Simulation of UV photoreactor for water disinfection in Eulerian framework

A novel computational fluid dynamic (CFD) modeling procedure was developed in order to simulate ultraviolet (UV) photoreactors in the Eulerian framework. In this procedure, the governing equations of radiation distribution, mass conservation, momentum conservation, and species mass conservation are solved together in order to determine the radiant energy field, velocity field, and the concentration profile of microorganisms at steady state conditions. The general method presented can be employed to derive the volumetric inactivation rate and the theoretical efficiency of a UV photoreactor. The integrated CFD model of UV photoreactor performance was successfully evaluated with experimental biodosimetry results. The verified procedure can be applied to the simulation and design optimization of UV photoreactors with different geometries and operating conditions.     

Neutron analysis of spent fuel storage installation using parallel computing and advance discrete ordinates and Monte Carlo techniques

In the United States, the Nuclear Waste Policy Act of 1982 mandated centralised storage of spent nuclear fuel by 1988. However, the Yucca Mountain project is currently scheduled to start accepting spent nuclear fuel in 2010. Since many nuclear power plants were only designed for -10 y of spent fuel pool storage, > 35 plants have been forced into alternate means of spent fuel storage. In order to continue operation and make room in spent fuel pools, nuclear generators are turning towards independent spent fuel storage installations (ISFSIs). Typical vertical concrete ISFSIs are -6.1 m high and 3.3 m in diameter. The inherently large system, and the presence of thick concrete shields result in difficulties for both Monte Carlo (MC) and discrete ordinates (SN) calculations. MC calculations require significant variance reduction and multiple runs to obtain a detailed dose distribution. SN models need a large number of spatial meshes to accurately model the geometry and high quadrature orders to reduce ray effects, therefore, requiring significant amounts of computer memory and time. The use of various differencing schemes is needed to account for radial heterogeneity in material cross sections and densities. Two P3, S12, discrete ordinate, PENTRAN (parallel environment neutral-particle TRANsport) models were analysed and different MC models compared. A multigroup MCNP model was developed for direct comparison to the SN models. The biased A3MCNP (automated adjoint accelerated MCNP) and unbiased (MCNP) continuous energy MC models were developed to assess the adequacy of the CASK multigroup (22 neutron, 18 gamma) cross sections. The PENTRAN SN results are in close agreement (5%) with the multigroup MC results; however, they differ by -20-30% from the continuous-energy MC predictions. This large difference can be attributed to the expected difference between multigroup and continuous energy cross sections, and the fact that the CASK library is based on the old ENDF/B-II library. Both MC and SN calculations were run in parallel on a BEOWULF PC-cluster (eight processors). Timing results indicate that the SN calculation yielded a detailed dose distribution at over 318,426 points in -164 h. Unbiased continuous energy MC required 214 h to calculate dose rates with a 1% relative error in only 18 regions on the surface of the cask. The biased A3MCNP calculations yields dose rates with -0.8% relative error in only 2.5 h on one processor. This study demonstrates that a parallel code, such as the 3-D parallel SN transport code, PENTRAN can solve a complex large problem, such as the storage cask, accurately and efficiently. Moreover, this calculation was performed on a relatively inexpensive PC-cluster. Possible inadequacies of the CASK cross section library still need to be evaluated.     

Effect of freezing on the dynamic mechanical response of hardened cement paste down to −60°C

The dynamic mechanical response (the internal friction, tanδ, and the E-modulus) in the temperature range from −60°C to 0°C reveals a sharp modulus change and a shallow tanδ peak. This “capillary transition” occurs only in specimens suspected of having capillary water; its strength is both a function of moisture content and the pore structure. The E-T curve in the transition region for a number of saturated specimens is fitted by an empirical equation; the parameters of this equation are correlated with the capillary porosity, defined in the conventional manner. One of these parameters is interpreted in terms of a maximum pore size. A theory is outlined for interpreting the E-T curve in terms of thermodynamic, composite mechanics, and pore structure parameters. The discovery of the “capillary transition” has made available a new technique for investigating the various, rather complicated aspects of the process of freezing within a porous solid.     

The Impact of Ventilation Rate and Partition Layout on the VOC Emission Rate: Time-Dependent Contaminant Removal

A numerical study has been carried out to predict the indoor air quality in a newly painted partitioned office and to assess the effect of ventilation rate and partition layout on the pre-ventilation time required to allow the contaminant concentration level to drop to an acceptable level. The air-flow pattern, the time history of the average contaminant concentration level in the occupied zone, and the pre-ventilation time were computed by a CFD code with a K-ε turbulence model. A displacement ventilation system, with different operating conditions, was used to remove contaminants in the office. The VB emission model, developed by Guo and Tichenor (1992), has been improved to include the influence of non-uniform concentration distribution on contaminant emission rate from the paints. The average contaminant (VOC) concentration levels in the occupied zone and in each chamber were recorded hourly until the average concentrations in the office were less then 0.0005 g/m³. It has been found that the non-uniform concentration distri- bution greatly influences the decay rate of VOC emissions. The time required for the average concentration in each chamber to reach the threshold level is different, since they are non-uniform. The time differences between the chambers having the highest and lowest concentrations were 3 to 8 hours. The results also show that the time required to reduce the concentration in the occupied zone to the threshold level is dependent on the partition layout. For the same ventilation rate, the required time is approximately 4-8 hours longer when using a side layout than when using a central layout.     

Optimization of ventilation systems in office environment,Part II: Results and discussions

Whether one considers the issues related to office workers' well-being and productivity or the issues from an energy and environmental perspective, there are clear evidences in favor of improving the quality of office environment. Part I of this paper proposed a simulation-based optimization approach by using computational fluid dynamics (CFD) techniques in conjunction with genetic algorithm (GA), with the integration of an artificial neural network (ANN) for response surface approximation (RSA) and for speeding up fitness evaluations inside GA loop. In this part, the results from data preparation for ANN model construction, ANN training and testing, and sensitivity analysis (regarding the impact of weighting factors in the objective function on the optimization results) are presented. Final optimization results indicate that the present choices of objective function and optimization approach are able to result in great improvement in the design and operation of ventilation systems in an office environment, with the goal of enhancing the thermal comfort and indoor air quality (IAQ) without sacrificing the energy costs of ventilation.     

An integrated weighted fuzzy multi-objective model for supplier selection and order scheduling in a supply chain

This paper presents a new weighted fuzzy multi-objective model to integrated supplier selection, order quantity allocation and customer order scheduling problem to prepare a responsive and order-oriented supply chain in a make-to-order manufacturing system. Total cost and quality of purchased parts as well as the reliability of on-time delivery of customer orders are regarded as the objectives of the model. On the other hand, flexible suppliers can contribute to the responsiveness and flexibility of entire supply chain in the face of uncertain customer orders. Therefore, a mathematical measure is developed for evaluating the volume flexibility of suppliers and is considered as the other objective of the model. Furthermore, by considering the effect of interdependencies between the selection criteria and to handle inconsistent and uncertain judgments, a fuzzy analytic network process method is used to identify top suppliers and consider as the last objective. In order to optimise these objectives, the decision-maker needs to decide from which supplier to purchase parts needed to assemble the customer orders, how to allocate the demand for parts between the selected suppliers, and how to schedule the customer orders for assembled products over the planning time horizon. Numerical examples are presented and computational analysis is reported.     

New comprehensive model based on virtual clusters and absorbing Markov chains for energy-efficient virtual machine management in cloud computing

The Journal of Supercomputing - Utilizing from energy-aware solutions along with maintaining service-level agreements is one of the most important research issues in cloud computing. In the...