Numerical investigation of two phase flow using automatic controller system and chaotic approach

Dissertation supervisor: Dr. Yuwen Zhang.Includes vita.An automatic and intelligent system to recognize the two-phase water-air flow regime in a vertical tube based on fuzzy logic and genetic algorithm is proposed. Two approaches, volume of fluid (VOF) and Eulerian model, were used for the numerical simulation of gas-liquid two-phase flow. Four different turbulence models, i.e., k-[epsilon] RNG, k-[epsilon] standard, Reynolds stress and k-[epsilon] realizable, were employed. Image processing procedure was implemented to obtain the flow pattern. It was found that the k-[epsilon] RNG gives best results for turbulence modeling and the fuzzy logic code predicts the flow pattern well. In addition, in the present study investigation of chaotic flow in a two and three dimensional closed-loop pulsating heat pipe has been carried out numerically. Constant temperature and heat flux boundary conditions have been applied for the heating (evaporator) section and only constant temperature for cooling (condenser) section. Water and ethanol were as used as working fluids. Volume of Fluid (VOF) method has been employed for two-phase flow simulation. Volume fraction of liquid and vapor in the pulsating heat pipe was investigated under different operating conditions. Approaches such as spectral analysis of temperature time series, correlation dimension, autocorrelation function, Lyapunov exponent and phase space reconstruction were used to investigate chaos in the pulsating heat pipe. Thermal resistance behavior was analyzed with respect to heating power and optimal points were found in case of thermal performance of the pulsating heat pipe. Spectral analysis of temperature time series using Power Spectrum Density showed existence of dominant peak in PSD diagram indicates periodic or quasiperiodic behavior in temperature oscillations at particular frequencies. Correlation dimension values for ethanol were higher than water under the same operating conditions. High values of correlation dimension referred to high frequency, small scale temperature oscillations, caused by miniature bubbles or short vapor plugs dynamically flowing in PHP tubes. Decay of autocorrelation function with respect to time indicated finite prediction ability of the system. Change in working fluid did not lead to any particular conclusion for ACF behavior. An O-ring structure pattern was obtained for reconstructed 3D attractor at periodic or quasi-periodic behavior of temperature oscillations.Includes bibliographical references (pages 167-176)

Experimental Investigation on the Ice Accretion Effects of Airplane Compressor Cascade of Stator Blades on the Aerodynamic Coefficients

In this paper the effects of ice accretion on the pressure distribution and the aerodynamic coefficients in a cascade of stator blades were experimentally investigated. Experiments were conducted on stage 67A type stator Controlled-Diffusion blades, which represent the mid-span of the first stage of the stator for a high-bypass turbofan engine. The measurements were carried out over a range of cascade angle of attack from 20° to 45° at Reynolds number of 500000. Experimental blade surface pressure coefficient distribution, lift and drag force coefficients, and momentum coefficients for clean blades were compared with those of the iced blades and the effects of ice accretion on these parameters were discussed. It is observed that the ice accretion on the blades causes the formation of flow bubble on the pressure side, downstream of the leading edge. By increasing the angle of attack from 20° to 35° , the bubble length decreases and the pressure coefficient increases inside the bubble region, constantly. In addition, for the iced blades the diffusion points at the suction side come closer to the trailing edge. In addition, it is found that by increasing the angle of attack up to 35° , the ice accretion has no significant effect on the lift coefficient but the drag coefficient increases comparing with the clean blades. More over at 40° and 45° , by increasing the flow interference effects between the blades, the iced blades experience higher lift and lower drag in comparison with the clean ones

Experimental Study of Air-Cooled Parallel Plate Fin Heat Sinks with and without Circular Pin Fins between the Plate Fins

Experiments were conducted to investigate forced convective cooling performance of an air cooled parallel plate fin heat sink with and without circular pin fins between the plate fins. The original parallel plate heat sink was fabricated consist of 9 parallel plates of length 53 mm with cross-sectional area of 1.4 mm in width by 20 mm height for each plate. The second heat sink has the same geometry of original one but with some circular pins between the plate fins. Thermal and hydrodynamics performances of the heat sinks have been assessed from the results obtained for the pressure drop, thermal resistance and overall performance with the free stream air velocity ranging from 4.7 to 12.5 m/s. Results show that the free stream air velocity has a significant effect on the thermal and hydrodynamics performance of the system. With increasing free stream velocity, the heat transfer coefficient increases and consequently the thermal resistance decreases while pressure drop increases due to higher inertial of fluid at higher velocities. Furthermore, at the same free stream air velocity the thermal resistance for the heat sink with circular pin is about 37.7% lower than that of the original heat sink

Experimental Study of Plasma Actuator Effects on Flow Field Separation Bubble around Blunt Flat Plate

In this paper, the air flow around a blunt flat plate with a rounded leading edge has been experimentally examined with and without the presence of a plasma actuator. Tests have been conducted with Reynolds numbers ranging from 104 to 105. Significant phenomena in this flow field is the flow separation at the leading edge of the body, which called separation bubble. There are two considerably dimensionless parameters in this experiment. One of them is the leading edge radius ratio to body thickness and other one is the ratio of maximum velocity induced by plasma actuator to free stream velocity. Geometries with the values of R/D=0, 1/16, 2/16, 4/16 were tested. For each geometry, the effectiveness of plasma actuator on the separation bubble is studied in different values of velocity ratio. The results show that, the effect of plasma actuator for the geometry with sharp edge (R/D=0), is negligible, while in geometry with rounded edge, the plasma actuator has significant effect on the separation bubble domain. This effectiveness is enhanced, by increasing of leading edge radius and velocity ratio, so that in rounded edge geometry (R/D=4/16) length of separation bubble is reduced about 75%

Experimental Investigation of Flow Control over an Ahmed Body using DBD Plasma Actuator

Ahmed body is a standard configuration of road vehicles and most of the studies of automobile aerodynamics are performed based on it. In this paper, the plasma actuator was used as an active flow control method to control the flow around the rear part of the Ahmed body with the rear slant angle of 25°. Experiments were carried out in a wind tunnel at two different velocities of U=10m/s and U=20m/s using steady and unsteady excitations. The hot-wire anemometer was used to measure the vortex shedding frequency at the downstream of the body. Pressure distribution was measured using 52 sensors and total drag force was extracted with a load cell. Furthermore, smoke flow visualization was employed to investigate the flow pattern around the body. The results showed that the plasma actuator was more effective on the pressure distribution and total drag force at the velocity of U=10m/s. In fact, by applying steady and unsteady excitations there was 7.3% and 5% drag reduction; respectively. While at the velocity of U=20m/s; the actuator had no significant effect on pressure distribution and total drag. As a remarkable result, the plasma actuator, especially in the steady actuation, has demonstrated its effectiveness on dispersing the longitudinal vortices and suppressing the separated flow on the rear slant at low velocities