A novel method for rapid dynamic thermodynamic simulation based on discrete event simulation

Abstract

This invention belongs to the field of dynamic simulation technology for heating pipe networks. It discloses a novel method for rapid dynamic thermal simulation based on discrete event simulation. This method considers heat loss and time delay of water in pipes to simulate heating pipe networks, accurately and quickly calculating the heat propagation of each pipe, playing a crucial role in the planning and optimization of pipe network systems. This is the first time discrete event simulation has been applied to the simulation of district heating pipe networks. It can effectively and accurately handle varying inlet temperatures and flow rates, and is also applicable under zero-flow conditions. The spatiotemporal discretization of variables is automatically determined by the input data. The calculation does not consider transmission delays and numerical temperature diffusion, enabling rapid calculation of heat loss and temperature drop. This model is highly efficient and flexible, achieving unprecedented efficiency while maintaining accuracy, and is suitable for various large-scale branched and ring-shaped pipe networks. This pipe model can be used for rapid dynamic thermal simulation of large-scale centralized heating pipe networks with varying time and spatial steps.

Description

Technical Field

[0001] This invention belongs to the field of dynamic simulation technology of heating pipe networks, specifically involving a novel rapid dynamic thermal simulation method based on discrete event simulation. Background Technology

[0002] As the heating area in northern my country continues to expand, the pressure on centralized heating systems to conserve energy and reduce consumption is also increasing year by year. Currently, heating systems suffer from weak situational awareness. On the one hand, the number of sensors cannot be excessive; on the other hand, due to the strong hydraulic-thermal coupling, large thermal inertia, and variable time delay characteristics of centralized heating systems, current dynamic simulation technologies suffer from common problems such as being limited to nodes, slow calculation speed, and insufficient accuracy, making it difficult to capture dynamic changes in operating conditions. Dynamic thermal simulation of heating systems plays a crucial role in improving the situational awareness of the pipeline network. However, research on the overall system optimization involving pipeline network operation is still relatively lacking. The obstacle to this research is the lack of fast and accurate network models capable of iterative calculation. Accurate simulation of heat propagation helps the pipeline network to precisely adjust according to the heat load. The propagation of heat in the pipeline network is affected by two main parameters: transmission time delay and heat loss. The speed of heat propagation is approximately equal to the flow velocity. Under typical operating conditions, the transmission time delay ranges from several minutes to several hours. Depending on the network structure, scale, and operating conditions, heat loss during the heating process accounts for 5% to 20% of the annual heat supply. Therefore, it is essential to establish an accurate dynamic thermodynamic simulation model of the heating system and to accurately calculate the heat propagation of each pipe.

[0003] Currently, the two main methods used for dynamic thermal simulation of heating pipe networks are the Eulerian method and the Lagrangian method. The Eulerian method uses a fixed mesh, where the mesh and fluid particles are independent, making it suitable for partial differential equations, finite volume methods, and finite difference methods. However, it is significantly affected by the time and spatial step size of the mesh; coarse meshes and fluctuating conditions reduce accuracy and stability. Furthermore, calculating fine meshes for large-scale heating networks requires significant resources and time, hindering timely optimization and control. The Lagrangian method, on the other hand, targets fluid particles, tracking the transport of a specific particle (control volume). It can be combined with the finite volume method, and the variable control volume allows for the feasibility of variable spatial step sizes. Due to the flexibility of spatial discretization, Lagrangian models are more flexible and accurate than models based on the Eulerian method. However, numerical errors caused by the complete mixing of outlet temperatures at different time steps are unavoidable. Moreover, existing studies using a uniform time step still require fine time discretization to accurately simulate rapid temperature changes.

[0004] To address the aforementioned issues, this invention implements a Lagrangian district heating pipeline model based on Discrete Event Simulation (DES), innovatively applying DES to the simulation of district heating networks. This model considers heat loss and time delay of water within the pipeline. It can effectively and accurately handle varying inlet temperatures and flow rates, and is also applicable under zero-flow conditions. The advantages of this method are: (1) the spatiotemporal discretization of variables is automatically determined by the input data; (2) adaptive variable time and spatial step sizes; (3) no numerical diffusion problem; and (4) rapid dynamic simulation of the heating network. Summary of the Invention

[0005] This invention provides a novel method for rapid dynamic thermal simulation based on discrete event simulation. It simulates heating pipe networks by considering the heat loss and time delay of water in pipes, and accurately and quickly calculates the heat propagation of each pipe, which plays a key role in the planning and optimization of pipe network systems.

[0006] The technical solution of this invention:

[0007] A novel method for rapid dynamic thermodynamic simulation based on discrete event simulation includes a transport time model, a temperature model, and a heat loss model.

[0008] (1) The main idea of ​​discrete event simulation is to treat the operation of the heating network as a combination of multiple consecutive events. Events can be viewed as state changes. Events can be created, canceled, and modified during the simulation. The order of occurrence is handled by an event queue, where events are ordered according to their predetermined activation times. Unlike time-driven simulation methods with fixed time steps, the simulation time in discrete event simulation jumps from the current event time to the next event time, which means that the time step depends on the activation of the event and is therefore of variable length.

[0009] The discrete event simulation model includes three types of events: inlet temperature change, flow rate change, and hydraulic front arrival at the pipe end. Inlet temperature change and flow rate change events can occur independently, provided by the input data. A hydraulic front arrival event is generated during the simulation. The flow rate remains constant between two flow rate change events.

[0010] When the inlet temperature or flow rate changes, a hydraulic front forms at the pipe inlet. The hydraulic front travels through the pipe until it reaches the outlet, constituting an arrival event. The sequence of hydraulic fronts is managed by a first-in, first-out (FIFO) queue. The hydraulic front that arrives at the outlet first is called the first front, and the most recently created is the last front. Initially, the queue is empty, and the pipe is initialized for steady-state operation based on constant flow rate and inlet temperature. While the FIFO queue is not empty, arrival events of the first front are always scheduled in the event queue. When a front reaches its first position, the activation time of the arrival event is calculated and updated when the flow rate changes.

[0011] The transport time model includes the transport time τ of the first water boundary and the arrival time t of the first water boundary. out and entry time t in The transport time τ′ of the new water boundary, and the arrival time t of the first new water boundary. out ′ and entry time t in Its specific structure is as follows:

[0012] The transport time τ at the first water boundary is its arrival time t. out and entry time t in The difference, that is:

[0013] τ=t out -t in

[0014] Assuming the initial flow velocity v > 0, the expected arrival time of the first water boundary is calculated as follows:

[0015] t out =t+s i / v

[0016] Among them, s i s is the remaining travel distance of the water boundary; when creating water boundary i, if a previous water boundary exists, s i It is defined as the distance to the previous (i-1) water boundary; if no previous water boundary exists, then s is... i Set to pipe length S; t is the current time;

[0017] When the flow velocity changes, the arrival time needs to be updated; the arrival time t of the new water boundary. out ′ is given by the following formula:

[0018] t out ′=t c +(t out -t)v / v′

[0019] Where v′ refers to the new flow velocity, t c The moment when the flow velocity changes;

[0020] When the flow velocity is t out and t out When the intervals between ′ remain constant, the entry time t of the new water boundary in The following relation exists:

[0021] t in ′=t in +(t out ′-t out )v out / v in

[0022] Among them, t in It is the initial entry time, v out v is the outflow velocity of the fluid within the pipe section. in The inflow velocity of the fluid within the pipe section;

[0023] The transport time τ′ at the new water boundary has the following relationship:

[0024]

[0025] Where, Δt=t out ′-t out It is the difference between the arrival time of the new water boundary and the arrival time of the previous water boundary;

[0026] (2) In the temperature model;

[0027] The water boundary temperature in the pipeline changes exponentially with time, gradually approaching the ambient temperature; the current temperature T of the water boundary when it reaches the outlet after a transport time τ. out The following relationship exists:

[0028]

[0029] Among them, T amb Here, R represents the ambient temperature, R represents the total thermal resistance between the water and the environment, ρ represents the density of water, and c... p T represents the specific heat capacity of water, A represents the flow area of ​​the pipe, and T represents the specific heat capacity of water. in It is the temperature of the fluid at the pipe inlet;

[0030] When a new water boundary is formed, a new inlet temperature is generated. Therefore, the inlet temperature with time t is a piecewise exponential function. The inlet temperature function between two breakpoints is described as follows:

[0031]

[0032] in, This refers to the time of the previous inlet temperature change event; k and α are two coefficients, whose initial values ​​are obtained by solving two systems of equations based on adjacent inlet boundaries; t represents the current time, T in ′(t) represents the inlet temperature interpolation at time t;

[0033] Correspondingly, as each water boundary reaches the outlet, the outlet temperature also changes. The inlet temperature follows a piecewise exponential function with time t, and the outlet temperature function between two breakpoints is described as follows:

[0034]

[0035]

[0036] Among them, vin v is the inlet velocity at the historical water boundary. out The exit velocity of the historical water boundary is given by α′ and β′, which are introduced coefficients, and the negative constant C = -1 / (ρc) p AR); It should be noted that instantaneous mode I means that the inlet temperature is at a given t i The temperature changes immediately within a short time and remains at that temperature until t. i+1 The next temperature change. Gradual mode G refers to the inlet temperature changing gradually in each time interval. The inlet temperature distribution is a piecewise exponential function when using exponential interpolation, and a piecewise linear function when using linear interpolation.

[0037] (3) The specific structure of the heat loss model is as follows: the inlet energy input submodule is connected in series with the internal energy change submodule of the medium inside the pipe, and the internal energy change submodule of the medium inside the pipe is connected in parallel with the heat loss submodule and the outlet energy output submodule, respectively, forming a heat loss model; the inlet energy input submodule transfers energy to the fluid inside the pipe, and the fluid transfers energy to the heat loss submodule and the outlet energy output submodule based on its own internal energy change and the energy received, and based on the energy difference with the outside or downstream; the total energy change of the four submodules is 0;

[0038] The heat loss model is established as follows:

[0039]

[0040] Where t0 and t end These are the start time and the end time, respectively; Q in and Q out These represent the inlet and outlet energies, respectively; E(t) represents the internal heat energy, which only exists between t0 and t1. end Time calculation;

[0041] Q in and Q out Calculated using the following formulas respectively:

[0042]

[0043]

[0044] The internal thermal energy between water boundaries i and i-1 is given by the following equation:

[0045]

[0046]

[0047] in, It is the time of generation of water boundary i. It is the activation time of the last inlet temperature change event when water boundary i-1 is generated; when water boundary i-1 is the last one, It equals time t.

[0048] The beneficial effects of this invention are as follows: This invention proposes a novel method for rapid dynamic thermal simulation based on discrete event simulation. It considers heat loss and time delay of water in pipelines to simulate heating networks, accurately and quickly calculating the heat propagation of each pipeline, playing a crucial role in the planning and optimization of pipeline systems. It innovatively applies discrete event simulation to the simulation of district heating networks. This model can effectively and accurately handle varying inlet temperatures and flow rates, and is also applicable under zero-flow conditions. The spatiotemporal discretization of variables is automatically determined by the input data, and the calculation does not consider transmission delays and numerical temperature diffusion, enabling rapid calculation of heat loss and temperature drop. This model is highly efficient and flexible, achieving unprecedented efficiency while maintaining accuracy, and is suitable for various large-scale branched and ring-shaped pipeline networks. This pipeline model can be used for rapid dynamic thermal simulation of large-scale centralized heating networks with varying time and spatial steps. Attached Figure Description

[0049] Figure 1 This diagram illustrates the construction of a novel method for rapid dynamic thermal simulation based on discrete event simulation.

[0050] Figure 2 This is a schematic diagram illustrating a discrete event simulation example.

[0051] Figure 3 This refers to the state changes of discrete events.

[0052] Figure 4 This is a transport time model.

[0053] Figure 5 The temperature distribution of a control volume as it passes through a pipe.

[0054] Figure 6 To simulate the energy balance of the pipeline during the process.

[0055] In the diagram: 1. Transport time model; 2. Temperature model; 3. Heat loss model; 4. Discrete event simulation model; 301. Inlet energy input submodule; 302. Internal energy change submodule of the medium inside the pipe; 303. Heat loss submodule; 304. Outlet energy output submodule. Detailed Implementation

[0056] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0057] A novel method for rapid dynamic thermodynamic simulation based on discrete event simulation includes transport time model 1, temperature model 2, and heat loss model 3.

[0058] Example:

[0059] like Figure 2 , Figure 3 As shown, the discrete event simulation example includes four events: an inlet temperature change at t1, two flow rate changes at t2 and t3, and a water boundary reaching t4. Figure 3 The corresponding status change was displayed.

[0060] At t1, the inlet temperature changes, creating the first and only front F1, and scheduling the corresponding arrival events based on the current flow rate (flow velocity).

[0061] At t2, the flow velocity changes, creating a second hydraulic front F2, and the arrival time of F1 is recalculated based on the new flow velocity.

[0062] At t3, the flow rate changes again, creating the hydraulic front F3, and the arrival time of F1 is recalculated.

[0063] At t4, the hydraulic front F1 arrives and will be removed from the first-in-first-out queue. F2 becomes the first front and, based on the current flow rate, will arrive as planned.

[0064] like Figure 4 As shown, assuming the initial flow velocity v > 0, the expected arrival time of the first hydraulic front is calculated as follows:

[0065] t out =t+s i / v

[0066] When the flow velocity is t out and t out When the intervals between ′ remain unchanged, the t in The following relation exists:

[0067] t in ′=t in +(t out ′-t out v3 / v1

[0068] Where t in v1 is the initial entry time, v3 is the fluid outflow velocity in the example pipe section, and v1 is the fluid inflow velocity in the example pipe section.

[0069] The new transport time τ′ has the following relationship:

[0070]

[0071] like Figure 5 As shown, the temperature of the control volume in the pipeline is an exponential function of the time it takes to asymptotically approach the ambient temperature.

[0072]

[0073] The function for calculating the outlet temperature is as follows:

[0074]

[0075]

[0076] like Figure 6 As shown, the inlet energy input 301 is connected in series with the internal energy change of the medium inside the pipe 302. The internal energy change of the medium inside the pipe 302 is connected in parallel with the heat loss 303 and the outlet energy output 304, forming a series connection to form heat loss model 3. The inlet energy input 301 transfers energy to the fluid inside the pipe. The fluid, based on its own internal energy change and the energy received, transfers energy to the heat loss 303 and the outlet energy output 304 according to the energy difference with the outside or downstream. The sum of the energy changes of the four is 0.

[0077] It should be noted that this invention is based on the following five assumptions: uniform flow; neglecting the difference between the thermal front and the flow front; constant ambient temperature in the pipe; negligible heat caused by friction between the pipe wall and water; and disregarding radial heat transfer.

[0078] It should be noted that the energy calculation is based on the ambient temperature. In linear interpolation, it is assumed that the inlet temperature is a piecewise linear function of t, and the outlet temperature distribution is approximately a piecewise linear function.

[0079] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A novel method for rapid dynamic thermodynamic simulation based on discrete event simulation, characterized in that, This novel method for rapid dynamic thermodynamic simulation includes a transport time model, a temperature model, and a heat loss model. (1) The transport time model includes the transport time τ of the first water boundary and the arrival time t of the first water boundary. out and entry time t in The transport time τ′ of the new water boundary, and the arrival time t of the first new water boundary. out ′ and entry time t in Its specific structure is as follows: The transport time τ at the first water boundary is its arrival time t. out and entry time t in The difference, that is: τ=t out -t in Assuming the initial flow velocity v > 0, the expected arrival time of the first water boundary is calculated as follows: t out =t+s i / v Among them, s i s is the remaining travel distance of the water boundary; when creating water boundary i, if a previous water boundary exists, s i It is defined as the distance to the previous (i-1) water boundary; if no previous water boundary exists, then s is... i Set to pipe length S; t is the current time; When the flow velocity changes, the arrival time needs to be updated; the arrival time t of the new water boundary. out ′ is given by the following formula: t out ′=t c +(t out -t)v / v′ Where v′ refers to the new flow velocity, t c The moment when the flow velocity changes; When the flow velocity is t out and t out When the intervals between ′ remain constant, the entry time t of the new water boundary in The following relation exists: t in ′=t in +(t out ′-t out )v out / v in Among them, t in It is the initial entry time, v out v is the outflow velocity of the fluid within the pipe section. in The inflow velocity of the fluid within the pipe section; The transport time τ′ at the new water boundary has the following relationship: Where, Δt=t out ′-t out It is the difference between the arrival time of the new water boundary and the arrival time of the previous water boundary; (2) In the temperature model; The water boundary temperature in the pipeline changes exponentially with time, gradually approaching the ambient temperature; the current temperature T of the water boundary when it reaches the outlet after a transport time τ. out The following relationship exists: Among them, T amb Here, R represents the ambient temperature, R represents the total thermal resistance between the water and the environment, ρ represents the density of water, and c... p T represents the specific heat capacity of water, A represents the flow area of ​​the pipe, and T represents the specific heat capacity of water. in It is the temperature of the fluid at the pipe inlet; When a new water boundary is formed, a new inlet temperature is generated. Therefore, the inlet temperature with time t is a piecewise exponential function. The inlet temperature function between two breakpoints is described as follows: in, This refers to the time of the previous inlet temperature change event; k and α are two coefficients, whose initial values ​​are obtained by solving two systems of equations based on adjacent inlet boundaries; t represents the current time, T in ′(t) represents the inlet temperature interpolation at time t; Correspondingly, as each water boundary reaches the outlet, the outlet temperature also changes. The inlet temperature follows a piecewise exponential function with time t, and the outlet temperature function between two breakpoints is described as follows: Among them, v in v is the inlet velocity at the historical water boundary. out The exit velocity of the historical water boundary is given by α′ and β′, which are introduced coefficients, and the negative constant C = -1 / (ρc) p AR); (3) The specific structure of the heat loss model is as follows: the inlet energy input submodule is connected in series with the internal energy change submodule of the medium inside the pipe, and the internal energy change submodule of the medium inside the pipe is connected in parallel with the heat loss submodule and the outlet energy output submodule, respectively, forming a heat loss model; the inlet energy input submodule transfers energy to the fluid inside the pipe, and the fluid transfers energy to the heat loss submodule and the outlet energy output submodule based on its own internal energy change and the energy received, and based on the energy difference with the outside or downstream; the total energy change of the four submodules is 0; The heat loss model is established as follows: Where t0 and t end These are the start time and the end time, respectively; Q in和 Q out These represent the inlet and outlet energies, respectively; E(t) represents the internal heat energy, which only exists between t0 and t1. end Time calculation; Q in and Q out Calculated using the following formulas respectively: The internal thermal energy between water boundaries i and i-1 is given by the following equation: in, It is the time of generation of water boundary i. It is the activation time of the last inlet temperature change event when water boundary i-1 is generated; when water boundary i-1 is the last one, It equals time t.

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