LAMINAR DEVELOPING FLOW IN THE ENTRANCE REGION OF ROTATING CURVED PIPES

Three-dimensional laminar flow in the entrance region of rotating curved pipes was investigated. The governing equations were written in an orthogonal curvilinear coordinate system and solved with a fully three-dimensional numerical method. The development of secondary flow, axial velocity, local and average friction factors for different cases of rotation were given and discussed in detail. The results show that rotation influences the flow structure and friction factor greatly and that the secondary flow is sink-type in the early stage of development and then turns to vortex structure. The average friction factor and the intensity of secondary flow have drastic decrease near the entrance. At some proper rotation, the average friction factor can be noticeably reduced.

LAMINAR FLOW AND HEAT TRANSFER IN THE DEVELOPING REGION OF HELICAL SQUARE DUCTS

Simultaneous development of the laminar flow and heat transfer in helicalsquare ducts was numerically studied. The governing equations were written in an orthogonal helicalcoordinate system and fully parabolized in the axial direction. Results were found out over a widerange of the governing parameters. Two axial velocity entries were taken into account. Thedevelopment of secondary flow, axial velocity and temperature distribution for the large Dean numberwere examined in detail and the effects of different governing parameters on the friction factorand- the Nusselt number were annlyzee. Many new and interesting conclusions were reached. Thepresent results reveal the nature of fluid flow and heat transfer in the developing region ofhelical square ducts.

THE NUMERICAL PREDICTION OF DEVELOPING TURBULENT FLOW IN 3-D RECTANGULAR DUCTS

Comparisons are made between experimental data and numerical predictions based on the k-e turbulent model of low Reynolds number applicable to developing turbulent flow in rectangular ducts of arbitrary aspect ratio.The numerical procedure utilizes the separated-layers finite-analytical method.The merits of the k-e turbulent model of low Reynolds number and the computation procedure are assessed by means of comparison with results,referred to that of the length-scale model and the full-Reynolds-stress model used in recent years.

Analysis of Entropy Generation in a Rectangular Porous Duct

In this paper, we have considered a fully developed flow of a viscous incompressible fluid in a rectangular porous duct saturated with the same fluid. The duct is heated from the bottom for forced and mixed convection. The Brinkman model is used to simulate the momentum transfer in the porous duct. Using the momentum and thermal energy equations, the entropy generation has been obtained due to the heat transfer, viscous and Darcy dissipations. It is found from the mathematical analysis that the entropy generation is double when the viscous as well as the Darcy dissipations terms are taken in the thermal energy equation in comparison when the viscous as well as the Darcy dissipations terms are not taken in the thermal energy equation. This result clearly shows that there is no need of taking the viscous and Darcy dissipations terms in the thermal energy equation to obtain the entropy generation.

STEADY ENTRANCE FLOW IN A STRAIGHT CIRCULAR PIPE WITH SLOWLY VARYING CROSS SECTION

A pertubation method for the steady entrance flow through a convergingdiverging pipe, which can model an arterial stenosis at the initial stage, is presented. Aset of formulas for velocity, pressure and wall shear stress distributions are obtained. Results show that the flow patterns of entrance flow are considerably different from those ofdeveloped flow in a converging-diverging pipe: the velocity distribution at the stenosis issmoother, the centerline velocity decreases, and the wall shear stress and pressure gradient increase in the entrance region. The length of entrance flow for the pipe is slighty lower than that for a straight one with constant cross section. The results are checked by thefinite difference method.

GAS-SOLIDS FLOW BEHAVIOR WITH A GAS VELOCITY CLOSE TO ZERO

In a 9.3 m high and 0.10 m i.d. gas-solids downflow fluidized bed （downer）, the radial and axial distributions of the local solids holdups and particle velocities along the downer column were measured with the superficial gas velocity set to zero. A unique gas-solids flow structure was found in the downer system with zero gas velocity, which is completely different from that under conditions with higher gas velocities, in terms of its radial and axial flow structures as well as its micro flow structure. The gas-solids flow pattern under zero gas velocity conditions, together with that under low gas velocity conditions, can be considered as a special regime which differs from that under higher gas velocity conditions. According to the hydrodynamic properties of the two regimes, they can be named the ＂dense annulus＂ regime for the flow pattern under zero or low gas velocity conditions and the ＂dense core＂ regime for that under higher gas velocity conditions.

Truncation method for calculating the resistance of ventilation air-conditioning duct systems under nonfully developed flow boundary conditions

Calculating the resistance of ventilation air-conditioning ducts under nonfully developed flow is a crucial problem that must be addressed. Based on the characteristics of the resistance in ventilation air-conditioning ducts, the truncation method-a computational method that is appropriate for nonfully developed flow boundary conditions-was proposed in this study. The resistance distributions in the upstream and downstream ducts from typical local components, including reducers, bends and tee ducts, were investigated. Using the resistance values of the local components under fully developed flow, the resistances that did not belong to nonfully developed flow were truncated and removed. Finally, the calculation steps of the proposed method were discussed, an engineering case study was presented, and the accuracy of the developed model was analyzed. The results showed that for the local components in the system (reducers, bends and tee ducts), their proportions of the total resistance exhibited similar trends under different width-to-height ratios. The resistance of these local components included upstream resistance, downstream resistance and their own resistance. The upstream resistance accounted for 2%–6% of the total resistance, whereas the downstream resistance of the reducers, bends and tee ducts accounted for 40%–60% of the total resistance. A functional relationship was established between the local resistance and cutoff distance of the reducers, bends and tee ducts. Hence, the truncation method can calculate the local resistance from the cutoff distance. Moreover, in the presented engineering case study, the error between the actual measured resistance values and those simulated with the truncation method was only 4.28%, which was far less than that of the results simulated with the traditional calculation methods (53.64%).

Natural convection flow in a vertical annulus with time-periodic thermal boundary conditions

This paper presents an analytical solution for natural convection flow in a vertical annulus due to time-periodic heating of annulus surfaces.Closed-form expressions for velocity,temperature,skin-friction,mass flow rate and rate of heat transfer which is expressed as Nusselt number are obtained by solving the present mathematical model after separating into steady component and periodic regime.The effects of pertinent parameters such as Strouhal number(St),Prandtl number(Pr)and radius ratio(λ)are shown with the aid of contour and line graphs.Results indicate that the role of Strouhal number and Prandtl number is to decrease fluid velocity,temperature and skin-friction.Furthermore,increase in Strouhal number increases the temperature phase-lag.

Numerical Simulation of Unidirectional Stratified Flow by Moving Particle Semi Implicit Method

Numerical simulation of stratified flow of two fluids between two infinite parallel plates using the Moving Particle Semi-implicit(MPS)method is presented.The developing process from entrance to fully development flow is captured.In the simulation,the computational domain is represented by various types of particles.Governing equations are described based on particles and their interactions.Grids are not necessary in any calculation steps of the simulation.The particle number density is implicitly required to be constant to satisfy incompressibility.The weight function is used to describe the interaction between different particles.The particle is considered to constitute the free interface if the particle number density is below a set point.Results for various combinations of density,viscosity,mass flow rates,and distance between the two parallel plates are presented.The proposed procedure is validated using the derived exact solution and the earlier numerical results from the Level-Set method.Furthermore,the evolution of the interface in the developing region is captured and compares well with the derived exact solutions in the developed region.

Some developments of hot accretion flow theory in the past ten years

The hot accretion flow model was re-discovered in 1994 by Narayan and collaborators.Intensive theoretical works have been conducted and significant progresses have been achieved.In this paper,we review several developments in the past ten years.This mainly includes the finding of outflow and convection and its dynamical effect on inflow;the direct electron heating by viscous dissipation;the effect of large scale toroidal magnetic fields in the inner region of the accretion flow;and the effect of global Compton scattering.Their observational applications are also introduced very briefly.

THE APPLICATION AND DEVELOPMENT OF TURBULENCE MODELS IN BUOYANT RECIRCULATING FLOWS

In this paper, a modified κ-ε turbulence model, a simplified algebraic stress model and a developed two-fluid model have been presented based on numerical modeling of turbulent buoyant recirculating flows. The calculated results by these models are in good agreement with experiments. However, the last model is much better for simulating gravity-stratified flows.