Subcooling Number Research Articles

Comparison analysis of density wave oscillation type two-phase flow instability of superheated and saturated boiling tubes

Most of previous conclusions about two-phase flow instability come from saturated two-phase flow boiling tubes (SatTFB). Superheated two-phase flow boiling tubes (SupTFB) can generate superheated steam, which will make the thermodynamic cycle efficiency higher and the steam generator or system simpler. However, there is no systematic mathematical analysis about the difference between the two-phase flow instabilities of SatTFB and SupTFB. Algebraic criteria composed of dimensionless numbers are deduced to easily determine their flow instability boundary. Different from six dimensionless numbers used in SatTFB, SupTFB have one more dimensionless number. It is superheated number or two-phase number. The verified model is applied to investigate the parameters’ effects and the transient characteristics of density wave oscillation (DWO) type two-phase flow instability. The effects of Froude number and friction number for SupTFB are the same as that for SatTFB. When system pressure is low, only the case with high inlet resistance coefficients can be stable in SupTFB. Thus for SupTFB at low system pressures, they become stable when increasing the inverse of Froude number (or inclination angle) or decreasing friction number, which do not have the nonmonotonic phenomenon as the SatTFB. For the added superheated steam region, increasing its friction number makes SupTFB unstable. The reasonable application range of the figure expressed by subcooling number and phase change number is the same for both SatTFB and SupTFB when analyzing parameters’ effects. Different from increasing continually for SatTFB, the ratio between oscillation period and transit time for the SupTFB first increases with subcooling numbers and then keeps unchanged at high subcooling numbers.

Experimental assessment of fluid-to-fluid scaling for critical heat flux investigations in rod bundles at low mass flow

Critical heat flux (CHF) is an essential topic in nuclear technology and for that reason it has been investigated during several decades. Novel nuclear reactor designs, such as self-pressurized small modular integral reactors (SMiR), represent a new challenge regarding this issue since they are designed to work at conditions for which traditional tools and procedures may cause too large uncertainties. In addition, traditional fluid-to-fluid modeling techniques for CHF tests have been derived from experiments mainly performed at PWR conditions and therefore its use is doubtful at different conditions.In this work we make use of water and Freon (CCl2F2) experimental data points obtained at low mass flow condition and relatively low subcooling representing natural circulation CAREM-25 SMiR at nominal pressure and at 10 MPa. The test section consists of 19-rods in a hexagonal array mimicking 1/6 of CAREM-25 fuel bundle. Traditional fluid-to-fluid mass flux scaling rules (derived by Ahmad and Katto) are then used to quantify its performance by comparing water and Freon data tests performed at similar conditions in a dimensionless space. Such a space is defined by the subcooling number, the phase change number and the so-called mass flux number. As a result of this part of the study it is concluded that the use of Katto’s scaling rule for the mass flux produces an average error in estimating water data of around 10 %. In addition, the use of Ahmad’s scaling rule causes an average error of about 20 % when estimating water data. Due to these modest performances, a dedicated mass flux scaling rule was derived based on the dimensionless Weber and liquid Reynolds numbers. The new scaling rule succeeded in reducing the average difference between water and Freon test data to less than 3 %. The use of the proposed scaling rule allowed treating the Freon and water data as a single dataset, which in turn enlarged the experimental domain. The proposed mass flux scaling rule is thus recommended for fluid-to-fluid CHF tests at low mass flux conditions and reduced subcooling numbers.

A Comparative Assessment on Different Aspects of the Non-Linear Instability Dynamics of Supercritical Fluid in Parallel Channel Systems

The thermal-hydraulic behavior of supercritical water reactors with a parallel channel configuration was examined through a non-linear instability analysis. This analysis was performed under wide-ranging conditions and aspects, including different working supercritical fluids, varied heat-flux and flow-rate conditions, and channel inclinations. The supercritical fluid (SCFs) dynamics were captured using the density, enthalpy, and velocity analytical approximation functions. The major findings show that both SCFs (water and carbon dioxide) experienced density wave oscillations at a low pseudo-subcooling number. Static instability characteristics were observed for supercritical water, at a relatively high subcooling number. Further, it was found that at different heat flux, the hotter channel makes the overall system more unstable, whereas, at equal heat flux, parallel channels perform similar to a single-channel system. However, the effect of the inclination angle was found to be negligible owing to supercritical pressure conditions. Moreover, stable and unstable limit cycles along with out-of-phase oscillation characteristics were observed in dynamic stability regions. The present model was also compared with experimental and numerical data. Moreover, co-dimension and numerical simulations were performed to confirm the observed non-linear characteristics. This study helps to enhance the heat transfer characteristics during safe operation of heated channel systems, such as nuclear reactors and solar thermal systems.

Non linear stability analysis of supercritical water cooled reactor: Parallel channels configuration

The present work is devoted to the study of non-linear dynamics of the supercritical water reactor (SCWR) with a parallel channel configuration. To fulfil this objective, point reactor kinetics has been coupled with a parallel channel thermal-hydraulic model. The reduced ordered model is developed by coupling point kinetics equations with one group of delayed neutrons, fuel heat transfer and 1-D fluid flow model. The existing studies are commonly limited to linear stability analysis, therefore, present work focused on to demonstrate the nonlinear dynamics of the SCWR with parallel channel reactor core. The stability boundaries are shown in thermal hydraulic (pseudo phase-change and pseudo subcooling number) and intrinsic reactivity feedbacks (Doppler and density reactivity feedback) parametric spaces. Several bifurcation characteristics associated to density wave oscillations namely, subcritical, supercritical and generalized Hopf bifurcations are observed, which is also confirmed by calculating first Lyapunov coefficients. Corresponding to these phenomena, several numerical simulations are carried out at different parametric values, which confirms the existence and nature of the limit cycle behavior associated with these bifurcations. In addition, parametric sensitivity analysis on the stability characteristics are also carried out.

Experimental investigation on flow boiling instability of R1233zd(E) in a parallel mini-channel heat sink for the application of battery thermal management

The present study focused on the flow boiling instability of R1233zd(E) in a 21 parallel mini-channel heat sink designed for the application on battery thermal management. Experiments were conducted under different conditions, including inlet subcoolings (2.5–8 K), mass fluxes (111–276 kg/m2 s), and heat fluxes (up to 12 W/cm2). A high speed camera with the frequency of 2000 fps was used to capture the unstable flow pattern. The results showed that the period and the initial position of reverse flow were shortened with the increase of heat flux, meanwhile four typical reverse flow patterns in the parallel channel were captured. The Parallel Channel Instability (PCI) featured by large inlet pressure oscillation was suppressed when the inlet subcooling was increased, while mass flux had limited influence on the suppression of PCI. The standard deviation of inlet pressure σ(pin) was increased obviously under the inlet subcooling of 5 K. Beside, faster elevation of σ(pin) was observed under 111 kg/m2 s when the heat flux was higher than 3.8 W/cm2. Finally, the flow instability maps were established based on heat flux, mass flux and dimensionless parameters of subcooling number Nsub and phase change number Npch. The stable, transition and instability regions were divided on the maps based on the operating conditions and the flow patterns captured during the experiment.

Onset of flow instability during subcooled flow boiling of seawater in a vertical annulus

The present study investigates the onset of flow instabilities during subcooled flow boiling of 3.5 wt% artificial seawater in a vertical annulus. All flow instability experiments are conducted by decreasing mass flux gradually under a specific condition of inlet temperature and heat flux. The results reveal that, comparing with de-ionized water, the artificial seawater is more susceptible to the flow instability and demonstrates 12–79% larger mass fluxes for onset of flow instability (OFI) due to the unique two-phase flow pattern of densely bubbly flow or bubble foam flow during the subcooled flow boiling of artificial seawater. After OFI, unstable bubble foam with slug bubbles resulting from foam ruptures is revealed in artificial seawater resulting in significant pressure drop oscillations at lower inlet temperature of 55 °C, 65 °C and 75 °C. The existing criteria or correlations, developed based on the data of de-ionized water, may not predict the OFI for artificial seawater. A new empirical correlation to predict the mass flux at OFI is proposed. A two-phase flow instability map is established on the plane of phase change number versus subcooling number.

Computational case study on upward two-phase flow in vertical circular pipe based on CFD approach: Multidimensional flow instabilities investigation

In this paper, a numeric scheme made up of the three-dimensional (3-D) Two-Fluid Model (TFM) and the Rensselaer Polytechnic Institute (RPI) wall boiling model for two-phase flow instabilities simulation was validated by comparing its results with the flow boiling and instability experimental data. Then two-phase flow instabilities in a vertical circular channel were studied based on the validated numeric scheme. The flow instability was analyzed by both the Lumped Parameter Approach (LPA) and the Distributed Parameter Approach (DPA). The average parameter oscillation characteristics and local distributions were analyzed. The transient responses of the local conditions during the flow instability were discussed. Besides, the stability of the vertical circular tube channel flow boiling system was studied with the stable map based on the subcooling number (Nsub) versus the phase change number (Npch). This study provides a new feasible scheme for the simulation of flow instability which also actualizes an improved understanding of multidimensional flow and heat transfer characteristics in the flow instabilities phenomenon.

Two-dimensional quantitative study of boiling flow evolution in vertical inner-heated annulus channel

Boiling flow is a multidimensional phenomenon because of its thermal nonequilibrium characteristic. Studying the evolution of boiling flow in both the axial and radial directions is crucial for the modeling and validation of Computational Fluid Dynamics (CFD) codes. A quantitive analysis is conducted on a boiling dataset with two-dimensional, high-resolution data measured by conductivity probes in a vertical inner-heated annulus. By tracing the bubble layer's development, three stages of the boiling flow evolution are recognized, and the dominant mechanisms of each stage are discussed. New concepts of the bubble sublayers, namely 80%, 50% void layers and 80%, 50% bubble-number layers, are created to identify the region where the majority of the bubbles are located. The previous bubble-layer thickness models are validated, and the flat model is observed to be more accurate in modeling the subcooled boiling flow than the triangular model. Limitations are observed in the results of the bubble-layer thickness model, and modifications are proposed in this paper to overcome the limit. After summarizing the twenty-two conditions in the boiling dataset, the bubble sublayers correspond to the void fraction or to the subcooling number and the boiling number. Correlations are developed to calculate the boundary locations of the sublayers from the parameters. In addition, the development of the bubble sublayers and the transition of the flow regimes are found to be related, where the flow regimes are identified by the criteria and by viewing the high-speed videos.

Scale analysis for water jet impingement over a horizontal flat plate under film boiling configuration

A scale analysis has been performed to investigate the film boiling heat transfer associated with cooling of a horizontal flat plate through water jet impinging. The forced convection film boiling of water is treated by classifying a wide spectrum of subcooling regime by two extreme situations of low and high subcooling dominated by mass evaporation and sensible heating respectively. A simple expression of heat ratio is derived for executing the classification. The ratio is derived by considering the heat and mass transport phenomena and vaporization effect. The radiation heat effect is considered, while obtaining the scale of wall Nusselt number for convective and total heat transfer under both low and high subcooling cases. The comparison of the developed correlations, based on scaling, are made for the wall superheat between 290 and 1100 °C and jet velocity of water between 0.4–3.2 m/s along with the variation of water subcooling between 5 and 45 °C. The proper coefficient of the scaling is developed by making overall comparison of the data from the analysis with experimental data to predict the heat transfer. The effect of liquid to vapor viscosity ratio indicates a substantially higher Nusselt number for the jet impingement of plate under film boiling condition. The density ratio influences the heat transfer only at low subcooling condition. A lower Jakob superheat number, higher Jakob liquid subcooling number, and higher jet Reynolds number increases Nusselt number.

Convective heat transfer by micro-encapsulated PCM in a mini-duct

The phase change materials (PCMs) have almost constant temperature during phase change. This property makes them favorable materials in the heat transfer process. This paper examines the heat transfer characteristics of these particles in a mini-channel using the two-phase model in the Eulerian-Lagrangian view. In this method, fluid is studied using a control volume method (SIMPLE for velocity and pressure domains), and particles are studied using the Lagrangian view. The study is performed for water-based fluid and Paraffin wax micro-particles with 30 μm diameter which have been covered by a thin layer of Fe3O4, with different volumetric concentrations from 0 to 10% and Subcooling number in the range of Sb = 0 to 0.8 and in a wide range of the Reynolds number from 250 to 1000. The results show that the most increase in the heat transfer rate occurs at Sb = 0. In this case and for Re = 1000 and the volumetric concentration of 10%, Nusselt number increases by 13%. This increase is small due to the absence of particles near the wall.

Pressure drop instability analysis in mini-channel evaporators under different magnitudes of gravity

Pressure drop instability in mini-scale channels has been observed in a wide temperature/pressure range, including both the room temperature fluids and cryogenic ones. In the last decade, this instability and corresponding oscillations of flow rate and temperature have attracted numbers of investigations. In this paper, the pressure drop instability in micro/mini-channels is theoretically analyzed. Frictional, accelerational and gravitational pressure drops are considered, and then experimental studies of ammonia in mini-channels are conducted. Based on the analysis, it is found that the stability criterion for pressure drop oscillation can be simplified as a maximum allowable inlet sub-cooling degree. If this maximum degree is not exceeded, the flow boiling will be absolutely stable without pressure drop instability at any heat fluxes. This maximum allowable inlet sub-cooling degree is mainly influenced by the latent heat, the kinematic viscosity ratio and the gravity. By using the phase change number and sub-cooling number, a dimensionless stability limit is proposed. It predicts the occurrence of pressure drop instability successfully, for both ammonia and water. Besides, the effects of gravitational and accelerational pressure drop on the flow instability are also studied. It is found that the accelerational pressure drop has little influence on the flow instability. But the gravitational effect can be significant. By introducing a newly proposed dimensionless number, a novel stability map containing gravity effects is established. The map shows that the gravitational pressure drop stabilizes the system in upward flow cases. But in downward flow cases, it destabilizes the system.

Non-linear analysis of nitrogen pressure drop instability in micro/mini-channels

Pressure drop instability of nitrogen flow boiling in micro/mini-channels challenges the application of nitrogen in the cooling & heat dissipation areas by causing unstable flows, oscillations of flow rate, temperature and pressure. In the present study, a non-linear analysis of nitrogen flow boiling instability is conducted. By introducing the structure number, a novel stability criterion consisting of 4 dimensionless numbers are proposed. Based on the criterion a stabilization map is obtained and validated with experimental data from different researchers. Influences of acceleration pressure drop, saturation pressure, channel length, diameter and length-diameter ratio on the stabilization map are investigated. It is found that acceleration pressure drop plays a significant role in the nitrogen flow boiling instability and has opposite effects to the frictional pressure drop. Increasing of saturation pressure generally stabilizes the nitrogen boiling flow by compressing the unstable region on the stabilization map. Besides, these is a maximum allowable subcooling number which ensures the flow boiling system to be inherently stabile if it is not exceeded. And it increases with the growth of saturation pressure. Furthermore, channels with the same length-diameter ratio have very similar stabilization map. Length-diameter ratio can be a significant parameter for further similarity analysis of nitrogen flow boiling instability.

Effect of throttling on the two-phase flow stability in an open natural circulation system

Low pressure natural circulation systems fit the concept of passive safety systems as useful feature to bring a nuclear power plant to a safe and stable state during a postulated accident without active systems. The operation is characterized by low mass fluxes and passing the known unstable two-phase region from activation to a continuous heat removal in the stable two-phase region. The occurring mass flow oscillations and the condensation induced water hammer phenomenon limited the performance of heat removal. These effects are highly undesirable because they may effect destruction of entire apparatus. The GENEVA test facility as an open natural circulation system represents the containment cooling condenser of the KERENATM reactor concept in the main dimensions. Initiated by the steam supply, the mass flow results from induced heat flux and is limited only by the internal pressure drops: the local, the frictional, the acceleration and the gravitational pressure drop across the flow path. The influence of the flow resistance on its unstable two-phase region and their oscillatory behavior are experimentally studied in detail. Its variation is summarized in stability maps plotted in the parameter plan of the heat flux and the subcooling number at the inlet of the heated section. Thereby, the dynamical evolution of the boiling boundary indicates the initiation of two-phase flow oscillations and the condensation induced water hammer phenomenon. The increasing local pressure drop at the inlet of the heated section reduces the velocity amplitudes but a clear reduction of the pressure surges could not be determined although these should be more pronounced due to decreasing buoyancy and increasing back flow.

On nuclear-coupled thermal-hydraulic instability analysis of Super-Critical-Light-Water-cooled-reactor (SCLWR)

The nonlinear stability analysis of a supercritical light water reactor (SCLWR) is presented using a nuclear-coupled thermal-hydraulic reduced-order model. The analytical model is developed by coupling 1. the point-kinetics equations with one group of delayed neutrons, 2. the fuel heat transfer and 3. a 1-D reduced order model which represents the heat absorption phenomenon during the coolant flow. Unlike the existing studies, which are limited to linear stability analysis, the primary objective of the work is to present the detailed nonlinear dynamics of the SCLWR system. The said goal is achieved at two levels. The first level is the linear stability analysis wherein the linear stability boundaries are shown in two sets of parameter space namely the two intrinsic reactivity feedbacks (Doppler reactivity feedback and density reactivity feedback) and the pseudo-phase-change number and pseudo subcooling number. The parametric effects show the sensitivity of the linear stability boundaries with the system parameters. In the second level, to discuss the nonlinear characteristics of the system, two types of Hopf bifurcations (subcritical and supercritical) are studied with the help of first Lyapunov coefficients of the system. Multiple numerical simulations are performed to verify the resultant limit cycle behavior associated with these bifurcations. Moreover, the occurrence of the generalized Hopf bifurcation is shown which represents the bifurcation between the subcritical Hopf and the supercritical Hopf regions. Further, inside the stable region of the linear stability boundary, the saddle-node bifurcation is found which represents the location of the turning point and the threshold of the globally stable region of the system.

Visual investigation on the boiling crisis post the flow instability in the channel with bypass

In this study, a visual experiment was conducted in the channel with bypass for the boiling crisis post the flow instability. Also, the trigger mechanism of boiling crisis was researched combined with the flow state and the two-phase interface morphology. Meanwhile, all the boiling crisis occurred in this study belonged to DO (Dry Out) type boiling crisis. As for the high liquid mass fluxes (Gl = 1500 kg/m2s, Gl = 2000 kg/m2s) and the large inlet subcoolings (Tsub = 40 K, Tsub = 30 K) in the channel of Din = 2.15 mm, the liquid film near the heating wall was rapidly broken up due to the intense evaporation of itself and the significant dragging of vapor core, which lead to the simultaneously occurrence of the flow instability and the boiling crisis. As for other working conditions, the boiling crisis subsequently occurred with the heat flux further increasing as the flow instability happened. Finally, a CHF model was established based on Confinement number (Nconf), Subcooling number (Nsub), Weber number (We) and the geometric structure characteristics of test-section (Lh/Din) with all the errors less than ±25%.

A study of the heat flux profile effect on parallel channel density wave oscillation in sodium heated heat exchanger

Because of the heat transfer coefficient varying much from single-phase to two-phase and changes of operation conditions, heat flux profiles in sodium heated heat exchanger(SHHX) should be very different from the usually studied uniform profile for analyzing the parallel channel density wave oscillation (PCDWO). In this paper, the typical heat flux profiles in SHHX were separated and its effects on PCDWO were studied. The results showed that the heat flux profile will have a distinct effect on PCDWO, and the marginal stability boundaries (MSB) will move clearly as heat flux profile changes. However, when the operation condition points of the SHHX were taken into account, in most cases the stability boundaries of different heat flux profiles would follow the movement of operation points, making the heat flux profile effect be a factor enhancing the SHHX stability. Because of the heat flux profile effects, impacts of many parameters on stability like water inlet temperature would be suppressed, and only the water mass flow, sodium mass flow and water pressure could have distinct effects. Further discussion was performed and the two phase region was found to be dominate in determining the heat flux profile effect, and the normalized boiling onset location was believed to be a better parameter than the subcooling number for analyzing the heat flux profile effect. It was found that the normalized boiling onset location at the intersection point of the MSB curves of two different heat flux profiles may not change when inlet resistance coefficient varied and might decrease with increase of outlet resistance coefficient, and that the stability could be strongly enhanced by large two phase normalized heat flux. On the basis, a criterion that can be used to estimate the effect of heat flux profile at different boiling onset locations for a certain heat flux profile, to compare roughly the relative stability of different heat flux profiles under different conditions or to analyze qualitatively the slopes of the MSB curves was proposed. This study will provide a better understanding of the heat flux profile effects on PCDWO.

Experimental study on natural circulation flow instability in parallel boiling channels under low pressure

Two-phase flow instabilities in parallel boiling channels under natural circulation condition were experimentally studied at low pressure (0.2–0.6 MPa). The parallel boiling channels composed of two different form channels, namely a 3 × 3 rod bundle channel and a tube channel, were used as the testing section. In the present experiment, the flow oscillation behaviors can be classified into three typical modes based on their frequency spectrum characteristics: (1) small amplitude out-of-phase flow instability induced by the type-1 density wave oscillations in parallel boiling channels, (2) large amplitude out-of-phase flow instability with reversal flow induced by the occurrence of geysering in tube channel and (3) a kind of compound flow instability superposed by geysering and natural circulation instability. The influence of thermal hydraulic parameters on the evolution process of above three flow instabilities were analyzed. The marginal stability boundaries presented by the subcooling number (Nsub) and phase change number (Npch) at different system pressures are obtained and all unstable flow occurs in the negative region of the outlet equilibrium quality in the present experiment. Besides, the effects of asymmetric heating between two parallel channels on NC flow instability were also studied. The asymmetric heating can change the NC flow distribution in parallel boiling channels and reduce the system stability. The occurrence of out-phase oscillations in parallel channels can be suppressed with the increasing of heating power asymmetry.

A study on the density wave oscillation relative stability turning point of uniform and cosine heat flux profiles in parallel channels

Existing studies have shown that heat flux profile is a key parameter influencing the system stability boundary, and in practical systems, heat flux profile along the flow channel can be very complex and would change with operation conditions. For the stability analysis of a certain system, instead of obtaining all the stability boundaries under various heat flux profiles, it may be more wise to find the stability boundary of the most conservative heat flux profiles, and in this case it is important to have a knowledge of the relative stability turning point (RSTP) of given profiles. In this paper we performed a study on the properties of parallel channel density wave oscillation (DWO) RSTP of two given heat flux profiles, mainly based on the most studied uniform and cosine profiles. A lot of different operation conditions were calculated to find the effects of operation parameters including inlet resistance coefficient, outlet resistance coefficient, mass flow and pressure on RSTP and to obtain the properties of RSTP characteristics of the profiles. Results showed that RSTP in the Nsub-Npch plane can change with inlet resistance coefficient, outlet resistance coefficient and mass flow, but the pressure effect may be ignorable. For zero outlet resistance coefficient, RSTP of the profiles studied in our paper would almost follow a constant ratio of the subcooling number (Nsub) and phase change number (Npch), and this can be used to determine the relative stability of the profiles conveniently. Also, it was found that outlet resistance coefficient increase would reduce the value of Nsub/Npch and the single phase length ratio difference of the profiles at RSTP.

Improvement of RELAP5 code for density wave instability analysis in parallel narrow rectangular channels

As one of the most common types of oscillatory instability, density wave oscillation (DWO) is of practical importance in the general analysis of two-phase boiling systems. Due to the confinement effect of narrow space, the two-phase phenomena in thin rectangular channels are probably different from those in conventional pipes with large diameters. As the widely accepted system analysis code, RELAP5 has the capability to simulate the DWO phenomena in narrow channels, but the accuracy is limited under the condition of different pressures, flow rates, or subcoolings. In this paper, modifications for related constitutive models are proposed based on single-variable sensitivity analysis. Two-phase friction model and nucleate boiling heat transfer model are selected as the most effective models to improve the predictability of RELAP5. The influence of both models mentioned above is analyzed and it reveals that they are related to two-phase pressure drop and the vapor void fraction distribution along the channel, respectively. The modified calculation is assessed against the experimental results under the wide range of pressure 1–10 MPa, and these comparisons are performed in the map of stability boundary with two dimensionless parameters, phase change number Npch and subcooling number Nsub. Then the heating power and the period of flow oscillation under threshold condition are assessed in detail. The results show that a better agreement can be obtained between the modified calculations and the measured experimental data.

Experimental investigation of the oscillation characteristics of natural circulation of R134A in a two-phase loop

Natural circulation (NC) loops were widely used in many fields, but still limited in the instability of the loop. The instability characteristics of NC are not fully understood, such as oscillation boundary, amplitude and frequency. To understand the oscillation characteristics, experiments of two-phase natural circulation were performed for R134a in a single loop. Two types of the oscillation were observed and identified by the characteristic frequency in their oscillation spectra. The characteristic frequencies and amplitudes of the oscillations were affected by the system pressure, the heating power and the evaporator inlet subcooling. For the type 1 oscillation, with the increase of the heating power, the main characteristic frequency increased, while the oscillation amplitude decreased; with the increase of the system pressure, the oscillation frequency decreased; with the increase of the subcooling, the frequency decreased. For the type 2 oscillation, with the increase of the heating power, the main characteristic frequency increased, the oscillation amplitude increased; with the increase of subcooling, the frequency decreased, and the amplitude increased. An instability map was built based on the dimensionless numbers: the subcooling number N_sub versus the phase change number N_pch. The stable region of the system was wider at low subcooling condition. Two types of the oscillation may occur at the same time, and the stable region disappeared when subcooling number higher than 3. Combining dimensionless analysis and spectrum analysis, we divided the two-phase natural circulation into five regions in the instability map. Through the oscillation spectrum, this study provided a deep perspective for the instability of two-phase natural circulation. Also, the experimental results of this study can lay the foundation for future research.