A combined elbow type supercritical U-tube heat exchanger and its design method

Abstract

The present invention relates to the technical field of supercritical fluid heat exchangers, and specifically to a combined elbow type supercritical U-tube heat exchanger and a design method thereof, wherein the heat exchanger comprises: a tube side outlet pipe, a tube side inlet pipe, a shell side inlet pipe, a shell side outlet pipe, a head, a pipe box, a combined U-tube, a common U-tube, a baffle, a shell, and a tube sheet. The combined U-tube and the common U-tube are arranged in the shell, the inner side is a common U-tube, and the rest of the peripheral tube bundles are combined U-tubes. Supercritical fluid passes through the tube bundle, and the inner wall of the shell and the outer wall of the tube bundle form a heat exchange cavity, in which a second fluid flows. The present invention can not only ensure that the assembly space and material cost are consistent with those of common U-tubes, but also greatly improve the local bending curvature of the return bend, thereby improving the heat transfer efficiency of the peripheral tube bundle.

Description

Technical Field

[0001] The invention relates to the technical field of supercritical fluid heat exchangers, and in particular to a combined elbow type supercritical U-tube heat exchanger and a design method thereof. Background Art

[0002] Global economic growth and industrialization have led to an increase in the use of fossil fuels. The production and consumption of fossil fuels have brought adverse environmental impacts to countries, including global warming, air pollution, and increased health risks. Transcritical organic Rankine cycle and supercritical organic Rankine cycle have become a research hotspot for recovering low-grade thermal energy due to their higher thermal efficiency and lower heat source matching temperature.

[0003] Shell and tube heat exchanger is one of the types of partition heat exchanger. It has a simple structure, is easy to manufacture, install, clean and maintain, is inexpensive, and is particularly suitable for cooling and condensing high-pressure fluids. Therefore, it is still widely used in chemical, petroleum, refrigeration, nuclear energy and power industries.

[0004] In the existing U-tube shell and tube heat exchangers suitable for supercritical fluids, the assembly method inside the shell is to arrange the U-tubes from the inside to the outside, resulting in the U-tubes closer to the periphery having a smaller curvature. This results in the supercritical fluid in the outer U-tube bundle being less affected by centrifugal force, and the heat transfer efficiency is weakened relative to the U-tube bundle with a larger internal curvature. The present invention proposes to flexibly treat this part of the U-tube with a smaller curvature by "partial bending": decomposing the 180° bend of the ordinary U-tube into two 90° bends, which can increase the local bending curvature of the bend part while keeping the assembly space and material costs unchanged, thereby improving the heat exchange capacity of the supercritical fluid in the bend area and its downstream.

[0005] The existing Chinese patent document CN201510663050.2 discloses a countercurrent spiral baffle U-tube bundle heat exchanger, which consists of a U-tube bundle core, a shell and a water chamber; the U-tube bundle core consists of a U-tube bundle, a spiral baffle assembly, a tube sheet, a tie rod sleeve assembly, and a longitudinal partition plate; the spiral baffle assembly is composed of a group of flat circular baffles and a group of arc-end strip baffles composed of inclined sections and left and right flattened sections in the upper and lower semi-cylindrical areas respectively; the two groups of baffles are provided with an overlapping area of ​​a row of tubes.

[0006] Due to its special thermophysical properties, the flow and heat transfer characteristics of supercritical fluids are different from those of working fluids under normal pressure. As the temperature rises, when approaching the large specific heat area, several important parameters that are often concerned in thermodynamics, such as thermal conductivity and specific heat capacity, will change from a smooth transition to a violent fluctuation, and the density and viscosity will decrease rapidly. This characteristic causes the supercritical fluid to be subjected to a violent buoyancy force in the heat exchanger, especially in the return bend of the U-shaped tube bundle, which is subject to the combined effect of buoyancy and centrifugal force.

[0007] The above-mentioned countercurrent spiral baffle U-tube bundle heat exchanger is more suitable for heat exchange of ordinary fluids. If it is used for heat exchange of supercritical fluids, since the curvature of the U-tube bundle on the periphery of the heat exchanger is small, the centrifugal force is small, and the comprehensive enhanced heat exchange effect of buoyancy and centrifugal force is limited; the present invention specifically solves the problem of low heat exchange efficiency of supercritical fluids in U-tubes with small curvature, and can be used under certain working conditions (Gr q / Gr th >35 and the dimensionless curvature r / R of a common U-shaped tube curve <0.0285) improves the heat transfer efficiency of the supercritical fluid in the peripheral U-tube. Summary of the invention

[0008] The object of the present invention is to provide a combined elbow supercritical U-tube heat exchanger and a design method thereof, so as to solve the problem that the U-tube of the above-mentioned ordinary U-tube shell and tube heat exchanger has a small outer curvature and a weak heat exchange capacity by improving the traditional heat exchanger.

[0009] In order to achieve the above technical objectives and the above technical effects, the present invention is implemented through the following technical solutions:

[0010] A combined elbow type supercritical U-tube heat exchanger comprises: a tube side outlet pipe, a tube side inlet pipe, a shell side inlet pipe, a shell side outlet pipe, a head, a pipe box, a combined U-tube, a common U-tube, a baffle, a shell and a tube sheet.

[0011] Furthermore, the combined U-shaped tube and the ordinary U-shaped tube are arranged in the shell, the inner side is the ordinary U-shaped tube, and the remaining peripheral tube bundles are all combined U-shaped tubes. Supercritical fluid passes through the tube bundle, and the inner wall of the shell and the outer wall of the tube bundle form a heat exchange cavity, and the second fluid flows through the heat exchange cavity.

[0012] Furthermore, the first fluid (i.e., supercritical fluid) flows into the pipe box from the tube side inlet pipe, and the pipe box evenly disperses the incoming flow into the ordinary U-tube and the combined U-tube. After completing heat exchange, the first fluid flows out from the tube side outlet pipe. The second fluid enters the shell side from the shell side inlet pipe, completes heat exchange with the supercritical fluid in the heat exchange cavity on the outside of the tube bundle and the inside of the tube wall, and then flows out from the shell side outlet pipe.

[0013] Furthermore, the baffle is arranged in the shell, dividing the interior of the shell into multiple sub-intervals, and the connection of each adjacent sub-interval is staggered and distributed on the upper and lower sides of the shell, guiding the shell-side fluid to flow up and down, enhancing turbulence, increasing the stroke of the medium in the shell-side cylinder, and improving the heat exchange efficiency. At the same time, the shell-side inlet pipe is arranged at the upper end of the shell, so that the flow directions of the two heat exchange media are opposite, further improving the heat exchange efficiency. The tube sheet is located on the inlet and outlet side of the tube bundle, separating the tube side and the shell side. The tube box is located at both ends of the heat exchange tube bundle, evenly distributing the supercritical fluid transported in the pipeline to each heat exchange tube and collecting the supercritical fluid together to send it out of the heat exchanger.

[0014] Furthermore, the combined U-tube and the common U-tube are fixed on the same tube sheet, the tube can be freely expanded and contracted, and no thermal stress will be generated when there is a temperature difference between the shell and the heat exchange tube bundle; the combined U-tube and the common U-tube are fixed in the fixing holes of the tube sheet by welding.

[0015] Furthermore, the dimensionless curvature of the outermost layer of the common U-tube can be as small as 0.015, and the dimensionless curvature of the innermost layer can be as large as 0.26; the inner radius r of the common U-tube and the combined U-tube is 10.3 mm, the wall thickness δ is 1.2 mm, the distance from the outer surface of the outermost layer of the heat exchange tube of the U-tube heat exchanger bundle to the inner wall of the shell is not less than 8 mm, and the spacing between the heat exchange tubes is not less than 1.25 times the inner diameter of the U-tube, that is, 20 heat exchange tubes can be installed in the shell.

[0016] On the other hand, the present invention provides a design method for a combined U-shaped tube bundle, comprising the following steps:

[0017] S1. Input boundary conditions of ordinary U-shaped tube;

[0018] S2. Calculate the wall temperature T according to Jackson's empirical correlation w ;

[0019] S3. According to T w Calculation of the dimensionless Grashof number Gr that characterizes the strength of buoyancy q / Gr th ;

[0020] S4, determine whether two criteria are met;

[0021] Principle 1: Gr q / Gr th >35;

[0022] Criterion 2: Dimensionless curvature r / R of ordinary U-shaped tube curve <0.0285;

[0023] S5. Improve the ordinary U-shape into a combined U-shape tube, and increase the dimensionless curvature by 0.07 to 0.13 on the original basis, while the total heat exchange length remains unchanged.

[0024] Furthermore, the boundary conditions in step S1 include boundary conditions and thermophysical information of the supercritical fluid in an ordinary U-tube, wherein the boundary conditions of the supercritical fluid include mass flow rate, inlet and outlet temperature, and pressure, and the thermophysical information includes dynamic viscosity, enthalpy, specific heat capacity at constant pressure, and Prandtl number.

[0025] Furthermore, in step S2, the wall temperature T of each section is predicted according to the Jackson empirical correlation formula. w , the specific steps are as follows:

[0026] S2.1. Divide the ordinary U-shaped tube into sections and calculate the mainstream temperature T of each section. b , mainstream temperature T b The enthalpy value H b The calculation formula is as follows:

[0027]

[0028] where q w is the wall heat flux, d i is the inner diameter of the U-tube, m is the mass flow rate at the inlet, Δy is the length of each section of the U-tube, and the mainstream temperature T of each section can be determined by constant pressure and enthalpy of each section. b ;

[0029] S2.2, iteratively calculate the heat transfer coefficient (HTC) according to Jackson's empirical correlation;

[0030] The Jackson empirical correlation in step S2.2 is as follows:

[0031]

[0032]

[0033] Where Nu is the Nusselt number, which is defined as Re is the Reynolds number, Pr is the Prandtl number, T pc is the critical temperature (K) at the corresponding pressure, T w is the wall temperature (K), ρ b is the density of the mainstream working fluid (kg / m 3 ), ρ w is the density of the working fluid at the wall (kg / m 3 ), is the average specific heat capacity (J / (kg·K)), expressed as Subscript b refers to mainstream and w refers to wall;

[0034] S2.3. When the iteration error is less than 1%, the calculated wall temperature T is output. w .

[0035] Furthermore, Gr in step S3 q / Gr th It is a quantitative criterion for the buoyancy of supercritical organic working fluids in horizontal flow, and can better predict the development of buoyancy along the flow direction during heat transfer. q and Gr th It is expressed by the following formula:

[0036]

[0037]

[0038] in according to To determine, b is the mainstream dynamic viscosity (Pa·s), λ b is the mainstream thermal conductivity (W / (m·K)), H b is the mainstream enthalpy (J / kg), H w is the enthalpy of the fluid near the wall (J / kg), ρ w is the density of the working fluid at the wall (kg / m 3 ), ρ b is the density of the mainstream working fluid (kg / m 3 ), subscript b refers to mainstream and w refers to wall.

[0039] Furthermore, there are two criteria for judging whether the ordinary U-shaped tube in step S4 can be optimized into a combined U-shaped tube to achieve enhanced heat exchange:

[0040] Principle 1: Gr q / Gr th >35;

[0041] Criterion 2: Dimensionless curvature r / R of ordinary U-shaped tube curve <0.0285;

[0042] In the U-tube bundle of the heat exchanger, any ordinary U-tube that meets the above criteria can be optimized into a combined U-tube to enhance heat transfer. The stronger the buoyancy of the supercritical fluid, the more obvious the benefit of enhanced heat transfer. q / Gr th =35 is the threshold value for the combined U-tube to enhance heat transfer, Gr q / Gr th When the temperature is higher than 35, the heat transfer enhancement effect begins to become obvious.

[0043] Dimensionless curvature r / R curve The dimensionless curvature r / R of a normal U-tube is the ratio of the inner diameter of the heat exchange tube to the curvature radius of the normal U-tube. curve<0.0285 (r is the inner radius of the common U-shaped tube, R curve is the radius of curvature of an ordinary U-tube), the heat transfer can be enhanced by improving the ordinary U-tube into a combined U-tube, and the dimensionless curvature r / R of an ordinary U-tube is curve The smaller it is, the more obvious the enhanced heat exchange effect will be after it is improved into a combined U-tube.

[0044] When an ordinary U-tube is improved into a combined U-tube, the local curvature increases too much, which will lead to an increase in pressure drop and an increase in the impact force of the supercritical fluid on the tube wall, affecting the safety of use; if the curvature increases too little, the heat exchange performance of the improved combined U-tube will be limited. Considering the safety and economy of the combined elbow supercritical U-tube heat exchanger, when the ordinary U-tube is improved into a combined U-tube, the dimensionless curvature increases by 0.07 to 0.13 on the original basis.

[0045] The length of the combined U-tube is the same as that of the ordinary U-tube. On the basis of the conventional U-tube, the combined U-tube improves the original 180° bend into two 90° bends and a partial straight tube, where r is the inner radius of the U-tube and the combined U-tube; L is the bend length of the U-tube, and R curve is the bending radius of the ordinary U-shaped tube, R curv0 is the bending radius of the combined U-shaped tube, H is the length of the straight tube portion of the combined U-shaped tube connecting the straight tube heating section and the return bend, L 0 The length of the straight pipe section that connects the two bends of the combined U-tube.

[0046] In order to meet the actual assembly space, the two U-tube geometric structures meet two constraints: the length of the bend along the way is consistent (to ensure the same heat exchange area) and the width of the bend is consistent (to ensure the same assembly space). The corresponding geometric relationship is:

[0047] H / L=(π-2) / 2π-(π-2) / 2×(R curv0 / L)

[0048] L 0 / L=(2 / π)-2×(R curv0 / L)

[0049] Beneficial effects of the present invention:

[0050] In a traditional U-tube heat exchanger, when the fluid passes through the curved part, the larger bending radius causes the centrifugal force on the fluid to decrease, resulting in the limited effect of buoyancy and centrifugal force on the ordinary U-tube with a smaller outer curvature, which in turn affects the enhanced heat exchange effect brought about by the combined effect of buoyancy and centrifugal force. The combined elbow design introduced in the present invention increases the number of elbows (two 90° bends instead of one 180° bend), so that the fluid is subjected to the combined effect of centrifugal force and buoyancy, thereby enhancing the local secondary flow intensity and thus enhancing the heat exchange in the bend range. Due to the increase in the curvature of the elbow part of the combined U-tube, the secondary flow intensity is significantly increased compared with the conventional U-tube, which increases the heat exchange peak value, thereby increasing the average heat exchange capacity within the bend range. The heat exchange enhancement of the sandwiched straight section is weakened due to the fading of the bend effect, but it will not be lower than the heat exchange level of the horizontal straight tube. This not only improves the heat exchange efficiency between the fluid and the tube wall, but also effectively destroys the thermal boundary layer, further increasing the heat transfer rate of the heat exchanger; the design of the new combined U-tube of the present invention, especially the constitutive improvement in the bending area, is intended to maintain or even improve the structural integrity and strength of the heat exchanger.

[0051] The design of the baffle guides the upward and downward flow of the shell-side fluid through staggered distribution, which not only increases the flow path of the fluid in the shell, but also promotes the change of thermodynamic properties of the fluid in different sub-areas, such as temperature and pressure, and improves the uniformity and efficiency of heat exchange; although the increase of local curvature can improve the heat exchange effect, it may also cause local temperature rise and pressure drop. The present invention balances the relationship between heat exchange efficiency and pressure drop by precisely controlling the increase of dimensionless curvature (between 0.07 and 0.13), thereby optimizing the overall performance of the heat exchanger without significantly increasing the system operating cost.

[0052] Through the carefully designed U-tube layout and size, the new heat exchanger achieves a significant improvement in performance without increasing the heat exchange area. This design allows for a more efficient pipe layout to improve performance within the same assembly space, which can save significant costs for large-scale industrial applications. The present invention allows existing heat exchangers to achieve significant performance improvements through relatively simple modifications, providing an economical and efficient solution for industrial sites.

[0053] The operating parameters of the heat exchange tube are determined by empirical correlation (Jackson correlation), such as wall temperature prediction, dimensionless Grashof number Gr q / Gr th The calculation ensures the scientificity and feasibility of the design method; Gr q / Gr th>35 is the threshold for the combined U-tube to enhance the heat transfer effect. At the same time, if the dimensionless curvature of the ordinary U-tube is less than 0.0285, it means that design improvements can be made. Such a quantitative threshold helps designers make more reasonable decisions and ensure the adaptability and efficiency of the designed heat exchanger under different working conditions.

[0054] Of course, any product implementing the present invention does not necessarily need to achieve all of the advantages described above at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for describing the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0056] Figure 1 It is a schematic diagram of the structure of the combined elbow type supercritical U-tube heat exchanger and the combined U-tube and ordinary U-tube of the present invention;

[0057] Figure 2 It is a design flow chart of the combined U-shaped tube in the present invention;

[0058] Figure 3 The heat flux q is 30kw / m 2 The mass flow rates G1, G2 and G3 are 400 kg / (s·m 2 )、800kg / (s·m 2 )、1000kg / (s·m 2 ) under the working condition of q / Gr th Trend chart;

[0059] Figure 4 The mass flow rate G is 1000 kg / (s·m 2 ), heat flux q1, q2, q3 are 50kw / m 2 、30kw / m 2 、10kw / m 2 Gr predicted by Jackson empirical correlation under the working conditions q / Gr th Trend chart;

[0060] Figure 5 The comparison of the heat transfer coefficient along the line between the ordinary U-tube with dimensionless curvatures of 0.015, 0.0285, and 0.26 respectively and the combined U-tube with dimensionless curvatures increased by 0.1 (i.e., 0.115, 0.1285, and 0.36 respectively);

[0061] In the figure: 1. tube side outlet pipe, 2. tube side inlet pipe, 3. shell side inlet pipe, 4. shell side outlet pipe, 5. head, 6. pipe box, 7. combined U-shaped tube, 8. ordinary U-shaped tube, 9. baffle, 10. shell, 11. tube sheet. DETAILED DESCRIPTION

[0062] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0063] Example 1

[0064] The combined elbow supercritical U-tube heat exchanger described in this embodiment includes: a tube side outlet pipe 1, a tube side inlet pipe 2, a shell side inlet pipe 3, a shell side outlet pipe 4, a head 5, a pipe box 6, a combined U-tube 7, a common U-tube 8, a baffle 9, a shell 10, and a tube sheet 11.

[0065] In this embodiment, the combined U-shaped tube 7 and the ordinary U-shaped tube 8 are arranged in the shell 10, the inner side is the ordinary U-shaped tube 8, and the remaining peripheral tube bundles are all combined U-shaped tubes 7. Supercritical fluid passes through the tubes, and the inner wall of the shell 10 and the outer wall of the tube bundle form a heat exchange cavity, and the second fluid flows through the heat exchange cavity.

[0066] In this embodiment, the first fluid (supercritical fluid) flows into the pipe box 6 from the tube side inlet pipe 2, and the pipe box 6 evenly disperses the incoming flow into the ordinary U-tube 8 and the combined U-tube 7. After completing heat exchange, the first fluid flows out from the tube side outlet pipe 1, and the second fluid enters the shell side from the shell side inlet pipe 3, completes heat exchange with the supercritical fluid in the heat exchange cavity on the outside of the tube bundle and the inside of the tube wall, and then flows out from the shell side outlet pipe 4.

[0067] In this embodiment, the baffle 9 is arranged in the shell 10, dividing the interior of the shell 10 into a plurality of sub-intervals, and the connection of each adjacent sub-interval is staggered and distributed on the upper and lower sides of the shell 10, guiding the shell-side fluid to flow up and down, enhancing turbulence, increasing the stroke of the medium in the shell-side cylinder, and improving the heat exchange efficiency. At the same time, the shell-side inlet pipe 3 is arranged at the upper end of the shell 10, so that the flow directions of the two heat exchange media are opposite, further improving the heat exchange efficiency. The tube sheet 11 is located at the inlet and outlet side of the tube bundle, separating the tube side and the shell side. The tube box 6 is located at both ends of the heat exchange tube bundle, evenly distributing the supercritical fluid transported in the pipeline to each heat exchange tube and collecting the supercritical fluid together to send it out of the heat exchanger.

[0068] In this embodiment, the combined U-shaped tube 7 and the ordinary U-shaped tube 8 are fixed on the same tube sheet 11, and the tubes can be freely expanded and contracted. When there is a temperature difference between the shell 10 and the heat exchange tube bundle, no thermal stress will be generated; the combined U-shaped tube 7 and the ordinary U-shaped tube 8 are fixed in the fixing holes of the tube sheet 11 by welding.

[0069] In this embodiment, the dimensionless curvature of the outermost layer of the common U-shaped tube 8 can be as small as 0.015, and the dimensionless curvature of the innermost layer can be as large as 0.26; the inner radius r of the common U-shaped tube 8 and the combined U-shaped tube 7 is 10.3 mm, the wall thickness is 1.2 mm, the distance from the outer surface of the outermost layer of the heat exchange tube of the U-shaped tube heat exchanger bundle to the inner wall of the shell 10 is not less than 8 mm, and the distance between the heat exchange tubes is not less than 1.25 times the inner diameter of the U-shaped tube, that is, 20 heat exchange tubes can be installed in the shell 10.

[0070] Example 2

[0071] like Figure 1 As shown, in the embodiment of the combined elbow supercritical U-tube heat exchanger, the dimensionless curvature of the ordinary U-tube in the outermost layer can be as small as 0.015, and the dimensionless curvature of the innermost layer can be as large as 0.26. The inner radius r of the ordinary U-tube and the combined U-tube is 10.3 mm, the wall thickness δ is 1.2 mm, the distance from the outer surface of the outermost heat exchange tube of the U-tube heat exchanger tube bundle to the inner wall of the shell is not less than 8 mm, and the distance between the heat exchange tubes is not less than 1.25 times the inner diameter of the U-tube, that is, 20 ordinary U-tubes can be installed in the shell, and the dimensionless curvature of these ordinary U-tubes gradually decreases from 0.26 in the innermost layer to 0.015 in the outermost layer from the inside to the outside.

[0072] Example 3

[0073] This embodiment describes the comparison of the enhanced heat exchange effect between the combined U-tube and the ordinary U-tube. Figure 2 As shown in the figure, the outermost ordinary U-shaped tube (dimensionless curvature is 0.015) is improved into two combined U-shaped tubes (dimensionless curvatures of U1 and U2 are 0.065 and 0.115 respectively). 2 , mass flow rate is 600kg / (s·m 2 ), comparison of heat transfer coefficients under the condition of inlet temperature 345K.

[0074] like Figure 2 As shown in the figure, the heat transfer capacity of the two combined U-tubes is stronger than that of the conventional U-tubes. The combined U-tube with r / R = 0.115 has a greater curvature of the elbow, so the strengthening effect is better than that of the combined U-tube with r / R = 0.065. In the entire influence range (the influence range after the bend and the exit, until the heat transfer level is restored to the same level as the conventional U-tube), the average heat transfer coefficient of U1 is increased by 10%, and the average heat transfer coefficient of U2 is increased by 13%.

[0075] Example 4

[0076] A design method for a combined U-shaped tube bundle described in this embodiment is as follows: Figure 2 As shown, this embodiment takes the supercritical working fluid R1234yf as an example, and improves the ordinary U-tube in the supercritical U-tube heat exchanger into a combined U-tube through the following steps.

[0077] S1. Input boundary conditions of ordinary U-tube;

[0078] As a preferred technical solution of the present invention, the boundary conditions in step S1 include the boundary conditions and thermophysical information of the supercritical fluid in an ordinary U-tube, wherein the boundary conditions of the supercritical fluid include mass flow rate, inlet and outlet temperature, and pressure, and the thermophysical information includes dynamic viscosity, enthalpy, constant-pressure specific heat capacity, and Prandtl number.

[0079] S2. Calculate the wall temperature T according to Jackson's empirical correlation w The specific steps are as follows:

[0080] S2.1. Divide the ordinary U-shaped tube into sections and calculate the mainstream temperature T of each section. b ;

[0081] The mainstream temperature T in step S2.1 b The enthalpy value H b The calculation formula is as follows:

[0082]

[0083] where q w is the wall heat flux, d i is the inner diameter of the U-tube, m is the mass flow rate at the inlet, and Δy is the length of each section of the U-tube. The mainstream temperature T of each section can be determined by constant pressure and enthalpy of each section. b .

[0084] S2.2, iteratively calculate the heat transfer coefficient (HTC) according to Jackson's empirical correlation;

[0085] The Jackson empirical correlation in step S2.2 is as follows:

[0086]

[0087]

[0088] Where Nu is the Nusselt number, which is defined as Re is the Reynolds number, Pr is the Prandtl number, T pc is the critical temperature (K) at the corresponding pressure, T w is the wall temperature (K), ρb is the density of the mainstream working fluid (kg / m 3 ), ρ w is the density of the working fluid at the wall (kg / m 3 ), is the average specific heat capacity (J / (kg·K)), expressed as The subscript b refers to mainstream and w refers to wall.

[0089] S2.3. When the iteration error is less than 1%, the calculated wall temperature T is output. w ;

[0090] From the expression of convection heat transfer coefficient Iteratively calculate the heat flux q c , when q c With q w When the error is less than 1%, the output wall temperature T w .

[0091] S3. According to T w Calculation of the dimensionless Grashof number Gr that characterizes the strength of buoyancy q / Gr th ;

[0092] G q / Gr th It is a quantitative criterion for the buoyancy of supercritical organic working fluids in horizontal flow, and can better predict the development of buoyancy along the flow direction during heat transfer. q and Gr th It is expressed by the following formula:

[0093]

[0094]

[0095] in according to To determine, b is the mainstream dynamic viscosity (Pa·s), λ b is the mainstream thermal conductivity (W / (m·K)), H b is the mainstream enthalpy (J / kg), H w is the enthalpy of the fluid near the wall (J / kg), ρ w is the density of the working fluid at the wall (kg / m 3 ), ρ b is the density of the mainstream working fluid (kg / m 3 ), subscript b refers to mainstream and w refers to wall.

[0096] S4. Determine whether two criteria are met:

[0097] Principle 1: Gr q / Gr th >35;

[0098] According to the above calculation, Gr under each working condition can be obtained q / Gr th The stronger the buoyancy of the supercritical fluid, the more obvious the benefit of enhanced heat transfer. q / Gr th =35 is the threshold value for the combined U-tube to enhance heat transfer, Gr q / Gr th When the temperature is higher than 35, the heat transfer enhancement effect begins to become obvious.

[0099] like Figure 3 and 4 As shown in the figure, the buoyancy criterion Gr calculated by Jackson empirical correlation formula under different combinations of mass flow rate G and heat flux density q is q / Gr th The distribution along the route. Figure 3 The shown value is q maintained at 30kw / m 2 The mass flow rates G1, G2, and G3 are 400 kg / (s·m 2 )、800kg / (s·m 2 )、1000kg / (s·m 2 ). Figure 4 The figure shows that G is maintained at 1000kg / (s·m 2 ) remains unchanged, and the heat flux densities q1, q2, and q3 are 50kw / m 2 、30kw / m 2 、10kw / m 2 Buoyancy criterion number Gr q / Gr th Affected by q and G, the dimensionless length x / d of the bend in a common U-shaped tube is between 70 and 270. Gr in this x / d range q / Gr th If it is greater than 35, criterion one is met. Figure 3 and 4 The only working conditions that meet criterion 1 are G1 and G2.

[0100] Criterion 2: Dimensionless curvature r / R of ordinary U-shaped tube curve <0.0285;

[0101] The dimensionless curvature r / R of a common U-shaped tube curve When <0.0285, the heat transfer can be enhanced by improving the ordinary U-tube into a combined U-tube, and the dimensionless curvature r / R of the ordinary U-tube curveThe smaller the value, the more obvious the enhanced heat transfer effect after the improvement into a combined U-tube; on the contrary, when r / R curve When >0.0285, the heat transfer enhancement brought by transforming the ordinary U-tube into the combined U-tube is not obvious, so it is not advisable to implement the transformation.

[0102] S5. Improve the ordinary U-shape into a combined U-shape tube, and increase the dimensionless curvature by 0.07 to 0.13 on the original basis, while the total heat exchange length remains unchanged.

[0103] The main view of the combined U-tube and the ordinary U-tube is as follows Figure 1 As shown. The length of the combined U-tube and the ordinary U-tube is the same. Based on the conventional U-tube, the combined U-tube improves the original 180° bend into two 90° bends and a part of the straight tube. Among them, r is the inner radius of the U-tube and the combined U-tube; L is the bend length of the U-tube, R curve is the bending radius of the ordinary U-shaped tube, R curv0 is the bending radius of the combined U-shaped tube, H is the length of the straight tube portion of the combined U-shaped tube connecting the straight tube heating section and the return bend, L 0 The length of the straight pipe section that connects the two bends of the combined U-tube.

[0104] In order to meet the actual assembly space, two constraints need to be met: the length of the bend along the way is consistent (to ensure the consistency of the heat exchange area) and the width of the bend is consistent (to ensure the consistency of the assembly space). The corresponding geometric relationship is:

[0105] H / L=(π-2) / 2π-(π-2) / 2×(R curv0 / L)

[0106] L 0 / L=(2 / π)-2×(R curv0 / L)

[0107] When ordinary U-tubes are improved into combined U-tubes, the local curvature increases too much, which will lead to an increase in pressure drop and an increase in the impact of the supercritical fluid on the tube wall, affecting the safety of use; if the curvature increases too little, the heat exchange performance of the improved combined U-tube will be limited. Considering the safety and economy of the combined elbow supercritical U-tube heat exchanger, when ordinary U-tubes are improved into combined U-tubes, the dimensionless curvature increases by 0.07 to 0.13 on the original basis, and the total heat exchange length remains unchanged. The heat exchange capacity of three ordinary U-tubes with different curvatures (located in the innermost, middle and outermost layers of the heat exchange tube bundle) and the improved combined U-tube are compared.

[0108] like Figure 5As shown in the figure, a, b, and c are the comparison of the heat transfer coefficients along the line of the ordinary U-shaped tube with dimensionless curvature equal to 0.015, 0.0285, and 0.26 and the combined U-shaped tube with dimensionless curvature increased by 0.1 (i.e., 0.115, 0.1285, and 0.36, respectively). The working condition is G = G1, i.e., 400kg / (m 2 ·s), the other conditions are p=3.55Mpa, q=30kW / m 2 , T in =345K. Figure 3 It can be seen that the buoyancy criterion number Gr in the curved section under this condition is q / Gr th >35, meeting the first design criterion.

[0109] When the curvature is 0.26 (the innermost U-tube), the overall heat transfer capacity of the combined U-tube and the ordinary U-tube is not much different, and the maximum heat transfer coefficient is only 5.42% different. When the curvature is 0.0285 (the middle U-tube), the combined U-tube has a double peak of heat transfer, the heat transfer capacity is enhanced in the bend section, and the heat transfer in the straight section is weakened, but in general, the heat transfer capacity of the combined U-tube is close to that of the ordinary U-tube at this time. When the curvature is 0.015 (the outermost U-tube), the combined U-tube has a double peak of heat transfer due to the increase in curvature, and the heat transfer is also greatly enhanced in the straight section of the combined U-tube, and the overall heat transfer capacity is increased by 1.15 times.

[0110] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific implementation methods described. Obviously, many modifications and changes can be made according to the content of this specification. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can understand and use the present invention well. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A combined elbow type supercritical U-tube heat exchanger, characterized in that: The heat exchanger comprises: a tube-side outlet pipe (1), a tube-side inlet pipe (2), a shell-side inlet pipe (3), a shell-side outlet pipe (4), a head (5), a pipe box (6), a combined U-shaped tube (7), a common U-shaped tube (8), a baffle (9), a shell (10), and a tube sheet (11); the combined U-shaped tube (7) and the common U-shaped tube (8) are arranged in the shell (10), the inner side is the common U-shaped tube (8), and the remaining peripheral tube bundles are all combined U-shaped tubes (7); a first fluid passes through the tube bundle, and the inner side wall of the shell (10) and the outer side wall of the tube bundle form a heat exchange cavity, and a second fluid flows through the heat exchange cavity; The dimensionless curvature of the outermost layer of the common U-shaped tube (8) is greater than or equal to 0.015, and the dimensionless curvature of the innermost layer is less than or equal to 0.26; the inner radius r of the common U-shaped tube (8) and the combined U-shaped tube (7) is 10.3 mm, the wall thickness is 1.2 mm, the distance from the outer surface of the outermost layer of the heat exchange tube of the U-shaped tube heat exchanger bundle to the inner wall of the shell (10) is greater than or equal to 8 mm, and the distance between the heat exchange tubes is not less than 1.25 times the inner diameter of the U-shaped tube; The combined U-shaped tube (7) is specifically: the original 180° bend is improved into two 90° bends and a part of a straight tube, wherein L is the bend length of the U-shaped tube, R curve is the bending radius of the ordinary U-shaped tube, R curv0 is the bending radius of the combined U-shaped tube, H is the length of the straight tube portion of the combined U-shaped tube connecting the straight tube heating section and the return bend, and L0 is the length of the straight tube portion of the combined U-shaped tube connecting the two return bends; In order to meet the real assembly space, the two U-tube geometric structures meet two constraints: the length of the bend along the same and the width of the bend are consistent. The corresponding geometric relationship is: H / L=(π-2) / 2π-(π-2) / 2×(R curv0 / L) L0 / L=(2 / π)-2×(R curv0 / L).

2. The combined elbow type supercritical U-tube heat exchanger according to claim 1, characterized in that: The first fluid flows into the pipe box (6) from the tube side inlet pipe (2), and the pipe box (6) evenly distributes the incoming fluid into the common U-shaped tube (8) and the combined U-shaped tube (7). After the first fluid completes heat exchange, it flows out from the tube side outlet pipe (1). The second fluid enters the shell side from the shell side inlet pipe (3), completes heat exchange with the first fluid in the heat exchange cavity on the outside of the tube bundle and the inside of the tube wall, and then flows out from the shell side outlet pipe (4).

3. The combined elbow type supercritical U-tube heat exchanger according to claim 1, characterized in that: The baffle (9) is arranged in the shell (10) to divide the interior of the shell (10) into a plurality of sub-areas, and the connection between adjacent sub-areas is staggered and distributed on the upper and lower sides of the shell (10). At the same time, the shell-side inlet pipe (3) is arranged at the upper end of the shell (10) so that the two heat exchange media flow in opposite directions. The tube sheet (11) is located at the inlet and outlet sides of the tube bundle to separate the tube side and the shell side. The tube box (6) is located at both ends of the heat exchange tube bundle to evenly distribute the first fluid transported in the pipeline to each heat exchange tube and collect the first fluid together to be sent out of the heat exchanger.

4. A design method for a supercritical U-tube heat exchanger combined with a U-tube bundle, characterized in that: The following steps are involved: S1. Input boundary conditions of ordinary U-tube; S2. Calculate the wall temperature T according to Jackson's empirical correlation w ; S3. According to T w Calculation of the dimensionless Grashof number Gr that characterizes the strength of buoyancy q / Gr th ; S4, determine whether two criteria are met; Principle 1: Gr q / Gr th >35; Criterion 2: Dimensionless curvature r / R of ordinary U-shaped tube curve <0.0285; S5. After the above criteria are met, the ordinary U-tube is improved to a combined U-tube, and the dimensionless curvature is increased by 0.07 to 0.13 on the original basis, and the total heat exchange length remains unchanged; The combined U-tube is specifically: the original 180° bend is improved into two 90° bends and a part of the straight tube, where L is the bend length of the U-tube, R curve is the bending radius of the ordinary U-shaped tube, R curv0 is the bending radius of the combined U-shaped tube, H is the length of the straight tube portion of the combined U-shaped tube connecting the straight tube heating section and the return bend, and L0 is the length of the straight tube portion of the combined U-shaped tube connecting the two return bends; In order to meet the real assembly space, the two U-tube geometric structures meet two constraints: the length of the bend along the same and the width of the bend are consistent. The corresponding geometric relationship is: H / L=(π-2) / 2π-(π-2) / 2×(R curv0 / L) L0 / L=(2 / π)-2×(R curv0 / L).

5. The design method of the combined U-shaped tube bundle according to claim 4, characterized in that: The boundary conditions in step S1 include boundary conditions and thermophysical information of the supercritical fluid in an ordinary U-tube, wherein the boundary conditions of the supercritical fluid include mass flow rate, inlet and outlet temperature, and pressure, and the thermophysical information includes dynamic viscosity, enthalpy, constant-pressure specific heat capacity, and Prandtl number.

6. The design method of the combined U-shaped tube bundle according to claim 4, characterized in that: In step S2, the wall temperature T of each section is predicted according to the Jackson empirical correlation formula. w , the specific steps are as follows: S2.

1. Divide the ordinary U-shaped tube into sections and calculate the mainstream temperature T of each section. b , mainstream temperature T b The enthalpy value H b The calculation formula is as follows: where q w is the wall heat flux, d i is the inner diameter of the U-tube, m is the mass flow rate at the inlet, Δy is the length of each section of the U-tube, and the mainstream temperature T of each section is determined by the constant pressure and the enthalpy value of each section. b ; S2.2, iteratively calculate the heat transfer coefficient (HTC) according to Jackson's empirical correlation; The Jackson empirical correlation in step S2.2 is as follows: Where Nu is the Nusselt number, which is defined as Re is the Reynolds number, Pr is the Prandtl number, T pc is the critical temperature (K) at the corresponding pressure, T w is the wall temperature (K), ρ b is the density of the mainstream working fluid (kg / m 3 ), ρ w is the density of the working fluid at the wall (kg / m 3 ), is the average specific heat capacity (J / (kg·K)), expressed as Subscript b refers to mainstream and w refers to wall; S2.

3. When the iteration error is less than 1%, the calculated wall temperature T is output. w .

7. The design method of the combined U-shaped tube bundle according to claim 4, characterized in that: The Gr in step S3 q / Gr th It is a quantitative criterion for the buoyancy of supercritical organic working fluids in horizontal flow, and can better predict the development of buoyancy along the flow direction during heat transfer; Gr q and Gr th It is expressed by the following formula: in according to To determine, b is the mainstream dynamic viscosity (Pa·s), λ b is the mainstream thermal conductivity (W / (m·K)), H b is the mainstream enthalpy (J / kg), H w is the enthalpy of the fluid near the wall (J / kg), ρ w is the density of the working fluid at the wall (kg / m 3 ), ρ b is the density of the mainstream working fluid (kg / m 3 ), subscript b refers to mainstream and w refers to wall.

Images