| 1. |
- Ghazanfari Holagh, Shahriyar, et al.
(författare)
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An experimental investigation on bubbles departure characteristics during sub-cooled flow boiling in a vertical U-shaped channel utilizing high-speed photography
- 2021
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Ingår i: Thermal Science and Engineering Progress. - : Elsevier BV. - 2451-9049. ; 22
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Tidskriftsartikel (refereegranskat)abstract
- Classified as a passive heat transfer enhancement technique, U-shaped channels are commonly encountered with applications associated with boiling heat transfer such as air conditioning systems, evaporators, and boilers. On the other hand, understanding heat transfer in flow boiling is heavily dependent upon bubbles dynamic behavior, which is influenced by flow and geometrical conditions. In this paper, bubbles departure characteristics are experimentally studied in upward sub-cooled flow boiling of distilled water in a vertical U-shaped channel. The influence of flow conditions, embracing heat flux, mass flux, and inlet sub-cooling on bubbles characteristics, encompassing departure diameter, growth and waiting times, and nucleation frequency, is investigated through conducting 68 experiments. All the experiments are carried out at atmospheric pressure over a Nichrome heating surface installed on the outer wall of the channel, with heat flux, mass flux, and inlet flow temperature in the ranges of 26.1–61.5 kW.m-2, 114–255kg.m-2s-1, and 1 to 8 °C. A high-speed camera is utilized to capture bubbles growth process and departure instance. The results reveal that as wall heat flux increases and mass flux and inlet sub-cooling decrease, bubbles departure diameter and frequency rise, while waiting time declines. Also, lower growth times are detected when wall heat flux and mass flux increase and inlet sub-cooling declines. Two new correlations are developed to predict present and previously published experimental data of departure diameter and nucleation frequency with mean standard deviations of 18.0% and 18.5%, correspondingly.
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| 2. |
- Shafiee, Mahmood, 1982, et al.
(författare)
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A Redundancy Optimization Model Applied to Offshore Wind Turbine Power Converters
- 2013
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Ingår i: PowerTech (POWERTECH), 2013 IEEE Grenoble. - 9781467356695
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Konferensbidrag (refereegranskat)abstract
- Unexpected failures, high operation and maintenance (O&M) cost, and low accessibility are critical issues for offshore wind farms. According to existing statistics, power converters are among the most critical components in offshore wind turbines, and suffer from a high failure rate. One efficient way to improve the reliability and availability of the converter system is by adding at least one independent redundant converter, which ensures that the system would still operate in case of a converter failure. However, the redundant converters will increase the system’s cost, volume, and weight. In this paper, we propose a cost-rate minimization model aiming to simultaneously determine the optimal allocation of redundant converters and the optimal number of the converters that are allowed to fail before sending a maintenance crew to the offshore platform. The optimal solution under system-level constraints is derived, and the conditions required to make using redundant converter system beneficial are discussed. Finally, the proposed model has been tested on data collected from an offshore wind farm database and the results are compared with a conventional wind turbine converter system.
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| 3. |
- Shafiee, Mahmood, 1982, et al.
(författare)
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Adapting an age-reduction model to extend the useful-life duration
- 2012
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Ingår i: Proceedings of 2012 International Conference on Quality, Reliability, Risk, Maintenance, and Safety Engineering, ICQR2MSE 2012, Chengdu, 15-18 June 2012. - 9781467307888 ; , s. 1055-1059
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Konferensbidrag (refereegranskat)abstract
- Most of the proposed age-reduction models was introduced and widely used in applications only for the case of monotonically increasing failure rates. However, from a practical perspective, there are several situations for which failure rates are decreasing or bathtub shaped. In these cases, the existing imperfect maintenance models may lead to the restoration of a system to the infant mortality period and can be considered as harmful. In this paper, we modify the usual assumptions on traditional age-reduction models to take into account the bathtub-shaped failure rates. Finally, we provide a numerical example to illustrate our result.
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| 4. |
- Shafiee, Mahmood, 1982, et al.
(författare)
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An optimal age–usage maintenance strategy containing a failure penalty for application to railway tracks
- 2016
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Ingår i: Proceedings of the Institution of mechanical engineers. Part F, journal of rail and rapid transit. - : SAGE Publications. - 0954-4097 .- 2041-3017. ; 230:2, s. 407-417
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Tidskriftsartikel (refereegranskat)abstract
- Railway track maintenance plays a key role in enhancing the reliability and safety of railway transportation, as it reduces the potential risks of defects and derailments. A railway track degrades over time, due to its cumulative usage (in terms of million gross tons) that results from traffic movements. When the cumulative usage reaches a failure threshold, the railway track breaks and has to be replaced with a new section. Moreover, the infrastructure owner charges the maintenance agent a specific penalty due to traffic disrruption and passenger dissatisfaction. To avoid such costly defects,the railway tracks must be preventively replaced at regular time intervals. In this paper, we propose an optimal bivariate(age–usage) maintenance strategy for railway tracks such that the average long-run maintenance cost per unit time is minimized. The proposed model is applied to 60E1 track on a small part of the Swedish heavy haul line ‘Malmbanan’, and the results are compared with two conventional age-based and usage-based maintenance policies. The results show that the proposed maintenance policy has a substantial potential to reduce the servicing costs of railway track maintenance.
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| 5. |
- Shafiee, Mahmood, 1982, et al.
(författare)
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An Optimal Number-Dependent Preventive Maintenance Strategy for Offshore Wind Turbine Blades Considering Logistics
- 2013
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Ingår i: Advances in Operations Research. - : Hindawi Limited. - 1687-9147 .- 1687-9155. ; Volume 2013 (2013)
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Tidskriftsartikel (refereegranskat)abstract
- In offshore wind turbines, the blades are among the most critical and expensive components that suffer from different types of damage due to the harsh maritime environment and high load. The blade damages can be categorized into two types: the minor damage, which only causes a loss in wind capture without resulting in any turbine stoppage, and the major (catastrophic) damage, which stops the wind turbine and can only be corrected by replacement. In this paper, we propose an optimal number-dependent preventive maintenance (NDPM) strategy, in which a maintenance team is transported with an ordinary or expedited lead time to the offshore platform at the occurrence of the N th minor damage or the first major damage, whichever comes first. The long-run expected cost of the maintenance strategy is derived, and the necessary conditions for an optimal solution are obtained. Finally, the proposed model is tested on real data collected from an offshore wind farm database. Also, a sensitivity analysis is conducted in order to evaluate the effect of changes in the model parameters on the optimal solution.
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| 6. |
- Shafiee, Mahmood, 1982, et al.
(författare)
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Maintenance models in warranty: A literature review
- 2013
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Ingår i: European Journal of Operational Research. - : Elsevier BV. - 0377-2217. ; 229:3, s. 561-572
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Tidskriftsartikel (refereegranskat)abstract
- Along with increasing the warranty period for complex systems, reducing the warranty servicing costs has become an issue of great importance to the manufacturers. One possible way to reduce the expected warranty servicing cost is by making sound decision on the product warranty and maintenance strategies. Therefore, warranties (basic warranty and extended warranty) and maintenance (corrective and preventive) are strongly interlinked and of great interest to both manufacturers and customers. This paper is the first identifiable academic literature review to deal with warranty and maintenance. It provides a classification scheme for the articles that link warranty and maintenance published between 2001 and 2011 covering 44 journals and proposes a taxonomy scheme to classify these articles. Nine hundred articles were identified for their relevance to warranty and were carefully reviewed. One-hundred and twenty-two articles were subsequently selected for their relevance to maintenance and included in the classification. (c) 2013 Elsevier B.V. All rights reserved.
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| 7. |
- Shafiee, Mahmood, 1982, et al.
(författare)
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Maintenance models in warranty: A literature review
- 2013
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Ingår i: European Journal of Operational Research. - : Elsevier BV. - 0377-2217. ; 229:3, s. 561-572
-
Tidskriftsartikel (refereegranskat)abstract
- Along with increasing the warranty period for complex systems, reducing the warranty servicing costs has become an issue of great importance to the manufacturers. One possible way to reduce the expected warranty servicing cost is by making sound decision on the product warranty and maintenance strategies. Therefore, warranties (basic warranty and extended warranty) and maintenance (corrective and preventive) are strongly interlinked and of great interest to both manufacturers and customers. This paper is the first identifiable academic literature review to deal with warranty and maintenance. It provides a classification scheme for the articles that link warranty and maintenance published between 2001 and 2011 covering 44 journals and proposes a taxonomy scheme to classify these articles. Nine hundred articles were identified for their relevance to warranty and were carefully reviewed. One-hundred and twenty-two articles were subsequently selected for their relevance to maintenance and included in the classification. (c) 2013 Elsevier B.V. All rights reserved.
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| 8. |
- Shafiee, Mahmood, 1982, et al.
(författare)
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Optimal burn-in and preventive maintenance warranty strategies with time-dependent maintenance costs
- 2013
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Ingår i: IIE Transactions. - : Informa UK Limited. - 0740-817X .- 1545-8830. ; 45:9, s. 1024-1033
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Tidskriftsartikel (refereegranskat)abstract
- This article considers the determination of the optimal burn-in time, the degree of preventive maintenance, and the preventive maintenance interval (or, equivalently, the number of preventive maintenance actions) for warranted products with time-dependent maintenance costs. The expected cost function is derived by adopting an appropriate age-reduction model and the determination of the optimal joint solution is discussed. Finally, the impact of providing a burn-in/preventive maintenance program is evaluated through numerical examples.
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| 9. |
- Shafiee, Mahmood, 1982, et al.
(författare)
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Optimal redundancy and maintenance strategy decisions for offshore wind power converters
- 2015
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Ingår i: International Journal of Reliability, Quality and Safety Engineering. - : World Scientific Publishing Company. - 0218-5393. ; 22:3
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Tidskriftsartikel (refereegranskat)abstract
- Analysis of field failure data collected from various wind farm databases indicates that the power converters are among the most critical components in offshore wind turbines, since they suffer from a high failure rate. One efficient approach to enhance the reliability and availability of the wind power systems is through using a redundant converter design, in which a set of power converters is placed together in parallel. The main advantage of a multiple parallel converter system is that the failure of one converter will not necessarily lead to the failure of the entire system. It may however increase the wind turbine’s acquisition cost, volume, and weight. In this paper, we propose an approach of joint redundancy and maintenance strategy optimization for offshore wind power converters, aiming to simultaneously determine the "optimal allocation of redundant converters" and the "optimal threshold number of converters that are allowed to fail before sending a maintenance crew to the offshore platform". The optimal solution under various system-level constraints (such as reliability, weight, and the available space in nacelle) is derived and the conditions required to make using a redundant system beneficial are discussed. The proposed design is applied to an offshore wind turbine system and its performance is evaluated using a Monte-Carlo simulation technique. Finally, the results are compared with the conventional power converter system and a sensitivity analysis is conducted in order to make the proposed approach applicable for the next generation of wind turbines.
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| 10. |
- Shafiee, Mahmood, 1982
(författare)
-
Warranty and Maintenance: under-researched areas
- 2012
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Ingår i: Proceedings of the 5th Asia-Pacific International Symposium on Advanced Reliability and Maintenance Modeling (APARM). ; , s. 414-421
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Konferensbidrag (refereegranskat)abstract
- Warranties (basic and extended) and maintenance (corrective and preventive) are strongly interlinked and of great interest to both researchers from different disciplines and practitioners in different industry sectors. The literature on both warranty and maintenance in separate isvery vast, but the bulk of the research that links these two topics is relatively narrow and discipline specific. This paper briefly reviews the articles that link warranty and maintenance published between 2001 and 2012. It will then discuss some new topics for future research.
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