Thermal conductivity and dynamic viscosity are two critical properties of nanofluids that indicate their heat transfer performance and flow. Nanofluids are prepared by dispersing mono or several organic or synthetic nanoparticles in selected base fluids to form mono or hybrid nanofluids. The qualitative and quantitative stability measurement of nanofluids will then be addressed, followed by a detailed discussion on how the dispersion of nanoparticles in water (W), ethylene glycol (EG), and the mixture of W:EG 60:40% by volume affects the thermal conductivity and dynamic viscosity ratio. The data comparison demonstrated that the thermal conductivity ratio increases with increasing normalized concentrations, the bulk temperature of nanofluids, and the smaller nanoparticle size. The dynamic viscosity ratio is multiplied by the normalized concentration increase. Nevertheless, as the bulk temperature climbed from 0 to 80°C, the dynamic viscosity ratio was scattered, and the dynamic viscosity ratio trend dropped with increasing particle size. While the majority of nanofluids enhanced thermal conductivity ratio by 20%, adding carbon-based nanoparticles to synthetic nanofluid increased it by less than 10%. The disadvantage of nanofluids is that they multiply the dynamic viscosity ratio of all nanofluids, which increase power consumption and reduces the efficiency of any mechanical system.

Hybrid systems are dynamic systems that arise out of the interaction of continuous state dynamics and discrete state dynamics. Switched systems, which are a type of hybrid system, have been given much attention by control systems research over the past decade. Problems with the controllability, observability, converseability and stabilizability of switched systems have always been discussed. In this paper, the trend in research regarding the stability of switched systems will be investigated. Then the variety of methods that have been discovered by researchers for stabilizing switched linear systems with arbitrary switching will be discussed in detail.

Among the various methods for enhancing heat transfer in a heat exchanger, a passive method of inserting a continuous swirler inside a heated tube provides a secondary flow along the fluid that reduces the thickness of the thermal boundary layer, thus increasing the efficiency of convection heat transfer performance. The research’s primary goal is to conserve energy, materials, and money by operating efficient heat exchanger equipment. However, the continuous swirler along the fluid flow creates a persistent obstruction, which amplifies the friction factor and increases the working fluid’s energy loss. As a result, this research presented the twisted delta winglet swirler (TDWS), a new design of a decaying swirler that uses delta winglets twisted to 180° to produce a swirling flow along the tube. The swirler comprises four twisted delta winglets arranged in a circle with a diameter 6% smaller than the tube and a length of L/D=2.2. It was placed at the entrance to a heated tube test section with a diameter of 0.016 m and a length of L/D=93.75. The Reynolds Stress Model was used to simulate the flow domain with a water-ethylene glycol mixture was chosen as the working fluid. TDWS transformed the uniform inlet flow from potential energy to high kinetic energy, resulting in a high intensity of swirling flow downstream of the circular tube up to L/D=46.88 before decaying and reaching a steady state. Compared to other decaying swirlers, TDWS obtained one of the lowest relative friction factors, 1.36, with this flow. The maximum global relative Nusselt number increased by only 11% because this value considered the area where the flow reached a steady state. Since the TDWS is a decaying swirler, the thermal-hydraulic performance reached unity along the tube. However, the optimal performance of TDWS in the plain tube with a length of L/D=93.75 can be found if the dimension or geometric configuration of the TDWS is modified, or two or more TDWS may be placed in an array orientation.

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