Small Heating Elements Research Articles

Increase factor for forced convection at small heating elements

Der konvektive Warmeubergang auf kleinen, flach montierten Heizelementen interessiert fur Mikroelektronik und thermische Sensoren. Der maximale Warmeubergang liegt im Anfangsbereich des uberstromten Heizelements mit der Lange l*. Der uber die Heizelementlange l* gemittelte Warmeubergangskoeffizient h* ist groser als der Warmeubergangskoeffizient h am gleichen Ort, wenn z. B. die ganze Platte oder der Zylinderumfang beheizt ist. Zur Charakterisierung dieses Sachverhalts wird der Uberhohungsfaktor K = h*/h eingefuhrt. Gegenubergestellt werden K‑Werte aus Grenzschichtrechnungen, Warmeubergangsmessungen, Kalibrierung von thermischen Wandschubspannungssensoren und numerischen Modellen (im Bereich von l* = 0,01 bis 4 mm). Der Uberhohungsfaktor K ist abhangig von der Uberstromlange l*, dem Warmeubergangskoeffizienten h und der thermischen Isolation.

Modelling and IR measurement of the electronic substrate thermal conductivity

The temperature distribution on a ceramic substrate with a small heating element in the middle has been measured by infrared thermography. By comparing the experimental data with a theoretical analysis, the thermal conductivity could be easily obtained.

Dynamics of single and multiple bubbles and associated heat transfer in nucleate boiling under low gravity conditions.

Experimental studies and numerical simulation of growth and lift-off processes of single bubbles formed on designed nucleation sites have been conducted under low-gravity conditions. Merging of multiple bubbles and lift-off processes during boiling of water in the parabola flights of KC-135 aircraft were also experimentally studied. The heating area of the flat heater surface was discretized and equipped with a number of small heating elements that were separately powered in the temperature-control mode. As such, the wall superheat remained nearly constant during the growth and departure of the bubbles, whereas the local heat flux varied during the boiling process. From numerical calculation it is found that peak of heat flux occurs locally at the contact line of bubble and heater surface. Dry conditions exist inside the bubble base area, which is characterized through a zero heat flux region in the numerical calculation and a lower heat flux period in the experimental results. During the merger of multiple bubbles, dry-out continues. In both the numerical calculations and experimental results, the bubble lift-off is associated with an apparent increase in heat flux. Wall heat flux variation with time and spatial distribution during the growth of a single bubble from numerical simulations are compared with experimental data.

Thermoelectric Voltage at Metallic Point Contacts From Nonequilibrium Effects

This paper presents results for thermoelectric voltage experiments carried out with metallic point contacts. The motivation for this work is the fundamental understanding of the nonequilibrium state between electrons and phonons, and for applying this phenomenon to a point temperature measurement of small heating elements like an LSI (Large-Scale Integration) electronic chip. The experiments were carried out by using gold, silver, copper, and tungsten point contacts under cryogenic conditions, and a heat flux through the point contact was applied via a temperature difference across the contact. In this condition, thermoelectric voltages appeared even when the entire open circuit consisted of only one material. This phenomenon was explained theoretically by considering the nonequilibrium effect between the phonon and electron temperatures, and the geometric effect at the point contact. Good agreement was found between the theory and the experiment.

Thermoelectric Voltage at Metallic Point Contact by Nonequilibrium Effect and Electron Tunneling Eeffct.

The motivation for this work is the desire to obtain a fundamental understanding of the nonequilibrium state between electrons and phonons, and to apply this phenomenon to a point temperature measurement of small heating elements such as an LSI. The experiment was carried out using a gold-gold point contact under cryogenic conditions, and a heat flux through the point contact was applied via the temperature difference across the contact. In this condition, thermoelectric voltages appeared even when the entire closed circuit consisted of the same material. In the theoretical analysis, the nonequilibrium effect and the geometric effect at the point contact were taken into account to explain the appearance of the thermoelectric voltage in a closed circuit made by the same material. Good agreement between the theory and the experiment was obtained.

Heat scan: a simple technique to study gratings in fibers

A simple and powerful technique to characterize gratings in fibers is described. It is based on scanning a small heating element along the fiber so as to disturb the phase relationship of the waves in a controlled way.

Acoustic scattering from fluid turbulence.

There has been considerable interest in the oceanographic community in whether it is possible to use high-frequency acoustic echo sounders to measure ocean microstructure. A laboratory program, the ocean acoustics turbulence study (OATS), has been developed to examine the nature of such scattering and to compare results with that of a model recently proposed by Goodman (1990). The fundamental theory underlying the model is Bragg scattering, which results in the scattered pressure field proportional to the Fourier transform of the temperature field. A small heating element is used to generate a buoyant turbulent plume. Such a relationship could allow the prediction of the three-dimensional turbulent wave-number spectrum provided a sufficiently wide dynamic range in Bragg wave-number space were measurable. By performing scattering experiments over a very wide range of angles (ϑ=5° to ϑ=160°) and frequencies (250 kHz to 1.4 MHz) it is possible to infer such a composite spectrum. Preliminary results indicate a turbulent wave-number spectrum of classical form (−11/3 power law) in agreement with that expected for a fully developed turbulent field.

A Study on Heat Transfer from Small Heating Elements in an Integrated Circuit Chip.

Heat transfer from small heating elements on a substrate has been studied experimentally and analytically. In one of the experiments, the heating elements are diodes in an actual IC chip. In another set of experiments, heaters are small thin films deposited on a glass plate. In both experiments, the substrates are cooled by an impinging jet of air or channel flow. The experimental data indicate that heat conduction from the heating element to the substrate is an important factor in the determination of the maximum temperature of the element, while the surface heat transfer determines the bulk temperature of the chip. The numerical analysis clarifies that a chip surface with an area much larger than the element size is necessary for the heat transfer to the air flow.

Acoustic scattering from a buoyant plume.

A recent model of acoustic scattering from ocean temperature microstructures (Goodman, 1990) has suggested that there are acoustic and environmental parameter regimes in which acoustic scattering from ocean temperature microstructures are significant enough to be observable. The fundamental theory underlying the model is Bragg scattering, which results in the scattered pressure field proportional to the Fourier transform of the temperature field. A new laboratory program, the Ocean Acoustics Turbulence Study (OATS), has been undertaken to examine the validity of this theory as well as the detailed structure of such scattering in a controlled environment. A small heating element is used to generate a buoyant plume. By varying input power, the plume can be made to go from laminar to unstable to turbulent. The approach of the OATS program is to perform a series of progressively more complicated scattering experiments, initially examining scattering from the near axisymmetric laminar plume, then examining scattering from isolated transient eddies that occur immediately after the plume is turned on, and finally, scattering from a turbulent plume. Scattering is performed for a bistatic scattering geometry from near-forward scattering ϑ=5° to ϑ=160°. Initial results from the scattering of the laminar plume will be reported. A comparison will be made of the two-dimensional Fourier transform of the temperature field calculated from the acoustics data and that computed from the temperature measurements. Data will also be presented on preliminary results of scattering from isolated eddies.

The determination of the energy stored in a metal during plastic deformation

A new apparatus for measurement of the stored energy is described and the results obtained are presented. The method involves the heating, in vacuo , of an annealed and a deformed specimen, of equal weight, by small heating elements located inside each specimen. During heating the specimens are kept at the same temperature as each other, and they are surrounded by a metal shield continuously adjusted to this temperature. The stored energy is calculated from measurements of the difference in the powers supplied to the specimens made by means of a differential wattmeter constructed for this purpose. The apparatus has proved to be capable of measuring, with an accuracy of ± 5 %, a difference of 0.1 cal/g in the energy supplied to the specimens in heating them through a temperature range of 240° C in a period of 40 min. Results are given for specimens of electrolytic copper deformed in torsion, and they show that the stored energy increases with deformation in a similar way to the torque equired to produce the deformation. Small amounts of some impurities appear to be responsible for large differences both in the recrystallization temperatures and in the shapes of the curves showing the release of the stored energy as a function of temperature, but the total stored energy is not changed markedly. However, in other cases impurities may produce a significant alteration in the stored energy. It is suggested that differences in composition are the cause of discrepancies between the present results and those of previous workers.