Search results

A domain‐adaptive technique which maps the unknown, time‐dependent, curvilinear geometry of annular liquid jets into a unit square is used to determine the steady state mass absorption rate and the collapse of annular liquid jets as functions of the Froude, Peclet and Weber numbers, nozzle exit angle, initial pressure and temperature of the gas enclosed by the liquid, gas concentration at the nozzle exit, ratio of solubilities at the inner and outer interfaces of the annular jet, pressure of the gas surrounding the liquid, and annular jet's thickness‐to‐radius ratio at the nozzle exit. The domain‐adaptive technique yields a system of non‐linearly coupled integrodifferential equations for the fluid dynamics of and the gas concentration in the annular jet, and an ordinary differential equation for the time‐dependent convergence length. An iterative, block‐bidiagonal technique is used to solve the fluid dynamics equations, while the gas concentration equation is solved by means of a line Gauss‐Seidel method. It is shown that the jet's collapse rate increases as the Weber number, nozzle exit angle, temperature of the gas enclosed by the annular jet, and pressure of the gas surrounding the jet are increased, but decreases as the Froude and Peclet numbers and annular jet's thickness‐to‐radius ratio at the nozzle exit are increased. It is also shown that, if the product of the inner‐to‐outer surface solubility ratio and the initial pressure ratio is smaller than one, mass is absorbed at the outer surface of the annular jet, and the mass and volume of the gas enclosed by the jet increase with time.

The gas assisted Iaser heating of engineering surfaces finds wide application in industry. Numerical simulation of the heating process may considerably reduce the cost spent on experimentation. In the present study, 2‐dimensional axisymmetric flow and energy equations are solved numerically using a control volume approach for the case of a gas assisted laser heating of steel surfaces. Various turbulence models including standard k‐ε, k‐ε YAP, low Reynolds number k‐ε and RSTM models are tested. The low Reynolds number k‐ε model is selected to account for the turbulence. Variable properties of both solid and gas are taken into account during the simulation. Air is considered as an assisting gas impinging the workpiece surface coaxially with the laser beam. In order to validate the presently considered methodology, the study is extended to include comparison of present predictions with analytical solution for the case available in the literature. It is found that the assisting gas jet has some influence on the temperature profiles. This effect is minimum at the irradiated spot center and it amplifies considerably in the gas side. In addition, account for the variable properties results in lower surface temperatures as compared to the constant properties case.

This study aims to develop an experimentally validated three-dimensional numerical model for predicting different flow patterns produced with a gas dynamic virtual nozzle (GDVN).

The physical model is posed in the mixture formulation and copes with the unsteady, incompressible, isothermal, Newtonian, low turbulent two-phase flow. The computational fluid dynamics numerical solution is based on the half-space finite volume discretisation. The geo-reconstruct volume-of-fluid scheme tracks the interphase boundary between the gas and the liquid. To ensure numerical stability in the transition regime and adequately account for turbulent behaviour, the k-ω shear stress transport turbulence model is used. The model is validated by comparison with the experimental measurements on a vertical, downward-positioned GDVN configuration. Three different combinations of air and water volumetric flow rates have been solved numerically in the range of Reynolds numbers for airflow 1,009–2,596 and water 61–133, respectively, at Weber numbers 1.2–6.2.

The half-space symmetry allows the numerical reconstruction of the dripping, jetting and indication of the whipping mode. The kinetic energy transfer from the gas to the liquid is analysed, and locations with locally increased gas kinetic energy are observed. The calculated jet shapes reasonably well match the experimentally obtained high-speed camera videos.

The model is used for the virtual studies of new GDVN nozzle designs and optimisation of their operation.

To the best of the authors’ knowledge, the developed model numerically reconstructs all three GDVN flow regimes for the first time.

This paper analyses numerically the effects of sinusoidal g—jitter on the fluid dynamics of, and mass transfer in, annular liquid jets. It is shown that the pressure and volume of the gases enclosed by the jet, the gas concentration at the jet’s inner interface, and the mass absorption rates at the jet’s inner and outer interfaces are sinusoidal functions of time which have the same frequency as that of the g—jitter. The amplitude of these oscillations increases and decreases, respectively, as the amplitude and frequency, respectively, of the g—jitter is increased. The pressure coefficient and the gas concentration at the jet’s inner interface are in phase with the applied g—jitter and the amplitude of their oscillations increases almost linearly with the amplitude of the g—jitter. The mass absorption rates at the jet’s inner and outer interfaces exhibit a phase lag with respect to the g—jitter.

Heat treatment by quenching of individual metallic parts with multiple impinging gas jets is an environmentally friendly alternative to conventional surface hardening and quenching in liquids. In the present investigation the gas flow field and simultaneous heat transfer process in gas quenching is studied by numerical simulation for surface treatment of a cylindrical sample geometry. Aim of the investigation is the evaluation of optimized flow conditions and nozzle arrangements to achieve: a maximum overall heat release (high integral heat transfer rates) to maximize the quenching efficiency; a local smooth distribution of the cooling process (spatially homogeneous heat transfer) for avoidance of spatial hardness variations. These aims are achieved by derivation of an optimized nozzle arrangement and appropriate operation conditions of the gas jet array with respect to the three dimensional sample geometry of a cylinder to be quenched.

Jet impingement onto surface finds wide application in industry. In laser processing an assisting gas jet is introduced either to shield the surface from oxidation reactions or initiating exothermic reaction to increase energy in the region irradiated by a laser beam. When an impinging gas jet is used for a shielding purpose, the gas jet enhances the convective cooling of the cavity surface. The convective cooling of the laser formed cavity surface can be simulated through jet impingement onto a cavity with elevated wall temperatures. In the present study, gas impingement onto a slot is considered. The wall temperature of the cavity is kept at elevated temperature similar to the melting temperature of the substrate material. A control volume approach is used to simulate the flow and temperature fields. The Reynolds Stress Turbulence model (RSTM) is employed to account for the turbulence. To examine the effect of cavity depth on the heat transfer characteristics, the depth is varied while keeping the cavity width constant. It is found that impinging jet penetrates into a cavity, which in turn, results in a stagnation region extending into the cavity. An impinging gas jet has considerable effect on the Nusselt number along the side walls of the cavity while the Nusselt number monotonically changes with varying cavity depth.

WHILE the technical part of the history of the aircraft gas turbine in Great Britain presents the features of success and failure familiar in technical progress, there is another part of the history which I believe can be described as an unqualified success. I refer to the habit of collaboration which was developed between the several technical teams in my own country, between Great Britain and the United States, and, later, between Great Britain and the British Dominions.

The paper's aim is to provide information on heat transfer and flow characteristics for a jet emerging from a conical nozzle and impinging onto the cylindrical, which resembles the laser heating process, for researchers and graduate students working in the laser processing area, which can help them to improve the understanding of the laser machining process.

A numerical scheme employing the control volume approach is introduced to model the flow and heating situations. The effect of jet velocity on the heat transfer rates and skin friction around the cylindrical cavity subjected to the jet impingement was investigated.

Increasing jet velocity at nozzle exit enhances the heat transfer rates from the cavity wall and modifies the skin friction at cavity wall, which is more pronounced as the cavity depth increases to 1 mm.

The effects of nozzle cone angle on the flow structure and heat transfer characteristics were not examined, which perhaps limits the general usefulness of the findings.

Very useful information are provided for the laser gas assisted processing, which has a practical importance in machining industry.

This paper provides original information for the effects of the gas jet velocity on the cooling rates of the laser produced cavity.

Gas jet assisting process finds wide application in industry due to its ability to alter the heat transfer characteristics of the region subjected to jet assisted processing. In the present study, jet impingement onto a cavity with elevated wall temperature is considered. The flow and heat transfer equations are solved numerically using a control volume approach. Reynolds Stress Turbulence Model is employed to account for the turbulence. The simulations are repeated for four cavity depths and two gas jet velocities. It is found that the stagnation zone moves slightly further into the cavity with increasing cavity depth. The flow generated behind the stagnation zone in the cavity influences the heat transfer characteristics in this region, particularly for the cavities with relatively large depth.

EVERYBODY travelling in air or water by its own power applies the reaction or “repulse” principle, that is to say, it either takes up parts of masses contained within itself or, by means of suitable organs, gathers up parts of the surrounding fluid medium and accelerates these masses at a speed greater than its own travelling speed, and this generally in the direction opposite to that in which it desires to travel; whilst in certain cases, in addition to the force produced by the repulse, a further force is obtained through the forward suction of the fluid medium. Devices intended to utilize only the negative pressure produced by suction, e.g. through lateral ejection by means of radial surfaces running at very high (five‐figure) r.p.m. have not, in spite of repeated endeavours, proved successful.