Analytical and numerical studies on a single-droplet evaporation and combustion under forced convection

Existing droplet evaporation/combustion mod- els in computational fluid dynamics （CFD） simulation of spray combustion are based on simplified 1-D models. Both these models and recently developed 3-D models of single- droplet combustion do not give the conditions for the different existing droplet combustion modes. In this paper, droplet evaporation and combustion are studied both analytically and numerically. In the analytical solution, a 2-D axisymmetric flow surrounding an evaporating and combusting droplet was considered. The governing equations were solved using an integral method, similar to the Karman-Pohlhausen method for solving boundary-layer flows with pressure gradient. The results give a local evaporation rate and flame radius in agree- ment with experimental results. In numerical simulation, 3-D combusting gas flows surrounding an ethanol droplet were studied. The prediction results show three modes of droplet combustion under different relative velocities, explaining the change in the evaporation constant with an increase in relative velocity observed in experiments. This implies that different droplet combustion models should be developed in simu- lating spray combustion. The predicted local evaporation rate and flame radius by numerical simulation are in agree- ment with the analytical solution in the range of azimuthal angles 0° 〈 θ 〈 90°. The numerical results indicate that the drag force of an evaporating and combusting droplet is much smaller than that of a cold solid particle, and thus the currently used drag models should be modified.

Experimental study of evaporation of sessile water droplet on PDMS surfaces

Evaporation of sessile water droplet on polydimethylsiloxane （PDMS） surfaces with three different curing ratios （5：1, 10：1, and 20：1） was experimentally investigated in this paper. We show that the constant contact radius （CCR） evaporation on surface with high curing ratio lasts longer than that with low curing ratio. We also measured Young＇s moduli of PDMS films by using atomic force microscopy （AFM） and simulated surface deformation of PDMS films induced by sessile water droplet. With increasing curing ratio of PDMS film, Young＇s modulus of PDMS film is getting lower, and then there will be larger surface deformation and more elastic stored energy. Since such energy acts as a barrier to keep the three-phase contact line pinned, thus it will result in longer CCR evaporation on PDMS surface with higher curing ratio.

Investigation of Droplet Evaporation on Copper Substrate with Different Roughness

In the present study,we investigated the evaporation process and deposition pattern of saline droplet on a copper substrate with different roughness under 40℃ambient temperature.These four substrates are classified as smooth surface and rough surface based on their droplet contact angles.It has been found in this study that the evaporation pattern of droplets has a strong relationship to substrate roughness.The thickness boundary of the evaporation pattern on a smooth surface is larger than that on a rough surface and the particles are closer to boundary and the tendency is more obvious on a smooth surface.The below factors contribute to the result.On the smooth surface,the contact angle of droplet increases as the roughness decreases.On the rough surface,the contact angle increases as the roughness increases.With contact angle decreasing,the evaporation rate at the boundary increases leading to the particles at the boundary more easily sedimentate.Moreover,the capillary flow is hindered by increasing the substrate roughness,while the Marangoni flow remains constant,resulting in more particles remain in the center of the droplet on the rough surface.To sum up,the coffee-ring formation is suppressed by increasing the substrate roughness on a copper substrate under 40℃temperature.

A new stationary droplet evaporation model and its validation

The liquid droplet evaporation character is important for not only combustion chamber design process but also high-accuracy spray combustion simulation. In this paper, the suspended droplets＇ evaporation character was measured in a quiescent high-temperature environment by micro high-speed camera system. The gasoline and kerosene experimental results are consistent with the reference data. Methanol, common kerosene and aviation kerosene droplet evaporation characteristics, as well as their evaporation rate changing with temperature, were obtained. The evaporation rate experimental data were compared with the prediction result of Ranz-Marshall boiling temperature model（RMB）, Ranz-Marshall low-temperature model（RML）, drift flux model（DFM）, mass analogy model（MAM）, and stagnant film model（SFM）. The disparity between the experimental data and the model prediction results was mainly caused by the neglect of the natural convection effect, which was never introduced into the droplet evaporation concept. A new droplet evaporation model with consideration of natural convection buoyancy force effect was proposed in this paper. Under the experimental conditions in this paper, the calculation results of the new droplet evaporation model were agreed with the experimental data for kerosene, methanol and other fuels, with less than 20% relative deviations. The relative deviations between the new evaporation model predictions for kerosene and the experimental data from the references were within 10%.

MATHEMATICAL MODELING OF NEAR-WALL FLOWS OF TWO- PHASE MIXTURE WITH EVAPORATING DROPLETS

In the framework of the two-continuum approach, using the matched asymptotic expansion method, the equations of a laminar boundary layer in mist flows with evaporating droplets were derived and solved. The similarity criteria controlling the mist flows were determined. For the flow along a curvilinear surface, the forms of the boundary layer equations differ from the regimes of presence and absence of the droplet inertia deposition. The numerical results were presented for the vapor-droplet boundary layer in the neighborhood of a stagnation point of a hot blunt body. It is demonstrated that, due to evaporation, a droplet-free region develops near the wall inside the boundary layer. On the upper edge of this region, the droplet radius tends to zero and the droplet number density becomes much higher than that in the free stream. The combined effect of the droplet evaporation and accumulation results in a significant enhancement of the heat transfer on the surface even for small mass concentration of the droplets in the free stream.

Evaporation Characteristics of Ethanol Diesel Droplets Containing Nanoparticles

Evaporation plavs an important role in the cylinder combustion process.so the study of fiel evaporation characteristics is very important to improve the combustion performance of internal combustion engine.In this study,ethanol diesel was used as the base fuel.The evaporation characteristics of droplets at different ambient temperatures(623 K.923 K)were investigated by adding alumina nanoparticles with different mass fractions(1%,2.5%)to ethanol diesel.The results show that the evaporation states of three kinds of droplets(et hanol diesel,base fuel with 1%alumina nanoparticles,base fuel with 2.5%alumina nanoparticles)are similar in the low temperature environment,and nanoparticles inhibit the evaporation of the droplets.At low temperature,the higher the concentration of nanoparticles is,the slower the evaporation rate is.However.in the high temperature environment,nanoparticles significantly promote droplet evaporation.and the phenomenon of micro explosion is obvious.By analvzine the evaporation processes of two different temperatures,we kuow that nanoparticles first.gather on the surface of the droplet,theu form a protective shell,and finally reduce the evaporation rate of the droplet at low temperature.In the high temperature environment,mucleation sites and bubbles are formed in the interior of the droplet:as the droplet is heated further,mucleation sites and bubbles first gather and then fuse:the bubbles gradually expand and burst rapidly.causing micro explosion of the droplets.and most of them are evaporated in the micro explosion.

Simulation on Evaporation and Motion of Atomized Droplets in Spray Dry Flue Gas Desulfurization Tower

The evaporation and motion of atomized droplets have an essential effect on the safe and efficient long-term operation of the desulphurization tower. Therefore, the two-phase flow model is established and solved by three-dimensional steady Reynolds-averaged Navier-Stokes equations;the droplets are tracked by Eulerian-Lagrangian method. The three factors, including inlet swirling flow of flue gas, initial droplet diameter, and inlet flue gas temperature, are analyzed to show the effects on the evaporation and motion of atomized droplets, respectively. The results show that the swirling flow of flue gas and initial droplet diameter dominate the penetration length of the atomized droplets and the mixing characteristic of droplets and flue gas. With the increase of droplet diameter, the length of droplet penetrating flue gas increases. When droplet diameter is 200 μm and inlet swirl number is 0.35, droplets completely penetrate the core area. Therefore, this is the maximum initial droplet diameter at the inlet swirl number of 0.35. The droplets evaporation time of initial 150 μm diameter is 85.5% longer than that of 50 μm droplets(0.35 of inlet swirl number). Increasing the inlet flue gas temperature can enhance the heat transfer. When inlet flue gas temperature rises from 483 K to 523 K, the evaporation time decreases by 33.8%. The results can be used to guide the optimization of droplets spray evaporation under practical operating conditions in the desulfurization tower.

Analysis of Spray Evaporation in a Model Evaporating Chamber:Effect of Air Swirl

Spray evaporation of liquid fuels in a turbulent flow is a common process in various engineering applications such as combustion.Interactions between fuel droplets(discrete phase)and fluid flow(continuous phase)have a considerable effect on liquid fuel evaporation.In this paper,both the single-and two-phase modeling of liquid fuel injection into a model evaporating chamber are presented.The influences of important issues such as turbulence models,coupling between gas phase and droplets,secondary break-up and air swirling on the current spray simulation are investigated.Accordingly,the shear stress transport turbulence model,Taylor analogy break-up and two-way coupling models are applied to simulate the two-phase flow.Atomization and spray of fuel droplets in hot air are modeled employing an Eulerian-Lagrangian approach.The current results show an acceptable agreement with the experiments.Adjacent the fuel atomizer,bigger droplets are detected near the spray edge and minor droplets are situated in the middle.With increasing the droplets axial position,the droplets diameter decreases with a finite slope.The smaller droplets have a deeper penetration,but their lifetime is smaller and they evaporate sooner.A linear relation between penetration and lifetime of smaller droplets is detected.Maximum droplet penetration and mean axial velocity of gas phase are observed for no air swirling case.The effect of variation of swirl number on the lifetime of droplets is almost negligible.By enhancing the swirl number,the uniformity of droplet size distribution is reduced and some large droplets are formed up in the domain.