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

J. Heat Transfer. 2019;141(4):041501-041501-9. doi:10.1115/1.4042779.

The computational fluid dynamics is an important methodology to study the characteristics of flows in nature and in a number of engineering applications. Modeling nonisothermal flows may be useful to predict the main flow behavior allowing the improvement of equipment and industrial processes. In addition, investigations using computational models may provide key information about the fundamental characteristics of flow, developing theoretical groundwork of physical processes. In the last years, the topic of phase change has been intensively studied using computational fluid dynamics due to the computational and numerical advances reported in the literature. Among several issues related to the phase change topic, direct contact condensation (DCC) is widely studied in the literature since it is part of a number of industrial applications. In the present work, DCC was studied using a mathematical and computational model with an Eulerian approach. The homemade code MFSim was used to run all the computational simulations in the cluster of the Fluid Mechanics Laboratory from the Federal University of Uberlandia (UFU). The computational model was validated and showed results with high accuracy and low differences compared to previous works in the literature. A complex case study of DCC with cross-flow was then studied and the computational model provided accurate results compared to experimental data from the literature. The jet centerline was well represented and the interface dynamic was accurately captured during all the simulation time. The investigation of the velocity field provided information about the deeply transient characteristic of this flow. The v-velocity component presented the more large variations in time since the standard deviation was subjected to a variation of about 45% compared to the temporal average. In addition, the time history of the maximum resultant velocities observed presented magnitude from 29 m/s to 73 m/s. The importance of modeling three-dimensional (3D) effects was confirmed with the relevance of the velocity magnitudes in the third axis component. Therefore, the Eulerian phase change model used in the present study indicated the possibility to model even complex phenomena using an Eulerian approach.

Commentary by Dr. Valentin Fuster

### Research Papers: Forced Convection

J. Heat Transfer. 2019;141(4):041701-041701-8. doi:10.1115/1.4042784.

Forced convection heat transfer is investigated from a thin disk in power-law fluids over wide range of conditions such as: Reynolds number, 1 ≤ Re ≤ 100, Prandtl number, 1 ≤ Pr ≤ 100, power-law index, 0.4 ≤ n ≤ 1.8, and disk thickness to diameter ratio, t/D = 0.01, 0.025, 0.05, and 0.075. The wide range of values of the power-law index spanned here covers both shear-thinning as well as shear-thickening fluid behavior. These results also elucidate the influence of the type of thermal boundary conditions, i.e., constant wall temperature condition (CWT) and constant heat flux condition (CHF) prescribed on the disk surface. Extensive results are presented in terms of the local and average Nusselt numbers to delineate the effect of each of the influencing parameters, Re, Pr, n, t/D for each thermal boundary condition. Limited results are also included here at vanishingly small values of the Peclet number to understand the behavior in the creeping flow condition. Finally, the present numerical results on the average Nusselt number have been consolidated in the form of a predictive equation to facilitate the interpolation of the present data for intermediate values of the parameters and/or a priori estimation of the average Nusselt number in a new application.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(4):041702-041702-15. doi:10.1115/1.4042841.

In internal cooling passages in a turbine blade, rib structures are widely applied to augment convective heat transfer by the coolant passing through over the ribbed surfaces. This study concentrates on perforated 90 deg ribs with inclined holes in a cooling duct with rectangular cross section, aiming at improving the perforated holes with additional secondary flows caused by inclined hole arrangements. Two sets of perforated ribs are used in the experiments with the inclined angle of the holes changing from 0 deg to 45 deg and the cross section are, respectively, circular and square. Steady-state liquid crystal thermography (LCT) is applied to measure the ribbed surface temperature and obtain corresponding convective heat transfer coefficients (HTCs). Two turbulence models, i.e., the kω shear stress transportation (SST) model and the detached eddy simulation (DES) model, are used in the numerical studies to simulate the flow fields. All the inclined cases have slightly larger overall averaged Nusselt number (Nu) than with straight cases. The enhancement ratio is approximately 1.85–4.94%. The averaged Nu in the half portion against the inclined direction is enlarged for the inclined hole cases. The inclined hole cases usually have smaller averaged Nu in the half portion along the inclined direction. For the straight hole case and small inclined angle case, the penetrated flows mix with the mainstream flows at the perforated regions. When the inclined angle is larger, the penetrated flows are pushed to the inclined direction and mixing with the approaching flows occurs just at the side of the inclined direction.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2019;141(4):041901-041901-13. doi:10.1115/1.4042771.

Enhancement of droplet heat transfer on a hydrophobic surface is examined via introducing the fin-like structures inside the droplet without altering the wetting state of the surface. A solution crystallization of polycarbonate surface is carried out and the functionalized silica particles are deposited onto the crystallized surface to create the hydrophobic surface characteristics. The ferrous particles (Fe2O3) are locally spread onto the hydrophobic surface and, later, manipulated by an external magneto-static force generating various configurations of fin-like structures inside the droplet. The droplet with fin-like structures is heated from the hydrophobic surface through introducing a constant temperature heat source. Flow and temperature fields inside the droplet are simulated in line with the experimental conditions. It is found that changing the configuration of the fin-like structures in the droplet modifies significantly the flow and temperature fields inside the droplet. The Bond number remains less than unity for all configurations of the fin-like structures while demonstrating the importance of the Marangoni current over the buoyancy current in the flow field. The presence of the fin-like structures lowers the difference between the fluid bulk and the minimum temperatures inside the droplet and improves considerably the heat transfer rates and the Nusselt number.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(4):041902-041902-8. doi:10.1115/1.4042780.

Numerical simulations are used to analyze the thermal performance of turbulent flow inside heat exchanger tube fitted with cross-cut twisted tape with alternate axis (CCTA). The design parameters include the Reynolds number $(5000, cross-cut width ratio $(0.7, cross-cut length ratio $(2, and twist ratio $(2. The objective functions are the Nusselt number ratio $(Nu/Nus)$, the friction factor ratio $(f/fs)$, and the thermal performance $(η)$. Response surface method (RSM) is used to construct second-order polynomial correlations as functions of design parameters. The regression analysis shows that heat transfer ratio decreased with increasing both the Reynolds number and the width to diameter ratio of the twisted tape. This means that the twisted tape has more influence on heat transfer at smaller inlet fluid velocities. Sensitivity analysis reveals that among the effective input parameters, the sensitivity of $Nu/Nus$ to the Reynolds number is the highest. The results reveal that thermal performance enhances with increasing the width to diameter ratio of the twisted tape $(b/D)$. The maximum thermal performance factor of 1.531 is obtained for the case of $Re=5000, b/D=0.9, s/D=2.5$, and $y/D=4$.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2019;141(4):042001-042001-15. doi:10.1115/1.4042776.

A detailed understanding of the flow of a liquid metal in a rectangular duct subject to a strong transverse magnetic field is vital in a number of engineering applications, notably for proposed blanket technologies for fusion reactors. Fusion reactors offer the potential for clean base-load energy and their development is now entering an engineering phase where the practical means by which the energy released can be converted into useful heat must be addressed. To such ends, this article considers the convective heat transfer processes for fully developed laminar magnetohydrodynamic (MHD) flows in rectangular ducts of the kind proposed in some blanket designs. Analytical solutions which incorporate the nonuniformity of peripheral temperature and heat flux and the effect of volumetric heating, are developed as functions of magnetic field strength and duct aspect ratio. A distinct feature of these MHD problems, not yet addressed in the literature, is that unlike the conventional characterization of heat transfer by a Nusselt number, it is necessary to generalize the concept to vectors and matrices of Nusselt coefficients, due to the extreme anisotropy of both the flow and heating. The new analytical results presented here capture more complex heat transfer behavior than non-MHD flows and in particular characterize the importance of aspect ratio. The importance of these new results lie not only in the improved understanding of this complex process but also in the provision of characterizations of convective heat transfer which underpin progress toward systems scale simulations of fusion blanket technology which will be vital for the realization of practical fusion reactors.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(4):042002-042002-6. doi:10.1115/1.4042785.

Pulse thermography (PT) is a nondestructive testing method in which an energy pulse is applied to a surface while the surface temperature evolution is measured to detect sub surface defects and estimate their depth. This nondestructive test method was developed on the assumption of instantaneous surface heating, but recent work has shown that relatively long pulses can be used to accurately determine defect depth in polymers. This paper examines the impact of varying input pulse length on the accuracy of defect depth quantification as a function of the material properties. Simulations using both thermoplastics and metals show that measurement error is dependent on a nondimensionalized pulse length. The simulation results agree with experimental results for three-dimensional (3D) printed acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) components. Analysis and experiments show that defects can be accurately detected with minor modification to the standard methods as long as the pulse ends before the characteristic defect signal is detected.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat Transfer in Manufacturing

J. Heat Transfer. 2019;141(4):042101-042101-12. doi:10.1115/1.4042778.

Cracking control is very important for laser drilling of yttria-stabilized zirconia (YSZ). In this paper, a thermomechanical model is presented to investigate what will be happened in the trepan drilling of YSZ by a picosecond pulsed laser. The thermal model is developed for describing the interaction between laser and YSZ material to obtain the transient temperature. The mechanical model is developed to calculate the stress distribution caused by the phase transformation based on the temperature results. The goal of the present study is to explore the intrinsic mechanism of cracking around the drilled hole from the aspect of the phase transformation in laser drilling of YSZ.

Commentary by Dr. Valentin Fuster

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

J. Heat Transfer. 2019;141(4):042201-042201-10. doi:10.1115/1.4042772.

Overall cooling effectiveness was determined for a full-coverage effusion cooled surface which simulated a portion of a double wall cooling gas turbine blade. The overall cooling effectiveness was measured with high thermal-conductivity artificial marble using infrared thermography. The Biot number of artificial marble was matched to real gas turbine blade conditions. Blowing ratio ranged from 0.5 to 2.5 with the density ratio of DR = 1.5. A variation of cooling arrangements, including impingement-only, film cooling-only, film cooling with impingement, and film cooling with impingement and pins, as well as forward/backward film injection, was employed to provide a systematic understanding on their contribution to improve cooling efficiency. Also investigated was the effect of reducing wall thickness. Local, laterally averaged, and area-averaged overall cooling effectiveness were shown to illustrate the effects of cooling arrangements and wall thickness. Results showed that adding impingement and pins to film cooling, and decreasing wall thickness increase the cooling efficiency significantly. Also observed was that adopting backward injection for thin full-coverage effusion plate improves the cooling efficiency.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(4):042202-042202-11. doi:10.1115/1.4042777.

This study investigates the effects of blowing ratio, density ratio, and spanwise pitch on the flat plate film cooling from two rows of compound angled cylindrical holes. Two arrangements of two-row compound angled cylindrical holes are tested: (a) the first row and the second row are oriented in staggered and same compound angled direction (β = +45 deg for the first row and +45 deg for the second row); (b) the first row and the second row are oriented in inline and opposite direction (β = +45 deg for the first row and −45 deg for the second row). The cooling hole is 4 mm in diameter with an inclined angle of 30 deg. The streamwise row-to-row spacing is fixed at 3d, and the spanwise hole-to-hole (p) is varying from 4d, 6d to 8d for both designs. The film cooling effectiveness measurements were performed in a low-speed wind tunnel in which the turbulence intensity is kept at 6%. There are 36 cases for each design including four blowing ratios (M = 0.5, 1.0, 1.5, and 2.0), three density ratios (DR = 1.0, 1.5, and 2.0), and three hole-to-hole spacing (p/d = 4, 6, and 8). The detailed film cooling effectiveness distributions were obtained by using the steady-state pressure-sensitive paint (PSP) technique. The spanwise-averaged cooling effectiveness are compared over the range of flow parameters. Some interesting observations are discovered including blowing ratio effect strongly depending on geometric design; staggered arrangement of the hole with same orientation does not yield better effectiveness at higher blowing ratio. Currently, film cooling effectiveness correlation of two-row compound angled cylindrical holes is not available, so this study developed the correlations for the inline arrangement of holes with opposing angles and the staggered arrangement of holes with same angles. The results and correlations are expected to provide useful information for the two-row flat plate film cooling analysis.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(4):042203-042203-9. doi:10.1115/1.4042781.

Two biomimetic synthetic jet (SJ) actuators were designed, manufactured, and tested under conditions of a jet impingement onto a wall. Nozzles of the actuators were formed by a flexible diaphragm rim, the working fluid was air, and the operating frequencies were chosen near the resonance at 65 Hz and 69 Hz. Four experimental methods were used: phase-locked visualization of the oscillating nozzle lips, jet momentum flux measurement using a precision scale, hot-wire anemometry, and mass transfer measurement using the naphthalene sublimation technique. The results demonstrated possibilities of the proposed actuators to cause a desired heat/mass transfer distribution on the exposed wall. It was concluded that the heat/mass transfer rate was commensurable with a conventional continuous impinging jets (IJs) at the same Reynolds numbers.

Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2019;141(4):042401-042401-13. doi:10.1115/1.4042770.

In this paper, a lattice Boltzmann (LB) model is established to simulate the gaseous fluid flow and heat transfer in the slip regime under the curved boundary condition. A novel curved boundary treatment is proposed for the LB modeling, which is a combination of the nonequilibrium extrapolation scheme for the curved boundary and the counter-extrapolation method for the macroscopic variables on the curved gas–solid interface. The established numerical model can accurately predict the velocity slip and temperature jump of the microscale gas flow on the curved surface, which agrees well with the analytical solution for the microcylindrical Couette flow and heat transfer. Then, the slip flow and the heat transfer over the single microcylinder are numerically studied in this work. It shows that the velocity slip and the temperature jump are obviously influenced by the Knudsen number and the Reynolds number, and the local Nusselt number depends on which gas rarefaction effect (velocity slip or temperature jump) is dominant. An increase in the Prandtl number leads to a decrease in the temperature jump, which enhances the heat transfer on the microcylinder surface. The numerical simulation of the flow and heat transfer over two microcylinders in tandem configuration are carried out to investigate the wake interference effect. The results show that the slip flow and heat transfer characteristics of the downstream microcylinder are influenced by the wake region behind the upstream cylinder as the spacing is small.

Commentary by Dr. Valentin Fuster

### Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2019;141(4):042501-042501-9. doi:10.1115/1.4042782.

In this study, free convection in a cavity with differentially heated wavy walls is numerically investigated in the presence of a magnetic source. Polyharmonic spline radial basis function (RBF) is utilized to discretize the governing dimensionless equations formulated by stream function-vorticity. The effects of dimensionless Hartmann number, Rayleigh number, the number of undulations, amplitude of wave, and the location of magnetic source are visualized in streamlines and isotherms as well as calculating average Nusselt number through the heated wall. Results show that primary vortex in streamlines is altered with the impact of magnetic source. The augmentation of undulations and amplitude causes convective heat transfer to decrease if Ra = 105. The impact of location of magnetic source is noted close to the top wall.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2019;141(4):042502-042502-9. doi:10.1115/1.4042812.

Numerical investigations have been carried out to predict the near-wall dynamics in indirect natural convection for air (Pr = 0.7) and water (Pr = 5.2). Near-wall flow structures appear to be line plumes. Three-dimensional laminar, steady-state model was used to model the problem. Density was formulated using the Boussinesq approximation. Flux scaling, plume spacing and plume lengths obtained numerically are found to have the same trend with the results available in the literature. Plume length and Nusselt number, Nu exhibits an increasing trend with an increase in Rayleigh number, RaH for both Pr fluids. The plume spacing is found to have an inverse relationship with RaH. The cube root of Rayleigh number based on plume spacing, $Raλ1/3$ is found to have a slight dependence on the dimensionless plume spacing, λ/H. Nu scales as $Nu∼CRaHn$, n =0.26 for air and n =0.3 for water. Heat transfer is thus found to be dominated by near-wall phenomenon. Nu shows a nonlinear relationship with $LpH/A$ and is found to be an accurate representation of heat transfer.

Commentary by Dr. Valentin Fuster

### Research Papers: Two-Phase Flow and Heat Transfer

J. Heat Transfer. 2019;141(4):042901-042901-18. doi:10.1115/1.4041830.

Recently, two-phase cryogenic flow boiling data in liquid nitrogen (LN2) and liquid hydrogen (LH2) were compared to the most popular two-phase correlations, as well as correlations used in two of the most widely used commercially available thermal/fluid design codes in Hartwig et al. (2016, “Assessment of Existing Two Phase Heat Transfer Coefficient and Critical Heat Flux on Cryogenic Flow Boiling Quenching Experiments,” Int. J. Heat Mass Transfer, 93, pp. 441–463). Results uncovered that the correlations performed poorly, with predictions significantly higher than the data. Disparity is primarily due to the fact that most two-phase correlations are based on room temperature fluids, and for the heating configuration, not the quenching configuration. The penalty for such poor predictive tools is higher margin, safety factor, and cost. Before control algorithms for cryogenic transfer systems can be implemented, it is first required to develop a set of low-error, fundamental two-phase heat transfer correlations that match available cryogenic data. This paper presents the background for developing a new set of quenching/chilldown correlations for cryogenic pipe flow on thin, shorter lines, including the results of an exhaustive literature review of 61 sources. New correlations are presented which are based on the consolidated database of 79,915 quenching points for a 1.27 cm diameter line, covering a wide range of inlet subcooling, mass flux, pressure, equilibrium quality, flow direction, and even gravity level. Functional forms are presented for LN2 and LH2 chilldown correlations, including film, transition, and nucleate boiling, critical heat flux, and the Leidenfrost point.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Heat Transfer. 2019;141(4):044501-044501-4. doi:10.1115/1.4042813.

A generalized effective medium theory (EMT) is proposed to account for the fractal structure of the dispersed phase in a dispersing medium under the dilute limit. The thermal conductivity of nanofluids with fractal aggregates is studied using the proposed model. Fractal aggregates are considered as functionally graded spherical inclusions and its effective thermal conductivity is derived as a function of its fractal dimension. The results are studied for self-consistency and accuracy within the limitations of the analytical approximations used.

Commentary by Dr. Valentin Fuster

### Discussion

J. Heat Transfer. 2019;141(4):045501-045501-1. doi:10.1115/1.4042783.

The present comment concerns some doubtful results in the above paper.

Commentary by Dr. Valentin Fuster