Base Bleed Unit Research Articles

Design and Analysis of a Base Bleed Unit for the Drag Reduction of a High-Power Rocket Operating at Transonic Speeds

In the present study, a passive flow device is considered for drag reduction purposes through implementation in a transonic high-power rocket. The high-power rocket serves as a reference platform that, apart from the operating conditions, enforces several constraints in terms of available volume and placement locations. A step-by-step methodology is suggested, where the unit is initially broken down into an inlet and an outlet component. The flow field is investigated by means of computational modeling (CFD), where the Reynolds-averaged Navier–Stokes equations are solved coupled with turbulence models that vary depending on the design phase and the individual component. In the first design phase, the best alternative configuration is selected for each component by comparing mass flow rates and discharge coefficients. In the second design phase, each component is analyzed in greater detail based on the first phase results. Indicatively, the protruding inlet diffuser-type channel is converted into a protruding inlet nozzle-type channel to avoid choked flow phenomena, and a nozzle geometry is selected as the outlet amongst the other considered scenarios. The two components are eventually integrated into a common base bleed unit and a final assessment is made. The computational results are used to predict the performance and trajectory of the rocket through a well-established trajectory software. The overall methodology is validated against full-scale test flight data. The results show that the base bleed unit developed in the framework of this study yields a drag reduction of approximately 15% at transonic speeds without impacting the rocket mass and stability.

Trajectory Analysis of a Solid Fuel Ramjet Propelling an Artillery Shell

This paper intends to estimate the possible range of a 155 mm artillery shell propelled using a ramjet in a fuel-rich propellant configuration. It also intends to obtain an optimum design of ramjet to maximize the range. A six degree of freedom trajectory simulation was developed to obtain the trajectories of the shell with a ramjet, which dynamically evaluates the off-design performance of the ramjet during the flight. A fuel-rich propellant was taken from literature for evaluating the performance of the ramjet. Various trajectories of shell were obtained at different initial thrusts and A/F ratios and with a constant combustion efficiency of 0.95. Results showed that with a 39-calibre gun, the range enhancement of around 105% compared to the current range of 24 km was obtained with a 155 mm artillery shell without any base bleed unit. This was achieved for the ramjet providing thrust equal to 1.2 times the drag at sea level with an initial A/F ratio of 11.

Investigation of thermal performance in dual‐fuel units

AbstractNumerical studies were carried out to investigate the thermal performance of the muzzle flow with strong shock discontinuities and gas–gas heat transfer mechanism in the base bleed unit (BBU) of an aerodynamic vehicle under the combined effects of muzzle flow and depressurization process. The process was modeled using the Navier–Stokes equations with chemical kinetics and interior ballistic equations. The results show that the muzzle phenomena with large temperature gradients and different wave structures appear after the supersonic vehicle enters into the free‐flight stage. The BBU also experiences a strong depressurization process with the release of high‐temperature propellant gas at 2200 K. However, the igniter combustion gas at a higher temperature of 3050 K in the BBU develops downstream and upstream repeatedly under the combined effects of the base wave and depressurization process, which makes the energy conversion process in the unit more complicated. With the projectile leaving the control of the muzzle flow, the BBU is full of the igniter combustion gas and the energy in the unit is uniformly distributed gradually. The high‐temperature area, at approximately 2750 K, propagates downstream along the axial and radial directions simultaneously until it finally dominates the entire unit. The maximum temperature of the combustion gas acting on the propellant surface is always 2740 K and the maximum heat flux remains approximately 1250 W/cm2 at later periods.

Evolution characteristics of under-expanded accompanying jet under extreme heat condition

This work is focused to study heat and mass transfer characteristics as well as evolution mechism of the wake flow field of the aerodynamic vehicle (ADV) with base-bleed unit (BBU) under extreme heat condition. Here, we have used a new type of annular nozzle for BBU to improve the structure of the wake flow field and provide more mass and energy for the wake flow field. Based on H2-CO reaction mechanism for the post-combustion from the base-bleed gas and supersonic inflow, numerical models of unsteady accompanying jet coupled with the burning of the propellant are established for annular and circular nozzles. Comparisons of chamber pressure and jet shape of computational results are done with experimental studies, and a reasonable match has been obtained in these comparisons. Further, an under-expanded supersonic annular jet with containing central jet develops into a subsonic annular flow, presenting some distinguishing features compared with the decay process of an under-expanded circular jet. The base pressure coefficient for the annular nozzle increases by 21.8% and the peak of average base temperature is improved by 15.4%, which means this new type of annular nozzle is more beneficial to the increase of heat energy and pressure of the ADV base.

Unsteady flow of igniter combustion-gas heating base bleed propellant by convection

The performance and launching accuracy of the vehicle with the base-bleed unit (BBU) depend strongly on the complex flow field characteristics and reignition delay time of the base bleed propellant in the combustion chamber of the BBU because the rapid depressurization process will occur in the BBU with a significantly altered flow field structure and induced extinguishment of the base bleed propellant located on both sides of the BBU when the vehicle is out of the muzzle at an initial time. However, the base bleed propellant will be reignited under the continuous convective heating effect of the igniter combustion-gas jet in the BBU at a later time. In this study, numerical simulations are carried out to investigate the coupling flow field characteristics and heating mechanism of the igniter of an actual BBU under a rapid depressurization process. Indeed, it is found that when the base bleed vehicle is out of the muzzle, the BBU experiences a rapid depressurization process due to the pressure difference between the combustion chamber of the BBU and the atmosphere, resulting in a strong unsteady flow field structure under the Kelvin–Helmholtz instability effect from the velocity difference between the igniter combustion-gas and gun propellant combustion-gas. Meanwhile, the high-temperature combustion-gas from the igniter is pressed on the end face of the igniter, and then, the axial expansion becomes quicker than the radial expansion with the development of the depressurization process. Afterward, the continuous convective heating effect of the igniter combustion-gas jet on the base bleed propellant occurs due to the direct contact of the high-temperature combustion-gas jet from the igniter with the surface of the base bleed propellant.

Effects of Mg/PTFE pyrotechnic compositions on reignition characteristics of base bleed propellants and heating mechanism

The effects of magnesium/polytetrafluoroethylene (Mg/PTFE) pyrotechnic compositions on the coupled flow field and reignition mechanism are important aspects governing the performance and range of base bleed projectiles (BBPs). Owing to a decrease in pressure and temperature when the BBP leaves the muzzle, rapid depressurization occurs, which extinguishes the base bleed propellant. The Mg/PTFE pyrotechnic composition pressed in the igniter of the base bleed unit (BBU) provides additional energy to the BBU via a chemical reaction. Thus, the extinguished base bleed propellant is reignited under the effect of high-temperature combustion gas jets from the igniter. In this study, a numerical analysis is conducted to evaluate the effects of PTFE and Mg granularity as well as Mg/PTFE pyrotechnic compositions. Owing to the rapid depressurization, the temperature and pressure was found to decrease for different Mg/PTFE pyrotechnic compositions. However, the depressurization time increased as the PTFE granularity increased, the Mg granularity decreased, and the Mg content increased. When the pressure in the combustion chamber of the BBU decreased to the atmospheric pressure, the combustion gas jets from the igniter expand upstream (rather than downstream). However, these combustion gas jets exhibit different axial and radial expansion characteristics depending on the pyrotechnic compositions used. The results show that the reignition delay time, td, of the base bleed propellant was 377.608, 94.27, 387.243, 523.966, and 221.094 ms for cases A–E, respectively. Therefore, it was concluded that the Mg/PTFE pyrotechnic composition of case B was the most beneficial for the reignition of the base bleed propellant, with the earliest addition of energy and mass to the BBP.

Solid Fuel rich Propellant Development for use in a Ramjet to Propel an Artillery Shell

This study describes the development of a fuel-rich propellant to be used in a solid fuel ramjet to provide active propulsion to a 155 mm artillery shell. Fuel-rich propellants consisting of aluminum, ammonium perchlorate and hydroxyl terminated polybutadiene were developed and their ballistic properties were measured to choose the appropriate fuel for the ramjet application. The attempts made were to enhance the burn rates of the propellant to provide required burn rates at lowest possible pressures in primary combustor of the ramjet. The propellant selection was done with reference of working time period of the base bleed unit, to calculate the required burn rate and corresponding pressure in primary combustor. It was observed that the fuel rich propellant of composition 35% ammonium perchlorate with 1 % Iron oxide embedded on it, 30 % mechanically activated aluminum with 10% polytetrafluoroethylene, and 25 % HTPB was found suitable for the above application. This provided the higher burn rates among all developed propellants with high pressure index of 0.58. This makes it suitable for the ramjet requiring higher burn rates at lower possible primary chamber pressures. The Young’s modulus and tensile strength of this propellant was measured to be 1.73 MPa and 0.24 MPa, respectively.

Optimization of artillery projectiles base drag reduction using hot base flow

The CFD numerical simulations were carried out to investigate the base drag characteristics of a projectile with base bleed unit with a central jet. Different base bleed grain types with different combustion temperatures were used. The goal was to find a way to effectively control the base flow for base drag reduction and optimisate the latter using an adequate CFD software. Axisymmetric, compressible, mass-averaged Navier-Stokes equations are solved using the k-? SST, transition k-kl-?, and RSM turbulence models. The various base flow characteristics are obtained by the change in the non-dimensionalized injection impulse. The results obtained through the present study show that there is an optimum bleed condition for all base bleed grains tested. That optimum is dependent on the temperature of the grain combustion products. The optimum reduces the total drag for 6,9% in the case of air injection at temperature of 300 K and reaches up to 28% in the case of propellant combustion products injection at almost 2500 K. Besides, the increasing of molecular weight has a role no less important than temperature of the combustion products in terms of base drag reduction.

Coupling characteristics of combustion-gas flows generated by two energetic materials in base bleed unit under rapid depressurization

The present study is devoted to investigate the physical characteristics of the coupling combusting-gas flows generated by gun propellant and pyrotechnics along with the effect of initial pressure on the extension behavior of the igniter combustion jet under the rapid depressurization process that the BBU (base bleed unit) undergoes when a BBP (base bleed projectile) is flying out of the muzzle. A semi-closed bomb is designed to reproduce the depressurization process. The near-nozzle pattern evolution of the exhaust plume is observed by using an HSC (high-speed camera). Based on the experiment, a two-dimensional axisymmetric model is established including the high-resolution upwind scheme AUSM+ and the finite-rate chemistry model (no turbulence-chemistry interaction). The cell-centered FVM (finite volume method) is employed to simulate the coupling characteristics of the two high-temperature combustion gases under sharp depressurization. The results show that as the pressure drops rapidly, the exhaust plume gradually transforms from highly underexpanded supersonic flow into a subsonic flow, in where the wave structure converts from Mach reflection to regular reflection. The periodic shock diamond and rhombic flame string are formed, and finally the flame becomes continuous. In the subsonic flow, the radial heat convection and diffusion downstream of the flame in BBU are stronger than upstream. During the depressurization process, the influence of initial NPR (nozzle pressure ratio) becomes more significant. As the initial NPR increases, the igniter flame squashes downstream more slowly, and the heat loss of the gas expansion accelerates, thus the radial heat convection and diffusion in BBU decrease.

Effects of Charge Size on Slow Cook‐Off Characteristics of AP/HTPB Composite Propellant in Base Bleed Unit

AbstractTo investigate the effects of charge size on the slow cook‐off characteristics of trilobite ammonium perchlorate (AP)/hydroxyl‐terminated polybutadiene (HTPB) composite propellant with slits, a cook‐off model of three‐dimensional base bleed unit is developed based on the two‐step decomposition reaction mechanism of AP/HTPB propellant at the external heating rates from 3.3 to 9 K/h. The charge length of AP/HTPB propellant is 72 mm with 43 mm to 53 mm in grain diameter, and the grain diameter is 43 mm with 72 mm to 90 mm in charge length, respectively. The results show that heating rate and charge size determine the ignition position of AP/HTPB propellant. The effects of charge size on the ignition position and cook‐off temperature are small at the heating rate of 3.3 K/h. As the heating rate increases to 4.6 K/h–6 K/h, with the grain diameter and charge length extending, the ignition position gradually moves from the inner wall to the surface near the slit, meanwhile the cook‐off temperature changes obviously. When the heating rate increases to 9 K/h, the ignition position of AP/HTPB propellant at different charge sizes all occurs on the outer interface between the propellant and slit, there is no obvious influence of different charge sizes on the ignition temperature of AP/HTPB propellant. Additionally, the law of ignition time of AP/HTPB propellant to charge size is the same at different heating rates.

항력감소분석을 위한 항력산출에 대한 연구

본 논문은 비행시험의 결과를 이용하여 항력 및 항력감소 분석을 위한 항력계수 산출기법에 대하여 기술하였다. 2차원 탄도운동방정식의 역산을 통하여 비행시험을 이용한 항력계수 산출방법에 대하여 정리하였으며 155 mm 탄의 비행시험을 통하여 이론적인 항력계수와 시험결과로부터 산출된 항력계수를 비교하여 항력계수 산출 기법에 대한 검증을 수행하였다. 항력감소제가 적용된 탄의 비행시험결과를 이용하여 항력계수 산출 및 항력감소 양상을 분석하였다. In this paper, determination method for drag force and drag coefficient from results of firing test is described. The drag force and drag coefficient are determined through inverse operation of 2-dimensional projectile equation of motion. Determination method was verified by comparing analytical drag coefficient with data from flight test. Analysis of drag coefficient and drag reduction was performed with the data of flight test using artillery projectiles with base bleed unit.

Three-Dimensional Simulation of Base Bleed Unit with AP/HTPB Propellant in Fast Cook-off Conditions

ABSTRACTIn this work, a three-dimensional unsteady heat transfer model of base bleed unit with trilobite ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene (HTPB) composite solid propellant is presented to analyze the cook-off characteristics. According to the two-step chemical reaction of AP/HTPB propellant, a small-scale cook-off test is established. A comparison of the experimental and calculated results is made to verify the rationality of the computation model. On this basis, a cook-off numerical simulation of the base bleed unit at the heating rates of 0.33, 0.58 and 0.83 K/s is presented to investigate the ignition and initiation characteristics. The results show that the ignitions occur on the head face of the AP/HTPB propellant and near the internal gas chamber in these conditions. As the heating rate increases, the runaway time decreases and the ignition temperature rises.

Numerical Study on Wake Flow Field Characteristic of the Base-Bleed Unit under Fast Depressurization Process

ABSTRACTNumerical analyses have been performed to study the influence of fast depressurization on the wake flow field of the base-bleed unit (BBU) with a secondary combustion when the base-bleed projectile is propelled out of the muzzle. Two-dimensional axisymmetric Navier-Stokes equations for a multi-component chemically reactive system is solved by Fortran program to calculate the couplings of the internal flow field and wake flow field with consideration of the combustion of the base-bleed propellant and secondary combustion effect. Based on the comparison with the experiments, the unsteady variation mechanism and secondary combustion characteristic of wake flow field under fast depressurization process is obtained numerically. The results show that in the fast depressurization process, the variation extent of the base pressure of the BBU is larger in first 0.9 ms and then decreases gradually and after 1.5 ms, it remains basically stable. The pressure and temperature of the base-bleed combustion chamber experience the decrease and pickup process. Moreover, after the pressure and temperature decrease to the lowest point, the phenomenon that the external gases are flowing back into the base-bleed combustion chamber appears. Also, with the decrease of the initial pressure, the unsteady process becomes shorter and the temperature gradient in the base-bleed combustion chamber declines under the fast depressurization process, which benefits the combustion of the base-bleed propellant.

An improvement of the base bleed unit on base drag reduction and heat energy addition as well as mass addition

Numerical simulations are carried out to investigate the base drag and energy characteristics of a base-bleed projectile with and without containing the effect of a post-combustion process for a boattailed afterbody in a supersonic flow, and then to analyze the key factor of drag reduction and pressure decreasing of base bleed projectile. Detailed chemistry models for H2CO combustion have been incorporated into a Navier-Stokes computer code and applied to flow field simulation in the base region of a base-bleed projectile. Detailed numerical results for the flow patterns and heat energy addition as well as mass addition for different conditions are presented and compared with existing experimental data. The results shows that, the post-combustion contributes to energy addition and base drag reduction up to 78% on account of that the heat energy released from the post-combustion using fuel-rich reaction products as the fuel in the wake region is much higher than for the corresponding cold bleed and hot bleed cases. In addition, the temperature distribution regularities are changed under post-combustion effect, presenting that the peak appears in a couple of recirculation regions. The fuel-rich bleed gas flows into the shear layer along the crack between these two recirculation regions and then those can readily burn when mixing with the freestream, thus causing component changes of H2 and CO in the base flowfield.

Numerical simulation of the muzzle flows with base bleed projectile based on dynamic overlapped grids

Numerical investigations of the launch process of a base bleed projectile from the muzzle to free-flight stage have been performed in this paper. A dynamic overlapped grids approach was applied to deal with the problems of a high-speed moving base bleed projectile. A high-resolution upwind scheme (AUSMPW+) and detailed reaction kinetics model were employed to solve the chemical non-equilibrium Euler equations for dynamic grids. A typical case was calculated for the verification of the dynamic overlapped grids approach and numerical methods solving the chemical non-equilibrium flows. After good agreement was achieved, the development process of muzzle flows with high-speed moving base bleed projectile and the rapid depressurization process in the combustion chamber of base bleed unit are discussed in detail. This present numerical study confirms that complicated transient phenomena exist in the early launch stages after the base bleed projectile moves from the muzzle of cannon to the free-flight stage. The base bleed unit of projectile undergoes a strong non-steady-state rapid depressurization. This paper is a significant investigation for understanding muzzle flows with base bleed projectile and rapid depressurization in the combustion chamber of base bleed unit, which can provide valuable reference data for research on combustion stability of the propellant in the combustion chamber.

EFFECT OF BASE BLEED DIMENSIONS ON THE BALLISTIC PERFORMANCE OF ARTILLERY PROJECTILES

ABSTRACTBase bleed unit is one of the active methods to increase the range of artillery projectiles. Ballistic performance of base bleed unit has been experimentally assessed using firing tests and wind tunnel experiments. Meanwhile, analytical and numerical studies have been carried out. In some of these studies, solid propellant is used as the source of the burnt gases ejected into the wake behind the projectile base. But, in other studies, different types of gases such as air, argon, hydrogen, and helium are ejected at different temperatures.In this paper, the effects of the main dimensions of 2-parts tubular base bleed grain unit on its ballistic performance are studied analytically. These dimensions are base bleed grain maximum radius, length, inner diameter of the grain, and exit diameter of base bleed unit. The study is applied to the base bleed unit which is installed to K307 155mm projectile. The study leads to a new method to control the ejected mass flow rate. This method is based on changing the exit diameter of base bleed unit in order to get higher injection parameter in the first few seconds of projectile flight and lower values in the remaining time of base bleed grain burning. Therefore, the base bleed projectile range is increased by 1.7 % when comparing with its counterpart which is supplied with base bleed unit having constant exit diameter and the same base bleed grain.

Study on Measurement and Simplified Model of Burning Rate of AP/HTPB Base Bleed Propellant under High Pressure

In order to understand the burning rate of AP/HTPB propellant of the base bleed unit, the p-t curves of combustion of AP/HTPB base bleed propellant are measured in the closed bomb. The burning rate formula of the propellant under 50-200MPa is obtained by fitting to the experimental results. Based on division zone combustion theory, a simplified model of the burning rate of AP/HTPB base bleed propellant under high pressure is established. The calculation results and the experimental results are in good agreement.

Ballistic Analysis of a Projectile Provided with Base Bleed Unit

An analysis has been performed to investigate the ballistic performance of the 155mm K307 extended range projectile provided with base bleed unit. Different tools were used to perform this analysis; including analytical modeling and available computational resources. Drag reduction modeling has been used to study the effect of different parameters on base bleed performance such as burning rate, number of base bleed grain parts and the grain dimensions. The study has showed that a better ballistic performance could be obtained when the exit mass flow rate of the burnt gases is relatively large at the first few seconds of the projectile flight. To change this mass flow rate along the trajectory, a newly developed technique has been introduced. The technique depends on filling the base bleed unit with a grain subjected to burning by two rates resulting in extending the projectile range.

Effect of Exhaust Nozzle Diameter on Internal Flow Field Characteristics in Base Bleed Unit Simulator

For the optimal design of base bleed unit, a 2D-axisymmetric unsteady model was established and the effects of different exhaust nozzle diameters on the internal flow field were studied. Results show that the larger the exhaust nozzle is, the higher the depressurization rate and the mass flux are at the earlier stage of depressurization, and the lower the pressure, the temperature, the density in the base bleed unit and the flow rate at the nozzle outlet are. But at the later stage, the depressurization rate and the mass flux are smaller instead. The radial temperature is generally unchanged in most part far from the burning surface and increases near the burning surface at the earlier stage of depressurization, whereas it is lowest at the axis and rises up quickly along the radial direction at the later stage. The radial density distributes just oppositely.