Translated Abstract
Flow and heat transfer in packed beds are the basic processes in many fields, such as nuclear energy, chemical industry and renewable energy. The investigation of flow and heat transfer is very important to understand the transport phenomena and transfer theory in packed beds. In the present thesis, the flow and heat transfer in structured packed beds are investigated on the pore scale. In the first, flow transitions in structured packed beds are studied with electrochemical methods. Then, a systematical study involving four contact point treatments is carried out for the local forced convective heat transfer analysis in the structured packed beds of spheres. The flow and heat transfer in packed beds under high temperature condition is also discussed. Finally, the flow instability as well as symmetry is discussed in detail in SC (simple cubic) packed bed. The major contents are as follows:
(1) The flow transitions are experimentally tested with the electrochemical technique in three different structured packed beds, including simple cubic (SC), body center cubic (BCC) and face center cubic (FCC) packing forms. Meanwhile, the effect of tube to particle diameter ratio N on flow transtion in SC packed beds is studied. The microelectrodes are placed at the tube wall and inner particle surfaces to test the local flow fluctuations at the pore level. According to the analysis of Fluctuating Rate (FR), three different flow regimes in the packed beds, including laminar, transition and turbulent flow regimes are identified. It is found that, the flow transition in SC packed bed is the latest while that in FCC packed bed is the earliest, considering both the end of laminar flow and onset of turbulent flow among the three structured packed beds. The transition flow regime of SC packed beds covers the range of 260＜Re＜430 at N=3. The transition flow regime occurs at Reynolds number ranging from 170 to 390 for most electrodes in simple cubic packed beds with N≥4. For BCC packed bed, the end of laminar flow occurs at about Re ≈130, the onset of turbulent flow of inner probes occurs at about Re ≈350 and that of tube wall probes at about Re ≈580. For FCC packed bed, the transition regime covers a range of 70＜Re＜250 for most electrodes.
(2) Both macroscopic and local flow and heat transfer characteristics for different packing forms (simple cubic, body center cubic and face center cubic packing forms) and different contact treatments (gaps, overlaps, bridges and caps modifications) are examined. The effects caused by the bridge size for the bridge treatment are discussed, and the numerical results are compared with available experiments in literatures. It is found that, the effects of contact treatments on the pressure drops are remarkable for different structured packing forms, especially when the porosity is relatively low, while such effects on the Nusselt numbers are relatively small. Among the four different contact modifications, the bridge method would give the most reasonable pressure drops and Nusselt numbers for all the structured packing forms. The local flow and heat transfer characteristics are sensitive to the methodology of contact modifications. The gaps and caps treatments would distort the local flow and temperature distributions, especially near the contact zones. While the local flow and temperature distributions from the overlaps and bridges treatments would be more reasonable. Based on both the macroscopic and local flow and heat transfer analyses, the bridge treatment is recommended. The effects caused by the bridge size in the bridge treatment are also remarkable. It is noted that, too small or too large bridge size would lead to unreasonable results for both the macroscopic and local flow and heat transfer analyses. A reasonable range of bridge diameter is found to be from 16% dp to 20% dp.
(3) Flow and heat transfer in the SC structured packed bed under high temperature condition are numerically studied. Due to large temperature difference in high temperature packed bed, the physical properties of fluid vary greatly during flowing through packed beds. The velocity and local pressure drops increase along the packed cells. Therefore, the distributions of velocity and pressure drops do not present periodical behavior, however, the temperature distributions are periodical. Under the same mass flow rate, the local Reynolds numbers decrease along the packed cells. The local pressure drops, local heat transfer coefficients and local friction factors are greater while local Nusselt numbers are smaller in the same packed cell when the temperature difference is larger. Local pressure drops and local heat transfer coefficients increase as local Reynolds numbers decrease in high temperature packed beds, which is contrast to that under the constant physical properties condition. However, the trends of dimensionless local friction factors and local Nusselt numbers with local Reynolds number agree with those under the constant physical properties condition.
(4) The flow instability as well as symmetry is discussed in detail in the SC (simple cubic) packed bed. Multi-periodic boundary conditions are used to simulate the periodic flow in a representative cell of SC paced bed. The time traces of the drag coefficients CD and monitor velocity are discussed to analyze flow instability. When Re≤225, the drag coefficients CD and monitor velocity are converging at a stable value as the calculation time increases, indicating that the flow is steady. When Re≥250, the drag coefficients CD and monitor velocity would present an oscillatory behaviour as the calculation time increases, indicating that the flow is physically unsteady. The velocity distributions and streamlines are also displayed to analyze the flow symmetry. When Re≤225, the velocity distributions and streamlines present symmetry. When Re≥250, the velocity distributions and streamlines are not symmetrical. The onsets of flow unsteasiness and flow asymmetry in SC packed bed are simultaneous. Therefore, the transition from steady-symmetric flow to unsteady-asymmetric flow in SC packed bed covers the range of 225＜Re＜250.
Translated Keyword
[Contact treatment, Flow transition, High temperature, Pore scale, Structured packed beds]
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