Multi-Evaporator Closed Loop Thermosyphon

A novel type of multi-evaporator Closed-Loop Two-Phase Thermosyphon has been designed and tested at different inclinations and heat input levels. The device consists in an aluminum tube (I.D./O.D. 3/5 mm), bended into a planar serpentine with five U-turns in the heated zone, with a 50 mm transparent section for the purpose of visualization. The tube is closed in a loop, evacuated and partially filled with FC-72, 50% vol. Each turn is equipped with an electric wiring heater and the peculiar location of the heating sections causes the fluid to circulate regularly in a preferential direction. The condenser zone is embedded into a heat-sink and cooled by fans blowing air at 20 °C. Sixteen T-type thermocouples are located on the external tube wall in the evaporator and condenser zones, while the fluid pressure is measured in the condenser zone adjacent to the transparent tube. The flow pattern is recorded by means of a high-speed camera, the device operational limits (start-up and dry-out heating levels at the different inclinations) are detected and the overall thermal performance is calculated for different inclination angles and heat power inputs. Thanks to the fluid flow motion stabilization in a preferential direction due to the peculiar position of the heating elements, in vertical position the device is able to dissipate heat fluxes higher than 57% with respect to the CHF limit for FC-72, keeping also the temperatures at the evaporator zone lower than 80 °C

A Novel Type of Multi-Evaporator Closed Loop Two Phase Thermo-syphon: Thermal Performance Analysis and Fluid Flow Visualization

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Faasimuutoslämmönvaihtimen soveltuvuus tehoelektroniikkakomponenttien jäähdytykseen

The development of power electronics continuously increases their power density and therefore creates challenges for their cooling. Conventional cooling methods, such as heatsinks, are no longer able to cope with the cooling of high heat densities, and therefore, new cooling methods are needed. Two-phase compact thermosyphon (COTHEX) cooling allows the cooling of higher heat densities without having to resort to using pumps. The standard COTHEX technology, however, cannot replace heatsinks due to their structure because it does not match with the cuboid shape of a heatsink. This thesis focuses on designing a thermosyphon concept for power electronics cooling that can replace a heatsink without the need to redesign the products in which they are installed. In this thesis, three thermosyphon based cooling elements were designed, and their structures were optimized to provide as effective cooling as possible with thermal simulation. The thermal performances of the optimized elements and a conventional heatsink of same size were then compared to determine the solution which most effectively transfers heat. The best thermosyphon cooling method was selected, and its features were studied in more detail. The most suitable thermosyphon cooling element substantially improved the cooling of the power electronic device, semiconductor, compared to a conventional heatsink of the same size. The selected thermosyphon construction was able to cool more effectively than the conventional heatsink at higher heat loads, higher surrounding temperatures and with lower air flows. The construction generates lower pressure drop and therefore allows higher air flow rates pass the finned structure of the cooling element. The coolant circulation enables stable heat distribution to the whole area of the baseplate in which the semiconductor is attached. Thus, thermosyphon technology provides heat transfer at the baseplate at a constant temperature, which offers many benefits. For example, even temperature distribution has a positive effect on the aging of semiconductor chips. This thesis developed a new thermosyphon type and analysed its thermal performance. The new thermosyphon gave such positive results that it is highly recommended to replace conventional heatsinks with thermosyphon technology in power electronics cooling.Tehoelektroniikan tehointensiteetti kasvaa jatkuvasti tekniikan kehittyessä mikä luo paineita tehoelektroniikan jäähdytykselle. Nykyisenä jäähdytysteknologiana käytetyt ripajäähdytteiset jäähdytyslevyt eivät enää kykene jäähdyttämään tarpeeksi tehokkaasti tehoelektronisia komponentteja ja siksi tarvitaan uusia jäähdytysmetodeja. Kompaktit faasimuutoslämmönvaihtimet (COTHEX) eli termosifonit mahdollistavat tehokkaamman jäähdytyksen ilman pumppuja. Jo käytössä oleva termosifonitekniikka ei kuitenkaan suoraan sovellu korvaamaan nykyisiä jäähdytyslevyjä, koska niiden muoto eroaa vahvasti suorakulmaisen särmiön muotoisista jäähdytyslevyistä. Tässä työssä suunnitellaan sellainen termosifonikonsepti tehoelektroniikan jäähdyttämiseen, joka voi korvata jäähdytyslevyn ilman että tuotetta, jonka sisälle se on asetettu, pitää muotoilla vahvasti uudelleen. Työhön ideoitiin kolme erilaista termosifonitekniikalla toimivaa jäähdytyselementtiä ja niiden rakenne optimoitiin lämpöteknisesti lämpösimulointiohjelmien avulla. Optimoitujen jäähdytyselementtien ja samankokoisen jäähdytyslevyn jäähdytystehoja verrattiin keskenään, jotta löydettäisiin tehokkain tapa siirtää lämpöä pois tehoelektronisesta komponentista. Paras termosifonitekniikalla toimiva elementti valittiin voittajakonseptiksi ja sen lämmönsiirrollisia piirteitä tutkittiin tarkemmin. Elementti paransi tehoelektronisen komponentin, puolijohdemoduulin, jäähdytystä merkittävästi, kun sitä verrattiin samankokoiseen jäähdytyslevyyn. Valittu termosifonirakenne jäähdytti paremmin niin korkeammissa lämpökuormissa ja ympäristön lämpötiloissa, kuin matalammissa jäähdytysilmamäärissä. Se tuottaa vähemmän vastapainetta ja siten päästää enemmän ilmaa lauhduttimen läpi perinteiseen jäähdytyslevyyn verrattuna. Termosifonin jäähdytysnesteen kierto mahdollistaa tasaisemman lämmön jakautumisen pohjalevyyn, johon puolijohdemoduuli on kiinnitettynä. Siten lämpötilajakauma pohjalevyssä on myös tasaisempi, mikä on todella toivottavaa puolijohteita käytettäessä. Termosifonitekniikan mahdollistama tasaisempi lämpötilajakauma pohjalevyssä esimerkiksi lisää puolijohteen käyttöikää. Tässä tutkimuksessa keksittiin uusi termosifonirakenne ja sen jäähdytyskykyä tutkittiin. Se tuotti merkittäviä parannuksia puolijohteiden lämmönsiirrossa, joten on hyvin suositeltavaa soveltaa lämpösifonitekniikkaa tehoelektroniikan jäähdytyksessä jäähdytyslevyjen korvaamiseksi

Irregular aluminium foam and phase change material composite in transient thermal management

PhD ThesisTraction systems generate high loads of waste heat, which need to be removed for efficient operations. A new transient heat sink is proposed, which is based on salt hydrate phase change material (PCM). The heat sink would absorb heat during the short stationary phase i.e. at stations in which the PCM melts, a process accelerated by aluminium foam as it increases the rate of heat transfer within the PCM. When the train moves, the PCM is solidified via a forced convection stack. This creates a passive and efficient thermal solution, especially once heat pipe is employed as heat conduit. At the outset, the characteristics of the foam needed to be accurately determined. The foam was uncommon as its pore morphology was irregular, therefore it was scanned in a medical computed tomography (CT) scanner, which allowed for the construction of a three dimensional (3D) model. The model accuracy was enhanced by software, resulting in an extremely useful analytical tool. The model enabled important structural parameters to be measured e.g. porosity and specific surface area, which were crucial for the subsequent thermal and fluid flow analyses. A defect dense region was also detected, the effect of which was further investigated. Interestingly in the volume devoid of this defect, the porosity and specific surface area were uniform. A test rig was constructed that mimicked liquid cooling (or in the planned application, heat pipe cooling) in power electronics. At the core was a heat sink of salt hydrate PCM, impregnated within the foam. The sink with its current specifications (with liquid cooling) was able to absorb a thermal load consistent from a group of 4-5 IGBTs, which dissipated a low power of 20W per module during stops. The heating period of 1600-3500s per cycle meant the sink could be fitted to intercity locomotives. The foam increased the effective thermal conductivity by a factor of 24, from 0.45 to 10.83 W/m.K. 3D volume averaged numerical simulation was validated by experiment, which could be used to facilitate scale up or redesign for further optimization. As well as a support structure for the storage component of the system, the foam could replace conventional fins in forced convection, adding value to the potential manufacturer of the system. Heat transfer coefficient calculation incorporated the actual surface area that was derived from the 3D model, a first for metal foam studies. Results have shown a good Nu/Re correlation, comparable with other metal foam works

HASTECS: Hybrid Aircraft: reSearch on Thermal and Electric Components and Systems

In 2019, transportation was the fastest growing sector, contributing to environmental degradation. Finding sustainable solutions that pollute less is a key element in solving this problem, particularly for the aviation sector, which accounts for around 2-3% of global CO2 emissions. With the advent of Covid-19, air traffic seems to have come to a fairly permanent halt, but this pandemic reinforces the need to move towards a "cleaner sky" and respect for the environment, which is the objective of the Clean Sky2 program (H2020 EU), the context in which the HASTECS project has been launched in September 2016

Local thermal cycles determination in thermosyphon-cooled traction IGBT modules reproducing mission profiles

6 páginas, 8 figuras.Temperature mapping in two IGBT modules cooled by a thermosyphon-based system is performed under realistic power mission profiles. The power mission profiles are inferred from a traction design tool results based on feedback data extracted from the field, in which the service line, the train characteristics, and its speed profile are taken into account. Thereby, the chips which are more prone to fail due to a temperature-activated failure are detected by means of the experienced thermal cycles.This work was carried out in the frame of the European project PORTES (Power Reliability for Traction Electronics, MTKI-CT-2004-517224).Peer reviewe