فصلنامه مکانیک هوافضا
سال چهاردهم شماره 3 (پیاپی 53، پاییز 1397)

This paper deals with utilization of the network method to fast analysis of the gas turbine combustors. In this method, a combustor is divided into several independent flows by some nodes and interconnecting elements. The fundamental equations which should be satisfied in the network method are conservation equations of mass and energy at each node and the pressure drop-flow rate correlation for each element. These equations constitute a system of equations of flow rate and pressure drop. Solving this system using a set of initial value gives pressure, flow rate, and density at each node that finally leads to a converging solution. The conventional relations for pipes and orifices are used for flow modeling. In order to combustion modeling, equilibrium assumption is applied in the primary and secondary combustion zones. The dilution effect is considered by entering the air from dilution holes into the flow of combustion products. The liner temperature is calculated by considering the effects of convection and radiation in the combustor. By the network method in a can type combustor, at the first step a cold flow analysis is performed and the flow rate and the pressure drop in each element are obtained. In the next step by including the combustion of kerosene and heat transfer, the distribution of flow rates, pressure, temperature, and species concentration of combustion products are calculated. The computation results are compared with the existing experimental data and an excellent agreement is seen. The quality of results and the solution procedure of the problem show that the network method is able to present a costly and accurate analysis to help the combustor designers

Cooling of blades is a challenge in advancement of gas turbines. Channels are used inside the blades to improve the heat transfer. Repeated ribs have significant impact on the heat transfer and pressure drop characteristics of a channel. In this study the effect of ribs with specified geometry is numerically investigated. In a relatively large range of Reynolds from 10000 to 80000, the fluid flow and heat transfer were simulated. The finite volume method was implemented. To model the turbulence, the standard k– ε model was used. The thermal boundary condition was the constant heat flux at the channel surfaces. The magnitude of the heat flux was assigned in a way to result in temperature difference from 10 K to 15 K. The results of current research were compared to the available experimental data under similar flow conditions. The coefficient of pressure drop was comparable with experimental data both in two dimensional and three dimensional cases. In the two dimensional simulation, the heat transfer coefficient was similar to the experimental data in the range of this study. In the three dimensional simulation, accuracy of the heat transfer coefficient in the case of low Reynolds was not satisfactory. However, it improved progressively as the Reynolds number increased. It was observed that, the maximum improvement in the heat transfer and pressure drop are about more than 200% and 300% respectively. Heat transfer augmentation could be explained by the presence of the secondary flows and disruption of the boundary layer growth due to the ribs.

Increasing environmental concerns in recent decades has caused a significant attention to pollutants emission and the ways to control them. Piston cavity diameter and dept effects on direct injection diesel engine performance and emissions were studied in this research. AVL FIRE 3D CFD software was used for engine cycle simulation in this studying. By applying the combustion, turbulence and pollution equations, the direct injection diesel engine closed cycle has simulated numerically. After mesh independency analyzing and simulation results validation with experimental data, the engine simulation was done for four different piston bowl geometries and the results such as incylinder pressure, heat release rate, temperature and emissions. Then the results were compared for all geometries. The results shown that there is less wet wall for geometry 4 that has large diameter and low dept and it can be concluded that incylinder mixing process and combustion for this geometry is better and it has good performance and low emission in comparison to other one. Also the results shown that nitrogen oxide emission reduced by reducing the diameter and increasing the depth of the piston bowl, but it increase unburned hydrocarbons, carbon monoxide and soot emissions.

In this paper, the optimization of the film cooling effectiveness over a rotating blade is performed using the laterally diffused hole. The three-dimensional numerical simulation of blade cooling is performed using RNG k-ε turbulence model for three rotating speeds, i.e., 0, 300 and 500 rpm. Results show that the film trajectory always inclines due to the Coriolis force acts in the centrifugal direction. The film cooling deflection becomes greater with increasing the rotational speed. The comparisons of results show that, for the laterally diffused hole, the mixing of the hot gas flow with the injected coolant flow is less that the cylindrical hole. Applying the laterally diffused hole consequences the increase of film cooling effectiveness by the 39, 38 and 35 percent at the rotating speeds of 0,300 and 500 rpm respectively.

Variable timing of valves is one of the effective factors on performance of a spark ignition (SI) engine. The time of opening, closing, duration of open state, and the amount of lift for inlet and exhaust valves are the parameters that must be considered in variable valve timing issue. These parameters influence thermodynamic performance factor and heat loss of an engine. In this paper at first, using K3PREP pre-processor, a structured moving mesh was generated for valves as well as for inner and outlet runners. Then, using KIVA-3V as the main solver, the variation effects of each valve timing parameters on the engine performance were studied. The simulation was done on a SI- engine. Comparison between simulation and experiment results indicates satisfactory agreement.

In designation process of liquid rocket engine, selection of cooling rout of thrust chamber have important role for assessing safety and reliability of the engine. Also the correct perception of internal flow characteristics in LRE thrust chamber, such as temperature and pressure, plays effective role in optimization of engine design to decrease consumption fuel and how to cooling the thrust chamber wall. One of the ways to cooling the thrust chamber wall to decrease the combustion temperature near the wall is injection extra fuel in this section. In this paper, numerical investigation of the effect of film cooling with mixed heat transfer on decrease the thrust chamber wall temperature in the LRE, relative to condition that only exist mixed heat transfer in the wall was performed.

The study of heat transfer in porous media has many applications in industry. In this work, heat transfer between an inert gas and a semi infinite porous media based on Singular Perturbation method and with using of Laplace Transform, has been studied. To solve this problem with this approach, the thermal conductivity is assumed to be small. Also, the solution is expressed in both inner and outer layer. For matching these two layers a matching method is suggested. This approach shows a good accuracy for small values of parameter, small thermal conductivity. One time the problem is solved by using the third kind boundary condition on the wall and one time this boundary condition is changed into first kind and then has been considered in problem. With comparing between this solution and the solution of using boundary condition of third kind shows the good accuracy of this approach, except near the wall that there occurred small differences between these two solutions. This idea can be used as a useful method in simplifying the analysis of the issues. But in cases that the good accuracy of solution or slop changes with high precision is important, this procedure may be caused approximate results.

In this article, aerodynamic stability of J79 compressor was analytically investigated. For this purpose, conservation equations of mass, momentum and energy were applied with several assumptions. By accomplishment of equations related matrix as well as investigating its eigenvalues, the effect of parameters on aerodynamic stability were assessed. Due to the complexity of the equations used in the matrix, the equations codes in MATLAB and the desired matrix have been provided. By applying the geometry and flow characteristics of J79 engine on the matrix and extract the relevant diagrams, the effect of some parameters on the aerodynamic stability of the engine is evaluated. According to the results, by reducing the mass flow rate through the compressor, the system tends to be working in an unstable condition. By increasing the ratio of upstream to downstream compressor duct length, the aerodynamic stability of the system will increase. Increasing the plenum volume will yield to a considerable reduction in stability. But by increasing the volume of compressor, the system stability will increase. Enhancing the combustion chamber temperature and also increasing the compressor flow area have favorable effects on system stability. The results of this study compared with previous outcome. Significant achievements were observed between both aforementioned results Relatively good alignment between the results has also been observed.