Flow pattern transformation identification method for different channel types

Abstract

The invention discloses a flow pattern transformation identification method for different channel types, and relates to the technical field of gas-liquid two-phase flow, and the method comprises the following steps: setting identification marks for different channel types of a vertical circular tube, a rectangular channel and a rod bundle channel; designing flow pattern transformation models of different channels; and inputting the identification mark of the channel in the input card, determining the type of the selected channel, and matching different void shares to judge the flow pattern characteristics. According to the method, the flow pattern transition points in different channel types of a vertical circular tube, a rectangular channel and a rod bundle channel can be determined, the flow pattern characteristics can be accurately judged, the accuracy is high, and the operation is convenient.

Description

Technical Field

[0001] The present invention relates to the technical field of gas-liquid two-phase flow, and more specifically, to a method for identifying flow pattern transitions of different channel types. Background Art

[0002] When gas and liquid are mixed and flow together, the different physical properties of the two phases make the gas-liquid two-phase flow very different from single-phase flow, and the physical mechanisms of two-phase flow are more complex. The deformability of the gas-liquid two-phase interface, the relative motion between the phases, and the variability of the phase distribution make the constitutive equation of two-phase flow more complex, and also lead to the complexity of its flow calculation. Due to the interaction between the two phases, the gas-liquid interface is prone to deformation, forming different combined interfaces, which in turn constitute different flow patterns. Different flow patterns have different flow and heat transfer characteristics. In the thermal-hydraulic calculation of actual two-phase flow systems, usually under the condition of determined flow patterns, calculation formulas for flow and heat transfer are established. Therefore, the study of two-phase flow patterns is the basic basis for two-phase flow and resistance calculation, and determining the conditions for flow pattern transitions and establishing a flow pattern transition model are the basis for thermal-hydraulic calculation. In addition, the characteristics of gas-liquid two-phase flow are affected by many factors, such as system pressure, gas flow rate, liquid flow rate, temperature, void fraction, flow direction, physical properties of the working fluid, channel shape and size, local resistance components, etc.

[0003] Gas-liquid two-phase flow in vertical circular tubes, rectangular channels, and rod bundle channels widely exists in many industrial fields such as power engineering, nuclear energy, chemistry, aerospace, and atomic energy. The flow patterns in different channels are also different. It is difficult to clearly understand the flow pattern transitions, and accurate pressure drop and heat transfer calculations cannot be carried out. Therefore, how to determine the flow pattern transition points in different channel types and then accurately judge the flow pattern characteristics is a technical problem that needs to be solved urgently by those skilled in the art. Summary of the Invention

[0004] In view of this, the present invention provides a method for identifying flow pattern transitions of different channel types, which solves the problems existing in the background art.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] A method for identifying flow pattern transitions of different channel types includes the following steps:

[0007] Set identification marks for different channel types of vertical circular tubes, rectangular channels, and rod bundle channels;

[0008] Design a flow pattern transition model for different channels;

[0009] Input the identification mark of the channel in the input card to determine the selected channel type, and match different void fractions to determine the flow pattern characteristics.

[0010] Optionally, the flow patterns in the vertical circular tube include bubbly flow, slug flow, annular flow, and mist flow. The flow pattern transition model for the vertical circular tube is divided into the following different cases:

[0011] 1) For the flow pattern transition from bubbly flow to slug flow, considering the difference in void fraction corresponding to the flow pattern transition at different mass fluxes G, linear interpolation is used for the transition region, and the expression is:

[0012]

[0013] The lower limit of the void fraction for the flow pattern transition from bubbly flow to slug flow in the vertical circular tube is α BS = 0.1, and the upper limit of the void fraction is α BS = 0.3. The void fraction at the transition point is related to the tube diameter and independent of the flow rate;

[0014] A dimensionless size parameter D* is introduced to correct the tube diameter of the vertical circular tube. Linear interpolation is used for the transition region, and Eq. (2) is rewritten to establish the transition criterion for bubbly flow to slug flow in the vertical circular tube:

[0015] α BS = max[0.3min(1, 0.107D * ), 0.1] (4);

[0016] In the formula: α BS represents the void fraction for the flow pattern transition from bubbly flow to slug flow, D represents the tube diameter of the vertical circular tube, used to express that the minimum value is limited not to be lower than 0.001; Gm represents the two-phase flow mass flow rate, m represents mixture, g represents the acceleration due to gravity, ρf represents the liquid phase density, ρg represents the gas phase density, and σ represents the surface tension;

[0017] 2) For the flow pattern transition from slug flow to annular flow, the lower limit of the void fraction for the flow pattern transition from slug flow to annular flow in the vertical circular tube is α SA = 0.8, and the upper limit of the void fraction is α SA = 0.9. Based on the liquid film reverse flow model and the vapor core entrained droplet model, the relationship for the flow pattern transition from slug flow to annular flow is established as:

[0018]

[0019] Among them,

[0020]

[0021] Considering the need for the transition of the interfacial drag coefficient, a transition interval of 0.05 is set, that is, at the void fraction α SA - 0.05 and α SAPerform weighted transition of the interfacial drag coefficient in between;

[0022] In the formula: α SA represents the void fraction at which the flow pattern transitions from slug flow to annular flow, D represents the diameter of the vertical circular tube, represents the critical void fraction of reverse flow, represents the critical void fraction of entrainment, vg represents the specific volume of the gas phase, g represents the acceleration due to gravity, Δρ represents the two-phase density difference, ρg represents the gas-phase density, and σ represents the surface tension;

[0023] 3) For the flow pattern transition from annular flow to mist flow, use the void fraction α AM = 0.9999 as the transition point from annular flow to mist flow.

[0024] Optionally, the flow patterns in the rectangular channel include bubbly flow, slug flow, annular flow, and mist flow. The flow pattern transition model for the rectangular channel is divided into the following different cases:

[0025] 1) For the flow pattern transition from bubbly flow to slug flow, the void fraction at which the bubbly flow in the rectangular channel transitions to slug flow is α BS = 0.195, and the void fraction at the transition point is independent of the tube diameter and mass flow rate;

[0026] 2) For the flow pattern transition from slug flow to annular flow, the void fraction at which the slug flow in the rectangular channel transitions to annular flow is α SA = 0.77, and the void fraction at the transition point is independent of the tube diameter and mass flow rate;

[0027] Considering the need for interfacial drag coefficient transition, set a transition interval of 0.05, that is, perform weighted transition of the interfacial drag coefficient between the void fractions of 0.765 - 0.77;

[0028] 3) For the flow pattern transition from annular flow to mist flow, use the void fraction α AM = 0.9999 as the transition point from annular flow to mist flow.

[0029] Optionally, the flow patterns in the rod bundle channel include bubbly flow, cap flow, cap churn flow, annular flow, and mist flow. The flow pattern transition model for the rod bundle channel is divided into the following different cases:

[0030] 1) For the flow pattern transition from bubbly flow to cap flow, in the air-water two-phase flow in the rod bundle channel, under various flow velocity conditions, the void fraction at which the bubbly flow transitions to cap flow is α BC = 0.186, and the void fraction is independent of the mass velocity;

[0031] 2) For the flow pattern transition from slug flow to slug-churn flow, flow pattern transition models from slug flow to slug-churn flow are established respectively under high flow rate and low flow rate conditions. Linear interpolation is used for the transition region, and the expression is as follows:

[0032]

[0033] In the formula: α CT represents the void fraction at which the flow pattern transitions from slug flow to slug-churn flow, and Gm represents the mass flow rate of the two-phase flow;

[0034] 3) For the flow pattern transition from slug-churn flow to annular flow, the void fraction at which the flow pattern transitions from slug-churn flow to annular flow in the rod bundle channel is α TA = 0.8, and the void fraction at the transition point is independent of the mass flow rate;

[0035] Considering the need for the transition of the interphase drag coefficient, a transition interval of 0.05 is set, that is, the interphase drag coefficient is weighted and transitioned between the void fractions of 0.75 - 0.8;

[0036] 4) For the flow pattern transition from annular flow to mist flow, the void fraction α AM = 0.9999 is used as the transition point from annular flow to mist flow.

[0037] As can be seen from the above technical solutions, compared with the prior art, the present invention discloses and provides a method for identifying flow pattern transitions of different channel types, which can determine the flow pattern transition points in different channel types such as vertical circular tubes, rectangular channels, and rod bundle channels, and then accurately judge the flow pattern characteristics, with high accuracy and convenient operation. Brief Description of the Drawings

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained according to the provided drawings without creative efforts.

[0039] Figure 1 is a flowchart of a method for identifying flow pattern transitions of different channel types provided by the present invention;

[0040] Figure 2 is the α-G characteristic diagram of the flow pattern transition region of the 10 mm vertical circular tube provided by the present invention;

[0041] Figure 3 is the α-G characteristic diagram of the flow pattern transition region of the 25 mm vertical circular tube provided by the present invention;

[0042] Figure 4The α-D* characteristic diagram of the transition from bubbly flow to slug flow in the vertical circular tube provided by the present invention BS -D* characteristic diagram;

[0043] Figure 5 The α-G characteristic diagram of the flow pattern transition region in the 4mm rectangular channel provided by the present invention;

[0044] Figure 6 The α-G characteristic diagram of the flow pattern transition region in the 6mm rectangular channel provided by the present invention;

[0045] Figure 7 The α-G characteristic diagram of the flow pattern transition region in the rod bundle channel provided by the present invention;

[0046] Figure 8 The α-G characteristic diagram of the transition from cap flow to cap churn flow in the rod bundle channel provided by the present invention CT -G characteristic diagram. Specific embodiments

[0047] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0048] The embodiments of the present invention disclose a method for identifying flow pattern transitions of different channel types, as Figure 1 shown, including the following steps:

[0049] Set identification marks for different channel types of vertical circular tubes, rectangular channels, and rod bundle channels;

[0050] Design flow pattern transition models for different channels;

[0051] Input the identification mark of the channel in the input card to determine the selected channel type and match different void fractions to determine the flow pattern characteristics.

[0052] In this embodiment, through the Figure 1 process shown, the R & D design of the flow pattern diagrams of different channels can be understood, the flow pattern transition points in different channel types of vertical circular tubes, rectangular channels, and rod bundle channels can be determined, and thus the flow pattern characteristics can be accurately judged. Next, the specific content of the R & D design will be elaborated in detail to further understand the technical solution of the present invention.

[0053] I. Flow pattern transition model of vertical circular tube

[0054] 1.1 Flow pattern diagram of vertical circular tube

[0055] In the existing literature, the flow patterns in vertical circular pipes are usually defined as bubbly flow, slug flow, churn flow, annular flow, mist flow, etc. In the existing thermal-hydraulic system programs, the RELAP5 program collectively refers to slug flow and churn flow as slug flow, while the TRACE program first classifies the flow patterns into four types according to the flow state: bubbly flow, Taylor bubble / slug flow (Taylor bubble flow for large-sized channels and slug flow for small-sized channels), annular flow, and mist flow, and uses corresponding interfacial resistance models for calculation under different flow pattern conditions.

[0056] It can be seen that in the existing mainstream thermal-hydraulic system programs, slug flow and churn flow are often classified as the same flow pattern for unified treatment. This is because: on the one hand, the boundary between slug flow and churn flow is relatively blurred, and it is difficult to uniformly define a clear transition region; on the other hand, churn flow can be regarded as a more churned slug flow, and the mechanical characteristics of its flow phase interface are very similar to those of slug flow. Therefore, in the process of modeling the flow pattern transition mechanism for vertical circular pipes in this embodiment, the flow patterns are mainly divided into bubbly flow, slug flow, annular flow, and mist flow.

[0057] Considering the possible influence of the mass velocity G on the flow pattern transition, the α-G characteristics of the flow pattern transition regions of 10-mm vertical circular pipes and 25-mm vertical circular pipes are compared as Figure 2 、 Figure 3 shown. Based on this, it can be known that the mass velocity G has almost no obvious influence on the flow pattern transition of these two sizes of vertical circular pipes. It should be noted that the pipe diameter has a greater influence on the transition point from bubbly flow to slug flow.

[0058] 1.2 Bubbly flow - slug flow pattern transition model

[0059] The flow pattern transition model proposed by Taitel et al. based on circular pipes has been widely applied to various types of pipes. Taitel considered the influence of pipe diameter, flow velocity, etc. on the flow pattern transition, and the corresponding model has also been applied to the system program RELAP5. Taitel believes that under the conditions of medium-sized pipe diameters and low flow velocities, when the void fraction α = 0.25, violent interactions between bubbles will occur and then coalesce to form slug flow. When the void fraction α < 0.2, the coalescence phenomenon is not obvious, and when the void fraction α > 0.35, there is almost no bubbly flow. Taitel believes that strong turbulent effects will break the bubbles into spherical bubbles smaller than 2 mm. The spherical bubbles are less affected by the wake during the rising process, and the swing amplitude of the bubbles is also smaller. The probability of random collisions between bubbles decreases. Therefore, the transition from bubbly flow to slug flow will be delayed until the void fraction α = 0.52. Taitel further considered the influence of channel size on the flow pattern transition and believed that when the slug velocity in the channel is less than the velocity of the deformed bubbles, there will be no bubbly flow in the channel. From this, the critical size D of the pipe where bubbly flow does not occur under normal temperature and pressure is calculated to be approximately 60 mm.

[0060] In addition, Mishima and Ishii obtained the void fraction α = 0.3 at the transition point from bubbly flow to slug flow in a circular tube through the maximum bubble arrangement model. Other scholars such as Das and Rattanyak et al. proposed a quantity balance model to determine the flow pattern transition point, and Levy proposed that the interfacial concentration reaches the maximum value at the transition from bubbly flow to slug flow to determine the flow pattern transition point. However, these methods require a large number of closure relationships and have not been widely used at present.

[0061] Therefore, in this embodiment, for the flow pattern transition from bubbly flow to slug flow, the RELAP5 program considers the difference in void fraction corresponding to the flow pattern transition under different mass flow rates G, and uses linear interpolation for the transition region. The expression is:

[0062]

[0063] In addition to the RELAP5 program, the COBRA-TF program uses a simpler flow pattern transition model. Both of these systems consider that when the void fraction α = 0.2, the transition between bubbly flow and slug flow occurs.

[0064] Combined with Figure 2 and Figure 3 it can be seen that in the air-water two-phase flow in vertical circular tubes with diameters of 10 mm and 25 mm, bubbly flow generally exists under various liquid flow velocity conditions. As the mass flow rate increases, the void fraction at the transition point from bubbly flow to slug flow does not change significantly, but the tube diameter has a significant impact on the void fraction at this transition point. Combining with the existing experimental data, it is considered that the lower limit of the void fraction for the flow pattern transition from bubbly flow to slug flow in a vertical circular tube is α BS = 0.1, the upper limit of the void fraction is α BS = 0.3. The void fraction at the transition point is related to the tube diameter and has nothing to do with the flow rate;

[0065] Introduce a dimensionless size parameter D* to correct the tube diameter of the vertical circular tube, use linear interpolation for the transition region, rewrite Equation (2), and establish a transition criterion for bubbly flow to slug flow in a vertical circular tube:

[0066] α BS = max[0.3min(1,0.107D * ),0.1] (4);

[0067] In the formula: α BS represents the void fraction at the flow pattern transition from bubbly flow to slug flow, D represents the tube diameter of the vertical circular tube, It is used to express that the defined minimum value cannot be lower than 0.001; Gm represents the mass flow rate of two-phase flow, m represents mixture; g represents the acceleration of gravity, ρf represents the liquid-phase density, ρg represents the gas-phase density, and σ represents the surface tension;

[0068] Among them, when D = 10 mm, D* = 3.68, and when D = 25 mm, D* = 9.20. The void fraction α of the transition point from slug flow to churn flow in a vertical circular tube varies with the dimensionless tube diameter BS -D* characteristics are as Figure 4 shown.

[0069] 1.3 Slug Flow - Annular Flow Pattern Transition Model

[0070] Continuing the flow pattern diagram framework in the RELAP5 program, the slug flow and churn flow are unified into the slug flow pattern in flow pattern modeling. For the model of the transition from slug flow to annular flow, generally two mechanisms are proposed in the prior art to describe the occurrence of annular flow: one is the liquid film reverse flow mechanism, and the other is the gas core entraining liquid droplets mechanism. Wallis believed that according to the flooding phenomenon, when the downward liquid film on both sides of the gas slug in slug flow reverses, the transition from slug flow to annular flow will occur. At the same time, Hewitt and Wallis' experiments found that when the dimensionless superficial gas velocity is greater than or equal to 1, annular flow will be formed, and this relationship is also applied to the system program RELAP5. Ishii, by analyzing the churn flow model and the liquid film reverse phenomenon, also obtained the annular flow pattern transition model caused by the liquid film reverse flow mechanism. Ishii added the void fraction to correct this model. Taitel proposed a flow pattern transition model of the gas core entraining liquid droplets, believing that when the gas velocity is high enough, the gas acts on the liquid droplets suspended in the gas core so that they can flow upward stably without forming a liquid bridge, thus forming a stable annular flow. This model is also applied to the system program RELAP5.

[0071] The RELAP5 program mainly based on the liquid film reverse flow mechanism and the gas core entraining liquid droplets mechanism to establish a flow pattern transition model from slug flow to annular flow. However, the void fraction at the transition point calculated by this relationship has a large difference under various working conditions. While the TRACE and COBRA-TF programs adopt a simpler flow pattern transition model. Both of these two system programs believe that when the void fraction α = 0.8, the transition between slug flow and annular flow occurs.

[0072] Combined with Figure 2 , Figure 3 it can be known that in the air-water two-phase flow in a 10 mm and 25 mm vertical circular tube, the void fraction α at the transition point from slug flow to annular flow SAIt varies between 0.75 and 0.9, and the experimental results for different circular tube sizes also vary. This may be due to the experimental error caused by the relatively ambiguous slug flow to annular flow transition phenomenon itself. As the mass flow rate increases, the void fraction at the slug flow to annular flow transition point in the vertical circular tube also changes to a certain extent. Combining with the existing experimental data, it is considered that the lower limit of the void fraction for the slug flow to annular flow pattern transition in the vertical circular tube is α SA = 0.8, and the upper limit of the void fraction is α SA = 0.9. Based on the liquid film reverse flow model and the vapor core entrained droplet model, the relationship for the slug flow to annular flow pattern transition is established as:

[0073]

[0074] where,

[0075]

[0076] Considering the need for the transition of the interfacial drag coefficient, a transition interval of 0.05 is set, that is, the interfacial drag coefficient is weighted and transitioned between the void fraction α SA - 0.05 and α SA ;

[0077] In the formula: α SA represents the void fraction at the slug flow to annular flow pattern transition, D represents the diameter of the vertical circular tube, represents the critical void fraction of reverse flow, represents the critical void fraction of entrainment, vg represents the gas specific volume, g represents the acceleration due to gravity, Δρ represents the two-phase density difference, ρg represents the gas density, and σ represents the surface tension.

[0078] 1.4 Annular flow - mist flow pattern transition model

[0079] Under the condition of mist flow occurring, the void fraction in the vertical circular tube is often very large. Therefore, in this embodiment, for the annular flow - mist flow pattern transition, the void fraction α AM = 0.9999 is used as the transition point from annular flow to mist flow.

[0080] II. Flow pattern transition model for rectangular channels

[0081] 2.1 Flow pattern map for rectangular channels

[0082] The rectangular channel is also a conventional channel, and its flow pattern definition is basically the same as that of the circular tube channel. Therefore, in the process of modeling the flow pattern transition mechanism for the rectangular channel in this embodiment, the flow patterns are mainly divided into bubbly flow, slug flow, annular flow, and mist flow. Mishima et al. conducted experiments on the flow patterns in narrow rectangular channels, and Hibiki et al. proposed a flow pattern transition model applicable to narrow rectangular channels in combination with circular tubes.

[0083] Considering the influence of the possible mass flow rate G on the flow pattern transition, the α-G characteristics in the flow pattern transition regions of the 4-mm and 6-mm rectangular channels are compared as Figure 5 , Figure 6 shown. Based on this, it can be seen that the narrow slit gap and the mass flow rate G have no significant influence on the flow pattern transition of these two-sized rectangular channels.

[0084] 2.2 Bubbly Flow-Slug Flow Pattern Transition Model

[0085] Combined with Figure 5 , Figure 6 it can be known that in the air-water two-phase flow in the 4-mm and 6-mm rectangular channels, bubbly flow generally exists under various liquid phase flow rate conditions. The void fraction α BS = 0.195 at which the flow pattern transition from bubbly flow to slug flow occurs does not change significantly due to the channel size. Moreover, in the two test sections, as the mass flow rate increases, the void fraction at the transition point from bubbly flow to slug flow does not change significantly. The transition from bubbly flow to slug flow occurs at α BS = 0.195. This conclusion has little difference from the flow pattern diagrams proposed by other scholars and is almost the same as the bubbly-slug transition point α BS = 0.2 in the COBRA-TF program.

[0086] In the rectangular channel flow pattern experiment, although the rectangular channel has a certain restrictive effect on the bubble movement due to the existence of the narrow slit, the transition point from bubbly flow to slug flow obtained from the experiment does not advance significantly. This may be because although the bubble movement is restricted, the bubbles will move freely towards the wide side, and the narrow slit does not have an obvious promoting effect on the interaction between bubbles.

[0087] Combined with the existing experimental data, in this embodiment, it is considered that the void fraction at which the flow pattern transition from bubbly flow to slug flow occurs in the rectangular channel is α BS = 0.195, and the void fraction at the transition point has nothing to do with the pipe diameter and mass flow rate.

[0088] 2.3 Slug Flow-Anular Flow Pattern Transition Model

[0089] Combined with Figure 5 , Figure 6 it can be known that in the air-water two-phase flow in the 4-mm and 6-mm rectangular channels, the void fraction α SAIt varies between 0.75 and 0.79, and the experimental results for different pipe sizes show slight differences, which may be caused by experimental errors due to the relatively ambiguous slug flow to annular flow transition phenomenon itself. As the mass flow rate increases, the void fraction at the slug flow to annular flow transition point in the vertical circular pipe does not change significantly. There are large differences in the transition lines from bubbly flow to slug flow proposed by different scholars, mainly because the definitions of slug flow and annular flow are not unified. The experimental results of this study suggest that the slug flow to annular flow transition occurs at α SA = 0.77, which is almost the same as the slug flow to annular flow transition point α SA = 0.8 in the TRACE and COBRA-TF programs.

[0090] Therefore, combining the existing experimental data, this embodiment believes that for the flow pattern transition from slug flow to annular flow, the void fraction at which the flow pattern transition from slug flow to annular flow occurs in the rectangular channel is α SA = 0.77, and the void fraction at the transition point is independent of the pipe diameter and mass flow rate;

[0091] Considering the need for the transition of the interfacial drag coefficient, a transition interval of 0.05 is set, that is, the interfacial drag coefficient is weighted and transitioned between the void fractions of 0.765 - 0.77.

[0092] 2.4 Annular flow - mist flow transition model

[0093] Under the conditions of mist flow, the void fraction in the rectangular channel is often very large. Therefore, for the flow pattern transition from annular flow to mist flow, this embodiment uses the void fraction α AM = 0.9999 as the transition point from annular flow to mist flow.

[0094] III. Flow pattern transition model for rod bundle channels

[0095] 3.1 Flow pattern map of rod bundle channels

[0096] Compared with the conventional flow channel, the flow pattern in the rod bundle channel is more rod bundle-like and is affected by many parameters, such as the superficial gas velocity, superficial liquid velocity, physical properties of the two phases, slip ratio, channel size, spacer grid, etc. Different scholars have defined different flow patterns in the rod bundle channel. Williams et al. studied the two-phase flow patterns in a 1×4 heated rod bundle channel at 2.86 MPa, 8.37 MPa, and 13.89 MPa. According to the visualization research results, the two-phase flow patterns were divided into bubbly flow, foam flow, slug flow, and annular flow. Zhou et al. carried out a study on the steam-water two-phase flow patterns in a 3×3 rod bundle channel at normal temperature and pressure. Similarly, based on the visualization results, four flow patterns were defined: bubbly flow, bubbly churn flow, churn flow, and annular flow. Venkateswararao et al. conducted air-water experiments on a rod bundle channel with 24 rods (including 8 incomplete rods) in a circular casing and defined four flow patterns: bubbly flow, slug flow, churn flow, and annular flow. Harvel et al. also divided the flow patterns in the rod bundle channel into bubbly flow, slug flow, churn flow, and annular flow. Mizutani et al. first realized the visualization of the flow patterns in the rod bundle sub-channel using a high-speed camera, optical fiber probe, FEP tube, etc., and defined three flow patterns: bubbly flow, churn flow, and annular flow, as well as a transitional flow pattern. Paranjape et al. obtained the flow pattern map in an 8×8 rod bundle channel based on the void fraction obtained from a flat-plate void meter and a neural network algorithm, and divided the flow patterns into bubbly flow, cap flow, cap churn flow, churn flow, and annular flow.

[0097] Although the names of the flow patterns in the rod bundle channel defined by each researcher are slightly different, they all include three flow patterns: bubbly flow, churn flow, and annular flow. The main difference lies in the definition of the transition region from bubbly flow to churn flow. Williams et al., Venkateswararao et al., and Harvel et al. defined it as slug flow, while Paranjape et al. defined it as cap flow and cap churn flow. Due to the interfacial instability of large bubbles and the existence of geometric obstacles, it is difficult to have stable slug bubbles in the rod bundle channel. Therefore, Liu et al. also adopted the flow pattern classification and definition of Paranjape et al. for the rod bundle channel and defined this region as cap flow and cap churn flow.

[0098] As Figure 7 shown, the α-G characteristic curve of the flow pattern transition region in the rod bundle channel is given. It can be seen that the mass velocity G has a certain influence on the flow pattern transition in the rod bundle channel.

[0099] 3.2 Bubbly Flow-Cap Flow Pattern Transition Model

[0100] With the increase of gas-phase flow velocity, small bubbles will gradually coalesce into cap-shaped bubbles, which may occupy a complete sub-channel and deform under the confinement of four rods. Due to the surface instability of large bubbles, small vapor bubbles do not form slug flow that occupies the entire channel cross-section in a conventional channel. Therefore, it can be considered that cap flow is the flow of deformed bubbles and deformed cap-shaped bubbles in a relatively stable manner in the continuous liquid phase. Different scholars have different flow pattern transition lines for the transition from bubbly flow to cap flow in rod bundles. Venkateswararao et al. developed a flow pattern transition model applicable to rod bundles based on the Taitel circular tube model and verified it with experimental results, and considered that the critical void fraction for the transition from bubbly flow to cap flow is 0.25. The Liu-Hibiki model believes that the critical void fraction for the transition from bubbly flow to cap flow will vary with the change of the gas-liquid density ratio.

[0101] Combined with Figure 7 It can be seen that for the flow pattern transition from bubbly flow to cap flow, in the air-water two-phase flow in rod bundles, under various flow velocity conditions, the void fraction for the flow pattern transition from bubbly flow to cap flow is α BC = 0.186, and the void fraction is independent of the mass flow rate.

[0102] 3.3 Cap flow - Cap churn flow pattern transition model

[0103] As the gas-phase flow rate continues to increase, the cap-like bubbles further increase, occupy more sub-channels, and even occupy all sub-channels, and the bubble length is also longer, similar to Taylor bubbles in a circular tube. As the gas-phase flow velocity further increases, the lateral migration between sub-channels is enhanced, and the cap-like bubbles no longer maintain stability. At the same time, in cap churn flow, large bubbles are difficult to occupy the entire flow channel, and there are often dispersed small bubbles on one side or around them, and the bubble motion is relatively disordered and unstable. Therefore, cap churn flow is significantly different from cap flow, and the change rules of the cross-sectional concentration of the two are also significantly different. Therefore, many scholars have defined the cap churn flow pattern.

[0104] It was first proposed by Schlegel et al. that the transition from cap flow to cap churn flow occurs when the void fraction is 0.51, and the flow pattern transition model established by Liu-Hibiki also adopted this criterion. Venkateswararao et al. believe that the void fraction for the transition from slug flow to churn flow is 0.335. In the air-water two-phase flow in rod bundles, in this experiment, the average void fraction α at the transition point from cap flow to cap churn flow at each flow rate measured by the sub-channel void meter CT = 0.39. Further combined with the change of the interface concentration measured by the probe, it is considered that when the mass flow rate is less than 2000 kg / (m2·s), the transition point α from cap flow to cap churn flow CT= 0.35; when the flow rate is greater than 2000 kg / (m2·s), there is no cap-shaped swirling flow within the measurement operating range, and the transition from the cap-shaped flow to the cap-shaped swirling flow is delayed. According to models such as Taitel, Schlegel, and Liu-Hibiki, it can be considered that when the flow rate is relatively large, the void fraction at the flow pattern transition from the cap-shaped flow to the cap-shaped swirling flow is α CT = 0.51.

[0105] Therefore, for the flow pattern transition from the cap-shaped flow to the cap-shaped swirling flow, in this embodiment, flow pattern transition models from the cap-shaped flow to the cap-shaped swirling flow are established respectively under two conditions of high flow rate and low flow rate, as Figure 8 shown, linear interpolation is used for the transition region, and the expression is as follows:

[0106]

[0107] In the formula: α CT represents the void fraction at the flow pattern transition from the cap-shaped flow to the cap-shaped swirling flow, and Gm represents the mass flow rate of the two-phase flow;

[0108] In the RELAP5 flow pattern map framework, there is no specific flow pattern option for the cap-shaped flow and the cap-shaped swirling flow in the rod bundle channel. Combining with the framework of the interfacial resistance model, the transition from the cap-shaped flow to the cap-shaped swirling flow is equivalent to the transition from the bubbly flow to the slug flow in a conventional channel. Therefore, the α CT in the rod bundle channel is used as the α BS in the conventional channel in the RELAP5 flow pattern map framework.

[0109] 3.4 Flow Pattern Transition Model from Cap-shaped Swirling Flow to Annular Flow

[0110] Combining the TRACE and COBRA-TF programs, for the flow pattern transition from the cap-shaped swirling flow to the annular flow, in this embodiment, it is considered that the void fraction at the flow pattern transition from the cap-shaped swirling flow to the annular flow in the rod bundle channel is α TA = 0.8, and the void fraction at the transition point is independent of the mass flow rate;

[0111] Considering the need for the transition of the interfacial resistance coefficient, a transition interval of 0.05 is set, that is, the interfacial resistance coefficient is weighted and transitioned between the void fractions of 0.75 - 0.8.

[0112] 3.5 Flow Pattern Transition Model from Annular Flow to Mist Flow

[0113] Under the conditions of the occurrence of mist flow, the void fraction in the rod bundle channel is often very large. Therefore, for the flow pattern transition from the annular flow to the mist flow, in this embodiment, the void fraction α AM = 0.9999 is used as the transition point from the annular flow to the mist flow.

[0114] In summary, this embodiment describes the research and development design of different channel flow pattern models, which is applicable to the development, testing, and maintenance of different channel flow pattern models in software autonomy projects. The research and development of different channel flow pattern models is mainly used to identify different channel types of vertical circular tubes, rectangular channels, and rod bundle channels, and then determine the flow pattern transition points between bubbly flow - slug flow and slug flow - annular flow in different channels.

[0115] Based on the flow pattern experimental results of two sizes of vertical circular tubes (inner diameters of 10 mm and 25 mm), two sizes of rectangular channels (4 mm and 6 mm), and a rod bundle channel (5×5 rod bundle with mixing wing spacer grids) in this embodiment, and combined with the flow pattern transition models proposed by other researchers and the flow pattern diagrams in the mainstream thermal - hydraulic system programs RELAP5 and COBRA - TF, a flow pattern transition model for different channels is established. The flow pattern transition points in different channel types of vertical circular tubes, rectangular channels, and rod bundle channels can be determined, and then the flow pattern characteristics can be accurately judged, with high accuracy and convenient operation.

[0116] The embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. For the same or similar parts among the embodiments, reference can be made to each other.

[0117] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for identifying flow pattern transitions of different channel types, characterized in that: The following steps are involved: Set identification marks for different channel types such as vertical circular tubes, rectangular channels, and rod bundle channels; Design flow pattern transition models for different channels; Enter the channel identification in the input card, determine the selected channel type, and match different cavitation fractions to determine the flow pattern characteristics.

2. The method for identifying flow pattern transitions of different channel types according to claim 1, characterized in that: The flow patterns of a vertical circular tube include bubbly flow, slug flow, annular flow and mist flow. The flow pattern transition model of a vertical circular tube is divided into the following different cases: 1) For the flow pattern transition from bubbly flow to slug flow, considering the difference in cavitation fraction corresponding to the flow pattern transition under different mass flow rates G, linear interpolation is used for the transition region, and the expression is: The lower limit of the cavitation fraction for the transition from bubbly flow to slug flow in a vertical circular tube is α BS =0.1, the upper limit of cavitation fraction is α BS =0.3, the cavitation fraction at the transition point is related to the pipe diameter and has nothing to do with the flow rate; The dimensionless size parameter D* is introduced to correct the diameter of the vertical circular tube, and linear interpolation is used in the transition region. Formula (2) is rewritten to establish the transition criterion from bubbling flow to slug flow in the vertical circular tube: a BS =max[0.3min(1,0.107D * ),0.1] (4); Where: α BS represents the cavitation fraction at which the bubbling flow changes to the slug flow, D represents the diameter of the vertical circular tube, It is used to express that the minimum value cannot be less than 0.001; Gm represents the mass flow rate of two-phase flow, m represents mixture; g represents the acceleration of gravity, ρf represents the liquid phase density, ρg represents the gas phase density, and σ represents the surface tension; 2) For the flow pattern transition from slug flow to annular flow, the lower limit of the cavitation fraction for the flow pattern transition from slug flow to annular flow in a vertical circular tube is α SA =0.8, the upper limit of cavitation fraction is α SA =0.9, based on the liquid film backflow model and the steam core entrained droplet model, the relationship between the transition from slug flow to annular flow is established as follows: in, Considering the transition needs of the phase resistance coefficient, a transition interval of 0.05 is set, that is, in the cavitation fraction α SA -0.05 and α SA Perform weighted transition of phase resistance coefficient between phases; Where: α SA represents the cavitation fraction of the flow pattern transition from slug flow to annular flow, D represents the diameter of the vertical circular tube, represents the critical cavitation fraction of backflow, represents the entrainment critical cavitation fraction, vg represents the gas phase volume, g represents the gravitational acceleration, Δρ represents the density difference between the two phases, ρg represents the gas phase density, and σ represents the surface tension; 3) For the flow pattern transition from annular flow to mist flow, the cavitation fraction α is used AM =0.9999 is the transition point from annular flow to mist flow.

3. The method for identifying flow pattern transitions of different channel types according to claim 1, characterized in that: The flow patterns of rectangular channels include bubbly flow, slug flow, annular flow and mist flow. The flow pattern transition model of rectangular channels is divided into the following different cases: 1) For the flow pattern transition from bubbly flow to slug flow, the cavitation fraction of the flow pattern transition from bubbly flow to slug flow in a rectangular channel is α BS =0.195, the cavitation fraction at the transition point has nothing to do with the tube diameter and mass flow rate; 2) For the flow pattern transition from slug flow to annular flow, the cavitation fraction of the flow pattern transition from slug flow to annular flow in the rectangular channel is α SA =0.77, the cavitation fraction at the transition point has nothing to do with the tube diameter and mass flow rate; Considering the transition needs of the interphase resistance coefficient, a transition interval of 0.05 is set, that is, the interphase resistance coefficient weighted transition is performed between the cavitation fraction of 0.765-0.77; 3) For the flow pattern transition from annular flow to mist flow, the cavitation fraction α is used AM =0.9999 is the transition point from annular flow to mist flow.

4. The method for identifying flow pattern transitions of different channel types according to claim 1, characterized in that: The flow patterns of the rod bundle channel include bubbly flow, cap flow, cap-shaped mixed flow, annular flow and mist flow. The flow pattern transition model of the rod bundle channel is divided into the following different cases: 1) Regarding the flow pattern transition from bubbly flow to cap-shaped flow, in the air-water two-phase flow in the rod bundle channel, under various flow velocity conditions, the cavitation fraction of the flow pattern transition from bubbly flow to cap-shaped flow is α BC =0.186, and the cavitation fraction is independent of the mass flow rate; 2) For the flow pattern transition from cap-shaped flow to cap-shaped mixed flow, the flow pattern transition model from cap-shaped flow to cap-shaped mixed flow is established under high flow velocity and low flow velocity respectively, and linear interpolation is used for the transition area. The expression is as follows: Where: α CT It indicates the cavitation fraction of the flow pattern transition from cap-shaped flow to cap-shaped mixed flow, and Gm indicates the mass flow rate of the two-phase flow; 3) For the flow pattern transition from cap-shaped mixed flow to annular flow, the cavitation fraction of the flow pattern transition from cap-shaped mixed flow to annular flow in the rod bundle channel is α TA =0.8, the cavitation fraction at the transition point has nothing to do with the mass flow rate; Considering the transition needs of the interphase resistance coefficient, a transition interval of 0.05 is set, that is, the interphase resistance coefficient weighted transition is performed between cavitation fraction 0.75-0.8; 4) For the flow pattern transition from annular flow to mist flow, the cavitation fraction α is used AM =0.9999 is the transition point from annular flow to mist flow.

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