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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

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

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: 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

Research Papers: Max Jacob 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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