A drop exposed to cross flow of air experiences sudden accelerations, which deform it rapidly, ultimately proceeding to disintegrate into smaller fragments. In this work, we examine the breakup of a drop as a bag film with a bounding rim, resulting from acceleration-induced Rayleigh–Taylor instabilities and characterized through the Weber number, We, representative of the competition between the disruptive aerodynamic force imparting acceleration and the restorative surface tension force. Our analysis reveals a previously overlooked parabolic dependence (∼We2) of the combination of dimensionless instability wavelengths (λ¯bag2/λ¯rim4λ¯film) developing on different segments of the deforming drop. Furthermore, we extend these findings to deduce the dependence of the average dimensionless drop sizes for the rim, ⟨D¯rim⟩, and bag film, ⟨D¯film⟩, individually, on We and see them decreasing linearly for the rim (∼We−1) and quadratically for the bag film (∼We−2). The reported work is expected to have far-reaching implications as it provides unique insight on destabilization and disintegration mechanisms based on theoretical scaling arguments involving the commonly encountered canonical geometries of a toroidal rim and a curved liquid film.

Lubricant oil is used in the rotary compressor to lubricate the bearings and to seal the clearance of sliding parts. However, a portion of lubricant oil in the upper cavity will ultimately exhaust from the compressor through the discharge tube, carried by the refrigerant. The accumulated lubricant in the tubes will deteriorate the heat transfer of the condenser and the evaporator, resulting in decreased efficiency. This work focuses on characterizing the pathway of lubricant droplets in a rotary compressor. In-situ measurements include lubricant spray penetration velocity at different phases in a rotating cycle and droplet velocities at different locations, i.e., lower cavity, above the rotor/stator, and at the discharge tube level, different frequencies, and different pressures via time-resolved shadowgraph. Particle Image Velocimetry (PIV) and Optical Flow Velocimetry (OFV) are adopted to quantify the velocities of spray and droplets. The penetration velocity of the lubricant spray in the lower cavity, horizontal swirling velocity above the rotor/stator and at the discharge tube level, and the vertical recirculation velocity above the rotor/stator are characterized. The results support a droplet pathway model in the rotary compressor that can guide the optimization of the next-generation rotary compressors.

Rotary compressors are widely used in split air conditioning units and have a significant global market share. However, the atomization of lubricant in the rotary compressor during discharge from the cylinder can cause negative effects on both heat transfer performance and reliability. Therefore, it is crucial to deepen the understanding of the behavior of atomized oil in the form of small droplets in order to improve the system's performance. In this study, lubricant droplets were analyzed at different locations, operating frequencies, and pressures using shadowgraph imaging. The droplets were extracted using the Canny operator and corrected using a Convolutional Neural Network, and their sizes were analyzed statistically. The results reveal that the droplet characteristic mean diameters increase with frequency and pressure, with larger sizes observed in the outer region above the rotor/stator and constant sizes in the radial direction at the discharge tube level. Additionally, the droplet's volume fraction increases with frequency, with higher values observed in the outer region above the rotor/stator and above the rotor/stator clearance at the discharge tube level. These findings provide valuable experimental data for setting up the boundary conditions used in computational fluid dynamics (CFD) simulations.

Characterization of the spray resulting from a 1 mm diameter liquid ethanol jet injection into a supersonic air crossflow is presented.The isentropic Mach number at the injection location is 1.94, while the momentum flux ratio is varied between 3 and 13.Steady and unsteady components of the penetration depth are retrieved using backlit imaging perpendicular to the spray at a frame rate of 20 kHz, coupled with a telecentric lens to accommodate the test section's width and ensure equal magnification through the spray's width.Spray width at specific axial locations downstream of injection is measured by imaging Mie scattered light from a planar laser sheet directed perpendicular to the flow direction so that a cross section of the spray is recorded by a camera placed at a known angle between camera axis and flow direction.

As hundreds of millions of Air conditioning (AC) systems are produced each year, and many of them use rotary compressors as the heat pump, optimizing the flow inside the rotary compressor to improve its reliability and efficiency becomes a key issue of the manufactures. Since the invention of the rotary compressor, its internal flow has been studied numerically with real models. However, a rotary compressor’s internal flow can be extremely complicated due to the complex internal structures’ geometry and high-speed moving parts, making it difficult to interpret the result by CFD simulation and repeat the simulation in different models. In our experiments for observing lubricant oil droplets above the rotor/stator in a rotary compressor, droplets’ movement reveals that two major effects control the gas flow in the compressor’s upper cavity. One is the swirling jet produced by the high-speed rotating rotor with no-slip condition on its sidewall. The other one is the rotating disk effect induced by the top of the high-speed rotating rotor. Either of them has been studied individually in different areas. For example, the swirling jet is often used in combustors while the rotating disk is applied in the viscous pump. However, the coupling of these two effects in the rotary compressor with different velocity ranges, size scales, and fluid properties has not been studied according to our best knowledge. In our simulation, a model that only consists of a simplified rotor, simplified stator, sidewall, and discharge tube (outlet) is built. Thus, the effect by small parts, such as the balance block and coils, is excluded. The rotor is set to rotate at 30, 60, and 90 Hz. Uniform velocity calculated with the theoretical flow rate and ambient pressure conditions are given at the inlet (rotor/stator clearance) and outlet, respectively. No-slip conditions are defined at other walls. Steady-state K-ω SST turbulence models are applied, and the cases are computed with OpenFoam. The CFD results show an inner recirculation zone above the rotor that creates a downward velocity component above the rotor and an outer circulation zone above the stator. The CFD result meets the observation of the droplets’ movement above the rotor/stator. With the CFD results and the experiment’s observations, we propose the model of the oil droplet’s path in the rotary compressor’s upper cavity, which can help reduce the exhausted lubricant oil droplets from the compressor.

The spray characteristics of standard and alternative aviation fuels created by a hybrid pressure-swirl airblast (HPSA) atomizer were investigated at conditions corresponding to engine cold start for an aviation gas turbine. Drop size and drop velocity of sprays from three different fuels, Jet-A (A-2), JP-5 (A-3), and a JP-5/farnesane blend (C-3) were investigated using a phase Doppler anemometry. The spatially resolved drop measurements were obtained at three axial locations downstream of the swirler exit plane and up to 30 mm in the radial direction from the spray centerline. Smaller D-10, D-32, and MMD were observed when increasing the pressure drop across the atomizer swirler (Delta P/P). Increasing Delta P/P also increased the mean axial velocity. No definitive trend was observed with viscosity on drop size, while liquid-gas surface tension showed a weak correlation with drop size due to its small variation in value among fuels. The spray unsteadiness was examined using interparticle arrival time distribution functions. A semi-empirical model was also developed using a phenomenological three-step model for the atomization process of the HPSA atomizer and was used to predict drop-size values at engine cold-start conditions in this work and near lean blowout conditions presented in earlier work.

A droplet subjected to external aerodynamic perturbations and fragmenting into smaller droplets is defined as secondary atomization. In the past decades, droplet breakup at low Ohnesorge numbers ( Oh < 0.1) had been characterized with unanimity in the community regarding its physics. However, droplet breakup at high Oh number, i.e. atomization where the influence of viscosity is significant, has not been studied in detail, and there is a poor agreement in conclusions by various researchers. This work presents a thorough analysis of the modes of deformation and breakup exhibited by a high viscous ( Oh = 0.5, Oh = 1) droplet subjected to continuous airflow ( We = 30-120). Several modes of droplet breakup are presented and transition case with no droplet fragmentation has been discovered. Size-velocity pdfs of droplets at different Ohnesorge numbers for varying Weber numbers were obtained by processing the images acquired using Digital in-line holography (DIH). Particle image velocimetry (PIV) was employed to characterize the airflow in cases where the droplets exhibited no breakup and cases with multiple bag formation. The percentage of minimum backflow velocity required for droplet breakup is also presented. ? 2021 Elsevier Ltd. All rights reserved.

Hundreds of millions of Air conditioning (AC) systems are produced each year. Many of them, especially small AC appliances, use rotary compressors as the system's heat pump due to their simple structure and high efficiency in a small system. Lubricant oil is used in the rotary compressor to lubricate the moving parts, such as the crankshaft and the rolling piston, and to seal the clearance between the sliding parts, e.g., the clearance between the rolling piston and the cylinder, and the vane and the cylinder. As the compressed refrigerant vapor is discharged from the cylinder through the discharge port, part of lubricant oil in the cylinder would be carried by the vapor and atomize into small droplets in the lower cavity during the discharge process, which is complicated and highly-coupled. Some of these oil droplets would ultimately be exhausted from the compressor and enter other parts in the system, reducing the compressor reliability and deteriorating the heat transfer of the condenser and the evaporator in the system. Our previous research studied the atomization of the lubricant oil during the discharge process in the compressor's lower cavity. However, the oil droplets' behavior downstream of the lower cavity is unknown. Thus, studying the oil droplets' behavior after passing through the rotor/stator can help understand how the rotor/stator would affect the droplet size distribution and movement, thus controlling the flow rate of escaped oil droplets. In this study, a hot gas bypass test rig is built to run a modified rotary compressor with sapphire windows right above the rotor/stator. The oil droplets' size distribution and movement along the radial direction are obtained at the shaft's rotating frequency of 30 and 60 Hz by shadowgraph. It is found that droplet size at 30 and 60 Hz varies little in the inner region of the rotor/stator clearance and would increase sharply above the clearance and keep increasing in the outer region of the clearance. More importantly, droplet velocity has a downward velocity component at the inner region and an upward velocity component at the outer region of the rotor/stator clearance. With the result of droplet size distribution and droplet velocity above the rotor/stator, we propose the model of the oil droplet's path above the rotor/stator, which can be understood as the coupling of a swirling jet and a rotating disk.

The spray characteristics of standard and alternative aviation fuels generated by a hybrid pressure-swirl airblast atomizer were studied at conditions corresponding to near-lean blowout conditions: ambient gas pressure Pvessel=2.07bar, fuel temperature Tfuel=322K, and atomizing gas temperature Tgas=394K. The candidate alternative fuels (C-1, C-5, C-7, and, C-8 synthetic blends) were compared to Jet-A (A-2). The drop size and drop velocity were measured using phase Doppler anemometry at three measurement planes downstream of the swirler exit. The representative diameters [D10, D32, and the mass mean diameter (MMD)] were used to characterize the drop size. The drop velocity was characterized using the arithmetic mean and the root mean square. An increase in the swirler pressure drop (Delta P/P) showed a decrease in the mean drop size and an increase in the magnitude of the axial drop velocity. Comparing the different fuels generally showed an increase in D10 for higher fuel viscosity. However, definitive trends in D32 and the MMD were not observed, based on fuel physical properties such as viscosity, surface tension, or density. A variation of the fuel injection pressure Delta PPilot showed no significant effect for both the standard and alternative fuels. The full cone angle (obtained using shadowgraph images) was found to not vary significantly with Delta PPilot or Delta P/P.

The rotary compressor is widely used in small air conditioners, and is the most important element in the system. It relies on eccentric rolling pistons that rotate at high speed to compress refrigerant in the cylinder. The lubricant oil in the rotary compressor is used for lubricating the bearing and sealing the clearance of sliding parts. However, the oil can experience complex and highly-coupled atomization processes when discharged from the cylinder, and part of oil droplets can exhaust from the rotary compressor by the refrigerant flow and reduce the efficiency and reliability of the compressor as a result. Thus, characterizing the behavior of oil droplets in the lower cavity of a rotary compressor where the atomization occurs is a major challenge for manufacturers who rely on CFD tools to predict the multiphase flow. By modifying a rotary compressor, the oil behavior in the lower cavity of a rotary compressor is observed and recorded by shadowgraphy. In the current phase, the number, size, and morphology of oil droplets are analyzed statistically with image processing method, which provides better understanding to the atomization mode in the lower cavity, the velocity of the mist of oil droplets is calculated with Optical Flow Velocimetry. The results can assist designers in improving the CFD analysis of compressors and ultimately reducing the Oil Discharge Rate (ODR).

A rotary compressor relies on an eccentric rolling piston, which rotates at high speed, to compress gas in the compression chamber. The oil in the rotary compressor is used for lubricating the bearing and sealing the clearance of sliding parts. However, the oil can exhaust from the rotary compressor by the refrigerant flow and reduce the reliability of the compressor as a result. Thus, studying the behavior of oil droplets distribution in a rotary compressor is a major challenge for manufacturers who rely on CFD tools to predict the multiphase flow. By modifying a rotary compressor, the oil behavior inside the cylinder is observed and recorded by a high-speed imaging system. In the current phase, multiple targeted locations, including the space between the bearing housing and the stator, and the space above the stator are measured in different conditions. The number, size, velocity, and morphology of oil droplets are analyzed based on multiple snapshots. The result can assist designers in improving the CFD analysis of compressors and ultimately reducing the oil discharge rate (ODR).

Dynamic stringy objects such as liquid rims and ligaments are frequently observed in important applications such as the multiphase breakup of fuel droplets. We develop a new method based on digital in-line holography to automatically measure complicated stringy objects. A static spring mounted on a rotator is measured to validate the effectiveness and accuracy of the method. The sections are extracted along the skeleton of the spring in a depth-of-field extended image and then sized and located as individual particles using a hybrid method. The surface points of sections are stitched together to visualize the entire spring. Local thickness errors smaller than 5.3%, and z errors smaller than 230 μm are achieved. This method is applied to characterize the spatial-temporal features of the liquid rim formed in the bag-type regime of the aerodynamic breakup of a falling drop. The evolution of the rim/ligament structures is continuously captured in seven frames, lasting in 1.58 ms. This Letter extends the application of digital holography as an effective 3D diagnostic tool.

Quantifying the early stage of bag-type breakup of droplet is an important way to study the mechanism of drop breakup, but remains a challenge due to the lack of spatial-temporal resolved diagnostic technique. High-speed digital in-line holography at 20 kHz is employed to characterize secondary droplets formed in bag rupture of an ethanol drop exposed in the gas stream. Droplets as small as 10 μm are resolved at the beginning of bag rupture at Weber number of 11. The velocities of secondary droplets can almost reach that of the gas stream. Then the thin wall shrinks to form wrinkles that will generate relatively larger secondary droplets with smaller velocities. Droplet diameters are statistically displayed and the relationship between velocity and diameter as well as time is analyzed. This will help the further understanding of fuel spray generation in gas turbine engines.

Over the last two decades there has been a renewed interest in employing shear-thinning non-Newtonian liquids for use with liquid rocket engines due to a desire for greater safety and performance. Since the difficulty with producing quality atomization is the main disadvantage for using non-Newtonian liquid propellants, further understanding of the breakup physics and accurate measurement of the spray characteristics is needed. The atomization produced by the like-on-like jet impingement of three different non-Newtonian liquids was experimentally investigated in this work using shadowgraphy and Phase Doppler Anemometry (PDA) at high strain rates (10(5)-10(6)), which correspond closely to the strain rates used in practical rocket engines. The three water-based solutions tested were of two different grades of Carboxymethylcellulose (CMC): 0.5 wt.-% CMC-7HF, 0.8 wt. -% CMC-7MF, and 1.4wt.-% CMC-7MF. The Bird-Carreau rheological model was used to characterize the non-Newtonian behavior and a generalized Carreau number was used as the primary parameter to characterize the liquid viscosity inside the orifice at injection. A generalized Bird-Carreau jet Reynolds number Re-j,Re-gen-BC and the jet Weber number We i were used as primary parameters to characterize the spray formation, sheet breakup length, drop size, and drop velocity. The unique spray formation a behavior of the three non-Newtonian liquids was ascribed to the relative influence of the inertial vs. viscous forces, primarily expressed through Re-j,Re-gen-BC. At many relatively high inertial force test conditions, drops in the spray were near spherical and could be reliably measured using PDA. Spray characteristics of intact sheet length, drop diameter, and drop velocity were observed to be strong functions of Re-j,Re-gen-BC. In addition to providing fundamental understanding to aid with injector design for liquid rocket engine applications using non-Newtonian liquids, the measurements presented in this work could be used to validate a future closed-form analytical non-Newtonian atomization model, which remains an open problem in non-Newtonian impinging jets literature. (C) 2018 Elsevier Ltd. All rights reserved.

A liquid drop undergoes aerodynamic deformation and breakup when it is exposed into a gas stream. Many techniques were used to measure the size and velocity of the secondary droplets while quantifying the rim/ligament still remains a challenge. An automatic method to extract the 3D properties of the toroidal rim in the bag breakup was recently developed based on digital in-line holography (DIH). To reduce the uncertainty caused by the out-of-focus overlap, a DIH configuration with a slightly rotated view is adopted here. The entire rim is reconstructed by stitching all the sections together. Holograms are recorded with a high-speed camera operated at 20 kHz to study the dynamic evolution of the rim in the bag breakup of an ethanol drop. Both the 3D visualization and z–y view reflect the rim’s structure development within 5.2 ms. The rim expands followed with disintegration into ligaments and relatively larger droplets. The volume of the rim is measured ∼ 95 % and that of the secondary droplets is ∼ 5 % of the initial drop volume before rim breakup. Then the volume of rim/ligament decreases after rim breakup which on the other hand increases the volume fraction of secondary droplets. The total volume of the rim/ligament and fragments is very close to the initial drop volume in most measurements except when the instant swelling happens in local atomization. Then the total measured volume decreases rapidly as the relatively large fragments move out of the field of view.

While there is no single analytical model that accurately predicts all stages and modes of secondary atomization, many groups have developed models that predict deformation and oscillation of a single, isolated drop. The TAB (Taylor Analogy Breakup) model was chosen for this investigation, mainly due to its widespread use by Liu and Reitz [1], Hwang et al. [2], Tanner [3], and Lee and Reitz [4], among others. Since the TAB model is also the foundation for many other analytical models, it will also be used here as a starting point for the development of a viscoelastic non-Newtonian model to predict droplet deformed radii, droplet deformation time, and velocity at deformation time for viscoelastic xanthan gum - DI water solutions. Three additional improvements are made to this viscoelastic TAB model: the first is a change to a TAB coefficient; the second to the equation for the drag coefficient, and the third modification is to the breakup criterion. This model uses Carreau rheology and Zimm relaxation time. Non-dimensional drop diameter and initiation times are plotted against We; model results are compared to experimental results for a range of xanthan gum solution concentrations. Results show fair agreement between experimental results and model results for non-dimensional drop diameter, with the best match at low XG concentration and low-to-medium We (10–30). It was also noted that increased viscoelasticity seems to increase this drop diameter. Good agreement between experimental data and model results has been seen for initiation time, with increased viscoelasticity increasing this parameter as well.

When a spherical liquid drop is subjected to a step change in relative gas velocity, aerodynamic forces lead to drop deformation and possible breakup into a number of secondary fragments. To investigate this flow, a digital in-line holography (DIH) diagnostic is proposed which enables rapid quantification of spatial statistics with limited experimental repetition. To overcome the high uncertainty in the depth direction experienced in previous applications of DIH, a crossed-beam, two-view configuration is introduced. With appropriate calibration, this diagnostic is shown to provide accurate quantification of fragment sizes, three-dimensional positions and three-component velocities in a large measurement volume. These capabilities are applied to investigate the aerodynamic breakup of drops at two non-dimensional Weber numbers, We, corresponding to the bag (We = 14) and sheet-thinning (We = 55) regimes. Ensemble average results show the evolution of fragment size and velocity statistics during the course of breakup. Results indicate that mean fragment sizes increase throughout the course of breakup. For the bag breakup case, the evolution of a multi-mode fragment size probability density is observed. This is attributed to separate fragmentation mechanisms for the bag and rim structures. In contrast, for the sheet-thinning case, the fragment size probability density shows only one distinct peak indicating a single fragmentation mechanism. Compared to previous related investigations of this flow, many orders of magnitude more fragments are measured per condition, resulting in a significant improvement in data fidelity. For this reason, this experimental dataset is likely to provide new opportunities for detailed validation of analytic and computational models of this flow. (C) 2017 Elsevier Ltd. All rights reserved.