Backflow Vortex Research Articles

Experimental and numerical study of the cooling characteristics of the double-walled cooling structure on integral support plate

The diffuser, rectifier, fuel injection rod, and flame holder in the afterburner are designed integrally, and a double-layer cooling structure is installed for the integral flame holder. Infrared thermography is employed to carry out the cooling efficiency test experiments under non-afterburner conditions, and the numerical simulation method is used to obtain the flow characteristics inside the integral afterburner. The effects of subtle geometrical parameter changes on the near-wall flow and wall-cooling effect are investigated. The results show that: The cooling air covers the film panel to form a film under the traction of the backflow vortex and spiral vortex behind the support plate, which provides effective cooling for the integral flame holder; the increase of the mass flow ratio will improve the cooling effectiveness of the double-wall, but it will cause additional pressure loss; impact spacing H has a small effect on the cooling effectiveness and relative pressure loss; reduced film aperture diameter df, increased aperture spacing S and aperture row angle β lead to a drop in the cooling capacity of the double-wall, whereas a reduced impact aperture diameter di leads to a certain degree of improvement in the cooling effectiveness of the double-wall.

Numerical study on unsteady characteristics of backflow vortex cavitation in an axial flow pump

Abstract Axial pumps play a crucial role in various industries such as agriculture, construction, and chemical engineering, exhibiting optimal efficiency under design conditions. However, in practical applications, these pumps often operate under off-design conditions due to environmental constraints, which would lead to a series of problems such as increased vibration, elevated noise levels, and a shortened pump lifespan, thereby endangering its safe and stable operation. This study addresses the issue by using a modified RNG k-ε turbulence model combined with the Zwart cavitation model to simulate the cavitation characteristics of an axial-flow pump. The calculated cavitation characteristics are consistent with the experimental results, proving the exactitude of the numerical approach. A noteworthy observation is the severe cavitation occurrences at the backflow vortex, termed backflow vortex cavitation. This phenomenon undergoes inception, development, and eventual collapse constantly. According to the severity of the backflow vortex cavitation, different periods are divided into the bimodal stage and small-peak stage. Significantly, it is identified that the backflow vortex cavitation interacts with the impeller blades, as the impeller moves, thereby inducing blade vibration, and noise, and exacerbating cavitation erosion. The research results offer a theoretical foundation that is beneficial to enhancing the safe and stable operation of axial flow pumps.

On the impact of operating condition and testing environment on the noise sources in an industrial engine cooling fan

Engine cooling fan noise is a relevant issue for manufacturers. It is well known that both the operating point and testing environment can affect the noise generation mechanisms and, consequently, the measured noise may change. Therefore, these aspects are investigated on a reference industrial fan, for which experimental data exists, using high-fidelity numerical simulations based on the lattice-Boltzmann method. Two operating conditions, namely the free blowing and the maximum efficiency ones, and three testing environments are analyzed: (i) a conventional semi-anechoic room, (ii) an ideal free field environment, and (iii) a testing environment resembling an anechoic aeraulic facility. For cases (i) and (ii) no pressure difference across the fan is imposed, while, for case (iii), a pressure difference across the fan can be imposed. For the latter, the impact of a fully reflective and fully absorbing wall separating the two regions upstream and downstream of the fan is analyzed. At free blowing conditions, the flow over the blades is largely separated. When the blade passes through a blockage region, because of the presence of a honeycomb-like structure needed for structural purposes, it experiences a prominent loading hump. The far-field noise, at a listener located along the axis of rotation, is therefore highly tonal, with a clear peak at the blade passing frequency tone. When the same fan is tested in a free field environment, it is found that there is a difference in the acoustic pressure at higher harmonics of the blade passing frequency due to the presence of flow recirculations in the anechoic room. Placing a thin wall across the fan increases the mass flow rate, for a given rotational speed, which results in a more severe flow separation over the blades and, therefore a higher tone prominence at the blade passing frequency. If the thin wall is modeled as a sound-absorbing wall, there is a drop of the overall sound pressure level of about 2 dBA. When the fan is tested at its maximum efficiency, i.e., nonzero pressure difference across the fan, it is found that the blockage effect is less relevant. The main noise generation mechanism is the back-flow vortex induced by the pressure difference across the fan interacting with the blade tip leading edge.

Numerical Study of Low-Specific-Speed Centrifugal Pump Based on Principal Component Analysis

The characteristics of pressure pulsations in centrifugal pumps have attracted considerable attention. In this study, principal component analysis is used to discuss the pressure pulsations in a centrifugal pump with a low specific speed, and the primary causes for these pressure pulsations are analyzed in conjunction with experimental results. The results indicate that principal component analysis effectively separates the primary modes that influence the flow field characteristics. An excessive wrap angle results in the formation of a backflow vortex on the working face of the blade. Obvious stratification of the zero-order modal pressure indicates that the geometric structure of the impeller is rational and that the transient flow field is stable. The second- and third-order modes are conjugates, and their dominant frequency coincides with the dominant rotating frequency of the impeller, indicating that the pulsations of a single channel are the primary component of the pressure pulsations. The primary frequency (148.54 Hz) of the pressure pulsations at monitoring points distributed across the volute is three times the rotational frequency (49.51 Hz) of the impeller. The different positions and sub-frequencies of the monitoring points mean that the principal component analysis can effectively identify the impeller-induced sub-frequency difference.

Study on mechanism of unsteady gas-liquid two-phase flow in liquid-ring vacuum pump

To investigate the gas-liquid two-phase flow mechanism in liquid-ring vacuum pump, the large-eddy simulation coupled with volume-of-fluid method (LES-VOF) was used to numerically simulate the unsteady gas-liquid flow characteristics in pump. The structure of gas-liquid flow within pump and its dynamic evolution characteristics were studied, and the hydraulic excitation characteristics induced by gas-liquid flow were analyzed. The result shows that the channel vortex inside the impeller on the pump suction side gradually flows out of the flow passage with the rotation of the impeller, and continuously mixes with the wake vortex at the outlet of the impeller and the separation vortex at the inner wall of the casing. The evolution and development of vortex structures with different intensities and scales near the inner wall of the suction side casing have a relatively large contribution to pressure fluctuation. The wake vortex at the impeller outlet of the exhaust side gradually enters the flow passage and dissipates gradually with time. There is an obvious high-intensity backflow vortex in the exhaust section due to the pressure difference between the flow channels near the leading edge of the exhaust port, which leads to obvious rotor-stator interaction in the exhaust section.

New type of cavitation instability with peculiar frequency characteristic in liquid rocket inducer

In a liquid rocket turbopump with low pressure and thin wall tanks, cavitation inevitably occurs at the inducer which is installed at the inlet of the turbopump. The cavities at each blade oscillate periodically, and the turbopump sometimes becomes unstable, called cavitation instability. Rotating cavitation is one of the common types of cavitation instability, in which the cavities at each inducer blade oscillate respectively and appear to propagate from blade to blade. However, the new type of cavitation instability which does not follow the conventional principle has been observed in our experiment with an inducer. Converting the frequency in the inertial frame to that in the rotational frame and organizing it in the cavitation number, it was found that the frequency of unsteady cavitation increases as the cavitation number decreases, and this is the peculiar point of this instability. Additionally, during the cavitation instability, a few numbers of backflow vortex cavities were observed and moved sub-synchronously from blade to blade. In addition, the tip leakage vortex cavitation on each blade also propagates like sub-synchronous rotating cavitation but in different propagation speed from that of the backflow vortex cavities. As a result, it was supposed that the new instability is one of the types of sub-synchronous rotating cavitation related to backflow vortex cavitation, tip leakage vortex cavitation, and interaction between cavity propagation and inducer blade.

Comparative study on stator corner separation vortex characteristics in axial-flow pump and tubular pump: To understand the effects of guide-vane cone diffusion on horn-like vortices

The guide-vane cone diffusion is a typical structural difference between an axial-flow pump and a tubular pump, which is shown as low diffusivity and high diffusivity respectively, but it is still unclear how this structural difference affects the vortex characteristics in stator corner separation flows. In this paper, a comparative study of stator corner separation flows in an axial-flow pump and a tubular pump was conducted, and the effects of guide-vane cone diffusion on vortical structures were clarified. Firstly, for the apparent vortical features, compared with the guide-vane of axial-flow pump with low diffusivity, the horn-like vortex in the guide-vane of tubular pump with high diffusivity has the features of smaller scale, weaker swirling strength, shorter evolution cycle and lower pressure fluctuations. Secondly, for the vortex dynamics mechanism, the guide-vane cone diffusion of tubular pump can cause additional pressure energy recovery, which leads to higher adverse pressure gradients, so it is easier to induce additional shroud backflow near the suction surface. In this coupled flow field of main flow, namely hub corner separation flow and shroud backflow, the streamwise periodic oscillation of the pressure function gradient ωS ·▽(▽p/ρ) is found, and it induces the unique vortex-street-like distributions of the deformational vorticity ωS and the rigid vorticity ωR . This physical effect causes a mutual competition between the horn-like vortex near the hub corner and the opposite backflow vortex near the shroud side. It is this competition effect originating from the guide-vane cone diffusion that greatly suppresses the development of the horn-like vortex in a tubular pump.

Numerical investigation on vortex dynamics characteristics of a large vertical centrifugal pump operating in the hump region under cavitation conditions

The large vertical centrifugal pump (LVCP) is widely used in major hydraulic engineering such as cross-region water transfer. This paper focuses on the hydraulic stability of LVCP under extremely operating conditions. The transient numerical simulations of cavitation in LVCP in the hump region are conducted based on delayed detached eddy simulation coupled with a homogeneous cavitation model. The main research is the effect of cavitation on the flow pattern and the vortex dynamics of the LVCP under stall conditions. The results of the numerical investigations show that the variation of the pump performance parameters is consistent with the experimental data under cavitation conditions. Under deep stall conditions, the H drop due to cavitation in the pump can happen earlier. Under the critical cavitation of the stall condition, backflow vortex cavitation, sheet cavitation and separation vortex cavitation can be observed at the impeller inlet. The vortex structures generated at the cavitation tail can result in worse flow patterns in the impeller. The vortex dynamics analysis shows that severe cavitation in the impeller deteriorates the inlet conditions of the vaned diffuser and induces sheet cavitation at the LE of vanes. The cavitations in the vaned diffuser accelerate the production and development of stall vortices and result in increased flow instability. The research in this paper can be helpful to improve the operation stability of LVCP.

Comparison of the Shutdown Transitions of the Full-Flow Pump and Axial-Flow Pump

In this study, a comparative analysis of the shutdown transitions of a full-flow pump and an axial-flow pump was carried out through numerical simulation and model tests. The UDF method was used to achieve control of the impeller rotational speed during shutdown. The results show that during the shutdown transition, the rate of decline of rotational speed, flow rate, and torque of the axial-flow pump are greater than those of the full-flow pump, so the axial-flow pump stops faster than the full-flow pump. The axial force of the axial-flow pump is significantly lower than that of the full-flow pump, and the maximum value of the radial force of the axial-flow pump is approximately 1.14 times that of the full-flow pump. Due to the influence of the clearance backflow vortex, the impeller inlet and outlet of the full-flow pump generate clearance backflow vortices in the near-wall area, resulting in the overall flow pattern of the impeller chamber being worse than that of the axial-flow pump and the hydraulic loss being greater than that of the axial-flow pump. The runaway speed and flow rate of the axial-flow pump are higher than those of the full-flow pump. Due to the influence of the clearance backflow, the range of the high entropy production rate at the suction side of the impeller of the full-flow pump is always larger than that of the axial-flow pump. The research results in this paper can provide theoretical support for the selection and operation of pumps in large low-head pumping stations.

Maximum Likelihood Identification of Backflow Vortex Instability in Rocket Engine Inducers

Abstract Bayesian estimation is applied to the analysis of backflow vortex instabilities in typical three- and four-bladed liquid propellant rocket engine inducers. The flow in the impeller eye is modeled as a set of equally intense and evenly spaced 2D axial vortices, located at the same radial distance from the axis and rotating at a fraction of the impeller speed. The circle theorem is used to predict the flow pressure in terms of the vortex number, intensity, rotational speed, and radial position. The theoretical spectra so obtained are frequency broadened to mimic the dispersion of the experimental results and parametrically fitted to the measured data by maximum likelihood estimation with equal and independent Gaussian errors. The method is applied to three inducers, tested in water at room temperature and different operating conditions. It successfully characterizes backflow instabilities using the signals of a single pressure transducer flush-mounted in the impeller eye, effectively bypassing the aliasing limitations and the data acquisition/reduction complexities of traditional multiple-sensor cross-correlation methods. The identification returns the estimates of the model parameters and their standard deviations, providing the information necessary for assessing the accuracy and statistical significance of the results. The flowrate is found to be the major factor affecting the backflow vortex instability, which, on the other hand, is rather insensitive to the occurrence of cavitation. The results are consistent with the data reported in the literature, as well as with those generated by the auxiliary models specifically developed for initializing the maximum likelihood searches and supporting the identification procedure.

The unsteadiness of tip leakage vortex breakdown and its role in rotating instability

The unsteadiness due to tip leakage vortex (TLV) breakdown was studied using a special experimental test campaign in parallel with numerical simulations. The back flow vortex (BFV), an isolated vortex caused by TLV spiral-type breakdown, was found to play a key role in rotating instability (RI). High-speed pressure transducers were used to measure the unsteady pressure field at the casing end wall of the blade in an isolated subsonic compressor rotor, which identified a low-frequency fluctuation at the near stall condition. A single-passage unsteady Reynolds-averaged Navier–Stokes simulation was used to study the evolution of unsteady flow structures, validated by the experimental measurements. Two distinct kinds of periodically unsteady flow were revealed by the simulations. A high-frequency fluctuation corresponding to 1.0 blade pass frequency (BPF) was caused by the spiral-type breakdown of the TLV. The other low-frequency fluctuation corresponding to 0.5BPF was caused by the feedback interaction between the BFV and the blade loading. The BFV was generated by the TLV breakdown, which was separated from the twisted vortex core of the TLV, and it moved downstream along the pressure side of the adjacent blade. A larger sized BFV reduced the local loading of the adjacent blade. The TLV was weakened as a consequence of the reduced loading, resulting in a smaller sized BFV. The blade tip loading was relatively less affected by the small sized BFV rather than the larger sized BFV. Therefore, the blade loading recovered and the size of the BFV increased, repeating the cycle. This feedback mechanism produced a pressure fluctuation with a frequency equal to 0.5BPF, which was closely related to RI.

Numerical Investigation of Rotating Instability Development in a Wide Tip Gap Centrifugal Compressor

In the current study, full-stage unsteady simulations were performed to investigate rotating instability inception mechanisms in a particularly large tip clearance centrifugal compressor with a vaneless diffuser and a volute. Four operating points along a speed line were analysed to understand the influence of the mass flow reduction on flow structures. Close to the peak efficiency, an unsteady interaction between the tip clearance vortices and splitter blades was observed. Considering other studies, the influence of the tip gap size was analysed. Then, a large-scale vortex shedding from the leading edges of the main blades was detected when the stage operated near the maximum pressure ratio. It was demonstrated that shed vortices were caused by the combination of the radial gradient of the tangential velocity under the tip vortex and the reverse backflow near the casing. Previous studies on axial compressors refer to these vortical structures as backflow vortices. These vortices cause a significant increase in the incidence angle in the tip region.

Transient numerical study of CO2 two-phase flow in a needle adjustable ejector

Steady-state numerical modeling is mostly used for predicting the flow and transport characteristics in an ejector, but it may not be able to reflect the actual performances of an ejector when operating condition varies. Moreover effects of needle movement on the performances of an ejector is not very clear. In this study, a transient heterogeneous mixture model was developed for a two-phase CO2 needle adjustable ejector with motive nozzle inlet in supercritical, critical or sub-critical regions. The dynamic mesh method was used to control the needle movement, and the unsteady coupling solution between the needle movement and the flow field is solved for the adjustable two-phase CO2 ejector. Modeling results show that the transient modeling can predict dynamic performances of CO2 ejector more accurately than the steady-state modeling. Based on the transient modeling, it is found that as the needle moves further into the motive nozzle throat, effective nozzle throat area decreases, expansion angle of motive flow increases then decreases, the velocity at the motive nozzle outlet decreases then increases, and entrainment ratio decreases, then increases sharply, then decreases, finally become negative. High pressure at the motive nozzle inlet leads to high entrainment ratio owning to high velocity at the motive nozzle throat outlet. A sudden increase of pressure at the motive nozzle inlet in different thermodynamic state regions (supercritical, sub-critical) leads to different dynamic performance responses and oscillation times of ejector. The mechanisms of boundary layer separation and backflow vortices as well as shock waves were explored to explain the suction flow chocking phenomena. This study is helpful to understand flow mechanisms inside the CO2 ejector, and improve optimal design and control of an adjustable two-phase CO2 ejector in variable operating conditions in practice.

Comparison of transient characteristics of positive and negative power-off transition process of “S” shaped bi-directional full-flow pump

In this study, the positive and negative power-off process of a bi-directional full-flow pump is investigated by model tests and numerical simulations, and the results show that under steady conditions, the head of the bi-directional full-flow pump in the positive direction is larger than in the negative direction. The positive power-off process of the bi-directional full-flow pump is slower than the negative power-off process. The clearance backflow rate of the bi-directional full-flow pump continues to drop during the power-off process until it remains stable under the runaway condition. The clearance backflow causes the vortex at the impeller inlet and the size of the vortex gradually decreases during the power-off process. Compared to the positive power-off process, the backflow vortex ratio is greater, and therefore, the flow pattern is poorer during the negative power-off process. The entropy production rate in the negative power-off process is significantly greater than that in the positive power-off process. The pumping condition has the largest hydraulic loss in the whole power-off process. The runaway rotational speed in the positive power-off process is higher than that in the negative power-off process, and the axial force in the positive runaway condition is 1.40 times greater than in the negative runaway condition. During the power-off process, the vibration and pressure pulsation in the negative operation is significantly greater than that in the positive operation, and the acceleration of the impeller vibration from large to small is the Y (vertical) direction, Z (axial) direction, and X (horizontal) direction. The research in this paper can provide an important reference for the design and operation of the bi-directional full-flow pump.

Effect of tip clearance flow on pressure oscillation and vortex patterns of a high-speed centrifugal pump

In a micro centrifugal pump with tip clearance working at high speed, the pressure fluctuation and complex vortex structures related to the tip leakage flow are conducted. The external characteristics calculated by the numerical simulation and experiment method agree well, and the maximum deviation of the simulated pump head and efficiency compared with the experimental data is less than 5%. The vortex structures induced by the tip clearance flow, which is less influenced by the flow conditions, can be divided into four categories, namely backflow vortex, passage vortex, primary tip clearance flow (PTLV), and separation TLV (STLV). The evolution of these vortex structures is described by the relative vorticity transport equation, using the relative vortex stretching, Coriolis force, and viscous diffusion. The PTLV starts from the corner of blade tip gap near the leading edge (LE), and breaks away into the STLV in the region of 0.7 R* to 0.8 R* owing to the pushing effect of the secondary tip leakage flow. To analyze the leakage flow quantitatively, the relative velocity in the gap region is decomposed. The results show that the local maximal and minimal values of the normal component correspond to the start position of the PTLV and the separation region of the STLV, respectively. Besides, the passage vortex has the greatest contribution to the pressure pulsation of the passage from main blade to main blade (MB). But the dominant frequency of the pressure alterations has little to do with the vortex, and mainly depends on the rotor-stator interaction (RSI) between the impeller and volute tongue. The results could be referred to improve the efficiency and operational stability of the high-speed centrifugal pumps.

Comparative studies on the propagation of rotating stall in a liquefied natural gas cryogenic submerged pump-turbine in both pump and turbine mode

With the tightening of liquefied natural gas (LNG) supply, unsteady stall flow under partial flow rate will likely cause significant difficulties with respect to the application of a cryogenic submerged pump-turbine (PT) at LNG-receiving terminals. In this study, the unsteady propagation characteristics of stall cells in pump mode (PM) and turbine mode (TM) of the PT are investigated numerically using the timescale-based hybrid turbulence model. The predicted performance curves show good consistency with on-site experimental data. As the rotating speed and fluid undercooling increase, the triggering of stall flow tends to a lower flow rate in PM, but this is reversed in TM. Under the superimposed action of separation and backflow vortex, stall flow in the impeller suffers from dynamic emergence to dissolution in PM, but quasi-static stall vortex is dominant in TM, benefiting from the rectification of the guide vane. By comparison, running in PM is prone to severe deep stall, and the stall cells have a higher propagation frequency and strength, which also induces greater local energy loss than in TM.

Evolution of unsteady vortex structures in the tip region of an axial compressor rotor

The evolution of unsteady vortex structures in the tip region of an axial compressor rotor is investigated based on delayed detached eddy simulation. The vortex structures are identified by the LTcri method, and the velocity fields are visualized by the particle tracing method. The results show that the evolution of the tip leakage vortex (TLV) can be divided into three phases: the generation phase, the development phase, and the dissipation phase. The unsteadiness of the flow field mainly appears in the dissipation phase as a consequence of the unsteady secondary tip leakage. There are three primary unsteady vortex structures caused by the tip leakage flow (TLF), and these vortex structures are related to each other as a feedback loop. The intermittent formation of the vortex ropes leads to the breakdown of the TLV and thus results in the roll-up of the backflow vortex (BFV) due to the radial velocity gradient. The secondary leakage of the BFV locally enhances the TLF jet and affects the formation of the vortex ropes in turn. This feedback loop causes the unsteady behavior of the TLF and has great impacts on the performance and stability of the compressors.

Impact of Operating Conditions and Axial Casing Grooves on the Evolution of Flow Structure Across Blade Rows in an Axial Compressor

Abstract Stereo particle image velocimetry (SPIV) measurements in a series of axial planes investigate the impact of operating conditions and semicircular axial casing grooves (ACGs) on the evolution of flow structure across multiple blade rows in an axial compressor. The field of view extends radially from the hub to the tip and circumferentially over entire blade passages. Previous studies in this machine have shown that the ACGs improve the stall margin significantly but reduce the peak efficiency. At pre-stall flowrate and without ACGs, intermittent reverse axial flow near the casing is induced by backflow vortices, tip leakage vortex (TLV), and the leakage flow extend upstream of the rotor leading edge. Inside the rotor, the tip region blockage, characterized by low axial and high circumferential momentum, expands radially inward as the flow evolves axially. This extreme non-uniformity diminishes rapidly within the stator. In addition to previously shown ACGs effects, the current data reveal that the flow jetting out from the groove upstream of the rotor generates axially aligned vortices on both sides of each jet. These vortices substantially reduce the flow non-uniformity over the entire passage by entraining the faster mid-span flow into the tip region. Near the best efficiency point, the jets become weaker, the blockage is confined to the tip region, and differences between the global flow structure with and without ACGs become subtle. However, interactions of the TLV with secondary flows entrained from the grooves into the passage expand the TLV signature, which has adverse effects on the compressor performance.

Analyzing the Inlet Gas Void Fraction on the Flow Characteristics for a Multiphase Pump Based on Cavitation Cases

Inlet gas void fraction (IGVF) affects the cavitation evolution in a multiphase pump and easily results in a drop of the head and efficiency when cavitation is more serious. In this paper, a numerical method was performed to qualitatively and quantitatively analyze the effect of the inlet gas void fraction on the pressure and velocity characteristics of the multiphase pump at different cavitation stages. The results show that with the increase of IGVF and the development of cavitation, the pressure in the impeller flow passage is reduced, and the pressure corresponding to the cavitation region drops sharply to the saturated vapor pressure. With the decrease of the cavitation coefficient, and due to the expulsion effect of the cavitation bubbles, the relative velocity in the cavitation region becomes larger. Because of the large pressure gradient at the end of the cavitation bubbles, the kinetic energy of the fluid is insufficient to overcome the effect of the inverse pressure gradient, resulting in a backflow vortex. Investigations on cavitation evolution in the multiphase pump at different IGVFs are of great significance for improving its performance.

Application of double-wall cooling structure in the integrated strut flame stabilizer

• After gas flows through the integrated strut, symmetrical spiral vortices and backflow vortices are formed behind the strut. • Under the pressure of spiral vortices and backflow vortices, the cool air flowing out of the double-wall covers the wall surface to cool it. • In the non-afterburner state, the cool air flowing out of the double-wall covers the wall in the form of film. In the afterburner state, the cool air flowing out of the double-wall covers the wall in the form of vortex film. • With the increase of mass flow ratio, the cooling efficiency of double-wall increases, the film adhesion decreases, and the scale of the film vortex increases. In this paper, the double-wall cooling structure is applied to the integrated strut flame stabilizer. The cooling performance and aerodynamic performance of the double-wall cooling structure under non-afterburner state and afterburner state are studied through numerical simulation. The flow behind the strut and the interaction between the film outflow and the main flow are analyzed in detail. The effect of mass flow ratio on the cooling performance of the double-wall is explored. The results show that the spiral vortexs and the backflow vortexs are the main components of the recirculation area behind the strut, and the flame burns stably in the recirculation area; under the pressure of the eddy current in the recirculation area, the cool air outflow covers the wall surface in the way of film or film vortex, which can effectively cool the wall surface; with the increase of mass flow ratio, the cooling efficiency of double-wall increases, the wall covering property of film decreases, and the scale of film vortex increases.