Flow-induced vibration analysis of conical rings used for heat transfer enhancement in heat exchangers

In the present work, flow-induced vibration characteristics of conical-ring turbulators used for heat transfer enhancement in heat exchangers are investigated experimentally. The conical-rings, having 10, 20 and 30 mm pitches, are inserted in a model pipe-line through which air is passed as the working fluid. Vortex-shedding frequencies and amplitude are determined and St-Re, Prms-Re variations are presented graphically. Flow-acoustic coupling is also analyzed experimentally. It is observed that as the pitch increases, vortex shedding frequencies also increase and the maximum amplitudes of the vortices produced by conical-ring turbulators occur with small pitches. In addition, the effects of the promoters on the heat transfer and friction factor are investigated experimentally for all the arrangements. It is found that the Nusselt number increases with the increasing Reynolds number and the maximum heat transfer is obtained for the smallest pitch arrangement.Turbulator Heat transfer enhancement Flow-induced vibration Vortex shedding

Effect of thickness on the dielectric properties and glass transition of plasma poly(ethylene oxide) thin films

Plasma poly(ethylene oxide) thin films at different thicknesses of 20, 100, 250, 500 nm were deposited by plasma-assisted physical vapor deposition on glass substrates between aluminum electrodes in capacitor form at 5 W plasma discharge power. The structural analyses were performed by Fourier transform in7frared spectroscopy. The dielectric properties such as dielectric constant kappa' and electric modulus M '' were defined by dielectric spectroscopy measurements. Dielectric spectroscopy measurements were performed in the angular frequency range of 10(-1)-10(6) rad/s and temperature range of 353-173 K range. The measurement results showed that alpha, beta, and gamma-relaxations, which are the expected relaxations in polymeric structures, are effective on total polarization in the investigated frequency and temperature range. Dielectric constant exhibits an increase till 500 nm, then reaches a saturation behavior. When resonance angular frequencies belonging to alpha-relaxation were fitted by Vogel-Fulcher-Tamman equation. It was observed that glass transition temperatures increase with decreasing thickness. These results support the influence of the dead layer to total polarization and the dynamics of the structure. Besides, it was shown that dielectric spectroscopy is a useful way to analyze the glass transition temperature in thin film form

Performance and flow-induced vibration characteristics for conical-ring turbulators

In the present study, the performance characteristics of the conical-ring turbulators are determined by means of the entropy-generation minimization method based on the second law and enhancement efficiency based on the first law of thermodynamics. The relations between these performance and flow-induced vibration characteristics have been examined. The maximum entropy generation, at the same Reynolds number, occurs in sequence by the conical rings with 10, 20 and 30 mm pitches, respectively. The conical rings are thermodynamically advantageous (Ns,aEntropy-generation number Enhancement efficiency Flow-induced vibrations

Thickness dependence of AC conductivity in TlSe thin films

Thalium selenide (TlSe), which has a lattice with tetragonal symmetry, is a member of the A(3)B(6) semiconductor group. The structure of TlSe is defined as chains where atoms inside are bonded with an ionic-covalent bond. TlSe thin films were deposited by thermal evaporation under a high vacuum on glass substrates. The structure of TlSe thin films is amorphous with a tetragonal structure. The AC conductivity measurements were operated via the measurements of capacitance and dielectric dissipation (tan delta) at room temperature. AC conductivity values change between 10(-11) and 10(-6) S/cm at the low-frequency side with decreasing thickness. Two different conduction regions were observed with increasing frequency. The region observed at the low-frequency side can be attributed to the motion of a chain-like part of the lattice, while the region observed at the high-frequency side can be attributed to side groups

Optimum design-parameters of a heat exchanger having hexagonal fins

In the present study, the effects of the heights and widths of the hexagonal fins, streamwise and spanwise distances between fins, and flow velocity on the heat and pressure-drop characteristics were investigated using the Taguchi experimental-design method. Nusselt number and friction factor were considered as performance statistics. An L18(21*37) orthogonal array was selected as the experimental plan for the five parameters mentioned above. While the optimum parameters were determined, the trade-off among goals was considered. First of all, each goal was optimized, separately. Then, all the goals were optimized together, considering the priority of the goals, and the optimum results were found to be a fin width of 14 mm, a fin height of 150 mm, spanwise distance between fins of 20 mm, and streamwise distance between fins of 20 mm for a flow velocity of 4 m/s.Heat-transfer enhancement Taguchi methods Cooling of electronics Hexagonal fins

Investigation of dielectric properties of PEGMEA/PEGDA nanocomposites containing in-situ photochemically prepared ?-Fe2O3 nanocrystals

Photopolymerization of PEGMEA/PEGDA monomer mixture consisting of FeCl3.6H2O metallic salt was performed using a Type I photoinitiator, namely 2-Hydroxy-4 '-(2-hydroxyethyl)-2-methyl propane (Irgacure 2959). Herein, we report the easy and one-step in-situ synthesis method of a rare e-Fe2O3 polymorph in a polymer matrix with different concentrations of Fe3+ salt. Thermal gravimetric analyses of the nanocomposites indicate an increase of char yield depending on both the synthesized nanoparticle concentration and good distribution of Nps. SEM images are also proof of formation of e-Fe2O3 Nps in polymer matrix. The obtained results indicate 4 and 20%w/w Fe3+ salt loading in the formulations resulted in higher char yield. Dielectric spectroscopy analysis and calculated glass transition temperatures show that there may be a critical ratio between 4 and 8% in the A1 and A2 samples, in comparison with the sample A3 which is the nanocomposite with 20% e-Fe2O3 Nps. A3 probably has excess free charges contributing to additional conduction

Effects of tapes with double-sided delta-winglets on heat and vortex characteristics

An experimental study was carried out for tapes with double-sided delta-winglets under different geometrical and flow parameters, including angles of attack (90°, 60° and 30°), winglet heights (8, 12 and 16 mm), pitch arrangements (25, 50 and 75 mm) and Reynolds numbers (3690, 10493 and 16906). By using the Taguchi experimental-design method, the optimum parameters of the turbulator were determined by obtaining the Nusselt number, friction factor, amplitude of fluctuation pressure of the vortices and the vortex-shedding frequency. First of all, each goal was optimized, separately. Then, all the goals were optimized together, considering the priority of the goals, and the optimum results were obtained at a Reynolds number of 16906, 25 mm pitch, 8 mm height of winglet and 30° angle of attack, so that vortex-shedding frequencies are maximised. For lower frequencies, optimum conditions occurred at a Reynolds number of 16906 for 75 mm pitch, 8 mm height of winglet and 60° angle of attack.Heat-transfer enhancement Delta-winglet Taguchi method Flow-induced vibration

Dielectric properties of plasma polymerized poly(ethylene oxide) thin films

Plasma polymerized poly(ethylene oxide) (pPEO) thin films were deposited by plasma assisted physical vapor deposition (PAPVD) with a constant thickness of 500 nm. The crosslinking density of pPEO films was controlled through the discharge power. Differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) were applied to analyze the structural peculiarities of these coatings. Both DSC and BDS detected an increase of the crosslink density with power, which correlated with the increasing dynamic glass transition temperature (T-g). It was shown that plasma power changes the structure of plasma polymer, and dielectric spectroscopy may be a suitable technique for structural analysis. (C) 2016 Elsevier B.V. All rights reserved

Dielectric properties of AuNPs/PEGMEA/PEGDA nanocomposite film prepared with an alpha-amino ketone by in-situ photochemical method

Dielectric properties and AC conductivity of in-situ formed AuNPs/PEGMEA/PEGDA nanocomposite film was investigated at room temperature. Frequency dependent dielectric constant shows that there are two main relaxation mechanisms for both samples. First of these mechanisms which correspond to dipolar polarization of long polymer chain was observed at frequencies between 0.1-10(3) Hz, 0.1-10(4) Hz for PEGMEA/PEGDA film and AuNPs/PEGMEA/PEGDA nanocomposite film, respectively. This mechanism has dielectric constant of 10.8, 14.1 for PEGMEA/PEGDA film and AuNPs/PEGMEA/PEGDA nanocomposite film, respectively. The second mechanism which was attributed to dipolar polarization of chain branches was observed at frequencies between 10(2)-10(6) Hz, 10(5)-10(7) Hz for PEGMEA/PEGDA film and AuNPs/PEGMEA/PEGDA nanocomposite film, respectively. This mechanism has dielectric constant of 9.6 for PEGMEA/PEGDA film and 10.3 for AuNps/PEGMEA/PEGDA nanocomposite film which cured in the presence of Irg-907 as Type I initiator by using medium pressure mercury light. These relaxation mechanisms can also be detected at frequency dependence of dielectric dissipation (tan delta) as Cole-Cole fitted relaxation peaks and of AC conductivity graphs as frequency independent (DC-like), and frequency dependent regions. DC-like region, it was thought that the main effect of AuNPs was observed, has values for AuNPs/PEGMEA/PEGDA nanocomposite film and PEGMEA/PEGDA film as 6x10(-7) and 2x10(-8) S/cm, respectively. In addition to these two polarization mechanisms, electrode polarization was observed at frequencies lower than 1 Hz for AuNps/PEGMEA/PEGDA nanocomposite film due to accumulation of AuNPs