Oxy-fuel combustion of pulverized coal: Characterization, fundamentals, stabilization and CFD modeling

The possible role of ammonia in a future energy infrastructure is discussed. The review is focused on the catalytic decomposition of ammonia as a key step. Other aspects, such as the catalytic removal of ammonia from gasification product gas or direct ammonia fuel cells, are highlighted as well. The more general question of the integration of ammonia in an infrastructure is also covered.

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Ammonia as a possible element in an energy infrastructure: catalysts for ammonia decomposition

Review: Direct ethanol fuel cells

The performance of heat exchangers can be improved to perform a certain heat-transfer duty by heat transfer enhancement techniques. In general, these techniques can be divided into two groups: active and passive techniques. The active techniques require external forces, e.g. electric field, acoustic or surface vibration, etc. The passive techniques require fluid additives or special surface geometries. Curved tubes have been used as one of the passive heat transfer enhancement techniques and are the most widely used tubes in several heat transfer applications. This article provides a literature review on heat transfer and flow characteristics of single-phase and two-phase flow in curved tubes. Three main categories of curved tubes; helically coiled tubes, spirally coiled tubes, and other coiled tubes, are described. A review of published relevant correlations of single-phase heat transfer coefficients and single-phase, two-phase friction factors are presented.

A review of flow and heat transfer characteristics in curved tubes

A review of high-temperature proton exchange membrane fuel cell (HT-PEMFC) system

Flooding is the limiting condition of stable countercurrent gas-liquid flow in columns and has been the subject of extensive research over a number of decades. An exhaustive review of the subject, as pertaining to unpacked columns, has been given by Bankoff and Lee.1 While a number of correlations exist in the literature to predict the onset of flooding, the topic of the mechanism of flooding has been debated and discussed widely in the recent literature. Briefly, there are two schools of thought on the flooding mechanism: (1) the formation and upward transport of large waves from the bottom of the column near the gas injection/liquid withdrawal point, and (2) the destabilization of the liquid film near the liquid inlet resulting in the formation of large waves. Direct visual evidence of formation of large waves at the liquid exit under flooding conditions has been reported by a number of researchers.2,3,4 However, this idea was opposed by Zabaras and Dukler.5 They measured the local instantaneous wall shear stress using an electrochemical method just below the liquid feed, as well as just above the liquid withdrawal point. They found that in all instances of countercurrent flow, including conditions along the flooding curve, the time-averaged wall shear stress was directed upward indicating that the velocity near the wall was downward. They also measured the instantaneous film thickness from two conductance probes located 0.053 m apart. Cross-correlation of these signals showed a well-defined peak at a measurable time delay even under flooding conditions. Zabaras and Dukler, therefore, concluded that waves were travelling downward even under flooding conditions and that they did not reverse direction at the flooding point. They explained the photographic observation of upward-travelling waves by McQuillan et al.3 as a transient phenomenon generated by the small depressurization used by McQuillan et al. to induce flooding. Thus, Zabaras and Dukler argued that the process of flooding was controlled by conditions that existed at the or just below the feed rather than at the liquid exit. Jayanti et al.6 attributed the existence of different mechanisms of flooding to the effect of tube diameter. In small diameter tubes, a coherent, ring-like wave could be formed around the inside wall of the tube which could then be transported upward by the gas. A flooding model, based on the formation of a large standing wave at the flooding point was developed by Shearer and Davidson.7 Jayanti et al. argued that such a wave would be very unstable in large diameter tubes and, hence, could not travel upward for long distances. Vijayan et al.8 conducted flooding experiments in tubes of 25, 67 and 99 mm inner dia. and confirmed visually the existence of upward travelling waves in the 25 mm dia. tube under flooding conditions. In the larger diameter tubes, the waves were observed to move only a short distance before collapsing. Similar behavior was also observed by Biage et al.9 in a duct of rectangular cross-section and by Watson and Hewitt10 in a tube of 82 mm internal dia. While there is, thus, a convergent view on the flooding mechanism, the careful experimental results of Zabaras and Dukler5 remain unexplained. Specifically, their observations that the wall shear stress always indicated a downward moving flow and that the film thickness measurements recorded no upward-moving waves need to be explained in a manner consistent with the recent visual observations of flooding mechanisms. Toward this end, film thickness measurements have been conducted under conditions in which different flooding mechanisms prevailed. The details of these studies and the results obtained are reported here.

On axial coherence of interfacial waves in countercurrent flow

During the 2011 nuclear catastrophe at Fukushima Daiichi, Unit 3 had a sharper increase in containment pressure than Unit 2, with thermal stratification of the suppression pool cited as one of the contributing factors. In the present work, the buoyancy-induced circulation consequent to steam condensation in a large, toroidal pool of water is studied using computational fluid dynamics (CFD) simulations with a view to understanding the role of important design parameters of the suppression pool system. The tunnelling phenomenon observed in the development of the thermal stratification process is delineated in terms of the establishment of a thermocline. The effects of the number of steam injection points and the cross-section of the pool on thermal stratification characteristics have been investigated through a number of case studies. In all the cases, the surface temperature, which is responsible for over-pressurization of the containment, is found to be significantly higher than the bulk pool temperature. Multiple injection points with the same overall steam flow rate are found to lead to higher surface temperatures due to a shortened circulation path. For the same volume of pool water, the simulations show that a deeper and narrower pool gives rise to significantly higher temperatures than a wider and shallower pool. This is attributed to the relatively deeper penetration of the buoyancy-induced circulation into the pool.

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CFD Study of Thermal Stratification in a Scaled-Down, Toroidal Suppression Pool of Fukushima Daiichi Type BWR

Numerical studies on the drag coefficient of a condensing ellipsoidal bubble

Improving Low-temperature Performance of Vanadium Flow Battery through Thermal Activation of Electrodes

The influence of internal circulation on the heat and mass transfer within dispersed fluid drops in translation was studied numerically using the computational fluid dynamics simulation tool ANSYS FLUENT. The three phases identified for heat/mass transfer are: the initial and final diffusion-dominated phases and a middle convection-dominated phase. Non-isothermal mass transfer simulations using temperature-dependent diffusion coefficients were carried out to investigate the influence of heating and cooling of the drop on the solute transfer. Mass transfer is slower compared to heat transfer and the final diffusion-dominated mass transfer occurs at the final temperature attained by the drop. Mass transfer is faster for drop heating than for drop cooling, as the final diffusion phase is rate controlling and significant solute transfer occurs at the final higher temperature. 

Sreenivas Jayanti

Papers

On axial coherence of interfacial waves in countercurrent flow

CFD Study of Thermal Stratification in a Scaled-Down, Toroidal Suppression Pool of Fukushima Daiichi Type BWR

Numerical studies on the drag coefficient of a condensing ellipsoidal bubble

Improving Low-temperature Performance of Vanadium Flow Battery through Thermal Activation of Electrodes

Non‐isothermal Mass transfer in Fluid Drops with Internal Circulation