Heated Target Surface Research Articles

Induction heating for desorption of surface contamination for high-repetition laser-driven carbon-ion acceleration

This study reports the first experimental demonstration of surface contamination cleaning from a high-repetition supply of thin-tape targets for laser-driven carbon-ion acceleration. The adsorption of contaminants containing protons, mainly water vapor and hydrocarbons, on the surface of materials exposed to low vacuum (>10−3 Pa) suppresses carbon-ion acceleration. The newly developed contamination cleaner heats a 5-μm-thick nickel tape to over 400 °C in 100 ms by induction heating. In the future, this heating method could be scaled to laser-driven carbon-ion acceleration at rates beyond 10 Hz. The contaminant hydrogen is eliminated from the heated nickel surface, and a carbon source layer—derived from the contaminant carbon—is spontaneously formed by the catalytic effect of nickel. The species of ions accelerated from the nickel film heated to various temperatures have been observed experimentally. When the nickel film is heated beyond ∼150 °C, the proton signal considerably decreases, with a remarkable increase in the number and energy of carbon ions. The Langmuir adsorption model adequately explains the temperature dependence of desorption and re-adsorption of the adsorbed molecules on a heated target surface, and the temperature required for proton-free carbon-ion acceleration can be estimated.

Numerical research on the flow and heat transfer characteristic of impingement cooling with return holes

Jet impingement cooling is usually used in zones with extremely high thermal loads. The major factor that limits impingement heat transfer uniformity is the crossflow. The impingement cooling with return hole is applied to reduce the negative effect of crossflow on the target surface heat transfer in present paper. The influences of jet-to-plate spacing (H/D), impingement target surface curvature (α), temperature ratio (TR) and Reynolds number on flow field and heat transfer performance are investigated in detail. The results indicate that the correlation between the overall area average Nusselt number of target surface and H/D, the overall area average Nusselt number decreases by approximately 5.76% with H/D increases by one unit. Increasing the impingement target surface curvature is beneficial to expand the spanwise heat transfer. The overall area average Nusselt number increases by 295.5% for Reynolds number varying from 10,000 to 50,000 at TR = 0.910, and the overall area average Nusselt number increases by 8.68% for the TR varying from 0.668 to 0.910 at Re = 30,000. The results indicate that impingement cooling with return hole can achieve high heat transfer coefficient and uniform heat transfer distribution.

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.

Characterization of Convective Transfer of Heat by Air Impingement using Single Swirling Jet with Insertion of Twisted Tapes by Computational Fluid Dynamics

This study showed the analysis of heat transfer by computational fluid dynamics (CFD) from a heated target surface by the use of jet of single swirl and impingement by air at various Reynolds numbers. The half-length downstream with insertion of twisted tape is applied in a nozzle body to get the swirl. The distance between twist ratio and nozzle to plate of twisted tape is found to be same as y = 2.93 and 21mm (H/D = 1). The characteristics of transfer of heat on heated surface are determined with varied Reynolds numbers like 12000, 17000, 22000 and 27000.

Numerical Investigation of Nondimensional Constant and Empirical Relation Representing Nusselt Profile Nonuniformity

The use of air jet impingement for cooling is the most expensive task as far as the efficiency of the component is concerned: not only this, but the generation of characteristic cooling over the target surface (heat sink) is of great significance in material processing firms. The conversion of the temperature profile to a Nusselt profile is widely used when the concern of nondimensionality comes into the picture. So, the present work reports the Nusselt profile for different impinging parameters, and the nonuniformities concerned with the profiles are evaluated. The standard deviation is the parameter used to represent the nonuniformity in the Nusselt profile for the present study. A nondimensional constant (ratio of Reynolds number and nozzle-target spacing) is defined to represent the start and end of nonuniformity in the Nusselt profile. When this constant exceeds a value of 6000, the nonuniformity in the profile ends. Also, an empirical relation representing the standard deviation is proposed in terms of the Reynolds number and nozzle-target spacing. The generation of turbulence vortices at the exit of the nozzle is responsible for the end of nonuniformity in the Nusselt profile.

Analysis of heat transfer and fluid flow of a slot jet impinging on a confined concave surface with various curvature and small jet to target spacing

Purpose This study aims to focus on the surface curvature, jet to target spacing and jet Reynolds number effects on the heat transfer and fluid flow characteristics of a slot jet impinging on a confined concave target surface at constant jet to target spacing. Design/methodology/approach Numerical simulations are used in this research. Jet to target spacing, H/B is varying from 1.0 to 2.2, B is the slot width. The jet Reynolds number, Rej, varies from 8,000 to 40,000, and the surface curvature, R2/B, varies from 4 to 20. Results of the target surface heat transfer, flow parameters and fluid flow in the concave channel are performed. Findings It is found that an obvious backflow occurs near the upper wall. Both the local and averaged Nusselt numbers considered in the defined region respond positively to the Rej. The surface curvature plays a positive role in increasing the averaged Nusselt number for smaller surface curvature (4-15) but affects little as the surface curvature is large enough (> 15). The thermal performance is larger for smaller surface curvature and changes little as the surface curvature is larger than 15. The jet to target spacing shows a negative effect in heat transfer enhancement and thermal performance. Originality/value The surface curvature effects are conducted by verifying the concave surface with constant jet size. The flow characteristics are first obtained for the confined impingement cases. Then confined and unconfined slot jet impingements are compared. An ineffective point for surface curvature effects on heat transfer and thermal performance is obtained.

Effect of dimple pitch and depth on jet impingement heat transfer over dimpled surface impinged by multiple jets

The effect of dimpled surface configurations on heat transfer is investigated for multiple jets issuing from orifices arranged in a 3 × 3 array. The Reynolds numbers during these studies maintained in the range of 5000–40000. To assess the influence of jet and dimple pitch, the studies are performed for different dimple and orifice pitches (p = 3d, 4d, and 5d). Along with these cases, dimple depths of 0.25d and 0.5d and eccentricity between the orifice and the dimple are also studied. The distance between the orifice plate and the test surface is varied from 1d to 6d. To characterize the steady-state heat transfer from the heated target surface to the impinging air jet, the thermal image of the target surface captured with a thermal imaging camera is analysed with thin metal foil heater technique. To evaluate the effectiveness of jet impingement cooling of the dimpled surface, the average Nusselt number and coefficient of variance (COV) are calculated over a defined pitch area of ± p. It is observed that for the jet orifice and dimple pitches of three nozzle diameter (p = 3d), the flat plate is more efficient than a dimpled surface. However, for greater pitches (p ≥ 4d), the dimpled surface performed much better in comparison with the flat plate.In case of the shallow dimpled surface (t/d = 0.25), best heat transfer performance is observed for the pitch of 5d. The coefficient of variance (COV) suggests that the Nusselt number is more uniformly distributed for a pitch of 3d.

Heat transfer and flow field measurements of a pulsating round jet impinging on a flat heated surface

Heat transfer and flow field characteristics of a pulsating round air jet, impinging on a flat heated target surface were measured and compared to those of a steady impinging jet. The jet was partially confined and the target surface stand-off distance equaled two nozzle diameters, D. Investigated Reynolds numbers ranged from Re = 4,606 to 13,513, while pulsating jet velocities varied nearly sinusoidal with Strouhal numbers ranging between 2×10−3< Sr<15.6×10−3. Results indicated that besides the well-known secondary Nusselt number peak at approximately 2D from the stagnation point, an additional distinct increase in Nusselt number was detected at approximately 4D from the stagnation point as a result of partial jet confinement. For the studied Re and Sr-number range, jet pulsation resulted in both enhanced and slightly reduced overall heat transfer. Our results indicated that the physical mechanism responsible for overall heat transfer enhancement or reduction due to jet pulsation is increased or reduced primary vortex generation Strouhal numbers (Srv), respectively. The latter were extracted from phase-locked particle image velocimetry measurements. Overall heat transfer modulation linearly depended on the ratio of the ensemble averaged pulsating jet and steady jet primary vortex generation Strouhal numbers, Sr¯v/Sr¯v,0. Overall heat transfer enhancement and reduction was obtained when Sr¯v/Sr¯v,0>1 and Sr¯v/Sr¯v,0<1, respectively.

Uniform Cooling of a Flat Surface by an Optimized Array of Turbulent Impinging Air Jets

The aim of this study is to investigate the uniform cooling of a hot isothermal heated target surface, using four turbulent impinging air jets. Eight parameters including the width of jets, the space between the inner jets, the space between inner and outer jets, the distance of jets from the plate, the impingement angle of jets, and the overall volumetric flow rate of the cooling air per unit depth of the nozzle are considered as design variables. The normalized standard deviation of the local Nusselt number from the desired Nusselt number is considered as the objective function. An optimization algorithm based on pattern search method is utilized to obtain the optimum array of the jets. Two different scenarios of the problem are considered, one with fixed normal impingement angles and the other with the optimized angles. Results show an almost uniform distribution of the local Nusselt number. Increasing the amount of desired Nusselt number for the case with fixed impinging angles results in a higher Reynolds number, a wider opening for outer jets and a reduction in jet to jet and jet to surface distances. However, changes in design parameters for the case with optimum impinging angles are erratic.

Experimental study on heat transfer characteristics of hybrid nanofluid impinging jets

Silver (Ag)-multiwall carbon nanotube/water hybrid nanofluids were used as a heat transfer media. The stability of the hybrid nanofluids was tested by comparing the matching ratios of different Ag particles and multiwall carbon nanotube particles. The optimal ratio of mixed solution and deionized water were selected at different mass fractions. The experimental study analyzes conventional impinging jets (CIJs) and swirling impinging jets (SIJs) on the conditions of different heat exchange surfaces, different length to diameter (L/D) values and different swirling ratios of twisted ribbons. The analysis also includes the effects of adding different dispersants on mixed nanofluids. The results shows that the thermal conductivity of Ag-multiwall carbon nanotube/water hybrid nanofluids is higher than that of multiwall carbon nanotube nanofluids. Furthermore, the heat transfer efficiency increases with the increasing of mass fraction. Under the same experimental conditions, the effect of heat transfer by swirling impinging jets was better than that of conventional impinging jets. The arc target surface heat transfer coefficient was higher, and L/D = 4 was the best nozzle size to target the surface space ratio. Finally, y/w = 3 was the best spiral structure of the built-in twisted ribbon.

Influence of target temperature on H alpha line of laser-induced silicon plasma in air

This study mainly discusses the effect of target temperature on the emission intensity of the H alpha line in laser-induced silicon plasma in an atmospheric environment. The sample was uniformly heated to temperatures ranging from room temperature (22 °C) to a high temperature (300 °C) with laser energy ranging from 5.5 mJ to 11.5 mJ. Emission lines of Si (I), Si (II), and H (I) were observed. The H alpha line was attributable to a small amount of water vapor derived from natural humidity in the surrounding air. The H alpha line was widely used to measure electron density in laser-induced plasma. The results show that an increase in the sample temperature improved the spectral intensity of the Si line, whereas the emission intensity of the H alpha line declined. The reduction in the H alpha line was due to lower air density near the heated target surface. This investigation enhances the understanding of laser-induced breakdown spectroscopy applications at higher sample temperatures.

Design, Test and Analysis of a Gyrotron Cavity Mock-Up Cooled Using Mini Channels

In 2016, we have designed, built, and finally tested at the FE200 facility in Le Creusot, France, a planar mock-up mimicking the water-cooled cylindrical resonance cavity of the European 170-GHz, 1-MW gyrotron to be used for electron cyclotron plasma heating in ITER. The aim of the mock-up is the characterization of the cooling capability of the cavity. A Glidcop target is heated with an electron beam gun with resulting peak heat fluxes relevant for the full-size cavity. Underneath the target surface, whose temperature is monitored by means of a pyrometer, a set of parallel semicircular mini channels, with a diameter of 1.5 mm, allow the flow of pressurized water, entering the mock-up at ~9 bar and 40 °C. Several thermocouples measure the target temperature, at different distances from the heated target surface. The experimental results show that the mock-up is capable to withstand heat fluxes of 21 MW/m2, while the cooling system keeps the heated surface below ~400 °C, for flow conditions comparable to those of the full-size cavity. The test results are used to first calibrate the uncertain model parameters and then, with frozen parameters, to validate a previously developed computational fluid dynamics model, showing good agreement with the experiment. In view of its reliability, this model might eventually be a useful tool for the simulation of the full-size gyrotron cavity operation.

Effect of Reynolds Number, Hole Patterns, and Hole Inclination on Cooling Performance of an Impinging Jet Array—Part I: Convective Heat Transfer Results and Optimization

This study comprehensively illustrates the effect of Reynolds number, hole spacing, jet-to-target distance, and hole inclination on the convective heat transfer performance of an impinging jet array. Spatially resolved target surface heat transfer coefficient distributions are measured using transient liquid crystal (TLC) measurement techniques, over a range of Reynolds numbers from 5000 to 25,000. Considered are effects of streamwise and spanwise jet-to-jet spacing (X/D, Y/D: 4–8) and jet-to-target plate distance (Z/D: 0.75–3). Overall, a test matrix of 36 different configurations is employed. In addition, the effect of hole inclination (θ: 0–40 deg) on the heat transfer coefficient is investigated. Optimal hole spacing arrangements and impingement distance are pointed out to maximize the area-averaged Nusselt number and minimize the amount of cooling air. Also included is a new correlation, based on that of Florschuetz et al., to predict row-averaged Nusselt number. The new correlation is capable to cover low Z/D ∼ 0.75 and presents better prediction of row-averaged Nusselt number, which proves to be an effective impingement design tool.

EFFECTS OF PIN FIN CONFIGURATIONS ON HEAT TRANSFER AND FRICTION FACTOR IN AN IMPROVED LAMILLOY COOLING STRUCTURE

In this study, numerical simulations are conducted to investigate the effects of pin fin location, pin fin diameter, and pin fin shape on the target and pin fin surfaces heat transfer as well as friction factor in an improved Lamilloy cooling structure. The pin fin normalized location is varied from 0.35 to 0.65 while the pin fin diameter is changing from 15 mm to 60 mm. Cone-shaped pin fins are introduced, and the root to roof diameter ratio of the cone-shaped pin fin is ranging from 0.5 to 2. The Reynolds number is between 10,000 and 50,000. Results of the target and pin fin surfaces Nu number, friction factor, and flow structures are included. For convenience of comparison, the Lamilloy cooling structure whose pin fin normalized location is 0.5 with a pin fin diameter of 30 mm is studied as the baseline. It was found that with increase of the pin fin normalized location, the heat transfer on the pin fin surfaces is gradually decreased while the friction factor shows a lower value as the pin fins are positioned either near the impingement center or the film holes. This trend is also found for increasing the pin fin diameter. In addition, the heat transfer on the pin fin surface is increased remarkably by using a coneshaped pin fin with a slight target surface heat transfer penalty. It was also found that by changing the pin fin location, pin fin diameter, and the pin fin shape, it may reach 7.6% higher values than the baseline thermal performance based on the target surface Nu number while it is 43.58% based on the pin fin surface. (Less)

A numerical investigation of dimple effects on internal heat transfer enhancement of a double wall cooling structure with jet impingement

Purpose The dimple is adopted into a double wall cooling structure which is widely used in hot gas components to increase the heat transfer effects with relatively low pressure drop penalty. The purpose of this paper is to study the effect of dimple depth and dimple diameter on the target surface heat transfer and the inlet to outlet friction factor. Design/methodology/approach The study is carried out by using the numerical simulations. The impingement flow is directly impinging on the dimple and released from the film holes after passing the double wall chamber. The ratio between dimple depth and dimple diameter is varied from 0 to 0.4 and the ratio between dimple diameter and impingement hole diameter is ranging from 0.5 to 3. The Reynolds number is between 10,000 and 70,000. Results of the target surface Nusselt number, friction factor and flow structures are included. For convenience of comparison, the double wall cooling structure without the dimple is considered as the baseline. Findings It is found that the dimple can effectively enhance the target surface heat transfer due to thinning of the flow boundary layer and flow reattachment as well as flow recirculation outside the dimple near the dimple rim especially for the large Re number condition. However, the stagnation point heat transfer is reduced. It is also found that for a large dimple depth or large dimple diameter, a salient heat transfer reduction occurs for the toroidal vortex. The thermal performance indicates that the intensity of the heat transfer enhancement depends upon the dimple depth and dimple diameter Originality/value This is the first time to adopt a dimple into a double wall cooling structure. It suggests that the target surface heat transfer in a double wall cooling structure can be increased by the use of the dimple. However, the heat transfer characteristic is sensitive for the different dimple diameter and dimple depth which may result in a different flow behavior

Achieving Heat Flux Uniformity Using an Optimal Arrangement of Impinging Jet Arrays

In order to achieve uniform distribution of heat flux over an isothermal heated target surface, a numerical algorithm is developed to obtain an optimized array of four laminar impinging slot jets. Root mean square deviation of the local Nusselt distribution from the desired Nusselt number is considered as the objective function. Jets' widths, jet-to-jet and jet-to-surface spacings, and the overall flow rate are chosen as design variables. Conjugate gradients method along with backtracking line search is applied to optimize the objective function calculated by numerical simulation for three different cases of Nu = 7, 9, and 11. For each of these desired Nusselt numbers, an almost uniform distribution of local Nusselt number with percent of root mean square error less than 2.5% is achieved in fewer than 12 iterations. An experimental study using a Mach–Zehnder interferometer (MZI) has been performed. The measured distribution of local Nusselt number is in good agreement with numerical results in all three optimal configurations.

COMPUTATIONAL INVESTIGATION OF DIMPLE EFFECTS ON HEAT TRANSFER AND FRICTION FACTOR IN A LAMILLOY COOLING STRUCTURE

Good heat transfer performance with a moderate pressure drop penalty contributes to the gas turbine engine lifetime and guaranteeing engine efficiency. In this study, the dimple effects for a Lamilloy® (Allison Advanced Development Corporation, Indiana, IN, USA) cooling structure on the heat transfer and friction factor are numerically investigated. The dimple is positioned directly under the jet impingement nozzle. The Reynolds number ranges from 10,000 to 70,000, the dimple normalized depth is between 0 and 0.3, and the dimple normalized diameter varies from 1 to 2.5. The results for the flow field, target surface heat transfer, pin fin surface heat transfer, friction factor, and solid domain outer-wall temperature are included. For comparison, a Lamilloy cooling structure without the dimple is considered as the baseline. The results show that the dimple significantly increases the local heat transfer due to flow reattachment and recirculation. With an increase in the normalized dimple depth, the heat transfer on the target surface is first augmented due to the increase of flow reattachment and recirculation, and then it is decreased due to the large toroidal vortex. However, an increase in the dimple depth results in reduction of the pin fin surface heat transfer. As the dimple diameter increases, the target surface heat transfer is also first augmented due to the increase in the flow reattachment and recirculation, and then it is decreased as the flow separation increases. The thermal performance indicates that the intensity of the heat transfer enhancement depends on the depth and diameter of the dimple.

Heat transfer in a channel with inclined target surface cooled by single array of centered impinging jets

An experimental investigation has been carried out to study the heat transfer characteristics in a channel with heated target plate inclined at an angle cooled by single array of equally spaced centered impinging jets for three different jet Reynolds numbers (Re=9300, 14400 and 18800). Air ejected from an array of orifices impinges on the heated target surface The target plate forms the leading edge of a gas turbine blade cooled by jet impingement technique. The work includes the effect of jet Reynolds numbers and feed channel aspect ratios (H/d = 5, 7, 9 where H=2.5, 3.5, 4.5 cm and d=0.5 cm) on the heat transfer characteristics for a given orifice jet plate configuration with equally spaced centered holes with outflow exiting in both directions (with inclined heated target surface). In general, It has been observed that, H/d=9 gives the maximum heat transfer over the entire length of the target surface as compared to all feed channel aspect ratios. H/d=9 gives 3% more heat transfer from the target surface as compared to H/d=5 (for Re=14400). Also, it has been observed that the magnitude of the averaged local Nusselt number increases with an increase in the jet Reynolds number for all the feed channel aspect ratios studied.

원형 다공배플이 있는 덕트에서의 열전달과 마찰계수에 관한 수치해석

The present numerical study was performed to determine how the two perforated baffles( Inclined angle=<TEX>$5^{\circ}$</TEX>; perforation diameter=2cm) placed at a rectangular duct affect heat transfer and associated friction factors. The parametric effects of perforated baffles(3, 6 and 12 holes) and flow Reynolds number ranging from 28,900 to 61,000 on the heated target surface are explored. As for the investigation of heat transfer behaviours on the local Nusselt number with two baffles placed at <TEX>$x/D_h=0.8$</TEX> and <TEX>$x/D_h=0.8$</TEX> of the edge baffles, it is evident that the average Nusselt number increases with increasing number of holes, but the friction factor decreases with an increase in the hole number placed at baffles. The numerical results by commercial code CFX 10.0 are confirmed with the experimental data.

Study of Heat Transfer Distribution in a Channel with Inclined Target Surface Cooled by a Single Array of Staggered Impinging Jets

An experimental investigation has been carried out to study the heat transfer characteristics in a channel with a heated target surface inclined at an angle, cooled by a single array of staggered impinging jets. The work encompasses the effect of three feed channel aspect ratios (5, 7, 9) and three exit outflow orientations (coincident with the entry flow, opposed to the entry flow, and both), and three Reynolds numbers (9400, 14,400, 18,800) on heat transfer. Results show that increasing the Reynolds number increases the heat transfer on the inclined target surface. The outflow orientations affect significantly the local heat transfer charactracistrics, through influencing the jet flow together with the crossflow in the impingement channel. The outflow orientation coincident with the entry flow and the outflow from both sides show better averaged Nusselt number values compared to outflow orientation opposed to the entry flow. The inclined surface affects the local Nusselt number distribution especially for the outflow orientation opposing the entry flow at the narrow region of the impingement channel. In general, the feed channel aspect ratio does not affect the Nusselt number distribution, except for outflow coincident with the entry flow. The local Nusselt number for aspect ratio 9 has been found to be greater than the Nusselt number for aspect ratio 5 by 11%. Additionally, for a given jet-orifice plate with staggered holes, the heat transfer is almost the same throughout the target surface for the outflow exiting in both directions.