Optimal Estimation of Thermal Diffusivity in a Thermal Energy Transfer Problem with Heat Generation, Convection Dissipation and Lateral Heat Flow

This work focuses on determining the coefficient of thermal diffusivity in a one-dimensional heat transfer process along a homogeneous and isotropic bar, embedded in a moving fluid with heat generation. A first type (Dirichlet) condition is imposed on one boundary and a third type (Robin) condition is considered at the other one. The parameter is estimated by minimizing the squared errors where noisy observations are numerically simulated at different positions and instants. The results are evaluated by means of the relative errors for different levels of noise. In order to enhance the estimation performance, an optimal design technique is chosen to select the most informative data. Finally, the improvement of the estimate is discussed when an optimal design is used.

Experimental Investigation of the Effect of Heat Spreading on Boiling of a Dielectric Liquid for Immersion Cooling of Electronics

This paper investigated the effect of heat spreading on the boiling of the Novec 649TM for two-phase immersion cooling of electronics. Reference pool boiling tests were performed by attaching a 25.4 by 25.4 mm square plate square copper plate to a same-sized heater, thus minimizing lateral heat spreading. Experimental measurements showed that the critical heat flux (CHF) happened at a heat flux of 17.4 ±0.8 W/cm2. Then, lateral heat spreading through the heat spreader was studied by attaching larger (47 mm by 47mm) spreaders with four different thicknesses to the copper plate. With an increase in the integrated heat spreader (IHS) thickness from 1 mm to 6 mm, the CHF increased by more than 60% at the saturation condition. One plate was a 1 mm-thick IHS removed from a commercial microprocessor. In this case, the CHF happens at 8.6 W/cm2 (50% lower compared to the reference case) in the saturation condition. At CHF, the boiling can be observed on the whole surface, with columns and slugs regime at the center and the fully developed nucleate boiling regime at the edges. This non-uniform boiling was more pronounced in sub-cooled conditions, in which the CHF occurred at the center while there were regions at the edges that had no boiling. Finally, the performance of a micro porous-coated IHS (with 3.15 mm thickness) was compared to the 6mm thick IHS. The thermal resistance was almost equal for powers above 200 W. This indicates that lateral heat spreading is a critical parameter for the thermal design of immersion cooling along with micro-porous coating.

An interferometric study of free convection in a window with a heated between-panes blind

An experimental study has been conducted to examine free convection in a window with an enclosed aluminum Venetian blind. The unique feature of this experiment was that the blind slats were heated electrically to simulate absorbed solar radiation. Centre-glass convective heat transfer measurements and temperature field visualization were obtained using a laser Mach-Zehnder interferometer. Measurements were made for three plate (glazing) spacings, three blind slat angles, three blind heat fluxes, and two plate temperature differences. It was found that a recently proposed simplified model, called the Reduced Slat Length (RSL) model, closely predicted the experimental results when the flow appeared to be laminar and steady. Under these conditions, the temperature field and lateral heat transfer was dominated by conduction. Under some conditions, evidence of highly unsteady/turbulent flow was observed. As expected, the RSL model performed poorly under these conditions.

An interferometric study of free convection in a window with a heated between-panes blind

An experimental study has been conducted to examine free convection in a window with an enclosed aluminum Venetian blind. The unique feature of this experiment was that the blind slats were heated electrically to simulate absorbed solar radiation. Centre-glass convective heat transfer measurements and temperature field visualization were obtained using a laser Mach-Zehnder interferometer. Measurements were made for three plate (glazing) spacings, three blind slat angles, three blind heat fluxes, and two plate temperature differences. It was found that a recently proposed simplified model, called the Reduced Slat Length (RSL) model, closely predicted the experimental results when the flow appeared to be laminar and steady. Under these conditions, the temperature field and lateral heat transfer was dominated by conduction. Under some conditions, evidence of highly unsteady/turbulent flow was observed. As expected, the RSL model performed poorly under these conditions.

Effects of skin heat conduction on aircraft icing process

During the aircraft icing process caused by super-cooled droplet impingement, the surface temperature and heat flux distributions of the skin would vary due to the solid substrate heat conduction. An unsteady thermodynamic model of the phase transition was established with a time-implicit solution algorithm, in which the solid heat conduction and the water freezing were analyzed simultaneously. The icing process on a rectangular skin segment was numerically simulated, and the variations of skin temperature distribution, thicknesses of ice layer and water film were obtained. Results show that the presented model could predict the icing process more accurately, and is not sensitive to the selection of time step. The latent heat released by water freezing affects the skin temperature, which in turn changes the icing characteristics. The skin temperature distribution would be affected notably by the boundary condition of the inner skin surface, the lateral heat conduction and thermal property of the skin. It was found that the ice accretion rate of the case that the inner surface boundary is in natural convection at ambient temperature is much smaller than that with constant ambient temperature there; due to the skin lateral heat conduction, the outer skin surface temperature increases first and then decreases with uneven distribution, leading to an unsteady ice accretion rate and uneven ice thickness distribution; a smaller heat conductivity would lead to a more uneven temperature distribution and a lower ice accretion rate in most regions, but the maximum ice thickness could be larger than that of higher heat conductivity skin. Therefore, in order to predict the aircraft icing phenomenon more accurately, it is necessary to consider the solid heat conduction and the boundary conditions of the skin substrate, instead of applying a simple boundary condition of adiabatic or a fixed temperature for the outer skin surface.

Investigation of Production of Nanofiber Nonwoven Fabric and its Thermal Properties

Nanofibers of polypropylene were produced by a modified melt-blowing method. The manufacturing method and thermal characteristics of fabricated nonwoven-fabric nanofibers were studied. Apparent thermal conductivity was measured as an evaluation of adiabatic properties, and a prediction model was developed with computational fluid dynamics (CFD) based on a one-dimensional computer-aided engineering method. In addition, we attempted to evaluate true thermal conductivity in consideration of lateral heat dissipation during measurement by thickness. Consequently, we determined the influence of the fiber diameter and thickness of the nonwoven fabric on the thermal conductivity and demonstrated that the proposed CFD model was effective for estimating the characteristics of the thermal conductivity of the nonwoven fabric.

Thermal model for 3-D integrated circuits with integrated MLGNR-based through silicon via

This paper applies the multi-layer graphene nanoribbon as a new prospective filler material for through silicon via to solve the complex heat problems in the 3-D integrated circuits. An equivalent thermal model for 3-D integrated circuits with the MLGNR-based through silicon via is presented in this work, which take lateral heat transfer of through silicon via into account. The experimental results show that the heat transfer performance of MLGNR-based through silicon via is better than the conventional Cu-based through silicon via. Furthermore, it is found that the temperature predicted by the proposed model are in good accordance with the ANSYS simulation, and the maximum relative deviation is less than 4.0%

Temperature log simulations in high-enthalpy boreholes

Temperature logs have important applications in the geothermal industry such as the estimation of the static formation temperature (SFT) and the characterization of fluid loss from a borehole. However, the temperature distribution of the wellbore relies on various factors such as wellbore flow conditions, fluid losses, well layout, heat transfer mechanics within the fluid as well as between the wellbore and the surrounding rock formation, etc. In this context, the numerical approach presented in this paper is applied to investigate the influencing parameters/uncertainties in the interpretation of borehole logging data. To this end, synthetic temperature logs representing different well operation conditions were numerically generated using our newly developed wellbore simulator. Our models account for several complex operation scenarios resulting from the requirements of high-enthalpy wells where different flow conditions, such as mud injection with- and without fluid loss and shut-in, occur in the drill string and the annulus. The simulation results reveal that free convective heat transfer plays an important role in the earlier evolution of the shut-in-time temperature; high accuracy SFT estimation is only possible when long-term shut-in measurements are used. Two other simulation scenarios for a well under injection conditions show that applying simple temperature correction methods on the non-shut-in temperature data could lead to large errors for SFT estimation even at very low injection flow rates. Furthermore, the magnitude of the temperature gradient increase depends on the flow rate, the percentage of fluid loss and the lateral heat transfer between the fluid and the rock formation. As indicated by this study, under low fluid losses (< 30%) or relatively higher flow rates (> 20 L/s), the impact of flow rate and the lateral heat transfer on the temperature gradient increase can be ignored. These results provide insights on the key factors influencing the well temperature distribution, which are important for the choice of the drilling data to estimate SFT and the design of the inverse modeling scheme in future studies to determine an accurate SFT profile for the high-enthalpy geothermal environment.