Heat transfer and pressure drop in fractal tree-like microchannel nets

Inspired by the fractal pattern of mammalian circulatory and respiratory systems, a new design of fractal branching channel net for cooling of electronic chips is studied in this paper. A comparison of the new design with the traditional parallel net shows that the new fractal branching channel net has a stronger heat transfer capability and requires a lower pumping power.     

A study on the hydraulic and thermal characteristics in fractal tree-like microchannels by numerical and experimental methods

This paper focused on the hydraulic and thermal characteristics of fractal tree-like microchannels with different ARs (aspect ratios) for the Reynolds numbers ranging from 150 to 1200. And the fractal microchannels with two branches and straight microchannels with the same heat transfer area were also compared in terms of the pressure loss, mean heat transfer coefficient, and COP (coefficient of performance). The experimental results showed that the fractal tree-like microchannels had a much higher heat transfer coefficient than that of straight microchannels at the cost of a much higher pump power. And for fractal and straight microchannels, the AR of microchannels was also found to have a relatively large impact on both the pressure loss and heat transfer.     

An experimental investigation to consider thermal methods efficiency on oil recovery enhancement

To study the profound impact of reservoir characteristics on flow regime in two‐phase condition, the effect of saturation and capillary pressure (Pc–S) should be taken into consideration in the porous medium. The purpose of this extensive experimental investigation is to inject the foaming agent and nitrogen gas, which is produced by the foam generator after waterflooding in a fractured reservoir to select the best optimum scenario. It has been elaborated that nitrogen of lower density and lower compressibility would provide a secondary gas cap at the top of the cores which causes to mobilize more oil volume in the unswept zones. The rupturing of foam considerably influences this phenomenon in the high permeable layers that have led the oil of low permeable layers to be mobilized in the presence of nitrogen.     

An experimental investigation of microchannel flow with internal pressure measurements

Experiments have been conducted to investigate discrepancies in previously published data for the pressure drop in microchannels. Straight channel test sections with integrated pressure sensors were developed with channel hydraulic diameters ranging from 25 to 100 μm. Compressible flow results for 6.8 < Re < 18,814 and incompressible flow results for 4.9 < Re < 2068 have been obtained. The results suggest that friction factors for microchannels can be accurately determined from data for standard large channels. The large inconsistencies in previously published data are probably due to instrumentation errors and/or improper accounting for compressibility effects.     

Laminar mixing, heat transfer and pressure drop in tree-like microchannel nets and their application for thermal management in polymer electrolyte fuel cells

The laminar convective heat transfer and pressure drop characteristics in tree-like microchannel nets are numerically investigated and compared to the corresponding characteristics in traditional serpentine flow patterns, by solving the Navier–Stokes and energy equation for an incompressible fluid with constant properties in three dimensions. A constant heat flux is applied to the walls of the square cross-sectional channels. The intrinsic advantage of tree-like nets with respect to both heat transfer and pressure drop is demonstrated. In addition, secondary flow motions initiated at bifurcations and their important role on thermal mixing are identified and discussed. Thermal management issues in polymer electrolyte fuel cells are addressed and in this context, the future employment of tree nets is recommended.     

新型分形树状冷却通道流动特性的研究

对一种新型树状分形冷却通道结构进行了较为系统的研究。引入了长度分形维数D和通道直径分形维数Δ,阐明了它们的意义和各自对通道结构的影响;分析了这种新型树状分形冷却通道结构的几何特征和阻力特性。计算结果表明,在换热面积和入口雷诺数相同的条件下,树状通道的压降相对于传统蛇型冷却通道有着明显的优势,意味着这个新型的换热结构有着巨大的应用潜力。     

An experimental investigation on the confined and elongated bubbles in subcooled flow boiling in a single microchannel

The characteristics of the confined bubble and elongated bubble in subcooled flow boiling in a single horizontal rectangular microchannel with hydraulic diameter Dh =1mm are studied experimentally.The channel with 1×1mm cross section is fabricated in a thin copper plate whose confinement number is Co=2.8 and the degassed deionized water is used as the working fluid.Visualization on the confined and elongated bubbles inside the microchannel is carried out by employing a high-speed CCD camera with a microlens.The recorded images are carefully analyzed to illustrate the behaviors of the confinement and elongation processes of the bubble.The boiling number is used as an adjustable parameter to regulate the operating conditions which is eventually found to take a vital role in the bubble elongation process.Two formation patterns of the confined and elongated bubble are identified and the interactions between the neighboring confined and elongated bubbles are elucidated.     

Efficiency of optimized bifurcating tree-like and parallel microchannel networks in the cooling of electronics

A bifurcating tree-like network consists of a single inlet channel, which bifurcates over several levels to uniformly distributed microchannels that are vertically connected to a second network for fluid return. Here we introduce a one-dimensional model that considers convective heat transfer from the solid into the liquid as well as entrance and mixing effects. The performance of the bifurcating network is compared with that of a parallel microchannel cold plate branching from a single tapered manifold channel in terms of a constant volume flow rate, pressure gradient, and required pumping power. We optimized both networks independently with regard to global boundary conditions for cooling microprocessors and found a significantly superior performance for the parallel channel cooler. For a constant flow rate, the parallel channel network achieves a more than fivefold higher performance coefficient than the bifurcating tree-like network, while almost four times more heat can be removed for a constant pressure gradient across the networks.     

Experimental investigation of titanium nanofluids on the heat pipe thermal efficiency

The enhancement heat transfer of the heat transfer devices can be done by changing the fluid transport properties and flow features of working fluids. In the present study, therefore, the enhancement of heat pipe thermal efficiency with nanofluids is presented. The heat pipe is fabricated from the straight copper tube with the outer diameter and length of 15, 600 mm, respectively. The heat pipe with the de-ionic water, alcohol, and nanofluids (alcohol and nanoparticles) are tested. The titanium nanoparticles with diameter of 21 nm are used in the present study which the mixtures of alcohol and nanoparticles are prepared using an ultrasonic homogenizer. Effects of %charge amount of working fluid, heat pipe tilt angle and %nanoparticles volume concentrations on the thermal efficiency of heat pipe are considered. The nanoparticles have a significant effect on the enhancement of thermal efficiency of heat pipe. The thermal efficiency of heat pipe with the nanofluids is compared with that the based fluid.     

An experimental investigation to augment the efficiency of photovoltaic panels by using longitudinal fins

The temperature of a photovoltaic (PV) panel has a negative effect on the generated power. As the solar irradiance that falls on the PV increases, the operating PV temperature rises, which leads to a decrease in electrical efficiency. Therefore, there arises a need to introduce a cooling system to minimize PV temperature. In this study, the simple passive cooling method of extending its surfaces with fins was used to reduce the PV temperature. Different numbers of longitudinal aluminum fins were attached to the bottom surface of a PV panel and their effects were examined under realistic weather conditions for Baghdad, Iraq. Results show that the use of the passive cooling method under natural convection will be more effective in reducing PV temperature before solar noon than after solar noon. The maximum power enhancement was about 2.5 W and occurred at solar noon when using 10 aluminum fins. The peak efficiency value of the PV panel with fin cooling was about 15.3% against 14% for the unfinned PV panel.     

Effects of minerals on steam distillation during thermal heavy-oil recovery: An experimental investigation

An experimental study was carried out to investigate the role of various clay and non-clay minerals present in reservoir formations on steam distillation process. Dead oil samples (100 g) of two heavy oil reservoirs with 30 g of water and 10 g of crushed rock mixed with different clay minerals were kept under steam pressure with 150, 200, and 250°C in a batch autoclave reactor for a period of 40 hours, and the results were compared with respect to the changes in the density, viscosity and chemical composition of remaining heavy oil. Three different clay minerals (bentonite, kaolinite, and sepiolite) were added to the crushed rock to observe their effects. Among these clay minerals, kaolinite had the greatest effect on steam distillation. Kaolinite has an inert surface compared to other clay minerals which can be considered as an catalytic effect to make easier the evaporation of the volatile components of heavy oil during steam distillation. On the other hand, bentonite which has a swelling property in the presence of water may not allow the entrance of oil molecules because of its low permeability has retard/decrease the evaporation of volatile components. In case of kaolinite addition, density and viscosity of remaining oil are the greatest comparing with the two other minerals added. In addition, the asphaltene content of the remaining oil after distillation increased compared to original oil sample for all added clay minerals.     

Numerical investigation of hydrodynamic and heat transfer performances of nanofluids in a fractal microchannel heat sink

Heat transfer and hydrodynamic performances for nanofluids, Al₂O₃‐water and SiO₂‐water, are numerically investigated with different nanoparticles’ volume fractions and the initial velocities in a fractal microchannel heat sink. The fractal microchannel is 100 μm × 100 μm in the inlet cross‐section, and the length at the 0th level is 2000 μm. A constant heat flux of 500 kW/m² was applied to the bottom wall of the fractal microchannel heat sink. The heat transfer and hydrodynamic performances of different cases are discussed in terms of the mean heat transfer coefficient, mean base temperature, pressure loss, thermal resistance, friction factor f/f₀, and COP/COP₀. Results indicate that increasing the initial velocity and nanoparticles’ volume fraction lead to an enhanced heat transfer at the expense of pressure loss. Al₂O₃‐water has a higher mean heat transfer coefficient and pressure drop than that of SiO₂‐water, a lower f/f₀, mean base temperature, thermal resistance, and COP/COP₀. Ultimately, as compared to pure water, the heat transfer coefficients of 4% Al₂O₃‐water increased by 7.53%, 7.80%, 8.00%, 8.14%, 8.16%, and 8.30%, and the pressure drops increased by 32.09%, 31.41%, 30.81%, 30.05%, 29.21%, and 28.58%, respectively, corresponding to the initial velocities by 4, 5, 6, 7, 8 and 9 m/s.     

Experimental investigation of thermal contact conductance at low temperature based on fractal description

An experimental investigation of thermal contact conductance was conducted with pressed pairs of aluminum alloy 5052 and stainless steel 304 over the low temperature range from 155 to 210 K, with nominal contact pressure from 1 to 7 MPa. The contact surfaces were prepared through bead blasting and characterized with the fractal dimension D and the parameter G of the Weierstrass–Mandelbrot function. The range of fractal dimension is 1.59–1.86 for aluminum and 1.56–1.92 for stainless steel. And the parameter G is in the magnitude of 10⁻⁷ m. From the measured results, thermal contact conductance over this temperature range (155–210 K) is less than that near or above room temperature (T > 300 K). The load sensitivity at low temperature is less than that at room temperature. The smaller fractal dimension D characterizes the rougher surface when G is on the same magnitude and results in the smaller value of the contact conductance and insensitivity to the contact pressure.     

An experimental investigation on the effects of surfactants on the thermal performance of hybrid nanofluids in helical coil heat exchangers

In the present study, the effects of surfactants on the thermal performance of the hybrid nanofluid (Alumina–Silver) at constant wall temperature and laminar flow have been experimentally studied in a helical coil heat exchanger. Different surfactants such as anionic Sodium Dodecyl Sulfate (SDS) and nonionic Poly Vinyl Pyrrolidone (PVP) in the concentration of range of 0.1–0.4 wt.% are employed. It is found that the thermal performance can be maximized by using the 0.2 vol.% hybrid nanofluid and 0.1 wt.% SDS anionic surfactant in the helical coil. The maximum thermal performance in the presence of hybrid Alumina–Silver nanofluid and SDS anionic surfactant is 16% higher than that of the pure distilled water. The presented results can have potential application in process intensification and optimum design of heat exchangers.     

Thermal characteristics of tree-shaped microchannel nets with/without loops

Several tree-shaped microchannel networks with/without loops are numerically examined and compared for application in cooling of electronic components. The physical model of microchannel electronic cooling system is set up with tree-shaped networks. The tree-shaped microchannel nets are embedded in a disk-shaped heat sink, which is attached to a chip to remove the heat dissipated by a chip. The effects of total branching level and loops on the thermal and flow performances of heat sink system are investigated numerically. Results show that tree-shaped nets with loops provide a great advantage when the structure experiences accidental damage to one or more channel segments since the loop assures continuity of coolant flow. Under blockage of some branches, the channel networks only experience an increase of pressure drop while maintaining the capability to remove the heat generated by the chip.     

非对称树状二分岔结构的有效导热系数

采用热电模拟的方法研究了非对称树状二分岔网络结构的有效导热系数,给出了此类结构有效导热系数的解析表达式。结果表明,此类结构的有效导热系数大小与结构本身的非对称率α,分岔处直径的幂律指数p以及分岔级数m有关,并分析了各参数对整个结构有效导热系数的具体影响。结果发现,当幂律指数p=2、非对称率α=1即为对称分岔时,整个结构的有效导热系数取得最大值,且等于组成该结构的材料本身的导热系数。     

Experimental investigation of microchannel coolers for the high heat flux thermal management of GaN-on-SiC semiconductor devices

Experiments on removing high heat fluxes from GaN-on-SiC semiconductor dies using microchannel coolers are described. The dies contain an AlGaN/GaN heterostructure operated as a direct current resistor, providing a localized heat source. The active dimensions of the heat source are sized to represent the spatially-averaged heat flux that would appear in microwave power amplifiers. A wide variety of microchannel materials and configurations are investigated, allowing a comparison of performance and the resulting GaN temperatures. Silicon and AlN microchannel coolers exhibit good performance at lower power densities (1000–1200 W/cm² over 3 × 5 mm² to 2 × 5 mm² active areas). Polycrystalline chemical vapor deposited (CVD) SiC microchannel coolers are found to be extremely promising for higher power densities (3000–4000 W/cm² over 1.2 × 5 mm² active areas with 120 °C GaN temperature). A hybrid microchannel cooler consisting of low-cost CVD diamond on polycrystalline CVD SiC exhibits moderately better performance (20–30%) than polycrystalline CVD SiC alone.     

Performance of serpentine channel based Li-ion battery thermal management system: An experimental investigation

Thermal management of large-scale Li-ion battery packs is of great significance to their safety and life cycle, which would impact their applicability in electric vehicles. Of the many strategies developed for this purpose, indirect liquid cooling has already demonstrated quite high potentials in thermal regulation of such battery systems. In this study, a compact lightweight serpentine wavy channel configuration was chosen to construct an indirect liquid cooling system for a battery module of cylindrical Li-ion cells. The serpentine channel has a number of six internal minichannels. Experimental test data were used to conduct a comprehensive thermal analysis to examine the highest temperature, the maximum temperature difference, and the heat accumulation percentages, and so forth within the battery pack. Results have revealed the ability of the cooling system to maintain the module temperature within appropriate working conditions for electric vehicle applications for most cycling tests including two driving cycles. Furthermore, the analysis insights raised by this study could be useful in understanding the cooling performance of the liquid-based thermal management systems for electric vehicles.     

An experimental study of flow boiling instability in a single microchannel

A simultaneous visualization and measurement study has been carried out to investigate stable and unstable flow boiling phenomena of deionized water in a single microchannel having a hydraulic diameter of 155 µm with a bottom Pyrex glass wall. Fifteen platinum serpentine microheaters, bonded on the Pyrex glass wall, were used to measure local instantaneous wall temperatures. At low mass flux, a syringe pump was used to drive the subcooled water passing through the microchannel. Stable and unstable flow boiling modes in the single microchannel are identified, and flow pattern maps in terms of heat flux and mass flux as well as in term of exit vapor quality are presented respectively. It was found that unstable flow boiling occurred in the single microchannel if the exit vapor quality x_e > 0.013.     

An experimental investigation on frosting characteristics of a microchannel outdoor heat exchanger in an air conditioning heat pump system for electric vehicles

To increase the driving range of electric vehicles in cold climate, air conditioning heat pump (ACHP) system is supposed to be the most effective solution. Working near 0°C with high humidity, the microchannel outdoor heat exchanger (OHX) in system would experience badly frosting process, like traditional residential heat pump system. It would lead to a significant reduction of system performance without defrosting in time. In this article, experimental investigation has been implemented on the frosting process of ACHP system of electric vehicles which is with a microchannel OHX. The phenomenon of frosting distribution was observed, the frosted part on surface shows uneven with various flows paths. The typical frosting characteristics of an outdoor microchannel heat exchanger were also obtained. In a self-designed three-heat exchanger ACHP system, the inlet and outlet refrigerant temperature of OHX as well as the outlet air temperature of system decrease with increasing frosting coverage rate. The frosting phenomenon was analyzed with variation of ambient temperature and humidity. System influence by frosting was also studied with under different ambient conditions. When OHX begins to frost, the heating capacity reduction of system under different ambient conditions were both increased but the differences in the coefficient of performance (COP) variations under different ambient conditions were small as frosting progressed.