A review of thermal conductivity of various nanofluids

Structural vibrations caused by a flowing fluid. Whenever a structure is exposed to a flowing fluid,…

Flow-induced vibration

Electronics cooling with nanofluids: A critical review

Experimental investigation of graphene oxide nanofluid on heat transfer enhancement of pulsating heat pipe

Recent advances on the fundamental physical phenomena behind stability, dynamic motion, thermophysical properties, heat transport, applications, and challenges of nanofluids

Visualization and comparative investigations of pulsating ferro-fluid heat pipe

In this study, an experimental investigation was conducted from a thermal performance standpoint on closed-loop pulsating heat pipes (CLPHPs) with four different fluids and their water-based binary mixtures as working fluids with volume mixing ratios of 3:1, 1:1, and 1:3. Ethanol and acetone as two types of fluids that are soluble in water and, to unprecedentedly compare the behavior of insoluble mixtures with the soluble ones as the working fluids, toluene and hexane as two types that are insoluble in water were used. Additionally, to predict the thermal performance of the pure, soluble binary, and insoluble binary fluids simultaneously for the first time, a correlation was derived.

Experimental investigation on thermal performance of closed loop pulsating heat pipes with soluble and insoluble binary working fluids and a proposed correlation

Pulsating heat pipes (PHPs) are one of the new devices used for cooling in several applications such as electronic and aerospace systems. Their low cost, effectiveness at various conditions, being equipped for passive energy conversion, and well distribution of temperature compared to conventional heat pipes are among the reasons of their popularity. To investigate the effect of surface tension of the working fluid on the behavior of PHPs, a copper heat pipe is fabricated with inner and outer diameters of 2 mm and 4 mm, respectively. Five different concentrations of cetrimonium bromide (C-Tab) surfactant are dissolved in water and are tested with a filling ratio of 50% (± 1%). A piece of glass is placed in the adiabatic section to make the flow visualizations possible. Thermal resistance and flow visualization results are compared. Visualization of the flow shows that by increasing the surfactant concentration, annular and semi-annular regimes can be observed at lower powers. It is also detected that by increasing the surfactant concentration, thermal resistance will decrease, while the maximum heat flux is reduced. This can be explained by thinner film thickness in the evaporator. The lowest thermal resistance was detected to be 0.44 K/W for the 0.25 g L−1 C-Tab concentration at 25 W heat input power which shows a significant improve of 77.5% compared to pure water at the same power.

Investigation and visualization of surfactant effect on flow pattern and performance of pulsating heat pipe

A study is carried out to investigate the effect of surface topology on the boundary layer and the near-wake flow around a rectangular cylinder with side-ratio of 2.5 and fully-round corners (half-circular leading and trailing edges) at a Reynolds number based on thickness of Red = 2, 500 . The topology is defined using Fourier modes with an amplitude of 5% of the thickness, along the perimeter only (2D geometry) as well as along the perimeter and the span (3D geometry) of the cylinder. The study is motivated by understanding the flow around parachute suspension lines, which exhibit geometrical features and parameters that, while not an exact replica of, are reasonably represented by the canonical geometry investigated. Single-component molecular tagging velocimetry is employed to measure the streamwise velocity profiles at different streamwise locations above the surface and in the wake of the cylinder. Profiles of the mean and the root-mean-square velocity are employed to analyze the development of the boundary layer and the separated flow on the cylinder at three positive angles of attack: α = 0°, 2° and 5°. Results show that the most significant effect of the surface topology is found when a peak in the topology resides at the leading edge (LE). In this case, separation occurs earlier, and the separated shear layer is thicker, displaced farther away from the cylinder and exhibits stronger unsteadiness than the smooth cylinder and the cylinder with topology with a valley at the LE. These observations hold for both the 2D and the 3D topology, with some differences between both seen at α = 0° and 2°. At α = 5°, these differences are not very significant, and the 2D profiles reasonably approximate those of the sectional 3D flow. Finally, examination of the streamwise evolution of the wake mean centerline velocity (?̅?c) at α = 0° demonstrates that the reverse-flow and the closure length in the near-wake are only affected slightly by the presence of surface topology for both 2D and 3D geometries. Farther downstream, the smooth-cylinder and the 3D topology exhibit similar ?̅?c streamwise evolution, leading to “far-wake” behavior that is different from the 2D geometry.

Influence of surface topology on boundary layer and near-wake behavior of rectangular cylinders

The main motivation behind this study is to understand the susceptibility of the suspension lines of Precision Airdrop Systems (PADS) to flow-induced vibrations by galloping instability. Such vibrations are undesirable as they negatively affect the controllability of PADS. Suspension cables of PADS have a cross-section of a roundcorner rectangle with surface geometrical features (topology) produced by the braided structure of the cables. Therefore, an experimental study is conducted to provide fundamental understanding of the effects of surface topology on the instability of round-corner rectangular cylinders to galloping. The study utilizes a baseline cylinder with smooth (topology-free) surface, a side (chord-to-width) ratio of 2.5 and fully-round corners. Results from the baseline model are compared against those from cylinders with the same geometry but with added surface topology. Fourier modes are employed to define different surface topologies with an amplitude of 5% of the cylinder width d along the perimeter only (2D geometry), as well as along the span (3D geometry). Boundary-layer-resolved measurements are conducted using single-component molecular tagging velocimetry at Reynolds numbers based on d of Red = 1, 100 and 2, 500, and cylinder angle of attack of 0°, 2° and 5°. A complementary investigation using direct force measurements is also carried out to determine the galloping stability characteristics of the cylinders examined. Results suggest that the topology could lead to destabilization to galloping when the associated geometrical features cause the flow to separate along the corner region of the cylinder, promoting earlier separation relative to the baseline smooth geometry. On the other hand, reattachment of the separated shear layer, which has a stabilizing effect on galloping, is promoted by the presence of spanwise topology variation (3D geometry) and increasing Red.

Kian Kalan

Papers

Visualization and comparative investigations of pulsating ferro-fluid heat pipe

Experimental investigation on thermal performance of closed loop pulsating heat pipes with soluble and insoluble binary working fluids and a proposed correlation

Investigation and visualization of surfactant effect on flow pattern and performance of pulsating heat pipe

Influence of surface topology on boundary layer and near-wake behavior of rectangular cylinders

Characterization of the Aerodynamics of Rectangular Cylinders with Surface Topology