MEASUREMENTS OF TIME-SPACE DISTRIBUTION OF CONVECTIVE HEAT TRANSFER TO AIR USING A THIN CONDUCTIVE-FILM
MEASUREMENTS OF TIME-SPACE DISTRIBUTION OF CONVECTIVE HEAT TRANSFER TO AIR USING A THIN CONDUCTIVE-FILM
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使用薄导电膜测量空气对流热传递的时空分布
DOI：
10.1299/kikaib.73.1906
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发表时间：
2007
期刊：
影响因子：
--
通讯作者：
Hajime Nakamura
中科院分区：
文献类型：
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作者：
Hajime Nakamura
发展了一种利用薄导电膜和高速红外热像仪测量空气换热时空特性的技术。本工作以2米厚的钛箔为试片，利用120赫兹红外热像仪对其表面温度进行了测量。通过将层流边界层的换热系数与数值分析的结果进行比较，证实了测量的准确性。为了验证该测量技术在实际测量中的适用性，对湍流边界层壁面上的非定常换热进行了研究。即使换热系数很低(h=10-20W/mK)，通过求解墙体内部的热传导方程，也可以恢复时间上30赫兹、空间波长4.5 mm的换热系数的时空分布。结果表明，湍流边界层近壁区形成的条纹反映了换热的时空行为。湍流边界层的统计数值，即脉动换热系数的均方根值和热纹的平均间距，与已有的数值模拟和实验结果吻合较好。导论对流换热一般具有非均匀和不稳定的性质，这反映在壁面附近的三维流动上。然而，大多数关于对流换热的实验研究都是以时间平均的方式或采用单点测量的方式进行的。这经常导致人们对热传递机制的理解很差。使用液晶(Iritani等人，1983)或使用红外热像仪(Hetsroni和Rozenblit，1994，以及Nakamura和Igarashi，2004,2006)，通过使用具有低热容的薄测试表面，开发了热传递的时空特性的测量技术。然而，这些测量的主要问题是由于测试表面的热容而导致的温度波动的衰减。此外，通过测试表面的横向传导衰减了空间温度分布的幅度。这些衰减相当大，特别是对换热系数较低的空气的换热。本文作者通过求解热传导方程研究了试验表面换热的频率响应和空间分辨率(Nakamura，2007)。图1(A)和(B)分别显示了可用红外热像仪检测的波动频率上限fmax和空间波长下限bmin，其中h是热传递系数的波动幅度，TIR0是黑体红外热像仪的噪声等效温差(NETD)。如图1所示，如果用一层极薄的导电膜作为受热面，用现在的红外热像仪(TIR0=0.025C)，可以观察到高达50赫兹，b 1 mm(Tw-T0=30C，h=3-5W/MK)的空气的非稳定热传递。本文采用导电膜和高速红外热像仪相结合的测量技术，对湍流引起的空气非定常换热进行了测量。通过求解墙体内部的热传导方程，恢复了由于热容量和侧向传导引起的衰减。术语bc：空间截止波长bmin：可检测空间波长下限c：比热fc：截止频率fmax：可检测波动频率上限h：换热系数L：壁面摩擦长度=/u q：热通量Re：基于动量厚度的雷诺数T：温度T0，Tw：自由流和壁面温度Tir0：黑体红外热像仪的噪声当量温差t：时间u0，u：自由流和壁面摩擦速度x，y，z：流向，垂直，和跨度坐标：厚度：导热系数：流体运动粘度：流体密度上标和下标()：平均值()均方根值
A measurement technique of time-space characteristics of heat transfer to air has been developed using a thin conductive film and a high-speed infrared thermograph. In this work, a titanium foil of 2 m thick was used as a test surface, and measured temperature on it by employing an infrared thermograph of 120 Hz. The accuracy of the measurements was confirmed by comparing the heat transfer coefficient of a laminar boundary layer to that of a numerical analysis. In order to verify the applicability of this measurement technique to practical measurements, unsteady heat transfer on the wall of a turbulent boundary layer was examined. It was possible to restore the timespace distribution of the heat transfer coefficient up to 30 Hz in time and 4.5 mm in spatial wavelength by solving the heat conduction equations inside the wall, even though the heat transfer coefficient was low ( h = 10 – 20 W/mK). The results showed that the time-space behavior of the heat transfer was clearly revealed, which was reflected by the streaks formed in the near-wall region of the turbulent boundary layer. The statistical values of the turbulent boundary layer, that is, rms value of the fluctuating heat transfer coefficient and mean spacing of the thermal streaks, agreed well to those of previous data of DNS and experiments. INTRODUCTION Convective heat transfer generally has a nature of nonuniformity and unsteadiness, which is reflected by a three-dimensional flow near a wall. However, most experimental studies concerning the convective heat transfer have been performed in a time-averaged manner or using one-point measurements. This frequently results in poor understandings on the mechanisms of the heat transfer. Measurement techniques for time-space characteristics of the heat transfer have been developed using liquid crystal (Iritani et al., 1983) or using infrared thermography (Hetsroni and Rozenblit, 1994, and Nakamura and Igarashi, 2004, 2006), by employing a thin test surface having low heat capacity. However, the major problem of these measurements is attenuation of the temperature fluctuation due to the heat capacity of the test surface. Also, lateral conduction through the test surface attenuates the amplitude of the spatial temperature distribution. These attenuations are considerably large, especially for the heat transfer to air for which the heat transfer coefficient is low. The present author investigated the frequency response and the space resolution of the heat transfer from a test surface by solving heat conduction equations (Nakamura, 2007). Figure 1 (a) and (b) shows the upper limit of the fluctuating frequency, fmax, and the lower limit of the spatial wavelength, bmin, respectively, which are detectable using infrared thermograph, where h is fluctuating amplitude of the heat transfer coefficient and TIR0 is noise equivalent temperature difference (NETD) of infrared thermograph for a black body. If an extremely thin conductive film, as indicated in Fig. 1, is used as the heated surface, the unsteady heat transfer to air is observable up to f 50 Hz and b 1 mm (at Tw – T0 = 30C and h = 3 – 5 W/mK) by employing an infrared thermograph of nowadays ( TIR0 = 0.025C). In this work, the measurement technique using a thin conductive film and a high-speed infrared thermograph was applied to measure the unsteady heat transfer to air caused by flow turbulence. The attenuation due to the heat capacity and the lateral conduction was restored by solving the heat conduction equations inside the wall. NOMENCLATURE bc : cutoff wavelength in space bmin : lower limit of spatial wavelength detectable c : specific heat fc : cutoff frequency fmax : upper limit of fluctuating frequency detectable h : heat transfer coefficient l : wall-friction length = /u q : heat flux Re : Reynolds number based on momentum thickness T : temperature T0, Tw : freestream and wall temperatures TIR0 : noise equivalent temperature difference of infrared thermograph for a black body t : time u0, u : freestream and wall-friction velocities x, y, z : streamwise, vertical, and spanwise coordinates : thickness : thermal conductivity : kinematic viscosity of fluid : density of fluid Superscripts and Subscripts ( ) : mean value ( )rms : root-mean-square value
影响因子：
4.6
作者：
Kong, H;Choi, H;Lee, JS
通讯作者：
Lee, JS