Application of nanofluids to a heat pipe liquid-block and the thermoelectric cooling of electronic equipment

Microprocessor power dissipation is constantly increasing. An increase in microprocessor size has also resulted in higher heat fluxes. The growth of information technology has rapidly increased over the past few years, causing an increase in the demand for a microprocessor that has a very high computing ability. The previous generation of central processing units (CPU) had 1.17 billion transistors planted in it, which indicates that a significant amount of heat was generated. The total heat dissipation resulting from a high end CPU is approximately 110-140 W, which will increase if the CPU voltage and frequency increase. Conventional air-cooled cooling systems are no longer adequate to remove these heat fluxes. For a number of applications, direct air-cooling systems will have to be replaced or enhanced by other high performance compact cooling techniques. In this study, the application of nanofluids as the working fluid on a heat pipe liquid-block combined with thermoelectric cooling is investigated. The type and effect of volume concentrations of nanofluids, coolant temperature, and thermoelectricsystem as heat pumps of a PC on the CPU’s temperature are considered. The results obtained from this technique are compared to those from other conventional cooling techniques. The heat pipe liquid-block combined with the thermoelectric system has a significant effect on heat transfer from the CPU. The higher thermal performance heat pipe liquid-block and thermoelectric cooled system with nanofluids proved its potential as a working fluid.     

Numerical simulation of heat transfer in a micro channel heat sinks using nanofluids

In this study, a numerical simulation of copper microchannel heatsink (MCHS) using nanofluids as coolants is presented. The nanofluid is a mixture of pure water and nanoscale metallic or nonmetallic particles with various volume fractions. Also, the effects of various volume fractions, volumetric flow rate and various materials of nanoparticles on the performance of MCHS have been developed. A three-dimensional computational fluid dynamics model was developed using the commercial software package FLUENT, to investigate the conjugate fluid flow and heat transfer phenomena in micro channel heatsinks. The results show that the cooling performance of a microchannel heat sink with water based nanofluid containing Al₂O₃ (vol 8%) is enhanced by about 4.5% compared with micro channel heatsink with pure water. Nanofluids reduce both the thermal resistance and the temperature difference between the top (heated) surface of the MCHS and inlet nanofluid compared with that pure water. The cooling performance of a micro channel heat sink with metal nanofluids improves compared with that of a micro channel heat sink with oxide metal nanofluids because the thermal conductivity of metal nanofluid is higher than oxide metal nanofluids. Micro channel heat sinks with nanofluids are expected to be good candidates as the next generation cooling devices for removing ultra high heat flux.     

Integrated microthermoelectric cooler for microfluidic channels

Precise fluid temperature control in microfluidic channels is a requirement for many lab-on-a-chip and microreactor devices, especially in biotechnology where most processes are highly temperature sensitive. We demonstrate the concept of a microthermoelectric cooler integrated into a microfluidic channel in order to give rapid and localised fluid cooling. The key aspect of this concept is the use of a second imbedded microfluidic channel that is used as a miniature heat sink. An analytical thermal model has been derived that couples thermoelectric effects with fluid heat-transfer rates from both the hot and cold connections. Using this model, the effect on cooling performance of varying the thermal resistance between the hot and cold connections and the fluid has been quantified, as well as the effect of substrate thermal conductivity. If the substrate thermal conductivity is too high, heat leakage renders the thermoelectric cooler ineffective. The optimum electrical current for cooling has been shown to be a function of the thermal resistance of the heat sink. For thermoelectric coolers there is competition between temperature reduction and cooling power. Using this fact, based on the final fluid temperature required, we have calculated the maximum flow rate that will achieve this. Finally, a prototype integrated microthermoelectric cooler has been fabricated and tested.     

Correlation for the fin Nusselt number of natural convective heat sinks with vertically oriented plate-fins

In this paper, a correlation for estimating the fin Nusselt number of natural convective heat sinks with vertically oriented plate-fins is suggested. For this purpose, extensive experimental investigations are performed for various channel widths, heights and input powers. A numerical simulation is conducted by using a commercial tool to verify the present experimental results and examine fluid flow and heat transfer characteristics of a natural convective heat sink. By comparison of the present study and the previous studies, it is shown that the present correlation is widely applicable for Elenbaas numbers between 0.5 and 2 × 10⁶ and is more accurate than previous correlations. Based on an asymptotic approach, it is found that the optimal channel width is independent of the heat sink height but dependent of the heat sink length, the difference between the heat sink base and the ambient temperatures, and fluid property.     

Experimental evaluation of cooling performance of circular heat sinks for heat dissipation from electronic chips using nanofluid

In this work, an experimental investigation on cooling performance of using nanofluid to replace the pure water as the coolant in a minichannel heat sink is conducted. The heat sink comprises of four circular channels with hydraulic diameter of 6 mm. Thermal and hydraulic performances of the nanofluid cooled minichannel heat sink are evaluated from the results obtained for the Nusselt number, friction factor, thermal resistance and pumping power, with the volume flow rate ranging from 0.3 to 1.5 L/min. The experimental results show that the nanofluid cooled heat sink outperforms the water-cooled one, having significantly higher average heat transfer coefficient. Despite the marked increase in dynamic viscosity due to dispersing the nanoparticles in water, the friction factor for the nanofluid-cooled heat sink is found slightly increased only.     

Numerical simulation for fin effect of a rectangular latent heat storage vessel packed with molten salt under heat release process

The present numerical study has dealt with the enhancement of latent heat Release by using plate type fins mounted on the vertical cooling surface in the rectangular vessel packed with molten salt as a latent heat storage material. It was found that the fin thickness and pitch exerted an influence on solidification heat transfer in a liquid layer of a nitric molten salt. The numerical results elucidated the flow pattern, velocity profile and heat transfer rate in the melted liquid layer.     

Parametric study on heat transfer enhancement and pressure drop of an internal blade tip-wall with pin-fin arrays

One way to cool gas turbine tips is to design serpentine passages with 180° turns inside the blades to fully utilize the coolant potential. It is therefore a desire to improve the cooling of the blade tips to ensure a long durability and safe operation. In the present work, a two-pass channel with a 180° turn and various arrays of pin-fins mounted internally on the tip-cap is considered. The effects of pin-fin height, diameter and pitches on the heat transfer enhancement and pressure drop are investigated numerically. The nominal ratio of height to diameter (H/D) of the pin-fins is 2, and the ratio of tip clearance to pin-fin height is about 10. The inlet Reynolds numbers based on hydraulic diameter are ranging from 100,000 to 600,000. Details of the three dimensional fluid flow and heat transfer over the pin-finned tips are presented. The overall performances of various tips are compared. It is found that due to the combination of turning, impingement and pin-fin crossflow, the heat transfer coefficient of the pin-finned tips is up to a factor of 2.1 higher than that of the smooth tip. This augmentation is achieved at the expense of a penalty of pressure drop around 30%. Results show that the magnitude of the heat transfer enhancement depends upon pin-fin configuration and arrangement. It is suggested that pin-fins are suitable to enhance the blade tip heat transfer and thus to improve the tip cooling.     

The microtube heat sink with tangential impingement jet and variable fluid properties

This paper presents the numerical investigation of the microtube heat sink with impingement jet feeding. The inlet channel covers only the quarter of the tube perimeter so the swirl flow is settled in the tubes and the heat transfer between the liquid flow and silicon substrate is improved. The water with the variable physical properties is used as the working fluid and laminar flow regime is considered. The proposed microtube heat sink with impingement jet feeding is compared with classic microtube heat sink in terms of temperature variation along the heated surface and temperature difference. The influence of the temperature dependent physical properties on the fluid flow and heat transfer is analyzed.     

Experimental investigation of flow and heat transfer for the ethanol-water solution and FC-72 in rectangular microchannels

Rapid development of super scale integration circuit (IC) provides unprecedented challenge to thermal control for aviation electronic equipments. To solve the problem of cooling electronic chips and devices for aircraft avionics, this paper experimentally investigated the characteristics of single-phase forced convection heat transfer and flow resistance in rectangular microchannels with two liquid coolants. One was 30% of ethanol–water solution, the most commonly used coolant in aviation. The other was FC-72, the latest coolant for electronic equipments. Based on the experimental data collected and those available in the open literature, comparisons and analyses were carried out to evaluate the influences of liquid velocity, supercooling temperature, microchannel structures and wall temperature etc. on the heat transfer behaviors. And the correlations of flow resistance and heat transfer characteristics were provided for the ethanol–water solution and FC-72 respectively. The results indicate transition from laminar to turbulent flow occurs at the Reynolds number of 750–1,250 for FC-72, and the behaviors of flow and heat transfer in rectangular microchannels strongly depend on the kind of coolant and geometric configuration of microchannels.     

Influence of heating load on heat transfer characteristics in micro-pin-fin arrays

Experimental investigations were carried out to explore the convective heat transfer in micro pin-fins with different aspect ratios, and the influence of heating load on Nusselt numbers in micro pin-fins with liquid water as working fluid were investigated. The mechanism of convective heat transfer in micro pin-fins at different heating load were studied by 3-D numerical investigations, and the relationships of thermal physical properties change, the end wall effect and axial thermal conduction with Nu numbers in micro pin-fins were analysed. It was found that the thickness of boundary layer was decreased as much as 33.3 % attributed to the destructive effect of thermal physical properties change, and convective heat transfer in the micro pin-fin channel was more than 20 % enhanced by the flow disturbance caused by the increase of temperature difference. The discrepancy of Nu in micro pin-fin channel with different aspect ratios reached 34.59 %, and this discrepancy was reduced by the increase of heating load. The maximum value of impact factors of dynamic viscosity and thermal conductivity on the Nu in micro-pin-fins reached 25.02 and 7.68 %, respectively.     

Flow and coupled heat transfer in the cavity between the rotor and stator

The turbulent flow and coupled heat transfer in the cavity between the rotor and stator is numerically simulated. Reynolds-averaged Navier-Stokes equations closed with equations of the k-ɛ turbulence model are used to calculate the viscous compressible gas flow characteristics and heat transfer; the unsteady heat conduction equation is used to calculate the temperature field in the metal. The influence of the mass flow rate of the coolant on the flow structure and efficiency of cooling of the rotor and stator walls is studied. The calculated results are compared with experimental data.     

Interaction of surface radiation with conjugate mixed convection from a vertical channel with multiple discrete heat sources

Important results of a numerical study performed on combined conduction–mixed convection–surface radiation from a vertical channel equipped with three identical flush-mounted discrete heat sources in its left wall are provided here. The channel has walls of identical height with the spacing varied by varying its aspect ratio (AR). The cooling medium is air that is considered to be radiatively transparent. The heat generated in the channel gets conducted along its walls before getting dissipated by mixed convection and radiation. The governing equations for fluid flow and heat transfer are considered without boundary layer approximations and are transformed into vorticity–stream function form and are later normalized. The resulting equations are solved, along with relevant boundary conditions, making use of the finite volume method. The computer code written for the purpose is validated both for fluid flow and heat transfer results with those available in the literature. Detailed parametric studies have been performed and the effects of modified Richardson number, surface emissivity, thermal conductivity and AR on various pertinent results have been looked into. The significance of radiation in various regimes of mixed convection has been elucidated. The relative contributions of mixed convection and radiation in carrying the mandated cooling load have been thoroughly explored.     

Experimental investigation of heat transfer and flow pattern from heated horizontal rectangular fin array under natural convection

This paper presents results of the experimental study conducted on heated horizontal rectangular fin array under natural convection. The temperature mapping and the prediction of the flow patterns over the fin array with variable fin spacing is carried out. Dimensionless fin spacing to height (S/H) ratio is varied from 0.05 to 0.3 and length to height ratio (L/H) = 5 is kept constant. The heater input to the fin array assembly is varied from 25 to 100 W. The single chimney flow pattern is observed from 8 to 12 mm fin spacing. The end flow is choked below 6 mm fin spacing. The single chimney flow pattern changes to sliding or end flow choking at 6 mm fin spacing. The average heat transfer coefficient (h_a) is very small (2.52–5.78 W/m² K) at 100 W for S = 5–12 mm. The h_a is very small (1.12–1.8 W/m² K) at 100 W for 2–4 mm fin spacing due to choked fin array end condition. The end flow is not sufficient to reach up to central portion of fin array and in the middle portion there is an unsteady down and up flow pattern resulting in sliding chimney. The central bottom portion of fin array channel does not contribute much in heat dissipation for S = 2–4 mm. The h_a has significantly improved at higher spacing as compared to lower spacing region. The single chimney flow pattern is preferred from heat transfer point of view. The optimum spacing is confirmed in the range of 8–10 mm. The average heat transfer results are compared with previous literature and showed similar trend and satisfactory agreement. An empirical equation has been proposed to correlate the average Nusselt number as a function of Grashof number and fin spacing to height ratio. The average error for this equation is ?0.32 %.     

Experimental studies on heat and mass transfer performance of a coiled tube absorber for R134a-DMAC based absorption cooling system

Absorber is an important component in vapor absorption refrigeration system and its performance has greater influence in overall efficiency of absorption machines. Falling film heat and mass transfer in an absorber is greatly influenced by fluid properties, geometry of heat exchanger and its operating parameters. This paper presents on the results of experimental studies on the heat and mass transfer characteristics of a coiled tube falling film absorber, using 1,1,1,2-Tetrafluroethane(R-134a) and N-N Dimethyl Acetamide (DMAC) as working fluids. The effects of film Reynolds number, inlet solution temperature and cooling water temperature on absorber heat load, over all heat transfer coefficient and mass of refrigerant absorbed are presented and discussed. Normalized solution and coolant temperature profiles and refrigerant mass absorbed along the height of absorber are also observed from the experimental results. The optimum over all heat transfer coefficient for R-134a–DMAC solution found to be 726 W/m²K for a film Reynolds number of 350. The R-134a vapour absorption rate is maximum in the normalized coil height of 0.6 to 1.     

高超声速溢流冷却实验研究

高超声速溢流冷却是一种新型的飞行器热防护方法,基本思想为:在高热流区布置溢流孔,控制冷却液以溢流方式流出,之后通过飞行器表面摩阻作用展布为液膜,形成热缓冲层以降低飞行器表面热流. 目前,溢流冷却技术还处于探索阶段,实现工程应用前还需开展大量的实验验证和机理研究工作. 本文首次开展溢流冷却的实验研究工作,采用热流测量、液膜厚度测量及液膜流动特性观测技术,搭建了完善的溢流冷却风洞实验平台,对溢流冷却热防护性能和高超声速条件下液膜流动规律进行了初步研究. 研究表明:(1) 高超声速流场中通过溢流能够在飞行器表面形成液膜并有效隔离外部高温气流,可降低飞行器表面热流率;(2) 楔面上的液膜前缘流动是一个逐渐减速的过程,增加冷却液流量液膜厚度变化不明显,但液膜前缘运动速度增大;(3) 液膜层存在表面波,在时间和空间方向发生演化,导致液膜厚度的微弱扰动;(4) 液膜层存在横向展宽现象,即液膜层宽度大于溢流缝宽度. 原因是液膜层与流场边界层条件不匹配,存在压力梯度,迫使冷却液向低压区流动,从而展宽液膜层,并且流量越高,横向展宽现象越明显.      

Effect of non-uniform heat source on MHD heat transfer in a liquid film over an unsteady stretching sheet

An analysis has been carried out to study the magnetohydrodynamic boundary layer flow and heat transfer characteristics of a laminar liquid film over a flat impermeable stretching sheet in the presence of a non-uniform heat source/sink. The basic unsteady boundary layer equations governing the flow and heat transfer are in the form of partial differential equations. These equations are converted to non-linear ordinary differential equations using similarity transformation. Numerical solutions of the resulting boundary value problem are obtained by the efficient shooting technique. The effects of magnetic and the non-uniform heat source/sink parameters on the dynamics are discussed. Findings of the paper reveal that non-uniform heat sinks are better suited for effective cooling of the stretching sheet. Skin friction coefficient and the local Nusselt number are also explored for typical values of magnetic and non-uniform heat source/sink parameters. The results are in excellent agreement with the earlier published works, under some limiting cases.     

Effects of varying Reynolds number, blowing ratio, and internal geometry on trailing edge cutback film cooling

Three-dimensional mean velocity and concentration fields have been measured for a water flow in a pressure side cutback trailing edge film cooling geometry consisting of rectangular film cooling slots separated by tapered lands. Three-component mean velocities were measured with conventional magnetic resonance velocimetry, while time-averaged concentration distributions were measured with a magnetic resonance concentration technique for flow at two Reynolds numbers (Re) differing by a factor of 2, three blowing ratios, and with and without an internal pin fin array in the coolant feed channel. The results show that the flows are essentially independent of Re for the regime tested in terms of the film cooling surface effectiveness, normalized velocity profiles, and normalized mean streamwise vorticity. Blowing ratio changes had a larger effect, with higher blowing ratios resulting in surface effectiveness improvements at downstream locations. The addition of a pin fin array within the slot feed channel made the spanwise distribution of coolant at the surface more uniform. Results are compared with transonic experiments in air at realistic density ratios described by Holloway et al. (2002a).     

开式热沉内冷刀具的设计及其导热性能分析

车削加工温度对工件的表面加工质量和刀具的使用寿命具有重要影响. 设计了一种开式热沉内冷刀具,计算了在实际加工工艺参数下刀具受到的切削力和前刀面上的热流密度,分析了刀具的结构强度;建立了刀具热-流-固耦合温度场模型,探讨了热稳态条件下刀具的温度场分布,以及刀片冷却液流道内热沉数量对刀具导热性能的影响规律,比较了在相同热源条件下开式热沉内冷刀具与其他内冷刀具的导热性能. 结果表明:对于刀片材料为硬质合金YT5的刀具,在热流密度为10 W/mm2的条件下,内置6个热沉的设计方案可获得最佳冷却效果,刀具的最高切削温度控制为187.1 ℃;与其他内冷刀具相比,开式热沉内冷刀具的最高切削温度降低了12.1 ℃.      

Numerical modeling of pin–fin micro heat exchangers

A micro heat exchanger (MHE) can effectively control the temperature of surfaces in high heat flux applications. In this study, several turbulence models are analyzed using a 3D finite element model of a MHE. The MHE consists of a narrow planar flow passage between flat parallel plates with small cylindrical pin fins spanning these walls. The pin fin array geometry investigated is staggered, with pin diameters of 0.5, 5.1 and 8.5 mm, height to diameter ratio of 1.0 and streamwise (longitudinal) and spanwise (transverse) to diameter ratios of 1.5 and 2.5, respectively. Pressure loss and heat transfer simulated results for 4,000 ≤ Re ≤ 50,000 are reported and compared with previously published numerical and experimental results. It was found that the flat micro pin fin overall thermal performance always exceeds that of the parallel plate counterpart (smooth channel) by a factor of as much as 2.2 for the 8.5 mm diameter pins, and by 4 for the 0.5 mm diameter pins in the investigated Reynolds number range. Further, among the six turbulence models investigated, the RNG model tends to be the best model to predict both the Nusselt number and the friction factor and capture the main feature of the flow field in MHE.     

Quantitative infrared investigation of local heat transfer in a circular finned tube heat exchanger assembly

This work deals with the local heat transfer coefficient evaluation over the fin of the second row of a staggered circular finned tube heat exchanger assembly. The coefficient distribution is determined by using a transient technique and by calculating the energy balance during the fin cooling. The calculation model takes into account radiation with the surrounding and lateral heat conduction into the material. The method uses infrared measurements and integration between time bounds that depend on space. It is proposed to choose the integration bounds with an original criterion based on local heat transfer. Validation is performed on the reference case consisting in a thin plate in an aerodynamically and thermally developing channel flow. Then, distributions of Nusselt number on the circular fin are presented for several Reynolds numbers. The high resolution of the whole method and set-up allow detecting thermal imprints of developing horseshoe vortices. These imprints are analyzed by following their angular evolution around the tube.