Pressure Oscillation Intensity Research Articles

A Pressure-Oscillation-Based RON Estimation Method for Spark Ignition Fuels beyond RON 100

Knock in spark ignition (SI) engines occurs when the air–fuel mixture in the combustion chamber ignites spontaneously ahead of the flame front, reducing combustion efficiency and possibly leading to engine damage if left unattended. The use of knock sensors to prevent it is common practice in modern engines. Another measure to mitigate knock is the use of higher-octane fuels. The American Society for Testing and Materials’ (ASTM) determination of the Research Octane Number (RON) and Motor Octane Number (MON) of spark ignition fuels has been based on measuring cylinder pressure rise at the onset of knock since its inception in the 1930s. This is achieved through a low-pass filtered pressure signal. Knock detection in contemporary engines, however, relies on measuring engine vibrations caused by high-frequency pressure oscillations during knock. The difference between conditions in which fuels are evaluated for their octane rating and the conditions that generate a knock intensity signal from the knock sensor suggests a potential difference between octane rating and the knock limit typically identified by a contemporary knock sensor. To address this disparity, a modified RON measurement method has been developed, incorporating pressure oscillation measurements. This test method addresses the historical lack of correlation between RON and high-frequency pressure oscillation intensity during knock. Using toluene standardization fuels (TSFs) as a reference, the obtained results demonstrate excellent high-frequency knock intensity-based RON estimations for gasoline. The method is able to differentiate between two fuels that share the same ASTM RON, associating them with a RON-like metric that is more aligned with their performance in a modern SI engine. This alternative method could potentially serve as a template for an upgrade to the existing ASTM RON method without significantly disrupting the current approach. Additionally, its capability to evaluate fuels beyond RON 100 opens the door to assessing a wider range of fuels for antiknock properties and the intensity of fuel oscillations during knocking combustion.

Study of engine knocking combustion using simultaneous high-speed shadowgraph and natural flame luminosity imaging

Engine knocking is a classical but significantly impactful combustion phenomenon that arouses unfailing research interests. An in-depth understanding of the knocking mechanism holds the key to the next-generation high-efficiency spark ignition engines adopting a high compression ratio. In this study, a novel 72 kHz imaging system of simultaneous shadowgraph and natural flame luminosity (NFL) was implemented on an optical engine. The key combustion parameters that affect the engine knock intensity were investigated based on the heat release analysis. The shadowgraph image velocimetry (SIV) was utilized to measure the in-cylinder flow field. The end gas autoignition process in the engine knocking combustion was studied under knock intensity as high as 18.1 bar. The results indicate that there is an “optimum” combustion phasing that produces the highest knock intensity. The cases with more end gas mass at knock onset, though having lower mean temperatures, generally produce higher knock intensity. The knock intensity can be significantly different under the same end gas mass and mean end gas temperature, possibly due to the different temperature gradients in the end gas. The simultaneous shadowgraph and NFL imaging succussed in differentiating the density variation zone in the end gas caused by the low-temperature heat release from the hot flame zone before auto-ignition. An intense density variation zone is witnessed around the center of the cylinder under higher engine knock intensity by the shadowgraph images, showing a dark region that oscillates together with the local pressure oscillations. The coupling among the local density variation, pressure oscillation, and NFL intensity oscillation is observed during the engine knocking combustion. The velocity distribution of the flow field indicates that the engine knocking combustion enhances the in-cylinder flow, which may cause more convective heat transfer loss and flow loss.

Experimental study on tail cavity structure and pressure characteristics of underwater vehicle with tail jet

The tail of an underwater vehicle with a tail jet often creates an attached tail cavity when moving at high speed. The strong coupling between the tail cavity and high-speed jet affects the kinematics characteristics of underwater vehicles. This paper compares the influence of Froude number (Fr) and ventilation coefficient (CQv) on the tail cavity at area ratio A/Ae = 92.5 and 236.7 by water tunnel experiment. It was found that at a small Fr, the critical ventilation coefficient of the transition from intact (IC) to partially broken (PBC) and PBC to pulsating foam (PFC) at A/Ae = 92.5 is greater than that of A/Ae = 236.7. The cavitation number (σc) at A/Ae = 92.5 is generally smaller than that of A/Ae = 236.7. With increased CQv, PBC's dimensionless time-averaged length (Lc‾/D) decreases sharply and then gradually flattens. The Lc‾/D of A/Ae = 236.7 is larger than that of A/Ae = 92.5. The fluctuation amplitude of the PBC length is smaller than that of the PFC, and the fluctuation frequency (fLc) of the PFC length is about 120 Hz. The change in the tail cavity will affect the pressure characteristics of the vehicle's bottom. With the increase of CQv, the peak frequency of the bottom pressure fluctuation (fpeak) will continue to rise in the PBC until the transition to the PFC, and the fpeak will remain at about 120 Hz. The correlation coefficient is 0.74 between the fLc and the fpeak. In addition, the pressure oscillation intensity at the vehicle's bottom (pbrms) is small in the IC region. When the tail cavity changes from PBC to PFC, pbrms increases gradually at first, then tends to be flat.

Experimental investigation on pressure oscillation induced by steam injection into subcooled water through an opposite nozzle

The pressure oscillation characteristics induced by steam jet condensation through an opposite nozzle were experimentally studied. Four condensation regimes, namely Chugging-I (Chug-I), Chugging-II (Chug-II), Condensation Oscillation-I (CO-I), and Condensation Oscillation-II (CO-II), were identified based on interfacial behaviors and corresponding pressure signals in the pipe and the water tank. In the Chug-I and Chug-II regimes, the probability density function (PDF) of the pressure signals in the pipe was highly asymmetric, while the PDF in the water tank was sharp unimodal distribution. The PDF in the CO-I regime was sharp unimodal distribution, symmetrical to 0 kPa. The PDF in the CO-II regime was asymmetrical dumpy distribution. In addition, the variations of pressure oscillation intensity in the pipe and the water tank were investigated. Finally, a modified correlation was proposed for predicting the dominant frequency of pressure oscillation, with the maximum deviation of ± 10 %.

Condensation oscillation characteristic of steam with non-condensable gas through multi-hole sparger at low mass flux

To study oscillation characteristic of steam and non-condensable gas direct contact condensation through multi-hole sparger at low mass flux, a series of experiments of pure steam and mixture gas condensation have been carried out under the conditions of steam mass flux of 20–120kg/m2s, water temperature of 20–95 °C and mass fraction of non-condensable gas of 0–5%. The regime map of pure steam condensation through multi-hole sparger is divided into steam chugging, separated bubble, aggregated bubble and escaping aggregated bubble. The bubbles behavior of synchronization in the same hole columns and desynchronized excitation between different hole columns can be found. The coalescence effect of mixture bubbles increases with water temperature and non-condensable gas content increasing. Pressure oscillation intensity of pure steam condensation first increases and then decreases with water temperature increasing, and increases with steam mass flux increasing. Pressure oscillation intensity of mixture gas condensation decreases with water temperature and non-condensable gas content increasing, which is significantly weaker than that of pure steam condensation. The oscillation dominant frequency decreases with the rise of water temperature and non-condensable gas content. The correlations for oscillation intensity and dominant frequency respectively are developed in pure steam and mixture gas condensation at low mass flux.

Experimental investigation of the pressure oscillations induced by subsonic steam jets under different vessel pressures

An experimental study has been conducted to investigate the characteristics of subsonic steam jet condensation pressure oscillations in the ranges of steam mass flux 50–90 kg/m2s, water temperature 20–90 °C and vessel pressure 0.1–0.4 MPa. High-frequency dynamic pressure and high-speed images of the steam bubble condensation process are recorded and analyzed. It is found that the steam bubble necking frequency increases with steam mass flux and decreases with water temperature. The vessel pressure also has an effect on the steam bubble necking frequency. The pressure oscillation intensity in the pipe increases with the steam mass flux in the condensation oscillation region, but the trend in the chugging region is opposite. The vessel pressure can strengthen the internal pipe pressure oscillation in the chugging region. The pressure oscillation intensity in the pool increases initially, but then decreases rapidly with the water temperature. The water temperature where the pressure oscillation intensity becomes maximum shifts lower with increasing vessel pressure. An equation correlating the Strouhal number, Reynolds number and Jakob number is developed and can predict the experimental data obtained under 0.1 MPa vessel pressure with a deviation within ± 10%. A dimensionless pressure term is added in the correlation to be applied for 0.2–0.4 MPa conditions and the maximum discrepancy between the predicted values and experimental results is within ± 15%.

Effect of active ignition on the end-gas autoignition with detonation combustion in confined space

In spark-ignition engines, the presence of knock significantly prevents the engines from further improving thermal efficiency. In this work, the effect of active ignition in the end-gas region on the end-gas autoignition with detonation combustion was optically observed in the self-designed constant volume combustion bomb (CVCB-TJU). The different combustion modes together with pressure oscillation were presented. The results indicate that advancing the ignition timing of the end-gas spark can reduce the pressure oscillation intensity and even suppress the occurrence of autoignition. In addition, the present work observed a statistically positive relationship between unburned mass fraction (UMF) and detonation intensity for the knock induced by developing detonation. The underlying influencing mechanism of end-gas ignition on auto-ignition was presented. The end-gas ignition influences the occurrence and intensity of detonation and pressure oscillation by two means: 1) reducing the UMF burned by autoignition, 2) influencing the flame acceleration, thereby reducing the flame tip velocity, and the subsequent shock intensity. Furthermore, the burned mass fraction (BMF) consumed by end-gas flame when jet flames emerge is applied to substitute the ignition timing when measuring its influence at different oxygen concentrations of 29% and 33%. It is found there exists a critical BMF above which the autoignition would be suppressed. And this critical value increases with increased oxygen concentration.

Characteristics of fluid and pressure oscillations induced by steam injected through a vertical blow down pipe under different vessel pressures

An experimental study has been conducted to investigate the characteristics of condensation oscillations induced by steam injected through a vertical blow down pipe. The inner pipe fluid oscillation phenomenon was observed at lower steam mass flux and water temperature conditions when chugging occurs. The inner pipe steam-water interface oscillation amplitude increases with the vessel pressure and decreases with steam mass flux and water temperature. The effect of the vessel pressure and steam mass flux on the interface oscillation frequency is prominent. The boundaries of the chugging condensation under different vessel pressures were identified. The pressure oscillation intensity in the pipe increases with the vessel pressure and decreases with the steam mass flux and water temperature. But in the pool, it first increases and then decreases with the water temperature. The peak values of the dynamic pressure were intensified by higher vessel pressure.

Experimental Study on the Impingement Characteristics of Self-Excited Oscillation Supercritical CO2 Jets Produced by Organ-Pipe Nozzles

Supercritical carbon dioxide (SCO2) jets are a promising method to assist drilling, enhance oil–gas production, and reduce greenhouse gas emissions. To further improve the drilling efficiency of SCO2 jet-assisted drilling, organ-pipe nozzles were applied to generate a self-excited oscillation SCO2 jet (SEOSJ). The impact pressure oscillation and rock erosion capability of SEOSJs under both supercritical and gaseous CO2 (GCO2) ambient conditions were experimentally investigated. It was found that the impact pressure oscillation characteristics of SEOSJs produced by organ-pipe nozzles are dramatically affected by the oscillation chamber length. The optimum range of the dimensionless chamber length to generate the highest impact pressure peak and the strongest pressure oscillation is within 7–9. The dimensionless pressure peak and the pressure ratio decreases gradually with increasing pressure difference, whereas the pressure oscillation intensity increases with increasing pressure difference and the increasing rate decreases gradually. The dominant frequency was observed to decrease monotonically with increasing chamber length but increases with the increase of pressure difference. Moreover, the comparison of impingement characteristics of SEOSJs under different ambient conditions showed that the values of dimensionless peak impact pressure are similar under the two ambient conditions, and the SEOSJ achieves higher pressure oscillation intensity and dominant frequency in SCO2 at the same pressure difference. The rock breaking ability of the SEOSJ is closely related to its axial impact pressure. The erosion depth and mass loss of sandstone caused by the organ-pipe nozzle with the best impact pressure performance is higher than those produced by other nozzles. The SEOSJ results in a deeper and narrower crater in SCO2 than in GCO2 under the same pressure difference. The reported results provide guidance for SEOSJ applications and the design of an organ-pipe nozzle used for jet-assisted drilling.

Oscillation characteristics of periodic condensation induced water hammer with steam discharged through a horizontal pipe

Condensation induced water hammer (hereafter, CIWH) phenomenon is widely observed in various industrial operations, causing severe damage to related equipment and personal safety. This study investigated the CIWH regime map and the oscillation characteristics of periodic CIWH under different pipe lengths when the steam was discharged into a water tank through a horizontal pipe. First, the CIWH regime maps were obtained under different pipe lengths, with distinct regimes of periodic CIWH and non-periodic CIWH. A criterion was proposed to predict the transitions from periodic to non-periodic CIWH. It was found that the occurrence frequency of periodic CIWH decreased with the increase in the pipe length. An empirical correlation of Strouhal number, non-dimensionalized occurrence frequency, was developed to predict the occurrence frequency. The correlation could reproduce more than 95 % of the experimental data with the error of -20.2 % to 23.7 %. The pressure oscillation intensity of periodic CIWH was analyzed. The oscillation intensity increased when the pipe length and steam mass flux increased, whereas it increased first and then decreased when pool temperature increased. Finally, the occurrence position of periodic CIWH was found to be nearly constant under the same test condition. The position moved downstream of the steam when the pipe length and Froude number increased, and Jakob number decreased. The average occurrence position was correlated with the error of -17.1 % to 18.4 %.

Study on pressure oscillation characteristics in a constant volume bomb

The pressure oscillation characteristics of a promising gasoline additive Di-isopropyl Ether (DIPE) were studied. The experiments were operated in a constant volume bomb (CVB) under different initial pressure (1, 3, 5 bar) and equivalence ratio (0.8–1.6). The experimental data acquisition system includes pressure, sound pressure and vibration detection system and Schlieren system. The experimental results show that the pressure oscillation intensity first increases and then decreases with increasing of the equivalence ratio, and the overall oscillation intensity increases with initial pressure. The sound pressure, vibration intensity and gray-scale brightness extracted from flame processing photos all have the same tendency. N2 is replaced by He and CO2 dilution gas with different blending ratios to investigate the change of oscillation. The effects of several factors’ behavior were studied. When Markstein length is low, the pressure oscillations become much easier to occur. The increase of Zeldovich number can decrease the intensity of oscillations. It is induced that the combustion instability is the main reason for pressure oscillations in constant volume bomb.

Numerical investigation on spray self-pulsation characteristics of a liquid-centered swirl coaxial injector

Self-pulsation characteristics of a liquid-centered swirl coaxial (LCSC) injector are investigated numerically based on the coupled volume of fluid VOF and Level Set (CLSVOF) model. Good agreements of simulation results and experiment data, in aspect for self-pulsation frequency and spray angle, are generally achieved. On the basis of the flow dynamics of the injector, mechanism of spay self-pulsation in recessed LCSC injector is proposed. The self-pulsation arises from the periodic blocking of annular gas by the liquid sheet. Self-pulsated “Christmas tree” spray patterns result from the periodic vortex rings generating and shedding around the liquid sheet. Theoretical analysis demonstrates that the self-pulsation frequency is almost the same as the frequency of the liquid sheet impacting the recess chamber wall, and the model may be able to predict the self-pulsation frequency. Pressure oscillation inside the air-core is closely related to the primary breakup of the liquid sheet, and the frequency is consistent with the self-pulsation frequency. Effects of the gas-liquid mass flow rate ratio (GLR) on the self-pulsation frequency and pressure oscillation frequency inside the air-core are negligible. The self-pulsation intensity peaks under conditions with the critical mixing flow pattern, while, the pressure oscillation intensity inside the air-core increases with the elevating of GLR. It seems that the spray self-pulsation could improve the spray angle.

Investigation on steam direct contact condensation injected vertically at low mass flux, part II: Steam–air mixture experiment

An experimental study is conducted on the condensation behavior, onset of chugging, fluid oscillation, and pressure oscillation when a steam–air mixture is injected vertically into subcooled water at a low mass flux. Air is present in the form of dispersive small bubbles after condensation at a low water temperature and in the shape of a toroid at a high water temperature. Bubble resonance is first observed experimentally; a large bubble is almost stationary below the pipe outlet and air is sustainably released by small bubbles from the upper surface. The upper air mass fraction of chugging is obtained and compared with a transient conduction–diffusion model and momentum balance model. The dependence of different parameters on the amplitude, frequency, and probability of fluid oscillation is analyzed. A new frequency correlation modified with probability is developed and shows good agreement with the experimental data. During bubble resonance, the wave shape of pressure oscillation is harmonic with the frequency at about 35 Hz, and the intensity is larger than that under pure steam conditions. When bubble resonance does not occur, the presence of air can reduce the pressure oscillation intensity.

Characteristics of entrapped bubbles of periodic condensation-induced water hammer in a horizontal pipe

Steam condensation-induced water hammer (CIWH) can cause great pressure oscillation, and the knowledge of CIWH is important to pipes and relevant equipment. In this work, an experiment was conducted to investigate the condensation process of entrapped bubbles for periodic CIWH and its pressure oscillation in a horizontal pipe. Results indicated that CIWH was caused by the condensation of entrapped bubbles, and the initial volume of entrapped bubbles of the periodic CIWH kept nearly unchanged when the test conditions were constant. The initial bubble volume increased with the increases in steam mass flux and water temperature. The pressure oscillation intensity of the CIWH increased with the increase in initial bubble volume but decreased with the increase in water temperature. Furthermore, the change rate of bubble volume gradually increased with the condensation of the entrapped bubbles. The final change rate of bubble volume increased with the increase in initial bubble volume but decreased with the increase in water temperature. Finally, a theoretical correlation, that is, a linear relationship between the pressure oscillation intensity and the final change rate of bubble volume, was proposed to calculate the pressure oscillation intensity of the CIWH. The predicted deviation ranged from −44% to 64%.

Experimental and theoretical study of pressure oscillation of unstable steam-air jet condensation in water in a rectangular channel

Pressure oscillation occurs during the steam jet condensation in water, notably in unstable flow pattern region. Consequently, pressure oscillation may risk the thermal system. In this work, we introduce a method of reducing the pressure oscillation intensity by adding a small amount of air into steam in unstable flow pattern region. The effects of the air content on the unstable flow patterns and pressure oscillation characteristics were studied experimentally. Notably, the air layer showed a periodic accumulation-and-dispersion phenomenon around the steam–water interface. This phenomenon resulted in a periodic and dramatic fluctuation of the interface at a relatively low air mass fraction. When the air mass fraction increased to a critical value, the flow pattern became stable due to a sharp reduction in condensation rate. Accordingly, the pressure oscillation intensity increased and then dropped at the critical air mass fraction, showing a convex-shaped distribution. The air mass fraction should exceed the critical value if air is added into steam to reduce the pressure oscillation intensity. Otherwise, the air could lead to a much stronger pressure oscillation. A theoretical model for the critical air mass fraction was established and could predict the critical air mass fraction within a deviation of ±14%.

Investigation on steam contact condensation injected vertically at low mass flux: Part I pure steam experiment

Steam direct contact condensation in subcooled water is an inevitable process in pressure suppression system of advanced light water reactor. Ten pure steam experiments have been performed in the range of steam mass flux 5–80 kg/m2 s and water temperature 19–96 °C. The steam condensation behave, fluid oscillation and pressure oscillation are recorded and analyzed. Prominent small bubbles are observed on the bubble surface due to the Rayleigh-Taylor instability. Vortex ring appears after aspherical bubble collapses. The upper threshold of chugging is identified by temperature data acquired continuously. The amplitude and frequency of fluid oscillation are obtained and compared with previous correlations. Two typical pressure wave shapes of continuous oscillation and pulse oscillation are identified. The pulse oscillation is induced by steam bubble collapse and observed at low water temperature. The maximum pressure pulse peak is about 1 MPa measured in the nozzle and 0.1 MPa in the pool. The pressure oscillation intensity in the water pool increases first, then decreases rapidly, and then increases slowly. It has a maximum at water temperature about 55–75 °C and the related temperature increases with the increment of the steam mass flux. The pressure oscillation dominant frequency is analyzed and compared with available correlations.

Experimental investigations on pressure oscillation induced by steam-air mixture gas sonic jets in subcooled water

Present paper aims to experimentally investigate pressure oscillation induced by steam-air mixture gas sonic jets in subcooled water. Dynamic pressure signals and jet images are recorded and analyzed. Experimental results show that: the pressure oscillation dominant frequency decreases with the rise of water temperature, inlet pressure and air mass fraction. Air mass fraction has a complex effect on pressure oscillation intensity. When water temperature is less than 55 °C, as the air mass fraction increases, the pressure oscillation intensity increases first, and then decreases slightly, and then increases quickly. When water temperature is more than 55 °C, as the air mass fraction increases, the pressure oscillation intensity decreases first, and then increases quickly. The interfacial fluctuation height and pressure oscillation intensity dependency to the air mass fraction is the same near the nozzle exit. In addition, the validity of previous analysis model for steam sonic jet pressure oscillation dominant frequency is evaluated. A modified function considering the effect of air mass fraction is developed. The predicted results of steam-air mixture gas sonic jet pressure oscillation dominant frequency agree well with the experimental results. More than 98% of the data points are inside the error band of ±20%.

Experimental study on combustion instability and attenuation characteristics in the lab-scale gas turbine combustor with a sponge-like porous medium

The present study has experimentally investigated the combustion instability and its attenuation characteristics in the lab-scale swirlstabilized premixed combustor with a sponge-like porous medium. Unlike the conventional premixed burners, this model combustor has the unique features including a porous dump plane and an acoustic cavity, which was devised for attenuating the combustion instability. When replacing the dump plane with a non-porous medium, the burner becomes the conventional design. In order to evaluate the damping effects of the porous medium on the unstable flame dynamics, various acoustic and photonic measurements and flame visualization were made. Special emphasis is given to the effects of the acoustic cavity length on the stabilization characteristics. Results showed that the model combustor with the porous dump plane and the acoustic cavity exhibited dramatic attenuation of the pressure oscillation intensity by up to about 40 %. The attenuation was increased with increasing the acoustic cavity length. It was also found that the attenuation is effective not only for the fundamental resonance but also for its higher harmonics. Based on the experimental results, detailed discussions are made for the combustion instability and its attenuation characteristics in the model gas turbine combustor with porous and nonporous media.

Effect of non-condensable gas on pressure oscillation of submerged steam jet condensation in condensation oscillation regime

The pressure oscillation is an important characteristic of direct contact condensation of steam in subcooled water. Many experimental works have been performed on pure steam submerged jet condensation. However, the effect of non-condensable gas content on the characteristics of the pressure oscillation is not yet fully understood. So, present paper aims to investigate the effect of air mass fraction on the characteristics of the pressure oscillation. Experimental results show that: for pure steam jets, the pressure oscillation dominant frequency decreases with the rise of water temperature and nozzle diameter. While it increases with the rise of steam mass velocity, which is consistent with the most of previous research results. The pressure oscillation intensity increases with the rise of water temperature, steam mass velocity and nozzle diameter. For air-steam mixture gas jets, the effect of water temperature and steam mass velocity on pressure oscillation characteristics is the same as pure steam jets. The pressure oscillation dominant frequency rapidly decreases with the rise of air mass fraction. However, air mass fraction has a complex effect on pressure oscillation intensity. As the air mass fraction increases, the pressure oscillation intensity rapidly increases at first, then slowly decreases and then slowly increases. In addition, new correlations for pressure oscillation dominant frequency and intensity are developed. The predicted results agree well with the experimental results.

Interface dynamics and pressure oscillation of stable steam jet condensation in water flow in a confined channel with the presence of non-condensable gas

Though direct contact condensation (DCC) has been widely used in many industrial fields, the noise and vibration caused by pressure oscillation becomes an unignorable problem during the operating of related equipments. So there is an urgent need to find methods to reduce noise and vibration. In the present work, experimental study was conducted on the dynamic of steam-air mixture condensation in water flow in a confined channel, expecting to reduce the pressure oscillation intensity by means of adding a proper amount of air into steam. The experiments were tested in stable flow pattern region with steam mass flux, water Reynolds number, water temperature and air mass fraction in the range of 350–600kg/m2s, 78,000–164,000, 30°C and 0–5%, respectively. The observation and image processing of interface behavior and flow pattern transitions provides insight into the pressure oscillation mechanism with non-condensable gas involved. Dynamic pressures on the wall of channel were obtained. The results showed that the DCC of steam-air mixture was determined by two important factors, e.g. gas velocity at nozzle outlet and air content. When gas had a higher velocity, the air layer around the interface would be destroyed so that the effect of air on condensation would be weakened, but the interface was relatively stable. With air content increased to a critical value, stable flow patterns would transform into unstable due to the increase of air quantity. Moreover, the flow pattern transitions could be reflected also by the variation of pressure oscillation intensity. When air content was lower than the critical value, the gathering of air around the interface decreased the fluctuation of interface penetration length, which resulted in the dramatic decrease of pressure oscillation intensity. However, with air content increased to be higher than the critical value, stable jet would transform into unstable, leading to a rapid increase of pressure oscillation intensity. In addition, the results of various test conditions showed that the critical air mass fraction increased with the increase of steam mass flux but decreased with the increase of water Reynolds number. Furthermore, an optimal air mass fraction which could minimize the pressure oscillation intensity under various test conditions was suggested.