Hot Wind Tunnel Research Articles

Pore-scale investigation of transpiration cooling based on triply periodic minimal surface

Triply periodic minimal surface (TPMS) can control the structural characteristic parameters through implicit functions, which possesses great significance for the customization of porous matrix topologies used in transpiration cooling. An innovative geometric reconstruction and meshing method is developed to reconstruct different TPMS porous matrix, a pore-scale numerical simulation of transpiration cooling coupled with mainstream is performed, and a hot wind tunnel experiment is developed to validate the numerical method. The main conclusions are as follows: The flow resistance loss of coolant within porous matrix is greatly influenced by the shape and size of pore throat, and due to the interaction between mainstream and coolant, higher flow resistance loss doesn't mean a higher coolant injection pressure. The influence of pore morphology on cooling efficiency is more obvious at higher coolant dosage, and considering the coolant injection pressure and cooling efficiency at the same time, the W(B) structure shows the best comprehensive performance. The generation of counter-rotating vortex pairs (CRVPs) in coolant film might lead to the increase in skin-friction coefficient on the structure surface, and the complex outflow angle of coolant can inhibit the generation of CRVPs, so for the case needs to reduce the skin-friction resistance, W(A) structure is recommended.

Triboelectric Polymer with High Thermal Charge Stability for Harvesting Energy from 200 °C Flowing Air

AbstractDue to the thermionic emission effect, the electron transferred to dielectric surface can be released into vacuum after the contact electrification (CE). Therefore, triboelectric nanogenerator (TENG) cannot maintain effective electrical output under high temperature conditions. In order to obtain high thermal charge stability, polyimide is modified with strong electron withdrawing groups like trifluoromethyl (CF3) and sulfone group (SO2) in backbone. The fluorinated polyimides (F‐PI) with a big band gap can provide a tribocharge density of 170 µC m−2 (4 times of common Kapton film) and become more negative than polytetrafluoroethylene in triboelectric series. In addition, BaTiO3 nanoparticles are doped in F‐PI film for inducing deep traps and interfacial polarizations for CE, which can further enhance the charge density (200 µC m−2) and thermal charge stability. Finally, a flutter‐driven TENG (FD‐TENG) is designed based on this BaTiO3‐doped F‐PI film to harvest wind energy and sense wind velocity. This FD‐TENG can maintain 32% of its output performance at 200 °C in comparison with room temperature, which is the highest thermal charge stability reported for triboelectric polymers. Therefore, this BaTiO3‐doped F‐PI has great application prospects for energy generation and motion detection in hot wind tunnel and many other harsh environments.

Exit-shape effect of film holes at pressure side on trailing edge cutback film cooling characteristics

Pressure side (PS) cutback structure has been widely employed in modern turbine blade trailing edge (TE) cooling design. However, hot gas flows around lip of TE slot to generate an impingement to cutback, resulting in a rapid decay of film cooling effectiveness and hence thermal damage of entire TE. Proper angled-ejections upstream of TE can suppress the downstream effectiveness-decay. Therefore, in present work, five types of film-holes were designed to discuss the effect of exit-shape of film-hole on TE film cooling characteristics, inlet boundary layer profile, fluid-interactions, discharge behaviors and total pressure losses. The studied models included cylindrical-hole, common and laidback fan-shaped film-holes, and converging slot-holes with two different exit-to-inlet area ratios ( ARs). The solo cutback cooling was chosen as a reference. Thermal tests were conducted in a hot wind tunnel featuring a simplified TE test section by infrared thermal technique. Engine-similar mainstream-to-coolant temperature ratio of 2.0 was controlled. Comparisons of experimental results revealed that the converging slot-holes can obtain the highest film cooling effectiveness at TE region and overall cooling effectiveness at PS film-plate, but the slightest increase of total pressure losses. Flow measurements using Particle Image Velocimetry technique indicated the converging slot-holes can modify the inlet boundary layer profile, suppress effectively the impingement of mainstream and change the flow separation features at cutback. In addition, applying shaped-holes can effectively suppress the flow ingestion under low coolant amounts. However, relative to the exit-shape effect, the coolant amount plays a more important role on the aerodynamic and thermal characteristics of TE cooling.

Experimental study of free-stream turbulence intensity effect on overall cooling performances and solid thermal deformations of vane laminated end-walls with various internal pin–fin configurations

Experimental investigations of mainstream turbulence intensity (Tu) effect on conjugate heat transfer performances, total pressure loss and solid thermal deformations of vane laminated cooling end-walls (LCEs) have been carried out under two engine-near mainstream-to-coolant temperature ratios of 1.4 and 2.0. Influences of parameters of internal elements of LCEs were discussed. A servicing film cooling end-wall (FCE) was chosen as a reference. Two typical Tus were designed, 2.2% and 10.1% (in the engine range). Measurements of fluid-thermal-solid coupling characteristics of cooled end-walls were implemented in a hot wind tunnel based on matched engine Biot number under three mass flow ratios. Generally, the elevated Tu exhibited a negative influence on overall cooling effectiveness and component reliability of cooled end-walls, total pressure loss in cascade and slight thermal deformations of LCEs. Moreover, the elevated Tu displayed a negative effect on the remarkable benefit of LCEs over FCE. The shape and diameter of pin-fins and the impingement-hole size played complex roles on overall effectiveness, surface thermal gradient, aerodynamic loss and component weight. However, the Tu effect on the aforementioned roles was slight. Overall, the LCE with diamond-shaped pin-fins and large impingement-holes was the most optimal design.

Experimental and numerical investigation of isolator in three-dimensional inward turning inlet

Numerical simulation about a three-dimensional inward turning inlet with isolator and combustor was performed to study the flow field structure and flow characteristics of shock train. Cold and hot model wind tunnel experiments were carried out to compare the effects of cold and hot condition on shock train characteristics. The result shows that no oscillation phenomenon of the shock train was observed in the cold model experiment, but oscillation back and forth occurred in the hot model experiment. The trend of pressure distributions are consistent in cold and hot condition before the combustor. But it changed in the combustor where the pressure presented a continuous rising trend in cold condition, while a trend of rising first and then descending in hot condition. Establishing the corresponding relationship between throttling ratio and gas-oil ratio is beneficial to predict the property of the isolator in hot condition according to the cold condition.

Self-pumping transpiration cooling with phase change for sintered porous plates

This study experimentally investigated self-pumping transpiration cooling in a hot wind tunnel. The coolant (liquid water) automatically and continuously flowed from the water tank to the heated porous surface without the use of any pump and then evaporated on the porous surface, thereby dissipating a large amount of heat. The self-pumping cooling system was stable and the temperature distributions were homogeneously maintained at approximately 373 K with a cooling efficiency of 86%. The entrainment ratio was approximately 0.4 in the mainstream conditions of this research. The unsteady and inhomogeneous problems in traditional transpiration cooling systems were successfully solved by this self-pumping transpiration cooling system. The particle diameter of the sintered porous plate had a slight influence on the coolant mass flow rate. However, the cooling system broke down when the particle diameter was too large to provide a sufficient capillary force. The coolant mass flow rate exhibited a rapid and automatic response to changes in the mainstream velocity and temperature, while the porous surface temperature remained constant. A Mathematical model was built to predict the coolant mass flow rate of this self-pumping transpiration cooling system coupled with the mainstream and the feasible particle size.

An experimental investigation on transpiration cooling performances using solid hydrogel as coolant

This paper presents an experimental investigation on a new conception of thermal protection techniques for hypersonic vehicles, namely, transpiration cooling using solidified water as coolant. In the work, a commercial superabsorbent polymer, SAP-50, was used to make swollen hydrogel, and then a thermogravimetric analysis test of the home-made hydrogel was conducted. To enhance the heat transfer inside the solid hydrogel, a copper foam was embedded. The transpiration cooling experiments were carried out with a porous flat plate specimen in a hot wind tunnel, and the cooling characteristics captured by an infrared thermal imaging system are exhibited and compared between the hydrogel and water, with and without the copper foam. The original thermal images and the comparisons reveal the following novel and valuable phenomena: (1) Solid hydrogel and liquid water can provide approximate transpiration cooling effect. (2) Different from liquid water cooling, the hydrogel coolant can provide a longer acting time, and get a more homogenous cooling effectiveness distribution. (3) In the transpiration cooling systems without coolant injection force, the copper foam installed in the coolant chamber can provide a better cooling performance and longer effective time.

An experimental investigation of cooling characteristics at a vane end-wall with a locally enhanced hole-layout

This paper presents an experimental investigation on the effect of film hole-layout at a turbine vane end-wall on the cooling performances, including the distributions of overall cooling effectiveness, uniformity of temperature field and highest temperature drop, using a hot wind tunnel and an Infrared Thermal Imaging System (ITIS). Two specimens were manufactured and used. One is named as “uniform hole-layout (UHL)”, where ten rows of film holes were uniformly arranged along the axial direction. However, UHL results in a non-uniform temperature distribution over the entire end-wall surface, and the low film coverage and bad reliability in the regions near leading edge (LE) and pressure side (PS) of the vane thereby. To improve the cooling performance in these regions, a new layout named as “locally enhanced hole-layout (LEHL)” was proposed. The difference from the previous experiments conducted under adiabatic assumption is that, the conjugate heat transfer is considered, and the conjugate-temperature distribution is used as the basis of design of LEHL. The comparisons of the cooling performances of two specimens at various mass flow ratios (MFRs) of coolant-to-mainstream revealed that: (1) from MFR = 0.5% to 1.5%, using LEHL, the area-averaged overall cooling effectiveness over the entire end-wall, in LE region and in the corner of PS can be increased by at least 32.3%, 34.2% and 26.2%, respectively; and (2) the temperature differences from PS to suction side (SS) and the temperature at the entire end-wall can be significantly reduced by LEHL. Therefore, the design strategy of new hole-layout using the conjugate-temperature is conducive to obtain the simultaneous improvement of film coverage and working life.

Experimental study of flow and heat transfer in rotary air preheaters with honeycomb ceramics and metal corrugated plates

The objective of this paper is to compare flow and heat transfer performance of rotary air preheaters consisting of honeycomb ceramics and metal corrugated plates. The experimental study was carried out by a bi-directional switching hot wind tunnel filled with single heat transfer matrix. The heat transfer and resistance empirical correlations, as well, the relationships between effectiveness and number of transfer unit were deduced from measured data. Under the same conditions, higher outlet air temperature and heat transfer effectiveness, lower pressure drop occurred in the metal heat transfer elements. The effects of switching time and flue gas velocity on flow and heat transfer performance of rotary air preheaters were analyzed. The experimental approach of bi-directional switching hot wind tunnel filled with single heat transfer matrix could be served as a performance analysis method for rotary air preheaters with different materials and passage shapes.

Loop thermosiphon as a feasible cooling method for the stators of gas turbine

A stator sample of a gas turbine was developed with loop thermosiphon structure to seek a feasible cooling method for high-temperature stators of gas turbine. The sample was comprised by 8 hollow vanes and a cooling tube. Its inner space was filled by sodium-potassium alloy (Na-K) to become a loop thermosiphon stator sample (LTSS). The thermosiphon structure could transfer the heat from the hot surfaces of the vanes heated by the high-temperature gas mainstream to its own cooling tube, which would be blown by the cold air in the external duct of a gas turbine. Heatproof ability of the LTSS was experimented at 1230°C in a hot wind tunnel to judge whether loop thermosiphon is a feasible cooling method. Results indicated the average wall temperature of 8 vanes was about 340°C lower than 1230°C.

Conjugate heat transfer investigation on the cooling performance of air cooled turbine blade with thermal barrier coating

A hot wind tunnel of annular cascade test rig is established for measuring temperature distribution on a real gas turbine blade surface with infrared camera. Besides, conjugate heat transfer numerical simulation is performed to obtain cooling efficiency distribution on both blade substrate surface and coating surface for comparison. The effect of thermal barrier coating on the overall cooling performance for blades is compared under varied mass flow rate of coolant, and spatial difference is also discussed. Results indicate that the cooling efficiency in the leading edge and trailing edge areas of the blade is the lowest. The cooling performance is not only influenced by the internal cooling structures layout inside the blade but also by the flow condition of the mainstream in the external cascade path. Thermal barrier effects of the coating vary at different regions of the blade surface, where higher internal cooling performance exists, more effective the thermal barrier will be, which means the thermal protection effect of coatings is remarkable in these regions. At the designed mass flow ratio condition, the cooling efficiency on the pressure side varies by 0.13 for the coating surface and substrate surface, while this value is 0.09 on the suction side.

Experimental and numerical study of an integrated air- and steam-cooled gas turbine vane

This paper proposes a new cooling method which integrates steam and air for gas turbine vane cooling with the aim to solve the problem of a very high thermal load at the trailing edge region of a steam-cooled gas turbine vane. The influence of coolant inlet conditions on the temperature and cooling effectiveness distribution of the experimental vane surface has been investigated in a hot wind tunnel test facility with a three-vane cascade. The test vane external aerodynamic and heat transfer were investigated experimentally and numerically. Two improved arrangement of cooling working fluid flow direction were designed to reduce the temperature gradient along the vane height. The following conclusions can be made: the thermal load at the trailing edge region is reduced for this integrated air and steam cooled gas turbine vane; the surface temperature uniformity is improved. Under an optimal thermal physical parameters of the cooling fluids, the cooling effectiveness of the trailing edge region and the leading edge region are 50% and 55%, respectively; the cooling effectiveness of the mid chord region is the highest and reaches 67%; The cooling method and structure have more influence on the cooling effectiveness compared with the cooling fluids inlet conditions.

Experimental investigations on overall cooling effect of ribbed channel with air bleeds

An experimental investigation on overall heat transfer performance of a rectangular channel, in which one wall has periodically placed oblique ribs to enhance heat exchange and cylindrical film holes to bleed cooling air, has been carried out in a hot wind tunnel at different mainstream temperatures, hot mainstream Reynolds numbers, coolant Reynolds numbers and blowing ratios. To describe the cooling effect of combined external coolant film with the internal heat convection enhanced by the ribs, the overall cooling effectiveness at the surface exposed in the mainstream with high temperature was calculated by the surface temperatures measured with an infrared thermal imaging system. The total mass flow rate of cooling air through the coolant channel was regulated by a digital mass flow rate controller, and the blowing ratio passing through the total film holes was calculated based on the measurements of another digital-type mass flow meter. The detailed distributions of overall cooling effectiveness show distinctive peaks in heat transfer levels near the film holes, remarkable inner convective heat transfer effect over entire channel surface, and visible conductive heat transfer effect through the channel wall; but only when the coolant Reynolds number is large enough, the oblique rib effect can be detected from the overall cooling effectiveness; and the oblique bleeding hole effect shows the more obvious trend with increasing blowing ratios. Based on the experimental data, the overall cooling effectiveness is correlated as the functions of Rem (Reynolds number of hot mainstream) and Rec (Reynolds number of internal coolant flow at entrance) for the parametric conditions examined.

F207 ガスタービン翼の高湿分冷却試験(OS-7 高温・高効率発電(1),一般講演,地球温暖化防止と動力エネルギー技術)

Experiments of gas turbine blades cooled by high-humidity air were performed to examine the effect on reducing cooling air for AHAT(Advanced Humid Air Turbine) system. Two types of hybrid cooling structure using dry air and high-humidity air as a coolant were considered. A hot wind tunnel was used to realize the real combustion gas condition. The experimental result shows this hybrid cooling structure has high cooling effectiveness compared with the conventional cooling structure.

Surface and Gas Temperature Measurements Along a Film Cooled Wall

Film cooling is widely used to protect critical sections of propulsion devices from extreme heat loads in the internal flowpath. Many studies have been conducted to formulate scaling laws characterizing the effectiveness of film cooling in a variety of configurations for the development of engineering correlations. There is an emerging demand for detailed measurements near film-cooled surfaces to support computational model development of these complex wall-bounded flows. In this study a unique hot wind tunnel facility was used to perform detailed measurements in a canonical two-dimensional thermally stratified wall-jet configuration. Wall temperature, adiabatic effectiveness, and turbulent flow measurements were performed over a range of carefully controlled inlet conditions. Far-field laboratory effectiveness measurements compare well with previously developed scaling laws; however, the effect of the ratio of mainstream to cooling stream temperature is significantly underpredicted. The micro-thermocouple probe used in this study provided mean flow temperatures and detailed turbulent statistics characterizing the nearwall mixing behavior. The mean flow temperature measurements in the far field demonstrate self-similar behavior whereas fluctuating temperatures indicate fully developed turbulence in the wall layer. The turbulent statistics provided in this study are especially useful for development and validation of computational models of film-cooled surfaces.

High-enthalpy hypersonic project at ONERA

This article deals with the research performed in the framework of the internal ONERA Hyperenthalpic Hypersonic Project. This project involved fifteen researchers specialised in different domains, which are the following: fundamental studies on shock/wave and shock/boundary layers interactions; study of the laminar/turbulent transition; real gas effects and associated physical modelling; the development of non-intrusive optical diagnostic methods usable in cold and hot wind tunnels; flow rebuilding in a hot shot wind tunnel such as F4; conception, realisation and validation of computational fluid dynamics (CFD) solvers.

An experimental study on fouling over a single tube

An experimental study to investigate the fouling phenomena over a single tube is carried out in the hot wind tunnel. The temperature of gas and cooling air, and the temperature in the fouling probe are measured as a function of time, probe position, gas temperature, particle feeding rate, cooling air flow rate, etc., giving the local and average heat transfer rate and fouling factor. The amount of foulant adhered to the probe has different values at the different positions on the probe surface, giving different heat transfer rate and fouling factors as a function of position on the probe surface. The fouling factor increases with the flow rate of cooling air and foulant feeding rate. The local fouling factor increases with the gas temperature but the average fouling factor does not depend much on the gas temperature above 700°C.

The Advanced Cooling Technology for the 1500°C Class Gas Turbines: Steam-Cooled Vanes and Air-Cooled Blades

It is very essential to raise the thermal efficiency of combined cycle plants from the viewpoint of energy saving and environmental protection. Tohoku Electric Power Co., Inc., and Toshiba Corporation in Japan have jointly studied the next generation of combined cycle systems using 1500°C class gas turbine. A promising cooling technology for the vanes using steam was developed. The blades are cooled by air, adopting the impingement cooling, film cooling, and so on. The cooling effectiveness was confirmed both for the vanes and the blades using a hot wind tunnel. This paper describes the design features of the vanes and the blades, and the results of the verification tests using the hot wind tunnel.

Particulate Two-Phase Flow Analysis for Fouling Prediction(I)-Design of Hot Wind Tunnel and Its Performance Experiment-

We designed the hot wind tunnel to reproduce the conditions of utility boiler and carried out its performance test, in order to investigate the particulate two-phase flow behaviour, the fouling and heat transfer characteristics to the heat exchanger. The hot wind tunnel introduces the control system to control the temperature in the test section. The particle is injected into the hot gas stream. The fouling probe (cylindrical tube) is positioned normal to the particulate gas-particle two-phase flow and cooled by the air. The temperature of gas and cooling air, and temperature in the fouling probe are measured as a function of time, giving the local and averaged heat transfer and fouling factor. The shape of particulate deposition adhered to the fouling probe is also observed.

The Transpired Turbulent Boundary Layer in Various Pressure Gradients and the Blow-Off Condition

An experimental study of the reduction in heat transfer to a transpiration-cooled flat surface subjected to pressure gradients (zero, negative, and positive) is presented for flow conditions similar to those encountered in gas turbines. The investigation is carried out for high injection rates and determines the blow-off conditions under which the boundary layer is lifted away from the wall by the transpired coolant. The study was conducted in a hot blow-down wind tunnel facility. The transient nature of the facility ensures that the wall remains isothermal. The Reynolds number, the ratio of the gas to wall temperatures, and the pressure gradient parameters K are chosen to be representative of the conditions found in advanced gas turbines. The effect of the pressure gradient was found to be small. However, a local strong acceleration can reduce the cooling effectiveness. The heat transfer rates or Stanton numbers on a solid surface downstream of a transpiration cooled wall are found to be sensitive to the upstream injection ratio (b) and to the pressure gradient parameter. The data indicate that the ratio of Stanton numbers with and without cooling is nonzero for values of the injection parameters larger than values obtained theoretically by Kutateladze. The predicted value of the critical injection ratio (bcr) determined from this study agrees well with the experimental data of Liepmann and Laufer for a free mixing layer, which is similar to a transpired boundary layer near blow-off as pointed out by Coles.