Volume 35, Issue 3 -

A novel injector has been designed and cold flow injection tests were performed in a modified supersonic wind tunnel. To complement these experimental studies three dimensional STAR-CCM+CFD simulations were developed. The pulse width may be varied, with options of injecting gas for 33 %, 50 % and 66 % of the injection period. The scramjet combustor environment is simulated in a supersonic wind tunnel through a backward facing step for secondary injection purposes and a 157.5 cm (62-inch) long test section. The gas in secondary injection is carbon dioxide and the primary flow is air. The simulations show a coupled interaction between the forcing from injection and the shear layer. Steady state static pressure measurements on the lower wall of the wind tunnel test section agree well with the simulated static pressure along the lower wall. The pulse width strongly impacts shear layer reattachment on the lower wall and varies between 2.4 and 4.3 step heights. Reduction in duty cycle from 66 % to 33 % at 1 kHz caused ~30 % reduction in the shear layer reattachments distance, which points to large scale mixing enhancement.

Entropy generation and energy efficiency of turbofan engines become greater concern in recent years caused by rises fuel costs and as well as environmental impact of aviation emissions. This study describes calculation of entropy generation for a two-spool CFM56-7B high-bypass turbofan widely used on short to medium range, narrow body aircrafts. Entropy generation and power analyses are performed for five main engine components obtaining temperature-entropy, entropy-enthalpy, pressure-volume diagrams at ≈121 kN take-off thrust force. In the study, maximum entropy production is determined to be 0.8504 kJ/kg K at the combustor, while minimum entropy generation is observed at the low pressure compressor component with the value of 0.0025 kJ/kg K. Besides, overall efficiency of the turbofan is determined to be 14 %, while propulsive and thermal efficiencies of the engine are 35 % and 40 %, respectively. As a conclusion, this study aims to show increase of entropy due to irreversibilities and produced power dimension in engine components for commercial turbofans and aero-derivative cogeneration power plants.

Separation in compressor cascade triggers flow loss and instability. This paper presents a passive flow control method by introducing a slot into the blade. The slot induces self-adapted jet, while the jet flow is used to suppress cascade’s separation. To study the flow control effect, experiments were conducted and flow field details were given by validated numerical simulations. The results show that a well-designed slot carries adequate jet airflow from pressure side (PS) to suction side (SS) due to the great pressure fall between the two sides. The jet airflow delays suction side separation within specific incidence angles, so the performance of cascade achieves considerable improvements. It enables to be concluded that the slot carries jet flow to SS separation zone, and then the self-adapted jet flow re-energizes low momentum fluid and suppresses vortices in the separation which are negative to the cascade flow.

The heat transfer performance of swirling impinging jets (SIJs) was investigated and compared with that of the conventional jet (CIJ). The swirling jets were induced by helical rod inserts (HRs) fitted with pipe nozzles. The helical rod inserts with two different rod diameter to nozzle diameter ratios (d/D) of 0.46 and 0.64 were employed for comparison. Jet-to-plate distance to nozzle diameter ratio (L/D) was varied from 1 to 5 while Reynolds number was fixed at 20,000. The temperature and Nusselt number distributions on the impinged plate were measured and evaluated using thermochromic liquid crystal (TLC) sheet and image processing technique. The experimental results showed that the swirling jet (SJ) at d/D=0.64 gave higher average heat transfer rate than the SJ at d/D=0.46 up to 6.41 % and the conventional jet (CJ) up to 35.05 %. In addition, Nusselt number peak of swirling jets increased as jet-to-plate distance decreased.

Air ejector diffusers are employed in gas turbine exhaust systems to cool the exhaust gases. These diffusers are developed as passive devices, which use the energy of the main flow to entrain the relatively cool ambient air through the annular slot openings. Multiple slot openings are provided along the length of ejector diffusers to lower the temperatures of the exhaust gases. This paper presents results of a 3-D numerical study carried out at a fixed Reynolds Number of 2.5×10 5 with a corresponding inlet Mach number of about 0.22 on three configurations of a non-circular ejector diffuser having an overall area ratio of 9. The three configurations being investigated are the best diffuser configurations established on the basis of cold flow studies reported in literature. The results are presented in terms of temperature distribution, entrainment mass flux rates and static pressure recovery. The results show that the higher number of slot openings improves cooling in the ejector diffuser as compared to thicker interfaces or inclination at the slot inlet.

Radial growth predictions of rotating labyrinth seals are conventionally obtained from one-dimensional analytical models. However, these predictions quantitatively differ within themselves by about 5-500 %. Simulations using three-dimensional finite element method (FEM) are carried out in this paper for a typical labyrinth seal, subjected to high rotational speed and temperature, for a range of radius-to-length ratio of the rotor. Taking the predicted values by FEM as reference, four analytical models are assessed and their errors are quantified. These errors are found to be independent of rotational speed and temperature but significantly vary with the radius-to-length ratio of the rotor. Based on this finding, simple analytical models, together with correction factor charts, are suggested.

This study proposes a new test approach for a ram air-driven turbo-refrigeration system. The flight performance of the air cycle turbo-refrigerator is indirectly verified through ground tests under various flight conditions, and the actual altitude performance is predicted through dimensional analysis and similarity based on the ground test results. In addition, correction of the Reynolds number is also considered for adequate prediction of performance. This study develops a turbo-refrigerator that consists of a radial turbine and a centrifugal compressor. As this simple system drives the reverse Brayton cycle, it acts as a refrigerator that relieves the heat load. Although the ram air and exhaust conditions are usually simulated in high altitude test facilities, they are very costly and not suitable for prototype development, which often demands requirement and specification changes. The proposed system is designed to deliver a cooling capacity over 2 kW through the heat exchanger at a Mach number of 0.8 and altitude up to 10 km. As a result of current study, the proposed test approach predicts that air cycle turbo-refrigeration system yields sufficient cooling capacity to comply with the requirements for the primary mission flight envelope under the hot day condition, despite slight performance degradation at high altitudes.

Numerical simulations have been performed on the film cooling characteristics of counter-inclined structures, which have advantage in manufacturing relative to the usually used parallel-inclined film-hole row structure, on a turbine vane leading edge model. Single row structure and dual-row structure with counter-inclined film holes were applied in the simulation of leading edge film cooling of turbine vane. The effect of jet-interaction between counter-inclined film-hole rows was studied. The distributions of film cooling effectiveness and heat transfer coefficient were obtained at blowing ratios of 1.0 and 2.0. The results of single row structure show that the film cooling performances of counter-inclined film-hole row are not weakened compared to the traditional parallel-inclined film-hole row structure. The film cooling effectiveness of the counter-inclined film-hole row structure decreases with the increase of blowing ratio, while the heat transfer coefficient increases. The jet-interaction in the dual-row film cooling structure has more notable influence on the film cooling effectiveness than the heat transfer coefficient. Compared to the single row case, the interactions between the upstream counter-blowing jets and the downstream jet improve the film coverage performance and reduce the heat transfer intensity of this downstream jet under larger blowing ratio condition.