Small Heat Source Research Articles

Experimental Investigation into Penetration of a Weak Fire Plume into a Hot Upper Layer

A hot upper layer pre-established under the ceiling of a room in a house due to a solar load in the summer or room heating in the winter may cause problems for fire detection by ceiling-mounted smoke detectors. A weak plume from a smoldering fire may be pushed back down by the hot layer, and be unable to reach a ceiling-mounted detector. In this study, based on the physics of plume penetration into a hot upper layer, a model for the penetration height was developed as a function of the distance of an upper layer from a fire source and the temperatures of the upper layer and the plume axis. A series of experiments were conducted to validate the model and determine the value of the coefficient involved in the model. Also measurements were made of the heat of combustion of smoldering fire of Test Fire 3 (TF3) prescribed in ISO/TC21/SC3 N301(Draft) using a cone calorimeter to see if a well-established formula for plume temperature is still usable for predicting penetration of a weak plume. Based on these results, it was confirmed that plume penetration height can be estimated even if the plume originates from a small heat source such as smoldering fire.

Optimal distribution of discrete heat sources on a plate with laminar forced convection

In this paper constructal theory is applied to the fundamental problem of how to arrange discrete heat sources on a wall cooled by forced convection. The global objective is to maximize the conductance between the discrete heated wall and the fluid. This is equivalent to minimizing the temperature of the hot spot on the wall, when the heat generation rate is specified. The mechanism by which the global objective is achieved is the generation of flow configuration, in this case the distribution of discrete heat sources. Two different analytical approaches are used: (i) large number of small heat sources, and (ii) small number of heat sources with finite length, which are mounted on a flat wall. Both analyses show that the heat sources should be placed nonuniformly on the wall, with the smallest distance between them near the tip of the boundary layer. If the Reynolds number is high enough, then the heat sources should be mounted flush against each near the entrance to the channel. The analytical results are validated by a numerical study of discrete heat sources that are distributed nonuniformly inside a channel formed by parallel plates.

小型筐体内におけるモデルパッケージからの複合伝熱(J01-5 筐体放熱設計,J01 エレクトロニクス実装における熱制御および信頼性評価)

This paper reports on measurements of conjugate heat transfer from a model package placed on a PCB located in a rectangular duct cooled by air. The model package used by the present experiment is isothermal model, which is composed of two copper plates sandwiching a foil heater and a heat flux sensor. The effects of the thermal conductivity of PCB and the other small heat sources around the model package on the conjugate heat transfer characteristics were studied experimentally. Using the concept of effective heat transfer area, a unique non-dimensional correlation is proposed, which can predict the maximum temperature for various thermal conductivity of PCB. The correlation is also applicable for two small heat sources around the package.

Simulation of Unsteady Small Heat Source Effects in Sub-Micron Heat Conduction

In compact transistors, large electric fields near the drain side create hot spots whose dimensions are smaller than the phonon mean free path in the medium. In this paper, we present a study of unsteady hot spot behavior. The unsteady gray phonon Boltzmann transport equation (BTE) is solved in the relaxation time approximation using a finite volume method. Electron-phonon interaction is represented as a heat source term in the phonon BTE. The evolution of the temperature profile is governed by the interaction of four competing time scales: the phonon residence time in the hot spot and in the domain, the duration of the energy source, and the phonon relaxation time. The influence of these time scales on the temperature is investigated. Both boundary scattering and heat source localization effects are observed to have considerable impact on the thermal predictions. Comparison of BTE solutions with conventional Fourier diffusion analysis reveals significant discrepancies.

Optimal distribution of discrete heat sources on a wall with natural convection

This paper is an application of the constructal method to the discovery of the optimal distribution of discrete heat sources cooled by laminar natural convection. The global objective is to maximize the global conductance between the wall and the fluid, or to minimize the hot-spot temperatures when the total heat generation rate and global system dimensions are specified. Two scenarios are investigated: (i) a large number of small heat sources mounted on a vertical wall facing a fluid reservoir, and (ii) a small number of finite-size heat sources mounted on the inside of the side wall of a two-dimensional enclosure. It is shown that the optimal distribution is not uniform (the sources are not equidistant), and that as the Rayleigh number increases the heat sources placed near the tip of a boundary layer should have zero spacings. In both (i) and (ii), the optimal configuration of the wall with discrete sources is generated by the pursuit of maximal global performance subject to global constraints.

Ultrasonic imaging in air using fan-beam tomography and electrostatic transducers

Air-coupled ultrasonic capacitance transducers operating at frequencies of up to 1 MHz have been employed in a fan-beam configuration for the cross-sectional tomographic imaging of temperature fields and flow fields in air, and the location of solid objects. Separate transmitter and receiver transducers were manufactured using thin polymer dielectric membranes and polished metal backplates, and used to acquire through-transmission data. The fan-beam reconstruction was developed in LabVIEW ® using a re-bin routine combined with a filtered backprojection algorithm and a difference technique to generate the cross-sectional images. The system was first used to reconstruct images showing the locations of solid objects positioned within the scanned region through interpretation of the arrival time of the transmitted ultrasound. The technique was then extended to image the temperature fields produced in air above a small heat source and the flow field produced by a nozzle connected to a regulated compressed air source. Reconstructed temperatures were within 4% of the measured background air temperature and 9% of the air temperature measured above the heat source. Reconstructed images of the flow field above a small nozzle were also presented, showing that the horizontal component of the flow velocity could be resolved using this method.

Axial steady free surface jet impinging over a flat disk with discrete heat sources

Abstract A free jet of high Prandtl number fluid impinging perpendicularly on a solid substrate of finite thickness containing small discrete heat sources on the opposite surface has been analyzed. Both solid and fluid regions have been modeled and solved as a conjugate problem. Equations for the conservation of mass, momentum, and energy were solved taking into account the transport processes at the solid–liquid and liquid–gas interfaces. The shape and location of the free surface (liquid–gas interface) was determined iteratively as a part of the solution process by satisfying the kinematic condition as well as the balance of normal and shear forces at this interface. The number of elements in the fluid and solid regions were determined from a systematic grid-independence study. A non-uniform grid distribution was used to adequately capture large variations near the solid–fluid interface. Computed results included velocity, temperature, and pressure distributions in the fluid, and the local and average heat transfer coefficients at the solid–fluid interface. Computations were carried out to investigate the influence of different operating parameters such as jet velocity, heat flux, plate thickness, and plate material. Numerical results were validated with available experimental data. It was found that the local heat transfer coefficient is maximum at the center of the disk and decreases gradually with radius as the flow moves downstream. The thickness of the disk as well as the location of discrete sources showed strong influence on the maximum temperature and the average heat transfer coefficient.

Effects of nozzle-inlet chamfering on pressure drop and heat transfer in confined air jet impingement

The effect of changing the nozzle geometry on the pressure drop and local heat transfer distribution in confined air jet impingement on a small heat source was experimentally investigated. Heat transfer and pressure drop measurements obtained with chamfered nozzles were compared to those obtained with a square-edged (non-chamfered) nozzle of the same diameter for different turbulent Reynolds numbers in the range of 5000–20,000, nozzle-to-target spacings, chamfer angles and chamfer lengths. The ratio of average heat transfer coefficient to pressure drop was enhanced by as much as 30.8% as a result of chamfering the nozzle, with narrow chamfering providing the better performance.

Temperature distribution as function of time around a small spherical heat source of local magnetic hyperthermia

A spherical region containing magnetic particles embedded in extended muscle tissue is taken as model of small breast carcinomas. Using analytically derived equations the spatial temperature distribution is calculated as function of the time for exposing to an alternating magnetic field. In vitro measurements with muscle tissue yielded such an agreement with the calculations that treatment of small tumors in slightly vascularized tissues on the base of mathematical predictions seems now to be more promising than in the past.

Experimental analysis of a confined transitional plume with respect to subgrid-scale modelling

Free convection flow of water induced by a small prismatic heat source in a confined space is studied in the present paper. The forcing of the flow is in the range of Rayleigh numbers where a spatial transition from laminar to turbulent flow can be observed.Velocity measurements were performed by means of Particle Tracking Velocimetry (PTV ) . From the velocity data a resolved and a subgrid field is obtained by means of spatial filtering. These data are analysed with respect to modelling consequences in Large-Eddy Simulation (LES) , constituting an a priori test.Astatistically steady flow has been obtained with a converged temporal mean and standard deviation as function of space. A low correlation of time mean model stresses with exact stresses is found. In the transitional region the inter-scale kinetic energy transfer, taken over a wavenumber corresponding to the laminar plume width, is found to possess a relative large standard deviation. In this region backscatter and forward scatter of kinetic energy, with respect to the mentioned wavenumber, are of equal importance. Application of a dynamic model to the filtered data yields a qualitative good representation of the exact inter-scale kinetic energy transfer.

Calculation of thermoelastic stresses in microelectronics

An overview is given of several situations in which thermoelastic stresses can occur in electronics. To gain insight into these situations, simple models are presented allowing analytical or semi-analytical solutions. A first model describes the thermoelastic behaviour of a substrate. A second model describes the attachment between layers. A third model describes thermoelastic effects around small heat sources.

Nozzle-geometry effects in liquid jet impingement heat transfer

Experiments were conducted to determine the effect of nozzle geometry (diameter and aspect ratio) on the local heat transfer coefficients from a small heat source to a normally impinging, axisymmetric, submerged and confined liquid jet of FC-77. A single jet with nozzle diameters in the range of 0.79–6.35 mm and up to seven different nozzle aspect ratios in the range of 0.25–12 were tested, at turbulent jet Reynolds numbers from 4000 to 23 000 and nozzle to heat source spacings of 1–14 jet diameters. The results indicate that at very small nozzle aspect ratios (Ild ⩽ 1), the heat transfer coefficients are the highest. As the aspect ratio is increased to values of 1–4, the heat transfer coefficients drop sharply, but with further increases in lid of up to 8–12, the heat transfer coefficients gradually increase. This effect is less pronounced as the nozzle to target spacing is increased. Possible explanations for these trends are provided in terms of flow separation at the nozzle entrance and its effect on the exit velocity profiles. The nozzle diameter also has a definite effect on the heat transfer coefficients.

Experimental and Theoretical Studies of Mist Jet Impingement Cooling

Experimental data and analytical predictions for air/liquid mist jet cooling of small heat sources are presented. The mist jet was created using a coaxial jet atomizer, with a liquid jet of diameter 190 μm located on the axis of an annular air jet of diameter 2 mm. The impingement surface was a square of side 6.35 mm. Experimental data were obtained with mists of both methanol and water. Surface-averaged heat fluxes as high as 60 W/cm2 could be dissipated with the methanol/air mist while maintaining the target surface below 70°C. With the water/air mist, a heat flux of 60 W/cm2 could be dissipated with the target surface at 80°C. Major trends in the data and model predictions have been explained in terms of the underlying hydrodynamic and heat transfer phenomena.

Confined and Submerged Liquid Jet Impingement Heat Transfer

The local heat transfer from a small heat source to a normally impinging, axisymmetric, and submerged liquid jet, in confined and unconfined configurations, was experimentally investigated. A single jet of FC-77 issuing from a round nozzle impinged onto a square foil heater, which dissipated a constant heat flux. The nozzle and the heat source were both mounted in large round plates to ensure axisymmetric radial outflow of the spent fluid. The local surface temperature of the heat source was measured at different radial locations (r/d) from the center of the jet in fine increments. Results for the local heat transfer coefficient distribution at the heat source are presented as functions of nozzle diameter (0.79 ≤ d ≤ 6.35 mm), Reynolds number (4000 to 23,000), and nozzle-to-heat source spacing (1 ≤ Z/d ≤ 14). Secondary peaks in the local heat transfer observed at r/d ≈ 2 were more pronounced at the smaller (confined) spacings and larger nozzle diameters for a given Reynolds number, and shifted radially outward from the stagnation point as the spacing increased. The secondary-peak magnitude increased with Reynolds number, and was higher than the stagnation value in some instances. Correlations are proposed for the stagnation and average Nusselt numbers as functions of these parameters.

Correlating Equations for Impingement Cooling of Small Heat Sources With Multiple Circular Liquid Jets

Experimental data have been obtained for liquid jet impingement cooling of small square heat sources resembling electronic integrated circuit chips. Both free-surface and submerged jet configurations have been studied for a range of velocities, nozzle diameters, and nozzle-to-heater separation distances, with water and a fluorocarbon liquid (3M FC-77) as coolants. Major trends in the data have been explained in terms of the underlying hydrodynamic and thermal phenomena. The data, obtained over parameter ranges applicable to the cooling of microelectronic chips, have been compared with the predictions of previously developed correlations for jet impingement heat transfer and substantial discrepancies between the data and the predictions have been noted. Based on the present data, two new correlating equations, one for free-surface and the other for submerged jet impingement, have been developed and presented.

Interferences of Peltier thermal waves produced in ohmic contacts upon integrated circuits

We have experimentally detected a Peltier effect produced by the current flow in ohmic contacts upon integrated circuits. These contacts are small size heat sources which are very useful for investigating integrated circuits. We have characterised their thermal properties with a high resolution interferometric laser probe by measuring absolute thermal expansions. Subpicometric resolution has allowed original imaging of semiconductor components : we have mapped the thermal waves created at ohmic contacts under normal operating conditions.

Correlating Equations for Impingement Cooling of Small Heat Sources With Single Circular Liquid Jets

Experimental data have been obtained for liquid jet impingement cooling of small square heat sources resembling electronic integrated circuit chips. Both free-surface and submerged jet configurations have been studied for a range of velocities, nozzle diameters, and nozzle-to-heater separation distances, with water and a fluorocarbon liquid (3M FC-77) as coolants. Major trends in the data have been explained in terms of the underlying hydrodynamic and thermal phenomena. The data, obtained over parameter ranges applicable to the cooling of microelectronic chips, have been compared with the predictions of previously developed correlations for jet impingement heat transfer and substantial discrepancies between the data and the predictions have been noted. Based on the present data, two new correlating equations, one for free-surface and the other for submerged jet impingement, have been developed and presented.

Cold fusion: What do we know? what do we think?

Considering the experiments which have been listed as convincing in Fig. 1, two groups have provided evidence for the existence of an unidentified small heat source. The low level of heat that is produced in the Texas A&M and Stanford work, can most plausibly be explained by a chemical explanation. The absence of helium and neutrons is consistent with this explanation, however, the high tritium level observed by Texas A&M is not.

Experimental Investigation of a Flexible Bellows Heat Pipe for Cooling Discrete Heat Sources

A computer model was developed to aid in the design of a flexible bellows heat pipe for cooling small discrete heat sources or arrays of small heat sources. This model was used to evaluate the operational characteristics and performance limitations of these heat pipes and to formulate and optimize a series of conceptual designs. Three flexible bellows heat pipes approximately 40 mm in length and 6 mm in diameter were constructed and tested using three different wick configurations. The test pipes were found to be boiling limited over most of the operating temperature range tested. Heat fluxes in excess of 200 W/cm2 were obtained and thermal resistance values of less than 0.7 °C/W were measured. Although the computer model slightly underestimated the experimentally determined transport limit for one of the wicking configurations, the remaining transport predictions were consistently within 8 percent of the experimental values.

Thermal structure of the ionosphere of Mars: Simulations with one- and two-dimensional models

One- and two-dimensional models of the thermal structure of the upper ionosphere of Mars have been constructed. The effect of heat flux saturation has been included. It is found that the dayside ion temperature observed by the Viking 1 retarding potential analyzers can be satisfactorily simulated by including small upper boundary and volume heat sources. The transport of heat by plasma flowing from the day- to the nightside makes no contribution to the ion and electron temperatures calculated for the nightside.