Porous Injector Research Articles

Experimental investigation on seeping gas film effectiveness in supersonic flow downstream of a porous injector

Based on seepage flow through porous media, the seeping gas film cooling method is an effective means to protect large areas of optical windows and other hot components of hypersonic vehicles. Here, the pressure-sensitive paint (PSP) technique was applied and experiments were conducted on seeping gas film effectiveness downstream of a porous injector. A Mach 3 wind tunnel was used to explore the influence of different cooling gas blowing ratios, gas types, and incoming boundary-layer conditions on film effectiveness. Results show film effectiveness decreases monotonically along the downstream wall; however, with blowing ratio increases the film effectiveness increases linearly. Once the blowing ratio exceeds 0.4% film effectiveness ascends nonlinearly, indicating that the contribution to film effectiveness per unit mass flow-rate promotion gradually decreases. In contrast, the non-uniformity of the film coverage on the downstream wall in the spanwise direction becomes more significant at a higher blowing ratio. For the same blowing ratio, helium, with a low molecular weight has a higher film effectiveness compared to nitrogen and carbon dioxide. With blowing ratio less than 0.4 %, the film effectiveness downstream of the porous injector under laminar flow conditions is almost twice that of turbulent flow. However, with blowing ratio above 0.5 %, the growth rate of the film effectiveness decreases dramatically to 50 %. Preliminary analysis suggests that this is caused by the complete transition of the laminar boundary layer to turbulent flow after passing through the porous wall under high blowing ratios, where the mixing effect of turbulence is fully manifest.

Cooling effects of the derived coolant-film layer from partitioned porous injectors for transpiration cooling

This study aims to reveal the cooling benefits of the derived coolant film from partitioned porous injectors for transpiration cooling. The near-wall flow development and the cooling performance are investigated based on various layouts of porous injectors. The numerical investigation is conducted using an in-house recursive regularized thermal lattice Boltzmann method (RR-TLBM). The validated RR-TLBM solver is accelerated by three Tesla V100 graphic processing units (GPUs) for the simulation of incompressible and coupled-domain transpiration cooling at high Reynolds numbers, based on high-resolution grids with 2.65 × 108 nodes. The results indicate that the near-wall flow shows high momentum with the partitioned porous injectors. And the mean velocity increases by more than 23 % with the same coolant consumption per unit blowing area, which contributes to the coolant-film development and cooling uniformity downstream. Moreover, the partitioned porous layout shows the possibilities of coolant reduction as compared to the single continuous porous injector in Case 1. A reduction of 43 % in total coolant consumption with the partitioned porous layout only results in a decrease of 6 % in cooling performance downstream. Additionally, the partitioned coolant injection improves local cooling performance by roughly 22 %, with better cooling homogeneity at the same total coolant consumption. Turbulent fluctuations are weakened in the partitioned coolant injection zones, which contributes to an improvement in cooling evenness. The partitioned porous layout can be an appropriate choice for transpiration cooling systems.

Improving leading edge cooling through transpiration with partitioned porous injectors and a jet

This study proposes a novel transpiration cooling layout that improves the cooling effect in the stagnation region and downstream zones for a sharp leading edge. Large-eddy simulation is conducted using our in-house recursive-regularized thermal lattice Boltzmann method (RR-TLBM) and high-resolution grids with 2.33 × 108 nodes. The RR-TLBM method shows low numerical dissipation, which is crucial in studying the transpiration-cooled leading edge with small-scale flow structures. A proper size of transpiration area in a single porous injector can extend the effective cooling area, with about 62.2 % of the wall area cooled at the same coolant consumption. Moreover, the opposing jet can reduce the local heat flux in the stagnation region, especially for the sharp leading edges. The proposed combinational cooling method shows an improvement of about 20 % in the cooling performance of the stagnation zone, while there is a 25 % improvement downstream of the stagnation region when compared to that of the porous layout with partitioned injectors. Furthermore, turbulent mixing can impair the improvement in cooling performance downstream, and the extended effective cooling area is related to low turbulent fluctuation. The RR-TLBM solver shows great potential in capturing the 3D coherent structures in the transpiration-cooled leading edge, based on high-resolution grids and a desktop-level computer with three Tesla V100 graphics processing units (GPUs).

Investigation of full-scale porous injector plate transpiration cooling coupled with combustion in high-thrust H2/O2 rocket engines

The high chamber pressure and large heat flux impose greater demands on the structure cooling and thermal protection of the future high-thrust H2/O2 rocket engine. To investigate the applicability of transpiration cooling on the full-scale injector plate in the high-thrust rocket engine, a numerical model is presented, in which the porous region and the mainstream hot gas region are directly coupled. The local equilibrium model and eddy dissipation model are considered. The numerical model is validated by transpiration cooling experiments for a subscale rocket engine injector plate. Then, the numerical simulation of transpiration cooling for the high-thrust H2/O2 rocket engine injector plate shows that the transpiration cooling with 13% of the total hydrogen mass flow rate can reduce the gas side temperature of the injector plate by about 150 K and reduce the thermal penetration depth to 9.2% of the plate thickness. Moreover, As the coolant mass flow rate increase from 4% to 13%, the gas side temperature of the plate decreases from 735 K to 668 K and the cooling effectiveness rises. The lower the coolant inlet temperature, the lower the plate temperature, but the higher the cooling effectiveness. Besides, the plate temperature drops with the decreases of the thermal conductivity of the porous media.

Transpiration-Cooling Heat Transfer Experiments in Laminar and Turbulent Hypersonic Flows

The design of a transpiration-cooled system requires detailed local heat transfer information on and in the vicinity of the porous injector; however, limited spatially resolved experimental studies exist, particularly in hypersonic flows. In this work, experiments were conducted in the University of Oxford’s high-density tunnel at Mach 6.1 in both laminar and turbulent regimes. Spatially resolved two-dimensional surface heat transfer measurements were acquired by imaging directly on and downstream of two microporous transpiration-cooled injectors (METAPOR® CE170 and zirconia) using high-speed infrared thermography. Whereas injection in the laminar regime results in a steady, monotonic reduction in heat transfer from the start of the injector, a flatter profile is present for the turbulent cases where turbulent mixing inhibits surface heat transfer reduction. It was found that a modification to existing relations from film theory successfully correlates the streamwise heat transfer distribution on the injector for different blowing rates of nitrogen and helium. A key result is that helium performs much better than reported in previous experiments. Finally, the downstream thermal effectiveness is characterized for turbulent flows. A collapse of the thermal effectiveness is achieved and a modified analytical correlation proposed.

Propellant Atomization for Porous Injectors

Porous injectors represent an alternative injection concept to coaxial injectors for rocket engine applications using gas/liquid propellant combinations such as liquid oxygen (LOX)/hydrogen (). This paper summarizes the main design features of porous injectors and proposes a mechanism of atomization for porous injectors that is considerably different to the atomization mechanism for coaxial injectors. The results of hot-fire test campaigns are presented, in which several parameters relevant to the injection process were varied: the injection velocities and momentum fluxes, the combustion chamber Mach number at the beginning of the nozzle contraction, and the LOX injector diameter. All hot-fire tests were conducted at P8 test facility for high pressure combustion research at the DLR site of Lampoldshausen with 50-mm-diameter combustion chambers operated with at sub- and supercritical chamber pressures between 30 and 100 bar. The results presented here are supporting the proposed mechanism of atomization and allow derivation of some general design guidelines for porous injectors.

Evaluation of the grid convergence for a rocket combustion chamber with a porous injector

A study of the grid convergence was carried out in order to quantify the mesh impact on simulations of rocket combustion chambers performed using the averaged Navier–Stokes equations. The present work is a continuation of previous studies on simulations of rocket combustion chambers with the porous injector head API-68. Turbulence is modelled by the SST turbulence model; turbulent combustion is modelled using the extended eddy-dissipation model. The grid convergence study is carried out for two injector configurations (one injector in the middle of the injector head and one injector near the sidewall) on five meshes for each configuration. Results of the work show that the dependences of the flame length and wall heat flux on mesh spacing are described well by a parabola. Using the found dependence on mesh spacing, spatial discretization errors have been evaluated. Analysis of the results shows that a reasonable accuracy of modelling can be reached using the current computational fluid dynamic model at a mesh spacing of around 30 μm in the flame area.

Disintegration process and performance of a coaxial porous injector

In order to understand the breakup performance of coaxial porous injectors, the sprays of coaxial porous injectors with two different porous material cylinder lengths were compared with those of conventional shear coaxial injectors. To allow comparison, the wall injection lengths were designed to be equivalent to the value of the recess depth. Cold flow sprays were visualized using back-lit photography methods and analyzed quantitatively with a laser diffraction apparatus, in order to study the effects of the momentum flux ratio and Weber number on the breakup for each type of injector. In case of the shear coaxial injector, the large liquid core was observed in low air mass flow rate condition. However, the destabilization of the liquid jet from the coaxial porous injector is almost complete within the inner region, near the injector face plate. Additionally, better breakup performance in low gas flow rate condition was obtained when the porous cylinder length decreased, while the shear coaxial injectors showed better breakup efficiency when the recess length increased. In conclusion, the different breakup process caused by the radial momentum in the inner region of the porous injector disintegrated the liquid core.

동축형 다공성재 분사기의 분열 메커니즘

동축형 다공성재 분사기에서는 기체가 분사기 중심을 지나는 액체 제트 주위를 둘러싼 다공성재를 통해 액체 제트를 향해 분사된다. 분사 방법의 차이로 인한 전단동축 분사기와 동축형 다공성재 분사기의 분열 메커니즘 차이를 살펴보기 위해, 전단동축 분사기와 동축형 다공성재 분사기를 사용하여 수류시험과 2-D 축대칭 수치해석을 진행하였다. 같은 유량조건에서의 가시화 이미지와 분무 평균입경을 비교하였으며, 수치해석을 통해 분사기 내부에서의 속도 분포가 액체 제트에 어떤 영향을 끼치는지 고찰하였다. 결과적으로, 기체의 유량이 늘어날수록 분사기 내부 속도가 낮음에도 동축형 다공성재 분사기의 미립화 및 혼합 성능이 전단동축 분사기에 비해 유리함을 확인할 수 있었다. In a coaxial porous injector, a gas propellant is injected through the porous cylinder surface to the liquid jet which is encircled by a porous cylinder. In this study, to observe the differences in disintegration mechanisms between a shear coaxial injector and a coaxial porous injector, cold-flow tests and 2-D axisymmetric numerical analysis have been carried out. The shadowgraph images and Sauter mean diameters were compared in similar experimental conditions, and the effects of velocity distributions at the inner injector region on the disintegration of liquid jet were investigated through the numerical calculations. As a result, in high air mass flow rate condition, the disintegration performance of coaxial porous injector is better than shear coaxial injector, in spite of a lower velocity at the inner injector region.

Effects of Wall-Injection Length on Spray and Combustion in a Coaxial Porous Injector

The mixing performance of an injector determines the combustion performance of a bipropellant rocket engine. In the previous study, a novel coaxial porous injector concept was developed to improve the mixing performance of a shear coaxial injector. It demonstrated a higher characteristic velocity efficiency than that of a typical shear coaxial injector. To clarify the efficiency-improving mechanism of a coaxial porous injector, the combustion flowfields of both injectors were visualized using a high-speed shadowgraph technique. The effect of geometry factors on the spray core length and the combustion performance as well as the differences in the reactive spray behavior between the coaxial porous injector and the shear coaxial injector were observed. The coaxial porous injector with a different wall-injection length showed higher characteristic velocity efficiencies, shorter spray core lengths, and thicker phase-change patterns at the spray core boundary. Changes in wall-injection length had a nonmonotonic effect on both the spray core length and the characteristic velocity efficiency. As opposed to the expected results, the coaxial porous injector with the highest combustion efficiency showed the longest spray core length. The phase-change pattern width, caused by the evaporation of liquid fuel droplets, was proportional to the efficiency.

Measurements and modelling of wall heat fluxes in rocket combustion chamber with porous injector head

A rocket combustion chamber with a porous injector head, which is a new concept for injecting propellants, is tested. The hot-fire tests are carried out using cryogenic oxygen and hydrogen at the mass ratio of oxidizer to fuel of 6 and at pressure of 80 bar. Pressures, wall temperatures, and wall heat fluxes are measured along the axis of the chamber. The wall heat fluxes are determined by the calorimetric method. The wall heat flux has a maximum at a distance of 50–100 mm from the injector plate. To interpret the experimental results, numerical simulations are performed using the commercial CFD code ANSYS CFX. The turbulent flow is modelled by the Favre-averaged Navier–Stokes equations and the shear-stress transport model. The turbulent combustion is modelled using two different models: the extended eddy-dissipation model and the extended coherent flame model. Results obtained with the different models are compared both with experimental data and with each other. The numerical results agree with the experimental data. The extended eddy-dissipation model gives the very good agreement which is achieved by applying additional parameters (‘Flame Temperature’ and ‘Extinction Temperature’) dependent on the mixture fraction.

Combustion Characteristics of a Coaxial Porous Injector

A coaxial porous injector was proposed for gas–liquid combustion for improved mixing performance compared to a conventional shear coaxial injector. The coaxial porous injector discharged a gas jet in a direction normal to the center liquid core, and it was surmised that the momentum transfer from the radial gas jet to the axial liquid jet occurred more efficiently than with a traditional shear coaxial injector. To validate the effectiveness of this idea, the combustion performances of a coaxial porous injector and a shear coaxial injector were compared by conducting a hot-firing test using nitrous oxide-ethanol as propellant. The momentum flux ratio at the injector tip was selected as a spraying condition index, and the characteristic velocity efficiency was considered as a combustion performance parameter. The coaxial porous injector demonstrated higher combustion efficiency than the conventional shear coaxial injector, especially at lower-momentum flux ratio conditions. In addition, the comparison of spray images between the shear coaxial and coaxial porous injectors under ambient conditions supported the results of the hot-firing experiment.

Breakup structure of two-phase jets with various momentum flux from a porous injector

Spray structure and atomization characteristics were investigated through a comparison of a porous and a shear coaxial injector. The porous injector shows better atomization performance than the shear coaxial injector. To increase atomization performance and mixing efficiency of two-phase jets, a coaxial porous injector which can be applicable to liquid rocket combustors was designed and tested. The characteristics of atomization and spray from a porous and a shear coaxial injector were characterized by the momentum flux ratio. The breakup mechanism of the porous injector is governed by Taylor-Culick flow and axial shear forces. Momentum of injected gas flow through a porous material which is composed of sintered metal is radically transferred to the center of the liquid column, and then liquid column is effectively broken up. Although the shapes of spray from porous and shear coaxial jets were similar for various momentum ratio, spray structures such as spray angle and droplet sizes were different. As increasing the momentum flux ratio, SMD from the porous injector showed smaller value than the shear coaxial injector

동축형 다공성재 분사기의 에탄올/아산화질소 연소성능

The gas jet from a coaxial porous injector for two-phase flows is discharged radially from the porous surface, which encloses the center liquid jet. Several hot-firing test using ethanol/nitrous oxide propellants was conducted to analyze the effect of oxidizer/fuel ratio on the combustion performance, and the uncertainty analysis was performed for the results. The characteristic velocity was affected by oxidizer/fuel ratio similarly with the results of CEA calculation except that the maximum characteristic velocity was appeared in the stoichiometric ratio. The characteristic velocity efficiency was increased as the oxidizer/fuel ratio increases.

다공성재를 통과하는 압축성 유체의 압력강하에 관한 실험적 연구

This study proposes the characteristics of the pressure drop in a compressible fluid through porous media for application to a porous injector in a liquid rocket engine in order to improve the uniformity of the drop size distribution and the mixing performance of shear coaxial injectors. The fluid through the porous media is a Non-Darcy flow that shows a Nonlinear relation between the pressure drop and the velocity at high speed and high mass flow rate. The pressure drop of the Non-Darcy flow can be derived using the Forchheimer equation that includes the losses of viscous and inertia resistance. The permeability and Ergun coefficient represented as a function of the pressure drop and pore size can be applied to the porous injector, where the fluid through the porous media is compressible. A generalized correlation between the pressure drop in relation to the pore size was derived.

웨버수 및 운동량 플럭스비에 따른 동축형 다공성재 분사기의 거시적 분무특성

The gas jet from a coaxial porous injector for two-phase flows is discharged from the porous surface, which encloses the center liquid jet, and the gas and liquid jet interact with each other physically. The wall injected gas jet transfers the radial momentum effectively while the radial gas jet develops to axial jet, and the performance of atomizing and mixing can be improved. In this study, the Weber number and the ratio of momentum flux were controlled by changing the gas injection area and the mass flow rate of the gas jet, and a study on the spray characteristics at the cold-flow test using water and air simulant was performed. It is concluded that the radial momentum transfer concept of a coaxial porous injector gives a positive effect on the atomization and mixing of the two-phase spray.

Analytical study of stationary temperature field in transpiration cooled porous wall

A one-dimensional heat conduction problem in a porous wall with a cooling fluid was considered theoretically. The case related to the porous injector head of rocket engine, where the heat flux is directed toward a coolant flow, was studied. The equations for the temperatures of permeable media and coolant were solved analytically. Using reasonable assumptions the equation system was simplified and the order of equations was reduced. The margins, where the used assumptions are valid, were defined. The solution is expressed in hyperbolic functions and shows that the flow of coolant leads to the increase in the temperature gradient near the hot side of porous wall. The expressions for the temperatures of wall and coolant at hot-gas-side wall are given. The thickness of wall, at which the temperature of the hot wall is independent on the wall thickness, was found.

다공성재를 이용한 동축형 분사기의 미립화특성

To improve the mixing and atomizing performance at the center region of the conventional coaxial shear injector spray, the concept of a coaxial porous injector was invented. This novel injection concept for liquid rocket engines utilizes the Taylor-Culick flow in the cylindrical porous tube. The 2-dimensional injector, which can be converted in three injection configurations, was fabricated, and several cold flow tests using water-air simulant propellant was performed. The hydraulic characteristics and the effects of a gas flow condition on the spray pattern and the Sauter mean diameter (SMD) was analyzed for each configuration. The atomizing mechanism of coaxial porous injector was different with the coaxial shear injector, and it was explained by the momentum of the gas jet, which is injected normally against the center liquid column, and by the secondary disintegration at the wavy interface of liquid jet, which was generated at the recessed region. The SMD of 2D coaxial porous injector, which has higher gas momentum, was measured and it shows better atomizing performance at the center and outer side of spray than the 2D coaxial shear injector.

Spot turbulence, breakup, and coalescence of bubbles released from a porous plug injector into a gas-stirred ladle

Many metallurgical processes are connected with gas injection into liquid metals for refining purposes. For this reason, considerable effort has been made during the past 2 decades to investigate gas-injection operations in steelmaking ladles. Numerous physical and mathematical models are available in the literature as well as experiments (most of them performed in the air-water model). In theoretical works, usually, the bubble size is assumed constant, but this approximation is good just at low gas flow rates. When the gas flow rate increases, three different types of bubble dispersion patterns are observed in experiments. This situation cannot be predicted by means of the turbulence multiphase models normally implemented in commercial CFD codes. Their results predict a smooth (and wrong) bubble-size increase and not a sudden transition from a pattern to another, as in experiments. In this articles a new idea for approaching the bubble turbulence in the ladle, called “spot turbulence,” is presented and comparison with experimental data shown.

Uniform Thin Film Electrodeposition onto Large Circular Wafer Substrates

A wafer-scale electrochemical flow cell specially suited for uniform thin-film electrodeposition is presented. The cell consists of a porous disk injector set parallel and coaxial to a working disk electrode with a narrow separation gap between them. The porous injector was designed to supply a uniform axial flow of electrolyte onto the surface of the working disk electrode. The mass transfer characteristics of the cell and the current distribution on the working disk electrode were studied for various operating conditions, geometric parameters and bath chemistries. The experimental results confirmed the simple scaling properties that were theoretically predicted. It has also been shown that this cell can provide nearly uniform current distribution on the working electrode under conditions of both migration and convective-diffusive mass transfer control. The results indicate that this cell is suitable for uniform thin-film electrodeposition onto large circular wafer substrates.