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

J. Heat Transfer. 2018;140(6):061501-061501-10. doi:10.1115/1.4038572.

We use the configuration of film boiling on a horizontal tube positioned in a stagnant pool of saturated diethyl carbonate (DEC, $(C2H5O)2CO$) to study DEC decomposition at temperatures up to 1500 K. The composition of bubbles that percolate through the liquid pool is measured and the results are used to infer the decomposition reactions. The results show that below tube temperatures of about 1100 K, the decomposition products are ethylene (C2H4), carbon dioxide (CO2), and ethanol (EtOH, C2H5OH) with a molar ratio nC2H4/nCO2∼1, which is consistent with a first-order decomposition process. At higher temperatures, nC2H4/nCO2 > 1 which is explained by an additional route to forming C2H4 from radicals in the system (created by EtOH decomposition) attacking DEC. The presence of H2, CO, CH4, and C2H6 in the product stream was noted at all temperatures examined with concentrations that increased from trace values at low temperatures to values comparable to the DEC unimolecular process at the highest temperatures. Formation of a carbon layer on the tube was observed but did not appear to influence the decomposition process. A scale analysis shows that the rate constant controls decomposition compared to the residence time, which has a weaker dependence on temperature.

Commentary by Dr. Valentin Fuster

### Research Papers: Forced Convection

J. Heat Transfer. 2018;140(6):061701-061701-12. doi:10.1115/1.4038841.

The present study is devoted to the problem of oscillatory convective flow in the presence of viscous dissipation around different positions of a sphere. The system of differential equations governing the flow phenomenon is transformed into dimensionless form by using suitable group of variables and then transformed into convenient form for integration by using primitive variable formulation. Numerical simulation based on finite difference method is carried out to analyze the mixed convection flow mechanism. Special focus is given on the transient shear stress and the rate of heat transfer characteristics and their dependency on various dimensionless parameters that is mixed convection parameter λ, Prandtl number Pr, dissipation parameter $N,$ and angular frequency parameter $ω$. The angles $X=30deg,90 deg$, and $360deg$ are the favorable positions around the sphere for different parameters, where the transient rate of shear stress and heat transfer is noted maximum. Later, the obtained results are presented graphically by using Tech Plot-360 and compared with the previous work given in the literature.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(6):061702-061702-9. doi:10.1115/1.4038843.

Heat transfer coefficients for turbulent pipe flow are typically envisioned as axially varying from very high values at the pipe inlet to a subsequent monotonic decrease to a constant fully developed value. This distribution, although well enshrined in the literature, may not be universally true. Here, by the use of high accuracy numerical simulation, it was shown that the initially decreasing values of the coefficient may attain a local minimum before subsequently increasing to a fully developed value. This local minimum may be characterized as an undershoot. It was found that whenever a turbulent flow laminarizes when it enters a round pipe, the undershoot phenomenon occurs. The occurrence of laminarization depends on the geometry of the pipe inlet, on fluid-flow conditions in the upstream space from which fluid is drawn into the pipe inlet, on the magnitude of the turbulence intensity, and on the Reynolds number. However, the presence of the undershoot does not affect the fully developed values of the heat transfer coefficient. It was also found that the Fanning friction factor may also experience an undershoot in its axial variation. The magnitude of the heat transfer undershoot is generally greater than that of the Fanning friction factor undershoot.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(6):061703-061703-15. doi:10.1115/1.4039085.

We consider convective heat transfer for laminar flow of liquid between parallel plates. The configurations analyzed are both plates textured with symmetrically aligned isothermal ridges oriented parallel to the flow, and one plate textured as such and the other one smooth and adiabatic. The liquid is assumed to be in the Cassie state on the textured surface(s) to which a mixed boundary condition of no-slip on the ridges and no-shear along flat menisci applies. The thermal energy equation is subjected to a mixed isothermal-ridge and adiabatic-meniscus boundary condition on the textured surface(s). We solve for the developing three-dimensional temperature profile resulting from a step change of the ridge temperature in the streamwise direction assuming a hydrodynamically developed flow. Axial conduction is accounted for, i.e., we consider the extended Graetz–Nusselt problem; therefore, the domain is of infinite length. The effects of viscous dissipation and (uniform) volumetric heat generation are also captured. Using the method of separation of variables, the homogeneous part of the thermal problem is reduced to a nonlinear eigenvalue problem in the transverse coordinates which is solved numerically. Expressions derived for the local and the fully developed Nusselt number along the ridge and that averaged over the composite interface in terms of the eigenvalues, eigenfunctions, Brinkman number, and dimensionless volumetric heat generation rate. Estimates are provided for the streamwise location where viscous dissipation effects become important.

Commentary by Dr. Valentin Fuster

### Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2018;140(6):062001-062001-11. doi:10.1115/1.4038853.

Production of heavy oil from deep/tight formation using traditional technologies (“cold” production, injection of hot steam, etc.) is ineffective or inapplicable. An alternative is electromagnetic (EM) heating after fracturing. This paper presents the results of a numerical study of heavy oil production from a well with hydraulic fracture under radiofrequency (RF) EM radiation. Two parameters ignored in our previous modeling studies, namely adiabatic effect and the thermal expansion of oil, are considered in the new formulation, while high gradients of pressure/temperature and high temperature occur around the well. The mathematical model calculates the distribution of pressure and temperature in the system of “well-fracture-formation.” The distribution of thermal heat source is given by the Abernetty expression. The mathematical model takes into account the adiabatic effect and the thermal expansion of heavy oil. The latter makes a significant contribution to heavy oil production. Multistage heavy production technology with heating is assumed and several stages are recognized: stage 1: “Cold” heavy oil production, stage 2: RF-EM heating, and stage 3: RF is turned off and “hot” oil production continues until the flow rate reaches its initial (before heating) value. These stages are repeated starting from the second stage. Finally, RF-EM heating technology is compared to “cold” production in terms of additional oil production and economics. When producing with RF-EM heating with power 60 kW (50 days in the second stages), the oil rate increased several times. Repeated RF-EM heating (25 days in the fourth stage) doubled the production rate. Near-well region temperature increased by ∼82 °C in the second stage with RF-EM heating. Temperature increased by ∼87 °C in the fourth stage with repeated RF-EM heating and production cycles. Economic analysis and evaluation of energy balance showed that the multistage production technology is more efficient; i.e., the lower the payback period, the greater the energy balance. With the increase in pressure difference, the payback period and energy balance increased linearly.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(6):062002-062002-11. doi:10.1115/1.4038602.

The motion of the gliding DC electric arc under the effect of magnetic field is investigated. The temperature distribution in the inside and the outside of the moving arc is computed. The temperature distribution for the fixed-spot arc is also obtained. It appears that the gas relative velocity inside the arc gives rise to heat convection, which has an impact on the arc motion. A practical analytical solution is derived using magneto gas dynamic equations in order to investigate the heat transfer occurring in the arc and its vicinity, to determine its characteristics, and to estimate its velocity when it is exposed to external and electrode-induced magnetic fields. Two methods are suggested: one for the free-burning arc and the other for arc burning between close surrounding walls.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(6):062003-062003-16. doi:10.1115/1.4038792.

Heat extraction and drop impact regimes occurring when a local portion of a horizontal flat-fan air mist impinges the active surface of a Pt disk hold at Tw from ∼60 to 1200 °C are investigated. Boiling curves comprise single-phase, nucleate boiling (NB), transition boiling (TB), and film boiling (FB). Mists are generated under wide ranges of water and air flow rates, and the disk is placed at center and off-center positions along the mist footprint major axis. Conditions generate a wide spectrum of water impact flux, w, droplet diameter, dd, droplet velocity, uzs, and impingement angle. Heat flux extracted, −q, along each boiling regime correlates very well with expressions involving Reynolds, Weber, and Jakob numbers evaluated in terms of local average characteristics of free nonimpinging mists—w, volume mean diameter, d30, normal volume weighted mean velocity, uz,v—and Tw; close estimation indicates that hydrodynamic and thermal forces are well accounted. During arrival of sparse parcels visualization of mist–wall interactions, using a high speed camera aided by laser illumination, allows determination of the predominance area diagram of droplet impact regimes in terms of normal impinging Weber number, Wez, and Tw. The regimes include stick, rebound, spread, and splash; the last subclassified as fine-, crown- and jet-atomization. Arrival of parcels in close succession is ubiquitous causing rapid surface flooding and leading to formation of discontinuous well agitated thick liquid films, which interacts longer with the surface than drops in sparse parcels, acting as heat sinks for longer periods of time.

Commentary by Dr. Valentin Fuster

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

J. Heat Transfer. 2018;140(6):062201-062201-10. doi:10.1115/1.4038691.

Jet impingement cooling has been extensively used in the leading edge region of a gas turbine blade. This study focuses on the effect of jet impinging position on leading edge heat transfer. The test model is composed of a semicylindrical target plate, side exit slots, and an impingement jet plate. A row of cylindrical injection holes is located along the axis (normal jet) or the edge (tangential jet) of the semicylinder, on the jet plate. The jet-to-target-plate distance to jet diameter ratio (z/d) is 5 and the ratio of jet-to-jet spacing to jet diameter (s/d) is 4. The jet Reynolds number is varied from 10,000 to 30,000. Detailed impingement heat transfer coefficient distributions were experimentally measured by using the transient liquid crystal (TLC) technique. To understand the thermal flow physics, numerical simulations were performed using Reynolds-averaged Navier–Stokes (RANS) with two turbulence models: realizable k–ε (RKE) and shear stress transport k–ω model (SST). Comparisons between the experimental and the numerical results are presented. The results indicate that the local Nusselt numbers on the test surface increase with the increasing jet Reynolds number. The tangential jets provide more uniform heat transfer distributions as compared with the normal jets. For the normal jet impingement and the tangential jet impingement, the RKE model provides better prediction than the SST model. The results can be useful for selecting a jet impinging position in order to provide the proper cooling distribution inside a turbine blade leading edge region.

Commentary by Dr. Valentin Fuster

### Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2018;140(6):062501-062501-19. doi:10.1115/1.4038842.

The problem of unsteady conjugate natural convection and entropy generation within a semicircular porous cavity bounded by solid wall of finite thickness and conductivity has been investigated numerically. The governing partial differential equations with the corresponding initial and boundary conditions have been solved by the finite difference method using the dimensionless stream function, vorticity, and temperature formulation. Numerical results for the isolines of the stream function, temperature, and the local entropy generation due to heat transfer and fluid friction as well as the average Nusselt and Bejan numbers, and the average total entropy generation and fluid flow rate have been analyzed for different values of the Rayleigh number, Darcy number, thermal conductivity ratio, and the dimensionless time. It has been found that low values of the temperature difference reflect the entropy generation, mainly in the upper corners of the cavity, while for high Rayleigh numbers, the entropy generation occurs also along the internal solid–porous interface. A growth of the thermal conductivity ratio leads to an increase in the average Bejan number and the average entropy generation due to a reduction of the heat loss inside the heat-conducting solid wall.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(6):062502-062502-11. doi:10.1115/1.4038541.

A high-temperature natural circulation loop (NCL) using supercritical carbon dioxide as loop fluid is modeled to study the effects of operating variables and relevant design parameters on loop performance. The steady-state system model duly considers the axial conduction through loop fluid as well as loop wall and heat transfer with surroundings. The heat source is considered to be a heater with controlled heat flux and the heat sink is modeled as an end heat exchanger with water as the external cold fluid. The governing conservation equations for mass, momentum, and energy are nondimensionalized and are solved numerically discretizing in finite volume method. The numerical results are validated against experimental results reported in the literature in terms of modified Grashof number (Grm) and Reynolds number (Re). Results show that heat loss to the ambient affects the loop performance significantly for the high-temperature loop. It is also observed that the heat input at which the circulation becomes maximum can be increased by increasing either the diameter and/or the loop height. However, better performance is obtained with larger diameter tubes instead of longer loop heights. Axial conduction is seen to have a negligible effect on the overall loop performance. Boussinesq approximation appears to be reasonable as the operating conditions of the supercritical loop are away from the critical point.

Commentary by Dr. Valentin Fuster

### Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2018;140(6):062701-062701-11. doi:10.1115/1.4038912.

In this work, the effect of applying different approximations for the scattering phase function on radiative heat transfer in pulverized coal combustion is investigated. Isotropic scattering, purely forward scattering, and a δ-Eddington approximation are compared with anisotropic scattering based on Mie theory calculations. To obtain suitable forward scattering factors for the δ-Eddington approximation, a calculation procedure based on Mie theory is introduced to obtain the forward scattering factors as a function of temperature, particle size, and size of the scattering angle. Also, an analytical expression for forward scattering factors is presented. The influence of the approximations on wall heat flux and radiative source term in a heat transfer calculation is compared for combustion chambers of varying size. Two numerical models are applied: A model based on the discrete transfer method (DTRM) representing the reference solution and a model based on the finite volume method (FVM) to also investigate the validity of the obtained results with a method often applied in commercial CFD programs. The results show that modeling scattering as purely forward or isotropic is not sufficient in coal combustion simulations. The influence of anisotropic scattering on heat transfer can be well described with a δ-Eddington approximation and properly calculated forward scattering factors. Results obtained with both numerical methods show good agreement and give the same tendencies for the applied scattering approximations.

Commentary by Dr. Valentin Fuster

### Research Papers: Thermal Systems

J. Heat Transfer. 2018;140(6):062801-062801-8. doi:10.1115/1.4038827.
FREE TO VIEW

The performance of a flat-plate solar collector is usually assessed by its efficiency. This efficiency is normally defined on a steady-state basis, which makes it difficult to correctly track the instantaneous performance of the collector in various case-studies. Accordingly, this paper proposes an improved definition of instantaneous efficiency of a flat-plate solar collector used as a part of a solar water heating system. Using a predeveloped model by the authors for such a system, the proposed efficiency-definition is examined and compared with the conventional one for specific case studies. The results show that the improved definition of efficiency records reasonable values, i.e., no over-range values are observed contrast to the case of conventional efficiency-definition. Furthermore, this suggested efficiency approximately coincides with the conventional one at a wide range of time, as long as the system is operating in the so-called trans-steady-state phase or when the system is off-operational provided that the instantaneous rate of heat stored in the heat transfer fluid (HTF) is less than or equal to zero. As a result, the improved efficiency-definition yields more realistic results in reflecting the performance of a flat-plate collector in an active solar water heating system and is recommended to be used.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Heat Transfer. 2018;140(6):064501-064501-4. doi:10.1115/1.4038835.

Experiments were conducted with graphene nanoplatelets (GNP) to investigate the relative benefit of the thermal conductivity increase in relationship to the potential detriment of increased viscosity. The maximum enhancement ratio for GNP nanofluid thermal conductivity over water was determined to be 1.43 at a volume fraction of 0.014. Based on GNP aspect ratios, the differential effective medium model is shown to describe the experimental results of this study when using a fitted interfacial resistance value of 6 × 10−8 m2 K W−1. The viscosity model of Einstein provided close agreement between measured and predicted values when the effects of temperature were included and the intrinsic viscosity model term was adjusted to a value of 2151 representative for GNP. Heat transfer in external flows in laminar regime is predicted to decrease for GNP nanofluids when compared to water alone.

Commentary by Dr. Valentin Fuster