A numerical study of temperature effects on hot wire measurements inside turbulent channel flows

A way to investigate turbulence is through experiments where hot wire measurements are performed. Analysis of the in turbulence of a temperature gradient on hot wire measurements is the aim of this thesis work. Actually - to author's knowledge - this investigation is the first attempt to document, understand and ultimately correct the effect of temperature gradients on turbulence statistics. However a numerical approach is used since instantaneous temperature and streamwise velocity fields are required to evaluate this effect. A channel flow simulation at Re_tau = 180 is analyzed to make a first evaluation of the amount of error introduced by temperature gradient inside the domain. Hot wire data field is obtained processing the numerical flow field through the application of a proper version of the King's law, which connect voltage, velocity and temperature. A drift in mean streamwise velocity profile and rms is observed when temperature correction is performed by means of centerline temperature. A correct mean velocity pro�le is achieved correcting temperature through its mean value at each wall normal position, but a not negligible error is still present into rms. The key point to correct properly the sensed velocity from the hot wire is the knowledge of the instantaneous temperature field. For this purpose three correction methods are proposed. At the end a numerical simulation at Re_tau =590 is also evaluated in order to confirm the results discussed earlier.

“Study of Electro-thermal Effects on PLA Materials Fed with AC Currents”

Given the rise in the emergence of new composite materials, their multifunctional properties, and possible applications in simple and complex structural components, there has been a need to unravel the characterization of these materials. The possibility of printing these conductive composite materials has opened a new area in the design of structural components which can conduct, transmit, and modulate electric signals with no limitation from complex geometry. Although several works have researched the behaviour of polymeric composites due to the immediate growth, however, the electrothermal behaviour of the material when subjected to varying AC applied voltage (Joule’s effect) has not been thoroughly researched. This study presents the characterization of the electrothermal behaviour of conductive composites of a polylactic acid matrix reinforced with conductive carbon black particles (CB-PLA). An understanding of this behaviour would contribute to the improved work in additive manufacturing of functional electro-mechanical conductive materials with potential application in energy systems, bioelectronics, etc. In this study, the electrothermal interplay is monitored under applied AC voltage, varying lengths, and filament printing orientations (longitudinal, oblique, and transverse). Each sample was printed using the fused deposition modeling technique such that each specimen has three different lengths (1L, 2L, 2.75L). To this end, deductions were made on properties that affect composite’s efficiency and life expectancy. The result of this study shows a great influence of printing orientation on material properties of 3D printed conductive composites of CB-PLA. The result also identifies the contribution of AC applied voltage to composites' stabilization time. This knowledge is important to provide experimental background for components' electrothermal interplay, estimate possible degradation and operating limits of composite structures when used in applications.

An experimental and modelling study of the capture of CO2 from gas mixtures with different techniques, with focus on multicomponent effects.

In the present work, the deviations in the solubility of CO2, CH4, and N2 at 30 °c in the mixed gases (CO2/CH4) and (CO2/N2) from the pure gas behavior were studied using the dual-mode model over a wide range of equilibrium composition and pressure values in two glassy polymers. The first of which was PI-DAR which is the polyimide formed by the reaction between 4, 6-diaminoresorcinol dihydrochloride (DAR-Cl) and 2, 2’-bis-(3, 4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). The other glassy polymer was TR-DAR which is the corresponding thermally rearranged polymer of PI-DAR. Also, mixed gas sorption experiments for the gas mixture (CO2/CH4) in TR-DAR at 30°c took place in order to assess the degree of accuracy of the dual-mode model in predicting the true mixed gas behavior. The experiments were conducted on a pressure decay apparatus coupled with a gas chromatography column. On the other hand, the solubility of CO2 and CH4 in two rubbery polymers at 30⁰c in the mixed gas (CO2/CH4) was modelled using the Lacombe and Sanchez equation of state at various values of equilibrium composition and pressure. These two rubbery polymers were cross-linked poly (ethylene oxide) (XLPEO) and poly (dimethylsiloxane) (PDMS). Moreover, data about the sorption of CO2 and CH4 in liquid methyl dietahnolamine MDEA that was collected from literature65-67 was used to determine the deviations in the sorption behavior in the mixed gas from that in the pure gases. It was observed that the competition effects between the penetrants were prevailing in the glassy polymers while swelling effects were predominant in the rubbery polymers above a certain value of the fugacity of CO2. Also, it was found that the dual-mode model showed a good prediction of the sorption of CH4 in the mixed gas for small pressure values but in general, it failed to predict the actual sorption of the penetrants in the mixed gas.