Comparison of Hydrodynamics and Mass Transfer in Airlift and Bubble Column Reactors Using CFD

Computational Fluid Dynamics (CFD) is used to compare the hydrodynamics and mass transfer of an internal airlift reactor with that of a bubble column reactor, operating with an air/water system in the homogeneous bubble flow regime. The liquid circulation velocities are significantly higher in the airlift configuration than in bubble columns, leading to significantly lower gas holdups. Within the riser of the airlift, the gas and liquid phases are virtually in plug flow, whereas in bubble columns the gas and liquid phases follow parabolic velocity distributions. When compared at the same superficial gas velocity, the volumetric mass transfer coefficient, k_La, for an airlift is significantly lower than that for a bubble column. However, when the results are compared at the same values of gas holdup, the values of k_La are practically identical.     

A LEVEL SET METHOD FOR SIMULATION OF RISING BUBBLE

A level set method, the TVD scheme of second-order upwind procedure coupled with flux-limiter, and SIMPLE algorithm were incorporated to simulate the flow and interracial motion of immiscible two-fluids with large density ratio and viscosity ratios, large topology distortion and surface tension. As a numerical example axi-symmetric rising bubbles were investigated. It is found that the method is numerically stable and has good convergence property and the results are in good agreement with other works.     

Dew- and bubble-point measurements for the mixtures of dichlorodifluoromethane and bromotrifluoromethane

The dew and bubble points for the mixtures of dichlorodifluoromethane (CCl₂F₂; R 12) and bromotrifluoromethane (CBrF₃; R13B1) were measured with the use of a constant-volume method coupled with an expansion procedure and visual observation of the meniscus at the vapor-liquid interface. In order to check the reliability of the apparatus used, vapor pressure measurements were conducted for carbon dioxide at 273.15 K and for two pure components, CCl₂F₂ and CBrF₃. Thirty-eight dew and bubble points of the CCl₂F₂+CBrF₃ system were determined for four different compositions of 0, 21, 45, and 70 mol % CBrF₃ in the range of temperatures from 299 to 384 K, pressures up to 4.2 MPa, and densities from 89 to 1228 kg · m^–1.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Access to rows and columns of a rectangular array in a concentricloop bubble memory

Conventional magnetic bubble memories do not allow efficient access to both rows and columns of a bidimensional array. We propose here a new chip organization based on two concentric loops, that does not require technological innovations to be built. Rows and columns of the array are naturally brought to the periphery of the inner loop, and are accessed through the outer loop.     

冒泡排序算法的改进

通过对传统冒泡排序算法的讨论，指出其效率不高的缺陷，提出了局部冒泡排序算法，并编程予以实现，其效率及性能较传统的冒泡排序算法有一定程度的提高。同时采用随机及特殊数据在计算机上对传统冒泡排序和局部冒泡排序算法进行了分析和性能对比测试，对局部冒泡排序算法的时间性能作出了评价，指出了局部冒泡排序算法的特点及优势。通过实验证明了局部冒泡排序算法较传统冒泡排序算法在时同性能上有了一定的改进。     

新垂直筛板帽罩单元传质性能的测试与分析

根据新垂直筛板帽罩单元两相流动的实际情况，在新双池模型〔３〕的基础上提出了罩内外两段组成传质及其各段传质的模型。通过对帽罩效率的测定，得出了罩内外传质单元数的关联式。     

EXPERIMENTAL INVESTIGATIONS ON BUBBLE COLLAPSE NEAR BOUNDARIES

Collapse of a spark-generated bubble near rigid or deformable surfaces is studied experimentally with a high speed cammera. The formation of a bubble wall jet has been observed and discussed. Results from experimental studies on the effect of liquid viscosity on bubble collapse near the rigid wall are also presented.     

垂熔制度对掺杂钨性能的影响

烧结的目的是通过高温作用得到接近理论密度和无孔隙的制品。而在掺杂钨中，需要形成合适的钾泡来稳定细微孔隙，以使钨丝具有高温蠕变强度；同时需去除钨中的杂质和无效掺杂剂。本文的研究表明，通过合理的垂熔烧结制度可使钨条密度最佳。     

Nonlinear identification and control of a high-purity distillation column: a case study

Identification and control of ill-conditioned, interactive and highly nonlinear processes pose a challenging problem to the process industry. In the absence of a reasonably accurate model, these processes are fairly difficult to control. Using a high-purity distillation column as an example, model identification and control issues are addressed in this paper. The structure of the identified models is that of the polynomial type nonlinear autoregressive models with exogenous inputs (NARX). While most of the work in this area has concentrated on linear models (one-time scale and two-time scale models), this work is aimed at identifying the inherent nonlinearities. Comparisons are drawn between the identified models based on statistical criteria (AIC etc.) and other validation tests. Simulation results are provided to demonstrate the closed-loop performance of the nonlinear ARX models in the control of the distillation column. The controller employed is based on a nonlinear model predictive scheme with state and parameter estimation.     

Experimental study of the flow boiling heat transfer enhancement and pressure drop due to the bubble behavior restricted by a screen sheet

A unique method previously proposed by the authors was applied to the heat transfer augmentation in the flow boiling field. In this method a screen sheet was placed on the horizontal heated surface where bubble nucleation occurred. Generated vapor bubbles were trapped between the screen and the wall, became flat, and moved along the surface. This restricted bubble behavior caused the heat transfer enhancement. Three types of screen sheet were tested in the present experiment and the effect of the screen on the heat transfer and two‐phase flow characteristics was investigated. In two of these cases, the screen was displaced upward by the bubble nucleation. Compared with the ordinary flow boiling case, heat transfer was enhanced by a factor of 1.2 to 6 within the present experimental range. Using a simple flow model, it was made clear that the effect of the height of the displaced screen was important in evaluating the increase in pressure drop. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 319–329, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10094