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Research Papers: Combustion and Reactive Flows

J. Heat Transfer. 2017;139(6):061201-061201-12. doi:10.1115/1.4035443.

This work numerically studies the effects of inlet air and fuel turbulators on the thermal behavior of a combustor burning the jet propulsion (JP) (kerosene-surrogate) fuel and its resulting pollutants emission including the nanoparticulate soot aerosols and aromatic compounds. To model the soot formation, the method employs a semi-empirical two-equation model, in which the transport equations for soot mass fraction and soot number density are solved considering soot nanoparticles evolutionary process. The soot nucleation is described using the phenyl route in which the soot is formed from the polycyclic aromatic hydrocarbons. Incorporating a detailed chemical mechanism described by 200 species and 6907 elementary reactions, the flamelets and their lookup table library are precomputed and used in the context of steady laminar flamelet model (SLFM). Thus, the current finite-volume method solves the transport equations for the mean mixture fraction and its variance and considers the chemistry–turbulence interaction using the presumed-shape probability density functions (PDFs). To validate the utilized models, a benchmark combustor is first simulated, and the results are compared with the measurements. Second, the numerical method is used to investigate the effects of embedding different inflow turbulators on the resulting flame structure and the combustor pollutants emission. The chosen turbulators produce mild to severe turbulence intensity (TI) effects at the air and fuel inlets. Generally, the results of current study indicate that the use of suitable turbulators can considerably affect the thermal behavior of a JP-fueled combustor. Additionally, it also reduces the combustor polycyclic aromatic hydrocarbon (PAH) pollutants emission.

Commentary by Dr. Valentin Fuster

Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2017;139(6):061501-061501-13. doi:10.1115/1.4035562.

Slug flow is an essential flow pattern observed in microchannels where its transition boundaries in microchannels are characterized by two complex hydrodynamic phenomena, the bubble confinement and the bubble coalescence. Slug flow may be classified in terms of bubble size into two major zones: isolated bubble zone and coalescence bubble zone. In this paper, a semi-analytical model is developed for predicting the main characteristics of isolated bubble zone for flow boiling in a horizontal microchannel. The influences of surface tension, shear, and inertial forces have been taken into account. The model is developed on the basis of drift flux model, and a fully developed slug unit is chosen as a control volume for deriving the equations of motion. The effects of main operating conditions, mass and heat fluxes, on bubble length and bubble frequency have been investigated. The boundaries of slug flow regime have been identified based on the most proper diabatic flow pattern maps available in the literature for the chosen database. The model has been validated using the database available in the literature for flow boiling of R134a and R245fa in 0.509 mm and 3.0 mm inner diameter horizontal mini-tubes, respectively, and over wide range of mass fluxes (300G1000kg/m2s). This study has shown that the mass flux has a significant effect on the slug length and the bubble frequency. The model gave a good agreement with the experimental data of bubble length and bubble frequency with a mean absolute error (MAE) of 18.0% and 27.34%, respectively.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(6):061502-061502-9. doi:10.1115/1.4035709.

This paper extends previous liquid film flow boiling studies by including the effect of an additional electrohydrodynamic (EHD) force, namely, the dielectrophoretic (DEP) force. Rather than using only EHD conduction pumping of the liquid film to electro-wet the heater surface, a localized nonuniform electric field above the heater surface is established to generate a DEP force for improved vapor bubble extraction during the nucleate boiling regime. The effects of liquid film height and applied potential are studied as a function of heater superheat and heat flux. A brief analytical study is also used to estimate the expected DEP force magnitude to explain the results. All of the above studies are also used to quantify the enhancement in heat transfer that can be achieved when heat transport systems are driven or augmented by these two EHD mechanisms. The results show remarkable enhancement of up to 1217% in boiling heat transfer coefficient at a given superheat when both mechanisms are used simultaneously. The experimental data are important for applications in thermal management in terrestrial and space conditions.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(6):061503-061503-11. doi:10.1115/1.4035711.

This paper presents the results from computational fluid dynamics (CFD) simulations of heat and mass transfer of pure vapor flowing and condensing in a vertical cylindrical condenser system at various inlet temperatures, mass flow rates, and operating pressure for the case where the vapor condensation is not completed inside the condenser tube. The heat and mass transfer inside the condenser tube is simulated as single phase flow, and the thin condensate film on the condensing surface is replaced by a set of boundary conditions that couple the CFD simulations inside the condenser tube and the coolant channel. The CFD results are compared with the experimental results, and good agreement has been found for the various measured temperatures. It is found that both the wall temperature and the heat flux vary significantly along the condenser tube, and it is necessary to consider the conjugate problem that consists of the whole condenser system (condenser plus coolant flow) in predicting the pure vapor condensation in a condensing system. The CFD results show that the heat flux along the condenser tube can be increasing for counter-flow condenser, and the condensate film may not be the main limiting factor in the pure vapor condensation. The results from the CFD simulations also show that the estimation of the interface shear stress cannot be based on the bulk velocity of the water vapor alone.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Exchangers

J. Heat Transfer. 2017;139(6):061801-061801-9. doi:10.1115/1.4035603.

A printed circuit heat exchanger (PCHE) was selected as the recuperator of supercritical carbon dioxide (S-CO2) Brayton cycle, and the segmental design method was employed to accommodate the rapid variations of properties of S-CO2. The local heat capacity rate ratio has crucial influences on the local thermal performance of PCHE, while having small influences on the frictional entropy generation. The heat transfer entropy generation is far larger than the frictional entropy generation, and the total entropy generation mainly depends on the heat transfer entropy generation. The axial conduction worsens the thermal performance of PCHE, which becomes more and more obvious with the increase of the thickness and thermal conductivity of plate. The evaluation criteria, material, and size of plate have to be selected carefully in the design of PCHE. The present work may provide a practical guidance on the design and optimization of PCHE when S-CO2 is employed as working fluid.

Commentary by Dr. Valentin Fuster

Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2017;139(6):062001-062001-8. doi:10.1115/1.4035446.

This paper experimentally investigates how different mass concentration and aspect ratio multiwall carbon nanotubes (MWCNTs) acetone nanofluid affects the heat transfer performance of a flat plate heat pipe (FPHP). Different mass concentration and aspect ratio MWCNTs-acetone nanofluids are prepared without any surfactants or additives using the two-step method. Aspect ratios of MWCNTs are 666 (M1) and 200 (M2), respectively, and their according mass concentrations are 0.002, 0.005, 0.01, and 0.015 wt. %, respectively. The thermal resistance and wall temperature of the FPHP are experimentally obtained when the above-mentioned nanofluids are used as working fluid. The results showed that different mass concentration affects the heat transfer performance, therefore, there is an optimal MWCNTs-acetone nanofluid mass concentration (about 0.005wt. %). Also, the results showed that the thermal resistances of the FPHP with M1-acetone nanofluid (0.005 wt. %) and M2-acetone nanofluid (0.005 wt. %) are reduced 40% and 16%, respectively. Based on the above experimental phenomenon, this paper discusses the reasons for enhancement and decrement of heat transfer performance of the different mass concentration. For the M1-acetone nanofluid, the investigated FPHP has a thermal resistance of 0.26 °C/W and effective thermal conductivity 3212 W/m k at a heat input of 160 W. For the M2-acetone nanofluid, the investigated FPHP has a thermal resistance of 0.33 °C/W and effective thermal conductivity 2556 W/m k at a heat input of 150 W. The nanofluid FPHP investigated here provides a new approach in designing a high efficient next generation heat pipe cooling devices.

Commentary by Dr. Valentin Fuster

Research Papers: Melting and Solidification

J. Heat Transfer. 2017;139(6):062301-062301-10. doi:10.1115/1.4035710.

Laser surface topography has great applications in mechanical, medical, and electrical industries. This paper proposes a mechanism for the shape formation of stainless steel by pulsed laser melting. A 2D axisymmetric finite element model considering the temperature-dependent surface tension is developed, in which the melt flow and free surface deformation are analyzed by using the normal and shear surface forces. The numerical results show that the molten flows toward the place of the greatest surface tension and the free surface deformation are dominated by the shear force (Marangoni effects), generated by the surface tension gradient during heating phase, and the normal stress, generated by the surface tension at the curved surface during the cooling period.

Commentary by Dr. Valentin Fuster

Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2017;139(6):062401-062401-8. doi:10.1115/1.4035441.

Microchannels have been studied extensively for a variety of heat transfer applications including electronic cooling. Many configurations of microchannels have been studied and compared for their effectiveness in terms of heat removal. Recently, the use of staggered pins in microchannels has gained considerable traction, since they can promote internal flow fluctuations that enhance internal heat transfer. Furthermore, staggered pins in microchannels have shown higher heat removal characteristics because of the continuous breaking and formation of the heat transfer fluid boundary layer. However, they also exhibit higher pressure drop because the pins act as flow obstructions. This paper presents numerical results of two characteristic staggered 100-μm pins (square and circular) in microchannels. The heat transfer performance of a single phase fluid (SPF) in microchannels with staggered pins, and the corresponding pressure drop characteristics are presented. Furthermore, a phase change material (PCM, n-eicosane) fluid was also considered by implementing the effective specific heat capacity model approach to account for the corresponding phase change process of PCM fluid. Comparisons of the heat transfer characteristics of single phase fluid and PCM fluid are presented for two different pin geometries and three different Reynolds numbers. Circular pins were found to be more effective in terms of heat transfer by exhibiting higher Nusselt number. Microchannels with circular pins were also found to have lower pressure drop compared to the square-pin microchannels.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;139(6):062402-062402-14. doi:10.1115/1.4035655.

This study focuses on microheat sinks with different staggered arrangements of micro pin fins (MPFs). A rectangular microchannel with the dimensions of 5000 × 1500 × 100 μm3 (l′ × w′ × h′) was considered for all the configurations while different MPF diameters, height over diameter ratio (H/D), and longitudinal and transversal pitch ratios (SL/D and ST/D) were considered in different arrangements. Using the ansys fluent 14.5 commercial software, the simulations were done for different Reynolds numbers between 20 and 160. A constant heat flux of 30 W/cm2 was applied through the bottom heating section. The performances of the microheat sinks were evaluated using design parameters, namely pressure drop, friction factor, Nusselt number, and thermal-hydraulic performance index (TPI). The effect of each geometrical parameter as well as wake-pin fin interaction patterns were carefully studied using the streamline patterns and temperature profiles of each configuration. The results reveal a great dependency of trends in pressure drops and Nusselt numbers on the wake region lengths as well as the local velocity and pressure gradients. Moreover, the wake region lengths mostly contribute to the increase in obtained pressure drop and Nusselt number with Reynolds number. Although an increase in the H/D and SL/D ratios results in an increase and a decrease in pressure drop, respectively, the effect on the Nusselt number depends on other geometrical parameters and Reynolds number. A larger ST/D ratio generally results in a decrease in the pressure drop and Nusselt number. Finally, while the friction factor decreases with Reynolds number, two different trends are seen for the TPI values of configurations with the H/D ratio of 1 and 2 (D = 100 and 50 μm). While the trend in the TPIs is increasing for Reynolds numbers between 20 and 40, it reverses for higher Reynolds numbers with a steeper slope in the configurations with the ST/D ratio of 1.5.

Commentary by Dr. Valentin Fuster

Research Papers: Porous Media

J. Heat Transfer. 2017;139(6):062601-062601-14. doi:10.1115/1.4035650.

Desiccant-coated energy wheels are rotary-air-to-air energy exchangers widely used in ventilation systems to reduce the energy consumption required in industrial environments and commercial buildings. In this study, the effects of silica gel microphysical properties, i.e., pore width (Pw), specific surface area (SA), and particle size (dp), on the moisture recovery efficiency (latent effectiveness) of energy wheels are investigated. Three silica gel samples with different particle size and pore width (55 μm–77 Å, 150 μm–63 Å, and 160 μm–115 Å) are selected to coat small-scale energy exchangers. The sorption performance of the exchangers is determined from their normalized humidity response to a step increase in the inlet humidity at different flow rates. The results demonstrate that the transient humidity response is mainly specified by the desiccant pore size distribution, specific surface area, and mass of the coating. The transient analytical model is used to calculate the latent effectiveness (ɛL) of the exchangers from the transient humidity response. It was found that the exchanger coated with the smallest pore width (63 Å) has the highest available surface area and the highest latent effectiveness. With almost the same particle size (dp = 150 μm and 160 μm), the latent effectiveness increases by 5% (at wheel speed 20 rpm and Re = 174) as the pore width reduces from 150 Å to 63 Å. Increasing the particle size from 55 μm to 150 μm with almost the identical pore width (Pw = 63 Å and 77 Å) results in a slight enhancement in the latent effectiveness. ɛL is also calculated for correlated data (Yoon–Nelson model) where the results agree within experimental uncertainty bounds.

Commentary by Dr. Valentin Fuster

Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2017;139(6):062701-062701-8. doi:10.1115/1.4035654.

Laser-assisted machining (LAM) is a growing trend in machining of hard to cut materials. In most experimental cases, LAM is carried out in two stages; first, laser and machine parameters are tuned to adjust the temperature at the material removal point (Tmr), and second, the cutting tool is engaged to cut the points that have already been heated by the laser. Alternatively, an analytical model for the prediction of temperature filed can replace lengthy experimentation needed for tuning the material removal temperature. This paper presents an analytical solution to the transient temperature field in a rotating cylinder subject to a localized laser heat source based on Green's functions. The analytical solution is validated by comparing the surface point temperatures to thermal experiments on DIN 1.7225 steel, which shows good agreement in trend and values. Furthermore, a finite element model is developed and verified by the results of the same experiments, providing a more detailed investigation on the performance of the analytical model. The developed analytical scheme can be used to readily calculate pointwise temperatures on workpiece surface and internal points which can be used as a tool for designing machining conditions.

Commentary by Dr. Valentin Fuster

Research Papers: Two-Phase Flow and Heat Transfer

J. Heat Transfer. 2017;139(6):062901-062901-13. doi:10.1115/1.4035649.

The paper introduces a novel two-phase heat transfer device (TPHTD) which is employed in the thermal management of light emitting diodes (LEDs). The heat transfer device structurally resembles a conventional loop heat pipe (LHP) without a compensation chamber, but operates very differently from it. The device is comprised of a central evaporator package and a circular coil that acts as a heat exchanger loop. The working fluid leaving the evaporator has a two-phase mixture quality of approximately 0.2. Having introduced the device, the paper delineates a mathematical model for predicting its thermal performance. The primary objective of the model is to provide a fundamental understanding of the operation of the device. A one-dimensional thermal resistance model (TRM) is utilized in modeling the evaporator. The paper presents a detailed discussion on obtaining these resistances from experiments conducted on the device. A correlation for the external heat transfer coefficient of the heat exchanger loop is proposed based on experiments and is found to be in good agreement with literature. The model predicts performance parameters such as board temperature, two-phase mixture quality, and saturation and subcooled temperatures (Tsat and Tsc) of the working fluid for different input thermal powers (Qtot). Based on experimental evidence, it is concluded that the majority of Qtot (∼75%) is utilized in phase change of the working fluid, and the rest reheats the working fluid from a lower subcooled temperature (Tsc) to the saturation temperature (Tsat) of the evaporator.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Heat Transfer. 2017;139(6):064501-064501-6. doi:10.1115/1.4035708.

In this paper, characteristics of turbulent flow and convective heat transfer of supercritical China RP-3 kerosene in a horizontal straight circular tube are studied experimentally, and the validity of Chilton–Colburn analogy is examined. Using a three-stage heating system, experiments are conducted at a fuel temperature range of 650–800 K, a pressure range of 3–4 MPa, and a Reynolds number range of 1 × 105–3.5 × 105. The Nusselt number and skin friction coefficient are calculated through control volume analysis proposed in this paper. Heat transfer enhancement and deterioration were observed in the experiments as well as the similar change of skin friction coefficient. The present results show that Chilton–Colburn analogy is also valid for turbulent flow and heat transfer of supercritical kerosene in horizontal straight circular tubes.

Commentary by Dr. Valentin Fuster

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