Buckling of jets in electrospinning

Various buckling instabilities of electrospinning jets were observed and compared with the buckling instabilities of uncharged fluid jets. Buckling instability arises due to jet compression at impingement on a collector surface and occurs independently of the electrical bending instability. The velocity, diameter, density and viscosity of the electrospinning jets are the key factors that determine the buckling frequency. The electrically charged jets impinging onto grounded, horizontal or inclined (wedge-like) electrodes moving laterally at a constant velocity are studied experimentally. Straight and bending (electrospinning) jets emerge at short and sufficiently long inter-electrode distances, respectively. The experiments show that both straight segment and bending jets, when impinging onto a counter-electrode, buckled and produced patterns of meandering deposits. In the case of bending electrospun jets these short-length buckling patterns were superimposed on the bending loops found in the deposits. Buckling-related and bending-related morphologies are easily distinguishable. The buckling patterns have frequencies of the order of 10⁵-10⁶ Hz, whereas the bending loops are formed at the frequencies of the order of 10³ Hz. The deposited buckling patterns include sinuous, zigzag-like, figures-of-eight, recurring curves, coiled and other structures that resembled many patterns created by uncharged jets of highly viscous fluids impinging a hard flat surface. In addition, several new morphologies which were not observed before with uncharged jets were found. The experimentally measured frequencies of the buckling patterns were compared to the theoretical predictions and a reasonable agreement was found.     

Analytical and numerical assessments of local overpressure from hydrogen gas explosions in petrochemical plants

Accurate prediction of pressure rise is important for safety assessments of a petrochemical plant in the event of an explosion accident. The sudden pressures arising from gas explosions at various hydrogen concentrations in air have been predicted analytically and numerically. These solutions were compared against experimental data. The analytical solution, based on the self‐similar solution for pointwise strong explosions in an open space, which assumed no energy loss and premixed fuel‐air mixture, reasonably predicted the explosive‐ignition detonation case while the numerical solutions were more suitable to model spark‐ignition deflagration cases that accounted for the effect of turbulence arising from three‐dimensionality and presence of obstacles in the computational domain. Comparison of both analytical and numerical results against experimental data indicates that their differences are within a 30% margin. The analytical model presented herein can be useful for field engineers who want conservative estimates of the overpressure resulting from explosive‐ignition detonation. Copyright © 2016 John Wiley & Sons, Ltd.     

Nanofiber coating of surfaces for intensification of drop or spray impact cooling

A novel enhancement of drop and spray cooling for microelectronic and radiological elements and server rooms requiring extremely high heat fluxes is proposed. The key idea of the method is to cover the heat transfer surfaces with electrospun non-woven polymer nanofiber mats. The mats are permeable for water drops. The enhanced efficiency of drop cooling in the presence of nanofiber mats observed experimentally results from full elimination of receding and bouncing of the drops, characteristic of the current spray cooling technology. Therefore, the drops evaporate completely, and the large cooling potential associated with the latent heat of water evaporation is more fully exploited. This is paradoxical: the best cooling can be provided by a “fur overcoat”! The proposed cooling method alone may lead to a breakthrough in further miniaturization of microelectronic chips, optical and radiological elements and accelerate the development of a new generation of computers. In order to check the suitability of different materials for the drop and spray cooling applications, the thermal and structural properties of nanofiber mats based on four different polymers have been measured over a wide temperature range. Based on the results of these measurements, the most suitable materials have been chosen.     

Analytical solution of stresses and material birefringence inoptical fibers with noncircular cladding

An analytical method for the calculation of thermal stress in polarization-maintaining optical fibers is presented. The method is based on the thermoelastic potential and is appropriate for structures with a noncircular stress-cladding boundary. The analytical solution is compared with the results of the finite element method (FEM) for the case of an elliptical boundary