Iranian Journal of Science and Technology Transactions of Mechanical Engineering
Volume:38 Issue: 1, 2014

In the present study fractal geometry is used to model frost growth around a cooled horizontal circular cylinder having constant surface temperature. Fractal geometries are very applicable in this area because phase changes such as melting and solidifications are simulated by conventional methods but frost formation is a most complicated coupled heat and mass transfer and conventional mathematical functions cannot capture the effects of all parameters on its growth and development because this process is influenced by many factors and it is a time dependent process. Data for fractal analysis and modeling are collected from the experimental measurements. In the present work a fractal geometry based on the Von Koch curve idea is used to model frost growth procedure, especially in frost thickness and density. Comparison is performed between fractal modeling and experimental measurements. Results show that fractal geometry is capable to model frost formation process over a cold horizontal cylinder. According to the obtained results and based on the developed model frost formation over cylinders with different diameters are modeled.

In the present work, a laminar forced convection flow over a backward-facing step (BFS) in a duct is investigated. This study concerns the practical application of entropy generation analysis in separated convective flow for the first time. Two different methods, (i) attaching a baffle to the channel top wall and (ii) employing suction for enhancing heat transfer performance, are compared by the second law criteria. Entropy generation in the flow domain which indicates the amount of irreversibilities in convective flow is computed numerically. Heat transfer and viscous dissipation are the two sources of entropy generation in convective flow. The set of governing equations defining conservations of mass, momentum and energy are solved numerically to calculate velocity and temperature profiles which are needed for estimating entropy generation. The SIMPLE algorithm is used to resolve the pressure-velocity coupling. Numerical expressions, in terms of entropy generation number (Ns) and Nusselt number (Nu) are derived in dimensionless forms. The total entropy generation is also calculated in different case studies for comparing the amount of irreversibilities. Numerical results revealed that using baffles and permeable walls with suction have considerable effects in enhancing heat transfer but the presence of baffle produces more irreversibilities than the suction technique.

The objective of this study is to implement a numerical method which is a combination of pseudo-spectral collocation method with a positive scaling factor and extrapolation for solving steady, laminar, incompressible, viscous and electrically conducting fluid of the boundary layer flow due to a constant temperature rotating disk subjected to a uniform suction and injection through its surface in the presence of a uniform transverse magnetic field. These equations are obtained from the Navier Stokes equations through the similarity transformations introduced by Von Karman in 1921. The proposed solution is equipped by the Chebyshev polynomials that have perfect properties to achieve this goal. This method solves the problem on the semi infinite domain without truncating it to a finite domain. In addition, the presented method reduces solution of the problem to solution of a system of algebraic equations. The obtained numerical solutions are verified by the previous results in the literature.

Three-dimensional laminar flow and heat transfer characteristics in smooth hexagonal ducts with equal sides have been numerically investigated in the Reynolds number range from 300 to 2000. The numerical solutions are obtained for both axially and peripherally constant wall temperature (T) and heat flux (H2) thermal boundary conditions for five different values of the duct angle ( = 30, 45, 60, 75, and 90o). Local Fanning friction factor and Nusselt number are obtained along the duct. Hydrodynamic and thermal entrance lengths have been determined. The accuracy of the results obtained in this study is verified by comparing the results with those available in the literature. Results show that duct geometry plays an important role on both flow and heat transfer characteristics. It is seen that the thermal entrance length for H2 boundary condition is greater than that for T boundary condition. Minimum hydrodynamic and thermal entrance lengths are obtained for  = 60o, regular duct. New correlations, important for the design of thermal equipment, are presented for the hydrodynamic and thermal entrance lengths, friction factor, and Nusselt number for 30o    90o and 300  Re  2000.

Increase in air pollution due to automotives is an important problem worldwide. Present experimental work concerns with the influence of inlet air temperature along with oxygen enriched hydrogen gas on reduction of exhaust emission and increasing the fuel economy of a DI diesel engine. Here, the oxygen enriched hydrogen gas was produced by the process of water electrolysis. When the potential difference is applied across the anode and the cathode electrodes of the electrolyzer, water is transmuted into oxygen enriched hydrogen gas. The produced gas was aspirated into the combustion process of petroleum diesel along with intake air at the flow rate of 4.6 liters per minute (lpm). The results are very promising. The fuel economy enhanced and simultaneously engine exhaust emissions by the addition of oxygen enriched hydrogen gas with change in inlet air charge temperature. In this investigation inlet air temperature was changed from normal operating temperature of 300C to 350C and 250C. When the flow rate of the gas mixture was 4.6 lpm with increased inlet air charge temperature of 350C, brake specific energy consumption of the test engine got decreased from 14.8 MJ/kWh to 12.72 MJ/kWh, by a decrease of 14.06%, and unburned hydrocarbon emission from 66 ppm to 51 ppm, by a decrease of 22.73%. Smoke emission reduced substantially from 42 HSU to 29 HSU, by a reduction of 30.95. However; the NOX emission got increased from 420 ppm to 496 ppm, i.e., by 18.1%.

In this study, the flow field of impinging jets in rarefied condition and high pressure ratios was investigated. Direct Simulation of Monte Carlo (DSMC) method was employed to find the flow field at two different Knudsen numbers (Kn) of 0.1 and 0.01. For each studied Knudsen number the effects of different nozzle-to-plate distances (L) are studied. For all cases a supersonic jet forms downstream of the nozzle. The results show that no shock occurs in the supersonic jet impinging on the plate in contrary to those in the continuum regime. The variations of molecular number density, axial velocity and Mach number on the center line were computed and discussed for two different Knudsen numbers. In general, in these conditions the gas expands from specified upstream condition to a lower background monotonically.

The thermal performances of the heat sink with un-uniform fin width designs with base plates and carbon nano tube coating were investigated experimentally and numerically. Realistic, manufacturable geometries are considered for minimizing thermal resistance at low velocity. The parameters include the Reynolds number (Re = 5000 - 25000), five fin width design (Type - a to Type - e), three base plates (copper, ccc and copper-diamond composite) and carbon nano tube coating. In this study, the effects of base plates and nano coating on thermal performance of heat sinks with un-uniform fin width are experimentally studied. Experimental results show that among the many design parameters such as base plates and nano coating, composite base plate has a more significant influence on the thermal performance of heat sinks. It is also found that there is potential for optimizing the un-uniform fin width heat sinks with base plates and nano coating.

Explosive cladding is a metal cladding technique, wherein restricted detonation impinges two or more metals to fuse together. On detonation, the chemical energy stored in the chemical explosive is converted instantaneously into kinetic energy, forcing the flyer plate to impinge obliquely with the base plate to craft a metallurgically strong bond. The kinetic energy available at the interface characterizes the interface microstructure and mechanical properties of explosive clad which depends on process parameters viz. explosive loading ratio, standoff distance and preset angle. Titanium-stainless steel 304L plates are explosively cladded with multi loading ratios, stand off distance and preset angle. Formation of smooth wavy interface is observed at lower explosive mass while formation of intermetallic compounds is observed at higher energetic conditions. Amplitude and wavelength of the interfacial waves are directly proportional to kinetic energy lost at the interface. The increase in mechanical strength of the explosive clads is also reported.

This paper addresses a Taguchi based method for optimization of process parameters in waterjet cleaning (WJCl). The experimental data was collected based on Taguchi L18 orthogonal array. The tests were conducted under varying waterjet pressure (P), nozzle traverse rate (V) and stand–off–distance (D). The effects of these input parameters are investigated on cleaning width (W), one of the most important characteristics in waterjet technique. In this regard, analysis of variance (ANOVA) and F–test have been used to evaluate the relative significance of process variables affecting cleaning width. Furthermore, using signal-to-noise (S/N) ratio, the optimal set of process parameters has been identified to maximize cleaning width. The optimization result is then verified against experimental data to evaluate the performance of Taguchi technique in determining the optimum levels of process parameters. This comparison clearly indicates that the Taguchi technique is quite effective in determining the best set of process parameters in waterjet cleaning.

In the present study, Al-WC (0, 5, 10, 15% weight fractions) nano composite was developed with WC particles as the reinforcement and Al particles as the base metal. The WC (99.5%, 20μm) is mixed (0,5,10 & 15% in weight fractions) with Al powder (99.5%, 6 μm) and milled separately for 45 h using high energy ball mill (Pulversitte 6, Fritsch-Germany,50 WC balls,10mm,b/p 20:1,300 rpm). Characterisation (every 9 h) is done using SEM (Hitachi, and SU1510-Japan), Zetasizer (Malvern, Nano ZS90 - UK) and AFM (XE 70, Park Systems - Korea).The particles reach the nano size after 45 h of milling. The raw powders are compacted (200,250,300kN), sintered (450ºC) and furnace cooled (for 48 h) to prepare the specimens of various aspect ratios (0.3, 0.32, 0.36, 0.38, 0.4, 0.42). The Nano indentation (2nN, AFM, XE 70, Park Systems, Korea), non-destructive (Fallon Ultrasonics,100MHZ, FUI1050, Canada) and destructive testing (UTM,1MN,5mm/min,10 kN increments) are carried out on all preforms to evaluate mechanical (axial, hydrostatic, hoop stresses and strains), physical (density, acoustic impedance, hardness) and elastic (Young’s, shear, bulk modulus and Poisson ratio) properties of the composites and then analysed (graphical analysis), and the composite having the best properties (Al-15WC, h/d 0.3,ρf 0.79) is identified. Then, its properties are fed into ANSYS Workbench software (Release14) and FEA analysis (Static structural,67500 nodes, Linear Solid185 type,38283 elements) is carried out on the piston (114.7mm³,1.83 kg), modelled using PRO-E software. The results show the substantial improvement of the properties of the Al-WC composites compared to the parent metal.

In this paper, vibration suppression and control of a smart flexible satellite maneuvering in a circular orbit are studied. The satellite is considered as a rigid hub and two flexible appendages with PZT (lead zirconate titanate) layers attached on them as sensors and actuators. Flexible satellite governing equations of motion are obtained using Lagrange Rayleigh- Ritz technique and assumed mode method; these dynamic equations of motion of the flexible satellite are nonlinear and coupled. A thorough look at the resulting equations reveals that the flexible satellite dynamics that include the appendage vibrations and its rigid maneuver occur in two different time scales. Therefore, the dynamics of the flexible satellite can be divided into two fast and slow subsystems using the singular perturbation theory. The slow and fast subsystems are associated with rigid maneuver and appendages vibrations, respectively. A hybrid controller is proposed which consists of an adaptive inverse dynamics for slow subsystem maneuvering control, and a Lyapunov based controller for vibration suppression of the fast subsystem. Use of adaptive controller allows us to cope with parameters uncertainty for the rigid motion of the system. Using the Lyapunov approach the stability of these hybrid controllers is studied. Finally, the whole system is simulated and the simulation results show the effective performance of the proposed hybrid controller.

Swimming microrobots are miniature machines which can be designed and fabricated using microelectromechanical systems (MEMS) technology. They can play a key role in many biomedical applications, such as controlled drug delivery, microsurgery, and diseases monitoring. Many researches have been carried out on micro swimming methodologies. Also, different propulsion mechanisms have been introduced for 1-DOF microswimmers. The objective of this work is to study a flagellar microswimmer with controlled maneuvers. The propulsion mechanism used in our design contains two prokaryotic flagella, rotating into the fluid media, leading to microrobot movement. In this study, we have tried to focus on dynamic modeling of the motion proposed for the swimming microrobot. Then, an appropriate control law was developed in order to control the microrobot maneuvers. The resistive-force theory was used in order to determine the hydrodynamic force created by the rotary motion of each flagellum into the fluid media. Feedback linearization method was used to control the motion of microrobot for tracking performance. The results obtained revealed that microrobot can be controlled in such a way that the desired maneuver can be performed by applying the designed controller.