Microchannel slug flow boiling heat transfer theoretical calculation method

Abstract

The invention provides a micro-channel slug flow boiling heat transfer theoretical calculation method, which is characterized in that a model innovatively divides a flow boiling process in a micro-channel into five areas, namely a bubble free growth area, a liquid slug area, a lengthened bubble area, a partial drying area and a complete drying area. In the free growth area, the model particularly introduces a free growth mechanism of bubbles starting from an extremely small size, and the mechanism allows the bubbles to freely expand according to the energy balance principle under the condition of not being influenced by external force until the bubbles reach physical limitation in the length or width direction of the micro-channel. Through refined simulation of the stage, the model can more truly reflect the morphological change and speed distribution of the bubbles in the growth process, and then a theoretical calculation basis is provided for a heat transfer module. The prediction capability of the model under the low steam quality condition is enhanced and the calculation precision is improved by considering the bubble free growth stage, the evaporation and shearing dual effects during liquid film thinning and the bubble flow and heat transfer change during the low capillary number.

Description

Technical Field

[0001] The invention relates to the field of heat theory calculation, in particular to a microchannel slug flow boiling heat transfer theory calculation method. Background Art

[0002] At present, the academic community generally tends to adopt a mechanism model when theoretically evaluating the boiling heat transfer characteristics of two-phase flow in microchannels. The core of this model is to predict the flow boiling heat transfer coefficient by simulating the real geometric configuration of the gas-liquid interface. Compared with the correlation model directly based on experimental data fitting, this method shows more significant advantages in applicability and accuracy. However, it is worth noting that the traditional modeling framework often uses micro-circular tubes as the basis for thermodynamic analysis. This setting ignores the diversity of actual application scenarios to a certain extent, especially in those channels with sharp corners such as square tubes, which are more common in actual applications.

[0003] Furthermore, existing models often simplify the bubble shape to an ideal circle when constructing it, and assume that this shape remains unchanged during the flow process. However, this simplification ignores the important influence of capillary forces on the initial bubble morphology, especially under conditions of extremely low steam quality, the actual shape of the bubble will deviate significantly from the circle. This neglect has led to severe limitations on the accuracy of the model in predicting related phenomena. In addition, the model also fails to consider the influence of steam shear force on the dynamic changes of the liquid film when dealing with the gradual thinning of the liquid film around the bubble. This omission also limits the model's predictive ability under complex flow conditions. Summary of the invention

[0004] The main purpose of the present invention is to provide a microchannel slug flow boiling heat transfer theoretical calculation method that effectively improves the calculation accuracy and has flexibility; at the same time, by comprehensively considering multiple physical factors and their interactions, the model shows stronger adaptability and stability when facing different working conditions, material properties and boundary conditions.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a microchannel slug flow boiling heat transfer theoretical calculation method, S1, based on steady flow, the calculation is divided into two stages: bubble growth module and heat transfer module, and the flow boiling process in the microchannel is divided into five key areas, namely, bubble free growth area, liquid plug area, elongated bubble area, partial drying area and complete drying area; S2. In the bubble free growth zone, a free growth mechanism of bubbles starting from a very small size is introduced, and bubbles expand freely according to the energy balance principle until they reach physical limits in the length or width direction of the microchannel; S3. When the bubble grows to the turning point where the length or width of the microchannel is limited, a bubble shape classification mechanism based on the capillary number Ca is introduced. When the capillary number Ca is low, the bubble interface presents a rectangular shape with rounded corners; if the capillary number Ca gradually increases, the bubble interface shape is circular; S4. In the bubble elongation region, the dual effects of liquid evaporation and steam shear force are comprehensively considered, and this detailed model is used to estimate the time required for the liquid film to go from wetting to drying.

[0006] In the preferred embodiment, the specific steps S1-S4 are: A1. Based on the fluid property data and microchannel hydraulic diameter in the NIST database , Liquid mass flow , heat flux and steam mass fraction at the measuring point As the initial value of the calculation; A1. The calculation framework is divided into a bubble growth module and a heat transfer module: Bubble growth module: establishes the bubble generation time constant based on the input parameters , the initial velocity of the bubble at the time of confinement is solved by the energy balance equation of bubble growth , combined with the fluid reaching saturation, that is The liquid velocity at the time is based on the actual measurement. Calculate the bubble velocity at the measuring point , and determine the passage time of the gas slug and the liquid slug and ; A2. Heat transfer module: introduction of capillary number As the basis for judging the bubble interface morphology, the critical value is used. as a benchmark; when When: the interface morphology is considered as a rectangle with rounded corners, and the initial film thickness is Determined based on the relationship between dimensionless film thickness and Ca; Based on the evaporation heat balance and the liquid flow continuity equation under shear, the thinning rate of the liquid film dδ / dt is calculated until the liquid film is completely evaporated, and the drying time t of the liquid film in the center of the elongated bubble area is obtained. pd,dry,L ; when When: the interface is considered circular and the calculation ; The time from when the liquid film is completely dry to when the liquid in the corner of the microchannel is completely dry Directly solve the heat balance equation; A3. By increasing the gas slug area residence time Compared with the above two types of drying time Compare and precisely divide the time periods of gas presence areas, including extending the bubble zone time , Partial drying time , Complete drying time , The slug zone time is the previously calculated liquid slug zone ; A4. Using the local heat transfer coefficient , where the wall temperature and the saturation temperature are represented according to the geometric configuration of the liquid present in the cross-section, solve the transient heat transfer performance of each region, and then obtain the average heat transfer coefficient of each region by integrating over time; A5. Finally, the average heat transfer coefficient at the time of model output , which is the time weight of the average heat transfer coefficient of each region.

[0007] In the preferred embodiment, the calculation method of step A1 is as follows: In the bubble growth module, the bubble generation time constant , the initial bubble velocity , the bubble velocity at the measurement point , the occupation times of the gas and liquid regions are respectively and .

[0008] In the preferred embodiment, the calculation method of step A2 is as follows: Introduce the capillary number as the criterion for judging the bubble interface morphology, and adopt the critical value Cac = 0.03 as the benchmark; When Ca < Cac, the interface morphology is regarded as a rectangle with rounded corners, and the initial film thickness δ0 is determined according to the relationship between the dimensionless film thickness and Ca; When Ca > Cac, the interface is regarded as a circle, and the time for the liquid film to be completely dried is calculated accordingly. The technical method is as follows: When , the initial liquid film thickness , the total change rate of the liquid film thickness with time is ; When , the initial liquid film thickness , the total change rate of the liquid film thickness with time is , use MATLAB to solve the value of to obtain , and calculate the time required for the liquid film at the corner to be completely dried .

[0009] In the preferred embodiment, the calculation method of step A4 is as follows: The calculation method of the average heat transfer coefficient of each region is as follows: The heat transfer coefficient of the slug zone , where the microchannel is heated on three sides, and the Nusselt number Nu is based on the aspect ratio of the microchannel Calculate, where the Nusselt number Nu is expressed as: ; The local heat transfer coefficient of the elongated bubble area is calculated according to different interface conditions. When the interface is a rounded rectangular interface, , when the circular interface is ; Calculation of heat transfer coefficient of liquid at the corner of the elongated bubble area: , the local heat transfer coefficient of the elongated bubble region for , the average heat transfer coefficient of the elongated bubble region is expressed as: ; Local heat transfer coefficient of liquid distribution geometry in local dry zone for , the average heat transfer coefficient of the local drying area is expressed as: ; Heat transfer coefficient of complete drying zone ; Calculate the time-averaged heat transfer coefficient at a specific position of the square microchannel after one cycle of flow ; The model can output the time-averaged heat transfer coefficient h, which is the time weight of the average heat transfer coefficient of each zone; Calculating prediction error .

[0010] In the preferred embodiment, when the capillary number of the fluid at the measuring point is greater than 0.03, the bubble is a circular interface with the following characteristics: The calculation formula for the initial liquid film thickness is: .

[0011] The total rate of change of liquid film thickness over time is: ; From this, it can be calculated that when the liquid film is completely dry size; The local heat transfer coefficient of the elongated bubble region is: ; The other calculation methods and calculation procedures are the same as the above calculation when the capillary number is less than or equal to 0.029.

[0012] The present invention provides a theoretical calculation method for microchannel slug flow boiling heat transfer, taking into account bubble growth and confinement: the model adds bubbles from free growth to confinement stage, and carefully estimates the initial bubble velocity, making the bubble dynamics simulation more accurate, providing a theoretical calculation basis for the heat transfer module.

[0013] Detailed description of low capillary number bubble shape: For low capillary number conditions, the model takes special account of the change in bubble shape. By introducing the actual geometric configuration of the liquid, the bubble morphology is dynamically adjusted to more realistically reflect the bubble behavior in actual flow.

[0014] Full integration of shear effect: The model incorporates the influence of fluid shear force on the thinning process of bubble liquid film into the core calculation framework. By introducing the analysis of shear force terms, the influence of shear effect on liquid film thickness and heat transfer efficiency is accurately simulated, thereby improving the applicability of the model under complex flow conditions.

[0015] The model can also be improved on the basis of the existing four-zone model and modified according to the present model. The required changes are to add the judgment of the capillary number Ca and add the above-mentioned parts to the corresponding steps. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be further described below in conjunction with the accompanying drawings and embodiments: Figure 1 It is the calculation flow chart of the new model of the present invention; Figure 2 These are two bubble interface shapes that appear when the bubbles are restricted in the present invention; Figure 3 It is a schematic diagram of judging each area of ​​the present invention; Figure 4 is a diagram showing the thinning change of the liquid film in an embodiment of the present invention; Figure 5 It is the cross-section mark of the elongated bubble region of the present invention when the capillary number is low; Figure 6 It is a cross-sectional marking diagram of a portion of the drying area of ​​the present invention.

[0017] In the figure: DETAILED DESCRIPTION Example 1 like Figures 1 to 6 As shown, a microchannel slug flow boiling heat transfer theoretical calculation method, S1, based on steady flow, the calculation is divided into two stages: bubble growth module and heat transfer module, and the flow boiling process in the microchannel is divided into five key areas, namely, bubble free growth area, liquid plug area, elongated bubble area, partial drying area and complete drying area; S2. In the bubble free growth zone, a free growth mechanism of bubbles starting from a very small size is introduced, and bubbles expand freely according to the energy balance principle until they reach physical limits in the length or width direction of the microchannel; S3. When the bubble grows to the turning point where the length or width of the microchannel is limited, a bubble shape classification mechanism based on the capillary number Ca is introduced. When the capillary number Ca is low, the bubble interface presents a rectangular shape with rounded corners; if the capillary number Ca gradually increases, the bubble interface shape is circular; S4. In the bubble elongation region, the dual effects of liquid evaporation and steam shear force are comprehensively considered, and this detailed model is used to estimate the time required for the liquid film to go from wetting to drying.

[0018] In the preferred embodiment, the specific steps S1-S4 are: A1. Based on the fluid property data and microchannel hydraulic diameter in the NIST database , Liquid mass flow , heat flux and steam mass fraction at the measuring point As the initial value of the calculation; A1. The calculation framework is divided into a bubble growth module and a heat transfer module: Bubble growth module: establishes the bubble generation time constant based on the input parameters , the initial velocity of the bubble at the time of confinement is solved by the energy balance equation of bubble growth , combined with the fluid reaching saturation, that is The liquid velocity at the time is based on the actual measurement. Calculate the bubble velocity at the measuring point , and determine the passage time of the gas slug and the liquid slug and ; A2. Heat transfer module: introduction of capillary number As the basis for judging the bubble interface morphology, the critical value is used. as a benchmark; when When: the interface morphology is considered as a rectangle with rounded corners, and the initial film thickness Determined based on the relationship between dimensionless film thickness and Ca; Based on the evaporation heat balance and the liquid flow continuity equation under shear, the thinning rate of the liquid film dδ / dt is calculated until the liquid film is completely evaporated, and the drying time t of the liquid film in the center of the elongated bubble area is obtained. pd,dry,L ; when When: the interface is considered circular and the calculation ; The time from when the liquid film is completely dry to when the liquid in the corner of the microchannel is completely dry Directly solve the heat balance equation; A3. By increasing the gas slug area residence time Compared with the above two types of drying time Compare and precisely divide the time periods of gas presence areas, including extending the bubble zone time , Partial drying time , Complete drying time , the liquid slug time is the liquid slug area calculated previously ; A4. Utilize the local heat transfer coefficient , where the wall temperature and the saturation temperature are represented according to the geometric configuration of the liquid present in the cross-section. Solve the transient heat transfer performance of each region, and then obtain the average heat transfer coefficient of each region by integrating over time; A5. Finally, when the model outputs the average heat transfer coefficient , this is the time weight of the average heat transfer coefficient of each region.

[0019] In the preferred solution, the calculation method in step A1 is as follows: In the bubble growth module, the bubble generation time constant , the initial bubble velocity , the bubble velocity at the measurement point , and the occupation times of the gas and liquid regions are respectively and .

[0020] Where: : The bubble generation time constant. : The characteristic dimension of the microchannel. : The latent heat of vaporization at the gas-liquid interface. : The vapor density. : The pressure. : The heat flux density.

[0021] : The initial bubble velocity. : The parameter related to the mass flow rate. : The liquid density. : The bubble velocity at the measurement point in the microchannel.

[0022] In the preferred solution, the calculation method in step A2 is as follows: Introduce the capillary number as the criterion for judging the bubble interface morphology, with a critical value of Cac = 0.03 as the benchmark; When Ca < Cac, the interface morphology is regarded as a rectangle with rounded corners, and the initial film thickness δ0 is determined according to the relationship between the dimensionless film thickness and Ca; When Ca > Cac, the interface is regarded as circular, and the time when the liquid film is completely dried is calculated accordingly. The technical method is as follows: When , the initial liquid film thickness , and the total change rate of the liquid film thickness with time is ; When , the initial liquid film thickness , and the total change rate of the liquid film thickness with time is . Use MATLAB to solve the value of when, and obtain , and calculate the time required for the liquid film at the corner to completely dry .

[0023] Where Ca is the capillary number, is the dynamic viscosity of the liquid, Surface tension, is the liquid film thickness, is a time related to the drying of the liquid film, : The time required for the liquid film at the corner to completely dry.

[0024] In the preferred embodiment, the calculation method of step A4 is: the calculation method of the average heat transfer coefficient of each zone is: Heat transfer coefficient of liquid plug area , where the microchannel is heated on three sides, and the Nusselt number Nu is calculated based on the aspect ratio of the microchannel. Calculate, where the Nusselt number Nu is expressed as: ; The local heat transfer coefficient of the elongated bubble area is calculated according to different interface conditions. When the interface is a rounded rectangular interface, , when the circular interface is ; Calculation of heat transfer coefficient of liquid at the corner of the elongated bubble area: , the local heat transfer coefficient of the elongated bubble region for , the average heat transfer coefficient of the elongated bubble region is expressed as: ; Local heat transfer coefficient of liquid distribution geometry in local dry zone for , the average heat transfer coefficient of the local drying area is expressed as: ; Heat transfer coefficient of complete drying zone ; Calculate the time-averaged heat transfer coefficient at a specific position of the square microchannel after one cycle of flow ; The model can output the time-averaged heat transfer coefficient h, which is the time weight of the average heat transfer coefficient of each zone; Calculating prediction error .

[0025] in, : Heat transfer coefficient of liquid plug area, : thermal conductivity of liquid, $Nu$: Nusselt number, : Microchannel hydraulic diameter, : microchannel height-to-width ratio (aspect ratio), : The local heat transfer coefficient of the elongated bubble zone changes with time, , : Time variation of the length parameter associated with the liquid at the corner of the elongated bubble region.

[0026] , : The heat transfer coefficient of the liquid at the corner of the elongated bubble zone changes with time.

[0027] : The heat transfer coefficient of the liquid film changes with time. : The average heat transfer coefficient of the elongated bubble region.

[0028] : The local heat transfer coefficient of the liquid distribution geometry in the local dry zone varies with time. , : The length parameter associated with the local dry zone varies with time.

[0029] : The heat transfer coefficient of the drying zone changes with time. : The average heat transfer coefficient of the local drying area.

[0030] : Heat transfer coefficient of completely dry zone. : Thermal conductivity of vapor.

[0031] : The time-averaged heat transfer coefficient calculated after one cycle of flow at a specific position of the square microchannel.

[0032] In the preferred embodiment, when the capillary number of the fluid at the measuring point is greater than 0.03, the bubble is a circular interface with the following characteristics: The calculation formula for the initial liquid film thickness is: .

[0033] The total rate of change of liquid film thickness over time is: ; From this, it can be calculated that when the liquid film is completely dry size; The local heat transfer coefficient of the elongated bubble region is: ; The other calculation methods and calculation procedures are the same as the above calculation when the capillary number is less than or equal to 0.029.

[0034] Example 2 Further illustrate with reference to Example 1, Figure 1-6 The structure shown, such as Figure 1As shown, the model calculation method in the present invention adopts the following steps: A more complete model calculation includes a bubble growth module and a heat transfer module. Take an experimental data point from the database, whose flow conditions and fluid properties are shown in Table 1. The flow conditions are the same as the experiment. The fluid properties at saturation temperature are obtained from the NIST database. The measured value of the heat transfer coefficient h exp = 28.836kW / m 2 ·K.

[0035] Table 1 Flow conditions and fluid properties

[0036] Note: h is the heated perimeter, this is three-sided heating, D h is the hydraulic diameter, R h is the hydraulic radius, R h = D h / 2;P sat is the saturation pressure, P c is the critical pressure.

[0037] In the calculation of the bubble growth module 100, the calculation results of each parameter are as follows: bubble generation time constant , initial bubble velocity ; Bubble velocity at the measuring point in the microchannel The predicted value of is: , the gas and liquid area occupation times are: .

[0038] In the calculation of the heat transfer module 200, according to the bubble velocity U at the measuring point v Calculate the capillary number at this time , which belongs to the low capillary number range, and the bubble interface shape is judged to be a rectangle with rounded corners; the initial liquid film thickness ; According to the total change rate of liquid film thickness over time under low capillary number: , Solving with MATLAB The value of t pd,dry,L = 0.09s, Figure 4 Display the change of film thickness over time. On the other hand, calculate the time required for the film in the corner to completely dry ;according to Figure 3 Schematic diagram to determine the actual occupation time of each area: t l =1.72ms,t e =2.75ms,t p =0,t f =0.

[0039] The heat transfer coefficient of each zone is calculated as follows: Heat transfer coefficient of liquid plug area kW / m 2 K, where the microchannel is heated on three sides, Nusselt number Nu: , β is the aspect ratio of the microchannel. This is a square tube model, so β=1.

[0040] According to the geometric configuration of the liquid in the cross section, such as Figure 5 As shown, calculate the heat transfer coefficient h of the liquid at the corner of the elongated bubble area lp They are:

[0041] Local heat transfer coefficient of the elongated bubble region for:

[0042] Therefore, the average heat transfer coefficient of the elongated bubble region is for: kW / m 2 ·K The geometric configuration of liquid distribution in the local dry area is as follows: Figure 6 As shown, its local heat transfer coefficient for:

[0043] Average heat transfer coefficient of the local drying area for:

[0044] Heat transfer coefficient of complete drying zone .

[0045] Finally, at a specific position of the square microchannel, the average heat transfer coefficient h calculated after one cycle is: kW / m 2 ·K Prediction Error , this embodiment predicts well.

[0046] Example 3 Further illustrate with reference to Examples 1-2, Figure 1-6 For the structure shown, when the capillary number of the fluid at the measuring point is > 0.03, the bubble is a circular interface. At this time, the calculation formula for the initial liquid film thickness is: , the total change rate of liquid film thickness over time is:

[0047] From this, we can calculate the time when the liquid film is completely dry t pd,dry,H The local heat transfer coefficient of the elongated bubble area is: , other calculation methods and calculation processes are the same and will not be repeated here.

[0048] Example 4 Further illustrate with reference to Examples 1-2, Figure 1-6 The structure shown is a more complete calculation method for the theoretical model of microchannel slug flow boiling heat transfer. Based on the assumption of steady flow, the calculation is divided into two stages: bubble growth module and heat transfer module.

[0049] Feature 1: The model innovatively divides the flow boiling process in the microchannel into five key areas, namely, the bubble free growth area, the liquid plug area, the elongated bubble area, the partial drying area and the complete drying area.

[0050] In the free growth zone, the model specifically introduces a free growth mechanism for bubbles starting from extremely small sizes. This mechanism allows bubbles to expand freely according to the energy balance principle without being affected by external forces until they reach physical limits in the length or width direction of the microchannel. Through the refined simulation of this stage, the model can more realistically reflect the morphological changes and velocity distribution of bubbles during the growth process, thereby providing a theoretical calculation basis for the heat transfer module.

[0051] Feature 2: When the bubble grows to the turning point where the microchannel length or width is limited, the model innovatively introduces a bubble shape classification mechanism based on the capillary number (Ca). Specifically, when the bubble begins to be limited by the channel width, the model will finely divide the bubble shape changes according to the capillary number at this time. When the capillary number Ca is low, the model predicts that the bubble interface is a rectangular shape with rounded corners; if the capillary number Ca gradually increases, the model predicts that the bubble interface shape is circular.

[0052] Feature 3: In view of the diversity of bubble shapes, the model adopts a differentiated strategy in the calculation of liquid film thinning and the estimation of the heat transfer coefficient of the elongated bubble area. When the capillary number is high, the bubble diameter is 2R; when the capillary number is low, the length and width of the bubble are both 2R, and the rounded corners are R / 2.

[0053] Feature 4: In the bubble elongation area, the dual effects of liquid evaporation and steam shear force are comprehensively considered, and this detailed model is used to estimate the time required for the liquid film to go from wetting to drying (the film thickness gradually decreases).

[0054] Using the fluid property data in the NIST database, the microchannel hydraulic diameter D h , liquid mass flow rate G, heat flux density q and steam mass fraction x at the measuring point are used as the initial values ​​for calculation.

[0055] The calculation framework is decomposed into two core modules: the growth module that only considers bubble growth and the heat transfer module that only considers heat exchange. First, the bubble generation time constant is determined based on the input parameters , and the initial velocity U at the moment when the bubble is restricted is solved through the energy balance equation of bubble growth v,o ; combining with the liquid velocity when the fluid reaches the saturation state (x = 0), the bubble velocity U at the measuring point is calculated based on the measured x value v , and the passing times t v and t l of the gas slug and the liquid slug are determined.

[0056] After that, further, the capillary number Ca (Ca = μ l U v / σ) is introduced as the criterion for judging the bubble interface morphology, and the critical value Cac = 0.03 is used as the benchmark.

[0057] When Ca < Cac, the interface morphology is regarded as a rectangle with rounded corners, and the initial film thickness δ0 is determined according to the relationship between the dimensionless film thickness and Ca; based on the evaporation heat balance and the liquid flow continuity equation under shear action, the liquid film thinning rate dδ / dt is calculated until the liquid film is completely evaporated, and the drying time t pd,dry,L (the calculation here is relatively complex and a calculation tool such as MATLAB is used) of the central liquid film in the elongated bubble region is obtained.

[0058] Conversely, when Ca > Cac, the interface is regarded as circular, and t pd,dry,H is calculated accordingly. At the same time, the time t pd,dry from the moment when the liquid film is completely dried to when the liquid at the microchannel corner is completely dried is directly solved by the heat balance equation; By comparing the residence time t v in the gas slug region with the above two types of drying times t pd,dry,(H / L) , the time period of the gas existence region is accurately divided, including the time t e in the elongated bubble region, the partial drying time t p , the complete drying time t f , and the liquid region time is the previously calculated liquid slug region t l ; the transient heat transfer performance of each region is solved by using the local heat transfer coefficient h(t) = q / (T w - T sat ) (where the wall temperature T w and the saturation temperature T sat are represented according to the geometric configuration of the liquid existing in the cross-section), and then the average heat transfer coefficient of each region is obtained by integrating over time; finally, the time-averaged heat transfer coefficient h can be output by the model, which is the time weight of the average heat transfer coefficient of each region.

[0059] The implementation of this patent can be improved on the basis of the existing four zones, and the changes required for improvement are: 1. The initial bubble velocity is calculated by combining the growth energy formula with the generation time constant t.

[0060] 2. At the measuring point, the relationship under adiabatic two-phase flow conditions is used to predict the initial bubble film thickness, and the interface judgment logic is added.

[0061] 3. In the calculation of liquid film thinning, in addition to considering the evaporation effect, the liquid continuity equation is introduced to calculate the shear force. The total liquid film shear rate is derived through the shear force balance condition at the gas-liquid interface, and then the time for the liquid film to completely dry is solved. 4. In the elongated bubble region, solve the equation for the local heat transfer coefficient when adding a low capillary number based on the difference in bubble interface shape.

[0062] The above embodiments are only preferred technical solutions of the present invention and should not be regarded as limiting the present invention. The protection scope of the present invention shall be the technical solutions recorded in the claims, including equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A theoretical calculation method for microchannel slug flow boiling heat transfer, characterized in that: S1. Based on steady flow, the calculation is divided into two stages: a bubble growth module and a heat transfer module, and the flow boiling process in the microchannel is divided into five key regions, namely, a bubble free growth region, a liquid slug region, an elongated bubble region, a partially dry region, and a completely dry region; S2. In the bubble free growth region, a free growth mechanism of bubbles starting from extremely small sizes is introduced, and they expand freely according to the principle of energy balance until they reach physical limitations in the length or width direction of the microchannel; S3. When the bubble grows to the turning point where it is limited in the length or width direction of the microchannel, a bubble shape classification mechanism based on the capillary number Ca is introduced. When the capillary number Ca is low, the bubble interface presents a rectangular shape with rounded corners; if the capillary number Ca gradually increases, the bubble interface shape is circular; S4. In the bubble elongation region, the dual effects of liquid evaporation and vapor shear force are comprehensively considered to estimate the time required for the liquid film to change from wet to dry with this refined model.

2. According to claim 1, a microchannel slug flow boiling heat transfer theoretical calculation method is characterized by: The specific steps of S1 - S4 are as follows: A1. Based on the fluid property data and microchannel hydraulic diameter in the NIST database , Liquid mass flow , heat flux And the steam mass fraction at the measuring point As the initial value of the calculation; A1. The calculation framework is divided into a bubble growth module and a heat transfer module: Bubble growth module: establishes the bubble generation time constant based on the input parameters , the initial velocity of the bubble at the time of confinement is solved by the energy balance equation of bubble growth , combined with the fluid reaching saturation, that is The liquid velocity at the time is based on the actual measurement. Calculate the bubble velocity at the measuring point , and determine the passage time of the gas slug and the liquid slug and ; A2. Heat transfer module: Introducing capillary number As the basis for judging the bubble interface morphology, the critical value is used. as a benchmark; when When: the interface morphology is considered as a rectangle with rounded corners, and the initial film thickness is Determined based on the relationship between dimensionless film thickness and Ca; Based on the evaporation heat balance and the liquid flow continuity equation under shear, the thinning rate of the liquid film dδ / dt is calculated until the liquid film is completely evaporated, and the drying time t of the liquid film in the center of the elongated bubble area is obtained. pd,dry,L ; when When: the interface is considered circular and the calculation ; The time from when the liquid film is completely dry to when the liquid in the corner of the microchannel is completely dry Directly solve the heat balance equation; A3. By increasing the gas slug area residence time Compared with the above two types of drying time Compare and precisely divide the time periods of gas presence areas, including extending the bubble zone time , Partial drying time , Complete drying time , the liquid slug time is the liquid slug area calculated previously ; A4. Using local heat transfer coefficient , where the wall temperature and saturation temperature According to the geometric configuration of the liquid in the cross section, the transient heat transfer performance of each area is solved, and then the average heat transfer coefficient of each area is obtained by integrating the time; A5. Average heat transfer coefficient at the final model output , which is the time weight of the average heat transfer coefficient of each zone.

3. The theoretical calculation method for microchannel slug flow boiling heat transfer according to claim 2, characterized in that: the calculation method of step A1 is: In the bubble growth module, the bubble generation time constant , initial bubble velocity , bubble velocity at the measuring point The gas and liquid region occupation times are and .

4. According to claim 2, a microchannel slug flow boiling heat transfer theoretical calculation method is characterized by: The calculation method of step A2 is: Introducing capillary number As the basis for judging the bubble interface morphology, the critical value Cac=0.03 is used as the benchmark; When Ca < Cac, the interface morphology is regarded as a rectangle with rounded corners, and the initial film thickness δ0 is determined according to the relationship between the dimensionless film thickness and Ca; When Ca > Cac, the interface is regarded as circular, and the time for the liquid film to be completely dry is calculated accordingly. The technical method is: when When the initial liquid film thickness The total change rate of liquid film thickness over time is ; when When the initial liquid film thickness The total change rate of liquid film thickness over time is , using MATLAB to solve When the value is , and calculate the time required for the liquid film at the corner to completely dry .

5. According to claim 2, a microchannel slug flow boiling heat transfer theoretical calculation method is characterized by: The calculation method of step A4 is: the calculation method of the average heat transfer coefficient in each region is: Heat transfer coefficient of liquid plug area , where the microchannel is heated on three sides, and the Nusselt number Nu is calculated based on the aspect ratio of the microchannel. Calculate, where the Nusselt number Nu is expressed as: ； The local heat transfer coefficient of the elongated bubble area is calculated according to different interface conditions. When the interface is a rounded rectangular interface, , when the circular interface is ; Calculation of the liquid heat transfer coefficient at the corner of the elongated bubble region: , the local heat transfer coefficient of the elongated bubble region for , the average heat transfer coefficient of the elongated bubble region is expressed as: ； Local heat transfer coefficient of liquid distribution geometry in local dry zone for , the average heat transfer coefficient of the local drying area is expressed as: ； Heat transfer coefficient of complete drying zone ; Calculate the time-averaged heat transfer coefficient at a specific position of the square microchannel after one cycle of flow ; The model can output the time-averaged heat transfer coefficient h, which is the time weight of the average heat transfer coefficient in each region; Calculating prediction error .

6. The microchannel slug flow boiling heat transfer theoretical calculation method according to claim 1 is characterized by: When the capillary number of the fluid at the measurement point is greater than 0.03, the bubble has a circular interface and has the following characteristics: the calculation formula for the initial liquid film thickness is: ； The total change rate of the liquid film thickness with time is: ； From this, it can be calculated that when the liquid film is completely dry size; The local heat transfer coefficient in the elongated bubble region is: ； Other calculation methods and calculation processes are the same as the above calculations when the capillary number is less than or equal to 0.029.

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