortex pump pressure pulsation

In order to reduce pressure pulsation of vortex pumps,the mantis shrimp was chosen as biological prototype and a bionic engineering model was developed from its abdominal segment grooves.Bionic mantis shrimp groove volute vortex pump models with different numbers of grooves were developed,and numerical simulation methods were used to calculate the models to study the effects of the volute grooves on the pressure pulsation of a vortex pump.The results show that a bionic groove volute could effectively improve the pressure pulsation of a vortex pump outlet,and reduce the pressure pulsation around the pump’s tongue and other internal points.The pressure pulsation under different conditions is impacted by shaft frequency and blade frequency.The bionic groove structure has little effect on the external characteristics of the pump,but could improve the static pressure,velocity distribution,and vortex structure of the flow field.Additionally,pressure pulsation of the whole vortex pump is reduced.

MASS TRANSFER IN TURBULENT PULSATING FLOWS

The effect of flow oscillation to the mass transfer between turbulent fluid and solid wall was investigatedby measuring the mass transfer rate between fluid and pipe wall with imposed oscillating flow usingelectrochemical method.The velocity and concentration field in the viscous sublayer which controls the mass trans-fer in such a process was simulated by a simple wave model of single harmonics.Experimental results confirmthat the flow oscillation has no influene on time averaged mass transfer rate,but the phase difference betweenphase averaged velocity field and concentration field shifts with the frequency of imposed oscillating flow.Numeri-cal analysis reveals that the concentration boundarylayer which is responsible for the mass transfer is muchthinner than the viscous sublayer which greatly weakens the influence of imposed oscillating flow on mass transfer.

Visual Study on Flow and Operational Characteristics of Flat Plate Closed Loop Pulsating Heat Pipes

This paper presents an experimental study including visualization on a flat plate closed loop pulsating heat pipes.It consists of a total of 40 channels with square cross section(2 mm×2 mm,165 mm long) machined directly on an aluminum plate(180 mm×120 mm×3 mm) covered by a transparent plate.The working fluid employed is ethanol.As a result,various flow patterns and their transitions are observed and found to be related to the fluid fill ratio,input heat load and the device orientation.Also the operational characteristics and working mechanism are discussed.

NUMERICAL PREDICTION OF PULSATING FLOW IN A LIQUID LINE WITH BRANCH OF HYDRAULIC SERVO SYSTEM

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Three-dimensional human thermoregulation model based on pulsatile blood flow and heating mechanism

A three-dimensional thermoregulation mathematical model of temperature fluctuations for the human body is developed based on predecessors＇ thermal models. The following improvements are necessary in real situations： ellipsoids and elliptical cylinders are used to adequately approximate body geometry, divided into 18 segments and five layers; the core layer consists of the organs; the pulsation of the heart cycle, the pulsatile laminar flow, the peripheral resistance, and the thermal effect of food are considered. The model is calculated by adopting computational fluid dynamics（CFD） technology, and the results of the model match with the experimental data. This paper can give a reasonable explanation for the temperature fluctuations.

Nonlinear Dynamics of Viscoelastic Pipe Conveying Pulsating Fluid Subjected to Base Excitation

Based on the Euler-Bernoulli beam theory and Kelvin-Voigt model,a nonlinear model for the transverse vibration of a pipe under the combined action of base motion and pulsating internal flow is established.The governing partial differential equation is transformed into a nonlinear system of fourth-order ordinary differential equations by using the generalized integral transform technique(GITT).The effects of the combined excitation of base motion and pulsating internal flow on the nonlinear dynamic behavior of the pipe are investigated using a bifurcation diagram,phase trajectory diagram,power spectrum diagram,time-domain diagram,and Poincare map.The results show that the base excitation amplitude and frequency significantly affect the dynamic behavior of the pipe system.Some new resonance phenomena can be observed,such as the period-1 motion under the base excitation or the pulsating internal flow alone becomes the multi-periodic motion,quasi-periodic motion or even chaotic motion due to the combined excitation action.

Lattice Boltzmann Simulation of One Particle Migrating in a Pulsating Flow in Microvessel

A lattice Boltzmann model of two dimensions is used to simulate the movement of a single rigid particle suspended in a pulsating flow in micro vessel The particle is as big as a red blood cell, and the micro vessel is four times as wide as the diameter of the particle. It is found that Segrd-Silberberg effect will not respond to the pulsation of the flow when the Reynolds number is relatively high. However, when the Reynolds number is low enough, Segrd-Silberberg effect disappears. In the steady flow, different initial position leads to different equilibrium positions. In a pulsating flow, different frequencies of pulsation also cause different equilibrium positions. Particularly, when the frequency of pulsation is closed to the human heart rate, Segrd-Silberberg effect presents again. The evolutions of velocity, rotation, and trajectory of the particle are investigated to find the dynamics of such abnormal phenomenon.

Two-Phase Flow Modeling in a Single Closed Loop Pulsating Heat Pipes

Mathematical modeling of pulsating heat pipes through ‘first’ principles is a contemporary problem which remains quite elusive. Simplifications and assumptions made in all the modeling approaches developed so far render them unsuitable for engineering design. In this paper, a more realistic modeling scheme is presented which provides considerable try for thought toward the next progressive step. At high enough heat flux level, closed loop pulsating heat pipes experience a bulk internal unidirectional fluid circulation. Under such a condition, conventional two-phase flow modeling in capillary tubes may be applied. This has been attempted for single-loop PHPs. A homogeneous model and a separated two-fluid flow model based on simultaneous conservation of mass, momentum and energy, have been developed for an equivalent ‘open flow’ system. The model allows prediction of two-phase flow parameters in each sub-section of the device thereby providing important insights into its operation. The concept of ‘void fraction constraint’ in pulsating heat pipe operation is introduced and its relevance to future modeling attempts is outlined.

Effects of Clearance around Square Pillar in Rectangular Enclosure on Cooling Performance of Pulsating Airflow

This study focuses on a development of heat transfer enhancement techniques using pulsating flow for thermal equipment such as electronic equipment and heat exchangers. In this report, the heat transfer performance of the pulsating airflow around the heating pillar mounted in the rectangular enclosure was investigated experimentally while changing the size of the clearance between the enclosure wall and the pillar. The pillar simulates the components mounted in thermal equipment such as fins and electrical components. The rectangular enclosure simulates an enclosure of electronic equipment and heat exchangers. The shape of the cross section of the pillar was square having sides 30 mm. The dimension of the width of the enclosure was changed from 50 mm to 80 mm. It was found that the heat transfer performance of the pulsating airflow became higher than that of the steady flow regardless of the dimension of the clearance. The heat transfer enhancement around heating components by the pulsating flow can be available regardless of the clearance around the components.

叶片数对无轴泵喷推进器性能的影响

为丰富无轴泵喷推进器噪声性能优化理论,研究了叶片数对无轴泵喷推进器性能的影响,重点探究了不同叶片数下无轴泵喷推进器内的压力脉动变化规律。基于计算流体力学中的有限体积法离散RANS方程,应用RNG k-ε湍流模型封闭计算模型,采用SIMPLEC进行压力-速度耦合,对不同叶片数下的无轴泵喷推进器内部流场进行了非定常数值模拟计算,并对计算结果进行了详细的对比分析。研究结果表明:随叶片数的增加,无轴泵喷推进器模型的扬程、效率和推力均逐渐增大,叶片表面压力分布更加均匀,叶轮轮缘和中部的压力脉动均明显降低,叶轮进出口主频处的压力脉动幅值从轮缘向中心逐渐减小。

一种适于脉动条件的气液分离器的设计与研究

文中针对在流量脉动条件下,无罩、圆柱状、“柱+锥”状稳涡罩对气液分离器的分离性能及流场特性产生的影响,进行了数值模拟和试验研究,提出了旋流场脉动衰减程度评价指标———脉动衰减率(ηp),其值大小与脉动衰减性能成反比。研究结果显示:在流量脉动条件下,“柱+锥”状稳涡罩对于提高旋流场稳定性的能力最为显著,脉动衰减率为0.01028,分离效率最高可达95.15%;同时,运用高速摄像技术对两相流场中气核运动及变化形态进行可视化观测,掌握了基于流量脉动条件下,旋流场内气核的运移规律,并将其划分为4个阶段(断续阶段、汇聚阶段、融合阶段、最终阶段),发现“柱+锥”状稳涡罩旋流器的气核向中心轴线聚集趋势显著,形成具有向上运动形态的空气柱,对旋流场内脉动流有明显的抑制作用。

基于Realflow的大力神波轮的设计研究

波轮式洗衣机通过波轮的正反转动作,带动桶内水的流动,形成一定规律的水流。波轮的结构对水流的强度具有较大的影响,因此如何设计一款具有最佳特征的波轮结构,成为了波轮产品的一大痛点。以大力神波轮为研究对象,介绍波轮的工作原理、洗涤系统和波轮参数设置,使用Realflow仿真软件对波轮进行有限建模以及水流仿真分析,得到一款具有最佳特征的波轮结构。利用CFD对波轮水流仿真的方法,可以极大减少波轮结构的无效设计,缩短波轮结构的设计周期,为波轮式洗衣机的水流设计提供一定的参考价值。

降速方式对混流泵压力脉动特性的影响

混流泵在降速过程中的内部流动特性变化对其性能和稳定性有重要影响。针对这一问题,该研究探究了混流泵在降速过程中内部流动的复杂性,尤其关注不同降速方式对泵内压力脉动特性的影响。通过构建多通道测试系统,收集了混流泵在不同降速方式下的外特性参数信号和泵内压力脉动信号。并采用非稳态信号处理技术,从时域、频域和时频三方面探究不同降速方式对叶轮出口和导叶内部的压力脉动特性的影响以及二者之间的相关性。结果表明,泵内压力脉动的峰值和样本熵的变化趋势与不同降速方式的幂指数呈正相关关系;在整个降速过程中叶轮出口压力脉动对导叶内压力脉动起主导作用;叶轮出口和导叶内部压力脉动之间的相干频率主要集中于80~120 Hz之间;在不同降速过程中叶轮出口和导叶内部的压力脉动之间强相关区域的频带范围保持不变,但是强相关区域在整个降速过程频带范围中的占比有所变化;对于具有固定初始转速和目标转速的降速过程,降速方式的改变不会影响叶轮出口和导叶内压力脉动的相干频率。研究结论可为揭示混流泵瞬态运行特性及提高混流泵降速运行中的稳定性提供参考。

基于能量守恒定律对涡轮流量计的脉动误差上限的估算

在管道输运当中,非稳态流会使涡轮流量计的计量结果产生偏差。为了指导脉动流工况下的涡轮流量计选型和修正,需估算脉动工况对涡轮流量计产生的最大脉动误差。忽略流体压力和温度变化影响,基于涡轮流量计的动态特性和能量守恒定律,在一个脉动周期内计算脉动流和定常流对叶轮的做功比值,推导得出正弦脉动流误差上限公式。基于6DOF模型和UDF建立了涡轮流量计CFD仿真模型,搭建了空气流实验平台,以工作级流量标准装置作为标准表,被校表和标准表间采用内置整流器的管段相连。对比定常流下的流量值,计算各脉动工况下的误差限值。综合各脉动工况点,上限公式与CFD仿真所得脉动误差上限的最大差值为0.281%,与空气流实验所得脉动误差上限的最大差值为0.224%。经CFD仿真和空气流实验结果对比验证,脉动误差上限公式较为准确,可以直接用于脉动频率大于10 Hz的工况下误差值的估计和修正。

脉动流通道内不同纵向涡发生器的颗粒污垢特性

换热器工作时介质中往往含有细小杂质粒子,容易沉积在通道中形成颗粒污垢。将传统脉动流与纵向涡发生器相结合,采用数值模拟方法对光滑通道、脉动流通道、纵向涡发生器通道以及脉动流结合纵向涡发生器通道的颗粒污垢特性进行对比分析,并且详细分析了脉动流结合纵向涡发生器通道中6种不同纵向涡发生器的影响。结果表明,相较于光滑通道,脉动流通道与纵向涡发生器通道的污垢热阻分别减小17%与22%,而脉动流结合纵向涡发生器通道能够减小45%,说明二者结合后的抑垢效果更好。对比脉动流结合6种不同纵向涡发生器发现相同类型下弯曲纵向涡发生器的抑垢效果优于未弯曲纵向涡发生器,通过与脉动流通道的污垢热阻渐近值对比发现抑垢效果由低到高为三角形12.8%、弯曲三角形14.7%、梯形22.0%、弯曲梯形23.8%、矩形29.4%、弯曲矩形33.0%,其中弯曲矩形纵向涡发生器的抑垢效果最好。