John J. J. Chen

An extension of the Lockhart-Martinelli theory of two phase pressure drop and holdup

Abstract The Lockhart-Martinelli model is extended for the case of seperated flow, enabling theoretical relationships to be developed for holdup and pressure loss. For the stratified flow case the analytical solution gives close agreement with pressure loss data and with the results of the analyses of both Johannessen and of Taitel and Dukler. The holdup relation which was derived gave good agreement with data for the situation where interfacial shear is unimportant. For the case of annular separated flow the analytical solution gives close agreement with pressure loss data for large diameter pipes where liquid surface effects are minimal. The holdup relation on the other hand was severely in error but an empirical modification did serve to give good agreement with experimental data. A further theoretical extension of the Lockhart-Martinelli approach enabled a general pressure loss correlation to be developed for annular type flows. Lack of systematic data for large diameter pipes, particularly for the steam-water case, hampers the application of the derivation, but, despite this draw-back, a general correlation is developed which accounts for the effect of pipe diameter and is useful for prediction of pressure loss in steam-water systems.

Effects of an oscillating interface on heat transfer

The effects of gas/liquid interfacial movement on heat transfer from a discrete heat source on a vertical wall to the liquid were investigated experimentally for two situations of interfacial oscillations using air-water systems: air/water interfacial oscillation of an oscillating water column and air/water interfacial wave in a water tank. Experiments on the oscillating water column have shown that the heat transfer near the air/water interface is strongly affected by the oscillating interface. The local heat transfer coefficient near the interface increases rapidly with the oscillation frequency compared to the heat transfer measured at a point totally submerged in the liquid but remote from the interface. The local heat transfer near an oscillating interface can be related to the periodic renewal of the boundary layer with the fresh bulk liquid. The influence due to the interfacial movement gradually diminishes towards a distance approximately twice the oscillation amplitude measured from the equilibrium gas/liquid interfacial position. It has also been found that the Nusselt number and the oscillatory Reynolds number correlate the heat transfer data reasonably well. Experimental results obtained using air/water interfacial waves have shown that the local heat transfer rate is similar to those obtained using the oscillating water column under the conditions of similar wave frequency and amplitude.

Simulation and experiment of the unsteady heat transport in the onset time of nucleation and crystallization of ice from the subcooled solution

Abstract Heat transfer is an unsteady process in the initial period of ice nucleation or phase transition from aqueous solution. During this period the latent heat of freezing increases the temperature in bulk solution monotonously until the system reaches equilibrium. Meanwhile heat can transfer from the solution to the environment or vise versa. The analysis of this unsteady heat transfer process leads to the establishment of a mathematical model, which is represented by two simultaneous differential equations. Using the Laplace transform and inverse transform, and incorporating the initial condition of ice nucleation, we obtained an analytical solution of this model. Further discussion of the model’s fitness by comparing to the experimental data leads to a recognition that ice fouling (or ice adhesion) on the cooler wall should be highlighted in estimating the heat transfer resistance at the very beginning of the ice formation. The model fits to the experimental data satisfactorily.

Development and validation of models for annealing furnace control from heat transfer fundamentals

Temperature in a continuous annealing furnace is studied by furnace modelling using two methods. A 3D model is used to investigate the temperature distribution of the steel strip that is being annealed and the furnace thermocouple probes, and information from the 3D model enables the construction of a highly simplified 1D/2D model, which predicts furnace and strip temperatures with very good agreement to the 3D model. The simple model has a very short solution time and is suitable for rapid simulation of alternative furnace operating conditions in order to optimise heat treatment quality, plant throughput and energy consumption.

COMPARISON OF MASS TRANSFER PERFORMANCE FOR VARIOUS SINGLE AND TWIN IMPELLERS

The mass transfer in the air-water system was investigated for various single- and twin-impeller systems including the standard Rushton impellers, pitched-blade impellers, concave-blade impellers and comb-blade impellers. For twin-impeller systems, the effects of the two impellers operating at various speed combinations, opposite impeller rotation and alternate impeller rotation were investigated. Results showed that although slightly better performance was achieved by using the concave-blade and comb-blade impellers, all the volumetric mass transfer coefficients, k L a , obtained were mainly determined by the specific power consumption ( P/V ) and the superficial gas velocity ( U s ) and correlated reasonably well by a single correlation due to van’t Riet. The possible reason for the better performance of the comb-blade impellers is discussed.

Multivariate statistical monitoring of the aluminium smelting process

Abstract This paper describes the development of a new ‘cascade’ monitoring system for the aluminium smelting process that uses latent variable models. This system is based on the changes of variability patterns within a feeding cycle which are used to provide indications of faults and their possible causes. The system has been tested offline using 31 data sets. The performance of the system to detect an anode effect has been compared with a typical latent variable model that monitors the change of behaviour at every time instant. The results show that the ‘cascade’ monitoring system is able to detect abnormal events. It was possible to relate each event with specific patterns associated with abnormalities thus facilitating later fault diagnosis.

Resistance due to the presence of bubbles in an electrolytic cell with a grooved anode

Bubble resistance in an electrolytic cell using a grooved anode and anodes with plane surfaces are measured and reported. The cell was operated at various inclination angles of the anode and at a range of anode-cathode distances (ACD). The comparison shows that bubble resistance obtained for a grooved anode is smaller than that for anodes with plane surfaces at the same operating conditions (i.e. inclination angle, current density, and equivalent ACD). Explanations for the reduction in the bubble resistance due to the use of a grooved anode are given. The use of grooved anodes may provide a significant reduction in the overall cell voltage drop, and thus increase the energy efficiency, provided the node material is not consumed during the process, causing the grooves to revert to a planar surface. The work reported in this paper is of particular significance to the aluminium smelting industry if a non-consumable inert anode is available.

Diagnosing faults in aluminium processing by using multivariate statistical approaches

Multivariate statistical approaches are expected to detect and diagnose faults effectively for complex materials processing including steel, iron, copper and aluminium processing. This advanced monitoring of materials processing is essential to address abnormal or faulty conditions in a timely manner. In the aluminium-smelting process, late diagnosis of abnormal conditions such as an anode effect can result in an increase of energy consumption and emission of greenhouse gases. In this article, a new statistical framework is proposed that is based on hierarchal diagnostic approach to diagnose two groups of faults, anode faults and non-anode faults. The system diagnoses faults by predicting the type of fault based on continuous, non-linear and multivariate process data using discriminant partial least squares (DPLS). The new system goes beyond the typical multivariate system in that it also includes the dynamic behaviour of the process during anode changing and alumina feeding. The results of performance evaluation of the new diagnosis system tested using real-data show that the system can diagnose the two groups of faults.

Composition and Thermal Analysis of Crust Formed from Industrial Anode Cover

When the anode cover is heated up in the reduction cell, the crust formation from the anode cover commences at the bottom and the process is driven by thermo-chemical processes. It is important to study the composition and thermal stability properties of the crust in order to understand the mechanisms of crust formation and deterioration. Several crust pieces were taken from industrial prebaked anode cells. A number of vertical crust sections were sampled from these pieces, and each section was analyzed for composition and phase change temperature. Results show that the bottom layer is enriched in cryolite, consistent with results published in the literature. The upper region was found to contain more chiolite. Crushed bath-based anode crust has higher CR than alumina based anode crust. The melting of chiolite in the crust leaves substantial macro-porosity there, which contributes to the absorption of NaAlF4 and the penetration of bath through it. The formation conditions of crystalline crust were discussed.

Increasing the Power Modulation Window of Aluminium Smelter Pots with Shell Heat Exchanger Technology

With power prices constantly rising, and varying aluminium prices requiring operating flexibility, the financial incentive for smelters to adopt a power modulation strategy is becoming larger. However, the power modulation window, in which a smelter can safely operate its reduction cells, is limited. The Light Metals Research Centre has developed the Shell Heat Exchanger (SHE) technology for controlling the heat dissipation from aluminium smelting pot shells. By varying the air flow through the SHE, the heat removal from the shell can be increased or decreased as desired, doubling the previous power modulation window or allowing power modulation with minimal disturbance to the pot thermal balance.