H. Arabnejad

Scalable and durable polymeric icephobic and hydrate-phobic coatings

Ice formation and accumulation on surfaces can result in severe problems for solar photovoltaic installations, offshore oil platforms, wind turbines and aircrafts. In addition, blockage of pipelines by formation and accumulation of clathrate hydrates of natural gases has safety and economical concerns in oil and gas operations, particularly at high pressures and low temperatures such as those found in subsea or arctic environments. Practical adoption of icephobic/hydrate-phobic surfaces requires mechanical robustness and stability under harsh environments. Here, we develop durable and mechanically robust bilayer poly-divinylbenzene (pDVB)/poly-perfluorodecylacrylate (pPFDA) coatings using initiated chemical vapor deposition (iCVD) to reduce the adhesion strength of ice/hydrates to underlying substrates (silicon and steel). Utilizing a highly-cross-linked polymer (pDVB) underneath a very thin veneer of fluorine-rich polymer (pPFDA) we have designed inherently rough bilayer polymer films that can be deposited on rough steel substrates resulting in surfaces which exhibit a receding water contact angle (WCA) higher than 150° and WCA hysteresis as low as 4°. Optical profilometer measurements were performed on the films and root mean square (RMS) roughness values of Rq = 178.0 ± 17.5 nm and Rq = 312.7 ± 23.5 nm were obtained on silicon and steel substrates, respectively. When steel surfaces are coated with these smooth hard iCVD bilayer polymer films, the strength of ice adhesion is reduced from 1010 ± 95 kPa to 180 ± 85 kPa. The adhesion strength of the cyclopentane (CyC5) hydrate is also reduced from 220 ± 45 kPa on rough steel substrates to 34 ± 12 kPa on the polymer-coated steel substrates. The durability of these bilayer polymer coated icephobic and hydrate-phobic substrates is confirmed by sand erosion tests and examination of multiple ice/hydrate adhesion/de-adhesion cycles.

CFD Simulation of Round Impinging Jet and Comparison With Experimental Data

In this work, fluid dynamics of a turbulent round impinging jet has been studied using Computational Fluid Dynamics (CFD) and the results have been compared with experimental data from the literature. The fluid was water with density of 1000 kg/m 3 and the average velocity of the submerged jet was kept constant at 10.7 m/s while the liquid viscosity varied from 1 cP to 100 cP. Different turbulence models including k-ε, k-ω and Reynolds Stress Model (RSM) have been employed in ANSYS FLUENT and the predicted axial and radial velocity profiles at various distances from the wall are compared with LDV data. It was observed that at locations away from the target wall, predicted velocities are comparable to the measured velocities for all the viscosities. However, near the wall, the deviation between the CFD predictions and experimental measurements become noticeable. The performance of k-ω model and RSM are found to be better than the k-ε model especially for the highest viscous fluid, but no model was found to be superior for all conditions and at all locations. NOMENCLATURE D Nozzle diameter H Standoff distance in submerged geometry um Maximum velocity at jet centerline before impingement; velocity at y = δm V Initial jet velocity x Coordinate parallel to the impingement plane y Coordinate normal to the impingement plane δ Boundary layer thickness δm Distance from wall to the maximum velocity line in the wall-jet region INTRODUCTION Impinging jets have been widely used in industry to enhance the process of heat and mass transfer. Thermal management is vital for electronic equipment and a challenging area for aerospace engineering and many other applications. Gas or liquid impinging jets are used to control the operating temperature of electronic circuits and their components. In aerospace engineering and turbine design, heat transfer and hydrodynamic calculations of jet impinged surfaces is of great importance. Controlling the mass transfer in jet deposition processes and erosion study of slurry jets are other examples of engineering problems which require prediction of the fluid behavior in jet impingement configurations (Arabnejad et al. 2015a; Arabnejad et al. 2015b; Mansouri et al. 2014; Mansouri et al. 2015). In addition to the direct use of a hydrodynamic solution of the impinged jets, heat transfer calculations are possible if velocity of the fluid near the wall is known. Different configurations of impinging jets have been studied in the literature using empirical, numerical or theoretical methods. Donaldson and Snedeker (1971) measured the heat transfer characteristics of a circular impinging jet and introduced a correction term to use the laminar heat transfer coefficient for turbulent flows. Elison and Webb (1994) studied local heat transfer experimentally for a liquid jet impinging a flat surface with uniform heat flux. Womac et al. (1993) conducted experimental investigations of liquid jet impingement cooling of square heat sources in free-surface and

Calculation of Turbulent Boundary Layer for a Slot Jet Impingement on a Flat Surface

The objective of this study is to characterize flow parameters for two-dimensional turbulent jets impinging on a flat surface. An integral form of the momentum equation has been used to obtain a hydrodynamic solution. The boundary layer was divided into three regions, stagnation zone, developing zone and fully developed zone for free-surface and free shear, and into two regions, stagnation and wall jet zone for submerged jet configurations. A nonlinear ordinary differential equation has been obtained for frictional velocity at each zone using a logarithmic velocity profile with Coles’s law of the wake and solved numerically to predict wall shear stress as well as boundary layer and momentum thicknesses. The proposed method is more straightforward and computationally less expensive in calculating the main flow parameters as compared to turbulent flow models such as RANS and LES. Predicted wall shear stresses for a submerged jet were compared to experimental data for different cases and showed agreement with experimental data.Copyright