﻿ 临近空间环境下封闭方腔内耦合换热特性<sup>*</sup>
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Conjugate heat transfer characteristics of enclosure cavity in near space environment
ZI Guicai, HE Weiliang
School of Astronautics, Beijing University of Aeronautics and Astronautics, Beijing 100083, China
Received: 2017-06-19; Accepted: 2017-10-13; Published online: 2017-12-20 13:48
Corresponding author. HE Weiliang, E-mail:heweiliang@buaa.edu.cn
Abstract: Aimed at the application of near space aerostats' load cabins, numerical simulation of natural convection, surface thermal radiation and heat conduction in a cubical enclosure cavity with a heat source in complex thermal boundary conditions was carried out. A model of near-space thermal environment was established considering the effects of convective heat transfer, infrared radiation and solar radiation. The diurnal variation of the thermal characteristics in the enclosure cavity was studied by introducing the external unsteady convection-radiation coupling thermal boundary conditions through the Fluent software's user-defined function (UDF). The effects of solid wall thickness, emissivity and thermal conductivity were discussed. The numerical results indicate that the average temperature change in the enclosure cavity is about 12.9 K during one day, and the temperature field distribution changes with the sun's position. The natural convection in the enclosure cavity is weak and the maximum temperature difference is 71.3 K at the same time. Increased thermal resistance and surface emissivity lead to weakening of natural convection in the cavity.
Key words: near space     enclosure cavity     conjugate heat transfer     natural convection     numerical simulation

1 封闭方腔数学建模 1.1 封闭方腔理论分析模型

 图 1 封闭方腔热分析模型 Fig. 1 Thermal analysis model of enclosure cavity

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Xi, sky为方腔表面对空的角系数，是该表面与水平面之间夹角θi(规定上表面与水平面的夹角为0)的函数[12]

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1.2 封闭方腔数值分析模型

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 图 2 X=0.28 m截面的网格示意图 Fig. 2 Schematic of grid at X=0.28 m section
 图 3 不同网格下沿直线X=Y=0.28 m的温度变化曲线 Fig. 3 Variation curves of temperature at X=Y=0.28 m for different grids
2 计算结果分析 2.1 环境昼夜变化的影响

 图 4 腔内温度极值和平均值随时间变化曲线 Fig. 4 Variation of extreme and average temperature in cavity with time
 图 5 不同时刻腔内温度和垂直速度分布云图 Fig. 5 Distribution contours of temperature and vertical velocity in cavity at different time

 图 6 0:00和12:00时刻Y=0.28 m截面垂直方向的速度矢量图 Fig. 6 Vertical velocity vector of Y=0.28 m section at 0:00 and 12:00

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 图 7 腔内顶面平均努赛尔数和空气温差变化曲线 Fig. 7 Variation curves of average Nusselt numbers and air temperature differences at top surface in cavity
2.2 内表面发射率的影响

 内表面发射率 辐射换热量/W 对流换热量/W 总换热量/W 0.2 2.99 1.92 4.91 0.4 3.30 1.73 5.03 0.6 3.43 1.65 5.08 0.8 3.47 1.63 5.10

0:00时刻，在不同内表面发射率下，X=0.28 m截面的温度(单位为K)和速度(单位为m/s)等值线如图 8所示，腔内温度极值(TmaxTmin)和平均温度(Tave)的变化如图 9所示，沿直线X=Z=0.28 m方向垂直速度Vz的变化如图 10所示。

 图 8 0:00时刻不同内表面发射率下X=0.28 m截面的温度和速度等值线 Fig. 8 Temperature and velocity contours at X=0.28 m section for different values of internal surface emissivity at 0:00
 图 9 0:00时刻腔内温度极值和平均温度随内表面发射率的变化 Fig. 9 Change of extreme temperature and average temperature in cavity with internal surface emissivity at 0:00
 图 10 0:00时刻不同内表面发射率下直线X=Z=0.28 m方向垂直速度变化曲线 Fig. 10 Variation curve of vertical velocity at X=Z=0.28 m for different values of internal surface emissivity at 0:00

2.3 腔壁导热系数的影响

 图 11 0:00时刻不同腔壁导热系数下X=0.28 m截面的温度和速度等值线 Fig. 11 Temperature and velocity contours at X=0.28 m section for different values of thermal conductivity at 0:00
 图 12 0:00时刻不同腔壁导热系数下直线X=Z=0.28 m方向温度和垂直速度的变化曲线 Fig. 12 Profiles of temperature and vertical velocity at X=Z=0.28 m for different values of thermal conductivity at 0:00
2.4 腔壁厚度的影响

 图 13 0:00时刻不同腔壁厚度下X=0.28 m截面的温度和速度等值线 Fig. 13 Temperature and velocity contours at X=0.28 m section for different values of solid wall thickness at 0:00
 图 14 0:00时刻不同腔壁厚度下直线X=Z=0.28 m方向温度和垂直速度的变化曲线 Fig. 14 Variation curve of temperature and vertical velocity at X=Z=0.28 m for different values of solid wall thickness at 0:00

 图 15 腔内平均温度为262 K时，导热系数和腔壁厚度的关系 Fig. 15 Thermal conductivity versus solid wall thickness when average temperature in cavity is 262 K
3 结论

1) 临近空间环境中，封闭方腔外的非均匀对流-辐射耦合热边界条件及其昼夜变化对腔内热特性的影响主要体现在腔内温度分布和空气流动状态上。腔内温度分布随太阳高度角和方位角变化而变化，温度较高的区域空气流速较快。弱导热系数的保温材料能有效降低外界环境的影响，使腔内平均温度昼夜变化很小，约为12.9 K，满足大部分设备的工作要求。

2) 临近空间环境中，方腔内空气流速很低，最大不超过0.55 m/s，换热能力较弱。同一时刻，腔内大部分区域温差不大，但在靠近热源的附近存在较大温度梯度。夜间温差最大，约为71.3 K，12：00时刻温差最小，约为67.9 K。如果方腔内设备发热量较大、工作温度要求苛刻，需要采取其他措施增加其向空气的散热，促进腔内温度分布均匀。

3) 腔内表面辐射效应增强，会削弱自然对流换热的强度，但总换热量增加。方腔内表面发射率增加，对腔内平均温度影响较小，但能促进腔内温度分布的均匀。

4) 腔壁热阻增加(导热系数减小或厚度增加)会削弱腔内自然对流的强度，同时降低外界环境对腔内温度的影响。

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#### 文章信息

ZI Guicai, HE Weiliang

Conjugate heat transfer characteristics of enclosure cavity in near space environment

Journal of Beijing University of Aeronautics and Astronsutics, 2018, 44(6): 1283-1293
http://dx.doi.org/10.13700/j.bh.1001-5965.2017.0412