An Optimal Arrangement Method for Evaporators with Bends in Transcritical Cycles

Abstract

The present invention discloses an optimization layout method for a bend-type evaporator in a transcritical cycle. The numerical simulation method is used to qualitatively and quantitatively analyze the magnitude of heat exchange enhancement of three layout methods in the horizontal U-shaped tube of the evaporator relative to the same type of straight tube. The results show that in the C layout method, the vortex directions generated by the centrifugal force and the buoyancy force in the bend are the same, so that the secondary flow effect is superimposed, and the heat exchange will be enhanced, and the C layout method has a more obvious advantage in mixed convection conditions with strong natural convection. The calculation results show that under the same operating conditions and dimensions of the heat exchanger in the evaporator, the heat exchange enhancement rate of the optimized A, B, and C layout methods is improved compared to the layout methods before optimization. Therefore, under the same heat exchange rate, the optimization of this layout method can effectively reduce the size of the equipment and make the structure more compact, which has guiding significance for the design of transcritical evaporators.

Description

Technical Field

[0001] The invention relates to the technical field of heat exchangers, and in particular to an optimization arrangement method of a bend-type evaporator in a transcritical cycle. Background Art

[0002] Under the background of dual carbon, it is urgent to efficiently utilize medium and low temperature waste heat, and the organic Rankine cycle is a widely used waste heat utilization technology. Compared with the subcritical organic Rankine cycle, the transcritical organic Rankine cycle crosses the two-phase region in the evaporator, making the narrow point temperature difference between the working fluid and the heat source smaller, so the matching of the supercritical fluid and the heat source in the transcritical cycle and the overall system performance are better than the subcritical cycle.

[0003] At present, whether in subcritical cycles or transcritical cycles, evaporators with bends are widely used, because the bends can make the heat exchange pipe arrangement more compact and the secondary flow generated by the centrifugal force will enhance the heat exchange. For example, the Chinese patent authorization announcement number CN103748414B discloses a single-pass evaporator, which includes: an inlet manifold; one or more inlet headers, which are fluidly connected to the inlet manifold; one or more tube stacks, wherein each tube stack includes one or more inclined evaporator tubes; one or more tube stacks are fluidly connected to one or more inlet headers; wherein the inclined tubes are inclined at an angle less than 90 degrees or greater than 90 degrees to the vertical line; one or more outlet headers, which are fluidly connected to one or more tube stacks; and an outlet manifold, which is fluidly connected to one or more outlet headers.

[0004] The evaporator provided by the above patent is designed to achieve the purpose of reducing or simplifying engineering design and manufacturing through different pipeline arrangements. However, the existing arrangement of the evaporator pipeline does not take into account that different pipeline arrangements will result in different heat exchange efficiencies. In order to facilitate overall processing and application, the heat exchange equipment will mostly be placed horizontally, and there are mostly three arrangements of horizontal flow U-tubes. The directions of the secondary flow vortexes generated by the centrifugal force in the bend are different in different arrangements. In addition, the supercritical fluid will also produce a secondary flow effect due to the buoyancy. Therefore, the secondary flow generated by the buoyancy and centrifugal force in the bend will be reconstructed, offset or superimposed under different arrangements. It can also be judged that the heat exchange performance of the supercritical fluid will be different under different arrangements. However, in the existing pipeline arrangements, it is not clearly stated which pipeline arrangement can achieve a higher heat exchange effect. Therefore, there is an urgent need for an optimized arrangement method for the bend-type evaporator in the market to achieve the purpose of improving the heat exchange effect of the evaporator. Summary of the invention

[0005] The purpose of the present invention is to provide an optimized layout method for a bend-type evaporator in a transcritical cycle, aiming to improve the problem that among the various existing pipeline layout methods, it is uncertain which pipeline layout method can achieve a higher heat exchange effect, and it is impossible to clearly provide a layout method that can effectively improve the heat exchange effect of the evaporator.

[0006] The present invention is achieved in that:

[0007] An optimization arrangement method for a bend-type evaporator in a transcritical cycle, the specific steps of the method are as follows:

[0008] S1: ICEM CFD software is used to perform three-dimensional modeling and mesh node division on the U-shaped tubes and the same type of straight tubes in the evaporator under the A layout, B layout and C layout, so as to establish a finite volume analysis model;

[0009] S2: Import the model described in the above steps into Ansys Fluent and set calculation parameters such as initial conditions, boundary conditions and monitoring conditions, wherein the monitoring conditions are used to determine whether the calculation has converged. The inner wall temperature of the tube can be obtained through these calculation parameters;

[0010] S3: The inner wall temperature calculated above is exported from Ansys Fluent. According to the division of the grid nodes in the U-shaped tube and straight tube models in S1, the wall temperature T of each grid node in the circumferential direction of a transverse section is obtained. After processing, the average wall temperature T of each circumferential section can be obtained. ave :

[0011]

[0012] Where n is the number of mesh nodes in the circumferential direction of a transverse section;

[0013] Then we get the fluid enthalpy H b :

[0014] H b =H in +4(qx / Gd)

[0015] Among them, H in is the inlet enthalpy, q is the wall heat flux, x is the distance in the flow direction, G is the mass flow rate, and d is the pipe diameter;

[0016] By H b The two parameters of the working pressure P can be used to determine the fluid temperature T under this condition in the REFPROP software. b ;The heat transfer coefficient of the tube is given by the formula:

[0017] h=q / (T ave -T b )

[0018] The heat transfer coefficient of the U-tube is calculated and defined as h U , the heat transfer coefficient of a straight tube is defined as h ST ;

[0019] Average heat transfer enhancement rate α ave By formula:

[0020] α ave =h U / h ST -1

[0021] Calculated;

[0022] S4: Compare with the straight tube under the same conditions, and obtain the heat transfer enhancement rate of the U-tube under the A arrangement, B arrangement and C arrangement relative to the same type of straight tube, and then quantitatively analyze the best arrangement.

[0023] Preferably, the finite volume analysis model in step S1 includes: the inner diameter of the tube, the bend radius and the tube length in the model information; and the cross-sectional node distance, the number of nodes and the number of axial nodes in the grid information.

[0024] Preferably, in step S1, a zero-gravity working condition is further set in the U-tube heat exchange to study the influence of the buoyancy effect on the heat exchange.

[0025] Preferably, in step S2, since the physical properties of the supercritical fluid change very dramatically near the pseudo-critical point, the method of importing UDF in Fluent is adopted to define the physical properties parameters of the supercritical fluid, including density, thermal conductivity, specific heat, viscosity and sound velocity.

[0026] Preferably, the initial conditions in step S2 include fluid inlet temperature, pressure, and fluid mass flow rate; boundary conditions include wall heat flux density; and monitoring conditions include monitoring of fluid outlet temperature and flow rate.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. Based on the numerical simulation method, we studied the heat transfer characteristics of U-tubes under three layouts with different curvatures and different buoyancy conditions. Finally, we determined the layout with the best heat transfer performance through qualitative and quantitative analysis.

[0029] 2. The use of numerical methods as an analysis tool can reduce the high cost of experiments. Model verification and grid independence verification were performed before calculation to ensure the authenticity and reliability of the calculation results. The optimized layout not only significantly improves the heat transfer performance of the same type of straight tubes, but also has better heat transfer performance than the U-tube before optimization, which can effectively improve the heat transfer performance of the transcritical evaporator. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a schematic diagram of arrangement A of the present invention;

[0031] Figure 2 It is a schematic diagram of arrangement B of the present invention;

[0032] Figure 3 is a schematic diagram of arrangement C of the present invention;

[0033] Figure 4 It is the average heat transfer coefficient with / without buoyancy in different arrangements under working condition 2 of the present invention. DETAILED DESCRIPTION

[0034] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0035] The following is a further description with reference to the accompanying drawings and specific embodiments:

[0036] Example 1

[0037] like Figure 1 , Figure 2 and Figure 3 As shown: This embodiment provides an optimization arrangement method of a bend-type evaporator in a transcritical cycle, and the specific steps of the method are as follows:

[0038] S1: ICEM CFD software is used to perform three-dimensional modeling and mesh node division of the U-shaped tubes and the same type of straight tubes in the evaporator under the A, B and C layouts to establish a finite volume analysis model; the finite volume analysis model includes: the tube inner diameter, bend radius and tube length in the model information; the cross-sectional node distance, node number and axial node number in the mesh information. In addition, a zero-gravity working condition is set in the U-shaped tube heat exchange to study the influence of buoyancy effect on heat exchange.

[0039] S2: Import the model described in the above steps into Ansys Fluent and set calculation parameters such as initial conditions, boundary conditions and monitoring conditions. Through these calculation parameters, the inner wall temperature of the tube can be obtained; and because the physical properties of supercritical fluids change dramatically near the pseudo-critical point, the method of importing UDF in Fluent is used to define the physical properties of supercritical fluids, including density, thermal conductivity, specific heat, viscosity and sound velocity. The initial conditions include fluid inlet temperature, pressure, and mass flow rate; boundary conditions include wall heat flux density; and monitoring conditions include monitoring of fluid outlet temperature and flow rate.

[0040] S3: The inner wall temperature calculated above is exported from Ansys Fluent. According to the division of the grid nodes in the U-tube and straight tube models in S1, the wall temperature T of each grid node in the circumferential direction of a transverse section is obtained. After processing, the average wall temperature t of each circumferential section can be obtained. ave :

[0041]

[0042] Where n is the number of mesh nodes in the circumferential direction of a transverse section;

[0043] Then we get the fluid enthalpy H b :

[0044] H b =H in +4(qx / Gd)

[0045] Among them, G in is the inlet enthalpy, q is the wall heat flux, x is the distance in the flow direction, G is the mass flow rate, and d is the pipe diameter;

[0046] By H b The two parameters of the working pressure P can be used to determine the fluid temperature T under this condition in the REFPROP software. b ;The heat transfer coefficient of the tube is given by the formula:

[0047] h=q / (T ave -T b )

[0048] The heat transfer coefficient of the U-tube is calculated and defined as h U , the heat transfer coefficient of a straight tube is defined as h ST ;

[0049] Average heat transfer enhancement rate α ave By formula:

[0050] α ave =h U / h ST -1

[0051] Calculated;

[0052] S4: By comparing with the straight tube under the same conditions, the heat transfer enhancement rate of the U-shaped tube under the A, B and C layouts relative to the same type of straight tubes is obtained, and then the best layout is quantitatively analyzed. In mixed convection with strong natural convection, the heat transfer enhancement rate of the C layout is nearly 32% higher than that of the A layout; the C layout can superimpose the secondary flow effects generated by the centrifugal force and buoyancy in the bend, and the heat transfer is significantly enhanced.

[0053] Example 2

[0054] This embodiment provides a numerical simulation laboratory result and result analysis of an optimized arrangement method of a bend-type evaporator in a transcritical cycle:

[0055] Table 1: Parameters corresponding to the four working conditions

[0056]

[0057] like Figure 1 , Figure 2 and Figure 3 As shown: After model verification, this embodiment uses a numerical simulation method to set up supercritical R134a horizontal flow heat exchange in U-tubes and straight tubes under arrangement modes A, B and C in working condition 1. In addition, a zero-gravity working condition is also set in the U-tube heat exchange to better study the influence of buoyancy effect on heat exchange.

[0058] like Figure 4 As shown: The average wall temperature and average heat transfer coefficient of the three U-tube arrangements and the straight tube in the four working conditions are obtained through steps S1, S2 and S3. Buoyancy has a deteriorating effect on heat transfer after the secondary flow is stable, but due to the existence of the centrifugal force of the bend, the fluid is coupled with the centrifugal force and buoyancy after entering the bend, which enhances the heat transfer. The heat transfer coefficients of the bend and downstream in the A, B and C arrangements are gradually higher than the heat transfer coefficients in the condition without buoyancy, which shows that buoyancy has an enhanced heat transfer effect in the bend.

[0059] Table 2: Heat exchange enhancement rate under different layouts in different working conditions

[0060]

[0061] As shown in Tables 1 and 2: In working condition 2, the curvature is relatively large, the centrifugal force is large, and the heat transfer enhancement rate of arrangement C in the bend is as high as 59%, which is 27% different from arrangement B; as the curvature decreases, the centrifugal force weakens, and the heat transfer enhancement rate of arrangement C in the bend is still as high as 51%, which is 32% different from the worst arrangement A. Through the above analysis, it can be seen that whether there is a buoyancy effect or different curvatures, the heat transfer effect of arrangement C is the best, and the greater the curvature, the more obvious the heat transfer enhancement.

[0062] It can be judged from the buoyancy criterion number that working conditions 3 and 4 are weak buoyancy conditions. Regardless of whether the curvature is large or small, the enhanced heat transfer effects of layout A, layout B, and layout C are significantly lower than those of the above two conditions, but the enhanced heat transfer rate under layout C is still the highest.

[0063] In arrangement A, the directions of the vortices caused by centrifugal force and buoyancy are perpendicular, and the coupling of the two will lead to the reconstruction of the vortices. The specific reconstruction effect depends on the relative size of the centrifugal force and the buoyancy, resulting in a significant difference in heat transfer between the two working conditions in (2). In arrangement B, the directions of the vortices caused by the two are completely opposite, which will lead to partial or complete cancellation of the secondary flow effect, resulting in poor heat transfer effect. In arrangement C, the directions of the vortices caused by the two are the same, which will lead to the superposition of secondary flow effects. This is why arrangement C is the best in the above working conditions.

[0064] Using the buoyancy criterion number Gr q / Gr th It can characterize the buoyancy effect in the horizontal tube. The study found that when 0 <Gr q / Gr th <100, the buoyancy effect is weak, and the convection heat transfer in the tube is mainly forced convection; when Gr q / Gr th When the curvature is greater than 100, the buoyancy gradually increases, and the tube will present a mixed convection form and the natural convection gradually becomes stronger. When forced convection heat transfer is the main form in the tube, regardless of the strength of the centrifugal force, the enhanced heat transfer effect of the three arrangements relative to the straight tube is not obvious, and the C arrangement is only slightly higher than the other two; when the tube is mixed convection, as the buoyancy increases, the enhanced heat transfer effect of the three arrangements will gradually increase with the increase of curvature, and the C arrangement will have a more prominent enhanced heat transfer effect relative to the other two forms, showing a greater advantage.

[0065] In summary, compared with the prior art, this invention studies the heat transfer characteristics of U-tubes under three arrangements with different curvatures and different buoyancy conditions based on numerical simulation methods, and finally determines the arrangement with the best heat transfer performance through qualitative and quantitative analysis. The use of numerical methods as an analysis method can reduce the high cost of experiments. Model verification and grid independence verification were performed before calculation to ensure the authenticity and reliability of the calculation results. The optimized arrangement not only significantly improves the heat transfer of the same type of straight tubes, but also has better heat transfer performance than the U-tube before optimization, which can effectively improve the heat transfer performance of the transcritical evaporator.

[0066] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for optimizing the layout of a bend-type evaporator in a transcritical cycle, characterized in that: The specific steps are as follows: S1: ICEM CFD software is used to perform 3D modeling and mesh node division of the U-shaped tubes and the same type of straight tubes in the evaporator under the A, B and C layouts to establish a finite volume analysis model; the finite volume analysis model includes: the tube inner diameter, bend radius and tube length in the model information; the cross-sectional node distance, node number and axial node number in the mesh information; S2: Import the model described in the above steps into Ansys Fluent and set the calculation parameters such as initial conditions, boundary conditions and monitoring conditions, wherein the monitoring conditions are used to determine whether the calculation has converged, and the inner wall temperature of the tube can be accurately obtained through these calculation parameters; the physical properties of the supercritical fluid are defined by importing UDF in Fluent, including density, thermal conductivity, specific heat, viscosity and sound velocity; the initial conditions include fluid inlet temperature, pressure, and fluid mass flow rate; the boundary conditions include wall heat flux density; the monitoring conditions include monitoring of fluid outlet temperature and flow rate; S3: The inner wall temperature obtained by the above simulation calculation is exported from Ansys Fluent. According to the division of the grid nodes in the U-shaped tube and straight tube model in S1, the wall temperature T of each grid node in the circumferential direction of a transverse section is obtained. After processing, the average circumferential wall temperature T of each section can be obtained. ave : Where n is the number of mesh nodes in the circumferential direction of a transverse section; Then we get the fluid enthalpy H b : H b =H in +4(qx / Gd) Among them, H in is the inlet enthalpy, q is the wall heat flux, x is the distance in the flow direction, G is the mass flow rate, and d is the pipe diameter; By H b The two parameters of the working pressure P can be used to determine the fluid temperature T under this condition in the REFPROP software. b ; The heat transfer coefficient of the tube is given by the formula: h=q / (T ave -T b ) The heat transfer coefficient of the U-tube is calculated and defined as h U , the heat transfer coefficient of a straight tube is defined as h ST ; Average heat transfer enhancement rate α ave By formula: a ave =h U / h ST -1 Calculated; S4: Compare with the straight tube under the same conditions, and obtain the heat transfer enhancement rate of the U-tube under the A arrangement, B arrangement and C arrangement relative to the same type of straight tube, and then quantitatively analyze the best arrangement.

2. The method for optimizing the arrangement of a bend-type evaporator in a transcritical cycle according to claim 1, characterized in that: In step S1, a zero-gravity working condition is also set in the U-tube heat exchange to study the influence of the buoyancy effect on the heat exchange.

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