Jet Hole Research Articles

Heat transfer in a rotating rectangular channel with impingement jet and film holes

The heat transfer characteristics in a rectangular channel under rotating conditions with impingement jet and film holes was studied. The key features of the geometry are multiple impingement jets and film holes in the leading edge model. The jet Reynolds number (Rej), jet rotation number (Roj) and buoyancy number (Buo) varied from 5000 to 10,000, from 0 to 0.24, and from 0 to 0.57, respectively. Four temperature ratio (TR) cases (0.07, 0.10, 0.13, and 0.16) and three channel orientations (β = 90, 135, and 180) are selected. For all experiment cases, the spacing of jet-to-jet (s/d = 3) and jet-to-target surface (l/d = 3) were held constant. In the non-rotating cases, the heat transfer level of the T2 region (stagnation point), impinged by the jet directly, is the strongest and about twice to the other regions. The overall heat transfer distribution almost does not change significantly as the Rej varied from 5000 to 10,000. The jet rotation number plays an important role under the rotation state. For the orientation of 135°, the Coriolis force deflects the jet and weakens the heat transfer of the medium radius in the T2 region. No obvious variation in the heat transfer distribution for different temperature ratio under non-rotating and rotating cases was observed. The Coriolis force generated by different rotation directions causes two influences on the flow structure, leading to the different heat transfer distribution. The overall heat transfer level for the orientation of 90° and 135° varies little (5%) with the rotation number, while that of 180° decreases much (25%).

Numerical investigation of a novel jet hole design for staggered array jet impingement cooling on a semicircular concave surface

Staggered array jet impingement cooling (JIC) on a semi-circular concave surface has investigated numerically. The main objective of this work is to examine elongated jet holes on heat transfer and flow characteristics of the staggered array JIC and to elucidate its applicability on cooling of a turbine blade. Jet holes were elongated to the target surface by the nozzles. Numerical computations were performed with different jet Reynolds numbers (5000≤Re≤25000), normalized confinement plate-to-target plate distances (1.0≤H/d≤8.0) and jet nozzle-to-target plate gap (0.5≤G/d≤6.0). Average Nusselt (Nu) number, local Nu number distributions on the surface, flow characteristics and Thermal Performance Factor (TPF) were comprehensively investigated. Numerical results were compared with the normal staggered array JIC. Results showed that local Nu number and average Nu number increases with decreasing G/d. The highest enhancement of heat transfer using elongated jet hole is obtained as 20.16% by decreasing G/d to 0.5 at H/d = 8.0. However, highest TPF is estimated as 1.11 at H/d = 8.0 by G/d = 2.0 for Re = 25,000. Decreasing of G/d provides more uniform heat transfer on the surface of interest in comparison with normal staggered array JIC. Consequently, elongating jet hole is a feasible heat transfer enhancement design for the staggered array JIC on a semicircular concave surface.

Sweeping Jet Film Cooling at High Blowing Ratio on a Turbine Vane

Abstract Experimental and numerical investigations were performed to study the effects of high blowing ratios and high freestream turbulence on sweeping jet film cooling. Experiments were conducted on a nozzle guide vane suction surface in a low-speed linear cascade at a range of blowing ratios of 0.5–3.5 and freestream turbulence of 0.6% and 14.3%. Infrared thermography was used to estimate the adiabatic cooling effectiveness. Thermal field and boundary layer measurements were conducted at a cross-plane at x/D = 12 downstream of the hole exit. Results were compared with a baseline 777-shaped hole and showed that the sweeping jet hole has improved cooling effectiveness at high blowing ratios (M > 1). The thermal field data revealed that the coolant separates from the surface at high blowing ratios for the 777-shaped hole while the coolant remains attached for the sweeping jet hole. Boundary layer measurements further confirmed that due to the sweeping motion of the jet, the effective jet momentum of the sweeping jet hole remains much lower than that of a 777-shaped hole. Thus, the coolant remains closer to the wall even at high blowing ratios. Large Eddy simulations (LES) were performed for both the sweeping jet and 777-shaped hole to evaluate the interaction between the coolant jet and the freestream in the near hole regions. Results showed that 777-shaped hole has a strong jetting at high blowing ratio that originates inside the hole breakout edges thus causing the jet to blow-off from the surface. In contrast, the sweeping jet does not show this behavior due to its internal geometry and flapping motion of the jet.

Numerical Simulation of Inverse Diffusion Combustion and Flow Characteristics in a Trapped Vortex Combustor

In this work, the cold and combustion flow characteristics of a trapped vortex combustor with inverse diffusion combustion method have been studied numerically, and the effects of four jet arrangement schemes on the vortex structure, temperature distribution, turbulence kinetic energy, OH mass fraction and combustion efficiency were compared. The results indicate that the vortex structures in cold state are basically the same with a double vortex structure for all the jet schemes. However, when the jet holes are arranged on the after-body wall, the vortex changes from a double vortex structure in cold state to a single vortex structure in combustion state, the combustion effect is not too large when the jet holes are arranged on the fore-body wall. The high-temperature distribution in cavity is mainly concentrated in the upper part, but when the jet holes are arranged on the after-body wall, the OH mass fraction distribution is higher than that when the jet holes are arranged on the fore-body wall. In addition, when the jet holes are arranged on the fore-body wall, complete combustion can be accelerated. In general, when the jet holes are arranged in the same position, the multi-jet scheme is more conducive to flame stability in cavity and the improvement of combustion and mixing performance than the coaxial jet.

A Novel Multi-Stage Impingement Cooling Scheme—Part II: Design Optimization

Abstract Cross flow and coolant maldistribution are the common design challenges for impingement cooling in modern gas turbine. This paper reports a novel multi-stage impingement cooling scheme for combustor liner. The design concept and general working mechanism are introduced in the Part I paper. This Part II paper presents the design flexibilities and optimization strategies. Conjugate heat transfer (CHT) analysis was conducted at a range of Reynolds numbers to assess the thermal performance, loss penalty, and the working mechanism behind. The results show that varying the jet hole diameter in each cooling stage can be an effective design optimization strategy in balancing the cooling requirement and loss penalty. Inter-stage bypass design is also another design flexibility offered by the multi-stage scheme to regulate the cooling air consumption at different stages. With these optimization strategies, the target surface temperature and local gradient can be effectively reduced with reasonable pressure loss with 50% reduction in the cooling air consumption compared to conventional single-stage impingement design. This multi-stage impingement concept can be practically applied to gas turbine combustor liner and turbine blade cooling.

Endwall film cooling holes design upstream of the leading edge of a turbine vane

Endwall film cooling upstream of the leading edge (LE) of a vane presents a relatively complex flow phenomenon due to the horseshoe vortices (HVs) generated at the LE region. Upstream of the LE region, the coolant flow has difficulties to eject out. This research work focuses on controlling the jet holes coolant coverage upstream of the LE region. Compound angle holes in staggered arrangement are introduced and applied in the LE region to increase the coolant coverage. Six different arrangements of cooling holes are designed, with variations from one row or two rows arrangements, parallel or staggered arrangements, normal cylindrical holes or compound angle holes. Besides cooling holes with different shapes and arrangements, effect of the blowing ratio (BR) and turbulence intensity (TI) are also considered. The BR ranges from 1 to 3 and TI ranges from 1.3% to 15%. The calculated results show that the film cooling holes upstream of the LE of a vane have significant cooling effects on both the vane surfaces and the endwall. At small BRs, the film cooling effectiveness (η) on the endwall is considerable. When the BR is increased, the η on the vane surfaces is increased more quickly and becomes dominant. The coolant coverage in the vane-endwall junction region are not affected by the mainstream turbulence intensity and almost keep the same with the varied turbulence intensities. A single row of cylindrical holes (Case 1) and two rows of compound angle holes with staggered arrangement (Case 5) have relatively high overall averaged cooling effectiveness compared with other cases at different BRs. In addition, the high averaged cooling effectiveness on the endwall and vane surfaces by Case 5 is not affected by a change of the BRs.

Study on the Hybrid Cooling of the Flame Tube in a Small Triple-Swirler Combustor

An experimental and numerical investigation is conducted to study the influence of different cooling schemes on the wall temperature of the flame tube in a small triple-swirler combustor in this paper. Two different cooling structures are adopted: the impingement-film and inclined multi-hole cooling structure (Scheme B, C), and the inclined multi-hole control group (Scheme A). The impact of parameters including inlet temperature (373–423 K), inlet Mach number (Ma) (0.12–0.18), and fuel–air ratio (FAR) (0.02–0.03) are discussed. The results show that the wall temperature of the flame tube rises with the increase in inlet temperature; as the inlet Mach number increases, the wall temperature (Scheme B, C) of the primary zone goes up and is distributed more uniformly; as FAR rises, the wall temperature in Scheme C is nearly unchanged, while it is increased in Scheme A and B. For the range of parameters considered in this study, the lowest wall temperature and the best cooling effect are observed in Scheme C. The experiment conducted on the impingement-film and inclined multi-hole structure shows a better cooling effect than that conducted on the traditional inclined multi-hole structure. Compared with the row number of multi-inclined holes, the diameter of jet hole has a more significant influence on the cooling effect.

Numerical Investigation of Heat Transfer and Pressure Force from Multiple Jets Impinging on a Moving Flat Surface

In this paper extensive numerical investigation of the heat transfer characteristics and the pressure force of jet impingement from the single row and multiple rows on a fixed and moving flat surface are reported. The computations were carried out over a wide range of parameters: relative nozzle-to-surface distance (H/d) from 0.5 to 6, relative nozzle to nozzle distances (S/d) from 4 to 10, jet angle from 45° to 90°, relative velocity ratio (Vplate/Vj) i.e. ratio of surface velocity to jet velocity from 0 to 1. The jet Reynolds number (Re) of 2,500, 3,400, 10,000, 20,000, and 23,000 and the number of jet rows of 1, 2, 4, and 8 have been used. It was found that the numerical accuracy by SST k-ω model is reasonably high to allow for a discussion of the main flow and heat transfer characteristics. The jet impingement heat transfer performance is generally enhanced with the increase of jet Reynolds number and jet angle and with the decrease of surface distance (H/d), jet distance (S/d) and the relative velocity ratio (Vplate/Vj) within the range examined. The pressure force coefficients on the impingement surface are relatively insensitive to Re number and the velocity ratio within the range examined, while it has highly dependent on H/d, S/d and jet angle. For multiple rows of aligned jet holes, the flow pattern exhibited a different shape due to the different intensity of the interference between adjacent air jets. The effect of multiple rows with regards to the impact on average Nu and pressure force coefficient for different geometry variations such as Re, H/d, S/d, VR and ɵ is negligible compared to the single row by approximately 9 and 13% in average respectively. Based on the computed results, equations of dimensionless parameters are correlated.

Influence of pre-chamber structure and injection parameters on engine performance and combustion characteristics in a turbulent jet ignition (TJI) engine

Turbulent jet ignition (TJI) has been proven to effectively improve the burning rate and expand the lean-burn limit of gasoline engines. In this work, based on a single-cylinder engine, injection parameters in the pre-chamber were investigated, combustion characteristics were studied, and pollutant emissions were optimized. Results show that the injection timing in the pre-chamber should be carefully controlled to generate a well-mixed air–fuel mixture and to avoid the diffusion of fuel from the pre-chamber to the main chamber. Therefore, the injection event should be set at the early stage of the compression stroke. Besides, excessive fuel supply in the pre-chamber should be avoided to get optimal engine performance and low pollutant emissions. The lowest indicated specific fuel consumption (ISFC) is achieved when the λ is between 1.5 and 1.6, because lean combustion reduces the heat dissipation but too lean combustion causes more fuel to be burned incompletely. Moreover, to further improve the engine performance and reduce the pollutant emissions, three types of optimized pre-chamber are tested, and results show that with lower pre-chamber volume, better engine performance and lower pollutant emissions are both realized because smaller pre-chamber volume leads to less heat dissipation and less fuel burning around stoichiometric equivalence ratio in the pre-chamber. Furthermore, the pre-chamber with one jet hole is effective to improve the burning rate and to expand the lean-burn limit compared with the 7-hole pre-chamber, and the lean-burn limit of λ is expanded to 2.2 by the optimized pre-chamber, when the minimum NOx of 0.15 g/kWh is achieved.

Secondary fuel jet strategies on mixing enhancement performance of rocket-based combined cycle engine

The mixed augmentation performance between secondary fuel jet and uneven supersonic inflow is a significant evaluation index of the rocket-based combined cycle engine. In this study, the effects of the secondary fuel jet on the flow field structure and mixing enhancement performance are discussed by changing both the secondary fuel jet angle and spacing of multiple rows of jet holes. The three-dimensional coupled implicit Reynolds averaged Navier-Stokes equations, SST k-ω turbulence model, and eddy dissipation concept (EDC) reaction model were adopted to evaluate the flow field structure with a sonic hydrogen jet. It is indicated that the 135° fuel jet has the largest penetration depth value, followed by the 90° fuel jet, whereas the 45° fuel jet has the smallest penetration depth value. However, the 45° fuel jet possesses the best mixing performance, it easily induces a pair of downstream counter-rotating vortex, and the oblique torque is further generated to aggravate the tearing ability to the jet. The three jet holes induce a stronger shock wave and form a larger barrier under the 90° jet. Additionally, when the spacing between the multiple rows of jet holes reduces, penetration depth is easily created. Further, when the spacing of the three rows of jet holes is larger, the fluctuation of the mixing coefficient increases.

High pressure jet drilling effect in chalk and alteration of local geomechanics properties surrounding the radial hole

This study presents an improved understanding of how high-velocity fluid jet affects the mechanical properties of chalk, building a solid ground for the borehole stability analysis. Our laboratory experiments on water and acid-aided water jet-drilling of chalk samples have shown that high-pressure jet drilling alters the local mechanical properties of the chalk, resulting in a significant weakening of strength properties (tensile, uniaxial/triaxial compressive and pore collapse stress) in close proximity of the jet hole. Acid-aided water has even more impact on chalk matrix as revealed by Scanning Electron Microscopy (SEM) analysis. Numerous micro-perforations are found on the surface of microsparites (calcite). However, no extensive dissolution of calcite cement was observed in the chalk matrix. Results show that mechanical properties degradation is the outcome of both dissolution of the cementation and micro-perforations created on the calcite surface.

Influence of synthetic jet in crossflow configuration on heat transfer enhancement

A two-dimensional computational model was developed to investigate the effect of synthetic jet interaction with crossflow in channel on the cooling of heat wall. A range of parametric studies by varying membrane oscillating amplitudes and inlet velocities of channel was conducted. The resulting complex, conjugate heat dissipation was analysed. The synthetic jet interacts with the channel flow and gives rise to heat dissipation enhancement at the heated wall. The upstream heat transfer deteriorates in the jet hole, and the heat transfer in the downstream region has improved.

Design and optimization of a water jet-based biomimetic antifouling model for marine structures

Marine structures, such as ship hulls and offshore platforms, are basic elements in marine engineering. Due to the harsh ocean environment, marine structures are prone to adhesion and corrosion by marine biofouling. The biomimetic antifouling technology has been recognized as the most promising solution to marine biofouling, while there is still a long way to go to take this technology outside of research laboratories. In order to develop practical biomimetic antifouling techniques, this work presents a new water jet-based biomimetic antifouling model for marine structures to prevent the enrichment of biofouling. First, a semi-empirical formula is proposed based on the Schlichting self-similar solution to determine the effective width of the water jet. Then, a numerical simulation model is established to investigate the effects of the jet parameters (such as the jet aperture, jet velocity, and jet hole spacing) on the water jet distribution. Subsequently, visualization experiments are carried out to compare and validate the numerical simulation results. Finally, the simulation data are used to train a genetic neural network to predict the effective jet coverage ratio. The optimal parameters of the antifouling model are obtained corresponding to the largest effective jet coverage ratio. The findings of this study deliver a practical biomimetic antifouling technique for marine structures.

Film cooling performance and flow field of compounded double jet holes with trench

Film cooling performance of a novel double jet configuration is numerically analysed. The proposed scheme has a primary compound angle hole along with a secondary hole compounded in the laterally opposite direction. The primary hole is provided with a trench at the coolant hole exit. Numerical analysis is performed using RANS equations along with standard k-ω turbulence model. The numerical scheme is validated through the excellent agreement between the available experimental data for tangential, compound, and crater designs having similar primary hole geometric parameters. It is seen that the proposed compounded double jet trench design (DJT) combines the advantages of compound angle, double jet and trenched configurations and provides better lateral and uniform coolant distribution, reduced jet lift off, and higher coolant attachment in the stream wise direction. The vortex mechanisms responsible for the coolant spread in near-hole and down stream regions are identified. The various flow structures that are responsible for higher effectiveness are compared with other configurations. The effect of free stream turbulence at various blowing ratios is studied and found that the increase of turbulence produced significant increase in lateral effectiveness at higher blowing ratios. This effect is predominant at far downstream locations of the coolant exit plane.

Thermal analysis of jet impingement on hemispherical protrusion on heated surface

ABSTRACT Jet impingement on absorber plate with hemispherical protrusion is taken up for heat and friction characteristics analysis, where the protrusions are provided exactly below the jet holes to provide impingement over the protrusions for generating larger turbulence. The geometric parameters selected for experimental analysis are Relative protrusion diameter to protrusion height ratio (), Relative streamwise pitch (), relative spanwise pitch (). The maximum value of Thermohydraulic performance parameter () achieved was 3.01. The Nusselt number () and friction factor () correlations are developed and second law analysis is carried out for the selected range of geometric and flow parameters.

Experimental and numerical investigation of jet impingement cooling using extended jet holes

In this study, jet impingement cooling on flat surface was investigated experimentally. The aim of this study is to elucidate the effect of extended jet holes on the heat transfer performance of the in-line array jet impingement configuration. The studies were performed under fully turbulent flow condition (16250≤Re≤32500). Local Nusselt number (Nu) distribution on the surface of interest was obtained experimentally by using Transient Liquid Crystals (TLC) method. Numerical investigations were conducted as the same configuration with the experimental method to explore the flow and heat transfer characteristics. SST k-ω with low-Re correction turbulence model was used for solving turbulence equations. Experimental and numerical studies were conducted on 1×6 (in-line array) jet impingement cooling configuration. Dimensionless jet to jet spacing (Xn/Dj), dimensionless jet plate to target plate spacing (Z/Dj) and dimensionless target plate width (Y/Dj) were taken as 5.0, 6.0 and 6.0, respectively. Five different Gj/Dj (1.0, 2.0, 3.0, 4.0 and 5.0) were investigated and the results were compared with orifice plate jet impingement configuration (Z/Dj=Gj/Dj=6.0). Average and local Nu number distributions, pressure drop of the system, flow characteristics and Performance Evaluation Criterion (PEC) were examined in detail. Numerical results were compared with the experimental data and it was obtained that SST k-ω turbulence model was able to accurately predict the average and local Nu number distributions on the surface of interest. The maximum average and local Nu numbers were obtained on the condition of Gj/Dj=2.0. Furthermore, PEC shows that the most feasible dimensionless nozzle to target plate gap was Gj/Dj=2.0 at all Re numbers.

Jet impingement cooling on a rib-roughened surface using extended jet holes

In this study, jet impingement cooling on a rib-roughened surface has been investigated experimentally. The aim of this study is to investigate the effect of extended jet holes on the heat transfer performance and flow characteristics of the jet impingement cooling on a rib-roughened surface. The studies have been conducted under turbulent flow condition (16,250 ≤ Rej ≤ 32,500). Transient Liquid Crystals (TLC) method has been employed to investigate the average and local Nusselt number (Nu) distributions on the surface of interest. Six inline arrays of jet impingement configuration have been examined as the jet impingement cooling system. Jet holes were extended towards the target surface with the nozzles. Various dimensionless nozzles to the target surface gaps (Gj/Dj = 1.0, 2.0, 3.0, 4.0 and 5.0) have been investigated. Rectangular cross-sectional ribs were located on the surface of interest for the augmentation of heat transfer. Experimental studies were conducted on the dimensionless rib height as Hr/Dj = 0.42. In addition, numerical studies were carried out to investigate the flow and heat transfer characteristics. The effect of various dimensionless rib heights (Hr/Dj) on convective heat transfer performance has also been investigated numerically. SST k-ω with low-Re correction turbulence model was used for solving turbulence equations. Average and local Nu number distributions, flow characteristics and Performance Evaluation Criterion (PEC) were examined in detail. Results were compared with the orifice plate (Gj/Dj = 6.0) jet impingement configuration. Results showed that SST k-ω turbulence model accurately reveals the experimental data. Application of extended jet holes is a feasible method for practical application of the jet impingement cooling, especially at relatively low dimensionless nozzle gap (Gj/Dj≤4.0). Furthermore, improperly designed rib height has been found to diminish heat transfer performance.

Mechanism study on drag reduction and thermal protection for the porous opposing jet in hypersonic flow

The porous opposing jet, one of the most practical strategies to reduce the aero-heating of forebody, is able to improve the overall characteristics of vehicle in supersonic/hypersonic flow. In order to reinforce the cognition about the drag reduction and thermal protection mechanism of porous opposing jet, the three-dimensional flowfield about porous jet is analyzed systematically by the numerical method. The obtained results show that there is a multi-stage shock train along the direction of blunt leading edge in the porous opposing jet flowfield. The interaction among adjacent shock waves is obvious. With the increase of jet total-pressure ratio, the shock stand-off distance enlarges and the vortex strength and recirculation region increase gradually. The flowfield structure is affected by the adjacent jet holes obviously and the blunt leading edge is covered by the recirculation region. That is very advantageous for hypersonic vehicle to achieve the effective thermal protection. The shapes about shock trains are almost the same on different Mach numbers conditions when PR keeps the same value. What's more, there is tight relationship between configuration parameters and free flow conditions to decide the flowfield structure, and there exists the optimal porous jet scheme to make the overall performance best.

Origin of spin–orbit misalignments: the microblazar V4641 Sgr

ABSTRACT Of the known microquasars, V4641 Sgr boasts the most severe lower limit (>52°) on the misalignment angle between the relativistic jet axis and the binary orbital angular momentum. Assuming the jet and black hole spin axes coincide, we attempt to explain the origin of this extreme spin–orbit misalignment with a natal kick model, whereby an aligned binary system becomes misaligned by a supernova kick imparted to the newborn black hole. The model inputs are the kick velocity distribution, which we measure customized to V4641 Sgr, and the immediate pre/post-supernova binary system parameters. Using a grid of binary stellar evolution models, we determine post-supernova configurations that evolve to become consistent with V4641 Sgr today and obtain the corresponding pre-supernova configurations by using standard prescriptions for common envelope evolution. Using each of these potential progenitor system parameter sets as inputs, we find that a natal kick struggles to explain the origin of the V4641 Sgr spin–orbit misalignment. Consequently, we conclude that evolutionary pathways involving a standard common envelope phase followed by a supernova kick are highly unlikely for V4641 Sgr. An alternative interpretation is that the jet axis does not reliably trace the black hole spin axis. Our results raise concerns about compact object merger statistics gleaned from binary population synthesis models, which rely on unverified prescriptions for common envelope evolution and natal kicks. We also challenge the spin–orbit alignment assumption routinely invoked to measure black hole spin magnitudes.

Three-dimensional simulations of detonation propagation in circular tubes: Effects of jet initiation and wall reflection

In the present work, using a high-resolution three-dimensional numerical analysis, the initiation and propagation mechanism of a detonation wave is studied in a circular tube with a hot jet initiation. The reactive Euler equations with a one-step two-species chemistry model are solved based on the structured adaptive mesh refinement technique. Influences of both a single hot jet and impinging double hot jets on the formation of the detonation wave are studied. For each case, the objective is to investigate the role of the tube wall on the initiation and propagation of the detonation wave. The result for both cases shows that the hot jet-induced bow shock forms a complex reflection structure in the circular tube. The reflection effect of the circular wall strengthens the shock and facilitates the formation of the Mach stem, which leads to the formation of the detonation wave. However, when the hot jet condition and the total area of jet hole remain the same, for the case of initiation using double hot jets, the reflection strength of the bow shocks weakens when the diameters of the hot jets become smaller. When using a single hot jet, the initiated detonation is overdriven and propagates in the two-headed mode. In this initiation mode, by increasing the inflow Mach number, a four-headed mode detonation front is formed, while in the case of impinging double hot jets, a four-headed mode detonation front is initiated directly.