Research Papers: Conduction

J. Heat Transfer. 2017;140(4):041301-041301-8. doi:10.1115/1.4038198.

A series expansion and an approximation have been derived for the temperature in the general case of transient heat conduction in a thin layer with a surface heat flux between two semi-infinite media at different uniform initial temperatures. Their temperature accuracy has been evaluated for two test cases in the field of thermoplastic shaping of polymers. The series enables quick yet fairly accurate thermal analysis of compression molding (CM) and injection molding (IM) and its vitrification rate and of fused deposition modeling™ (FDM) and its bead welding and vitrification rate.

Commentary by Dr. Valentin Fuster

Research Papers: Forced Convection

J. Heat Transfer. 2018;140(4):041701-041701-10. doi:10.1115/1.4038339.

This paper presents a novel transient method for calibrating heat transfer gauges for convective wall heat flux measurements in high enthalpy flows. The new method relies on the transient heating of sensor substrates under rapid exposure to a hot flow in order to obtain the necessary reference heat flux. Compared to previous calibration facilities, the new facility is simple, inexpensive, easy to adapt for different flow configurations and sensor geometries, and quick to run across a wide range of conditions. In this paper, the design of the new calibration facility is described and the transient calibration method explained. The method is demonstrated by calibrating a Gardon gauge at convective heat transfer coefficients between 300 and 600 W/m2 K. Typical facility data and calibration results are presented in terms of voltage-heat flux sensitivity and calibration correction ratio. These results are shown to agree with theoretical estimates within the estimated calibration uncertainty.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(4):041702-041702-13. doi:10.1115/1.4038338.

An experimental program is presented of heated tension springs in an external crossflow over a range of laminar Reynolds numbers, spring stretch ratios, and angles of attack. Extensive measurements of the forced convection heat transfer of helical wire within a wind tunnel reveal an interesting nonmonotonic dependence on angle of attack. Computational fluid dynamics (CFD) simulations, showing good agreement with the experimental data, are used to explore the behavior and gain a better understanding of the observed trends. A dimensionless correlation is developed that well captures the experimental and CFD data and can be used as an efficient computational tool in broader applications.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Exchangers

J. Heat Transfer. 2018;140(4):041801-041801-8. doi:10.1115/1.4038419.

Air-side heat transfer and flow friction characteristics of four different fin patterns suitable for flat tube bank fin heat exchangers are investigated experimentally. The fin patterns are the fin with six dimples, the fin with nine dimples, the double louvered fin, and the fin with delta-winglet vortex generators (VGs). The corresponding plain fins (plain fin I and plain fin II) are used as the references for evaluating the thermal performances of these fin patterns under identical pump power constraint. The performance of the fin with the six dimples is better than that with nine dimples. The performance of the fin with delta-winglet VGs is better than that of the double louvered fin, and the performance of the latter is better than that of the fins with six or nine dimples. In the tested Reynolds number range, the heat transfer enhancement performance factor of the fin with six dimples, the fin with nine dimples, the double louvered fin, and the fin with delta-winglet VGs is 1.2–1.3, 1.1–1.2, 1.3–1.6, and 1.4–1.6, respectively. The correlations of Nusselt number and friction factor with Reynolds number for the fins with six/nine dimples and the double louvered fin are obtained. These correlations are useful to design flat tube bank fin heat exchangers.

Commentary by Dr. Valentin Fuster

Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2017;140(4):042001-042001-14. doi:10.1115/1.4038193.

In published literature, only very limited studies were carried out for low-temperature biogas upgrading for CO2/CH4 mixture separation due to the freeze-out of CO2 under low temperature, which causes several operational problems. Therefore, the present study aims to provide in-depth analysis for a low-temperature distillation process of a typical model of biogas mixture (CH4 + CO2) to tackle the problem of freezing. The process has been optimized by means of varying distillation column feed pressure, temperature and CO2 concentration, reflux ratio, feed stage number, and produced methane purity to lower the risk of CO2 freezing in the column. The modeling results reveal a substantial feature of the low-temperature process that it can capture CO2 in liquid phase with a purity of 99.5 mol % as a valuable byproduct for transport. Additionally, it is found that increasing the column reflux ratio mitigates the risk of CO2 freeze-out allowing the column to reach higher CH4 purities (up to 97 mol %) without CO2 solidification. Moreover, the occurrence of CO2 freeze-out in the column is not affected within a relatively wide range of feed CO2 concentrations. The low-temperature technique can serve as a new promising approach for biogas upgrading overcoming the risk of CO2 frosting.

Commentary by Dr. Valentin Fuster

Research Papers: Jets, Wakes, and Impingment Cooling

J. Heat Transfer. 2017;140(4):042201-042201-12. doi:10.1115/1.4037641.

Motivated by the need to further improve film cooling in terms of both cooling effectiveness and coolant coverage area, the mist/air film cooling scheme is investigated through experiments using fan-shaped holes over an extended downstream length in this study. Both an existing wind tunnel and test facility, used in previous work, have been retrofitted. The first modification was extending the length of the flat plate test section to cover longer distances downstream of the injection holes, up to X/D = 100, in order to investigate whether mist cooling can be harnessed farther downstream where single-phase film cooling is not effective. X represents the axial distance downstream of the cooling hole of diameter D. The second modification was to incorporate fan-shaped (diffusion) holes which are proven to have a higher film cooling efficiency, than cylindrical holes. The objective is to investigate whether mist can further enhance the film cooling performance of the already highly effective fan-shaped holes. A phase Doppler particle analyzer (PDPA) system is employed to measure the droplet size, velocity, and turbulence information. An infrared camera and thermocouples are both used for temperature measurements. Part I is focused on the heat transfer result on the wall. The results show that, at low blowing ratios when the film is attached to the surface, the enhancement of the mist film cooling effectiveness, compared to the air-only case, on the centerline of the hole ranges from 40% in the near hole region to over 170% at X/D = 100. Due to the diffusive nature of the fan-shaped hole, the laterally averaged enhancement is on par with that on the centerline. The significant enhancement over the extended downstream distance from X/D = 40–100 is attributed to the evaporation time needed to evaporate all of the droplets. Each droplet acts as a cooling sink and flies over a distance before it completely vaporizes. This “distributed cooling” characteristic allows the water droplets to extend the cooling effects farther downstream from the injection location. At higher blowing ratios, when the cooling film is lifted off from the surface, the cooling enhancement drops below 40%. Although the enhancement in the near hole region X/D < 40 is about 20% lower than that achieved by using the cylindrical holes, the magnitudes of the mist adiabatic film cooling effectiveness using fan-shaped holes are still much higher than those of the cylindrical holes. Part II of this study is focused on analyzing the two-phase droplet multiphase flow behavior to explain the fundamental physics involved in the mist film cooling.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(4):042202-042202-11. doi:10.1115/1.4037642.

A phase Doppler particle analyzer (PDPA) system is employed to measure the two-phase mist flow behavior including flow velocity field, droplet size distribution, droplet dynamics, and turbulence characteristics. Based on the droplet measurements made through PDPA, a projected profile describing how the air–mist coolant jet flow spreads and eventually blends into the hot main flow is prescribed for both cylindrical and fan-shaped holes. The mist film layer consists of two layers: a typical coolant film layer (cooling air containing the majority of the droplets) and a wider droplet layer containing droplets outside the film layer. Thanks to the higher inertia possessed by larger droplets (>20 μm in diameter) at the injection hole, the larger droplets tend to shoot across the coolant film layer, resulting in a wider droplet layer than the coolant film layer. The wider droplet layer boundaries are detected by measuring the droplet data rate (droplet number per second) distribution, and it is identified by a wedge-shaped enclosure prescribed by the data rate distribution curve. The coolant film layer is prescribed by its core and its upper boundary. The apex of the data rate curve, depicted by the maximum data rate, roughly indicates the core region of the coolant film layer. The upper boundary of the coolant film layer, characterized by active mixing with the main flow, is found to be close to relatively high values of local Reynolds shear stresses. With the results of PDPA measurements and the prescribed coolant film and droplet layer profiles, the heat transfer results on the wall presented in Part I are re-examined, and the fundamental mist-flow physics are analyzed. The three-dimensional (3D) droplet measurements show that the droplets injected from the fan-shaped holes tend to spread wider in lateral direction than cylinder holes and accumulate at the location where the neighboring coolant film layers meet. This flow and droplet behavior explain the higher cooling performance as well as mist-enhancement occurs between the fan-shaped cooling holes, rather than along the hole's centerline as demonstrated in the case using the cylindrical holes.

Topics: Drops , Coolants
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(4):042203-042203-9. doi:10.1115/1.4038210.

The widely used gas turbine combustor double-walled cooling scheme relies on very small pedestals. In a combustor it is impractical for computational fluid dynamics (CFD) to resolve each pedestal individually as that would require a very large amount of grid points and consequent excessive computation time. These pedestals can be omitted from the mesh and their effects captured on the fluid via a pedestal subgrid-scale (SGS) model. The aim is to apply the SGS approach, which takes into account the effects on pressure, velocity, turbulence, and heat transfer, in an unstructured CFD code. The flow inside a two-dimensional (2D) plain duct is simulated to validate the pedestal SGS model, and the results for pressure, velocity, and heat transfer are in good agreement with the measured data. The conjugate heat transfer inside a three-dimensional (3D) duct is also studied to calibrate the heat source term of the SGS model due to the pedestals. The resolved flow in the combustor pedestal tile geometry is numerically investigated using Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) in order to first assess the viability of the RANS and LES to predict the impinging flow and second to provide more validation data for the development of the SGS pedestal correlations. It is found that the complexity of such a flow, with high levels of curvature, impingement, and heat transfer, poses a challenge to the standard RANS models. The LES provides more details of the impinging flow features. The pedestal model is then applied to the complete tile to replace the pedestals. The results are close to both the fully resolved CFD and the measurements. To improve the flow features in the impingement zone, the first two rows were resolved with the mesh and combined with the SGS modeling for the rest of the tile; this gave optimum results of pressure, velocity, and turbulence kinetic energy (TKE) distribution inside the pedestal cooling tile.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(4):042204-042204-10. doi:10.1115/1.4038421.

Jet impingement cooling is widely used in many industrial applications due to its high heat transfer capability and is an option for advanced high power density systems. Jet impingement cooling with supercritical pressure fluids could have much larger heat transfer rates combining with the large fluid specific heat near the pseudocritical point. However, the knowledge of its flow and heat transfer characteristics is limited. In this study, the flow and the local and average heat transfer characteristics of jet impingement cooling with supercritical pressure fluids were studied experimentally with carbon dioxide first. An integrated thermal sensor chip that provided heating and temperature measurements was manufactured using micro-electro-mechanical systems (MEMS) techniques with a low thermal conductivity substrate as the impingement cooled plate. The experiment system pressure was 7.85 MPa, which is higher than the critical pressure of carbon dioxide of 7.38 MPa. The mass flow rate ranged from 8.34 to 22.36 kg/h and the Reynolds number ranged from 19,000 to 68,000. The heat flux ranged from 0.02 to 0.22 MW/m2. The nozzle inlet temperature ranged from lower to higher than the pseudocritical temperature. Dramatic variations of the density at supercritical pressures near the heating chip were observed with increasing heat flux in the strong reflection and refraction of the backlight that disappeared at inlet temperatures higher than the pseudocritical temperature. The local heat transfer coefficient near the stagnation point increased with increasing heat flux while those far from the stagnation point increased to a maximum with increasing heat flux and then decreased due to the nonuniformity of jet impingement cooling. The heat transfer is higher at inlet temperatures lower than the pseudocritical temperature and the surface temperature is slightly higher than the pseudocritical temperature due to the dramatic changes in the fluid thermo-physical properties at supercritical pressures.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(4):042205-042205-7. doi:10.1115/1.4038422.

Numerical study of nanofluid jet impingement cooling of a partially elastic isothermal hot surface was conducted with finite element method. The impingement surface was made partially elastic, and the effects of Reynolds number (between 25 and 200), solid particle volume fraction (between 0.01 and 0.04), elastic modulus of isothermal hot surface (between 104 and 106), size of the flexible part (between 7.5 w and 25 w), and nanoparticle type (spherical, cylindrical, blade) on the fluid flow and heat transfer characteristics were analyzed. It was observed that average Nusselt number enhances for higher Reynolds number, higher values of elastic modulus of flexible wall, smaller size of elastic part, and higher nanoparticle solid volume fraction and for cylindrical shaped particles. It is possible to change the maximum Nusselt number by 50.58% and 33% by changing the elastic modulus of the hot wall and size of elastic part whereas average Nusselt number changes by only 9.33% and 6.21%. The discrepancy between various particle shapes is higher for higher particle volume fraction.

Commentary by Dr. Valentin Fuster

Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2018;140(4):042401-042401-13. doi:10.1115/1.4038415.

In this study, different nanofluids (NFs) were developed by mixing a molten salt mixture (60% NaNO3–40% KNO3) with 1.0 wt % of silica–alumina nanoparticles using different methods. These NFs can be used as thermal energy storage materials in concentrating solar plants with a reduction of storage material if the thermal properties of the base fluid are increased. New mixing procedures without sonication were introduced with the aim to avoid the sonication step and to allow the production of a greater amount of NF with a procedure potentially more suitable for large-scale productions. For this purpose, two mechanical mixers and a magnetic stirrer were used. Each NF was prepared in aqueous solution with a concentration of 100 g/l. The effect of different concentrations (300 g/l and 500 g/l) was also studied with the most effective mixer. Specific heat, melting temperature, and latent heat were measured by means of differential scanning calorimeter. Thermal conductivity and diffusivity in the solid state were also evaluated. The results show that the highest increase of the specific heat was obtained with 100 g/l both in solid (up to 31%) and in liquid phase (up to 14%) with the two mechanical mixers. The same NFs also showed higher amount of stored heat. An increase in thermal conductivity and diffusivity was also detected for high solution concentrations with a maximum of 25% and 47%, respectively. Scanning electron microscopy (SEM) and energy-dispersive X-ray analyses revealed that the grain size in the NFs is much smaller than in the salt mixture, especially for the NF showing the highest thermal properties increase, and a better nanoparticles distribution is achieved with the lowest concentration. NFs with enhanced thermal properties can be synthesized in a cost-effective form in high concentrated aqueous solutions by using mechanical mixers.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(4):042402-042402-10. doi:10.1115/1.4038233.

A numerical investigation of a single highly confined bubble moving through a millimeter-scale channel in the absence of phase change is presented. The simulation includes thermal boundary conditions designed to match those of completed experiments involving bubbly flows with large numbers of bubbles. The channel is horizontal with a uniform-heat-generation upper wall and an adiabatic lower boundary condition. The use of a Lagrangian framework allows for the simulation of a channel of arbitrary length using a limited computational domain. The liquid phase is a low-Reynolds-number laminar flow, and the phase interactions are modeled using the volume-of-fluid (VOF) method with full geometric reconstruction of the liquid/gas interface. Results are presented for three bubble diameters, which include two levels of confinement within the channel and two liquid flow rates. Bubble shape and speed closely match experimental observations for each bubble size and liquid flow rate. Nusselt numbers in the bubble wake for all configurations follow a power law relationship with distance behind the bubble. Important dynamical structures include a pair of vortical structures at the rear of the bubble associated with the primary heat transfer enhancement and a pair of prominent liquid jets oriented in the transverse direction on either side of the bubble.

Commentary by Dr. Valentin Fuster

Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2017;140(4):042501-042501-9. doi:10.1115/1.4038132.

A liquid fluoride salt experiment has been constructed and used to acquire natural convection heat transfer data. The experiment used FLiNaK salt in a small cell that included a cylindrical electrical heater, 1.27 cm in diameter, oriented vertically in a FLiNaK bath. Thermocouples internal to the heater were used, along with FLiNaK temperature measurements and heater power measurements, to determine natural circulation heat transfer coefficients. These data were acquired for Rayleigh numbers ranging from 3 × 105 to 8 × 109 and salt temperatures from 560 °C to 640 °C. Test results show that measured heat transfer coefficients are consistent with conventional natural convection heat transfer coefficients for cylinders, but the average error can be as high as 20% using these correlations. Correlations developed by fitting the data for this experiment show much lower errors (<10%).

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(4):042502-042502-12. doi:10.1115/1.4038192.

Natural convection of a water-based nanofluid in a partially open triangular cavity with a local heat source of constant temperature under the effect of Brownian diffusion and thermophoresis has been analyzed numerically. Governing equations formulated in dimensionless stream function and vorticity variables on the basis of two-phase nanofluid model with corresponding initial and boundary conditions have been solved by finite difference method. Detailed study of the effect of Rayleigh number, buoyancy-ratio parameter, and local heater location on fluid flow and heat transfer has been carried out. It has been revealed that an increase in the buoyancy force magnitude leads to homogenization of nanoparticles distribution inside the cavity. A growth of a distance between the heater and the cavity corner illustrates the heat transfer enhancement.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(4):042503-042503-12. doi:10.1115/1.4038420.

Natural convection during solidification of liquids is known to impact the freezing characteristics and also lead to defect formation. In this study, we report the findings of real-time interferometric observation of bottom-cooled solidification of pure water in a cubical cavity. The results show first quantitative evidence of full-field thermal history during solidification, clearly depicting the anomalous expansion of water below 4 °C. Furthermore, based on the strength of natural convection, characterized by the Rayleigh number, we identify and report four distinct regimes of solidification, namely—conduction dominated, early convection, front instability, and sustained convection. A critical Rayleigh number that initiates instability in the solidifying front has been proposed, which is significantly different from conventional calculations of Rayleigh number relating to the initiation of flow. The study shows full-field quantitative evidence of a well-known phenomenon and provides a further understanding of flow driven nonhomogeneities in the solidifying interfaces.

Commentary by Dr. Valentin Fuster

Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2017;140(4):042701-042701-6. doi:10.1115/1.4038231.

In nuclear packed pebble beds, it is a fundamental task to model effective thermal conductivity (ETC) of thermal radiation. Based on the effective heat transfer cells of structured packing, a short-range radiation model (SRM) and a subcell radiation model (SCM) are applied to obtain analytical results of ETC. It is shown that the SRM of present effective heat transfer cells are in good agreement with the numerical simulations of random packing and it is only slightly higher than empirical correlations when temperature exceeds 1200 °C. In order to develop a generic theoretical approach of modeling ETC, the subcell radiation model is presented and in good agreement with Kunii–Smith correlation, especially at very high temperature ranges (over 1500 °C). Based on SCM, one-dimensional (1D) radial heat transfer model is applied in the analysis of the HTTU experiments. The results of ETC and radial temperature distribution are in good agreement with the experimental data.

Commentary by Dr. Valentin Fuster

Research Papers: Two-Phase Flow and Heat Transfer

J. Heat Transfer. 2017;140(4):042901-042901-9. doi:10.1115/1.4038190.

A novel inorganic aqueous solution (IAS) is shown to have a better heat transfer performance than water when used as the working fluid in copper-made phase-change heat transfer devices. First, the physical properties of IAS are measured and compared to those of water. Another, a chemical analysis is performed, and the chemical reactions involved between IAS and the copper surface are listed and categorized by their contributions to the heat transfer performance. In addition, a capillary rise test is performed to show how each chemical contributes to the improvement of the surface wettability. Last, using IAS in copper-made phase-change heat transfer devices is discussed, and the main focus is how IAS improves the heat transfer performance by a smaller thermal resistance and a larger critical heat flux. The conclusion is validated by thermo-siphon tests at different inclination angles.

Commentary by Dr. Valentin Fuster

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