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### Guest Editorial

J. Heat Transfer. 2016;138(11):110301-110301-2. doi:10.1115/1.4033972.
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Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2016;138(11):112001-112001-12. doi:10.1115/1.4033614.

The steady three-dimensional stagnation-point flow and heat transfer of a dusty fluid toward a stretching sheet is investigated by using similarity solution approach. The freestream along z-direction impinges on the stretching sheet to produce a flow with different velocity components. The governing equations are transformed into ordinary differential equations by introducing appropriate similarity variables and an exact solution is obtained. The nonlinear ordinary differential equations are solved numerically using Runge–Kutta fourth-order method. The effects of the physical parameters like velocity ratio, fluid and thermal particle interaction parameter, ratio of freestream velocity parameter to stretching sheet velocity parameter, Prandtl number, and Eckert number on the flow field and heat transfer characteristics are obtained, illustrated graphically, and discussed. Also, a comparison of the obtained numerical results is made with two-dimensional cases existing in the literature and good agreement is approved. Moreover, it is found that the heat transfer coefficient and shear stress on the surface for axisymmetric case are larger than nonaxisymmetric case. Also, for stationary flat plat case, a similarity solution is presented and a comparison of the obtained results is made with previously published results and full agreement is reported.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(11):112002-112002-8. doi:10.1115/1.4033537.

Convective heat transfer from a fluid to a surface is an approximately linear function of driving temperature if the properties within the boundary layer are approximately constant. However, in environments with large driving temperatures like those seen in the hot sections of gas turbine engines, significant property variations exist within the boundary layer. In addition, radiative heat transfer can be a significant contributor to the total heat transfer in a high-temperature environment such that it can not be neglected. As a result, heat transfer to the surface becomes a nonlinear function of driving temperature and the conventional linear heat flux assumption cannot be employed to characterize the convective heat transfer. The present study experimentally examines the nonlinearity of convective heat flux on a zero-pressure-gradient flat plate with large freestream to wall-temperature differences. In addition, the need to account for the radiative component of the overall heat transfer is highlighted. Finally, a method to account for the effects of both variable properties and radiation simultaneously is proposed and demonstrated. Overall, the proposed technique provides the means to quantify the independent contributions of radiative and variable property convective heat transfer to the total conductive heat transfer to or from a surface in a single experiment.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(11):112003-112003-4. doi:10.1115/1.4033880.

The thermal conductivity of five samples of crude oil and one sample of gas condensate was measured by the transient hot-wire technique. The measurements were made along isotherms (245, 250, 273, 295, 320, 336, and 373 K) in the pressure range from atmospheric pressure up to 1000 MPa and along isobars (at 0.1, 100, 200, 300, 400, 500, and 1000 MPa) in the temperature range 245–450 K. It was observed that the thermal conductivity of the samples investigated strongly depends on the pressure and rises with increasing pressure for all the temperatures. At a certain pressure, the temperature coefficient of thermal conductivity reverses from negative to positive. The pressure at which this reversal was observed varied in the range of 300–380 MPa.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(11):112004-112004-10. doi:10.1115/1.4033998.

Infrared thermal imaging based on active thermal excitations has been widely used for nondestructive evaluation (NDE) of materials. While the experimental systems have remained essentially the same during the last few decades, development of advanced data-processing methods has significantly improved the capabilities of this technology. However, many limitations still exist. One fundamental limitation is the requirement, either explicitly or implicitly, of the tested material to be homogeneous such that detected thermal contrasts may be used to determine an average material property or attributed to flaws. In this paper, a new thermal tomography (TT) method is introduced, which for the first time can evaluate heterogeneous materials by directly imaging their thermal-property variations with space. It utilizes one-sided flash thermal-imaging data to construct the three-dimensional (3D) distribution of thermal effusivity in the entire volume of a test sample. Theoretical analyses for single and multilayer material systems were conducted to validate its formulation and to demonstrate its performance. Experimental results for a ceramic composite plate and a thermal barrier coating (TBC) sample are also presented. It was shown that thermal diffusion is the primary factor that degrades the spatial resolution with depth for TT; the spatial resolutions in the lateral and axial directions were quantitatively evaluated.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(11):112005-112005-6. doi:10.1115/1.4033971.

The present study explores the influence of viscous dissipation, Joule heating, and double dispersion on unsteady, free convective magnetohydrodynamics (MHD) flow of an incompressible Casson fluid over a vertical cone and flat plate saturated with porous medium subject to variable viscosity and variable electrical conductivity. The governing coupled, nonlinear partial differential equations are solved by Crank–Nicolson method. The effects of various significant parameters on flow, heat, and mass transfer characteristics are displayed in the form of figures and tables. The results indicate that the presence of variable viscosity parameter meagerly accelerates the fluid flow. It is observed that heat transfer is enhanced for increasing the thermal dispersion parameter and Eckert number.

Commentary by Dr. Valentin Fuster

### Research Papers: Melting and Solidification

J. Heat Transfer. 2016;138(11):112301-112301-11. doi:10.1115/1.4033700.

In this paper, horizontal solidification of gallium in a rectangular cavity was studied both experimentally and numerically. Two three-dimensional (3D) numerical models related to different numerical approaches were built. The first is a single-domain (SD) model based on the volume-of-fluid (VOF) method. This model also takes into account the presence of a mushy zone. The second model is a multidomain (MD) one; it includes two different meshes for the two phases and uses Stephan's boundary condition to determine the front velocity. The 3D models were tested under various thermal boundary conditions and compared with experimental results obtained in an appropriate experimental setup. The experimental setup included an ultrasonic Doppler velocimeter (UDV) for noninvasive measurements of the velocities in the liquid part of the metal, liquid–solid interface position and profile, its displacement, and longitudinal mean velocity. For determining the boundary influence, both 3D and 2D models were built. The comparison was carried out for the solidification front location and shape and the velocity and temperature fields. In general, the 3D numerical model gave more accurate results than the 2D model with respect to the experiments results. Although the MD model is more complicated to build and requires more computational efforts than the VOF model, the 3D MD model provides the most accurate results in comparison with current experiments.

Commentary by Dr. Valentin Fuster

### Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2016;138(11):112501-112501-9. doi:10.1115/1.4031921.

Natural circulation loop (NCL) is simple and reliable due to the absence of moving components and is preferred in applications where safety is of foremost concern, such as nuclear power plants and high-pressure thermal power plants. In the present study, optimum operating conditions based on the maximum heat transfer rate in NCLs have been obtained for subcritical as well as supercritical fluids. In recent years, there is a growing interest in the use of carbon dioxide (CO2) as loop fluid in NCLs for a variety of heat transfer applications due to its excellent thermophysical environmentally benign properties. In the present study, three-dimensional (3D) computational fluid dynamics (CFD) analysis of a CO2-based NCL with isothermal source and sink has been carried out. Results show that the heat transfer rate is much higher in the case of supercritical phase (if operated near pseudocritical region) than the subcritical phase. In the subcritical option, higher heat transfer rate is obtained in the case of liquid operated near saturation condition. Correlations for optimum operating condition are obtained for a supercritical CO2-based NCL in terms of reduced temperature and reduced pressure so that they can be employed for a wide variety of fluids operating in supercritical region. Correlations are also validated with different loop fluids. These results are expected to help design superior optimal NCLs for critical applications.

Commentary by Dr. Valentin Fuster

### Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2016;138(11):112701-112701-11. doi:10.1115/1.4033699.

The discrete ordinates method is a popular and versatile technique for solving the radiative transport equation, a major drawback of which is the presence of ray effects. Mitigation of ray effects can yield significantly more accurate results and enhanced numerical stability for combined mode codes. When ray effects are present, the solution is seen to be highly dependent upon the relative orientation of the geometry and the global reference frame. This is an undesirable property. A novel ray effect mitigation technique of averaging the computed solution for various reference frame orientations is proposed.

Commentary by Dr. Valentin Fuster

### Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2016;138(11):111501-111501-8. doi:10.1115/1.4033796.

Droplets evaporation and boiling crisis of ethanol water solution were studied experimentally. At intensive nucleate boiling within a droplet, most evaporation relates to an increase in the area of the wetting droplet surface and only 10–20% of evaporation relates to the effect of diffusion and a change in the thermal–physical coefficients. In alcohol solution with mass salt concentration C0 = 25–35%, maximal instability of the bubble microlayer is observed. The critical heat flux behaves nonmonotonously due to changes in mass alcohol concentration in the solution, and there are two extrema. The maximal value of sustainability coefficient at droplets evaporation of ethanol solution corresponds to C0 of 25–30%. The heat transfer coefficient of ethanol water solution of droplet in the suspended state decreases with a rise of wall overheating and spheroid diameter. Experimental dependence of the vapor layer height on wall overheating at boiling crisis was observed. The height of this layer at Leidenfrost temperature was many times higher than the surface microroughness value. The liquid–vapor interface oscillates, and this extends the transitional temperature zone associated with a droplet's boiling crisis.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(11):111502-111502-12. doi:10.1115/1.4033743.

In this paper, we report on the recent development of an advanced microscale heat sink, termed as piranha pin fin (PPF). A 200 μm deep microchannel embedded with PPFs was fabricated and tested. Fluid flow and heat transfer performance were evaluated with HFE7000 as the working fluid. The surface temperature, pressure drop, heat transfer coefficient, and critical heat flux (CHF) conditions were experimentally obtained and discussed. A 676 W/cm2 CHF was achieved based on the heater area and at an inlet mass flux of 2460 kg/m2 s. Microchannels with different PPF configurations were investigated and studied for different flow conditions. It was found that a microchannel with PPFs can dissipate high heat fluxes with reasonable pressure drops. Flow conditions and PPF configuration played important roles for both fluid flow and heat transfer performances. These studies extended knowledge and provided useful reference for further PPF design in microchannel for flow boiling.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2016;138(11):111901-111901-16. doi:10.1115/1.4033747.

Detailed heat transfer and flow field investigations behind a surface-mounted slitted trapezoidal rib have been performed using liquid crystal thermography (LCT) and particle image velocimetry (PIV). In the accomplished experiments, the effects of varying the chamfering angle over the trailing edge of a rib with a centrally placed longitudinal continuous slit carrying an open area ratio equivalent to 25% were studied. The chamfering angle has been varied from 0 to 20 deg in a step of 5 deg. Experiments were carried out for four different Reynolds numbers ranging in between 9400 and 61,480, which were based upon the hydraulic diameter of the rectangular duct. The motive behind the present work is to systematically study the effect of change in chamfering angle of a trapezoidal rib with a centrally placed continuous slit over the flow and heat transfer parameters. Emphasis was made to identify the flow parameters responsible for augmentation in surface heat transfer coefficients (HTCs). Results are presented in terms of mean and rms velocity fields, stream traces, Reynolds stress, vorticity, and surface- and spanwise-averaged augmentation Nusselt number distribution. The reattachment length and the average augmentation Nusselt number have been evaluated for all of the different configurations. Entire configurations under selected range of Reynolds number led to the rise in heat transfer enhancement as against the flat surface without the rib. It is observed that slitted ribs cause shorter reattachment length and better heat transfer enhancement in the downstream vicinity of the rib. Further, the recirculation area behind the rib is enlarged to the point of spanning the nearby downstream vicinity of the rib ($x/e<4$), which signifies the zone of maximum heat transfer enhancement due to the effect of flow coming out of the slit. Salient critical points and foci of secondary recirculation patterns are extracted, which provides clues to the physical process occurring in the flow, which were responsible for the mixing enhancement behind slitted trapezoidal rib geometries.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2016;138(11):112401-112401-8. doi:10.1115/1.4033642.

It has recently been suggested that the accommodation coefficient of nano-aluminum/alumina particles may be significantly smaller than previously assumed. This result has significant implications on the heat transfer and performance of the nanoparticles in combustion environments. Currently, the accommodation coefficient has been deduced only after assuming a combustion model for the nano-aluminum particle and changing the accommodation coefficient to fit experimental temperature data. Direct measurement is needed in order to decouple the accommodation coefficient from the assumed combustion mechanism. Time-resolved laser-induced incandescence (TiRe-LII) measurements were performed to measure the accommodation coefficient of nano-alumina particles in various gaseous environments. The accommodation coefficient was found to be 0.03, 0.07, and 0.15 in helium, nitrogen, and argon, respectively, at 300 K and 2 atm in each environment. These values represent upper limits for the accommodation coefficient as it is expected to decrease with increasing ambient temperature. The values are similar to what has been seen for other metallic nanoparticles and significantly smaller than values used in soot measurements. The results will allow for additional modeling of the accommodation coefficient extended to other environments and support previous measurements of high combustion temperatures during nano-aluminum combustion.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(11):112402-112402-6. doi:10.1115/1.4033954.

A better understanding of submicron-scale heat transfer is rapidly gaining interest due to the complex phenomena involved in nanometer scales. We discuss the role of interfacial resistance, in particular that of curvature effects, and the possibility of achieving high temperatures inside the particles without creating a phase transition in the surrounding fluid. The heat transfer from a heated nanoparticle into surrounding fluid is studied using molecular dynamics (MD) simulations. The results show that the particle size and wetting strength between the nanoparticle–liquid influence the heat transfer characteristics. The interfacial conductance and Kapitza length for a model solid–liquid interface were calculated. Both quantities are found to be strongly dependent on particle size and temperature. Smaller nanoparticles are observed to have a stronger bonding with the interfacial fluid when the temperature of the particle is higher, while larger nanoparticles have better affinity with the liquid at lower temperatures.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(11):112403-112403-11. doi:10.1115/1.4033955.

Heat transfer and fluid flow through different microchannel geometries in the transitional regime (rarefied flow) are analyzed by means of direct simulation Monte Carlo (DSMC) simulations. Four types of three-dimensional microchannels, intended to be used as expansion slots in microresistojet concepts, are investigated using nitrogen as working fluid. The main purpose is to understand the impact of the channel geometry on the exit velocity and the transmission coefficient, parameters which are well known to affect directly the thruster performance. Although this analysis can be applied in principle to several possible microfluidics scenarios, particular focus is given to its application in the field of space propulsion for micro-, nano-, and picosatellites, for which the requirements ask for low thrust levels from some micronewtons to a few millinewtons and moderate specific impulse, as well as a low power consumption in the order of a few watts. Analysis shows that the thrust produced by one single microchannel can be increased by about 480% with a careful selection of the channel geometry, decreasing at the same time the specific impulse by just 5%, with a power consumption decrease of more than 66.7%.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Heat Transfer. 2016;138(11):114501-114501-6. doi:10.1115/1.4033809.

In the current paper, the nonlinear fin problem with temperature-dependent thermal conductivity and heat transfer coefficient is revisited. In this problem, it has been assumed that the heat transfer coefficient is expressed in a power-law form and the thermal conductivity is a linear function of temperature. It is shown that its governing nonlinear differential equation is exactly solvable. A full discussion and exact analytical solution in the implicit form are given for further physical interpretation and it is proved that three possible cases may occur: there is no solution to the problem, the solution is unique, and the solutions are dual depending on the values of the parameters of the model. Furthermore, we give exact analytical expressions of fin efficiency as a function of thermogeometric fin parameter.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2016;138(11):114503-114503-8. doi:10.1115/1.4034039.

This paper presents the motion of unsteady gravity-induced nanofluid flow containing gyrotactic micro-organisms along downward vertical convectively heated surface subject to passively controlled nanofluid. Considering the influence of temperature on the dynamic viscosity during convection and nature of thermal conductivity during heat conduction processes, these thermophysical properties are treated as linear functions of temperature. The governing equations are nondimensionalized by using suitable similarity transformation. The dimensionless nonlinear coupled PDEs are solved using a new pseudo-spectral technique called paired quasi-linearization method (PQLM). Convergence tests and residual error analysis are also presented to validate the accuracy, solution error, and computational convergence. The proposed PQLM yields accurate results which are obtained after a very few iterations. Minimum coefficients of $(ξ/xRex)Shx$ with Sc are obtained at final steady stage.

Commentary by Dr. Valentin Fuster

### Technical Brief: Technical Brief

J. Heat Transfer. 2016;138(11):114502-114502-4. doi:10.1115/1.4033811.

In order to improve the temperature uniformity inside the battery, the effects of partially utilizing metal and nonmetal materials on the heat sink of an air-cooled Lithium-ion (Li-ion) battery module were studied. Aluminum and aluminum foam as heat conductors and ceramic, and ceramic foam as insulators were examined using two-dimensional transient numerical simulation. The effects of the length of utilizing each material to the total length of the battery pack from the inlet by assuming that the other part of the heat sink is aluminum were investigated. The results showed that using aluminum foam and ceramic as part of the heat sink decreases the temperature uniformity of the battery pack. However, using the ceramic foam at the inlet section of the heat sink improves the temperature uniformity of the battery significantly. Furthermore, partially inserting the aluminum foam inside the air flow channel from outlet was investigated, and significant enhancement on the temperature uniformity of the battery pack was found. Overall, higher temperature reduction and higher temperature uniformity were achieved inside the battery pack using the combination of both ceramic and aluminum foams.

Commentary by Dr. Valentin Fuster