Heat transfer coefficients in concentric annuli

**Authors:**Dirker, Jaco**Date:**2012-09-05**Subjects:**Nusselt number , Heat -- Transmission**Type:**Thesis**Identifier:**uj:9597 , http://hdl.handle.net/10210/7019**Description:**M.Ing , The geometric shape of a passage's cross-section has an effect on its convective heat transfer capabilities. For concentric annuli, as cross section, the diameter ratio of the annular space plays an important role. The purpose of this investigation was to find a . correlation that will accurately predict heat transfer coefficients at the inner wall of smooth concentric annuli for the flow of water. Experiments were conducted on water under turbulent flow conditions for a wide range of diameter ratios. The Wilson plot method was used to determine the heat transfer coefficients from which a correlation was developed that could be used to predict the heat transfer coefficients. It was found that the correlation predicted Nusselt numbers accurately within 3% of measured values for diameter ratios between a = 1.7 and a = 5.1 and a Reynolds numbers range of 4 000 to 30 000.**Full Text:**

**Authors:**Dirker, Jaco**Date:**2012-09-05**Subjects:**Nusselt number , Heat -- Transmission**Type:**Thesis**Identifier:**uj:9597 , http://hdl.handle.net/10210/7019**Description:**M.Ing , The geometric shape of a passage's cross-section has an effect on its convective heat transfer capabilities. For concentric annuli, as cross section, the diameter ratio of the annular space plays an important role. The purpose of this investigation was to find a . correlation that will accurately predict heat transfer coefficients at the inner wall of smooth concentric annuli for the flow of water. Experiments were conducted on water under turbulent flow conditions for a wide range of diameter ratios. The Wilson plot method was used to determine the heat transfer coefficients from which a correlation was developed that could be used to predict the heat transfer coefficients. It was found that the correlation predicted Nusselt numbers accurately within 3% of measured values for diameter ratios between a = 1.7 and a = 5.1 and a Reynolds numbers range of 4 000 to 30 000.**Full Text:**

Heat-extraction from solid-state electronics by embedded solids with application to integrated power electronic

**Authors:**Dirker, Jaco**Date:**2008-11-19T05:28:30Z**Subjects:**Solid-state electronics , Power electronics , Heat conduction , Heat transmission**Type:**Thesis**Identifier:**uj:14733 , http://hdl.handle.net/10210/1739**Description:**D. Ing. , Power electronics refers to electronic processing of electrical power. In this process the electrical power is controlled by switching of power semiconductor devices as well as electromagnetically stored in electrical and magnetic fields while the energy flow is directed through selective conduction paths. The losses/power-efficiency of all these actions is of paramount importance in the processing. The lack for standardisation and the absence of a modular approach is a barrier to the development of more compact systems. Recently more research resources have been invested in development of integrated power electronic modules as an attempt to solve this problem. By integrating power electronic components, an increase in the power density is achieved, which unfortunately also leads to higher internal heat-generation and higher operating temperatures. This has an unfavourable effect on electronic behaviour and the reliability of the structures. In order to maintain the advances made in volume reduction of integrated power electronics, efficient and cost effective methods for removing heat is of essence. In this investigation the performance of rectangular cross-section embedded solid-state heatextraction inserts to increase thermal heat spreading and the reduction of steady-state peak temperatures was evaluated theoretically and experimentally. Theoretically, the cross sectional aspect ratio of such inserts was thermally optimised for a wide range of dimensional, thermal, and material property conditions. Possible materials investigated for use as heat extractors in power electronics include aluminium nitride, beryllium oxide, and synthetic diamond. The presence of interfacial thermal resistance was theoretically found to have a significant detrimental influence on the thermal performance of an integrated heat-extraction system and should be minimised as far as possible. For conditions commonly found in integrated power passives, continuous embedded heatextraction layers are proposed. Theoretically it is shown that such inserts can aid in the increase of power density by reducing the temperature increase per unit volume of heat-generation. Experimental test results corresponded closely with the theoretically expected allowed increase in heat-generation that could be accommodated due to the heat-extraction action of the inserts. As an experimental system, insertion of aluminium nitride into ferrite in an integrated electromagnetic power passive module was investigated. An increase of 187% in the effective power density could be achieved due to the presence of aluminium nitride heat-extraction layers embedded into ferrite. Preliminary magnetic flux density optimisation, in terms of the volume fraction occupied by a parallel-layered heat-extraction system, was performed for a wide range of heat-extraction materials, and interfacial resistance values.**Full Text:**

**Authors:**Dirker, Jaco**Date:**2008-11-19T05:28:30Z**Subjects:**Solid-state electronics , Power electronics , Heat conduction , Heat transmission**Type:**Thesis**Identifier:**uj:14733 , http://hdl.handle.net/10210/1739**Description:**D. Ing. , Power electronics refers to electronic processing of electrical power. In this process the electrical power is controlled by switching of power semiconductor devices as well as electromagnetically stored in electrical and magnetic fields while the energy flow is directed through selective conduction paths. The losses/power-efficiency of all these actions is of paramount importance in the processing. The lack for standardisation and the absence of a modular approach is a barrier to the development of more compact systems. Recently more research resources have been invested in development of integrated power electronic modules as an attempt to solve this problem. By integrating power electronic components, an increase in the power density is achieved, which unfortunately also leads to higher internal heat-generation and higher operating temperatures. This has an unfavourable effect on electronic behaviour and the reliability of the structures. In order to maintain the advances made in volume reduction of integrated power electronics, efficient and cost effective methods for removing heat is of essence. In this investigation the performance of rectangular cross-section embedded solid-state heatextraction inserts to increase thermal heat spreading and the reduction of steady-state peak temperatures was evaluated theoretically and experimentally. Theoretically, the cross sectional aspect ratio of such inserts was thermally optimised for a wide range of dimensional, thermal, and material property conditions. Possible materials investigated for use as heat extractors in power electronics include aluminium nitride, beryllium oxide, and synthetic diamond. The presence of interfacial thermal resistance was theoretically found to have a significant detrimental influence on the thermal performance of an integrated heat-extraction system and should be minimised as far as possible. For conditions commonly found in integrated power passives, continuous embedded heatextraction layers are proposed. Theoretically it is shown that such inserts can aid in the increase of power density by reducing the temperature increase per unit volume of heat-generation. Experimental test results corresponded closely with the theoretically expected allowed increase in heat-generation that could be accommodated due to the heat-extraction action of the inserts. As an experimental system, insertion of aluminium nitride into ferrite in an integrated electromagnetic power passive module was investigated. An increase of 187% in the effective power density could be achieved due to the presence of aluminium nitride heat-extraction layers embedded into ferrite. Preliminary magnetic flux density optimisation, in terms of the volume fraction occupied by a parallel-layered heat-extraction system, was performed for a wide range of heat-extraction materials, and interfacial resistance values.**Full Text:**

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