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Convective heat transfer in a rotary kiln was studied as a function of operating parameters. The experiments were carried out in a steel kiln of 0.19 m in diameter and 2.44 m in length. The operating parameters covered included gas flow rate, solid throughput, rotational speed, degree of solid holdup, inclination angle, particle size and temperature. To minimize radiation effects, air was used as the heating medium and maximum inlet air temperatures were limited to 650 K. Ottawa sand was used in all the runs except in the study of the effect of particle size where limestone was employed. The experiments were conducted under conditions where the bed height along the kiln was maintained constant and the bed was in the rolling mode. Both the heat transfer coefficients from the gas to the solids bed and the gas to the rotating wall were found to be significantly influenced by gas flow rate. Increasing rotation al speed increases the gas to bed heat transfer, but decreases the gas to wall heat transfer. The former effect is relatively small. The effect of degree of fill was slightly negative in the gas to solids bed heat transfer, and insignificant in the heat transfer from the gas to wall. The effects of inclination angle, solid throughput, particle size and temperature were found negligible over the range tested. One of the major findings in this study is that contrary to suggestions in the literature, the coefficients for gas to bed heat transfer are about an order of magnitude higher than those for gas to wall. The higher coefficients for gas to solids bed are attributed to two factors, the underestimation of the true area by basing coefficients on the plane chord area and the effect on the gas film resistance of the rapid particle velocity on the bed surface. The experimental data were correlated in a form suitable for design purposes, and the results compared with meager data available in the literature. A mathematical model was developed for convective heat transfer from the gas to a rolling solids bed. The model requires the knowledge of the gas to particle heat transfer coefficient and the rolling velocity of the aerated particles. The model gives a reasonable prediction of the gas to bed coefficient in a rotary kiln using values of the gas to particle coefficient taken from the literature. The required data on the surface velocity of particles was obtained in a lucite kiln of the same size. Residence time distribution of particles was also studied briefly to verify that solids were nearly in axial plug flow. A simple mathematical model of a rotary kiln heat exchanger is presented. This model predicts gas, solids and wall-temperatures in a kiln as a function of the kiln design and operating parameters using the heat transfer correlations developed in this work. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

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本論文由兩部分組成。第一部分介紹了關於瑞利－伯纳德對流的兩項工作。第一項工作在五個側向寬高比不同的矩形對流槽中研究了空間約束效應對流體動力學和傳熱效率的影響。實驗發現，整體流場隨著約束強度的增加而明顯減弱，而且大尺度環流的流向反轉變得更為頻繁。令人驚訝的是，儘管流動變慢，總體傳熱效率卻顯著地提高。仔細分析表明，空間約束改變了邊界層中羽流的形態和動力學特性，表現為形成的羽流結構更為有序和充滿活力，從而導致邊界層變得更薄更均勻，傳熱效率由此得以提高。第二項工作研究了不同溫度邊界條件對湍流熱對流的影響。實驗在兩個對流槽中進行：其中一個在上邊界固定溫度而在下邊界固定熱通量（HC 對流槽），另一個則在兩個邊界都保持恆定溫度（CC對流槽）。研究發現，在恆定溫度的邊界條件下，溫度邊界層的厚度對瑞利數的依賴關係滿足于1/3的標度律，區別于在恆定熱通量下的行為（標度律近似為2/7）。此外，CC 對流槽中的流場強度平均比HC對流槽中的要強∼9% ，其流向反轉頻率也要快1.5倍。我們把這些現象歸咎於不同溫度邊界條件下羽流的發射強度不同。 / 論文的第二部分對另外一種重要的熱對流系統，水平對流，進行了實驗研究。在加熱和冷卻位于系統上邊界同一高度的條件下，我們觀測到了一個貫穿整個對流槽的大尺度環流。通過染料示蹤表明，流場中的迴流并不一定需要損耗能量來跨越溫度邊界層，因此可以更有效地維持其流動強度。對於熱傳輸效率以及溫度邊界層厚度隨瑞利數的變化關係，實驗觀測到了一個0.3的標度律，不同于經常被報道的理論值（0.2），這可能是因為在高瑞利數下流動狀態發生了轉變。本研究的结果表明，表面温度梯度不仅可以驱动大尺度环流，而且其強度也比人们普遍认为的要强。 / This thesis consists of two self-contained parts, both related to the topic of thermal convection. / In the first part, two sets of studies on turbulent Rayleigh-Bénard convection are presented. / The first study addresses the confinement effects on the heat transport and flow dynamics in quasi-2D geometry. The experiments were conducted in five rectangular cells with the same height and length, while the width being varied to produce a lateral aspect ratio Γ ranging from 0.6 to 0.1. As expected, with decreasing Γ, i.e. increasing the level of confinement, the overall flow slows down and more plumes travel through the bulk region, thus resulting in a large-scale circulation with more frequent reversals. Surprisingly, despite a slower flow, the global heat transport efficiency enhances significantly. Detailed examinations from experiment and simulation show that this enhancement is brought about by the changes in the dynamics and morphology of the thermal plumes in the boundary layers: the confined geometry produces more coherent and energetic plume clusters that in turn result in more uniform and thinner thermal boundary layers. This study demonstrates how changes in turbulent bulk flow can influence the boundary layer dynamics and shows that the quasi-2D geometry is very different from the true 2D and also the 3D systems. / The second study investigates the influence of thermal boundary conditions. Two experimentally achievable configurations are examined; one is fixed-flux at the bottom boundary and fixed-temperature at the top (HC cell), while the other is fixed-temperature at both boundaries (CC cell). It is found that, rather surprisingly, the Rayleigh number dependence of the thermal boundary layer thickness follows a 1/3 power law approximately under the condition of constant temperature, in contrast to the behavior in the case of constant flux (close to a 2/7 scaling). It is further found that the flow strength is on average ∼ 9% larger in the CC cell, and its flow reversal frequency is ∼ 1.5 faster than that for the HC case, which may be understood as changes in the plume emission dynamics. / The second part deals with horizontal convection. The experiment was conducted with a long apparatus (∼ 2.4 m) with the heating and cooling imposed over the top boundary. Flow visualization studies revealed a full penetrating circulation and suggest that the returning flow does not necessarily overcome the stratification to penetrate the entire thermal BL, in such a way that it can “save its kinematic energy thus supports the motion more efficiently. Both the heat transport efficiency and thermal BL thicknesses are found to follow a 0.3 power law, in contrast to the often-reported value of 0.2. This is likely caused by a flow-regime transition in the high Ra number range. This study demonstrates experimentally that a surface temperature gradient is capable of driving a large-scale circulation and its strength is much stronger than what is generally believed by the oceanography community. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Huang, Shidi = 瑞利-伯納德對流及水平對流的實驗研究 / 黃仕迪. / Thesis (Ph.D.) Chinese University of Hong Kong, 2013. / Includes bibliographical references (leaves 97-110). / Abstracts also in Chinese. / Huang, Shidi = Ruili-Bonade dui liu ji shui ping dui liu de shi yan yan jiu / Huang Shidi.

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