Eccentric Annular Ducts Research Articles

The developing flow characteristics of water - ethylene glycol mixture based Fe3O4 nanofluids in eccentric annular ducts in low temperature applications

• The effect of nanofluids’ lower viscosity values on heat transfer was investigated. • Annular flow propagation of water-ethylene glycol based Fe 3 O 4 nanofluid was studied. • Maximum heat transfer enhancement was obtained for highest concentration. • Addition of the nanoparticles resulted in delay to reach steady-state conditions. • The performance evaluation criteria, pressure drop and exergy analysis were studied. Natural circulation loops with double pipe heat exchangers at heating and cooling ends have a potential to be used in the refrigeration systems as an alternative to suction line heat exchangers. The heat transfer capability of such natural circulation loops depends on the geometrical parameters as well as thermophysical properties of the working fluid. This study aims to investigate the effect of water-ethylene glycol mixture based Fe 3 O 4 nanofluids (0.01, 0.05 and 0.1 vol.%) on the annular flow propagation and heat transfer in the annuli of double pipe heat exchanger at low pressure side of the refrigeration cycle. In addition to increased non-dimensional velocity values due to the lower viscosity and higher non-dimensional temperature values with expanded temperature gradient, improved heat transfer by nanofluids shows that they can be used as secondary heat transfer fluids at low-pressure side in refrigeration systems. Although the maximum transferred (13.6% improvement compared to base fluid) heat observed for the highest concentration, the nanofluids with smallest concentration has the minimum pressure drop value (25% reduction compared to base fluid) and the highest performance evaluation criteria (PEC) value (PEC = 1.08) with tiny increase in exergy destruction (1.45% compared to base fluid).

Laminar fluid flow in concentric annular ducts of non-conventional cross-section applying GBI method

Fluid flow in concentric or eccentric annular ducts have been studied for decades due to large application in medical sciences and engineering areas. This paper aims to study fully developed fluid flow in straight ducts of concentric annular geometries (circular with circular core, elliptical with circular core, elliptical with elliptical core, and circular with elliptical core) using the Galerkin-based Integral method (GBI method). The choice of method was due to the fact that in the literature it is not applied in ducts of cross-sections of the annular shape with variations between circular and elliptical. Results of different hydrodynamics parameters such as velocity distribution, Hagenbach factor, Poiseuille number, and hydrodynamic entrance length, are presented and analyzed. In different cases, the predicted hydrodynamic parameters are compared with results reported in the literature and a good concordance was obtained.

Addressing Mismatch Between the Peripheral and Local Nusselt Number for Non-Axisymmetric Flow Conditions: Redefining the Mean Temperature

For non-axisymmetric conditions, the traditional fluid mean (bulk) temperature at a tube cross section, subjected to constant heat flux, can be higher than the temperature of a point on the wall. This results in a mismatch between the direction of applied heat flux and the temperature difference resulting in (misleading) negative peripheral heat transfer coefficient and Nusselt number (Nu). The resulting cross-sectional Nu (average of peripheral Nu values) differs from that calculated from average surface temperature and mean temperature. To address this gap in the literature, a detailed procedure is investigated based on a radial mean temperature of individual lines calculated from the lowest temperature spot in the cross-section to wall points, physically satisfying the mechanism of actual heat transfer from hot to cold spots. Each radial line’s mean temperature is used to calculate a peripheral Nu. These peripheral and average Nu are always positive and matches with the cross-sectional Nu that is calculated using the traditional method – single average cross-sectional temperature. This method is verified for different duct geometries including circular, square, and rectangular. The method also succeeds in the estimation of the peripheral and cross-sectional Nu values for eccentric annular ducts that are inherently non-axisymmetric and have negative Nu values according to the published literature.

Laminar flow and convective heat transfer of non-Newtonian fluids in doubly connected ducts

A hybrid numerical–analytical solution based on the Generalized Integral Transform Technique (GITT) is obtained for laminar heat and fluid flow of power-law non-Newtonian fluids inside doubly connected ducts. The mathematical formulation is constructed in the cylindrical coordinates system in such a way that the solid surfaces are described in terms of internal and external radii as functions of the angular coordinate, thus avoiding discontinuities in the boundary conditions. An annular doubly connected duct of arbitrary geometric configuration is considered for the analysis of the fully developed velocity field, as well as for the temperature field under thermally developing flow with boundary conditions of prescribed wall temperature. For illustration purposes, the case of eccentric annular ducts is more closely analyzed in order to demonstrate the ability of the GITT approach in dealing with such class of problems. Numerical results for the velocity field, the product of the Fanning friction factor-Reynolds number, temperature field and Nusselt numbers were produced for different values of the governing parameters, i.e., eccentricity, radii ratio and power-law indices. Such results were examined against previously reported ones, providing critical comparisons in order to illustrate the adequacy of the employed integral transform approach.

Thermal processing of viscous non-Newtonian fluids in annular ducts: effects of power-law rheology, duct eccentricity, and thermal boundary conditions

Forced convection heat transfer in fully developed laminar flows of power-law fluids in eccentric annular ducts is computationally analyzed. With an insulated outer surface, the heating or cooling on the inner surface is modeled by two fundamental boundary conditions – uniform axial heat flux ( H1 ) and constant wall temperature ( T ) – commonly encountered in thermal processing applications. Numerical solutions for the velocity and temperature distributions, isothermal frictions factors, and Nusselt numbers for annular ducts of varying aspect ratios (0.2⩽r ∗⩽0.8) and inner core eccentricity (0⩽ε ∗⩽0.6) are presented for both shear-thinning (0.2⩽ n<1) and shear-thickening (1< n⩽1.8) fluids. Due to the geometric asymmetry of the eccentric annular cross-section, the flow tends to stagnate in the narrow section and have higher peak velocities in the wide section. This induces greater non-uniformity in the temperature field, and degradation in the average heat transfer coefficient. The nonlinear shear behavior of the fluid further aggravates the flow and temperature maldistribution, which produces a significantly anomalous thermal-hydraulic performance.

Entrance flow in eccentric annular ducts

AbstractA control‐volume‐based solution of the complete set of Navier‐Stokes equations for the laminar, three‐dimensional developing flow in straight, eccentric, cylindrical annular ducts is described. Numerical results for velocity and pressure development, pressure defect and entrance lengths are presented for a wide range of duct parameters, i.e. relative eccentricity ϵ and radius ratio γ. The present results match very well with earlier numerical solutions for the limiting cases of developing flow in concentric ducts and fully developed flow in eccentric ducts. Comparison with earlier approximate results for developing flow in eccentric ducts indicates that the approximate model predicts the velocity and pressure development with an error of about 10%. However, the development length predicted by the approximate model is grossly in error. The pressure defect and development length in eccentric ducts are very high compared with their counterparts in concentric ducts. The pressure defect, development length and maximum velocity increase with the radius ratio for eccentric ducts, while the reverse is true for concentric ducts. Also, the apparent friction factor decreases as the eccentricity increases.

A numerical solution of laminar developing flow in eccentric annular ducts

Laminar incompressible flow in straight ducts of eccentric annular cross section is analyzed. Numerical methods are used to solve an approximate hydrodynamic model, which was developed from the bipolar coordinate representation of the Navier-Stokes equations. Solutions obtained for the proposed model compare favorably with those of two alternative models and with published solutions for the concentric annulus and the circular tube. Results are presented for five different geometries.

A numerical solution of developing temperature for laminar developing flow in eccentric annular ducts

The energy equation expressed in bipolar coordinates is used to determine the temperature distribution in the thermal entrance region of an eccentric annular duct. An implicit alternating-direction method is used in the numerical solution. The analysis of the hydrodynamic entrance region, which provides the velocity distributions needed for the thermal solution, was obtained from a published solution by the present authors. A published Graetz solution for an eccentric annulus and a published combined thermal and hydrodynamic entrance region solution for the circular tube are used in the verification of the present solution. In the present analysis 17 combinations of fundamental thermal boundary conditions, Prandtl number, and annular geometry are considered. The annular geometry with equal relative eccentricity and radius ratio of 0.5 is used to study the effects of eccentricity and Prandtl number on the fluid temperature and surface heat flux distributions.

Experiments on turbulent-flow phenomena in eccentric annular ducts

The investigation to be described here is a wide-ranging experimental study aimed at determining both the details of the flow field and the pressure drop and friction factor characteristics for turbulent flow in eccentric annular ducts. The experiments were performed utilizing three annular ducts of different diameter ratios; in each case the eccentricity was varied from zero (concentric annulus) to unity (walls in contact). To provide the broadest possible perspective, the measurements of the velocity field are presented in three different ways. First, contour maps showing lines of constant velocity are constructed. From these are deduced circumferential distributions of the local shear stress on the bounding walls. Velocity profiles along lines normal to the walls are represented in terms of both law-of-the-wall variables and defect-law variables. Neither of these representations affords complete agreement with the universal circular-tube distributions. In general, the defect law provides a somewhat closer correlation of the results for the eccentric annulus with those for the tube. The experimental findings do not substantiate a prior analytical model which assumes that the universal law of the wall applies on all lines normal to the bounding walls of the annulus. Friction factors, based on static pressure measurements, are shown to decrease with increasing eccentricity. The measured friction factors are in fair agreement with those of analysis. Hydrodynamic development lengths, deduced from entrance-region pressure data, are found to increase with increasing eccentricity. Circumferential pressure variations also increase with eccentricity.

Results of laminar flow analysis and turbulent flow experiments for eccentric annular ducts

Results of laminar flow analysis and turbulent flow experiments for eccentric annular ducts