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

J. Heat Transfer. 2017;139(10):101501-101501-9. doi:10.1115/1.4036599.

The effect of surface roughness on the pool boiling heat transfer of water was investigated on superhydrophilic aluminum surfaces. The formation of nanoscale protrusions on the aluminum surface was confirmed after immersing it in boiling water, which modified surface wettability to form a superhydrophilic surface. The effect of surface roughness was examined at different average roughness (Ra) values ranging from 0.11 to 2.93 μm. The boiling heat transfer coefficients increased with an increase in roughness owing to the increased number of cavities. However, the superhydrophilic aluminum surfaces exhibited degradation of the heat transfer coefficients when compared with copper surfaces owing to the flooding of promising cavities. The superhydrophilic aluminum surfaces exhibited a higher critical heat flux (CHF) than the copper surfaces. The CHF was 1650 kW/m2 for Ra = 0.11 μm, and it increased to 2150 kW/m2 for Ra = 0.35 μm. Surface roughness is considered to affect CHF as it improves the capillary wicking on the superhydrophilic surface. However, further increase in surface roughness above 0.35 μm did not augment the CHF, even at Ra = 2.93 μm. This upper limit of the CHF appears to result from the hydrodynamic limit on the superhydrophilic surface, because the roughest surface with Ra = 2.93 μm still showed a faster liquid spreading speed.

Commentary by Dr. Valentin Fuster

### Research Papers: Forced Convection

J. Heat Transfer. 2017;139(10):101701-101701-8. doi:10.1115/1.4036734.

In this paper, the determination of convective heat transfer coefficient under actual convection processes is proposed by using thermoelectric modules. The thermoelectric modules are positioned where cooling/heating processes take place. Based on the Seebeck effect and energy balance, voltage signals are mathematically related to the convective heat transfer coefficient in real time. In experiments, convective heat transfer coefficients of airflow in a wind tunnel are determined under heating/cooling processes at various wind speeds. The relative mean difference of the convective heat transfer coefficients between the proposed methodology and empirical formula is 2.31%. For real-time implementation, convective heat transfer coefficients of a copper plate, which is exposed to outdoor conditions during a whole day, are determined to predict copper plate temperatures from a governing equation. The performance of temperature prediction is confirmed by a coefficient of determination R2 of 0.9992. Analytical and experimental results show the effectiveness of the proposed thermoelectric modules in determining the convective heat transfer coefficient for air under actual cooling/heating conditions, in time.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(10):101702-101702-10. doi:10.1115/1.4036729.

Convective heat transfer in rotating disk systems is of great importance in many engineering applications. Despite the high practical relevance, there have been only a small number of experimental investigations regarding the influence of the Prandtl number larger than unity. Ever since Dorfman's pioneering work more than 50 years ago, various analytical works about the heat transfer of a rotating disk have been published. However, this study is a novelty because measurements of the laminar convective heat transfer over a free rotating disk for a wide range of Prandtl number up to $Pr=5000$ are presented. The accuracy of the employed experimental apparatus was assessed by heat transfer measurements in air, for which reliable literature data are widely available. Natural convection effects and temperature-dependent physical properties have been taken into consideration using the property-ratio method. The experimental results are in excellent agreement with analytical self-similar solutions and the theoretical correlation of Lin and Lin. The applicability of frequently used heat transfer correlations is assessed by the means of the new experimental data.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(10):101703-101703-10. doi:10.1115/1.4036620.

This study includes experimental and three-dimensional numerical analysis of conjugate steady-state laminar forced ferroconvection of Newtonian incompressible ferrofluid through a horizontal circular pipe under constant heat flux and in presence of transverse magnetic field. The magnetic field was applied by two fixed parallel magnet bars at the beginning of the tube. To validate the thermohydrodynamic characteristics obtained by numerical results, appropriate experimental setup with accurate instrumentations was conducted. Based on presence and absence of porous media and solid rod inside of pipe, six conditions were compared for quantifying the heat transfer enhancement and effectiveness. Governing equations were discretized by finite volume method (FVM) and solved using the semi-implicit method for pressure linked equations (SIMPLE) algorithm and computational fluid dynamic (CFD) techniques. It was found that magnetic field, porous media, and solid rod increase heat transfer and pressure loss in the pipe such that solid rod has the best effect on heat transfer and worst effect on effectiveness.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Exchangers

J. Heat Transfer. 2017;139(10):101801-101801-12. doi:10.1115/1.4036618.

In this paper, four types of plate-fin heat exchangers applied in 200 kW microturbines are investigated. Multi-objective optimization algorithm, NSGA-II (nondominated sorting genetic algorithm (GA)), is employed to maximize the efficiency of the recuperator and minimize its total cost, simultaneously. Feasible ranges of pressure drop, Reynolds number, and recuperator efficiency are obtained according to a penalty function. The optimizations are conducted for rectangular fin, triangular fin, louver fin, and offset strip fin recuperators with cross and counter flow arrangements. The results of each optimization problem are presented as a set of designs, called “Pareto-optimal solutions.” Afterward, for the designs, cycle efficiency and net present value (NPV) are compared based on technical and economic criteria, respectively. Maximum cycle efficiency occurring in a recuperator with louver fin and counter flow arrangement is found to be 38.17%. Finally, the optimum designs are compared based on nondominated sorting concept leading to the optimal solutions.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2017;139(10):102001-102001-9. doi:10.1115/1.4036619.

The number of launches of nano- and pico-satellites has significantly increased over the past decade. Miniaturized subsystems, such as micropropulsion, for these classes of spacecraft are rapidly evolving and, in particular, micro-resistojets have shown great potential of applicability. One of the key points to address in the development of such devices is the propellants selection, since it directly influences the performance. This paper presents a methodology for the selection and characterization of fluids that are suitable for use as propellants in two micro-resistojet concepts: vaporizing liquid micro-resistojet (VLM) and the low-pressure micro-resistojet (LPM). In these concepts, the propellant is heated by a nonchemical energy source, in this case an electrical resistance. In total 95 fluids have been investigated including conventional and unconventional propellants. A feasibility assessment step is carried out following a trade-off using a combination of the analytical hierarchy process (AHP) and the Pugh matrix. A final list of nine best-scoring candidates has been analyzed in depth with respect to the thermal characteristics involved in the process, performance parameters, and safety issues. For both concepts, water has been recognized as a very promising candidate along with other substances such as ammonia and methanol.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(10):102002-102002-9. doi:10.1115/1.4036573.

For nonlinear transient heat transfer system, a fuzzy adaptive predictive inverse method (FAPIM) is proposed to inverse transient boundary heat flux. The influence relationship matrix is utilized to establish time-varying linear prediction model of the temperatures at measurement point. Then, the predictive and measurement temperatures are used to inverse the heat flux at current moment by rolling optimization. A decentralized fuzzy inference (DFI) mechanism is established. The deviation vector of the predictive temperature is adopted to conduct decentralized inference by a set of fuzzy inference units, and then, the influence relationship matrix is updated online to guarantee the adaptive ability of the prediction model by weighting fuzzy inference components. FAPIM is utilized to inverse the unknown heat flux of a heat transfer system with temperature-dependent thermal properties, which has shown that the inverse method has better adaptive ability for the inverse problems of nonlinear heat transfer system.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(10):102003-102003-15. doi:10.1115/1.4036509.

Film cooling effectiveness is closely dependent on the geometry of the hole emitting the cooling film. These holes are sometimes quite expensive to machine by traditional methods, so 3D printed test pieces have the potential to greatly reduce the cost of film cooling experiments. What is unknown is the degree to which parameters like layer resolution and the choice among 3D printing technologies influence the results of a film cooling test. A new flat-plate film cooling facility employing oxygen-sensitive paint (OSP) verified by gas sampling and the mass transfer analogy and measurements both by gas sampling and OSP is verified by comparing measurements by both gas sampling and OSP. The same facility is then used to characterize the film cooling effectiveness of a diffuser-shaped film cooling hole geometry. These diffuser holes are then produced by a variety of additive manufacturing (AM) technologies with different build layer thicknesses. The objective is to determine if cheaper manufacturing techniques afford usable and reliable results. The coolant gas used is CO2 yielding a density ratio (DR) of 1.5. Surface quality is characterized by an optical microscope that measures surface roughness. Test coupons with rougher surface topology generally showed delayed blow off and higher film cooling effectiveness at high blowing ratios (BR) compared to the geometries with lower measured surface roughness. At the present scale, none of the additively manufactured parts consistently matched the traditionally machined part, indicating that caution should be exercised in employing additively manufactured test pieces in film cooling work.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(10):102004-102004-9. doi:10.1115/1.4036601.

This paper presents significant advances in the analytical calculation of the low-temperature lattice thermal conductivity in finite crystals. It shows that an accurate prediction of the direction-dependent lattice thermal conductivity can be obtained at low temperatures when Houston's method is used to account for the anisotropy of the Brillouin zone in the calculation of the phonon spectrum. It also provides an approach to predict from a spatial-dependent Boltzmann equation the rate at which phonons are scattered by the sample boundary in the presence of intrinsic scattering mechanisms, which is crucial for the calculation of the lattice thermal conductivity in finite crystals.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(10):102005-102005-8. doi:10.1115/1.4036728.

The local mass transfer over dissolving surfaces was measured at pipe Reynolds number of 50,000, 100,000, and 200,000. Tests were run at multiple time periods for each Reynolds number using 203 mm diameter test sections that had gypsum linings dissolving to water in a closed flow loop at a Schmidt number of 1200. The local mass transfer was calculated from the decrease in thickness of the gypsum lining that was measured using X-ray-computed tomography (CT) scans. The range of Sherwood numbers for the developing roughness in the pipe was in good agreement with the previous studies. The mass transfer enhancement (Sh/Shs) was dependent on both the height ($ep−v$) and spacing ($λstr$) of the roughness scallops. For the developing roughness, two periods of mass transfer were present: (i) an initial period of rapid increase in enhancement when the density of scallops increases till the surface is spatially saturated with the scallops and (ii) a slower period of increase in enhancement beyond this point, where the streamwise spacing is approximately constant, and the roughness height grows more rapidly. The mass transfer enhancement was found to correlate well with the parameter ($ep−v/λstr$)0.2, with a weak dependence on Reynolds number.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2017;139(10):102401-102401-11. doi:10.1115/1.4036643.

Design of effective microcooling systems to address the challenges of ever increasing heat flux from microdevices requires deep examination of real-time problems and has been tackled in depth. The most common (and apparently misleading) assumption while designing microcooling systems is that the heat flux generated by the device is uniform, but the reality is far from this. Detailed simulations have been performed by considering nonuniform heat load employing the configurations U, I, and Z for parallel microchannel systems with water and nanofluids as the coolants. An Intel® Core i7-4770 3.40 GHz quad core processor has been mimicked using heat load data retrieved from a real microprocessor with nonuniform core activity. This study clearly demonstrates that there is a nonuniform thermal load induced temperature maldistribution along with the already existent flow maldistribution induced temperature maldistribution. The suitable configuration(s) for maximum possible overall heat removal for a hot zone while maximizing the uniformity of cooling have been tabulated. An Eulerian–Lagrangian model of the nanofluids shows that such “smart” coolants not only reduce the hot spot core temperature but also the hot spot core region and thermal slip mechanisms of Brownian diffusion and thermophoresis are at the crux of this. The present work conclusively shows that high flow maldistribution leads to high thermal maldistribution, as the common prevalent notion is no longer valid and existing maldistribution can be effectively utilized to tackle specific hot spot location, making the present study important to the field.

Commentary by Dr. Valentin Fuster

### Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2017;139(10):102501-102501-4. doi:10.1115/1.4036727.

The problem of steady mixed convection boundary layer flow on a cooled vertical permeable circular cylinder embedded in a fluid-saturated porous medium is studied. Here, we evaluate the flow and heat transfer characteristics numerically for various values of the governing parameters and demonstrate the existence of dual solutions beyond a critical point.

Commentary by Dr. Valentin Fuster

### Research Papers: Porous Media

J. Heat Transfer. 2017;139(10):102601-102601-12. doi:10.1115/1.4036574.

Compared to the regular (monodisperse) porous medium (MDPM) with one porosity scale, the bidisperse porous medium (BDPM) has two porosity scales, which may enhance the heat transfer capability. This work investigates the forced convective heat transport through a circular pipe filled with a BDPM. The two-velocity two-temperature model is utilized to describe the flow and temperature fields for both the fracture phase (macropores) and the porous phase (the matrix with micropores). The bidispersion effect is taken into account by altering the permeability of the porous phase in the medium. Analytical solutions of the velocities and temperatures for both phases are derived under the constant wall heat flux boundary condition. The local Nusselt number and heat transfer performance (HTP) are also developed to investigate how the bidispersivity affects the thermal characteristics over a wide range of parameter space.

Commentary by Dr. Valentin Fuster

### Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2017;139(10):102701-102701-8. doi:10.1115/1.4036616.

Several studies have validated that diffusive Fourier model is inadequate to model thermal transport at submicron length scales. Hence, Boltzmann transport equation (BTE) is being utilized to improve thermal predictions in electronic devices, where ballistic effects dominate. In this work, we investigated the steady-state thermal transport in a gallium nitride (GaN) film using the BTE. The phonon properties of GaN for BTE simulations are calculated from first principles—density functional theory (DFT). Despite parallelization, solving the BTE is quite expensive and requires significant computational resources. Here, we propose two methods to accelerate the process of solving the BTE without significant loss of accuracy in temperature prediction. The first one is to use the Fourier model away from the hot-spot in the device where ballistic effects can be neglected and then couple it with a BTE model for the region close to hot-spot. The second method is to accelerate the BTE model itself by using an adaptive model which is faster to solve as BTE for phonon modes with low Knudsen number is replaced with a Fourier like equation. Both these methods involve choosing a cutoff parameter based on the phonon mean free path (mfp). For a GaN-based device considered in the present work, the first method decreases the computational time by about 70%, whereas the adaptive method reduces it by 60% compared to the case where full BTE is solved across the entire domain. Using both the methods together reduces the overall computational time by more than 85%. The methods proposed here are general and can be used for any material. These approaches are quite valuable for multiscale thermal modeling in solving device level problems at a faster pace without a significant loss of accuracy.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Heat Transfer. 2017;139(10):104501-104501-6. doi:10.1115/1.4036598.

The effects of the uniform heat flux and a linear velocity-slip on the heat transfer phenomena of spheres in Newtonian fluids are numerically investigated using semi-implicit marker and cell (SMAC) method implemented on a staggered grid arrangement in spherical coordinates. The solver is thoroughly benchmarked through domain independence, grid independence, and comparison with literature. Further extensive results are obtained in the range of conditions as: Reynolds number, Re = 0.1–200; Prandtl number, Pr = 1–100; and dimensionless slip number, λ = 0.01–100. The results are presented and discussed such that the isotherm contours and the local and average Nusselt numbers of isoflux spheres with velocity-slip at the interface are compared with their isothermal spheres counterparts under identical conditions. Briefly, the results indicate that the average Nusselt numbers of isoflux spheres are large compared to those of isothermal spheres under identical conditions. Finally, an empirical correlation is developed for the average Nusselt numbers of the spheres in Newtonian fluids with velocity-slip and the uniform heat flux conditions along the fluid–solid sphere interface.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(10):104502-104502-4. doi:10.1115/1.4036688.

Thermomagnetic convection of a ferrofluid flow induced by the internal magnetic field around a vertical current-carrying wire was theoretically analyzed and experimentally validated for the first time. The Nusselt number for a heated 50-μm diameter wire in a ferrofluid was measured for different electrical currents and fluid temperatures. The experimental results are in a good agreement with the proposed scaling analysis. We found that increasing the current will increase the Nusselt number nonlinearly and ultimately enhances the heat transfer capability of the induced ferrofluid flow. We observed that the thermomagnetic convection becomes dominant, if large enough currents are applied.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(10):104503-104503-5. doi:10.1115/1.4036692.

Effects of nanostructured defects of a copper solid surface on bubble growth in liquid argon have been investigated through a hybrid atomistic-continuum (HAC) method. The same solid surfaces with five different nanostructures, namely, wedge defect, deep rectangular defect (R-I), shallow rectangular defect (R-II), small rectangular defect (R-III), and no defect were modeled at the molecular level. Liquid argon was placed on top of hot solid copper with a superheat of 30 K after equilibration was achieved with computational fluid dynamics–molecular dynamic (CFD–MD) coupled simulation. Phase change of argon on five nanostructures has been observed and analyzed accordingly. The results showed that the solid surface with wedge defect tends to induce a nanobubble more easily than the others, and the larger the size of the defect, the easier it is for the bubble to generate.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(10):104504-104504-5. doi:10.1115/1.4036690.

The asymptotic behavior of laminar forced convection in a circular duct for a Herschel–Bulkley fluid of constant properties is analyzed. The viscous dissipation and the axial heat conduction effects in the fluid are both considered. The asymptotic bulk and mixing temperature field, and the asymptotic values of the bulk and mixing Nusselt numbers are determined for every boundary condition, enabling a fully developed region. In particular, it is proved that whenever the wall heat flux tends to zero, the asymptotic Nusselt number is zero. The obtained results are compared to other existing solutions in the literature for Newtonian and non-Newtonian cases.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(10):104505-104505-6. doi:10.1115/1.4036645.

An entropy generation analysis of steady boundary layer flow of viscous fluid with variable properties over an exponentially stretching sheet is presented. The basic nonlinear partial differential equations that govern the flow are reduced to ordinary differential equations by using appropriate transformations. Numerical solutions are obtained by using shooting technique along with Runge–Kutta method. Expressions for the dimensionless volumetric entropy generation rate $(NG)$ and Bejan number are also obtained. The effects of different dimensionless emerging parameters on entropy generation number $(NG)$ and Bejan number $(Be)$ are investigated graphically in detail.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(10):104506-104506-4. doi:10.1115/1.4036617.

This work presents a study on double-diffusive free convection in a porous square cavity using the thermal equilibrium model. Transport equations are discretized using the control-volume method, and the system of algebraic equations is relaxed via the SIMPLE algorithm. The effect of ks/kf on average Nusselt and Sherwood values was investigated. Results show that increasing ks/kf affects Nuw and Shw boosting mass transfer at the expense of reducing overall heat transport across the enclosure.

Commentary by Dr. Valentin Fuster

### Discussion

J. Heat Transfer. 2017;139(10):105501-105501-1. doi:10.1115/1.4036733.

In Ref. [1], the Marangoni number M is defined as Display Formula

(1)$M=γω=μa$

Commentary by Dr. Valentin Fuster