Thermal evaluation of the use of porous ceramic plates on ventilated façades—part II: Thermal behavior

The effects of porous ceramic plates on the thermal behavior of ventilated façades were evaluated, and the results are presented herein. Thermal behavior in a ventilated façade of specimens containing 40 wt% of lime mud and a firing temperature of 1100°C was evaluated and compared with a commercial porcelain ceramic tile, which was the reference material. An experimental apparatus was designed to evaluate the thermal performance of the studied ventilated façades. The results revealed that the ventilated façade composed of the studied porous ceramic tiles produced a greater reduction in temperature between the external environmental and the interior of a box representing a building (ΔT5) of 65.7°C, compared with the façade composed of the commercial porcelain ceramic tiles (ΔT5 = 56.0°C) and even the traditional façade (ΔT5 = 49.1°C). Thus, porous ceramic tiles based on byproducts are promising candidates for ventilated façade systems.     

Bubble Formation at a Rotating Cylindrical Surface in Cross‐Flowing Liquid

A novel method of gas sparging from a rotating cylinder is proposed, which prevents against formation of large attached gas cavities in cross‐flowing liquids including those flowing downwards. Experimental and theoretical results regarding critical rotation speed necessary to remove the attached cavity, bubble formation process and size distribution of the produced bubbles in a low viscosity system (air‐water) are presented in this study.     

Modelling and Thermal Analysis of Tray‐layered Fruits inside Ventilated Packages during Forced‐air Precooling

Ventilated packaging is widely used in the forced‐air precooling practice for horticultural produce. Fresh fruits are living organisms which are sensitive to temperature in turn related to airflow and heat transfer inside package. In this study, a transient mathematical model considering heat of respiration and evaporation is developed to predict the thermal response of tray‐layered fruits in ventilated packages during forced‐air precooling. Specifically, the heat source is combined with the energy conservation equation and loaded into numerical solution by User Defined Function (UDF). Temperature profiles of three variously distributed circular and oblong vents in three different patterns (spaced, paralleled and crossed stacking) are simulated, separately. The results show that the heat source affects fruit cooling process, and the layered fruit in paralleled stacking pattern tends to be cooled better than others. Furthermore, the results indicate that vertical oblong vent could improve the longitudinal and lateral airflow, while non‐central vent design could greatly improve the overall cooling performance. Definitely a triangular distribution of three circular vents was superior to laterally distributed centre vents with 66.5% higher uniformity and 2.5°C lower of the highest temperature. Compared with the three identical vertical oblong vent conditions, vent design with one hand hole and two side vertical oblong vents can be cooled more uniformly with an increase of 6.5%. It is revealed that vents with large major‐to‐minor axis ratio could be applied to balance airflow and ease cooling differences for a rapid but uniform cooling. Experimental validations were performed for Sim2, Sim4 and Sim6, Sim8, Sim9, and good agreement was obtained considering the five vent conditions with the error less than 3.5°C but coordinated later (within the limits of the experimental uncertainty).Thus the numerical model can be used to predict and optimize temperature distribution within precooling packages. Copyright © 2016 John Wiley & Sons, Ltd.     

Numerical study on the influence of centroid-to-buoyancy center ratio on the motion stability of supercavitating vehicle

In order to study and solve the influence of the change of the position of the centroid of the supercavitating vehicle on the motion stability, the influence law of the position of the centroid of the supercavitating vehicle on the motion stability is found, and the quantitative criterion that can guide the design of the supercavitating vehicle is summarized. On the basis of theoretical analysis, the dynamic mesh technology combined with the volume of fluid multiphase flow model and the rigid body six degrees of freedom motion calculation model is used to carry out the supercavitating vehicle motion and supercavitating flow field coupling numerical simulation research for supercavitating vehicles with different cavitator shapes. The results show that the position relationship between the centroid position and the buoyancy position has a great influence on the motion stability of the vehicle. When the center of buoyancy is before the center of mass, the vehicle is unstable; when the center of buoyancy is behind the center of mass, the vehicle sails stably, and when the position of buoyancy action coincides with the center of mass, the amplitude of the tail beat of the vehicle is the smallest, and the navigation is the most stable, which is the best position of the centroid. In this article, the concept of centroid-to-buoyancy center ratio is proposed. The so-called centroid-to-buoyancy center ratio refers to the ratio of the length from the centroid to the top of the head of the supercavitating vehicle to the length from the center of buoyancy to the top of the head of the vehicle. When the centroid-to-buoyancy center ratio fluctuates in the range of [0.8–1], it is an ideal condition for the stable navigation of the vehicle. When the centroid-to-buoyancy center ratio is in the range of (0, 0.8), the tail beat frequency is high, which accelerates the kinetic energy consumption and the rapid instability of the vehicle. Therefore, under the same constraint conditions, it is not that the closer the center of mass is, the better the stability of the vehicle motion is. The optimal centroid position is not a certain “point,” but a “periodic fluctuation interval” that changes with the buoyancy center.     

EXPERIMENTAL RESEARCH ON THE SHAPE CHARACTERS OF NATURAL AND VENTILATED SUPERCAVITATION

A series of projectile and water-tunnel experiments were conducted to investigate the shape characters of natural and ventilated supercavitation. It was found that the shape and dimensions of both the natural supercavitation and the ventilated supercavitation are similar through the comparison of them when the cavitation number is small and equal, and the contour of both the natural supercavitation and the ventilated supercavitation can be calculated effectively with the Savchenko formula. The gravity effect can induce the asymmetry of ventilated supercavitation, and the asymmetry is more apparent as the Froude number is smaller. The empirical formula for calculating the axial deformation of ventilated supercavitation was corrected and expanded based on experiment data. The evolution rules of both the natural supercavitation and the ventilated supercavitation were described, and the vortex frequency of natural cavitation was obtained. In addition, the hysteretic aspect was observed between the development and the fall process of ventilated cavitation.     

新型旋流风冷冷渣器

在研究国内外已有循环流化床锅炉冷渣器的基础上,提出新型冷渣器结构模型,通过冷渣器内部的流动传热分析及模拟计算表明,新型冷渣器具有结构简单、传热和体积效率高、气固比小的特点,可很好地满足循环流化床锅炉的应用要求。     

头支撑下空化器外形对通气超空泡的影响

目前,空化器外形对超空泡形态的影响研究都是在空化器前的流场未被干扰时进行的,当水洞试验因采用头部支撑方式而影响了空化器前的流场时,通气超空泡形态随空化器外形的变化规律尚不明确。本文应用计算流体力学(CFD)仿真软件FLUENT,参照头支撑下通气超空泡生成的试验模型,建立了随空化器直径、连接杆直径和空化器锥角变化的仿真模型。在通气量一定的情况下,通气超空泡的长度和长径比随头支撑连接杆的增大而增大,随空化器直径的增大而减小,随空化器锥角的减小而增大,并深入研究了空化器外形对空泡尺寸的影响规律,为水洞超空泡试验在头支撑方式下空化器外形的设计提供参考。     

混合对流作用下通风小室内流动换热的数值模拟

本文对一个侧壁受定热流作用的通风小室在两种气流组织型式下的混合对流换热情况进行了数值模拟,计算得到了流场,温度场及混合对流条件下室内的换热情况等随雷诺数Ｒａ的变化规律。     

Numerical Simulation for Ventilated Supercavitation High Speed Underwater Vehicle

Supercavitation is a revolutionary technique to achieve high drag reduction for underwater vehicle. It can help us to break through the conventional speed barrier. This article presents a numerical algorithm for ventilated supercavitation flow field based on mixture multiphase flow model, briefs the calculation results and compares them with that tested in high-speed water tunnel and towing tank. The mathematical model, its numerical calculation method, computational region and boundary conditions are discussed in detail. Some pertinent nondimensional parameters about the ventilated supercavitation, such as geometrical configuration of supercavity, drag coefficient and ventilation rate are investigated. Reynolds number is selected to predict gas ventilation rate instead of Froude number. Finally, based on the test and simulation results, a semi-empirical formula of the ventilation rate estimation suitable for different conical angle caritators is proposed.     

基于ALE方法的弹性圆柱壳入水时的流固耦合模拟

为了获得头型及材料弹性对圆柱壳入水过程中头部变形、压力分布、入水空泡形态及入水运动状态的影响,采用ALE方法,基于有限元软件ANSYS\LS-DYNA,对平头、120°锥角、90°锥角弹性圆柱壳的入水过程进行了数值模拟,对平头弹性圆柱实体和平头刚性圆柱壳进行计算,并对模拟结果进行对比。结果表明:入水过程中空泡直径随着锥角的增大而增大,锥角越大,圆柱壳上表面的变形越大,最大变形出现的时间越早:平头弹性圆柱壳在0.1 ms时出现最大变形0.84 mm,120°锥角弹性圆柱壳在0.3 ms时出现最大变形0.54 mm,90°锥角弹性圆柱壳在0.5 ms时出现最大变形0.43 mm; 下表面受到的压力越大,圆柱壳的速度衰减越快; 平头圆柱壳下表面的变形与振动频率大于上表面; 上表面的变形是由惯性引起的,下表面的变形是由流体冲击力引起的。