In this paper, turbulent data obtained from the Damxung site during the Secondary Tibetan Plateau Science Experiment (TIPEX) in 1998 are used to study the characteristics of the turbulent spectra, turbulence transport...In this paper, turbulent data obtained from the Damxung site during the Secondary Tibetan Plateau Science Experiment (TIPEX) in 1998 are used to study the characteristics of the turbulent spectra, turbulence transport, and the dissipation rates of turbulent kinetic energy, temperature variance, and humidity variance in the middle area of the Tibetan Plateau. The turbulent spectra of wind velocity, potential temperature, and humidity satisfy the-2/3 power law in the high frequency range. Horizontal transportation of heat and water vapor is negligible compared with vertical transportation under strong unstable conditions, and as the stability parameter z/L increases (where z is the observational height, and L is the Monin Obukhov length), horizontal transportation becomes dominant under near-neutral, neutral, and stable conditions. The non-dimensional temperature and humidity variances are 20% less than the temperature and humidity gradient variances. These deficits appear to increase as the absolute stability parameter increases. Moreover, the effects of turbulence transportation and pressure variance exist throughout the entire stability region.展开更多
Small concentrations of a high-molecular-weight polymer have been used to create so-called ＂elastic tur- bulence＂ in a micro-scale serpentine channel geometry. It is known that the interaction of large elastic stres...Small concentrations of a high-molecular-weight polymer have been used to create so-called ＂elastic tur- bulence＂ in a micro-scale serpentine channel geometry. It is known that the interaction of large elastic stresses created by the shearing motion within the fluid flow with streamline curvature of the serpentine geometry leads initially to a purely-elastic instability and then the generation of elastic turbulence. We show that this elastic turbulence enhances the heat transfer at the micro-scale in this geometry by up to 300% under creeping flow conditions in comparison to that achieved by the equivalent Newtonian fluid flow.展开更多
The recent works on the development of computational mass transfer （CMT） method and its applications in chemical process simulation are reviewed. Some development strategies and challenges in future research are als...The recent works on the development of computational mass transfer （CMT） method and its applications in chemical process simulation are reviewed. Some development strategies and challenges in future research are also discussed.展开更多
A computational mass transfer model is proposed for predicting the concentration profile and Murphree efficiency of sieve tray distillation column. The proposed model is based on using modified c＇2 -εc＇ two equatio...A computational mass transfer model is proposed for predicting the concentration profile and Murphree efficiency of sieve tray distillation column. The proposed model is based on using modified c＇2 -εc＇ two equations formulation for closing the differential turbulent mass transfer equation with improvement by considering the vapor injected from the sieve hole to be three dimensional. The predicted concentration distributions by using proposed model were checked by experimental work conducted on a sieve tray simulator of 1.2 meters in diameter for desorbing the dissolved oxygen in the feed water by blowing air. The model predictions were confirmed by the experimental measurement. The validation of the proposed model was further tested by comparing the simulated result with the performance of an industrial scale sieve tray distillation column reported by Kunesh et al. for the stripping of toluene from its water solution. The predicted outlet concentration of each tray and the Murphree tray efficiencies under different operating conditions were in agreement with the published data. The simulated turbulent mass transfer diffusivity on each tray was within the range of the experimental result in the same sieve column reported by Cai et al. In addition, the prediction of the influence of sieve tray structure on the tray efficiency by using the proposed model was demonstrated.展开更多
To understand the local atmosphere and heat transfer and to facilitate the boundary-layer parameterization of numerical simulation and prediction, an observational campaign was conducted in the Eastern Himalayas in Ju...To understand the local atmosphere and heat transfer and to facilitate the boundary-layer parameterization of numerical simulation and prediction, an observational campaign was conducted in the Eastern Himalayas in June 2010. The local atmospheric properties and near-surface turbulent heat transfers were analyzed. The local atmosphere in this region is warmer, more humid and less windy, with weaker solar ra- diation and surface radiate heating than in the Middle Himalayas. The near-surface turbulent heat transfer in the Eastern Himalayas is weaker than that in the Middle Himalayas. The total heat transfer is mainly contributed by the latent heat transfer with a Bowen ratio of 0.36, which is essentially different from that in the Middle Himalayas and the other Tibetan regions.展开更多
Derivation of bulk transport coefficients helps solving land surface processes. A similarity-based method for determining the turbulent transfer (including the flux exchange, the vertical distribution of wind and pote...Derivation of bulk transport coefficients helps solving land surface processes. A similarity-based method for determining the turbulent transfer (including the flux exchange, the vertical distribution of wind and potential temperature) in the atmospheric surface layer is presented. Comparisons with iterative schemes (Businger, 1971) are given to demonstrate the advantages of the calculation methods.展开更多
Based on the HEIFE 1988 and 1990 pilot observations,an analysis on the turbulence structure of Gobi surface layer,mainly on the similarity formulations of wind and temperature variances,the spectra and cospectra chara...Based on the HEIFE 1988 and 1990 pilot observations,an analysis on the turbulence structure of Gobi surface layer,mainly on the similarity formulations of wind and temperature variances,the spectra and cospectra characteristics,is presented.The phenomenon of downward water vapor flux over Gobi desert in daytime is confirmed in both observations,this and the well-known‘oasis effect’are two sides of a local mesoscale circulation.展开更多
The water-heat transfer process between land and atmosphere in Haibei alpine meadow area has been systematically observed.A multi-layer coupling model for land-atmosphere interaction was presented with special attenti...The water-heat transfer process between land and atmosphere in Haibei alpine meadow area has been systematically observed.A multi-layer coupling model for land-atmosphere interaction was presented with special attention paid to the moisture transfer in leaf stomata under unsaturated condition.A profound investigation on the physical process of turbulent transfer inside the vegetation has been performed with a revised formula of water absorption for root system.The present model facilitates the study of vertically distributed physical variables in detail.Numerical simulation was conducted according to the transfer process of Kinesia humility meadow in the area of Haibei Alpine Meadow Ecosystem Station,CAS.The calculated results agree well with observation.展开更多
The horizontal convection in a square enclosure driven by a linear temperature profile along the bottom boundary is investigated numerically by using a finite difference method.The Prandtl number is fixed at 4.38,and ...The horizontal convection in a square enclosure driven by a linear temperature profile along the bottom boundary is investigated numerically by using a finite difference method.The Prandtl number is fixed at 4.38,and the Rayleigh number Ra ranges from107 to 1011.The convective flow is steady at a relatively low Rayleigh number,and no thermal plume is observed,whereas it transits to be unsteady when the Rayleigh number increases beyond the critical value.The scaling law for the Nusselt number Nu changes from Rossby’s scaling Nu~Ra^(1/5)in a steady regime to Nu~Ra^(1/4)in an unsteady regime,which agrees well with the theoretically predicted results.Accordingly,the Reynolds number Re scaling varies from Re~Ra^(3/11)to Re~Ra^(2/5).The investigation on the mean flows shows that the thermal and kinetic boundary layer thickness and the mean temperature in the bulk zone decrease with the increasing Ra.The intensity of fluctuating velocity increases with the increasing Ra.展开更多
The heat transfer enhancement(HTE) in tubular heat exchangers fitted with vortex-generator(VG) inserts is experimentally investigated. The studied four parameters and ranges are: winglets-pitch ratio(1.33, 2.67, and 4...The heat transfer enhancement(HTE) in tubular heat exchangers fitted with vortex-generator(VG) inserts is experimentally investigated. The studied four parameters and ranges are: winglets-pitch ratio(1.33, 2.67, and 4),winglets-length ratio(0.33, 0.67, and 1), winglets-width ratio(0.2, 0.4, and 0.6), and Reynolds number(5200to 12200). The testing fluids are the water and Cu–water nanofluid at the volumetric fraction of 0.2%. The results obtained on HTE, pressure drop, and performance evaluation criterion(PEC) are compared with those for water in a smooth tube. It is found that the VG inserts with lower winglets-pitch ratio and higher winglets-length/width ratios present higher values of HTE and pressure drop. Over the range studied, the maximum PEC of 1.83 is detected with the Cu–water nanofluid inside the tube equipped with a VG insert at the winglets-width ratio of0.6 for the maximum Reynolds number, when the heat transfer rate and pressure drop are 1.24 times and 2.03 times of those in the smooth tube. Generalized regression equations of the Nusselt number, friction factor, and PEC are presented for the tubular heat exchangers with the VG inserts for both water and Cu–water nanofluid.It is concluded that the main advantage of the VG inserts is their simple fabrication and considerable performance, particularly at higher Reynolds number.展开更多
Simulations of adsorption process using the Reynolds mass flux model described in Part I of these serial articles are presented. The object of the simulation is the methylene chloride adsorption in a packed column(0.0...Simulations of adsorption process using the Reynolds mass flux model described in Part I of these serial articles are presented. The object of the simulation is the methylene chloride adsorption in a packed column(0.041 m id,packed with spherical activated carbon up to a length of 0.2 m). With the Reynolds mass flux model,breakthrough/regeneration curves, concentration and temperature as well as the velocity distributions can be obtained. The simulated results are compared with the experimental data reported in the literature and satisfactory agreement is found both in breakthrough/regeneration curves and temperature curves. Moreover,the anisotropic turbulent mass diffusion is characterized and discussed.展开更多
Separation process undertaken in packed columns often displays anisotropic turbulent mass diffusion. The anisotropic turbulent mass diffusion can be characterized rigorously by using the Reynolds mass flux(RMF) model....Separation process undertaken in packed columns often displays anisotropic turbulent mass diffusion. The anisotropic turbulent mass diffusion can be characterized rigorously by using the Reynolds mass flux(RMF) model.With the RMF model, the concentration and temperature as well as the velocity distributions can be simulated numerically. The modeled Reynolds mass flux equation is adopted to close the turbulent mass transfer equation,while the modeled Reynolds heat flux and Reynolds stress equations are used to close the turbulent heat and momentum transfer equations, so that the Boussinesq postulate and the isotropic assumption are abandoned. To validate the presented RMF model, simulation is carried out for CO2 absorption into aqueous Na OH solutions in a packed column(0.1 m id, packed with 12.7 mm Berl saddles up to a height of 6.55 m). The simulated results are compared with the experimental data and satisfactory agreement is found both in concentration and temperature distributions. The sequel Part II extends the model application to the simulation of an unsteady state adsorption process in a packed column.展开更多
Turbulent forced convective heat transfer and flow con figurations in a square channel with wavy-ribs inserted diagonally are examined numerically. The in fluences of the 30° and 45° flow attack angles for w...Turbulent forced convective heat transfer and flow con figurations in a square channel with wavy-ribs inserted diagonally are examined numerically. The in fluences of the 30° and 45° flow attack angles for wavy-ribs, blockage ratio, R B= b/H = 0.05–0.25 with single pitch ratio, R P= P/H = 1 are investigated for the Reynolds number based on the hydraulic diameter of the square channel, Re = 3000–20000. The use of the wavy-ribs, which inserted diagonal in the square channel, is aimed to help to improve the thermal performance in heat exchange systems.The finite volume method and SIMPLE algorithm are applied to the present numerical simulation. The results are presented on the periodic flow and heat transfer pro files, flow con figurations, heat transfer characteristics and the performance evaluations. The mathematical results reveal that the use of wavy-ribs leads to a higher heat transfer rate and friction loss over the smooth channel. The heat transfer enhancements are around 1.97–5.14 and 2.04–5.27 times over the smooth channel for 30° and 45° attack angles, respectively. However, the corresponding friction loss values for 30° and 45° are around 4.26–86.55 and 5.03–97.98 times higher than the smooth square channel, respectively. The optimum thermal enhancement factor on both cases is found at R B= 0.10 and the lowest Reynolds number, Re = 3000, to be about 1.47 and 1.52, respectively, for 30° and 45° wavy-ribs.展开更多
This study evaluates the spatial distributions in the quality of momentum and sensible heat fluxes,and determines the turbulent transfer characteristics with quality-controlled observations.The research is based on ra...This study evaluates the spatial distributions in the quality of momentum and sensible heat fluxes,and determines the turbulent transfer characteristics with quality-controlled observations.The research is based on raw turbulence data collected over a Gobi surface in the Dunhuang area in June 2004.The results indicate that part of the momentum fluxes are of poor quality in the daytime and nighttime.The poor quality of the momentum fluxes in the daytime is mainly attributed to the development of turbulence.The footprint reveals that,in general,the momentum fluxes and sensible heat fluxes can be measured well in the east and west upwind sectors under unstable conditions.The relationship between the non-dimensional standard deviation of the wind components and atmospheric stability follow the "1/3 power law",which supports the Monin-Obukhov similarity theory.Moreover,this study identifies a clear disturbance in the measurements surrounding the Gobi surface.The momentum roughness length of z0m=0.59 mm is determined after excluding such disturbance,and the additional resistance during the daytime is proposed to be an average of 3.1,although its actual value is highly scattered.This study discusses the applicability of several thermodynamic parameterization schemes for the Gobi surface.The results show that the scheme κB-1=3.1 can represent well the summer diurnal turbulent heat transfer.展开更多
Performance evaluation of heat transfer devices can be based on the overall entropy production in these devices. In our study we therefore provide equations for the systematic and detailed determination of local entro...Performance evaluation of heat transfer devices can be based on the overall entropy production in these devices. In our study we therefore provide equations for the systematic and detailed determination of local entropy production due to dissipation of mechanical energy and due to heat conduction, both in turbulent flows. After turbulence modeling has been incorporated for the fluctuating parts the overall entropy production can be determined by integration with respect to the whole flow domain. Since, however, entropy production rates show very steep gradients close to the wall, numerical solutions are far more effective with wall functions for the entropy production terms. These wall functions are mandatory when high Reynolds number turbulence models are used. For turbulent flow in a pipe with an inserted twisted tape as heat transfer promoter it is shown that based on the overall entropy production rate a clear statement from a thermodynamic point of view is possible. For a certain range of twist strength there is a decrease in overall entropy production compared to the case without insert. Also, the optimum twist strength can be determined. This information is unavailable when only pressure drop and heat transfer data are given.展开更多
Direct Nmerical Simulation （DNS） of turbulent heat transfer in a wall-normal rotating channel flow has been carried out for the rotation number Nr from 0 to 0.1, the Reynolds number 194 based on the friction velocit...Direct Nmerical Simulation （DNS） of turbulent heat transfer in a wall-normal rotating channel flow has been carried out for the rotation number Nr from 0 to 0.1, the Reynolds number 194 based on the friction velocity of non ro taring case and the half-height of the channel, and the Prandtl number 1. The objective of this study is to reveal the effects of rotation on the characteristics of turbulent flow and heat transfer. Based on the present calculated results, two typical rotation regimes are identified. When 0 〈 Nr 〈 0.06, turbu lence and thermal statistics correlated with the spanwise veloc ity fluctuation are enhanced since the shear rate of spanwise mean flow induced by Coriolis force increases; however, the other statistics are suppressed. When Nr 〉 0.06, turbulence and thermal statistics are suppressed significantly because the Coriolis force effect plays as a dominated role in the rotating flow. Remarkable change of the direction of near wall streak structures based on the velocity and temperature fluctuations is identified.展开更多
This study deals with the turbulent structure in the surface layer over the Qinghai-Xizang Plateau.Using gradient transfer and heat balance methods we have determined the nondimensional coefficient 1/(?)_m(?)h in the ...This study deals with the turbulent structure in the surface layer over the Qinghai-Xizang Plateau.Using gradient transfer and heat balance methods we have determined the nondimensional coefficient 1/(?)_m(?)h in the expression of turbulent transfer coefficient for sensible heat (K_h).It is found that the results are in good agreement with the 1/(?)_m(?)_h obtained by Pruitt,et al.The K_h at a height of 1m under cloudy and cloudless conditions is calculated.Finally,the ratio of K_h to momentum turbulent coefficient over the plateau is compared with those over plains.展开更多
Direct numerical simulation of vertical rotating open-channel flow with heat transfer has been carried out for the rotation number Nτfrom 0 to 0.1,the Prandtl number 1,and the Reynolds number 180 based on the frictio...Direct numerical simulation of vertical rotating open-channel flow with heat transfer has been carried out for the rotation number Nτfrom 0 to 0.1,the Prandtl number 1,and the Reynolds number 180 based on the friction velocity of non-rotating flow and the height of the channel.The ob jective of this study is to reveal the effect of rotation on the characteristics of turbulent flow and heat transfer,in particular near the free surface and the wall of the open-channel.Statistical quantities,e.g.,the mean velocity,temperature and their fluctuations,turbulent heat fluxes,and turbulence structures,are analyzed.The depth of surface-influenced layer decreases with the increase of the rotation rate.In the free surface-influenced layer,the turbulence and thermal statistics are suppressed due to the effect of rotation.In the wall-influenced region,two typical rotation regimes are identified.In the weak rotation regime with 0<Nτ<0.06 approximately,the turbulence and thermal statistics correlated with the spanwise velocity fluctuation are enhanced since the shear rate of spanwise mean flow induced by Coriolis force increases;however,the other statistics are suppressed.In the strong rotation regime with Nτ>0.06,the turbulence and thermal statistics are suppressed significantly because the Coriolis force effect plays a dominant role in the rotating flow.To elucidate the effect of rotation on turbulent flow and heat transfer,the budget terms in the transport equations of Reynolds stresses and turbulent heat fluxes are investigated.Remarkable change of the direction of streak structures based on the velocity and temperature fluctuations is discussed.展开更多
A structure-based mass-transfer model for turbulent fluidized beds （TFBs） was established according to mass conservation and the balance of mass transfer and reaction. Unlike the traditional method, which assumes a ...A structure-based mass-transfer model for turbulent fluidized beds （TFBs） was established according to mass conservation and the balance of mass transfer and reaction. Unlike the traditional method, which assumes a homogeneous structure, this model considered the presence of voids and particle clusters in TFBs and built correlations for each phase. The flow parameters were solved based on a previously proposed structure-based drag model. The catalytic combustion of methane at three temperatures and ozone decomposition at various gas velocities were used to validate the model. The TFB reactions com- prised intrinsic reaction kinetics, internal diffusion, and external diffusion. The simulation results, which compared favorably with experimental data and were better than those based on the average method, demonstrated that methane was primarily consumed at the bottom of the bed and the methane concentration was closely related to the presence of the catalyst. The flow and diffusion had an important effect on the methane concentration. This model also predicted the outlet concentrations for ozone decomposition, which increased with increasing gas velocity, lnterphase mass transfer was presented as the limiting step for this system. This structure-based mass-transfer model is important for the industrial application of TFBs.展开更多
To investigate the convective heat transfer of nanofluids, experiments were performed using silver-water nanofluids under laminar, transition and turbulent flow regimes in a horizontal 4.3 mm inner-diameter tube-in-tu...To investigate the convective heat transfer of nanofluids, experiments were performed using silver-water nanofluids under laminar, transition and turbulent flow regimes in a horizontal 4.3 mm inner-diameter tube-in-tube counter-current heat transfer test section. The volume concentration of the nanoparticles varied from 0.3% to 0.9% in steps of 0.3%, and the effects of thermo-physical properties, inlet temperature, volume concentration, and mass flow rate on heat transfer coefficient were investigated. Experiments showed that the suspended nanoparticles remarkably increased the convective heat transfer coefficient, by as much as 28.7% and 69.3% for 0.3% and 0.9% of silver content, respectively. Based on the experimental results a correlation was developed to predict the Nusselt number of the silver-water nanofluid, with ＋10% agreement between experiments and prediction.展开更多
基金support of the National Natural Science Foundation of China(41075005)the Research Fund for the Doctoral Program of Higher Education(20110001130010)R&D Special Fund for Public Welfare Industry (Meteorology) by Ministry of Finance and Ministry of Science and Technology(GYHY201006014) in the present study
文摘In this paper, turbulent data obtained from the Damxung site during the Secondary Tibetan Plateau Science Experiment (TIPEX) in 1998 are used to study the characteristics of the turbulent spectra, turbulence transport, and the dissipation rates of turbulent kinetic energy, temperature variance, and humidity variance in the middle area of the Tibetan Plateau. The turbulent spectra of wind velocity, potential temperature, and humidity satisfy the-2/3 power law in the high frequency range. Horizontal transportation of heat and water vapor is negligible compared with vertical transportation under strong unstable conditions, and as the stability parameter z/L increases (where z is the observational height, and L is the Monin Obukhov length), horizontal transportation becomes dominant under near-neutral, neutral, and stable conditions. The non-dimensional temperature and humidity variances are 20% less than the temperature and humidity gradient variances. These deficits appear to increase as the absolute stability parameter increases. Moreover, the effects of turbulence transportation and pressure variance exist throughout the entire stability region.
基金financial support from The Higher Committee for Education Development in Iraq and The Iraqi Ministry of Higher Education and Scientific Research
文摘Small concentrations of a high-molecular-weight polymer have been used to create so-called ＂elastic tur- bulence＂ in a micro-scale serpentine channel geometry. It is known that the interaction of large elastic stresses created by the shearing motion within the fluid flow with streamline curvature of the serpentine geometry leads initially to a purely-elastic instability and then the generation of elastic turbulence. We show that this elastic turbulence enhances the heat transfer at the micro-scale in this geometry by up to 300% under creeping flow conditions in comparison to that achieved by the equivalent Newtonian fluid flow.
基金Supported by the National Science Foundation of China（20736005）.ACKNOWLEDGEMENTSThe authors acknowledge the assistance from thestaff in the State Key Laboratories of Chemical Engineering （Tianjin University）.
文摘The recent works on the development of computational mass transfer （CMT） method and its applications in chemical process simulation are reviewed. Some development strategies and challenges in future research are also discussed.
基金Supported by the National lqatural Science Foundation of China （20736005）.
文摘A computational mass transfer model is proposed for predicting the concentration profile and Murphree efficiency of sieve tray distillation column. The proposed model is based on using modified c＇2 -εc＇ two equations formulation for closing the differential turbulent mass transfer equation with improvement by considering the vapor injected from the sieve hole to be three dimensional. The predicted concentration distributions by using proposed model were checked by experimental work conducted on a sieve tray simulator of 1.2 meters in diameter for desorbing the dissolved oxygen in the feed water by blowing air. The model predictions were confirmed by the experimental measurement. The validation of the proposed model was further tested by comparing the simulated result with the performance of an industrial scale sieve tray distillation column reported by Kunesh et al. for the stripping of toluene from its water solution. The predicted outlet concentration of each tray and the Murphree tray efficiencies under different operating conditions were in agreement with the published data. The simulated turbulent mass transfer diffusivity on each tray was within the range of the experimental result in the same sieve column reported by Cai et al. In addition, the prediction of the influence of sieve tray structure on the tray efficiency by using the proposed model was demonstrated.
基金financed by the Ministry of Science and Technology of the People's Republic of China (Grant No.2009CB421403)the Chinese Academy of Sciences (Grant No. KZCX3-YW-Q11-01the National Natural Science Foundation of China (GrantNo.40905067)
文摘To understand the local atmosphere and heat transfer and to facilitate the boundary-layer parameterization of numerical simulation and prediction, an observational campaign was conducted in the Eastern Himalayas in June 2010. The local atmospheric properties and near-surface turbulent heat transfers were analyzed. The local atmosphere in this region is warmer, more humid and less windy, with weaker solar ra- diation and surface radiate heating than in the Middle Himalayas. The near-surface turbulent heat transfer in the Eastern Himalayas is weaker than that in the Middle Himalayas. The total heat transfer is mainly contributed by the latent heat transfer with a Bowen ratio of 0.36, which is essentially different from that in the Middle Himalayas and the other Tibetan regions.
基金Key National Scientific Project for the"9^(th)-five year"economic plan(No.96-908-02-04-2)
文摘Derivation of bulk transport coefficients helps solving land surface processes. A similarity-based method for determining the turbulent transfer (including the flux exchange, the vertical distribution of wind and potential temperature) in the atmospheric surface layer is presented. Comparisons with iterative schemes (Businger, 1971) are given to demonstrate the advantages of the calculation methods.
基金The HEIFE program is supported by National Natural Science Foundation of China,the Chinese Academy of Sciences,the Special Committee for WCRP of the Geodesy Council,Japanese Ministry of Education,Science and Culture.
文摘Based on the HEIFE 1988 and 1990 pilot observations,an analysis on the turbulence structure of Gobi surface layer,mainly on the similarity formulations of wind and temperature variances,the spectra and cospectra characteristics,is presented.The phenomenon of downward water vapor flux over Gobi desert in daytime is confirmed in both observations,this and the well-known‘oasis effect’are two sides of a local mesoscale circulation.
文摘The water-heat transfer process between land and atmosphere in Haibei alpine meadow area has been systematically observed.A multi-layer coupling model for land-atmosphere interaction was presented with special attention paid to the moisture transfer in leaf stomata under unsaturated condition.A profound investigation on the physical process of turbulent transfer inside the vegetation has been performed with a revised formula of water absorption for root system.The present model facilitates the study of vertically distributed physical variables in detail.Numerical simulation was conducted according to the transfer process of Kinesia humility meadow in the area of Haibei Alpine Meadow Ecosystem Station,CAS.The calculated results agree well with observation.
基金the National Natural Science Foundation of China(Nos.11988102,92052201,11972220,11825204,91852202,and 11732010)the China Postdoctoral Science Foundation(No.2020M681259)the Key Research Projects of Shanghai Science and Technology Commission(Nos.19JC1412802 and 20ZR1419800)。
文摘The horizontal convection in a square enclosure driven by a linear temperature profile along the bottom boundary is investigated numerically by using a finite difference method.The Prandtl number is fixed at 4.38,and the Rayleigh number Ra ranges from107 to 1011.The convective flow is steady at a relatively low Rayleigh number,and no thermal plume is observed,whereas it transits to be unsteady when the Rayleigh number increases beyond the critical value.The scaling law for the Nusselt number Nu changes from Rossby’s scaling Nu~Ra^(1/5)in a steady regime to Nu~Ra^(1/4)in an unsteady regime,which agrees well with the theoretically predicted results.Accordingly,the Reynolds number Re scaling varies from Re~Ra^(3/11)to Re~Ra^(2/5).The investigation on the mean flows shows that the thermal and kinetic boundary layer thickness and the mean temperature in the bulk zone decrease with the increasing Ra.The intensity of fluctuating velocity increases with the increasing Ra.
文摘The heat transfer enhancement(HTE) in tubular heat exchangers fitted with vortex-generator(VG) inserts is experimentally investigated. The studied four parameters and ranges are: winglets-pitch ratio(1.33, 2.67, and 4),winglets-length ratio(0.33, 0.67, and 1), winglets-width ratio(0.2, 0.4, and 0.6), and Reynolds number(5200to 12200). The testing fluids are the water and Cu–water nanofluid at the volumetric fraction of 0.2%. The results obtained on HTE, pressure drop, and performance evaluation criterion(PEC) are compared with those for water in a smooth tube. It is found that the VG inserts with lower winglets-pitch ratio and higher winglets-length/width ratios present higher values of HTE and pressure drop. Over the range studied, the maximum PEC of 1.83 is detected with the Cu–water nanofluid inside the tube equipped with a VG insert at the winglets-width ratio of0.6 for the maximum Reynolds number, when the heat transfer rate and pressure drop are 1.24 times and 2.03 times of those in the smooth tube. Generalized regression equations of the Nusselt number, friction factor, and PEC are presented for the tubular heat exchangers with the VG inserts for both water and Cu–water nanofluid.It is concluded that the main advantage of the VG inserts is their simple fabrication and considerable performance, particularly at higher Reynolds number.
基金Supported by the National Natural Science Foundation of China(21376163)
文摘Simulations of adsorption process using the Reynolds mass flux model described in Part I of these serial articles are presented. The object of the simulation is the methylene chloride adsorption in a packed column(0.041 m id,packed with spherical activated carbon up to a length of 0.2 m). With the Reynolds mass flux model,breakthrough/regeneration curves, concentration and temperature as well as the velocity distributions can be obtained. The simulated results are compared with the experimental data reported in the literature and satisfactory agreement is found both in breakthrough/regeneration curves and temperature curves. Moreover,the anisotropic turbulent mass diffusion is characterized and discussed.
基金Supported by the National Natural Science Foundation of China(21376163)
文摘Separation process undertaken in packed columns often displays anisotropic turbulent mass diffusion. The anisotropic turbulent mass diffusion can be characterized rigorously by using the Reynolds mass flux(RMF) model.With the RMF model, the concentration and temperature as well as the velocity distributions can be simulated numerically. The modeled Reynolds mass flux equation is adopted to close the turbulent mass transfer equation,while the modeled Reynolds heat flux and Reynolds stress equations are used to close the turbulent heat and momentum transfer equations, so that the Boussinesq postulate and the isotropic assumption are abandoned. To validate the presented RMF model, simulation is carried out for CO2 absorption into aqueous Na OH solutions in a packed column(0.1 m id, packed with 12.7 mm Berl saddles up to a height of 6.55 m). The simulated results are compared with the experimental data and satisfactory agreement is found both in concentration and temperature distributions. The sequel Part II extends the model application to the simulation of an unsteady state adsorption process in a packed column.
基金Supported by College of Industrial Technology,King Mongkut's University of Technology North Bangkok,Thailand
文摘Turbulent forced convective heat transfer and flow con figurations in a square channel with wavy-ribs inserted diagonally are examined numerically. The in fluences of the 30° and 45° flow attack angles for wavy-ribs, blockage ratio, R B= b/H = 0.05–0.25 with single pitch ratio, R P= P/H = 1 are investigated for the Reynolds number based on the hydraulic diameter of the square channel, Re = 3000–20000. The use of the wavy-ribs, which inserted diagonal in the square channel, is aimed to help to improve the thermal performance in heat exchange systems.The finite volume method and SIMPLE algorithm are applied to the present numerical simulation. The results are presented on the periodic flow and heat transfer pro files, flow con figurations, heat transfer characteristics and the performance evaluations. The mathematical results reveal that the use of wavy-ribs leads to a higher heat transfer rate and friction loss over the smooth channel. The heat transfer enhancements are around 1.97–5.14 and 2.04–5.27 times over the smooth channel for 30° and 45° attack angles, respectively. However, the corresponding friction loss values for 30° and 45° are around 4.26–86.55 and 5.03–97.98 times higher than the smooth square channel, respectively. The optimum thermal enhancement factor on both cases is found at R B= 0.10 and the lowest Reynolds number, Re = 3000, to be about 1.47 and 1.52, respectively, for 30° and 45° wavy-ribs.
基金supported by National Basic Research Program of China (Grant No. 2009CB421405)National Natural Science Foundation of China (Grant No. 40730952)Open Fund of Regional Climate-Environment Research for Temperate East Asia,Chinese Academy of Sciences
文摘This study evaluates the spatial distributions in the quality of momentum and sensible heat fluxes,and determines the turbulent transfer characteristics with quality-controlled observations.The research is based on raw turbulence data collected over a Gobi surface in the Dunhuang area in June 2004.The results indicate that part of the momentum fluxes are of poor quality in the daytime and nighttime.The poor quality of the momentum fluxes in the daytime is mainly attributed to the development of turbulence.The footprint reveals that,in general,the momentum fluxes and sensible heat fluxes can be measured well in the east and west upwind sectors under unstable conditions.The relationship between the non-dimensional standard deviation of the wind components and atmospheric stability follow the "1/3 power law",which supports the Monin-Obukhov similarity theory.Moreover,this study identifies a clear disturbance in the measurements surrounding the Gobi surface.The momentum roughness length of z0m=0.59 mm is determined after excluding such disturbance,and the additional resistance during the daytime is proposed to be an average of 3.1,although its actual value is highly scattered.This study discusses the applicability of several thermodynamic parameterization schemes for the Gobi surface.The results show that the scheme κB-1=3.1 can represent well the summer diurnal turbulent heat transfer.
文摘Performance evaluation of heat transfer devices can be based on the overall entropy production in these devices. In our study we therefore provide equations for the systematic and detailed determination of local entropy production due to dissipation of mechanical energy and due to heat conduction, both in turbulent flows. After turbulence modeling has been incorporated for the fluctuating parts the overall entropy production can be determined by integration with respect to the whole flow domain. Since, however, entropy production rates show very steep gradients close to the wall, numerical solutions are far more effective with wall functions for the entropy production terms. These wall functions are mandatory when high Reynolds number turbulence models are used. For turbulent flow in a pipe with an inserted twisted tape as heat transfer promoter it is shown that based on the overall entropy production rate a clear statement from a thermodynamic point of view is possible. For a certain range of twist strength there is a decrease in overall entropy production compared to the case without insert. Also, the optimum twist strength can be determined. This information is unavailable when only pressure drop and heat transfer data are given.
基金Project supported by the National Natural Science Foundation of China (Grant Nos:90405007 ,10302028,10125210) ,Specialized Research Fund for the Doctoral Programof Higher Education (Grant No :20020358013),the China NKBRSF Pro-ject (Grant No :2001CB409600) ,and the Hundred-Talent Programof the Chinese Academy of Sciences
文摘Direct Nmerical Simulation （DNS） of turbulent heat transfer in a wall-normal rotating channel flow has been carried out for the rotation number Nr from 0 to 0.1, the Reynolds number 194 based on the friction velocity of non ro taring case and the half-height of the channel, and the Prandtl number 1. The objective of this study is to reveal the effects of rotation on the characteristics of turbulent flow and heat transfer. Based on the present calculated results, two typical rotation regimes are identified. When 0 〈 Nr 〈 0.06, turbu lence and thermal statistics correlated with the spanwise veloc ity fluctuation are enhanced since the shear rate of spanwise mean flow induced by Coriolis force increases; however, the other statistics are suppressed. When Nr 〉 0.06, turbulence and thermal statistics are suppressed significantly because the Coriolis force effect plays as a dominated role in the rotating flow. Remarkable change of the direction of near wall streak structures based on the velocity and temperature fluctuations is identified.
文摘This study deals with the turbulent structure in the surface layer over the Qinghai-Xizang Plateau.Using gradient transfer and heat balance methods we have determined the nondimensional coefficient 1/(?)_m(?)h in the expression of turbulent transfer coefficient for sensible heat (K_h).It is found that the results are in good agreement with the 1/(?)_m(?)_h obtained by Pruitt,et al.The K_h at a height of 1m under cloudy and cloudless conditions is calculated.Finally,the ratio of K_h to momentum turbulent coefficient over the plateau is compared with those over plains.
文摘Direct numerical simulation of vertical rotating open-channel flow with heat transfer has been carried out for the rotation number Nτfrom 0 to 0.1,the Prandtl number 1,and the Reynolds number 180 based on the friction velocity of non-rotating flow and the height of the channel.The ob jective of this study is to reveal the effect of rotation on the characteristics of turbulent flow and heat transfer,in particular near the free surface and the wall of the open-channel.Statistical quantities,e.g.,the mean velocity,temperature and their fluctuations,turbulent heat fluxes,and turbulence structures,are analyzed.The depth of surface-influenced layer decreases with the increase of the rotation rate.In the free surface-influenced layer,the turbulence and thermal statistics are suppressed due to the effect of rotation.In the wall-influenced region,two typical rotation regimes are identified.In the weak rotation regime with 0<Nτ<0.06 approximately,the turbulence and thermal statistics correlated with the spanwise velocity fluctuation are enhanced since the shear rate of spanwise mean flow induced by Coriolis force increases;however,the other statistics are suppressed.In the strong rotation regime with Nτ>0.06,the turbulence and thermal statistics are suppressed significantly because the Coriolis force effect plays a dominant role in the rotating flow.To elucidate the effect of rotation on turbulent flow and heat transfer,the budget terms in the transport equations of Reynolds stresses and turbulent heat fluxes are investigated.Remarkable change of the direction of streak structures based on the velocity and temperature fluctuations is discussed.
文摘A structure-based mass-transfer model for turbulent fluidized beds （TFBs） was established according to mass conservation and the balance of mass transfer and reaction. Unlike the traditional method, which assumes a homogeneous structure, this model considered the presence of voids and particle clusters in TFBs and built correlations for each phase. The flow parameters were solved based on a previously proposed structure-based drag model. The catalytic combustion of methane at three temperatures and ozone decomposition at various gas velocities were used to validate the model. The TFB reactions com- prised intrinsic reaction kinetics, internal diffusion, and external diffusion. The simulation results, which compared favorably with experimental data and were better than those based on the average method, demonstrated that methane was primarily consumed at the bottom of the bed and the methane concentration was closely related to the presence of the catalyst. The flow and diffusion had an important effect on the methane concentration. This model also predicted the outlet concentrations for ozone decomposition, which increased with increasing gas velocity, lnterphase mass transfer was presented as the limiting step for this system. This structure-based mass-transfer model is important for the industrial application of TFBs.
基金King Mongkut’s University of Technology Thonburi, the Thailand Research Fund, the Office of the Higher Education Commissionthe National Research University Project
文摘To investigate the convective heat transfer of nanofluids, experiments were performed using silver-water nanofluids under laminar, transition and turbulent flow regimes in a horizontal 4.3 mm inner-diameter tube-in-tube counter-current heat transfer test section. The volume concentration of the nanoparticles varied from 0.3% to 0.9% in steps of 0.3%, and the effects of thermo-physical properties, inlet temperature, volume concentration, and mass flow rate on heat transfer coefficient were investigated. Experiments showed that the suspended nanoparticles remarkably increased the convective heat transfer coefficient, by as much as 28.7% and 69.3% for 0.3% and 0.9% of silver content, respectively. Based on the experimental results a correlation was developed to predict the Nusselt number of the silver-water nanofluid, with ＋10% agreement between experiments and prediction.
