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101.
Mehrdad Hosseini Kalajahi Samrand Rash Ahmadi Samad Nadimi Bavil Oliaei 《The International Journal of Advanced Manufacturing Technology》2013,69(1-4):687-704
In this study, thermal modeling and finite element simulation of electrical discharge machining (EDM) has been done, taking into account several important aspects such as temperature-dependent material properties, shape and size of the heated zone (Gaussian heat distribution), energy distribution factor, plasma flushing efficiency, and phase change to predict thermal behavior and material removal mechanism in EDM process. Temperature distribution on the cathode has been calculated using ANSYS finite element code, and the effect of EDM parameters on heat distribution along the radius and depth of the workpiece has been obtained. Temperature profiles have been used to calculate theoretical material removal rate (MRR) from the cathode. Theoretically calculated MRRs are compared with the experimental results, making it possible to precisely determine the portion of energy that enters the cathode for AISI H13 tool steel. Also in this paper, the effect of EDM parameters on MRR has been investigated by using the technique of design of experiments and response surface methodology. Finally, a quadratic polynomial regression model has been proposed for MRR, and the accuracy of this model has been checked by means of analysis of residuals. 相似文献
102.
H. Afsoos Biria M. A. Lashteh Neshaei A. Ghabraei M. A. Mehrdad 《Frontiers of Structural and Civil Engineering》2015,9(1):82
Marine structures, such as Groynes, Sea walls and Detached Breakwaters, are constructed in coast of area to improve coast stability against bed erosions due to changing wave and current pattern. Marine mechanisms and interaction with the hydraulic structures need to be intensively studied. Groynes are one of the most prominent structures that are used in shore protection and littoral sediment. The main hydraulic function of the groyne is to control the long shore current and littoral sediment transport. This structure can be submerged and provide the necessary beach protection without negative aesthetic impact. However, for submerged structures adopted for beach protection, the shoreline response to these structures is not well understood. The objective of this study is to predict sediment transport in the vicinity of submerged groyne and comparison with non-submerged groyne focusing on a part of the coast at Dahane Sar Sefidrood, Guilan Province, Iran, where serious coast erosion has been occurred. The simulations were designed using a one-line model which can be used as a first approximation of shoreline prediction in the vicinity of groyne. The results of the proposed model are compared with experimental data to determine the shape of the coast. The results of predicted beach deformation show that when submerged groyne construct in the beach, sediment accumulation will be slightly less than the non-submerged groyne; because transfer coefficient for the submerged groyne is more than non-submerged groyne. This result will cause more sediment passing on submerged groyne. Finally, the result of the present study show that using submerged groyne is an efficient way to control the sediment and beach erosion without causing severe environmental effect on the coast. 相似文献
103.
One of the fundamental issues in gas insulated substations (GIS) which has destructive effects on GIS equipment is the very fast transient over-voltages (VFTOs). This paper models a 400/230 kV substation in order to study the effects of VFTO extensively implemented on EMTP-RV. In addition, the application of ferrite rings for suppressing VFTOs is assessed thoroughly. The main advantage of this paper is its new proposed algorithm according to the ferrite ring frequency dependent modeling that is validated with experimental results. This paper examines the effects of three compositions of the ferrite ring on VFTO suppression. Moreover, it estimates the dimension of the ferrite ring based on the SF6 gas insulation withstand and the maximum effect of ferrite rings on VFTO suppression constraint with the COMSOL multiphysics software. Furthermore, it gains VFTO attenuated percentages due to the installation of the ferrite ring in different GIS nodes. Finally, it analyzes the offered VFTO amendment technique in various GIS switching scenarios. 相似文献
104.
We have investigated for the first time the impact of electron overflow on the performance of nanowire light-emitting diodes (LEDs) operating in the entire visible spectral range, wherein intrinsic white light emission is achieved from self-organized InGaN quantum dots embedded in defect-free GaN nanowires on a single chip. Through detailed temperature-dependent electroluminescence and simulation studies, it is revealed that electron leakage out of the device active region is primarily responsible for efficiency degradation in such nanowire devices, which in conjunction with the presence of nonradiative surface recombination largely determines the unique emission characteristics of nanowire light-emitting diodes. We have further demonstrated that electron overflow in nanowire LEDs can be effectively prevented with the incorporation of a p-doped AlGaN electron blocking layer, leading to the achievement of phosphor-free white light-emitting diodes that can exhibit for the first time virtually zero efficiency droop for injection currents up to ~2200 A/cm(2). This study also provides unambiguous evidence that Auger recombination is not the primary mechanism responsible for efficiency droop in GaN-based nanowire light-emitting diodes. 相似文献
105.
Alireza Riasi Ahmad Nourbakhsh Mehrdad Raisee 《Canadian Metallurgical Quarterly》2010,136(11):929-934
An effective numerical method to compute hydraulic resonance in pressurized piping of hydropower systems is presented. For this purpose, the impulse response method is used, i.e., a unit pressure impulse is introduced at the downstream waterway as an exciter. The method of characteristics is used to solve the governing equations for the hydraulic transient and to get the pressure response of the system in the time domain. The discrete Fourier transform is used to compute the frequency response of the system which gives the resonance frequencies in the hydropower plant system. To increase the accuracy of the results, unsteady friction is incorporated into the methodology. The influence of the unsteady friction, wave speed, and power on the pressure response diagram is investigated for the waterway of a multiunit hydropower plant which has been recently installed. Computed results agree very well with those obtained from the standard method of the characteristics. 相似文献
106.
Pulsed jets in different configuration are potentially considered for enhancing transport phenomenon generally. Flow and temperature field in a pulsed impinging jet are simulated numerically by solving the governing equations using the control volume method. Ensemble Averaging Method as well as Phase Averaging has been employed for reporting the results in this study. In order to simulate a pulsating jet, inlet velocity profile was exerted as a time dependent sinusoidal and step signals. The results of this simulation showed an oscillatory jet could lead to an increase in jet development and its cross section with the wall and also a more uniform Nusselt profile would be obtained compared to the steady jet. For parametric investigations and extracting flow and thermal characteristics of a pulsed impinging jet, the effects of various parameters including flow frequency and amplitude and heat flux frequency were considered. It has been seen that Nusselt number varies by the changes in frequency, amplitude and the type of the excitation. It has been shown that the oscillating impinging jet has a better performance rather than the steady case when the excitation amplitude and frequency increase. Finally, it is also observed how a thermal field is going to respond with two pulsating inputs. 相似文献
107.
108.
This article reports the outcome of our recent effort in prediction of gas slip flow through micro- and nanochannels. Slip nitrogen flow through short (length to height ratio of 20) and long (length to height ratio of 2,500) microchannels is analyzed and discussed using the spectral element method and the generalize slip model boundary condition of Karniadakis and Beskok. The well-known curvature in pressure distribution, due to the compressible behavior of the gas flow, is observed. Comparison of numerical results with the experimental and direct simulation Monte Carlo (DSMC) data as well as previous reported numerical results showed that the generalized slip model is able to produce reliable results for both short and long microchannels. 相似文献
109.
110.
Mehrdad Lahouti Peter J. Peterson 《Journal of the science of food and agriculture》1979,30(2):136-142
Crop plants accumulated chromium from nutrient solutions labelled with either 51CrIII or 51CrVI and retained approximately 98 % of the elements in the roots. Of the 9 crop species examined, cauliflower plants accumulated the most chromium in both shoots and roots and mung bean seedlings the least. Species differences of approximately 10-fold in the tops and 5-fold in the roots were recorded. The roots of each species supplied with 51CrIII contained more chromium than those supplied with 51CrVI but the reverse was found in the shoots. There was a greater proportion of soluble chromium in plants supplied with 51CrVI than in those supplied with 51CrIII, the major soluble compound being tentatively identified as trioxalato-CrIII. Little chromium was associated with cell organdies or soluble proteins. 相似文献