Segmented Microchannels Research Articles

Thermal Performance Analysis of Micro Pin Fin Heat Sinks under Different Flow Conditions

Due to microscale effects, the segmented microchannels or micro pin fin heat sinks emerged as a high thermal management solution. In this context, the present work analyzes the influence of different heights of square micro pin fins with an aligned array and investigates their influence on pressure drop and heat transfer behavior. The HFE-7100 is used as the working fluid, and the pressure drop and surface temperature behavior are analyzed for different mass fluxes and inlet subcooling. The single-phase flow was analyzed numerically using the computational fluid dynamics (CFD) software ANSYS FLUENT® for comparing the simulation results with the experimental data, showing that the highest micro pin fins configuration provides a more uniform and lowest wall temperature distribution compared to the lowest configuration. There is a good agreement between the experimental results and the numerical analysis, with a mean absolute error of 6% for all the considered parameters. For the two-phase flow condition, experimental tests were performed, and for the highest subcooling, an increase in mass flux causes an enhancement in the heat transfer for low heat flux; by increasing heat flux, there is a gradual predominance of boiling heat transfer over convection as the heat transfer mechanism. The pressure drop drastically increases with the vapor amount flowing into the system, regardless of the pin fin height; the boiling curves for the higher fin height show a much smaller slope and a smaller wall superheat than the fin with the smallest height, and consequently, a high heat transfer performance. A larger region of the heat sink is filled with vapor for lower inlet subcooling temperatures, degrading the heat transfer performance compared to higher inlet subcooling temperatures.

Experimental Study on Flow Boiling Characteristics in Continuous and Segmented Microchannels with Vapor Venting Membrane

Flow boiling in microchannels is one of the promising techniques for heat dissipation occurred in micro devices. However, the rapid bubble growth must be suppressed, which leads to serious boiling instabilities, high pressure drop, and low heat transfer coefficient. The addition of porous hydrophobic membrane has proven an effective method to remove the vapor in-site in the literature. However, the effects of heat sink’s topological structures on the vapor venting are still a research gap. The present study experimentally investigates the influence of Polytetrafluoroethylene (PTFE) membrane on fluid flow pattern, pressure drop, vapor venting performance, and heat transfer characteristics of flow boiling in the continuous and segmented heat sinks. Results show that the vapor venting membrane can reduce the pressure drop and increase the heat transfer coefficient effectively by decreasing the exit vapor quality, especially in combination with the segmented structures. The interconnection area as a space for bubble growth and coalescence is beneficial for vapor venting due to increased vapor pressure and quantity. Following the enhanced vapor discharge, the fluctuation of pressure drop is further weakened, which is conducive for the safe operation of heat sink.

Prospects for using two-phase micro-size systems for high heat flux removal

Heat transfer in active systems for high heat removal based on the micro-channels and hybrid micro-channel/micro-jet is considered. The application of these systems allows significantly increasing the critical heat flux for a dense arrangement of the heat stressed equipment. The characteristics of heat transfer and critical heat flux during subcooled flow boiling of water in the micro-channel heat sink and during micro-jet impingement in narrow channel are obtained. The experiments are performed for the horizontal segmented microchannels with a cross section of 340×2000 μm2 made on the top of copper target and for impingement micro-jet cooling of the copper target in the gap of 1000 μm. It has been found that, compared with impingement micro-jet cooling in similar condition, the micro-channel cooling is more effective for high heat flux removal although it creates the considerably high wall temperature.

Comparative study of flow boiling heat transfer and pressure drop of HFE-7000 in continuous and segmented microchannels

The flow boiling heat transfer and pressure drop characteristics of HFE-7000 were experimentally investigated in two test sections with continuous and segmented structures. Each microchannel had seven parallel microchannels with the total length of each channel being 110 mm. The cross sectional area of each channel was 2 mm × 1 mm (width × height) with a rib width of 0.5 mm. The 10 mm long transversely connected zones were set every 30 mm along the flow direction in the segmented one. The experiments all used the same saturated inlet temperature of 50 ± 1 °C with mass fluxes from 300 to 500 kg/m2 s and heat fluxes from 30 to 300 kW/m2. The results showed that the interconnected areas in the segmented microchannels improved the heat transfer rates that were related to the flow patterns and fluid-structure effects. For high vapor qualities, the segmented microchannels delayed the heat transfer deterioration seen in the continuous microchannels and the critical heat flux was significantly increased. The frictional pressure drop in the segmented microchannels was less for these experimental conditions before dry out occurred. The interconnected areas in the segmented microchannels also reduced the frictional pressure drop fluctuations, especially the instabilities during the bubbly and slug flow regimes.

Flow and heat transfer characteristics of microchannel flow boiling enhancement with channel configurations

The heat dissipation problem encountered in the process of the development of new electronic equipment and mechanical components towards miniaturization has become the bottleneck of continuous progress in the field of mechanical and electronic control engineering, and the flow boiling heat transfer technology in microchannel may provide the effective cooling guarantee for this purpose. Given the reasonable operating temperature ranges of devices and the diversity of coolant evaporation temperature and pressure choices, refrigerants have become an important option, especially for zeotropic mixtures, which can not only modulate the properties of pure components, but also present the unique phase change heat transfer characteristics with great application prospects. In the present study, the flow boiling heat transfer research of R134a/R245fa zeotropic mixtures as well as their pure compartments in parallel multi-microchannel with continuous and segmented channel configurations were experimentally studied. Each microchannel test section consisted of seven parallel channel passages with the same total length of 110 mm. The cross-sectional area of each channel passage was 2 mm × 1mm (width×height) with the slab width of 0.5 mm. What difference from continuous microchannel was that 10 mm-long interconnected area was arranged every 30 mm along the channel length in segmented one. The experiments were carried out at the same inlet saturation temperature of 26 °C under the conditions comprising the heat flux range of 20− 350 kW/m2 and mass flux range of 300− 400 kg/(m2 s), respectively. In order to obtain more accurate data matching relationship, the analysis of heat transfer performance focused on the region near the outlet of the test sections. It’s worth noting that the multi-microchannel should be treated as fin effects during the heat transfer data reduction. The ribbed slabs could be considered as the rectangular straight fins either for continuous or segmented multi-microchannel. The results showed that compared with continuous microchannel, the heat transfer performance was slightly enhanced for pure components while slightly suppressed for zeotropic mixtures in segmented microchannel at lower effective heat flux. With the increase of effective heat flux, the heat transfer suppression effect of zeotropic mixtures was gradually weakened. The interconnected area was helpful to delay the occurrence of the transfer deterioration and improve the flow boiling CHF regardless of pure or zeotropic refrigerants. Besides, the flow boiling pressure drop in segmented microchannel was obviously reduced for zeotropic mixtures and their pure components. The smaller the liquid-vapor density ratio of test refrigerants, the higher the reduction of pressure drop with the help of interconnected area. For zeotropic mixtures, the flow boiling pressure drop was smaller at lower effective heat flux and the increasing trend of pressure drop on heat flow is slower.

Experimental study on flow boiling heat transfer for pure and zeotropic refrigerants in multi-microchannels with segmented configurations

In this paper, the flow boiling heat transfer characteristics for pure refrigerants of R134a, R245fa and their binary zeotropic mixtures with three blending ratios (R134a/R245fa: 10/90, 30/70 and 70/30 by wt%) were experimentally investigated in the aluminum multi-microchannels with segmented configuration. Each multi-microchannel test plate consisted of seven parallel channel passages with the same total length of 110 mm and cross-sectional area of 2 mm × 1 mm (width × height), while the 10 mm-long interconnected area was arranged every 30 mm along the channel length in segmented one. For each refrigerant working fluid, the experiments were performed at the same inlet evaporating temperature of 26 °C under conditions of the heat flux and mass flux ranging from 20 to 350 kW/m2 and 300 to 400 kg/m2 s, respectively. According to the comparative study, R134a working fluid presented higher heat transfer coefficient than R245fa, but it was more prone to local dry out spots in advance. The small addition of volatile component of R134a was found to be beneficial for improving the heat transfer coefficient at higher heat flux as well as the flow boiling CHF, despite the common feature of degraded heat transfer coefficient for test zeotropic mixtures in most conditions. Compared with continuous microchannels, the heat transfer performance was enhanced for pure refrigerants but suppressed for zeotropic mixtures in segmented microchannels, however, in which the interconnected area was helpful to delay the occurrence of the transfer deterioration and improve the flow boiling CHF regardless of pure or zeotropic refrigerants.

Numerical investigation for optimizing segmented micro-channel heat sink by Taguchi-Grey method

The scale-down trend increases the chips’ density and the high power handling capability generates unnecessary heat which can disrupt the reliability of the electronic devices. Therefore, various types of cooling solution have been proposed to enhance heat dissipation from the electronic devices. One of the solution is using inexpensive straight-channel heat sink. However, the presence of large temperature gradient between the upstream and downstream in the straight-channel can shorten the life span of the device and subsequently reduce the reliability. In this study, a novel segmented micro-channel is introduced to improve the thermal performance of the straight-channel heat sink. Computational fluid dynamic analysis are performed to investigate the performance of the micro-channel heat sink. The bottom of the heat sink is subjected to a constant heat flux condition and water is used as a coolant. Following that, Taguchi-grey method is applied to optimize the design of the segmented micro-channel. The effect of fin width, fin length, fin transverse distance, number of segments, channel width and mass flow rate on the specific performance, variation of temperature and pressure drop are investigated. The results indicate that a three segments of segmented micro-channel, fin width-1 mm, fin length-2 mm, fin transverse distance-5 mm and channel width-1 mm have successfully enhance the heat transfer performance with minimum pressure drop. It is also found that the optimized micro-channel heat sink is able to cool the chip with heat flux of 800 W to 56.6 °C and pumping power of 0.13 W using 15 gs−1 of water.

The Belle II imaging Time-of-Propagation (iTOP) detector

High precision flavor physics measurements are an essential complement to the direct searches for new physics at the LHC ATLAS and CMS experiments. Such measurements will be performed using the upgraded Belle II detector that will take data at the SuperKEKB accelerator. With 40x the luminosity of KEKB, the detector systems must operate efficiently at much higher rates than the original Belle detector. A central element of the upgrade is the barrel particle identification system. Belle II has built and installed an imaging-Time-of-Propagation (iTOP) detector. The iTOP uses quartz optics as Cherenkov radiators. The photons are transported down the quartz bars via total internal reflection with a spherical mirror at the forward end to reflect photons to the backward end where they are imaged onto an array of segmented Micro-Channel Plate Photo-Multiplier Tubes (MCP-PMTs). The system is read out using giga-samples per second waveform sampling Application-Specific Integrated Circuits (ASICs). The combined timing and spatial distribution of the photons for each event are used to determine particle species. This paper provides an overview of the iTOP system.

Experimental study of heat transfer enhancement with segmented flow in a microchannel by using molecule-based temperature sensors

This paper presents an experimental study of heat transfer enhancement with liquid-gas two-phase segmented microchannel flows by utilizing a novel technique of molecule-based temperature sensors. A serpentine microchannel was constructed and steady liquid and gas segments of ethanol and air were successfully produced at various flow rates. A molecule-based temperature sensor made of Tris(2,2′-bipyridyl) ruthenium was utilized in the liquid segments by dissolving in ethanol as temperature-sensitive fluid and mixing in a dope to produce temperature-sensitive paint in order to retrieve fluid and surface temperature data, respectively. Three cases of different liquid and gas flow rates were tested. The heat transfer enhancements of these three cases were carefully examined under constant heat flux thermal boundary conditions, and the increased heat transfer was evaluated in comparison with the single-phase flow, where only liquid was injected in the microchannel. An improvement of up to 30% in the averaged Nu number of the microchannel flow was determined for Case 3, with a high gas flow rate and short liquid segments. Velocity and vorticity profiles in the liquid segments at different gas and liquid flow rates were also studied using a micro-particle image velocimetry technique. Stronger circulation was observed in Case 3, which confirmed the results from the heat transfer analysis, namely that more efficient heat transfer occurs with strong circulation in short liquid segments when a high gas flow rate is applied. The results of the present study provide detailed temperature profiles in segmented flow as well as the evolution of the Nu number from the entrance to the exit of a serpentine microchannel.

Sample Dispersion for Segmented Flow in Microchannels with Rectangular Cross Section

Hydrodynamic dispersion in microchannels can be significantly reduced by segmentation with a second immiscible phase. We address the effect of microchannel cross section on the dispersion of analytes in a segmented gas-liquid flow of alternating bubbles and liquid segments. Channels of square or nearly square cross section are considered. A significant fraction of the liquid surrounds the bubbles and wets the channel walls in the form of films or menisci. This stagnant fraction of the liquid remains when gas and liquid segments flow by, and it is connected to the liquid within the liquid segments by diffusion only and it effectively increases dispersion. We design and fabricate a microchip with integrated analyte injection and detection to investigate the effects of the influence of the stagnant liquid in segmented flow through square microchannels on the analyte bandwidth. The measured data and a corresponding model confirm the experimental trends and suggest operating conditions at which the unwanted effect of dispersion in segmented microchannel flow is minimized. Dispersion is least when the liquid flow rate is greater than the gas flow rate, and the optimum ratio of the two flow rates slightly increases with increasing bubble velocity.

Unstructured 3D PIC simulations of the flow in a retarding potential analyzer

A three-dimensional particle in cell (PIC) code is developed on unstructured tetrahedral grids. Zero-order (Nearest Grid Point) and linear (volume) particle charge weighting and force interpolation schemes are incorporated. A parametric investigation of the numerical heating in unstructured 3D PIC simulations is presented. The PIC methodology is applied to the simulation of the flow inside the segmented microchannel of a directional-Retarding Potential Analyzer. Simulations show the flow characteristics of the ions and electrons and provide estimates of the collected current by the microplate.