Starting transients in second throat vacuum ejectors for high altitude testing facilities
Introduction
The static testing of rocket nozzle is one of the critical and mandatory test procedures for any space mission involving rocket propulsion. Among the static testing of various rocket stages, the most challenging is the testing of the upper stage rocket motors. This is because the upper stage rocket motor is subjected to vacuum pressure condition and maintaining such low vacuum pressure in a test chamber is a challenging task. The conventional method of using vacuum pumps may not provide the required vacuum level due to the enormous mass flux ejected by the rocket motor into the vacuum chamber. An alternative method for creating the low back pressure is by pumping out the fluid from the vacuum chamber using the nozzle jet itself. It is well known that a high momentum nozzle jet (primary flow) inducts and entrains the surrounding fluid (secondary flow) due to the momentum exchange between the two streams. This principle can be used to create a vacuum condition at the nozzle exit by exhausting the nozzle jet into a confined chamber with upstream closed and such devices are known as the vacuum ejectors or zero secondary flow ejectors [1], [2], [3]. In general, the high-altitude test facilities (HAT) used for testing upper stage rocket motors employs the vacuum ejector principle for maintaining the required vacuum level at nozzle exit. A schematic of a typical high altitude testing facility is shown in Fig. 1(a). As shown in Fig. 1(a), the nozzle will be kept in a vacuum chamber (secondary chamber) and is connected to a diffuser system for the pressure recovery. Two configurations of ejector-diffuser systems are commonly used for HAT systems, 1) straight ejector diffuser (SED) and 2) the second throat ejector diffuser (STED) system [4]. The schematic of the two configurations can be found in Fig. 1(a) and 1(b), respectively. In straight ejector diffuser (SED), a straight constant area duct is used as the diffuser section and in second throat ejector diffuser (STED) system a convergent area section followed by a constant area duct is used as the diffuser section. Both these configurations utilise the shock cells developed in the duct for the pressure recovery.
It is seen from the literature that the ejector system in high altitude testing (HAT) facility operates in two modes during the initial transient starting phase, where the stagnation pressure in the combustion chamber builds up to a steady state. During this transient stage, the stagnation pressure () at the inlet of the rocket nozzle increases and the jet expands continuously. In the first operation mode of the ejector, the jet boundary of the nozzle plume is not attached to the outer wall and this mode is called the un-started mode [4]. However, at a specific inlet stagnation pressure , the underexpanded jet coming from nozzle impinges on the outer wall and produce shock cells. These shock cells help in pressure recovery and the outer duct downstream to the impingement point acts as a diffuser. The condition at which the supersonic jet attaches with the outer duct is therefore referred as the starting of the diffuser and the corresponding nozzle inlet stagnation pressure () is called the starting pressure of the diffuser [4], [5]. As the diffuser attains the started mode, the jet boundary seals the vacuum chamber from the diffuser downstream and blocks further induction of fluid from the vacuum chamber. As a result of this, the minimum vacuum chamber pressure can be expected at the onset of started mode operation. To predict the starting pressure ratio and minimum vacuum pressure, various theoretical models have been proposed in the past and experimental validations have been done [6], [7], [8]. It is observed that the starting pressure ratio depends on the geometrical parameters like the ratio of diffuser length to diameter (L/D), the diffuser diameter to nozzle throat [9] and the annular gap between the nozzle exit and the diffuser. It has been reported by German and Bauer [3] that the starting pressure ratio decreases with increase in diffuser length to diameter ratio (L/D) and at a critical L/D ratio, the starting mode pressure ratio attains a minimum value and thereafter increases with further increase in L/D ratio. Annamalai et al. [9] carried out various full scale rocket motor tests in high altitude testing facility using both hot and cold flow jets. Their studies proposed a scaling criterion which related the effect of diffuser length in determining the starting mode conditions. It should also be noted that the starting mode pressure ratio increases with increase in diffuser diameter to throat diameter (D/d) ratio [10]. Past studies have reported that the starting mode pressure decreases with the use of second throat ejector diffuser systems [11].
In the past, there have been many optimisation studies on the ejector-diffuser systems to improve the performance of vacuum generation for high altitude testing facility. To optimise second throat ejector diffuser Kumaran et al. [12] carried out various steady-state simulations and studied the different parameters of second throat ejector diffuser such as the effect of nozzle position and supersonic cone convergent angle. They found out that the starting pressure of diffuser is hardly affected by the change in convergent angle and the nozzle position. However, their studies are based on steady-state simulations and did not captured the entire transient start-up operation. Many past studies have also reported that size of vacuum chamber does not affect the starting pressure of the diffuser [13], [14]. Park et al. [15] investigated the effect of pre-evacuation of the vacuum chamber and found that pre-evacuation expedites the starting process. The hot fluid from the reverse flow in the diffuser can destroy the complex instruments placed in the vacuum chamber and also affect the vacuum pressure. Hence, there have been attempts in the past to minimise the backflow into the vacuum chamber [16], [17]. Lijo et al. [16] used orifice plate at the inlet of the diffuser as backflow arrester and got improvement in the performance of straight ejector diffuser for a certain range of pressure ratio. Ashok et al. [17] successfully tried few backflow arresters such as rocket motor with thick nozzle lip and ring-type backflow arresters.
Apart from the parametric and performance enhancement studies, there have also been many studies which reported the fluid dynamics during the ejector-diffuser start-up process in high altitude testing facility. Past studies have shown that a non-uniform vacuum generation process exists in high altitude testing during the vacuum ejector start-up process [18]. During the initial stage of un-started mode, the vacuum generation progress with large scale pressure oscillations in the vacuum chamber [18]. This is followed by a rapid evacuation stage which ends with the attainment of the started mode. After the started mode, any further increase in jet total pressure leads to destruction in vacuum level in the vacuum chamber. In order to understand the non-monotonic vacuum generation process, it is important to understand the complex fluid dynamics existing in the transient start-up phase. Park et al. [19] studied the evacuation process in high altitude testing and reported that a plume blow back exists during the trailing transient of the straight ejector diffuser. It has been observed that recirculation zone is formed at nozzle exit which opposes the flow from vacuum chamber [19]. Mittal et al. [20] reported the existence of repeated flow reversals leading to pressure oscillations during the initial stage of evacuation. However, these studies did not reveal the entire dynamics of recirculation bubble during the during the start-up process, since these studies didn't consider the transient pressure ramp-up process. This has been addressed by the recent studies by Arun et al. [21], [22] in which the dynamics of the recirculation bubble during the entire start-up process has been experimentally and numerically investigated. The transient simulations carried out by Arun et al. [22] revealed that the initial pressure oscillations during the un-started mode are due to the large-scale oscillations in the recirculation bubble. However, these studies were carried out using a straight circular nozzle with straight ejector diffuser and the actual high-altitude testing facility operates with convergent-divergent (C-D) nozzle. A recent study by Fouladi et al. [23] reported that the starting transients in the second throat ejector diffuser using C-D nozzle depends on the shock separation pattern inside the nozzle. The experimental and numerical study carried out by Fouladi et al. [24] revealed that the vacuum chamber pressure during the start-up and shut down process are different and there exists a hysteresis process. This hysteresis in vacuum pressure occurs due to the difference in the shock separation patterns existing in the nozzle during the starting and shutdown process [24].
It can be seen from all these past studies that the effect of geometric parameters on the high-altitude testing (HAT) performance have been fairly understood. A general understanding on the fluid dynamics in the diffuser during the entire start-up region is also fairly understood. However, the past studies which investigated the fluid dynamics during the start-up process majorly concentrated on the high-altitude testing configuration with straight ejector diffuser system. The few fundamental studies which investigated the starting transients in the second throat ejector diffuser system were majorly carried out with steady state simulations without considering transient pressure ramp-up process and does not revealed the complete transient evolution of the flow field. Moreover, many complex flow physics in the second throat ejector diffuser systems, such as the reason for the rapid evacuation stage, transient flow behaviour with variation in geometric parameters etc. are still not properly understood. Hence, the present work aims to bridge this gap by investigating the transient flow evolution during the entire start-up process in high altitude testing facility with second throat ejector diffuser system using CFD simulations. The nature of the start-up process with change in various geometric configurations like, convergence angle and nozzle position are also investigated. The study is expected to bring much more insight into the complex fluid dynamics during the start-up process in high altitude testing system and its effect on various geometric parameters which will be useful in better optimisation of the system.
Section snippets
Computational details
The computational domain for the present study consists of an axisymmetric second throat ejector diffuser system, as shown in Fig. 2. The flow field is simulated by solving the axi-symmetric transient compressible Reynolds averaged Navier-Stokes equation, using ANSYS FLUENT 2019. Ideal gas equation is taken as the state equation. All the governing equations along with the state equations are presented from Eq. (1) to Eq. (4). The flow turbulence is modelled using SST k-ω turbulence model, in
Results and discussion
This section consists of two parts. The first part discusses about the vacuum chamber starting transients during the primary nozzle stagnation pressure ramp-up process and the associated fluid dynamics. The second part discusses about the staring process with various parametric configurations like, convergent cone angle and the nozzle position.
Conclusions
Numerical studies on second throat vacuum ejector diffuser system for high altitude testing facilities revealed that the vacuum generation process consists of four stages. The first three stages consist of the well-established gradual evacuation process, a transition region and the rapid evacuation stage, respectively. It is found that the rapid evacuation is followed by the reappearance of a fourth gradual evacuation stage, which is not been reported in many past studies. During the first
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This research project is funded by the SEED grant from IIT Jodhpur, India (I/SEED/AKR/20190025).
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