Abstract The experiment was performed to measure the flow-induced vibration of tubes in cross flow and to study the effect of different experimental parameters, natural frequency, tube patterns and positions of the missing tube, on the tube vibration. The vibration mode of structures was investigated by root-mean-square values of tube displacements, dominant response frequencies, power spectral densities and orbits. The vibration signals in various velocities were used to analyze the mode and mechanism of the tube vibration by examining the interrelations between power spectral densities and orbits of the tube vibration. It was found that the stability thresholds were higher at the higher natural frequencies of the tubes and approach angles of the flow. Missing tube would affect the stability of local flow field, so that the surrounding tubes became more unstable. For each array pattern, the dominant response frequency of tube was changed with the flow velocity. It should be associated with the flow field, the mode and mechanism of vibration. As the tube frequency jumped, it should be the change of vibrating mode and mechanism. From the spectra and orbits of the tube vibrations, it can be found that the dominant frequencies are more complex at the high flow velocity.

Abstract Pool boiling process is frequently encountered in a number of engineering applications. However, it is difficult to exactly predict the heat transfer coefficient. This is because the nucleate pool boiling phenomenon is rather complex and influenced by many factors, such as surface roughness, areas of heater, material, geometry, arrangement of heated rods, and refrigerants, etc. The key boiling parameters (bubble dynamics data) such as bubble departure frequency, diameter and active nucleation site density will be varied in such different heated surface resulting in the different effect of heat transfer. This study was performed experimentally. R-134a was used as refrigerants, and the present study is aim at providing the pool boiling data for smooth and plasma coating enhanced tube bundles. It is expected that the surface condition, amount of test tubes, geometric of bundles and different heat flux can affect the nucleate boiling heat transfer in certain degree. In addition were calculated and developed that heat transfer coefficients and relevant corrections. Furthermore, more fundamental of the physical phenomenon can be obtained. According to the results of experiments, Boiling curves and calculations of the bundle factors and geometry factors were subsequently secured. The enhanced heat transfer coefficients with coated tube bundles are 1.1-2.0 times higher than smooth tube bundles. The 1.5-2.3 and 1.1-3.8 bundle factors obtained from the smooth tube bundles and coated tube bundles respectively. The geometry factors were about 1 for all arrangements studied herein. Thermal design data of a flooded type evaporator of high performance as well as more and further physical insight of the above-stated nucleate boiling heat transfer can be acquired. The results will hopefully be helpful not only for the academia but for the industry.

Transport Membrane Condenser (TMC) is an innovative technology based on the property of a nano-scale porous material which can extract both waste heat and water from exhaust gases. This technology tremendously improves the efficiency of boilers and gas/coal combustors by lowering waste heat and increasing water recovery. Contaminants in the flue gases, such as CO2, O2, NOx, and SO2 are inhibited from passing through the membrane by the membrane’s high selectivity. The condensed water through these tubes is highly pure and can be used as the makeup water for many industrial applications. The goal of this research is to investigate the heat transfer, condensation rate, pressure drop and overall performance of crossflow heat exchangers. In this research, a numerical model has been developed to predict condensation of water vapor over and inside of nano-porous layers. Both capillary condensation inside the nanoscale porous structure of the TMC and the surface condensation were considered in the proposed method using a semi-empirical model. The transport of the water vapor and the latent heat of condensation were applied in the numerical model using the pertinent mass, momentum, turbulence and energy equations. By using the proposed model and simulation procedure, the effect of various inlet parameters such as inlet mass flow rate, inlet temperature, and water vapor content of the inlet flow on the performance of the cross-flow TMC heat exchanger was studied to obtain the optimum performance of the heat exchangers at different working conditions. The performance of the TMC heat exchangers for inlet flue gas rate 40 to 120 kg/h, inlet water rate 60 to 140 kg/h, inlet flue gas relative humidity 20 to 90%, and tube pitch ratio 0.25 to 2.25 has been studied. The obtained results show that the water condensation flux continuously increases with the increase of the inlet flue-gas flow rate, water flow rate, and the flue-gas humidity. The total heat flux also follows the same trend due to the pronounced effect of the latent heat transfer from the condensation process. The water condensation flux and the overall heat transfer increase at the beginning for small values of the tube pitches and then decreases as the tube pitch increases furthermore. In addition to the cross-flow TMC heat exchangers, the performance of a shell and tube TMC heat exchanger for high pressure and temperature oxy-combustion applications has been investigated. The performance analysis for a 6-heat exchanger TMC unit shows that heat transfer of the 2-stage TMC unit is higher than the 2-stage with the same number of the heat exchanger in each unit.

The present study considers steady laminar two-dimensional incompressible flow over both in-line and staggered flat tube bundles used in heat exchanger applications. The effects of various independent parameters, such as Reynolds number (Re), Prandtl number (Pr), length ratio (L/Da), and height ratio (H/Da), on the pressure drop and heat transfer were studied. A finite volume based FORTRAN code was developed to solve the governing equations. The scalar and velocity variables were stored at staggered grid locations. Scalar variables (pressure and temperature) and all thermophysical properties were stored at the main grid location and velocities were stored at the control volume faces. The solution to a one-dimensional convection diffusion equation was represented by the power law. The locations of grid points were generated by the algebraic grid generation technique. The curvilinear velocity and pressure fields were linked by the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm. The line-by-line method, which is a combination of the Tri-Diagonal Matrix Algorithm (TDMA) and the Gauss-Seidel procedure, was used to solve the resulting set of discretization equations. The result of the study established that the flow is observed to attain a periodically fully developed profile downstream of the fourth module. The strength increases and the size of the recirculation gets larger as the Reynolds number increases. As the height ratio increases, the strength and size of the recirculation decreases because the flow has enough space to expand through the tube passages. The increase in length ratio does not significantly impact the strength and size of the recirculation. The non-dimesionalized pressure drop monotonically decreased with an increase in the Reynolds number. In general, the module average Nusselt number increases with an increase in the Reynolds number. The results at Pr = 7.0 indicate a significant increase in the computed module average Nusselt number when compared to those for Pr = 0.7. The overall performance of in-line configuration for lower height ratio (H/Da = 2) and higher length ratio (L/Da = 6) is preferable since it provides higher heat transfer rate for all Reynolds numbers except for the lowest Re value of 25. As expected the staggered configurations perform better than the in-line configuration from the heat transfer point of view.

Cette thèse s’intéresse à la faisabilité de la mise en place de modèles d’ordre réduit pour l’étude des vibrations sous écoulement au sein de faisceaux tubulaires d’échangeurs de chaleur. Ces problématiques sont cruciales car les systèmes étudiés sont des éléments majeurs des centrales nucléaires civiles et des chaufferies embarquées dans les sous-marins.Après avoir rappelé le fonctionnement et les risques vibratoires existants au sein des échangeurs de chaleur, des calculs complets d’écoulement et de vibrations sous écoulement ont été effectués, d’abord pour un tube seul en milieu infini, puis pour un faisceau de tubes. Ces calculs ont été menés avec l’outil CFD Code_Saturne. La méthode de réduction de modèle POD (Proper Orthogonal De-composition) a été appliquée au cas des écoulements avec la structure laissée fixe.Les résultats obtenus montrent l’efficacité de la méthode pour ces configurations, moyennant l’introduction de méthodes de stabilisation pour l’écoulement au sein du faisceau. La méthode POD-multiphasique, permettant d’adapter la méthode POD à l’interaction fluide-structure, a ensuite été appliquée. Les grands déplacements d’un cylindre seul dans la zone d’accrochage (lock-in) ont été correctement reproduits par cette méthode de réduction de modèle. De même, on montre que les grands déplacements d’un cylindre en milieu confiné dans un faisceau de tubes sont fidèlement reconstruits.Enfin, l’extension de l’utilisation de la réduction de modèle aux études d’évolution paramétrique a été testée. Nous avons d’abord utilisé la technique considérant une base POD unique pour reproduire des écoulements à divers nombres de Reynolds autour d’un cylindre seul. Les résultats confirment la prédictivité bornée à une gamme de paramètres de cette méthode. Enfin, l’interpolation de bases POD pré-calculées pour une famille de paramètres donnés, utilisant les variétés de Grassmann et permettant de générer de nouvelles bases POD, a été testée sur des cas modèles. / The objective of this thesis is to study the ability of model reduction for investigations of flow-induced vibrations in heat exchangers tube bundle systems.These mechanisms are a cause of major concern because heat exchangers are key elements of nuclear power plants and on-board stoke-holds.In a first part, we give a recall on heat exchangers functioning and on vi-bratory problems to which they are prone. Then, complete calculations leaded with the CFD numerical code Code_Saturne are carried out, first for the flow around a single circular cylinder (fixed then elastically mounted) and then for the case of a tube bundle system submitted to cross-flow. Reduced-order method POD is ap-plied to the flow resolution with fixed structures. The obtained results show the efficiency of this technique for such configurations, using stabilization methods for the dynamical system resolution in the tube-bundle case.Multiphase-POD, which is a method enabling the adaptation of POD to fluid-structure interactions, is applied. Large displacements of a single cylinder elastically mounted under cross-flow, corresponding to the lock-in phenomenon,are well reproduced with this reduction technique. In the same way, large displace-ments of a confined moving tube in a bundle are shown to be faithfully recon-structed.Finally, the use of model reduction is extended to parametric studies. First,we propose to use the method which consists in projecting Navier-Stokes equations for several values of the Reynolds number on to a unique POD basis. The resultsobtained confirm the fact that POD predictability is limited to a range of parameter values. Then, a basis interpolation method, constructed using Grassmann mani-folds and allowing the construction of a POD basis from other pre-calculated basis,is applied to basic cases.

[pt] Neste trabalho, foi desenvolvido um modelo termodinâmico para uma caldeira flamotubular, de três passes, operando em regime permanente com gás natural boliviano. Foi testado o impacto no cálculo da temperatura no final de cada passe de várias correlações de troca de calor encontradas na literatura. Um planejamento composto central permitiu avaliar simultaneamente o impacto da variação de seis parâmetros (diâmetro dos tubos no primeiro passe e nos dois passes seguintes, comprimento da caldeira, número de tubos no segundo e terceiro passe e porcentagem de excesso de ar na combustão) e obter um modelo polinomial empírico a fim de otimizar as eficiências energética e exergética de caldeira. Um teste de malha foi realizado com temperaturas no final da chama, câmara de combustão, segundo e terceiro passes e apontou que, a partir de 200 divisões em cada passe o impacto do volume não é mais significativo na temperatura. O modelo foi validado com dados experimentais, observando-se que a diferença entre os valores experimentais e teóricos das temperaturas do primeiro e segundo passes é inferior a 22,9 graus C e menor que 43,9 graus C no terceiro passe. Com os modelos polinomiais robustos obtidos com o planejamento de experimentos, foi possível observar os parâmetros mais significativos, tanto para as eficiências energética e exergética, assim como para a destruição de exergia. São por ordem decrescente de importância: o excesso de ar; as duas contribuições associadas ao comprimento da caldeira; o número de tubos nos segundo e terceiro passes e; o diâmetro dos tubos no segundo e terceiro passe e o diâmetro do tubo no primeiro passe. Além disso, foi possível quantificar o trade-off entre o aumento da superfície de troca de calor e a manutenção do nível de turbulência do escoamento. / [en] In this work, a thermodynamic model was developed for a three-pass firetube boiler operating in a steady state with Bolivian natural gas. The sensitivity in the temperature profile of various heat exchange correlations found in the literature was tested. A central composite design made it possible to simultaneously evaluate the impact of the variation of six parameters (diameter of tubes in the first and two subsequent passes, boiler length, number of tubes in the second and third passes and percentage of excess air in combustion) and obtain an empirical polynomial model to optimize boiler energy and exergetic efficiencies. A mesh test was performed with temperatures at the end of the flame, combustion chamber, second and third passes and pointed to from 200 divisions in each pass the impact of volume is no longer significant on temperature. The model was validated with data obtained experimentally, observing that the difference between the experimental and theoretical values of the temperatures of the first and second passes is less than 22.9 degrees C and less than 43.9 degrees C in the third pass. With the robust polynomial models obtained with the design of experiments, it was possible to observe the most significant parameters, both for energy and exergetic efficiencies, as well as for exergy destruction. They are, in decreasing order of importance: excess air; the two contributions associated with the length of the caldera; the number of tubes in the second and third passes and the diameter of the tubes in the second and third pass and the diameter of the tube in the first pass. In addition, it was possible to quantify the trade-off between increasing the heat exchange surface and maintaining the flow turbulence level.

The Master´s thesis is dealing with water-cooled condensers and is splitted into three main parts. The first section provides general knowledge about condensers and heat exchangers, including the basic information of hydrophobic surfaces. These surfaces were used during verifying the impact of hydrophobicity on the heat exchange in water steam. The second section describes an experimental testing. It shows the difference between hydrophobic and nonhydrophobic surfaces and their impact on the heat exchange. The third and the last part of this thesis is design and calculation of water-cooled condenser. The design of the condenser is supported by drawings placed in attachement.