Two shell-and-tube heat exchangers (STHXs) using continuous helical baffles instead of segmental baffles used in conventional STHXs were proposed, designed, and tested in this study. The two proposed STHXs have the same tube bundle but different shell configurations. The flow pattern in the shell side of the heat exchanger with continuous helical baffles was forced to be rotational and helical due to the geometry of the continuous helical baffles, which results in a significant increase in heat transfer coefficient per unit pressure drop in the heat exchanger. Properly designed continuous helical baffles can reduce fouling in the shell side and prevent the flow-induced vibration as well. The performance of the proposed STHXs was studied experimentally in this work. The heat transfer coefficient and pressure drop in the new STHXs were compared with those in the STHX with segmental baffles. The results indicate that the use of continuous helical baffles results in nearly 10% increase in heat transfer coefficient compared with that of conventional segmental baffles for the same shell-side pressure drop. Based on the experimental data, the nondimensional correlations for heat transfer coefficient and pressure drop were developed for the proposed continuous helical baffle heat exchangers with different shell configurations, which might be useful for industrial applications and further study of continuous helical baffle heat exchangers. This paper also presents a simple and feasible method to fabricate continuous helical baffles used for STHXs.

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In this letter, a compact ultrawideband polarization diversity MIMO antenna system with dual band‐notched characteristics is proposed. The proposed antenna element consists of a metal ground with a stepped slot etched on it. The stepped slot is fed by a 50‐Ω microstrip line on the top layer. Dual notched bands (5G WLAN and 8G X band) are achieved by etching an open ended slot and a split ring resonator in the ground plane, respectively. Two identical antenna elements are arranged orthogonally and fed perpendicularly to obtain polarization diversity and enhance port isolation. A simple Y‐shaped defected ground structure is etched in the ground plane to suppress mutual coupling between two antennas and improve the impedance matching. Diversity performance in term of envelop correlation coefficient (ECC) and mean effective gain (MEG) are studied. The simulated and measured results show good agreement, which demonstrates that the proposed MIMO antenna system can be a good candidate for UWB MIMO system applications. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:626–630, 2016

Dielectric materials can store and release electrical energy quickly and efficiently and have potential applications in the fields of rail transportation, air and space detection, and electromagnetic weapons. However, the most promising dielectric polymer composites under research suffer either from unsatisfactory energy density (U e ) or from increasing the U e at the cost of energy efficiency (η). Herein, by the solution casting method, a nanocomposite film is fabricated by introducing trace selfassembly phase-transitioned lysozyme (PTL) modified boron nitride nanosheets (mBNNS) into a blend matrix consisting of poly(vinylidene fluoride−hexafluoropropylene) P(VDF−HFP) and poly(methyl methacrylate) (PMMA). The results suggest that PTL helps improve the interfacial compatibility of the corresponding nanocomposites via hydrogen-bonding interaction effectively. The nanocomposite film with 5 wt % mBNNS shows remarkably enhanced breakdown strength (E b ) of ∼500 MV/m and U e of 14.9 J/cm 3 , which are 166% and 244% of the blend matrix, respectively. Meanwhile, η of the nanocomposite film reaches ∼71% because of the clipping effect of linear PMMA on the large ferroelectric crystal phase of P(VDF−HFP) and the barrier effect of the highly insulating two-dimensional (2D) mBNNS, which effectively reduces the relaxation and leakage losses. Our research results show that by using a low-loss matrix and trace high-insulation 2D nanosheets, it is possible to achieve dielectric materials with high η and high U e at the same time.

A compact coplanar waveguide (CPW) fed ultrawideband (UWB) slot antenna for multiple‐input‐multiple‐output (MIMO)/diversity applications is presented in this letter. The antenna consists of two coplanar waveguides (CPWs) feeding circular radiating elements and a protruded stub, which is placed at 45° between them. The circular radiating elements are arrange orthogonally and fed perpendicularly to obtain polarization diversity. Notched band at X‐band communication downlinks is achieved by etching split ring resonator slots in the radiators and the other notched band at WLAN system is achieved by the collaboration of arc‐shaped strips and the protruded stub. The protruded stub not only can help to achieve band rejection but also can reduce mutual coupling between two ports. As all of the structures are printed in one layer, it can be very easily to be integrated with RF front‐end circuits. Simulated and measured results show that the proposed antenna meets the 10‐dB impedance bandwidth from 3.4 to 12 GHz and isolation better than 15 dB at most of the band. Good diversity performance in terms of envelope correlation coefficient and the ratio of mean effective gain demonstrates that the antenna is quite suitable for MIMO UWB applications. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:989–994, 2016