Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Heat and fluid flow in high-power LED packaging and applications

Light emitting diodes, LEDs, historically have been used for indicators and produced low amounts of heat. The introduction of high brightness LEDs with white light and monochromatic colors have led to a movement towards general illumination. The increased electrical currents used to drive the LEDs have focused more attention on the thermal paths in the developments of LED power packaging. The luminous efficiency of LEDs is soon expected to reach over 80 lumens/W, this is approximately 6 times the efficiency of a conventional incandescent tungsten bulb. Thermal management for the solid-state lighting applications is a key design parameter for both package and system level. Package and system level thermal management is discussed in separate sections. Effect of chip packages on junction to board thermal resistance was compared for both SiC and Sapphire chips. The higher thermal conductivity of the SiC chip provided about 2 times better thermal performance than the latter, while the under-filled Sapphire chip package can only catch the SiC chip performance. Later, system level thermal management was studied based on established numerical models for a conceptual solid-state lighting system. A conceptual LED illumination system was chosen and CFD models were created to determine the availability and limitations of passive air-cooling.

Thermal management of LEDs: Package to system

A numerical model of passive and active behavior of skeletal muscles

Recent advances in MEMS-based micro heat pipes

Recently, the high-brightness LEDs have begun to be designed for illumination application. The increased electrical currents used to drive LEDs lead to thermal issues. Thermal management for LED module is a key design parameter as high operation temperature directly affects their maximum light output, quality, reliability and life time. In this review, only passive thermal solutions used on LED module will be studied. Moreover, new thermal interface materials and passive thermal solutions applied on electronic equipments are discussed which have high potential to enhance the thermal performance of LED Module.

https://iopscience.iop.org/article/10.1088/1674-4926/32/1/014008/pdf

A review of passive thermal management of LED module

Two-phase cooling of light emitting diode for higher light output and increased efficiency

Electrical-thermal-luminous-chromatic model of phosphor-converted white light-emitting diodes

The diode forward voltage method with pulsed currents was widely used for monitoring junction temperature (Tj) of light-emitting diodes (LEDs). However, a thermal transient effect (TTE) was observed by the pulsed currents and consequent errors were introduced. Thermoelectric physics was conducted to explain this phenomena and a group of experiments was used to reveal the TTE during Tj measurement for high-voltage (HV) LEDs. In addition, an improved pulse-free direct junction temperature measurement method was conducted for HV LEDs to reduce the errors and to achieve in situ Tj measurement with dc currents, simpler setups, and a less step sequence.

Thermal Transient Effect and Improved Junction Temperature Measurement Method in High-Voltage Light-Emitting Diodes

The increased electrical currents used to drive light emitting diode (LED) cause significant heat generation in the solid state lighting (SSL) system. As the temperature will directly affect the maximum light output, quality, reliability and the life time of the SSL system, thermal management is a key design aspect in terms of cost and performance. Particularly for consumer SSL system, natural convection cooling is cheaper and more reliable than the forced air cooling heat sinks. Although with less efficiency, natural convection heat sink is a good compromise between economy and thermal performance of SSL systems. In this work, the thermal performance of two geometrically different passive heat sink designs for consumer SSL applications is numerically simulated. The heat sink performance is simulated for two orientations: LED up and LED down orientation. Simulation runs for the two designs at the two orientations, in order to investigate the thermal performance of the heat sinks with natural convection cooling. Meanwhile, the radiation effect is considered. With passive cooling, the natural convection plays an important role, and results show that if free ambient air flow is blocked by the heat sink design and the performance reduces considerably. Furthermore, the volume of free air in the luminaire is expected to have significant impact to the heat sink thermal performance. Therefore, the thermal performance for different volumes of luminaire enclosures is also investigated in this work. To analyze the modeling results, a straightforward calculation of the thermal resistance between the LED junction and the environment is applied. In the results, the thermal resistances of LED junction to environment decreases but the air velocity gradually increases with the increasing luminaire volume. In conclusion, although more study is needed for validation of the optimal volume and shape for natural convection, the results in this work can already be used to guide the design of luminaires. In future work, the simulation on real model of bulb or luminaires will be applied to investigate what extend designs based on natural convection and radiation principles can be exploited to manage the LED junction temperature.

A.W.J. Gielen

Papers

A review of passive thermal management of LED module

Two-phase cooling of light emitting diode for higher light output and increased efficiency

Electrical-thermal-luminous-chromatic model of phosphor-converted white light-emitting diodes

Thermal Transient Effect and Improved Junction Temperature Measurement Method in High-Voltage Light-Emitting Diodes

Numerical modeling of thermal performance: Natural convection and radiation of solid state lighting