GCRIS

Mixing in microfluidic systems is a challenging problem since the flow is almost always laminar hence the transport of species depends only on diffusion. In this thesis, a passive micromixer is designed for mixing two miscible liquids in a micro channel. The mixer utilizes series of throttles, which reduce the diffusion length, placed along the mixing channel. Although there are many fabrication techniques to manufacture microfluidics, the throttles constituting the micromixer are specially designed to be fabricated by micromilling. Here, micromilling is particularly chosen because of its flexibility and productivity compared to other microfluidic fabrication methods. For design purposes, firstly the parameters affecting the mixing performance are determined. These parameters are defined as the Reynolds number (Re), non-dimensional throttle size and the number of throttles. Then, micromixers are simulated using computational fluid dynamics tools available in COMSOL Multiphysics at different parameter levels. As a result, it is found that micromixers with 15 to 20 throttles can achieve a mixing efficiency greater than 80% in very low Re flows. After optimizing the design, the micromixer is improved by shifting the throttles off the axis of the mixing channel. It is seen that the improvement increases the mixing efficiency to 84%.

The aim of the study is to get the best and the most efficient design of Solar Chimney Power Plant (SCPP) considering different chimney and collector parameters. SCPP is one of the most promising applications of solar energy because of the high reliability of the system in addition to the low cost of long-term energy production and low maintenance needs. Because of the reasons above, it has been a focus for researchers to increase efficiency of such systems. Solar chimney power plant is a thermal system containing three basic components; the solar collector, the solar chimney and the wind turbine. SCPP has basic working principles which are the effect of greenhouse, chimney drag and conversion to kinetic energy. The air below the semi-transparent collector (glass) is heated up by the solar radiation which is coming from the top of the collector surface. The heated air moves by buoyancy force to the center of the collector where the turbine is located. The heated air leads to a flow through the turbine and drives the turbine which rotates a generator and electric energy is obtained from that rotation. In this study, a 2-D computational fluid dynamics (CFD) analysis for the entire system of solar chimney power plant is done by using commercial program ANSYS 18.2. After verifying the CFD results of the numerical model with the results of the experimental work and obtaining almost identical results, the main parameters like the height of tower, the diameter of the tower, the tower configuration, the diameter of the collector, the height of collector and the inclination angle of the collector which can effect on performance of SCPP has been changed and the influence of varying these parameters on the effectiveness and performance of the solar chimney system is studied. It is found through this study that all of the chimney and the collector parameters which are mentioned above have a direct impact on the effectiveness and performance of SCPP. Depending on the numerical results, it is found that the height and the diameter of the tower is considered as the most effective parameters which can affect largely on the value of the output power of SCPP and the height and diameter of chimney equal to 3.5 m and 25 cm are the best values considering the performance of the chimney. Also results shows that the diverging chimney configuration is the best tower geometry that can be utilized and it can improve and increase the performance of SCPP largely. The numerical findings shows that the increase of solar collector diameter leads to increasing the output power and the best value for the diameter of the collector is 400 cm (the maximum value). Furthermore, it is concluded that the height and inclination angle of the solar collector has an important effect on the output power and the best design for solar collector parameters is 6 cm height of the solar collector with 0 degree for the inclination angle of the solar collector.

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