Enhancing heat transfer at the micro-scale using elastic turbulence

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.

The Local Atmosphere and the Turbulent Heat Transfer in the Eastern Himalayas

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.

Direct numerical simulation of viscoelastic-fluid-based nanofluid turbulent channel flow with heat transfer

Our previous experimental studies have confirmed that viscoelastic-fluid-based nanofluid(VFBN) prepared by suspending nanoparticles in a viscoelastic base fluid(VBF, behaves drag reduction at turbulent flow state) can reduce turbulent flow resistance as compared with water and enhance heat transfer as compared with VBF. Direct numerical simulation(DNS) is performed in this study to explore the mechanisms of heat transfer enhancement(HTE) and flow drag reduction(DR) for the VFBN turbulent flow. The Giesekus model is used as the constitutive equation for VFBN. Our previously proposed thermal dispersion model is adopted to take into account the thermal dispersion effects of nanoparticles in the VFBN turbulent flow. The DNS results show similar behaviors for flow resistance and heat transfer to those obtained in our previous experiments. Detailed analyses are conducted for the turbulent velocity, temperature, and conformation fields obtained by DNSs for different fluid cases, and for the friction factor with viscous, turbulent, and elastic contributions and heat transfer rate with conductive, turbulent and thermal dispersion contributions of nanoparticles, respectively. The mechanisms of HTE and DR of VFBN turbulent flows are then discussed. Based on analogy theory, the ratios of Chilton–Colburn factor to friction factor for different fluid flow cases are investigated, which from another aspect show the significant enhancement in heat transfer performance for some cases of water-based nanofluid and VFBN turbulent flows.

A METHOD FOR DETERMINING TURBULENT TRANSFER IN THE ATMOSPHERIC SURFACE LAYER

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.

TURBULENCE STRUCTURE AND TRANSFER CHARACTERISTICS IN THE SURFACE LAYER OF HEIFE GOBI AREA

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.

OBSERVATION AND MODELING FOR TERRESTRIAL PROCESSES IN ALPINE MEADOW

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.

Horizontal convection in a rectangular enclosure driven by a linear temperature profile

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.

DIRECT NUMERICAL SIMULATION OF TURBULENT HEAT TRANSFER IN A WALL-NORMAL ROTATING CHANNEL FLOW

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.

EXPERIMENTAL ANALYSIS OF THE TURBULENT TRANSFER COEFFICIENT FOR SENSIBLE HEAT IN THE SURFACE LAYER OVER THE QINGHAI-XIZANG PLATEAU

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.

Direct Numerical Simulation of Vertical Rotating Turbulent Open-Channel Flow with Heat Transfer

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.

Simulation with a structure-based mass-transfer model for turbulent fluidized beds

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.

Convective heat transfer of nanofluids with correlations

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.

Experiment on Dust Flux During Duststorm Periods over Desert Area

The present study investigates the characteristics of turbulent transfer and the conditions for dust emission and transport using the dust concentration and micrometeorological data obtained during dust events occurring in the spring of 2004 over the Hunshandake desert area. The turbulent exchange coefficients and turbulent fluxes of momentum and heat are calculated. The relationships between dust flux, friction velocity, and wind speed are also explored. The results show that thermal turbulence is dominant during daytime of non-dusty days. The dynamic turbulence increases obviously and the sensible heat flux reduces by different degrees during dust events. There is an efficient downward transfer of momentum before duststorm occurrence, and both the dynamic turbulence and the thermal turbulence are important in the surface layer. The dynamic turbulence even exceeds the thermal turbulence during severe duststorm events. The values of dust flux vary in the range of -5 5, -30 30, and -200-300 μg m^-2 s^-1 during non-dusty days, blowing dust, and duststorm events, respectively. A slight upward transport of dust is observed during non-dusty days. The dust flux gradually varies from positive to negative during duststorm periods, which indicates the time evolution of dust events from dust rising to stably suspending and then deposition. The dust flux is found to be proportional to u＊^3. The threshold values of wind speed and friction velocity are about 6 and 0.4 m s^-1, respectively.