Xinjun Wang

Numerical Investigation of Heat Transfer and Flow Characteristics in a Steam-Cooled Square Ribbed Duct

In order to increase entry gas temperature and improve the efficiency of gas turbine, steam is used as a coolant instead of air. Much research has been carried out on the closed circuit steam cooling of vanes substituted with film-cooling using compressor air in recent years. Furthermore, by studying the steam flow and heat transfer characteristics in rib ducts, this investigation focuses on establishing the basis of steam cooling technology application in complex flow field of internally-cooled turbine vane. In this paper, a report and assessment of RSM method based on SSG turbulence model is performed with commercial computational fluid dynamics software ANSYS CFX. The numerical results of heat transfer coefficient and friction factors in square channels with 90 degree rib turbulators for Reynolds numbers of 10 000, 30 000 and 60 000 are compared with the experimental data from Han’s. It is found that the obtained heat transfer coefficient distributions and friction factors match well with SSG turbulence model. In addition, the heat transfer distribution and pressure drop of steam-cooled ducts are predicted under the same work conditions by using dry real gas model. The Reynolds number could be correlated with the Nusselt number. The impact of steam physical properties on heat transfer performance are researched detailedly by respectively changing the steam superheat and entry pressure. The results indicate that the RSM method with a suitable turbulence model is valuable for the air-cooled and steam-cooled duct with the acceptable engineering accuracy (less than 20%). Comparing the cooling efficiency between steam and air under the same operation condition, the advantage of using cooling steam is evident than using cooling air. Furthermore, the efficiency of the whole gas turbine system will be greatly improved through using the closed loop steam cooling system. Changing the steam superheat and entry pressure, it has little effect on the steam flow and heat transfer characteristics. Increasing the steam overheat would raise the friction factor. Contrarily, enhancing the entry pressure would decrease the friction factor.Copyright

Computational fluid dynamics analysis for effect of length to diameter ratio of nozzles on performance of pre-swirl systems

In a direct-transfer pre-swirl system, cooling air expands through stationary pre-swirl nozzles and flows through the cavity to the receiver holes located in the rotating turbine disc for blade cooling. This paper investigates the effect of the length-to-diameter ratios of pre-swirl nozzles on the performance of a direct-transfer pre-swirl system in a rotor-stator cavity. The commercial code CFX 12.1 is used to solve the Reynolds-averaged Navier–Stokes equations using the SST turbulence model. Computations are performed for seven length-to-diameter ratios, L/D = 1, 2, 3, 4, 5, 6, and 7, a range of pre-swirl ratios, 0.5 < β p <2.0, and varying turbulent flow parameters, 0.12 < λ T  < 0.36. The rotational Reynolds number for each case is 106. The computational fluid dynamics model presented in this paper is validated with the experimental results available in the literature. The nozzle exit flow angle α decreases as length-to-diameter ratio L/D increases for L/D < 1/tan θ (θ is pre-swirl nozzle angle). When L/D > 1/tan θ, α is approximately invariant and below θ. The discharge coefficient C d,b for the receiver holes reaches a peak as the fluid in the rotating core is in synchronous rotation with the receiver holes. For small turbulent flow parameters λ T , a peak of C d,b can be observed as L/D = 3. For large turbulent flow parameters λ T , the shift of the position for a peak occurs. When L/D = 3, the synchronous rotation can be achieved with the smallest value for turbulent flow parameters λ T . The adiabatic effectiveness of the system increases with turbulent flow parameters λ T . A peak of Θ b,ad is seen as L/D = 3 for each case. When L/D < 1/tan θ, there is a significant increase in Θ b,ad , especially for L/D < 2, with an increase in L/D. When L/D is increased further, a slight decrease occurs. Both performance parameters show that the optimum value for all cases can be achieved as L/D = 3, which is slightly above 1/tan θ.

Flow and Heat Transfer of Air and Steam in Internal Cooling Passages of Turbine Blade

Numerical prediction of three-dimensional flow and heat transfer of air and steam are presented for serpentine cooling channels by using the commercial software CFX. The results show that SSG model is the best turbulence model for the ribbed channels. A study of Grid Generation was performed for flow and heat transfer in serpentine cooling channels, with the same turbulence model. And the results show that the space between the first node and the wall surface (Δy) is 0.0001 mm and the grid density is 1.3 or Δy of 0.001 mm and grid density of 1.2 is the appropriate choice for grid generation. Ribbed channels are not sensitive to mesh generation compared with smooth passages. With the same inlet flux, steam heat transfer efficiency is higher than that of air about 15–20%; steam superheat degree is not the key factor for heat transfer, but it had an effect on flow resistance. Compared with smooth channels, ribbed channels reduce the impact of the turn; the best heat transfer regions appear downstream of the turn. V-type ribs have better heat transfer performance than the parallel type ribs; the highest heat transfer occurs in the section between the ribs.Copyright

Experimental Study on Effects of Slot Hot Blowing on Secondary Water Droplet Size and Water Film Thickness

The effects of the slot hot blowing of the hollow stator blades on the size of secondary water droplets and the thickness of the water film were experimentally investigated in this paper. The experiment was carried out on the turbine blade cascades in a wet air tunnel with an inlet air wetness fraction of 7.9%, an outlet air velocity of 170 m/s, a slot width of 1.0 mm, and a slot angle of 45 deg to blade surface. The Malvern droplet and particle size analyzer was utilized to measure the secondary water droplet size and distribution downstream of the hollow stator blades in the experimental tests. The experimental results show that the maximum diameter and Sauter mean diameter of the secondary water droplets were reduced greatly, and the water droplet size distribution became narrower. The larger blowing pressure difference resulted in the smaller secondary water droplets and the narrower water droplet size distributions. In addition, the efficiency of water separation from the hollow stator blade surfaces was higher for slot on the suction side than that of the pressure side case. Another simplified experimental test was also carried out on the flat plate to investigate the effect of slot hot blowing on the thickness of the water film downstream of the slot. The conductivity probes were used to measure the thickness of the water film downstream and upstream of the blowing slot. The results show that the slot hot blowing reduced the thickness of the water film downstream of the slot, which was affected by the blowing pressure difference and temperature difference between the hot blowing air and the main airflow. In conclusion, the slot hot blowing of the hollow stator blades has reduced the size of the secondary water droplets and secondarily has evaporated a little water film on the blade surfaces. Both effects are beneficial to reduce the erosion damage to the rotor blades.

Effect of droplet characteristics and rotation speed on the flow and heat transfer characteristics of mist/air cooling in a rotating ribbed two-pass rectangular channel

The flow and heat transfer characteristics of mist/air cooling in the rotating ribbed two-pass rectangular channel are numerically investigated using the CFD software ANSYS-CFX. In this article, a comparison in heat transfer performance between the mist/air cooling and the air-only cooling is performed. Additionally, the effect of the initial mist diameter, temperature, velocity and the channel rotation speed on the mist/air cooling performance is analysed. The results show that the droplet flow distance and Nusselt number of the mist/air cooling increase as the initial mist temperature decreases. In addition, as the initial mist diameter decreases, the diameter of mist on the whole channel decreases, resulting in the higher heat transfer, whilst the mist concentration also decreases, leading to the lower heat transfer. Therefore, there is an optimal initial mist diameter which makes the heat transfer performance best. Nevertheless, the droplet movement and heat transfer performance of mist/air cooling are nearly insensitive to the initial mist velocity. It is also noted that the Coriolis force increases with the channel rotation speed, causing the flow deflection becomes more obvious. Consequently, as the channel rotation speed increases, in the first passage the averaged Nusselt number on the trailing wall increases, and that on the leading wall decreases, while the trend in the second passage is reversed.

CFD analysis of mist/air film cooling on a flat plate with different hole types

ABSTRACT The flow and heat transfer mechanisms of mist/air film cooling are studied in this paper with three hole types under three blowing ratios. The velocity vectors and three-dimensional contours of the vortices are shown in this paper in order to investigate the mist/air cooling characters with different hole types. Both the span averaged and centerline film cooling effectiveness are studied in this paper to compare the cooling performance of three cooling hole types. Mist/air cooling performances of cylindrical hole, fan-shaped hole, and console hole are studied numerically. Flow structures of mist/air jets are studied. Both centerline and span averaged film cooling effectiveness are studied. Results show that the kidney vortex uplifts the mists further away from the flat plate and the antikidney vortex uplifts the mists near the lateral edge of the hole gradually. Mists enhancement on cooling effectiveness is significantly impaired at a higher blowing ratio in cylindrical hole and fan-shaped hole cases. The cooling effectiveness can maintain a relatively high value at each blowing ratio in console hole cases.

Effects of Curvature on the Film Cooling Effectiveness of Double-Jet Film Cooling

The effect of curvature on the film cooling characteristics of Double-Jet Film Cooling (DJFC) was numerically investigated using three-dimensional Reynolds-Averaged Navier-Stokes (RANS). The low-Reynolds number shear stress transport (SST) model was employed as the turbulence closure model. Six different curved surfaces and a flat surface were tested numerically. The blowing ratios were from 0.66 to 1.99, and the compound injection angle with respect to the cooled surface was 30 degree. The blowing ratios and the curvature of cooled surface have crucial effects on the film cooling effectiveness. The numerical results show that there are two peek value of the averaged film cooling effectiveness along the mainstream direction. The results also indicate that the film cooling effectiveness of a specified curved surface depends on the reasonable selection of the slope of curved surface and blowing ratios.Copyright

Study on the Sampling Quality of Wetness Measurement Probe for Thermodynamic Methods

Software FLUENT was applied to conduct the numerical calculations of the sampling velocity at the sampling nozzle inlet of the wetness measurement probe and the trajectories of water droplets in the steam flow. The steam wetness of samples and the percentage of the droplets with different diameters entering the sampling nozzle were ascertained. The results showed that wetness measurement probe affected the flow of vapor phase at some degrees. Especially, there was a deflection of stream line nearby the sampling nozzle. It was showed that the isokinetic sampling could not be accomplished because of the viscosity of vapor. The larger the angle between the steam flow direction and the center line of the sampling nozzle was, the lower the average sampling velocity at sampling nozzle inlet section was. The percentage of water droplets captured by sampling nozzle increased with the augmentation of water droplet diameters. When the water droplet diameter was 5μm, the sampling nozzle would capture all water droplets in the corresponding area of the sampling nozzle inlet. When the sampling nozzle was dead against the upper stream, the wetness of sample extracted by sampling nozzle was lower than that of the measured steam. In contrast, the wetness of sample was larger than that of the measured steam when the angle between the sampling nozzle and upper stream was ± 5° or ± 10° respectively. The results have showed that the wetness error increased with the augmentation of sampling nozzle diameters, the vapor velocity and the angle between upper steam and center line of the sampling nozzle.Copyright

Effect of trench width and blowing ratio on double-jet film cooling embedded in trenches

In order to meet the problem of material damage induced by high turbine inlet temperature, reliable means of improving film cooling effectiveness are increasingly being sought by researchers involved in the study of gas turbines. Numerical investigation of the effects of trench width and blowing ratio on double-jet film cooling embedded in trench has been conducted using three-dimensional Reynolds-averaged Navier–Stokes (RANS) model. The trench is perpendicular to the mainstream flow direction. The commercial computational fluid dynamics (CFD) solver ANSYS 11.0 has been applied and shear stress transport (SST) turbulence model with gamma theta transitional model is used for turbulence model in simulations. The reliability of the numerical approach has been demonstrated by comparison of numerical results and experimental data. The blowing ratios are from 0.70 to 2.05, and the trench widths range from 5.83D to 7.92D. To investigate the film cooling effectiveness and heat transfer coefficient, the cooled surfaces have been given by adiabatic condition and fixed temperature, respectively. It is observed that the film cooling effectiveness has been greatly improved for double-jet with trench. In the vicinity region of film holes, the influence of the trench on the heat transfer coefficient is remarkable. The trench width and blowing ratio have important effects on cooling performance and heat transfer. For a given trench width, an optimum blowing ratio must be selected with an overall consideration of film cooling effectiveness and heat transfer coefficient.

Numerical Investigation on the Flow and Heat Transfer Characteristics of the Superheated Steam in the Wedge Duct With Pin-Fins

By using the CFX software, the three-dimensional flow and heat transfer characteristics in the cooling duct with pin-fin in the blade trailing edge were numerically simulated. The effects of pin-fin arrangements, Reynolds number, steam superheat degrees, streamwise pin density and convergence angle of the wedge duct on the flow and heat transfer characteristics were analysed. The results show that the Nusselt number on the endwall and pin-fin surfaces as well as the pin-fin row averaged Nusselt number increase with the increasing of Reynolds number, while it decreased with the with the increasing of X/D. The pressure drop increases with the increasing of Reynolds number while decreases with the increasing of X/D in the wedge duct. The degree of superheat has little effect on the pressure loss in the wedge duct. A comprehensive analysis and comparison show that the highest thermal performance is reached in the wedge duct when the value of X/D is 1.5.Copyright