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### Research Papers

J. Thermal Sci. Eng. Appl. 2019;11(5):051001-051001-12. doi:10.1115/1.4042584.

The analysis of fluid flow and heat transfer characteristics of double turbulent jet flow impinging on a stationary and moving plate has been numerically studied. Unsteady-state two-dimensional incompressible turbulent forced convection flow is considered for present analysis. Turbulence is modelled by the Reynolds-averaged Navier–Stokes (RANS) equation with the kε model and enhanced wall treatment. The governing equations are solved using a finite volume based commercial solver. The results for the effect of single jet and double jet, jet Reynolds number, plate velocity, location, and center spacing between the two jets on flow and heat transfer characteristics are reported. The results show that the enhancement of heat transfer is $32.70%$ for the double jet compared with the single jet impingement on a stationary plate. As significant enhancement of heat transfer is observed with an increase in the second jet Reynolds number and plate velocity. The results show that the size and shape of the recirculation zones between jets are greatly altered with respect to spacing between the jets to the plate and the center distance between the jets. The results show that the enhancement of heat transfer is $37.3%$ for moving plate velocity due to a decrease in the spacing between the jets and the plate from 6 to 4. Results show that the local peak Nusselt number is influenced by the plate velocity. These results are validated by experimental and numerical results available in the literature.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051002-051002-10. doi:10.1115/1.4042586.

The purpose of this study is to examine the instabilities of a two-dimensional mixed convection boundary layer flow induced by an impinging ascending flow on a heated horizontal cylinder. A significant amount of works is done in recent years on this problem because of its wide range of applications. However, they did not check the stability of the flow in the face of small disturbances that occur in reality. For this, we adopt the linear stability theory by first solving the steady basic flow and then solving the linear perturbed problem. Thus, the governing equations of the basic flow are reduced to two coupled partial differential equations and solved numerically with the Keller-Box method. The corresponding steady solution is obtained, by varying the position along the cylinder’s surface, for different values of Richardson number (λ) and Prandtl number (Pr), up to, respectively, 3000 and 20. To examine the onset of thermal instabilities, the linear stability analysis is done using the normal mode decomposition with small harmonic disturbances. The Richardson number λ is chosen as the control parameter of these instabilities. The resulting eigenvalue problem is solved numerically by the use of the pseudospectral method based on the Laguerre polynomials. The computed results for neutral and temporal growth curves are depicted and discussed in detail through graphs for various parametric conditions. The critical conditions are illustrated graphically to show from which thermodynamic state, the flow begins to become unstable. As a main result, from ξ = 0 to ξπ/3, we found that forced and mixed convection flow cases are linearly stable in this region. However, in free convection case (λ > 100), it appears that the stagnation zone is the most unstable one and then the instability decreases along the cylinder’s surface up to the limit of its first third, thus giving the most stable zone of the cylinder. Beyond ξ ≈ 1.2, strong instabilities are noted also for low values of Richardson number, and the flow tends to an unstable state even in the absence of thermal effect, i.e., hydrodynamically unstable Ri = 0, probably due to the occurring of the boundary layer separation.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051003-051003-11. doi:10.1115/1.4042588.

This article presents a numerical study of particle deposition in two fluids, i.e., liquid and droplet flow in a single row tube bundle heat exchanger. The tubes in the heat exchanger are modeled as heating sources. Two level-set functions are used to capture the liquid-droplet interface and the liquid-deposit front. The effects of different parameters, including Damköhler number, thermal conductivity of the deposit, viscosity of the liquid, and the heating power of the tube on the flow and heat transfer, are investigated. The deposit profiles on the tube surface are analyzed. Comparison is made for the averaged Nusselt number for the case without and with deposition. It is found that the tube surface has a thicker deposit at the upstream facing side compared with that of the downstream facing side. Generally, the heat transfer rate reduces with the growth of the deposit. Under certain conditions, heat transfer can be increased because of the increase in fluid velocity due to blockage of the flow area by the deposit. The averaged Nusselt number oscillated temporally in response to the droplet movement across the tube. Generally, the temperature at the liquid-deposit front decreases with thicker deposit formed. The averaged Nusselt number along the liquid-deposit front increases to a critical value initially, and it starts to decrease with the growth of the deposit.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051004-051004-9. doi:10.1115/1.4042589.

Lignocellulosic woody biomasses such as rhododendron (RD), ash tree (AT), and hybrid poplar (HP) were heated under N2 at 200 °C and 400 °C, which are regarded as outside the range of efficient torrefaction temperatures. Also, several Turkish brown coals were carbonized at 750 °C for comparison. The obtained biochars/chars were characterized by scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR), and thermal analysis. Combustion reactivity of the raw samples and the chars was estimated using the burning profiles. Burning kinetics was established by the Borchardt and Daniels (B&D) kinetic analysis method that was based on the evaluation of the differential scanning calorimetry (DSC) data. Ignition index (Ci), burnout index (Cb), comprehensive combustibility index (S), and burning stability index (DW) were considered to evaluate the combustion performance. It was concluded that although treatment at 200 °C did not lead to considerable changes on the biomass structure, the combustion performance of the treated biomass became highly improved in comparison with the raw biomass. However, treatment at 400 °C led to serious variations in the biomass structure mainly due to reduction in O content and volatiles so that the fuel properties and the burning characteristics were affected, and the combustion performance was negatively influenced.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051005-051005-10. doi:10.1115/1.4042855.

Natural-convection cooling with oil or other fluids of high Prandtl number plays an important role in many technical applications such as transformers or other electric equipment. For design and optimization, one-dimensional (1D) flow models are of great value. A standard configuration in such models is flow between vertical parallel plates. Accurate modeling of heat transfer, buoyancy, and pressure drop for this configuration is therefore of high importance but gets challenging as the influence of buoyancy rises. For increasing ratio of Grashof to Reynolds number, the accuracy of one-dimensional models based on the locally forced-flow assumption drops. In the present work, buoyancy corrections for use in one-dimensional models are developed and verified. Based on two-dimensional (2D) simulations of buoyant flow using finite-element solver COMSOL Multiphysics, corrections are derived for the local Nusselt number, the local friction coefficient, and a parameter relating velocity-weighted and volumetric mean temperature. The corrections are expressed in terms of the ratio of local Grashof to Reynolds number and a normalized distance from the channel inlet, both readily available in a one-dimensional model. The corrections universally apply to constant wall temperature, constant wall heat flux, and mixed boundary conditions. The developed correlations are tested against two-dimensional simulations for a case of mixed boundary conditions and are found to yield high accuracy in temperature, wall heat flux, and wall shear stress. An application example of a natural-convection loop with two finned heat exchangers shows the influence on mass-flow rate and top-to-bottom temperature difference.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051006-051006-8. doi:10.1115/1.4043091.

Free convection from an upward facing radial heat sink with fins at an equal angular gap attached to an isothermal base has been investigated numerically. The governing equations in primitive variables were changed to vorticity-vector potential formulation, and an in-house code was developed using finite difference technique. To close the computational domain, two pseudo boundaries were considered. Length, height, and number of fins strongly influence the rate of heat transfer while the fin thickness has a marginal role. As the fin length increases, the rate of heat transfer first increases and then remains almost unaffected. However, the active length of the fins depends on the strength of buoyancy. Heat transfer continuously increases with fin height but with diminishing effect. Adding more number of fins has two opposing effects. It provides more surface area for convection, but at the same time, the induced air is unable to reach the interior of the heat sink making the inner portion of the fins inoperative. As a result of these two opposing influences, heat transfer increases in the beginning and then decreases as more fins are added. This article suggests various fin parameters to achieve maximum cooling. In addition, one can estimate the rate of cooling to be achieved by any radial heat sink.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051007-051007-9. doi:10.1115/1.4042591.

The shell condenser is one of the key components of underwater vehicles. To study its thermal performance and to design a more efficient structure, a computational model is generated to simulate condensation inside straight and helical channels. The model combines empirical correlations and a MATLAB-based iterative algorithm. The vapor quality is used as a sign of the degree of condensation. Three calculation models are compared, and the optimal model is verified by a comparison of simulated results and available experimental data. Several cases are designed to reveal the effects of various inlet conditions and the diameter-over-radius (Dh/R) ratio. The results show that the inlet temperature and mass rate significantly affect the flow and heat transfer in the condensation process, the heat transfer capabilities of the helical channels are much better than that of the straight channel, and both the heat transfer coefficient and total pressure drop increase with the decrease of Dh/R. This study may provide a useful reference for performance prediction and structural design of shell condensers used for underwater vehicles and may provide a relatively universal prediction model for condensation in channels.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051008-051008-12. doi:10.1115/1.4042857.

In many convective liquid–vapor phase-change heat transfer engineering applications, cryogenic fluids are widely used in industrial processes, spacecraft and cryosurgery systems, and so on. For example, cryogens are usually used as liquid fuels such as liquid hydrogen, liquid methane, and liquid oxygen in the rocket industry, liquid nitrogen and helium are frequently used to cool superconducting magnetic device for medical applications. In these systems, proper transport, handling, and storage of cryogenic fluids are of extreme importance. Among all the cryogenic transport processes performed in room temperatures, quenching, also termed chilldown, is an unavoidable initial, transient phase-change heat transfer process that brings the system down to the cryogenic condition. The Leidenfrost temperature or rewet temperature that signals the end of film boiling is practically considered the completion point of a quenching process. Therefore, rewet temperature has been considered the most important parameter for the engineering design of cryogenic thermal management systems. As most of the previous correlations for predicting the Leidenfrost temperature and the rewet temperature have been developed for water, they are shown to disagree with recent liquid nitrogen pipe chilldown experiments in upward and downward flow directions over a wide range of flow rates, pressures, and degrees of inlet subcooling. In addition to a complete review of the literature, two modified correlations are presented, one based on bubble growth and another based on the theoretical maximum limit of superheat. Each correlation performs well over the entire dataset.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051009-051009-13. doi:10.1115/1.4043006.

Heat transfer often occurs effectively from horizontal elements of relatively complex shapes in natural convective cooling of electronic and electrical devices used in industrial applications. The effect of complex surface shapes on laminar natural convective heat transfer from horizontal isothermal polygons of hexagonal and octagonal flat surfaces facing upward and downward of different aspect ratios has been numerically investigated. The polygons’ surface is embedded in a large surrounding plane adiabatic surface, where the adiabatic surface is in the same plane as the surface of the heated element. For the Boussinesq approach used in this work, the density of the fluid varies with temperature, which causes the buoyancy force, while other fluid properties are assumed constants. The numerical solution of the full three-dimensional form of governing equations is obtained by using the finite volume method-based computational fluid dynamics (CFD) code, FLUENT14.5. The solution parameters include surface shape, dimensionless surface width, different characteristic lengths, the Rayleigh number, and the Prandtl number. These parameters are considered as follows: the Prandtl number is 0.7, the Rayleigh numbers are between 103 and 108, and for various surface shapes the width-to-height ratios are between 0 and 1. The effect of different characteristic lengths has been investigated in defining the Nusselt and Rayleigh numbers for such complex shapes. The effect of these parameters on the mean Nusselt number has been studied, and correlation equations for the mean heat transfer rate have been derived.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051010-051010-12. doi:10.1115/1.4042587.

A numerical model has been developed to simulate the growth of an equaixed binary alloy dendrite under the combined effect of thermal anisotropy and forced convection. A semi implicit–explicit approach is used where the velocity and pressure fields are solved implicitly using the SIMPLER algorithm, while energy and species conservation equations are treated explicitly. The effect of thermal anisotropy present in the solid crystal is implemented by the addition of a departure source term in the conventional isotropic heat transfer based energy equation. The departure source represents the anisotropic part of the diffusive term in the isotropic heat transfer based energy equation. Simulations were performed to find the relative effect of convection strength and thermal anisotropy on the growth rate and morphology of a dendrite. Subsequently, parametric studies were conducted to investigate the effect of thermal anisotropy ratio, inlet flow velocity, undercooling temperature, and the relative strength of the thermal to mass diffusivity ratio by analyzing the variation of the equilibrium tip velocity of the top and left arms, the arm length ratio (ALR), and the equivalent grain radius. Based on simulations, a chart has been developed, which demarcates different regimes in which convection or thermal anisotropy is the most dominant factor influencing the dendrite growth rate. The model has also been extended to study the growth of multiple dendrites with random distribution and orientation. This can be useful for the simulation of microstructure evolution under the combined effect of convection and thermal anisotropy.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051011-051011-9. doi:10.1115/1.4042590.

Air-cooled condenser (ACC) design methodologies use empirical correlations that are unable to account for the complex flow phenomena associated with ACCs. Numerical models are seen as an alternative evaluation tool. This paper details the development of a modeling strategy for an ACC in the computational fluid dynamics (CFD) code of OpenFOAM. The axial flow fan is modeled using the extended actuator disk model (EADM) and validated using the B2a-fan. A good agreement between experimental and numerical results are noted for the volumetric flow rates expected in the ACC operating range. The A-frame heat exchanger is also validated using the empirical data. The ACC operating point is numerically and analytically determined. An overprediction of the numerical results to the analytical solution is attributed to the presence of kinetic energy recovery and validated using experimental results. A numerical recovery coefficient of 0.527 is measured and correlates well with the experimentally determined coefficient of 0.553.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051012-051012-11. doi:10.1115/1.4042856.

Earth air tunnel heat exchanger (EATHE) is a capable and quite simple passive technique which may be utilized for space cooling/heating using the constant temperature of underground subsoil. However, it could not gain much attraction as a heating/cooling system as it requires larger trench lengths to accommodate longer pipes. Larger trench lengths involve huge excavation cost and a sufficiently large piece of land. The length of the trench needed can be reduced substantially by adopting a proper pipe layout. In the present study, the performance of U-shaped, slinky-coil, and helical-coil pipe layouts of an EATHE system is compared numerically using ANSYS FLUENT 15.0. Results reveal that the temperature drop and heat transfer rate per unit trench length are higher in the slinky-coil pipe layout than in U-shaped and helical-coil pipe layouts. After 12 h of continuous operation, the effectiveness of the EATHE system with U-shaped, slinky-coil, and helical-coil pipe layouts is obtained as 0.60, 0.80, and 0.78, respectively. The study reveals that the selection of pipe layout for the EATHE system mainly depends on temperature drop EATHE is capable of giving, heat transfer rate, pumping power required, and ease of fabrication and installation as all these factors directly affect the initial and recurring capital investment for the EATHE system.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051013-051013-23. doi:10.1115/1.4042858.

In this article, the natural convection process in a two-dimensional cold square enclosure is numerically investigated in the presence of two inline square heat sources. Two different heat source boundary conditions are analyzed, namely, case 1 (when one heat source is hot) and case 2 (when two heat sources are hot), using the in-house developed flexible forcing immersed boundary–thermal lattice Boltzmann model. The isotherms, streamlines, local, and surface-averaged Nusselt number distributions are analyzed at ten different vertical eccentric locations of the heat sources for Rayleigh number between 103 and 106. Distinct flow regimes including primary, secondary, tertiary, quaternary, and Rayleigh–Benard cells are observed when the mode of heat transfer is changed from conduction to convection and heat sources eccentricity is varied. For Rayleigh number up to 104, the heat transfer from the enclosure is symmetric for the upward and downward eccentricity of the heat sources. At Rayleigh number greater than 104, the heat transfer from the enclosure is better for downward eccentricity cases that attain a maximum when the heat sources are near the bottom enclosure wall. Moreover, the heat transfer rate from the enclosure in case 2 is nearly twice that of case 1 at all Rayleigh numbers and eccentric locations. The correlations for heat transfer are developed by relating Nusselt number, Rayleigh number, and eccentricity of the heat sources.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051014-051014-7. doi:10.1115/1.4042859.

Calcium chloride hexahydrate (CaCl2·6H2O) is an attractive candidate as a phase change material for supplemental cooling in air-cooled thermal power-plant since it has a low-phase transition temperature of 29.3 °C and a relatively large volumetric energy storage density of 289 MJ/m3. The volumetric energy storage density is approximately double the energy densities of comparable paraffins with similar melting points. However, calcium chloride hexahydrate often requires high degree of supercooling to initiate solidification and long-term thermal stability has impeded the adoption of calcium chloride hexahydrate in the latent heat thermal energy storage system. There are only a few literatures which report on long-term stability of pure calcium chloride hexahydrate accurately. In this paper, the effects of sodium chloride and strontium chloride in mitigating supercooling in calcium chloride hexahydrate over 1000 melt–freeze cycles and thermal stability at elevated temperatures were studied in large sample size. Since there is not much data available on calcium chloride hexahydrate with nucleating additives, the current data available do not provide an insight into the effects of thermal cycling on supercooling. Therefore, this study also aims to measure the reliability of calcium chloride hexahydrate and report it, in terms of variations in melting temperature, supercooling, energy storage density, and change in mass over 1000 melt–freeze cycles. The results have shown that strontium chloride as heterogenous nucleators reduces supercooling by 2.5 °C and survived up to 1000 melt–freeze cycles (i.e., 2.7 years).

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051015-051015-9. doi:10.1115/1.4043386.

The present research work is undertaken to develop ASHRAE like standard rating charts for currently used refrigerants R-134a and R-410A and their potential low global warming potential (GWP) substitutes R-1234yf and R-32, respectively. A self-adjustable mass prediction algorithm has been developed using an averaging technique. Based on this, a matlab code dynamically linked to refprop v. 9.0 software has been developed that solves governing equations of mass, momentum, and energy. Two-phase flow inside the capillary tube is assumed homogeneous and metastability is ignored in the proposed model. The proposed numerical models are in good agreement with the available experimental data with overall percentage mean deviation is less than 6%. Coil diameter plays an important role in adjusting the mass flow rate in the helical capillary tube. Coiling of capillary tube causes an increase in friction pressure drop and a reduction in refrigerant mass flow rate. It has been found that the mass flow rate reduces by about 5% as coil diameter is reduced from 120 to 20 mm.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051016-051016-10. doi:10.1115/1.4043089.

Several seawater desalination technologies have been developed and widely used during the last four decades. In the current investigation, a new approach to the seawater desalination process is presented, which utilizes microencapsulated phase change materials (MEPCMs) and thin film evaporation. In this process, the MEPCMs were placed into hot seawater. Then, the hot seawater and the MEPCMs containing the liquid phase change material (PCM) were ejected into a vacuum flash chamber. A thin liquid film of seawater was formed on the surface of the MEPCM, which subsequently vaporized. This evaporation significantly increased the evaporation heat transfer and enhanced the desalination efficiency. Film evaporation on MEPCM surfaces decreased their temperature by absorbing sensible heat. If their temperature was lower than the phase change temperature, the MEPCM would change phase from liquid to solid releasing the latent heat, resulting in further evaporation. The MEPCMs were then pumped back into the hot seawater, and the salt residue left on the MEPCMs could be readily dissolved. In this way, the desalination efficiency could be increased and corrosion reduced. A mathematical model was developed to determine the effects of MEPCM and thin film evaporation on desalination efficiency. An analytical solution using Lighthill's approach was obtained. Results showed that when MEPCMs with a radius of 100 µm and a water film of 50 µm were used, the evaporation rate and evaporative capacity were significantly increased.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051017-051017-8. doi:10.1115/1.4043385.

Slug flow is a common flow pattern in the liquid–liquid two-phase flow in microchannels. It is an ideal pattern for mass transfer enhancement. Many factors influence the slug formation such as the channel geometries (channel widths, channel depth), flow rates of the two phase, and physical properties. In this paper, in order to investigate the liquid–liquid two-phase slug formation in a T-junction microchannel quantitatively, the volume of fluid (VOF) method is adopted to simulate the whole slug formation process. With the validated model, the effects of the disperse phase channel width, channel depth, and two-phase flow rate ratio on slug formation frequency and slug size (slug volume and slug length) are analyzed with dimensionless parameters. Dimensionless parameters include the disperse-to-continuous phase channel width ratio, height-to-width ratio, and two-phase flow rate ratio. Results show that both the channel geometry and two-phase flow rate ratio have a significant influence on slug formation. Compared with the conventional slug formation stages, a new stage called the lag stage emerges when the disperse phase channel width decreases to half of the continuous phase channel width. When the channel depth decreases to one third of the continuous phase channel width, the flow patterns become unstable and vary with the two-phase flow rate ratio. Moreover, empirical correlations are proposed to predict the slug formation frequency. The correlation between slug formation frequency and slug volume is quantified.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051018-051018-5. doi:10.1115/1.4043466.

The main goal of this work is to obtain semi-analytical solutions for the geometric optimization of fins considering the presence of a primary surface. Optimized fin dimensions devoted to finned surfaces are already available in the literature; however, these solutions have as assumption a known fin base temperature. In contrast, in many practical situations, employed fins are attached to a primary surface that has restricted dimensions due to design requirements. In these cases, the primary surface dimensions are kept fixed and the known condition is a temperature or convective condition prescribed on the primary surface boundary. For such scenarios, to the best of our knowledge, there are no simple analytical expressions derived in the literature. Thus, reasoning on this main problem, in the present work, analytical optimum expressions are derived in order to deal with this kind of applications. The obtained analytical equations are subsequently compared with optimum results acquired from numerical calculations. In summary, both analytical and numerical results show a good agreement. The main contribution of this work is to provide closed solutions for the mentioned optimization problem, thus allowing the construction of simple charts, facilitating the corresponding fin design process.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051019-051019-8. doi:10.1115/1.4043469.

The horizontally oriented jet flame induced by rectangular source impinging upon the opposite wall is actually common in the chemical industry, but the related studies are limited. In this paper, the computational fluid dynamics codes are carried out to investigate the temperature profile in thermal impinging flow of the horizontally oriented methane jet flame with rectangular source, which the rectangular orifice is 400 mm2 with three different aspect ratios (L/W = 1, 2, 4); besides, the jet velocities vary from 27.5 m/s to 125 m/s. As the horizontally oriented methane jet flame impinges on the vertical plate in front of the fuel orifice directly, the vertical temperature along the opposite plate is focused on. Results show that the temperature near the impingement point is the same for different jet velocities, but the temperature along the vertical direction is larger with increasing fuel jet velocity. Moreover, the orifice aspect ratio has a significant effect on the temperature, which increases with the aspect ratio at a given position for the momentum-controlled flame. The effective heat release rate on the basis of unburned fuel and ellipse flame shape hypothesis is put forward to correlate the temperature profile. Finally, a new correlation to illustrate the vertical temperature rising along the opposite plate is proposed in light of the orifice aspect ratio and fuel jet velocity, and the predictions obtained by the proposed model agree well with the numerical results, which is applicable for the horizontally oriented flame with rectangular source impinging upon the opposite wall.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051020-051020-9. doi:10.1115/1.4043533.

An experimental device was designed to perform the thermal and dynamic study of natural convection airflow in an open vertical channel. The two side walls of the vertical channel are made of Plexiglas allowing the visualization of the flow via the particle image velocimetry (PIV) method. For the two other vertical walls, one is heated at a constant temperature, and the other is insulated with a 9-cm thick polystyrene insulation. The dynamic characterization of convection is carried out by nonintrusive measurements (PIV), and thermal phenomena are analyzed using nonintrusive heat flux instrumentation (simultaneous temperature and velocity measurements have been carried out across the channel at different elevations). Moreover, this study deals with the influence of the Rayleigh number on the measured vertical velocity profiles as well as the thermal flux densities recorded along the heated wall. To do this, different values of the modified Rayleigh numbers were considered in the interval with the channel aspect ratio of A = 5 and A = 12.5. The obtained Nusselt number values have been compared successfully with those of the literature. The impacts of the Rayleigh number and the aspect ratio on the velocity profiles and the convective and radiative heat transfer have been examined.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051021-051021-9. doi:10.1115/1.4043548.

The ultra compact combustor (UCC) aims to increase the thrust-to-weight ratio of an aircraft gas turbine engine by decreasing the size, and thus weight, of the engine’s combustor. The configuration of the UCC as a primary combustor enables a unique cooling scheme to be employed for the hybrid guide vane (HGV). A previous effort conducted a computational fluid dynamics (CFD) analysis that evaluated whether it would be possible to cool this vane by drawing in freestream flow at the stagnation region of the airfoil. Based on this study, a cooling scheme was designed and modified with internal supports to make additive manufacturing of the vanes possible. This vane was computationally evaluated comparing the results with those of a solid vane and hollow vane without cooling holes as a demonstration of the improvements offered by this design. Furthermore, the effects of the internal support structure were deemed beneficial to surface cooling when evaluated through comparisons of internal pressure distribution and overall effectiveness. Following the computational study, the vane was manufactured and experimentally evaluated with the results compared to those of an uncooled solid vane. The experimental results validated the computational analysis and demonstrated through pressure and temperature measurements that the cooled vane had a reduced surface temperature compared to the uncooled vane and that pressure distributions supported coolant flow through film-cooling holes.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):051022-051022-6. doi:10.1115/1.4043625.

We have developed the novel oxygen-enriched burner using the self-induced oscillating phenomenon of jet flow which has excellent radiation heat transfer characteristics. We revealed that the flame property suitable for radiation heat transfer is obtained by extending the oscillation cycle. By optimizing the connecting tube length and diameter, we got the stable oscillating flame in 1 s-oscillation period. At this time, the relationship between the burner structure including the connecting tube and the oscillation period can be easily evaluated by the dimensionless tube length and Strouhal number. In addition, combining multi-staged combustion technology, the burner can improve heat transfer efficiency by 9.1% compared with the conventional oxygen-enriched burner. We confirmed that the burner has a low NOx emission level below 55 ppm.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Thermal Sci. Eng. Appl. 2019;11(5):054501-054501-6. doi:10.1115/1.4042854.

In this study, an analytical approach is used to find expressions for the closure time in freezing processes for spheres and cylindrical tubes. The starting point is the well-known two-phase Stefan problem. A new characteristic solution is established for extending the theory of constant heat flux ratio. Next, temperature profiles are assumed and substituted into the interface equation, which are then solved for the inward freezing process to get the closure time. Plots are generated to compare the new expressions to previously published experimental results of closure time. The new analytical approximations give reasonable outcomes as discussed in this paper. This paper demonstrates a general approach that can be further applied to different types of phase-change problems.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):054502-054502-7. doi:10.1115/1.4042592.

Phase-change materials (PCMs) are a useful alternative to more traditional methods of thermal management of various applications. PCMs are materials that absorb large amounts of latent heat and undergo solid-to-liquid phase change at near-constant temperature. The goal of the research is to experimentally investigate the thermal properties of a novel shape-stabilized PCM/HDPE composite extruded filament. The extruded filament can then be used in a 3D printer for custom PCM/HDPE shapes. The PCM used in the study is PureTemp PCM 42, which is an organic-based material that melts around 42 °C. Four PCM/HDPE mixtures were investigated (all percentages by mass): 20/80, 30/70, 40/60, and 50/50. Preliminary findings include differential scanning calorimeter (DSC) measurements of melting temperature and latent heat as well as scanning electron microscope (SEM) pictures of filament composition.

Commentary by Dr. Valentin Fuster

### Discussion

J. Thermal Sci. Eng. Appl. 2019;11(5):055501-055501-1. doi:10.1115/1.4042585.

The present discussion concerns some doubtful results included in the above paper.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):055502-055502-1. doi:10.1115/1.4042910.

The authors used a different form than the commonly used Boussinesq approximation of the axial momentum equation to model mixed convection over an inclined surface. The common Boussinesq approximation form of the governing equations was used by Chen et al. [1] for regular (instead of nano) fluids. The authors proceeded to present their analyses based on this uncommon form of the governing equations.

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):055503-055503-2. doi:10.1115/1.4043092.

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

Commentary by Dr. Valentin Fuster
J. Thermal Sci. Eng. Appl. 2019;11(5):055504-055504-1. doi:10.1115/1.4043534.

The present discussion queries some results included in the above paper.

Commentary by Dr. Valentin Fuster