Metin Renksizbulut

A detailed examination of gas and liquid phase transient processes in convective droplet evaporation

A finite volume numerical technique has been used to model the evaporation of an n-heptane droplet with an initial Reynolds number of 100 in air at 800 K, 1 atm. The effects of variable thermophysical properties, liquid phase motion and heating, and transient variations in droplet size and velocity are included in the analysis. With appropriate corrections for the effects of variable properties and liquid phase heating, quasi-steady correlations are shown to predict accurately the transient histories of the drag coefficient and Nusselt and Sherwood numbers. For the case investigated here, the transient effects of importance were the variation in droplet velocity, the decline in the liquid phase velocities, and the rise in the droplet surface and volume average temperatures. In spite of the transient rise in the droplet temperature, the nature of the liquid phase heating, as characterized by the liquid Nusselt number, was found to remain constant during most of the droplet lifetime.

A MASS TRANSFER CORRELATION FOR DROPLET EVAPORATION IN HIGH-TEMPERATURE FLOWS

The analogy between heat and mass transfer is strong, but not exact, for droplet evaporation in high-temperature air streams at intermediate Reynolds numbers. Convection enhances mass transfer, while evaporation is self-inhibitive due to its blowing effect. In this paper, the following mass transfer correlation is developed: Shf(1 + BM)0.7 = 2 + 0.87Rem12Scf13 (10 <Rem < 2000), which accounts for the effects of convection, droplet heating, surface blowing and variable thermophysical properties. It is shown that this correlation agrees well with the results of detailed numerical analyses involving single and multicomponent droplets, as well as experimental data for various fluids from different sources.

Laminar Flow and Heat Transfer in the Entrance Region of Trapezoidal Channels With Constant Wall Temperature

Simultaneously developing three-dimensional laminar flow and heat transfer in the entrance region of trapezoidal channels have been investigated using numerical methods in the Reynolds number range from 10 to 1000. The principal and secondary velocity fields, the temperature field, and all associated heat and momentum exchange parameters have been examined. The present results for the fully developed flow region of the channels compare well with the available literature. In the entrance region, it is observed that the axial velocity profiles develop overshoots near the walls and particularly at the channel comers. It is shown that boundary-layer type of approximations, which lead to Reynolds-number-independent Poiseuille and Nusselt numbers, can be used for Reynolds numbers over 50 and after a few hydraulic diameters from the channel inlet. It is also shown that hydrodynamic entrance lengths calculated with methods based on fully developed flow data are grossly in error. New correlations are proposed for the entrance length, and for the friction and heat transfer coefficients.

Multicomponent droplet evaporation at intermediate Reynolds numbers

Abstract The convective evaporation of a binary hydrocarbon droplet (decane-hexadecane) in air at 1000 K and at a pressure of 10 atmospheres has been studied using numerical methods. All transient effects including droplet size and velocity variations, heat and mass transfer within the liquid phase, and thermophysical property variations with temperature and concentration in both phases are included in the analysis. As the rate controlling process, liquid phase mass transfer is examined in detail. It is demonstrated that the existing drag coefficient, Sherwood number, and Nusselt number correlations originally developed for single-component droplets can be used for multicomponent droplets as well.

Experimental investigation on cellular breakup of a planar liquid sheet from an air-blast nozzle

The cellular breakup phenomenon is investigated experimentally for a planar liquid sheet from an air-blast nozzle. The dominant sinuous wave growing spatially downstream forms complicated cellular structures of perforated thin films and surrounding ligaments. Several characteristic parameters are measured from photographic images and compared with linear temporal analysis. The dominant wavelength is proportional to the inverse square of the relative velocity between air and liquid. The estimated breakup time matches the growth time of the most unstable wave, while the breakup length corresponds to a product of breakup time and liquid velocity. Numerical simulation shows a substantially reduced mean effective velocity near flow reattachment region of the air stream. Air turbulence seems to play a major role on initial perturbations of cellular breakup in the given nozzle configuration. The measured spatial growth rates are always less than linear predictions due to deviation from the linear regime at higher amplitudes.

Transient deformation and evaporation of droplets at intermediate Reynolds numbers

Abstract The early life histories of isolated n -heptane droplets injected into 1000 K air at 1 and 10 atm with initial Reynolds and Weber numbers of 100 and 2, respectively, are reported. A numerical model is used to predict the transient droplet shape, and the velocity, pressure, temperature and concentration fields in both phases. Initially spherical droplets show strongly damped oscillations at frequencies within 25% of the theoretical natural frequency of Lamb (1932). Circulation within the droplets is responsible for the observed strong damping and promotes the formation of prolate shapes for surfactant-free droplets. The computed heat and mass transfer rates are well predicted by existing quasi-steady correlations.

Investigating a methanol spray flame interacting with an annular air jet using phase-Doppler interferometry and planar laser-induced fluorescence

Abstract An experimental investigation of the interaction of an annular air jet with a methanol spray flame was conducted using phase Doppler interferometry (PDI) to measure fuel droplet size and velocity as well as gas-phase velocity, and planar laser-induced fluorescence (PLIF) to determine instantaneous and time-averaged reaction zone location, as well as quantitative OH concentration. Temperature measurements were made using Pt/Pt-10% Rh thermocouples. PLIF imaging of the OH radical showed that the spray flame had a dual reaction zone structure with annular air off but a single reaction zone with annular air on. Peak OH concentrations were measured to be 5400 PPM, regardless of the annular air flow rate. The OH concentration peaks always occurred on the fuel-lean side of the reaction zone, while temperature peaks occurred on the fuel-rich side. Gas-phase velocity vector fields show that annular air tends to channel the flow towards the centreline, and large droplet velocity vector fields show that the large droplets follow this trend as well, suggesting that the annular air jet assists in confining large spray droplets. Overall visible flame length is reduced by over 50% with annular air, providing a flame well-suited to compact combustion chambers.

Electro-Osmotic Flow in Reservoir-Connected Flat Microchannels With Non-Uniform Zeta Potential

The effects of non-uniform zeta potentials on electro-osmotic flows in flat microchannels have been investigated with particular attention to reservoir effects. The governing equations, which consist of a Laplace equation for the distribution of external electric potential, a Poisson equation for the distribution of electric double layer potential, the Nernst-Planck equation for the distribution of charge density, and modified Navier-Stokes equations for the flow field are solved numerically for an incompressible steady flow of a Newtonian fluid using the finite-volume method. For the validation of the numerical scheme, the key features of an ideal electro-osmotic flow with uniform zeta potential have been compared with analytical solutions for the ionic concentration, electric potential, pressure, and velocity fields. When reservoirs are included in the analysis, an adverse pressure gradient is induced in the channel due to entrance and exit effects even when the reservoirs are at the same pressure. Non-uniform zeta potentials lead to complex flow fields, which are examined

Numerical solution of deforming evaporating droplets at intermediate Reynolds numbers

Abstract A finite volume numerical model, using a nonorthogonal adaptive grid, has been developed to examine both steady deformed and transient deforming droplet behavior. The model has been tested by comparison with existing numerical solutions and experimental data. Computations of the steady state evaporation of n-heptane droplets in high-temperature air (T∗ ∞ = 1000 K, 10 ≤ Re∞ ≤ 100, We∞ ≤ 10) show deformed oblate shapes with major axes perpendicular to the mean flow direction. Using volume equivalent diameters, predictions based on existing Nusselt and Sherwood number correlations for spherical droplets are in good agreement with the numerical results. A new correlation is presented for the drag coefficient of deformed vaporizing droplets.

Transient three-dimensional heat transfer from rotating spheres with surface blowing

Transient heat transfer and thermal patterns around a rotating spherical particle with surface blowing are studied numerically for Reynolds numbers in the range 10⩽Re⩽300 and non-dimensional angular velocities up to Ω=1. This range of Reynolds number includes three distinct wake regimes: steady and axisymmetrical, steady but non-symmetrical, and unsteady with vortex shedding. The Navier–Stokes and energy equations for an incompressible viscous flow are solved numerically by a finite-volume method in a three-dimensional and time-accurate manner. The transient aspects of the thermal wakes associated with the aforementioned wake regimes have been explored. An interesting feature associated with particle rotation and surface blowing is that they can affect the near wake structure in such a way that an unsteady three-dimensional flow with vortex shedding develops at lower Reynolds numbers as compared to flow over a solid sphere in the absence of these effects, and thus, the temperature distributions around the particle are significantly affected. Despite the fact that particle rotation brings about major changes locally, the surface-averaged heat transfer rates are not influenced appreciably even at high rotational speeds; consequently, it is shown that the total heat transfer rates associated with rotating spheres with surface blowing can be calculated from heat transfer correlations developed for flow over evaporating droplets.