Solar-fuel fired cogeneration plants—Sizing and costing model for molten salt storage systems

The large-scale utilization of solar energy will be facilitated by economical and efficient energy storage. The proposed energy storage systems have been critically reviewed, and capital cost estimates compared on a common basis. A model for sizing an energy storage system is proposed and used to determine the size range of practical interest. Based on selection criteria and relevant data two storage systems have been investigated: an all sodium system and a molten salt system. The design equations, cost estimates, and correlations indicate that, for the energy storage systems developed to date, in the capacity range of 700–2100 MWh, a molten salt, two-tank isolated-type system is the most cost effective and technically feasible for a solar, central receiver, hybrid cogeneration plant. At the extremes of the above range the unit capital cost for the molten salt storage system was found to be 22.8–26.7 $/kWh of stored energy, compared to 43.0–45.4 $/kWh for the sodium storage system.     

Numerical simulation of dispersed gas-liquid flows

This paper reviews the available information on numerical simulation of dispersed gas-liquid flows. Emphasis is on informing the reader about various aspects of constructing simulation models rather than giving an exhaustive literature review. The information is organised in a way so as to provide answers to the following questions: how to formulate model equations? how to select suitable algorithms and numerical techniques to solve these model equations? how to translate these into workable computer codes? how to use such codes for simulating flows in industrial equipment? Though greater emphasis is given to dispersed gas-liquid flows, the methodology can be applied to any multi-phase problem. Special features of multi-phase flow simulation over single-phase simulation are highlighted. A case of gas-liquid flow in a bubble column is presented as an illustration for the general methodology. The simulated mean flow field agrees reasonably with the experimental data. Properly validatedcfd codes thus can serve as a useful tool for design engineers of multi-phase systems. Some of the common pitfalls in using simulation codes for design are also discussed. This review is expected to give an overall idea about the present state-of-art of two-phase simulation in industrial equipment.     

The variable property grain model applied to the zinc oxide-hydrogen sulfide reaction

The variable property grain model, including the effect of grain diffusion resistance, has been applied to experimental results from the reaction of hydrogen sulfide with zinc oxide. Grain radius is assumed to vary under the combined effects of sintering and extent of reaction. Property variations are correlated by the specific surface area, an easily measured quantity. All model parameters with the exception of the grain diffusion coefficient have been evaluated by literature correlations or independent experimental measurements. Significant improvement in the match with experimental data, as compared to the constant property grain model, has been achieved.     

CHARACTERIZATION OF GAS-LIQUID FLOWS IN RECTANGULAR BUBBLE COLUMNS USING CONDUCTIVITY PROBES

Unsteady gas-liquid flows in bubble columns are comprised of various flow processes occurring with varying length and time scales and govern mixing and transport processes. In the present work, we have characterized dynamic and time-averaged properties of gas-liquid flows in rectangular bubble columns using conductivity probes. The development of a single-tip conductivity probe, data processing methodology, and photographic validation procedure is discussed in detail. The effect of superficial gas velocity and aerated liquid height-to-width (H/W) ratio on voidage fluctuations and time-averaged gas holdup was investigated. The experimental data presented here can help in understanding the dynamics of various flow processes and validating computational fluid dynamics based models.     

Scheduling light-trails on WDM rings

We consider the problem of scheduling communication on optical WDM (wavelength division multiplexing) networks using the light-trails technology. We seek to design scheduling algorithms such that the given transmission requests can be scheduled using a minimum number of wavelengths (optical channels). We provide algorithms and close lower bounds for two versions of the problem on an n$n$ processor linear array/ring network. In the stationary   version, the pattern of transmissions (given) is assumed to not change over time. For this, a simple lower bound is c$c$, the congestion or the maximum total traffic required to pass through any link. We give an algorithm that schedules the transmissions using O(c+logn)$O(c+logn)$ wavelengths. We also show a pattern for which Ω(c+logn/loglogn)$\Omega (c+logn/loglogn)$ wavelengths are needed. In the on-line   version, the transmissions arrive and depart dynamically, and must be scheduled without upsetting the previously scheduled transmissions. For this case we give an on-line algorithm which has competitive ratio Θ(logn)$\Theta (logn)$. We show that this is optimal in the sense that every on-line algorithm must have competitive ratio Ω(logn)$\Omega (logn)$. We also give an algorithm that appears to do well in simulations (for the classes of traffic we consider), but which has competitive ratio between Ω(log²n/loglogn)$\Omega ({log}^{2}n/loglogn)$ and O(log²n)$O\left({log}^{2}n\right)$. We present detailed simulations of both our algorithms.     

CFD predictions of flow near impeller blades in baffled stirred vessels: Assessment of computational snapshot approach

The present article summarizes simulations of turbulent flow generated by a Rushton turbine (six blades with disc) and a downflow pitched blade turbine (four blades, 45° inclined) using a computational snapshot approach. The computational snapshot approach proposed by Ranade and Dommeti was extended and generalized to suit impellers of any shape. The approach was implemented using a commercial CFD code, FLUENT (Fluent Inc., USA). Mean flow and turbulence characteristics were computed by solving the Reynolds averaged Navier-Stokes equations combined with the standard k - l turbulence model. The QUICK discretization scheme (with SUPERBEE limiter function) was used to discretize all the governing equations. Preliminary numerical experiments were carried out to identify adequate grid resolution. The predicted results were compared with the comprehensive data set available in the literature. Simulated results show a pair of trailing vortex behind the blades of a turbine. The results were also compared quantitatively in the near-impeller region with the published experimental data and published simulated results using other approaches. The simulations have captured most of the key features of near-impeller flows with sufficient accuracy. The results and conclusions drawn from this study will have important implications for extending the applicability of CFD models to simulate complex stirred reactors.