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# Accepted Manuscripts

BASIC VIEW  |  EXPANDED VIEW
Technical Brief
J. Heat Transfer   doi: 10.1115/1.4038480
A Phase change heat transfer from liquid to gas is studied in nanoscopic framework using molecular dynamics. Water on structured Si substrate is observed from molecular viewpoint after employing heat flux at a constant rate. Initially, we observe that water settles down on the substrate occupying the free space within the notch to obtain its static shape maintaining intra molecular configuration based on attractive and repulsive forces in neighboring hydroxyl bonds. Upon applying heat flux we observe that molecular vibration increases which repels neighbors to make the packing loose. Molecular dilution initiated at the notch and then proceeds to the rest domain. Progressive loosening of the molecules leads to the formation of vapor bubbles which increase in size with time. The rate of growth of this bubble is studied as a function of surface geometry parameters such as notch height, notch width, notch type and notch spacing. Present simulations enrich the knowledge of surface characteristics on boiling heat transfer from fundamental principle in the molecular domain.
TOPICS: Heat transfer, Boiling, Molecular dynamics, Bubbles, Water, Heat flux, Engineering simulation, Vibration, Geometry, Molecular configurations, Shapes, Vapors, Vacuum, Packing (Shipments), Simulation, Packings (Cushioning)
Technical Brief
J. Heat Transfer   doi: 10.1115/1.4038477
Non Equilibrium Molecular Dynamics (NEMD) simulations have been performed to understand the evaporation of a liquid droplet in the presence of a solid nanoparticle. The influence of solid-liquid interaction strength ($\varepsilon_{sl}$) on the evaporation properties was addressed. The system consists of a solid nanoparticle (Platinum) engulfed in a droplet (Argon) in Argon vapor environment. After the equilibration of this nanoparticle embedded droplet with its vapor, the boundary of this system is heated continuously to evaporate the droplet. It is observed that the addition of a nanoparticle to the droplet resulted in a slower evaporation rate when compared to that of a pure droplet. It was found that the evaporation rate of the droplet is decreased with increasing solid-liquid interaction strength ($\varepsilon_{sl}$) and those liquid atoms around the solid nanoparticle with higher $\varepsilon_{sl}$ are able to delay evaporation even at higher temperature owing to its decreased interfacial resistance. In order to analyse further on the vibrational coupling of the solid and liquid atoms, the vibrational density of states (VDOS) of the solid atoms is studied. It is observed that for higher values of $\varepsilon_{sl}$, the particle is able to retain a structured layer of liquid even at high temperature and also a higher heat input is necessitated to break the interaction strength of the liquid molecules around the solid nanoparticle, which makes it possible in delaying the complete evaporation of the droplet.
TOPICS: Drops, Evaporation, Nanoparticles, Atoms, Vapors, Particulate matter, Simulation, Equilibrium (Physics), Molecular dynamics, Density, Heat, Temperature, Engineering simulation, Delays, Platinum, Interfacial thermal resistance, High temperature
research-article
J. Heat Transfer   doi: 10.1115/1.4038481
Most of the existing empirical correlations for wall heat transfer during flow boiling show a limited predictability stemming mainly from so-called the suppression and enhancement factors, which are introduced to describe the boiling heat transfer hypothetically by a combination of nucleate pool boiling and single phase forced convection. There is no physical basis strongly supporting the determination of these factors. This study, to avoid such limitations, presents a distinctive approach to the modeling of wall boiling heat transfer utilizing the physical concept of wall heat flux partitioning. A new correlation of local boiling heat transfer coefficient is composed of primary heat transfer mechanisms of transient conduction and forced convection. Heat transfer areas of these mechanisms replace the suppression and enhancement factors in the new correlation and are determined empirically by dimensionless analysis. Based on an experimental database of 3,187 points collected for saturated boiling of various working fluids flowing inside channels of different configurations, the new correlation is obtained and compared with existing correlations widely used. The evaluation highlights much better predictability of the present correlation. While the other correlations show relatively large scattering with over 30% deviation from the experimental data, the newly proposed correlation shows an excellent agreement with a deviation of less than 10%. The good predictability would be from the well-structured physical basis and make the new correlation promising in practical boiling heat transfer analysis.
TOPICS: Boiling, Flow (Dynamics), Heat transfer, Forced convection, Modeling, Databases, Nucleate pool boiling, Transient heat transfer, Heat flux, Heat transfer coefficients, Scattering (Physics), Fluids, Radiation scattering, Electromagnetic scattering
research-article
J. Heat Transfer   doi: 10.1115/1.4038478
Natural convection heat transfer from a vertical hollow cylinder suspended in air has been analyzed numerically by varying the Rayleigh number (Ra) in laminar (10^4 = Ra = 10^8) regime. The simulations have been carried out by changing the ratio of length to pipe diameter (L/D) in the range of 1<=L/D<=20. Full conservation equations have been solved numerically for a vertical hollow cylinder suspended in air using algebraic multi-grid solver of FLUENT 13.0. The flow field around the vertical hollow cylinder have been observed to be pretty interesting and visually stimulating for small and large L/D. It has been found that the average Nusselt number(Nu) for vertical hollow cylinder suspended in air increases with the increase in Rayleigh number and the Nu for both the inner and the outer surface also increases with Ra . However, with the increase in L/D, average Nu for the outer surface increases almost linearly whereas the average Nu for the inner surface decreases quadratically and attains asymptotic value at lower L/D for low Ra. In this study the effect of parameters like L/D and Ra on Nu are analyzed, and a correlation for average Nusselt number has been developed for laminar regime. These correlations are accurate to the level of ±6% .
TOPICS: Heat transfer, Natural convection, Numerical analysis, Cylinders, Rayleigh number, Engineering simulation, Simulation, Algebra, Pipes, Flow (Dynamics)
research-article
J. Heat Transfer   doi: 10.1115/1.4038479
The present work establishes an improved experimentally-validated analysis to predict performance and exergy-related parameters of a mechanical draft cooling tower involving wooden splash fills. Unlike earlier studies, which account for the effect of at most three tower inlet parameters for the exergy analysis, the present study simultaneously considers all five inlet parameters affecting the tower exergy performance. To simultaneously predict outlet air and water conditions, an optimization algorithm involving discrete functions of dry and wet bulb temperatures is used in conjunction with the mathematical model derived from mass and energy conservations within the control volume involving Bosnjakovic correlation. From practical point of view, five inlet parameters such as; dry-bulb temperature, relative humidity, water temperature, water and air flow rates are selected for the exergy analysis. Thereafter, the influence of all inlet parameters on the tower performance is analyzed on various important exergy-related factors. The quantitative analysis reveals that the inlet air humidity, water inlet temperature and the inlet water mass flow rate significantly influence the air and water exergy changes. The present study also reveals that among the five inlet parameters, the water temperature, air humidity and air mass flow rate are primarily responsible for the exergy destruction. Furthermore, it is observed that the second law efficiency is mainly governed by the inlet air flow rate. The present study is proposed to be useful for selecting the tower inlet parameters to improve exergy performance of mechanical cooling towers.
TOPICS: Exergy, Cooling towers, Water, Temperature, Water temperature, Air flow, Flow (Dynamics), Exergy analysis, Optimization algorithms
research-article
J. Heat Transfer   doi: 10.1115/1.4038449
A general expression has been obtained to estimate thermal conductivities of both stochastic and periodic structures with high solid thermal conductivity. An air layer partially occupied by slanted circular rods of high thermal conductivity was considered to derive the general expression. The thermal conductivity based on this general expression was compared against that obtained from detailed 3-dimensional numerical calculations. A good agreement between two sets of results substantiates the validity of the general expression for evaluating the stagnant thermal conductivity of the periodic structures. Subsequently, this expression was averaged over a hemispherical solid angle to estimate the stagnant thermal conductivity for stochastic structures such as a metal foam. The resulting expression was found identical to the one obtained by Hsu et al., Krishnan et al., and Yang and Nakayama. Thus, the general expression can be used for both stochastic and periodic structures.
TOPICS: Thermal conductivity, Periodic structures, Rods, Metal foams
research-article
J. Heat Transfer   doi: 10.1115/1.4038451
We focus on the entropy generation minimization for the flow of a viscoelastic fluid through a parallel plate microchannel, under the combined influences of applied pressure gradient, interfacial slip and conjugate heat transfer. We use the simplified Phan-Thien-Tanner model (s-PTT) to represent the rheological behaviour of the viscoelastic fluid. Using the thermal boundary conditions of third kind, the transport equations are solved analytically to obtain the velocity and temperature distributions in the flow field, which are further used to calculate the entropy generation rate in the analysis. In this study, the influential role of following dimensionless parameters on entropy generation rate is examined: the viscoelastic parameter, slip coefficient, channel wall thickness, thermal conductivity of the wall, Biot number and Peclet number. We show that, there exists particular value of the above mentioned parameters, leading to a minimum entropy generation of the system. We believe that results of this analysis could be of helpful in optimum design of microfluidic system/devices, typically used in thermal management such as in electronic device, micro reactor, micro heat exchanger etc.
TOPICS: Entropy, Viscoelastic fluids, Flow (Dynamics), Heat transfer, Wall thickness, Microchannels, Rheology, Thermal conductivity, Microfluidics, Design, Heat exchangers, Boundary-value problems, Pressure gradient, Temperature distribution, Thermal management
research-article
J. Heat Transfer   doi: 10.1115/1.4038415
In this study different nanofluids were developed by mixing a molten salt mixture (60% NaNO3-40% KNO3) with 1.0 wt.% of silica-alumina nanoparticles using different methods. New mixing procedures without sonication were introduced with the aim to avoid the sonication step and to allow the production of a greater amount of nanofluid with a procedure potentially more suitable for large-scale productions. For this purpose, two mechanical mixers and a magnetic stirrer were used. Each nanofluid was prepared in aqueous solution with a concentration of 100g/l. The effect of different concentrations (300g/l and 500g/l) was also studied with the most effective mixer. Specific heat, melting temperature and latent heat were measured by means of Differential Scanning Calorimeter (DSC). Thermal conductivity in the solid state was also evaluated. The results show that the highest increase of the specific heat was obtained with 100 g/l both in solid (up to 31%) and in liquid phase (up to 14%) with the two mechanical mixers. The same nanofluids also showed higher amount of stored heat. An increase in thermal conductivity and diffusivity was also detected for high solution concentrations with a maximum of 25% and 47% respectively. SEM and EDX revealed a smaller grain size in the nanofluids respect to salt mixture. A better nanoparticles distribution is achieved with the lowest concentration. Nanofluids with enhanced thermal properties can be synthesized in a cost-effective form in high concentrated aqueous solutions by using mechanical mixers.
TOPICS: Concentrating solar power, Nanofluids, Thermal conductivity, Nanoparticles, Specific heat, Heat, Temperature, Melting, Thermal properties, Grain size, Latent heat
research-article
J. Heat Transfer   doi: 10.1115/1.4038420
Natural convection during solidification of liquids is known to impact the freezing characteristics and also lead to defect formation. In this study, we report the findings of real-time interferometric observation of bottom-cooled solidification of pure water in a cubical cavity. The results show first quantitative evidence of full-field thermal history during solidification, clearly depicting the anomalous expansion of water below 4°C. Furthermore, based on the strength of natural convection, characterized by the Rayleigh number, we identify and report four distinct regimes of solidification, namely - conduction dominated, early convection, front instability and sustained convection. A critical Rayleigh number that initiates an instability in the solidifying front has been proposed, which is significantly different from conventional calculations of Rayleigh number relating to the initiation of flow. The study shows full-field quantitative evidence of a well-known phenomenon and provides a further understanding of flow driven non-homogeneities in the solidifying interfaces.
TOPICS: Density, Convection, Solidification, Water, Rayleigh number, Natural convection, Flow (Dynamics), Freezing, Heat conduction, Cavities
research-article
J. Heat Transfer   doi: 10.1115/1.4038421
In this study, the flow field and the local and average heat transfer characteristics of jet impingement cooling with supercritical pressure fluids were studied experimentally with carbon dioxide firstly. An integrated thermal sensor chip that provided heating and temperature measurements was manufactured using MEMS (Micro-Electro-Mechanical Systems) techniques with a low thermal conductivity substrate as the impingement cooled plate. The experiment system pressure was 7.85 MPa, which is higher than the critical pressure of carbon dioxide of 7.38 MPa. The mass flow rate ranged from 8.34 to 22.36 kg/h and the Reynolds number ranged from 19000 to 68000. The heat flux ranged from 0.02 to 0.22 MW/m2. The nozzle inlet temperature ranged from lower to higher than the pseudocritical temperature. Dramatic variations of the density at supercritical pressures near the heating chip were observed with increasing heat flux in the strong reflection and refraction of the backlight that disappeared at inlet temperatures higher than the pseudocritical temperature. The local heat transfer coefficient near the stagnation point increased with increasing heat flux while those far from the stagnation point increased to a maximum with increasing heat flux and then decreased due to the non-uniformity of jet impingement cooling. The heat transfer is higher at inlet temperatures lower than the pseudocritical temperature and the surface temperature is slightly higher than the pseudocritical temperature due to the dramatic changes in the fluid thermo-physical properties at supercritical pressures.
TOPICS: Impingement cooling, Carbon dioxide, Temperature, Heat flux, Pressure, Flow (Dynamics), Heat transfer, Fluids, Heating, Microelectromechanical systems, Thermal conductivity, Nozzles, Heat transfer coefficients, Sensors, Temperature measurement, Reflection, Reynolds number, Refraction, Density
research-article
J. Heat Transfer   doi: 10.1115/1.4038422
Numerical study of nanofluid jet impingement cooling of a partially elastic isothermal hot surface was conducted with finite element method. The impingement surface was made partially elastic and effects of Reynolds number (between 25 and 200), solid particle volume fraction (between 0.01 and 0.04), elastic modulus of isothermal hot surface (between 10$^4$ and 10$^4$), size of the flexible part (between 7.5$w$ and 25$w$) and nanoparticle type (spherical, cylindrical, blade) on the fluid flow and heat transfer characteristics were analyzed. It was observed that average Nusselt number enhances for higher Reynolds number, higher values of elastic modulus of flexible wall, smaller size of elastic part and higher nanoparticle solid volume fraction and for cylindrical shaped particles. It is possible to change the maximum Nusselt number by 50.58$\%$ and 33$\%$ by changing the elastic modulus of the hot wall and size of elastic part whereas average Nusselt number changes by only 9.33$\%$ and 6.21$\%$. The discrepancy between various particle shapes is higher for higher particle volume fraction.
TOPICS: Cooling, Nanofluids, Particulate matter, Elastic moduli, Reynolds number, Nanoparticles, Impingement cooling, Blades, Fluid dynamics, Heat transfer, Finite element methods, Shapes
research-article
J. Heat Transfer   doi: 10.1115/1.4038423
Selective Laser Melting (SLM) is a widely used powder-based additive manufacturing process. However, it can be difficult to predict how process inputs affect the quality of parts produced. Computational modeling has been used to address some of these difficulties, but a challenge has been accurately capturing the behavior of the powder in a large, bed-scale model. In this work, a multi-scale melting model is implemented to simulate the melting of powder particles for SLM. The approach employs a particle-scale model for powder melting to develop a melt fraction-temperature relationship for use in bed-scale simulations of SLM. Additionally, uncertainties from the particle-scale are propagated through the relationship to the bed scale, thus allowing particle-scale uncertainties to be included in the bed-scale uncertainty estimation. Relations, with uncertainty, are developed for the average melt fraction of the powder as a function of the average temperature of the powder. The utility of these melt fraction-temperature relations is established by using them to model phase change using a continuum bed-scale model of the SLM process. It is shown that the use of the developed relations captures partial melt behavior of the powder that a simple melting model cannot. Furthermore, the model accounts for both uncertainty in material properties as well as packing structure in the final melt fraction-temperature relationship, unlike simple melting models. The developed melt fraction-temperature relations may be used for bed-scale SLM simulations with uncertainty due to particle effects.
TOPICS: Particulate matter, Modeling, Uncertainty, Melting, Temperature, Simulation, Engineering simulation, Additive manufacturing, Packings (Cushioning), Materials properties, Lasers, Computer simulation, Packing (Shipments)
Technical Brief
J. Heat Transfer   doi: 10.1115/1.4038425
This paper presents a novel adaptive thermal management technique to improve the efficiency of Solid State Power Amplifier (SSPA) for Geo Synchronous Satellites. The thermal management for space segment is very important as it determines the reliability of the satellite. The Microwave Power Amplifiers (MPAs), either Travelling Wave Tube Amplifiers (TWTAs) or SSPAs, are the maximum power consuming and heat dissipative elements in the satellite and their power efficiency determines weight, volume, cost and reliability of the satellite. So it is necessary to improve the efficiency of SSPA. A novel technique is presented which improves the efficiency of the SSPA and hence, saving of costly DC power generation on-board and reduction of the heat dissipation.
TOPICS: Thermal management, Satellites, Heat, Reliability, Energy dissipation, Energy efficiency, Energy generation, Microwaves, Traveling waves, Weight (Mass)
Technical Brief
J. Heat Transfer   doi: 10.1115/1.4038419
Air-side heat transfer and flow friction characteristics of four different fin patterns used in flat tube bank fin heat exchangers are investigated experimentally. These fin patterns are the fin with six dimples, the fin with nine dimples, the double louvered fin, and the fin with delta-winglet vortex generators (VGs). The counterpart plain fins (plain fin I and plain fin II) are selected as references for evaluating the heat transfer enhancement performance of the studied fin patterns under identical pump power constraints. The fin with the six dimples behaves better in terms of heat transfer performance than that with nine dimples for the studied cases. The heat transfer performance of the fin with delta-winglet VGs is better than that of the double louvered fin, and the performance of the latter is better than that of the fins with six and nine dimples. Correspondingly, in the tested range of Reynolds numbers, the thermal performance factors of the four different fin patterns, the fin with six dimples, the fin with nine dimples, the double louvered fin, and the fin with delta-winglet vortex generators (VGs), are 1.4-1.6, 1.3-1.6, 1.2-1.3 and 1.1-1.2, respectively. The correlations of Nusselt number and friction factor with Reynolds number for the fins with six/nine dimples and the double louvered fin are obtained. These correlations would be used to design flat tube bank ?n heat exchangers.
TOPICS: Heat transfer, Heat exchangers, Fins, Generators, Reynolds number, Friction, Vortices, Flow (Dynamics), Design, Pumps
research-article
J. Heat Transfer   doi: 10.1115/1.4038359
The basic theme of this investigation is to analyze heat and mass transport for three dimensional (3D) stagnation flow of nanofluid caused by an exponentially stretched surface when water is treated as base fluid. In this study we invoked the boundary layer phenomena and suitable similarity transformation of exponential character, as a result our three dimensional non-linear equations of momentum and energy are transmuted into nonlinear and non-homogeneous differential equations involving ordinary derivatives. Final equations are than puzzled out by applying homotopy analysis technique. Interesting outcomes of aggressing parameters involved in this study, effecting profiles of temperature field and velocity are explained in detail. Different aspects of skin friction coefficient as well as Nusselt number are calculated at the end. Graphical results of involved parameters appearing in considered nanofluid are presented separately.
TOPICS: Heat transfer, Nanofluids, Stagnation flow, Water, Nonlinear equations, Momentum, Heat, Temperature, Fluids, Skin friction (Fluid dynamics), Boundary layers, Differential equations, Performance
research-article
J. Heat Transfer   doi: 10.1115/1.4038339
This paper presents a novel transient method for calibrating heat transfer gauges for convective wall heat flux measurements in high enthalpy flows. Previously only steady-state methods have been considered. These require cooling, which adds complexity and expense to the experimental design. The new method is simple, inexpensive, easy to adapt for different flow configurations and sensor geometries and quick to run across a wide range of conditions. The theory of the convection sensitivity of circular foil gauges is summarised as a basis for the design of the new calibration facility. The experimental design, method and data processing techniques are then presented along with some preliminary experimental results.
TOPICS: Flow (Dynamics), Heat transfer, Gages, Calibration, Enthalpy, Transients (Dynamics), Convection, Heat flux, Experimental design, Steady state, Sensors, Cooling, Design
research-article
J. Heat Transfer   doi: 10.1115/1.4038338
An experimental program is presented of heated tension springs in an external cross-flow over a range of laminar Reynolds numbers, spring stretch ratios, and angles of attack. Extensive measurements of the forced-convection heat transfer of helical wire within a wind tunnel reveal an interesting non-monotonic dependence on angle of attack. Computational fluid dynamics (CFD) simulations, showing good agreement with the experimental data, are used to explore the behavior and gain a better understanding of the observed trends. A dimensionless correlation is developed that well-captures the experimental and CFD data and can be used as an efficient computational tool in broader applications.
TOPICS: Heat transfer, Springs, Computational fluid dynamics, Engineering simulation, Forced convection, Tension, Wind tunnels, Cross-flow, Reynolds number, Wire, Simulation
research-article
J. Heat Transfer   doi: 10.1115/1.4038231
In nuclear packed pebble beds, it is a fundamental task to model effective thermal conductivity (ETC) of thermal radiation. Based on the effective heat transfer cells of structured packing, a short-range radiation model (SRM) and a sub-cell radiation model (SCM) are applied to obtain analytical results of ETC. It is shown that the SRM of present effective heat transfer cells are in good agreement with the numerical simulations of random packing and it is only slightly higher than empirical correlations when temperature exceeds 1200°C. In order to develop a generic theoretical approach of modeling ETC, the sub-cell radiation model is presented and in good agreement with Kunii-Smith correlation, especially at very high temperature ranges (over 1500°C). Based on SCM, one-dimensional radial heat transfer model is applied in the analysis of the HTTU experiments. The results of effective thermal conductivity and radial temperature distribution are in good agreement with the experimental data
TOPICS: Thermal radiation, Thermal conductivity, Modeling, Heat transfer, Radiation (Physics), Packing (Shipments), Packings (Cushioning), Computer simulation, Temperature distribution, High temperature, Temperature
research-article
J. Heat Transfer   doi: 10.1115/1.4038233
A numerical investigation of a single highly confined bubble moving through a millimeter-scale channel in the absence of phase change is presented. The simulation includes thermal boundary conditions designed to match those of completed experiments. The channel is horizontal with a uniform-heat-generation upper wall and an adiabatic lower boundary condition. The use of a Lagrangian framework allows for the simulation of a channel of arbitrary length using a limited computational domain. The liquid phase is a low-Reynolds-number laminar flow, and the phase interactions are modeled using the Volume-of-Fluid method with full geometric reconstruction of the liquid/gas interface. Results are presented for three bubble diameters, which include two levels of confinement within the channel, and two liquid flow rates. Bubble shape and speed closely match experimental observations for each bubble size and liquid flow rate. Nusselt numbers in the bubble wake for all configurations follow a power law relationship with distance behind the bubble. Important dynamical structures include a pair of vortical structures at the rear of the bubble associated with the primary heat transfer enhancement and a pair of prominent liquid jets oriented in the transverse direction on either side of the bubble.
TOPICS: Pressure, Flow (Dynamics), Heat transfer, Bubbles, Channel flow, Boundary-value problems, Simulation, Wakes, Jets, Fluids, Laminar flow, Heat, Shapes
research-article
J. Heat Transfer   doi: 10.1115/1.4038210
The widely used gas turbine combustor double-walled cooling scheme relies on very small pedestals. In a combustor it is impractical for CFD to resolve each pedestal individually as that would require a very large amount of grid points and consequent excessive computation time. These pedestals can be omitted from the mesh and their effects captured on the fluid via a pedestal sub-grid scale (SGS) model. The aim is to apply the SGS approach, which takes into account the effects on pressure, velocity, turbulence and heat transfer, in an unstructured CFD code. The flow inside a 2-D and 3-D plain duct is simulated to validate the pedestal SGS model and the results for pressure, velocity and heat transfer are in good agreement with the measured data. The resolved flow in the combustor pedestal tile geometry is numerically investigated using RANS and LES in order to first assess the viability of the RANS and LES to predict the impinging flow and second to provide more validation data for the development of the SGS pedestal correlations. The LES provides more details of the impinging flow features. The pedestal SGS approach is then applied to the complete tile to replace the pedestals. The results are close to both the fully resolved CFD and the measurements.
TOPICS: Flow (Dynamics), Impingement cooling, Modeling, Combustion chambers, Tiles, Computational fluid dynamics, Pressure, Reynolds-averaged Navier–Stokes equations, Heat transfer, Cooling, Fluids, Turbulence, Computation, Ducts, Geometry, Gas turbines