An experimental study of turbulent forced convection heat transfer to water flowing in a vertical annular passage is reported in this paper. The study investigates the influence of eccentricity (ranging from 0% to 80%) and diameter ratio (ranging from 1.5 to 4.0) upon the heat transfer phenomena occurring at the inner boundary of the annular passage. Dimensionless heat transfer parameters calculated from measurements made at the two locations corresponding to the maximum and minimum separation of the inner and outer boundaries of the annular passage are correlated in terms of the Reynolds number, the eccentricity and the diameter ratio. Analysis of the correlations indicates that eccentricity affects the heat transfer phenomena occurring at the two locations on the inner boundary of the annular passage in different fashions; increasing eccentricity causes the heat transfer to increase at the location corresponding to the maximum separation of the boundaries and causes the heat transfer to decrease at the location corresponding to the minimum separation of the boundaries. The magnitude of the increase or decrease in heat transfer is dependent upon the diameter ratio; at a particular level of eccentricity, the greater variations in heat transfer occur at the smaller diameter ratios. Ranges in which eccentricity does not influence heat transfer are found in connection with the larger diameter ratios. Moody friction factors calculated from measurements made with concentric annular passages are correlated as a function of Reynolds number. / Thesis / Master of Engineering (ME)

The ultimate objective of the research which led to this thesis is to increase the efficacy and safety of radiofrequency catheter ablation (RFCA) of cardiac tissue. The purpose of RFCA is to carefully heat selected locations in the heart. The resulting thermal injury creates lesions which prevent the generation or propagation of arrhythmias. The ability to predict the appropriate amount of energy required at any ablation site is essential to increasing the efficacy and safety of RFCA. The research documented in this thesis focuses on the development of an approximate thermal model of the time-dependent temperature profile within the myocardium during an RFCA procedure. It is anticipated that this model will ultimately give electrophysiologists the ability to accurately titrate energy delivery in clinical situations. The approximate thermal model uses a convective boundary condition to account for convective cooling of the myocardial surface. This model also uses a point source rather than the complicated heat generation function that accounts for the spatial variation of the voltage in the cardiac tissue. A C program was written to evaluate the engineering model. The effect of the convection coefficient (h), the depth at which the point source is located (zo), and the power dissipation rate (P) on the 50 ˚C isotherm in the cardiac tissue is shown. The accuracy of the approximate model depends greatly on the values of these three parameters. Rigorous three-dimensional numerical modeling was done in order to validate the engineering model. The numerical model was done using a commercial computational fluid dynamics (CFD) package. This software solved the steady, incompressible Reynolds-Averaged Navier-Stokes (RANS) equations—along with the Reynolds-Averaged energy transport equation—using an unstructured, segregated, pressure-based finite-volume procedure. This model is different from other numerical RF ablation models in that it took into account the turbulent flow of the blood. It also accounted for the effect of the flow past the electrode and the spatially varying heat generation function. The heat generation function was found from the solution of the Laplace equation to find the voltage distribution in the tissue. The three unknown parameters governing the approximate thermal model were changed manually and good fits of the approximate model with the numerical model resulted, proving that the engineering model can accurately predict the size of the 50 ˚C isotherm in the cardiac tissue.

The hydrophobicity of a surface is defined as the degree to which it repels water molecules, and the internal contact angle that the droplet makes with the surface is a measure of the hydrophobicity. Contact angles less than 90° occur on hydrophilic surfaces, while contact angles greater than 90° occur on hydrophobic surfaces. If a surface's contact angle is greater than 120° the surface is commonly defined as superhydrophobic (SH). Superhydrophobicity is accomplished through a combination of microscale surface roughness and water repellant surface chemistry. The roughness creates cavities, or pockets, of vapor underneath the droplet which act to increase the effects of surface tension and lead to increased contact angles. The cavity fraction, F_c, of a surface is a measure of the surface roughness and is defined as the ratio of the projected cavity area to the projected total area of the surface. This thesis investigates the effects of varying cavity fraction, F_c, and substrate temperature, T_s, on heat transfer to evaporating water droplets. Distilled water droplets of nominally 3 mm in diameter were placed on heated SH substrates of varying F_c (0.5, 0.8, and 0.95). A smooth hydrophobic surface was included in the experiments for comparative purposes. The temperature of the surface was held constant at temperatures ranging from 60 to 230°C while the droplet evaporated. Measurements of droplet temperature and size were taken throughout the evaporation process using CCD and infrared camera images. These images were analyzed to yield heat transfer rates for the various surface types and surface temperatures studied. At temperatures below the saturation point of water, average droplet temperatures and heat transfer rates decrease with increasing cavity fraction. Differences in heat transfer rate between substrates increase with substrate temperature. Nusselt number decreases as cavity fraction is increased. Cavity fractions less than about 0.5 show only modest differences in Nusselt number between surfaces. As cavity fraction approaches unity, differences in Nusselt number become amplified between surfaces. At temperatures above the saturation point of water, boiling behavior on SH surfaces deviates dramatically from that of smooth untextured surfaces. Average heat transfer rates decrease with increasing cavity fraction. Nucleate boiling is delayed to highter superheats than normal or is not observed. The Liedenfrost point is advanced to lower superheats as cavity fraction is increased. Similar heat transfer rates are observed beyond the Leidenfrost point.

[pt] Este trabalho prossegue a investigação sobre convecção forçada com ebulição de película em tubos verticais, na qual o regime de escoamento consiste de gotículas de líquido dispersas em vapor superaquecido. Um procedimento de cálculos já existe (baseado em um tamanho único médio de gota que sofre colapso e evapora ao longo do tubo) para determinar os fenômenos cinéticos e de transferência de calor. O principal objetivo do presente trabalho é introduzir uma mais realista distribuição nos tamanhos de gota no procedimento de cálculo com o intuito de determinar a validade do uso do tamanho único de gota para representar a real distribuição. Dados experimentais já existentes para ebulição de película de nitrogênio (70.00 menor do que G menor do que 190.000 ibm/hr/ft2 e 5.000 menor do que q / A menor do que 25.000 Btu/hr/ft2 em tubos de diâmetro interno de 0.228 menor do que D menor do que 0.462 polegadas e 8 pés de comprimento) foram usados no desenvolvimento do modelo analítico tanto para o estudo com tamanho único de gota como para tamanho múltiplo de gota. Para tamanho múltiplo de gota, o critério do número de Weber foi utilizado para prever o colapso da gota que se divide em 9 gotas menores de vários tamanhos. Os resultados do estudo com tamanho múltiplo de gota forneceram resultados semelhantes aos com tamanho único de gota, indicando de gota para descrever o fenômeno da ebulição de película com escoamento disperso. Também está incluída uma compilação de propriedade do nitrogênio, necessárias para os estudos deste trabalho. / [en] This work further investigates forced convection film boiling in vertical tubes, in whitch the flow regime consist of dispersed liquid droplets in a superheated vapor. Calculation procedures already exist (based on a single average size droplet that breaks up and evaporates as it flows along tube) for determining the kinetic and heat transfer phenomenas. The principal objective of the present work is to introduce a more realistic distribution of droplet in a superheated vapor. Calculation procedure already axist (based on a single average size droplet that braks up and evaporates as it flows along tube) for determining the kinetic and heat transfer phenomenas. The principal objective of the present work is to introduce a more realistic distribution of droplet sizes into the calculation procedure in order to determine the validity of using a single droplet size to represent a real distribution. Previous experimental data for film boiling of nitrogen (70.000 less than G less than 190.00 lbm/hr/ft2 and 50.000 less than q/As25.000 btu/br/ft2 in tubes of 0.22 less than D less than 0.462 inch diameter and 8 feer long) were usad in developing analytical models for both the single droplet size and multiple droplet size studies. For the multiple droplet size distribution, a critical Weber number was utilized to predict the breakup of a droplet into approximately 9 smaller droplets of various sizes. The results of the multiple droplets of various sizes. The results of the multiple droplet study compare well with those of the single droplet studies, thus indicating the validity of using the single droplet size model for predicting phenomena in dispersed flow boiling.

Heat transfer and pressure data were performed and reported on two different rigs. The first rig has an aspect ratio of (19:1) with two different inlet conditions and the second rig is composed of two different aspect ratio channels, (1:8) and (1:4). Rib turbulators were used as a flow disruptor scheme to enhance the heat transfer and friction factor. Rib aspect ratios ranging from (1:1) to (1:5) rib-height-to-width ratio were used. The first channel rib-width-to-pitch (Wr/P) ratio was kept at 1/2 where flow was kept at relatively low Reynolds numbers, between 3000 and 13000. Results from the current tests showed that existing correlations could be used for high aspect ratio channels in predicting the effectiveness of the cooling scheme. Two different inlet conditions were tested; one was arranged so that the flow was hydrodynamically fully-developed at the entrance of the heated section, while the other uses an abrupt entrance from bleeding off mass flow from a horizontal channel. The heat transfer augmentation (compared to a well known and accepted correlation proposed by Dittus-Boelter) in these channels are extremely high with an average of 350% to 400%. However, this was accompanied by a substantial increase in the pressure drop, causing the overall thermal performance to increase between twenty to thirty percent. The second channel rib-width-to-pitch ratio (Wr/P) ranges from 0.1, 0.3, and 0.5; the flow conditions are tested from 20,000 to 40,000 Reynolds number. Correlations for heat transfer and friction augmentation of the test data was also given. The test shows large rib blockage ratio does not demonstrate the best thermal performance; however it does give a high heat transfer augmentation ranging from 200 to 300 percent for both aspect ratios depending on the width of the used ribs.

An analytical model is developed to quantify the heat transfer to droplets impinging on heated superhydrophobic (SH) surfaces. Integral analysis is used to incorporate an apparent temperature jump at the superhydrophobic surface as a boundary condition. This Thesis considers the scenario of both isotropic and anisotropic slip, as would be realized on post-cavity style and rib-cavity style SH surfaces. This thermal model is combined with a hydrodynamic model which incorporates velocity slip at the surface. Use of the two models allows determination of the overall cooling effectiveness, a metric outlined in contemporary work. The effect of varying velocity slip and temperature jump is determined for impact Weber numbers ranging from 20 to 150 and surface temperatures ranging from 60 to 100°C. The model results are compared to experiments and good agreement is shown. Heat transfer to a drop impacting superhydrophobic surfaces is decreased when compared to conventional surfaces. A correlation function for the total heat transfer (cooling effectiveness) as a function of relevant parameters is found for isotropic surfaces with a good fit. Anisotropic rib-cavity surfaces are compared to isotropic surfaces to explore the impact of anisotropic slip on the cooling effectiveness, with similar trends seen to that for isotropic surfaces. It's determined that anisotropic surfaces can be modeled with minimal error as an isotropic surface with a temperature jump length equal to the anisotropic surface's average temperature jump length.

<p> An experimental study of forced convection heat transfer and friction in water flowing in a vertical tube is reported in this thesis. The study investigates the effect of coiled wire turbulence promotors of various pitch to diameter ratios (ranging from 1.00 to 5.50) upon the Nusselt Prandtl modulus Nu/Pr^1/3 and Fanning friction factor f. The investigation is carried out for three different wire sizes, 0.052 in., 0.063 in., 0.072 in. respectively.</p> <p> Analysis of the various dimensionless numbers computed from the measurements of the present study indicates that the heat transfer increases by as much as 250% for low values of pitch to diameter ratio, though at the cost of a much larger increase in pressure drop. Consequently, the tubes using coiled wire turbulence promotors can be employed with advantage for cases where pumping power is not the dominating factor and reduction in weight and size of the equipment are more important.</p> <p> The experimental data are empirically correlated in terms of the ratio of Nusselt numbers for the tubes with turbulence promoters to the empty tube as a function of Reynolds number and pitch to diameter ratio. Nu/Nuo = K(Re)^a (H/D)^-0.3 </p> <p> To evaluate the net effect of coiled wire turbulence promotors, the ratio j/f = [Nu/RePr^1/3]/f is plotted against Reynolds number. The curves for the coiled wire turbulence promotors fell below the theoretical curve for the empty tube indicating that coiled wire turbulence promotors are not advantageous in terms of heat transfer per unit pressure drop.</p> / Thesis / Master of Engineering (MEngr)

Detonation-based combustion systems are desired for propulsion and power systems due to their ability to provide high thermal efficiency and enable supersonic flight. Detonation combustion in hypersonic flows has traditionally been realized using an oblique detonation wave. However, oblique detonation realization and stabilization in combustion systems is challenging. This communication presents an alternative realization of a detonation mode of combustion through a reacting Mach stem. The detonation is experimentally realized in a hypersonic reacting facility, which is optimized for Mach 5 flow at the combustor inlet and includes a 2D-wedge to stabilize hypersonic reactions at high-enthalpy flow conditions. The Mach stem detonation is analyzed with simultaneous 30 kHz schlieren and chemiluminescence imaging, which reveals the coupling between the Mach stem and the reaction. Further confirmation is provided by comparing the Mach number of the reacting Mach stem with the Chapman-Jouguet (CJ) detonation Mach number. It is found that the Mach number of the reacting Mach stem reaches 94% of the CJ detonation Mach number, confirming that the reacting Mach stem realizes a detonation mode of combustion.

Three existing condensation heat transfer models are validated using 544 experimental data points for pure refrigerants and refrigerant mixtures. The Cavallini et al. (1999) model predicts well with the pure-refrigerant data sets. However, the Cavallini et al. (1999) model fails to predict the refrigerant-mixture data sets. The Yu and Koyama (1998) model, which is applicable for the pure refrigerants only, fails to predict most of the R22 data sets. The Kedzierski and Goncalves (1999) model, which is applicable for both pure refrigerants and refrigerant mixtures, yields relatively high mean absolute deviations for most of the pure-refrigerant data sets. The Kedzierski and Goncalves (1999) model does not account for the mass transfer thermal resistance in refrigerant mixtures. A new pure-refrigerant model and a new refrigerant-mixture semi-empirical model have been developed. Both the new models successfully predict the experimental data for pure refrigerant and for refrigerant mixtures.

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