Year:2009 | Volume:43 | Issue:1 (119)|Papers Count:12

In this research, heat transfer enhancement and simultaneous effect of that on pressure drop inside condensers with twisted tape inserts are investigated. A refrigeration system is designed for attaining to maximum level of heat transfer with minimum pressure drop. The test condenser is a double pipe heat transformer with inner and outer diameter of 10.7mm and 12.7mm for internal pipe, respectively. The test tube’s length is 104mm. Experiments are performed for a plain tube and four tubes with twisted tape inserts with different twist ratios of 6, 9, 12 and 15. Also data are collected for mass velocities of 56.44, 69.49, 82.38, 96.52, 113.32 and 130.9kg/m2s. In conclusion, data analysis showed that insertion of twisted tapes inside tubes has increased the heat transfer and pressure drop by as much as 40% and 240% above the plain tube values on a nominal area basis, respectively.

In this paper a special sandwich panel with a sinusoidal core under distributed load on upper and lower plates has been studied. By drawing free body diagrams, and using energy method and Castagiliano theory the forces and moments involved in the core has been determined. Then, the stresses, normal and horizontal displacements due to above forces and moments are calculated. In order to ensure the accuracy of the analysis, the results obtained are verified by using ANSYS software. Finally, by use of displacement as a constraint for sandwich panel, optimization is carried out for this problem by genetic algorithm and MOEA toolbox in MATLAB environment. The results obtained from optimization and other analysis show a very good agreement.

This paper describes an upper bound analysis of cold forward extrusion of spur gears with modified tooth profile. Spur gear geometrical parameters such as module, number of teeth, pressure angle, bore radius and addendum modification factor were input to a computer program written in Visual Basic. Then spur gear, billet and extrusion die cavity were modeled automatically in SolidWorks. For upper bound analyses, half pitch of a tooth was considered as a deformation unit and it has been divided into twelve regions and it is assumed that material flow in each region is axisymmetric. Comparison between present theoretical results and the experimental data of other researchers’ work were carried out to illustrate the validity of this proposed model. Finally effects of various parameters on the extrusion load were studied.

In the present work, the effect of fluid’s elasticity was investigated on the hydrodynamic instability of Blasius flow. To determine the critical Reynolds number as a function of the elasticity number, a two-dimensional linear temporal stability analysis was invoked. The viscoelastic fluid is assumed to obey the Walters’ B fluid model for which base flow velocity profiles were fortunately available from literature. Neglecting terms nonlinear in the perturbation quantities, a Generalized Orr-Sommerfeld equation was obtained incorporating an elastic term. The eigenvalue problem so obtained was solved numerically using the Chebychev collocation-point method. Based on the results obtained in this work, fluid’s elasticity is predicted to have a destabilizing effect on the Blasius flow.

Roughness elements or turbulence promoters have been widely used to enhance heat transfer in cooling passages of modern gas turbine blades. Although such ribs substantially enhance heat transfer, the heat transfer coefficient is reduced immediately at corner downstream of each rib, creating hot spots. To remove such hot spots some of the ribs can be detached from the channel walls. In this paper, this idea is investigated using numerical methods. In this study, turbulent flow and heat transfer through three types of channels, namely: 1) a channel with detached ribs from a wall 2) a channel with detached ribs from both walls and 3) a channel with alternative attached-detached ribs on both walls have been investigated. Computations presented in this research have been obtained with the linear and non-linear k-e models. The numerical method in this work is finite volume methodology and simple algorithm. The governing equations are discretized in a semi-staggered grid system. In all equations the convective terms are approximated using Hybrid scheme. The numerical results for the channel with detached ribs close to one principal wall showed that both the linear and non-linear k-e models are able to predict the length and width of the wake downstream of the detached ribs, although both models produce weaker wakes compared with experimental data. Both models "specially the non-linear k-e model" predict lower stream-wise velocity and turbulent intensities closed to the ribbed wall. As a result, both turbulence models "and specially the non-linear k-e model" substantially under-predict the wall heat transfer. For the channel with ribs detached close to both walls, both turbulence models produce better heat transfer predictions though still predict lower heat transfer levels. Finally, for the channel with alternative attached-detached ribs, both turbulence models fail to predict reliable heat transfer levels in first half of the channel but return acceptable Nusselt levels in the second half of the channel.

Understanding how a cutout influences the load bearing capacity and buckling behavior of cylindrical shells is fundamental in the design of structural components used in automobiles, aircrafts, and marine structures. In this article, simulation and analysis of steel cylindrical shells with various lengths, include quasi elliptical cutout, subjected to axial compression were systematically carried out using finite element numerical method and the investigation examined the influence of the cutout size, cutout angle and the shell aspect ratio (L/D) on the buckling, and postbuckling responses of the moderately thick steel cylindrical shells. For several specimens an experimental investigation was also carried out via an INSTRON 8802 servo hydraulic machine. And the results obtained from the experiments were compared with numerical results. A very good accordance was observed between the results obtained from the finite element simulation and the experiments. Finally, corresponding to experimental and numerical results, an equation was presented for finding buckling load of such structures.

This paper investigates the forming of sheet metal forming under cycling loading by considering the Bauschinger effect. Different proposed plasticity models which can handle this kind of deformation process have been reviewed in details. For instance, isotropic, kinematic and combined forms in the linear and non-linear cases including the well-known Yoshida and Chaboche's models have been studied. The ability of the best proposed model has been examined in the forming of reverse cup drawing and the obtained results have been compared with some other experimental results. The normalized axial stress, von-mises stress and the punch forces for both first and second stages have been calculated for different materials and thicknesses.

Effect on microstructure of boundary layer by disturbing the flow field such as imposing suction, blowing and employing variety forms of surface roughnesses are subjects of most interested research field in heat transfer enhancement. In this context, some experimental techniques are employed by research workers. While the experimental techniques explore overall fluid behavior, the numerical methods unveil details of flow structure, exploring causes to manage advanced techniques for enhancement. In present work, effect of a rectangular rod, it’s aspect ratio and distance from a flat plate on local heat transfer enhancement are studied. The study focuses more on details of flow structure and contribution that each of flow fields may make in enhancement. As a result of insertion of the rod into flow field, a stagnation point, a jet and recirculation zone form. More study, reveals that the stagnation point, while being a cause of local drop in heat transfer, forms a high pressure point driving part of boundary layer flow through the gap between the rod and the flat plate, causing wash away of local turbulent boundary layer and instead grows a new boundary layer inside the gap. As an effect, local heat transfer coefficient (LHTC) increases abruptly to a maximum but falls as new boundary layer thickenes along the gap. After the gap, a vortex forms. Its size is a function of obstacle height as well as strength of the jet flow. The vortex, thougth less enhancement imposes into the heat transfer coeficient compared to the jet, its effects widens along the flat plate. If compared to single flate plate, an overall heat transfer enhancement of more than 40% over the affected area is achieved. In the study, Navier- Stocks equations together with energy equation are solved. To handel turbulence development into the flow field, low Reynolds number Launder-Sharma turbulence model is employed. The model is expected to deal with wake generating flow with good accuracy. All equations are expanded over physical domain using SIMPLE procedure in 2 dimentional physical domain. To maintain accuracy for low Reynolds number turbulent models, fine meshing near the wall is necessary. Four grids are set inside the laminar sublayer, i.e. one on the wall, one at y+ =11 and two more in between these two.  Therefore each grid point has a distace of Dy= 1´10-4mm from the other grid in y direction.

In this work the effect of the accuracy of a FAE detonation modeling on the generated blast wave is investigated. First, a one-dimensional numerical simulation with a reduced chemical kinetics of C2H2-O2-Ar, involving 25 elementary reactions, is used as the base model. The properties of the blast calculated with this model is compared with those of simpler models, the similarity solution of Taylor, the constant volume (CV) explosion model, and the CJ-burn model. It is found that the result of Taylor's similarity model is in very good agreement with the result of the base model. The blast properties that calculated with the CJ-burn model are also in close agreement with those of the base and Taylor's models. However, the CV model prediction shows considerable difference with the base model. Considering the computational cost and the accuracy, the Taylor's model is recommended as the favorite model for the calculation of the FAE blast properties.

Optimization of machining parameters is very important and the main goal in every machining process. Surface finishing prediction is a pre-requirement to establish a center for automatic machining operations. In this research, a neuro-fuzzy approach is used in order to model and predict the surface roughness in dry turning. This approach has both the learning capability of neural network and linguistic representation of complex and indefinite phenomena in <if-then> lingual phrases forms. A model which represents the influence of machining parameters and tool properties on surface roughness is established first. Then, this model is edited via the usage of results of training data. Finally, the efficiency of neuro-fuzzy model is evaluated via the comparison between the model's output and the output of surface roughness obtained from the theoretical formula.

In this paper the effect of site conditions, which are Altitude, Temperature and Relative Humidity and also Heat Dissipation Capacity are investigated on the optimized design of natural draft dry cooling towers with specific kind of heat exchangers, known as Forgo T60 type. A genetic algorithm program has been developed for optimized design of these cooling towers. Objective function includes both cost and performance of the tower. The optimized dimensions of the tower, the number of heat exchangers and mass flow rate of water and air are the results of this optimization program. The results show it is possible to find the variation of the optimized value of each design variable versus the variations of the site conditions and heat dissipation capacity. Also finding an optimum for all design parameters for any site condition is possible, however it is very dependent on how exact the cost analysis is.

The present paper deals with the prediction of three-dimensional fluid flow and heat transfer in rib-roughened ducts of square cross-section. Such flows are of direct relevance to the internal cooling system of modern gas turbine blades. The main objective is to assess how a recently developed variant of a cubic non-linear k-e model (proposed by Craft et al. (1999)), that has been shown to produce reliable thermal predictions through axi-symmetric and plane two-dimensional ribbed passages (Raisee et al. (2004)), can predict flow and heat transfer characteristics through more complex three-dimensional ribbed ducts. To fulfil this objective, the present paper discusses turbulent air flow and heat transfer through two different configurations, namely: (I) a square duct with “in-line” ribs normal to the flow direction at Re = 5.0´104 and (II) a square duct with normal ribs in a “staggered” arrangement at Re = 1.0´105. In this paper the flow and thermal predictions of the linear k-e model (EVM) are also included, as a set of baseline predictions. Both turbulence models have been used with the form of length-scale correction term to the dissipation rate originally proposed by ‘Yap’ and also a differential version of this term, ‘NYP’. The mean flow predictions show that both linear and non-linear k-e models can successfully reproduce most of the measured data for stream-wise and cross-stream velocity components. Moreover, the non-linear model, which is sensitive to turbulence anisotropy, is able to produce better results for the turbulent stresses. As far as heat transfer predictions are concerned, it was found that both EVM and NLEVM2, the more recent variant of the non-linear k-e, with the algebraic length-scale correction term, overestimate the measured Nusselt numbers for both geometries examined. While the EVM with the differential length-scale correction term underestimates heat transfer levels, the Nusselt number predictions with the NLEVM2 and the ‘NYP’ term are in close agreements with the measured data. Comparisons with our earlier work, Iacovides and Raisee (1999), show that the NLEVM2 thermal predictions are of similar quality to those of a second-moment closure. This modified version of the non-linear k-e model, that in earlier studies was shown to improve thermal predictions in axi-symmetric and plane ribbed passages, is thus now found to also produce reasonable heat transfer predictions in three-dimensional ribbed ducts.