Research Papers: Conduction

J. Heat Transfer. 2018;140(9):091301-091301-13. doi:10.1115/1.4040085.

Modeling of steady-state thermal conduction for crack and v-notch in anisotropic material remains challenging. Conventional numerical methods could bring significant error and the analytical solution should be used to improve the accuracy. In this study, crack and v-notch in anisotropic material are studied. The analytical symplectic eigen solutions are obtained for the first time and used to construct a new symplectic analytical singular element (SASE). The shape functions of the SASE are defined by the obtained eigen solutions (including higher order terms), hence the temperature as well as heat flux fields around the crack/notch tip can be described accurately. The formulation of the stiffness matrix of the SASE is then derived based on a variational principle with two kinds of variables. The nodal variable is transformed into temperature such that the proposed SASE can be connected with conventional finite elements (FE) directly without transition element. Structures of complex geometries and complicated boundary conditions can be analyzed numerically. The generalized flux intensity factors (GFIFs) can be calculated directly without any postprocessing. A few numerical examples are worked out and it is proven that the proposed method is effective for the discussed problem, and the structure can be analyzed accurately and efficiently.

Commentary by Dr. Valentin Fuster

Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2018;140(9):091501-091501-10. doi:10.1115/1.4039397.

Heat transfer and pressure drop characteristics of R-134a boiling in a chevron-patterned brazed plate heat exchanger (BPHE) are studied experimentally. With corrugated BPHE channels having hydraulic diameter of 3.4 mm and low refrigerant mass flux, boiling near the micro-macroscale transition is speculated. Heat exchanger performance is characterized with varying mass flux (30–50 kgm−2s−1), saturation pressure (675 kPa and 833 kPa), heat flux (0.8 and 2.5 kWm−2), and vapor quality (0.1–0.9). The two-phase refrigerant heat transfer coefficient increases with heat flux as often observed during nucleate boiling. It also weakly increases with saturation pressure and the associated lower latent heat during convective boiling; heat transfer is improved by the decreased liquid film thickness surrounding confined bubbles inside the narrow BPHE channels, which is the main characteristic of microscale boiling. As often observed in macroscale boiling, the inertial forces of the liquid and vapor phases cause an unsteady annular film, leading to premature partial dryout. The onset of dryout is accelerated at the lower saturation pressure, due to increased surface tension, another microscale-like characteristic. Higher surface tension retains liquid in sharp corners of the corrugated channel, leaving lateral surface areas of the wall dry. Two-phase pressure drop increases with mass flux and vapor quality, but with decreasing saturation pressure. Dryout decreases the friction factor due to the much lower viscosity of the gas phase in contact with the wall. Several semi-empirical transition criteria and correlations buttress the current analyses that the thermal-fluidic characteristics peculiar to BPHEs might be due to macro-microscale transition in boiling.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):091502-091502-11. doi:10.1115/1.4039911.

This paper presents corrections for existing hydrodynamic instability-based critical heat flux (CHF) models in pool boiling by taking into account the effect of the viscosity, geometry and size of the liquid–vapor interface. Based on the existing literature, the Kelvin–Helmholtz (KH) theory, used by the most commonly adopted CHF models, can lead to noticeable errors when predicting the instability conditions. The errors are mainly due to the inaccuracy of the inviscid flow assumptions and the oversimplification of the interface geometry. In addition, the literature suggests the most unstable condition predicted by the viscous correction for viscous potential flow (VCVPF) theory for the cylindrical interfaces best match the observed air column breakup conditions in water. In this paper, the most unstable instability conditions predicted by the VCVPF theory are used to correct the existing CHF models. The comparison between the existing and corrected CHF models suggests that the corrected models always predict a higher CHF value. In addition, the corrected Zuber model predicts similar CHF value to the Lienhard and Dhir model. The comparison with experimental data suggests that the correction to the Zuber model can increase its prediction accuracy in most cases, but not necessary for the Lienhard and Dhir model. When compared to experimental CHF data for boiling cryogens at different pressures, the corrected CHF models are consistently more accurate than the original CHF models.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):091503-091503-11. doi:10.1115/1.4040083.

In this work, the condensation of refrigerants on a single, high-density, low-fin tube and full-sized shell and tube condensers were investigated experimentally. The low-fin tube had an external fin density of 56 fins per inch (fpi) and fin height 1.023 mm. Another three-dimensional (3D) finned tube was also tested for comparison. The condensing heat transfer coefficient of the refrigerant R134a was first investigated outside a single horizontal tube at saturation temperature of 40 °C. The overall heat transfer coefficients of the two tubes were similar in magnitude. The condensing heat transfer coefficient of the low-fin tube was 16.3–25.2% higher than that of 3D enhanced tube. The experiments of the two condensers mounted with low-fin and 3D enhanced tubes were then conducted in centrifugal and screw chiller test rigs. It was found that chillers with the two different condensers generally had the same refrigeration capacity under the same experiment conditions. The refrigeration capacity of the screw chiller was smaller. It had fewer tube rows and elicited fewer inundation effects owing to the falling condensate. The heat transfer coefficients of the condensers with R134a in centrifugal chillers equipped with high-density low-finned tubes were higher than those in the screw chillers. The total number of tubes for low-fin tube condensers, in the two chillers, was reduced by approximately 15% compared with the use of domestic advanced condensers equipped with the 3D enhanced tubes.

Commentary by Dr. Valentin Fuster

Research Papers: Forced Convection

J. Heat Transfer. 2018;140(9):091701-091701-9. doi:10.1115/1.4039765.

The paper gives a comprehensive study on the space fractional boundary layer flow and heat transfer over a stretching sheet with variable thickness, and the variable magnetic field is applied. Novel governing equations with left and right Riemann–Liouville fractional derivatives subject to irregular region are formulated. By introducing new variables, the boundary conditions change as the traditional ones. Solutions of the governing equations are obtained numerically where the shifted Grünwald formulae are applied. Good agreement is obtained between the numerical solutions and exact solutions which are constructed by introducing new source items. Dynamic characteristics with the effects of involved parameters on the velocity and temperature distributions are shown and discussed by graphical illustrations. Results show that the velocity boundary layer is thicker for a larger fractional parameter or a smaller magnetic parameter, while the temperature boundary layer is thicker for a larger fractional parameter, a smaller exponent parameter, or a larger magnetic parameter. Moreover, it is thicker at a smaller y and thinner at a larger y for the velocity boundary layer with a larger exponent parameter while for the velocity and temperature boundary layers with a smaller weight coefficient.

Commentary by Dr. Valentin Fuster

Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2018;140(9):092001-092001-10. doi:10.1115/1.4039759.

A two-year study was conducted to engage undergraduate mechanical engineering students to approach heat transfer education in an active, hands-on manner and excite them to pursue research and graduate studies in the field. Physical workshops were designed and implemented into junior level heat transfer classes, allowing students to feel and observe heat transfer using heat flux and temperature sensors that provided real-time data. These instruments, coupled with open-ended, challenge-based pedagogy, provided opportunities for students to explore important heat transfer concepts, such as the differences between heat and temperature. The conceptual knowledge of the students was assessed through concept-specific questions. These results were compared to those of a control group who took the traditional lecture without the workshops. The results yielded significantly higher scores for the experimental group in the first year but much less of a difference in the second year, which added video-enhanced workshops in place of the purely hands-on workshops. In addition to concept questions, surveys taken by the students reveal that the students much preferred the workshops in either form over not having them. They also believed the workshops strongly enhanced their learning by giving them a real, hands-on experience.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):092002-092002-7. doi:10.1115/1.4039913.

For the heat transfer of pebble or granular beds (e.g., high temperature gas-cooled reactors (HTGR)), the particle thermal radiation is an important part. Using the subcell radiation model (SCM), which is a generic theoretical approach to predict effective thermal conductivity (ETC) of particle radiation, particle-scale investigation of the nuclear packed pebble beds filled with monosized or multicomponent pebbles is performed here. When the radial porosity distribution is considered, the ETC of the particle radiation decreases significantly at near-wall region. It is shown that radiation exchange factor increases with the surface emissivity. The results of the SCM under different surface emissivity are in good agreement with the existing correlations. The discrete heat transfer model in particle scale is presented, which combines discrete element method (DEM) and particle radiation model, and is validated by the transient experimental results. Compared with the discrete simulation results of polydisperse beds, it is found that the SCM with the effective particle diameter can be used to analyze behavior of the radiation in polydisperse beds.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):092003-092003-4. doi:10.1115/1.4040084.

The seeded supersonic oven originally used to produce sodium clusters was incapable for lithium clusters that should be produced at higher temperatures. Ultimately, we designed a new compact stainless steel (SS) oven with thicker walls and constructed two molybdenum alloy (TZM) heaters for this new oven. The newly designed SS oven and heaters have been tested with liquid lithium, and the tested results demonstrated they can successfully work at ∼1000 °C, and a deposition layer of lithium was observed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):092004-092004-7. doi:10.1115/1.4039910.

Invisibility has recently been achieved in optics, electromagnetics, acoustics, thermotics, fluid mechanics, and quantum mechanics; it was realized through a properly designed cloak structure with unconventional (anisotropic, inhomogeneous, and singular) material parameters, which limit practical applications. Here, we show, directly from the solution of Laplace's equation, that two or more conventional (isotropic, homogeneous, and nonsingular) materials can be made thermally invisible by tailoring the many-particle local-field effects. Our many-particle thermal invisibility essentially serves as a new class of invisibility with a mechanism fundamentally differing from that of the prevailing cloaking-type invisibility. We confirm it in simulation and experiment. As an application, the concept of many-particle thermal invisibility helps us propose a class of many-particle thermal diodes: the diodes allow heat conduction from one direction with invisibility, but prohibit the heat conduction from the inverse direction with visibility. This work reveals a different mechanism for thermal camouflage and thermal rectification by using composites, and it also suggests that besides thermotics, many-particle local-field effects can be a convenient and effective mechanism for achieving similar controls in other fields, e.g., optics, electromagnetics, acoustics, and fluid mechanics.

Commentary by Dr. Valentin Fuster

Research Papers: Jets, Wakes, and Impingment Cooling

J. Heat Transfer. 2018;140(9):092201-092201-14. doi:10.1115/1.4039763.

Within the framework of scale resolving simulation techniques, this paper considers the application of the stress-blended eddy simulation (SBES) model to pressure side (PS) film cooling in a high-pressure turbine nozzle guide vane. The cooling geometry exhibits two rows of film cooling holes and a trailing edge cutback, fed by the same plenum chamber. The blowing conditions investigated were in the range of coolant-to-mainstream mass flow ratio (MFR) from 1% to 2%. The flow regime resembles that in a real engine (exit isentropic Mach number of Ma2is = 0.6), but also low speed conditions (Ma2is = 0.2) were considered for comparison purposes. The predicted results were validated with measurements of surface adiabatic effectiveness and instantaneous off-wall visualizations of the flow field downstream of cooling holes and cutback slot. The focus is on SBES ability of developing shear layer structures, because of their strong influence on velocity field, entrainment mechanisms and, thus, vane surface temperature. Special attention has been paid to the development and dynamics of coherent unsteadiness, since measured values of shedding frequency were also available for validation. SBES provided significant improvement in capturing the unsteady physics of cooling jet-mainstream interaction. The effects of changes in flow regime and blowing conditions on vortex structures were well predicted along the cutback surface. As regards the cooling holes, the high speed condition made it difficult to match the experimental Kelvin–Helmholtz breakdown in the shear layer, in the case of high velocity jets.

Commentary by Dr. Valentin Fuster

Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2018;140(9):092401-092401-9. doi:10.1115/1.4039904.

This is a theoretical exploration of the magnetohydrodynamic Carreau fluid in a suspension of dust and graphene nanoparticles. Graphene is a two-dimensional single-atom thick carbon nanosheet. Due to its high thermal conductivity, electron mobility, large surface area, and stability, it has remarkable material, electrical, optical, physical, and chemical properties. In this study, a simulation is performed by mixing of graphene + water and graphene + ethylene glycol into dusty non-Newtonian fluid. Dispersion of graphene nanoparticles in dusty fluids finds applications in biocompatibility, bio-imaging, biosensors, detection and cancer treatment, in monitoring stem cells differentiation, etc. Graphene + water and graphene + ethylene glycol mixtures are significant in optimizing the heat transport phenomena. Initially arising set of physical governing partial differential equations are transformed to ordinary differential equations (ODEs) with the assistance of similarity transformations. Consequential highly nonlinear ODEs are solved numerically through Runge–Kutta Fehlberg scheme. The computational results for nondimensional temperature and velocity profiles are presented through graphs. Additionally, the numerical values of friction factor and heat transfer rate are tabulated numerically for various physical parameter obtained. We also validated the present results with previous published study and found to be highly satisfactory. The formulated model in this study reveals that heat transfer rate and wall friction is higher in mixture of graphene + ethylene glycol when compared to graphene + water.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):092402-092402-12. doi:10.1115/1.4039909.

In this paper, we have reported the effects of Hall current on magnetohydrodynamics (MHD) unsteady heat and mass transfer of Casson nanofluid flow through a vertical plate in the presence of radiation and chemical reaction. The model equations have been used for the Casson nanofluid and they include the effects of thermophoresis and Brownian motion. Then, the obtained model equations have been transformed into nondimensional form by the usual mathematical procedure of transformation and the resultant nondimensional couple of partial differential equations are solved by explicit finite difference technique. Then, the obtained results are plotted after stability test by using the graphical software tecplot-9 and these results indicate the fluid flow, temperature, and concentration distributions which are significantly invaded by the variation of different dimensionless parameters such as magnetic parameter, Schmidt number, thermal Grashof number, Lewis number, Prandtl number, mass Grashof number, Dufour number, thermophoresis parameter, Brownian motion parameter, chemical reaction, and radiation parameter on velocity, temperature, and concentration along with the skin friction coefficient, Nusselt number, and Sherwood number. Further, the results have been discussed also with the help of graphs. Furthermore, it is observed that with the increase of the Casson parameter, velocity puts down, whereas by increasing the heat generation parameter, the temperature profiles are decreased.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):092403-092403-9. doi:10.1115/1.4040141.

Manifold-microchannel (MM) combinations used on heat transfer surfaces have shown the potential for superior heat transfer performance to pressure drop ratio when compared with chevron-type corrugations for plate (frame) heat exchangers (PHEs). This paper presents an advanced genetic algorithm (GA)-based procedure for analyzing and optimizing the MM-based PHE. One distinctive feature of the implementation is the blended variable formulation for the chromosomes to allow the use of continuous variables rather than the bitwise variables in standard GA methods. The resulting GA procedure is particularly well suited for PHEs for several reasons, including the fact that it does not require continuous variables or functional dependence on the design variables. In addition, the computational effort required for the GA technique in the current implementation scales linearly with the number of design variables, making it appropriate for MM-based PHEs, which have several variables. The computed results compare well with experimental data and show better performance compared to conventional PHEs of the same volume utilizing chevron corrugations. Although a full-scale computational fluid dynamics (CFD) analysis may give more accurate results than the semi-empirical approach used in this paper, the former cannot efficiently support rapid concept de-selection during the preliminary stage of design. Optimization based on CFD also can usually not support discontinuous functions. To improve the fidelity of the current analysis, a discrete, finite-volume-type, one-dimensional (1D) reduced-order modeling is carried out, in addition to a purely bulk approach. Our discrete approach obviates the need for the є-NTU-type models.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):092404-092404-10. doi:10.1115/1.4040082.

Thermal effects in monolayer graphene due to an electron flow are investigated with a direct simulation Monte Carlo (DSMC) analysis. The crystal heating is described by simulating the phonon dynamics of the several relevant branches, acoustic, optical, K and Z phonons. The contribution of each type of phonon is highlighted. In particular, it is shown that the Z phonons, although they do not enter the scattering with electrons, play a non-negligible role in the determination of the crystal temperature. The phonon distributions are evaluated by counting the emission and absorption processes during the MC simulation. The crystal temperature raise is obtained for several applied electric fields and for several positive Fermi energies. The latter produces the effect of a kind of n-doping in the graphene layer. Critical temperatures can be reached in a few tens of picoseconds posing remarkable issues regarding the cooling system in view of a possible application of graphene in semiconductor devices. Moreover, a significant influence of the lattice temperature on the characteristic curves is observed only for long times, confirming graphene rather robust as regards the electrical performance.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):092405-092405-9. doi:10.1115/1.4040144.

We theoretically explore the influence of end-group chemistry (bond stiffness and mass) on the interfacial thermal conductance at a gold–alkane interface. We accomplish this using the nonequilibrium Green's function (NEGF) coupled with first principle parameters in density functional theory (DFT) within the harmonic approximation. Our results indicate that the interfacial thermal conductance is not a monotonic function of either chemical parameters but instead maximizes at an optimal set of mass and bonding strength. This maximum is a result of the interplay between the overlap in local density of states (LDOS) of the device and that in the contacts, as well as the phonon group velocity. We also demonstrate the intrinsic relationship between the diffusive mismatch model (DMM) and the properties from NEGF, and provide an approach to get DMM from first principles NEGF. By comparing the NEGF-based DMM conductance and range of conductance while altering the mass and bonding strength, we show that DMM provides an upper bound for elastic transport in this dimension-mismatched system. We thus have a prescription to enhance the thermal conductance of systems at low temperatures or at low dimensions where inelastic scattering is considerably suppressed.

Commentary by Dr. Valentin Fuster

Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2018;140(9):092501-092501-11. doi:10.1115/1.4039764.

Natural circulation loop (NCL) based secondary fluid systems are simple, reliable, and inexpensive due to the absence of any moving components such as pumps. Water-based NCLs are widely used in applications such as solar collectors and nuclear reactors. Also, most of the studies on NCLs do not consider the three-dimensional (3D) variation of the field variables. In the subject work, 3D steady flow simulation of water based, single-phase rectangular NCL with isothermal source and sink has been carried out to study the effects of different design and operating parameters such as loop height, temperature lift, in plane and out of plane tilt angles on the rate of heat transfer, and the rate of entropy generation due to both fluid flow and heat transfer. The rate of entropy generation due to both heat transfer and fluid flow for turbulent flow regimes in a NCL is calculated for a wide range of design and operating parameters. In turbulent flow regimes, the rate of entropy generation due to fluid flow is significant although the rate of entropy generation due to heat transfer is dominant. All the above-mentioned design and operating parameters have significant effect on the rate of entropy generation and the rate of heat transfer as well. With increases in loop height and temperature lift, the rate of entropy generation increases. As the tilt angle increases in the XY plane, the rate of the entropy generation initially increases but after certain tilt angle it starts decreasing.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):092502-092502-10. doi:10.1115/1.4039905.

Present research paper investigates the transient laminar free convective supercritical carbon dioxide flow past a semi-infinite vertical cylinder using numerical methods. Two new thermodynamic models for the supercritical fluid (SCF) flow are considered. Based on these models, for supercritical carbon dioxide, two new equations for thermal expansion coefficient are obtained on the basis of Redlich–Kwong equation of state (RK-EOS) and Van der Waals equation of state (VW-EOS). Based on the calculated values of thermal expansion coefficient, it is shown that not only RK-EOS is closer to experimental values but also gives greater accuracy when compared to VW-EOS validating RK-EOS as suitable model for predicting natural convective properties of carbon dioxide under supercritical condition. The governing equations of SCF flow are solved numerically using Crank–Nicolson implicit finite difference scheme. Numerical simulations are performed for carbon dioxide in the region of its critical point. Results in subcritical, supercritical, and near-critical regions are shown graphically and discussed for different physical parameters. From the obtained numerical results, it is clear that the steady-state time increases for the increasing values of reduced temperature and reduced pressure for carbon dioxide in supercritical region.

Commentary by Dr. Valentin Fuster

Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2018;140(9):092701-092701-10. doi:10.1115/1.4039992.

An inverse radiation-conduction analysis is performed for simultaneous estimation of the thermal properties in an absorbing, emitting, and linear-anisotropically scattering medium with spatially variable refractive index. The discrete ordinates method in conjugation with finite volume method is adopted to solve the direct problem. The conjugate gradient method (CGM) is employed to simultaneously estimate the conduction-radiation parameter, optical thickness, single scattering albedo, scattering phase function, and the wall emissivities from the knowledge of the exit radiation intensities over the boundaries. The effects of these parameters and the measurement errors on the precision of the inverse analysis are investigated. Results show that the proposed inverse approach can successfully retrieve the unknown parameters for different refractive index profiles.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Heat Transfer. 2018;140(9):094501-094501-8. doi:10.1115/1.4039554.

A vector–matrix differential equation is formulated using normal mode analysis from the governing equations of a three-dimensional anisotropic half space in presence of heat source and gravity. The corresponding solution is obtained with the help of eigenvalue approach. Numerical computations for displacement, thermal strain and stress component, temperature distribution are evaluated and presented graphically.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):094502-094502-7. doi:10.1115/1.4039641.

In this research paper, fully developed natural convection flow in a vertical parallel plate's micro-channel in the presence of viscous dissipation is theoretically examined by using a perturbation series method. The effects of velocity slip and temperature jump are taken to consideration. Due to the presence of viscous dissipation, the momentum and energy equations are coupled system of ordinary differential equations. The influences of Knudsen number, fluid wall interaction parameter, and viscous dissipation on the flow formation and heat transfer aspects are demonstrated through graphs and tables. This result indicates that increasing the value of rarefaction parameter decreases the effect of viscous dissipation on the Nusselt number. Furthermore, it is found that the effects of rarefaction parameter as well as buoyancy parameter on temperature and velocity are significantly pronounced in the case of symmetric heating

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):094503-094503-5. doi:10.1115/1.4039912.

This paper reports the changes made in the flow and heat transfer characteristics of a closed enclosure in the presence of sidewalls with symmetrical linear heating. The flow inside the enclosure is primarily driven by a centrally placed discrete heater with thermal radiation included at all surfaces involved. Finite volume method-based computational results corresponding to the resulting steady-state were obtained. The factors causing augmentation and suppression of heat transfer are discussed for two types of sidewall heating. Moreover, it is found that the role of radiation is well stronger than convection in determining the total heat transfer rate when the sidewall heating is decreasing with height.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(9):094504-094504-5. doi:10.1115/1.4039761.

In this work, a numerical study is conducted to investigate film cooling of a corrugated surface. A conjugate heat transfer analysis is carried out, accounting for the presence of thermal barrier coating (TBC) and gas radiation. The Mach number of mainstream flow is maintained at Ma = 0.6, while cold stream Mach number is varied from 0.3 to 0.58, and density ratio is kept 4. From this study, it is observed that the overall film cooling effectiveness increases by a value ranging from 0.10 to 0.15 with the use of TBC. The hot side metallic wall temperature increases in the range of 100–150 °C when the effect of gas radiation is considered. It is also found that the film cooling effectiveness decreases with decrease in the cold side Mach number.

Commentary by Dr. Valentin Fuster

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