Droplet Kinetic Energy Research Articles

Water droplet can mitigate dust from hydrophobized micro-post array surfaces

Water droplet rolling motion over the hydrophobized and optically transparent micro-post array surfaces is examined towards dust removal pertinent to self-cleaning applications. Micro-post arrays are replicated over the optically transparent polydimethylsiloxane (PDMS) surfaces. The influence of micro-post array spacing on droplet rolling dynamics is explored for clean and dusty surfaces. The droplet motions over clean and dusty micro-post array surfaces are monitored and quantified. Flow inside the rolling droplet is simulated adopting the experimental conditions. Findings reveal that micro-post gap spacing significantly influences droplet velocity on clean and dusty hydrophobized surfaces. Air trapped within the micro-post gaps acts like a cushion reducing the three-phase contact line and interfacial contact area of the rolling droplet. This gives rise to increased droplet velocity over the micro-post array surface. Droplet kinetic energy dissipation remains large for plain and micro-post arrays with small gap spacings. A Rolling droplet can pick up dust particles from micro-post array gaps; however, few dust residues are observed for large gap spacings. Nevertheless, dust residues are small in quantity over hydrophobized micro-post array surfaces.

Respiratory droplet resuspension near surfaces: Modeling and analysis

Knowing the environmental spreading pathway of COVID-19 is crucial for improving safety practices, particularly for health care workers who are more susceptible to exposure. This paper focuses on the possible secondary transmission due to resuspension of virus-laden droplets from common surfaces, which several studies have shown to be possible under external disturbances. Such disturbances could be body motion during walking, running, clothes removal, or airflow in the environment. In this paper, a three-dimensional two-phase model is utilized to study respiratory droplet resuspension dynamics on various surfaces due to sudden agitation. The velocity range and variation during walking, surgical glove removal, and dropping an object are studied experimentally. A parametric study is performed to characterize the effects of droplet size and surface wettability on the minimum initial droplet velocity required for detachment from surfaces. The results are reported as average droplet velocity during the detachment process, total detachment time, and detached droplet volume. The obtained results indicate that respiratory droplets larger than 200 μm can detach from typical surfaces due to normal daily activities. Droplets are partially separated from hydrophilic surfaces with contact angle ≤90°, while the entire droplet is detached from hydrophobic surfaces with contact angle >90°. Furthermore, the minimum initial droplet velocity to induce the resuspension depends on the droplet size. Droplet velocity immediately after detachment is a function of droplet size, initial droplet velocity, and surface wettability. Bigger droplets have larger detached volume percentage as well as higher velocity after detachment compared to smaller droplets. Finally, a higher initial velocity is needed to separate droplets from hydrophilic surfaces as compared to hydrophobic surfaces. In accordance with the results, the droplet minimum initial velocity to cause detachment is 2 m s−1, while our experiments show that surface velocity can reach up to 3 m s−1 during normal human activities. We also develop an analytical model to predict the required kinetic energy to detach droplets from different surfaces, which is in good agreement with numerical results. The mechanism of droplet detachment is dictated by a competition between droplet kinetic energy induced by surface motion and surface energy due to droplet–surface interaction as well as droplet–vapor and surface–vapor interactions. We believe that the results of this fundamental study can potentially be used to suggest proper surface wettability and safe motion that reduce respiratory droplet resuspension from various surfaces.

Hydrodynamic constraints on the energy efficiency of droplet electricity generators

Electric energy generation from falling droplets has seen a hundred-fold rise in efficiency over the past few years. However, even these newest devices can only extract a small portion of the droplet energy. In this paper, we theoretically investigate the contributions of hydrodynamic and electric losses in limiting the efficiency of droplet electricity generators (DEG). We restrict our analysis to cases where the droplet contacts the electrode at maximum spread, which was observed to maximize the DEG efficiency. Herein, the electro-mechanical energy conversion occurs during the recoil that immediately follows droplet impact. We then identify three limits on existing droplet electric generators: (i) the impingement velocity is limited in order to maintain the droplet integrity; (ii) much of droplet mechanical energy is squandered in overcoming viscous shear force with the substrate; (iii) insufficient electrical charge of the substrate. Of all these effects, we found that up to 83% of the total energy available was lost by viscous dissipation during spreading. Minimizing this loss by using cascaded DEG devices to reduce the droplet kinetic energy may increase future devices efficiency beyond 10%.

Droplet Rolling Dynamics over a Hydrophobic Surface with a Minute Width Channel.

Unidirectional and stabilize droplet rolling over hydrophobic surfaces is critical for self-cleaning applications of large areas. Introducing minute size channels on hydrophobic surfaces in the droplet rolling direction can minimize droplet wobbling and enables unidirectional rolling. The droplet rolling behavior over an inclined hydrophobic surface having a minute size channel is investigated. The flow field developed inside the droplet fluid is numerically simulated in a three-dimensional domain pertinent to experimental conditions. Experiments are carried out using a high-speed facility to monitor and evaluate droplet motion over channeled and flat hydrophobic surfaces. The findings revealed that predictions of the droplet translational velocity and those obtained from the experiments are in good agreement. The presence of a minute channel on the hydrophobic surface gives rise to droplet fluid inflection into the minute channel, which in turn modifies the center of mass of the droplet during rolling. This lowers the droplet wobbling height and enables the droplet to roll unidirectionally along the channel length. Enlarging the channel width on the hydrophobic surface increases droplet kinetic energy dissipation while reducing the droplet rolling speed. The complex flow structures formed in the droplet fluid modifies the pressure along the droplet centerline; however, the droplet fluid pressure remains almost the same order as the Laplace pressure in the upper region of a rolling droplet over the channeled hydrophobic surface.

Analysis of Droplet Characteristics and Kinetic Energy Distribution for Fixed Spray Plate Sprinkler at Low Working Pressure

HighlightsAn experiment was conducted to investigate the hydraulic performance of a fixed spray plate sprinkler (FSPS).A model was developed for estimating the cumulative kinetic energy of the FSPS under a moving system.The droplet characteristics and kinetic energy distribution were affected by the working pressure and FSPS structure.A high cumulative kinetic energy could lead to a low water infiltration rate into the soil.Abstract. The kinetic energy of droplets from a fixed spray plate sprinkler (FSPS) has a substantial influence on runoff and soil erosion, as well as on the energy consumption of moving sprinkler irrigation systems. To determine the droplet characteristics and kinetic energy of an FSPS, an experiment was conducted to investigate the effects of working pressure, plate structure, and nozzle size on the droplet diameter, velocity, and kinetic energy. Two plates (the FSPSB with a concave trajectory and deep grooves in the blue plate, and the FSPSY with a flat trajectory and shallow grooves in the yellow plate) were used in the tests. The cumulative kinetic energy and water depth were calculated for a single sprinkler moving in a straight line. The results show that the FSPSB, which had deeper grooves in the plate, produced a larger droplet diameter than the FSPSY, with shallow grooves in the plate. The droplet landing velocities presented a logarithmic relationship with the droplet diameter, and velocities increased with an increase in droplet diameter. The peak specific power (SP) value of the FSPSB was 1.14 to 16.76 times that of the FSPSY. When the working pressure was less than 150 kPa, the peak SP of the FSPSB remained at a high level. With an increase in working pressure, the peak SP of the FSPSB initially increased and then decreased, while the peak SP of the FSPSY increased. The cumulative kinetic energy of the FSPSB was higher than that of the FSPSY under mobile spray conditions. Compared with the cumulative water depth, the cumulative kinetic energy of the FSPSB increased and then decreased as the working pressure increased for the same applied water volume. Because the soil had a lower infiltration rate under the FSPSB, surface ponding was more likely to occur with the FSPSB than with the FSPSY at low working pressure. Keywords: Cumulative kinetic energy, Droplet size, Specific power, Sprinkler irrigation, Working condition.

A universal single electrode droplet-based electricity generator (SE-DEG) for water kinetic energy harvesting

Harvesting water kinetic energy is a prospective way to obtain sustainable power. However, conventional water energy harvesting technologies are restricted by specific operating locations and fabrication materials. These techniques urgently need to be improved and supplemented with new methods. Here, we report a novel and universal single electrode droplet-based electricity generator (SE-DEG) that allows converting water droplet kinetic energy into electricity power on any surfaces of artificial or natural materials such as PTFE, glass, wood, aloe, cicada wing, stone and etc. This functionality is endowed by a special configuration characterized by the location of the electrode on the top of dielectric material rather than underlying as in the conventional triboelectric nanogenerator (TENG). Such special configuration can effectively harness the triboelectric charges generated without going through the electrostatic induction process between bottom electrode and the surface in the case of TENG. We envision that SE-DEGs represent a new dimension in ability to harvest abundant hydrodynamic energy everywhere. • A universal SE-DEG represents a new dimension in ability to harvest abundant hydrodynamic energy everywhere. • The universal preparation makes the operating locations and fabrication materials of the generator unrestricted. • The design of the electrode can harness the triboelectric charges without going through electrostatic induction process.

Quantifying wind-driven rain on a heritage facade through computational fluid dynamics

The Chennai Central Station is a heritage structure and a stellar landmark - its Neo-Gothic visage is an iconic structure in the heart of this thriving metropolis requiring preservation. A new computational fluid dynamical code is developed to map an ensemble of rain droplet trajectories during Chennai's most vulnerable northeast monsoon season. This involved developing a Random Walk Model for drop trajectories in inhomogeneous turbulence (Hunt et al., 2011). A look up table of the catch ratios of nonvertical droplet showers is presented (Blocken and Carmeliet, 2004). The extent of curing on the lime mortar facade is recommended based on the quantification of micro dents formed by the impinging droplets vis-a-vis the modelled droplet kinetic energy fluxes (Khare et al., 2014). Finally, a feasible curing method and other additional measures are suggested for the soft and permeable surface of the lime mortar (Moropoulou et al., 2005), in order to enhance the life of the heritage structure for the years to come.

Spray Cooling Heat Transfer above Leidenfrost Temperature

This study considers spray cooling starting at surface temperatures of about 1200 °C and finishing at the Leidenfrost temperature. Cooling is in the film boiling regime. The paper uses experimental techniques for the study of which spray parameters are necessary for good prediction of spray cooling intensity. The research is based on experiments with water and air-mist nozzles. The following spray parameters were measured together with a heat transfer coefficient: water flowrate, water impingement density, impact pressure, droplet size and velocity. Derived parameters as droplet kinetic energy, droplet momentum and droplet Reynolds number are used in the tested correlations as well. Ten combinations of spray parameters used for correlation functions for the heat transfer coefficient (HTC) are studied and discussed. Correlation functions for prediction of HTC are presented and it is shown which spray parameters are necessary for reliable computation of HTC. The best results were obtained when the parameters impact pressure and water impingement density were used together. It was proven that the correlations based only on water impingement density, which are the most frequent in literature, can not provide reliable results.

The potential of wire feed pulsation to influence factors that govern weld penetration in GMA welding

Derivative welding processes are in many cases capable of altering phenomena that determine fundamental aspects of weld bead formation. Some of these evolutions act over the wire feed dynamics. However, in this scenario, the effects of the wire feed pulsation on the weld bead formation governing factors have not been fully explored yet. Therefore, this work aimed at examining how a wire feed pulsation approach affects the droplet transfer in gas metal arc welding and how its interaction with the molten pool defines the weld bead penetration. Bead-on-plate weldments were produced by varying the wire feed pulsation frequency, yet keeping the same levels of arc energy and wire feed speed, with the power source operating in constant voltage and current modes. To assess the droplet transfer behavior, high-speed imaging was used. The geometry of the weld beads was compared in terms of fusion penetration. The results showed that an increase in the wire feed pulsation frequency intensifies the detachment frequency of the droplets, being possible to accomplish a stable metal transfer with them straightly projected toward the weld pool, which contributed to a centralized-increased penetration profile. Based on a descriptive model, it was demonstrated that the increase in droplet momentum or kinetic energy, due to the wire feed pulsation, was not enough to justify the penetration enhancement. It was concluded that the wire feed dynamics can also stimulate surface tension variations in the weld pool and therefore disrupt the behavior of its mass and heat convection, supporting fusion penetration.

Dynamics of splashed droplets impacting wheat leaves treated with a fungicide.

Wheat is threatened by diseases such as leaf rust. One significant mechanism of disease spread is the liberation and dispersal of rust spores due to rainsplash. However, it is unclear to what extent the spore-laden splashed droplets can transmit the disease to neighbouring leaves. Here, we show that splashed droplets either bounce or stick, depending on the orientation of the leaf and whether the surface of the leaf has been treated with a fungicide. A scaling model revealed that bouncing was enabled when the droplet's kinetic energy exceeded its pinning energy to the surface. Our findings indicate that, ironically, the application of fungicide to protect a wheat plant may also facilitate pathogen spread and infection by making leaves sticky to spore-laden droplets.

A water droplet motion energy harvester with wafer-level fabrication method

Water droplets are ubiquitous in nature and harvesting droplet kinetic energy has attracted a great deal of attention to meet the increasing worldwide energy demands. In this work, we report a water droplet motion energy harvester with the wafer-level fabrication method, which is based on the water–solid surface contact electrical double layer effect. A theoretical model is established to illuminate the water droplet motion energy harvesting mechanism. Guided by the model, an energy harvester is developed based on Micro Electro-Mechanical Systems compatible process technology, which achieve a wafer-level fabrication. The output performance of the energy harvester has been optimized by decreasing the electrode width and its interval. Experimental results also show that the output voltage can be improved by increasing the droplet volume and decreasing the ionic concentration. The optimized energy harvester exhibits an open circuit output peak voltage of 57.5 mV. This research provides an in-depth theoretical study and a practical guidance on water droplet kinetic energy harvesting.

Dynamics of droplet impingement on bioinspired surface: insights into spreading, anomalous stickiness and break-up.

Inspired by the self-cleaning ability of lotus leaves and stickiness (towards water) of rose petals, we investigate the droplet impact dynamics on such bioinspired substrates. Impact studies are carried out with water droplets for a range of impact velocities on glass, PDMS and soft lithographically fabricated replicas of the lotus leaf and rose petals, which exhibit near identical wetting properties as that of the original biological entities. In this work, we investigate the spreading, dewetting and droplet break-up mechanisms subsequent to impact. Surprisingly, the rose petal and lotus leaf replicas manifest similar impact dynamics. The observation is extremely intriguing and counterintuitive, as rose petal and its replicas are sticky in contrast to lotus leaves. However, these observations are based on experiments performed with sessile water droplets. By contrast, in the current study, we find that rose petal replicas exhibit non-sticky behaviour at the short time scale s similar to that exhibited by lotus leaf replicas. Air entrapment in the micrometre features of bioinspired surfaces prevent frictional dissipation of droplet kinetic energy, leading to contact edge recession. We have also unveiled interesting universal physics that govern the spreading, recession of the contact edge and subsequent break-up modes (ligament or bulb-ligament) of the droplet.

Effects of Liquid Viscosity on Agricultural Nozzle Droplet Parameters

To discuss the effects of pesticide viscosity on the atomisation characteristics of an agricultural nozzle, glycerite with different mass fractions was prepared to replace the pesticide. First, the atomisation region of the nozzle was meshed and sized. Second, the speed and kinetic energy of the droplets at different positions in the atomisation region were measured by Phase Doppler Anemometry. The results demonstrated that the Sauter mean diameter, volume mean diameter and arithmetic mean diameter of droplets first decreased and then increased gradually in the axial direction of the atomisation region. Surface waves of a certain pattern were formed on the liquid surface, which was ejected by the disturbance of external air resistance. As the distance increased, the amplitude increased and the wave crest was broken into small droplets. These droplets then collided and agglomerated into large droplets under the effect of gravity. Droplets had an approximately symmetric distribution on the radial direction of the atomisation region, and the droplets were small in the middle and large at the two ends. The droplet size was positively related to the radial distance. Compared with the droplet speed at the two ends, the droplet speed at the axis was higher and the droplet size was smaller. Moreover, the kinetic energy of the droplets along the axial direction decreased sharply and then increased slowly. Droplets with high viscosity at the near end of the nozzle had small kinetic energy, and the effects of liquid viscosity on the atomisation characteristics of a nozzle could not be neglected. The droplet kinetic energy slightly increased at the far end.

Energy-Based Water Droplet Impact Erosion Studies of Laser-Treated Austenitic and Martensitic Steels and their Applications

Abstract High-power diode laser (HPDL) surface treatments of high-manganese austenitic and high-chromium martensitic steel round specimens have been carried out, and their resistance to water droplet impact erosion (WDIE) in air was evaluated as per ASTM G73-10, Standard Test Method for Liquid Impingement Erosion Using Rotating Apparatus. This is because the water droplets always travel along with a fluid medium, either steam or air, and these are influenced by inertial, Coriolis, and centrifugal forces, which are missing in a vacuum. Because of these forces, the water droplets elongate and may break before hitting a target, resulting in WDIE damages entirely different from those in a vacuum. The WDIE damages on the test specimens were observed on the leading edge towards the suction side, where the boundary layer is attached, and the pressure gradients are high. These are similar to those occurring on the blades of an axial flow compressor of a gas turbine, a low-pressure steam turbine, and a helicopter rotor, because their leading edges are round. The WDIE testing is based on water droplet kinetic energy (KEd) and kinetic energy flux (KEfd). It has been proved experimentally that the product of KEd and KEfd can predict WDIE damages in a material, and this product can be used to compare the WDIE test results of different materials received from different laboratories. It is observed from the test results that the resistance to WDIE of HPDL-treated high-manganese austenitic steel (Hadfield’s steel) has reduced drastically, whereas that of high-chromium martensitic stainless steel has improved manifold. Fine microstructure and increased martensitic contents in high-chromium martensitic stainless steel after HPDL treatment are the main reasons for its improved performance, whereas the coarse microstructure having microcracks in HPDL-treated Hadfield’s steel is responsible for poor performance.

Splashing characteristics of diesel exhaust fluid (AdBlue) droplets impacting on urea-water solution films

The rapid implementation of Selective-Catalytic-Reduction (SCR) technologies into light passenger and commercial vehicles, led to the omission of fundamental research creating several reliability issues that are largely related to the research field of droplet dynamics. In this study, the splashing behaviour of an AdBlue droplet impacting onto thin urea-water solution films is experimentally investigated over a range of impact parameters. In particular, the crown-type splashing threshold, the number of fingers and the characteristics of the ejected secondary droplets are evaluated for various drop impact velocities, wall-film thicknesses and urea concentrations in the liquid film. The results show that impact parameters that are able to enhance the energy dissipation in the wall-film, e.g. film thickness and viscosity, influence negatively the intensity of splashing. On the other hand, as the droplet kinetic energy increases or the wall-film thickness decreases, more energy is available to intensify the splashing outcome, and consequently, the upward ejected secondary droplet volume. The obtained trends are correlated in simple empirical expressions providing a remarkable industrial design tool, and they are also compared to the state-of-the-art literature of single- and binary-droplet/wall-film interactions aiming to draw generalised theories and paradigms that will support the connection between SCR applications and academic research.

Numerical simulation of VPPA-GMAW hybrid welding of thick aluminum alloy plates considering variable heat input and droplet kinetic energy

In order to realize the high-efficiency and high-quality welding of thick aluminum alloy plates, a paraxial variable-polarity plasma arc–gas metal arc welding (VPPA-GMAW) hybrid welding method was developed in the present study. The target material was a thick plate of 7A52 aluminum alloy, which is widely used in the aerospace field because it offers high specific strength and high corrosion resistance. The thermal action characteristics of paraxial VPPA-GMAW hybrid welding during direct-current electrode-negative and direct-current electrode-positive phases were considered. A variable combined heat source model was developed to investigate the hybrid welding thermal process. In the model, the variable-polarity plasma arc (VPPA) was treated as two curve-rotated body heat sources with different distribution parameters for electrode-negative and electrode-positive durations; the two heat sources were loaded periodically to express the heat input form. The GMAW arc was described as a double-ellipsoidal heat source, the overheated droplet was described as a uniform-body heat source, and the impact effect of the overheated droplet on the weld pool was considered by accounting for its kinetic energy. This heat source model was employed to conduct a finite-element analysis of hybrid welding temperature profiles. The numerical simulation results indicated that the VPPA provided deep penetration in the hybrid welding. The subsequent GMAW increased the penetration achieved by the VPPA and filled the weld pool to achieve deep-penetration welding. Experiments were performed to measure weld geometries and thermal cycles, and the weld pool shape was captured by high-speed camera. The calculated weld bead geometry, weld pool shape and thermal cycles showed excellent agreement with the experimental results.

Droplet Motion Model and Simulation of a Complete Fluidic Sprinkler

Abstract. Simulation and experimental verification of the spray trajectory of a complete fluidic sprinkler was performed. Through further analysis of the forces moving droplets in the air and by exploring the relationship between the air drag coefficient and droplet trajectory, a droplet motion model was established. The velocity of a spray nozzle outlet is a gamma distribution, and the droplet deviation angle is a uniform distribution with a range of 0° to 360° when the sprinkler rotates uniformly for a full circle of spraying. The droplet size distribution in the spray area is approximately log-normal, the wind speed and direction follow a normal distribution, and the initial droplet angle is 27°. Based on a random simulation theory, the droplet trajectories were simulated using MATLAB software. The simulated results show that, compared to the tested value, the error between the simulated and measured values for the average droplet diameter decreased from 53% to 2% with increasing distance. The simulated values for droplet cumulative frequency and droplet speed were nearly the same as the tested value with only a small error. For both droplet distribution and kinetic energy, the relative error was less than 20%. Therefore, the stochastic simulation method can simulate the sprinkler droplet distribution characteristics. Keywords: Complete fluidic sprinkler, Droplet breakup mechanism, Droplet distribution, Droplet motion trajectory.

Imparting Icephobicity with Substrate Flexibility.

Ice accumulation hinders the performance of, and poses safety threats for, infrastructure both on the ground and in the air. Previously, rationally designed superhydrophobic surfaces have demonstrated some potential as a passive means to mitigate ice accretion; however, further studies on material solutions that reduce impalement and the contact time for impacting supercooled droplets (high viscosity) and can also repel droplets that freeze during surface contact are urgently needed. Here we demonstrate the collaborative effect of substrate flexibility and surface micro/nanotexture on enhancing both icephobicity and the repellency of viscous droplets (typical of supercooled water). We first investigate the influence of increased viscosity (spanning from 0.9 to 1078 mPa·s using water-glycerol mixtures) on impalement resistance and the droplet-substrate contact time after impact. Then we examine the effect of droplet partial solidification on recoil and simulate more challenging icing conditions by impacting supercooled water droplets (down to -15 °C) onto flexible and rigid surfaces containing ice nucleation promoters (AgI). We demonstrate a passive mechanism for shedding partially solidified (recalescent) droplets-under conditions where partial solidification occurs much faster than the natural droplet oscillation-which does not rely on converting droplet surface energy into kinetic energy (classic recoil mechanism). Using an energy-based model (kinetic-elastic-capillary), we identify a previously unexplored mechanism whereby the substrate oscillation and velocity govern the rebound process, with low areal density and moderately stiff substrates acting to efficiently absorb the incoming droplet kinetic energy and rectify it back, allowing droplets to overcome adhesion and gravitational forces, and recoil. This mechanism applies for a range of droplet viscosities, spanning from low- to high-viscosity fluids and even ice slurries, which do not rebound from rigid superhydrophobic substrates. For a low-viscosity fluid, i.e., water, if the substrate oscillates faster than the droplet spreading and retraction, the action of the substrate is decoupled from the droplet oscillation, resulting in a reduction in the droplet-substrate contact time.

A study of the splash phenomenon of water drops on wood – Emitted droplet velocity and kinetic energy

The dynamic prompt splash behavior of water drop impinging upon a natural wood surface (Shorea spp.) was studied in this work. The water drop images during the impacting process were taken sequentially to study the properties of the emitted droplets such as the critical Weber number, droplet number, shape, velocity, size, and kinetic energy. For prompt splashing on a Shorea spp. surface, the emitted droplets formed not only during their advancing wetting process but also during the drop receding process. The emitted droplets that formed at the early stage usually had greater velocity but significant deformation. The kinetic energy of the emitted droplets varied with great scattering at every impact height, but the average kinetic energy (KEavg) of the droplets varied linearly with the impact height: KEavg (erg)=0.0017 (m0gH), implying that every emitted droplet had approximately 0.17% of the initial potential energy of the impacting water drop. Furthermore, the number of emitted droplets depended linearly on the Weber number: N=0.304 (We–77).

LMB simulation of head-on collision of evaporating and burning droplets in coalescence regime

In this paper, collision of two identical evaporating or burning droplets is simulated in coalescence regime. To do this, based on Lee’s two phase flow framework a lattice Boltzmann scheme is proposed for evaporation and combustion simulation of liquid fuel droplet. The effect of evaporation rate on collision outcome is investigated by changing Stefan number for various Weber numbers. Variation of droplet kinetic energy and viscous energy loss as well as changes in droplet oscillations after coalescence is studied for different evaporation rates. Comparing collision of evaporating and non-evaporating droplets with the same surface tension, revealed that the evaporation process reduces the surface tension effect. An effective surface tension is introduced to measure the effect of evaporation rate on collision process. The effective surface tension is a function of Stefan number, Weber number and time. It decreases by increasing Stefan or Weber number but increases with time. For collision of burning droplets, it was revealed that the combustion increases the kinetic energy of droplets before collision. Intensified velocity vortexes, which is made by the high temperature flame zone, is most likely the cause of droplet energy increment before collision.