Stabilization of Mixed Convective Air Flow Driven by a Heated Circular Disk Embedded in the Bottom of a Horizontal Rectangular Duct by Inclining its Bottom Plate
林 清 發
|關鍵字:||金屬有機化學氣相沈積;縱向渦流;橫向渦卷流;回流;metal-organic chemical vapor deposition (MOCVD);longitudinal vortex roll;transversal vortex roll;return flow|
|摘要:||本論文研究藉由流場可視化及暫態溫度量測之方法，探討底板傾斜逐漸增加主流場速度對於圓形底部加熱面之梯型管道其渦流結構之穩定性，梯型管道之寬高比可藉由調整底板傾斜角度使得入口20之寬高比至出口處可達22.5或30，實驗所得結果將與底板未傾斜之水平管道做比較，主要著重於底板傾斜對於縱向渦流(longitudinal roll)、橫向渦流(transverse roll)及迴流(return flow)結構之影響；實驗操作參數範圍雷諾數介於3.7到79.7之間，雷利數則由9,040到24,000，針對底板傾斜角度0.34°及0.97°來探討穩定及暫態之渦流結構。
In the present study an experiment combining flow visualization and temperature measurement is carried out to investigate the possible stabilization of the buoyancy driven mixed convective vortex flow of air by the main flow acceleration associated with the inclination of the bottom plate of a horizontal rectangular duct. The vortex flow is driven by a heated circular disk embedded in the bottom plate. Specifically, the duct is tapered to become trapezoidal so that its aspect ratio at the duct inlet is 20 and gradually raised to 22.5 or 30 at the exit of the duct. The results in the trapezoidal duct are compared with those in the horizontal rectangular duct. Particular attention is paid to delineating the spatial changes of the longitudinal, transverse and return vortex air flow structures with the plate inclination angle and to how the bottom plate tilting possibly suppresses and stabilizes these vortex flows. Experiments are conducted for the Reynolds number at the duct inlet ranging from 3.7 to 79.7 and the Rayleigh number from 9,040 to 24,000 for the two inclined angles of the bottom plate, 0.34° and 0.97° covering the steady and time-dependent vortex flows. The results from the present study indicate that in the rectangular duct the steady, regular longitudinal vortex rolls (L-rolls) driven by the circular heated plate at low buoyancy-to-inertia ratios are induced at more upstream locations in the duct core, which are completely opposite to those induced in a rectangular duct with a uniformly heated bottom. The results in the trapezoidal duct show a delay in the onset of L-rolls and the effective suppression of the buoyancy driven unstable longitudinal vortex flow by the bottom plate inclination. Besides, more rolls can be induced due to the increase in the aspect ratio of the duct with the axial distance and the rolls are smaller, when compared with those in the rectangular duct. In the trapezoidal duct the transverse vortex rolls (T-rolls) driven by the circular heated disk do not have the same convection speed and oscillation frequency when moving downstream. Furthermore, the T-rolls in the trapezoidal duct would not reduce their spanwise length and the amplitude of flow oscillation decays slowly in the streamwise direction. Additionally, the onset of T-rolls is delayed and the unstable T-rolls can be regularized by the bottom plate inclination. We further note that the bottom plate inclination can effectively delay the onset of the return flow and weaken it slightly. Empirical equations are provided to correlate the present data for the wavelength, oscillation frequency and convection speed of the T-rolls and for the onset of return flow in the trapezoidal duct with its bottom inclined at the large angle of 0.97°. A flow regime map is also given to illustrate the effects of the bottom plate inclination on the appearance of various longitudinal vortex flow patterns.