Accepted Manuscripts

Fabio Villa, Marco Marengo and Joël De Coninck
J. Heat Transfer   doi: 10.1115/1.4041708
Heat pipe characteristics are linked to the surface properties of the diabatic surfaces, and, in the evaporator, surface properties influence both the onset boiling temperature (TONB) and the critical heat flux (CHF). In this work the effect of surface wettability in pool boiling heat transfer is studied in order to understand if there could be a path to increment heat pipe thermal performance. This work analyses the effects of surface wettability on boiling (tested fluid is pure water) and proposes a new super-hydrophobic polymeric coating [1], which can have a very important effect in improving the heat pipe start-up power load and increasing the thermal performance of heat pipes when the flux is lower than the critical heat flux. The polymeric coating is able to reduce the TONB (-11% from 117°C to about 104°C) compared with the uncoated surfaces, as it inhibits the formation of a vapour film on the solid-liquid interface, avoiding CHT conditions up to maximum wall temperature (125°C). This is realized by the creation of a heterogeneous surface with SHS zones dispersed on top of a hydrophilic surface (stainless steel surface). The proposed coating has an outstanding thermal resistance: No degradation of SH properties of the coating has been observed after more than 500 thermal cycles.
TOPICS: Heat pipes, Coating processes, Coatings, Critical heat flux, Boiling, Surface properties, Cycles, Pool boiling, Stainless steel, Thermal resistance, Wall temperature, Water, Stress, Temperature, Heat transfer, Fluids
Ruming Pan, Bachirou Guene Lougou, Yong Shuai, Guohua Zhang and Hao Zhang
J. Heat Transfer   doi: 10.1115/1.4041707
In this paper, heat transfer modeling of a high-temperature porous-medium filled solar thermochemical reactor for hydrogen and synthesis gas production is investigated. The numerical simulation is performed using a 3D numerical model and surface-to-surface radiation model coupled to Rosseland approximation for radiation heat transfer. The effects of operating conditions and the porous structural parameters on the reactor thermal performance were investigated significantly. It was found that large axial temperature gradient and high-temperature distribution throughout the reactor were strongly dependent on the operating conditions. The inlet gas temperature has remarkable effects on the temperature distribution. The thermal performance of porous-medium filled solar thermochemical reactor could be improved by preheating the inlet gas up to 393.15 K. Moreover, a correlation was established between the protective gas inlet velocity and the porosity of porous media. The temperature difference decreased with the increase in the porosity of the inner cavity of the reactor. In contrast to the front and back part of the inner cavity of the reactor, higher temperature distribution could be obtained in the porous region by increasing the average cell diameters of porous media.
TOPICS: Heat transfer, Modeling, Solar energy, High temperature, Porous materials, Radiation (Physics), Computer simulation, Temperature, Cavities, Porosity, Temperature distribution, Hydrogen, Temperature gradient, Syngas, Approximation
Pedro Resende, Alexandre M. Afonso, Carlos Pinho and Mohsen Ayoobi
J. Heat Transfer   doi: 10.1115/1.4041623
Combustion characteristics at small scales have been studied continuously due to the potential applications in portable power devices. It is known that heat release impacts at small scales result in different flame behavior as compared to conventional scales. The impacts of geometry, stoichiometry, flow rates, wall temperatures, etc. are widely studied in literature. However, dilution impacts still need to be further studied due to its important role on controlling the flame behavior and subsequent pollutants emissions at these scales. In this work, premixed hydrogen/air combustion is simulated at an axis-symmetric micro channel (diameter D=0.8mm and length L=10mm), where detailed chemical kinetics are implemented in simulations. The heat transfer on the wall is considered by imposing a hyperbolic temperature profile on the wall, where the wall temperature increases from 300 K at the inlet to 1300 K at the outlet. With this setup, a range of equivalence ratios including a typical fuel-lean regime (?=0.7), stoichiometric regime (?=1.0) and and two cases at an ultra-rich regime (?=2.0 and ?=3.0) are investigated. For each equivalence ratio, excess dilution (using N2) is introduced to the mixture and its impact is compared with other cases. With that, the impacts of dilution variations on the combustion characteristics of premixed hydrogen/air are investigated for different equivalence ratios. More specifically, several incidents such as flame dynamics, flame stabilization, extinctions and NOx emissions are studied for the aforementioned operating conditions.
TOPICS: Combustion, Hydrogen, Nitrogen oxides, Emissions, Flames, Wall temperature, Pollution, Microchannels, Geometry, Stoichiometry, Temperature profiles, Fuels, Simulation, Engineering simulation, Dynamics (Mechanics), Flow (Dynamics), Heat, Heat transfer, Chemical kinetics
Ramin Zakeri, Ramin Kamali-Moghadam and Mahmood Mani
J. Heat Transfer   doi: 10.1115/1.4041552
A new chemical model in the DSMC algorithm, entitled by Modified Collision Energy (MCE), has been developed for simulation of reactive rarefied flows without some limitations of the conventional macroscopic models. Determination of correct values of the experimental parameters for computing the Arrhenius reaction rate is a serious challenge in some macroscopic chemical reaction models such as Total Collision Energy (TCE) and General Collision Energy (GCE). The proposed MCE model in the present study is a reliable method to properly determine values of these parameters for all types of gases with the intrinsic properties of the particles and without need of any experimental data. Extraction of constant parameters has been carried out usingthe analytical method and numerical Quantum Kinetics (QK) or Modified Quantum Kinetics (QK) models. The proposed MCE method has been evaluated infourtest cases,including assessment of the reaction rate in equilibrium and non-equilibrium conditions, study of the rarefied flow along the stagnation line and investigation of the hypersonic gas flow over the axial symmetry blunt nose. The results show that the proposed method has desirable accuracy without using any experimental parameters.The MCE method can also be used to calibrate the macroscopic reactive models such as the TCE and GCE.
TOPICS: Collisions (Physics), Algorithms, Equilibrium (Physics), Flow (Dynamics), Gases, Particulate matter, Simulation, Gas flow, Chemical reactions
Pooya Navid, Shirin Niroomand and Carey J. Simonson
J. Heat Transfer   doi: 10.1115/1.4041557
Saturation of the water vapor is essential to form frost inside a permeable membrane. The main goal of this paper is to develop a numerical model that can predict temperature and humidity inside a membrane in order to show the location and time of saturation. This numerical model for heat and mass transfer is developed to show that frost formation may be prevented or delayed by controlling the moisture transfer through the membrane which is the new approach in this paper. The idea is to simultaneously dry and cool air to avoid saturation conditions and thereby eliminate condensation and frosting in the membrane. Results show that saturation usually occurs on side of the membrane with the highest temperature and humidity. The numerical model is verified with experimental data and used to show that moisture transfer through the membrane can delay or prevent frost formation.
TOPICS: Delays, Membranes, Computer simulation, Temperature, Condensation, Water vapor, Mass transfer, Heat
Baghir Suleimanov, Hakim Abbasov, Fuad Valiyev, Rayyat Ismayilov and Shie-Ming Peng
J. Heat Transfer   doi: 10.1115/1.4041554
The thermal conductivity of microfluids comprising Ni3(µ3-ppza)4Cl2 metal string complex (MSC) microparticles in an aqueous glycerol solution was investigated using the transient hot-wire method. A comparative analysis of the thermal-conductivity enhancements of microfluids and nanofluids revealed that the best results were achieved using microparticles of monocrystalline metal string complexes Ni3(µ3-ppza)4Cl2 as well as Ni5(µ5-pppmda)4Cl2 micro- and copper nanoparticles. Compared to the base fluid, the thermal-conductivity enhancements were 72% for Ni3-water-glycerol, 53% for Cu-water-glycerol, and 47% for Ni5-water-glycerol. It is shown that the high thermal-conductivity enhancement achieved with Ni3 microfluids is a result of higher stability in compare with nanofluid due to the lower density of the microparticles and the formation of particle assemblies. Therefore, the formation of hydrogen bonds between the MSC particles (through their organic fragments) and water molecules, take place. Colloidal structure of Ni3-microfluids has a significant impact on it's thermophysical properties.
TOPICS: Metals, Particulate matter, String, Thermal conductivity, Water, Microparticles, Nanofluids, Hydrogen bonds, Transients (Dynamics), Nanoparticles, Wire, Copper, Density, Stability, Fluids
Ali Saleh Alshomrani and Malik Zaka Ullah
J. Heat Transfer   doi: 10.1115/1.4041553
This article presents Darcy-Forchheimer three dimensional (3D) flow of water-based carbon nanotubes (CNTs) with heterogeneous-homogeneous reactions. A bi-directional linear extendable surface has been employed to create the flow. Flow in porous space is represented by Darcy-Forchheimer expression. Heat transfer mechanism is explored through convective heating. Equal diffusion coefficients are considered for both auto catalyst and reactants. Results for single-wall (SWCNT) and multi-wall (MWCNT) carbon nanotubes have been presented and compared. The diminishment of partial differential framework into nonlinear ordinary differential framework is made through suitable transformations. Optimal homotopy scheme is used for arrangements development of governing flow problem. Optimal homotopic solution expressions for velocities and temperature are studied through plots by considering various estimations of physical variables. The skin friction coefficients and local Nusselt number are analyzed through plots. Our findings depict that the skin friction coefficients and local Nusselt number are enhanced for larger values of the nanoparticles volume fraction.
TOPICS: Flow (Dynamics), Carbon nanotubes, Skin friction (Fluid dynamics), Nanoparticles, Catalysts, Water, Heating, Multi-walled carbon nanotubes, Single-walled carbon nanotubes, Temperature, Diffusion (Physics), Heat transfer
Ivan Voytkov, Roman S. Volkov and Pavel A. Strizhak
J. Heat Transfer   doi: 10.1115/1.4041556
The optical techniques (Particle Image Velocimetry, Laser-Induced Phosphorescence, Planar Laser-Induced Fluorescence) are used to study unsteady and inhomogeneous temperature and velocity fields of a gas-vapor mixture forming in the immediate vicinity of rapidly evaporating water droplets. Experiments involve various arrangements of several (two, three, and five) water droplets in a heated air flow. We establish the dependencies of the temperature and velocity of a gas-vapor mixture in the trace of each droplet on the heating time, velocity and temperature of the air flow, initial dimensions and droplet arrangement scheme. Distinctive features of the synergistic effect of a droplet group on their temperature and aerodynamic traces are identified. Longitudinal and transversal dimensions of the aerodynamic and thermal traces of evaporating droplets are established. The length of the temperature trace of one droplet equals 10-12 of its radii, and the width of the temperature and aerodynamic trace of a droplet is no larger than its diameter.
TOPICS: Temperature, Vapors, Drops, Evaporation, Water, Lasers, Dimensions, Air flow, Fluorescence, Phosphorescence, Particulate matter, Heating
Technical Brief  
Edward Sun, Jun Ma, Srinivasa M. Salapaka and Sanjiv Sinha
J. Heat Transfer   doi: 10.1115/1.4041555
The recent development of flexible sensors that can measure temperatures at the surface of the skin opens novel possibilities for continuous health monitoring. Here, we investigate such sensors as 3-omega thermometers to non-invasively detect deep dermal dehydration. Using numerical simulations, we calculate the temperature rise at the sensor at heating frequencies from 10 mHz to 10 Hz at varying levels of dehydration. The heating power in each case is limited to avoid burn injury. Our results indicate that 10-100 mHz frequencies are necessary to detect deep dermal dehydration. We show that the root mean square difference in temperature rise between normal and dermally dehydrated skin can be as high as 250 mK, which is detectable using lock-in techniques. Thermal contact resistance between the sensor and skin can dominate the signal when the resistance exceeds ~10^-3 Km^2/W. This work provides quantitative limits for sensing human dehydration using non-invasive sensors that measure the thermal conductivity of the skin structure.
TOPICS: Temperature measurement, Skin, Sensors, Temperature, Heating, Contact resistance, Locks (Waterways), Thermometers, Wounds, Computer simulation, Thermal conductivity, Signals
TaeGyu Kim and Hyun-Ung Oh
J. Heat Transfer   doi: 10.1115/1.4041558
The feasibility of using a liquid metal with a high thermal conductivity as a functional fluid for realizing a variable conductance radiator for space applications was proposed and investigated. The variable thermal conductivity of the radiator can be achieved by moving the liquid metal using a magneto-hydraulic pump between the two reservoirs in accordance with the temperature conditions of the on-board equipment. The liquid metal radiator proposed in this study is much more effective for saving heater power under cold condition while effectively dissipating heat to deep space under hot condition. The thermal behavior of the liquid metal radiator was demonstrated using the ambient thermal tests under cooling and insulation modes of the radiator. The performance of the proposed variable conductance radiator was evaluated by comparing it with that of the conventional radiator whose conductivity value is fixed.
TOPICS: Liquid metals, Electrical conductance, Thermal conductivity, Pumps, Insulation, Heat, Temperature, Cooling, Fluids, Reservoirs
Matthew Ralphs, Chandler Scheitlin, Robert Y. Wang and Konrad Rykaczewski
J. Heat Transfer   doi: 10.1115/1.4041539
Thermally conductive soft composites are in high demand and aligning the fill material is a potential method of enhancing their thermal performance. In particular, magnetic alignment of nickel particles has previously been demonstrated as an easy and effective way to improve directional thermal conductivity of such composites. However, the effect of compression on the thermal performance of these materials has not yet been investigated. This work investigates the thermal performance of magnetically aligned nickel fibers in a soft polymer matrix under compression. The fibers orient themselves in the direction of the applied magnetic field and align into columns, resulting in a 3x increase in directional thermal conductivity over unaligned composites at a volume fraction of 0.15. Nevertheless, these aligned fiber columns buckle under strain resulting in an increase in the composite thermal resistance. These results highlight potential pitfalls of magnetic filler alignment when designing soft composites for applications where strain is expected such as thermal management of electronics.
TOPICS: Fibers, Fillers (Materials), Polymer composites, Buckling, Compression, Thermal resistance, Composite materials, Nickel, Thermal conductivity, Design, Polymers, Particulate matter, Magnetic fields, Electronics, Thermal management
S.Y. Misyura and Vladimir Morozov
J. Heat Transfer   doi: 10.1115/1.4041323
Evaporation of layers of aqueous solutions of salts (LiBr, CaCl2, NaCl, MgCl2, BaCl2, CsCl) is studied experimentally. Experimental data are compared with evaporation of the water layer. The solution is placed on a horizontal surface of a cylindrical heating section. Experiments on surface crystallization of salts are carried out. For aqueous solutions of salts LiBr, LiCl and CaCl2 there is an extremum for the heat transfer coefficient al. For water and for solutions of salts NaCl and CsCl the extremum is absent. The first factor is a decreasing function of time, and the second factor is an increasing function of time. For the water layer, both factors continuously increase with time, and the maximum evaporation rate corresponds to the final stage of evaporation. The heat balance for interface layer is made up. The role of the free gas convection in the heat balance strongly depends on the salt concentration and varies with the rise of evaporation time. For low salt concentrations the influence of free convection in the gas phase on heat transfer in the liquid phase can be neglected; however, for high concentrations this effect is comparable with other factors. The curves for the rate of crystallization have been built. More than two times difference between the experiment and the calculation is associated with the kinetics of dendritic structures.
TOPICS: Evaporation, Water, Heat, Crystallization, Convection, Heat transfer, Heating, Heat transfer coefficients, Natural convection
Amirhossein Mostafavi, Shunkei Suzuki, Sumeet Changla, Aditya Pinto, Shigetoshi Ipposhi and Donghyun Shin
J. Heat Transfer   doi: 10.1115/1.4041241
Recent studies have shown that doping nanoparticles into a molten salt eutectic can induce salt molecules to form a stelliform nanostructure that can enhance the effective heat capacity of the mixture. This phenomenon can result from a unique characteristic of a eutectic molten salt system, which can self-form a nanostructure on a nanoscale solid surface. Hence, such an enhancement was only observed in a molten salt eutectic. Similarly, a stelliform nanostructure can be artificially synthesized and dispersed in other liquids. Mixing polar-ended molecules with a nanoparticle in a medium can induce the polar-ended molecules ionically bonded to a nanoparticle to form a stelliform nanostructure. Hence, this may enhance the effective heat capacity of the mixture. In this study, we disperse various nanoparticles and polar-ended materials into a sodium acetate trihydrate at different ratios to explore the effect of nanoparticle type and concentration as well as polar-ended materials and their concentrations on the resultant heat capacity of sodium acetate trihydrate. The result shows the specific heat capacity was the highest with silica nanoparticle at 1 % concentration of weight and polar-ended material at 4 % concentration.
TOPICS: Specific heat, Sodium, Nanoparticles, Heat capacity, Nanoscale phenomena, Weight (Mass)
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
Salam Hadi and Mustafa Rahomey
J. Heat Transfer   doi: 10.1115/1.4039642
Numerical simulations are carried out for fluid flow and natural convection heat transfer induced by a temperature difference between a hot inner cylinder with different geometries (i.e. circular; triangular; elliptic; rectangular; and rhombic) and a cold outer square enclosure filled with nanofluid superposed porous-nanofluid layers. The Darcy-Brinkman model is applied for the saturated porous layer with nanofluid. Moreover, the transport equations (mass, momentum, and energy) are solved numerically using the Galerkin weighted residual method by dividing the domain into two sets of equations for every layer with incorporating a non-uniform mesh size. The considered domains in this investigation are closely examined over a wide range of Rayleigh number (103 = Ra= 106), Darcy number (10-5 = Da = 10-1), the thickness of porous layer (0% = Xp = 100%), thermal conductivity ratio (1 = Rk = 20) and nanoparticle volume fraction (0 = ? = 0.1), respectively. The nanofluid is considered to be composed of Cu-nanoparticle and water as a base fluid. The results showed that the obtained total surfaces-averaged Nusselt numbers of the enclosure, in all cases, at the same operating conditions, the rate of heat transfer from the enclosure which the triangular cylinder is located inside is better. Also, as the thickness of the porous layer is increased from 20% to 80%, the free convection performance will decrease significantly (to about 50%) due to the hydrodynamic properties of the porous material.
TOPICS: Natural convection, Circular cylinders, Cylinders, Nanofluids, Heat transfer, Nanoparticles, Thermal conductivity, Momentum, Fluid dynamics, Temperature, Fluids, Porous materials, Computer simulation, Rayleigh number, Water

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