An investigation is made of the effects of the change of the porosity of the gas diffuser layer (GDL) on the performance of a proton exchange membrane (PEM) fuel cell. The analysis of fuel cell performance with non-uniform porosity of GDL is a necessity because the presence of liquid water in the GDL leads to a non-uniformly distributed porosity in the GDL. To implement this performance analysis, a half-cell model which considers the oxygen mass fraction distribution in the gas channel, the GDL and the catalyst layer, and the current density and the membrane phase potential in the catalyst layer and the membrane is investigated. Four continuous functions of position are employed to describe the porosity, and differential equations are derived based on oxygen transportation and Ohm's law for proton migration and solved numerically. Results show that a fuel cell embedded with a GDL with a larger averaged porosity will consume a greater amount of oxygen, so that a higher current density is generated and a better fuel cell performance is obtained. This explains partly why fuel cell performance deteriorates significantly as the cathode is flooded with water (i.e. to give a lower effective porosity in the GDL). In terms of the system performance, a change in GDL porosity has virtually no influence on the level of polarization when the current density is medium or lower, but exerts a significant influence when the current density is high. This finding supports the scenario proposed by previous studies that the polarization at high current density corresponds mainly to mass transfer through (or the concentration activation of) the membrane assembly.

n contrast to traditional networks, software-defined networking (SDN) decouples the control plane from the data plane to enhance programmability and flexibility of network control [1]. In SDN, the control plane is a logically centralized controller, which communicates with the data plane via a control channel, say the OpenFlow protocol standardized by the Open Networking Foundation (ONF) [2]. SDN is suitable for managing connectivity because the controller has a global view of the network. If deploying network function virtualization (NFV) modules [3] as SDN services is desired, the controller will have to handle numerous control messages and incoming packets from the data plane to classify them or even inspect packet payloads.This work presents an extended SDN architecture to reduce the traffic overhead to the controller and support NFV by service chaining [4]. We designed a two-tier mechanism to classify traffic on the data plane instead of the control plane as much as possible. In the first tier, a classification module, which inspects the TCP/IP and application headers, is added on the switch. If this module is unable to determine the policy to be applied, a deep packet inspection (DPI) module in the second tier serves as an NFV module and analyzes the packets that cannot be classified in the first tier. The controller determines the services to be invoked based on the classification results, and makes the decision about service routing. Both modules will retain the decision, so subsequent packets will be directly forwarded to the NFV modules designated by the controller. We implemented intrusion prevention as the example for evaluating the extended architecture.The rest of this article is organized as follows. We first present the background of this work. Then we present the extended architecture, and evaluate its performance. Finally, we conclude this work.

No abstract

No abstract

No abstract