Research Papers: Conduction

J. Heat Transfer. 2017;140(2):021301-021301-10. doi:10.1115/1.4037542.

This paper provides a solution for two-dimensional (2D) heating over a rectangular region on a homogeneous plate. It has application to verification of numerical conduction codes as well as direct application for heating and cooling of electronic equipment. Additionally, it can be applied as a direct solution for the inverse heat conduction problem, most notably used in thermal protection systems for re-entry vehicles. The solutions used in this work are generated using Green's functions (GFs). Two approaches are used, which provide solutions for either semi-infinite plates or finite plates with isothermal conditions, which are located a long distance from the heating. The methods are both efficient numerically and have extreme accuracy, which can be used to provide additional solution verification. The solutions have components that are shown to have physical significance. The procedure involves the derivation of previously unknown simpler forms for the summations, in some cases by virtue of the use of algebraic components. Also, a mathematical identity given in this paper can be used for a variety of related problems.

Commentary by Dr. Valentin Fuster

Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2017;140(2):021501-021501-10. doi:10.1115/1.4037154.

During a severe accident, ex-vessel cooling may pose a risk for larger-powered reactors. The current in-vessel retention (IVR) (through ex-vessel cooling) capability may not be sufficient for the larger-powered reactors, and critical heat flux (CHF) conditions may eventually lead to vessel failure. A manner in which the CHF can be increased is by applying a structured surface design on the outer surface of the reactor pressure vessel (RPV). A simple design proposed in this work is the pin–fin. An experimental investigation was performed to observe the effect of the pin–fin on CHF with a downward-facing heated surface in flow boiling conditions. A reduced pressure of approximately 0.05 MPa allowed for saturation at approximately 81 °C. A range of flow rates corresponding to mass flux of 202–1456 kg/m2 s were applied in the experiments. The results showed an increase in the CHF when compared to a bare surface. An average CHF enhancement of 61% was observed from the finned surface. An enhancement of approximately 19% was observed in the heat transfer coefficient. As seen in nanoparticle/nanofluid enhancement, an increase in the CHF also leads to an increase in the superheat. Even though an increase in the CHF had been observed, the CHF for the finned and bare surfaces occurred at approximately similar superheat.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(2):021502-021502-9. doi:10.1115/1.4037644.

The increasing demand for designing effective cooling solutions in high power density electronic components has resulted in exploring advanced thermal management strategies. Over the past decade, phase-change cooling has received widespread recognition due to its ability to dissipate large heat fluxes while maintaining low temperature differences. In this paper, a radial flow boiling configuration through a central inlet was studied. This configuration is particularly suited for chip cooling application. Two heat transfer surfaces with (a) radial microchannels, and (b) offset strip fins were fabricated and their flow boiling performance with distilled water was obtained. Furthermore, the effect of the liquid flow rate on the boiling performance and enhancement mechanisms was also investigated in this study. At a flow rate of 240 mL/min, a maximum heat flux of 369 W/cm2 at a wall superheat of 49 °C and a pressure drop of 59 kPa was achieved with the radial microchannels, while the offset strip fins achieved a maximum heat flux of 618 W/cm2 at a wall superheat of 20 °C. Increasing the flow rate to 320 mL/min resulted in a heat flux of 897 W/cm2 demonstrating the potential of using a radial configuration for enhancing the boiling performance. The increase in flow cross-sectional area was shown to be responsible for the reduced pressure drop when compared to straight microchannel configurations. The high-speed imaging incorporated in each test provided valuable insight and understanding into the flow patterns and underlying mechanism in these geometries. With the ease of implementation, highly stable flow, and further optimization possibilities with different microchannel and taper configurations, the radial geometry is expected to provide significant performance enhancement well beyond a critical heat flux (CHF) of 1 kW/cm2.

Commentary by Dr. Valentin Fuster

Research Papers: Forced Convection

J. Heat Transfer. 2017;140(2):021701-021701-12. doi:10.1115/1.4037498.

Numerical simulation of three-dimensional turbulent flow and heat transfer was performed in a multipass rectangular (AR = 2:1) rotating cooling channel with and without turning vane in the hub region under various flow conditions, with two different Reynolds numbers of 10,000 and 25,000, two different channel orientations of 45-deg and 90-deg, and the rotation number varies from 0 to 0.2. This study shows that the addition of the turning vane in the hub turn region does not cause much impact to the flow before the hub. However, it significantly alters the flow reattachment and vortex distribution in the hub turn region and after the hub turn portion. The local heat transfer is deeply influenced by this complex flow field and this turning vane effect lasts from the hub turn region to the portion after it.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(2):021702-021702-15. doi:10.1115/1.4037581.

In this study, a new coupled surface shape design (SSD) methodology named direct design method is presented for the solution of problems containing different types of convection heat transfer in which a specific distribution of either heat flux or temperature is given instead of the shape of a boundary. In the proposed method, the governing equation, without using any mathematical transformation for the physical domains, is manipulated so that the grid generation, solving fluid flow, and heat transfer as well as shape updating can all be carried out simultaneously. Five different inverse shape design problems containing different types of convection heat transfer are solved by the proposed method. All the problems are also solved using the ball-spine algorithm (BSA), which is a recently developed de-coupled algorithm, for the sake of comparison. In all problems, the effects of using different under-relaxation parameters are investigated and the capability of both approaches is compared with each other. The results show that the proposed coupled method can solve the problems better than the BSA in the sense that the direct design method converges sooner than the BSA when the same under-relaxation parameter is used for both methods. Also, it is shown that the computational cost of solving a SSD problem using the direct design method is slightly greater than solving an analysis problem.

Commentary by Dr. Valentin Fuster

Research Papers: Heat and Mass Transfer

J. Heat Transfer. 2017;140(2):022001-022001-9. doi:10.1115/1.4037501.

A novel method that incorporates uncertainty quantification (UQ) into numerical simulations of heat transfer for a 9 × 9 square array of spent nuclear fuel (SNF) assemblies in a boiling water reactor (BWR) is presented in this paper. The results predict the maximum mean temperature at the center of the 9 × 9 BWR fuel assembly to be 462 K using a range of fuel burn-up power. Current related modeling techniques used to predict the heat transfer and the maximum temperature inside SNF assemblies rely on commercial codes and address the uncertainty in the input parameters by running separate simulations for different input parameters. The utility of leveraging polynomial chaos expansion (PCE) to develop a surrogate model that permits the efficient evaluation of the distribution of temperature and heat transfer while accounting for all uncertain input parameters to the model is explored and validated for a complex case of heat transfer that could be substituted with other problems of intricacy. UQ computational methods generated results that are encompassing continuous ranges of variable parameters that also served to conduct sensitivity analysis on heat transfer simulations of SNF assemblies with respect to physically relevant parameters. A two-dimensional (2D) model is used to describe the physical processes within the fuel assembly, and a second-order PCE is used to characterize the dependence of center temperature on ten input parameters.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(2):022002-022002-11. doi:10.1115/1.4037748.

The state of the art of thermally self-excited oscillatory heat pipe technology is briefly mentioned to emphasize that there exists no oscillating/pulsating heat pipe (OHP/PHP) suited to long-distance heat transport. Responding to such conditions, this study actively proposes a newly devised conceptually novel type of OHP/PHP. In that heat pipe, the adiabatic section works as it were the dream pipe invented by Kurzweg. This striking quality of the proposed new-style OHP/PHP produces high possibilities of long-distance heat transport. To support such optimistic views, an originally planned mathematical model is introduced for feasibility studies. Hydraulic considerations have first been done to understand what conditions are required for sustaining bubble-train flows in a capillary tube of interest. Theoretical analysis has then been made to solve the momentum and energy equations governing the flow velocity and temperature fields in the adiabatic section. The obtained analytical solutions are arranged to give algebraic expressions of the effective thermal diffusivity, the performance index combined with the tidal displacement, and the required electric power. Computed results of those three are displayed in the figures to demonstrate the realizability of that novel OHP.

Commentary by Dr. Valentin Fuster

Research Papers: Jets, Wakes, and Impingment Cooling

J. Heat Transfer. 2017;140(2):022201-022201-12. doi:10.1115/1.4037499.

A combined transpiration and opposing jet cooling method was experimentally investigated for protecting porous struts with microslits in the leading edge. Schlieren images showed that this cooling method significantly affects the stability of the flow field and the profile of the detached shock wave. Three different states of flow fields were observed when increasing the coolant injection pressure of a strut having a 0.20 mm wide microslit. The detached bow shock was pushed away by the opposing jet; it then became unstable and even disappeared when the coolant injection pressure was increased. Combined transpiration and opposing jet cooling could effectively cool the entire strut, especially the leading edge. The leading edge cooling efficiency increased from 3.5% for the leading edge without a slit to 52.8% for the leading edge with a 0.20 mm wide slit when the coolant injection pressure was 0.55 MPa. Moreover, combined transpiration and opposing jet cooling with nonuniform injection distribution made the strut temperature distribution more uniform and caused the maximum temperature to decrease compared to standard transpiration cooling.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(2):022202-022202-10. doi:10.1115/1.4037540.

An experimental study using the liquid crystal thermography technique is conducted to investigate the convective heat transfer performance in jet impingement cooling using various porous media configurations. Aluminum porous foams are used in the present study. Four impinging jet configurations are considered: jet impingement (1) without porous media, (2) over the porous heat sink, (3) with porous obstacle case, and (4) through porous passage. These configurations are evaluated on the basis of the convective heat transfer enhancement for two different Reynolds numbers of 400 and 700. Jet impingement with porous heat sink showed deterioration in the average Nusselt number by 9.95% and 18.04% compared to jet impingement without porous media configuration for Reynolds numbers of 400 and 700, respectively. Jet impingement with porous obstacles showed a very negligible enhancement in the average Nusselt number by 3.48% and 2.73% for Reynolds numbers of 400 and 700, respectively. However, jet impingement through porous passage configuration showed a maximum enhancement in the average Nusselt number by 52.71% and 74.68% and stagnation Nusselt numbers by 58.08% and 53.80% compared to the jet impingement without porous medium for Reynolds numbers of 400 and 700, respectively. Within the porous properties considered, it is observed that by decreasing the permeability and porosity the convective heat transfer performance tends to increase.

Commentary by Dr. Valentin Fuster

Research Papers: Melting and Solidification

J. Heat Transfer. 2017;140(2):022301-022301-9. doi:10.1115/1.4037776.

The classical problem of inward solidification heat transfer inside a spherical capsule, with an application to thermal energy storage (TES), was revisited in the presence of nano-enhanced phase change materials (NePCM). The model NePCM samples were prepared by dispersing graphite nanoplatelets (GNPs) into 1-tetradecanol (C14H30O) at loadings up to 3.0 wt %. The transient phase change, energy retrieval, and heat transfer rates during solidification of the various NePCM samples were measured quantitatively using a volume-shrinkage-based indirect method. The data reduction and analysis were carried out under single-component, homogeneous assumption of the NePCM samples without considering the microscale transport phenomena of GNPs. It was shown that the total solidification time becomes monotonously shorter with increasing the loading of GNPs, in accordance with the increased effective thermal conductivity. The maximum relative acceleration of solidification was found to be more than 50% for the most concentrated sample, which seems to be appreciable for practical applications. In addition to enhanced heat conduction, the possible effects due to the elimination of supercooling and viscosity growth were elucidated. The heat retrieval rate was also shown to be increased monotonously with raising the loading of GNPs, although the heat storage capacity is sacrificed. Despite the remarkable acceleration of the solidification time, the use of a high loading (e.g., 3.0 wt %) was demonstrated to be possibly uneconomical because of the marginal gain in heat retrieval rate. Finally, correlations for the transient variations of the melt fraction and surface-averaged Nusselt number were proposed.

Commentary by Dr. Valentin Fuster

Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2017;140(2):022401-022401-9. doi:10.1115/1.4037541.

The effect of nanoparticle coating on the performance of a miniature loop heat pipe (mLHP) is experimentally investigated for heat inputs of 20–380 W using distilled water as the working fluid. Applications include the cooling of electronic devices such as circuit breaker in low voltage switch board and insulated gate bipolar transistor. Physical vapor deposition method is used to coat the nanoparticles on the evaporator surface for different coating thicknesses of 100 nm, 200 nm, 300 nm, 400 nm, and 500 nm, respectively. An optimum filling ratio (FR) of 30% is chosen for the analysis. Experimental findings show that the nanoparticle coating gives a remarkable improvement in heat transfer of the heat pipe. An average reduction of 6.7%, 11.9%, 17.2%, and 22.6% in thermal resistance is observed with coating thicknesses of 100 nm, 200 nm, 300 nm, and 400 nm, respectively. Similarly, enhancements in evaporator heat transfer coefficients of 47%, 63.5%, 73.5%, and 86% are noted for the same coating thicknesses, respectively. Evaporator wall temperature decreased by 15.4 °C for 380 W with a coating thickness of 400 nm. The repeatability test ensures the repeatability of experiments and the stability of coatings in the long run.

Commentary by Dr. Valentin Fuster

Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2017;140(2):022501-022501-8. doi:10.1115/1.4037537.

Natural convection heat transfer from an array of horizontal rectangular fins on a vertical flat plate in non-Newtonian power-law fluids has been studied. The underlying physical principles affecting heat transfer were studied using comprehensive solutions obtained from numerical investigations. Heat transfer to the power-law fluid was found to depend on the fluid rheology (power-law index) and significantly on the geometric parameters (interfin spacing, fin length) as well. The dependence was quantified using the Nusselt number (Nu) and fin effectiveness (Q/Q0). The present study shows that compared to a fin analyzed in isolation, the spatial arrangement of multiple fins relative to one another in an array does have a significant effect on the flow field around subsequent fins in power-law fluids. Therefore, the average heat transfer coefficient of the natural convection system is affected significantly. The variation of Nu with the dimensionless fin length (l/L), dimensionless interfin spacing (S/L), and fluid power-law index (n) was plotted. The dependence was found to be counter intuitive to expectations based on studies for natural convection from vertical flat plates to power-law fluids. In the present study involving fins, shear-thinning fluids (n < 1) show a decrease in heat transfer and shear-thickening fluids (n > 1) show an enhancement in heat transfer for higher l/L values. The results of the study may be useful in the design of natural convection systems that employ power-law fluids to enhance or control heat transfer.

Commentary by Dr. Valentin Fuster

Research Papers: Radiative Heat Transfer

J. Heat Transfer. 2017;140(2):022701-022701-10. doi:10.1115/1.4037496.

The aim of this study is to present a reduced spectral line-based weighted sum of gray gases (SLW) model to simulate the radiation heat transfer in nongray media at high temperatures. Inverse approach is used to divide the absorption cross section band into a clear gas with one gray gas and two gray gases, which are called the S-1 and S-2 approaches, respectively. The unknown absorption cross sections are determined from the knowledge of measured total incident intensities received by wall surfaces. In order to simulate the exact solution of radiation heat transfer in nongray gaseous media, the discrete transfer method (DTM) in combination with S-20 model is used, where the nongray medium is replaced with a set of a clear gas and 20 gray gases. The inverse problem is formulated as an optimization problem to minimize a least square objective function, which is solved by the conjugate gradient method (CGM). The accuracy of the present method is verified by comparing with previous researches and the S-20 approach with a large number of gray gases. The effects of noisy data on the inverse solution are investigated by considering an extreme case with large measurement error. The results show that the unknown absorption cross sections are retrieved well, even for noisy data.

Commentary by Dr. Valentin Fuster

Research Papers: Thermal Systems

J. Heat Transfer. 2017;140(2):022801-022801-9. doi:10.1115/1.4037194.

Transient thermal behavior modeling and simulation is a key issue in predicting flight performance of stratospheric lighter-than-air (LTA) vehicles, such as airships or balloons. To reduce computational load of the transient thermal model without significant loss of accuracy, first this paper adopted an analytical model of view factor from the element surfaces to the Earth and constructed a full distributed parameter transient thermal model. Then, the full model was validated by comparing the predictions obtained from the full model with the flight experimental data. The comparison results show that the divergence of the predicted average internal gas temperatures from the flight data is about 0.4%, and the divergence of the predicted envelop temperatures from the flight data is less than 2.4%. Furthermore, considering that the effect of the net radiation heat transfer among the inner surface enclosure on average internal gas temperature is far less than radiation heat transfer of the outer surfaces, the full model was simplified by omitting radiant heat exchange within the inner surface enclosure. The accuracy of the simplified model was investigated by comparing the predictions of average internal gas temperature and skin temperature distribution between the simplified model and full model under various conditions, such as flight time, altitude, and different external skin thermal properties. The comparison results indicate that the simplified model agrees well with the full model. The discrepancies of the predicted average internal gas temperature between the two models are less than 0.3% under most conditions, and the discrepancies of the predicted temperature distribution between the two models are also acceptable when the LTA vehicle, especially with low absorptivity/emissivity ratio coatings, operates at about 20 km altitude.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(2):022802-022802-17. doi:10.1115/1.4037534.

A novel solid–gas thermochemical sorption thermal energy storage (TES) system for solar heating and cooling applications operating on four steady-state flow devices and with two transient storage tanks is proposed. The TES system stores solar or waste thermal energy in the form of chemical bonds as the working gas is desorbed from the solid. Strontium chloride–ammonia is the working solid–gas couple in the thermochemical sorption TES system. Strontium chloride–ammonia has a moderate working temperature range that is appropriate for building heating and cooling applications. The steady-state devices in the system are simulated using Aspen Plus, and the two transient components are simulated using the ENGINEERING EQUATION SOLVER (EES) package. Multiple cases are examined of different heat and cold production temperatures for both heating and cooling applications for a constant thermal energy input temperature. Energy and exergy analyses are performed on the system for all simulated cases. The maximum energy and exergy efficiencies for heating applications are 65.4% and 50.8%, respectively, when the heat is generated at a temperature of 87 °C. The maximum energy and exergy efficiencies for cooling applications are 29.3% when the cold production temperature is 0 °C and 22.9% when it is −35 °C, respectively. The maximum heat produced per mass of the ammonia produced, for 100% conversion of the reactants in the chemical reaction, is 2010 kJ/kg at a heat production temperature of 87 °C, and the maximum cold energy generated is 902 kJ/kg at a temperature of 0 °C. Finally, the system is modified to operate as a heat pump, and energy and exergy analyses are performed on the thermochemical heat pump. It is found that the maximum energy and exergy coefficients of performance (COP) achieved by upgrading heat from 87 °C to 96 °C are 1.4 and 3.6, respectively, and the maximum energy and exergy efficiencies are 56.4% and 79.0%, respectively.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2017;140(2):022803-022803-7. doi:10.1115/1.4037643.

This paper proposes a general method combining evolutionary algorithm and decision-making technique to optimize the structure of a minichannel heat sink (MCHS). Two conflicting objectives, the thermal resistance θ and the pumping power P, are simultaneously considered to assess the performance of the MCHS. In order to achieve the ultimate optimal design, multi-objective genetic algorithm is employed to obtain the nondominated solutions (Pareto solutions), while technique for order preference by similarity to an ideal solution (TOPSIS) is employed to determine which is the best compromise solution. Meanwhile, both the material cost and volumetric flow rate are fixed where this nonlinear problem is solved by applying the penalty function. The results show that θ of Pareto solutions varies from 0.03707 K W−1 to 0.10742 K W−1, while P varies from 0.00307 W to 0.05388 W, respectively. After the TOPSIS selection, it is found that P is significantly reduced without increasing too much θ. As a result, θ and P of the optimal MCHS determined by TOPSIS are 35.82% and 52.55% lower than initial one, respectively.

Commentary by Dr. Valentin Fuster

Research Papers: Max Jakob Award Paper

J. Heat Transfer. 2017;140(2):023001-023001-6. doi:10.1115/1.4037497.

Experimental results for the Sherwood number variation near the lateral edge of the active surface of a smooth, finite-width flat plate in turbulent boundary layer flow are presented. Using naphthalene sublimation, local mass transfer rates are found for two different free stream velocities. A semi-empirical correlation of the experimental data is presented, allowing calculation of the increase of mass transfer near the edge and the size of the region affected by the lateral edge. The effect is shown to scale more so with the diffusion thickness than the boundary layer thickness.

Commentary by Dr. Valentin Fuster

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