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### Research Papers: Conduction

J. Heat Transfer. 2019;141(7):071301-071301-8. doi:10.1115/1.4043568.

A generalized solution for a two-dimensional (2D) transient heat conduction problem with a partial-heating boundary condition in rectangular coordinates is developed. The solution accommodates three kinds of boundary conditions: prescribed temperature, prescribed heat flux and convective. Also, the possibility of combining prescribed heat flux and convective heating/cooling on the same boundary is addressed. The means of dealing with these conditions involves adjusting the convection coefficient. Large convective coefficients such as 1010 effectively produce a prescribed-temperature boundary condition and small ones such as 10−10 produce an insulated boundary condition. This paper also presents three different methods to develop the computationally difficult steady-state component of the solution, as separation of variables (SOV) can be less efficient at the heated surface and another method (non-SOV) is more efficient there. Then, the use of the complementary transient part of the solution at early times is presented as a unique way to compute the steady-state solution. The solution method builds upon previous work done in generating analytical solutions in 2D problems with partial heating. But the generalized solution proposed here contains the possibility of hundreds or even thousands of individual solutions. An indexed numbering system is used in order to highlight these individual solutions. Heating along a variable length on the nonhomogeneous boundary is featured as part of the geometry and examples of the solution output are included in the results.

Commentary by Dr. Valentin Fuster

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

J. Heat Transfer. 2019;141(7):071501-071501-10. doi:10.1115/1.4043302.

This paper deals with the numerical study of the combined heat and mass exchanges in the process of direct evaporative cooler, from a porous media of laminar air flow between two parallel insulated walls. The numerical model implements momentum, energy, and mass conservation equations of humid air and water flow incorporating non-Darcian model in the porous region. The finite volume method is used for the mathematical model resolution, and the velocity–pressure coupling is treated with the SIMPLE algorithm. The main objective of this study is to examine the influences of ambient conditions and the porous medium properties (porosity and porous layer thickness) on the direct evaporative cooling performance from a porous layer. The major results of this study demonstrate that the porous evaporative wall could, in a satisfying manner, reduce the bulk air temperature. The better cooling performance can be achieved for lower air mass flow at the entrance and relative humidity. Additionally, the evaporative cooler is more effective for a high porosity and a thick porous medium, with an improvement achieving 23% for high porosity.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(7):071502-071502-11. doi:10.1115/1.4043300.

This paper reports the experimental data of boiling heat transfer coefficients for the ammonia–lithium nitrate mixture in a laminar falling film. The analyzed heat exchanger consists of a shell with an internal helical coil. More than one hundred test runs were carried out in steady-state conditions to determine the boiling heat transfer coefficients at generation temperatures, concentrations, and mass flow rates typical of absorption cooling systems of capacities between 5 and 10 kW. Ammonia vapor was produced at generation temperatures between 80 °C and 105 °C obtaining boiling heat transfer coefficients between 85 and 340 W/m2K. Semi-empirical correlations were used by diverse authors to correlate the experimental data. A new correlation was proposed with which the best adjustments were obtained. Also, the influence of the heat flux, the refrigerant solution mass flow rates, and the exit vapor qualities were analyzed in the boiling heat transfer coefficients.

Commentary by Dr. Valentin Fuster

### Research Papers: Experimental Techniques

J. Heat Transfer. 2019;141(7):071601-071601-9. doi:10.1115/1.4043381.

We demonstrate the functionality of a new active thermal microchip dedicated to the temperature calibration of scanning thermal microscopy (SThM) probes. The silicon micromachined device consists in a suspended thin dielectric membrane in which a heating resistor with a circular area of 50 μm in diameter was embedded. A circular calibration target of 10 μm in diameter was patterned at the center and on top of the membrane on which the SThM probe can land. This target is a resistive temperature detector (RTD) that measures the surface temperature of the sample at the level of the contact area. This allows evaluating the ability of any SThM probe to measure a surface temperature in ambient air conditions. Furthermore, by looking at the thermal balance of the device, the heat dissipated through the probe and the different thermal resistances involved at the contact can be estimated. A comparison of the results obtained for two different SThM probes, microthermocouples and probes with a fluorescent particle is presented to validate the functionality of the micromachined device. Based on experiments and simulations, an analysis of the behavior of probes allows pointing out their performances and limits depending on the sample characteristics whose role is always preponderant. Finally, we also show that a smaller area of the temperature sensor would be required to assess the local disturbance at the contact point.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(7):071602-071602-12. doi:10.1115/1.4043572.

A device for measuring a plurality of material properties is designed to include accurate sensors configured to consecutively obtain thermal conductivity, electrical conductivity, and Seebeck coefficient of a single sample while maintaining a vacuum or inert gas environment. Four major design factors are identified as sample-heat spreader mismatch, radiation losses, parasitic losses, and sample surface temperature variance. The design is analyzed using finite element methods for high temperature ranges up to 1000 °C as well as ultra-high temperatures up to 2500 °C. A temperature uncertainty of 0.46% was estimated for a sample with cold and hot sides at 905.1 and 908.5 °C, respectively. The uncertainty at 1000 °C was calculated to be 0.7% for a ΔT of 5 °C between the hot and cold sides. The thermal conductivity uncertainty was calculated to be −8.6% at ∼900 °C for a case with radiative gains, and +8.2% at ∼1000 °C for a case with radiative losses, indicating the sensitivity of the measurement to the temperature of the thermal guard in relation to the heat spreader and sample temperature. Lower limits of −17 and −13% error in thermal conductivity measurements were estimated at the ultra-high temperature of ∼2500 °C for a single-stage and double-stage radiation shield, respectively. It is noted that this design is not limited to electro-thermal characterization and will enable measurement of ionic conductivity and surface temperatures of energy materials under realistic operating conditions in extreme temperature environments.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Exchangers

J. Heat Transfer. 2019;141(7):071801-071801-9. doi:10.1115/1.4043570.

Recently, energy saving problem attracts increasing attention from researchers. This study aims to determine the optimal arrangement of a tube bundle to achieve the best overall performance. The multi-objective genetic algorithm (MOGA) is employed to determine the best configuration, where two objective functions, the average heat flux q and the pressure drop Δp, are selected to evaluate the performance and the consumption, respectively. Subsequently, a decision maker method, technique for order preference by similarity to an ideal solution (TOPSIS), is applied to determine the best compromise solution from noninferior solutions (Pareto solutions). In the optimization procedure, all the two-dimensional (2D) symmetric models are solved by the computational fluid dynamics (CFD) method. Results show that performances alter significantly as geometries of the tube bundle changes along the Pareto front. For the case 1 (using staggered arrangement as initial), the optimal q varies from 2708.27 W/m2 to 3641.25 W/m2 and the optimal Δp varies from 380.32 Pa to 1117.74 Pa, respectively. For the case 2 (using in-line arrangement as initial), the optimal q varies from 2047.56 W/m2 to 3217.22 W/m2 and the optimal Δp varies from 181.13 Pa to 674.21 Pa, respectively. Meanwhile, the comparison between the optimal solution with maximum q and the one selected by TOPSIS indicates that TOPSIS could reduce the pressure drop of the tube bundle without sacrificing too much heat transfer performance.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(7):071802-071802-6. doi:10.1115/1.4043620.

A new oscillating heat pipe (OHP) charged with hybrid fluids can improve thermal performance. The key difference in this OHP is that it uses room temperature liquid metal (Galinstan consisting of gallium, indium, and tin) and water as the working fluid. The OHP was fabricated on a copper plate with six turns and a 3 × 3 mm2 cross section. The OHP with hybrid fluids as the working fluid was investigated through visual observation and thermal measurement. Liquid metal was successfully driven to flow through the OHP by the pressure difference between the evaporator and the condenser without external force. Experimental results show that while added liquid metal can increase the heat transport capability, liquid metal oscillation amplitude decreases as the filling ratio of liquid metal increases. Visualization of experimental results show that liquid metal oscillation position and velocity increase as the heat input increases. Oscillating motion of liquid metal in the OHP significantly increases the heat transfer performance at high heat input. The lowest thermal resistance of 0.076 °C/W was achieved in the hybrid fluids-filled OHP with a heat input of 420 W. We experimentally demonstrated a 13% higher heat transfer performance using liquid metal as the working fluid compared to an OHP charged with pure water.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2019;141(7):071901-071901-11. doi:10.1115/1.4043212.

The flow around a rectangular cylinder mounted in the vicinity of a hot wall is numerically studied at a Reynolds number of 200. While the cylinder chord-to-height ratio C/W is varied from 2 to 10, the gap distance G from the wall to the cylinder is changed from 0.25 to 6.25. The focus of this study is given on the dependence of G/W and C/W on the heat transfer from the wall and associated physics. The variation in the Strouhal number is presented as a function of C/W. It is observed that the effect of G/W on the vortex dynamics and heat transfer is much more than that of C/W. Based on the dependence of the vortex dynamics and heat transfer on G/W, we have identified four distinct flows: no vortex street flow (G/W <0.75), single-row vortex street flow (0.75 ≤ G/W ≤1.25), inverted two-row vortex street flow (1.25 < G/W ≤2.5), and two-row vortex street flow (G/W >2.5). At the single-row vortex street flow, the two opposite-sign vortices appearing in a jetlike flow carry heat from the wall to the wake and then to the freestream. The maximum heat transfer is achieved at the single-row vortex street flow and 8% increase occurs at C/W =2, G/W =0.75–1.25.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(7):071902-071902-12. doi:10.1115/1.4043305.

The goal of this study is to evaluate the computational fluid dynamic (CFD) predictions of friction factor and Nusselt number from six different low Reynolds number k–ε (LRKE) models namely Chang–Hsieh–Chen (CHC), Launder–Sharma (LS), Abid, Lam–Bremhorst (LB), Yang–Shih (YS), and Abe–Kondoh–Nagano (AKN) for various heat transfer enhancement applications. Standard and realizable k–ε (RKE) models with enhanced wall treatment (EWT) were also studied. CFD predictions of Nusselt number, Stanton number, and friction factor were compared with experimental data from literature. Various parameters such as effect of type of mesh element and grid resolution were also studied. It is recommended that a model, which predicts reasonably accurate values for both friction factor and Nusselt number, should be chosen over disparate models, which may predict either of these quantities more accurately. This is based on the performance evaluation criterion developed by Webb and Kim (2006, Principles of Enhanced Heat Transfer, 2nd ed., Taylor and Francis Group, pp. 1–72) for heat transfer enhancement. It was found that all LRKE models failed to predict friction factor and Nusselt number accurately (within 30%) for transverse rectangular ribs, whereas standard and RKE with EWT predicted friction factor and Nusselt number within 25%. Conversely, for transverse grooves, AKN, AKN/CHC, and LS (with modified constants) models accurately predicted (within 30%) both friction factor and Nusselt number for rectangular, circular, and trapezoidal grooves, respectively. In these cases, standard and RKE predictions were inaccurate and inconsistent. For longitudinal fins, Standard/RKE model, AKN, LS and Abid LRKE models gave the friction factor and Nusselt number predictions within 25%, with the AKN model being the most accurate.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(7):071903-071903-12. doi:10.1115/1.4043675.

An experimental and numerical study has been conducted on heat transfer and turbulent flow structure in channels with novel hybrid structures with miniature V-shaped ribs and dimples on one wall. One miniature V-shaped rib was arranged immediately upstream each individual dimple to form the hybrid structure, which aims at inducing additional near-wall secondary flow interacting with the dimple vortex flow and further improving the heat transfer. Steady-state convective heat transfer experiments were done to obtain the heat transfer and pressure loss of the turbulent flow over the surfaces with the miniature V rib-dimples for the Reynolds numbers from 18,700 to 60,000. In addition, the turbulent flow structure in the V rib-dimpled channels has been predicted by carrying out numerical computations. The experimental results indicated that the overall heat transfer enhancement of the miniature V rib-dimpled channels can be increased by up to about 60.0% compared with the counterpart of the dimpled only channel, and by about 23.0% compared with the counterpart of the miniature V ribbed only channel. The miniature V ribs showed appreciable effects on the heat transfer and pressure loss characteristics for the turbulent flow over the V rib-dimpled surfaces. The numerical computations showed that the miniature V rib upstream each dimple produced strong near-wall downwashing secondary flow, which significantly changed the flow patterns and intensified the turbulent flow mixing inside and outside the dimple and above the surrounding wall. These unique near-wall flow characteristics generated a significant heat transfer improvement in both the magnitude and the uniformity.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2019;141(7):072001-072001-11. doi:10.1115/1.4043571.

Desalination systems can be conceptualized as power cycles in which the useful work output is the work of separation of fresh water from saline water. In this framing, thermodynamic analysis provides powerful tools for raising energy efficiency. This paper discusses the use of entropy generation minimization for a spectrum of desalination technologies, including those based on reverse osmosis (RO), humidification–dehumidification (HDH), membrane distillation (MD), electrodialysis (ED), and forward osmosis (FO). Heat and mass transfer are the primary causes of entropy production in these systems. The energy efficiency of desalination is shown to be maximized when entropy generation is minimized. Equipartitioning of entropy generation is considered and applied. The mechanisms of entropy generation are characterized, including the identification of major causes of irreversibility. Methods to limit discarded exergy are also identified. Prospects and technology development needs for further improvement are mentioned briefly.

Topics: Entropy , Water , Membranes , Heat , Pressure
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(7):072002-072002-7. doi:10.1115/1.4043677.

The governing equations of two-temperature generalized magneto-thermoelasticity with hydrostatic initial stress are specialized in two dimensions and are solved for surface wave solutions. The appropriate solutions in a half-space are obtained which satisfy relevant radiation condition and boundary conditions at thermally insulated as well as isothermal surface. The frequency equation of Rayleigh wave is obtained. The frequency equation is also reduced for limiting cases of small thermal coupling and small reduced frequency. Velocity of propagation and amplitude-attenuation factor of Rayleigh wave are computed for a numerical example. To illustrate the dependence of velocity and amplitude-attenuation factor upon two-temperature parameter, initial stress parameter thermal relaxation time and magnetic field, the numerical results are shown graphically.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(7):072003-072003-11. doi:10.1115/1.4043676.

The temperature-dependent dynamics of a negative temperature coefficient (NTC) thermistor conducting variable electric current is modeled using the differential approach. The thermistor is assumed to follow the Steinhart–Hart resistance-temperature equation. The developed mathematical model consists of a nonlinear differential-algebraic equations system, and it was analyzed by the Adomian decomposition method (ADM) and its time-marching version known as the multistage Adomian decomposition method (MADM) as well as the Dormand–Prince (DP) numerical method. Five sets of experiments were conducted on five different NTC thermistors and the laboratory measurements were compared with the model predictions. It is demonstrated that the proposed model, when combined with the MADM, can accurately simulate the thermal behavior of the NTC thermistors. The MADM reproduces the experimental temperature dynamics of the five NTC thermistors with an average absolute relative error of about 2.601% while the corresponding errors for the DP method and the classic ADM are 8.122% and 51.255%, respectively. Also, it is shown that the MADM is highly efficient in terms of computational efficiency and it is approximately 6.5 times faster than the classic DP method, when tuned appropriately.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer in Manufacturing

J. Heat Transfer. 2019;141(7):072101-072101-5. doi:10.1115/1.4043170.

The thermal conditions like temperature distribution and heat fluxes during metal cutting have a major influence on the machinability, the tool life time, and the metallurgical structure of the work piece material. Though numerous analytical and experimental efforts have been developed in order to understand the thermal conditions in metal cutting, many questions still prevail. So, the exact form, distribution, and intensity of heat sources in the primary and secondary shear zone, which may describe the observed temperature distributions, are not explored to a satisfactory extend. On the other hand, the influence of the material properties like friction coefficient, heat conductivity, and shear strength is not yet fully understood. Another essential question is the heat flux partition among chip, work piece, and tool depending on process parameters and material. The particular novelty of the current investigation is a new methodological approach using modern thermal measurement system and postprocessing methods in order not only to measure the entire temperature field in the orthogonal cutting zone but also to calculate the affiliated heat flow distribution in the cutting process. Thus, the cutting process is treated as energy conversation process of the governing mechanical power into sensible heat. This point of view offers compatibility across process parameters and materials, thus new possibilities for process design.

Topics: Heat , Temperature , Cutting
Commentary by Dr. Valentin Fuster

### Research Papers: Melting and Solidification

J. Heat Transfer. 2019;141(7):072301-072301-8. doi:10.1115/1.4043621.

Toward accelerated latent heat storage, the unconstrained melting heat transfer in spherical capsules was revisited experimentally in the presence of nano-enhanced phase-change materials (NePCMs), with an emphasis on the influence of capsule size on the rates of melting, heat transfer, and latent heat storage. It was shown that increasing the size of the spherical capsule leads to two competing effects, i.e., thicker molten layer in the close-contact melting (CCM) region and stronger natural convection. However, the NePCM with a high loading (3 wt % graphite nanoplatelets (GNPs)) is not preferred for all capsule sizes as a result of the significantly deteriorated heat transfer in both CCM and natural convection, because the dramatic viscosity growth at such a high loading leads to increased thermal resistance across the molten layer and loss of natural convection that overweigh the increased thermal conductivity. The 1 wt % NePCM sample was exhibited to be able to facilitate latent heat storage for two cases, i.e., in the smallest capsule having a radius of 14.92 mm at a higher wall superheat of 30 °C and in the intermedium 24.85 mm capsule at a lower wall superheat of only 10 °C. It was suggested that a relatively low loading of a specific NePCM can cause a faster rate of latent heat storage over the baseline case of the matrix phase-change material (PCM), if the capsule size (and the wall superheat) can be adjusted properly to regulate the molten layer thickness and the intensity of natural convection.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2019;141(7):072401-072401-9. doi:10.1115/1.4043174.

The need for higher energy efficiency is driving the space conditioning and refrigeration industry toward the use of advanced technologies. Vapor compression cycles work with a mixture of refrigerant and lubricant, and although oil in heat exchangers affects negatively the performances of the system, its presence is unavoidable. The studies of colloidal solutions showed promising results to enhance the heat transfer capabilities of the liquids in which nanoparticles are dispersed, and current experimental research revealed that the increase in thermal conductivity is not enough to explain the enhancements observed in heat transfer. In order to further understand the nanoparticles' contribution to the heat exchange phenomena, this paper analyses slip mechanisms that nanoparticles can be affected by when dispersed in the liquid phase of a high-viscosity oil–refrigerant mixture undergoing evaporation inside a horizontal tube. The study was conducted for Al2O3 nanoparticles dispersed in refrigerant R410A and polyolester (POE) oil at different mass concentrations. Depending on the flow regime, the slip mechanisms related to Brownian motion and thermophoresis were found to provide the largest contributions to nanoparticles' redistribution within the liquid phase of the mixture.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(7):072402-072402-12. doi:10.1115/1.4043569.

In this study, thermal resistance of a closed-loop oscillating heat pipe (OHP) is investigated using experimental tests and artificial intelligence methods. For this target, γFe2O3 and Fe3O4 nanoparticles are mixed with the base fluid. Also, intelligent models are developed to predict the thermal resistance of the OHP. These models are developed based on the heat input into evaporator section, the thermal conductivity of working fluids, and the ratio of the inner diameter to length of OHP. The intelligent methods are multilayer feed-forward neural network (MLFFNN), adaptive neuro-fuzzy inference system (ANFIS) and group method of data handling (GMDH) type neural network. Thermal resistance of the heat pipe (as a measure of thermal performance) is considered as the target. The results showed that using the nanofluids as working fluid in the OHP decreased the thermal resistance, where this decrease for Fe3O4/water nanofluid was more than that of γFe2O3/water. The intelligent models also predicted successfully the thermal resistance of OHP with a correlation coefficient close to 1. The root-mean-square error (RMSE) for MLFFNN, ANFIS, and GMDH models was obtained as 0.0508, 0.0556, and 0.0569 ($°C$/W) (for the test data), respectively.

Commentary by Dr. Valentin Fuster

### Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2019;141(7):072501-072501-9. doi:10.1115/1.4043673.

Mixed convection heat transfer characteristics from heat source located symmetrically inside square enclosure and cooled by Al2O3/water-based nanofluid flow was experimentally investigated. The configuration was subjected to high levels of natural convection and low rates of nanofluid flow. The nanofluid thermophysical properties were characterized using the available correlations in the literatures except the viscosity which was measured and correlated in terms of the nanoparticles loading ratios. Comparative analysis indicated that the application of nanofluid could not guarantee heat transfer enhancement in configurations dominated by natural convection. Exception heat transfer enhancement was only found when very low nanoparticles loading ratio was applied. Instead, heat transfer degradation was found especially in the cases of highest nanoparticles loading ratios. Alternatively, heat transfer enhancement was observed when the forced convection effect was substantial at the highest nanofluid flow rate. The present conclusions were justified and correlated to the findings reported in the literature.

Commentary by Dr. Valentin Fuster

### Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2019;141(7):072701-072701-8. doi:10.1115/1.4043573.

We show numerically the phase change material Ge3Sb2Te6 (GST) with special configuration as a heat modulator in the regime of near-field radiative heat transfer (NFRHT). The ability of GST to allow ultrafast reversible switch between two phases endows it great potential in practical modulation application. By designing silicon carbide (SiC) nanoholes (NHs) filled with GST, this configuration could achieve a considerable modulation effect and large near-field radiative heat flux. The underlying mechanism can be explained by the observation that the entire configuration supports either hyperbolic modes or surface phonon polaritons (SPhPs) resonance modes and even the combination of both modes, thereby resulting in the remarkable modulation effect. In addition, the effects of the volume filling factor and graphene coverage are also investigated at the vacuum gap distance of 100 nm. With graphene coverage, the modulation factor can be further improved to as high as 0.72 achieved at the volume filling factor of 0.6 with temperature difference of 20 K. The proposed configuration has the potential to effectively modulate heat in the near-field regime for designing heat modulation applications in the future.

Commentary by Dr. Valentin Fuster