﻿ 星载电子器件用空气射流散热特性
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Characteristic of air jet impingement cooling performance for electronic equipment of satellite
LIU Liangtang, WANG Anliang
School of Astronautics, Beijing University of Aeronautics and Astronautics, Beijing 100191, China
Abstract:On the base of the preparative stage of ventilation control system design for high power electronic equipment of satellite during ground tests, the study on optimization design of the cooling performance was done, and the numerical simulation was carried out on air jet impingement cooling system with different structural parameters. Results show that the diameter of nozzle, the distance between nozzle and heat surface, the air jet inclination angle and jet velocity directly affect the performance of the cooling system. And the optimization design results on the quantitative non dimensional parameters are analyzed. The study can be used to optimize heat dissipation for electronic equipment with a heat-flux density of about 1 kW/m2, and provide a reference for convection thermal control system design and ground tests for high-power electric equipment of satellite.
Key words: high power electronic equipment     satellite     ventilation cooling     air jet     optimization design

1 简化模型

 图 1 散热系统结构示意图Fig. 1 Schematic diagram of cooling system structure
2 数值分析方法

2.1 控制方程

2.2 湍流模型

k为湍动能;μt为湍动黏度系数,它不是物性参数.依据求解μt的方程数目,湍流模型可分为Zero-Equation模型、Spalart-Allmaras模型、Two-Equation模型(包括:标准k-ε模型、RNG k-ε模型、Realizable k-ε模型).针对空气射流散热问题,本文选用Realizable k-ε模型,其数学描述如下.

kε的输运方程为

Realizable k-ε模型的湍动黏度系数为

2.3 求解说明

1) 边界条件.

① 喷嘴.采用速度入口边界条件,喷嘴长度设置成1cm,喷嘴直径D为优化参数之一.喷出流体的初始温度设定为20℃.

② 冲击面.为功率器件表面,其热边界条件设置为固定加热功率23W.

③ 出口.采用压力出口边界条件,出口表压设置成0Pa,即为1个标准大气压.

2) 网格模型及无关性验证.

 图 2 计算区域的网格划分Fig. 2 Grids generation of computational region

 图 3 计算模型的网格无关性验证Fig. 3 Verification of independence of grids in computation model

3) 近壁处理.

4) 努塞尔数Nu.

Nu作为一个无量纲参数,用于表征对流换热的强烈程度.对于本文所研究的冲击射流换热问题,冲击面上驻点区平均Nu(下文统称为驻点区Nu)可通过下面公式计算得到:

 图 4 近壁模型的网格划分Fig. 4 Grids generation of near-wall model
2.4 计算方法检验

 图 5 空气射流冲击长方形针肋热沉换热示意图Fig. 5 Sketch map of rectangular pin-fin heat sink under air jet impingement

 图 6 实验数据与仿真数据的对比Fig. 6 omparison between experimental and simulation data

 参数 Hb/mm Hf/mm L/mm C1/mm D1/mm Cw/mm Dw/mm 行数 列数 数值 5 15 50.8 2.6 5 5 2 8 7

3 结果分析与讨论 3.1 流场特性

 图 7 射流冲击速度场分布Fig. 7 Velocity field of jet impingement

3.2 无量纲结构参数对散热性能的影响

 图 8 无量纲参数对Nu数的影响Fig. 8 Effect of non-dimensional parameters on Nu

1) 随着ζ的增加,驻点区Nu单调递增,且整体增加幅度越来越小,当ζ由0.18变为0.36时,驻点区Nu平均增加的百分比高达280%,而当ζ由0.54变为0.72时,驻点区Nu平均增加的百分比降至40%.这是因为随着ζ的增加,喷嘴的出口直径D增大,射流冲击面积(即驻点区面积)增大,总风量增加,系统散热性能增强,但是当ζ持续增加时,射流冲击面积与有效换热面积的比值也在不断增加,总风量的有效利用率在不断降低,所以随着ζ的持续增加,驻点区Nu、系统散热性能增加趋于平缓.

2) 驻点区Nuψ的变化规律比较复杂,分以下两组情况:当ψ≤5时,驻点区Nuψ的减小而逐渐增大,且在ψ趋于无穷小时,将出现下降趋势.当ψ＞5时,驻点区Nu的变化受ζ影响,在ζ≤0.36时,驻点区Nuψ的增加呈现出先增加再减小的变化,在ζ>0.36时,驻点区Nuψ呈现单调递减变化.

 图 9 p*随ψ的变化曲线Fig. 9 Curves of p* changing with ψ

ψ＞5时,换热面基本处于射流核心区之外,射流对驻点区的冲击强度变化不明显,此时湍流度的变化成为影响驻点区散热性能的主要因素[12].当射流初始湍流度不是很高时(如ζ≤0.36的工况),则射流湍流度会随着ψ的增加而逐步提高,直至到达峰值,然后又逐渐减小.受湍流度变化的影响,驻点区散热性能也会随着ψ的增加而逐步提高至峰值.之后,由于射流的到达速度和湍流度的逐渐下降,驻点区散热性能也开始呈现下降趋势,最终形成非单调性变化.但当射流初始湍流度足够高时(如ζ＞0.36的工况),则可以避免上述非单调性现象,使得驻点区散热性能随ψ单调递减变化.

3.3 喷嘴出口风速对散热性能的影响

ζ＝0.36、ψ＝10时,散热系统驻点区Nu随喷嘴出口风速V的变化曲线如图 10所示.

 图 10 Nu随V的变化曲线Fig. 10 Curves of Nu changing with V

3.4 射流倾斜角α对散热性能的影响

 图 11 换热面压强场分布图Fig. 11 Pressure field of heat surface

 图 12 Nu随α的变化Fig. 12 Curves of Nu changing with α
4 结 论

1) 驻点区的散热系数随直径比ζ的增加而增加,且当ζ>0.54时,增加趋势逐渐趋于平缓.

2) 当ζ＞0.36时,散热系数随ψ的增加单调递减;当ζ≤0.36时,散热系数呈现出随间距比ψ的增大先减小再增大,然后再减小的趋势,在ψ=5~7的范围内取得极小值,在ψ=16~22的范围内取得极大值.可据此优化设计喷嘴直径与射流间距.

3) 射流出口速度越大,散热性能越强.但是其负面影响为压力损失也在不断增加.所以在射流散热设计时要均衡考虑两方面的影响,出口速度最大不要超过40m/s.

4) 在其他条件不变时,垂直射流的散热性能要优于其他倾斜角的射流散热.

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#### 文章信息

LIU Liangtang, WANG Anliang

Characteristic of air jet impingement cooling performance for electronic equipment of satellite

Journal of Beijing University of Aeronautics and Astronsutics, 2015, 41(8): 1553-1559.
http://dx.doi.org/10.13700/j.bh.1001-5965.2014.0596