Forced Convection Environments Research Articles

Experimental comparison of opposed and concurrent flame spread in a forced convective microgravity environment

Flame spread experiments in both concurrent and opposed flow have been carried out in a 5.18-s drop tower with a thin cellulose fuel. Flame spread rate and flame length have been measured over a range of 0–30 cm/s forced flow (in both directions), 3.6–14.7 psia, and oxygen mole fractions 0.24–0.85 in nitrogen. Results are presented for each of the three variables independently to elucidate their individual effects, with special emphasis on pressure/oxygen combinations that result in earth-equivalent oxygen partial pressures (normoxic conditions). Correlations using all three variables combined into a single parameter to predict flame spread rate are presented. The correlations are used to demonstrate that opposed flow flames in typical spacecraft ventilation flows (5–20 cm/s) spread faster than concurrent flow flames under otherwise similar conditions (pressure, oxygen concentration) in nearly all spacecraft atmospheres. This indicates that in the event of an actual fire aboard a spacecraft, the fire is likely to grow most quickly in the opposed mode as the upstream flame spreads faster and the downstream flame is inhibited by the vitiated atmosphere produced by the upstream flame. Additionally, an interesting phenomenon was observed at intermediate values of concurrent forced flow velocity where flow/flame interactions produced a recirculation downstream of the flame, which allowed an opposed flow leading edge to form there.

Thermal analysis of a flip chip ceramic ball grid array (CBGA) package

The function of an electronic cooling package is to dissipate heat to ensure proper operation and reliability. The flip chip ball grid array package is probably the most suitable package for high-level thermal performance applications. A high thermal performance flip chip ceramic ball grid array (FC-CBGA) package with an aluminum silicon carbide (AlSiC) lid and one without lid were evaluated using the computational fluid dynamics (CFD) technique. This paper compares the thermal performance of a 35 × 35 mm FC-CBGA package with three different die sizes of 5 × 5 mm, 15 × 15 mm and 20 × 20 mm. The performance of a lid fitted with different heat sinks was investigated in standard JEDEC defined natural and in forced convection environments. Thermal measurements were performed using a functional application specific integrated circuit (ASIC) chip, in compliance with the JEDEC standards. Excellent agreement was found between the numerical results and the measured data. Improved thermal performance was observed with a lidded package as compared to the unlidded one. However, no significant improvement was observed between lidded and unlidded packages when fitted with a heat sink subjected to forced convection. This paper also discusses the package thermal budget estimate with and without heat sinks. Printed circuit board and package top surface temperature patterns were measured using an infrared thermal camera. The usefulness of the thermal characterization parameter is demonstrated in system level applications. Parametric studies were carried out to understand the effect of die size, radiation effect, gird size variations and airflow rate on die junction temperature and package thermal resistance. This study also incorporates the effects of substrate, lid, die and PCB temperatures for different die sizes in natural and forced convection environments.

Effect of gravity on methanol diffusion flames burning within a forced convective environment

Characteristics of methanol diffusion flames burning under three different configurations with respect to the directions of forced and natural convective flow fields are presented for zero and normal gravity conditions. Combustion of a spherical methanol particle and of a methanol film within a mixed convective environment, have been numerically simulated. The effects of gravity on aiding, opposing and perpendicular flow are presented. The gravitational field considerably changes the flow field, flame shapes and fuel burning rates, especially for the opposing flow case.

A New Multicomponent Diffusion Formulation for the Finite-Volume Method: Application to Convective Droplet Combustion

A new multicomponent formulation, appropriate for use with the finite-volume method, has been developed to describe mass diffusion velocities accurately. The new formulation is applied in a quasi-steady numerical model for n-heptane fuel droplet combustion in a forced-convection environment. Results obtained using the complete formulation are compared to the results obtained under various assumptions. Using a single binary diffusion coefficient produces results for extinction velocity, maximum temperature, flame dimensions, evaporation constant, and drag coefficient that are significantly different from the results obtained using the complete formulation. Neglecting thermal diffusion (Soret effect) causes only minor changes (less than 2%).

Transport and dynamics of liquid oxygen droplets in supercritical hydrogen streams

The transport and dynamics of an isolated liquid oxygen (LOX) droplet in a supercritical hydrogen stream has been numerically studied based on the complete conservation equations in axisymmetric coordinates. The approach employs a unified treatment of general fluid thermodynamics and transport, and accommodates rapid variations of fluid properties in the transcritical regime. Surface tension of the droplet is ignored in consistency with the supercritical thermodynamic condition. The analysis allows for a systematic investigation into droplet behaviour over broad ranges of fluid thermodynamic states and ambient flow conditions. Detailed flow structures and transport phenomena are examined to reveal various key mechanisms underlying droplet vaporization in a supercritical forced convective environment. In addition, correlations of droplet lifetime and drag coefficient are established in terms of fluid properties, pressure, and free-stream Reynolds number.

Numerical research on a vaporizing fuel droplet in a forced convective environment

A comprehensive numerical model is developed to study a vaporizing n-heptane droplet in a forced convective environment under different temperature conditions, which includes high-pressure effects, liquid phase internal circulation, variable thermophysical properties, solubility of inert species into the liquid phase, and gas and liquid phase transients. Numerical predications of time histories of the dimensionless diameter square of a vaporizing fuel droplet within a zero gravity environment are in very good agreement with the micro-gravity experimental data. The numerical results show that at higher ambient pressure (such as 4 MPa) the droplet swells initially due to the heat-up of the cold droplet and its subsequent regression rate is far from following the d 2-law during the early stages of droplet evaporation. However, at the ambient pressure of 0.1 MPa, the droplet swells are not obvious. The droplet presents an almost d 2-law behavior in later stages of droplet evaporation for all considered pressures (up to 4 MPa). The numerical results also show that the droplet lifetime decreases with increasing ambient temperature. For example, the droplet lifetime at the ambient temperature 1200 K can be only 50–60% of the droplet lifetime at the ambient temperature 600 K. The final penetration distance of the vaporizing droplet decreases almost linearly with the ambient temperatures considered.

A study of compact thermal model topologies in CFD for a flip chip plastic ball grid array package

A previously validated detailed model of a 119-pin flip-chip plastic ball grid array (FC-PBGA) package was created and validated against experimental data for natural convection and forced convection environments. Next, two compact models mere derived, a two-resistor model (created using the JEDEC-standard based computational approach), and a multiresistor model (created using the DELPHI optimization approach that was boundary condition independent within engineering accuracy). The compact models were placed in natural convection and forced convection (velocities of 1 and 2 m/s) environments with and without a heatsink. Based on the agreement obtained between the detailed model and compact model simulations, the accuracy and validity of the two compact models was assessed. Of the two compact thermal models considered, the Delphi multiresistor model provided the same predictive estimates (within 5%) as simulations involving a detailed thermal model of the package in natural and forced convection environments both with and without attached heatsinks. Some thermal modeling issues were addressed with respect to implementation of compact thermal models with attached heatsinks.

00/01952 The cost of uncertainty in capacity expansion problems

Lithium-ion batteries (LIBs) are replacing the Nickel–Hydrogen batteries used on the International Space Station (ISS). Knowing that LIB efficiency and survivability are greatly influenced by temperature, this study focuses on the thermo-electrochemical analysis of LIBs in space orbit. Current finite element modeling software allows for advanced simulation of the thermo-electrochemical processes; however the heat transfer simulation capabilities of said software suites do not allow for the extreme complexities of orbital-space environments like those experienced by the ISS. In this study, we have coupled the existing thermo-electrochemical models representing heat generation in LIBs during discharge cycles with specialized orbital-thermal software, Thermal Desktop (TD). Our model's parameters were obtained from a previous thermo-electrochemical model of a 185 Amp-Hour (Ah) LIB with 1–3 C (C) discharge cycles for both forced and natural convection environments at 300 K. Our TD model successfully simulates the temperature vs. depth-of-discharge (DOD) profiles and temperature ranges for all discharge and convection variations with minimal deviation through the programming of FORTRAN logic representing each variable as a function of relationship to DOD. Multiple parametrics were considered in a second and third set of cases whose results display vital data in advancing our understanding of accurate thermal modeling of LIBs.

Long-term studies on tetragonal lysozyme crystals grown in quiescent and forced convection environments

The growth of tetragonal hen lysozyme crystals in the size range 150-300 μm was studied using digital microscopy; the size and orientation of the growing crystals were estimated from the geometry of the ideal tetragonal lysozyme crystal. At a confidence level above 99%, statistical analyses indicate the (110) face growth rates of crystals grown in quiescent conditions are not inhibited by weak buoyancy-driven natural convection. Yet similar analyses of crystals subjected to a weak forced flow of the same magnitude indicate a statistically significant decrease in growth rate with time. This apparent paradox probably results from mass transport limitations within the crystal growth cell. Mathematical models of fluid mixing inside the growth chamber suggest that crystal growth is limited by the rate at which protein molecules are transported to crystals growing on the walls of the chamber. Our experiments also reveal a large variation in the growth rates of crystals within a nominally homogeneous population. The local environment of the crystal may account for some of the variation, but the mechanisms are not understood.

Thermal enhancements for a thin film chip carrier

The development of a thermal enhancement technique targeted for single-chip, thin-film carriers is described. The test package consisted of a 7.5-mm by 7.5-mm chip bonded to a high lead count organic tape substrate using tape automated bonding (TAB). Because of high chip power requirements, a thermal enhancement consisting of a heat sink bonded to the chip with a thermally conductive epoxy was developed. The objectives were to develop the appropriate heat sink, thermal adhesive, and assembly process for the heat management system. The thermal performance of the package was evaluated in both natural and forced convection environments. Reliability measurements were taken to ensure the chip to heat sink interface and the overall package withstood the required stresses without significant performance deterioration. These stresses included standard accelerated thermal cycling (ATC), ship shock, mechanical torque and vibration, and chemical exposure. >

Effects of Radiation on Enhanced Electronic Cooling

The radiation component of heat transfer has been shown to provide a significant portion of the power dissipation capability for certain applications of enhanced surfaces in electronic cooling. Experimental work using two surface coatings has shown that the radiation interchange can be increased in natural-convection cooled environments for both horizontally and vertically oriented heat sinks. It was also shown that, to a lesser degree, the radiation component of heat transfer is effective in a forced convection environment. Based on experimental data from an oxidized aluminium heat sink, the radiation component was evaluated using the geometry constraints of a heat sink enclosed within a system environment consisting of gray reradiation walls. The equations used were then modified to simulate a change in heat-sink surface finish. The results of this investigation indicate that, for an aluminium heat sink, in the test configuration evaluated, the radiation component in a natural convection environment dissipated approximately 5% of the total heat flow.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Liquid Crystal Thermography and Its Application to the Study of Convective Heat Transfer

A thermographic technique is presented that allows visual determination of both qualitative and quantitative heat transfer and fluid flow information to be obtained on heated objects placed in forced convection environments. The technique employs cholesteric liquid crystals as the temperature sensing agent. The liquid crystals indicate temperature by exhibiting brilliant changes in color over discrete, reproducible temperature ranges. The technique has been used to quickly and easily obtain information on the variation of the Nusselt number on a right circular cylinder placed in a crossflow of air. In addition to yielding precise quantitative heat transfer information, the liquid crystal thermographic technique afforded the opportunity to visually observe the effects of flow separation, the separation bubble region, the turbulent boundary layer, and the turbulent wake on the surface temperature of the heated cylinder. The experimental results obtained using the liquid crystal thermographic technique are in close agreement with results obtained by other investigators who have used standard measuring techniques.