Browsing by Subject "Convection"
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Item The effect of radial convection on cell proliferation Iin bone tissue engineering(2009-04) Shao, WeiruLarge bone defects are frequently encountered during surgeries. Traditional methods of repair are limited by bone graft availability and increased surgical morbidity. Tissue engineered bone tissue has many clinical advantages. However, its current technology is limited by implant size and lacks of immediate nutritional perfusion once the tissue is implanted. Objective: To sustain cell growth and proliferation in a three-dimensional scaffold unit with radial convective flow. Material and Methods: Fetal rat calvarial cells were harvested and loaded into 1x1 cm hydroxyapatite cylinders. Microperforated hollow fibers were placed at the center of the cylinders to generate radial convective flow with oxygenated cell culture medium under hydraulic pressure. Live cell densities within the blocks were determined after 8 days of convection. Results: Radial convection sustained cell growth and proliferation better than simple diffusion at all three zones of the cylinders: center, outer, and rim. Conclusion: Radial convective flow is capable of supporting cellular function and proliferation in small scaffold units. The design of the radial convection units and their system parameters are validated by this study. The results are very useful to devise future tissue engineering studies involving radial convective flow.Item Mixed convection in horizontal fluid-superposed porous layers(2013-08) Dixon, John MarkMixed convection in horizontal fluid-superposed porous layers is studied in the following work. Much research has been done in the field of natural, mixed, and forced convection in a porous layer. Several studies have investigated natural and forced convection in a two-domain system that includes a porous and a fluid layer, but mixed convection has not been addressed. This problem can be found in many natural and engineering applications. Some examples include beach sand, human lungs, bread, gravel, soil, rock, packed bed reactors, fiberglass insulation, thermal energy storage systems, electronic cooling, crude oil extraction, nuclear reactors, and the list goes on. The present study is motivated by the wide range of applications and seeks to fill the gap in the literature regarding mixed convection. The problem considers a long, narrow channel that is partially filled with a porous layer and has a fluid layer above the porous layer. The channel is partially heated on the bottom and cross flow along the length of the channel is added in varying degrees. The problem is studied at a fundamental level, with the governing equations being derived, non-dimensionalized, discretized, and solved numerically. The two layers are treated as a single domain and the porosity is used as a switching parameter, causing the governing equations to transition from an extended form of the Darcy-Brinkman-Forchheimer equation in the porous layer to the Navier-Stokes equations in the fluid layer. This method avoids the need for interfacial boundary conditions to be explicitly defined at the interface between the two domains. Several dimensionless numbers are varied and their effects on the overall Nusselt number of the system are documented. The parameters varied include the Peclet number, the Rayleigh number, the porous layer height ratio, the Darcy number, the Prandtl number, and the conductivity ratio between the solid and fluid phases. In addition, the impact of the various additional terms in the extended form of Darcy's law is investigated and documented as well. The conductivity ratio, Darcy number, porous layer height ratio, Rayleigh number, and Peclet number all have a strong effect on the overall Nusselt number of the system, while the Prandtl number, the Brinkman term, the Forchheimer term, and the convective terms have a negligible effect. A critical Peclet number was observed, where the Nusselt number is a minimum, and was shown to be proportional to the Rayleigh-Darcy number and inversely proportional to the porous layer height ratio. A critical porous layer height ratio was also found, where the Nusselt number is a minimum, and was shown to be proportional to the Rayleigh-Darcy number and inversely proportional to the Peclet number. The streamlines capture the transition from the natural convection regime to the forced convection regime. In the transition region the flow patterns have characteristics of both domains. The isotherms capture the plume flow and show the influence of the cross flow on the shape and character of the plume. An experimental apparatus is designed in order to collect data over a similar range of parameters as explored numerically. The average error between the numerical and experimental results is 30%, with a peak of 67%. The numerical results show good agreement with the experimental data within the bounds of uncertainty. The experimental results confirm the presence of a critical Peclet number. However, they do not show the same trends at intermediate porous layer height ratios. The effect of the porous layer height ratio, η=h_p⁄H, on the Nusselt number is shown to be small in the range of η = 0.5 to η = 1 and large in the range of η = 0 to η = 0.5. Also, the transition to the forced convection regime occurs earlier for the numerical results than it does for the experimental results. This points towards future research opportunities that focus on the lower range of porous layer height ratio values.Item Thermal convection at high Rayleigh numbers in compressed gases.(2007-12) Srinivasan, VinodThis study focuses on the heat transfer relationship for turbulent convection in a layer of fluid heated from below. Results are presented in the form of the Nusselt number as a function of the Rayleigh number, for Rayleigh numbers ranging from 2 × 10 9 to 3.1 × 10 12 . High Rayleigh numbers are attained by pressurizing nitrogen, argon and krypton to pressures of up to 80 bar. The experimental apparatus is designed with close attention to the effects of conduction through the insulating sidewalls at low Rayleigh number, and to the effects of variable properties that may affect the Boussinesq approximation at high Rayleigh numbers. The results show a relationship between the Nusselt and Rayleigh numbers that is close to a power-law with an exponent of 1/3 for the Rayleigh number. There is no visible transition, or incipient transition to a power-law regime with an exponent of 1/2, as has been theoretically predicted by some investigators. It is argued that various other values of the exponent that are found in the literature are either due to conduction effects at low Rayleigh numbers (leading to a lower exponent) or variable properties (leading to a higher exponent) at high Rayleigh numbers. However, the precise mechanism of energy transport that leads to an exponent of 1/3 remains unclear. While a large-scale recirculation has been observed in experimental apparatuses, there remains uncertainty as to the manner in which this flow affects the stability of the thermal boundary layer. Local temperature measurements were taken using a 76μ m thermocouple probe. The temperature measurements are at significant variance from the expected temperature distribution in the thermal boundary layer. Conduction effects in the probe are shown to be significant. Temperature statistics measured with the probe show some averaging over high frequencies.