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A numerical investigation is performed for double-jet film cooling (DJFC) on a semi-cylindrical leading edge under four momentum flux ratios. Three rows of film cooling holes are distributed on the leading edge, wherein DJFC units are applied at ± 30 deg lines and the film cooling holes at the stagnation line remain in cylindrical shape in the baseline case. Totally, nine cases of DJFC units are designed by altering the spanwise spacing, streamwise spacing, and streamwise injection angle, while keeping the spanwise injection angle unchanged. The results show that proper layout of DJFC unit produces a “branched” spreading feature of jet trajectories, attributed to the formation of the anti-kidney vortex pair. Evaluated in the spatially averaged results on the semi-cylindrical leading-edge surface, DJFC could increase the adiabatic film cooling effectiveness up 20% at I = 1.3 with respect to the baseline case. Among the current geometric parameters in the DJFC unit, the spanwise spacing is an important parameter affecting the jet spreading feature. In relative to the spanwise spacing, the streamwise injection angle shows a weaker influence on vortical structures in the downstream flowfield. A larger spanwise spacing produces a greater adiabatic film cooling effectiveness but also a little bigger heat transfer coefficient. A similar trend is also demonstrated for the streamwise injection angle. The streamwise spacing has nearly no influence on the spatially averaged heat transfer coefficient. A smaller streamwise spacing is more promising for increasing adiabatic film cooling effectiveness.

The structural optimization of a specific laminated cooling structure is conducted under gas turbine combustor representative aero-thermal conditions, with the aim at improving its overall cooling ...

Combined with infrared thermography experiments, large-eddy simulation was used for studying trench film cooling on C3X vane model at the mainstream Reynolds number of 2.5 × 105 based on the chord ...

This study employs large eddy simulation to investigate the impinging heat transfer of the pulsed chevron jet on a semi-cylindrical concave plate at f = 20 Hz and H/d = 4. The instantaneous temperature of the target plate is highly associated with the behavior of the pulsed jet. The temperature sharply decreases once it is covered by the cooling wall jet and then sharps increase only when the jet injection is off. The pulsation plays a significant role in the formation and evolvement of the coherence structure. At the time of t = 0.1 φ, the pulsed jet has covered the stagnation region. The stripe-shaped coherence structure is vertical to the ring-shaped secondary vortex. The secondary vortex becomes broken and unsteady as moving downstream. When the jet is off, the wall jet moves downstream because of inertia. There is no remnant cooling wall jet covering the target plate. There is no recirculation zone along the curvilinear direction during the duty cycle, while the recirculation zone is formed only when the jet injection is off. The hot wall jet along the curvilinear direction is entrained through large-scale recirculation and then returns to the target plate. The influencing area of the recirculation zone of the pulsation becomes larger and more closed to the jet mainstream compared with the steady jet. The existence of pulsation enlarges the spreading width of the jet core compared with a steady jet. The proportion of velocity and pressure in primary frequency dominates the pulsed jet, and the proportion in higher order can be neglected.

A series of experimental tests are performed for the pulsating jet impingement heat transfer by varying the Reynolds number (5000 ≤ Re ≤ 15000), operation frequency (5 Hz ≤ f ≤ 40 Hz) and dimensionless nozzle-to-surface distance (2 ≤ H/d ≤ 10) while fixing the duty cycle as DC = 0.5. The maximum uncertainty in the measurement of Nusselt number is estimated to be about ±7%. Meanwhile, numerical simulations are performed to demonstrate the instantaneous flow field of the pulsating jet impingement. Particular attention is paid to examine the influence of transmission chamber on the pulsating jet impingement heat transfer. The results show that by using an additional transmission chamber, the pulsating jet impingement heat transfer is enhanced. For example, the circumferentially-averaged Nusselt number around the stagnation point (x/d ≤ 2) is increased up 8%~16% by the adding of a transmission chamber in related to the baseline case under Re = 10000 and H/d = 6. Due to the presence of transmission chamber, the exiting jet velocity profile at the orifice outlet is varied. In related to the baseline case, the transmission chamber makes the time-averaged ejecting velocity in the central zone increase but decrease in the edge zone. As the peak velocity in the central zone of orifice outlet is effectively increased by the use of transmission chamber, the jet impinging velocity approaching to the target surface is also strengthened, resulting in a stronger jet impingement in the vicinity of the stagnation point.

A numerical investigation is performed to study the effects of transverse trenches on the film cooling performances of single row film cooling holes on the turbine guide vane, under the engine-repr...

Numerical simulations are carried out to address further insights into the effects of round-to-slot film cooling holes on a multi-row film-cooled turbine blade under representative engine-simulated conditions by adopting both adiabatic and conjugate heat transfer CFD models. In the current study, only the alteration of film-hole shape at suction and pressure sides is concerned while the showerhead film cooling holes at the leading edge remain cylindrical or baseline shape. Four film-hole shapes are taken into consideration, including converging round-to-slot hole (RTSH-1), equivalent-area round-to-slot hole (RTSH-2), diffusing round-to-slot hole (RTSH-3) and fan-shaped hole (FSH). Beside, a series of computations are also performed to the individual single-row film cooling by setting the coolant-feeding condition as its corresponding one in the multi-row film cooling situation. Viewed from the comprehensive influence of film-hole shape on film cooling performance, such as coolant feeding usage amount, enthalpy loss coefficient and cooling effectiveness, it is suggested that RTSH-2 should be the most favourable shape. RTSH-1 leads to the smallest coolant usage and enthalpy loss coefficient but its cooling effectiveness is low. Although RTSH-3 could present the highest cooling effectiveness, it produces the largest coolant usage and cascade enthalpy loss coefficient.

An experimental and numerical investigation is performed in the current study to further explore the convective heat transfer performance by a vertically-oriented piezoelectric fan in the presence of channel flow. The effects of velocity ratio and fan tip-to-heated surface clearance are taken into considerations. It is illustrated that the presence of channel flow has an innegligible influence on the vibration amplitudes of the piezoelectric fan under large channel flow velocities. In the presence of channel flow, the vortical structures at the edges of vibrating fan are certainly suppressed, especially under large velocity ratios. On the other hand, the vortical streaming flow mixes with the channel flow to form a long stripe of vortical structure downstream of the fan vibration envelope. Under small velocity ratios, the impingement role of streaming flow along fan tip is still dominated and simultaneously the channel flow passing through the vibration envelope is effectively pulsated. Therefore, combined flows generally produce heat transfer enhancement around the fan vibration envelope related to the pure vibrating fan, especially at a small non-dimensional tip-to-surface gap. While under large velocity ratios, the impingement role of streaming flow induced by a vibrating fan is seriously weakened by the strong channel flow. The convective heat transfer produced by combined flows in the fan vibration envelope is generally reduced in comparison with pure piezoelectric fan. Related to the pure channel flow, the combined flows effectively improve the convective heat transfer, especially downstream of the fan vibration envelope.

A CFD-based multi-objective optimization is performed for improving the film cooling performance of the laidback fan-shaped holes on the suction surface of a turbine guide vane under a typical blowing ratio of M = 1.5. Among the main geometric parameters, the inclination angle (α), lateral expansion angle (β) and forward expansion angle (γ) are selected as the design variables, with respective lower and upper bounds of (25°, 55°), (10°, 20°) and (3°, 15°) in turns. Two independent objective functions that are simultaneously optimized are selected as the spatially-averaged adiabatic film cooling effectiveness (ranging from s/d = 0 to s/d = 12) and the discharge coefficient. By using a variant of non-dominated sorting genetic algorithm (NSGA-II) coupled with the RBFNN-based surrogate model, the Pareto front of optimal solutions is obtained, providing a variety of options for seeking the maximum spatially-averaged adiabatic film cooling effectiveness, the maximum discharge coefficient, or the compromise of both aspects. The optimized results show that the optimal geometers of (α, β, γ) are (50.3°, 19.5°, 9.8°), (25°, 18.7°, 11.8°) and (27.3°, 19°, 5.1°) for achieving the most maximum film cooling effectiveness, the most maximum discharge coefficient and the compromise of both aspects, respectively. In general, a large lateral expansion angle of the laidback fan-shaped film-cooling hole is necessary in the shape optimization for all of the optimal options. However, with regard to the other design variables, their selections are very distinct following the optimal option. Further, the influence role of optimal fan-shaped geometry on the film cooling performance is illustrated according to the detailed flow and thermal behaviors.

An experimental and numerical investigation is performed to explore the effects of blade shape on the convective heat transfer performance induced by piezoelectric fans. Five blade types are taken into consideration corresponding to the ones presented by Lin et al. [28], including one baseline type with a rectangular shape (Type-A), two rectangular shapes with wider widths (Type-B and Type-C), and two trapezoidal shapes in divergent (Type-D) and convergent (Type-E). All the blades are attached to the same piezoelectric patch and have the same exposed length. The vibration tests show that the blade shape has a significant influence on the vibrating characteristics of piezoelectric fan. Related to the baseline Type-A, Type-B and Type-C make the first-order resonance frequency a little descent. Type-D makes its first-order resonance frequency far less than the baseline type but Type-E is opposite. From the numerical simulations, the vortical structures induced by different blades vibrating at their respective resonance frequencies are illustrated. It is found that Type-B and Type-C produce stronger vortical flow although they have a little less vibrating frequency than the baseline Type-A. For Type-E, as its vibration frequency is obviously larger than Type-A, the scale of vortex shedding from the vibrating fan seems much stronger. In comparison with baseline type of piezoelectric fan, the location of highly local heat transfer zone moves from the center to both edges of fan-tip vibration envelope of fan-tip vibration envelope with the increase of blade width. In general, the blade types like Type-C and Type-E are suggested to be the favorable shapes for achieving better convective heat transfer performance. However, a little larger power consumption for actuating the piezoelectric fan is paid in relative to the baseline blade shape.

Due to the lack of studies on the thermal and hydraulic performances of roughed surfaces subjected to the piezoelectric fan in the presence of forced channel flow, an experimental investigation is performed for a micro pin-finned heat sink integrated with a specific piezoelectric fan in the current study. Some influence factors, such as characteristic Reynolds number of vibrating fan, fan tip-to-heated surface clearance and channel flow Reynolds number, are taken into considerations for two specific micro pin-fin configurations having the same pin-fin height. Particular attention is focused on evaluating the role of piezoelectric fan in the design of an active heat sink. It is confirmed that the micro pin-fin surface with a larger pin diameter (H250D400) provides a stronger heat transfer enhancement but greater additional flow losses than H250D250. The integration of piezoelectric fan into micro pin-fin heat sink is a highly efficient means for improving thermal performance. Its role on heat transfer enhancement behaves more significantly under a higher characteristic Reynolds number of vibrating fan and smaller fan tip-to-surface distance. With the presence of piezoelectric fan, the overall thermal performance parameter is generally bigger than the corresponding one without the piezoelectric fan except for the micro pin-fin type H250D400 under high channel flow Reynolds numbers. The linear relationships of heat transfer coefficient and overall thermal performance parameter are derived with respect to the total Reynolds number combining the channel flow and piezoelectric fan. It is demonstrated that the influence weighting of piezoelectric fan on the overall thermal performance parameter is weaker than that on the pure thermal performance of micro pin-fin heat sink.

Large eddy simulation (LES) was performed for film cooling from converging slot-holes (consoles) at different blowing ratio of M = 1.0–3.0, and the Reynolds number, Re = 21719, based on the mainstream velocity and inlet hole diameter. In time-averaged flow fields of film cooling, counter-rotating vortex pairs (CRVPs) and anti-CRVPs are dominant structures. CRVPs result in the deterioration of film cooling performance, while the effect of anti-CRVPs is opposite. Instantaneous coherent structures for consoles including jet vortex pairs, anti-jet vortex pairs, hairpin vortexes, and leading- and trailing-edge vortexes were analyzed in detail. Jet vortex pairs are firstly generated in consoles, and then enter the mainstream channel, finally evolve into hairpin vortexes. The head, horizontal leg and vertical leg of hairpin vortexes correspond to roller vortexes, CRVPs and upright wake vortexes respectively. Anti-jet vortex pairs are the sources of anti-CRVPs. By time-frequency analysis of pressure signals, it was found that the signals from downstream of the hole centerline exhibit periodicity. The dominant frequency always increases with the increase of the blowing ratio, however, the corresponding amplitude of power spectrum increases firstly, and then decreases.

An experimental investigation is performed to study the effect of film-hole configurations on the blade tip film cooling performance in a five-blade linear cascade. Six film-hole configurations are taken into consideration under four typical blowing ratios. In Type-A, Type-B and Type-C, the film holes are arranged in a single row along the middle-camber line with different hole-to-hole pitches. Type-D, Type-E and Type-F have the same film-hole number of 13 as Type-B. For Type-D, the film holes are also arranged in a row, but they are located close to the suction-side squealer. For Type-E, the film holes are arranged in two rows. For Type-F, two rows of 8 holes are concentrated at the leading edge, and the rest 5 holes are arranged in a line at the middle chord region of the blade tip. Besides, some numerical simulations are conducted to provide detailed coolant jet trajectory. The results show that film-hole arrangement has a great impact on the tip film-cooling effectiveness. The coolant issuing from middle-camber holes are suctioned by the vortex structures inside squealer cavity to flow towards the pressure side, producing good film coverage at the local zone between the middle-camber line and pressure-side squealer for Type-A, Type-B and Type-C. For Type-F, as more film holes are concentrated at the leading edge, the film coverage on the front tip surface is well improved compared to the other film-hole arrangements. Under the same coolant usage, Type-F obtains the most favorable film cooling performance except for the trailing edge, and Type-C produces wider film coverage extending to trailing edge.

Single-objective optimization for improving adiabatic film cooling effectiveness is performed for single row of round-to-slot film cooling holes on a flat surface by using CFD analysis and surrogate approximation methods. Among the main geometric parameters, dimensionless hole-to-hole pitch (P/d) and slot length-to-diameter (l/d) are fixed as 2.4 and 2 respectively, and the other parameters (hole height-to-diameter ratio, slot width-to-diameter and inclination angle) are chosen as the design variables. Given a wide range of possible geometric variables, the geometric optimization of round-to-slot holes is carried out under two typical blowing ratios of M = 0.5 and M = 1.5 by selecting a spatially-averaged adiabatic film cooling effectiveness between x/d = 2 and x/d = 12 as the objective function to be maximized. Radial basis function neural network is applied for constructing the surrogate model and then the optimal design point is searched by a genetic algorithm. It is revealed that the optimal round-to-slot hole is of converging feature under a low blowing ratio but of diffusing feature under a high blowing ratio. Further, the influence principle of optimal round-to-slot geometry on film cooling performance is illustrated according to the detailed flow and thermal behaviors.

A numerical investigation is performed to study the serrated trenched-hole film cooling performance on the convex and the concave surfaces, with the use of a plenum coolant-fed mode. In current study, the relevant curvature of curved surface is varied as 2r1/d = 30, 60 and 90 at the convex side, 2r2/d = 90, 120 and 180 at the concave side. In addition, three coolant injection angles (such as θ = 30°, 35° and 45°) are considered in a specific curved channel. The blowing ratio defined in terms of the local primary flow velocity is ranged from 0.25 to 1.5. From the present study, a critical parameter (I0), in the form of Icos2θ, is preliminarily identified to be around 0.5 for the serrated trenched-hole film cooling. When Icos2θ I0. In particular, this critical value for the serrated trenched-hole film cooling is obviously less than the unity that previously identified in the cylindrical-hole film cooling situation. The relevant curvature shows nearly rare influence on the film cooling effectiveness under small blowing ratios, either on the convex surface or the concave surface. However, under high blowing ratios, it has a pronounced impact on the film cooling effectiveness, in particular at the concave side. Under M = 1.5, the area-averaged adiabatic film cooling effectiveness in the region between s/d = 10 and s/d = 20 is increased up to 33% by the smallest relevant curvature case with respect to the biggest relevant curvature case. The coolant injection angle plays an important role on the mutual interaction of jet-in-corssflow of the serrated trenched-hole film cooling. The reduction in the coolant injection angle leads to an obvious film cooling enhancement, either on the convex surface or the concave surface. Moreover, the enhancing effect led by the reduction of coolant injection angle behaves more pronouncedly as the blowing ratio increases.

With the booming performances of the gas turbine engine, the turbine vane of the gas turbine engine experiences more extreme thermal environment with supersonic flows. The film cooling appl...

A series of numerical computations are conducted to investigate the film cooling from a row of round-to-slot holes on a flat surface. Particular attention is paid to explore effect of slot width (0.2 ≤ s/d ≤ 0.5) on the film cooling performances of round-to-slot holes with a fixed slot length of l/d = 2.0 under typical blowing ratios ranged from 1.0 to 2.0. Besides, several experimental tests are also performed to obtain the film cooling effectiveness and discharge coefficient of the round-to-slot holes with specific geometries. The results show that the coolant flow turning towards both lateral sides due to the transitional flow passage of film-hole is dominated inside the round-to-slot hole. Owing to the specific interaction mechanism between the coolant jet issued from round-to-slot holes and the primary flow, the hot primary flow is mainly entrained into the coolant layer from the film-hole centerline. In general, the diffusing round-to-slot hole shows its favorable feature on the film cooling enhancement near the film hole. However, beyond a certain distance, the converging round-to-slot hole achieves a little higher laterally-averaged film cooling effectiveness than the diffusing round-to-slot hole. The converging round-to-slot hole with s/d = 0.2 produces higher laterally-averaged heat transfer coefficient than the cylindrical hole. While the diffusing round-to-slot hole produces less laterally-averaged convective heat transfer coefficient than the cylindrical hole beyond x / d = 10. The discharge coefficient decreases significantly as the slot width reduces.

Experimental tests are carried out to investigate the convective heat transfer performances on a flat surface around the vibration envelope of a vertically-oriented piezoelectric fan in the presence of cross flow. Distinct behaviors of convective heat transfer are illustrated under the present conditions of piezoelectric-fan excitation voltage (U = 50, 150, 250 V) or characteristic velocity (u PF = 0.83, 1.67, 2.34 m/s) fan tip-to-heated surface gap (G = 3, 5, 7 mm) and cross flow velocity (u CH = 0.94, 1.56 m/s). In addition, three-dimensional flow field simulations are conducted to illustrate the instantaneous flow fields around the vibrating fan. By comparing with the pure piezoelectric fan, the vortex induced by the vibrating fan is pushed downward by the cross flow and a series of vortices are displayed down the vibrating fan. It is confirmed that the presence of cross flow is contributive to the improvement of convective heat transfer in the rear zone downstream fan vibration envelope. The impingement role of streaming flow induced by piezoelectric fan is reduced by the presence of cross flow in the fan vibration envelope. On the other hand, the oscillating movement of the piezoelectric fan promotes the disturbance intensity of cross flow passing through the fan vibration envelope. These two aspects make the conjugated convective heat transfer in the vicinity of fan vibration envelope complicated. In general, the convective heat transfer in the vicinity of fan vibration envelope is mostly improved by the combined action of fan-excited steaming flow and cross flow in the situation where the piezoelectric fan is placed very close to the heated surface.

Experimental investigations were carried out to study the conjugated convective heat transfer on the leading edge of a conical wall subjected to external cold flow and internal hot jet impingement by a single chevron nozzle. The geometric effects, including the chevron penetration depth (p/d ranging from 0.1 to 0.2) and chevron length (l/d ranging from 0.1 to 0.3) on the conjugated convective heat transfer performances were experimentally analyzed for a typical 6-chevrons nozzle under non-dimensional jet-to-leading edge distance (H/d) of 2–4 and jet Reynolds number (Rej) of 7800–39,400. The results show that the chevron jet is proved to be capable of improving the heating effectiveness in the vicinity of the conical surface leading edge, especially under a small jet Reynolds number. For the specified zone with a chordwise length of 5d apart from the leading edge, the area-averaged heat effectiveness could be increased approximately 20% by the chevron nozzle in relative to the conventional nozzle. The heat transfer enhancement is improved with the increase of chevron penetration length for a fixed chevron length or the decrease of chevron length for a fixed chevron penetration length. The influence of chevron penetration depth or the chevron length on the specified area-averaged heating effectiveness becomes weaker gradually as the jet Reynolds number increases. For the current conditions, the non-dimensional jet-to-leading edge distance seems to have little influence on the specified area-averaged heating effectiveness.

Experimental and numerical research is performed to illustrate the effects of pin-fin shapes on mesh-fed slot film cooling performance on a flat-plate model. Three types of pin-fin shapes (such as circular, elliptical, and drop-shaped) with the same cross-sectional area are taken into consideration. The results show that a pair of counter rotating vortices is still generated for the mesh-fed slot film cooling scheme due to the strong “jetting” effect of coolant flow at the slot outlet. As the coolant jet ejecting from mesh-fed slot is capable of establishing more uniform film layer over the protected surface, the kidney vortices are illustrated to have weakly detrimental role on the film cooling performance. By the shaping of pin fins, the uniformity of coolant flow exiting mesh-fed slot is improved in comparison to the baseline case of circular shape, especially for the elliptical-shape pin-fin array. Therefore, the jetting effect of coolant flow is alleviated for the elliptical and drop-shaped pin-fin meshes when compared to the circular pin-fin mesh. In general, the pin-fin shape has nearly no influence on cooling effectiveness immediately downstream the film cooling-hole outlet. However, beyond x/s = 5, the elliptical and drop-shaped pin fins are demonstrated to be advantageous over the circular pin fins.

The heat transfer characteristics of a single synthetic jet (SJ) issuing from planar lobed orifice are investigated experimentally in current study. Two specific planar-lobed orifices (petal-shaped orifice and arch-shaped orifice) and two typical targets (flat target and semi-cylindrical concave target) are taken into consideration. For each lobed orifice, three configurations are included. In the petal-shaped orifices, the lobe numbers are chosen as N=4, 6 and 8 in turns. While in the arch-shaped orifices, the lobe number is fixed as N=6 but the lobe aspect ratios are altered as AR=1, 2 and 3 respectively. All lobed orifices have the same exit area with the baseline round orifice. The experimental tests are performed under a series of operational frequencies (ranging from 10Hz to 25Hz) and dimensionless jet-to-target distances (ranging from 2 to 14 for flat target, and from 6 to 14 for concave target). Under the present conditions, SJ impingement heat transfer enhancement is mostly achieved by the planar lobed orifices when compared to the baseline round orifice, which depends tightly on the lobe shape and target shape. Among current petal-shaped orifices, the N=6 petal-shaped orifice is demonstrated to be a most promising orifice configuration. With regard to current arch-shaped orifices, a moderate lobe aspect ratio is demonstrated to be the superior. When compared to the flat target, SJ impingement heat transfer on the concave target is seriously reduced, regardless of orifice shapes. Interestingly, the relationship of SJ impingement heat transfer on the concave target, between the N=6 petal-shaped orifice and AR=2 arch-shaped orifice, is found contrary to that on the flat target. On the concave target, the N=6 petal-shaped orifice does not show its advantages on SJ heat transfer enhancement as appeared on the flat target. However, the AR=2 arch-shaped orifice still shows its positive role on SJ heat transfer enhancement obviously in comparison to the baseline round orifice.

The aerodynamic and cooling characteristics of the endwall were studied by numerical and experimental methods. Two film-hole configurations, i.e. conventional film-hole arrangement and partitioned film-hole arrangement, were taken into consideration. The partitioned film-hole arrangement is proposed based on the analysis of the heat load and the near wall flow field of the endwall. The results show that the high heat transfer coefficient region on the endwall pressure side is consistent with the passage vortex core line, but independent of the pressure side separation line of the horseshoe vortex. Accordingly, the endwall is divided into five regions based on the suction side separation line of the horseshoe vortex and the passage vortex core line. The five regions include upstream-passage wedge region, leading edge region, pressure side region, downstream region of the pressure side and downstream region of the suction side. Compared with the conventional film-hole arrangement, the partitioned film-hole arrangement weakens the mixing of the coolant jet and the mainstream, thus reducing the energy loss and the total pressure loss at the outlet of the cascade. More importantly, the partitioned film-hole arrangement provides much uniform film coverage and a significant increase of the area-averaged film-cooling effectiveness by using less cooling air. This is derived from the fact that the partitioned film-hole arrangement has more film holes in the pressure side of the endwall with high back pressure, which improves the distribution of coolant on the endwall.

Large eddy simulation was applied for studying the flow and heat transfer characteristics in the rotating cavity with de-swirl nozzles. Simulations were performed at the rotational Reynolds number of 1.4 × 105 and nondimensional flow rate of 4040. Compared with vertical nozzle, de-swirl nozzle generates lower swirl ratio, pressure loss and Nusselt number. Distributions of radial velocity show that de-swirl nozzle generates lower velocity in the Ekman boundary layer, but higher velocity in the core region. In the corner region of shroud and jet, the ‘dead zone’ of coherent vortices exists for vertical nozzle. However, for de-swirl nozzle, the ‘dead zone’ disappears due to the impact of inclined injection, and accordingly, the scale of time-averaged counter-rotating vortex pair is reduced effectively. In the middle and lower sections of cavity, plenty of vortex stripes are formed both for vertical and de-swirl nozzles; however, in the Ekman layer, turbulent kinetic energy and Reynolds normal stress for vertical nozzle is much higher than de-swirl nozzle. The distributions of turbulent energy spectrum and energy integral length scale show that, for de-swirl nozzle, large-scale eddies take the dominant role in middle section, while small-scale eddies dominate the flow fields in upper and lower sections of cavity.

This study focuses on the temperature-variation effects in the piston-driven synthetic jet impingement and its influence on the definition of heat transfer coefficient. For this purpose, three test runs are conducted, including temperature variation test in the free synthetic jet, temperature variation test in the presence of an impinging target, and heat transfer test. Five operational frequencies (ranging from f = 5 Hz to f = 25 Hz) and seven dimensionless jet-to-surface distances (ranging from H/d = 2 to H/d = 14) are considered in the current tests. Due to the friction heat generation from the actuator itself and the warm air suction from the ‘synthesized’ formation, the temperature of synthetic jet experiences apparent variation during its working process. At the worst, the temperature rise could reach up to 20 °C with respect to the ambient temperature. As a consequence, the selection of the reference temperature shows a significant influence on the Nusselt number definition for the synthetic jet impingement, especially under small jet-to-surface distances and high operational frequencies. At the worst, the Nusselt number defined in the jet temperature or the adiabatic wall temperature maybe twice times of that defined in the ambient temperature. As the jet temperature and the adiabatic wall temperature are affected by too many process parameters, therefore, the accurate determination of them is really a challenging problem in the applications, which remains further identifications.

The convective heat transfer characteristics under a normally synthetic jet impingement driven by a piston actuator are investigated experimentally in the present study. Particular attentions are paid to present the detailed local and laterally-averaged heat transfer comparison between synthetic jet and continuous jet, as well as the effect of orifice shape on the synthetic jet impingement. Three jet orifices including single-round, single-square, and single-rectangular are designed to have the approximately same exit area. In additional, heat transfer regimes for the synthetic jet impingement in the situation of large stroke length to jet-to-surface spacing are further identified. In the present, the equivalent convective heat transfer coefficient is defined in terms of the difference between target temperature and ambient temperature. The results show that continuous jet exhibits stronger local heat transfer than the synthetic jet in the vicinity of stagnation point. However, the synthetic jet produces much flatter and more uniform local heat transfer coefficient distributions over the surface, showing its advantage over the continuous jet on laterally-averaged convective heat transfer enhancement at a larger jet-to-surface spacing. For the piston-driven synthetic jet featured by low excitation frequency and large stroke length, there is evidence for a power law relationship between stagnation Nusselt number and jet Reynolds number with an exponent of approximately 0.32. Two kinds of heat transfer regimes are observed and a critical ratio of stroke length to jet-to-surface spacing is identified as 18 approximately regardless of orifice shape. The orifice shape has a moderate effect on the convective heat transfer. The single-rectangular synthetic jet produces a slightly better stagnation point heat transfer than the other orifices. The synthetic jet originated from the round-hole orifice seems to introduce favorable overall convective heat transfer achievement. The advantage of synthetic jet impingement in comparison to the corresponding continuous jet is relatively degraded for the square-hole orifice.

Computational and experimental studies were conducted to investigate the conjugated convective heat transfer produced by single row of impinging jets inside a confined channel with non-uniform initial crossflow issued from discrete holes. The effects of the crossflow-to-jet Reynolds number ratio and crossflow holes arrangement (inline or staggered in relative to the impinging jet holes) on the jet impingement behaviors were studied providing the same total cooling air mass flowrate. Particular attention was paid to examine the tab-excited jet impingement behaviors in a confined channel with non-uniform crossflow. The results show that the tabbed excitation on the improvement of convective heat transfer in the vicinity of impingement region is not as well as the situation where no initial crossflow is present. For the inline arrangement of crossflow holes, the location corresponding to the peak laterally-averaged Nusselt number is moved downstream and the peak laterally-averaged Nusselt number is decreased significantly under larger crossflow-to-jet Reynolds number ratios. While for the staggered arrangement of crossflow holes, the location corresponding to the peak laterally-averaged Nusselt number is not affected by the crossflow but the peak laterally-averaged Nusselt number is decreased monotonously with the increase of crossflow-to-jet Reynolds number ratio. Under small crossflow-to-jet Reynolds number ratio, the initial crossflow arrangement has little influence on the laterally-averaged Nusselt number distribution. Among three crossflow-to-jet Reynolds number ratios ranged from 0.5 to 2, the maximum relative difference of peak laterally-averaged Nusselt numbers between two different crossflow arrangements is occurred under crossflow-to-jet Reynolds number ratio of 1.

Convective heat transfer on the rib-roughened wall impinged by a row of air jets inside semi-confined channel was experimentally investigated. Four rows of transverse ribs were arranged in the wall-jet zone downstream from the impinging jet stagnation to enhance heat transfer. Three typical rib configurations, including orthogonal ribs, V-shaped ribs and inverted V-shaped ribs, were considered under different non-dimensional jet-to-target distances ranging from 1 to 3 diameters and impinging jet Reynolds numbers ranging from 6000 to 30,000. The results show that the rib-roughened wall enhances the convective heat transfer up to 30% in the ribbed region by comparison with the smooth wall under the same jet Reynolds number. Among three rib configurations, the inverted V-shaped rib seems to be advantageous on the convective heat transfer enhancement, especially at lower jet-to-target spacing. The ribs on the impinging target do provide stronger convective heat transfer in the wall-jet region, but at greater expense of pressure drop inside the channel. At the jet-to-target spacing ratio of 1, the flow coefficient of the rib-roughened channel is decreased 5%–10% in related to the smooth channel.

Numerical investigation is performed to further address the effects of geometric parameters and excitation frequency of the actuator on the synthetic jet fluidic characteristics by utilizing a two-dimensional unsteady Reynolds-averaged Navier–Stokes model. The vibrating diaphragm is modeled as a movable wall varying in sinusoidal mode. Computations are carried out by using FLUENT software with the coupled user definition function (UDF) describing the diaphragm movement. The results show that the geometric parameters of the actuator, such as the cavity depth and diameter, as well as orifice thickness and diameter, have important influences on the synthetic jet fluidic characteristics. The velocity output could be maximized if the geometric parameters of the synthetic jet actuator are designed to ensure that the cavity acoustic Helmholtz resonance frequency is coincided with the diaphragm excitation frequency. For a fixed actuator cavity, when the diaphragm excitation frequency is consistent with the Helmholtz resonance frequency of actuator cavity, the relative pressure insides the cavity is obviously great during the ejection stroke while low during the suction stroke. In the presented actuator parameters, the synthetic jet is enhanced as the decrease of cavity depth for the fixed orifice. The change of cavity diameter in the vicinity of corresponding cavity acoustic resonance diameter has relatively weaker influence on the synthetic jet. When the diaphragm is excited at high frequency, small orifice diameter will restrict the ejection and suction capacity of the synthetic jet actuator.

Experimental study was conducted to investigate the heat transfer produced by single row of impinging jets inside a confined channel with different tab orientations of the triangular tabs at the jet exits. The effects of the tab number, tab orientation angle and tab penetration length on the jet impingement heat transfer behaviors were experimentally tested under nozzle-to-plate spacing of 1–3 diameters and jet Reynolds number of 10,000–20,000. A numerical computation was also carried out on the simulation of impinging jet flow fields to reveal the trends and differences observed in the heat transfer measurements. The results show that the presence of tabs increases the jet core velocity and induces array pairs of vortices, and hence enhances the heat transfer in the impingement region over the no-tab case. The tabs oriented upwards or downwards at 45° can better improve the heat transfer than the normal inclination case. The heat transfer enhancement produced by the tabs increases with the tab penetration length for the given range of 0.12–0.23 diameter. For the arrays of 4–12 triangular tabs, the array of 8 tabs seems to produce more effective heat transfer enhancement.

A numerical research on the film cooling performance of a single row of converging slot-holes (console) on the blade suction side in an engine-simulated environment was carried out, in which the Reynolds number was arranged from 400,000 to 600,000 and the blowing ratio was arranged from 0.5 to 3. A comparison in contrast to a cylindrical hole was made and the effects of major factors on the film cooling effectiveness and aerodynamic loss were explored, including the film hole location, blowing ratio and primary flow Reynolds number. For the console cooling geometry, the interaction between the coolant jet from inclined console and the mainstream flow results in secondary vortices with a sense of rotation opposite to the kidney pair, which makes the coolant jet from the console be of the flow mechanism for suppressing normal penetration. Additional corner vortices are also observed in the intersection of two consoles on the convex surface. When the film holes are located upstream of the channel throat, the level of aerodynamic loss for the console is obviously less than the cylindrical holes. At higher blowing ratios, more coolant jet momentum is transferred to the tangent and lateral flow of the coolant jet issued from the consoles, resulting in a film cooling enhancement. The console row shows great potential in the blade film cooling application, especially favorable when it is located upstream of the channel throat.

A three-dimensional numerical study on the flow and heat transfer characteristics over a rotating disk surface with discrete pins was conducted by the use of RNG k–e turbulent model. And some experiments were also made for validation. The effects of rotating angular speed and pin configuration on the temperature maps and convective heat transfer characteristics on the rotating surface were analyzed. As the increase of rotating velocity, the impingement of pumping jet on the centre of rotating disk becomes stronger and the transition from laminar to turbulent occurs at the outer radius of rotating disk, which resulting in heat transfer enhancement. The pins on the disk make the pumping action of a rotating disk weaker. Simultaneously, they also act as perturbing elements to the cyclone flow near the rotating disk surface, making the overall heat transfer to be enhanced. The needle pins have higher convective heat transfer capacity than the discrete ring pins with the same extend pin areas.

Experimental investigation was conducted in the current study to investigate the film cooling performance of a single row of consoles on a large-scale curved-plate model simulating the blade suction side, and an aided numerical study was also made to reveal the detailed film cooling features of consoles on the curved surface. A comparison in contrast to cylindrical hole was made. For the console cooling geometry, the interaction between coolant jet from inclined console and the mainstream flow results in reasonable vortices configuration to reduce mainstream–coolant interaction. And additional corner vortices are observed in the intersection of two consoles on the convex surface. The console row shows great potential in the blade film cooling application to produce high cooling effectiveness, especially at large blowing ratios. But this is at the expense of greater pressure drop. And the heat transfer coefficient ratio for the console is a little bigger than conventional cylindrical hole. Furthermore, the effects of major factors on the film cooling effectiveness, heat transfer coefficient and discharge coefficient were explored, including film hole location, blowing ratio and primary flow Reynolds number.

The conjugate heat transfer characteristics under a normally synthetic jet driven by piston actuator and horizontally forced flow were investigated experimentally. The effects of excitation frequency and orifice shape on the flow and heat transfer characteristics are explored. The piston-driven actuator produces fairly big velocity at higher operating frequency and the time-averaged orifice velocity is almost proportional to the frequency when the piston reciprocates at relatively low frequency (from 8 Hz to 24 Hz). The orifice shape shows very weaker effect on the exit velocity of synthetic jet, given the same orifice exit area, but has an important influence on the interaction between the normally synthetic jet and horizontally forced flow. The convective heat transfer capacity is enhanced under the conjugate action of a synthetic jet and forced flow, especially significant at higher excitation frequency of the synthetic jet. For all the orifice shapes, the peak laterally-averaged convective heat transfer coefficient is increased up to 100% in relative to individual action of the forced flow when the piston reciprocates at 24 Hz. Such detailed heat transfer results about the conjugate heat transfer characteristics have not been reported earlier and are expected to be useful for understanding the effects of excitation frequency and orifice shape on the conjugated heat transfer characteristics.

Experimental investigations on the film cooling characteristics of parallel-inlet film holes are carried out. There are four kinds of film hole arrangements: one-row, two-row(staggered arrangement), two-row(aligned arrangement) and three-row arrangement. Experimental results show that film hole arrangement and blowing ratios have great effects on film cooling effectiveness and heat transfer coefficient(HTC). Film cooling effectiveness is increased with the increase of blowing ratios, but the HTC is decreased with the increment of blowing ratio. The film cooling effectiveness of single-row film hole( d = 7 mm) is the best of all test pieces. As for staggered arrangement design, its HTC is larger than that of aligned arrangement at the same position. In addition, the HTC is increased with the decrement diameter of film hole for all kinds of film hole arrangement patterns. For film cooling, a smaller HTC means a better film cooling effectiveness.