scholarly journals Pressure-Driven Nitrogen Flow in Divergent Microchannels with Isothermal Walls

2021 ◽  
Vol 11 (8) ◽  
pp. 3602
Author(s):  
Amin Ebrahimi ◽  
Vahid Shahabi ◽  
Ehsan Roohi
Keyword(s):  
Heat Transfer ◽  
Fluid Flow ◽  
Heat Flow ◽  
Gas Flow ◽  
Flow Direction ◽  
Direct Simulation Monte Carlo ◽  
Velocity Slip ◽  
Nitrogen Flow ◽  
Nonequilibrium Effects ◽  
Pressure Driven

Gas flow and heat transfer in confined geometries at micro-and nanoscales differ considerably from those at macro-scales, mainly due to nonequilibrium effects such as velocity slip and temperature jump. Nonequilibrium effects increase with a decrease in the characteristic length-scale of the fluid flow or the gas density, leading to the failure of the standard Navier–Stokes–Fourier (NSF) equations in predicting thermal and fluid flow fields. The direct simulation Monte Carlo (DSMC) method is employed in the present work to investigate pressure-driven nitrogen flow in divergent microchannels with various divergence angles and isothermal walls. The thermal fields obtained from numerical simulations are analysed for different inlet-to-outlet pressure ratios (1.5≤Π≤2.5), tangential momentum accommodation coefficients, and Knudsen numbers (0.05≤Kn≤12.5), covering slip to free-molecular rarefaction regimes. The thermal field in the microchannel is predicted, heat-lines are visualised, and the physics of heat transfer in the microchannel is discussed. Due to the rarefaction effects, the direction of heat flow is largely opposite to that of the mass flow. However, the interplay between thermal and pressure gradients, which are affected by geometrical configurations of the microchannel and the applied boundary conditions, determines the net heat flow direction. Additionally, the occurrence of thermal separation and cold-to-hot heat transfer (also known as anti-Fourier heat transfer) in divergent microchannels is explained.

Slip Gas Flow and Heat Transfer in Confined Porous Media With Different Shape Cylinders

2021 ◽  
Author(s):  
Ammar Tariq ◽  
Zhenyu Liu
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Gas Flow ◽  
Velocity Slip ◽  
Heat Transfer Analysis ◽  
Variable Boundary ◽  
Flow And Heat Transfer ◽  
Multiple Relaxation Time ◽  
Permeability Increase ◽  
Boltzmann Method

Abstract With the recent advances in micro devices, an accurate gas flow and heat transfer analysis become more relevant considering the slip effect. A micro-scale, multiple-relaxation-time (MRT) lattice Boltzmann method with double distribution function approach is used to simulate flow and heat transfer through circular- and diamond-shaped cylinders at the porescale level. The velocity slip and temperature jump are captured at the boundaries using a non-equilibrium extrapolation scheme with the counter-extrapolation method. A pore-scale domain of micro-cylinders comprised of circle and diamond shape are studied. It is found that the permeability increases linearly with an increase in Knudsen number for both circular- and diamond-shaped cylinders. However, the permeability increase for circular obstacle is larger than that of the diamond one. A larger surface area for diamond cylinder will offer more resistance to flow, hence resulting in lower values. For heat transfer, the Nusselt number shows an increase with increasing Reynolds number, however, it decreases with the increase in porosity. Nusselt number values are found to be higher for a circular obstacle. A variable boundary gradient for circular obstacle could be a possible explanation for this difference.

Effects of a flow mode transition on natural convection heat transfer in a heat tracing enclosure

2018 ◽  
Vol 233 (3) ◽  
pp. 489-499 ◽  
Author(s):  
CJ Ho ◽  
GN Sou ◽  
CM Lai
Keyword(s):  
Heat Transfer ◽  
Fluid Flow ◽  
Flow Field ◽  
Numerical Study ◽  
Flow Direction ◽  
Convection Heat Transfer ◽  
Flow Mode ◽  
Mode Transition ◽  
Flow And Heat Transfer ◽  
Lower Variation

This paper presents a numerical study of transient buoyancy-induced fluid flow and heat transfer between two horizontal, differentially heated pipelines inside a circular, air-filled enclosure. Numerical simulations based on the finite difference method were conducted to investigate the flow mode transition of the buoyant airflow and its effects on the heat transfer characteristics of the pipelines. The results indicate that the fluid flow complexity and the heat transfer of air between the pipelines are strongly affected by the Rayleigh number. When Ra = 6 × 105 and 1.2 × 106, both the flow field and the temperature distribution exhibit periodic variations with different patterns. The former ( Ra = 6 × 105) is a complete alteration of the flow direction from clockwise to counterclockwise, whereas the latter is a variation in the flow field strength that varies between strong and weak. The latter has a lower variation frequency than that of the former.

Magnetogasdynamic Flow and Heat Transfer in a Microchannel

2011 ◽  
Author(s):  
Huei Chu Weng
Keyword(s):  
Magnetic Field ◽  
Heat Transfer ◽  
Joule Heating ◽  
Electromagnetic Force ◽  
Gas Flow ◽  
Velocity Slip ◽  
Flow And Heat Transfer ◽  
Electric And Magnetic Field ◽  
Flow Through ◽  
Flow Drag

The presence of current flow in an electric and magnetic field results in electromagnetic force and joule heating. It is desirable to understand the roles of electromagnetic force and joule heating on gas microflow and heat transfer. In this study, a mathematical model is developed of the pressure-driven gas flow through a long isothermally heated horizontal planar microchannel in the presence of an external electric and magnetic field. The solutions for flow and thermal field and characteristics are derived analytically and presented in terms of dimensionless parameters. It is found that an electromagnetic driving force can be produced by a combined non-zero electric field and a negative magnetic field and results in an additional velocity slip and an additional flow drag. Also, a joule heating can be enhanced by an applied positive magnetic field and therefore results in an additional temperature jump and an additional heat transfer.

The UGKS Simulation of Microchannel Gas Flow and Heat Transfer Confined Between Isothermal and Nonisothermal Parallel Plates

2020 ◽  
Vol 142 (12) ◽  
Author(s):  
Lianfu Dai ◽  
Huiying Wu ◽  
Jun Tang
Keyword(s):  
Heat Transfer ◽  
Cross Section ◽  
Gas Flow ◽  
Flow Direction ◽  
Channel Height ◽  
Parallel Plates ◽  
Gas Temperature ◽  
Gas Velocity ◽  
Microscopic Mechanism ◽  
Flow And Heat Transfer

Abstract The unified gas kinetic scheme (UGKS) is introduced to simulate the near transition regime gas flow and heat transfer in microchannel confined between isothermal and nonisothermal parallel plates. The argon gas is used and its inlet Knudsen number (Knin) ranges from 0.0154 to 0.0715. It is found that: (1) both microchannel gas flows under isothermal and nonisothermal parallel plates display a trend of speed acceleration and temperature decrease along flow direction, for which the microscopic mechanism explanation is first proposed; (2) inlet gas streamlines under nonisothermal plates condition deviate from the parallel distributions under isothermal plates condition due to the dual driving effects of pressure drop along flow direction and temperature difference along cross section; (3) gas temperature, pressure, density and viscosity distributions along cross section under nonisothermal plates condition deviate from the parabolic distributions under isothermal plates condition, while the gas velocity keeps the parabolic distribution due to the effect of Knudsen layer; (4) as channel height increases or channel length and gas molecular mean free path decrease, the gas temperature distribution along cross section under nonisothermal plates condition tends to transition from linear to curve one due to the decreasing effect of heat transfer along cross section and increasing effect of gas acceleration along flow direction, this transition from linear to curve one becomes more obvious along flow direction. (5) the gas velocity under nonisothermal plates condition decreases with the increase of inlet gas temperature (Tin), lower plate temperature (Tl) and Knin, while the gas temperature increases with the increase of Tin, Tl and Knin.

Heat Transfer Enhancement in Evaporating Mini-Layers: Effects of an Inert Gas Flow

2007 ◽  
Author(s):  
C. S. Iorio ◽  
O. A. Kabov
Keyword(s):  
Heat Transfer ◽  
Flow Rate ◽  
Thermal Instability ◽  
Gas Flow ◽  
Flow Direction ◽  
Inert Gas ◽  
Transfer Enhancement ◽  
Capillary Stress ◽  
Volatile Liquid ◽  
Thermo Physical Properties

When a layer of volatile liquid is subject to a flow of inert gas, a non-uniform distribution of the evaporation rate is generated all along the interface. Being evaporation stronger at the inlet boundary of the layer, because of the maximal efficiency of the inert gas flow in removing vapor from the interface, a thermal gradient along the interface is generated. Two opposite mechanisms regulate the movement of the interface: the shear stress of the gas that entrains the interface in the direction of the flow and the thermo-capillary stress that forces the interface to move against the flow direction. Moreover, because of the overall cooling of the interface due to the evaporative process, a gradient normal to the interface is also created. It results in a potentially unstable situation that is strongly influenced by the flow rate of inert gas, the layer thickness and the liquid thermo-physical properties. The goal of the present work is to study numerically if and how the dynamic evolution of the liquid layer is driven by the above-mentioned mechanisms. The main results concern the evaluation of the influence of the thermal instability patterns, eventually generated by the concurrent action of non-uniform evaporation and thermo-capillary motion, on the heat transfer at the bottom liquid. The distribution of temperature and velocity in the gas and liquid bulk phase for different mass flow rate of inert gas has also been of interest.

Rarefied gas flow behavior in micro/nanochannels under specified wall heat flux

2015 ◽  
Vol 26 (08) ◽  
pp. 1550087 ◽  
Author(s):  
Mojtaba Balaj ◽  
Hassan Akhlaghi ◽  
Ehsan Roohi
Keyword(s):  
Heat Transfer ◽  
Boundary Condition ◽  
Heat Flux ◽  
Gas Flow ◽  
Rarefied Gas ◽  
Flow Behavior ◽  
Direct Simulation Monte Carlo ◽  
Wall Heat Flux ◽  
Linear Distribution ◽  
Thermal Transpiration

In this paper, we investigate the effects of convective heat transfer on the argon gas flow through micro/nanochannels subject to uniform wall heat flux (UWH) boundary condition using the direct simulation Monte Carlo (DSMC) method. Both the hot wall (q wall > 0) and the cold wall (q wall < 0) cases are considered. We consider the effect of wall heat flux on the centerline pressure, velocity profile and mass flow rate through the channel in the slip regime. The effects of rarefaction, property variations and compressibility are considered. We show that UWH boundary condition leads to the thermal transpiration. Our investigations showed that this thermal transpiration enhances the heat transfer rate at the walls in the case of hot walls and decreases it where the walls are being cooled. We also show that the deviation of the centerline pressure distribution from the linear distribution depends on the direction of the wall heat flux.

scholarly journals Numerical analysis of heat transfer in air-water heat exchanger with microchannel coil

2019 ◽  
Vol 95 ◽  
pp. 02004
Author(s):  
Vladimir Glazar ◽  
Anica Trp ◽  
Kristian Lenic ◽  
Fran Torbarina
Keyword(s):  
Heat Transfer ◽  
Mathematical Model ◽  
Fluid Flow ◽  
Heat Exchanger ◽  
Numerical Analysis ◽  
Flow Direction ◽  
Heat Fluxes ◽  
Experimental Investigations ◽  
Flow And Heat Transfer ◽  
Working Parameters

This paper presents numerical analysis of fluid flow and heat transfer in the heat exchanger with microchannel coil (MCHX). In accordance with previously published experimental results, 3D mathematical model has been defined and appropriate numerical simulation of heat transfer has been performed. Geometry and working parameters of cross-flow air-water heat exchanger with microchannel coil, installed in an open circuit wind tunnel and used in experimental investigations, have been applied in numerical analysis in order to validate the mathematical model. 3D model with air and water fluid flow and heat transfer domains has been used, as it gives more precise results compared to models that assume constant temperatures or constant heat fluxes on the pipe walls. Developed model comprised full length of air and water flows in the heat exchanger. Due to limitations of computational capacity, domain has been divided in multiple computational blocks in the water flow direction and then solved successively using CFD solver Fluent. Good agreement between experimentally measured and numerically calculated results has been obtained. The influence of various working parameters on heat transfer in air-water heat exchanger has been studied numerically, followed with discussion and final conclusions.

Study of Temperature and Velocity Distribution of Rarefied Gas Flow in Micro-Nano Channels

2009 ◽  
Author(s):  
Hadi Ghezel Sofloo ◽  
Alireza Shams ◽  
Reza Ebrahimi
Keyword(s):  
Heat Transfer ◽  
Gas Flow ◽  
Rarefied Gas ◽  
Direct Simulation Monte Carlo ◽  
Ideal Gas ◽  
Gas Density ◽  
Channel Size ◽  
Flow And Heat Transfer ◽  
Nonideal Gas ◽  
Gas Effect

This paper deals with simulation of transport phenomena in micro and nano pores. The number of cavities and the cavity radius were estimated by using Henry’s law for adsorption of Argon onto ZSM-5 and NaX zeolites. This work showed both of zeolites have pores with average size less than 1 nm. Then with using micro-nano channel assumption instead of micro-nano pores, gas flow and heat transfer were investigated. Subsonic nonideal gas flow and heat transfer for different Knudsen number are investigated numerically using the Direct Simulation Monte Carlo method modified with a consistent Boltzamnn algorithm. The collision rate is also modified based on the Enskog theory for dense gas. It is shown that nonideal gas effect becomes significant when the gas becomes so dense that the ideal gas assumption breaks down. The results also show that the nonideal gas effect is dependent not only on the gas density, but also the channel size. A higher gas density and a smaller channel size lead to a more significant nonideal gas effect. The nonideal gas effect also causes lower skin friction coefficients and different heat transfer flux distributions at the wall surface.

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