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Numerical and Theoretical Investigation on CoolingPerformance of Radiators in the Power Transformers
Mälardalen University, School of Sustainable Development of Society and Technology.
(English)In: IEEE transactions on energy conversion, ISSN 0885-8969Article in journal (Refereed) Submitted
Abstract [en]

In this paper, heat transfer and fluid flow in a cooling system of power transformers have been studied. In order to simplify the study, a single radiator element is chosen which is composed of two welded identical steel plates. Also based on symmetrical property of radiator, just one quarter of it, was modelled and simulated. It is well known that phenomenon like Foucault currents generate unwanted loss in form of heat in the transformers. From engineering and design points of view, it is desired to establish efficient cooling in transformers efficiently in order to increase their operation life. The main objective in the current paper is to study the current situation of cooling system and its characteristics to identify the weakness points of installed cooling system. Also a new configuration for oil channels based on pin fin technology was studied to evaluate its capability for cooling efficiency improvement in this special case. Result can be used for optimization of power transformer cooling system in future studies. To accomplish these goals, a basic model based on theoretical correlation and a numerical study was performed for radiator model in the ONAN state. The physical properties of the cooling oil were calculated from technical drawing obtained from Iran Transfo Company. The finite volume method was used to solve the fluid and heat transfer governing equations in the steady state. Results show that there is a good agreement between available data, theoretical model and CFD model for oil channel in the current configuration. Heat transfer in ONAN state is within acceptable range, but the oil entrance and outlet passages should be redesigned to get better flow behaviour.  The radiator height has a direct effect on outlet temperature; however the maximum reduction in the outlet temperature would 7%, when the height is doubling. The radiator equipped with internal pin fins shows better heat transfer behaviour, while the pressure drop through the channels remains in acceptable range.

National Category
Energy Systems
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-14407OAI: oai:DiVA.org:mdh-14407DiVA: diva2:512480
Available from: 2012-03-27 Created: 2012-03-27 Last updated: 2013-01-24Bibliographically approved
In thesis
1. Numerical Analysis of Heat Transfer and Fluid Flow in Heat Exchangers with Emphasis on Pin Fin Technology
Open this publication in new window or tab >>Numerical Analysis of Heat Transfer and Fluid Flow in Heat Exchangers with Emphasis on Pin Fin Technology
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

One of the most important industrial processes is heat transfer, carried out by heat exchangers in single and multiphase flow applications. Despite the existence of well-developed theoretical models for different heat transfer mechanisms, the expanding need for industrial applications requiring the design and optimization of heat exchangers, has created a solid demand for experimental work and effort. This thesis concerns the use of numerical approaches to analyze and optimize heat transfer and fluid flow in power generation industry, with emphasis on pin fin technology.

This research begins with a review on heat transfer characteristics in surfaces with pin fins. Different pin fins shapes with various flow boundaries were studied, and thermal and hydraulic performances were investigated. The impact of parameters such as inlet boundary conditions, pin fin shapes, and duct cross-section characteristics on both flow and heat transfer were examined. Two important applications in power generation industry were considered for this study: power transformer cooling, and condenser for CO2 capturing application in oxy-fuel power plants. Available experimental data and correlations in the literature have been used for models validation. For each case, a model based on current configuration was built and verified, and was then used for optimization and new design suggestions. All numerical modeling was performed using commercial CFD software. A basic condenser design was suggested and examined, supplemented by the use of pin fin technology to influence the condensation rate of water vapour from a CO2/H2O flue gas flow. Moreover an extensive review of numerical modeling approaches concerning this condensation issue was conducted and presented.

The analysis results show that the drop-shaped pin fin configuration has heat transfer rates approximating those of the circular pin configuration, and the drop-shaped pressure losses are less than one third those of the circular. Results for the power transformer cooling system show those geometrical defects in the existing system are easily found using modeling. Also, it was found that the installation of pin fins in an internal cooling passage can have the same effect as doubling the radiator’s height, which means a more compact cooling system could be designed.

Results show that a condensation model based on boundary layer theory gives a close value to experimental correlations. Considering a constant wall temperature, any increase in CO2 concentration results in lower heat transfer coefficients. This is a subsequence of increased diffusivity resistance between combustion gas and condensing boundary layer. Also it was shown that sensitivity of heat transfer rate to inlet temperatures and velocity values decreased when these parameters increased.

The application of numerical methods concerning the condensation process for CO2 capturing required significant effort and running time as the complexity of multiphase flow was involved. Also data validation for the CO2/H2O condenser was challenging since this is quite a new application and less experimental data (and theoretical correlations) exist. However, it is shown that models based on numerical approaches are capable of predicting trends in the condensation process as well as the effect of the non-condensable CO2 presence in the flue gas.

The resulting data, conclusions, applied methodology can be applied to the design and optimization of similar industrial heat exchangers, such as oil coolers which are currently working at low efficiency levels. It can also be used in the design of electronic components, cooling of turbine blades, or in other design applications requiring high heat flux dissipation. Finally, the finding on water vapour condensation from a binary mixture gas can be referenced for further research and development in this field.

Place, publisher, year, edition, pages
Mälardalen University, 2012
Series
Mälardalen University Press Dissertations, ISSN 1651-4238 ; 98
Keyword
CFD, Heat exchanger, Pin fin technology, Cooling system, Power transformers, CO2
National Category
Energy Engineering
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-14409 (URN)978-91-7485-062-8 (ISBN)
Public defence
2012-04-13, Delta, Västerås, 10:00 (English)
Opponent
Supervisors
Available from: 2012-03-28 Created: 2012-03-28 Last updated: 2012-04-05Bibliographically approved

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Citation style
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