Polydimethylsiloxane (PDMS)-based microfluidic channel with integrated commercial pressure sensors

The precise characterisation of boiling in microchannels is essential for the optimisation of applications requiring two phase cooling. In this paper polydimethylsiloxane (PDMS) is employed to make microchannels for characterising microboiling. In particular the material properties of PDMS facilitate rapid prototyping and its optical transparency provides the capability to directly view any fluid flow. The production of microchannels is complicated by the need to integrate custom made sensors. This paper presents a PDMS microfluidic device with integrated commercial pressure sensors, which have been used to perform a detailed characterisation of microboiling phenomena. The proposed approach of integrating commercial pressure sensors into the channel also has potential applications in a range of other microsystems

Experimental study on local heat transfer coefficients and the effect of aspect ratio on flow boiling in a microchannel

Flow boiling in integrated microchannel systems is a cooling technology that has received significant attention in recent years as an effective option for high heat flux microelectronic devices as it provides high heat transfer and small variations in surface temperature. However, there are still a number of issues to be addressed before this technology is used for commercial applications. Amongst the issues that require further investigation are the two-phase heat transfer enhancement mechanisms, the effect of channel geometry on heat transfer characteristics, two-phase flow instabilities, critical heat flux and interfacial liquid-vapour heat transfer in the vicinity of the wall. This work is an experimental study on two-phase flow boiling in multi- and single-rectangular microchannels. Experimental research was performed on the effect of the channel aspect ratio and hydraulic diameter, particularly for parallel multi-microchannel systems in order to provide design guidelines. Flow boiling experiments were performed using deionised water in silicon microchannel heat sinks with width-to-depth aspect ratios (�) from 0.33 to 3 and hydraulic diameters from 50 μm to 150 μm. The effect of aspect ratio on two-phase flow boiling local heat transfer coefficient and two-phase pressure drop was investigated as well as the two-phase heat transfer coefficients trends with mass flux for the constant heat fluxes of 151 kW m-2, 183 kW m- 2, 271 kW m-2 and 363 kW m-2. Wall temperature measurements were obtained from five integrated thin nickel film temperature sensors. An integrated thin aluminium heater enabled uniform heating with a small thermal resistance between the heater and the channels. The microfabricated temperature sensors were used with simultaneous high-speed imaging and pressure measurements in order to obtain a better insight related to temperature and pressure fluctuations caused by two-phase flow instabilities under uniform heating in parallel microchannels. The results demonstrated that the aspect ratio of the microchannels affects flow boiling heat transfer coefficients. However, there is not clear trend of the aspect ratio on the heat transfer coefficient. Pressure drop was found to increase with increasing aspect ratio. Wide microchannels but not very shallow, with � = 1.5 and �ℎ = 120 μm, have shown good heat transfer performance, by producing modest two-phase pressure drop of maximum 200 mbar for the highest heat flux and heat transfer coefficients of 200 kW m-2 during two-phase flow boiling conditions. For the high aspect ratio, values of 2 and 3 two-phase flow boiling heat transfer coefficients were measured to be lower compared to aspect ratio of 1.5. Microchannels with aspect ratios higher than 1.5 produced severe wall temperature fluctuations for high heat fluxes that periodically reached extreme wall temperature values in excess of 250 ˚C. The consequences of these severe wall temperature and pressure fluctuations at high aspect ratios of 2 and 3 resulted in non-uniform flow distribution and temporal dryout. Abrupt increase in two-phase pressure drop occurred for � > 1.5. The effect of the inlet subcooling was found to be significant on both heat transfer coefficient and pressure drop. Furthermore, the effects of bubble growth on flow instabilities and heat transfer coefficients have been investigated. Although the thin film nickel sensors provide the advantage of much faster response time and smaller thermal resistance compared to classic thermocouples, they do not allow for full two-dimensional wall temperature mapping of the heated surface. An advanced experimental method was devised in order to produce accurate two-dimensional heat transfer coefficient data as a function of time. Infrared (IR) thermography was synchronised with simultaneous high-speed imaging and pressure measurements from integrated miniature pressure sensors inside the microchannel, in order to produce two-dimensional (2D) high spatial and temporal resolution two-phase heat transfer coefficient maps across the full domain of a polydimethylsiloxane (PDMS) microchannel. The microchannel was characterised by a high aspect ratio (� = 22) and a hydraulic diameter of 192 μm. The PDMS microchannel was bonded on a transparent indium tin oxide (ITO) thin film coated glass. The transparent thin film ITO heater allowed the recording of high quality synchronised high - speed images of the liquid-vapour distribution. This work presents a better insight into the two-phase heat transfer coefficient spatial variation during flow instabilities with two-dimensional heat transfer coefficient plots as a function of time during the cycles of liquid-vapour alternations for different mass flux and heat flux conditions. High spatial and temporal resolution wall temperature measurements and pressure data were obtained for a range of mass fluxes from 7.37 to 298 kg m-2 s-1 and heat fluxes from 13.64 to 179.2 kW m-2 using FC-72 as a dielectric liquid. 3D plots of spatially averaged two-phase heat transfer coefficients at the inlet, middle and outlet of the microchannel are presented with time. The optical images were correlated, with simultaneous thermal images. The results demonstrate that bubble growth in microchannels differs from macroscale channels and the confinement effects influence the local two-phase heat transfer coefficient distribution. Bubble nucleation and axial growth as well as wetting and rewetting in the channel were found to significantly affect the local heat transfer physical mechanisms. Bubble level heat transfer coefficient measurements are important as previous researchers have experimentally investigated local temperature and high speed visualisation in bubbles during pool boiling conditions and not flow boiling. The effect of the confined bubble axial growth to the two-phase heat transfer coefficient distribution at the channel entrance was investigated at low mass fluxes and low heat fluxes. The 3D plots of the 2D two-phase heat transfer coefficient with time across the microchannel domain were correlated with liquid-vapour dynamics and liquid film thinning from the contrast of the optical images, which caused suspected dryout. The 3D plots of heat transfer coefficients with time provided fine details of local variations during bubble nucleation, confinement, elongated bubble, slug flow and annular flow patterns. The correlation between the synchronised high-resolution thermal and optical images assisted in a better understanding of the heat transfer mechanisms and critical heat flux during two-phase flow boiling in microchannels

Experimental and Numerical Investigation of Micro/Mini Channel Flow-Boiling Heat Transfer with Non-Uniform Circumferential Heat Fluxes at Different Rotational Orientations

Flow-boiling of Perfluorohexane (FC-72) in horizontal micro/mini channels was investigated experimen- tally and numerically at different rotational orientations in terms of gravity. One-sided uniform channel heating was considered experimentally for rotational angles ranging from 0 °(heating from below) to 180 °(heating from above) in increments of 30 °. The micro/mini channel had a high aspect ratio of 10 (5 mm x 0.5 mm) and a hydraulic diameter of 909 μm. In-channel flow visualisations were recorded and heat transfer coefficients were determined for mass fluxes of 10, 20 and 40 kg/m 2 s at a saturation temperature of 56 °C. Suitable heat fluxes were applied to span the onset of nucleate boiling to near dry-out conditions within the channel. It was found that the rotational angle had a significant influence on the heat transfer performance due to its influence on bubble detachment. Bottom-heated cases (0 °orientation) resulted in local heat transfer coefficients that were up to 201% higher than for any other rotational orientation. Channel orientations of 60 °(slanted heating surface) and 90 °(heating from the side) generally produced the lowest local heat transfer coefficients. Insight into the influence of the grav- itational orientation on single-bubble growth within the nucleation and detachment region was obtained via two- and three-dimensional numerical simulations. Bubble behaviour after detachment and its effect on heat transfer were also investigated transiently until detachment. The numerical simulations mirrored the experimental trends and it was found that the presence of growing bubbles interrupted the velocity streamlines and the thermal boundary layer downstream of the nucleation sit

Ébullition dans les micro canaux : influence du rapport d'aspect sur le transfert thermique local

Flow boiling in microchannels is a promising technology for cooling of small-scale devices such as electronic chips, power rectifiers, radar arrays, chemical microreactors that require dissipating heat fluxes of several MW m-2 while maintaining constant temperature at the surface. Although flow boiling in macroscale provides higher performance than single-phase or pool boiling heat transfer, the advantages in microscale have not been yet completely justified. This study aims to assist in the better understanding of some outstanding issues regarding flow instabilities, two-phase heat transfer mechanisms and early dryout that occur in microchannels while increasing their aspect ratios (α). Fully integrated and instrumented silicon multimicrochannel heat sinks of width-to-height aspect ratios from 0.3 to 3 and hydraulic diameters ℎ from 50 to 150 μm were developed in order to fully characterise their local heat transfer performance during flow boiling. Local wall temperature measurements were obtained from five thin nickel film temperature sensors with simultaneous pressure measurements and flow visualisation from the top. Uniform heating was achieved with a thin aluminium heater integrated at the back of the microchannels. The effect of , mass flux, inlet subcooling temperature and bubble dynamics on two-phase flow boiling local heat transfer coefficient and pressure drop were investigated for constant heat fluxes. Severe pressure and temperature fluctuations in excess of 250 °C were measured at high microchannels. The heat sinks with microchannels of = 1.5 and ℎ = 120 μm, achieved the maximum heat transfer performance. High spatial and temporal resolution wall temperature maps were obtained with advanced thermography technique, synchronised with simultaneous high-speed imaging and pressure measurements from integrated miniature piezoresistive pressure sensors inside a high aspect ratio (α= 22) transparent Polydimethylsiloxane (PDMS)-based microchannel of ℎ =192 μm. The aim was to produce accurate two-dimensional (2D) high spatial and temporal resolution two-phase heat transfer coefficient maps across the full domain of a single microchannel using FC-72 dielectric liquid. The novel PDMS based microchannel provided measurements in the vicinity of the wall due to the transparency of PDMS to midwave infrared radiation. Synchronised flow visualisation images were related with liquid-vapour distribution of the channel base and were correlated with the two-phase heat transfer coefficient maps in order to elucidate flow boiling instabilities, film thinning during bubble confinement and wetting / rewetting phenomena during annular flow pattern.L’ébullition en micros canaux est une technique de refroidissement très prometteuse pour les composants en microélectronique. Ces derniers, de plus en plus miniaturisés, nécessitent souvent la dissipation de densités de flux importantes, pouvant atteindre quelques MW m-2 pour maintenir des températures acceptables. L’objet de cette étude est de mieux comprendre les instabilités des écoulements, le transfert thermique par changement de phase tout comme l’effet des rapports de forme (a) sur l’apparition de sites de nucléation à la surface de micro canaux. L’analyse des échanges convectifs locaux lors de l’ébullition a été réalisée dans le cas de micro canaux de rapport de forme allant de 0.3 à 3 et de diamètres hydrauliques allant de 50 à 150 μm. Le banc d’essai a été instrumenté de manière à pourvoir mesurer simultanément les températures de surface à l’aide capteurs en film mince de nickel, de capteurs de pressions instantanées et de caméra rapide pour la visualisation du phénomène d’ébullition. Le chauffage du fluide a été réalisé à l’aide du dépôt en couche mince d’un film résistif en aluminium directement appliqué à la surface des micro canaux. L’étude expérimentale a permis d’analyser les phénomènes de changement de phase par ébullition, du transfert thermique local ainsi que la chute de pression de l’écoulement. En particulier, le travail expérimental a permis de mettre en évidence les effets sur le transfert, du rapport de forma a, de la température de sous refroidissement du fluide à l’entrée des canaux et de la dynamique de formation et de grossissement des bulles. Des fluctuations importantes de pression et de températures ont été enregistrées pour des températures de surfaces avoisinant les 250 °C. Les micro canaux avec a= 1.5 et Dh =120 μm, correspondent à la configuration la plus performante. Les mesures par thermographie infrarouge (IR) combinées à la visualisation par caméra rapide et aux mesures des fluctuations de pressions par capteurs piézoresitifs, ont été réalisées dans le cas de canaux en Polydimethylsiloxane (PDMS) de grand rapport de forme (a = 22) et de diamètre hydraulique Dh =192 μm. L’objectif était d’identifier des cartographies bidimensionnelles et instationnaires de coefficients d’échanges convectifs dans le cas d’un micro canal utilisant un fluide diélectrique le FC -72. La double visualisation par thermographie infrarouge et par caméra CCD rapide a permis de corréler la dynamique de l’ébullition, et notamment le grossissement des bulles, l’asséchement et ou le mouillage des parois, aux coefficients d’échanges convectifs locaux

Local heat transfer measurements and aspect ratio influence during flow boiling in microscale

L’ébullition en micros canaux est une technique de refroidissement très prometteuse pour les composants en microélectronique. Ces derniers, de plus en plus miniaturisés, nécessitent souvent la dissipation de densités de flux importantes, pouvant atteindre quelques MW m-2 pour maintenir des températures acceptables. L’objet de cette étude est de mieux comprendre les instabilités des écoulements, le transfert thermique par changement de phase tout comme l’effet des rapports de forme (a) sur l’apparition de sites de nucléation à la surface de micro canaux. L’analyse des échanges convectifs locaux lors de l’ébullition a été réalisée dans le cas de micro canaux de rapport de forme allant de 0.3 à 3 et de diamètres hydrauliques allant de 50 à 150 μm. Le banc d’essai a été instrumenté de manière à pourvoir mesurer simultanément les températures de surface à l’aide capteurs en film mince de nickel, de capteurs de pressions instantanées et de caméra rapide pour la visualisation du phénomène d’ébullition. Le chauffage du fluide a été réalisé à l’aide du dépôt en couche mince d’un film résistif en aluminium directement appliqué à la surface des micro canaux. L’étude expérimentale a permis d’analyser les phénomènes de changement de phase par ébullition, du transfert thermique local ainsi que la chute de pression de l’écoulement. En particulier, le travail expérimental a permis de mettre en évidence les effets sur le transfert, du rapport de forma a, de la température de sous refroidissement du fluide à l’entrée des canaux et de la dynamique de formation et de grossissement des bulles. Des fluctuations importantes de pression et de températures ont été enregistrées pour des températures de surfaces avoisinant les 250 °C. Les micro canaux avec a= 1.5 et Dh =120 μm, correspondent à la configuration la plus performante. Les mesures par thermographie infrarouge (IR) combinées à la visualisation par caméra rapide et aux mesures des fluctuations de pressions par capteurs piézoresitifs, ont été réalisées dans le cas de canaux en Polydimethylsiloxane (PDMS) de grand rapport de forme (a = 22) et de diamètre hydraulique Dh =192 μm. L’objectif était d’identifier des cartographies bidimensionnelles et instationnaires de coefficients d’échanges convectifs dans le cas d’un micro canal utilisant un fluide diélectrique le FC -72. La double visualisation par thermographie infrarouge et par caméra CCD rapide a permis de corréler la dynamique de l’ébullition, et notamment le grossissement des bulles, l’asséchement et ou le mouillage des parois, aux coefficients d’échanges convectifs locaux.Flow boiling in microchannels is a promising technology for cooling of small-scale devices such as electronic chips, power rectifiers, radar arrays, chemical microreactors that require dissipating heat fluxes of several MW m-2 while maintaining constant temperature at the surface. Although flow boiling in macroscale provides higher performance than single-phase or pool boiling heat transfer, the advantages in microscale have not been yet completely justified. This study aims to assist in the better understanding of some outstanding issues regarding flow instabilities, two-phase heat transfer mechanisms and early dryout that occur in microchannels while increasing their aspect ratios (α). Fully integrated and instrumented silicon multimicrochannel heat sinks of width-to-height aspect ratios from 0.3 to 3 and hydraulic diameters ℎ from 50 to 150 μm were developed in order to fully characterise their local heat transfer performance during flow boiling. Local wall temperature measurements were obtained from five thin nickel film temperature sensors with simultaneous pressure measurements and flow visualisation from the top. Uniform heating was achieved with a thin aluminium heater integrated at the back of the microchannels. The effect of , mass flux, inlet subcooling temperature and bubble dynamics on two-phase flow boiling local heat transfer coefficient and pressure drop were investigated for constant heat fluxes. Severe pressure and temperature fluctuations in excess of 250 °C were measured at high microchannels. The heat sinks with microchannels of = 1.5 and ℎ = 120 μm, achieved the maximum heat transfer performance. High spatial and temporal resolution wall temperature maps were obtained with advanced thermography technique, synchronised with simultaneous high-speed imaging and pressure measurements from integrated miniature piezoresistive pressure sensors inside a high aspect ratio (α= 22) transparent Polydimethylsiloxane (PDMS)-based microchannel of ℎ =192 μm. The aim was to produce accurate two-dimensional (2D) high spatial and temporal resolution two-phase heat transfer coefficient maps across the full domain of a single microchannel using FC-72 dielectric liquid. The novel PDMS based microchannel provided measurements in the vicinity of the wall due to the transparency of PDMS to midwave infrared radiation. Synchronised flow visualisation images were related with liquid-vapour distribution of the channel base and were correlated with the two-phase heat transfer coefficient maps in order to elucidate flow boiling instabilities, film thinning during bubble confinement and wetting / rewetting phenomena during annular flow pattern

Translation as a means of introducing themes and ideology in Konstantin Thotokis' original literary work

This dissertation examines the literary translations of Konstantin Theotokis (1872-1923), a Corfiot writer, poet and translator. Theotokis translated from many different languages as well as a wide range of genres. For instance, his work includes excerpts from the Bhagavat Ghita and the Ramayana, plays by Kalidasha, Shakespeare and Aristophanes, poetry by Pindar, Horatius, Catullus, Goethe, Schiller and Heine, short texts by Dostoyevsky, Tolstoy, Turgenev and Gorki, ancient Greek and Latin philosophy (Plato, Vergil, Lucretius), Gustav Flaubert’s Madame Bovary. Theotokis also produced ample original literary work, much of which holds a significant place in modern Greek literature. The fact that his translations comprise more than half of the total bulk of his writings, and cover the whole period of his life and work as an author, indicates that he viewed translation as a process parallel with literary writing and even vital to it. Although he is traditionally considered as a naturalist, his work also bears influences from romanticism, Indian literature and philosophy, ancient Greek and Latin poets and so on, so it is evident that he has drawn inspiration from the texts he translated. In my thesis, apart from examining the choices Theotokis made in his translations (in terms of language, metre, style etc.), I also demonstrate how they formed and transformed his literary identity and how they served as a means of introducing certain themes and ideological elements into modern Greek literature. In addition, the thesis discusses Theotokis’ affiliation to the Heptanesian School (a group of scholars and authors based mainly in Corfu) and his ideological and literary kinship with Dionysios Solomos, the national poet of Greece. Finally, an important aspect of this dissertation is that it explores the use of the demotic Greek by Theotokis, his attitude towards the Language Issue and his linguistic theory.Στόχοι της παρούσας διατριβής είναι οι εξής: Πρώτο, να μελετηθεί και να αξιολογηθεί με επιστημονικούς όρους το σύνολο της μεταφραστικής εργασίας του Θεοτόκη, κάτι που ως τώρα έχει γίνει μόνο σποραδικά και μόνο για μεμονωμένα μεταφράσματα. Δεύτερο, να τοποθετηθεί η μεταφραστική εργασία του Θεοτόκη στο ιστορικό της πλαίσιο και να αναδειχθεί η συνάφειά της με το μεταφραστικό πρόγραμμα του σολωμικού κύκλου και των συνεχιστών του. Τρίτο, να γίνει διαχρονική μελέτη σύνολου του έργου του Θεοτόκη, μεταφραστικού και πρωτότυπου, ώστε να αναδειχθούν οι επιρροές του από όλα τα λογοτεχνικά και ιδεολογικά ρεύματα. Τέταρτο, να μελετηθούν συστηματικά οι διακειμενικές σχέσεις μεταξύ πρωτότυπου και μεταφραστικού έργου του Θεοτόκη, με σκοπό να αναδειχθεί η σημαντική συμβολή της λογοτεχνικής μετάφρασης στη δημιουργία πρωτότυπης λογοτεχνίας. Όλα τα παραπάνω έχουν ως τελικό σκοπό τη σύνθεση μιας ολοκληρωμένης εικόνας της συγγραφικής ταυτότητας του Θεοτόκη, θεωρώντας ως αναπόσπαστο κομμάτι της την ιδιότητα του μεταφραστή