Inter-jet Spacing Research Articles

Separation control with elliptical air-jet vortex generators

The flow organisation of air-jet vortex generators (AJVGs) of elliptical cross sections and their control effectiveness on a 24^{circ }-compression-ramp-induced shock-wave/boundary-layer interaction was analysed on the basis of experiments at M_{infty } = 2.52 and Re_{theta _c} = 8225. We investigated a circular orifice and two elliptical orifices of aspect ratios 0.5 and 2; all characterised by the same hydraulic diameter. Measurements of separation lengths from oil-flow visualisation and PIV reveal that elliptical AJVGs achieve a 25% reduction in total separation length, which constitutes a strong improvement over the 17% reduction achieved with the commonly used circular AJVGs. The jet-induced structures from elliptical AJVGs penetrate on average 25% farther into the boundary layer. However, the lateral spread is limited to a maximum value equal to the inter-jet spacing in the control array, which highlights the onset of jet/jet interactions between adjacent jets in the array. A consequence of these interactions is better flow entrainment for the elliptical cases, as observed in the mean boundary-layer velocity profiles and an improved turbulent mixing (indicated by an increase in Reynolds-shear-stress magnitude).

In-Flame And In-Cylinder Flow/Turbulence Measurements Near The Glow Plug Using Flame Image Velocimetry And Particle Image Velocimetry In An Optical Compression-Ignition Engine

The present study implements two diagnostic methods based on tracking of seeded olive oil droplets (PIV: particle image velocimetry) and pattern changes detected in high-speed flame movies (FIV: flame image velocimetry) in a small-bore optical diesel engine. For each measurement, a total of 100 engine cycles are recorded and processed to address the inherent cyclic variations. The ensemble-averaged flow fields and turbulence intensity distribution extracted from individual cycles via the spatial filtering method are discussed with a particular interest in the influence of glow plug on flow and turbulence, i.e. rigid body and fluid interaction. The PIV results show a swirl flow structure forms and rotates with its centre shifted towards the exhaust side, leading to an asymmetric swirl structure. By comparing a PIV laser plane tilted towards the glow plug and a 10 mm horizontal plane below the cylinder head with no glow flow-plug interaction, it is observed that the flow-plug interaction causes the flow winding around the plug tip to generate complex flow structures and new vortices downstream of the plug. The tilted plane and 10-mm plane show similar bulk flow magnitude distribution patterns; however, the flow-plug interaction generates high turbulence in the tilted plane right downstream of the plug tip where new vortices form, which lasts for a few crank angles. The spatially averaged flow magnitude and turbulence intensity are measured higher in the 10-mm plane where there is no flow-plug interaction, suggesting the increased turbulence is a localised behaviour. The flame-plug interaction is also investigated during the combustion event using the FIV method. The level of flame-plug interaction is adjusted by changing the inter-jet spacing angle of two nozzle holes with one case showing high interaction and the other displaying low interaction. From the FIV measurements, the most significant effect of the flame-plug interaction is observed as the further penetration of the wall bounced flame for the high interaction case. This is due to the glow plug as a rigid body blocking the swirl flow and promoting the flame penetration back towards the centre of the combustion chamber upon the piston-bowl wall impingement. The measured turbulence intensity is also higher thanks to the enhanced wall bounced flame in addition to more significant flame-plug interaction at the interface.

Influence of the inter-jet spacing of diesel sprays on the combustion development

Influence of the inter-jet spacing of diesel sprays on the combustion development

The influence of inter-jet spacing and jet-swirl interaction on flame image velocimetry (FIV) derived flow fields in a small-bore diesel engine

This study applies Flame Image Velocimetry (FIV) to show the in-flame flow field development with an emphasis on the jet-jet interaction and jet-swirl interaction phenomena in a single-cylinder small-bore optically accessible diesel engine. Two-hole nozzle injectors with three different inter-jet spacing angles of 45°, 90° and 180° are prepared to cause different levels of jet-jet interaction. The engine has a swirl ratio of 1.7, which is used to evaluate jet-swirl interaction of the selected 180° inter-jet spacing nozzle. High-speed soot luminosity imaging was performed at a high frame rate of 45 kHz for the FIV processing. For each inter-jet spacing angle, a total of 100 individual combustion cycles were recorded to address the cyclic variations. The ensemble averaged flow fields are shown to illustrate detailed flow structures while the Reynolds decomposition using spatial filtering is applied to analyse turbulence intensity. The results showed reduced bulk flow magnitude and turbulence intensity at smaller inter-jet spacing, suggesting the two opposed wall-jet heads colliding immediately after the jet impingement on the wall can cause flow suppression effects. This raised a concern on the mixing as lower inter-jet spacing creates more fuel-rich mixtures in the jet-jet interaction region. Despite lower flow magnitude, the cyclic variation was also estimated higher for narrower inter-jet spacing, which is another drawback of the significant jet-jet interaction. Regarding the jet-swirl interaction, the wall-jet head penetrating on the up-swirl side showed lower bulk flow magnitude as the counter-flow arrangement suppressed the flow, similar with the narrower interact-jet spacing results. However, the turbulence intensity was measured higher on the up-swirl side, suggesting the relatively weaker swirl flow vectors opposed to the penetrating wall-jet head could in fact enhance the mixing.

Computational Fluid Dynamics Analysis of Impingement Heat Transfer in an Inline Array of Multiple Jets

Amid all convective heat transfer augmentation methods employing single phase, jet impingement heat transfer delivers significantly higher coefficient of local heat transfer. The arrangement leading to nine jets in square array has been used to cool a plate maintained at constant heat flux. Numerical study has been carried out using RANS-based turbulence modeling in commercial CFD Fluent software. The turbulent models used for the study are three different “k-ε” models (STD, RNG, and realizable) and SST “k-ω” model. The numerical simulation output is equated with the experimental results to find out the most accurate turbulence model. The impact of variation of Reynolds number, inter-jet spacing, and separation distance has been considered for the geometry considered. These parameters affect the coefficient of heat transfer, temperature, and turbulent kinetic energy related to flow. The local “h” values have been noticed to decline with the rise in separation distance “H/D.” The SST “k-ω” model has been noticed to be in maximum agreement with the experimental results. The average value of heat transfer coefficient “h” reduces from 210 to 193 W/m2K with increase in “H/D” from 6 to 10 at “Re” = 9000 and S/D of 3. As per numerical results, inter-jet spacing “S/D” of 3 has been determined to be the most optimum value.

Understanding the effect of inter-jet spacing on lift-off length and ignition delay

In recent years, injectors with multiple outlet-holes and smaller diameters have been implemented to reduce pollutant emissions during the combustion development. Nevertheless, increasing the number of holes has also decreased the spacing between the jets, which could lead to interactions between sprays affecting their behavior, a phenomenon that has not been defined in detail. In this sense, this study analyzes the influence of inter-jet spacing on the ignition delay and lift-off length for a wide range of boundary conditions. To this end, two multi-hole diesel injectors were manufactured with different distributions of the outlet orifices, which allowed the study of the development of an isolated spray and a spray with an inter-jet spacing of 30∘ during the same injection event. Additionally, the results were compared to the development of sprays under inter-jet spacing values of 36∘ and 45∘. The results showed that the spray with smaller inter-jet spacing had a slightly larger ignition delay at low chamber temperature and density conditions. Regarding the lift-off length, reducing the inter-jet spacing decreased the lift-off length values, these reductions being more noticeable after a threshold of proximity between sprays was achieved. In particular, the spray with an inter-jet spacing value of 30∘ had considerably lower lift-off length values than those with an inter-jet spacing of 36∘ and 45∘, which in turn had closer lift-off length values than those obtained with the isolated spray. Finally, a novel empirical correlation was developed for the lift-off length considering the influence of inter-jet spacing, thus contributing to the understanding of the effect of the interaction between sprays in the combustion process.

Flame image velocimetry analysis of reacting jet flow fields with a variation of injection pressure in a small-bore diesel engine

This study measures in-flame flow fields in a single-cylinder small-bore optical diesel engine using Flame Image Velocimetry (FIV) applied to high-speed soot luminosity movies. Three injection pressures were tested for a two-hole nozzle injector to cause jet-wall interaction and a significant jet-jet interaction within 45° inter-jet spacing. The high-pressure fuel jets were also under the strong influence of a swirl flow. For each test condition, soot luminosity signals were recorded at a high framing rate of 45 kHz with which the time-resolved, two-dimensional FIV post-processing was performed based on the image contrast variations associated with flame structure evolution and internal pattern change. A total of 100 combustion events for each injection pressure were recorded and processed to address the inherent cyclic variations. The ensemble-averaged flow fields were used for detailed flow structure discussion, and Reynolds decomposition using a spatial filtering method was applied to obtain high-frequency fluctuations that were found to be primarily turbulence. The detailed analysis of flow fields suggested that increased injection pressure leads to enhanced jet flow travelling along the bowl wall and higher flow vectors penetrating back towards the nozzle upon the impingement on the wall. Within the jet-jet interaction region, the flow vectors tend to follow the swirl direction, which increases with increasing injection pressure. The FIV also captured a turbulent ring vortex formed in the wall-jet head, which becomes larger and clearer at higher injection pressure. A vortex generated in the centre of combustion chamber was due to the swirl flow with its position being shifted at higher injection pressure. The bulk flow magnitude indicated significant cyclic variations, which increases with injection pressure. The turbulence intensity is also enhanced due to higher injection pressure, which primarily occurs in the wall-jet head region and the jet-jet interaction region.

Computational Fluid Dynamics Analysis of Aerodynamics and Impingement Heat Transfer From Hexagonal Arrays of Multiple Dual-Swirling Impinging Flame Jets

Abstract Radiative furnaces pose significant thermal inertia and single impinging flames have been observed to cause occurrence of hotspots on the target surface. Multiple burners arranged in suitable array configuration represent one of the plausible solutions for more uniform heat transfer. In this study, computational fluid dynamics (CFD) simulations have been carried out for multiple swirling impinging flames arranged in a hexagonal array configuration. The turbulence chemistry interactions in the flame field are solved numerically using renormalization group (RNG) based k–ε/eddy dissipation model (EDM) framework. Comparison of co-and-counter-swirling configurations has been studied for interactions and spent gas release mechanism. Multiple swirling impinging flames undergo strong interactions resulting in distortions of recirculation zones (RCZ) for all the surrounding except central flame. Co-swirling flames result in development of higher turbulence in the interaction regions as compared to counter-swirl case. Results indicate that some flames in counter-swirl case are underutilized due to the fluid dynamics developed in the system and co-swirling hexagonal array configuration is a better arrangement for effective heating of target surface. Effect of interjet spacing (S/Dh = 5, 7, and 9) and separation distance (H/Dh = 3, 5, 7, and 9) studied for co-swirl case revealed that peak heat fluxes decreased with increasing interjet spacing and separation distance. Central flame represented a region of low heat flux and this region has been noticed to expand in size for increasing interjet spacings. Suppression of central flame has been observed to be maximum for minimum separation distance.

A Numerical Investigation on Fluid Dynamics and Heating Characteristics of Co-and-Counterrotating Multiple Swirling Impinging Flames Arranged in 3 × 3 Inline Array

Abstract Swirling impinging flame jets possess the potential of uniformly heating the target surface. Numerical simulations have been carried out for multiple reacting swirling flows arranged in square 3 × 3 array. The turbulence chemistry interactions in the flow field are modeled using RNG based k-ε/ eddy dissipation model (EDM) framework. Co-and-counterrotating configurations have been compared for interactions and spent gas release mechanism at fixed inter-jet spacing and separation distance. Multiple swirling impinging flames undergo strong interactions resulting in distortions of recirculation zones (RCZs) for all the surrounding but central flame. Co-swirling flames result in development of higher turbulence in the interaction regions as compared with counter-swirl case. Tilted heat flux contours have been noticed for co-swirling impinging flames, whereas heat flux impressions are straight for counter-swirl case. Effect of inter-jet spacing (C/Dh = 5, 7, and 9) and separation distance (H/Dh = 3, 5, 7, and 9) studied for co-swirl case revealed that peak heat fluxes decreased with increasing inter-jet spacing and separation distance. With increase in inter-jet spacing, the width of central flame increased due to increased suppression effect induced by the recirculating gases. Increase in separation distance resulted in decrease of width of central flame and thermal dilution taking place led to decrease in heat flux magnitudes.

Study of flow patterns and impingement heat transfer for an annular array of eight co-rotating dual-swirling flames

Performance of gas burners can be optimized by improvements in their design and proper selection of operating parameters associated with combustion and heat transfer mechanisms. Array of multiple swirl burners is one such configuration where uniformity in heat transfer can be increased significantly. The current study presents experimental and numerical investigations conducted for flow patterns and impingement heat transfer developed in an annular arrangement of eight co-rotating dual swirling impinging flames. Impingement heat flux distribution has been studied experimentally using analytical inverse heat conduction procedure (IHCP). Effect of change in separation distance and inter-jet spacing has been studied for interactions and subsequent impingement heat transfer characteristics. Turbulent co-swirling flames portray intense mixing developed at the interaction regions due to strong interactions. Presence of impingement plate causes large scale changes in the flow-field. Numerical simulation conducted under reacting conditions predicted formation of asymmetric and distorted recirculation zones. Inner flames have been observed to deflect from their straight upright positions. Adjacent co-swirling flows merge together and develop circulation of flow at the inner periphery of the annulus. High heat fluxes observed at the target surface are corresponding to interaction regions and tend to cluster around the geometric centre of the array. Reversed downward flow developed at the geometric centre represents region devoid of heating. Averaged heat fluxes registered at the impingement plate decreased with increasing inter-jet spacing and separation distance. Maximum uniformity in the heat transfer has been observed at inter-jet spacing, S/Dh of 4 and separation distance, H/Dh of 6.

Experimental and computational analysis of a row of three co-swirling impinging flames

Detailed flow field and heat transfer characteristics developed in a row of three co-rotating dual swirling impinging flames have been investigated experimentally as well as numerically. Impingement heat transfer and pressure distribution has been studied experimentally at different separation distances and inter-jet spacings. Inverse heat conduction procedure (IHCP) has been used for estimating heat fluxes on the front side of impingement plate. Turbulence induced mixing results in strong interactions amongst adjacent flames causing deflections of inner flames of the burners situated at sides of central burner. Numerical simulation predicted formation of asymmetric recirculation zones for side flames. Symmetric interactions taking place for central flame produced two equal recirculation lobes for central flame. Behavior of central inner flame has been observed to be dependent on the value of inter-jet spacing used. Suppression of central inner flame tend to occur at higher inter-jet spacings due to recirculating products. Impingement pressure distribution is observed to be consistent with the heat flux distribution. Averaged heat fluxes registered at the impingement plate due to the central flame are higher in magnitude to those pertaining to each of the side flames.

Study of flow field and heat transfer characteristics for an interacting pair of counter-rotating dual-swirling impinging flames

Experimental and numerical studies have been conducted to investigate the flow field and heat transfer characteristics of a pair of counter rotating dual-swirling flames impinging on a flat surface. Effect of separation distance (H/Dh = 3, 4, 6 and 8), inter-jet spacings (S/Dh = 4, 7 and 10) and Reynolds number on flow interactions and impingement heat transfer have been studied under stoichiometric conditions. The inner non-swirling flames are seen to bend towards interacting side due to asymmetric interactions developed in a pair of dual-swirling flames. Commercial computational fluid dynamics (CFD) code (FLUENT®) has been used to simulate the interacting dual-swirling impinging flames. Numerical simulation predicted the deflection of inner flames to be primarily due to large distorted RCZ developed from asymmetric interactions. Comparisons of co-and-counter swirling configurations suggest that the presence of plate has a stronger influence on flow structure of impinging flames than sign of rotation. High resolution heat fluxes have been obtained using steady state 1-D heat balance from temperature readings obtained on rear side of quartz impingement plate through IR thermal camera. 1-D heat balance has been corrected for lateral conduction taking place inside the impingement plate using an iterative procedure. For counter swirl case, cross-flow leaves as a straight flow along the transverse direction due to the nature of interactions. Cross-flow causes heat flux contours in the interaction region to be asymmetric and preferential heating is observed along the transverse direction. The increase in Re(o) causes wider flame spread and severe interactions result in increased tendency for preferential heating of the impingement plate. High average heat fluxes are obtained at smallest H/Dh and S/Dh whereas minimum relative deviation is observed at S/Dh = 10 for H/Dh = 4.

Effect of Interactions on Flow Field and Heat Transfer Characteristics for Three Corotating Dual Swirling Flames Impinging on a Flat Surface

ABSTRACTDetailed flow field and heat transfer characteristics of an array of three corotating dual swirling impinging flames arranged in a triangular configuration have been investigated experimentally as well as numerically. Effect of varying separation distance and inter-jet spacing on interactions and impingement heat transfer has been studied at fixed Reynolds numbers under stoichiometric conditions. Adjacent co-swirling flames developed strong interactions resulting in high heat transfer due to increased mixing and turbulence. Bending of inner non-swirling flames takes place toward geometric center of the array due to asymmetric interactions. Numerical simulation also predicted asymmetric interactions resulting in the formation of large distorted recirculation bubble in each dual-swirling flame. Interacting co-swirling flames resulted in the formation of circulating patterns of flow at the geometric center of array. The effect of inter-jet spacing on average heat flux is observed to be more significant than separation distance.

Heat transfer and fluid flow characteristics of a pair of interacting dual swirling flame jets impinging on a flat surface

Experimental and numerical studies have been conducted to investigate the flow field and heat transfer characteristics of a pair of dual interacting swirling flames impinging on a flat surface. Commercial computational fluid dynamics (CFD) code (FLUENT®) has been used to simulate the interacting isothermal swirling impinging jets. Inverse heat conduction procedure (IHCP) has been used to calculate the impingement heat fluxes from the surface temperatures captured by Infra-red camera. Effect of separation distance (H/Dh = 2.5, 4, 6 and 8) and inter-jet spacings (S/Dh = 4, 6, 8 and 10) have been studied at various Reynolds numbers (Re(o) = 7000, 9000, 11000, 13,000 and Re(i) = 700, 1000, 1300) under stoichiometric conditions. Strong interactions between adjacent dual swirling flames result in high heat transfer due to increased mixing and turbulence in the interaction region. The inner non-swirling flames are seen to deflect towards interacting side due to asymmetric interactions. Numerical simulation predicted this deflection to be primarily due to large recirculation bubble developed from asymmetric interactions. Tilted cross-flow, emerging from interaction region has been observed due to momentum exchange taking place between cross-flow and swirling flames (jets). Area weighted average of local heat flux and relative deviation from averaged value has been calculated at various H/Dh and S/Dh. High average heat fluxes are obtained at smallest H/Dh and S/Dh. It has been concluded that for a system of burners considered for the present study, H/Dh = 2.5 and S/Dh = 8 is the optimum configuration on the basis of minimum relative deviation.

Performance Analysis of a Multiple Micro-Jet Impingements Cooling Model

The present study investigates the thermal performance of a multiple micro-jet impingements model for electronics cooling. The fluid flow and heat transport characteristics were investigated for steady incompressible laminar flow by solving three-dimensional (3D) Navier-Stokes equations. Several parallel and staggered micro-jet configurations (ie. inline 2 Å~ 2, 3 Å~ 3 and 4 Å~ 4 jets, and staggered five-jet and 13-jet arrays with the jet diameter to the channel height ratios from 0.25–0.5) were analyzed at various flow rates for the maximum temperature rise, pressure drop, heat-transfer coefficient, thermal resistance, and pumping power characteristics. The parametric investigation was carried out based on the number of jets and the jet diameters at various mass flow rates and jet Reynolds numbers. Temperature uniformity and coefficient of performance were evaluated to find out the trade-off among the various designs investigated in the present study. The maximum temperature rise and the pressure drop decreased with an increase in the number of jets except in the case of staggered five-jet array. A higher temperature uniformity was observed at higher flow rates with a decrease in the coefficient of performance. The performance parameters, such as thermal resistance and pumping power, showed a conflicting nature with respect to design variables (viz. jet diameter to stand-off ratio and interjet spacing or number of jets) at various Reynolds numbers within the laminar regime.

Scaling of Off-Equatorial Jets in Giant Planet Atmospheres

Abstract In the off-equatorial region of Jupiter’s and Saturn’s atmospheres, baroclinic eddies transport angular momentum out of retrograde and into prograde jets. In a statistically steady state, this angular momentum transfer by eddies must be balanced by dissipation, likely produced by magnetohydrodynamic (MHD) drag in the planetary interior. This paper examines systematically how an idealized representation of this drag in a general circulation model (GCM) of the upper atmosphere of giant planets modifies jet characteristics, the angular momentum budget, and the energy budget. In the GCM, Rayleigh drag at an artificial lower boundary (with mean pressure of 3 bar) is used as a simple representation of the MHD drag that the flow on giant planets experiences at depth. As the drag coefficient decreases, the eddy length scale and eddy kinetic energy increase, as they do in studies of two-dimensional turbulence. Off-equatorial jets become wider and stronger, with increased interjet spacing. Coherent vortices also become more prevalent. Generally, the jet width scales with the Rhines scale, which is of similar magnitude as the Rossby radius in the simulations. The jet strength increases primarily through strengthening of the barotropic component, which increases as the drag coefficient decreases because the overall kinetic energy dissipation remains roughly constant. The overall kinetic energy dissipation remains roughly constant presumably because it is controlled by baroclinic conversion of potential to kinetic energy in the upper troposphere, which is mainly determined by the differential solar radiation and is only weakly dependent on bottom drag and barotropic flow variations. For Jupiter and Saturn, these results suggest that the wider and stronger jets on Saturn may arise because the MHD drag on Saturn is weaker than on Jupiter, while the thermodynamic efficiencies of the atmospheres are not sensitive to the drag parameters.

The shortening of lift-off length associated with jet–wall and jet–jet interaction in a small-bore optical diesel engine

Jet–wall and jet–jet interactions are important diesel combustion phenomena that impact fuel–air mixing, flame lift-off and pollutant formation. Previous studies to visualise a wall-interacting jet in heavy-duty diesel engines suggested that the shortening of lift-off length could occur due to the recirculated hot combustion products that are entrained back into incoming diesel jet. The significance of this effect, known as re-entrainment, can be higher in small-bore engines due to shorter nozzle-to-wall distance and increased wall curvature. In this study, we performed hydroxyl chemiluminescence imaging using an intensified CCD camera and high-speed imaging of natural soot luminosity using a CMOS camera in an automotive-size optical diesel engine. To provide detailed understanding of the reacting jet under the influence of re-entrainment as well as jet–jet interaction, various jet trajectories were investigated using one and two-hole injectors coupled with a modified piston that allows (1) the identification of the shortening of lift-off length, (2) the measurements of lift-off lengths for varying degrees of jet–wall interactions and (3) the clarification on inter-jet spacing effects on the lift-off lengths. Findings from the measurements support the re-entrainment theory because the shortening of lift-off length occurs only before the end of injection when the strong jet momentum induces the ambient gas entrainment. The shortening also shows good correspondence with the penetration of the recirculated jet head back towards the centre of the combustion chamber. Simultaneous imaging of the bowl-wall-interacting jet and cylinder-liner-wall-interacting jet depicts a shorter lift-off length for the bowl-wall-interacting jet, which further supports the importance of re-entrainment on diesel combustion. Inter-jet spacing effects on the lift-off length are also studied utilising two-hole injectors with two inter-jet spacing angles (51.4° and 102.8°). A narrower spacing between the jets results in the shorter lift-off length of the primary jet due to the higher jet–jet interaction even if short injection duration is used to suppress the re-entrainment. For tested conditions of this study, the shortening of lift-off length appears to be more sensitive to jet–jet interaction than to the re-entrainment caused by the jet–wall interaction.

Experimental Study of Two Impinging Jets Aligned With a Crossflow

Laser Doppler measurements provide information on the flowfield created by twin impinging jets aligned with a low velocity crossflow. The experiments were carried out for a Reynolds number based on the jet exit conditions of Rej = 4.3 × 104, an impingement height of 20.1 jet diameters and for a velocity ratio between the jet exit and the crossflow VR = Vj/Uo of 22.5, and an inter-jet spacing of S = 6D. The results show a large penetration of the first (upstream) jet that is deflected by the crossflow and impinges on the ground, giving rise to a ground vortex due to the collision of the radial wall and the crossflow that wraps around the impinging point like a scarf. The second jet (located downstream) is not so affected by the crossflow in terms of deflection, but due to the downstream wall jet that flows radially from the impinging point of the first jet it does not reach the ground. The results indicate a new flow pattern not yet reported so far, that is most relevant for a VSTOL aircraft operating in ground vicinity with front wind or small forward movement may result in enhanced under pressures in the aft part of the aircraft causing a suction down force and a change of the pitching moment towards the ground.

Performance analysis and design optimization of micro-jet impingement heat sink

This study evaluated a silicon-based micro-jet impingement heat sink for electronic cooling applications. First, the pressure-drop and thermal characteristics were investigated for steady incompressible and laminar flow by solving three-dimensional Navier–Stokes equations, and the performance enhancement was carried out through parametric and optimization studies. Several parallel and staggered micro-jet configurations consisting of a maximum of 16 jet impingements were tested. The effectiveness of the micro-jet configurations, i.e. inline 2 × 2, 3 × 3 and 4 × 4 jets, and staggered 5-jet and 13-jet arrays with nozzle diameters 50, 76, and 100 μm, were analyzed at various flow rates for the maximum temperature-rise and pressure-drop characteristics. A design with a staggered 13-jet array showed the best performance among the various configurations investigated in the present study. The design optimization based on three-dimensional numerical analysis, surrogate modeling and a multi-objective evolutionary algorithm were carried out to understand the thermal resistance and pumping power correlation of the micro-jet impingement heat sink. Two design variables, the ratio of height of the channel and nozzle diameter, and the ratio of nozzle diameter and interjet spacing, were chosen for design optimization. The global Pareto-optimal front was achieved for overall thermal resistance and required pumping power of the heat sink. The Pareto-optimal front revealed existing correlation between pumping power and thermal resistance of the heat sink. Of the range of Pareto-optimal designs available, some representative designs were selected and their functional relationships among the objective functions and design variables were examined to understand the Pareto-optimal sensitivity and optimal design space. A minimum of 66 °C of maximum-temperature-rise was obtained for a heat flux of 100 W/cm2 at a pressure drop of about 24 kPa.

Influence of jet–jet interactions on the lift-off length in an optical heavy-duty DI diesel engine

Several investigations have reported that the lift-off length on diesel jets depends strongly on the ambient temperature. The spacing between adjacent jets is thereby expected to influence the lift-off length, as it affects the amount of hot, burned gases present between the jets. Measurements on an 8-hole nozzle in an optical diesel engine showed that the lift-off length can be transient at all times between the start and end of injection. This is attributed to varying in-cylinder temperature and especially to the presence of hot combustion products in the gases entrained into the jets. The effect of inter-jet angle on lift-off length was investigated using symmetric and asymmetric nozzle cups. Decreasing the inter-jet angle produces shorter lift-off length. The lift-off length showed a weaker dependence on the ambient temperature in the engine than predicted by an empirical expression established in a constant-volume combustion vessel. These findings indicate that experiments in such vessels may not capture all features of the conditions in engines. The lift-off length tended to be 15% shorter on the downswirl (leeward) side of the jet. A strong interaction between the effects of the inter-jet spacing and the inlet temperature on the lift-off length was found. All these effects are attributed to the presence of hot gases between the jets.