Jan Berghmans

A generic methodology for deriving compact dynamic thermal models, applied to the PSGA package

A novel methodology for synthesizing compact, boundary condition independent, dynamic thermal models is represented. The resulting compact resistor/capacitor network accurately predicts the dynamic junction temperature response under any arbitrary set of external cooling conditions. The network is derived in two successive steps. First, a boundary condition independent resistor network is synthesized using steady-state finite element data for a large set of practical boundary conditions. Next, the general resistor network is expanded with discrete thermal capacitors featuring the thermal mass of the package. The value of the capacitors and their exact location within the resistor network is determined using frequency response finite element data for a limited set of boundary conditions. This paper focuses primarily on the second step, i.e., synthesis of the dynamic (capacitive) network elements. The synthesis method is successfully demonstrated for two types of polymer stud grid array (PSGA) packages, the standard PSGA and the thermally enhanced PSGA. It is shown that the thermal mass of a PSGA package can be lumped into five discrete thermal capacitances. For both the standard and the thermally enhanced PSGA, the generic compact dynamic network models can predict time dependent junction temperature profiles within an accuracy of 5%.

Experimental study of the pressure and temperature dependence on the upper flammability limit of methane/air mixtures

The flammability limits of methane/air mixtures are investigated experimentally at pressures up to 5500 kPa and temperatures up to 200°C. Two different criteria based on the maximum explosion pressure are used to define the flammability limit, the tangent criterion and the min-max criterion. It is shown that the min-max criterion should be used to determine the upper flammability limit (UFL), because the tangent criterion underestimates the UFL at initial pressures higher than ambient. In the pressure-temperature range tested second order pressure dependences and linear temperature dependences of the UFL are found. The temperature dependence of the UFL is influenced by the initial pressure which is in contrast with previous findings.

Comparison of two standard test methods for determining explosion limits of gases at atmospheric conditions

A comparison is made between two internationally accepted methods to determine the explosion limits of gases at atmospheric pressure and room temperature (20 l sphere - DIN 51649). Significant differences (about 1 vol.%) in the upper explosion limits (UEL) values are found for four hydrocarbons tested. A new criterion is proposed which leads to close agreement between the UEL values obtained by the two methods.

Pressure dependence of the auto-ignition temperature of methane / air mixtures

Abstract The results of experiments conducted to determine the auto-ignition temperature (AIT) and the cool flame temperature (CFT) of methane/air mixtures at elevated pressures (200–4700 kPa) and for concentrations from 30 up to 83 vol.% are reported. The experiments were performed in a closed spherical vessel with a volume of 8 dm 3 . It is shown that methane/air mixtures can react spontaneously for methane concentrations far outside the flammability limits. Cool flames are observed for methane concentrations higher than 40 vol.%. The AIT and CFT are strongly pressure-dependent and decrease with increasing pressures. A Semenov correlation is developed for the CFT as a function of the pressure. The upper explosion limit (UEL) of methane/air mixtures at elevated pressure and temperature is linked with the CFT. It is shown that for certain conditions of pressure and temperature, the flammability range increases considerably even for a small temperature rise.

Electrostatic fields and the maximum heat flux

Abstract A theoretical study is made of the effect of electrostatic fields upon the maximum heat flux during pool boiling on a large horizontal cylindrical heater. Use is made of the hydrodynamic approach to the maximum heat flux. Good agreement is obtained between the theoretical predictions and the experimental observations.

Experimental study of the flammability limits of toluene–air mixtures at elevated pressure and temperature

The flammability limits of toluene-air mixtures are experimentally determined at pressures up to 500 kPa and temperatures up to 250 degrees C in a closed spherical vessel. The results at atmospheric pressure are compared with the results obtained in a glass tube. The flammability limits depend linearly upon temperature. A twilight zone characterized by weak pressure rises is observed for toluene at all pressures, while soot is formed at elevated pressures only. The explosion characteristics of toluene are compared with those of methane. Despite their chemical differences, the explosion characteristics of toluene and methane are similar.

A simple blast wave model for bursting spheres based on numerical simulation

This paper deals with blast waves generated by a bursting spherical vessel filled with a pressurized gas. A large number of numerical simulations are performed. From the results of the simulations, a simple model comparable to the TNT-equivalent model is derived. The new blast wave model consists of a single relationship between the scaled peak overpressure and the scaled distance. Moreover, a closed-form expression for the explosion efficiency as function of the initial velocity only is provided.

Design of an optimal heat-sink geometry for forced convection air cooling of Multi-Chip Modules

An assessment is made of the limits to the cooling capability of forced convection air cooling of multi-chip modules. Therefore heat-sinks of different geometries are analysed, using a new design criterion which involves both the effects of heat transfer performance and pressure drop. It is shown that, when pressure drop is taken into account, plate fin heat-sinks have a better cooling performance than offset-strip fin or pin fin heat-sinks. The dimensions of a plate-fin heat-sink are optimized using an analytical model. The optimal plate-fin heat-sink has a cooling performance comparable to what is normally achieved by integral water cooling. To confirm the thermal performance of the optimal plate fin heat-sink, measurements are carried out in a low speed windtunnel.

The influence of an electric field on the heat transfer rate during film boiling of stagnant fluids

This paper describes the influence of an electric field on the heat transfer rate during film boiling of stagnant fluids (pool boiling) on a horizontal surface. Modelling the influence of an electric field on the heat transfer during film boiling requires the knowledge of the equilibrium shape of the liquid-vapour interface. This equilibrium shape is calculated using an iterative solution technique. In each iteration step a fourth order Runge-Kutta technique is used to calculate the shape of the interface and a boundary element method is used to calculate the electric field. It is shown that the vapour bubbles formed on this interface become elongated in the presence of an electric field. This effect is accounted for in the existing heat transfer models in order to describe the influence of the electric field on the heat transfer coefficient during film boiling.

Modelling of thermal vias in thin film multichip modules

A semi-analytical model is presented for evaluating the thermal resistance of via networks used in thin film multichip modules. Correlations between dimensionless groups were derived from numerical data. Therefore, the via network was divided in three basic elements. This allows one to reconstruct the network as a series and parallel connection. The results of this semi-analytical model are compared with those of an existing analytical model and a 3D finite element model. Good agreement between the three models was obtained. The heat conduction efficiency of a staggered thermal via network is defined and the influence of several parameters was investigated.<<ETX>>