Experimental on the liquid cooling system with thermoelectric for personal computer

In the present experimental investigation, the liquid cooling in the micro channel fin heat sink with and without thermoelectric for central processor unit (CPU) of personal computer. The micro channel heat sinks with two different channel height are fabricated from the aluminum with the length, the width and the base thickness of 28, 40, 2?mm respectively. The de-ionized water is used as coolant. Effects of channel height, coolant flow rate, and run condition of PC on the CPU temperature are considered. The liquid cooling in micro-rectangular fin heat sink with thermoelectric is compared with the other cooling techniques. The thermoelectric has a significant effect on the CPU cooling of PC. The experiments are performed at no load and full load conditions within 60?min after steady state, which the mass flow rate are 0.023, 0.017 and 0.01?kg/s. The results heat transfer rate increase with increasing coolant flow rate and higher channel. When comparing with the other cooling system, cooling system with thermoelectric gives the highest efficiency. However, thermoelectric has the high or low heat transfer rate from heat rejected and cooling capacity conditions.     

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.     

Experimental study on convective heat transfer of TiO2 nanofluids

In this study, nanofluids with different TiO₂ nanoparticle concentrations were synthesized and measured in different constant heat fluxes for their heat transfer behavior upon flowing through a vertical pipe. Addition of nanoparticles into the base fluid enhances the forced convective heat transfer coefficient. The results show that the enhancement of the convective heat transfer coefficient in the mixture consisting of ethylene glycol and distilled water is more than distilled water as a base fluid.     

Turbulent forced convection heat transfer of non-Newtonian nanofluids

Forced convection heat transfer of non-Newtonian nanofluids in a circular tube with constant wall temperature under turbulent flow conditions was investigated experimentally. Three types of nanofluids were prepared by dispersing homogeneously γ-Al₂O₃, TiO₂ and CuO nanoparticles into the base fluid. An aqueous solution of carboxymethyl cellulose (CMC) was used as the base fluid. Nanofluids as well as the base fluid show shear-thinning (pseudoplastic) rheological behavior. Results indicate that the convective heat transfer coefficient of nanofluids is higher than that of the base fluid. The enhancement of the convective heat transfer coefficient increases with an increase in the Peclet number and the nanoparticle concentration. The increase in the convective heat transfer coefficient of nanofluids is greater than the increase that would be observed considering strictly the increase in the effective thermal conductivity of nanofluids. Experimental data were compared to heat transfer coefficients predicted using available correlations for purely viscous non-Newtonian fluids. Results show poor agreement between experimental and predicted values. New correlation was proposed to predict successfully Nusselt numbers of non-Newtonian nanofluids as a function of Reynolds and Prandtl numbers.     

Experimental investigation of forced convective heat transfer coefficient in nanofluids of Al2O3/EG and CuO/EG in a double pipe and plate heat exchangers under turbulent flow

Nanofluid is the term applied to a suspension of solid, nanometer-sized particles in conventional fluids; the most prominent features of such fluids include enhanced heat characteristics, such as convective heat transfer coefficient, in comparison to the base fluid without considerable alterations in physical and chemical properties. In this study, nanofluids of aluminum oxide and copper oxide were prepared in ethylene glycol separately. The effect of forced convective heat transfer coefficient in turbulent flow was calculated using a double pipe and plate heat exchangers. Furthermore, we calculated the forced convective heat transfer coefficient of the nanofluids using theoretical correlations in order to compare the results with the experimental data. We also evaluated the effects of particle concentration and operating temperature on the forced convective heat transfer coefficient of the nanofluids. The findings indicate considerable enhancement in convective heat transfer coefficient of the nanofluids as compared to the base fluid, ranging from 2% to 50%. Moreover, the results indicate that with increasing nanoparticles concentration and nanofluid temperature, the convective heat transfer coefficient of nanofluid increases. Our experiments revealed that in lower temperatures, the theoretical and experimental findings coincide; however, in higher temperatures and with increased concentrations of the nanoparticles in ethylene glycol, the two set of results tend to have growing discrepancies.     

The experimental study on flat plate heat pipe of magnetic working fluid

An effective thermal spreader can achieve uniform heat flux distribution and thus enhance heat dissipation of heat sinks. Flat plate heat pipe is one of the highly effective thermal spreaders. Magnetic fluid is liquid and can be moved by the force of magnetic field. Therefore, the magnetic fluid is suitable to be used as the working fluid of flat plate heat pipes which have a very small gap between evaporation and condensation surfaces. We prepared a disk-shaped wickless flat plate heat pipe, and the distance between evaporation and condensation surfaces is only 1 mm. From experimental study, the effect of heat flux and working fluid ratio on the performance of flat plate heat pipe is presented. Also we compared the experimental results between the performance of water and magnetic fluid as working fluids.     

Heat transfer characteristics of a two-phase closed thermosyphons using nanofluids

The heat transfer characteristics 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 heat transfer characteristics of two-phase closed thermosyphon (TPCT) with iron oxide-nanofluids are presented. The TPCT is fabricated from the copper tube with the outer diameter and length of 15, 2000 mm, respectively. The TPCT with the de-ionic water and nanofluids (water and nanoparticles) are tested. The iron oxide nanoparticles with mean diameter of 4-5 nm were obtained by the laser pyrolysis technique and the mixtures of water and nanoparticles are prepared using an ultrasonic homogenizer. Effects of TPCT inclination angle, operating temperature and nanoparticles concentration levels on the heat transfer characteristics of TPCT are considered. The nanoparticles have a significant effect on the enhancement of heat transfer characteristics of TPCT. The heat transfer characteristics of TPCT with the nanofluids are compared with that the based 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.     

Heat transfer enhancement using Al2O3/water nanofluid in a two-phase closed thermosyphon

A two-phase closed thermosyphon (TPCT) is a device for heat transmission. It consists of an evacuated-close tube filled with a certain amount of working fluid. Fluids with nanoparticles (particles smaller than 100 nm) suspended in them are called nanofluids that they have a great potential in heat transfer enhancement. In the present study, we combined two mentioned techniques for heat transfer enhancement. Nanofluids of aqueous Al₂O₃ nanoparticles suspensions were prepared in various volume concentration of 1–3% and used in a TPCT as working media. Experimental results showed that for different input powers, the efficiency of the TPCT increases up to 14.7% when Al₂O₃/water nanofluid was used instead of pure water. Temperature distributions on TPCT confirm these results too.     

Natural convection cooling of a localised heat source at the bottom of a nanofluid-filled enclosure

This article presents a numerical study of natural convection cooling of a heat source embedded on the bottom wall of an enclosure filled with nanofluids. The top and vertical walls of the enclosure are maintained at a relatively low temperature. The transport equations for a Newtonian fluid are solved numerically with a finite volume approach using the SIMPLE algorithm. The influence of pertinent parameters such as Rayleigh number, location and geometry of the heat source, the type of nanofluid and solid volume fraction of nanoparticles on the cooling performance is studied. The results indicate that adding nanoparticles into pure water improves its cooling performance especially at low Rayleigh numbers. The type of nanoparticles and the length and location of the heat source proved to significantly affect the heat source maximum temperature.     

The effect of real viscosity on the heat transfer of water based Al2O3 nanofluids in a two-sided lid-driven differentially heated rectangular cavity

The heat transfer and fluid flow behavior of water based Al₂O₃ nanofluids are numerically investigated inside a two-sided lid-driven differentially heated rectangular cavity. Physical properties which have major effects on the heat transfer of nanofluids such as viscosity and thermal conductivity are experimentally investigated and correlated and subsequently used as input data in the numerical simulation. Transport equations are numerically solved with finite volume approach using SIMPLEC algorithm. It was found that not only the thermal conductivity but also the viscosity of nanofluids has a key role in the heat transfer of nanofluids. The results show that at low Reynolds number, increasing the volume fraction of nanoparticles increases the viscosity and has a deteriorating effect on the heat transfer of nanofluids. At high Reynolds number, the increase in the viscosity is compensated by force convection and the increase in the volume fraction of nanoparticles which results in an increase in heat transfer is in coincidence with experimental results.     

Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under turbulent flow in a helically dimpled tube

An experimental investigation on the convective heat transfer and friction factor characteristics in the plain and helically dimpled tube under turbulent flow with constant heat flux is presented in this work using CuO/water nanofluid as working fluid. The effects of the dimples and nanofluid on the Nusselt number and the friction factor are determined in a circular tube with a fully developed turbulent flow for the Reynolds number in the range between 2500 and 6000. The height of the dimple/protrusion was 0.6 mm. The effect of the inclusion of nanoparticles on heat transfer enhancement, thermal conductivity, viscosity, and pressure loss in the turbulent flow region were investigated. The experiments were performed using helically dimpled tube with CuO/water nanofluid having 0.1%, 0.2% and 0.3% volume concentrations of nanoparticles as working fluid. The experimental results reveal that the use of nanofluids in a helically dimpled tube increases the heat transfer rate with negligible increase in friction factor compared to plain tube. The experimental results showed that the Nusselt number with dimpled tube and nanofluids under turbulent flow is about 19%, 27% and 39% (for 0.1%, 0.2% and 0.3% volume concentrations respectively) higher than the Nusselt number obtained with plain tube and water. The experimental results of isothermal pressure drop for turbulent flow showed that the dimpled tube friction factors were about 2-10% higher than the plain tube. The empirical correlations developed for Nusselt number and friction factor in terms of Reynolds number, pitch ratio and volume concentration fits with the experimental data within ±15%.     

Experimental and theoretical research on capillary limit of micro heat pipe with compound structure of sintered wick on trapezium-grooved substrate

Since heat flux increases sharply yet cooling space in microelectronic and chemical products gradually decreases, a micro heat pipe has been an ideal device for heat transfer for high heat-flux products, and its performance depends largely on its capillary limit. This study proposed an integrated utilization of the advantages of lower backflow resistance to working fluid in trapezium-grooved-wick micro heat pipes and greater capillary force in sintered-wick micro heat pipes; first the factors that are crucial to both types’ heat transfer performances were analyzed, and then mathematical modeling was built for capillary limit of a micro heat pipe with the compound structure of sintered wick on trapezium-grooved substrate, and finally heat transfer limits for micro heat pipes with a trapezium-grooved wick, a sintered wick and with a compound structure were tested through experiments. Both the theoretical analysis and experimental results show that for a micro heat pipe with proposed compound structure, its capillary limit is superior to that of a micro heat pipe with a simplex sintered wick or trapezium-grooved wick.     

Experimental investigation on flow and heat transfer performance of a novel heat fin-plate radiator for electronic cooling

Within the electronics industry, high degree of integration and enhanced performance has led to high heat dissipation electronic devices. This has identified the future development of very high heat flux components. In this paper, a novel and high efficient diffusion welded heat fin-plate radiator (HFPR) was proposed and designed. Various parameters affect the thermal performance of HFPR. The effect of three parameters: the working fluid filling ratios (8% < FR < 70%), the vacuum degrees (0.001 Pa < VD < 0.1 Pa), and the air flow velocities (0.5 m/s < u < 6 m/s) were investigated experimentally. Using distilled water and ethanol as working fluids, a series of tests were carried out to find the influence of the above parameters on steady-state heat transfer characteristics of HFPR. The experimental results indicated that the filling ratio and vacuum degree had a significant influence on thermal performance of HFPR. Also compared with cooling performance using distilled water and ethanol, the HFPR cooling component using distilled water had a stronger heat dissipation capacity for the same filling ratio. The results also can provide a basis for optimal design of HFPR structure.     

Boiling heat transfer characteristics of nanofluids in a flat heat pipe evaporator with micro-grooved heating surface

An experimental study was performed to understand the nucleate boiling heat transfer of water–CuO nanoparticles suspension (nanofluids) at different operating pressures and different nanoparticle mass concentrations. The experimental apparatus is a miniature flat heat pipe (MFHP) with micro-grooved heat transfer surface of its evaporator. The experimental results indicate that the operating pressure has great influence on the nucleate boiling characteristics in the MFHP evaporator. The heat transfer coefficient and the critical heat flux (CHF) of nanofluids increase greatly with decreasing pressure as compared with those of water. The heat transfer coefficient and the CHF of nanofluids can increase about 25% and 50%, respectively, at atmospheric pressure whereas about 100% and 150%, respectively, at the pressure of 7.4 kPa. Nanoparticle mass concentration also has significant influence on the boiling heat transfer and the CHF of nanofluids. The heat transfer coefficient and the CHF increase slowly with the increase of the nanoparticle mass concentration at low concentration conditions. However, when the nanoparticle mass concentration is over 1.0 wt%, the CHF enhancement is close to a constant number and the heat transfer coefficient deteriorates. There exists an optimum mass concentration for nanofluids which corresponds to the maximum heat transfer enhancement and this optimum mass concentration is 1.0 wt% at all test pressures. The experiment confirmed that the boiling heat transfer characteristics of the MFHP evaporator can evidently be strengthened by using water/CuO nanofluids.     

Analysis of Sakiadis flow of nanofluids with viscous dissipation and Newtonian heating

The combined effects of viscous dissipation and Newtonian heating on boundary layer flow over a moving flat plate are investigated for two types of water-based Newtonian nanofluids containing metallic or nonmetallic nanoparticles such as copper (Cu) and titania (TiO₂). The governing partial differential equations are transformed into ordinary differential equations through a similarity transformation and are solved numerically by a Runge-Kutta-Fehlberg method with a shooting technique. The conclusions are that the heat transfer rate at the moving plate surface increases with the increases in the nanoparticle volume fraction and the Newtonian heating, while it decreases with the increase in the Brinkmann number. Moreover, the heat transfer rate at the moving plate surface with Cu-water as the working nanofluid is higher than that with TiO₂-water.     

Sedimentation and convective boiling heat transfer of CuO-water/ethylene glycol nanofluids

The convective boiling characteristics of dilute dispersions of CuO nanoparticles in water/ethylene glycol as a base fluid were studied at different operating conditions of (heat fluxes up to 174 kW m^?2, mass fluxes range of 353–1,059 kg m^?2 s^?1 and sub-cooling level of 343, 353 and 363 K) inside the annular duct. The convective boiling heat transfer coefficients of nanofluids in different concentrations (vol%) of nanoparticles (0.5, 1, and 1.5) were also experimentally quantified. Results demonstrated the significant augmentation of heat transfer coefficient inside the region with forced convection dominant mechanism and deterioration of heat transfer coefficient in region with nucleate boiling dominant heat transfer mechanism. Due to the scale formation around the heating section, fouling resistance was also experimentally measured. Experimental data showed that with increasing the heat and mass fluxes, the heat transfer coefficient and fouling resistance dramatically increase and rate of bubble formation clearly increases. Obtained results were then compared to some well-known correlations. Results of these comparisons demonstrated that experimental results represent the good agreement with those of obtained by the correlations. Consequently, Chen correlation is recommended for estimating the convective flow boiling heat transfer coefficient of dilute CuO-water/ethylene glycol based nanofluids.     

共线式热电发电机在侧面散热情况下的性能

热电材料是一种环境友好型功能材料,其可以实现热能与电能的相互转化,在热电发电、热电制冷中具有许多应用.传统的热电发电机为$\pi$型结构,要求热电腿的长度相等,在某些情况该结构不利于热电发电机的优化设计.热电发电机在高温工况下会引起强烈的热应力甚至应力集中,从而缩短了其工作寿命.另外,热电发电机的工作温度于环境温度,这样必然会有一部分热量散失到环境中,从而影响热电发电机的性能.针对该现象,本文建立了考虑散热的新型共线式热电发电机模型,该模型的热电腿可以独立进行优化,基于有限元方法,对考虑侧面散热的共线式热电发电机进行了仿真模拟,分析了其在狄利克雷边界条件下的热电性能和力学性能,得到了热电发电机的温度场、电势场、应力场,探究了不同强度的对流散热系数对热电发电机热电性能和力学性能的影响.结果表明,对流散热会降低热电发电机的能量转化效率,当对流换热系数达到~100W/(m$^{2}\cdot$\textcelsius) 时,效率为~0.0479,该值比绝热状态的转化效率0.066 7 低28%.对流散热使热电发电机侧面热损失增加,降低了热应力.在实际应用中,应合理优化设计隔热系统,提高能量的转化效率.      

An analytical solution to non-axisymmetric heat transfer with viscous dissipation for non-Newtonian fluids in laminar forced flow

Forced convection heat transfer in a non-Newtonian fluid flow inside a pipe whose external surface is subjected to non-axisymmetric heat loads is investigated analytically. Fully developed laminar velocity distributions obtained by a power-law fluid rheology model are used, and viscous dissipation is taken into account. The effect of axial heat conduction is considered negligible. The physical properties are assumed to be constant. We consider that the smooth change in the velocity distribution inside the pipe is piecewise constant. The theoretical analysis of the heat transfer is performed by using an integral transform technique – Vodicka’s method. An important feature of this approach is that it permits an arbitrary distribution of the surrounding medium temperature and an arbitrary velocity distribution of the fluid. This technique is verified by a comparison with the existing results. The effects of the Brinkman number and rheological properties on the distribution of the local Nusselt number are shown.     

Nucleate boiling heat transfer in aqueous solutions with carbon nanotubes up to critical heat fluxes

In this study, pool boiling heat transfer coefficients (HTCs) and critical heat fluxes (CHFs) are measured on a smooth square flat copper heater in a pool of pure water with and without carbon nanotubes (CNTs) dispersed at 60 °C. Tested aqueous nanofluids are prepared using multi-walled CNTs whose volume concentrations are 0.0001%, 0.001%, 0.01%, and 0.05%. For the dispersion of CNTs, polyvinyl pyrrolidone polymer is used in distilled water. Pool boiling HTCs are taken from 10 kW/m² to critical heat flux for all tested fluids. Test results show that the pool boiling HTCs of the aqueous solutions with CNTs are lower than those of pure water in the entire nucleate boiling regime. On the other hand, critical heat flux of the aqueous solution is enhanced greatly showing up to 200% increase at the CNT concentration of 0.001% as compared to that of pure water. This is related to the change in surface characteristics by the deposition of CNTs. This deposition makes a thin CNT layer on the surface and the active nucleation sites of the surface are decreased due to this layer. The thin CNT layer acts as the thermal resistance and also decreases the bubble generation rate resulting in a decrease in pool boiling HTCs. The same layer, however, decreases the contact angle on the test surface and extends the nucleate boiling regime to very high heat fluxes and reduces the formation of large vapor canopy at near CHF. Thus, a significant increase in CHF results in.