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J. Heat Transfer. 2012;134(8):080901-080901-1. doi:10.1115/1.4006672.
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Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):080902-080902-1. doi:10.1115/1.4006675.
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Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):080903-080903-1. doi:10.1115/1.4006679.
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Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):080905-080905-1. doi:10.1115/1.4006719.
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Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):080906-080906-1. doi:10.1115/1.4006720.
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Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):080907-080907-1. doi:10.1115/1.4006721.
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Topics: Wetting , Water
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):080908-080908-1. doi:10.1115/1.4006722.
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Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):080910-080910-1. doi:10.1115/1.4006724.
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Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):080911-080911-1. doi:10.1115/1.4006725.
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Topics: X-rays , Metal foams , Imaging
Commentary by Dr. Valentin Fuster

Evaporation, Boiling, and Condensation

J. Heat Transfer. 2012;134(8):081501-081501-11. doi:10.1115/1.4006161.

An experimental study is conducted to investigate the effects of inlet restriction (orifice) on flow boiling instability in a single horizontal microtube. The test-section is composed of a stainless steel tube with an inner diameter of 889 μm, and a length of 150 mm. Experiments are performed for three different orifice configurations with 20%, 35%, and 50% area ratio. Mass flux is varied from 700 to 3000 kg/m2  · s, whereas the heat flux is varied from 6 to 27 W/cm2 . The dielectric coolant FC-72 is selected as the working fluid. In the absence of an orifice at the inlet, four oscillation types are observed at the onset of flow instability; it is also noticed that the frequency of the oscillations increases with increasing heat flux, while the amplitude remains constant. The addition of an orifice at the inlet helps stabilizing the flow without generating significant pressure drop at the same operating condition as the microtube without orifice. The 20% area ratio orifice shows better performance at low mass fluxes (<1000 kg/m2  · s). Whereas, at high mass fluxes (>2000 kg/m2  · s), 50% and 35% area ratio orifices are efficient in stabilizing the flow or delaying the onset of flow instability. Therefore, selecting the area ratio of the orifice depends on the operating condition. A small area ratio orifice is preferably used at low mass fluxes, whereas a large area ratio orifice is more suitable for high mass fluxes.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):081502-081502-10. doi:10.1115/1.4006206.

Critical heat flux (CHF) characteristics in submerged jet impingement boiling of water on a heated copper surface are investigated at subatmospheric conditions. Data are reported at a fixed surface-to-nozzle diameter ratio of 23.8 and a fixed surface-to-nozzle height of 6 nozzle diameters. Three subatmospheric pressures of 0.176 bars, 0.276 bars, and 0.477 bars are considered, corresponding to fluid saturation temperatures of 57.3 °C, 67.2 °C, and 80.2 °C and liquid-to-vapor density ratios of 8502, 5544, and 3295, respectively. At each pressure, CHF for varying jet Reynolds numbers (Re) in the range 0–14,000 are compared for two different surface finishes of roughness average values of 123 nm and 33 nm. The CHF enhancement observed with increasing Re is depicted in a nondimensional CHF map. Existing correlations available in the literature, which are out of range of the current experimental conditions, are found to poorly predict the obtained CHF data. A CHF correlation that captures the entire experimental data set within an average error of ±3% and a maximum error of ±13% is developed. The effect of fluid subcooling on submerged jet CHF is studied at the lowest pressure of 0.176 bars. Subcooled jet CHF is found to be well predicted from saturated jet CHF by using a typical subcooled pool boiling CHF correction factor.

Commentary by Dr. Valentin Fuster

Experimental Techniques

J. Heat Transfer. 2012;134(8):081601-081601-9. doi:10.1115/1.4006205.

Plasma deposited amorphous fluorocarbon polymers find use in biopassivation, and as low-friction coatings, adhesion promoters, and interlayer dielectrics. Here, we exploit their ease of deposition into ultrathin layers (<50 nm thick) to explore their potential as thermal storage elements. We design and fabricate a microcalorimeter for measuring the heat capacity of thin fluorocarbons. Conventional thin film calorimetry assumes adiabatic conditions that lead to large errors as film thickness decreases. We propose a new data analysis procedure that incorporates a one-dimensional solution of the transient heat diffusion equation to account for conduction losses. The data for films with thicknesses in the range 12–27 nm reveal a lowering of the melting point and an increase in the volumetric heat capacity with decreasing thickness. We attribute this to change in the carbon to fluorine ratio in the films’ composition. The volumetric heat capacity approximately doubles at room temperature as the film thickness decreases from 27 nm to 12 nm.

Commentary by Dr. Valentin Fuster

Forced Convection

J. Heat Transfer. 2012;134(8):081701-081701-10. doi:10.1115/1.4006748.

The effect of an unsteady stator wake (simulated by wake rods mounted on a spoke-wheel wake generator) on the modeled rotor blade is studied using the pressure sensitive paint (PSP) mass-transfer analogy method. Emphasis of the current study is on the midspan region of the blade. The flow is in the low Mach number (incompressible) regime. The suction (convex) side has simple angled cylindrical film-cooling holes; the pressure (concave) side has compound angled cylindrical film-cooling holes. The blade also has radial shower-head leading edge film-cooling holes. Strouhal numbers studied range from 0 to 0.36; the exit Reynolds number based on the axial chord is 530,000. Blowing ratios range from 0.5 to 2.0 on the suction side and 0.5 to 4.0 on the pressure side. Density ratios studied range from 1.0 to 2.5, to simulate actual engine conditions. The convex suction surface experiences film-cooling jet lift-off at higher blowing ratios, resulting in low effectiveness values. The film coolant is found to reattach downstream on the concave pressure surface, increasing effectiveness at higher blowing ratios. Results show deterioration in film-cooling effectiveness due to increased local turbulence caused by the unsteady wake, especially on the suction side. Results also show a monotonic increase in film-cooling effectiveness on increasing the coolant to mainstream density ratio.

Commentary by Dr. Valentin Fuster

Heat Exchangers

J. Heat Transfer. 2012;134(8):081801-081801-7. doi:10.1115/1.4006163.

A method and practical results are presented for finding the geometries of fixed volume plate fins for maximizing dissipated heat flux. The heat transfer theory used in optimization is based on approximate analytical solutions of conjugated heat transfer, which couple conduction in the fin and convection from the fluid. Nondimensional variables have been found that contain thermal and geometrical properties of the fins and the flow, and these variables have a fixed value at the optimum point. The values are given for rectangular, convex parabolic, triangular, and concave parabolic fin shapes for natural and forced convection including laminar and turbulent boundary layers. An essential conclusion is that it is not necessary to evaluate the convection heat transfer coefficients because convection is already included in these variables when the flow type is specified. Easy-to-use design rules are presented for finding the geometries of fixed volume fins that give the maximum heat transfer. A comparison between the heat transfer capacities of different fins is also discussed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):081802-081802-10. doi:10.1115/1.4006108.

Competitive cycles must have a minimal initial cost and be inherently efficient. Currently, the supercritical carbon dioxide (S-CO2 ) Brayton cycle is under consideration for these very reasons. This paper examines one major challenge of the S-CO2 Brayton cycle: the complexity of heat exchanger design due to the vast change in thermophysical properties near a fluid’s critical point. Turbulent heat transfer experiments using carbon dioxide, with Reynolds numbers up to 100 K, were performed at pressures of 7.5–10.1 MPa, at temperatures spanning the pseudocritical temperature. The geometry employed nine semicircular, parallel channels to aide in the understanding of current printed circuit heat exchanger designs. Computational fluid dynamics was performed using FLUENT and compared to the experimental results. Existing correlations were compared, and predicted the data within 20% for pressures of 8.1 MPa and 10.2 MPa. However, near the critical pressure and temperature, heat transfer correlations tended to over predict the heat transfer behavior. It was found that FLUENT gave the best prediction of heat transfer results, provided meshing was at a y+  ∼ 1.

Commentary by Dr. Valentin Fuster

Heat Transfer Enhancement

J. Heat Transfer. 2012;134(8):081901-081901-8. doi:10.1115/1.4005740.

Synthetic jets are piezo-driven, small-scale, pulsating devices capable of producing highly turbulent jets formed by periodic entrainment and expulsion of the fluid in which they are embedded. The compactness of these devices accompanied by high air velocities provides an exciting opportunity to significantly reduce the size of thermal management systems in electronic packages. A number of researchers have shown the implementations of synthetic jets on heat transfer applications; however, there exists no correlation to analytically predict the heat transfer coefficient for such applications. A closed form correlation was developed to predict the heat transfer coefficient as a function of jet geometry, position, and operating conditions for impinging flow based on experimental data. The proposed correlation was shown to predict the synthetic jet impingement heat transfer within 25% accuracy for a wide range of operating conditions and geometrical variables.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):081902-081902-9. doi:10.1115/1.4006166.

Heat transfer and pressure characteristics in a rectangular channel with pin-fin arrays of partial detachment from one of the endwalls have been experimentally studied. The overall channel geometry (W = 76.2 mm, E = 25.4 mm) simulates an internal cooling passage of wide aspect ratio (3:1) in a gas turbine airfoil. With a given pin diameter, D = 6.35 mm = ¼E, three different pin-fin height-to-diameter ratios, H/D = 4, 3, and 2, were examined. Each of these three cases corresponds to a specific pin array geometry of detachment spacing (C) between the pin tip and one of the endwalls, i.e., C/D = 0, 1, 2, respectively. The Reynolds number, based on the hydraulic diameter of the unobstructed cross-section and the mean bulk velocity, ranges from 10,000 to 25,000. The experiment employs a hybrid technique based on transient liquid crystal imaging to obtain the distributions of the local heat transfer coefficient over all of the participating surfaces, including the endwalls and all the pin elements. Experimental results reveal that the presence of a detached space between the pin tip and the endwall has a significant effect on the convective heat transfer and pressure loss in the channel. The presence of pin-to-endwall spacing promotes wall-flow interaction, generates additional separated shear layers, and augments turbulent transport. In general, an increase in detached spacing, or C/D, leads to lower heat transfer enhancement and pressure drop. However, C/D = 1, i.e., H/D = 3, of a staggered array configuration exhibits the highest heat transfer enhancement, followed by the cases of C/D = 0 and C/D = 2, i.e., H/D = 4 or 2, respectively.

Commentary by Dr. Valentin Fuster

Heat and Mass Transfer

J. Heat Transfer. 2012;134(8):082001-082001-10. doi:10.1115/1.4006208.

The fluid flow and conjugate heat and mass transfer across a hollow fiber membrane tube bundle used for liquid desiccant air dehumidification are investigated. In this process, humid air flows across the fiber bank and salt solution flows inside the fibers packed in a shell. They exchange heat and moisture through the membranes. To overcome the difficulties in the direct modeling of the whole tube bundle, a representative cell, which comprises of a single fiber, a solution stream inside the fiber, and an air stream flowing across the fiber, is selected as the calculation domain. The liquid flow inside the fibers is assumed to be laminar due to the low Reynolds numbers, while the air flow across the bank is considered to be turbulent as a result from the disturbances from the numerous fibers. The governing equations for fluid flow and heat and mass transfer in the two flows and in the membrane are coupled together and solved numerically with a self-built code. Experimental work on hollow fiber membrane-based liquid desiccant air dehumidification is performed to validate the model. The fundamental data on friction factor, Nusselt and Sherwood numbers on both the shell and the tube sides are then obtained for Re = 300–600.

Commentary by Dr. Valentin Fuster

Jets, Wakes, and Impingment Cooling

J. Heat Transfer. 2012;134(8):082201-082201-11. doi:10.1115/1.4006155.

A combined experimental and numerical investigation of the heat transfer characteristics within arrays of impinging jets with rib-roughened surfaces is presented. Two configurations are considered: One with an inline arrangement of jets and ribs oriented perpendicular to the direction of cross-flow and one with a staggered arrangement of jets and broken ribs aligned with the direction of cross-flow. For both cases, the jet Reynolds number is 35,000, the separation distance measures H/D = 3, the spent air is routed through one exit contributing to the maximum cross-flow condition, and the rib height and width is both 1 D. The experiments are carried out in perspex models using the transient liquid crystal method. Local jet temperatures are measured at several positions on the impingement plate to account for an exact evaluation of the heat transfer coefficient. In addition to the measurements, a numerical analysis using the commercial CFD software package ANSYS CFX is conducted. Heat transfer predictions are compared with those obtained from experiments with regards to local distributions as well as averaged quantities. A good overall agreement is found but discrepancies for local values need to be accepted. The present investigation also emphasizes that configurations including rib roughness elements should be compared based on the amount of transferred heat flux in order to account for the area enlarging effect. This allows a correct evaluation of the thermal performance.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):082202-082202-12. doi:10.1115/1.4006167.

The effects of the Coriolis force are investigated in rotating internal serpentine coolant channels in turbine blades. For complex flow in rotating channels, detailed measurements of the heat transfer over the channel surface will greatly enhance the blade designers’ ability to predict hot spots so coolant may be distributed more effectively. The present study uses a novel transient liquid crystal technique to measure heat transfer in a rotating, radially outward channel with impingement jets. A simple case with a single row of constant pitch impinging jets with the crossflow effect is presented to demonstrate the novel liquid crystal technique and document the baseline effects for this type of geometry. The present study examines the differences in heat transfer distributions due to variations in jet Rotation number, Roj , and jet orifice-to-target surface distance (H/dj  = 1,2, and 3). Colder air, below room temperature, is passed through a room temperature test section to cause a color change in the liquid crystals. This ensures that buoyancy is acting in a similar direction as in actual turbine blades where walls are hotter than the coolant fluid. Three parameters were controlled in the testing: jet coolant-to-wall temperature ratio, average jet Reynolds number, Rej , and average jet Rotation number, Roj . Results show, such as serpentine channels, the trailing side experiences an increase in heat transfer and the leading side experiences a decrease for all jet channel height-to-jet diameter ratios (H/dj ). At a jet channel height-to-jet diameter ratio of 1, the crossflow from upstream spent jets greatly affects impingement heat transfer behavior in the channel. For H/dj  = 2 and 3, the effects of the crossflow are not as prevalent as H/dj  = 1: however, it still plays a detrimental role. The stationary case shows that heat transfer increases with higher H/dj values, so that H/dj  = 3 has the highest results of the three examined. However, during rotation the H/dj  = 2 case shows the highest heat transfer values for both the leading and trailing sides. The Coriolis force may have a considerable effect on the developing length of the potential core, affecting the resulting heat transfer on the target surface.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):082203-082203-14. doi:10.1115/1.4006109.

This paper delineates the results of an investigation on the combined effects of buoyancy and surface-radiation on heat transfer from an isothermal surface, subjected to a confined submerged impinging air-jet issuing from a slot-nozzle. The nondimensionalized governing equations are solved using the stream function-vorticity approach and an upwind finite-difference technique, employing the radiosity-irradiation formulation for surface-radiation. The effects of jet Reynolds number, dimensionless nozzle-to-heater distance, radiation-flow interaction parameter, Richardson number, and surface-emissivity, on the convective, radiative, and total Nusselt numbers, are analyzed for 100 ≤ Red ≤ 900, 1 ≤ H ≤ 8, 0.1 ≤ NRF,d ≤ 2, 0.01 ≤ Rid ≤ 10, and 0.05 ≤ ɛ ≤ 0.85. It was found that the radiation-flow interaction parameter was most influential in affecting the radiative Nusselt number and, hence, the total heat transfer from the impingement surface. In contrast to a substantial enhancement in the net radiative component of the overall heat transfer in both the stagnation region and the regions downstream for an increase in ɛ over the range considered, the convective counterpart was found to be suppressed. The effect of increase in Rid on the heat transfer in the stagnation region was found to be negligible; however, an adverse effect on the net radiation from the heater was observed. Increase in NRF,d resulted in an increase in contribution of radiation to the total heat transfer by about 25% in the stagnation region, while over 40% in the wall-jet region for low values of H, and to about 15% in the stagnation region to over 50% in the wall-jet region for relatively larger values of H. With increase in H, both convective and radiative Nusselt numbers decreased over most of the upstream regions of the heater, while the magnitude of local radiative Nusselt numbers increased over the regions closer to the outlet. For sufficiently large values of jet Reynolds number or large values of dimensionless nozzle-to-heater distance, a small recirculation region was found to occur over the heater at a certain distance downstream of the stagnation point, where distribution of the contribution of radiation to the overall heat transfer from the heater results in a local maxima reaching about 60–80% for specific combinations of controlling parameters.

Commentary by Dr. Valentin Fuster

Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2012;134(8):082401-082401-12. doi:10.1115/1.4006169.

A sub-micron thermal transport model based on the phonon Boltzmann transport equation (BTE) is developed using anisotropic relaxation times. A previously-published model, the full-scattering model, developed by Wang, directly computes three-phonon scattering interactions by enforcing energy and momentum conservation. However, it is computationally very expensive because it requires the evaluation of millions of scattering interactions during the iterative numerical solution procedure. The anisotropic relaxation time model employs a single-mode relaxation time, but the relaxation time is derived from detailed consideration of three-phonon interactions satisfying conservation rules, and is a function of wave vector. The resulting model is significantly less expensive than the full-scattering model, but incorporates directional and dispersion behavior. A critical issue in the model development is the role of three-phonon normal (N) scattering processes. Following Callaway, the overall relaxation rate is modified to include the shift in the phonon distribution function due to N processes. The relaxation times so obtained are compared with the data extracted from equilibrium molecular dynamics simulations by Henry and Chen. The anisotropic relaxation time phonon BTE model is validated by comparing the predicted thermal conductivities of bulk silicon and silicon thin films with experimental measurements. The model is then used for simulating thermal transport in a silicon metal-oxide-semiconductor field effect transistor (MOSFET) and leads to results close to the full-scattering model, but uses much less computation time.

Commentary by Dr. Valentin Fuster

Natural and Mixed Convection

J. Heat Transfer. 2012;134(8):082501-082501-5. doi:10.1115/1.4006159.

The steady fully developed mixed convection flow between two vertical parallel plates with asymmetrical thermal and nanoparticle concentration conditions at the walls filled by a nanofluid is studied. The nanofluid model used in this paper takes into account the Brownian diffusion and the thermophoresis effects, and the analysis is based on analytical solutions. Thus, analytical expressions for the fully developed velocity, temperature, and nanoparticle concentration profiles as well as for the Nusselt and Sherwood numbers at the left wall of the channel are given. A numerical solution has been also obtained and compared with the analytical solution, the agreement being very good.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):082502-082502-9. doi:10.1115/1.4006170.

Natural heat convection within an annular annulus bounded by two horizontal vertically eccentric long cylinders has been investigated. The annulus inner wall has been heated and maintained at either constant wall temperature CWT or constant heat flux CHF while the outer wall is cooled and maintained at constant temperature. The induced buoyancy driven flow and the associated heat convection are predicted through solving numerically the full conservation equations for mass, momentum, and energy using Fourier spectral method. Beside Rayleigh and Prandtl numbers, the heat convection process in the annulus depends on the annulus radius ratio and eccentricity (normalized by the radius difference). The study considered a moderate range of Rayleigh numbers up to 105 while Prandtl number is fixed at 0.7. The radius ratio is considered up to 3.2 while the eccentricity is varied between − 0.65 and + 0.65. The study has revealed that at certain radius ratio for a given Rayleigh number and eccentricity, the heat transfer is minimum in case of CWT and the mean inner wall temperature is maximum in case of CHF. The study has also shown, in the range considered for controlling parameters, that multiple convection cells only exist in case of CWT and only for positive eccentricity. Moreover, the study has shown that the present numerical solution of the pure conduction problem is almost identical with the newly presented analytical solution which confirms the high accuracy of the numerical solution.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2012;134(8):082503-082503-8. doi:10.1115/1.4006158.

This study numerically explores the effect of presence of micro-encapsulated phase change material (MEPCM) on the heat transfer characteristics of a fluid in a rectangular cavity driven by natural convection. The natural convection is generated by the temperature difference between two vertical walls at constant temperatures. The phase change material (PCM) melts in the vicinity of the hot wall and solidifies near the cold wall. Unlike the pure fluids, the heat transfer characteristics of MEPCM slurry cannot be simply presented in terms of corresponding dimensionless controlling parameters such as Rayleigh number. In the presence of phase change particles, the controlling parameters’ values change significantly due to the local phase change. The numerical results show significant increase in the heat transfer coefficient (up to 80%) at the considered operating conditions. This increase is a result of the MEPCM latent heat and the increased volumetric thermal expansion coefficient due to MEPCM volume change during melting.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Heat Transfer. 2012;134(8):084501-084501-4. doi:10.1115/1.4006101.

The natural or free convection in a polygonal duct with a heated core is solved by eigenfunction expansion and boundary collocation. The optimal sizes of the core for maximum flow or maximum energy transport are determined.

Commentary by Dr. Valentin Fuster

Announcements

J. Heat Transfer. 2012;134(8):088001-088001-1. doi:10.1115/1.4006726.
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Commentary by Dr. Valentin Fuster

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