Design and evaluation of an LQI-based beaconless routing protocol for a heterogeneous MSN

Wireless Networks - In a mobile sensor network with a mobile sink, choosing the next hop depends on the current location of the sink. This necessitates a frequent update of routing paths within the...     

Analytically investigating the characteristics of a high‐temperature unitized regenerative solid oxide fuel cell

A unitized regenerative solid oxide fuel cell (URSOFC) can be considered as a next‐generation power source and a storage device in the future since it can generate electricity in the SOFC mode and also produce H₂/O₂ in the solid oxide electrolyzer cell (SOEC) mode. In this paper, a two‐dimensional axisymmetric model is developed to simulate the characteristics of a URSOFC. The performance curves for an in‐house button URSOFC under different operating temperatures of 600, 700, and 800 °C are measured to validate the present model. Both the measured data and the prediction results reveal the beneficial effects of higher temperature on the cell performance. Based on the results of numerical simulations, the majority of the fuel gas is consumed at the interface of the electrolyte and the electrode, causing a great fuel concentration gradient near the interface. In addition, the predicted cell performance curves in both the SOFC and the SOEC modes correspond well with the measured data, demonstrating the applicability of this model in a button URSOFC. Copyright © 2013 John Wiley & Sons, Ltd.     

Numerically investigating fire suppression mechanisms for the water mist with various droplet sizes through FDS code

With rapid progress in computer capability recently, it becomes feasible to investigate the sophisticated phenomena related to the fire, especially for the interaction of fire and water spray, by way of the computational fluid dynamics (CFD). In this paper, a fire simulation CFD code_FDS is used to numerically investigate the different droplet sizes on the fire suppression/extinguishment mechanisms. The CFD models adopted in the FDS are first assessed against the previous experimental work of Kim and Ryou. The droplet size interested is varied from 100 μm to 1000 μm that is located within the droplet size range for a water mist. Based on the sensitivity simulations with different droplet sizes, the dependency of fire extinguishing time on the discharged droplet size can be obtained. The fire extinguishing time decreases with the decreasing droplet size for a mist with relatively fine droplet size since both the evaporation cooling and the oxygen displacement are the dominant mechanisms of fire suppression. However, this trend is reverse for a mist with larger-size droplets for the sake that the direct cooling of flame is the major suppression mechanism. These conclusions are also confirmed by comparing the simulation distributions of gas temperature, oxygen concentration, and steam concentration after the mist actuation and just before the fire extinguishment.     

Experimentally and numerically investigating cell performance and localized characteristics for a high-temperature proton exchange membrane fuel cell

This paper is to experimentally and numerically investigate the cell performance and the localized characteristics associated with a high-temperature proton exchange membrane fuel cell (PEMFC). Three experiments are carried out in order to study the performance of the PEMFC with different operating conditions and to validate the numerical simulation model. The model proposed herein is a three-dimensional (3-D) computational fluid dynamics (CFD) non-isothermal model that essentially consists of thermal–hydraulic equations and electrochemical model. The performance curves of the PEMFC predicted by the present model agree with the experimental measured data. In addition, both the experiments and the predictions precisely demonstrate the enhanced effects of inlet gas temperature and system pressure on the PEMFC performance. Based on the simulation results, the localized characteristics within a PEMFC can be reasonably captured. These parameters include the fuel gas distribution, liquid water saturation distribution, membrane conductivity distribution, temperature variation, and current density distribution etc. As the PEMFC is operated at the higher current density, the fuel gas would be insufficiently supplied to the catalyst layer, consequently causing the decline in the generation of power density. This phenomenon is so called mass transfer limitation, which can be precisely simulated by the present CFD model.     

Numerical simulation of a heat sink embedded with a vapor chamber and calculation of effective thermal conductivity of a vapor chamber

This study presents a numerical investigation of a whole set of thermal module, including a plate-fin heat sink embedded with a vapor chamber, subject to the influence of concentrated heat sources. Within the vapor chamber, the internal vapor is assumed as a common heat-transfer interface between the wicks. CFD simulations of the integrated heat sink are carried out with this assumption. The calculated results are in good agreement with the experiments, and show a maximum difference of 6.3% for the hotspot temperature rises. It is found that the area of the heat source has an important influence to the performance of the vapor chamber. The major spreading resistance of the vapor chamber comes from the bottom wall, where a concentrated heat source is applied. In addition, the isotropic and orthotropic approaches are proposed to calculate the effective thermal conductivities of the vapor chamber. By approximating the vapor chamber as a conduction plate, the effective conductivity can be obtained from the analytical solutions of the spreading resistances. The vapor chamber can reduce the spreading resistances sufficiently by its excellent lateral thermal spreading effect, which can be interpreted by the orthotropic approach.     

Predicting the accumulated number of plugged tubes in a steam generator using statistical methodologies

A steam generator (SG) plays a significant role not only with respect to the primary-to-secondary heat transfer but also as a fission product barrier to prevent the release of radionuclides. Tube plugging is an efficient way to avoid releasing radionuclides when SG tubes are severely degraded. However, this remedial action may cause the decrease of SG heat transfer capability, especially in transient or accident conditions. It is therefore crucial for the plant staff to understand the trend of plugged tubes for the SG operation and maintenance. Statistical methodologies are proposed in this paper to predict this trend. The accumulated numbers of SG plugged tubes versus the operation time are predicted using the Weibull and log–normal distributions, which correspond well with the plant measured data from a selected pressurized water reactor (PWR). With the help of these predictions, the accumulated number of SG plugged tubes can be reasonably extrapolated to the 40-year operation lifetime (or even longer than 40 years) of a PWR. This information can assist the plant policymakers to determine whether or when a SG must be replaced.     

Experiment and simulation investigations for effects of flow channel patterns on the PEMFC performance

Experiments and simulations are presented in this paper to investigate the effects of flow channel patterns on the performance of proton exchange membrane fuel cell (PEMFC). The experiments are conducted in the Fuel Cell Center of Yuan Ze University and the simulations are performed by way of a three‐dimensional full‐cell computational fluid dynamics model. The flow channel patterns adopted in this study include the parallel and serpentine flow channels with the single path of uniform depth and four paths of step‐wise depth, respectively. Experimental measurements show that the performance (i.e. cell voltage) of PEMFC with the serpentine flow channel is superior to that with the parallel flow channel, which is precisely captured by the present simulation model. For the parallel flow channel, different depth patterns of flow channel have a strong influence on the PEMFC performance. However, this effect is insignificant for the serpentine flow channel. In addition, the calculated results obtained by the present model show satisfactory agreement with the experimental data for the PEMFC performance under different flow channel patterns. These validations reveal that this simulation model can supplement the useful and localized information for the PEMFC with confidence, which cannot be obtained from the experimental data. Copyright © 2007 John Wiley & Sons, Ltd.     

On security of wireless sensor networks: a data authentication protocol using digital signature

Guaranteeing end-to-end data security in wireless sensor networks (WSNs) is important and has drawn much attention of researchers over past years. Because an attacker may take control of compromised sensor nodes to inject bogus reports into WSNs, enhancing data authenticity becomes a necessary issue in WSNs. Unlike PCREF (Yang et al. in IEEE Trans Comput 64(1):4–18, 2015) (LEDS, Ren et al. in IEEE Trans Mobile Comput 7(5):585–598, 2008), digital signature rather than message authentication polynomials (message authentication codes) is adopted by our protocol in en-route filtering. Keeping the advantages of clusters in PCREF and overcoming the drawbacks in LEDS, an enhanced and efficient cluster-based security protocol is proposed in this paper. The proposed protocol can guarantee end-to-end data authentication with the aid of digital signature and exhibits its effectiveness and efficiency through security analysis and performance analysis. Our analytical results show that the proposed protocol significantly outperforms the closely related protocols in the literature in term of security strength and protocol overhead.     

NUMERICAL SIMULATIONS OF ELECTRO-SLAG REMELTING PROCESS

Flow and heat transfer characteristics in the electro-slag remelting process (ESR) are important in manufacturing steel of good quality. An integrated numerical model is developed to compute the flow field and the temperature distribution inside ESR units with a metal pool profile which is solved simultaneously. In addition to the conservative equations of mass, momentum, energy, and turbulent properties, Maxwell's equations are employed to obtain the electromagnetic field by either AC or DC power supply. The results include the effects of power supply type, current amplitude, casting rate, and flow field patterns (laminar or turbulent) on flow and heat transfer characteristics. Different flow patterns and turbulent properties have been predicted using a pool profile close to the real one for AC and DC power supplies. The present model concludes that the casting rate and current amplitude are very effective in affecting the pool shape.     

Investigating the burning characteristics of electric cables used in the nuclear power plant by way of 3-D transient FDS code

Burning characteristics of electrical cables are one of the key parameters for the fire hazard assessment of nuclear power plants (NPPs) since the cables are the essential sources of fire in the plants. A three-dimensional (3-D) transient computational fluid dynamics (CFD) code_FDS is adopted in this paper to simulate these characteristics related to the cable burning. Being one of the NRC licensing fire codes, the FDS includes the thermal-hydraulic equations, the turbulence model and the chemical combustion model, etc. In order to assess the CFD fire models used in this code, a burning test using the control cable with the outer jacket of polyvinylchloride (PVC) and the inner insulation of cross-linked polyethylene (XLPE) is conducted. The measured parameters associated with the burning characteristics include the heat release rate (HRR), O₂ depletion, and CO and CO₂ production, etc. Except the amount of O₂ consumption, the predicted transient behaviors of other parameters can reproduce the measured data. Based on the chemical combustion model in the FDS code, this discrepancy may be essentially resulted from the default value of hydrogen fraction (H_frac) contained in the soot since the soot yield for the burning of PVC material is high enough that the uncertainty in the H_frac value has a prominent effect on the amount of O₂ consumption. This explanation can be confirmed by a benchmark calculation for simulating a burning test with the polymethylmethacrylate (PMMA) fuel of low-soot yield. The present simulation works can provide the useful information for the plant staff or the researcher as they would perform the fire hazard analysis in the NPPs using the FDS code.