Liwen Jin

A benchmark study on the thermal conductivity of nanofluids

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

Heat transfer of oscillating and steady flows in a channel filled with porous media

Experiments in steady and oscillating flows were conducted to investigate the heat transfer in a porous channel. The test section was designed to have an adiabatic plate channel filled with porous media. Two types of foam materials viz. copper and aluminum with different PPI (Pores Per Lineal Inch) and thermal conductivity were used. For both steady and oscillating flows, the surface temperature, pressure drop and flow velocity were measured and the local and averaged Nusselt numbers were calculated. The effects of thermal conductivity and permeability of different foam materials on heat transfer were analyzed. The uniformity of the local temperature distributions for both steady and oscillating flows were compared

Forced convection air cooling in porous graphite foam for thermal management applications

Rapid development in the design of electronic packages for modern high-speed computers has led to the demand for effective methods of chip cooling. The primary concerns in thermal management applications are high thermal conductivity, large specific surface area and low weight. Graphite foams which possess predominantly spherical pores with smaller openings between the cells constitute a novel highly-conductive porous material for electronic cooling applications. These foams can be produced with bulk thermal conductivities almost equivalent to dense aluminum alloys with only 1/5 the weight of solid aluminum material. Motivated by the thermal and physical properties of graphite foam, experiments were performed to assess the cooling performance of such foams for thermal management applications. A test facility was developed for experimental studies under constant heat flux heating condition. The graphite foam heat sinks were fabricated into different structures, which were designed to utilize the porous properties of the foam for heat removal. Heat transfer characteristics including local temperature and Nusselt number distributions for steady flow through the tested heat sinks were measured and a correlation of length-averaged Nusselt number in terms of Prandtl and Reynolds numbers was obtained. The findings of this study show that graphite foam heat sinks with appropriate structure can offer good heat removal with relatively low pressure drop.

Convective Heat Transfer in Graphite Foam Heat Sinks With Baffle and Stagger Structures

Highly conductive porous media have recently been considered for enhanced cooling applications due to their large internal contact surface area, which promotes convection at the pore level. In this paper, graphite foams that possess high thermal conductivity but low permeability are investigated for convection heat transfer enhancement using air as coolant. Two novel heat sink structures are designed to reduce the fluid pressure drop. Both experimental and numerical approaches are adopted in the study. The experimental data show that the designed structures significantly reduce flow resistance in graphite foams while maintaining relatively good heat removal performance. The numerical results obtained based on the local thermal nonequilibrium model are validated by experimental data and show that the inlet air flow partially penetrates the structured foam walls, while the remaining air flows tortuously through slots in the structure. Flow mixing, which is absent in the block graphite foam, is observed in the freestream area inside the designed structure. It can be concluded that graphite foams with appropriately designed structures can be applied as air-cooled heat sinks in thermal management applications.

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 Performance of Metal Foam Heat Sinks Subjected to Oscillating Flow

This paper reports the results of an experimental investigation on the heat transfer performance of metal foam as a heat sink subjected to oscillating flow. The measured pressure drops, velocities, and surface temperatures of oscillating flow through aluminum 40 PPI foam are presented in detail. The calculated cycle-averaged local temperatures and Nusselt numbers for different kinetic Reynolds numbers were analyzed. The variation of total heat transfer rate with a kinetic Reynolds number suggests that oscillating flow at relative low frequency has a substantial effect on heat transfer enhancement in a metal foam heat sink. A comparison of the length-averaged Nusselt numbers between oscillating and steady flows indicates that higher heat transfer rates can be obtained in metal foams subjected to oscillating flow. The relation between pumping power and total heat transfer rates for the oscillating flow cooling system was also analyzed with a view to designing a novel heat sink using metal foam. The results show that high heat transfer performance of metal foam heat sinks subject to oscillating flow can be obtained with moderate pumping power

Numerical simulations of forced convection in novel cylindrical oblique-finned heat sink

In this paper, forced convection heat transfer of laminar flow through novel cylindrical discrete oblique fin heat sinks were simulated by Computational Fluid Dynamics method (CFD). The single channel structure was imported into FLUENT v.12.1 as a physical model to be simulated. The various aspect ratio and length ratios of discrete oblique fin arrays were investigated, and the optimized results were achieved. According to the results, the average Nusselt number for the copper heat sink could increase by as much as 73.5% and the average convective thermal resistance for the novel cylindrical micro-channel reduces by as much as 61.7% in comparison with the conventional micro-channel. The wall temperature and temperature gradient of cylindrical oblique fin heat sink are also much lower than those of conventional straight fin heat sink. It was concluded that the novel cylindrical microchannel heat sink is superior to conventional cylindrical microchannel heat sink. Lastly, the guidelines for optimizing cylindrical heat sink structure in different applications were also discussed.

Enhanced heat transfer and reduced pressure drop using stepped fin microchannels

Experiments on flow boiling were conducted in straight and stepped fin microchannels. The test vehicles were made from copper with a footprint area of 25mm × 25mm. The microchannels were formed by wire cut Electro Discharge Machining process and have surface roughness (Ra) of about 2.0 µm. Tests were performed on channels having nominal width of 300 µm and a nominal aspect ratio of 4 over different mass velocity range and inlet temperature of 90°C. It was observed that the two phase pressure drop across the stepped fin microchannel heat sink was significantly lower as compared to its straight counterpart. Moreover the pressure drop and wall temperature fluctuations were seen reduced in the stepped fin microchannel heat sink. It was also noted that the stepped fin microchannel heat sink had a better heat transfer performance than the straight microchannel heat sink, under similar operating conditions. This phenomenon in stepped fin microchannel heat sink is explained based on its improved flow boiling stability that reduces the pressure drop oscillations, temperature oscillations and hence partial dry out, by allowing the bubbles to expand span wise and hence flow downstream with less resistance.

Experimental Study and High Speed Visualization of Flow Boiling Characteristics in Silicon Microgap Heat Sink

Flow boiling in microgap heat sink is very attractive for high-performance electronics cooling due to its high heat transfer rate and easy fabrication process. In absence of thermal interface material between the active electronic component and a microgap cold plate, significant reduction in interface thermal resistance and enhancement in heat transfer rate can be achieved. In earlier studies by these authors, encouraging results have been obtained using microgap heat sink as it can potentially mitigate flow instabilities, flow reversal and maintain uniform wall temperatures over the heated surface. So, more work should be carried out to advance the fundamental understanding of the two-phase flow heat transfer associated with microgap heat sink and the underlying mechanisms. In this study, local flow boiling phenomena in different microgap sizes have been investigated experimentally. Experiments are performed in silicon based microgap heat sink having microgap depth ranging from 80 μm to 500 μm, using deionized water with 10 °C subcooled inlet temperature. The effects of mass flux and heat flux on heat transfer coefficient and pressure drop characteristics are examined by using different mass fluxes ranging from 400 kg/m2s to 1000 kg/m2s and effective heat flux varying from 0 to 100 W/cm2. Apart from these experimental investigations, simultaneous high speed visualizations are conducted to observe and explore the mechanism of flow boiling in microgap. Confined slug and annular boiling are observed as the two main heat transfer mechanisms in microgap. Moreover, experimental results show that flow boiling heat transfer coefficients are dependent on gap size, and the lower the gap size, higher the heat transfer coefficient.Copyright

Parametric study of pool boiling from porous graphite foams in dielectric liquids

An experimental investigation of pool boiling on porous graphite foams is presented in this paper. A compact thermosyphon was developed to conduct the experiments using different types of graphite foams as evaporator inserts. Two dielectric liquids viz. FC-12 and HFE-700 were used as phase change coolants to investigate the effects of pore diameter and thermal conductivity of the foams on pool boiling heat transfer. It is found that the boiling heat transfer is affected by phase change coolant types. The difference of thermophysical properties between FC-12 and HFE-7000 resulted in different boiling characteristics on porous graphite foams. The experimental results indicate that thermal conductivity and pore diameter exhibit an interrelated effect on boiling performance. The compromise of these foam properties affects the bubble departure frequency which dominates the boiling heat transfer coefficient. This study demonstrates that the designed air-cooled thermosyphon can maintain the heater surface temperature below 85°C at heat flux of 112 W/cm2