0

### Research Papers: Forced Convection

J. Heat Transfer. 2018;140(11):111701-111701-14. doi:10.1115/1.4040481.

This paper presents a numerical and experimental study of a turbulent flow of air in a T-bifurcation. This configuration corresponds to a stator containing radial vents oriented vertically to the rotor–stator air gap in electrical machines. Our analysis focuses on the local convective heat transfer over the internal surface of the vents under a turbulent mass flow rate. To model the cooling installation in this region, computational fluid dynamics simulations and an experiment using particle image velocimetry (PIV) are performed. The resulting flow generally produces recirculation zones in various channels. The effect of the flow ratio and diameter of the bifurcation on the dynamic and thermal behavior of the flow is also examined. In this study, we apply a numerical approach based on the k$ω$ shear stress transport (SST) turbulence model (using the commercial software, “comsolmultiphysics”) to numerically solve the Navier–Stokes equations and energy equation of the system under consideration. We describe the different hypotheses necessary to formulate the equations governing the problem, initial conditions, and boundary condition. The velocity in the bifurcation calculated using the simulation is compared with that obtained by the experiment and it reveals a good agreement. The effect of the branch diameter of the bifurcation and flow ratio on the heat transfer is specifically analyzed in this research work.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(11):111702-111702-7. doi:10.1115/1.4040613.

Fully developed forced convective heat transfer within a channel filled with a functionally graded metal foam matrix was investigated analytically for the case of constant wall heat flux. A series of functionally graded metal foam matrices of the same mass (i.e., the same solidity) were examined in views of their heat transfer performances. The porosity either increases or decreases toward the heated wall following a parabolic function. Among the metal foam matrices of the same mass, the maximum heat transfer coefficient exists for the case in which the porosity decreases toward the heated wall (i.e., more metal near the wall). The heat transfer coefficients in such channels filled with a functionally graded metal foam matrix are found 20–50% higher than that expected from the increase in the effective thermal conductivity. Hence, functionally graded metal foam matrices are quite effective to achieve substantially high heat transfer coefficient with an acceptable increase in pressure drop.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Exchangers

J. Heat Transfer. 2018;140(11):111801-111801-11. doi:10.1115/1.4040609.

Wavy fins have been considered as an alternative of the straight fins in compact heat exchangers (CHEs) for better heat transfer performance, which can be augmented by considering vortex generators (VGs). This work is related to numerical investigation and optimization of corrugation height of fin and angle of attack of delta winglet type VGs in a wavy fin-and-tube heat exchanger. For this purpose, three-dimensional (3D) Reynolds-averaged Navier-Stokes analysis and a multi-objective genetic algorithm (MOGA) with surrogate modeling are performed. Numerical simulation is carried out to study the effect of delta winglets with varying the corrugation height of wavy fin in three rows of tubes with staggered tube arrangements. The corrugation height (H) and angle of attack (α) vary from 0.3 mm to 1.8 mm and 15 deg to 75 deg, respectively. Results are illustrated by investigating the flow structures and temperature contours. Results show that increasing the corrugation height of wavy fin and angle of attack of delta winglets enhances the heat transfer performance of heat exchanger while friction factor is also increased. Employing delta winglets has augmented the thermal performance for all corrugation heights and superior effect is observed at a higher corrugation. To achieve a maximum heat transfer enhancement and a minimum pressure drop, the optimal values of these parameters (H and α) are calculated using the Pareto optimal strategy. For this purpose, computational fluid dynamics (CFD) data, a surrogate model (neural network), and a multi-objective GA are combined. Results show that optimal orientation of delta winglets with respect to corrugation height can improve both the thermal and hydraulic performance of the heat exchanger.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2018;140(11):112001-112001-8. doi:10.1115/1.4040610.

The purpose of the new formulas, Cml, CmlK, and CmlY, which express the slowest char combustion rate, is to show the controlling mechanism of single coal burning. Oxygen diffusion through the boundary layer (as a result of releasing volatile matter from coal) to the char surface is the slowest step rate and can also represent as the rate determining. This step has not yet been taken into account in the literature and may effect incomparable decisions between numerical and experimental results of coal combustion studies. In the 1920s, Wilhelm Nusselt found the coal combustion equation for a single coal, which is based on initial coal diameter, and its burning time, or Nusselt square law (NSL). Also, the burning constant in NSL expressed oxygen partial pressure and the ambient temperature level. Nevertheless, recent studies according to char combustion have explained the effect of coal density on char combustion. Consequently, to help understand the slowest rate of char combustion, NSL as well as ordinary char combustion equations can be used together to establish the rate-determining factor. For this purpose, in this study, the slowest step of the char reaction rate is given as “Cml” of stable position for single coal particle, “CmlK” and “CmlY” for a coal particle in a motion.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(11):112002-112002-9. doi:10.1115/1.4040257.

To simulate heat transfer processes through periodic devices with nonuniform wall temperature distributions, we propose to decompose the regular temperature into two parts: namely the transient part and the equilibrium part. These two parts can be solved independently under their individual wall and inlet/outlet conditions. By calculating the flow field and the two component functions in one periodic module, one can easily generate the distributions of regular temperature in one or multiple modules. The algorithm and implementation are described in details, and the method is discussed thoroughly from mathematical, physical, and numerical aspects. Sample simulations are also presented to demonstrate the capacity and usefulness of this method for future simulations of thermal periodic flows using various numerical schemes.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(11):112003-112003-16. doi:10.1115/1.4040347.

We study the effects of particle-turbulence interactions on heat transfer in a particle-laden turbulent channel flow using an Eulerian–Lagrangian simulation approach, with direct numerical simulation (DNS) for turbulence and Lagrangian tracking for particles. A two-way coupling model is employed in which the momentum and energy exchange between the discrete particles and the continuous fluid phase is fully taken into account. Our study focuses on the modulations of the temperature field and heat transfer process by inertial particles with different particle momentum Stokes numbers ($St$), which in a combination of the particle-to-fluid specific heat ratio and the Prandtl number results in different particle heat Stokes numbers. It is found that as $St$ increases, while the turbulent heat flux decreases due to the suppression of wall-normal turbulence velocity fluctuation, the particle feedback heat flux increases significantly and results in an increase in the total heat flux. The particle thermal feedback effect is illustrated using the instantaneous structures and statistics of the flow and temperature fields. The mechanisms of heat transfer modulation by inertial particles are investigated in detail. The budget of turbulent heat flux is examined. Moreover, by taking advantage of the ability of numerical simulation to address different momentum and heat processes separately, we investigate in detail the two processes of particles affecting heat transfer for the first time, namely the direct effect of particle thermal feedback to the fluid (i.e., heat feedback) and the indirect effect of the modulation of turbulent velocity field induced by the particles (i.e., momentum feedback). It is found that the contribution of heat transfer from turbulent convection is reduced by both heat and momentum feedback due to the decrease of the turbulent heat flux. The contribution of heat transfer from particle transport effects is barely influenced by the momentum feedback, even if $St$ is large and is mainly affected by the heat feedback. Our results indicate that both heat and momentum feedback are important when the particle inertia is large, suggesting that both feedback processes need to be taken into account in computation and modeling.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(11):112004-112004-9. doi:10.1115/1.4040786.

Hot wire anemometer (HWA) measurements of turbulent gas flow have been performed in upward forced convection experiments at pressures ranging from 0.6 MPa to 6.3 MPa and fluid temperatures ranging from 293 K to 673 K. The results are relevant to deteriorated turbulent heat transfer (DTHT) and flow laminarization in strongly heated gas flows which could occur in gas-cooled very high temperature reactors (VHTRs).2 The HWA signals were analyzed to directly confirm the occurrence of flow laminarization phenomenon due to strong heating. An X-probe was used to collect radial and axial velocity fluctuation data for pressurized air and pure nitrogen flowing through a circular 16.8 mm diameter flow channel in a 2.7 m long graphite test section for local Reynolds numbers varying from 500 to 22,000. Analyses of the Reynolds stresses and turbulence frequency spectra were carried out and used as indicators of laminar, transition, or fully turbulent flow conditions. Low Reynolds stresses indicated the existence of laminar or transitional flow until the local Reynolds number reached a large value, ∼11,000 to 16,000, much higher than the conventional Re = 4000–5000 for transition to fully turbulent flow encountered in pipe flows. The critical Reynolds number indicating the completion of transition approximately doubled as the pressure was increased from 0.6 MPa to 2.8 MPa.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(11):112005-112005-10. doi:10.1115/1.4040784.

Heat transfer properties of two expanded polystyrene (EPS) samples of similar density, one without (white) and one with graphite opacifier particles (gray), are compared. Tomographic scans are used to obtain cell sizes of the foams. Using established models for closed-cell polymer foams, the extinction coefficient and the effective thermal conductivity are obtained. The effect of opacifiers is modeled using (1) an effective refractive index for the polystyrene walls within a cell model for the EPS and (2) a superposition of extinction due to a particle cloud upon extinction predicted by the cell model, where particles are modeled as oblate spheroids, or equivalent volume, surface, or hydraulic diameter spheres. Modeled effective conductivities are compared with measurements done on a guarded hot-plate apparatus at sample mean temperatures in the range from 0 °C to 40 °C. Typically, cells of the gray EPS are about 40% larger than those of the white EPS and the cell walls in the gray EPS are thicker. The refractive index mixing model and the model with graphite opacifier particles as oblate spheroids overpredict extinction, however, the mean error in the effective conductivity predicted by the oblate spheroids model is only 2.7%. Equivalent volume/surface sphere models underpredict extinction, but still yield a low mean error in effective conductivity of around 4%. While the oblate spheroids model has a lower mean error, the computationally less expensive equivalent volume or equivalent surface models can also be recommended to model the inclusions.

Commentary by Dr. Valentin Fuster

### Research Papers: Jets, Wakes, and Impingment Cooling

J. Heat Transfer. 2018;140(11):112201-112201-7. doi:10.1115/1.4040612.

In the present paper, numerical study of flow and heat transfer properties of RP-3 kerosene at liquid and supercritical conditions in an impingement model is conducted with renormalization group (RNG) $k−ε$ turbulence model and a ten-species surrogate of kerosene. The independence of grids is first studied, and the numerical results are compared with experimental data for validation. Characteristics of flow and heat transfer of kerosene flow in the impingement model are studied with different inlet mass flow rates and different inlet temperatures. The velocity and temperature field show similar profile compared to that of air impingement. The heat transfer rates increase first with the increasing of inlet temperature and then decrease suddenly when the inlet temperature is 500 K.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2018;140(11):112401-112401-16. doi:10.1115/1.4039903.

Enhancing spray cooling with surface structures is a common, effective approach for high heat flux thermal management to guarantee the reliability of many high-power, high-speed electronics and to improve the efficiency of new energy systems. However, the fundamental heat transfer enhancement mechanisms are not well understood especially for nanostructures. Here, we fabricated six groups of nanowire arrayed surfaces with various structures and sizes that show for the first time how these nanostructures enhance the spray cooling by improving the surface wettability and the liquid transport to quickly rewet the surface and avoid dry out. These insights into the nanostructure spray cooling heat transfer enhancement mechanisms are combined with microstructure heat transfer mechanism in integrated microstructure and nanostructure hybrid surface that further enhances the spray cooling heat transfer.

Topics: Cooling , Sprays , Nanowires
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(11):112402-112402-9. doi:10.1115/1.4040706.

Multiphase flow phenomena in single micro and minichannels have been widely studied. Characteristics of two-phase flow through a large array of microchannels are investigated here. An air–water mixture is used to represent the two phases flowing through a microchannel array representative of those employed in practical applications. Flow distribution of the air and water flow across 52 parallel microchannels of 0.4 mm hydraulic diameter is visually investigated using high-speed photography. Two microchannel configurations are studied and compared, with mixing features incorporated into the second configuration. Slug and annular flow regimes are observed in the channels. Void fractions and interfacial areas are calculated for each channel from these observations. The flow distribution is tracked at various lengths along the microchannel array sheets. Statistical distributions of void fraction and interfacial area along the microchannel array are measured. The design with mixing features yields improved flow distribution. Void fraction and interfacial area change along the length of the second configuration, indicating a change in fluid distribution among the channels. The void fraction and interfacial area results are used to predict the performance of different microchannel array configurations for heat and mass transfer applications. Results from this study can help inform the design of compact thermal-fluid energy systems.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(11):112403-112403-9. doi:10.1115/1.4040147.

This paper presents fundamental research on the hydrodynamics and heat transfer surrounding a single elongated bubble during flow boiling in a circular microchannel. A continuum surface force (CSF) model based on the volume of fluid (VOF) method is combined with the thermocapillary force to explore the effects of thermocapillarity for flow boiling in microchannels. To validate the self-defined codes, a two-phase thermocapillary-driven flow and a Taylor bubble growing in a capillary tube are studied. Results of both test cases show good convergence and agreement with data from the earlier literature. The bubble motion and the local heat transfer coefficient (HTC) on the heated wall with respect to time are discussed. It is found that for large Marangoni number (case 3), variation of surface tension has affected the bubble shape and temperature profile. The thermocapillary effect induces convection in a thin liquid film region, which augments the HTCs at specified positions. The numerical investigation also shows that the average HTC increased by 6.7% in case 3 when compared with case 1. Thus, it is very important to study further the effects of themocapillarity and the Marangoni effect on bubble growth in microchannels.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(11):112404-112404-14. doi:10.1115/1.4040782.

In the present paper, efforts has been made to study the thermophysical performance (properties) of N photovoltaic thermal flat plate collectors coupled with double slope solar still (N-PVT-FPC-DSSS) and operating with helically coiled heat exchanger. The analysis has been performed for the optimized concentration of NPs (Al2O3 0.107%; TiO2 0.093%; and CuO 0.131%) and optimized basin fluid (base fluid/nanofluid) mass (50 kg) for different weather conditions of the month May (New Delhi). The Nusselt number (Nu) and Rayleigh number (Ra) are functions of thermophysical properties of nanofluids and strongly influence the natural convective heat transfer coefficient in the solar still. Therefore, these numbers have also been investigated for base fluid and Al2O3, TiO2, and CuO–water-based nanofluids in detail. Significant enhancement in natural convective heat transfer coefficient (Al2O3 67.03%; TiO2 63.56%; and CuO 71.23%) and Nusselt number (Al2O3 119.72%; TiO2 98.64%; CuO 151.62%) has been observed. The monthly productivity of the hybrid system found to be higher by using nanofluids (320.77 kg TiO2; 338.23 kg Al2O3, and 355.46 CuO) as expected from the heat transfer results. Moreover, the comparative study between the proposed hybrid system and passive DSSS has been carried out.

Commentary by Dr. Valentin Fuster

### Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2018;140(11):112501-112501-11. doi:10.1115/1.4040614.

Two-dimensional computational fluid dynamics simulations of mixed convection heat transfer through aluminum metal foams partially filled in a vertical channel are carried out numerically. The objective of the present study is to quantify the effect of metal foam thickness on the fluid flow characteristics and the thermal performance in a partially filled vertical channel with metal foams for a fluid velocity range of 0.05–3 m/s. The numerical computations are performed for metal foam filled with 40%, 70%, and 100% by volume in the vertical channel for four different pores per inch (PPIs) of 10, 20, 30, and 45 with porosity values varying from 0.90 to 0.95. To envisage the characteristics of fluid flow and heat transfer, two different models, namely, Darcy Extended Forchheirmer (DEF) and Local thermal non-equilibrium, have been incorporated for the metal foam region. The numerical results are compared with experimental and analytical results available in the literature for the purpose of validation. The results of the parametric studies on vertical channel show that the Nusselt number increases with the increase of partial filling of metal foams. The thermal performance of the metal foams is reported in terms of Colburn j and performance factors.

Commentary by Dr. Valentin Fuster

### Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2018;140(11):112701-112701-11. doi:10.1115/1.4040482.

Foams, three-dimensional (3D)-printed cellular and honeycomb structures, and very oblate particles dispersed in a matrix are the examples of heterogeneous media with thin-wall morphology. Phase boundaries can also be considered by this approach. Statistical description is proposed to estimate the effective radiative properties of such media. Three orientation models are studied: (i) isotropic, (ii) surface elements parallel to a plane, and (iii) surface elements parallel to an axis. Radiative transfer equations (RTEs) are obtained and analyzed in the framework of the homogeneous phase approach (HPA) and the multiphase approach (MPA). Analytical expressions are obtained for the absorption, extinction, and scattering coefficients, the scattering phase function, and the radiative thermal conductivity for very oblate particles dispersed in an absorbing scattering matrix. The reflective properties of the platelets and their preferential orientation can be used to optimize the radiative thermal conductivity.

Commentary by Dr. Valentin Fuster

### Research Papers: Thermal Systems

J. Heat Transfer. 2018;140(11):112801-112801-8. doi:10.1115/1.4040707.

An experimental analysis for parallel connected two identical counter flow Ranque–Hilsch vortex tubes (RHVT) with different nozzle materials and numbers was conducted by using compressed air as a working fluid in this paper. Heating and cooling performance of vortex tube system (circuit) and the results of exergy analysis are researched comprehensively according to different inlet pressure, nozzle numbers, and materials. Nozzles made of polyamide plastic, aluminum, and brass were mounted into the vortex tubes individually for each case of experimental investigation with the numbers of nozzles 2, 3, 4, 5, and 6. The range of operated inlet pressure 150–550 kPa with 50 kPa variation. The ratio of length–diameter (L/D) of each vortex tube in the circuit is 14 and the cold mass fraction is 0.36. Coefficient of performance (COP) values, heating, and cooling capacity of the parallel connected RHVT system were evaluated. Further, an exergy analysis was carried out to evaluate the energy losses and second law efficiency of the vortex tube circuit. The greatest thermal performance was obtained with aluminum-six-nozzle when taking into account all parameters such as temperature difference, COP values, heating and cooling capacity, and exergy analysis.

Commentary by Dr. Valentin Fuster

### Research Papers: Max Jacob Award Paper

J. Heat Transfer. 2018;140(11):113001-113001-20. doi:10.1115/1.4039644.

Gas turbines have been extensively used for aircraft engine propulsion, land-based power generation, and industrial applications. Power output and thermal efficiency of gas turbines increase with increasing turbine rotor inlet temperatures (RIT). Currently, advanced gas turbines operate at turbine RIT around 1700 °C far higher than the yielding point of the blade material temperature about 1200 °C. Therefore, turbine rotor blades need to be cooled by 3–5% of high-pressure compressor air around 700 °C. To design an efficient turbine blade cooling system, it is critical to have a thorough understanding of gas turbine heat transfer characteristics within complex three-dimensional (3D) unsteady high-turbulence flow conditions. Moreover, recent research trend focuses on aircraft gas turbines that operate at even higher RIT up to 2000 °C with a limited amount of cooling air, and land-based power generation gas turbines (including 300–400 MW combined cycles with 60% efficiency) burn alternative syngas fuels with higher heat load to turbine components. It is important to understand gas turbine heat transfer problems with efficient cooling strategies under new harsh working environments. Advanced cooling technology and durable thermal barrier coatings (TBCs) play most critical roles for development of new-generation high-efficiency gas turbines with near-zero emissions for safe and long-life operation. This paper reviews basic gas turbine heat transfer issues with advanced cooling technologies and documents important relevant papers for future research references.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Heat Transfer. 2018;140(11):114501-114501-6. doi:10.1115/1.4040785.

This paper deals with the influence of overlapped-quadruple twisted tape (OQT) and typical quadruple twisted tape (QT) on the enhancing heat transfer rate in a heat exchanger tube. The effects of tube orientation (co-tapes and counter tapes), tape arrangement (cross and parallel arrangements), and overlapped twist ratio (yo/y = 0.625, 0.75, and 0.875) were investigated. The experiments were performed in the turbulent flow regime (5000 ≤ Re ≤ 20,000) under uniform wall heat flux boundary condition, using air as the testing fluid. The experimental results indicates that all OQTs consistently yield higher Nusselt number and thermal performance than QTs. The tapes in cross arrangement show better heat transfer enhancement and thermal performance than the ones in parallel arrangement. Heat transfer, friction loss, and thermal performance increase with decreasing overlapped twist ratio. The maximum thermal enhancement factor (TEF) of 1.58 is achieved by the use of overlapped-quadruple counter tapes in cross arrangement (CC-OQTs) with yo/y = 0.625 at the lowest Reynolds number of 5000.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(11):114502-114502-6. doi:10.1115/1.4040781.

Phase change materials (PCMs) are commonly used in many applications, including the transient thermal management of electronics. For many systems, paraffin-based PCMs are used with suspended nanoinclusions to increase their effective thermal conductivity. The addition of these materials can have a positive impact on thermal conductivity, but can also increase the viscosity in the liquid phase. In this paper, the impact of different nanoinclusions and surfactants on the dynamic viscosity of a common paraffin wax PCM is quantified in order to determine their suitability for thermal energy storage applications. The effect of the nanoparticles on the viscosity is found to be a function of the nanoparticle type with multiwalled carbon nanotubes (MWCNT) yielding the greatest increase in viscosity. The addition of both nanoparticle and surfactant to the base PCM is found to affect the viscosity even when the loading levels of the nanoparticles or surfactant alone are not enough to affect the viscosity, thus the combination must be carefully considered in any heat transfer application.

Commentary by Dr. Valentin Fuster

### Discussion

J. Heat Transfer. 2018;140(11):115501-115501-1. doi:10.1115/1.4040541.

The present comment concerns some doubtful results included in the discussed paper.

Commentary by Dr. Valentin Fuster