R. Arun Kumar

Abstract: This paper aims to investigate the secondary flow characteristics and the associated vacuum generation caused with increase in the primary pressure ramping in zero-secondary flow ejectors. The sudden expansion of the primary jet into the diffuser during the ejector start-up results in flow separation from the shear layer formed between the primary and inducted flows and produces large recirculation bubbles in the top and bottom sides of the jet. These recirculation bubbles cause an induced flow from ambient air into the diffuser duct as well. The fluid supply from the reverse flow due to the shear layer separation and the induced flow from ambient air provide a counter momentum against fluid entrainment from a vacuum chamber. As a result of this, the initial vacuum generation process progresses in a slow rate. Thereafter, the primary jet expansion reaches a critical level and a rapid vacuum generation can be seen. It is found that with the jet expansion reaching a critical level, the fluid supply from the reverse flow is suddenly entrained back into the main jet at the maximum jet expansion point. This suddenly reduces the counter-momentum which has been prohibiting the entrainment of fluid from the vacuum chamber and results in rapid evacuation. This is followed by a stage in which the vacuum chamber pressure is increasing due to the attainment of a constant Mach number at the diffuser inlet and the jet pressure ramping. It is found that the secondary flow dynamics and the vacuum generation processes in rectangular and round ejectors show a close resemblance.

Abstract: An experimental study has been carried out to investigate the nature of transients in vacuum ejector flows during start-up and the dynamics in flow characteristics. The results show that the secondary stream induction progresses with non-uniform rates with the ramping primary jet pressure during start-up. The initial evacuation period is subjected to gradual and highly perturbed secondary fluid entrainment. In this phase, the secondary stream induction by the shear layer is asymmetric leading to an un-even vacuum generation in the secondary chamber. In the second phase, the secondary pressure fluctuations are found to be ceased for a critical primary jet pressure followed by a rapid induction of the secondary fluid till the primary jet expands to the diffuser wall. The transition from the first phase to the second phase is caused by the secondary stream flow choking in the diffuser. Following the second phase, a stable stage exists in the third phase in which the vacuum pressure decreases only marginally. Any further attempt to increase the secondary chamber vacuum level beyond the third phase, by increasing the primary jet total pressure, results in flow reversal into the secondary chamber, spoiling the already achieved vacuum level. In the fourth phase of start-up, a complicated shock interaction transformation from a Mach reflection (MR) to regular reflection (RR) occurs within the diffuser. It is also observed that the primary jet pressures for the minimum secondary chamber pressure, the minimum secondary pressure, and the primary pressure for MR-RR transformation decrease initially with increase in diffuser length and then increase. It is found that the decreasing and increasing trends are caused by the pressure recovery and Fanno effects, respectively.

Abstract: This study investigates the shock transformation in an underexpanded jet in a confined duct when the jet total pressure is increased. Experimental study reveals that the Mach reflection (MR) in the fully underexpanded jet transforms to a regular reflection (RR) at a certain jet total pressure. It is observed that neither the incident shock angle nor the upstream Mach number varies during the MR–RR shock transformation. This is in contradiction to the classical MR–RR transformations in internal flow over wedges and in underexpanded open jets. This transformation is found to be a total pressure variation induced transformation, which is a new kind of shock transformation. The present study also reveals that the critical jet total pressures for MR–RR and RR–MR transformations are not the same when the primary pressure is increasing and decreasing, suggesting a hysteresis in the shock transformations.

Abstract: The design of a suitable separator is an effective approach to enhance the performance as well as the safety of a rechargeable battery. The conventional glass fiber separator has electrolyte leakage due to the random distribution of pores in the structure. The design of a gel polymer electrolyte with phosphorus containing compound is considered to be safer for the operation of a rechargeable sodium ion battery. Hence, we have developed a gel polymer using hydroxyapatite, a calcium phosphate-based compound in poly (vinylidene fluoride-hexafluoropropylene)-poly (butyl methacrylate) blend membrane by a simple solution casting technique. The developed membrane has an ionic conductivity of 1.086 × 10−3 S cm−1 with an electrochemical stability of up to 4.9 V, good porosity and electrolyte uptake, thereby making it a promising electrolyte to be used in a rechargeable sodium ion battery. To demonstrate its feasibility, the electrochemical properties of Na3V2(PO4)3/C are investigated using the prepared gel polymer electrolyte. The sodium ion cell using gel polymer electrolyte exhibits a specific capacity of 97 mAh g−1 at 4 C which is about 33.5% enhancement in specific capacity when compared to the cell with the conventional glass fiber membrane. This study illustrates the feasibility of using gel polymer electrolyte as a replacement to the existing glass fiber separator.

Abstract: In recent times, shock tube flows have been widely employed in many micro-scale devices in the fields of propulsion technology, micro-heat engines, particle delivery systems, and so on. The very small length scales in such micro-shock tubes make the flow physics more complicated compared to the ordinary macro-shock tubes. The major differences in the flow features are the profound influences of wall effects and rarefaction effects. The rarefaction effect alters the boundary layer structure by imparting additional velocity and thermal gradients to the wall-bounded fluid. These phenomena can strongly affect the micro-shock tube flow characteristics such as shock–contact wave speeds, wave propagations, hot and cold zone properties. The main objective of the present work is to produce a detailed understanding on the wave propagation characteristics in a micro-shock tube under rarefied conditions using computational fluid dynamics methods. The shock–contact interface movement under different operating conditio...

Abstract: A low area ratio rectangular supersonic gaseous ejector is subjected to parametric evaluation to calculate the performance parameters like stagnation pressure ratio, compression ratio, entrainment ratio and the mixing parameter known as non-mixed length for a wide range of operating conditions by varying the secondary flow rate. The operating conditions are achieved by varying the design Mach number of the primary flow nozzle, the total pressure of the primary flow and the secondary flow rate. Air is used as the working fluid in both the primary and secondary flow. The ejector is operated in the mixed regime. Mach number ratio is used as the non-dimensionalization parameter, and fully-expanded jet height is used as the scaling variable to collapse the huge set of obtained data for parametric studies. With variation in the secondary flow rate, staging is observed in the compression ratio, entrainment ratio, and also in the non-mixed length. Variation in the entrainment ratio and non-mixed length are observed to be linear, and it scales well with the fully-expanded jet height when there is a deficit in the secondary flow. Also, when there is no secondary flow, the non-mixed length is observed to be 80% lower in comparison with the case, where the secondary flow is uncontrolled. Schlieren visualization and wall static pressure measurements supplement the findings.

Abstract: This article describes experiments to investigate the fluid-to-wall interaction downstream of a highly underexpanded jet, with a pressure ratio of 120, confined in a channel. Heat transfer induced by Joule-Thomson cooling, which is a real gas effect in such a configuration, has critical implications on the safety of pressurised gas components. This phenomenon is challenging to model numerically due to the requirement to implement a real gas equation of state, the large range of (subsonic and supersonic) velocities, the high turbulence levels and the near-wall behaviour. An experimental setup with simple geometry and boundary conditions, and with a wide optical access was designed and implemented. It consisted of a high-pressure gas reservoir at controlled temperature and pressure, discharging argon through a nozzle into a square channel. This facility was designed to allow for a steady-state expansion from over 120 bar to atmospheric pressure for over 1 min. The choice of fluid, pressure and temperature regulation system, and the implementation of a high pressure particle seeding system are discussed. The gas dynamics of this flow was then investigated by two separate optical techniques. Schlieren measurements were used to locate the position of the Mach disk, and planar particle image velocimetry (PIV) was used to measure the turbulent velocity field in the regions of lower velocity downstream. Mie scattering images also indicated the presence of a condensed argon phase in the supersonic region as expected from previous studies on nucleation. The observed location of the sharp interface at the Mach disk was found to be in excellent agreement with the Crist correlation. Rapid statistics were derived from the PIV measurements at 3 kHz. The recirculation zone was found to extend about 4 channel heights downstream, and in the region between 2 and 3 channel heights downstream, a continuous deceleration on the centerline velocity was observed in line with the narrowing of the recirculation zone. The first and second velocity moments as well as Reynold stresses were quantified, including pdf distributions. In addition, a sensitivity and repeatability analysis, an evaluation of the PIV random uncertainty, as well as an estimation of errors induced by particle inertia were performed to allow for a full quantitative comparison with numerical simulations.

Abstract: This paper aims to investigate the secondary flow characteristics and the associated vacuum generation caused with increase in the primary pressure ramping in zero-secondary flow ejectors. The sudden expansion of the primary jet into the diffuser during the ejector start-up results in flow separation from the shear layer formed between the primary and inducted flows and produces large recirculation bubbles in the top and bottom sides of the jet. These recirculation bubbles cause an induced flow from ambient air into the diffuser duct as well. The fluid supply from the reverse flow due to the shear layer separation and the induced flow from ambient air provide a counter momentum against fluid entrainment from a vacuum chamber. As a result of this, the initial vacuum generation process progresses in a slow rate. Thereafter, the primary jet expansion reaches a critical level and a rapid vacuum generation can be seen. It is found that with the jet expansion reaching a critical level, the fluid supply from the reverse flow is suddenly entrained back into the main jet at the maximum jet expansion point. This suddenly reduces the counter-momentum which has been prohibiting the entrainment of fluid from the vacuum chamber and results in rapid evacuation. This is followed by a stage in which the vacuum chamber pressure is increasing due to the attainment of a constant Mach number at the diffuser inlet and the jet pressure ramping. It is found that the secondary flow dynamics and the vacuum generation processes in rectangular and round ejectors show a close resemblance.

Abstract: This study investigates the shock transformation in an underexpanded jet in a confined duct when the jet total pressure is increased. Experimental study reveals that the Mach reflection (MR) in the fully underexpanded jet transforms to a regular reflection (RR) at a certain jet total pressure. It is observed that neither the incident shock angle nor the upstream Mach number varies during the MR–RR shock transformation. This is in contradiction to the classical MR–RR transformations in internal flow over wedges and in underexpanded open jets. This transformation is found to be a total pressure variation induced transformation, which is a new kind of shock transformation. The present study also reveals that the critical jet total pressures for MR–RR and RR–MR transformations are not the same when the primary pressure is increasing and decreasing, suggesting a hysteresis in the shock transformations.

Abstract: Ejectors have many applications in aerospace and energy conversion technologies. An ejector is a passive device that pumps a secondary fluid by energy augmentation from a primary fluid. The performance of the ejector is primarily dependent on the complex gas dynamic interactions between the primary and secondary flows in a variable area duct. The maximum mass flow rate of the ejector in the critical flow regime is limited by choking of both the primary and secondary flows. This paper focuses on experimental investigations on the mixing characteristics of the ejector having an area ratio of 2 in the critical flow regime for a range of stagnation pressure ratios varying between 5.49 and 11.12 and a primary Mach number of 1.5, 2.0, and 2.5. The gas dynamic flow field is visualized using non-invasive high-speed schlieren and Mie-scattering techniques. Mie-scattering images are used to gain insight into the mixing characteristics and estimate the non-mixed length. The optical measurements are complimented with the wall static pressure measurements. The experimental performance data are compared with two well-established analytical models. The characterization of the mixing in the critical operating regime of the ejector using optical tools is being reported here for the first time. An important outcome of the paper is the observation of a significant increment in the non-mixed length to the tune of 55% increase in the critical flow regime in comparison to the mixed flow regime.