The fluid flow channels of modern heat exchangers are often equipped with different flow disturbance elements which enhance the convective heat transfer coefficient in each channel. These structural or surface roughness elements induce enhanced flow mixing and convective heat transfer at low Reynolds numbers (500 < Re < 2200) by fluid mixing near the channel walls and increasing the surface area. These elements, however, are accompanied by higher pressure drops in comparison to hollow smooth channels (without inserts). The Run-Around Membrane Energy Exchanger (RAMEE) system is an air-to-air energy recovery system comprised of two remote liquid-to-air membrane energy exchangers (LAMEEs) coupled by a pumped liquid desiccant loop. LAMEEs use semi-permeable membranes that are permeable to water vapor, but impermeable to liquid water. The membranes separate the liquid desiccant from the air flow channels, while still allowing both heat and water vapor transfer. The air channels are equipped with turbulence enhancing inserts which serve dual purposes: (a) to support the adjacent flexible membranes, and (b) to enhance the convective heat and mass transfer. This research experimentally investigates the increase in the air pressure drop and average convective heat transfer coefficient after an air-side insert is installed in a Small-scale wind tunnel for exchanger insert testing (WEIT) facility that is designed to simulate the air channels of a LAMEE and to measure all the properties required to determine the flow friction factor and Nusselt number. Experiments are conducted in the test section under steady state conditions at Reynolds numbers between 900 and 2200 for a channel with and without inserts. The 500-mm-long test section has a rectangular cross section (5 mm wide and 152.4 mm high) and is designed to maintain a specified constant heat flux on each side wall. The flow is laminar and hydrodynamically fully developed at the entrance of the test section and, within the test section, thermal development occurs. Nine different insert panels are tested. Each insert is comprised of several plastic rib spacers, each aligned parallel to the stream-wise direction, and several cross-bars aligned normal to the flow direction. The plastic rib spacers are placed either 30 mm, 20 mm or 10 mm apart, and the distance between the cylindrical bars is either 30 mm, 45 mm, 60 mm or 90 mm. The measured convective heat transfer coefficient and the friction factor have uncertainties that are less than ±7% and ±11%, respectively. It is found that the Nusselt number and friction factor are dependent on the insert geometry and the Reynolds number. An empirical correlation is developed for the inserts to predict Nusselt number and friction factor within an air channel of a LAMEE. The correlations are able to determine the Nusselt number and the friction factor within ±9% and ±20% of the experimental data. Results show the flow insert bar spacing is the most important factor in determining the convective heat transfer improvement. As an application of the experimental data in this thesis, the experimental and the numerical results from a LAMEE which has an insert in each airflow channel are presented. The results show that the insert within the air channel of the LAMEE is able to improve the total effectiveness of the LAMEE by 4% to 15% depending on the insert geometry and air flow Reynolds number and operating inlet conditions for the exchanger.

Fluid mixing is a challenging problem in laminar flow systems. Even in microfluidic systems, diffusion is often negligible compared to advection in the flow. The idea of chaotic advection can be applied in these systems to enhance mixing efficiency. Topological chaos can also lead to efficient and rapid mixing. In this dissertation, an approach to enhance fluid mixing in laminar flows without internal rods is demonstrated by using the idea of topological chaos. Periodic motion of three stirrers in a two-dimensional flow can lead to chaotic transport of the surrounding fluid. For certain stirrer motions, the generation of chaos is guaranteed solely by the topology of that motion and continuity of the fluid. This approach is in contrast to standard techniques. Appropriate stirrer motions are determined using the Thurston-Nielsen classification theorem, which also predicts a lower bound on the complexity of the dynamics in the flow. Work in this area has focused largely on using physical rods as stirrers, but the theorem also applies when the ``stirrers'' are passive fluid particles. In this thesis, topological chaos is theoretically and numerically investigated in lid-driven cavities and rectangular channels without internal rods. When a two-dimensional incompressible Newtonian flow is in the Stokes flow regime, the stream function satisfies the two-dimensional biharmonic equation. When the flow occurs in a lid-driven cavity with solid side walls, this equation can be solved using a method that is similar to the traditional Fourier expansion but uses an asymptotic approximation for the sum of high order terms. When the flow occurs between two infinite plates with spatially periodic boundary conditions, an exact solution in a rectangle with finite width, which represents the flow in this infinitely-wide cavity, can be obtained by using the principle of superposition. A fully developed, three-dimensional flow in a rectangular channel can be decomposed into an unperturbed Poiseuille flow in the axial direction and a lid-driven cavity secondary flow in the cross section. This model can be applied to numerically simulate either pressure-driven flow in a rectangular channel with surface grooves or electro-osmotic flow in a rectangular channel with variations in surface potential. In this dissertation, the occurrence of topological chaos in unsteady two-dimensional flows as well as steady three-dimensional flows without internal rods is demonstrated. For appropriate choices of boundary velocity on the top and/or bottom walls, there exist three periodic points in the flows that produce a chaos-generating motion. In steady flow through a three-dimensional rectangular channel, the axial direction plays the role of time and the periodic points lie on streamtubes that "braid" the surrounding fluid as it moves through the duct. When appropriate motion is applied on the boundary of the wide cavity or channel, topological chaos can also be generated in the flow. The stretching rate of non-trivial material lines in all these flows agrees with the prediction of the lower bound of topological entropy provided by the Thurston-Nielsen theorem. Ghost rod structures are found and analyzed in the lid-driven cavity and rectangular channel flows with solid side walls. The results suggest that the no-slip boundary condition on the stationary internal surfaces is one of the reasons for poor mixing in steady laminar three-dimensional flows considered previously with solid braided internal rods. / Ph. D.

The objective of current study was to numerically investigate the flow and heat transfer characteristics in a stationary one pass rectangular channel (AR=4:1) with circular and elliptical pin fin arrays. Two types of elliptical pin fins (a SEF and an N fin whose minor axis length is equal to the diameter of the circular fin) were used. The analysis was performed with an array of six rows of staggered pin fins in the streamwise direction for Reynolds numbers (Re) of 10,000, 20,000, 30,000, 40,000 and 50,000. 3-D, steady simulations were performed using the low Reynolds number k-omega SST turbulence model in the FLUENT CFD code. The data predicted by the current numerical model showed favorable agreement with the experiments in the validation study. It was observed that SEF array produces minimum pressure loss and the highest thermal performance. It was also observed that N fin array produces minimum hot spots and the highest channel averaged Nusselt number ratio values.

Yes / A sudden dam failure is usually simulated by the rapid removal of a gate in laboratory tests and numerical simulations. The gate-opening time is often determined according to the Lauber and Hager instantaneous collapse criterion (referred to as Lauber-Hager criterion) established for a rectangular open channel with a dry bed. However, this criterion is not suitable for non-rectangular channels or initial wet-bed conditions. In this study, the effect of the gate-opening time on the wave evolution is investigated by using the large eddy simulation (LES) model. The instantaneous dam break, namely the dam break without a gate, is simulated for comparison. A gate-opening criterion for generating dam-break flow in non-rectangular wet bed channel is proposed in this study, which can be used as an extension of the Lauber-Hager criterion and provides a more comprehensive and reasonable estimate of the gate opening time. / National Natural Science Foundation of China (Grant No: 51879179), the Open Fund from the State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University (SKHL1809) and Sichuan Science and Technology Program (No. 2019JDTD0007).

O escoamento hidrodinâmico é descrito por meio das equações da continuidade e da quantidade de movimento em conjunto com as condições iniciais e de contorno. Neste trabalho de pesquisa, o escoamento não permanente na saída de um canal retangular curto de declividade nula, resultante da abertura rápida de uma comporta foi experimentalmente investigado. Para determinar o coeficiente de pressão e o comprimento do trecho a montante da comporta com distribuição de pressão não hidrostática, usou-se um modelo unidimensional, descrito pelas equações de Saint Venant, para representar o fenômeno. Utilizou-se o método de Lax-Wendroff para obter as curvas características do fenômeno e compará-las com os resultados obtidos experimentalmente. Várias comparações foram feitas possibilitando a avaliação de valores para os parâmetros que definem a região com distribuição de pressão não hidrostática. Os resultados calculados para os valores de parâmetros constantes, estão de acordo com os resultados experimentais. Os resultados experimentais ajustam-se melhor quando os valores dos parâmetros variam no decorrer do tempo. / The hydrodynamic flow is described by means of the equations of the continuity and momentum together with the initial conditions, the boundary conditions. In this research work, the unsteady flow at the exit of a short rectangular channel of zero slope, resulting from the fast opening of a floodgate was investigated experimentally. For determining the pressure coefficient and the length of the upstream reach of the floodgate with non-hydrostatic pressure distribution, a one-dimensional model described by Saint Venant equations is used to represent the phenomenon. The Lax-Wendroff method was used to obtain the curves characteristics of the phenomenon and to compare them with the results obtained experimentally. Several comparisons were made to facilitate the evaluation of values for the parameters that define the region with non-hydrostatic pressure distribution. The results calculated for the values of constant parameters are in agreement with the experimental results. The experimental results are better adjusted when the values of the parameters vary in elapsing of the time.

This thesis presents the study of air bubble motion in stagnant water and in upward flowing water through a vertical narrow rectangular channel of 2.1X66.5 mm cross section by using image processing and analyzing techniques. The investigated bubble size range is 2 - 70 mm in area equivalent bubble diameter and mean water flow velocity is 0-25 cm/s. This study focuses on the quantitative assessment of bubble size (perimeter, area, volume, width and height), shape, path and rising velocity. The observed bubble shapes consist of circular, ellipse, ellipse wobbling, hat wobbling, cap-hat, cap and cap-bullet types. Ellipse wobbling and hat wobbling type bubble region show transition from ellipsoidal to circular cap region. The results of the ellipse bubble tests were compared with the rise velocity correlation of ellipsoidal bubbles in infinite medium and an empirical correlation for two dimensional ellipse bubbles was obtained. The cap bubble rise velocities measured in this study were compared with the correlations in literature and a new empirical correlation which is different from those given in the literature was obtained from the experimental results of this study. The trends of the cap bubble deformation which is defined as bubble height to bubble width ratio, for various water streams are almost similar in behavior. Therefore, a correlation was obtained from the measured cap bubble deformation values in water stream. The experimental results show that the bubble relative velocity under co-current flow conditions for area equivalent bubble diameter to channel width ratio, &amp / #955 / ae&lt / 0.6 is less than that under stagnant water condition. After bubble shapes reach to the slug type (&amp / #955 / ae &amp / #8805 / 0.6), relative bubble velocities pass over the terminal bubble velocities in stagnant water. If the water velocity is increased further, the bubble relative velocity increases in the slug bubble region. Rising velocities of slug bubbles were analyzed using the experimental data and a new correlation for slug bubble rise velocity was proposed for flowing water condition.

O escoamento hidrodinâmico é descrito por meio das equações da continuidade e da quantidade de movimento em conjunto com as condições iniciais e de contorno. Neste trabalho de pesquisa, o escoamento não permanente na saída de um canal retangular curto de declividade nula, resultante da abertura rápida de uma comporta foi experimentalmente investigado. Para determinar o coeficiente de pressão e o comprimento do trecho a montante da comporta com distribuição de pressão não hidrostática, usou-se um modelo unidimensional, descrito pelas equações de Saint Venant, para representar o fenômeno. Utilizou-se o método de Lax-Wendroff para obter as curvas características do fenômeno e compará-las com os resultados obtidos experimentalmente. Várias comparações foram feitas possibilitando a avaliação de valores para os parâmetros que definem a região com distribuição de pressão não hidrostática. Os resultados calculados para os valores de parâmetros constantes, estão de acordo com os resultados experimentais. Os resultados experimentais ajustam-se melhor quando os valores dos parâmetros variam no decorrer do tempo. / The hydrodynamic flow is described by means of the equations of the continuity and momentum together with the initial conditions, the boundary conditions. In this research work, the unsteady flow at the exit of a short rectangular channel of zero slope, resulting from the fast opening of a floodgate was investigated experimentally. For determining the pressure coefficient and the length of the upstream reach of the floodgate with non-hydrostatic pressure distribution, a one-dimensional model described by Saint Venant equations is used to represent the phenomenon. The Lax-Wendroff method was used to obtain the curves characteristics of the phenomenon and to compare them with the results obtained experimentally. Several comparisons were made to facilitate the evaluation of values for the parameters that define the region with non-hydrostatic pressure distribution. The results calculated for the values of constant parameters are in agreement with the experimental results. The experimental results are better adjusted when the values of the parameters vary in elapsing of the time.

The heat transfer and friction augmentation in the fully developed portion of a 2:1 aspect ratio rectangular channel with orthogonal ribs at channel Reynolds numbers of 20,000, 30,000, and 40,000 is studied both experimentally and computationally. Ribs are applied to the two opposite wide walls. The rib aspect ratio is varied systematically at 1, 3, and 5, with a constant rib height and constant rib pitch (rib-pitch-to-rib-height ratio of 10). The purpose of the study is to extend the knowledge of the performance of rectangular channels with ribs to include high aspect ratio ribs. The experimental investigation is performed using transient Thermochromic Liquid Crystals technique to measure the distribution of the local Nusselt numbers on the ribbed walls. Overall channel pressure drop and friction factor augmentation is also obtained with the experimental setup. A numerical simulation is also performed by solving the 3-D Reynolds-averaged Navier-Stokes equations using the realizable-k-Greek lowercase letter episilon] turbulence model for closure. Flow visualization is obtained from the computational results as well as numerical predictions of local distributions of Nusselt numbers and overal channel pressure drop. Results indicate that with increasing rib width, the heat transfer augmentation of the ribbed walls decreases with a corresponding reduction in channel pressure drop.

Yes / Natural earth-bounded channel flows usually subject to various sidewall turbulences, i.e. in the form of secondary currents, due to non-constant channel shapes at different sections. This paper investigates an improved Shiono-Knight model (SKM) by combining it with a Multi-Zonal (MZ) method (proposed by Pu, 2019) to represent lateral flow turbulence and secondary currents in different shapes of open channel, i.e. rectangular and trapezoidal. By applying the proposed analytical model to both rectangular and trapezoidal channel flows, we have inspected different streamwise velocity characteristics across transverse direction generated by their sidewalls in order to provide crucial fundamental understanding to real-world natural flow system. The proposed model has also been validated via various experimental data conducted in national UK Flood Channel Facility (UK-FCF). It has been observed that the trapezoidal channel has created a larger sidewall zone where secondary current can affect flow velocity; however, the intensity of the secondary flow in trapezoidal channel has been found lesser than that of the rectangular channel. By improving the modelling of natural flow at sidewall, the studied approach could be adapted into different existing analytical models to improve their accuracy.