S.Z. Shuja

Laser-induced thermal stresses on steel surface

Abstract In laser heat treatment of steels, a thin surface layer of austenite forms during heating and subsequent phase change process in the cooling period. However, thermal stress develops due to high-temperature gradient attainment in the surface vicinity which in turn results in microcrack development at the surface. The present study is carried out to compute the temperature profiles due to step input pulse laser radiation and determine the resulting thermal stresses. The study is extended to include three-step input pulses having the same energy content. This provides the comparison for the influence of the pulse length on the resulting thermal stresses. To validate the theoretical predictions, an experiment is conducted to irradiate the AISI 4142 steel surface by an Nd–YAG laser. Microphotography and EDS analysis of the heated regions are carried out. It is found that considerable thermal stress is eveloped at the workpiece surface due to attainment of high-temperature gradient in this region. In addition, microcracks are observed at the surface of the irradiated spot.

Local entropy generation in an impinging jet: minimum entropy concept evaluating various turbulence models

Second law analysis techniques have been widely used to evaluate the sources of irreversibility in fluid flow systems. The same technique may be used to evaluate the various turbulence models. In the present study, a local entropy generation rate is computed for a fluid jet impinging on a heated wall. The standard k–e, low-Reynolds number k–e and two Reynolds stress models are introduced to account for the turbulence. A numerical scheme employing a control volume approach is used to solve the governing equations. The predictions are, then, compared with the experimental findings in the literature. The local volumetric entropy generation in the region close to the stagnation point is used to evaluate the turbulence models. The entropy generation gives information about the magnitude of viscous dissipation in the flow field. The minimum energy concept alone may not be used to evaluate the various turbulence models, in which case, the experimental measurements are accompanied with the results of entropy analysis.

Laser short-pulse heating of surfaces

The Fourier theory of heating is not applicable to the short-pulse type of laser heating due to the assumptions made in the theory. On the other hand, two-equation and kinetic theory models offer an improved solution to the problem. Consequently, the present study compares the predictions of one-equation (Fourier heating model), two-equation, and kinetic theory models for the laser heating pulses of 10-9, 10-10 and 10-11 s lengths. The physical significance of the predictions are described and the discrepancies among the findings are discussed. It is found that all the models employed in the present study predict similar temperature profiles in the substrate for a nanosecond laser heating pulse. As the pulse length shortens such as to 10-10 and 10-11 s, the one-equation model predicts excessive temperature rise in the surface vicinity; however, two-equation and kinetic theory models predict similar temperature profiles. In this case, electron temperature rises rapidly while the lattice temperature increase slows down.

Second law analysis of a swirling flow in a circular duct with restriction

Abstract The present study is conducted to examine the entropy generation and second law analysis for the laminar flow passing through a circular duct with restriction and swirl. The governing fluid and energy equations are solved numerically for the combination of the conditions of restriction and swirling. The dimensionless quantities for the entropy generation, heat transfer and irreversibility are developed. The influence of Prandtl number, restriction and swirl on the dimensionless quantities and merit number are discussed. It is found that the irreversibility increases with increasing Prandtl number. The effect of swirling and restriction on the dimensionless quantities is more pronounced at high Prandtl numbers.

Modeling of laser heating of solid substance including assisting gas impingement

Laser heat treatment finds wide application in industry because of its precision operation, its ability to be used for local heating, and its low cost. In general, an assisting gas jet is introduced coaxially with the laser beam for shielding the region treated from the oxygen. To explore the effects of the gas jet on the heating mechanism, it is essential to perform a simulation of the process. The present study is conducted to simulate three-dimensional laser heating of steel substrate when subjected to impinging gas. The gas jet is considered to impinge to the workpiece surface coaxially with the laser beam. The k-e model with and without corrections and the Reynolds stress model are tested under conditions of constant heat flux introduced from the solid wall. As a result and in accordance with previous studies, the low-Re k-e model is selected to account for the turbulence. However, the transient Fourier heat conduction equation is considered to compute the temperature profiles in the solid substrate....

Confined swirling jet impingement onto an adiabatic wall

Abstract Impinging swirling jets generate interesting flow fields and depending on the magnitude of the swirl velocity, circulation cells develop in the region close to the solid wall. Moreover, axial momentum of the jet is influenced by the magnitude of the swirl velocity. This, in turn, results in considerable entropy generation in the flow field. In the present study, confined swirling jet impingement onto an adiabatic wall is investigated. The flow and temperature fields are computed numerically for various flow configurations. Different jet exit velocity profiles are considered and their effects on the flow field are examined. The entropy production due to different flow configurations is computed and the irreversibility ratios due to fluid friction and heat transfer are determined. It is found that the jet axis tilts towards the radial direction as swirl velocity increases and reducing the velocity profile number enhances the entropy generation due to heat transfer. The irreversibility ratio variation with the velocity profile number behaves opposite for the fluid friction and heat transfer.

GAS JET IMPINGEMENT ON A SURFACE HAVING A LIMITED CONSTANT HEAT FLUX AREA: VARIOUS TURBULENCE MODELS

The orthogonal jet impingement method is used in the processing industry to achieve intense heating, cooling, or drying rates. The present study examines jet impingement on a surface having a constant heat flux over a limited area. Air is considered as the impinging gas, and the process is simulated with a two-dimensional axisymmetric form of the governing conservation equations. Four turbulence models, including standard k-epsilon, low Reynolds number k-epsilon, and two Reynolds stress models, are introduced to account for the turbulence. A numerical scheme employing the control volume approach is introduced when discretizing the governing equations. To validate the theoretical results, the flow properties predicted from the present study are compared with the previously reported experimental findings. It is found that the standard k-epsilon model predicts excessive kinetic energy generation in the vicinity of the stagnation region, which in turn, results in excessive heat transfer and lowering of the te...

Mixed convection in a square cavity due to heat generating rectangular body

Flow in the cavity with heat generating body finds wide domestic and industrial applications. The heat transfer characteristics and the irreversibility generated in the cavity depend on mainly the cavity size, aspect ratio of the heat generating body, and inlet/exit port locations. In the present study, effect of exit port locations on the heat transfer characteristics and irreversibility generation in a square cavity with heat generating body is investigated. A numerical simulation is carried out to predict the velocity and temperature fields in the cavity. To examine the effect of solid body aspect ratio on the heat transfer characteristics two extreme aspect ratios (0.25 and 4.0) are considered in the analysis. Fifteen different locations of exit port are introduced while air is used as an environment in the cavity. It is found that non‐uniform cooling of the solid body occurs for exit port location numbers of 13 and beyond. In this case, heat transfer reduces while irreversibility increases in the cavity. These findings are valid for both aspect ratios of the solid body.

Optimal fin geometry based on exergoeconomic analysis for a pin-fin array with application to electronics cooling

Exergoeconomic analysis for a pin-fin array involves the achievement of a balance between the entropy generation due to heat transfer and pressure drop, while considering the unit cost of entropy generation. This process yields the optimum fin operation parameters based on minimum cost. In this study, analytical equations are presented considering the cost of operation for a pin-fin array. The solution of these equations would give the optimum fin diameter and length that result in a fin array with minimum operational cost. In addition, the influence of important fin thermal, physical, geometrical and cost parameters on the optimum diameter and length is presented in graphical form for quick calculations and easy interpretation. The presented results are subjected to the constraint that L/D is of the order of 1 or greater than 1. A case is also presented to demonstrate the use of the model for conditions typically found in cooling of electronic components.

Pulsative heating of surfaces

Abstract Laser repetitive pulse heating is one of the research fields in laser heating of engineering surfaces. The effect of assisting gas jet on the laser heating process is usually omitted in the previous studies. The present study examines the gas jet assisted laser repetitive pulse heating of steel surfaces. The flow and temperature fields are predicted numerically using the control volume approach. The two-dimensional axisymmetric heat conduction equation is solved for the solid substance, while continuity, momentum, energy and equation of state are considered for the impinging air, which is introduced coaxially with the laser beam and orthogonal to the workpiece surface as the assisting gas. The low Reynolds number k–e model is used to account for the turbulence in the flow field. The solid and gas properties are considered as variable. The study is extended to include three pulse types and two gas jet velocities. It is found that the thermal integration for the repetitive pulses of high cooling periods is unlikely and the effect of gas jet velocity is not substantial on the resulting solid side temperature profiles.