Non-equilibrium evolution and characteristics of the serrated microchannel hydrogen knudsen compressor

The hydrogen Knudsen compressor has great potential to transport hydrogen and provide the required pressure in MEMS and microfluidic systems. The microchannel composed of cold and hot serrated surfaces is beneficial to the temperature control of the multistage Knudsen compressor. In the present study, a serrated hydrogen Knudsen compressor model is established initially, and the non-equilibrium evolution is numerically studied by using the method of N–S equations with the slip boundary. The key factors affecting the non-equilibrium evolution are comprehensively analyzed. The flow behaviors and performance of the serrated hydrogen Knudsen compressor in different times are studied. It is found that the main factors affecting the non-equilibrium evolution are the thermal expansion flow, thermal transpiration flow, and Poiseuille flow. Meanwhile, the serrated structure affects the local flow in the serrated microchannel at different times. Under the interaction of the thermal transpiration flow and the Poiseuille flow, the pressure difference between the two containers first increases rapidly and then decreases slowly, and finally approaches 1886 Pa. The research reveals the flow mechanisms of the hydrogen Knudsen compressor in the non-equilibrium evolution, which provides theoretical support for the safety and reliability of the hydrogen Knudsen compressor.     

The hydrogen flow characteristics of the multistage hydrogen Knudsen compressor based on the thermal transpiration effect

Multistage hydrogen Knudsen compressor based on the thermal transpiration effect has very exciting prospect for the hydrogen transmission in the micro devices. Understanding of the hydrogen flow characteristic is the key issue for the designs and applications of the hydrogen energy systems. Firstly, the numerical models of the multistage hydrogen Knudsen compressor are established. The distributions of the rarefaction, velocity and temperature at different stages of the hydrogen flow are calculated and presented. Moreover, the dimensional pressure increases of the hydrogen gas flow are analyzed, and the flow behaviors in the microchannel and the connection channel are discussed. Secondly, the numerical simulation at different connection channel height is implemented, and the hydrogen gas flow characteristics in the connection are analyzed. Especially, the performances of the pressure drop in the connection channel under different channel heights are studied, and the hydrogen gas compression characteristics of different cases are compared and discussed. Also, the effect of the connection channel height on the hydrogen gas pressure increase in the microchannel is investigated. The studies presented in this paper could be greatly beneficial for the hydrogen detection and transmission.     

Characteristics of thermal transpiration effect and the hydrogen flow behaviors in the microchannel with semicircular obstacle

The thermal transpiration effect has great potential applications for the hydrogen energy. In this paper, the thermal transpiration effect and the hydrogen flow behaviors are studied in the microchannel with the semicircular obstacles. Firstly, the slip boundary model is used in the simulation of the flow performance in the microchannel. The validity of the model at different Kn is verified by comparing with some previous work. Then, the hydrogen flow characteristics of the thermal transpiration effect with the semicircular obstacle are investigated. The result shows that as the size of the semicircular obstacle increases, the hydrogen flow path of the thermal transpiration effect becomes longer, and the temperature gradient decreases. As the characteristic length of the hydrogen flow decreases, there is an obviously negative influence on the thermal transpiration flow. A deeper analysis shows that the thermal driven flow and the pressure driven flow will produce y-component velocity, which leads to a backflow under the effect of semicircles, and the semicircular obstacles make the Knudsen layer spread to the channel center.     

Constructal design and thermal analysis of microchannel heat sinks with multistage bifurcations in single-phase liquid flow

Based on constructal theory, five different cases with multistage bifurcations are designed as well as one case without bifurcations, and the corresponding laminar fluid flow and thermal performance have been investigated numerically. All laminar fluid flow and heat transfer results are obtained using computation fluid dynamics, and a uniform wall heat flux thermal boundary condition is applied all heated surfaces. The inlet velocity ranges from 0.66 m/s to 1.6 m/s with the corresponding Reynolds number ranging from 230 to 560. The pressure, velocity, temperature distributions and averaged Nusselt number are presented. The overall thermal resistances versus inlet Reynolds number or pumping power are evaluated and compared for the six microchannel heat sinks. Numerical results show that the thermal performance of the microchannel heat sink with multistage bifurcation flow is better than that of the corresponding straight microchannel heat sink. The heat sink with a long bifurcation length in the first stage (Case 1A) is superior. The usage of multistage bifurcated plates in microchannel heat sink can reduce the overall thermal resistance and make the temperature of the heated surface more uniform (Case 3). It is suggested that proper design of the multistage bifurcations could be employed to improve the overall thermal performance of microchannel heat sinks and the maximum number of stages of bifurcations is recommended to be two. The study complements and extends previous works.     

Thermal transpiration effect on the mass transfer and flow behaviors of the pressure-driven hydrogen gas flow

Thermal transpiration is a rarefied gas effect that drives the gas flow creeping in a microchannel due only to an imposed temperature gradient, which is often encountered in the hydrogen-transportation microfluidic applications such as proton exchange membrane fuel cell (PEMFC). Because of its impact on the pressure-driven flow behavior in the microchannel, this pumping phenomenon needs to be studied in designing and improving microfluidic devices for hydrogen transportation. However, so far little literature has discussed the thermal transpiration effects on the flow behaviors under normal boundary conditions. In this paper, a DSMC-SPH coupled multiscale approach is proposed on the study of the thermal transpiration effect on hydrogen gas multiscale flow behaviors. Various wall temperature distributions are used under a pressure-driven condition. The remarkable influence of thermal transpiration on the multiscale hydrogen gas flow are investigated and discussed. Since the thermal transpiration effect is often occurred in hydrogen transportation, the present simulation results can provide significant insights for designing and improving proton exchange membrane fuel cell (PEMFC).     

An experimental investigation of forced convective cooling performance of a microchannel heat sink with Al2O3/water nanofluid

Experiments were conducted to investigate forced convective cooling performance of a copper microchannel heat sink with Al₂O₃/water nanofluid as the coolant. The microchannel heat sink fabricated consists of 25 parallel rectangular microchannels of length 50 mm with a cross-sectional area of 283 μm in width by 800 μm in height for each microchannel. Hydraulic and thermal performances of the nanofluid-cooled microchannel heat sink have been assessed from the results obtained for the friction factor, the pumping power, the averaged heat transfer coefficient, the thermal resistance, and the maximum wall temperature, with the Reynolds number ranging from 226 to 1676. Results show that the nanofluid-cooled heat sink outperforms the water-cooled one, having significantly higher average heat transfer coefficient and thereby markedly lower thermal resistance and wall temperature at high pumping power, in particular. Despite the marked increase in dynamic viscosity due to dispersing the alumina nanoparticles in water, the friction factor for the nanofluid-cooled heat sink was found slightly increased only.     

Three dimensional channel effect on the flow characteristics and the performance of hydrogen Knudsen compressors

As a new type of the micro fluidic device, Knudsen compressor can provide the potential utilizations on the hydrogen transport in the micro systems. Considering actual structure of the compressor is three-dimensional, flow characteristic studies are the key issue for the performance predictions. Firstly, the model of three-dimensional Knudsen compressor is built, and the validity of the model is proved by comparison with the experimental result. Secondly, the flow behaviors in the three-dimensional model is investigated, and the distributions of pressure and velocity are investigated. Also, the performance of the hydrogen Knudsen compressor in two-dimensional structure and three-dimensional structure are compared and discussed. Thirdly, the three-dimensional hydrogen Knudsen compressors with different width are analyzed, and the pressure increase in different cases of the hydrogen Knudsen compressors are studied.     

Effect of the microchannel obstacles on the pressure performance and flow behaviors of the hydrogen Knudsen compressor

The hydrogen Knudsen compressor has potential applications on the hydrogen transmission for the microdevices and systems. In this paper, the numerical model of the hydrogen Knudsen compressor was established, combining the NS continuity equations with the slip boundary conditions. The effect of structures on the performance of the hydrogen Knudsen compressor is studied by generating different obstacles in the microchannels. This paper is mainly concerned on the rectangular and the triangular obstacles, and the influence of the obstacles length and height are investigated, respectively. The Knudsen number distribution and the rarefaction of the hydrogen gas flow are analyzed. Also, the characteristic of the pressure increase for the compressor under different parameters are investigated and discussed. The effect of the structure parameters on the flow velocity distributions are detailed described, as well as the velocity contour and the vortex distributions. Moreover, the variation of the Knudsen layers of the hydrogen gas flow in the hydrogen Knudsen compressor is presented, and the key factor of the Knudsen layers is analyzed and discussed. The results is significantly beneficial for the applications and designs of hydrogen Knudsen compressor.     

Thermal performance of plate-fin vapor chamber heat sinks

Since vapor chambers exhibit excellent thermal performance, they are suited to use as bases of heat sinks. This work experimentally studies the thermal performance of plate-fin vapor chamber heat sinks using infrared thermography. The effects of the width, height and number of fins and of the Reynolds number on the thermal performance are considered. Experimental data are compared with corresponding data for conventional aluminum heat sinks. The results show that generated heat is transferred more uniformly to the base plate by a vapor chamber heat sink than by a similar aluminum heat sink. Therefore, the maximum temperature is effectively reduced. The overall thermal resistance of the vapor chamber heat sink declines as the Reynolds number increases, but the strength of the effect falls. The effect of the fin dimensions on the thermal performance is stronger at a lower Reynolds number. At a low Reynolds number, a suitable number of fins must be chosen to ensure favorable thermal performance of the vapor chamber heat sink. However, at a high Reynolds number, the thermal performance improves as the fin number increases.     

3D study of temperature drop behavior of subsonic rarefied gas flow in microchannel

3D Numerical study of temperature variation for subsonic rarefied gas flow in a microchannel is carried out using an in-house MPI-based parallelized DSMC code. The temperature drop in the microchannel decreases with an increase in the aspect ratio whereas it increases with an increase in the pressure ratio, the cross-aspect ratio (CAR), and the Knudsen number. 3D and 2D simulations results are compared and effect of the CAR and Knudsen number are brought out. Finally, a correlation that predicts the temperature drop is formulated along with a list of conditions that ensures a near isothermal flow.     

Slip flow and heat transfer in axially moving micro-concentric cylinders

The hydrodynamics and thermal behaviors of fluid flow in axially moving micro-concentric cylinders are investigated analytically. Effects of Knudsen number, velocity and radius of the cylinders on the microchannel hydrodynamics and thermal behaviors are investigated. It is found that as Kn increases the slip in the hydrodynamic and thermal boundary condition increases. The slip and the jump at the inner surface are much larger than that of the outer one. When the outer cylinder velocity approaches the inner cylinder one, the slip velocity vanishes. Also, the effect of the variation of U₁ on the temperature jump for adiabatic outer surface is insignificant.     

Reducing energy availability losses with open parallel microchannels embedded in a micropatterned surface

This article develops a new technique of reducing exergy losses of external viscous flow over surfaces, based on optimized microchannels embedded within the surface. The rate of entropy production and loss of available optimized energy are formulated by an integral solution and modified Blasius profiles of boundary layer flow. The optimized number of microchannels, width and height of each microchannel and spacing between microchannels involve a selective compromise between added heat exchange due to surface area, together with reduced friction through slip conditions within each microchannel. Mixed Knudsen numbers across each microchannel require simultaneous modelling of both slip‐flow and no‐slip conditions at the wall. Results involving the minimal entropy production and optimized microchannel profiles are presented and compared to other benchmark results involving classical macro‐scale configurations. Copyright © 2005 John Wiley & Sons, Ltd.     

Effect of inclined and low magnetic field in gaseous slip flow in two‐dimensional rectangular microchannel using first‐order boundary conditions

In the present work, the effect of an oriented low magnetic field on near‐continuum gaseous slip flow inside a two‐dimensional rectangular microchannel has been studied using first‐order boundary conditions. The flow was assumed to be compressible, laminar, and steady. The governing equations were solved analytically to obtain the solutions of velocity, temperature, and the pressure of the flow. The influence of different parameters such as Knudsen number, aspect ratio, Hartmann number, and pressure ratio were studied and analyzed. It was found that the electric and magnetic field with an inclined angle had significant effects on the flow properties. The results showed that the velocity increases and the temperature decreases as the inclination angle of the magnetic field decreases. The velocity increases as the Knudsen number, pressure ratio, and aspect ratio increase, while it decreases with increasing of the Hartmann number. The temperature decreases with increasing of the Knudsen number, pressure ratio, and aspect ratio, while the temperature increases as the Hartmann number increases. The results of the present study were validated with published results in the literature.     

Thermal hydraulic performance of a double-layer microchannel heat sink with channel contraction

The present study investigates numerically the effects of vertically tapered and converging channel of a double-layered microchannel heat sink (DL-MCHS) on its thermal and hydraulic performance. The problem is solved using a three-dimensional conjugate heat transfer model with counterflow between upper and lower channels. The taper effect is studied with different channel height contraction ratio R_h. Numerical results generally shows vertical taper effect of the channel contribute to higher thermal performance as compared to conventional design of straight channel but the pumping power required increases as the channel height contraction ratio increases. Thermal hydraulic performance factor η is employed to assess the sustainability of the taper design of microchannels by considering both thermal and hydraulic performances. The results of thermal enhancement factors are below unity, indicating that the DL-MCHS with channel height contraction design is not a sustainable solution as compared to the DL-MCHS with conventional straight channel design when hydraulic performance is taken into account.     

An experimental investigation on the thermal efficiency of fractal tree-like microchannel nets

In this paper, a fractal tree-like microchannel net heat sink (20 mm × 20 mm × 1.4 mm) for cooling of electronic chips was fabricated on a silicon wafers by advanced MEMS technology. The length, width and height of the entrance microchannel were 10 mm, 800 μm and 25 μm, respectively. The fractal dimension D and the circulation number m of the fractal tree-like microchannel net were 2 and 4, respectively. It is confirmed experimentally that the thermal efficiency (defined as heat transfer rate per unit power required) of such a fractal tree-like microchannel heat sink is much higher than that of the traditional parallel microchannel heat sink for the same heat transfer rate, the same temperature difference and the same inlet velocity.     

Effect of tip clearance on the cooling performance of a microchannel heat sink

In this paper, the effect of tip clearance on the cooling performance of the microchannel heat sink is presented under the fixed pumping power condition. The thermal resistance of a microchannel heat sink is defined for evaluating its cooling performance. The effect of tip clearance is numerically investigated by increasing tip clearance from zero under the fixed pumping power condition. From the numerical results, the optimized tip clearance is determined, for which the thermal resistance has a minimum value. Finally, we show that the presence of tip clearance can improve the cooling performance of a microchannel heat sink when tip clearance is smaller than a channel width.     

Analysis of laminar heat transfer in micro-Poiseuille flow

The present study examines laminar forced convective heat transfer of a Newtonian fluid in a microchannel between two parallel plates analytically. The viscous dissipation effect, the velocity slip and the temperature jump at the wall are included in the analysis. Both hydrodynamically and thermally fully developed flow case is examined. Either the hot wall or the cold wall case is considered for the two different thermal boundary conditions, namely the constant heat flux (CHF) and the constant wall temperature (CWT). The interactive effects of the Brinkman number and the Knudsen number on the Nusselt numbers are analytically determined. Different definitions of the Brinkman number based on the definition of the dimensionless temperature are discussed. It is disclosed that for the cases studied here, singularities for the Brinkman number-dependence of the Nusselt number are observed and they are discussed in view of the energy balance.     

Performance characteristics of a microchannel heat sink using TiO2/water nanofluid and different thermophysical models

In this paper, the performance of a microchannel heat sink using TiO₂/water nanofluid is experimentally investigated. The dimensions of the microchannel are 500 μm width, 800 μm height, and 40 mm length, where the number of flowing channels is 40.The effects of uncertainties in thermophysical properties on the Nusselt number and friction factor are investigated by using three different sets of thermophysical models, which are based on experimental and theoretical relations. It is concluded that the use of the model which is based on experimental data is very important to estimate the friction factor, while the use of different models to calculate of thermal conductivity has no considerable effect on the prediction of Nusselt number.     

Thermal management of methanol reforming reactors for the portable production of hydrogen

Three-dimensional numerical simulations were performed to address the thermal management issues associated with the design of a methanol reforming microchannel reactor for the portable production of hydrogen. The design of the reactor was fundamentally related to the direct coupling of reforming and combustion reactions by performing them on opposite sides of dividing walls in a parallel flow configuration. Effective autothermal operation was achieved through a combination of microchannel reactor technology with heat exchange in a direction perpendicular to the reacting fluid flow. Computational fluid dynamics simulations and thermodynamic analysis were carried out to investigate the effect of various design parameters on the characteristics of the generation, consumption, and exchange of thermal energy within the system. The results indicated that the ability to control temperature and temperature uniformity is of great importance to the performance of the system. The degree of temperature uniformity favorably affects the autothermal operation of the reactor. Temperature uniformity of the reactor can be improved by controlling the rate of heat transfer through a variety of factors such as wall thermal conductivity, fluid velocities, and dimensions. High wall thermal conductivity would be greatly beneficial to the performance of the system and the temperature uniformity of the reactor.     

Thermal-fluid characteristics of plate-fin heat sinks cooled by impingement jet

This work experimentally and numerically studies the thermal-fluid characteristics of plate-fin heat sinks under impingement cooling by adjusting the impinging Reynolds number, the impingement distance, and the fin dimensions. The parameters and the ranges under consideration are the impinging Reynolds number (Re = 5000–25,000), the impingement distance (Y/D = 4–28), the fin width (W/L = 0.08125–0.15625) and the fin height (H/L = 0.375–0.625). The results show that the heat transferred by the heat sink increases with the impinging Reynolds number. The thermal performance can be improved significantly even at low impinging Reynolds number. However, the improvement becomes indistinct as the impinging Reynolds number increases. The thermal resistance declines as the impingement distance increases, and is minimal at Y/D = 20 for various impinging Reynolds numbers. Additionally, the thermal resistance increases as the impingement distance increases further. Increasing the fin width can effectively reduce the thermal resistance. However, as the fin width increases beyond a particular value, the thermal resistance increases dramatically. Reducing the thermal resistance by increasing the fin height depends on a suitable impinging Reynolds number and fin width. Therefore, the effect of the fin height is weaker than that of the impinging Reynolds number or the fin width.