Local heat transfer process and pressure drop in a micro-channel integrated with arrays of temperature and pressure sensors

One of the most important components in micro-fluidic system is the micro-channel which involves complicated flow and transport process. This study presents micro-scale thermal fluid transport process inside a micro-channel with a height of 37 μm. The channel can be heated on the bottom wall and is integrated with arrays of pressure and temperature sensors which can be used to measure and determine the local heat transfer and pressure drop. A more simplified model with modification of Young’s Modulus from the experimental test is used to design and fabricate the arrays of pressure sensors. Both the pressure sensors and the channel wall use polymer materials which greatly simplify the fabrication process. In addition, the polymer materials have a very low thermal conductivity which significantly reduces the heat loss from the channel to the ambient that the local heat transfer can be accurately measured. The air flow in the micro-channel can readily become compressible even at a very low Reynolds number condition. Therefore, simultaneous measurement of both the local pressure drop and the temperature on the heated wall is required to determine the local heat transfer. Comparison of the local heat transfer for a compressible air flow in micro-channel is made with the theoretical prediction based on incompressible air flow in large-scale channel. The comparison has clarified many of the conflicting results among different works.     

Acoustically generated flows in microchannel flexural plate wave sensors: Effects of compressibility

Acoustically generated flowfields in flexural plate wave sensors filled with a Newtonian liquid (water) are considered. A computational model based on compressible flow is developed for the sensor with a moving wall for pumping and mixing applications in microchannels. For the compressible flow formulation, an isothermal equation of state for water is employed. The velocity and pressure profiles for different parameters including flexural wall frequency, channel height, amplitude of the wave and wave length are investigated for four microchannel height/length geometries. It is found that the flowfield becomes pseudo-steady after sufficient number of flexural cycles. Both instantaneous and time averaged results show that an evanescent wave is generated in the microchannel. The predicted flows generated by the FPWs are compared with results available in the literature. The proposed device can be exploited to integrate micropumps with complex microfluidic chips improving the portability of micro-total-analysis systems.     

A multilayered microfluidic system with functions for local electrical and thermal measurements

Microfluidic systems have been extensively applied in research of chemistry, biology and fluidic dynamics. In these applications, local and precise measurements are often crucial for reliable results. We demonstrate here a multilayered, multifunctional microfluidic platform with embedded electrodes open to the microchannel and thermocouple sensors underneath the microchannel that are suitable for local electrical and thermal measurements, respectively. We demonstrate that precise transport measurements with ac excitation frequency up to 1 MHz can be performed for electrolytes in centimeter-long microchannels. Local temperature sensing of the fluids in the microchannels can also be performed on this system. Such system can be either used to characterize local electrical and thermal properties of fluids, or applied to the study of thermal related electrokinetic phenomena, such as joule heat generation in dc conductance or temperature dependence of electrical transport.     

微通道换热器的数值模拟和结构优化

为提高微通道换热器的换热效率,利用COMSOL耦合求解流动和传热方程,对微通道换热器换热特征进行数值模拟.通过分析微通道换热器的温度、微通道的入口与出口的压差以及微通道换热器的总热阻等参数,对其换热性能进行评估.优化微换热器的几何结构可以有效提高换热性能.数值模拟结果表明：当微通道的高宽比为0.8、微通道与间隔的宽度比为0.6、微通道数为71时热阻最小,换热性能最佳.     

Seed bubbles trigger boiling heat transfer in silicon microchannels

The smooth channel surface of microsystems delays boiling incipience in heated microchannels. In this paper, we use seed bubbles to trigger boiling heat transfer and control thermal non-equilibrium of liquid and vapor phases in parallel microchannels. The test section consisted of a top glass cover and a silicon substrate. Microheater array was integrated at the top glass cover surface and driven by a pulse voltage signal to generate seed bubbles in time sequence. Each microheater corresponds to a specific microchannel and is located in the microchannel upstream. Five triangular microchannels with a hydraulic diameter of 100 μm and a length of 12.0 mm were etched in the silicon substrate. A thin platinum film was deposited at the back surface of silicon chip with an effective heating area of 4,500 × 1,366 μm, acting as the main heater for the heat transfer system. Acetone liquid was used. With the data range reported here, boiling incipience was not initiated if wall superheats are smaller than 15°C without seed bubbles assisted. Injection seed bubbles triggers boiling incipience and controls thermal non-equilibrium between liquid and vapor phases successfully. Four modes of flow and heat transfer are identified. Modes 1, 2, and 4 are the stable ones without apparent oscillations of pressure drops and heating surface temperatures, and mode 3 displays flow instabilities with apparent amplitudes and long periods of these parameters. The four modes are divided based on the four types of flow patterns observed in microchannels. Seed bubble frequency is a key factor to influence the heat transfer. The higher the seed bubble frequency, the more decreased non-equilibrium between two phases and heating surface temperatures are. The seed bubble frequency can reach a saturation value, at which heat transfer enhancement attains the maximum degree, inferring that a complete thermal equilibrium of two phases is approached. The saturation frequency is about a couple of thousand Hertz in this study.     

Size effect on heat transfer in micro gas sensors

Gas gap is usually used as an important thermal insulation in micro gas sensors to reduce the heating power. The heat transport through the gap consists of two parts, heat conduction by air and thermal radiation between surfaces. It is usually regarded that thermal radiation through the gap is negligible compared with conductive heat transfer by air. This work investigates the heat transport by thermal radiation and heat conduction through a broad size range of gas gaps from one nanometer to dozens of micrometers. The result shows that thermal radiation is the major way of heat transfer when the gap is less than 20 nm, which will result in unexpected high energy consumption in the process of minimization. The equivalent thermal conductivity of thermal radiation is computed and a partition map is depicted to demonstrate the relative importance of radiation and conduction on different gap scales under dissimilar surface temperatures. A practical gas sensor heated by a micro hotplate (MHP) is thermally analyzed. The calculation shows that extra energy consumption comes forth as the gap distance reduces to several tens of nanometers.     

Suspended bimaterial microchannel resonators for thermal sensing of local heat generation in liquid

Suspended bimaterial microchannel resonator devices have been fabricated to measure the thermal behaviors of small biological molecules and individual cells in liquid. A resonant microbridge structure embeds this microfluidic channel in its interior. The fabrication process is based on the creation of buried channels in silicon-on-insulator wafers. For the bimaterial bridge structure layers of SiO₂ and SiN_x were used. This bimaterial resonant bridge with internal microfluidic channel could be employed as a very sensitive calorimeter, since the tensile stress generated by bimaterial effect in the heated bridge, produces a shift of resonant frequency. A laser beam was used to heat the center of the bridge resonator with the microchannel filled by water and the corresponding resonant frequency variations were evaluated. The measured sensitivity for the local heat at the center of the bridge is 8.6 ppm/μW in atmospheric condition.     

V型微通道热沉的 流体流动与传热问题研究

V型微通道热沉具有体积小、流速小、散热效率高等优点,是将多个DL线阵组装为面阵并实现高性能冷却封装的良好解决方案.本文采用计算流体力学软件Fluent建立了V型微通道的数值模型,研究了其中的流体流动与传热问题.仿真结果表明,设计的V型微通道可满足激光二极管线阵的散热要求.仿真分析结果与V型微通道热沉样品的模拟热源加载实验测试数据对比,吻合较好,证明了数值仿真的有效性.     

Pressure loss in constriction microchannels

Constriction devices contain elements inserted into the fluid stream, which change the local streamwise flow area. One such element is the orifice-like obstruction with sharp corners, a back-to-back abrupt contraction and expansion, which could trigger flow separation. A series of microchannels, 40 μm × 1 μm × 4000 μm in nominal dimensions, with constriction elements at the centers of the channels has been fabricated using standard micromachining techniques. The channel widths at the constriction sections varied from 10 μm to 34 μm, with pressure sensors integrated in each channel. Nitrogen gas was passed through the microdevices under inlet pressure up to 50 psi. The mass flow rates were measured for all the devices as a function of the pressure drop. A monotonic decrease of the flow rate with decreasing constriction-gap width was observed. The pressure distribution along the microchannel with the smallest constriction gap showed a pressure drop across the constriction element. Both mass flow rate and pressure measurements indicate that flow separation from the constriction sharp corners could occur     

Numerical analysis of heat transfer in a manifold microchannel heat sink with high efficient copper heat spreader

A manifold microchannel heat sink integrated with a high efficient copper heat spreader is presented. A series analysis of three-dimensional fluid flow and heat transfer performance in this mirochannel heat sink and conventional structure are performed by CFX commercial software package. The temperature difference along the flow direction in the new microchannel heat sink is less than that of the conventional microchannel heat sink due to effect of the transverse channel arrays. The maximum heat flux input of the new microchannel heat sink increases 75% more than the conventional structure with the flow rate of 1 m/s. The new design has better heat transfer characteristics than conventional one for the full range of flow rates considered.     

Fabrication, Assembly, and Testing of Cu- and Al-Based Microchannel Heat Exchangers

Metal-based microchannel heat exchangers (MHEs) are of current interest due to the combination of high heat transfer performance and improved mechanical integrity. Efficient methods for fabrication and assembly of functional metal-based MHEs are essential to ensure the economic viability of such devices. In this paper, the results on fabrication, assembly, and heat transfer testing of Cu- and Al-based MHE prototypes are reported. Efficient fabrication of Cu- and Al-based high-aspect-ratio microscale structures (HARMSs) has been achieved through molding replication using surface-engineered metallic mold inserts. Replicated metallic HARMSs were assembled through eutectic bonding to form entirely Cu- and Al-based MHE prototypes, on which heat transfer tests were conducted to determine the average rate of heat transfer from electrically heated Cu blocks placed outside the MHEs to water flowing within the molding replicated microchannel arrays. Experimentally observed heat transfer rates are higher as compared to those from previous studies on microchannel devices with similar geometries. The potential influence of microchannel surface profile on heat transfer rates is discussed. The present results illustrate the potential of metal-based MHEs in wide-ranging applications.$hfill$[2007-0170]      

Computational fluid dynamics for effects of coolants on on-chip cooling capability with carbon nanotube micro-fin architectures

Carbon nanotubes (CNTs) have shown a broad promising application in high mechanical strength and electronic structure. In this work, the effects of coolants on heat transfer capability of on-chip cooling with CNTs Micro-fin Architectures was studied, and the two-dimensional computational fluid dynamics (CFD) simulations have been done for a series of material parameters of coolants in this paper. The influences of thermal conductivity, density, specific heat and viscosity on cooling have been obtained in the case studies. The results demonstrate that pressure drop between the inlet and outlet of the cooling device is dependent on coolant’s density and viscosity. Consequently, it will be necessary to find out a good balance between heat transfer capability and pressure drop. The simulation results also indicate that the heat sink capability will be better if there are more fin rows in the microchannel.     

Hydrodynamics in deformable microchannels

We report experimental investigations into deformable rectangular microchannels using pressure drop, wall deformation and microparticle image velocimetry analysis. The available theoretical framework for the deformation of the wall is reviewed, and a physical explanation is obtained for the non-dimensional deformable parameter α through scaling analysis based on the thick-plate approximation. A new theoretical model based on the thick-plate approximation is developed and compared against the experimentally obtained values. The effect of the thickness of the microchannel on the deformation is also discussed. The bulging effects of the microchannel are probed along the length of the channel, and the reduction in the pressure drop is compared against a non-deforming channel. We observe a larger effect of the deformation near the microchannel inlet than that toward the end of the channel. Furthermore, we observe an upper limit to the maximum deformation of the microchannel walls, and the increased flow rate in the channel does not contribute to any further deformation and thus the reduction in the overall pressure drop. Overall, the observations from a multitude of experiments in the current study will aid in the development of microfluidic systems for Lab-on-Chip applications using soft lithography and also demonstrate the measures to be taken during the fabrication of any polydimethylsiloxane-based microfluidic systems.     

A microchannel heat exchanger design for microelectronics cooling correlating the heat transfer rate in terms of Brinkman number

Microchannel heat exchangers are a well known device in the application of microelectronics cooling. In this paper, liquid microchannel heat exchangers were designed and investigated with varying channel width in order to find the maximum cooling efficiency when combined with pumping performance. A recently developed correlation of heat transfer rate in terms of Nusselt number and Brinkman number was adopted to predict cooling efficiency of the microchannel heat exchanger and was compared with the experimental results. Conventional heat transfer theories and numerical commercial code were also used to predict the cooling efficiency. The measured minimum thermal resistance of the microchannel heat exchanger showed a good agreement with the prediction from the new correlation, whereas calculation results from conventional theories and numerical code showed large divergence. It can be seen that the microchannel heat exchanger can be optimized when combined with pumping performance. In addition, the new correlation of heat transfer rate in terms of Brinkman number can be quite a useful tool in design of microchannel heat exchanger.     

A Novel Benzocyclobutene-Based Device for Studying the Dynamics of Heat Transfer During the Nucleation Process

A novel microelectromechanical device has been developed to study the details of the heat transfer mechanisms involved at the nucleation site for the nucleate boiling process. This device enables quantifying the magnitude, time period of activation, and specific areas of influence of different mechanisms of heat transfer from the surface with a resolution several times greater than previously reported. This is achieved through the use of an array of embedded temperature sensors within a carefully designed dual-layer (silicon and benzocyclobutene) wall which allows for the accurate calculation of local heat flux, circumventing difficulties encountered when using existing methods. The sensors are radially distributed around the nucleation site. Heat is supplied to the wall by a thin film heater fabricated on the outer nonwetted surface. Single bubbles are generated at the center of the array while the temperatures and the bubble images are recorded with a sampling frequency of 8 kHz. The temperature data provided the necessary thermal boundary conditions to numerically calculate the surface heat flux with an unprecedented radial resolution of 22-40 mum. Fabrication, characterization, and the ability of the developed device to elucidate the heat transfer aspects of the nucleation process are demonstrated.     

Design and simulation of a thermo transfer type MEMS based micro flow sensor for arterial blood flow measurement

Thermo transfer type MEMS (Micro Electro Mechanical System) based micro flow sensing device have promising potential to solve the limitation of implantable arterial blood flow rate monitoring. The present paper emphasizes on modeling and simulation of MEMS based micro flow sensing device, which will be capable of implantable arterial blood flow rate measurement. It describes the basic design and model architecture of thermal type micro flow sensor. A pair of thin film micro heaters is designed through MEMS micro machining process and simulated using CoventorWare; a finite element based numerical code. A rectangular cross section micro channel has been modeled where in micro heater and thermal sensors are embedded using the same CoventorWare tools. Some promising and interesting results of thermal dissipation depending upon very small amount of flow rate through the micro channel are investigated. It is observed that measuring the variation of temperature difference between downstream and upstream, the variation of fluid flow rate in the micro channel can be measured. The numerical simulation results also shows that the temperature distribution profile of the heated surface depends upon microfluidic flow rate i.e. convective heat transfer is directly proportional to the microfluidic flow rate on the surface of the insulating membrane. The simplified analytical model of the thermo transfer type flow sensor is presented and verified by simulation results, which are very promising for application in arterial blood flow rate measuring in implantable micro devices for continuous monitoring of cardiac output.     

Microfluidics analysis of nanoparticle mixing in a microchannel system

The microfluidics of controlled nanodrug delivery to living cells in a representative, partially heated microchannel was analyzed, using a validated computer model. The objective was to achieve uniform nanoparticle exit concentrations at a minimum microchannel length with the aid of simple static mixers, e.g., a multi-baffle-slit or perforated injection micro-mixer. A variable wall heat flux, which influences the local nanofluid properties and carrier-fluid velocities, was added to ensure that mixture delivery to the living cells occurred at the required (body) temperature of 37°C. The results show that both the baffle-slit micro-mixer and the perforated injection micro-mixer aid in decreasing the microchannel length while achieving uniform nanoparticle exit concentrations. The injection micro-mixer not only decreases best the system’s dimension, but also reduces the system power requirement. The baffle-slit micro-mixer also decreases the microchannel length; however, it may add to the power requirement. The imposed wall heat flux aids in enhanced nanoparticle and base-fluid mixing as well.     

MEMS壁剪应力传感器热效应的数值模拟

借助计算流体动力学(CFD)商业软件FLUENT,采用数值模拟的方法,对基于MEMS的壁剪应力传感器热交换效应进行了分析。计算结果表明:在壁剪应力传感器的热膜下方加入真空腔或者空气腔是十分必要的。针对水流中测量的计算结果显示,真空腔和空气腔在整个计算区域的温度场分布以及对流体的传热效率的差别不大,而空腔可以明显地减小底层的热损失,这对提高剪应力传感器的灵敏度是十分有利的。此外,MEMS壁剪应力传感器的尺寸效应对传热效率也存在影响。     

Spatially resolved temperature measurement in microchannels

Non-intrusive local temperature measurement in convective microchannel flows using infrared (IR) thermography is presented. This technique can be used to determine local temperatures of the visualized channel wall or liquid temperature near this wall in IR-transparent heat sinks. The technique is demonstrated on water flow through a silicon (Si) microchannel. A high value of a combined liquid emissivity and substrate overall transmittance coupled with a low uncertainty in estimating this factor is important for quantitative temperature measurement using IR thermography. The test section design, and experimental and data analysis procedures that provide increased sensitivity of the detected intensity to the desired temperature are discussed. Experiments are performed on a 13-mm long, 50 μm wide by 135 μm deep Si microchannel at a constant heat input to the heat sink surface for flow rates between 0.6 and 1.2 g min⁻¹. Uncertainty in fluid temperature varies from a minimum of 0.60°C for a Reynolds number (Re) of 297 to a maximum of 1.33°C for a Re of 251.     

Efficient heat transfer enhancement by elastic turbulence with polymer solution in a curved microchannel

Heat and mass transfer in microscale flows are limited due to extremely low Reynolds number (Re). In a curved microchannel, however, complex flow behaviors, such as elastic instability and elastic turbulence, can be induced via viscoelastic fluid at vanishingly low-Re conditions, which is of great potential to enhance the heat transfer performance. The influence of elastic instabilities and turbulence on heat dissipation of exothermic components is experimentally investigated in this study. The heat transfer performance of both viscoelastic (polymer solutions) and Newtonian (sucrose solutions) fluid flows in a curved microchannel with a square cross section is experimentally characterized. Titanium–platinum (Ti–Pt) thin films embedded at the bottom wall of the polydimethylsiloxane (PDMS) microchannel serve as both microheater and temperature sensor. For viscoelastic fluids, the spectrum of outlet temperature fluctuation in broad frequency (f) region fits the power law of f ^?1.1. Heat transfer enhancement due to the elastic turbulence in a curved microchannel is thereby identified by the drastic growth of the Nusselt number (Nu, the ratio of convective to conductive heat transfer normal to the boundary) with the increase in the Weissenberg number (Wi, the ratio of elastic stress to viscous stress). The mechanism of heat transfer enhanced by the convection effect of elastic turbulence is also elucidated.