Salınımlı akış koşullarında açık hücreli metal köpükte akış ve ısı geçişinin deneysel ve sayısal olarak incelenmesi

Oscillating flow and heat transfer in porous media is encountered in many engineered systems such as heat pipes, regenerators, Stirling engines, cooling units of nuclear power plants and reciprocating internal combustion engines. Due to substantial heat removal rates, there has been interest in using oscillatory flow in porous media for cooling high-power-density high-speed electronic components, as well. Heat transfer due to oscillating flow in traditional porous media (e.g. packed spheres) has been studied before. Metal (aluminum, copper, etc.) foams are relatively new class of porous materials. They have extremely large surface area density, up to 10000 m2/m3, and very high porosity, around 90%. The shape of cells of metal foams can be regarded as tetrakaidecahedra. Oscillating flow and heat transfer of air in metal foam has also been studied. However, heat transfer due to oscillating flow of water in metal foam has never been studied. In terms of flow and heat transfer in porous media, air and water, as working fluids, are very different: in water, an added momentum and heat transport mechanism called dispersion is important, while it is negligible for air flow. There is also a big difference in the effective thermal conductivity when the porous medium is saturated with air compared to water. This is in addition to the difference in Prandtl number and other thermophysical properties for the two fluids. There is a difference in compressibility between the two fluids and an expected splashing for the case of oscillating water flow. These differences are expected to produce vastly different flow field and temperature distribution in metal foam. In the current study, a 20-ppi (pores per inch) cylinder-shaped aluminum metal foam core with a porosity of 87% was tested under the conditions of steady and oscillating water flow. The core was made of 6101-T6-aluminum alloy and it was brazed into an aluminum tube with the designation code of 6061-T6. The test section was deliberately brazed in order to avoid high thermal contact resistance. Before installing the foam as a test section, the test setup was tested hydrodynamically as a qualification study. The porous media used for this step were packed beds of 1- and 3-mm steel spheres with a cylindrical bulk volume similar to that of the metal foam. All of the porous-media flow regimes reported in literature were found. Therefore the system was proven to reveal not only turbulent flow regime in porous media, but also regimes observed at flow conditions even with the slightest fluid motion, namely pre-Darcy and Darcy regimes. The packed bed was replaced by the metal foam and care was taken so as to prevent any leakage. For steady-state flow experiments, water inlet from an elevated tank and directly from the network were used as constant pressure sources for low and high flow rates, respectively. The system was set to be open rather than a closed loop. For measuring pressure loss, differential pressure sensors with changeable ranges were used. For the flow rate measurements, mass scales with different ranges and accuracies along with a stopwatch were used. A quadratic relation between pressure loss and velocity was observed as expected. The pressure gradients were modified obtain linear curves with slopes varying from zone to zone. These zones with different slopes denoted different regimes. Four different regimes, namely pre-Darcy, Darcy, Forchheimer and turbulent regimes, were identified along with the transitions among them. Two important foam parameters, permeability and form drag coefficient were calculated, and proven to have different values, for each regime. Finally, the already-reduced pressure gradient-velocity couples were modified with the purpose of displaying the relationship between non-dimensional quantities, which were the friction factor and Reynolds number. This step ensured that the square root of the permeability calculated in the Darcy regime was a viable characteristic length for Reynolds number. This idea had been originated from the fact that the flow crept encapsulating the ligaments of the foam and was related well to internal morphology of the foam. The oscillating flow experiments involved the use of a reciprocating mechanism, which also resulted in a closed system. Because the flow was transient in nature, the revolutions of the oscillation-generating mechanism were recorded with respect to time for velocity calculations. Besides, rather than a differential pressure sensor, two pressure transmitters located at both ends of the foam were used to avoid inertia-induced errors. The runs were completed with combinations multiple frequencies and flow displacements. The data was acquired using a data logger. Two frequency zones were identified. In the low frequency zone, the pressures had counteracting behaviors, whereas in the high frequency zone, those values were in parallel, still with a certain pressure difference. In both of the zones, the friction factors were higher than those of the steady-state flows, and lower than those found previously oscillating water flow experiments in packed beds of spheres. For the steady-state heat transfer experiments, holes were drilled along the wall of the foam to measure wall temperatures. Constant heat flux was introduced through the wall. In addition, inlet and outlet water temperatures were measured. The bulk temperatures were calculated using averaged temperatures from these two ends and the wall temperature distribution. The velocities were in Darcy, Forchheimer and transition regimes. Nusselt number for each velocity case was calculated using the corresponding wall and bulk temperatures. In addition to thermal entry lengths, exit lengths were also found. The thermal entry lengths were contrasted to their counterparts in literature as well as the Nusselt numbers. The Nusselt numbers in the fully developed region for Darcy flow matched the analytical solutions extremely well for the thermal non-equilibrium approach. The steady-state heat transfer mechanism was turned into a closed system using the same oscillation generator for the oscillating heat transfer test. The cycle-averaged wall temperature distribution was observed to be symmetric under oscillation. Therefore, only one half of the axial domain was studied. The cycle-averaged Nusselt number and temperature distribution were obtained. It was observed that the temperatures were lower for higher displacements and frequencies. The distribution was also more uniform. Nusselt number was correlated with respect to the kinetic Reynolds number. The steady-state flow and heat transfer were modeled in three dimensions and simulated using ANSYS finite volume tools. The flow results were in good agreement with their experimental counterparts. Therefore numerical approximations were made for hydrodynamic entry length since this value is extremely hard to determine experimentally. The heat transfer results on the other hand exhibited divergence from experimental findings at high velocities. This result was attributed to the fact that the current built-in model was originally for traditional porous media, not high-porosity foam. The oscillating flow and heat transfer were modeled in two dimensions because the propagation of the temperature required an extensive CPU time. The flow results matched the experimental values well. However, there was a significant mismatch of heat transfer results, exceeding the experimental uncertainty. This was a result of the same problem reported above and inadequate mixing due to the current turbulence model in the numerical tools.