Entrance Region Of Rectangular Duct Research Articles

An experimental study of pressure drop and friction factor for laminar and transition opposing mixed convection in entrance region of rectangular duct

The laminar and transition opposing mixed convection was experimentally investigated when the air flow was in the entrance region through a rectangular duct. The pressure drops were measured under different air flowrates, convective heat fluxes and inclination angles, with the maximum value less than 6.0 Pa. The friction factors were calculated and analyzed under different Reynolds numbers, Grashof numbers and inclination angles. The results indicated that the pressure drops increased with the mean air velocity, and the friction factors showed different characteristics when the Reynolds numbers were larger or smaller than 1500. Finally, the correlations of friction factors were fitted considering the influences of Reynolds number, Grashof number, inclination angle and length to diameter ratio, which fitted the experimental data with the deviations less than ±10.0%.

Development of secondary flow and convective heat transfer in isothermal⧹iso-flux rectangular ducts rotating about a parallel axis

The present study investigates rotation-induced secondary flow and mixed convection heat transfer in the entrance region of rectangular ducts rotating about a parallel axis. Boussinesq approximation is invoked to take into account the centrifugal buoyancy. Mechanisms of the secondary vortex development in ducts of isothermal or iso-flux walls are explored by a vorticity transport analysis. Numerical solutions of the parabolized velocity–vorticity formulation of Navier–Stokes⧹Boussinesq systems are used to corroborate the above theoretical analysis and provide evidence for the relevance of axial variation of transport rates to the evolution of the secondary vortices. The present results also reveal that the rotational effect in an iso-flux duct is more significant than that in an isothermal flux. In a fully developed flow region, the iso-flux case can still retain a secondary vortex effect, while the effect vanishes in isothermal ducts.

The developing Nusselt numbers for slug flow in rectangular ducts

A rigorous solution is obtained for the 3-D temperature field in the thermal entrance region of rectangular ducts, for Newtonian fluids in slug flow, with constant wall heat flux and fluid longitudinal heat flux (H2 boundary conditions). The solution is presented as a series of the transverse Cartesian co-ordinates, where the coefficients depend on both the longitudinal co-ordinate and the different combinations of heated and adiabatic walls. The thermal entrance lengths and the Nusselt numbers are calculated as functions of both the Graetz number and the aspect ratio. The temperature profiles, the thermal entrance lengths, and the Nusselt numbers are presented and discussed in tables and graphs.

EFFECTS OF WALL TRANSPIRATION ON MIXED CONVECTION HEAT TRANSFER IN RECTANGULAR DUCTS ROTATING ABOUT A PARALLEL AXIS

A detailed numerical study, using the vorticity-velocity method, has been conducted to examine the flow and heat transfer characteristics of wall transpiration on mixed convection in the entrance region of rectangular ducts rotating about an axis parallel to the duct axis. Thermal boundary conditions of uniform heat flux (VHF) and uniform wall temperature (UWT) were considered. The investigation was presented for various parameters, wall Reynolds number Rew, rotational Reynolds number J, rotational Grashof number GrΩ, and aspect ratio γ. The predicted results disclosed that the wall transpiration effect has considerable impact on the flow and heat transfer characteristics. Results also showed that both ƒ Re and Nu are enhanced with an increase in J or GrΩ, except for the range of J < 400 or GrΩ< 1000. Wall transpiration effects are more significant for the UWT case than for the UHF case. It is also noted that the secondary vortex induced by the buoyancy effect in the entrance region was stronger for the UWT case than for the UHF case. In addition, the strength of the secondary vortex is weaker for the case of the parabolic velocity condition at the inlet than for the uniform velocity condition at the inlet.

Heat (Mass) Transfer Characteristics in the Entrance Region of Rib-Roughened Rectangular Ducts.

Experimental study has been conducted on local heat (mass) transfer characteristics in the entrance region of rectangular ducts with/without rib-roughened long side walls using the naphthalene sublimation technique. Three rib heights and four rib pitches have been tested. In the smooth duct, the laminar boundary layers are first formed near the duct entrance and then begin to transit to turbulent ones from the duct corners. The turbulent region develops from the corner toward the duct center as the flow proceeds downstream ; therefore, the local mass transfer rates change in both the streamwise and spanwise directions. In the rib-roughened duct as well, the local mass transfer rates vary in not only the streamwise but also the spanwise directions very near the duct entrance ; the location of the maximum local mass transfer rate caused by the flow reattachment moves upstream as the adjacent wall is approached. It has been found that the influence of rib height on the local mass transfer rate is more pronounced near the corner than in the duct center region.

Laminar mixed convection in the entrance region of horizontal rectangular ducts

SUMMARY A generally applicable finite element procedure for the prediction of laminar mixed convection in horizontal straight ducts of arbitrary cross-section is presented. The procedure, based on the parabolized simplification of the complete Navier-Stokes equations and on the Boussinesq approximation of the buoyancy terms, is validated through comparisons of computed results with the available literature data for mixed convection in the entrance region of a rectangular duct of aspect ratio a=2. Uniform heating at different sides is considered as the thermal boundary condition, although the proposed formulation allows specification of most thermal boundary conditions of practical interest.

Steady Laminar Flow in the Entrance Region of Rectangular Ducts : Supplement

Steady Laminar Flow in the Entrance Region of Rectangular Ducts : Supplement

Steady Laminar Flow in the Entrance Region of Rectangular Ducts

Steady Laminar Flow in the Entrance Region of Rectangular Ducts

Experimental study on heat transfer of combined free and forced laminar convection in thermal entrance region of horizontal rectangular ducts.

Experimental studies by the electrochemical method were made to investigate the heat transfer problems of combined free and forced laminar convection for high Pr fluid in the thermal entrance region of rectangular ducts with the aspect ratio Υ=0.5, where the boundary condition of uniform wall temperature has been used. Experimental data for the local and mean Nu were compared with analytical solutions obtained by developing Leveque''s method and the numerical one, and the following conclusions were reached: i) for the region of forced convection in which the effect of free convection approaches zero, the experimental results are well-correlated with the analytical results obtained by developing Leveque''s method; ii) for the region where the effect of free convection exists, the experimental results are in satisfactory agreement with the numerical ones; and iii) for the onset of free convection effects, both experimental and numerical results are also in good agreement with the original correlative equation suggested by Ou et al.

Incompressible Laminar Flow in the Entrance Region of a Rectangular Duct

Incompressible laminar flow in entrance region of rectangular duct allowing direct computation of eigenvalues

Experiments on Laminar Flow Development in Rectangular Ducts

The development of the laminar velocity and pressure fields in the hydrodynamic entrance region of rectangular ducts has been explored experimentally. Duct cross sections having aspect ratios of 5:1 and 2:1 were employed in the investigation; air was the working fluid. It was found that the development of the pressure field is much more rapid than that of the velocity field. The entrance length, relative to pressure development, is representable as (z/De)/Re = 0.02 for both ducts. The incremental pressure drop due to the development of the flow was deduced from the experimental data as being approximately equal to one velocity head. The axial development of the velocity field is illustrated by a sequence of velocity profiles measured along the symmetry lines of the cross section. The flow development in the 5:1 duct is found to be somewhat more rapid than in the 2:1 duct. Comparisons of the experimental results are made with available predictions of analysis, all of which are based on approximate models of the flow field. In general, the analyses over predict the incremental pressure drop due to flow development. The development of the velocity field appears to be reasonably well described by analysis.