Analysis method of horizontal tube falling film evaporation

Abstract

The invention discloses an analysis method for falling film evaporation of a horizontal pipe, which comprises the following steps: S1, establishing a geometric model of the horizontal pipe, and simulating a gas-liquid interface; s2, based on the simulated gas-liquid interface, establishing a turbulence model for the liquid outside the horizontal pipe; and S3, based on the gas-liquid interface and the turbulence model, performing heat conduction simulation to obtain a simulation model of horizontal tube falling film evaporation. According to the analysis method, software is used for carrying out simulation research on the flow characteristic and the heat exchange characteristic in the horizontal pipe outer falling film evaporation process, a VOF method is combined with a UDF (User Defined Function) program, a gas-liquid interface and a turbulence model are simulated through a geometric model, and the flow characteristic and the heat exchange characteristic of the horizontal pipe outer falling film evaporation process are analyzed. The method can effectively explore the influence of different surface structures (such as fin structures with circular arc, trapezoid, triangle and the like) of the evaporation pipe on the falling film flow evaporation characteristic outside the horizontal pipe, so that a user can conveniently design the fin structures of the horizontal pipe according to own requirements, and the optimal design of the horizontal pipe is realized.

Description

Technical Field

[0001] The invention relates to an analysis method of falling film evaporation, in particular to a falling film evaporation analysis with fins added to the outer wall of a heat exchange tube. Background Art

[0002] Horizontal tube falling film evaporation is an efficient phase change heat transfer technology, commonly used in seawater desalination, refrigeration heat pumps, petrochemical and other industrial fields. Compared with the full liquid evaporator, the falling film evaporator has the advantages of less working fluid consumption, small equipment size, and high heat transfer coefficient. In the past few decades, many scholars have conducted a lot of research on the falling film flow and heat transfer behavior on the surface of horizontal tubes, aiming to improve the heat transfer performance of falling film evaporators.

[0003] The flow behavior of the liquid film outside the horizontal tube is closely related to the heat transfer characteristics. Nu first proposed a calculation formula for the thickness of the liquid film outside the tube by solving the momentum energy equation of the falling film; Benjamin et al., based on Nu's theoretical research, introduced the influencing factors of the gas-liquid interface fluctuation and proposed a new calculation model. The flow pattern in the horizontal tube falling film evaporation can be divided into droplet flow, columnar flow, and sheet flow. The transition of the flow pattern between tubes was predicted by Mitrovic in the early years, and then Ambruster et al. supplemented the flow pattern characteristics; in addition, Hu et al. clearly defined the flow pattern characteristics Re outside the horizontal tube through experimental observations. When Re = 150~200, the flow pattern transitions from droplet to columnar; when Re = 315~600, the flow pattern transitions from columnar to sheet.

[0004] The flow characteristics of falling film evaporation in horizontal tubes are an important basis for the development of falling film evaporation technology. On this basis, scholars have conducted many studies on the heat transfer characteristics of falling film evaporation through experiments and simulations. Xu's falling film flow heat transfer performance experiment shows that when Re is in the range of 200~2500, the heat transfer coefficient outside the tube increases with the increase of spray density and evaporation temperature, and decreases slightly with the increase of heat transfer temperature difference between the tube wall and the fluid outside the tube. The total heat transfer coefficient increases significantly with the increase of saturation temperature and decreases with the increase of heat transfer temperature difference. However, other researchers have reached different conclusions. Fletcher et al. found that the heat transfer coefficient outside the tube first increases and then decreases slightly with the increase of spray density. This is because the thickness of the liquid film also increases accordingly, which hinders heat exchange. Shen et al. also obtained similar conclusions. Mu et al. found that the heat transfer coefficient increases slightly with the increase of saturation temperature, but then decreases because the thermal resistance of thermal conductivity increases with the increase of saturation temperature. Gherhard's experimental test shows that the heat transfer coefficient increases significantly with the decrease of the tube diameter, but when nucleate boiling occurs, the effect of the change in tube diameter is weakened. Moeykens and Schlager's experiment came to the opposite conclusion. They believe that the increase in tube diameter makes the liquid film thickness outside the tube thinner, the thermal resistance of thermal conductivity decreases, and the total heat transfer coefficient outside the tube increases. Liu believes that the size of the tube diameter is not related to the heat transfer coefficient. The different experimental conditions and working conditions of many scholars will affect the results obtained, and it is difficult to unify the conclusions.

[0005] The application of enhanced heat transfer technology and the improvement of the heat transfer efficiency of the evaporator are the consistent pursuits of researchers. By changing the shape of the tube cross section, a more uniform liquid film distribution outside the tube and a thinner liquid film can be achieved, thereby achieving a better heat transfer effect outside the tube. In this regard, many workers have done a lot of research. Qi proved that the elliptical tube has better heat and mass transfer effects than the circular tube by combining numerical simulation and experiments, and the total heat transfer coefficient is increased by 1.2 times. Zhao et al. experimentally simulated the change law of the liquid film on the inclined elliptical tube and found that the tube wall on the lower side, especially the upper half of the end, is prone to dry wall areas, but the elliptical tube has good heat and mass transfer characteristics and good pressure drop characteristics. Zhang et al. found through numerical simulation that the liquid film of the elliptical tube is more uniform, and the minimum thickness of the liquid film appears at the 90° circumferential position. Luo conducted experimental and simulation studies on egg-shaped tubes and drop-shaped tubes and concluded that the heat transfer coefficient outside the tube was 11.5%~16.2% and 5.0%~14.0% higher than that of the circular tube, respectively. The reason is that the dimensionless temperature of the egg-shaped tube and the drop-shaped tube is lower and the thermal boundary layer is thinner. However, the evaporation and condensation model provided by Fluent was used in the simulation to simulate the phase change in the entire domain. Fluent's own evaporation and condensation model has limitations, and the generation of bubbles can only be explained by the theory of nucleate boiling. The conclusion has a certain reference role, but it cannot explain the evaporation phenomenon of falling film evaporation in horizontal tubes in industrial production.

[0006] In summary, in the horizontal tube falling film evaporation system, changing the tube shape can improve the flow characteristics of the liquid film outside the tube. In addition, the heat transfer enhancement on the outer surface of the tube can also be achieved by increasing the heat exchange area, increasing the fluid velocity, intensifying the disturbance inside the fluid, and increasing the number of vaporization cores during evaporation.

[0007] By adding fins to the outer wall of the heat exchange tube for wall strengthening treatment, the researchers have conducted the following research. Mohamed et al. experimentally studied the falling film evaporation of water outside the spiral groove tube. The results showed that the liquid film thickness of the spiral groove tube with a pitch of 3mm was lower than that of the circular tube, and its Nu increased by 45%. The smaller the pitch, the more obvious the strengthening effect; Wang et al. solved the mathematical model and found that when the height of the spiral groove tube rib increased from 0.2mm to 0.3mm, the mass transfer and heat transfer rates were 18% and 30% higher than those of the smooth tube, respectively. Putilin and Rifert et al. experimentally found that the heat transfer coefficient of the longitudinal groove tube was 30%-90% higher than that of the smooth circular tube; but Sabin and Poppendiex showed that the longitudinal groove tube did not work for enhanced heat transfer. Comparing the two literatures, it was found that the groove depth of the latter was more than ten times shallower than that of the former, that is, the groove that was too shallow was equivalent to only increasing the surface roughness of the tube, and had little effect on heat transfer at low spray density, that is, the groove structure size had a greater influence on the liquid film flow characteristics. Therefore, further attention needs to be paid to the influence of the groove structure size. Roques and Thome experimentally studied three types of enhanced tubes, Gewa-B, Turbo-Bii, and High-Flux, under different conditions. The results showed that the heat transfer performance of the High-Flux tube was three times that of the first two. Zhao et al. experimentally studied the falling film evaporation and pool boiling outside the typical enhanced tube and found that the total heat transfer coefficient of the enhanced tube was 3 times higher than that of the smooth tube, and the shell-side falling film evaporation heat transfer coefficient increased by 2.1~4.9 times.

[0008] In summary, compared with changing the tube shape, adding fins to the outer wall of the heat exchange tube can better improve the heat transfer coefficient outside the tube and is more valuable for research. Summary of the invention

[0009] The invention provides an analysis method for falling film evaporation of a horizontal tube, which realizes the analysis of the falling film evaporation effect of adding fins to the horizontal tube.

[0010] The present invention provides an analysis method for horizontal tube falling film evaporation, comprising:

[0011] S1. Establish a geometric model of the horizontal tube to simulate the gas-liquid interface;

[0012] S2. Based on the simulated gas-liquid interface, a turbulence model is established for the liquid outside the horizontal tube;

[0013] S3. Based on the gas-liquid interface and turbulence model, heat conduction simulation is performed to obtain a simulation model of horizontal tube falling film evaporation.

[0014] Furthermore, the step S1. establishing a geometric model of the horizontal pipe includes:

[0015] S101. Obtaining a cross-sectional structure of a single horizontal tube;

[0016] S102. Simplifying the cross-sectional structure model to obtain a simplified two-dimensional region; the two-dimensional region is a half-side region of a single horizontal tube cross-sectional structure, and at least one side of the half-side region coincides with a vertical centerline of the horizontal tube cross-sectional structure;

[0017] S103. Simulate spraying conditions to determine the gas-liquid interface in the two-dimensional region.

[0018] Furthermore, the simulated spraying condition in S103 includes:

[0019] Establish the spray environment formula, specifically:

[0020]

[0021]

[0022] Where Γ is the spray density on one side of the spray port; u is the liquid inlet velocity; ρ is the liquid density; w is the half-side spray width of the horizontal pipe; and v is the dynamic viscosity.

[0023] Furthermore, the S2. establishes a turbulence model for the liquid outside the horizontal tube based on the simulated gas-liquid interface, including:

[0024] S201. Based on the simulated gas-liquid interface, the liquid phase and the gas phase are volume segmented;

[0025] S202. Determine the heat transfer of the evaporation process and the condensation process respectively;

[0026] S203. Determine the mass change between the liquid phase and the gas phase based on the heat transfer;

[0027] S204. Establish a turbulence model.

[0028] Furthermore, the S201. based on the simulation of gas-liquid interface, the liquid phase and gas phase are volume segmented, including: using VOF model to dynamically track the flow behavior of the gas-liquid free interface; the model regards each phase mixture as a fluid, the flow variables and physical properties are all volume average values, and the sum of each volume fraction in each control body is 1.

[0029]

[0030] In the formula, α l , α gare the volume fractions of liquid and gas phase, respectively.

[0031] Furthermore, the S202. respectively determining the heat transfer conditions of the evaporation process and the condensation process includes:

[0032] When the temperature of the computational grid near the interface is greater than the saturation temperature of the liquid, the mass of the liquid phase begins to transfer to the gas phase, which is an evaporation process; otherwise, it is a condensation process. The specific formula for this process is:

[0033] Evaporation process:

[0034] ;

[0035] Condensation process:

[0036] ;

[0037] In the formula, and are the phase change rates of liquid phase and gas phase respectively; the phase change coefficient r is 0.001 / s, and are the evaporation coefficient and condensation coefficient respectively; and are the densities of the liquid and gas phases, respectively.

[0038] Furthermore, the S203. determining the mass change between the liquid phase and the gas phase based on the heat transfer condition includes:

[0039] Establish the continuity equation:

[0040] ;

[0041] ;

[0042] In the formula, is the mass source term transferred from liquid phase to vapor phase.

[0043] Furthermore, the step S204. establishing a turbulence model includes:

[0044] Building the momentum equation:

[0045] (6)

[0046] In the formula, ρ is the average density of the vapor and liquid phases; μ is the dynamic viscosity; p is the average pressure of the vapor and liquid phases; is the volume force generated by liquid tension; v' is the pulsating velocity; the relationship between Reynolds stress and average velocity gradient established by eddy viscosity assumption is used to deal with the turbulent pulsating value Reynolds stress term , specifically:

[0047]

[0048] In the formula, is the turbulent viscosity, is the average speed, is the symbol for "Kronecker delta", when i = j, =1; when i ≠ j, =0, k is the turbulent kinetic energy;

[0049] The energy equation is further established as:

[0050] ;

[0051] Where E is the internal energy; λ is the average thermal conductivity; T is the average temperature of the vapor and liquid phases; and Q is the energy source term.

[0052] Furthermore, the S3. performs heat conduction simulation based on the gas-liquid interface and turbulence model to obtain a simulation model of horizontal tube falling film evaporation to achieve relevant analysis, including:

[0053] The k equation, ω equation and the transport equation of specific dissipation rate are established, including:

[0054] k equation:

[0055] ;

[0056] Omega equation:

[0057] ; ;

[0058] ;

[0059] ;

[0060] ;

[0061] Where ω is the specific dissipation rate; S is the modulus of the average strain rate tensor; and are k and The turbulent Prandtl number.

[0062] The present invention uses a variety of software to simulate the flow characteristics and heat transfer characteristics of the falling film evaporation process outside the horizontal tube. The VOF method combined with the UDF (user defined function) program can effectively explore the influence of different surface structures of the evaporation tube (arc, trapezoid, triangle) on the falling film flow evaporation characteristics outside the horizontal tube, and deeply studies the variation law of the average heat transfer coefficient of the micro-fin tube with different structural parameters (rib height, rib width, rib number), so as to facilitate users to design the horizontal tube fin structure according to their own needs and realize the optimal design of the horizontal tube. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 A schematic diagram of a two-dimensional region according to an embodiment of the present invention;

[0064] Figure 2 This is a schematic diagram of grid independence verification according to an embodiment of the present invention;

[0065] Figure 3 This is a schematic diagram of model accuracy verification of an embodiment of the present invention;

[0066] Figure 4 It is a schematic diagram of the geometric model of the micro-fin tube of the present invention;

[0067] Figure 5 is a graph of average heat transfer coefficients of different rib heights according to an embodiment of the present invention;

[0068] Figure 6 This is a graph of average heat transfer coefficients of different rib widths according to an embodiment of the present invention;

[0069] Figure 7 Graph showing average heat transfer coefficients for different numbers of ribs in an embodiment of the present invention. DETAILED DESCRIPTION

[0070] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only embodiments of a part of the present invention, rather than all embodiments.

[0071] The present invention provides an analysis method for horizontal tube falling film evaporation, comprising:

[0072] S1. Establish a geometric model of the horizontal tube to simulate the gas-liquid interface;

[0073] Among them, the geometric model of the horizontal tube can be processed to simulate the simulated gas-liquid interface formed by the falling film evaporation of the spray liquid after it is sprayed onto the surface of the horizontal tube, so that the horizontal tube with different fin structures can be specifically analyzed and simulated;

[0074] S2. Based on the simulated gas-liquid interface, a turbulence model is established for the liquid outside the horizontal tube;

[0075] Among them, when the horizontal tube has a fin structure, the flow characteristics of the liquid on different fin structures can be determined by establishing a turbulence model, and then adapted to the horizontal tubes with different fin structures to perform corresponding characteristic analysis;

[0076] S3. Based on the gas-liquid interface and turbulence model, heat conduction simulation is performed to obtain a simulation model of horizontal tube falling film evaporation.

[0077] The embodiment of the present invention uses software to simulate the flow characteristics and heat transfer characteristics of the falling film evaporation process outside the horizontal tube. The VOF method is combined with the UDF (user defined function) program. Through the geometric model, simulated gas-liquid interface, and turbulence model, the influence of different surface structures of the evaporation tube (such as fin structures with arc, trapezoidal, triangular, etc. cross-sections) on the falling film flow evaporation characteristics outside the horizontal tube can be effectively explored. The variation law of the average heat transfer coefficient of the micro-fin tube with different structural parameters (rib height, rib width, rib number) is deeply studied, so as to facilitate users to design the horizontal tube fin structure according to their own needs and realize the optimal design of the horizontal tube.

[0078] Optionally, the step S1. establishing a geometric model of a horizontal pipe includes:

[0079] S101. Obtaining a cross-sectional structure of a single horizontal tube;

[0080] S102. Simplifying the cross-sectional structure model to obtain a simplified two-dimensional region; the two-dimensional region is a half-side region of a single horizontal tube cross-sectional structure, and at least one side of the half-side region coincides with a vertical centerline of the horizontal tube cross-sectional structure;

[0081] S103. Simulate spraying conditions to determine the gas-liquid interface in the two-dimensional region.

[0082] Among them, this embodiment takes a single tube with a diameter of 25.4 mm commonly used in industry as the research object, and the horizontal tube falling film evaporation diagram and boundary conditions are as follows Figure 1 As shown in the figure, the computational domain is the 2D area of ​​the 15mm*35.4mm rectangle minus half of the tube. The diameter of the liquid distributor is 2mm, the spray height is 5mm, the water inlet temperature is set to the saturation temperature of 333.15K, and the wall is a constant temperature wall of 335.15K. The corresponding angle of the top of the tube is 0°, and the corresponding angle of the bottom of the tube is 180°. The liquid is sprayed from the top spray hole at a certain spray density. The liquid flows out and falls freely to the top of the tube. Under the action of gravity, viscosity and surface tension, the liquid flows downward along the outer wall of the tube, forming a liquid film around the tube. Under the condition of constant wall temperature or constant heat flux density, heat is transferred from the tube wall to the inside of the liquid film, and the gas-liquid interface begins to evaporate.

[0083] Optionally, the simulating spraying condition in S103 includes:

[0084] Establish the spray environment formula, specifically:

[0085]

[0086]

[0087] Where Γ is the spray density on the half side of the spray port, ; u is the liquid inlet velocity, ; ρ is the liquid density; w is the half-side spray width of the horizontal pipe, m; v is the dynamic viscosity, .

[0088] It is assumed that the fluid properties remain unchanged during the entire flow process. Since the falling film flow outside the tube is symmetrical, in order to save computing resources and time, Figure 1 As shown, half of the computational area is selected for modeling and then expanded to the entire area.

[0089] Optionally, the S2. establishing a turbulence model for the liquid outside the horizontal tube based on a simulated gas-liquid interface includes:

[0090] S201. Based on the simulated gas-liquid interface, the liquid phase and the gas phase are volume segmented;

[0091] S202. Determine the heat transfer of the evaporation process and the condensation process respectively;

[0092] S203. Determine the mass change between the liquid phase and the gas phase based on the heat transfer;

[0093] S204. Establish a turbulence model.

[0094] Optionally, the S201. performs volume segmentation of the liquid phase and the gas phase based on the simulation of the gas-liquid interface, including: using the VOF model to dynamically track the flow behavior of the gas-liquid free interface; the model regards each phase mixture as a fluid, and the flow variables and physical property parameters are all volume average values, and the sum of the volume fractions of each item in each control body is 1.

[0095] (1)

[0096] In the formula, α l , α g are the volume fractions of liquid and gas phase, respectively.

[0097] The VOF model is used to dynamically track the flow behavior of the gas-liquid free interface. The model regards each phase mixture as a fluid, and the flow variables and physical parameters are all volume averages, and the sum of the volume fractions of each control volume is 1.

[0098] In particular, the S202. respectively determines the heat transfer conditions of the evaporation process and the condensation process, including: when the temperature of the calculation grid near the interface is greater than the saturation temperature of the liquid, the liquid phase mass begins to transfer to the gas phase, which is an evaporation process; otherwise, it is a condensation process.

[0099] Among them, due to the limitations of the evaporation and condensation model of Fluent software itself, the bubble generation inside the liquid film does not conform to the evaporation phenomenon of industrial production. Therefore, this paper uses UDF to write the mass conversion model proposed by Lee to express the heat and mass transfer occurring at the gas-liquid interface. Based on the VOF model, the geometric reconstruction method of the vapor-liquid interface is used to obtain the flux flowing through the interface, and the real-time fitting and tracking of the interface is realized.

[0100] The controlled phase change occurs at the gas-liquid interface. Whether the phase change occurs in the computational domain depends on the superheat. When the computational grid temperature near the interface is greater than the saturation temperature of the liquid, the liquid mass begins to transfer to the gas phase, which is an evaporation process; otherwise, it is a condensation process. This process can be described as:

[0101] Evaporation process (T>Tsat):

[0102] (2)

[0103] Condensation process (T <Tsat):

[0104] (3)

[0105] In the formula, and are the phase change rates of liquid phase and gas phase respectively; the phase change coefficient r is 0.001 / s, and are the evaporation coefficient and condensation coefficient respectively; and are the liquid and gas phase densities, kg / m 3 .

[0106] In particular, the step S203. determining the mass change between the liquid phase and the gas phase based on the heat transfer condition includes:

[0107] Establish the continuity equation:

[0108] (4);

[0109] (5);

[0110] In the formula, is the mass source term transferred from liquid phase to vapor phase, kg·m -3 ·s -1 .

[0111] In particular, the step S204. establishing a turbulence model includes:

[0112] Building the momentum equation:

[0113] (6)

[0114] Where ρ is the average density of the vapor and liquid phases, ; μ is the dynamic viscosity, ; p is the average pressure of vapor and liquid phases, Pa; is the volume force generated by liquid tension, ; v' is the pulsation speed, ; The relationship between Reynolds stress and average velocity gradient established by the eddy viscosity assumption proposed by Boussinesq is used to deal with the turbulent pulsation value Reynolds stress term :

[0115] (7) In the formula, is the turbulent viscosity, is the average speed, is the symbol for "Kronecker delta", when i = j, =1; when i ≠ j, =0, k is the turbulent kinetic energy;

[0116] The energy equation is further established as:

[0117] (8)

[0118] Where E is the internal energy, ; λ is the average thermal conductivity, ; T is the average temperature of the vapor and liquid phases, K; Q is the energy source term, .

[0119] Optionally, the S3. performs heat conduction simulation based on the gas-liquid interface and turbulence model to obtain a simulation model of horizontal tube falling film evaporation to achieve relevant analysis, including:

[0120] The k equation, ω equation and the transport equation of specific dissipation rate are established.

[0121] Among them, existing research shows that the SST k-ω turbulence model can more accurately express the liquid film flow outside the horizontal tube and reflect the shear transport effect of the fluid near the wall. The k equation, ω equation and its specific dissipation rate transport equation are shown in Equations (9) to (14).

[0122] k equation:

[0123] (9)

[0124] Omega equation:

[0125] (10);

[0126] (11);

[0127] (12);

[0128] (13);

[0129] (14);

[0130] Where ω is the specific dissipation rate; S is the modulus of the average strain rate tensor; and are k and The turbulent Prandtl number, the empirical closed function , , , and For the detailed expression of SST k- The empirical constants of the model are as follows:

[0131] =6, =0.024, =1, , =1.168, =0.31

[0132] It should be noted that the parameters λ, v, ρ, μ, T, and E not specified in equations (4) to (14) are all averaged according to the volume fractions of the vapor and liquid phases. For example, the calculation method of the density ρ is shown in equation (15):

[0133] (15)

[0134] In the momentum equation, the volume force due to surface tension is for:

[0135] (16)

[0136] Where, σ is the tension coefficient of the gas-liquid interface, which is 0.066Nm-1; is the liquid phase curvature of the gas-liquid interface, m-1, satisfying

[0137] (17)

[0138] Where ω is the specific dissipation rate; S is the modulus of the average strain rate tensor; and are k and The turbulent Prandtl number.

[0139] When discretizing the general equations, the pressure-velocity coupling adopts the pressure implicit PISO algorithm with a splitting operator, the interpolation method of the pressure term uses PRESTO, the discretization of the volume fraction uses Geo-Reconstruct, and the convection term and diffusion term use the second-order upwind format and central difference format, respectively.

[0140] The heat transfer performance of the falling film evaporation in the horizontal tube of the embodiment of the present invention is characterized by the heat transfer coefficient. The heat transfer coefficient is divided into two types: the average heat transfer coefficient and the local heat transfer coefficient. The average heat transfer coefficient characterizes the heat transfer performance of the entire horizontal tube, while the local heat transfer coefficient characterizes the heat transfer performance of the local area outside the horizontal tube. The local heat transfer coefficient h is obtained by formula (18), and the average heat transfer coefficient h is av Obtained by formula (19).

[0141] (18)

[0142] (19)

[0143] Where q is the wall heat flux, W / m 2 ; T w is the wall temperature; T sat It indicates the saturation temperature value under the current working conditions, that is, the evaporation temperature, K; F is the area of ​​the heat exchange surface.

[0144] In order to verify the accuracy and reliability of the numerical calculation of the embodiment of the present invention, the smooth pipe =0.1165kg / (ms) working condition, the average heat transfer coefficient outside the tube calculated with different numbers of grids is compared, as shown in Figure 2 When the number of grids exceeds 36,000, the deviation of the calculated value of the average heat transfer coefficient outside the tube is less than 1%, so the accuracy of the calculation results can be guaranteed by using 36,000 grids. Figure 3 This is the relationship curve between the average heat transfer coefficient and the spray density when water undergoes falling film evaporation outside a smooth tube. Figure 3 It can be seen that the maximum error between the numerical calculation results and Parken's experimental results is less than 8%. The error increases with the increase of spray density, which is because the flow of the liquid film is relatively not very stable after the disturbance is strengthened. In general, the experimental and numerical simulation results are in good agreement, so it can be considered that the calculation model in this paper is relatively reliable.

[0145] In the present invention, the core of the horizontal tube enhanced heat transfer technology lies in how to thin the thermal boundary layer, and the main methods adopted include: special treatment of the heat exchange surface, disturbance of the flow field near the wall, etc. To this end, the analysis method of this embodiment can establish a corresponding analysis model for horizontal tubes with micro-ribs of different shapes added to the surface, and study the falling film flow heat transfer characteristics of three types of micro-rib tubes (arc micro-ribs, trapezoidal micro-ribs, and triangular micro-ribs) through numerical simulation methods, and compare them based on the average heat transfer coefficient outside the tube to determine the most suitable micro-rib tube type.

[0146] Figure 4 In the figure, b, c, and d are the geometric models of arc, trapezoidal, and triangular micro-fin tubes, respectively. The base tube diameter of the micro-fin tube is the same as that of the smooth tube, both of which are 25.4 mm. The wall surface of the micro-fin tube can be divided into the base tube surface and the micro-fin wall surface. The micro-fin wall surface is further divided into the upper wall surface facing the flow and the lower wall surface facing the leeward. The tip of the micro-rib is the rib peak. The first rib peak is located directly below the spray hole. The concave area between adjacent micro-ribs is the inter-rib area. The geometric dimensions of the micro-fin tube are characterized by the rib height H, rib width W, and rib number N. The distance from the rib peak to the base tube wall is the rib height H, the straight-line distance between the two points of the rib bottom is the rib width W, the sum of all micro-ribs in the calculation area is the rib number N, and the micro-ribs are evenly distributed along the circumference of the base tube.

[0147] The arc-shaped micro-fin tube has limited heat transfer improvement in the entire research range. Although the average heat transfer coefficient of the trapezoidal micro-fin tube is greatly improved at low spray density, it is not as good as the triangular fin when the spray density increases. The range has better heat transfer effect, so this embodiment selects triangular micro-fins as the research object. Specifically, the embodiment of the present invention takes the horizontal tube with H=0.4mm, W=1.2mm, and N=32 as an example, sets the spray density in the range of 0.0581~0.2458kg / (ms), and studies the falling film evaporation capacity of the triangular micro-fin tube.

[0148] The triangular micro-fin tube significantly improves the falling film evaporation heat transfer performance due to its special geometric structure. =0.0581~0.2458kg / (ms), T sat =333.15K, ∆T=2K, a corresponding model can be established based on the analysis method of the embodiment of the present invention according to actual research needs. Figure 5-7 For example, the embodiment of the present invention studies the changes in the triangular rib height, rib width, and rib number and the related characteristics of the heat transfer outside the tube. In the stage of establishing the geometric model, multiple geometric models are established from the three aspects of the triangular rib height, rib width, and rib number, and the subsequent analysis method is used to analyze the change laws of the heat transfer coefficient outside the tube, the liquid film thickness and distribution, and the velocity temperature field distribution, and the like. Figure 5 , 6, 7, and the average heat transfer coefficient variation law for different rib heights, rib widths, and rib numbers. Based on Figure 5 , 6 , 7 to obtain the optimal structural parameters of the micro-fin tube.

[0149] In summary, the method of the present invention establishes a two-dimensional micro-fin tube model and simulates the influence of structural parameters (such as fin height, fin width, and fin number) on the falling film evaporation flow and heat transfer characteristics. The accuracy of the model has been verified by data from previous literature.

[0150] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that after reading the specification of this application, the technicians can still modify or replace the specific implementation mode of the present invention with equivalents, but these modifications or changes do not deviate from the scope of protection of the pending claims of the present application.

Claims

1. An analytical method for horizontal tube falling film evaporation, characterized in that: include: S1. Establish a geometric model of the horizontal tube to simulate the gas-liquid interface; S2. Based on the simulated gas-liquid interface, a turbulence model is established for the liquid outside the horizontal tube; S3. Based on the gas-liquid interface and turbulence model, heat conduction simulation is performed to obtain a simulation model of horizontal tube falling film evaporation.

2. The analysis method of horizontal tube falling film evaporation according to claim 1, characterized in that: S1. Establishing a geometric model of a horizontal pipe includes: S101. Obtaining a cross-sectional structure of a single horizontal tube; S102. Simplifying the cross-sectional structure model to obtain a simplified two-dimensional region; the two-dimensional region is a half-side region of a single horizontal tube cross-sectional structure, and at least one side of the half-side region coincides with a vertical centerline of the horizontal tube cross-sectional structure; S103. Simulate spraying conditions to determine the gas-liquid interface in the two-dimensional region.

3. The analysis method of horizontal tube falling film evaporation according to claim 2, characterized in that: The simulated spraying condition in S103 includes: Establish the spray environment formula, specifically: Where Γ is the spray density on one side of the spray port; u is the liquid inlet velocity; ρ is the liquid density; w is the half-side spray width of the horizontal pipe; and v is the dynamic viscosity.

4. The analysis method of horizontal tube falling film evaporation according to claim 1, characterized in that: S2. establishes a turbulence model for the liquid outside the horizontal tube based on a simulated gas-liquid interface, including: S201. Based on the simulated gas-liquid interface, the liquid phase and the gas phase are volume segmented; S202. Determine the heat transfer of the evaporation process and the condensation process respectively; S203. Determine the mass change between the liquid phase and the gas phase based on the heat transfer; S204. Establish a turbulence model.

5. The analysis method of horizontal tube falling film evaporation according to claim 4, characterized in that: The step S201. based on the simulated gas-liquid interface, volume segmenting the liquid phase and the gas phase comprises: The VOF model is used to dynamically track the flow behavior of the gas-liquid free interface; this model regards each phase mixture as a fluid, and the flow variables and physical properties are all volume averages, and the sum of the volume fractions of each item in each control body is 1. In the formula, α l , α g are the volume fractions of liquid and gas phase, respectively.

6. The analysis method of horizontal tube falling film evaporation according to claim 4, characterized in that: The step S202. respectively determining the heat transfer conditions of the evaporation process and the condensation process, comprises: When the temperature of the computational grid near the interface is greater than the saturation temperature of the liquid, the mass of the liquid phase begins to transfer to the gas phase, which is an evaporation process; otherwise, it is a condensation process. The specific formula for this process is: Evaporation process: ； Condensation process: ； In the formula, and are the phase change rates of liquid phase and gas phase respectively; the phase change coefficient r is 0.001 / s, and are the evaporation coefficient and condensation coefficient respectively; and are the densities of the liquid and gas phases, respectively.

7. The analysis method of horizontal tube falling film evaporation according to claim 4, characterized in that: The step S203. determining the mass change between the liquid phase and the gas phase based on the heat transfer condition includes: Establish the continuity equation: ； ； In the formula, is the mass source term transferred from liquid phase to vapor phase.

8. The analysis method of horizontal tube falling film evaporation according to claim 4, characterized in that: The step S204. establishing a turbulence model comprises: Building the momentum equation: ； In the formula, ρ is the average density of the vapor and liquid phases; μ is the dynamic viscosity; p is the average pressure of the vapor and liquid phases; is the volume force generated by liquid tension; v' is the pulsating velocity; the relationship between Reynolds stress and average velocity gradient established by eddy viscosity assumption is used to deal with the turbulent pulsating value Reynolds stress term , specifically: ； In the formula, is the turbulent viscosity, is the average speed, is the symbol for "Kronecker delta", when i = j, =1; when i ≠ j, =0, k is the turbulent kinetic energy; The energy equation is further established as: ； Where E is the internal energy; λ is the average thermal conductivity; T is the average temperature of the vapor and liquid phases; and Q is the energy source term.

9. The analysis method of horizontal tube falling film evaporation according to claim 1, characterized in that: S3. Based on the gas-liquid interface and turbulence model, heat conduction simulation is performed to obtain a simulation model of horizontal tube falling film evaporation to achieve relevant analysis, including: The k equation, ω equation and the transport equation of specific dissipation rate are established to simulate the heat conduction and obtain the simulation model of falling film evaporation in horizontal tube to realize the relevant analysis.

10. The method for analyzing horizontal tube falling film evaporation according to claim 1, characterized in that: The k equation, ω equation and the transport equation of their specific dissipation rate specifically include: k equation: ； Omega equation: ； ； ； ； ； Where ω is the specific dissipation rate; S is the modulus of the average strain rate tensor; and are k and The turbulent Prandtl number.

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