Kai-Shing Yang

Thermal spreading resistance characteristics of a high power light emitting diode module

Abstract In this study, effects of the dimensions and the thermal conductivity of the substrate on the heat transfer characteristics of a LED module are investigated. The total thermal resistance corresponding to a LED module operating at different power levels is measured using a method following JESD51-1 and JESD51-14 standards. In addition, a finite element method (FEM) numerical simulation is carried out to analyze the heat transfer phenomena in the LED module. It is found that, for the current experimental conditions, the importance of the thermal spreading resistance effect increases with decreasing substrate thickness and/or increasing input power of the LED module, which corresponds to an increase in the total thermal resistance and correspondingly a higher chip temperature. Experimental and numerical results show that the thermal spreading resistance and thus the chip temperature can be reduced by increasing the substrate thickness or by utilizing materials with high lateral thermal conductivities (directionally-dependent) for the substrate. In consequence, for LED modules with the same substrate thickness, using graphite composite to replace aluminum as the substrate material reduces the spreading resistance by nearly 14% in the current study.

An Experimental Investigation of Micro Pulsating Heat Pipes

Si-based micro pulsating heat pipes (μPHP) charged with HFE-7100 were tested at several heating powers with two orientations, θ = 0° and 90°. The width of the channel is 0.8 mm in a μPHP having uniform channels, and 1.0 mm or 0.6 mm in the other μPHP. The depth of each channel is 0.25 mm. The overall size of each μPHP is 60 mm × 10 mm × 1.25 mm. Both visual observation and temperature response of the present μPHPs at various conditions were performed. The performance was compared between two μPHPs having either uniform channels or non-uniform channels at difference heating powers. Results showed that both μPHPs could not start the pulsating two-phase flow in the channel of μPHPs as the μPHPs were operated horizontally at heating power ranging from 1 W to 7 W, except when the μPHP having non-uniform channels was tested at heating power of 7 W. Unlike the failure start-up for horizontal arrangement of μPHPs, μPHPs with a vertical arrangement shows a significant start-up phenomenon for both μPHPs with uniform and non-uniform channels due to the assistance in the start-up of both μPHPs arising from gravity force.

A comparative study of nozzle/diffuser micropumps with novel valves.

This study conducts an experimental study concerning the improvement of nozzle/diffuser micropump design using some novel no-moving-part valves. A total of three micropumps, including two enhancement structures having two-fin or obstacle structure and one conventional micro nozzle/diffuser design, are made and tested in this study. It is found that dramatic increase of the pressure drops across the designed micro nozzles/diffusers are seen when the obstacle or fin structure is added. The resultant maximum flow rates are 47.07 mm3/s and 53.39 mm3/s, respectively, for the conventional micro nozzle/diffuser and the added two-fin structure in micro nozzle/diffuser operated at a frequency of 400 Hz. Yet the mass flow rate for two-fin design surpasses that of conventional one when the frequency is below 425 Hz but the trend is reversed with a further increase of frequency. This is because the maximum efficiency ratio improvement for added two-fin is appreciably higher than the other design at a lower operating frequency. In the meantime, despite the efficiency ratio of the obstacle structure also reveals a similar trend as that of two-fin design, its significant pressure drop (flow resistance) had offset its superiority at low operating frequency, thereby leading to a lesser flow rate throughout the test range.

Heat Transfer and Flow Pattern Characteristics for HFE-7100 Within Microchannel Heat Sinks

This study investigates the heat transfer characteristics and flow pattern for the dielectric fluid HFE-7100 within multiport microchannel heat sinks with hydraulic diameters of 480 μm and 790 μm. The test results indicate that the heat transfer coefficient for the smaller channel is generally higher than that of the larger channel. It is found that the heat transfer coefficients are roughly independent of heat flux and vapor quality for a modest mass flux ranging from 200 to 400 kg m−2 s−1 at a channel size of 480 μm and there is a noticeable increase of heat transfer coefficient with heat flux for hydraulic diameters of 790 μm. The difference arises from flow pattern. However, for a smaller mass flux of 100 kg m−2 s−1, the presence of flow reversal at an elevated heat flux for hydraulic diameters of 480 μm led to an appreciable drop of heat transfer coefficient. For a larger channel size of 790 μm, though the flow reversal is not observed at a larger heat flux, some local early partial dryout still occurs to offset the heat flux contribution and results in an unconceivable influence of heat flux. The measured heat transfer coefficients for hydraulic diameters of 790 μm are well predicted by the Cooper correlation. However, the Cooper correlation considerably underpredicts the test data by 35–85% for hydraulic diameters of 480 μm. The influence of mass flux on the heat transfer coefficient is quite small for both channels.

Natural convection heat transfer from square pin fin heat sinks subject to the influence of orientation

An experimental study is conducted on natural convection heat transfer from square pin fin heat sinks subject to the influence of orientation. A total of six pin fin heat sinks with various arrangements are tested under controlled environment. Test results show that the upward facing orientation yields the highest heat transfer coefficient, followed by the sideward facing and the downward facing ones. The heat transfer coefficients for upward facing are about 0% ~ 5% greater than those for sideward facing and are 0% ~ 20% greater than those for downward facing. Besides, the effect of orientation becomes less pronounced as the population density is gradually increased. It is also found that the heat transfer coefficient reveals a maximum at a fin height nearby 2 mm or at a fin density of 1 pin/cm2. The effect of fin height and fin density can be termed as a composite parameter, namely the finning factor which represents the ratio of total surface to the base surface. The heat transfer coefficient initially increases with the finning factor, attains a maximum around 1.6-1.7, and then decreases. Although the heat sinks may not necessarily surpass the un-finned base plate in the heat transfer coefficient, the heat sinks still yield smaller convection resistances than the un-finned base plate

Experimental investigation of high performance thermal module with dimple vortex generators

Pressure drop and heat transfer characteristics of different dimple vortex generators arrangement are examined in this study. A total of five heat sinks were made and tested, including plain fin, dimple, two-group dimple, oblique dimple gap 4–12 fin, oblique dimple gap 6–12 fin. The tested results indicate that the pressure drop for dimple fin geometry is significantly higher than other fin types, followed by the two group dimple fin. The fin structure with oblique dimple shows slightly increase of pressure drop as compared to the plain fin surface. The results show that more complicated fin structure will lead to higher pressure drop. The observed IR image of the temperature measurement of the test fin configurations also proves the measurements. The results suggest that the fin with dimple vortex generators is more beneficial than that of plain fin geometry. The oblique dimple fin is especially useful for air-cooling applicable for electronic devices to achieve effective augmentations without suffering from significant pressure penalty.

Performance of the LED array panel in a confined enclosure

This study performs an experimental study concerning performance of an LED array under natural convection. A total of 270 1-W LEDs having an efficiency of 75% are used to simulate a large LED panel. The size of the cooling heat sink is 348 mm × 558 mm having vertical plate fin configuration with a fin spacing of 9.33 mm. The effect of shrouding, including shrouding above the heat sink and the blockages being placed at the entrance and exit are investigated. For the shrouding effect above heat sink, the heat transfer coefficient is first slightly increased with the rise of shroud distance and peaks at a shroud distance of 20 mm, followed by a marginal decrease of heat transfer coefficient with the shroud distance. On the other hand, a continuous but a very small increase of the heat transfer coefficients with the inlet or outlet blockage distance is seen, yet no plateau is seen as opposed to the effect of shroud distance above heat sink. For the same blockage distance, normally the heat transfer coefficient for those being blocked at the exit is slightly higher than that of inlet blockage. The maximum temperature within the heat sink does not take place at the central position; rather it peaks at the quarter position of the heat sink due to edge effect and likely higher air flow at the center position. The average heat transfer coefficient reaches 6 W/m K as the upper shield is removed.

Heat transfer of vapor chamber with different types of microchannels

ABSTRACT Two 40-mm × 35-mm × 1.525-mm micro vapor chambers were fabricated by inductively coupled plasma etching on silicon substrates and tested in this study. One vapor chamber exhibited convergent microchannels, whose widest and narrowest width were 0.3 mm and 0.1 mm, respectively, and the other exhibited discontinuous microchannels having width of 0.3 mm. Those micro vapor chambers that were filled with deionized water in a filling ratio of approximately 48% were tested with various titled angles and input powers. The results showed that the thermal performance of the vapor chamber having discontinuous microchannels was inferior because the spacing between microchannelend and the micropostin both condensing and evaporating sections was too far to return the condensed fluid from condensing section to the evaporating section of the vapor chamber. On the contrary, the convergent microchannel in the other vapor chamber enhanced capillary force, so that the condensed liquid could be successfully forced from the condensing section to the evaporating section even with top heating mode (–90°). The thermal resistance of the vapor chamber having convergent micro channels with top heating mode was 2.08°C/W at 22 W, while the thermal resistance of the vapor chamber with horizontal heating mode was 1.46°C/W at 28 W.

An investigation of thermal spreading device with thermal via in high power LEDs

A detailed numerical simulation of the performance of thermal module having “thermal via” is made in this study. The results indicate the temperature distributions from the numerical simulation are significantly affected by spreading resistance. The filled thermal via can considerably improve the performance of thermal module. For further explanation of the significant drop of junction and thermal resistance at sub-mount with thermal via, the detailed thermal resistances distribution in the thermal module are further examined. It is found that the significant drop of thermal resistance is mainly from package level with the help of thermal via filled in submount. However, the effect of PCB on the thermal resistance is quite different for the simulated geometries, a “maximum” ratio of thermal resistance of PCB had occurred. The thermal resistance of heat sink remains the same for all simulated case. With further adding the thermal via, the effect of heat sink on the overall resistance will become more and more pronounced.

Heat Transfer and Pressure Drop Characteristics for HFE-7100 Within Microchannel Heat Sinks

This study examines the heat transfer and pressure drop characteristics of the dielectric fluid HFE-7100 within multiport microchannel heat sink having a square configuration rectangular with a hydraulic diameter of 460 μm. For a lower mass flux of 100 or 200 kg/m2 ·s, it is found that the heat transfer coefficients are roughly independent of heat flux and vapor quality provided that no flow reversal occurs. However, with the presence of flow reversal at an elevated heat flux, appreciable drop of heat transfer coefficient is encountered. The flow reversal also plays a significant role in the overall pressure drop. Without flow reversal, the pressure drop for higher heat flux always exceeds that of lower heat flux due to acceleration contribution. However, the presence of flow reversal may offset the contribution of acceleration and results in a negligible effect of heat flux. For a higher mass flux like 300 kg/m2 ·s, the heat transfer coefficients are virtually independent of vapor quality and heat flux.Copyright