A numerical study is conducted to compare the current most popular flow field configurations, porous, biporous, porous with baffles, Toyota 3D fine-mesh, and traditional rectangular flow field. ...Operation at high current densities is considered to elucidate the effect of the flow field designs on the overall heat transfer and liquid water removal. A comprehensive 3D, multiphase, nonisothermal computational fluid dynamics model is developed based on up-to-date heat and mass transfer sub-models, incorporating the complete formulation of the Forchheimer inertial effect and the permeability ratio of the biporous layers. The porous and baffled flow field improves the cell performance by minimizing mass transport losses, enhancing the water removal from the diffusion layers. The baffled flow field is chosen for optimization owing to the simple design and low manufacturing cost. A total of 49 configurations were mutually compared in the design of experiments to show the quantitative effect of each parameter on the performance of the baffled flow field. The results elucidate the significant influence of small geometry modifications on the overall heat and mass transfer. The results of different cases have shown that water saturation can be decreased by up to 33.59% and maximal temperature by 7.91 °C when compared to the reference case which is already characterized by very high performance. The most influencing geometry parameters of the baffles on the cell performance are revealed. The best case of the 49 studied cases is further optimized by introducing a linear scaling factor. Additional geometry modifications demonstrate that the gain in performance can be increased, but at a cost of higher pressure drop and increased design complexity. The conclusions of this work aids in the development of compact and high-performance proton exchange membrane fuel cell stacks.
This paper presents the proposal for the concept design of hybrid Offshore Patrol Vessels (OPV) that are suitable for countries with budget constraints. Many countries cannot afford two different ...classes of ships to answer the current civil and military challenges. Therefore, the proposed approach might be the most acceptable solution for these countries. The concept design covers hybrid OPVs that could include traditional navy capabilities such as anti-ship and anti-air warfare in addition to offshore patrol tasks. This enables a hybrid concept design model that incorporates major tasks of both patrol and corvette classes of ships. The solutions are achieved by applying a multi-attribute concept design procedure that was developed earlier for a generic naval vessel design. The procedure is customized to suit this purpose by applying appropriate design variables, parameter space constraints, updated ship propulsion systems, weapons, other equipment, and design requirements i.e., required areas, volumes, and electricity. The results are the Pareto optimal set of solutions that are used as initial designs in the subsequent phases of the design of the hybrid OPV. The earlier developed software named “MonakoPRB” is updated to suit the above-mentioned changes in the design procedure to generate hybrid OPV concept design solutions.
EXPERIMENTAL INVESTIGATION OF MARINE ENGINE EXHAUST EMISSIONS Mihanović, Luka; Jelić, Maro; Radica, Gojmir ...
Energy sources. Part A, Recovery, utilization, and environmental effects,
12/2021, Volume:
ahead-of-print, Issue:
ahead-of-print
Journal Article
Peer reviewed
The physicochemical process of formation of the four main pollutant emissions from diesel engines (carbon monoxide-CO, hydrocarbons-HC, particulate matter-PM and nitrogen oxides-NOx), quantities of ...produced exhaust emissions and their harmful impact are some of the most important factors for the establishment of stricter emission regulations and for improvement of marine engines. In this paper, the conducted experiment and analysis of exhaust emissions are explained. Experimental research provided on a four stroke main engine during a typical voyage of the ship shows that the quantity of NOx emissions is 3.42 g/kWh, which is lower than the permissible limit of 11.75 g/kWh. The engine complies with the International Maritime Organization (IMO) Class II emission limits; however, it does not comply with Class III emission limits. The results show that depending on the different operating conditions the concentration of CO
2
in exhaust gases varies from 3,2% to 6,6%, the content of CO varies from 170 ppm to 233 ppm and NO varies from 690 ppm to 1120 ppm. The necessity of implementing an onboard emission measurement method is emphasized. Recommendations are given on the possibility of improving the performance of the analyzed engine system.
This paper presents the research on reliability of high-speed radial marine diesel engine Zvijezda M 504 B2 based on experimental data of malfunctions on the engine and time required for repair. ...Exploitation reliability and failure intensity were calculated from operational data collected from the engine log book. Results were tested by computer program to determine the relevance of the obtained results. Mathematically calculated reliability model of high-speed radial marine diesel engine Zvijezda M 504 B2 showed continuous increasing function of the intensity of failure and the fact that the reliability of the engine can be reliably approximated by Weibull distribution. Based on the obtained results it has been shown that this distribution, regardless of its complexity, should be used in practice when calculating reliability of engines with similar constant and growing malfunction’s intensity. The conclusion is that proposed distribution enables a better depiction of the observed technical system and the impact of aging of components on the system reliability.
The Effects of Turnover on Expert Effort Estimation Karna, Hrvoje; Gotovac, Sven; Vicković, Linda ...
Journal of Information and Organizational Sciences,
01/2020, Volume:
44, Issue:
1
Journal Article, Paper
Peer reviewed
Open access
Turnover of the personnel represents a serious issue for management of software projects. The buildup of competences and phasing in of the people into the project requires both time and effort. This ...paper presents a case study of a large in-house agile software development project. The research goal was to determine the effects that turnover has on the expert effort estimation. In order to do this, paper examines relations across empirical data on a studied project. Study findings are the following: a) it is necessary to distinguish types of turnover, b) the general and planned turnover do not necessarily have a negative effect on estimation accuracy, and c) the unplanned turnover can have a significant negative impact on the reliability of the estimates and therefore should be treated with special attention. Results suggest that these facts should be taken into account both by the management and human resources.
This paper analyses the organisation and scope of activities of the Coast Guard of the Republic of Croatia. The Coast Guard of the Republic of Croatia is a military organisation with police ...authorities and is an integral part of the Croatian Navy. It is a task force whose basic purpose is to provide support to state administration bodies in charge of surveillance and protection of Croatian strategic, economic and other general national interests on the eastern side of the Adriatic Sea. It is necessary to stress that the Croatian Coast Guard is trained not only to carry out protection of national maritime interests but also to carry out combat tasks and to participate in joint operations in the national and international environment. This paper contributes in stressing the importance of the role of the Croatian Coast Guard and of investments in equipment and in human resource with the aim of maintaining maritime
safety on the Adriatic Sea.
The paper presents the initial case of a research of analysing maintenance interval adjustment in the shipping industry. This case is an example of the adjustment of the maintenance interval carried ...out by ship's crew on starting air compressor system. The example is found and followed through the records in the computerized Planned Maintenance System. Maintenance interval adjustment proposal and actual adjustment were carried out based on user's experience, after rechecking maintenance data and actual machinery condition, without theoretical approach or analysis. That process created new maintenance plan which is in use on the system for past ten years. Missing actions in the process, i.e. theoretical analysis of the initial and modified maintenance, are performed using the MA-CAD method with a modified reliability testing and are described in the paper. Results obtained by the theoretical analysis are compared with the requested and performed modification of the Maintenance Plan and a conclusion about performed action has been derived.
As part of the research on high speed radial diesel engines, an analysis had been conducted on the use of the operating systems of the missile boats and comparison of the results with the ...manufacturer’s recommendations. A graphic model had been set up (block – diagram) on high speed radial ship diesel engine. On the basis of research and empirical data collected on defects which cause the inability to use the engine, the total exploitation reliability of complex technical system and the statistical characteristics of the life of the system had been defined, starting from the assumption that the life of the technical system tested belongs to the Weibull distribution. Research results had been achieved by means of the simulation method (the simulation model) and then were tested with the use of the contemporary computer applications. By defining the levels of operational capability, the levels of reliability had eventually been set up and they need to be ensured through modified maintenance procedures so that the investigated technical system could meet the required criteria. In the research of new samples, the needed intervals of preventive replacement of critical system parts will be determined so that the demanded reliability would be ensured.
In order to increase efficiency, security, reliability and availability of systems, while simultaneously reducing operational cost of maintenance, use of new information technologies continues to ...grow steadily. New information technologies are implemented as an integral part of ship systems or as an independent system, such as cloud computing, RFID (Radio Frequency Identification) technology, diagnostic technology, software agents, etc. New information technologies allow optimal use of computer (general purpose, dedicated or industrial) aided ship systems, which results in the reduction of crew sizes and more efficient maintenance of ship systems. Finally, they allow shipping companies and marine equipment manufacturers to offer more competitive prices together with ensured level of reliability and availability of ship systems.
The Regulations for Preventing Collisions at Sea (COLREG) is a collection of international legal standards adopted in order to avoid collisions at sea. Comprehensive knowledge of these rules is ...required for all who sail on the sea. The ACTs project (Avoiding Collisions at Sea) investigates the current problems in applying these Rules and intends to develop a new online course making it simpler and easier to understand them. A COLREG questionnaire has been drawn up for research purposes. The purpose of the questionnaire is checking which rules are more difficult to understand. In this paper, the authors analyse the level of understanding of the Rules in a group of secondary maritime school students. The survey results indicate that many of the basic principles of COLREG are neither understood nor applied.
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