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Flow control is an important issue in the performance of aircrafts and many fluid mechanical devices. In recent years, synthetic jet actuators have shown good promise in delivering active flow control. Unsteady three dimensional incompressible laminar numerical studies were performed on single and two-orifice synthetic jet actuators in order to investigate the benefits of dual circular orifice synthetic jets over a single one for higher circulation production. The flow features of synthetic jets and the circulation of the vortex rings from one orifice and two-orifice actuators are examined and compared in this paper. Two dominant parameters in two-orifice actuators are the orifice diameter and spacing between them. The effects of these parameters on the vortex rings produced were investigated in detail. It was found that the generation of two distinct and non-interacting vortex rings has direct dependency on the spacing between the orifices. It is important that the vortex rings do not interact with each other to prevent any circulation cancellation, and in this regard, the present study suggests that a minimum spacing related to the diameter of the orifice (Do) and the dimensionless stroke length (L) is required (~1.5 L*Do). The present study also demonstrates that it is possible to increase circulation by more than 160 percent with a fixed input energy as a result of having two orifices with smaller diameters but higher dimensionless stroke lengths using two-orifice actuators. NomenclatureD = diameter of diaphragm, cavity or orifice f = oscillation frequency of diaphragm h = height of orifice H = height of cavity L = stroke length n = number of orifices r = radius of diaphragm, cavity or orifice Re = Reynolds number u = instantaneous velocity U = characteristic velocity ∆ = peak-to-peak displacement at the diaphragm centre ߥ = molecular kinematic viscosity Г = circulation ߩ = density Superscripts . = derivative with respect to time -= time average ˜ = space average Subscripts c = cavity value o = orifice value 1

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In this paper efficiency enhancement of a small scale closed solar thermal Brayton cycle is investigated by combining it to a simple organic Rankine cycle. Brayton power cycles are generally known as the enabling technology for high temperature solar power towers due to their higher efficiencies compared to other power cycles. Unlike conventional solarthermal plants, which concentrate the sun's energy to generate steam for driving a turbine, the Brayton thermodynamic does not use water. Instead, the concentrated solar energy is used to heat compressed air, which then expands through a gas turbine to generate power. Irreversible loss in compressor and turbine, the operating temperature of solar collector and recuperator effectiveness are the main features that limit the net power output of the system which should be considered and analyzed. The exhaust of the gas turbine is still at high temperature that should be cooled down before entering the compressor. Thus, this heat can be utilized to operate a low temperature Rankine cycle and increase the system efficiency and power generation.Operating points of off the shelf micro-turbines and steam turbine with parabolic solar dish concentrator of various concentrating ratios are considered. Thermodynamic analysis is applied, by using the first and second law of thermodynamics, to obtain the optimum temperature of solar collector, minimum irreversibility rates to maximize the efficiency and net power output of the system at various steady-state conditions.Results show that for the closed solar thermal Brayton cycle the maximum overall first law efficiency of the system can be increased of more than 5% by combining a simple Rankine cycle to recover the exhaust heat and a significant 20% increase in the second law efficicency. The system efficiency is related to the solar concentration ratio with an optimum operating temperature and the choice of micro-turbine. On this basis, both the overall efficiency and the total output power may reach their maximum value by optimizing the pressure ratio. In a small scale closed solar thermal Brayton cycle combined by a Rankine cycle with a micro turbine operating at its highest compressor efficiency, the operating conditions can be optimized in such a way that the system produces maximum net power output or having the highest overall efficiency.