共查询到20条相似文献,搜索用时 109 毫秒
1.
2.
采用格子Boltzmann方法对可变形腔体内自然对流问题进行数值研究,给出平均努赛尔数的经验关系式.腔体左壁加热长度分为左壁面的整个区域(H)和左壁面的中间区域(0.5H)两种情况,右壁向外界环境开放,上下边界绝热且可以上下移动,以此调节右出口尺寸.主要研究瑞利数(104 ≤ Ra ≤ 106),右出口尺寸(1.0H ≤ L ≤ 2.0H),左壁加热尺寸(Lh=0.5H或Lh=H)对腔体内等温线、流线、局部努塞尔数和平均努赛尔数的影响.结果表明:腔体内换热随着瑞利数的增大越来越强烈,表现为椭圆形准静止区域更加靠近上绝热壁,且热分层厚度逐渐变小,平均努赛尔数增加.而右出口尺寸的增加,对于两种加热尺寸下腔内的换热效果有不同程度影响,其中与加热尺寸为左壁面的全部区域Lh=H相比,加热尺寸为左壁面的中间情况Lh=0.5H时,右侧开口尺寸的增加对换热效果的影响不显著.此外,左壁加热尺寸为0.5H时显示出比加热尺寸为H时更高的平均传热效率.最后,针对不同的加热尺寸,提出加热面平均努赛尔数与Ra数及右壁面开口尺寸L*之间函数关系的经验预测,拟合效果满足工程实践与设计需要. 相似文献
3.
4.
5.
一、引言 在各种颗粒状原料的热等离子体加工过程中,努曾数(定义为气体粒于的平均自由程长度与颗粒直径之比)可能在广泛的范围内变化.如在近年来受到重视的低压等离子体喷涂中,努曾数可以达到一以至更大的数值.低压下原料颗粒的加热往往比大气压下困难,我们曾采用所谓的“热传导势跳跃近似”,分析过小努曾数(<0.5左右)下压力对热等离子体向颗粒传热的影响,指出努曾效应是低压下颗粒加热恶化的原因。对于努曾 相似文献
6.
采用计算流体动力学的方法, 研究了微通道内气体在速度滑移和随机表面粗糙度耦合作用下的流动特性. 其中, 利用二阶速度滑移边界条件描述气体的边界滑移, 利用分形几何学建立随机粗糙表面. 研究发现, 综合考虑二阶速度滑移边界条件和随机表面粗糙度在较大的平均Knudsen数范围内 (0.025-0.59) 得到的计算结果与实验数据符合得很好, 而一阶速度滑移边界条件只在平均Knudsen数较小时(<0.1)符合实验结果. 随机表面粗糙度对气体在边界处的滑移有显著影响, 相对粗糙度越大, 速度滑移系数越小. 并针对计算结果, 给出了滑移系数与相对粗糙度近似满足的关系. 随机粗糙表面对气体流动过程中的压强、速度、Poiseuille数也有显著影响.
关键词:
随机表面粗糙度
二阶速度滑移边界条件
分形
微通道 相似文献
7.
8.
在血管壁施加第三类边界条件是计算血液与组织间对流换热的一种近似计算方法.为分析其可行性,用有限元数值模拟方法计算血管分支结构中血液与组织的对流换热,得到不同流速和半径下分支血管内血液的截面平均Nu数沿管长的变化曲线.结果表明,血管树中分支血管的Nu数变化幅度不大,且趋于稳定值的速度很快.以相同边界条件下包含简单血管系统的舌体为例,分别用近似方法和完全耦合计算方法,进行血液流场和舌体温度场模拟.通过比较计算结果,得出两种方法得到的温度场分布趋势基本相同;用完全耦合计算方法得到的舌体温度略高于用近似方法得到的舌体温度,两者差值小于0.2℃. 相似文献
9.
10.
采用格子玻尔兹曼方法对有三种恒温热源(圆形、三角形、方形)参与的圆管内纳米流体(铜-水)自然对流进行数值研究。主要研究瑞利(Ra)数,纳米颗粒体积分数以及热源几何形状等控制参数对纳米流体的流动与传热的影响。结果发现纳米颗粒体积分数的增加有利于强化传热,且在Ra数较小时,平均努塞尔(Nu)数增加的幅度要优于Ra数较大的情况。在所研究的控制参数范围内,方形热源的平均Nu数最大。根据数值结果给出不同热源表面的平均Nu数、纳米颗粒体积分数、Ra数三者之间的函数关系式,该函数关系可为此类工程的设计提供理论指导。 相似文献
11.
根据二维非稳态层流的质量和动量守恒方程,研究强声波作用下煤颗粒周围气体的振荡流动特性.入射波的振幅远大于颗粒特征长度,声雷诺数小于20.根据通用微分方程的解,详细分析不同声雷诺数与斯特劳哈尔数下,颗粒壁面的流场分布、轴向压力梯度、切向应力及分离角的分布,发现在低频(~50 Hz)时,颗粒壁面轴向压力梯度、切向应力及流动分离角的分布主要受曲率效应影响,其变化规律与振荡速度的幅值变化相对应;在高频时(~5 000 Hz),颗粒壁面轴向压力梯度、切向应力及流动分离角的分布同时受到曲率效应和流动加速度的影响.为进一步研究强声波强化煤颗粒燃烧提供理论基础. 相似文献
12.
针对频率为500 Hz~3000 Hz和声压级为110 d B~133 d B的声场作用对铜球在空气中自然冷却的传热特性的影响,通过热电偶测温的方法,分析铜球温度梯度的分布与声场声压级、频率以及铜球直径的关系。结果表面,当频率f一定时,随着声压级的增加,铜球的传热效果得到明显增强,对于直径为5 mm的铜球,在133 d B声场中传热系数最大增加了25%。当声压级一定时,在频率范围中存在某一频率,此时铜球的传热系数最大,此特殊频率随着声压级的增大而增大。当铜球的直径为5 mm时,可以在低频段观测到声流效应的影响,而当铜球的直径为10 mm、15mm时,很难在低频段辨别出声流效应的影响。所得结论为声波应用于电站锅炉中,强化煤颗粒燃烧提供了依据。 相似文献
13.
This study discusses a possibility for enhancement of heat transfer between solids and ambient gas by application of powerful acoustic fields. Experiments are carried out by using preheated Pt wires (length 0.1-0.15 m, diameter 50 and 100 micro m) positioned at the velocity antinode of a standing wave (frequency range 216-1031 Hz) or in the path of a travelling wave (frequency range 6.9-17.2 kHz). A number of experiments were conducted under conditions of gas flowing across the wire surface. Effects of sound frequency, sound strength, gas flow velocity and wire preheating temperature on the Nusselt number are examined with and without sound application. The gas phase heat transfer rate is enhanced with acoustic field strength. Higher temperatures result in a vigorous radiation from the wire surface and attenuate the effect of sound. The larger the gas flow velocity, the smaller is the effect of sound wave on heat transfer enhancement. 相似文献
14.
An experimental investigation was made to study heat transfer in a pipe which is oscillated about an axis that is parallel to, but offset from, the pipe axis. Air was used as working fluid. The experimental setup was designed so as to provide oscillating motion of a test pipe. The measurement systems were installed on the oscillating section. For both steady and oscillating flows, the bulk air temperature and wall temperature, pressure drop, and frequency were measured. The parameters for this study were chosen as Reynolds number from 5,000 to 20,000 and oscillating frequencies from 10 to 20 Hz. The variations of Nusselt number versus these parameters were determined and presented graphically. Heat transfer enhancement of 42% was achieved at constant pumping power for oscillatory flow. 相似文献
15.
Forced convection heat transfer from a helically coiled heat exchanger embedded in a packed bed of spherical glass particles was investigated experimentally. With dry air at ambient pressure and temperature as a flowing fluid, the effect of particle size, helically coiled heat exchanger diameter, and position was studied for a wide range of Reynolds numbers. It was found that the particle diameter, the helically coiled heat exchanger diameter and position, and the air velocity are of great influence on the convective heat transfer between the helically coiled heat exchanger and air. Results indicated that the heat transfer coefficient increased with increasing the air velocity, increasing helically coiled heat exchanger diameter, and decreasing the particle size. The highest heat transfer coefficients were obtained with the packed-bed particle size of 16 mm and heat exchanger coil diameter of 9.525 mm (1/4 inch) at a Reynolds number range of 1,536 to 4,134 for all used coil positions in the conducted tests. A dimensionless correlation was proposed for Nusselt number as a function of Reynolds number, particle size, coil size, and coil position. 相似文献
16.
An experimental setup was built to study the influence of conduction heat loss on the convective heat transfer performance enhanced by an impinging jet in cross-flows. Results revealed that the conduction heat loss ratio (Ec/E) is between 12.0% and 40.1%, and it decreases nonlinearly with the ratio of jet-to-cross-flow velocity. The relative Nusselt number increases with the ratio of jet-to-cross-flow velocity. The maximum peak value and the average are 8.1 and 6.4, respectively. The distribution of the relative Nusselt number seems to be flattened by assuming a constant conduction heat loss ratio. 相似文献
17.
18.
In this article, distilled water and CuO particles with volume fraction of 1%, 2% and 4% are numerically studied. The steady state flow regime is considered laminar with Reynolds number of 100, and nano-particles diameters are assumed 20 nm and 80 nm. The hydraulic diameter and the length of equilateral triangular channel are 8 mm and 1000 mm, respectively. The problem is solved for two different boundary conditions; firstly, constant heat flux for all sides as a validation approach; and secondly, constant heat flux for two sides and constant temperature for one side (hot plate). Convective heat transfer coefficient, Nusselt number, pressure loss through the channel, velocity distribution in cross section and temperature distribution on walls are investigated in detail. The fluid flow is supposed to be one-phase flow. It can be observed that nano-fluid leads to a remarkable enhancement on heat transfer coefficient. Furthermore, CuO particles increase pressure loss through the channel and velocity distribution in fully developed cross section of channel, as well. The computations reveal that the size of nano-particles has no significant influence on heat transfer properties. Besides, the study shows a good agreement between provided outcomes and experimental data available in the literature. 相似文献
19.
圆管层流脉冲流动对流换热数值分析 总被引:3,自引:0,他引:3
对等热流和等壁温边界条件下圆管内层流脉冲流动对流换热问题进行了数值模拟。在等热流边界条件下的数值计算结果与理论解吻合很好。计算结果表明:在等热流和等壁温边界下脉冲流动可引起速度、温度以及努塞尔数随时间波动,振幅越大,脉冲频率越小,波动越大。但它们的时均值均等于在相同雷诺数下稳态流动的值,脉冲流动不能强化换热。 相似文献
20.
The understanding of the flow and heat transfer processes for fluid through micro- and nanochannels becomes imperative due to its wide application in micro- and nano-fluidic devices. In this paper, the method to simulate the convective heat transfer process in molecular dynamics is improved based on a previous study. With this method, we simulate a warm dense fluid flowing through a cold parallel-plate nanochannel with constant wall temperature. The characteristics of the velocity and temperature fields are analysed. The temperature difference between the bulk average temperature of fluid and the wall temperature decreases in an exponential form along the flow direction. The Nusselt number for the laminar flow in parallel-plate nanochannel is smaller than its corresponding value at macroscale. It could be attributed to the temperature jump at the fluid–wall interface, which decreases the temperature gradient near the wall. The results also reveal that the heat transfer coefficient is related to the surface wettabilities, which differs from that in the macroscopic condition. 相似文献