Dryout Behavior Research Articles

Thermomechanical Degradation of Thermal Interface Materials: Accelerated Test Development and Reliability Analysis

Abstract Due to the inherently low adhesive strength and structural integrity of polymer thermal interface materials (TIMs), they present a likely point of failure when succumbed to thermomechanical stresses in electronics packaging. Herein, we present a methodology to quantify TIM degradation through an accelerated and repeatable mechanical cycling technique. The testing apparatus incorporated a steady-state thermal conductivity measurement system, consistent with ASTM 5470-06, with added displacement actuation and force sensing to provide controlled cyclic loading between −20 N and 20 N. Additionally, a novel optical technique was utilized to observe void formation, pump-out, and dry-out behavior during cycling, in order to correlate the thermal performance with physical behaviors of different TIMs under cyclic stress. Of the two different pastes analyzed, cyclic testing was found to degrade the thermal performance of the less viscous TIM by increasing its interfacial resistance. Optical qualitative measurements revealed the breakdown of the TIM structure at the interface, which indicated the formation of voids due to TIM degradation. Applying this testing method for future TIM development could help in optimizing TIM structure for particular package applications.

The effect of permeability enhancement on dry-out behavior of CA- and microsilica gel-bonded castables as determined by NMR

This investigation deals with refractory monolithic materials that are broadly used in thermal treatment facilities as they are necessary e.g. for iron and steel, glass and cement production, thereby withstanding temperatures between 600 and 2000 °C. In the special case of hydraulic bond refractory castables, the components must be mixed with water for two reasons: firstly, to obtain a mouldable suspension; and secondly, to achieve a green strength via the hydraulic reaction of calcium aluminate cement that is high enough to enable a secure refractoriness of the concrete formwork. Prior to their first use in production, castables must have their pore water and hydraulic bond water carefully removed in order to avoid explosive spalling that can cause severe damages inside the furnaces.In this study, we investigate the one-dimensional drying behavior of two specific refractory castable compositions, a microsilica-containing low- and a no-cement castable (LCC/NCC) during first heat-up in the temperature regime between 20 and 300 °C. First results were already presented in a prior publication that demonstrate a specialized high-temperature Nuclear Magnetic Resonance (NMR) setup capable of continuously measuring moisture and temperature profiles on 74 mm-long cylindrical samples, without touching or moving the sample [1].In this paper we explore how the use of permeability-enhancing agents (fibers and MIPORE 20) beneficially affects the drying behavior and consequently allows higher heating rates. We also demonstrate that the NMR technique as applied here is sensitive enough to resolve differences in the dry-out behavior if said additives are used in the castable formulations.Our results demonstrate that incorporation of fiber and MIPORE 20 significantly alters the dry-out behavior. In particular, it can be resolved that as the fibers begin to melt, there is a noticeable increase in permeability that results in faster drying, as well as a decrease of the drying front temperature and therefore the generated maximum pressure.

A Study on Liquid Droplet and Dryout Behavior on Simulated Fuel Rod Heater under High Pressure Conditions

A Study on Liquid Droplet and Dryout Behavior on Simulated Fuel Rod Heater under High Pressure Conditions

高圧条件における加熱棒に生ずる液膜とドライアウトの挙動に関する研究

高圧条件における加熱棒に生ずる液膜とドライアウトの挙動に関する研究

The COOLOCE experiments investigating the dryout power in debris beds of heap-like and cylindrical geometries

The COOLOCE test facility has been used for experimental investigations of the coolability of porous core debris beds with different geometries. The main objective of the experiments was to compare the dryout behavior of conical (heap-like) and top-flooded cylindrical (evenly distributed) debris bed configurations in order to investigate the effect of geometry on the coolability of the debris bed. The experimental debris beds simulate the possible outcomes of melt discharge from the reactor pressure vessel and the formation of a core debris bed in a deep water pool during a severe accident. The results suggest that if the two debris bed configurations have equal height, the coolability of the conical bed is improved compared to the cylindrical bed due to the multi-dimensional infiltration of water through the surface of the cone. However, in case the conical and cylindrical debris beds have equal diameter and volume, the dryout power density of the conical configuration is lower than that of the cylindrical configuration by approximately 50%. This is due to the greater height of the conical configuration which leads to increased heat flux in the upper parts of the conical debris bed. According to the present results, the effect of the increased debris bed height is greater than the effect of multi-dimensional flooding. Simulations show the differences between the two-phase flow behavior of the two geometries, and the resulting difference in dryout development.

Measurement of surface dryout near heating surface at high heat fluxes in subcooled pool boiling

The authors have conducted measurements of liquid–vapor behavior in the vicinity of a heating surface for saturated and subcooled pool boiling on an upward-facing copper surface by using a conductance probe method. A previous paper [A. Ono, H. Sakashita, Liquid–vapor structure near heating surface at high heat flux in subcooled pool boiling, Int. J. Heat Mass Transfer 50 (2007) 3481–3489] reported that thicknesses of a liquid rich layer (a so-called macrolayer) forming in subcooled boiling are comparable to or thicker than those formed near the critical heat flux (CHF) in saturated boiling. This paper examines the dryout behavior of the heating surface by utilizing the feature that a thin conductance probe placed very close to the heating surface can detect the formation and dryout of the macrolayer. It was found that the dryout of the macrolayer formed beneath a vapor mass occurs in the latter half of the hovering period of the vapor mass. Two-dimensional measurements conducted at 121 grid points in a 1-mm × 1-mm area at the center of the heating surface showed that the dryout commences at specific areas and spreads over the heating surface as the heat flux approaches the CHF. Furthermore, transient measurements of wall void fractions from nucleate boiling to transition boiling were conducted under the transient heating mode, showing that the wall void fraction has small values (<10%) in the nucleate boiling region, and then steeply increases in the transition boiling region. These findings strongly suggest that the macrolayer dryout model is the most appropriate model of the CHF for saturated and subcooled pool boiling of water on upward facing copper surfaces.

Measurements of Surface Dryout at High Heat Flux in Subcooled Pool Boiling

The authors have conducted measurements of liquid-vapor behavior in the vicinity of a heating surface for saturated and subcooled pool boiling on an upward-facing copper surface by using a conductance probe method. The previous paper (Transactions of JSME, Series B, pp. 2951-2958, vol. 70, no. 699 (2004)) reported that thicknesses of a liquid rich layer (a so-called macrolayer) forming in subcooled boiling are comparable to or thicker than those formed near the CHF in suturated boiling. This paper examines the dryout behavior of the heating surface by utilizing the feature that a thin conductance probe very close to the heating surface can detect the formation and dryout of the macrolayer. It was found that the dryout of the macrolayer formed beneath a vapor mass occurs in the latter half of hovering period of the vapor mass. Two-dimensional measurements conducted at 121 grid points in a 1 mm×1 mm area at the center of the heating surface showed that the dryout commences at specific areas and spreads over the heating surface as the heat flux approaches the CHF. Furthermore, transient measurements of wall void fractions from nucleate boiling to transition boiling were conducted under the transient heating mode, showing that the wall void fraction takes small values (< 10%) in nucleate boiling region, and then steeply increases in transition boiling region. All these findings strongly suggest that the macrolayer dryout model is the most appropriate model of the CHF for saturated and subcooled pool boiling of water on the upward copper surface.

Boiling dryout on flat surfaces covered with bead-packed structures

Visual observations were conducted to focus on the dynamic interfacial behavior associated with dryout and rewetting during boiling on surfaces covered with porous structures made of staggered glass beads. Associated interfacial behavior was significantly affected by the bead-packed structure. Intensive wetting of the liquid replenishment protected heated surface from full dryout. Even at extremely high heat fluxes, intermittent rewetting of heated surface can also be established due to rather unsteady behavior of dry areas. Based on these unique characteristics, an introductory model was proposed to perform a theoretical analysis and explore the dryout process inside the pore structure. The theoretical results were compared with experimental data, and the present model provides a good explanation of the fundamental mechanisms and predicts the important influences of the bead-packed structure on dryout behavior.

Fibras metálicas e sua influência no comportamento mecânico de concretos refratários durante a secagem

A adição de fibras poliméricas pode auxiliar no aumento da resistência à explosão de concretos refratários por meio de dois mecanismos principais: aumento de permeabilidade ou reforço mecânico. Já as firas metálicas apresentam um potencial de reforço para a secagem, por manterem valores altos de resistência mecânica e de módulo de elasticidade na faixa de temperaturas crítica, entre 150 e 200 ºC. Neste trabalho, estudou-se o comportamento mecânico e de secagem de concretos contendo fibras metálicas curtas e longas, comparando-os com fibras poliaramídicas (PAr). As fibras metálicas aumentaram consideravelmente a energia de fratura, porém foram pouco eficientes na contenção do dano de secagem. O volume unitário dessas fibras, três ordens de grandeza superior ao da PAr, resulta em uma menor quantidade de fibras por unidade de volume. Por isso, solicitações mecânicas na matriz são menos beneficiadas pelo reforço gerado por essas fibras. Os resultados mostram a importância de considerar a escala das solicitações para definir a geometria dos elementos de reforço, a fim de otimizar seu desempenho.

A comparison of numerical models for evaporative two-phase flow in a self-heated porous medium

Two different numerical models using the finite difference method (FDM) for one-component time-dependent two-phase flows in a porous medium are investigated: the iterative four-variable model (I4VM) and the direct three-variable model (D3VM). The former includes the pressure gradient and uses the iterative method to solve a system of flow equations, whereas for the latter, the formulation without the pressure gradient is simultaneously solved using the algorithm for tri-tridiagonal equations of three dependent variables. The steady-state solution as well as the unsteady results obtained by two models are compared only for the low heat generation rate below the dryout limit. For the high heat generation rate the effects of two numerical models on the time-dependent flow and dryout behavior up to incipient dryout are discussed in terms of liquid volumetric fraction and liquid superficial velocity distributions. It was found that the I4VM is numerically more stable for the case of strongly nonlinear physical models (e.g. the Ergun constants model of Fand, R. M., Kim, B. Y. K., Lam, A. C. and Phan, R. T., Resistance to the flow of fluids through simple and complex porous media whose matrices are composed of randomly packed spheres. J. Fluids Engng, 1987, 109, 268–274) and enables us to analyze those, whereas the D3VM is advantageous for fast analysis of the weakly nonlinear model (e.g. the Ergun constants model of Macdonald, I. F., El-Sayed, M. S., Mow, K. and Dullien, F. A. L., flow through porous media—the Ergun equation revisited. Ind. Engng Chem. Fundam., 1979, 18, 199–208). Finally, a comparative evaluation of both numerical models is presented.

Effect of a Heat Flux Spike on the Downstream Dryout Behavior

The effect of a short hot patch on the downstream dryout behavior was investigated in heated tubes cooled by Freon-12 at conditions equivalent (on the basis of liquid to vapor density ratio) to 69 bar in steam-water. The hot patch, located well upstream from the heated end, generated a heat flux spike which resulted in a local dry patch. Following an increase in downstream surface heat flux the dry patch would often spread in the downstream direction even though the heat flux was only a small fraction of the measured critical heat flux with uniform heating. A force balance on the dry patch boundary showed that, for certain conditions, spreading of the dry patch in the downstream direction is inevitable. This spreading however can be limited by interfacial instability, droplet-wall interaction and axial conduction.