被動式渦流產生器對微流道中奈米流體傳熱增強的數值模擬

Numerical simulations on heat transfer enhancement of nanofluids in microchannel using a passive vortex generator

李永斌(YONG-BIN LEE)

指導教授 ： 廖川傑

中原大學/工學院/機械工程學系/碩士(2022年)

https://doi.org/10.6840/cycu202201709

摘要

隨著科技進步，微小化與集成化為電子晶片及電子系統的主流趨勢，使得單位面積下的散熱需求越來越高，因此關於可動結構於流道內散熱效果提升受到廣泛的探討。其中，在微流道內加入渦激振動(vortex-induced vibration , VIV)物體將產生渦流結構，此將有助熱傳效率之提升。本文採用沉浸邊界法(Immersed-boundary method, IBM)結合運動方程對彈性安裝在微流道中的渦流產生器(Vortex generator, VG)進行數值研究，並將其用於增強流道中奈米流體的對流傳熱。此外，本文將使用Buongiorno的兩相混合模型模擬微流道內奈米流體之對流熱傳，該模型考慮奈米顆粒於載液間的布朗運動及熱泳擴散。透過改變奈米流體濃度、雷諾數、質量比及衰減速度等重要參數，探討渦激振動的特性對微流道內熱流場之影響，並找出增強熱傳的關鍵因素。
首先探討有無VG以及VG安裝模式對流場與熱傳的影響，發現彈性安裝VG會使熱傳增強及機械損失提升。接著研究彈性安裝VG衰減速度的影響，發現彈性安裝VG隨衰減速度變化而改變的頻率與振幅顯著影響漩渦脫落狀態及位置，進而影響漩渦與熱邊界層的交互作用，使得不同衰減速度有不同熱傳增強。再來探討奈米濃度的影響，透過與無奈米流體的數值相除，計算Lock-in範圍重疊時的紐塞數(Nusselt number, Nu)增幅與壓降增幅，最終得出一整體效率，借此觀察不同濃度與兩種安裝模式在加入奈米粒子後的效益差。並由此得知有著58%的最大整體效率，但效益隨濃度增加而降低。同時發現低雷諾數條件下彈性安裝VG與固定VG相比的整體效率最大增幅約6%。接下來研究彈性安裝VG的質量比，發現流道內的彈性安裝VG隨質量比降低Lock-in範圍往低衰減速度，因而有較高的振盪頻率，使得紐塞數與壓降最大值一同提升，最終有著約62%的最大整體效率。最後研究不同發熱段溫度的影響，發現溫度對於渦激振動行為幾乎沒有影響，但隨溫度增高，奈米流體的熱傳導係數增加的同時紐塞數也增加。

關鍵字

渦激振動 ； 流固耦合 ； 沉浸邊界法 ； 奈米流體 ； 增強熱傳

並列摘要

With the advancement of science and technology, miniaturization and integration have become the mainstream trends of electronic chips and electronic systems, resulting in higher and higher heat dissipation requirements per unit area. Therefore, the improvement of the heat dissipation effect of movable structures in the flow channel has been widely discussed. Among them, adding vortex-induced vibration (VIV) body into the microchannel will generate a vortex structure, which will help improve the heat transfer efficiency. In this thesis, the Immersed-boundary method (IBM) combined with the equation of motion is used to numerically study a vortex generator (VG) elastically installed in a microchannel and use it to enhance the convective heat transfer of nanofluids in the channel. In addition, this thesis will use Buongiorno's two-phase mixing model to simulate the convective heat transfer of nanofluids in microchannels, which considers the Brownian motion and thermophoretic diffusion of nanoparticles in the carrier liquid. By changing the important parameters such as nanofluid concentration, Reynolds number, mass ratio and reduced velocity, the influence of the response characteristics of VIV on the heat flow field in the microfluidic channel is discussed, and the key factors for enhancing heat transfer are found out.
Firstly, the influence of the presence or absence of VG and the installation mode of VG on the flow field and heat transfer is discussed, and it is found that elastically-mounted VG will enhance the heat transfer and the mechanical penalty. Then, the influence of the reduced velocity of elastically-mounted VG is studied, and it is found that the frequency and amplitude of elastically-mounted VG changed with the change of reduced velocity significantly affected the vortex shedding state and position. Therefore, the interaction between the vortex and the thermal boundary layer is affected, and finally, different reduced velocity have different heat transfer enhancements. Next, the effect of nanofluid concentration is investigated, and it is found that with the increase of concentration, Nusselt number (Nu) and pressure drop increased together. By dividing with the value of pure water, calculate the increase in the number of new plugs and the increase in pressure drop when the Lock-in range overlaps, and finally obtain an overall efficiency. In this way, it is observed that different concentrations and two installation modes have poor efficiency after adding nanoparticles. From this, it is known that there is a maximum overall efficiency of 58%, but the benefit decreases with increasing concentration. At the same time, it is found that the overall efficiency of elastically-mounted VG compared with stationary VG under low Reynolds number conditions has a maximum increase of about 6%. Next, we study the mass ratio of elastically-mounted VG and find that the elastically-mounted VG in the flow channel decreases with the mass ratio of the Lock-in range to a low decay rate. As a result, there is a higher oscillation frequency, which increases the maximum value of Nu and pressure drop together, and finally has a maximum overall efficiency of about 62%. Finally, the influence of temperature in different heating sections is studied, and it is found that temperature has little effect on VIV behavior. But with the increase of temperature, the thermal conductivity of the nanofluid increases and Nu also increases.

並列關鍵字

Vortex-induced vibration ； Fluid-Structure Interaction ； Immersed-boundary method ； Nanofluid ； Heat-transfer enhancement

參考文獻

[1] R. Mahajan, C. P. Chiu, and G. Chrysler, "Cooling a Microprocessor Chip," Proceedings of the IEEE, vol. 94, no. 8, pp. 1476-1486, 2006.

[2] Y. M. Chung and K. H. J. J. H. T. Luo, "Unsteady heat transfer analysis of an impinging jet," Journal of Heat Transfer, vol. 124, no. 6, pp. 1039-1048, 2002.

[3] S. Ahmed, V. J. I. J. o. H. Carey, and M. Transfer, "Effects of gravity on the boiling of binary fluid mixtures," International Journal of Heat and Mass Transfer, vol. 41, no. 16, pp. 2469-2483, 1998.

[4] D. B. Tuckerman and R. F. W. J. I. E. d. l. Pease, "High-performance heat sinking for VLSI," IEEE Electron Device Letters, vol. 2, no. 5, pp. 126-129, 1981.

[5] M. Meis, F. Varas, A. Velázquez, and J. M. Vega, "Heat transfer enhancement in micro-channels caused by vortex promoters," International Journal of Heat and Mass Transfer, vol. 53, no. 1-3, pp. 29-40, 2010.