摘要
随着超导技术的发展，高温超导电缆在电力输运中逐渐得到重视并进行了广泛的研究。由于波纹管具有良好的柔韧性和收缩性，在高温超导电缆中得到应用。波纹管内的流动压力损失参数是高温超导电缆低温容器的主要的设计参数，这是因为压力损失决定了高温超导电缆的结构强度、循环压力以及液氮泵的功率。所以研究波纹管内的流动特性具有重要意义。为此，对通径为6mm，8mm和10mm的波纹管内液氮、氮气和水的流动特性进行了实验研究，为了更好分析波纹管内的流动特性，对波纹管内液氮的流动特性进行了数值计算。 为了进行波纹管内液氮流动特性的实验研究，制作了波纹管实验段，实验段的真空绝热结构，搭建了实验台。测量了液氮在通径为8mm波纹管内的流动压降。分析发现：在10000～25000的雷诺数范围内，液氮在波纹管内流动具有波动性，压力损失随雷诺数的增大而增大，要远大于光管的压力损失。
为了对比分析不同流体在波纹管内的流动特性，搭建了氮气和水在波纹管内流动实验台。测量了氮气和水在通径为6mm，8mm和10mm的波纹管内流动压降。分析表明：在4000～40000的雷诺数范围内，压力损失随质量流量的增大而增大。在同一质量流量下，压力损失由大到小依次为DN6mm﹥DN8mm﹥DN10mm。压力损失随雷诺数的增大而增大。在同一雷诺数下，压力损失由大到小依次为DN6mm﹥DN
8mm﹥DN10mm。波纹管的摩擦系数要高于光管的摩擦系数。摩擦系数随波纹高度的减小而减小。
对比发现：在波纹管内液氮、氮气和水的流动特性类似。不同的是：在同雷诺数下，液氮的压力损失介于水的压力损失和氮气的压力损失之
I
间，但是水的摩擦系数却要小于氮气的摩擦系数。
为了进一步研究液氮在波纹管内的流动特性，应用FLUENT软件对液氮在波纹管内的流动特性进行了数值计算，得到了液氮在不同波纹管内流动的压力、速度分布，并对结果进行了详细的分析。分析表明：压力损失随入口速度的增大而增大；在波纹管的波纹的前端和后端具有不同的压力梯度分布，并且后端处的压力要高于前端处的，在波纹内形成了涡旋流；在所考察的40000～200000雷诺数范围内，雷诺数对摩擦系数的影响不大，而波纹尺寸对摩擦系数具有主要的影响。摩擦系数随s/t 的增大而减小。
液氮是冷却超导体的最佳工质。高温超导体的形状多种多样，其中包括丝状，出于紧凑的考虑，对高温超导细丝的冷却可采用毛细管。为此，本文还研究了毛细管管径以及倾角对其内的加热丝与液氮的换热效果的影响。应用FLUENT软件对0°、30°、60°、90°倾角下管径为1.2mm 和2.0mm的毛细管内
的加热丝与液氮的换热情况进行了数值计算，得到了管内液氮的速度、温度以及加热丝的温度分布情况。计算结果表明倾角为30°和60°的换热效果最好，大管径的换热情况要比小管径的换热效果好。
关键词：波纹管，湍流，压力损失，摩擦系数，毛细管，自然对流换热
II
FLOW AND HEAT TRANSFER OF LIQUID NITROGEN IN CORRUGATED TUBES AND CAPILLARY
TUBES
ABSTRACT
With the development of the superconductor technology, high temperature superconducting (HTS) cable has been developed. However, the hydraulic characteristics in the HTS cable must be well investigated to design the transmission line.Especially the pressure drop in the transmission line is an important parameter to design, because the pressure drop decides the length of the transmission line and the pump power. The corrugated tube is used in the HTS power because of its flexility and contractibility. However the friction factor in corrugated tubes has been not well studied. So it is impo
rtant to study the flow characteristics in corrugated tubes. They are investigated systematically in the paper.
Experiments have been conducted to study the pressure drop characteristics of liquid nitrogen, nitrogen gas and water in three corrugated tubes with the diameters of 6mm, 8mm and 10mm. The friction factors are compared with the conventional correlations over a Reynolds numbers of 4000-40000. The pressure drop increases with the mass flux increases. The pressure drop increases with the Reynolds numbers increase. The friction factors of the corrugated tubes are higher than those of the smooth tubes. The corrugation geometries have big influences on the flow. The friction factor decreases with the depth of corrugation decreases.
We make comparison between liquid nitrogen, nitrogen gas and water
III
flow characteristics in corrugated tubes. And find that the pressure drop of liquid nitrogen is between water and the nitrogen gas at the same Reynolds numbers. While the friction factor of water is smaller than the nitrogen gas at the same Reynolds numbers. The thermo-physical properties have big influences on the flow.
The detailed flow characteristics of the liquid nitrogen in corrugated tubes are investigated by numerical simulation for different inlet fluid velocities and tube geometries, i.e. different sizes of the pitch and depth of corrugation. The pressure and velocity field contour plots are obtained and analyzed. The pressure drop increases with the velocity increases. The results of friction factor show that the influences of pitch and corrugation depth are significant but the dependency on the Reynolds number is not severe. The friction factor decreases while the s/t increases.
Liquid nitrogen is the most competitive potential substance for cooling superconductor. Superconductor may be any shape, including thin wire.Capillary tube is a compact choice when designing superconducting thin wire cooler.Natural convection in liquid nitrogen around a thin wire heater placed in the center of the capillary is numerically studied. We mainly discuss the effect of the diameter sizes and the inclination angles of the capillary tubes on the heat transfer performance. The inclination angles of the capillary tubes are 0°, 30°, 60° and 90° with the diameters of 1.2mm and 2.0mm. The numerical simulations are performed with the software-FLUENT. The typical distribution of the velocity and temperature of the liquid nitrogen in the capillary tubes is obtained. The results show that the heat transfer performance is better when the inclination angles are 30° and 60° and the heat transfer performance in the 2.0mm diameter capillary tube is also better than that in 1.2mm diameter capillary tube.
KEY WORDS: corrugated tube, turbulent flow, pressure drop, friction factor, capillary tube, natural convection
IV
符号说明英文字母
A面积，m2
d管径，mm
f Darcy摩擦系数
L波纹管道长度，mm g重力加速度， m/s2 m 质量流率， kg/s G质量流量，kg/m2 s Q漏热量，W
p
∆压降， Pa
T温度， K
Re雷诺数
硅气凝胶
T
∆温差，K
a总适应系数
V体积流量，3m s
e
F辐射因子
υ速度，m s
2
1−
F形状因子
柴火无烟灶p压力，Pa
山楂提取物
p
c比热容，)k
kg
/(
J⋅
r
p Prandtl数
fg
h气化潜热，kJ/kg
h换热系数，W/m2 K H
j传热因子
Nu Nusselt数
k总换热系数，W/m2 K s波纹的宽度，mm
t波纹的高，mm
Eu欧拉数希腊字母
µ动力粘度系数，Pa s⋅
ν运动粘度系数，2m s σ斯蒂芬-玻尔兹曼常数ε表面辐射率
λ平均热导率，()
W m K⋅α毛细管的倾角
λ导热系数，()
W m K⋅ρ密度，3
kg m
∆当量粗糙度 mm
δ误差
下标运行网
i 内表面o 外表面in 进口out 出口
LN 液氮苯胺的制备
max 最大值
N 仪表的
f 浮子的
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