Poh Seng Lee

Experimental study on laminar heat transfer in microchannel heat sink

Experiment results have been obtained for single-phase laminar forced convection in rectangular microchannels. Microchannel heat sinks (having a total projected channel area of 1.0 cm by 1.0 cm) with 64 channels of width 54 /spl mu/m and height 215 /spl mu/m each were built and tested. Tests were performed with deionized water as the cooling fluid, where the liquid flow rate ranged from 0.5 cc/s to 10 cc/s. Results of the system performances were presented in terms of overall pressure drop and local thermal resistance. Also, the associated convective heat transfer was reported in terms of the friction factor and the Nusselt number. The experimentally obtained results agree reasonably well with the classical developing channel flow theory.

Experimental Investigation of Heat Transfer in Microchannels

Heat transport in microchannels is experimentally investigated to explore the validity of classical correlations for conventional-sized rectangular channels in predicting the thermal behavior and the onset of transition in microchannels. The microchannels considered range in width from 194 μm to 534 μm, with the channel depth being nominally five times the width in each case. Ten microchannels were machined into a 2.54 cm by 2.54 cm copper substrate for each test piece. The experiments were conducted with deionized water, with the Reynolds number ranging from approximately 300 to 3500. The results show that the heat transfer in microchannels is satisfactorily predicted with a classical, continuum approach. However, the applicable classical correlations need to be chosen carefully to match the boundary and entrance length conditions imposed in the experiment.Copyright

Hotspot Mitigating With Obliquely Finned Microchannel Heat Sink—An Experimental Study

Sectional oblique fins are employed, in contrast to continuous fins, in order to modulate the flow in a microchannel heat sink. The breakage of continuous fin into oblique sections leads to reinitialization of boundary layers and generation of secondary flows that significantly enhance the cooling performance of the heat sink. In addition, an oblique finned microchannel heat sink has the flexibility to tailor local heat transfer performance by varying its oblique fin pitch. Clusters of oblique fins at higher density can be created in order to promote a greater degree of boundary layer redevelopment and secondary flow generation to provide more effective cooling at the high heat-flux region. Thus, the variation of oblique fin pitch can be exploited for hotspot mitigation. Experimental studies of a silicon chip with two hotspot scenarios show that the temperature hike and the temperature difference for the enhanced microchannel heat sink with variable pitch are reduced by as much as 17.1 °C and 15.4 °C, respectively. As a result, temperature distribution across the silicon chip is more uniform. In addition, the associated pressure drop penalty is much smaller than the achieved heat transfer enhancement, rendering it as an effective hotspot mitigating strategy for the single-phase microchannel heat sink.

Hot-Spot Thermal Management With Flow Modulation in a Microchannel Heat Sink

Recesses created in the lid of a microchannel heat sink can serve to modulate the flow, resulting in local and global heat transfer enhancement. Numerical analysis of laminar flow and heat transfer in such a modified microchannel heat sink has shown an augmentation of heat transfer without an added penalty of increased pressure drop. The presence of the recesses reduces the overall flow friction and thus pressure drop. The flow expansion into the recesses and the subsequent contraction into the downstream region causes significant local enhancement in heat transfer. Both the maximum and average wall temperatures are decreased as a result. The heat transfer is locally enhanced, by as much as 150% in the regions just downstream of the recesses due to the re-initialization of boundary layers as the flow re-enters the microchannels. The potential for hot-spot mitigation in microelectronics devices using this approach is discussed.© 2005 ASME

Nucleate Boiling in Microchannels

Dong Liu, Poh-Seng Lee and Suresh V. Garimella Purdue University, West Lafayette, Indiana 47907-2088 An understanding of bubble motion and evolution during nucleate boiling is necessary for the analysis of convective heat transfer rates in microchannels. Highspeed photography is used in this study to reveal the complex bubble dynamics during nucleate boiling in copper microchannels of hydraulic diameter 384 μm (275 μm wide and 636 μm high) and 25.4 mm length. Deionized water flows through the microchannels at a velocity of 0.68 m/s (Re = 735) and an inlet temperature of 86.5°C. The exit pressure is maintained at 1.05 bar. A constant heat flux of 16 W/cm is applied at the bottom of the microchannel heat sink. A high-speed digital video camera is used to observe the boiling process at 4,000, 8,000 and 15,000 frames per second. The images shown looking down into the microchannels reveal the transient processes of nucleation, growth, subsequent departure and interaction of bubbles from nucleation sites on the bottom wall of the channel. The measured bubble radius indicates a linear evolution with time. These results are useful in proposing predictive models for boiling heat transfer in microchannel heat sinks. 0 ms

Performance Emission and Economic Analysis of Preheated CNSL Biodiesel as an Alternate Fuel for a Diesel Engine

In the present work, CNSL (cashew nut shell liquid), a byproduct of cashew industry, has been considered as an economically viable alternate source for producing biodiesel. Further, the feasibility and economic viability of using CNSLME (cashew nut shell liquid methyl ester or CNSL biodiesel) directly in a diesel engine has been analyzed. CNSL is trans-esterified in double stages due to its high free fatty acid content and instead of using produced CNSLME in blend with diesel; it was directly used in a single cylinder diesel engine by preheating it. By this measure, the higher viscosity of biodiesel is reduced and the experimental investigation tends to show that both the performance and emission of preheated CNSLME has been improved. At an inlet fuel temperature of 80°C, the CNSLME discerns a 20% increase in BTE (brake thermal efficiency), 66% and 52% decrease in CO and HC emission, respectively, than unheated CNSLME. To purport the economic viability of CNSL, a detailed economic analysis has been conducted. From the analysis, it is inferred that CNSL is a low cost feedstock for biodiesel production among the other existing edible and non-edible oils considered so far.

A pump-free microfluidic 3D perfusion platform for the efficient differentiation of human hepatocyte-like cells†

The practical application of microfluidic liver models for in vitro drug testing is partly hampered by their reliance on human primary hepatocytes, which are limited in number and have batch‐to‐batch variation. Human stem cell‐derived hepatocytes offer an attractive alternative cell source, although their 3D differentiation and maturation in a microfluidic platform have not yet been demonstrated. We develop a pump‐free microfluidic 3D perfusion platform to achieve long‐term and efficient differentiation of human liver progenitor cells into hepatocyte‐like cells (HLCs). The device contains a micropillar array to immobilize cells three‐dimensionally in a central cell culture compartment flanked by two side perfusion channels. Constant pump‐free medium perfusion is accomplished by controlling the differential heights of horizontally orientated inlet and outlet media reservoirs. Computational fluid dynamic simulation is used to estimate the hydrostatic pressure heads required to achieve different perfusion flow rates, which are experimentally validated by micro‐particle image velocimetry, as well as viability and functional assessments in a primary rat hepatocyte model. We perform on‐chip differentiation of HepaRG, a human bipotent progenitor cell, and discover that 3D microperfusion greatly enhances the hepatocyte differentiation efficiency over static 2D and 3D cultures. However, HepaRG progenitor cells are highly sensitive to the time‐point at which microperfusion is applied. Isolated HepaRG cells that are primed as static 3D spheroids before being subjected to microperfusion yield a significantly higher proportion of HLCs (92%) than direct microperfusion of isolated HepaRG cells (62%). This platform potentially offers a simple and efficient means to develop highly functional microfluidic liver models incorporating human stem cell‐derived HLCs. Biotechnol. Bioeng. 2017;114: 2360–2370.

Influence of nanoscale geometry on the dynamics of wicking into a rough surface

The dynamics of imbibition into the roughness of a surface was investigated with hexagonal arrays of anisotropic nanofins fabricated with interference lithography and metal assisted chemical etching. It was found that viscous drag caused by the nanofins is similar to that caused by open nano-channels of equal length and height containing the same volume of liquid. In addition, the energy dissipated by form drag for a given driving pressure was determined to be directly proportional to the volume of fluid between nanofin planes that are flat and normal to the imbibition direction.

Experimental investigation of microgap cooling technology for minimizing temperature gradient and mitigating hotspots in electronic devices

Hotspots can be generated by non-uniform heat flux condition over the heated surface due to higher packaging densities and greater power consumption of high-performance computing technology in military systems designs. Because of this hotspot within a given chip, local heat generation rate exceed the average value on the chip and increase the peak temperature for a given total power generation which degrades the reliability and performance of equipments. Two phase microgap cooling technology is promising to minimization of temperature gradient and reduction of maximum temperature over the heated surface of the device because of unique boiling mechanism in microgap: confined flow and thin film evaporation. The present study aims to experimentally investigate the applicability of microgap cooling technology for minimizining temperature gradient and mitigating hotspots from the heated surface of electronic device. Experiments are performed in silicon based microgap heat sink having a range of gap dimension from 200 µm – 400 µm. Encouraging results have been obtained using microgap channel cooler for hotspots mitigation as it maintain uniform and low wall temperature over the heated surface.

Heat Transfer Enhancement in Microchannels Incorporating Slanted Grooves

The ever-increasing density, speed, and power consumption of microelectronics has led to a rapid increase in the heat fluxes which need to be dissipated in order to ensure their stable and reliable operation. The shrinking dimensions of electronics devices, in parallel, have imposed severe space constraints on the volume available for the cooling solution, defining the need for innovative and highly effective compact cooling techniques. Microchannel heat sinks have the potential to satisfy these requirements. However, significant temperature variations across the chip persist for conventional single-pass parallel flow microchannel heat sinks since the heat transfer performance deteriorates in the flow direction in microchannels as the boundary layers thicken and the coolant heats up. To accommodate higher heat fluxes, enhanced microchannel designs are needed. The present work presents an idea to enhance the single-phase convective heat transfer in microchannels. The proposed technique is passive, and does not require additional energy to be expended to enhance the heat transfer. The idea incorporates the generation of a spanwise or secondary flow to enhance mixing and hence decrease fluid temperature gradients across the microchannel. Slanted grooves can be created on the microchannel wall to induce the flow to twist and rotate thus introducing an additional component to the otherwise laminar flow in the microchannel. Numerical results are presented to demonstrate the effectiveness of such an enhanced microchannel heat sink. The heat transfer was found to increase by up to 12% without incurring substantial additional pressure drops.Copyright