Analysis on impulse characteristics of pulse detonation engine with nozzles

In order to study the influence of different nozzle configurations on the gas–liquid two-phase pulse detonation engine (PDE) propulsion performance, the measurement system based on the tunable diode laser absorption spectroscopy (TDLAS) technology is built to measure the velocity and the temperature, while the high frequency dynamic pressure sensor is used to measure the nozzle gas pressure. Based on the momentum principle, the contribution mechanism of unsteady gas jet on thrust is obtained indirectly by TDLAS data. The results show that the impulses of PDE with non-nozzle, convergent nozzle, divergent nozzle and convergent–divergent nozzle are 1.95, 2.08, 1.85 and 2.16 N∙s within 20 ms of the exhaust period, respectively. The analysis reveals that the impulses of PDE with convergent and convergent–divergent nozzles are larger than that with non-nozzle, while the impulse of PDE with divergent nozzle is smaller than that with non-nozzle. The research results in this paper can provide reference for the design of nozzles for PDE.

The influence of friction on gas parameters in the minimum section of nozzles

Processes of gas flow in nozzles, accompanied by the release of frictional heat, are presented in the form of polytropic processes. The polytropic process index n determines the degree of irreversibility of the gas flow process caused by the release of frictional heating. Relations are obtained to calculate the flow rate and thermodynamic properties of gas in the minimum section of the Laval nozzle and in the outlet section of the convergent nozzle at a pressure behind the nozzle less than the critical pressure. The gas calculated parameters (pressure, temperature, specific volume, velocity, cross-sectional area) in the minimum cross-section differ from the recommended values in the reference literature [1]. In particular, the gas pressure in the minimum cross section turns out to be higher than the critical pressure recommended in [1].

Improvement of the Combustion Completeness of Hydrogen Jet Flames within a Mesoscale Tube under Zero Gravity

Microjet hydrogen flames can be directly used as micro heat sources or can be applied in micro propulsion systems. In our previous study, under zero gravity and without an active air supply, the combustion completeness of hydrogen jet flames within a mesoscale tube with an inner diameter of 5 mm was very low. In this study, we were dedicated to improving the combustion efficiency by using a convergent nozzle (tilt angle was around 68°) instead of the previous straight one, and the exit diameter was 0.8 or 0.4 mm. The numerical results demonstrate that the maximum combustion efficiency in the case of d= 0.8 mm was only around 15%; however, the peak value for the case of d = 0.4 mm was around 36%. This happened because with d = 0.4 mm, the fuel jet velocity was around four times that of the d = 0.8 mm case. Hence, the negative pressure in the combustor of d = 0.4 mm decreased to a much lower level compared to that of d = 0.8 mm, which led to an enhancement of the air entrainment ratio. However, the highest combustion efficiency of d = 0.4 mm was still below 36%; therefore, a slightly larger tube or an even smaller nozzle exit diameter will be necessary for further improvements to the combustion efficiency.

Oxygen concentration distribution in a pulse detonation engine with nozzle–ejector combinational structures

Pulse detonation engines (PDEs) with three different types of nozzle–straight ejector combinational structures at three different ejector positions were simulated by the unsteady 2-D axisymmetric method to understand the influence of nozzle–ejector combinational structures on the performance of PDEs. Three types of nozzles included the straight nozzle, convergent nozzle, and convergent–divergent (CD) nozzle. Three ejector positions were considered according to the ratio of the distance between the nozzle outlet and the ejector inlet to the diameter of PDEs (Δx/d). Propane was used as the fuel and air as the oxidizer. The simulation results indicated that for the PDE with the straight nozzle, it took the shortest time for high-temperature burnt gas to exhaust from the detonation tube. For the PDE with the CD nozzle, the time at which the ejector was filled with external air was the fastest. Within the time range of t = 0–10 ms, the ejected air was less than the original air in the ejector among all the nine combinational structures. The maximum ejected air was obtained with the convergent nozzle, followed by the CD nozzle, and the minimum with the straight nozzle. For certain nozzles, the maximum air was ejected at the ejector position of Δx/d = +1, followed by the ejector position of Δx/d = 0, and the minimum at the ejector position of Δx/d = −1. For the convergent nozzle–ejector combinational structure, the air ejection speed was the fastest. Oxygen concentration distribution in the PDE with the CD nozzle was more uniform along the axial direction than the other nozzles.

Comparison Between the Effects of Zero and Non-Zero Flow Acceleration on the Entropy Wave Decay: An Experimental Study

Entropy wave, as the convecting hot spot, is one of the sources of combustion instabilities, which is less explored through the literature. Convecting in a highly turbulent flow of a combustor, entropy waves may experience some levels of dissipation and deformation. In spite of some earlier investigations in the zero acceleration flow, the extent of the wave decay has not been clear yet. Further, there exist no results upon the wave decay in non-zero accelerated flows. This is of crucial importance, as the wave passes through the end nozzle of the combustor or gas turbine stages. The current experiment, therefore, compares the wave decay in both flow of constant and variable bulk velocity, meaning, respectively, a uniform pipe and a convergent nozzle. The comparison will aid the theoretical models to reduce complexity by simplifying the relations of non-zero acceleration flow to those of no acceleration, as followed by the earlier effective-length method. Reynolds number and inlet turbulence intensity are considered as the governing hydrodynamic parameters for both investigated flows. The entropy wave is generated by an electrical heater module and detected using fast-response thermocouples. The results show that the entropy wave variation is point-wise and frequency-dependent. The accelerated flow of the nozzle is generally found to be more dissipative in comparison with the zero acceleration flow.

Elliptical Tabs as a Mixing Enrichment Tool for Jet Nozzles

Mixing efficiency helps to thrust gain and jet noise reduction. Aerospace and aviation research communities around the world are constantly demanding cleaner, quieter and more efficient commercial aircrafts. Though there are many sources of noise during flight, one such is deficient mixing of air at the rear of the nozzle, uneven expansion of nozzle and corresponding engine noise is quite dominant. The mixing proficiency is achieved by mixing of cold air at the surroundings with the hot air that exits through the nozzle. To reduce the engine noise, elliptical tabs are employed at the exit of convergent nozzle. These are passive noise controllers which create vortices to the existing flow and entrains cold mass from the surroundings in order to reduce noise level. This paper is concerned with the various shapes and sizes of tabs to test the mixing efficiency at different Mach numbers. Further, it is evaluated by both qualitative and quantitative analysis of shadowgraph system and the pressure identifying method using some of the propitious pressure scanner.

Rotational Effect on Flow Field and Thermal Characteristics of a Turboexpander for Helium Liquefaction System: A Numerical Perspective

Cryogenic turboexpander is an essential component to produce the refrigeration effect in various helium liquefaction systems. The convergent nozzle and small-scale radial inflow turbine (turboexpander) are the important components that are responsible for increasing the performance of the cycle. In this paper, an optimum preliminary design approach of the turbine and nozzle is explained using real gas properties. Initially, the Sobol method is used to determine the sensitivity indices and optimized range of ten important nondimensional and geometrical variables for better performance of the radial turbine. Three turboexpanders of a modified Collins cycle-based helium liquefaction system have been designed considering the optimized ranges. The proposed method improves the isentropic efficiency and power output of the turbine up to 3.86% and 5.14%, respectively, as compared to the initial design. Hereafter, a comparative three-dimensional numerical analysis is conducted to characterize the flow physics and thermal properties of three turboexpander systems (16 bar and 40 K, 6 bar and 20 K, and 16 bar and 10 K). The thermal and fluid flow properties such as temperature, Prandtl number, static enthalpy, entropy, velocity vectors, Reynolds number, and turbulence kinetic energy are determined at different spans and streamwise locations. Moreover, the present numerical results are also verified with the experimental and numerical results obtained from the existing literature. The study highlights the optimal range of design variables for helium turbine, the methodology for helium liquefaction system, and the numerical analysis to understand the flow physics and thermal properties of helium near its boiling point.

Impact of Bubble Size on Flow Response to Transient Pressure Drop Through Converging Nozzle

For homogenous two-phase bubbly flows, the theoretical speed of sound is dramatically reduced at moderate void fractions to speeds much lower than the speed of sound for either single phase. This theoretical speed of sound would suggest a propensity for bubbly flows to reach choked conditions when traveling through a convergent nozzle. However, for a bubbly flow to be considered homogenous requires assumptions that may not be realized in practical applications. In this experimental study, a bubbly flow was sent through a convergent nozzle before entering a large chamber. By setting steady flow conditions upstream and then reducing the chamber pressure via a vacuum pump, the transient response in terms of gas and liquid flow rates and upstream channel pressure was determined. The bubble size was carefully varied from ∼0.3–1 mm while holding gas and liquid flow rates constant in order to study how bubble size affects the transient flow characteristics. High-speed imaging was used for measuring the bubbles. Experiments were also conducted at two gas-liquid mass flow ratios. Results are presented to demonstrate the impact of bubble size and gas-liquid ratio on the transient response of upstream gas and liquid flow rates, upstream pressure and exit Mach number to the lowering of pressure downstream of the convergent nozzle. Results are presented both for flows that remained in the bubbly regime and for flows that transitioned to an annular flow regime during a trial.