Accepted Manuscripts

Lovedeep Sahota, V.S. Gupta and G.N. Tiwari
J. Heat Transfer   doi: 10.1115/1.4040782
In present paper, efforts has been made to study the thermo-physical characteristics of N photovoltaic thermal flat plate collectors coupled with double slope solar still (N-PVT-FPC-DSSS) and operating with helically coiled heat exchanger. The analysis has been performed for the optimized concentration of NPs (Al2O3 0.107%; TiO2 0.093%; and CuO 0.131%) and optimized basin fluid (basefluid/nanofluid) mass (50kg) for different weather conditions of the month May (New Delhi). The Nusselt number (Nu) and Rayleigh number (Ra) are functions of thermo-physical properties of nanofluids and strongly influence the natural convective heat transfer coefficient in the solar still. Therefore, these numbers have also been investigated for basefluid and Al2O3, TiO2 and CuO- water based nanofluids in detail. Significant enhancement in natural convective heat transfer coefficient (Al2O3 67.03%; TiO2 63.56%; and CuO 71.23%) and Nusselt number (Al2O3 119.72%; TiO2 98.64%; CuO 151.62%) has been observed. The monthly productivity of the hybrid system found to be higher by using nanofluids (320.77kg TiO2; 338.23kg Al2O3, and 355.46 CuO) as expected from the heat transfer results. Moreover, the comparative study between the proposed hybrid system and passive DSSS has been carried out.
TOPICS: Solar stills, Nanoparticles, Nanofluids, Convection, Heat exchangers, Flat plates, Water, Heat transfer, Fluids, Rayleigh number
Mark Kedzierski, Lingnan Lin and Donggyu Kang
J. Heat Transfer   doi: 10.1115/1.4040783
This paper quantifies the pool boiling performance of R134a, R1234yf, R513A, and R450A on a flattened, horizontal reentrant cavity surface. The study showed that the boiling performance of R134a on the Turbo-ESP exceeded that of the replacement refrigerants for heat fluxes greater than 20 kWm-2. On average, the heat flux for R1234yf and R513A was 16 % and 19 % less than that for R134a, respectively, for R134a heat fluxes between 20 kWm-2 and 110 kWm-2. The heat flux for R450A was on average 57 % less than that of R134a for heat fluxes between 30 kWm-2 and 110 kWm-2. A model was developed to predict both single-component and multi-component pool boiling of the test refrigerants on the Turbo-ESP surface. The model accounts for viscosity effects on bubble population and uses the Fritz (1935) equation to account for increased vapor production with increasing superheat. Both loss of available superheat and mass transfer resistance effects were modeled for the refrigerant mixtures. For most heat fluxes, the model predicted the measured superheat to within ± 0.31 K.
TOPICS: Cavities, Pool boiling, Heat, Flux (Metallurgy), Refrigerants, Heat flux, Turbochargers, Bubbles, Boiling, Mass transfer, Vapors, Viscosity
Zahir Shah, Saeed Islam, Hamza Ayaz and Saima Khan
J. Heat Transfer   doi: 10.1115/1.4040415
The present research aims to examine the micropolar nanofluid of Casson fluid between two parallel plates in a rotating system with effects of thermal radiation. The effects of Hall current on the micropolar nanofluids are have been taken into account. The flow of micropolar nanofluid has been assumed in steady state. The rudimentary governing equations have been changed to a set of differential nonlinear and coupled equations using suitable similarity variables. An optimal approach has been used to acquire the solution of the modelled problems. The convergence of the method has been shown numerically. The impact of the Skin friction on velocity profile, Nusslet number on temperature profile and Sherwood number on concentration profile have been studied. The influences of the Hall currents, rotation, Brownian motion and thermophoresis analysis of micropolar nanofluid have been mainly focused in this work. Moreover, for comprehension the physical presentation of the embedded parameters have been plotted and deliberated graphically.
TOPICS: Flow (Dynamics), Heat, Mass transfer, Fluids, Plates (structures), Nanofluids, Steady state, Temperature profiles, Rotation, Brownian motion, Skin friction (Fluid dynamics), Thermal radiation, Hall effect
Francisco Valentin, Narbeh Artoun and Masahiro Kawaji
J. Heat Transfer   doi: 10.1115/1.4040786
Hot wire anemometer (HWA) measurements of turbulent gas flow have been performed in upward forced convection experiments at pressures ranging from 0.6 MPa to 6.3 MPa and fluid temperatures ranging from 293 K to 673 K. The results are relevant to deteriorated turbulent heat transfer (DTHT) and flow laminarization in strongly heated gas flows which could occur in gas ­cooled Very High Temperature Reactors. The HWA signals were analyzed to directly confirm the occurrence of flow laminarization phenomenon due to strong heating. An X­-probe was used to collect radial and axial velocity fluctuation data for pressurized air and pure nitrogen flowing through a circular 16.8 mm diameter flow channel in a 2.7 m long graphite test section for local Reynolds numbers varying from 500 to 22,000. Analyses of the Reynolds stresses and turbulence frequency spectra were carried out and used as indicators of laminar, transition or fully turbulent flow conditions. Low Reynolds stresses indicated the existence of laminar or transitional flow until the local Reynolds number reached a large value, ~11,000 to 16,000, much higher than the conventional Re = 4,000­ ~ 5,000 for transition to fully turbulent flow encountered in pipe flows. The critical Reynolds number indicating the completion of transition approximately doubled as the pressure was increased from 0.6 MPa to 2.8 MPa.
TOPICS: Flow (Dynamics), Graphite, High pressure (Physics), High temperature, Turbulence, Reynolds number, Stress, Gas flow, Wire, Temperature, Spectra (Spectroscopy), Fluids, Very high temperature reactors, Forced convection, Pipe flow, Nitrogen, Probes, Signals, Turbulent heat transfer, Heating, Pressure
Technical Brief  
Professor Smith Eiamsa-ard and P. Samruaysin
J. Heat Transfer   doi: 10.1115/1.4040785
This paper deals with the influence of overlapped-quadruple twisted tape (OQT) and typical quadruple twisted tape (QT) on the enhancing heat transfer rate in a heat exchanger tube. Effects of tube orientation (co-tapes and counter tapes), tape arrangement (cross and parallel arrangments) and overlapped twist ratio (yo/y = 0.625, 0.75 and 0.875) were investigated. The experiments were performed in turbulent flow regime (5,000 ?Re?20,000) under uniform wall heat flux boundary condition, using air as the testing fluid. The experimental results indicates all overlapped-quadruple tapes (OQTs) consistently yield higher Nusselt number and thermal performace than typical quadruple tapes (QTs). The tapes in cross arrangement show better heat transfer enhancement and thermal performance than the ones in parallel arrangement. Heat transfer, friction loss and thermal performance increase with decreasing overlapped twist ratio. The maximum thermal enhancement factor (TEF) of 1.58 is achieved by the use of overlapped-quadruple counter tapes in cross arrangement (CC-OQTs) with yo/y = 0.625 at the lowest Reynolds number of 5000.
TOPICS: Heat transfer, Fluids, Turbulence, Reynolds number, Heat exchangers, Testing, Boundary-value problems, Heat flux, Friction
Technical Brief  
Rebecca Weigand, Kieran Hess and Amy Fleischer
J. Heat Transfer   doi: 10.1115/1.4040781
Phase change materials (PCMs) are commonly used in many applications, including the transient thermal management of electronics. For many systems paraffin-based phase change materials are a popular choice due to their high latent heat of fusion. However, paraffin-based materials exhibit low thermal conductivities and it is common to use suspended nanoinclusions to increase their effective thermal conductivity and surfactants to keep the nanoinclusions in suspension. The addition of these materials can have a positive impact on thermal conductivity, but can also increase the viscosity in the liquid phase. As increases in viscosity can negatively impact heat transfer by inhibiting convection currents, it is important to quantify this effect for these popular materials. In this paper the impact of different nanoinclusions and surfactants on the dynamic viscosity of a common paraffin wax PCM is quantified in order to determine their suitability for thermal energy storage applications. The nanoenhanced materials are found to be Newtonian in nature and to decrease in viscosity as temperature increases. The effect of the nanoparticles on the viscosity is found to be a function of the nanoparticle type with multi walled carbon nanotubes (MWCNT) yielding the greatest increase in viscosity. The addition of both nanoparticle and surfactant to the base PCM is found to affect the viscosity even when the loading levels of the nanoparticles or surfactant alone are not enough to affect the viscosity, thus the combination must be carefully considered in any heat transfer application.
TOPICS: Viscosity, Paraffin wax, Experimental analysis, Surfactants, Energy storage, Nanoparticles, Phase change materials, Heat transfer, Multi-walled carbon nanotubes, Thermal conductivity, Electronics, Currents, Thermal energy storage, Thermal management, Temperature, Transients (Dynamics), Convection, Heat of fusion
Dhruv C. Hoysall, Khoudor Keniar and Dr. Srinivas Garimella
J. Heat Transfer   doi: 10.1115/1.4040706
Multiphase flow phenomena in single micro- and mini-channels have been widely studied. Characteristics of two-phase flow through a large array of microchannels are investigated here. An air-water mixture is used to represent the two phases flowing through a microchannel array representative of those employed in practical applications. Flow distribution of the air and water flow across 52 parallel microchannels of 0.4 mm hydraulic diameter is visually investigated using high-speed photography. Two microchannel configurations are studied and compared, with mixing features incorporated into the second configuration. Slug and annular flow regimes are observed in the channels. Void fractions and interfacial areas are calculated for each channel from these observations. The flow distribution is tracked at various lengths along the microchannel array sheets. Statistical distributions of void fraction and interfacial area along the microchannel array are measured. The design with mixing features yields improved flow distribution. Void fraction and interfacial area change along the length of the second configuration, indicating a change in fluid distribution among the channels. The void fraction and interfacial area results are used to predict the performance of different microchannel array configurations for heat and mass transfer applications. Results from this study can help inform the design of compact thermal-fluid energy systems.
TOPICS: Heat, Two-phase flow, Microchannels, Flow (Dynamics), Porosity, Water, Design, Energy / power systems, Photography, Slug flows, Statistical distributions, Thermofluids, Mass transfer, Fluids, Multiphase flow
Hüseyin Kaya, Fahrettin Günver, Onuralp Uluer and Volkan Kirmaci
J. Heat Transfer   doi: 10.1115/1.4040707
An experimental analysis for parallel connected two identical counter flow Ranque-Hilsch vortex tubes with different nozzle materials and numbers was conducted by using compressed air as a working fluid in this paper. Heating and cooling performance of vortex tube system (circuit) and the results of exergy analysis are researched comprehensively according to different inlet pressure, nozzle numbers and materials. Nozzles made of polyamide plastic, aluminum and brass were mounted into the vortex tubes individually for each case of experimental investigation with the numbers of nozzles 2,3,4,5 and 6. The range of operated inlet pressure 150 kPa - 550 kPa with 50 kPa variation. The ratio of length - diameter (L/D) of each vortex tube in the circuit is 14 and the cold mass fraction is 0.36. Coefficient of performance (COP) values, heating and cooling capacity of the parallel connected RHVT system were evaluated. Further an exergy analysis was carried out to evaluate the energy losses and second law efficiency of the vortex tube circuit. The greatest thermal performance was obtained with aluminum-six-nozzle when taking into account all parameters such as temperature difference, COP values, heating and cooling capacity and exergy analysis.
TOPICS: Heating and cooling, Nozzles, Vortices, Exergy analysis, Circuits, Pressure, Aluminum, Brass (Metal), Energy dissipation, Experimental analysis, Compressed air, Flow (Dynamics), Temperature, Fluids
Giuseppe Romano and Alexie Kolpak
J. Heat Transfer   doi: 10.1115/1.4040611
Nanostructured semiconducting materials are promising candidates for thermoelectrics due to their potential to suppress phonon transport while preserving electrical properties. Modeling phonon-boundary scattering in complex geometries is crucial for predicting materials with high conversion efficiency. However, the simultaneous presence of ballistic and diffusive phonons challenge the development of models that are both accurate and computationally tractable. Using the recently developed first-principles Boltzmann transport equation (BTE) approach, we investigate diffusive phonons in nanomaterials with wide mean-free-path (MFP) distributions. First, we derive the short MFP limit of the suppression function, showing that it does not necessarily recover the value predicted by standard diffusive transport, challenging previous assumptions. Second, we identify a Robin type boundary condition describing diffuse surfaces within Fourier's law, extending the validity of diffusive heat transport in terms of Knudsen numbers. Finally, we use this result to develop a hybrid Fourier/BTE approach to model realistic materials, obtaining good agreement with experiments. These results provide insight on thermal transport in materials that are within experimental reach and open opportunities for large-scale screening of nanostructured thermoelectric materials.
TOPICS: Phonons, Nanostructures , Boundary-value problems, Model development, Heat, Scattering (Physics), Semiconductors (Materials), Radiation scattering, Electromagnetic scattering, Electrical properties, Mean free path, Modeling, Nanomaterials
Junde Li
J. Heat Transfer   doi: 10.1115/1.4040647
A partial differential-integral equation has been derived to connect vapor condensations and the development of condensation film in both the tangential and axial directions in a horizontal circular condenser tube. A high order explicit numerical scheme is used to solve the strongly nonlinear equation, and a simple strategy is applied to avoid possible large errors from high order numerical differentiation when the condensate becomes stratified. A set of correlations covering both laminar and turbulent film condensations have been selected to connect the film thickness with the rate of condensation, and thus allow for the predictions of local heat transfer coefficients. The predicted heat-transfer coefficients of film condensation for refrigerant R134a and water vapor in horizontal circular mini- and macro-tubes, respectively, have both been compared with experimental results and results from the simulations of film condensations using computational fluid dynamics, and very good agreement has been found. Some of the predicted film condensations are well into the strong stratification regime, and the results show that, in general, the condensate is close to annular near the inlet of the condenser tube and becomes gradually stratified as the condensate travels further away from the inlet for all the simulated conditions. The results also show that condensate in the mini-tubes becomes stratified much earlier than that in the macro-tubes.
TOPICS: Condensation, Water vapor, Heat transfer, Vapors, Turbulence, Simulation, Condensed matter, Computational fluid dynamics, Engineering simulation, Modeling, Condensers (steam plant), Errors, Film condensation, Film thickness, Nonlinear equations, Refrigerants, Heat transfer coefficients
Yunfei Xing, Fengquan Zhong and Xinyu Zhang
J. Heat Transfer   doi: 10.1115/1.4040612
In the present paper, numerical study of flow and heat transfer properties of RP-3 kerosene at liquid and supercritical conditions in a impingement model are conducted with RNG k-e turbulence model and a 10-species surrogate of kerosene. The independence of grids is first studied and the numerical results are compared with experimental data for validation. Characteristics of flow and heat transfer of kerosene flow in the impingement model are studied with different inlet mass flow rates and different inlet temperatures. The velocity and temperature field shows similar profile compared to that of air impingement. The heat transfer rates increase first with the increasing of inlet temperature and then decrease suddenly when the inlet temperature is 500K.
TOPICS: Flow (Dynamics), Temperature, Heat transfer, Turbulence, Impingement cooling, Aviation
Banjara Kotresha and N Gnanasekaran
J. Heat Transfer   doi: 10.1115/1.4040614
Two dimensional CFD simulations of mixed convection heat transfer through aluminum metal foams partially filled in a vertical channel are carried out numerically. The objective of the present study is to quantify the effect of metal foam thickness on the fluid flow characteristics and the thermal performance in a partially filled vertical channel with metal foams for a fluid velocity range of 0.05 - 3 m/s. The numerical computations are performed for metal foam filled with 40%, 70% and 100% by volume in the vertical channel for four different PPIs of 10, 20, 30 and 45 with porosity values varying from 0.90 to 0.95. To envisage the characteristics of fluid flow and heat transfer, two different models namely Darcy Extended Forchheirmer and Local thermal non-equilibrium have been incorporated for the metal foam region. The numerical results are compared with experimental and analytical results available in the literature for the purpose of validation. The results of the parametric studies on vertical channel show that the Nusselt number increases with the increase of partial filling of metal foams. The thermal performance of the metal foams is reported in terms of Colburn j and performance factors.
TOPICS: Heat transfer, Computational fluid dynamics, Mixed convection, Metal foams, Fluid dynamics, Porosity, Computation, Engineering simulation, Fluids, Foams (Chemistry), Aluminum, Simulation, Equilibrium (Physics)
Xiaohui Bai, Fujio Kuwahara, Moghtada Mobedi and Akira Nakayama
J. Heat Transfer   doi: 10.1115/1.4040613
Fully developed forced convective heat transfer within a channel filled with a functionally graded metal foam matrix was investigated analytically for the case of constant wall heat flux. A series of functionally graded metal foam matrices of the same mass (i.e. the same solidity) were examined in views of their heat transfer performances. The porosity either increases or decreases towards the heated wall following a parabolic function. Among the metal foam matrices of the same mass, the maximum heat transfer coefficient exists for the case in which the porosity decreases towards the heated wall (i.e. more metal near the wall). The heat transfer coefficients in such channels filled with a functionally graded metal foam matrix are found 20 to 50 % higher than that expected from the increase in the effective thermal conductivity. Hence, functionally graded metal foam matrices are quite effective to achieve substantially high heat transfer coefficient with an acceptable increase in pressure drop.
TOPICS: Convection, Metal foams, Heat transfer coefficients, Porosity, Pressure drop, Heat flux, Heat transfer, Metals, Thermal conductivity
Wei Li, Tariq Amin Khan, Weiyu Tang and W. J. Minkowycz
J. Heat Transfer   doi: 10.1115/1.4040609
Wavy fins have been considered as an alternative of the straight fins in compact heat exchangers for better heat transfer performance which can be augmented by considering vortex generators. This work is related to numerical investigation and optimization of corrugation height of fin and angle of attack of delta type vortex generator in a wavy fin-and-tube heat exchanger. For this purpose three-dimensional Reynolds-averaged Navier-Stokes analysis and a multi-objective genetic algorithm with surrogate modeling is performed. Numerical simulation is carried out to study the effect of delta winglets with varying the corrugation height of wavy fin in a three rows of tubes with staggered tube arrangements. The corrugation height (H) and angle of attack (?) varies from 0.3mm to 1.8mm and 15o to 75o, respectively. Results are illustrated by investigating the flow structures and temperature contours. Results show that increasing corrugation height of wavy fin and angle of attack of delta winglet enhances the heat transfer performance of heat exchanger while friction factor is also increased. Employing delta winglets has augmented the thermal performance for all corrugation heights and superior effect is observed at a higher corrugation. To achieve a maximum heat transfer enhancement and a minimum pressure drop, the optimal values of these parameters (H and ?) are calculated using the Pareto optimal strategy.
TOPICS: Heat exchangers, Optimization, Vortices, Generators, Heat transfer, Fins, Modeling, Computer simulation, Flow (Dynamics), Friction, Temperature, Genetic algorithms, Pressure drop
Cemil Koyunoglu
J. Heat Transfer   doi: 10.1115/1.4040610
The purpose of the new formulas, Cemil, CemilS, and CemilT, which express the slowest char combustion rate, is to show the controlling mechanism of single coal burning. The piecemeal step rate is also known as the rate that determines the combustion. Oxygen diffusion through the boundary layer (as a result of releasing volatile matter from coal) to the char surface is the slowest step rate, and can also represent as the rate determining. This step has not yet been taken into account in the literature, and may effect incomparable decisions between numerical and experimental results of coal combustion studies. In the 1920s, Wilhelm Nusselt found the coal combustion equation for a single coal, which based on initial coal diameter, and its burning time, or Nusselt Square Law (NSL). Also, the burning constant in NSL expressed oxygen partial pressure and the ambient temperature level 2. Nevertheless, recent studies according to char combustion have explained the effect of coal density on char combustion. Consequently, to help understand the slowest rate of char combustion, NSL as well as ordinary char combustion equations can be used together to reveal the rate determining factor. For this purpose, in this study, the slowest step of the char reaction rate is given as "Cemil", of stable position for single coal particle, "CemilT" and "CemilS" for a coal particle in motion.
TOPICS: Coal, Combustion, Particulate matter, Oxygen, Matter, Boundary layers, Density, Pressure, Temperature, Diffusion (Physics)
Asterios Pantokratoras
J. Heat Transfer   doi: 10.1115/1.4040541
The present comment concerns some doubtful results included in the above paper.
Ali Riahi, Julien Pellé, Lillia Chouchane, Souad Harmand and Sadok Ben Jabrallah
J. Heat Transfer   doi: 10.1115/1.4040481
This paper presents a numerical and experimental study of a turbulent flow of air in a Tbifurcation. This configuration corresponds to the radial vents placed in the stator vertically to the rotor-stator air gap in the electrical machines. Indeed, our analysis focuses on the local convective heat transfer on the vents internal surface under a turbulent mass flow rate. In order to dimension the cooling installation of this region, computational fluid dynamics (CFD) simulations and an experiment using particle image velocimetry (PIV) have been carried out. The resulting flow generally being the seat of recirculation zones in the various channels. The influence of the flow ratio and the diameter of the bifurcation on the dynamic and thermal behavior of the flow was. To carry out this study, we considered a numerical approach based on the KW-SST turbulence model (using commercial software, "Comsol Multiphysics"), allowing to numerically solve the Navier-Stokes equations and the energy equation in the system under consideration. We describe the different hypotheses necessary to formulate the equations governing the problem, the initial conditions, and the limits. The velocity in the bifurcation obtained using the simulation is compared with that obtained by the experiment and reveals a good agreement. The effect of branch diameter of the bifurcation and the effect of the flow ratio of heat transfer have been particularly analyzed in this paper.
TOPICS: Flow (Dynamics), Heat transfer, Cooling, Machinery, Particulate matter, Turbulence, Dimensions, Air flow, Simulation, Navier-Stokes equations, Computational fluid dynamics, Convection, Rotors, Bifurcation, Computer software, Stators, Vents
Andrey Gusarov
J. Heat Transfer   doi: 10.1115/1.4040482
Additively-manufactured cellular and honeycomb structures, foams, and very oblate particles dispersed in matrix are the examples of heterogeneous media with thin-wall morphology. Phase boundaries can also be considered by this approach. Statistical description is proposed to estimate the effective radiative properties of such media. Three orientation models are studied: (i) isotropic, (ii) surface elements parallel to a plane, (iii) surface elements parallel to an axis. Radiative transfer equations are obtained for the three models in the framework of the homogeneous phase approach and the multiphase approach. The radiative thermal conductivity is calculated for a bundle of circular rods, a packed pebble bed, and an open-cell metal foam. The results generally agree with the reference calculations by other methods. Small difference can be explained by different approaches to scattering and assumptions about the temperature distribution. Attenuation of light in a skin tissue is calculated by the diffusion approximation. The attenuation coefficient generally agrees with the reference Monte Carlo simulation. The difference observed at certain combination of parameters can be due to the assumption of regular arrangement of vessels at the Monte Carlo simulation.
TOPICS: Radiative heat transfer, Thin wall structures, Simulation, Radiation scattering, Honeycomb structures, Electromagnetic scattering, Thermal conductivity, Biological tissues, Approximation, Metal foams, Rods, Skin, Temperature distribution, Vessels, Diffusion (Physics), Scattering (Physics), Foams (Chemistry), Particulate matter
Taolue Zhang, Jayaveera Muthusamy, Dr. Jorge L. Alvarado, Anoop Kanjirakat and Reza Sadr
J. Heat Transfer   doi: 10.1115/1.4040393
The objective of this study was to visualize and simulate the thermal physical process during double droplet train impingement for three different horizontal impact spacings (S = 0.65 mm, 1.2 mm and 2 mm). Two identical HFE-7100 droplet trains were produced using a piezoelectric droplet generator at a frequency of 6000 Hz with a corresponding droplet Weber number of 312. A translucent sapphire substrate with a thin film ITO coating was used as heater in the experiments. The heat transfer and hydrodynamics of double droplet train impingement have been visualized using IR thermal imaging and high speed optical imaging techniques, respectively. The double droplet train impingement process was also simulated numerically using the Coupled Level Set-Volume of Fluid (CLS-VOF) approach with dynamic mesh adaption (DMA). Humps were observed both numerically and experimentally between two adjacent impact craters due to the interactions caused by the impinging droplet trains. It was found that the hump height decreased when impact spacing increased. IR images show that higher impact spacing leads to better heat transfer performance, which could be due to the lower hump height at greater impact spacing conditions. It was also observed that higher impact spacing leads to better thermo-hydrodynamics within and outside the impingement zone. In summary, results show that horizontal impact spacing plays a significant role in double droplet train impingement cooling. This work was supported by the National Priority Research Program of the Qatar National Research Fund, Grant No.: NPRP 6-1304-2-525.
TOPICS: Hydrodynamics, Heat transfer, Drops, Visualization, Trains, Imaging, Thermohydrodynamics, Thin films, Generators, Sapphire, Impingement cooling, Fluids, Coating processes, Coatings
Dong Hwan Shin, Yeonghwan Kim, Jin Sub Kim, Do Won Kang, Jeong Lak Sohn and Jungho Lee
J. Heat Transfer   doi: 10.1115/1.4040394
Flow visualization was performed to give a physical insight with vortical structures of an axisymmetric impinging jet on a concave surface. High-speed imaging was employed to get clear images with a laser light sheet illumination. An axisymmetric jet is issued into quasi-ambient air through a straight pipe nozzle with fully-developed velocity profile. A regular vertical pattern of an axisymmetric jet was observed with different flow entrainment rate. While an impinged jet turns to convert a wall jet along a concave surface, the flow interaction between the large-scale toroidal vortex and the concave surface was observed in the transition between the stagnation and wall jet zone. The ring-shaped wall eddies induced from a pair of toroidal vortices were also appeared to diverge into the radial direction along the concave surface. As the jet Reynolds number increases, small-scale vortices can be developed to a large-scale toroidal vortex. The location in which a large-scale toroidal vortex strikes is generally identical to the location where the secondary peak in heat transfer occurs. The frequency of large scale toroidal vortex on concave surface is found to be nearly similar as that of wall jet on flat surface. As the nozzle-to-target spacing (L/D) increases, it becomes shorter due to the loss of jet momentum. The flow behavior of axisymmetric impinging jet on a concave surface can be helpful to design the internal passage cooling for gas turbine blade.
TOPICS: Flow visualization, Vortices, Flow (Dynamics), Nozzles, Pipes, Design, Gas turbines, Heat transfer, Cooling, Lasers, Eddies (Fluid dynamics), Reynolds number, Blades, Imaging, Momentum

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