Heat transfer enhancement in vertical tubes plays an important role on the thermal performance of many heat exchangers and thermal devices.In this work,laminar mixed convection of airflow in a vertical dimpled tube wa...Heat transfer enhancement in vertical tubes plays an important role on the thermal performance of many heat exchangers and thermal devices.In this work,laminar mixed convection of airflow in a vertical dimpled tube was numerically investigated.Three-dimensional elliptical governing equations were solved using the finite-volume technique.For a given dimpled pitch,the effects of three different dimple heights(h/D=0.013,0.027,0.037) have been studied at different Richardson numbers(0.1,1.0 and 1.5).The generated vortex in the vicinity of the dimple destructs the thermal boundary layer and enhances the heat transfer.Therefore,lower wall temperature is seen where the dimples are located.Fluid flow velocity at the near-wall region significantly increases because of buoyancy forces with the increase of Richardson numbers.Such an acceleration at the near-wall region makes the dimples more effective at higher Richardson number.Using a dimpled tube enhances the heat transfer coefficient.However,the pressure drop is not important.For instance,in the case of Ri=1.5 and h/D=0.037,20% gains in the heat transfer enhancement only costs2.5% in the pressure loss.In general,it is recommended using a dimpled tube where the effects of buoyancy forces are important.展开更多
The fabrication of superhydrophobic surfaces and the studies on water flow characteristics therein are of great significance to many industrial areas as well as to science and technology development. Experiments were ...The fabrication of superhydrophobic surfaces and the studies on water flow characteristics therein are of great significance to many industrial areas as well as to science and technology development. Experiments were car- ried out to investigate slip characteristics of water flowing in circular superhydrophobic microtubes within lam- inar flow region. The superhydrophobic microtubes of stainless steel were fabricated with chemical etching- fluorination treatment. An experimental setup was designed to measure the pressure drop as function of water flow rate. For comparison, superhydrophilic tubes were also tested. Poiseuille number Po was found to be smaller for the superhydrophobic microtubes than that for superhydrophilic ones. The pressure drop reduc- tion ranges from 8% to 31%. It decreases with increasing Reynolds number when Re 〈 900, owing to the transition from Cassie state to Wenze] state. However, it is almost unchanged with further increasing Re after Re 〉 900. The slip length in superhydrophobic microtubes also exhibits a Reynolds number dependence similarly to the pressure drop reduction. The relation between slip length and Darcy friction factor is theoretically analyzed with consideration of surface roughness effect, which was testified with the experimental results.展开更多
The asymptotic behaviour of laminar forced convection in a circular duct, for a Herschel-Bulkley fluid with constant properties, is analysed by taking into account the viscous dissipation effects. The axial heat condu...The asymptotic behaviour of laminar forced convection in a circular duct, for a Herschel-Bulkley fluid with constant properties, is analysed by taking into account the viscous dissipation effects. The axial heat conduction in the fluid is neglected. The asymptotic temperature field and the asymptotic value of the Nusselt number are determined for every boundary condition that allows a fully developed region. Comparisons with other existing solutions for Newtonian and non-Newtonian cases are presented.展开更多
Experimental data are presented for the void fraction and the shear stresses of stratified gas-liquid flow in a pipe. A new technique was used to measure the interface shear stress. The interfacial shear stress was de...Experimental data are presented for the void fraction and the shear stresses of stratified gas-liquid flow in a pipe. A new technique was used to measure the interface shear stress. The interfacial shear stress was determined by using two methods: a momentum balance of gas and an extrapolation of the Reynolds shear stress profile at the gas-liquid interface. A new formula , relating to the interfacial friction factor with the void fraction and superficial gas Reynold number, was developed to predict the interface shear stress . The predicted values are in good agreement with experimental data.展开更多
This paper deals with the problem of theoretical identification of the residence time distribution （RTD） characteristics of a straight pipe at laminar pulsatile flow, if tracer diffusion can be neglected. This situa...This paper deals with the problem of theoretical identification of the residence time distribution （RTD） characteristics of a straight pipe at laminar pulsatile flow, if tracer diffusion can be neglected. This situation is typical for micro-apparatuses （e.g. fluidic element） and also for flow in large arteries. Residence time distribution based on velocity profiles at pulsatile flow of a Newtonian liquid in a rigid pipe will be derived theoretically and compared with the well known results for a constant flow rate E（τ） = τ-^2/2τ^3 at τ 〉 τ^-/2, where E （τ） is differential distribution, x is residence time and τ^- is the mean residence time. The following part of the paper deals stimulus response experimental techniques using tracers. The principal problem related to laminar and convection dominated pulsatile flows is discussed： Can the impulse response also be identified with the actual residence time distribution in the case of variable flow？ The general answer is no, and differences between RTD and impulse responses are evaluated as a function of the frequency and amplitude of pulsatile flows.展开更多
A boundary element method for simulating thermocapillary convection in a two-layer immiscible fluid system with flat and free interface has been developed.The divergence theorem is applied to the non-linear convective...A boundary element method for simulating thermocapillary convection in a two-layer immiscible fluid system with flat and free interface has been developed.The divergence theorem is applied to the non-linear convective volume integral of the boundary element formulation with the pressure penalty function.Consequently,velocity gradients are eliminated and the complete formulation is written in terms of velocity.This avoids the difficulty of convective discretizations and provides considerable reductions in storage and computational requirements while improving accuracy.In this paper,we give the influence of different parameters(Marangoni number, Reynolds number)on thermocapillary convection in cavity with two-layer immiscible fluids.As shown by the numerical results,when the physical parameters between liquid encapsulant and melt are chosen appropriately, the detrimental flow in the bottom melt layer can be greatly suppressed.The influence of the free interface on thermocapillary convection is also shown.展开更多
When sonic nozzles of significantly smaller diameter are used as standard flow meters,the critical back pressure ratio is affected by the boundary layer at the nozzle throat.However,the effect of the boundary layer on...When sonic nozzles of significantly smaller diameter are used as standard flow meters,the critical back pressure ratio is affected by the boundary layer at the nozzle throat.However,the effect of the boundary layer on choking criteria is still controversial.Then,the choking phenomenon of a convergent nozzle flow has been experimentally investigated using four convergent nozzles with the same diameter followed by a straight pipe of a variable length.As a result,it is shown that the critical back pressure ratio is smaller than that for the steady one-dimensional is-entropic flow and decreases as the boundary layer thickness increases.Moreover,the main flow Mach number at the nozzle exit is supersonic when the back pressure ratio is equivalent to the choking condition,and the Mach number increases as the boundary layer thickness increases.展开更多
文摘Heat transfer enhancement in vertical tubes plays an important role on the thermal performance of many heat exchangers and thermal devices.In this work,laminar mixed convection of airflow in a vertical dimpled tube was numerically investigated.Three-dimensional elliptical governing equations were solved using the finite-volume technique.For a given dimpled pitch,the effects of three different dimple heights(h/D=0.013,0.027,0.037) have been studied at different Richardson numbers(0.1,1.0 and 1.5).The generated vortex in the vicinity of the dimple destructs the thermal boundary layer and enhances the heat transfer.Therefore,lower wall temperature is seen where the dimples are located.Fluid flow velocity at the near-wall region significantly increases because of buoyancy forces with the increase of Richardson numbers.Such an acceleration at the near-wall region makes the dimples more effective at higher Richardson number.Using a dimpled tube enhances the heat transfer coefficient.However,the pressure drop is not important.For instance,in the case of Ri=1.5 and h/D=0.037,20% gains in the heat transfer enhancement only costs2.5% in the pressure loss.In general,it is recommended using a dimpled tube where the effects of buoyancy forces are important.
基金the National Natural Science Foundation of China(20476014,51376030)
文摘The fabrication of superhydrophobic surfaces and the studies on water flow characteristics therein are of great significance to many industrial areas as well as to science and technology development. Experiments were car- ried out to investigate slip characteristics of water flowing in circular superhydrophobic microtubes within lam- inar flow region. The superhydrophobic microtubes of stainless steel were fabricated with chemical etching- fluorination treatment. An experimental setup was designed to measure the pressure drop as function of water flow rate. For comparison, superhydrophilic tubes were also tested. Poiseuille number Po was found to be smaller for the superhydrophobic microtubes than that for superhydrophilic ones. The pressure drop reduc- tion ranges from 8% to 31%. It decreases with increasing Reynolds number when Re 〈 900, owing to the transition from Cassie state to Wenze] state. However, it is almost unchanged with further increasing Re after Re 〉 900. The slip length in superhydrophobic microtubes also exhibits a Reynolds number dependence similarly to the pressure drop reduction. The relation between slip length and Darcy friction factor is theoretically analyzed with consideration of surface roughness effect, which was testified with the experimental results.
文摘The asymptotic behaviour of laminar forced convection in a circular duct, for a Herschel-Bulkley fluid with constant properties, is analysed by taking into account the viscous dissipation effects. The axial heat conduction in the fluid is neglected. The asymptotic temperature field and the asymptotic value of the Nusselt number are determined for every boundary condition that allows a fully developed region. Comparisons with other existing solutions for Newtonian and non-Newtonian cases are presented.
基金Supported by the National Natural Science Foundation of China(No.59236130).
文摘Experimental data are presented for the void fraction and the shear stresses of stratified gas-liquid flow in a pipe. A new technique was used to measure the interface shear stress. The interfacial shear stress was determined by using two methods: a momentum balance of gas and an extrapolation of the Reynolds shear stress profile at the gas-liquid interface. A new formula , relating to the interfacial friction factor with the void fraction and superficial gas Reynold number, was developed to predict the interface shear stress . The predicted values are in good agreement with experimental data.
文摘This paper deals with the problem of theoretical identification of the residence time distribution （RTD） characteristics of a straight pipe at laminar pulsatile flow, if tracer diffusion can be neglected. This situation is typical for micro-apparatuses （e.g. fluidic element） and also for flow in large arteries. Residence time distribution based on velocity profiles at pulsatile flow of a Newtonian liquid in a rigid pipe will be derived theoretically and compared with the well known results for a constant flow rate E（τ） = τ-^2/2τ^3 at τ 〉 τ^-/2, where E （τ） is differential distribution, x is residence time and τ^- is the mean residence time. The following part of the paper deals stimulus response experimental techniques using tracers. The principal problem related to laminar and convection dominated pulsatile flows is discussed： Can the impulse response also be identified with the actual residence time distribution in the case of variable flow？ The general answer is no, and differences between RTD and impulse responses are evaluated as a function of the frequency and amplitude of pulsatile flows.
基金Project supported by the National Natural Science Foundation of China
文摘A boundary element method for simulating thermocapillary convection in a two-layer immiscible fluid system with flat and free interface has been developed.The divergence theorem is applied to the non-linear convective volume integral of the boundary element formulation with the pressure penalty function.Consequently,velocity gradients are eliminated and the complete formulation is written in terms of velocity.This avoids the difficulty of convective discretizations and provides considerable reductions in storage and computational requirements while improving accuracy.In this paper,we give the influence of different parameters(Marangoni number, Reynolds number)on thermocapillary convection in cavity with two-layer immiscible fluids.As shown by the numerical results,when the physical parameters between liquid encapsulant and melt are chosen appropriately, the detrimental flow in the bottom melt layer can be greatly suppressed.The influence of the free interface on thermocapillary convection is also shown.
文摘When sonic nozzles of significantly smaller diameter are used as standard flow meters,the critical back pressure ratio is affected by the boundary layer at the nozzle throat.However,the effect of the boundary layer on choking criteria is still controversial.Then,the choking phenomenon of a convergent nozzle flow has been experimentally investigated using four convergent nozzles with the same diameter followed by a straight pipe of a variable length.As a result,it is shown that the critical back pressure ratio is smaller than that for the steady one-dimensional is-entropic flow and decreases as the boundary layer thickness increases.Moreover,the main flow Mach number at the nozzle exit is supersonic when the back pressure ratio is equivalent to the choking condition,and the Mach number increases as the boundary layer thickness increases.
