In-pipe enhanced heat transfer element structure of ORC (organic Rankine cycle) waste heat recovery system

Abstract

The invention belongs to the technical field of waste heat utilization equipment, and particularly relates to an in-pipe enhanced heat transfer element structure of an ORC waste heat recovery system. Fluid in the pipe is divided through the spiral twisted belt, so that the fluid spirally moves in the twisting direction of the twisted belt; the spiral direction of the spiral twisted belt is opposite to that of the twisted sheet, so that reverse spiral shear flow is formed, turbulence disturbance is enhanced, and the thickness of a thermal boundary layer is reduced; the inverted triangular tooth grooves are formed in the surfaces of the two axial sides of the twisting piece along the spiral track, local vortex generation is optimized, and flow resistance is reduced. Fluid in a pipe wall area is disturbed through the layered turbulent flow structure so as to thin a boundary layer, the transfer speed of heat between the fluid and the wall face is increased, and then the heat exchange effect of the heat exchanger is remarkably improved. The device can be directly matched with an existing ORC system standard heat exchange tube, customized transformation is not needed, the cost is low, and installation and use are convenient and fast.

Description

Technical Field

[0001] The invention belongs to the technical field of waste heat utilization equipment, and in particular relates to an in-tube enhanced heat transfer element structure of an ORC waste heat recovery system. Background Art

[0002] At present, the main energy consumed is still fossil fuels. With the rapid development of industrial technology, energy consumption is rapid, and standard coal consumption accounts for 54.5% of the total energy consumption. However, the energy utilization efficiency is only about 30%, and a lot of medium and low temperature heat energy below 300℃ is directly discharged into the atmosphere, resulting in the energy system being unable to take into account both economic and environmental protection, and the ability to optimize operation is not high. Therefore, in response to the energy, economic and environmental dilemmas, the rational use of medium and low temperature waste heat is of great significance to reducing non-renewable energy consumption, reducing carbon emissions and protecting the environment.

[0003] Organic Rankine Cycle (ORC) is a power cycle that uses low-boiling-point organic working fluids instead of water to effectively convert waste heat into electrical energy. The component with the highest loss rate has a great influence on the thermal and economic performance of the whole system. The traditional ORC evaporator adopts smooth tube or simple twisted belt structure, with large thermal boundary layer thickness and low heat transfer coefficient (usually less than 500W / (m 2 ·K)), leading to the evaporator The loss rate is as high as the total system Therefore, it is necessary to design an enhanced heat transfer element structure in the tube of the ORC waste heat recovery system to solve the above technical problems. Summary of the invention

[0004] The purpose of the present invention is to improve the ORC waste heat recovery system evaporator in the prior art In order to solve the problem of high heat loss rate, an in-tube enhanced heat transfer element structure of an ORC waste heat recovery system is provided.

[0005] A heat transfer enhancement element structure in a tube comprises a spiral twisted belt and a twisted plate. The spiral twisted belt and the twisted plate are arranged concentrically in the tube. The twisted plate is the main component for generating longitudinal vortex in the central area. The spiral twisted belt is a thin belt structure that spirally extends around the twisted plate along the evaporator tube wall. The spiral directions of the spiral twisted belt and the twisted plate are opposite.

[0006] Furthermore, the inner diameter of the tube is D, the axial lengths of the spiral twisted band and the twisted sheet are both H, and H / D is 4-12.

[0007] Furthermore, the radial width of the spiral twisted band is 0.15D to 0.25D, and the radial width of the twisted sheet is 0.25D to 0.35D.

[0008] Furthermore, the helix angle of the spiral twisted tape is 35° to 55°, and the helix angle of the twisted sheet is 50° to 70°, where the helix angle is the angle between the central axis of the spiral twisted tape / twisted sheet and the direction of fluid flow; the spiral directions of the spiral twisted tape and the twisted sheet are opposite, so as to form a reverse spiral shear flow, enhance turbulent disturbance, and reduce the thickness of the thermal boundary layer.

[0009] Furthermore, the axial surfaces of both sides of the twisting blade are provided with tooth grooves along a spiral trajectory to optimize the generation of local vortexes and reduce flow resistance.

[0010] Furthermore, the tooth groove is an inverted triangle, the tooth groove depth is 0.07D to 0.16D, the tooth groove top surface width is 0.08D to 0.18D, and the ratio of the tooth groove depth to the top surface width is 1:0.5 to 1:1.2, avoiding stress concentration and fatigue crack propagation due to local geometric mutations.

[0011] Furthermore, the tooth grooves are distributed at equal intervals along the spiral track of the twisting plate, the interval between adjacent tooth grooves is 0.075H to 0.1H, and the overlap rate of axial projections of adjacent tooth grooves is 20% to 40%.

[0012] An ORC waste heat recovery system with an in-tube enhanced heat transfer element structure comprises an evaporator, an expander, a condenser, a liquid storage tank and a working fluid pump which are sequentially connected to form a closed loop, and a generator connected to the expander; the in-tube enhanced heat transfer element structure is installed in the heat exchange tube inside the evaporator.

[0013] Furthermore, after the organic working fluid is heated into saturated or superheated steam through the heat exchange tube in the evaporator, it enters the expander, where it releases energy by reducing the pressure to drive the generator to generate electricity. The expanded exhaust steam enters the condenser and is condensed into liquid. The liquid organic working fluid enters the liquid storage tank, and returns to the evaporator after the pressure is increased by the working fluid pump, completing an organic Rankine cycle.

[0014] Furthermore, the heat transfer enhancement element structure in the heat exchange tube is processed in sections, and after being disassembled into a plurality of spoiler elements, they are connected end to end and concentrically arranged in the heat exchange tube to form a toothed spiral geometric structure.

[0015] The beneficial effects of the present invention are:

[0016] The present invention divides the fluid in the tube by a spiral twisted band, so that the fluid makes a spiral motion along the twisting direction of the twisted band; the spiral twisted band is opposite to the spiral direction of the twisted blade, so as to form a reverse spiral shear flow, enhance turbulent disturbance, and reduce the thickness of the thermal boundary layer; and the inverted triangular tooth grooves are provided along the spiral trajectory on the axial surfaces of the twisted blade to optimize the local vortex generation and reduce the flow resistance. The present invention disturbs the fluid in the tube wall area through a layered disturbance structure to thin the boundary layer, speed up the heat transfer speed between the fluid and the wall, and thus significantly improve the heat exchange effect of the heat exchanger. The present invention can be directly adapted to the existing standard heat exchange tubes of the ORC system without customized modification, and has low cost and is easy to install and use. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is the schematic diagram of the ORC waste heat recovery system.

[0018] Figure 2 Schematic diagram of an in-tube enhanced heat transfer element structure in the present invention.

[0019] Figure 3 The figure is an axonometric view of an in-tube enhanced heat transfer element structure in the present invention.

[0020] Figure 4 This is a right side view of an in-tube enhanced heat transfer element structure in the present invention.

[0021] Description of reference numerals:

[0022] 1. Evaporator; 2. Expander; 3. Generator; 4. Condenser; 5. Liquid storage tank; 6. Working fluid pump; 7. ORC waste heat recovery system; 8. Spiral twisted belt; 9. Twist blade; 10. Tooth groove; 11. Heat exchange tube wall; 12. First spoiler element; 13. Second spoiler element; 14. Third spoiler element; 15. Fourth spoiler element; 16. Heat exchange tube. DETAILED DESCRIPTION

[0023] The present invention is further described below in conjunction with the accompanying drawings.

[0024] like Figure 2 and Figure 3 As shown, a structure of an enhanced heat transfer element in a tube includes a spiral twisted tape 8 and a twisted sheet 9. The spiral twisted tape 8 and the twisted sheet 9 are arranged concentrically in the tube. The twisted sheet 9 is the main component for generating longitudinal vortex in the central area. The spiral twisted tape 8 is a thin strip structure that spirally extends around the twisted sheet 9 along the evaporator tube wall.

[0025] The spiral angle of the spiral twisted tape 8 is 35° to 55°, and the spiral angle of the twisted sheet 9 is 50° to 70°. The spiral angle is the angle between the central axis of the spiral twisted tape 8 / twisted sheet 9 and the flow direction of the fluid. The spiral directions of the spiral twisted tape 8 and the twisted sheet 9 are opposite, so as to form a reverse spiral shear flow, enhance turbulent disturbance and reduce the thickness of the thermal boundary layer.

[0026] The inner diameter of the tube is D, the axial lengths of the spiral twisted band 8 and the twisted sheet 9 are both H, and H / D is 4-12.

[0027] The radial width of the spiral twisted band 8 is limited to 0.15D-0.25D to ensure that it can effectively divide the core fluid and form a stable spiral shear flow, while avoiding excessive blockage of the flow channel and causing a surge in pressure drop; the radial width of the twisted blade 9 is 0.25D-0.35D, which not only ensures the strength of the longitudinal vortex in the central area, but also reduces metal consumables. The spiral angle of the spiral twisted band 8 is set to 35°-55°, within which the coordination of the fluid shear effect and the axial flow can be taken into account; the spiral angle of the twisted blade 9 is 50°-70°. Through single variable control experiments, when the spiral direction is opposite, the thermal boundary layer disturbance efficiency is increased by 25% to 30% compared with the same direction structure.

[0028] The surfaces of both sides of the axial direction of the twisting plate 9 are provided with tooth grooves 10 along the spiral trajectory to optimize the local vortex generation and reduce the flow resistance. The tooth grooves 10 are inverted triangles, the depth of the tooth grooves 10 is 0.07D to 0.16D, the width of the top surface of the tooth grooves 10 is 0.08D to 0.18D, and the ratio of the depth to the top surface width of the tooth grooves 10 is 1:0.5 to 1:1.2, to avoid stress concentration and fatigue crack propagation due to local geometric mutations. The tooth grooves 10 are periodically distributed along the axial direction of the twisting plate 9, and the spacing between adjacent tooth grooves 10 is 0.075H to 0.1H, to ensure the continuity of local vortex generation.

[0029] According to the theory of material mechanics and fracture mechanics, the depth-to-width ratio of the inverted triangle tooth groove, which is the ratio of the depth to the top width, directly affects the stress distribution at the tooth root. When the depth-to-width ratio exceeds 1:1.2, the radius of curvature at the root of the tooth groove is too small, which is easy to cause stress concentration; when the depth-to-width ratio is lower than 1:0.5, the tooth groove is too shallow, which weakens the turbulent disturbance effect. Through orthogonal tests and finite element analysis FEA verification, a depth-to-width ratio of 1:0.5 to 1:1.2 can balance heat transfer enhancement and structural reliability.

[0030] like Figure 1 As shown, an ORC waste heat recovery system with an in-tube enhanced heat transfer element structure includes an evaporator 1, an expander 2, a condenser 4, a liquid storage tank 5 and a working fluid pump 6 which are sequentially connected to form a closed loop, and a generator 3 connected to the expander 2; the in-tube enhanced heat transfer element structure is installed in the heat exchange tube 16 inside the evaporator 1.

[0031] The organic working fluid undergoes isobaric heat exchange in the evaporator 1, and is heated to saturated or superheated steam in the evaporator 1 by recovering medium and low-grade heat energy as the heat source of the ORC waste heat recovery system 7. The organic working fluid that has completed heat exchange in the evaporator 1 enters the expander 2, where the pressure is reduced, driving the generator 3 to generate electricity and convert it into electrical energy. The expanded exhaust steam enters the condenser 4, where it is condensed into liquid after heat exchange with cooling water. The liquid organic working fluid enters the liquid storage tank 5, enters the working fluid pump 6 to increase the pressure, and is then sent to the evaporator 1, completing an organic Rankine cycle.

[0032] like Figure 2 As shown, the heat transfer enhancement element structure in the heat exchange tube 16 is processed in sections, and after being disassembled into multiple spoiler elements, they are connected end to end and concentrically arranged in the heat exchange tube 16 to form a toothed spiral geometric structure.

[0033] In one embodiment of the present invention, the heat transfer enhancement element structure in the tube includes a first spoiler element 12, a second spoiler element 13, a third spoiler element 14, and a fourth spoiler element 15. The four groups of spoilers are arranged concentrically along the inner axis of the heat exchange tube wall 11, forming a toothed spiral geometric structure. Different from the old traditional twisted tape, the spoiler element of the present invention is made by twisting two different rectangular thin plates.

[0034] The traditional twisted tape itself is huge in size, consumes a lot of metal material, and has a small flow area in the heat exchange pipe. For some non-Newtonian fluids with more impurities, blockage is very likely to occur in the heat exchange tube. At the same time, due to the single flow channel segmentation method, it is impossible to take into account the turbulent disturbances in the tube core and the tube wall area at the same time, resulting in limited thinning effect of the thermal boundary layer. In the present invention, the spiral twisted tape 8 will divide the fluid in the tube, so that the fluid will move in a spiral along the twisting direction of the twisted tape, disturb the fluid in the tube wall area to thin the boundary layer, speed up the heat transfer speed between the fluid and the wall, and thus significantly improve the heat exchange effect of the heat exchanger. The twisted blade 9 is much smaller in width than the traditional twisted tape, so the twisted blade 9 consumes relatively less metal. The principle is that the fluid is disturbed by the shape resistance of the protrusion and generates a vortex at the core of the tube, and the fluid is periodically disturbed in the tube wall area, thereby obtaining a higher heat transfer efficiency.

[0035] The in-tube heat transfer enhancement element provided by the present invention is used to improve the performance of the evaporator 1 and enhance the convection heat transfer in the tube, and fully utilize the different heat transfer enhancement mechanisms of the twisted sheet 9 and the spiral twisted belt 8, thereby improving the overall heat transfer effect of the heat exchange tube 16.

[0036] When the traditional ORC evaporator uses smooth tubes, the heat transfer coefficient is less than 500W / (m 2 ·K), while the heat transfer coefficient of a simple spiral twisted ribbon is only increased to 600-800W / (m 2·K), and the flow resistance increases by about 30% to 50%. Compared with the prior art, the present invention improves the heat transfer coefficient to 800-1300W / (m 2 ·K), while the flow resistance only increases by 16% to 25%. At the same time, as one of the most convenient heat transfer enhancement technologies, it can be directly adapted to the existing ORC system standard heat exchange tubes (such as ASTM B88 copper tubes or ASME SA214 stainless steel tubes) without customized modifications. This design not only provides flexible space for the engineering and technical transformation of old heat exchangers, but also reduces labor installation costs and investment costs, further improving the convenience of installation and use. The ORC system efficiency is significantly improved through low-cost structural transformation, which is suitable for medium and low temperature (80-300℃) scenarios such as industrial waste heat, geothermal power generation, ship waste heat recovery, and data center cooling systems, and has the potential for large-scale commercialization.

[0037] The above description is only a preferred embodiment of the present invention and is 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. An in-tube enhanced heat transfer element structure, characterized by: The invention comprises a spiral twisted band (8) and a twisted plate (9); the spiral twisted band (8) and the twisted plate (9) are arranged concentrically in the tube; the twisted plate (9) is a main component for generating longitudinal vortex in the central area; the spiral twisted band (8) is a thin band structure that spirally extends around the twisted plate (9) along the evaporator tube wall; and the spiral directions of the spiral twisted band (8) and the twisted plate (9) are opposite.

2. The in-tube enhanced heat transfer element structure according to claim 1, characterized in that: The inner diameter of the tube is D, the axial lengths of the spiral twisted band (8) and the twisted plate (9) are both H, and H / D is 4 to 12.

3. The in-tube enhanced heat transfer element structure according to claim 2, characterized in that: The radial width of the spiral twisted band (8) is 0.15D to 0.25D, and the radial width of the twisted sheet (9) is 0.25D to 0.35D.

4. The in-tube enhanced heat transfer element structure according to claim 3, characterized in that: The spiral angle of the spiral twisted band (8) is 35° to 55°, and the spiral angle of the twisted plate (9) is 50° to 70°. The spiral angle is the angle between the central axis of the spiral twisted band (8) / twisted plate (9) and the flow direction of the fluid. The spiral directions of the spiral twisted band (8) and the twisted plate (9) are opposite, so as to form a reverse spiral shear flow, enhance turbulent disturbance, and reduce the thickness of the thermal boundary layer.

5. The in-tube enhanced heat transfer element structure according to claim 4, characterized in that: The axial surfaces of the twisting blade (9) are provided with tooth grooves (10) along a spiral track, so as to optimize the generation of local vortices and reduce flow resistance.

6. The in-tube enhanced heat transfer element structure according to claim 5, characterized in that: The tooth groove (10) is an inverted triangle, the depth of the tooth groove (10) is 0.07D to 0.16D, the width of the top surface of the tooth groove (10) is 0.08D to 0.18D, and the ratio of the depth to the top surface width of the tooth groove (10) is 1:0.5 to 1:1.2, thereby avoiding stress concentration and fatigue crack propagation due to local geometric mutation.

7. The in-tube enhanced heat transfer element structure according to claim 6, characterized in that: The tooth grooves (10) are distributed at equal intervals along the spiral track of the twisting plate (9), the interval between adjacent tooth grooves (10) is 0.075H to 0.1H, and the overlap rate of the axial projections of adjacent tooth grooves is 20% to 40%.

8. An ORC waste heat recovery system with the in-tube enhanced heat transfer element structure of claim 1, characterized in that: The invention comprises an evaporator (1), an expander (2), a condenser (4), a liquid storage tank (5) and a working fluid pump (6) which are sequentially connected to form a closed loop, and a generator (3) connected to the expander (2); the in-tube enhanced heat transfer element structure is installed in a heat exchange tube (16) inside the evaporator (1).

9. The ORC waste heat recovery system with an in-tube enhanced heat transfer element structure according to claim 8, characterized in that: After the organic working medium is heated to saturated or superheated steam through a heat exchange tube (16) in an evaporator (1), it enters an expander (2), releases energy by reducing the pressure in the expander (2), and drives a generator (3) to generate electricity. The expanded exhaust steam enters a condenser (4) and is condensed into a liquid state. The liquid organic working medium enters a liquid storage tank (5), is increased in pressure by a working medium pump (6), and then returns to the evaporator (1), completing an organic Rankine cycle.

10. The ORC waste heat recovery system with an in-tube enhanced heat transfer element structure according to claim 9, characterized in that: The heat transfer enhancement element structure in the heat exchange tube (16) is processed in sections and is disassembled into a plurality of flow-disrupting elements, which are then connected end to end and arranged concentrically in the heat exchange tube (16) to form a toothed spiral geometric structure.

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