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

There is increasing urgency towards integration of renewable sources into electricity generation so as to minimize greenhouse gas (GHG) emissions. Renewable power sources are highly specific in prevalence, both regionally and temporally, and their utilization at utility scale for round-the-clock power supply poses the problem of matching power generation capacity and power demand at every instant of time. In the present work, we propose a model that integrates geographically distributed and diverse power sources with geographically spread-out power consumption centres and distributed and diverse electrical energy storage systems through transmission and storage loss models. For a regional electricity grid, the model computes, over a given day, the minute-by-minute power demand from and power generation in all districts, extracts matching power from a hierarchy of power sources so as to minimize transmission and storage losses, and stores/draws excess power in/from a variety of power storage options. For the specific case of the South Indian Power Grid (catering to about 285 million people in South India), the model shows that about 240 GWh of new electrical storage is required for round-the-clock power supply from non-GHG power sources.Graphical abstract 

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An energy-mix model for round-the-clock power supply in a decarbonized electricity generation scenario: case study of South India

Numerical Investigation of Conjugate Heat Transfer and Thermal Stresses of Additively Manufactured Tubes in Turbulent Flow regime

For large-scale stationary energy storage applications, flow batteries are gaining attention all over the world. Numerous studies have been done on flow batteries since their invention. Almost all the studies are based on the constant current cycling of flow batteries. In the present work, we explore a different perspective of a flow battery and characterize the power, energy, and efficiency characteristics of a 5-kW scale vanadium redox flow battery system through constant power cycling tests. Different ratios of charge power to discharge power characteristics of solar, wind, and peak shaving applications have been incorporated in the test protocol. It is shown that, over the range of testing, the round-trip energy efficiency and the fractional energy utilization depend linearly on the power at which the battery is charged or discharged.

Constant-Power Characterization of a 5 kW Vanadium Redox Flow Battery Stack

Hydrodynamic analysis of a novel circular, split-serpentine planar flow field

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.

Sreenivas Jayanti

Papers

An energy-mix model for round-the-clock power supply in a decarbonized electricity generation scenario: case study of South India

Numerical Investigation of Conjugate Heat Transfer and Thermal Stresses of Additively Manufactured Tubes in Turbulent Flow regime

Constant-Power Characterization of a 5 kW Vanadium Redox Flow Battery Stack

Hydrodynamic analysis of a novel circular, split-serpentine planar flow field

On Axial Coherence of Interfacial Waves in Countercurrent Flow (R&D Note)