abstract: Near-field thermal radiation occurs when the distance between two surfaces at different temperatures is less than the characteristic wavelength of thermal radiation. While theoretical studies predict that the near-field radiative heat transfer could exceed Planck’s blackbody limit in the far-field by orders of magnitudes depending on the materials and gap distance, experimental measurement of super-Planckian near-field radiative heat flux is extremely challenging in particular at sub-100-nm vacuum gaps and few has been demonstrated. The objective of this thesis is to develop a novel thermal metrology based on AFM bi-material cantilever and experimentally measure near-field thermal radiation. The experiment setup is completed and validated by measuring the near-field radiative heat transfer between a silica microsphere and a silica substrate and comparing with theoretical calculations. The bi-material AFM cantilever made of SiNi and Au bends with temperature changes, whose deflection is monitored by the position-sensitive diode. After careful calibration, the bi-material cantilever works as a thermal sensor, from which the near-field radiative conductance and tip temperature can be deduced when the silica substrate approaches the silica sphere attached to the cantilever by a piezo stage with a resolution of 1 nm from a few micrometers away till physical contact. The developed novel near-field thermal metrology will be used to measure the near-field radiative heat transfer between the silica microsphere and planar SiC surface as well as nanostructured SiC metasurface. This research aims to enhance the fundamental understandings of radiative heat transfer in the near-field which could lead to advances in microelectronics, optical data storage and thermal systems for energy conversion and thermal management. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2019

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Quenching technique is widely used in industrial applications as it enhances the mechanical properties of metals such as hardness and tensile strength. This technique consists of a heating process followed by fast cooling which results in different microstructures that enhance the metal behavior. Current competitive market in metal field requires the implementation of advanced and optimizing techniques by means of efficient and sustainable quenching techniques. Furthermore, cooling by multiple array of water jets offers wide range of cooling rate control and consequently the achievement of the desired properties. Quenching cooling rate for a rotary cylinder by multiple impinging jets is investigated in this experimental study. A rotating steel cylinder is heated up to 700°C by an induction heater and cooled down in short time by an array of water impinging jets in order to study quenching process of the test specimen by the impinging jet technique. This fast cooling has been found to be a crucial parameter that enhances the characteristics of steel thoroughly. The magnitude of its influence has been previously studied in water pools cooling techniques. Consequently, a further understanding of quenching technique is aimed in this study by the variation of different parameters: the multiple jet’s pattern (inline and staggered), jet-to-jet spacing (S/d=4 and 6), rotational speed (10-70rpm) and water subcooling temperature (55-85K) that have been studied in 10 experiments. Running of the experiments have been done with the help of different programs such as LabVIEW and NiMAX. Measurements of the temperature along the cylinder has been carried out by using some embedded thermocouples that have been connected to the DAQ.  Results from the study revealed faster cooling with rotation speed 30rpm since the contact between hot surface and impinged water jet is improved for lower speeds. However, rotation speed10rpm results experienced negative effects. In addition, jet-to-jet spacing S/d = 4 caused higher cooling rate than S/d = 6 since the impinged water from neighbor jets lead to higher interaction between water fronts and consequently a more uniform cooling. Furthermore, significant differences have been found in temperature drop between points located closer to the center of the cylinder and the ones beneath the cooling surface. Regarding the multiple array configuration of nozzles, staggered configuration revealed more uniform cooling over the surface due to the fact that placement of the jets led to a better distribution of the impinged water in the measurement line. The effect of higher subcooling temperature in agreement with previous studies results in which higher cooling rate and more drastic temperature drop. The aim of this study is to make a better understanding of the multiple water impinging jets quenching technique in order to make further research in the area of enhancing the mechanical properties of steel by understanding effect of the quenching parameters and their characteristics in order to optimize the quenching technique for different applications.

Air cooled heat exchangers are used in many technology applications. Compactness, functionality and prize is decision maker for many future customers. Current development of compact heat exchangers is evident. Mostly used technology - expanded copper tube on aluminium fin is fluently changed by all aluminium heat exchangers with sophisticated heat exchange surface. Heating, Ventilating and Air-Conditioning with Refrigeration industry (HVAC-R) uses more and more extrude aluminium profiles as the main structure for all size heat exchangers. Similarly to Automotive industry HVAC-R uses minichannels as the basis of their products. It is not a new technology, it is technology proved through many years use in car industry. It is also true that current well known minichannel technology is not the last step. New designs and production lines are currently developed as well. All these new technologies have one unsolved thing together – how will be the new technologies easily computed through models. Current correlations which were used till now are obsolete, because of system miniaturization mainly. It is not easy to adapt old macro channel correlations on minichannel or microchannels where is not possible to diminish capillarity effects, different flow structures, etc. All these difficulties causes in model changes, software changes or iterative procedures upgrades. It is also evident that a lot of experimental exercises have to be done to examine heat exchanger behaviour ad characteristics. Because of precise computation of minichannel heat exchangers need or its geometry optimization is dissertation theme aimed to compare computation methodologies. PhD thesis is also aimed to check functionality of proposed computation system used simple similarity model. All proposed methods are compared to experimentally measured data and as the outcome of thesis are these data used to develop new computation basis.

Schopnost predikovat tepelné toky do stěn v oblasti spalování, konstrukce pecí a procesního průmyslu je velmi důležitá pro návrh těchto zařízení. Je to často klíčový požadavek pro pevnostní výpočty. Cílem této práce je proto získat kvalitní naměřená data na experimentálním zařízení a využít je pro validaci standardně využívaných modelů počítačového modelování turbulentního vířivého difúzního spalování zemního plynu. Experimentální měření bylo provedeno na vodou chlazené spalovací komoře průmyslových parametrů. Byly provedeny měření se pro dva výkony hořáku – 745 kW a 1120 kW. Z měření byla vyhodnocena data a odvozeno nastavení okrajových podmínek pro počítačovou simulaci. Některé okrajové podmínky bylo nutné získat prostřednictvím dalšího měření, nebo separátní počítačové simulace tak jako například pro emisivitu, a nebo teplotu stěny. Práce zahrnuje několik vlastnoručně vytvořených počítačových programů pro zpracování dat. Velmi dobrých výsledků bylo dosaženo při predikci tepelných toků pro nižší výkon hořáku, kde odchylky od naměřených hodnot nepřesáhly 0.2 % pro celkové odvedené teplo a 16 % pro lokální tepelný tok stěnou komory. Vyšší tepelný výkon však přinesl snížení přesnosti těchto predikcí z důvodů chybně určené turbulence. Proto se v závěru práce zaměřuje na predikce vířivého proudění za vířičem a identifikuje několik problematických míst v použitých modelech využívaných i v komerčních aplikacích.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19703号 / 工博第4158号 / 新制||工||1641(附属図書館) / 32739 / 京都大学大学院工学研究科航空宇宙工学専攻 / (主査)教授 吉田 英生, 准教授 岩井 裕, 教授 鈴木 基史, 教授 江口 浩一 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Mould powders are synthetic slags that contain mixtures of silica (SiO2), lime (CaO), sodium oxide (Na2O), fluorspar (CaF2), and carbon (C). When heated to elevated temperatures these powders liquefy and float on the liquid steel in the mould. Mould oscillation helps the liquid flux to penetrate the tiny gap between the mould and the newly formed solid steel shell. In this position the liquid flux partially solidifies against the water cooled mould, while a small portion of the flux remains liquid next to the steel shell to provide lubrication between the moving parts. Effective horizontal heat transfer in the mould is critical for solidifying the liquid steel inthe mould. This process is largely influenced by the thickness and the nature of the flux layer that infiltrates the mould/shell gap. When casting titanium stabilised stainless steels the alloying element reacts with the molten flux, ultimately changing the behaviour of the flux. During the casting process, titanium from the liquid steel reacts with the molten flux producing solids at high temperatures known as perovskite (CaTiO3). Research has shown that perovskite reduces the lubrication capabilities of casting fluxes leading to detrimental effects on product quality while posing a serious threat of machine damage (breakout). The focus of this study is to investigate the effect of titanium pickup on the solidification nature of mould flux and the consequences on horizontal heat transfer. To achieve this, an experimental setup was constructed to simulate the behaviour of mould flux during continuous casting. Analyses of the test flux indicated that the liquid flux closest to the cold side (mould) instantly froze to produce a glassy solid structure. Closer to the hot side (steel shell), solid particles such as perovskite, cuspidine (Ca4Si2O7F2), olivine (Ca,Mg,Mn)2SiO4 and nepheline (Na2O.Al2O3.(SiO2)2) could be identified. Similar solid particles were also found in a slag rim sample taken during the industrial casting of 321- titanium stabilised stainless steel using SPH-KA1 mould powder. Further investigations of the crystalline flux layers showed the entrapment of many tiny gas bubbles during solidification. This porous structure acted as a thermal heat barrier limiting horizontal heat transfer. Experimental testing on 3.0 and 6.0mm flux thickness revealed that the overall thermal conductivity of mould flux decreased as the flux porosity increased. Larger amounts of gas entrapment (in the solid flux structure) resulted in higher thermal resistances which ultimately reduced the heat transfer capabilities of the flux. A second heat barrier, which has a far more dominating effect on the overall heat transfer, is created on mould surface during flux solidification. This thermal contact resistance is also found to be the result of entrapped gas bubbles. Experimental results concluded that the effect of titanium pickup on heat transfer is primarily overshadowed by the larger effect of the thermal contact resistance that is formed during mould flux solidification. The contact resistance in combination with gas entrapment in the solid crystalline structure is considered to be the key factors preventing horizontal heat transfer during continuous casting. / Dissertation (MEng (Metallurgical Engineering))--University of Pretoria, 2006. / Materials Science and Metallurgical Engineering / MEng / unrestricted

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