M. O. Budair

Nano-second laser pulse heating and assisting gas jet considerations

Abstract The nano-second laser pulse gas assisting processing method offers considerable advantages in the heat treatment process. In the present study gas assisted nano-second pulse laser heating of a stationary surface is considered. The gas, which is air, impinging onto the workpiece surface is modelled using flow equations, while the low Reynolds number k – ϵ model is employed to account for the turbulence. A heat conduction equation is introduced for the solid heating. A numerical scheme using a control volume approach is employed when discretizing the governing equations. The simulation is repeated for two gas jet velocities. To validate the present predictions, an analytical solution accommodating the convection losses is introduced for the workpiece heating. It is found that the temperature profiles predicted from the simulations agree well with the analytical solutions. Moreover, impinging gas jet velocity has no significant effect on the temperature distribution in the workpiece. As the heating progresses, the equilibrium heating initiates, in which case the internal energy gain of the solid increases at an almost constant rate.

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.

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....

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...

JET IMPINGEMENT ONTO A HOLE WITH CONSTANT WALL TEMPERATURE

Jet impingement onto a hole finds application in process industries. In the present study, a jet impingement onto a hole with a constant wall temperature is examined. The governing flow and heat transfer equations are solved numerically using a control-volume approach. The Reynolds stress turbulence model (RSTM) is employed to account for the turbulence. In the simulations, four hole wall temperatures and two jet velocities are considered. The Nusselt number ratio (ratio of the Nusselt number predicted to the Nusselt number obtained for a fully developed turbulent flow based on the hole entrance Reynolds number) is computed and the mass flow ratio (ratio of mass flow rate through the hole to mass flow emanating from the nozzle) is determined. It is found that increasing hole wall temperature extends the stagnation region close to the top edge of the hole. This, in turn, results in hot gas opposing the impinging jet. Therefore, the mass flow ratio falls with increasing hole wall temperature. The Nusselt number ratio improves at the hole inlet and falls as the axial distance along the hole wall increases toward the hole exit. A Nusselt number ratio of greater than 1 resulted for all the situations employed in the present study.

Influence of Conical and Annular Nozzle Geometric Configurations on Flow and Heat Transfer Characteristics due to Flow Impingement onto a Flat Plate

ABSTRACT Jet impingement onto a flat surface is considered, and the influence of the geometric configurations of conical and annular nozzles on the flow structure, heat transfer rates, and skin friction are examined in relation to laser machining applications. The governing equations of flow are solved numerically using a control-volume approach. Four cone angles of each nozzle are accommodated while keeping the nozzle height and exit area the same in the simulations, in accordance with laser machining applications. Jet emerging from the pipe and impinging onto a flat plate is accommodated to compare the flow and heat transfer characteristics due to the jet emerging from the nozzles. It is found that nozzle cone angles have significant effect on the flow structure in the region close to the flat plate, which in turn modify the heat transfer rates from the plate surface. Moreover, increasing cone angle results in radial acceleration of the flow in both the viscous sublayer and the turbulent boundary layer, which, in turn, enhances the heat transfer rates considerably as compared to a pipe flow situation.

Investigation into a confined laminar swirling jet and entropy production

A confined laminar swirling jet is an interesting research topic due to flow and temperature fields generated in and across the jet. In the present study, a confined laminar swirling jet is studied, and flow and temperature fields are simulated numerically using a control volume approach. In order to investigate the influence of the jet exiting (exiting the nozzle and inleting to the control volume) velocity profiles on the flow and heat transfer characteristics, eight different velocity profiles are considered. To identify each velocity profile, a velocity profile number is introduced. Entropy analysis is carried out to determine the total entropy generation due to heat transfer and fluid friction. Merit number is computed for various swirling velocities and velocity profiles. It is found that swirling motion expands the jet in the radial direction and reduces the jet length in the axial direction. This, in turn, reduces the entropy generation rate and improves the Merit number. Increasing velocity profile number enhances the entropy production rate, but improves the Merit number.

Flow through a protruding bluff body–heat and irreversibility analysis

Abstract Flow over a protruding bluff body finds wide application in many engineering fields. The modeling of the conjugate heating process due to bluff body gives insight into the physical process involved and minimizes the experimental cost. In the present study, the transient conjugate heat transfer analysis of the bluff body subjected to low Reynolds number flow is considered. The governing flow and energy equations are solved numerically using a control volume approach. The heat transfer characteristics are examined through the normalized Stanton number. The entropy generation due to fluid friction and heat transfer in the fluid system is computed and ratio of rate of heat transfer to irreversibility generated is obtained. It is found that the heat transfer in the region of top and bottom edges of the bluff body is enhanced due to the convection effect. The heat transfer to irreversibility ratio decreases sharply as the heating progresses.

Entropy analysis of a flow past a heat-generated bluff body

Cooling of a bluff body is a topic of interest for many engineers and scientists. Forced convection over the bluff body generates flow separation, which in turn affects the heat transfer characteristics and increases the irreversibilities involved in the system. In the present study, flow over a rectangular solid body with constant heat flux is considered. The governing flow and energy equations are solved in two-dimensional space numerically using a control volume approach. In order to investigate the effect of the fluid properties on the heating process, three different fluids are taken into account. These are air, ethylene glycol and therminol. To determine the irreversibilities involved in the system, entropy analysis is carried out. It is found that fluid properties have considerable effect on the entropy generation. The entropy generation due to heat transfer well exceeds the entropy generation due to fluid friction. The surface temperature of the solid body highly depends on the cooling fluid employed. Copyright