Volume 36, Issue 4 -

The design and validation of a Compact Crossover Diffuser (CCD) to replace the size-limited radial diffuser and axial de-swirl cascade of an existing Micro Gas Turbine (MGT) is discussed. A CCD strives to combine the performance of a channel diffuser with the operating range and efficiency of a vaneless diffuser. The development of a one-dimensional Mean-Line Code (MLC) is presented, which aids the designer in preliminary design and performance evaluation of the CCD. Design graphs indicating the performance effects of changing the primary design variables are developed and shown. The MLC is numerically validated using Computational Fluid Dynamics (CFD). Good agreement is seen between the MLC and CFD results, predicting the design point PR ss(2-4) to within 1.4 %. A CFD optimized CCD was manufactured and tested. Agreement between the CFD and experimental results for PR ts(0-4) is within 7.58 % at 106 kRPM. A numerically predicted increase in PR ts(0-4) from 3.31, to 3.53, to 3.83 is seen for the vaneless-, MLC optimized-, and CFD optimized-design respectively. An experimental increase of 82.3 % in engine thrust and 80.0 % in total-to-static pressure recovery across the compressor stage was measured when retrofitting the BMT120KS with a new impeller and CCD.

With the increasing requirements of aeroengine performance and working stability, the primary research task of the rotor system dynamics is to build a rotor system model that can reflect the actual situation and obtain the calculation results which can reflect the real dynamic characteristics of the rotor system. In this paper, the finite element analysis model of a power turbine rotor was established, and the dynamic model and dynamic characteristics of the complex rotor system were studied. The results indicated that the finite element model could reflect the real dynamic characteristics of the power turbine rotor. For the given design rotational speed, the critical speed had enough margin and the rotor system worked safely and smoothly. This research provided a reference and theoretical basis for the calculation of the dynamic characteristics of the similar rotor system.

Miniature air blast atomizers are intended for use in small jet engines. The typical miniature atomizer with rotating air outside the pressure swirl nozzle was investigated by theoretically and experimentally. The equations which describe the flow of rotating hollow liquid jet in a gaseous environment, where there is difference between outer and inner pressures of the liquid film were developed. The effects of surface tension, gravity forces, and inner-outer pressure difference on liquid film modes and its disintegration were considered. A set of the relevant equations was solved numerically. The initial conditions for the equations: pressure distribution inside and outside of the ‘onion’ as function of outer rotating air and liquid parameters, were obtained experimentally. The numerical results were verified by the experimental study. It was shown that exists two shapes of liquid film nearby nozzle exit – ‘onion’ and open film. Shape of the liquid film for the given nozzle pressure drop depends mainly on air pressure difference outside and inside the film. Two resulting spray modes are distinguished in droplet characteristics drastically. The results can be applied for estimation of the operational parameters of air-blast atomizers that operate with very low liquid pressure drop.

The time error of detonation acoustic in process of detonation formation and propagation in a multi-cycle gas-liquid two-phase pulsed detonation engine is experimentally investigated. Results from the tests show that before the detonation wave escapes through the open-end of PDE tube, the maximum average arrival time error of detonation acoustic is achieved in the process of overdriven detonation. After detonation wave exists of PDE tube, arrival time error at 0 deg is greater than the other directivity angles in all distances and increases dramatically first and then almost stays at a certain value. The filling fraction has a major impact on the time error of detonation acoustic. With filling fraction increasing, there are increases in arrival time error and interval time error. Arrival time error with the highest filling fraction at 30 deg is much greater than other filling fraction. The convergent nozzle exhibits a marked suppression in the time error of detonation acoustic, where the maximum reductions of 62.02 percent and 56.13 percent are obtained in arrival time error and interval time error respectively.

Affected by rotor–stator interaction and unstable inner flow, asymmetric pressure distributions and pressure fluctuations cannot be avoided in centrifugal pumps. To study the pressure distributions on volute and front casing walls, dynamic pressure tests are carried out on a centrifugal pump. Frequency spectrum analysis of pressure fluctuation is presented based on Fast Fourier transform and steady pressure distribution is obtained based on time-average method. The results show that amplitudes of pressure fluctuation and blade-passing frequency are sensitive to the flow rate. At low flow rates, high-pressure region and large pressure gradients near the volute tongue are observed, and the main factors contributing to the pressure fluctuation are fluctuations in blade-passing frequency and high-frequency fluctuations. By contrast, at high flow rates, fluctuations of rotating-frequency and low frequencies are the main contributors to pressure fluctuation. Moreover, at low flow rates, pressure near volute tongue increases rapidly at first and thereafter increases slowly, whereas at high flow rates, pressure decreases sharply. Asymmetries are observed in the pressure distributions on both volute and front casing walls. With increasing of flow rate, both asymmetries in the pressure distributions and magnitude of the pressure decrease.

Emission characteristic of an aero gas turbine combustor has been studied under fuel-rich condition. The combustor has been designed to operate at rich fuel-air mixture and may experience very high mixture up to equivalence ratio of 3.0 in the primary zone in the event of atomizer deterioration and flow passage distortion or during some maneuver. The concentration of major engine emission/pollutant constituents at combustor exhaust such as CO, NOx, soot, unburned hydrocarbons and atmospheric pollutants such as CO2 and water vapor has been analyzed at fuel-rich high equivalence ratio using computational model in CFX. The pollutant concentrations are found to be increased with increase in equivalence ratio. Atomization quality in terms of fuel droplet size is found to have a significant contribution in the concentration of various species at combustor exit.

Film cooling performance of one row of cylindrical holes integrated with saw-tooth slots was numerically studied at blowing ratios of 0.5, 1.0 1.5 and 2.0 respectively. The saw-tooth slot concept combines the advantages both of easy machining for the slot and of the high film cooling effectiveness caused by the anti-vortex induced by the shaped hole. The film holes have an inclination angles of 30°, length to diameter ratio of 4 and pitch to diameter ratio of 3. The corner angles of the saw-tooth are 60°, 90°, 120°, 150° and 180° respectively. The 180° corner angle corresponds to a standard transverse slot. The emphasis of this other is on the influence of the corner angles of the saw-tooth on film cooling effectiveness. The flow field and thermal field were obtained to explain the mechanism of film cooling performance improvement by the saw-tooth slot. The results show that the numerical data agrees with the experimental values for the cylindrical holes. Relatively small corner angles generate uniform local film cooling effectiveness and high spanwise averaged film cooling effectiveness due to the coolant ejected from the hole smoothly flowing into the slot. The effect of corner angles strongly depends on blowing ratios. The increase of x/D decreases the differences of film cooling effectiveness between various corner angles. At low blowing ratios, an anti-vortex can be found with the spanwise angle of 60° and 120°. At high blowing ratios, an anti-vortex can be found with the spanwise angle of 60°.

A new control scheme is proposed for aircraft engines, where the object is to transfer a regulated output (called main output) between two set-points, with the additional requirement that a set of outputs (called auxiliary outputs) remains within prescribed ranges. According to the prior outcomes, no assurance exists to protect output limits in transient regime by use of a traditional min-max scheme with linear compensators. To achieve a satisfied control performance for a corresponding output, each limitation loop is design separately. However, some auxiliary outputs still exceed their limits when other loop regulators are active or overshoot occurs. To overcome this problem, a linear matrix inequalities (LMI) approach is developed by considering the limit values in regulator design of other loops and the condition for no overshoot of auxiliary outputs. Finally, practical applicability is further demonstrated by numerical simulation. The results show that the designed controller can achieve a satisfactory dynamic and steady performance and keep the auxiliary outputs within constraints.

Two- and three-dimensional direct numerical simulation (DNS) of turbine cascade flow at low Reynolds number with heat transfer are performed using high-order finite difference method. Two-dimensional laminar computation which is used to construct the initial flow of three-dimensional DNS fails to predict Stanton number on the second half of suction side where the flow is turbulent in experiment. In three-dimensional DNS, transition is triggered by periodic blow-and-suction disturbances. Numerical experiments show that phase randomness of the disturbance is not necessary to trigger the transition which can be induced by disturbances with fixed phases. In a range of time fundamental frequency of disturbance, when increasing the frequency, transition moves downstream. When time fundamental frequency is big enough, transition disappears. With fixed space phases, time phases and selected time fundamental frequency, time averaged pressure and Stanton number distributions of three-dimensional DNS coincide with the experimental datum. Averaged velocity and temperature, Root-Mean-squares (RMS) of velocity pulse，temperature pulse, Reynolds shear stress and heat flux are extracted from the DNS database. All statistics agree well with experimental and theoretical results which verify the accuracy of present database.

The variable geometry turbine (VGT) has been widely used in different fields due to its higher efficiency and lower fuel consumption at part-load. However, the flow field in a VGT is characterized by the leakage flow through the radial clearance of rotational vane compared with a general fixed vane turbine. Numerical simulations are conducted on a linear turbine cascade to reduce the leakage flow based on passive control method. The aerodynamic performances and flow fields are compared for four kinds of tip configuration firstly. Then, the effect of variable geometry on the linear turbine cascade aerodynamic performance is investigated for five installation angles ranging from –5 to 5 deg. In addition, the development patterns and trends of the tip leakage vortex and the passage vortex are analyzed. The results show that the squealer tip and the rotating axis have a significant impact on suppressing leakage flow. The leakage flow rate has a tendency to decrease, and the total pressure coefficient is gradually increased when installation angle ranges from –5 to 5 deg. The interactions between tip leakage vortex and passage vortex leads to the different trends on leakage flow at various installation angles and axial sections.