Optimal selection of annulus radius ratio to enhance heat transfer with minimum entropy generation in developing laminar forced convection of water-Al2O3 nanofluid flow

Heat transfer and entropy generation of developing laminar forced convection flow of water-Al_2O_3 nanofluid in a concentric annulus with constant heat flux on the walls is investigated numerically. In order to determine entropy generation of fully developed flow, two approaches are employed and it is shown that only one of these methods can provide appropriate results for flow inside annuli. The effects of concentration of nanoparticles, Reynolds number and thermal boundaries on heat transfer enhancement and entropy generation of developing laminar flow inside annuli with different radius ratios and same cross sectional areas are studied. The results show that radius ratio is a very important decision parameter of an annular heat exchanger such that in each Re, there is an optimum radius ratio to maximize Nu and minimize entropy generation. Moreover, the effect of nanoparticles concentration on heat transfer enhancement and minimizing entropy generation is stronger at higher Reynolds.

Effect of Eccentricity and Radius Ratio on Fluid Flow and Heat Transfer Inside an Eccentric Semicircular Enclosure

The Problem of laminar natural convective heat transfer inside an eccentric semicircular enclosure of different radius ratio and eccentricity is investigated numerically. At the same time, combined effect of the radius ratio and eccentricity on fluid flow is also observed with isothermal upper and lower surface. Here laminar, steady natural convection heat transfer are predicted for radius ratio R*=1.75, 2.0, 2.25, 2.5. Simulation was carried out for a range of eccentricity, ε=0.0 to 0.6. Governing equations are solved using finite volume method with a body fitted grid with collocated variable arrangement for a range of Grashof number 101-107 based on R0.Results are presented in the form of constant stream function, isothermal lines, local Nusselt number and average Nusselt number at different angular position. Eccentricity has little dominance on heat transfer rate. But significant effect of eccentricity is observed on flow field. Radius ratio has significant effect on natural convection heat transfer as wen as on flow field. At higher eccentricity, bi-cellular now is observed with one crescent-shape vortex at narrower cross section.This crescent shaped vortex is broken down into two cells with the increase of radius ratio that means transition Grashof number for bi-cellular now to tri-cellular flow is decreased with the increase of radius ratio. Eccentricity also has the same effect on flow field. Eccentricity has little effect on heat transfer but with the increase of radius ratio, average heat transfer rate increases.

Investigation of block foundations resting on soil-rock and rock-rock media under coupled vibrations

In the present study,the dynamic response of block foundations of different equivalent radius to mass（R;/m） ratios under coupled vibrations is investigated for various homogeneous and layered systems.The frequency-dependent stiffness and damping of foundation resting on homogeneous soils and rocks are determined using the half-space theory.The dynamic response characteristics of foundation resting on the layered system considering rock-rock combination are evaluated using finite element program with transmitting boundaries.Frequencies versus amplitude responses of block foundation are obtained for both translational and rotational motion.A new methodology is proposed for determination of dynamic response of block foundations resting on soil-rock and weathered rock-rock system in the form of equations and graphs.The variations of dimensionless natural frequency and dimensionless resonant amplitude with shear wave velocity ratio are investigated for different thicknesses of top soil/weathered rock layer.The dynamic behaviors of block foundations are also analyzed for different rock-rock systems by considering sandstone,shale and limestone underlain by basalt.The variations of stiffness,damping and amplitudes of block foundations with frequency are shown in this study for various rock—rock combinations.In the analysis,two resonant peaks are observed at two different frequencies for both translational and rotational motion.It is observed that the dimensionless resonant amplitudes decrease and natural frequencies increase with increase in shear wave velocity ratio.Finally,the parametric study is performed for block foundations with dimensions of 4 m × 3 m × 2 m and 8m×5m×2m by using generalized graphs.The variations of natural frequency and peak displacement amplitude are also studied for different top layer thicknesses and eccentric moments.

Prediction of plastic zone size around circular tunnels in non-hydrostatic stress field

This paper discusses the calculation of plastic zone properties around circular tunnels to rock-masses that satisfy the Hoek–Brown failure criterion in non-hydrostatic condition,and reviews the calculation of plastic zone and displacement,and the basis of the convergence–confinement method in hydrostatic condition.A two-dimensional numerical simulation model was developed to gain understanding of the plastic zone shape.Plastic zone radius in any angles around the tunnel is analyzed and measured,using different values of overburden(four states)and stress ratio(nine states).Plastic zone radius equations were obtained from fitting curve to data which are dependent on the values of stress ratio,angle and plastic zone radius in hydrostatic condition.Finally validation of this equation indicate that results predict the real plastic zone radius appropriately.

Heat Transfer Analysis for Dropwise-Filmwise Hybrid Surface of Steam on Vertical Plate

A model is proposed to predict and evaluate the heat transfer characteristics of dropwise-filmwise hybrid surface.This is approached by modifying the original drop-size distribution,which is defined for fully dropwise condensation(DWC)and making it applicable for dropwise-filmwise condensation(DFC).The modification is achieved by simulation work to determine two parameters:the area fraction occupied by large drops f and the ratio of the maximum radius of newly formed drops to that of larger drops γ.Simulation results show a good agreement with the literature for fully DWC and provide a correlation for each parameter with respect to DWC region width in the DFC hybrid surface.The present model evaluates the heat transfer performance of DFC by utilizing these correlations.A comparison is made between the proposed model with experimental work from the literature and results show a good agreement.While changing filmwise condensation(FWC)region width significantly affects the overall heat transfer performance,utilizing smaller width to that of DWC region has the advantage over fully DWC.Furthermore,surface renewal within the hydrophobic region is controlled by adjusting the DWC region width.When the ratio of drop maximum diameter to DWC region width is unity,surface renewal is achieved by drops merging to adjacent FWC regions only.When the ratio is less than unity,surface renewal is achieved by sweeping of departing drops within the hydrophobic region and by merging.For each case,an optimum DWC region width which corresponds to the maximum DFC heat flux is defined.