Inlet Resistance Research Articles

Experimental investigation and modeling of onset of condensation induced water hammer in equal-height-difference natural circulation system

Condensation induced water hammer (CIWH) in equal-height-difference natural circulation systems (EHDNCS) can lead to significant damage to associated equipment. To investigate the mechanism and the prediction of the onset of CIWH in EHDNCS, a small-scale experiment of EHDNCS was conducted under different water temperatures, heat transfer powers, and resistance coefficients of the water-side inlet. This study analyzed the thermal hydraulics, flow regimes, and the onset mechanism of CIWH through temperature and pressure measurement, U-Net image segmentation, and analytical modeling. Four distinct flow regimes were observed in the horizontal pipe: single-phase flow, non-periodic bubble, periodic bubble, and CIWH. Flow regime maps based on water temperature and heat transfer power were obtained under different inlet resistance coefficients. When the steam bubble length is smaller than the pipe length, no CIWH occurs. When the steam bubble length reaches the pipe outlet to form stratified flow, CIWH occurs at a high steam channel height. The occurrence of stratified flow and reverse flow of subcooled water in the steam channel causes the onset of CIWH in EHDNCS. A dimensionless criterion has been developed to predict the onset of CIWH when the saturated steam-water mixture is discharged through a horizontal pipe in EHDNCS. This criterion effectively predicted the transition from periodic bubble to CIWH in the experiment.

Two-phase flow instability characteristics of HTGR Once Through Steam Generators

High Temperature Gas-cooled Reactors (HTGR) have the advantage of inherent safety compared with most Gen III reactors. They also have very high temperature, which make the plant have high electricity generation efficiency and have has the potential for hydrogen production. Once Through Steam Generator (OTSG) is responsible for transferring high temperature heat from primary loop helium to secondary loop composed of water and steam. The evaporation tubes of HTGR OTSG are very long due to the low heat transfer coefficient of helium convection over tube bundles. A Two-phase flow instability may occur in HTGR OTSG, which can cause pulsation of the mass flow rate, system pressure and temperature. This will not only interfere with the control system, but also cause thermal fatigue damage in the structures. Thus, it is important to predict and avoid two-phase flow instability in HTGR OTSG. Three types of flow instabilities are investigated for HTGR OTSG using both a linear analytical model and a nonlinear numerical model. They are flow excursion (or Ledinegg instability), density wave oscillation and pressure drop oscillation. For the nonlinear numerical model, RELAP5 software is adopted. Flow excursion and pressure drop oscillation are discussed based on the predicted hydraulic characteristic curve of HTGR OTSG (pressure drop vs. mass flow rate curve). Density wave oscillation is investigated using the transient variation of the outlet mass flow rate. For the linear analytical model, a set of ordinary differential equations are established using integration and small perturbation methods. The variables of differential equations are the length of the subcooling water region, the length of both the subcooling water and saturated two-phase regions, the average density of the saturated two-phase region and the inlet mass flux. The stability of OTSG is investigated using the eigenvalues of the coefficient matrix of the ordinary differential equations. Based on the results calculated using the linear analytical model and nonlinear numerical model, there is no flow excursion, pressure drop oscillation or density wave oscillation at the designed partial and full power levels (30%, 50%, 75% and 100%). Thus, HTGR OTSG can operate stably and have a sufficient safety margin at the designed power levels. Density wave oscillation will happen when decreasing system pressure. However, increasing inlet resistance coefficient can avoid density wave oscillation effectively. The tube lengths’ influences on the stability boundary of OTSG are also investigated and it shows no strong dependency.

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.

Study of density wave instability in natural circulation with parallel channels under inclined and heaving conditions

Nuclear reactors used in warships, commercial ships and floating nuclear power plants will move synchronously with the carrier due to the action of waves or surges. Under ocean conditions, flow oscillations due to additional external motions can be superimposed with flow instabilities in the system resulting in larger amplitude oscillations or even resonances. Sustained compound oscillation will lead to fatigue failure of components, deteriorate local heat transfer, and affect the safe operation of the reactor system. The PNCMC (Program for Natural Circulation under Motion Condition) program is utilized in this research to explore the density wave instability (Type-I) in parallel channels during ocean circulation, and stability boundaries under different thermal-hydraulic parameters as well as ocean conditions to support the safe operation of the system. Through our study, it is found that under the condition of small heating channel inlet resistance coefficient, the unstable boundary powers in vertical case of natural circulation according to the order from small to large are the initial boundary of the parallel three-channels out-of-phase oscillation, the initial boundary of system's density wave oscillation, the boundary of the terminated system oscillation of density wave, as well as boundaries of three parallel channels where out-of-phase oscillations terminate, respectively; The inclined condition's stable boundary is generally lower than that in the vertical case. Unstable boundary power for the natural circulation in the heaving situation according to the order from small to large are the initial boundary of density wave oscillation in whole system are the initial boundary of oscillation of the density wave in system, the initial boundary of the parallel three-channels out-of-phase oscillation, boundary for three parallel-channels where out-of-phase oscillations terminate, and the boundary of the terminated system density wave oscillation, respectively; The oscillations caused by heaving motions, the system oscillations of density wave, as well as oscillations out of phase between the channels were overlapped to cause flow oscillations of very complex compound inside the heating parallel channels. At the same time, the increase of the resistant coefficient at inlet of heating channels can significantly suppress out-of-phase oscillation between the channels.

Application of Simulated Annealing Algorithm in Core Flow Distribution Optimization

Core flow distribution is closely related to the thermal–hydraulic performance and safety of reactors. For natural circulation reactors with a limited driving force, flow distribution optimization is of particular significance, which can be contrived by suitably assigning the inlet resistance of a core assembly channel in reactor design. In the present work, core flow distribution optimization during the fuel life cycle is regarded as a global optimization problem. The optimization objective is to minimize the maximal outlet temperature difference of assembly channels during the fuel life cycle, while the input variable is the inlet resistance coefficient of each assembly channel. The simulated annealing algorithm is applied to the optimization code. The results show that the maximal outlet temperature difference is significantly reduced after optimization, and the resultant core outlet temperature distribution becomes more uniformed. Further evaluation indicates that the optimal solution has good applicability and stability under different reactor conditions. A comparison of the optimization objective function using different temperature difference definitions is also studied in the current study.

THE EFFECTS OF SYSTEM PARAMETERS ON THE INCEPTION OF FLASHING USING RELAP5

Natural circulation systems (NCS) are prone to instabilities, they are more unstable compared to forced circulation systems as a result of the low driving head and nonlinearity of the system. Flashing instability is prominent in these systems at low pressure due to the presence of a long nonheated riser to enhance the driving force. RELAP5 code has been identified as one of the Best Estimate (BE) codes used in the analysis of the systems. The studies by different authors showed that the effects of system parameters on instability generally have gained the attention of researchers in recent years. However, these studies have not discussed how these parameters prevent or enhance the commencement of flashing instability. This we sought to achieve in this work, by zeroing in on the inception of flashing instability, by comparing what happened at different heat fluxes. The objective of this work is to investigate the effects of system parameters on the inception of flashing instability in a natural circulation system using the RELAP5 code. With the increase in inlet subcooling and height of the riser, flashing moved to the outlet of the riser, stabilizing the system. However, an opposite effect was observed with an increase in the inlet resistance.

The Effects of System Parameters on Inception of Flashing using Relap5

Natural circulation systems (NCS) are prone to instabilities, they are more unstable compared to forced circulation systems as a result of the low driving head and nonlinearity of the system. Flashing instability is prominent in these systems at low pressure due to the presence of a long nonheated riser to enhance the driving force. RELAP5 code has been identified as one of the Best Estimate (BE) codes used in the analysis of the systems. The studies by different authors showed that the effects of system parameters on instability generally have gained the attention of researchers in recent years. However, these studies have not discussed how these parameters prevent or enhance the commencement of flashing instability. This we sought to achieve in this work, by zeroing in on the inception of flashing instability, by comparing what happened at different heat fluxes. The objective of this work is to investigate the effects of system parameters on the inception of flashing instability in a natural circulation system using the RELAP5 code. With the increase in inlet subcooling and height of the riser, flashing moved to the outlet of the riser, stabilizing the system. However, an opposite effect was observed with an increase in the inlet resistance.

Research on multi-objective optimization method for flow distribution of natural circulation reactor during its life-cycle

A reasonable flow distribution can flatten the temperature distribution at the core outlet and reduce the temperature fluctuation, improving the economy and safety of the reactor. Different from a forced circulation reactor, the flow distribution of a natural circulation reactor is automatically adjusted according to the relationship between the fuel assembly power and inlet resistance. In this work, first, a local optimal flow distribution calculation model is constructed based on the closed parallel multi-channel model. Second, a Multi-objective comprehensive evaluation based on the optimal time zone is proposed to find an optimal flow distribution scheme that satisfies multiple requirements during the entire life-cycle. Third, the flow distribution scheme of a small lead-bismuth fast reactor with long life and natural circulation (SPALLER-100) is studied. The Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) algorithm is chosen as the evaluation method with three evaluating indicators for calculation. Ultimately, the subchannel code is selected to verify the flow distribution scheme. The analysis results show that the scheme based on the power distribution of 3182 days performs the best. Compared with the scheme designed by the beginning of life (BOL), the maximum outlet temperature difference reduced by 30 K, the standard deviation of the maximum outlet temperature difference reduced by 41%, and maximum output power increased by 2.35%. The results are in good agreement with the SUBCHANFLOW code, and the maximum relative error is less than 4.5%.

Theoretical analysis of the flow stability of HTGR supercritical steam generators using dimensionless numbers

High Temperature Gas-cooled Reactors (HTGR) have the advantages of inherent safety and high temperature. Due to the high temperature, adoption of supercritical steam generator will further increase the power generation efficiency of HTGR. Flow instability may happen in supercritical steam generators due to acute changes in physical properties near the critical region. A theoretical model is established, verified and adopted to analyze the flow stability of HTGR supercritical steam generators. Secondary side supercritical water is divided into three regions. Partial differential governing equations describing supercritical systems are simplified to linear ordinary differential equations using integral and small disturbance linearization. The variables of differential equations are inlet velocity, the length of region 1 and the total length of region 1 and region 2. Routh stability criterion is adopted to determine supercritical system stability. Algebraic criterion is obtained from the coefficient matrix using Routh stability criterion. Similar to subcritical system, supercritical system stability is determined by algebraic criterion expressed by six dimensionless numbers. They are the inlet resistance coefficient, outlet resistance coefficient, sub-pseudo-critical number (similar to subcooling number in subcritical system), true trans-pseudo-critical number (similar to phase change number in subcritical system), friction number and Froude number. The actual supercritical steam generators of HTGR belong to the system with high inlet resistance coefficients and high friction numbers. Supercritical steam generators of HTGR (with high inlet resistance coefficients) can operate stably and have enough safety margin at full power level and partial power levels. Supercritical steam generators (with high inlet resistance coefficients and high friction numbers) become stable when system pressure, the inverse of Froude number (or angle inclination) and tube inner diameter increase. They become unstable when friction number (or friction factor) and tube length increase. The reasonable application range of the figure expressed by true trans-pseudo-critical number (NTPC) and sub-pseudo-critical number (NSUBPC) is discussed when analyzing parameters’ effects. The conclusions about parameters’ effect in supercritical systems are the same as that in subcritical systems. Previous contradictory conclusions about some parameters’ effects (friction number, tube length and tube inner diameter) are explained. The main reason is that the effect of friction numbers on supercritical systems is different at different inlet resistance coefficients.

Ejector mode performance improvement of rocket-based combined cycle engine designed for dual-ramjet mode

Rocket-based combined cycle engine integrating multi-mode operation accomplishes an excellent performance under various flight speed conditions. Generally researchers selected dual-ramjet mode as the cruise and design point with adjusting heat addition method to improve the performance under other modes. To analyze the difficulties during the multi-mode design, a unilateral expansive flowpath through central rocket ejection was investigated to analyze the effects of the mixing ratio of oxygen and fuel and the mass flow rate of the primary rocket plume. The results indicate that the mixing ratio increase can significantly strengthen the entrainment amount. There is an optimal operating rate to obtain the highest entrainment ratio due to Fabri choking limit induced by a confined path width. However, the excessively divergent afterbody based on dual-ramjet results in the under-expansion issue of the exhaust gas and negative thrust of the nozzle. The multistage ejection method can effectively overcome this shortage by compressing the exhaust gas. But with the flight speed increasing, the existence of upstream bow shock induces high inlet resistance. Adding extra duct can effectively solve the problem by decreasing inlet convergent ratio and sucking more secondary flow for combustion.

A preliminary analysis on optimization of core flow distribution

Optimization of core flow distribution is closely related with thermal safety, and a good match between core flow distribution and power distribution in reactor core is helpful for the promotion of thermal-hydraulic design, especially for the reactor with natural circulation whose driving force is limited. Flow distribution can be contrived by the reasonable resistance arrangement of core inlet in reactor design. In this study, an automatic solution search algorithm on optimizing inlet resistance distribution of reactor core is designed and established, and also an automatic core flow distribution optimization code coupled with COBRAIIIC code is developed in place of the method by skill and experience of user. A natural circulation reactor is selected to be tested, and the results show that the coolant temperature difference of assembly channel outlets at a specific time point of fuel cycle life can be eliminated by optimizing the core flow distribution. Further, for the flow distribution optimization during fuel cycle life which could be regarded as a global optimization problem with range limits of independent variable, simulated annealing algorithm is applied to solve it. A good optimization result that the coolant temperature difference of the assembly flow channel outlets during a fuel cycle life is reduced effectively is obtained, which provides a support for further enhancement of reactor system performance.

РАЗРАБОТКА АЛГОРИТМА ПОИСКА НЕИСПРАВНОСТИ ПРИ УДАЛЕННОЙ ДИАГНОСТИКЕ

Remote diagnostics of vehicles is currently becoming increasingly widespread. The application of this concept will allow analyzing data from pre-installed sensors, without interfering with the vehicle design, doing it remotely and displaying only the necessary information to maintain the proper level of the vehicle’s technical condition. To develop an algorithm for remote diagnostics carried out during the operation of machinery without installing additional sensors on the engine, using diagnostic indicators, the retardation method, and monitoring fuel consumption rate, the authors carried out the engine diagnostics at the idling speed. To analyse the technical state of the engine and to test the developed algorithm, two faults were intentionally sequentially introduced: air inlet resistance (“Clogged fi lter”) and no fuel supply into the cylinder (“Faulty injector”). The developed algorithm aims at distinguishing faults in the internal combustion engine associated with mechanical losses and the deterioration of its technical and economic performance. An experimental installation on the basis of the diesel engine of internal combustion IVECO F4HE9687P*J101 was used for approbation of the developed algorithm. This installation can examine the operation of technically serviceable engines having intentionally introduced defects. The experimental results have proved the adequacy of the developed diagnostic algorithm and the feasibility of its introduction.

A Numerical Analysis of the Effect of Impeller Rounding on Centrifugal Pump as Turbine

Pump as turbine (PAT) is one of the micro-hydro system components. It can be used alone as a generator in remote areas without power supply, and in hydraulic networks instead of pressure-reducing valves. However, its hydraulic optimization still remains an open research problem. One of the optimization techniques is the rounding of the sharp edges at the blade periphery. Existing studies are mostly based on prototype experiments to obtain the optimization effect. In order to more intuitively analyze the influence of this structural optimization on the internal flow of PAT, this paper uses the CFD method to study the influence of the leading edge of the centrifugal pump blade and the fillet of the impeller on the turbine performance. By simulating the PAT performance at different flow rates and speeds, the internal hydraulic performance changes caused by the inverted circular blade are analyzed. Simulation results show that, under various operating conditions, the impeller inverted circle improves the efficiency of PAT to different degrees. At the speed of 1500 rpm, the efficiency is most obviously improved, which can reach 8.09%. Internal flow results show that the efficiency increases along with the decrease in impeller inlet resistance and the flow separation region in the impeller. This paper provides an effective method for studying the PAT hydraulic optimization problem.

The Flow Instability Analysis for Steam Generator for China Experimental Fast Reactor (CEFR)

Steam generator plays an important role in nuclear power plant, especially in a fast reactor. As a barrier to separate the high temperature sodium and high pressure water/steam, it is important to make sure the safety of the steam generator. For this purpose, it is necessary to guarantee that there is no flow instability happened in the tubes of the steam generator. In order to ensure that the steam generator is stable in all conditions, this paper uses the method of Khabenskii to calculate the minimum inlet resistance coefficient to prevent flow instability in China Experimental Fast Reactor (CEFR) steam generator. The needed inlet resistance coefficient calculated is 1001.2. In addition, this paper analysis the effect of mass flow rate, pressure, subcooling and thermal power on flow instability in steam generator of CEFR. The result demonstrates that the increase of flow rate, pressure and subcooling and the decrease of thermal power will stabilize the flow instability in CEFR steam generator.

Effects of Direct Contact Condensation on Flow Characteristics of Natural Circulation System at Low Pressure

Direct contact condensation (DCC) as a complex thermal-hydraulic phenomenon has been performed a great deal of research under forced flow conditions. It is an interesting challenge to study the phenomenon in a natural circulation system (NCS). In this paper, the flow behaviors along with DCC phenomena were experimentally investigated by visualization method in a NCS at low pressure. Moreover, the influence of initial parameters including heat flux, inlet temperature and resistance of the heated section on the NCS was analyzed in detail. The experimental results revealed that two types of DCC phenomena were found in the NCS. When Type I DCC occurs, the interface wave will be formed due to the hydraulic jump caused the reverse flow of subcooled water. Interface wave easily contributes to occurring Type II DCC due to the Kelvin-Helmholtz instability. In addition, the NCS tends to be more unstable with the increase in the heat flux or the inlet resistance. The influence of inlet temperature on the flow rate should comprehensively consider the changes of outlet parameters and subcooled degree of the steam and the subcooled water.

Study on Effects of Inlet Resistance on the Efficiency of Scroll Expander in Micro-Compressed Air Energy Storage System

As an important part of a micro-compressed air energy storage system, the scroll expander directly affects the efficiency of the whole energy storage system. The effects of resistance on the efficiency of scroll expander caused by inlet structure and size are discussed with theory analysis and experimental methods in this paper. Micro-compressed air energy storage system has aroused widespread attention because of its pollution-free, high flexibility, in the community, remote areas power supply. Comprehensive experimental work with the selections of different size and structure of the air inlet of the scroll expander was performed with the cutting angle of air inlet chamber of the scroll expander. The results of the experiments are discussed on how exergy efficiency and inlet flow of the scroll expander were affected resulting from the cutting angles dissected. The results show that a maximum value exists for exergy efficiency of the scroll expander. Therefore, the exergy efficiency of the scroll expander can be effectively improved by enlarging the air inlet port dimension and modifying the size of air chamber.

Research of two‐phase density wave instability in reactor core channels with rolling motion

In this study, two-phase density wave instability in parallel-twin rectangular channels was investigated with axially nonuniform heat profiles in the reactor core combined with static and rolling conditions. A parallel-channel thermal-hydraulic model was built using the method of two-phase homogeneous flow developed in previous work, while the drift-flux approach for void fraction and profile-fit model for subcooled boiling were implemented. Although the rolling condition was chosen as the typical motion in this work, the additional force caused by the motion with six degrees of freedom was derived. The theoretical analysis was performed based on the method of small power perturbation for parallel-twin rectangular channels. The flow oscillation caused by rolling was studied for different system parameters, including inlet resistance, exit resistance, pressure, and axial heating profile. The influence on flow instability of rolling parameters such as period, amplitude, and distance between channels was analyzed. The results showed that it would destabilize the system if a larger additional force was generated by rolling parameters. The influence of different axial heat profiles on flow instability was also studied under inlet-peaked, cosine-shape, and outlet-peaked heat fluxes. The coupling effect of rolling motion and axial nonuniform heating was finally studied. The stability boundaries under different conditions were compared to the inherent boundary under the static condition with uniform heating. The results indicated that the influence of nonuniform heating was more evident and should be paid more attention to.

Natural circulation two-phase flow characteristics in a rod bundle channel under stable and rolling motion conditions

Natural circulation two-phase flow characteristics are studied under stable and rolling motion conditions in a rod bundle channel. The mass quality, flow rate and frictional pressure drop variation results with increasing heat flux are given and the effects of system pressure and inlet subcooling are analyzed. The flow rate decline is observed under high mass quality conditions since the frictional pressure drop is large enough. The flow instability characteristics for different stages with increasing heat flux are given. The main reason for the flow instability characteristics variation is that the dominant factor changes from subcooled boiling to the pressure drop oscillation. The experimental results indicate that increasing the system pressure and the inlet resistance of the test section, including moving the compressible volume to a downstream position, can delay the onset of the flow instability. The time-average flow parameters under rolling motion conditions are mainly influenced by the stable mass quality and the rolling parameters. For high stable mass quality conditions, the time-average two-phase flow parameters decrease a little compared to stable results. With the stable mass quality decreasing, the time-average mass quality gradually increases with the rolling frequency. For further lower stable mass quality conditions, the time-average values are even larger than stable values. For the frictional resistance calculation, a few correlations are evaluated and recommended. Additionally, new correlations are fitted for stable and time-average two-phase frictional pressure drop prediction in high mass quality conditions.

Flow instability analysis of natural circulation under ERVC condition for SMR

In this work, the internal natural circulation flow of ERVC system in SMR is calculated by the thermal-hydraulic code RELAP5. Both the transient characteristics and the flow instability are analyzed in detail to reveal the flow mechanism. Firstly, to verify the applicability of RELAP5 models, multiple cases of steady-state natural circulation with low heating power are simulated by comparing with the REPEC experiment. Next, a sliced model of ERVC loop is built to obtain the transient phenomena of natural circulation. As the circulation loop has no cold source, the inlet temperature of the heating section will keep rising to transform the flow patterns into stable-oscillating-stable in turns. During this transition, the flow is dominated by the subcooled boiling of heating section, and the mass flux is about 60–160 kg/m2·s. Based on the features of flow oscillation, the unstable flow patterns can be further divided into low-frequency and high-frequency oscillations. Both of them belong to the density wave oscillation, and their trajectory diagrams can present the attributes of the limit cycle. Finally, in the plane formed with the inlet subcooling (horizontal axis) and heating power (vertical axis), the boundaries of flow instability are divided. The instability domain change with the effects of heating power, back pressure, inlet resistance coefficient, and rising section height is discussed. The results indicate that increasing the back pressure will compress the instability domain. Reducing the inlet resistance coefficient will shift the instability boundaries towards the direction of power increase. When lengthening the height of the rising section, the flow can be gradually dominated by the flashing with the decrease of inlet subcooling, which may also introduce another instability.

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.