Large eddy simulations were performed for film cooling on scaled-up C3X turbine vane at the nominal blowing ratio of M=0.5~1.5, and the Reynolds number, Re=3000, based on the mainstream inlet velocity and hole diameter. On the pressure surface, large-scale coherent structures including hairpin vortexes and horseshoe vortexes are generated in film-cooling flow fields. Hairpin vortexes promote the mixture between hot mainstream and coolant jet and degrade cooling performance. The anti-entrainment of horseshoe vortexes improves the lateral-covering capability of coolant jet in the near-field region and results in the formation of a pair of low-temperature strips wrapped around the hole at high blowing ratio. On the suction surface, the transition of the boundary layer takes place in the downstream of cascade throat but in the upstream of the discharged hole, plenty of broken vortexes dominate film-cooling flow fields. The distribution of turbulent kinetic energy also indicates that the coolant jet from the suction surface generates higher turbulent intensity than that from the pressure surface. For pressure signals for film cooling on the suction surface, small-scale and random fluctuation takes the dominant role. For pressure signals for film cooling on the pressure surface, a dominant frequency corresponding to Strouhal number St≈2.1 both exists at low and high blowing ratios. The film-cooling system on the suction surface exhibits a higher random degree than that on the pressure surface.
Due to concerns over CO2 emissions and higher efficiency requirements future power generation systems with stationary gas turbines are projected to utilize carbon-free fuels such as ammonia and hydrogen at increasingly high pressure ratios and turbine inlet temperatures. This raises concerns whether conventional approaches for estimating the working fluid properties and for heat balance calculations are appropriate under such conditions. Herein, we therefore investigate the effect of several simplifying assumptions for the working fluid and the combustion scheme often made. We find that at high temperatures and equivalence ratios chemical reactions during the expansion of the gas should be considered, in particular at equivalence ratios close to unity. The extent to which chemical reactions occur during expansion in the turbine requires further investigations, as it could have severe consequences for the heat balances and output calculation of the turbine, as well as the concentration of pollutants such as NOx in the exhaust gas.
Urea Selective Catalytic Reduction (urea SCR) system is widely used for diesel engine to reduce the emission of NOx by NH3 which is provided by a hydrolysis of urea water. Concentration distribution of NH3 in an exhaust pipe is an important factor for improvement of the SCR efficiency and prevention of NH3 slip and urea deposit. Therefore, it is necessary to measure two-dimensional (2D) concentration of NH3 in detail. The purpose of this study is to develop the real-time two cross-sectional measurements technology of NH3 concentration using the computed tomography-tunable diode laser absorption spectroscopy (CT-TDLAS). Theoretical NH3 concentration distribution which was reconstructed by CT agreed to CFD results and quadruple pipe’s results showed good resolution by 14th order reconstruction. Therefore, this method has enough resolution and accuracy for measuring the concentration distribution of NH3. And this method was employed in a bent pipe model demonstrated a urea SCR system. The experimental results of two cross-sectional 2D concentration of NH3 show differences of the concentration distribution of NH3 each cross-section and flow pattern like swirl flow. It was found that CT-TDLAS was an effective method to measure concentration distribution of NH3 and observe characteristics of flow. In addition, observing flow pattern enable to validate CFD results, and it helps to improve efficiency of after treatment system.
Cooling technology plays an important role on the safe operation of hydro-generator. The natural circulation evaporative cooling system of hydro-generator stator busbar was proposed in China, which has the advantages of safety, energy-saving and high-efficiency and can meet better the requirements of generator cooling, compared with the traditional cooling technologies. In this paper, the thermal-hydraulic characteristics of this new cooling system under different loop heights were studied theoretically and experimentally, revealing the intrinsic relationship between the loop height and the system cooling performance. The increase of loop height can cause the mass flow rate increases gradually, enhancing the self-driven force and cooling capacity of system, and also makes the ratio of two phase region decrease and the system pressure increase, resulting in the rise of the wall temperature of the stator busbar. Hence the loop height should be selected optimally to ensure the adequate cooling capacity of system and the good cooling effects. Compared with the temperature of stator busbar by using the air cooling technology, it’s shown that those by utilizing evaporative cooling technology can be reduced greatly, indicating that this new cooling system has great potential of application for larger capacity hydro-generator. The results in this paper can also provide some reference for other two-phase natural circulation systems.
This study focuses on the application of OH planar laser-induced fluorescence (OH-PLIF) in high-pressure rocket combustion conditions, up to 7.0 MPa. The signal to noise ratio of PLIF degrades in high-pressure combustion owing to effects such as line broadening and interference from intense chemiluminescence. The OH(2,0) band excitation method was applied to obtain the OH(2,1) fluorescence emitted near 290 nm and filter out the intense OH(0,0) band chemiluminescence emitted near 308 nm. The gaseous H2/O2 (GH2/GO2) jet diffusion flame was formed using a recessed coaxial shear injector. The GH2/GO2 injection Reynolds number, Re (ReH2/ReO2 ≈ 2320/22800–4660/45600), was varied to examine the variation of the flame structure and reaction zone thickness under each pressure condition Pc, and Re injection condition. In addition, the variation of the experimentally derived full width at half maximum (FWHM) of the radial OH distribution, δOH, with the Damköehler number, Da, was compared with that of the simulated FWHM of the OH mole fraction, δOH-SIM. The OH distribution was clearly observed in the instantaneous PLIF image while eliminating the intense OH chemiluminescence even in the highest pressure condition of 7.0 MPa, which is a pressure higher than any of the previous OH-PLIF studies conducted on rocket combustion. The flame structure showed the typical characteristics of a turbulent jet diffusion flame and depended on Re rather than on the chamber pressure Pc. The variation of δOH with Da corresponded qualitatively with δOH-SIM and showed the characteristics of flame stretch in the vicinity of the injector.
Time-domain (TD) pulse laser is employed as the measurement laser to solve the inverse problem of radiative heat transfer and phase change in the participating medium. The finite volume method and an improved stochastic particle swarm optimization (ISPSO) algorithm are employed as the direct and inverse problem algorithms, respectively. Moreover, an optimal selection principle of the TD dimensionless boundary temperature measurement signals based on the principle component analysis (PCA) approach is proposed to improve the retrieval accuracy in retrieving the Stefan number St and the conduction to radiation parameter N. Results show that the TD dimensionless boundary temperature measurement signals obtained within dimensionless time selected within [0.05tp*, 0.8tp*] helps with the improvement in the retrieval accuracy. Moreover, compared with the SPSO algorithm, the ISPSO can avoid local optima and improve convergence accuracy, and reasonable results can be obtained even with 5% random measurement errors. As a whole, the present methodology provides a reliable and effective technique to study the inverse problem of radiative heat transfer and phase change in participating medium.
The HCCI engine can be a possible potential engine technology that gives high performance with fewer dangerous exhaust emissions. In HCCI engines, low-temperature combustion of lean, highly premixed charge effectively reduces the NOX and soot. The HCCI combustion allows utilization of a wide variety of fuels and provides higher thermal efficiency like conventional diesel. However, there are several practical challenges observed in HCCI combustion which limits HCCI engine operations to part-load conditions only. These challenges are; combustion phase control, abnormal pressure rise, and high levels of HC and CO emissions, cold start and homogeneous charge preparation, etc. The thermal and fuel stratification strategies, coupled to combustion phase control are introduced to address these issues. This paper reviews the methods to achieve the in-cylinder thermal and fuel stratifications, implementation with combustion phasing control parameters such as fuel injection timings, intake temperature, DI ratio, and EGR rate, and their effects on engine performance, and emissions.