Effect of driving frequency on electron heating in capacitively coupled RF argon glow discharges at low pressure

A one-dimensional（1D） fluid model on capacitively coupled radio frequency（RF） argon glow discharge between parallel-plates electrodes at low pressure is established to test the effect of the driving frequency on electron heating. The model is solved numerically by a finite difference method. The numerical results show that the discharge process may be divided into three stages： the growing rapidly stage, the growing slowly stage, and the steady stage. In the steady stage,the maximal electron density increases as the driving frequency increases. The results show that the discharge region has three parts： the powered electrode sheath region, the bulk plasma region and the grounded electrode sheath region. In the growing rapidly stage（at 18 μs）, the results of the cycle-averaged electric field, electron temperature, electron density, and electric potentials for the driving frequencies of 3.39, 6.78, 13.56, and 27.12 MHz are compared, respectively. Furthermore,the results of cycle-averaged electron pressure cooling, electron ohmic heating, electron heating, and electron energy loss for the driving frequencies of 3.39, 6.78, 13.56, and 27.12 MHz are discussed, respectively. It is also found that the effect of the cycle-averaged electron pressure cooling on the electrons is to ＂cool＂ the electrons; the effect of the electron ohmic heating on the electrons is always to ＂heat＂ the electrons; the effect of the cycle-averaged electron ohmic heating on the electrons is stronger than the effect of the cycle-averaged electron pressure cooling on the electrons in the discharge region except in the regions near the electrodes. Therefore, the effect of the cycle-averaged electron heating on the electrons is to ＂heat＂ the electrons in the discharge region except in the regions near the electrodes. However, in the regions near the electrodes, the effect of the cycle-averaged electron heating on the electron is to ＂cool＂ the electrons. Finally, the space distributions of the electron pressure cooling the electron ohmic heating and the electron heating at 1/4 T, 2/4 T, 3/4 T, and 4/4 T in one RF-cycle are presented and compared.

Conjugate Heat Transfer Predictions of Gas Turbine Hot Walls Jets Cooling: Influence of Short Hole Grid Resolutions Using Computational Fluid Dynamics*

Short hole investigations relevant to gas turbine (GT) hot walls cooling heat transfer techniques, were carried out using computational fluid dynamics (CFD) combined with conjugate heat transfer (CHT) code. The CFD software are commercial ones: ICEM for grid modelling and ANSYS Fluent for the numerical calculation, where symmetrical application prevails. The CFD CHT predictions were undertaken for Nimonic-75 metal walls with square (152.4 mm) arrays of 10 holes, whereby the lumped heat capacitance method was applied in order to determine the surface average heat transfer coefficient (HTC), h (W/m2 K) and the dimensionless Nusselt number, Nu. The major parameters considered for the short hole geometries are the pitch to diameter, X/D and length to diameter, L/D ratios and both were varied with range of D values, but X of 15.24 mm and L of 6.35 mm kept constant. Also applied, are variable mass flux, G (kg/s∙m2) and were used in predicting the flow aerodynamics in the short holes. The predictions were for classic thermal entry length into a round hole, as vena contracta, flow separation and reattachment dominates the holes, hence the development of thermal profile through the depth of the GT hot walls. Additionally, the acceleration of the flow along the wall surfaces as it approaches the holes, was a significant part of the overall heat transfer. This was shown to be independent of the hole length, even though the L/D parameter is a critical component to enhanced heat transfer. The CFD CHT predictions showed that validation of the HTC h, Nu and pressure loss, ∆P are in better agreement with measured data and within reasonable acceptance. The ∆P agreement signifies that the aerodynamics were predicted correctly, which is also the reason why the HTC expressed per wall hole approach surface area and Nu were better predicted. This illustrates how effective and efficient the wall internal heat transfer cooling is for gas turbine hot wall heat transfer using airflow jets cooling.