Oscillating heat pipes (OHPs) have been actively investigated since their inception due to their ability to manage high heat/heat fluxes. The OHP is a passive, wickless, two-phase heat transfer device that relies on pressure driven fluid oscillations within a hermetically-sealed serpentine channel structure. The cyclic phase-change heat transfer drives additional sensible heat transfer, and this combination causes OHPs to have high effective thermal conductivities. Many strides have been made, through both experimentation and modeling, to refine the design and implementation of OHPs. However, the main objective in OHP research has been to better understand the thermodynamic and fluid mechanic phenomena so as to enhance OHPs' thermal performance. The current work presents methods for using OHP in thermal-to-electric energy harvesting, which would allow for ‘dual-purpose’ OHP applications in which thermal management can be combined with work output. Energy harvesting occurred when a portion of the thermally-driven fluidic motion was used to generate a voltage either by electromagnetic induction or by a piezoelectric transducer imbedded in an OHP tube. For the induction approach, two methods were used to create the time-varying magnetic field required for induction. In the first, a ferrofluid was used as the OHP working fluid. Because the magnetic dipoles of the nanoparticles are randomly aligned naturally, two static, external ‘bias’ magnets were required to create a uniform magnetic field to align the particle dipoles for a non-zero magnetic flux change through a coaxial solenoid. The second method used a small rare-earth magnet confined inside a set length of an OHP channel that had a coaxial solenoid. As the OHP working fluid moved inside the harvesting channel, a portion of the fluid's momentum was transferred to the magnet, causing it to oscillate. For the piezoelectric approach, a narrow piezoelectric transducer was placed in a bow-shaped configuration along the inside of an OHP channel. Passing fluid would deflect the piezo creating a potential difference across its leads, which protruded out of the channel walls. All three of these methods successfully produced a voltage while retaining the excellent thermal performance synonymous with OHPs.

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La miniaturisation toujours plus poussée des composants électroniques génère des contraintes thermiques de plus en plus importantes. Les boucles diphasiques à pompage thermo-capillaire ou LHP suscitent actuellement un intérêt croissant en raison de leurs bonnes performances thermiques, de leur fiabilité et de leur géométrie permettant une grande souplesse d’implantation. Cependant, une meilleure compréhension des phénomènes en jeu dans ces systèmes est essentielle pour optimiser leur conception et prédire leur comportement de manière fiable. Dans ce travail, un modèle analytique est développé qui intègre les paramètres de fonctionnement d’une LHP, afin d’étudier leur influence en fonction des conditions opératoires. Son originalité principale réside dans la détermination précise de la répartition des différents flux thermiques dans l’évaporateur. Une étude de sensibilité est menée pour évaluer les influences de la résistance de contact entre la structure capillaire poreuse et l’enveloppe de l’évaporateur, de la conductivité thermique équivalente du matériau poreux, du coefficient d’accommodation lié aux transferts de chaleur par évaporation et des coefficients de transfert thermique entre la paroi et le milieu ambiant ou la source froide. Cette analyse montre que les paramètres mentionnés ci-dessus peuvent être estimés individuellement, en comparant le modèle à des données expérimentales judicieusement choisies. Un banc expérimental a également été conçu et fabriqué. Partiellement transparent, il permet l’observation de la position des phases liquide et vapeur au cours du fonctionnement. Les influences de la puissance thermique appliquée, de la présence de gaz incondensables et de la température de la source froide sont analysées. Aux puissances thermiques élevées, un régime d’ébullition nucléée est observé dans le réservoir, qui se traduit par une augmentation des flux parasites vers le réservoir donc une dégradation des performances de la LHP. Plusieurs phénomènes oscillatoires sont également observés et corrélés aux observations visuelles des écoulements. Enfin, différents régimes de condensation sont observés et les mécanismes conduisant au détachement des bulles dans le condenseur sont décrits. Un modèle numérique a été développé, afin de simuler le comportement du banc expérimental en se rapprochant au plus près de ses caractéristiques géométriques et thermophysiques. La comparaison entre les prédictions du modèle et les données expérimentales montre les carences des modèles de pertes de charges dans les écoulements diphasiques, pour la configuration étudiée. Les transferts de chaleur et de masse dans l’évaporateur sont analysés, ainsi que l’influence de l’apparition de l’ébullition dans le réservoir et celle de la conductivité thermique de l’enveloppe de l’évaporateur. Les résultats mettent également en évidence l’importance de la conduction thermique longitudinale dans les canalisations dans le cas d’un matériau conducteur. / The increasing development of electronics leads to higher constraints regarding their thermal management. Loop heat pipes (LHP) become more and more attractive because they offer thermal efficiency, reliability and large implementation flexibility. However, a better understanding of the physical phenomena involved within them is required in order to optimise their design and predict accurately their operation. An analytical model is developed to highlight the main parameters of a LHP and their influence depending on the operating conditions. Its main originality lies in a thorough consideration of heat transfer in the evaporator. A sensitivity analysis is conducted to study the influence of the contact thermal resistance between the wick and the body of the evaporator, of the effective thermal conductivity of the wick, of the accommodation coefficient linked to the evaporation heat transfer and of the heat transfer with the ambient and with the heat sink. This analysis shows that these parameters can be individually and separately estimated by comparing the model to a set of well-chosen experimental data. An experimental setup is designed and built. It is partially transparent, to observe the location of the liquid and vapour phases in operating conditions. The effects of the heat input, non-condensable gases and of the heat sink temperature are discussed. Nucleate boiling is observed inside the reservoir for high heat fluxes. This phenomenon increases significantly the parasitic heat flux towards the reservoir and therefore decreases the performance of the LHP. Several oscillating phenomena are also observed and correlated to the flow patterns. Finally, distinct condensation regimes are investigated and the mechanisms leading to the bubble detachment in the condenser are discussed. A numerical model is developed in accordance with the geometrical and thermophysical characteristics of the experimental setup. The model is compared with the experimental data. The comparison shows the lack of accuracy of the two-phase pressure drops models in this configuration. Heat and mass transfer in the evaporator are discussed and the effects of boiling in the reservoir and of the thermal conductivity inside the evaporator casing are investigated. The results highlight the importance of the longitudinal thermal conduction inside the tube in the case of conductive materials.

Designing turbines for either aerospace or power production is a daunting task for any heat transfer scientist or engineer. Turbine designers are continuously pursuing better ways to convert the stored chemical energy in the fuel into useful work with maximum efficiency. Based on thermodynamic principles, one way to improve thermal efficiency is to increase the turbine inlet pressure and temperature. Generally, the inlet temperature may exceed the capabilities of standard materials for safe and long-life operation of the turbine. Next generation propulsion systems, whether for new supersonic transport or for improving existing aviation transport, will require more aggressive cooling system for many hot-gas-path components of the turbine. Heat pipe technology offers a possible cooling technique for the structures exposed to the high heat fluxes. Hence, the objective of this dissertation is to develop new radially rotating heat pipe systems that integrate multiple rotating miniature heat pipes with a common reservoir for a more effective and practical solution to turbine or compressor cooling. In this dissertation, two radially rotating miniature heat pipes and two sector heat pipes are analyzed and studied by utilizing suitable fluid flow and heat transfer modeling along with experimental tests. Analytical solutions for the film thickness and the lengthwise vapor temperature distribution for a single heat pipe are derived. Experimental tests on single radially rotating miniature heat pipes and sector heat pipes are undertaken with different important parameters and the manner in which these parameters affect heat pipe operation. Analytical and experimental studies have proven that the radially rotating miniature heat pipes have an incredibly high effective thermal conductance and an enormous heat transfer capability. Concurrently, the heat pipe has an uncomplicated structure and relatively low manufacturing costs. The heat pipe can also resist strong vibrations and is well suited for a high temperature environment. Hence, the heat pipes with a common reservoir make incorporation of heat pipes into turbo-machinery much more feasible and cost effective.

Thesis (MSc)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: A tunable narrow-bandwidth vacuum ultra violet laser source was developed and characterised. Two-photon resonant four-wave sum-frequency mixing of two pulsed dye laser beams in magnesium vapour was used to generate the VUV laser light. A heat pipe oven with a concentric design was incorporated to provide a magnesium vapour column of around 30 cm in length with a sufficiently stable temperature and appropriate vapour pressure for efficient VUV production. This is a longer nonlinear medium length than previously produced in our laboratory using a crossed heat pipe oven. The longer medium facilitated the production of VUV laser light of higher intensity than was previously obtainable. High resolution laser induced fluorescence spectra of carbon monoxide in a supersonic gas jet was recorded using the tunable VUV laser light produced in the crossed heat pipe oven. Experimental parameters were optimised and adjusted for the selective detection of the forbidden singlet-triplet transitions which typically have longer lifetimes than singlet-singlet transitions. Transitions from the X¹Σ⁺(v = 0) ground state to the e³Σ⁻( v = 5) triplet state were recorded, and accurate wavelength for the spectral lines were determined. Laboratory wavelengths for these lines have not been measured previously. Accurate wavelength for the weak forbidden spectral lines of CO are important in astrophysical applications, for example determining column densities of interstellar gas clouds. / AFRIKAANSE OPSOMMING: 'n Afstembare smal bandwydte vakuum ultraviolet laser bron is ontwikkel en gekarakteriseer. Twee-foton resonante vier-golf som-frekwensie vermenging van twee gepulseerde kleurstoflaserbundels in 'n magnesium damp is gebruik om die VUV laser lig te produseer. 'n Hittepyp oond met 'n konsentriese ontwerp is in gebruik geneem om a magnesium damp kollom van ongeveer 30 cm in lengte te voorsien waarvan die temperatuur voldoende stabiel is en die dampdruk toepaslik is vir effektiewe VUV produksie. Dit is 'n langer nie-liniêre medium as wat in die verlede deur 'n kruis-hittepyp oond voorsien is. Die langer medium het dit moontlik gemaak om VUV laser lig van hoër intensiteit te produseer as wat tot dusver bereikbaar was. Hoë resolusie laser geinduseerde fluoresensie spektra van koolstof monoksied in a supersoniese gasstraal is opgeneem met die hulp van die afstembare VUV laser lig geproduseer in die kruis-hittepyp oond. Eksperimentele parameters is geoptimeer en verstel vir die selektiewe waarneming van die verbode singlet-triplet oorgange wat tipies langer leeftye besit in vergelyking met singlet-singlet oorgange. Oorgange vanaf die X¹Σ⁺(v = 0) grond toestand na die e³Σ⁻( v = 5) triplet toestand is opgeneem en akkurate golflengtes vir die spektrale lyne is bepaal. Laboratorium golflengtes het tot dusver nie bestaan vir hierdie lyne nie. Akkurate golflengtes vir die swak verbode spektrale lyne van CO het belangrike toepassings in astrofisika soos die bepaling van die kollom digtheid van interstellêre gas wolke.

This thesis presents an analytical and experimental investigation into the capillary wicking limitation of an array of pillars. Commercial and nanopillar wicks are examined experimentally to assess the effects of micro and nanoscale capillary forces. By exerting a progressively higher heat flux on the wick, a maximum achievable mass flow was observed at the capillary limit. Through the balance of capillary and viscous forces, an ab initio analytical model is also presented to support the experimental data. Comparison of the capillary limit predicted by the analytical model and actual limit observed in experimental results are presented for three baseline wicks and two nanowicks. / text

Impulse technology is a process in which water is removedfrom a wet paper web by the combined action of mechanicalpressure and intense heat. This results in increased dewateringrates, increased smoothness on the roll side of the sheet, andincreased density. Although the potential benefits of impulsepressing have been debated over the past thirty years, itsindustrial acceptance has been prevented by web delamination,which is defined as a reduction in the z-directional strengthof paper. This thesis deals with the mechanism of heat transfer withphase change during impulse pressing of wet paper. The resultsof four complementary experimental studies suggest that littleor no steam is formed in an impulse nip prior to the point ofmaximum applied load. As the nip is unloaded and the hydraulicpressure decreases, hot liquid water flashes to steam. Weadvance the argument that the force expressed upon flashing canbe used to displace liquid water, in a mechanism similar tothat originally proposed by Wahren. Additionally, modelexperiments performed in a novel experimental facility suggestthat the strength of flashing-assisted displacement dewateringcan be maximized by controlling the direction of steam venting.If this solution could be exploited in a commercially viableimpulse press, delamination would cease to be an issue ofconcern. The thesis includes a study of the web structure ofdelaminated paper. Here, we characterized delaminated paper bythe changes in transverse permeability and cross-sectionalsolidity profiles measured as a function of pressingtemperature. We found no evidence that wet pressing and impulsepressing induced stratification in non-delaminated sheets andconcluded that the parabolic solidity profiles observed weredue to capillary forces present during drying. Further, thepermeability of mechanically compressed never-dried samples wasfound to be essentially constant for pressing temperatureslower than the atmospheric boiling point of water and toincrease significantly at higher pressing temperatures. Wepropose this to be a result of damage to the cell wall materialdue to flashing of hot liquid water in the fiber walls andlumina. Finally, we present a method and an apparatus formeasurement of the thermal properties of water-saturated paperwebs at temperatures and pressures of interest for commercialhigh-intensity processes. After validation, the method wassuccessfully applied to measure the thermal conductivity,thermal diffusivity and volumetric heat capacity ofwater-saturated blotter paper as functions of temperature andsolids content. Here, we found that the thermal conductivityincreased with solids content in the range from 30%\ to 55%,which is in conflict with the commonly stated assumptions of adecreasing trend. We propose that this discrepancy could be dueto the thermal conductivity of air-free fibers wetted byunpressable water only, being significantly different from thatof dry cellulose.

The heat transfer characteristics of airflows in serpentine cooling channels in stator vanes of gas turbines and the novel QuTech® Heat Pipe (QTHP) for electronic cooling applications were studied. The cooling channels are modeled as smooth and roughened two-pass trapezoidal channels with a 180° turn over a range of Reynolds numbers between about 10,000 and 60,000. The naphthalene sublimation technique and the heat and mass transfer analogy were applied. The results showed that there was a very large variation of the local heat (mass) transfer distribution in the turn and downstream of the turn. The local heat (mass) transfer was high near the end wall and the downstream outer wall in the turn and was relatively low in two regions near the upstream outer wall and the downstream edge at the tip of the divider wall in the turn. The variation of the local heat (mass) transfer was larger with ribs on two opposite walls than with smooth walls. The regional average heat (mass) transfer was lower in the turn and higher in the entire channel with the flow entering the channel through the larger straight section than when the flow was reversed. The pressure drop across the turn was higher with the flow entering the channel through the larger channel than when the flow was reversed. Thermal performance of the QuTech® Heat Pipe was identified over a range of inclination angles between 90° and -90° and thermal mechanism of the QTHP was studied with GC-MS, ICP-OES, XRD, XPS, and DSC. This study resulted in the following findings: the performance of the QTHP was severely dependent on gravity; the QTHP utilizes water as working fluid; there were inorganic components such as Na, K, P, S, and Cr, etc.; and the vaporization temperature of the working fluid (mostly water) was lower than the boiling temperature of pure water. This was due to the presence of inorganic salt hydrates in the QTHP. It may be concluded that thermal performance of heat pipes increases with additional latent heat of fusion energy and energy required to release water molecules from salt hydrates.

Heat pipe is a passive two phase device capable of transferring large rates of heat with a minimal temperature drop. It is a sealed tube with a wick structure lined in it and with a working fluid inside the tube. It consists of three parts: an evaporator, a condenser and an adiabatic section. The heat pipes are widely used in electronics cooling and spacecraft applications. Although they can transfer large rate of heat in a short range, they have operating limits, namely: the capillary limit, the viscous limit, the entrainment limit, the sonic limit and the boiling limit. These limits determine the heat transfer capacity of the heat pipe. The properties of the working fluid, the structure of the wick, the orientation of the pipe, the length and the diameter of the tube etc. are the parameters that affect the limits. In this study, an analytical 1-D heat pipe model is formed and a computer code is prepared in order to analyze the effects of the parameters on the heat transfer capacity of a heat pipe. Water, Ammonia and Mercury are investigated as working fluids for different operating temperature ranges. The software is tested for a typical application for each working fluid.