Simulation and Optimization of the Operation Efficiency of Heat Exchange Units in Intelligent Heating Systems Based on Entransy Dissipation Thermal Resistance

So as to investigate heat exchange efficiency of heat exchange units under different operating conditions, this paper builds a simulation analysis model for heat exchange units in the framework of intelligent district heating systems based on the Modelica unified modeling language and the MWorks platform. Analyze the real-time operating efficiency of heat exchange units using entransy dissipation thermal resistance as an evaluation indicator of heat exchange efficiency. This paper analyzes the trend of heat exchange efficiency changes of heat exchange units when the primary side flow rate, secondary side flow rate, primary side inlet temperature, secondary side inlet temperature, and secondary side bypass pipeline valve opening change. The simulation results verify that when the primary side and the secondary side flow match, the entransy dissipation thermal resistance is the smallest, and its heat transfer efficiency is the highest. Under the simulation condition where the convective heat transfer coefficient is constant, the entransy dissipation thermal resistance is independent of the inlet temperature changes on the primary and secondary sides. The minimum entransy dissipation thermal resistance could be achieved, which represents the optimal real-time heat exchange efficiency state, when the valve opening of the mixed water heat exchange unit is 45% under the specific flow conditions.


Introduction
Based on the goal of carbon peaking and carbon neutrality, reducing energy consumption is a key task of dual carbon targets.Building energy consumption is a major contributor to the increase in energy consumption.In heating systems, the heat exchange efficiency of heat exchange units is one of the main factors affecting energy consumption.Guo et al. proposed a new heat transfer performance evaluation index entransy for performance optimization in the heat transfer process [1] .Fu conducted research and analysis on the optimization of plate-fin heat exchangers using the entransy dissipation theory [2] .The rapid development of simulation theory and technology has made it the most important means of verification after theoretical and experimental research.Huang used MATLAB software to model plate heat exchangers and conducted research and analysis on their operational optimization [3] .Zhao used the MWorks modeling platform based on Modelica language to simulate and optimize the plate-fin heat exchanger [4] .
At present, the use of entransy dissipation of thermal resistance as a real-time operational evaluation index of heat exchanger performance in simulation is not common.Some simulation software uses signal block diagrams for graphic modeling, which cannot actually reflect the connection relationship between components.Modelica language [5] is an object-oriented, non-causal relationship simulation modeling language with bi-directional data flow.It is suitable for the unified modeling of multi-domain systems involving hydraulic-thermal coupling and real-time control under the conditions of intelligent heating [6] .

Simulation Model
This article uses the modeling platform MWorks [7] based on Modelica language to conduct a simulation analysis of the heat exchanger unit.Firstly, a model of the heat exchanger unit is established, which includes the heat exchanger components and the entransy dissipation thermal resistance components.Two types of heat exchangers are constructed: a traditional heat exchanger unit and a mixed water heat exchanger unit.The model is used to simulate and analyze the relationship between flow rate and temperature changes and the entransy dissipation thermal resistance.The simulation analysis is used to obtain the flow rate and inlet temperature range with higher heat exchange efficiency.The mixed water heat exchange unit model is used to simulate and analyze the opening of the secondary bypass valve and the entransy dissipation thermal resistance, exploring the heat exchange efficiency under different proportions of the station internal heat exchange cycle and the external pipe network cycle.Due to space limitations, not all model component equations are listed.

2.1.Heat Exchanger Component
An important part of a heat exchange unit is the heat exchanger.The heat exchange unit is divided into a traditional heat exchange unit and a mixed water heat exchange unit [8] .

2.1.1.Conventional Heat Exchange Unit Module
The traditional heat exchange unit is a secondary heating system connected by a plate heat exchanger.The traditional heat exchange unit is shown in Figure 1.The difference between the mixed water heat exchange unit and the traditional heat exchange unit is that the secondary heating system of the mixed water heat exchange unit can be divided into two subsystems: the internal heat exchange system and the external piping system.The division of the two systems depends on their different locations and purposes.The main function of the station's internal heating system is to provide certain equipment and conditions for heat exchange between the primary and secondary pipe networks, while the station external pipe network system is responsible for the transmission and distribution of circulating hot water to meet the heating needs of heat users [9] .The schematic diagram of the mixed water heat exchange unit is shown in Figure 2.

2.2.Entransy dissipation of thermal resistance Components
Entransy is a physical quantity used to describe the heat transfer capacity of an object.Based on the new concept of entransy, this dissipation is called " entransy dissipation [10] ".The entransy dissipation thermal resistance is a thermal resistance redefined under the entransy theory, and the smaller the entransy dissipation thermal resistance, the better the heat transfer performance, which can naturally serve as an evaluation index for heat transfer performance.The equation for entransy dissipation thermal resistance in plate heat exchangers is: . ( Due to the bypass pipeline in the mixed water heat exchanger unit, the entransy dissipation thermal resistance of the mixed water heat exchanger unit is higher than that of the traditional heat exchanger unit by an additional mixed water thermal resistance.Consequently, the total entransy dissipation thermal resistance equation for the mixed water heat exchanger unit is: (

Simulation results and analysis
This paper uses the established heat exchange unit model for simulation analysis.It is assumed that the in primary and secondary side flow, primary and secondary side inlet temperature in the heat exchange unit change, and the opening degree the bypass pipeline adjustment valve of the mixed water heat exchange unit model changes.The relationship between its variable and entransy dissipation thermal resistance is explored.

The relationship between the primary side flow change and entransy dissipation thermal resistance
When the primary flow rate changes, the other parameters in the heat exchanger remain constant, and the simulation data is shown in Table 1.Table 1.Simulation date value table The simulation analysis shows that when the primary side flow rate of the heat exchange unit changes, the secondary side flow rate is 60 Kg/s, the valve opening of the secondary side bypass pipeline of the mixed water heat exchange unit is set to 20%, and the primary side flow rate is set to 40, 60 Kg/s.The simulation results are shown in Figure 3 and Figure 4.
Figure 3 and Figure 4 represent when the primary side flow rate increases from 40 Kg/s to 45 Kg/s, the entransy dissipation thermal resistance of the traditional heat exchanger unit decreases from 2.96×10 - 6 K/W to 2.9×10 -6 K/W, while the entransy dissipation thermal resistance of the mixed water heat exchanger unit decreases from 2.88×10 -6 K/W to 2.86×10 -6 K/W.The decrease in entransy dissipation thermal resistance means that the heat exchange efficiency is improved.When the primary side flow rate is 45 Kg/s, the real-time heat exchange efficiency of the heat exchanger unit is higher than that of the 40 Kg/s condition.At this flow rate condition, the real-time heat exchange efficiency of the mixed water heat exchanger unit is higher than that of the traditional heat exchanger unit.

3.2.The relationship between the secondary side flow change and entransy dissipation thermal resistance
During the simulation analysis of heat exchanger units, with changes in the secondary side flow rate, the primary side flow rate was set to 40 Kg/s, and the bypass valve of the mixed water heat exchanger unit on the secondary side was set to 20%.The secondary side flow rate was set to switch between 50 Kg/s and 55 Kg/s.The simulation results are shown in Figure 5 and Figure 6. Figure 6.Entransy dissipation thermal resistance.
Figure 5 and Figure 6 represent that as the secondary side flow rate increases from 50 Kg/s to 55 Kg/s, the entransy dissipation thermal resistance of the traditional heat exchanger increases from 2.89×10 -6 K/W to 2.92×10 -6 K/W, and that of the mixed water heat exchanger increases from 2.85×10 -6 K/W to 2.87×10 -6 K/W.The entransy dissipation thermal resistance varies with the secondary side flow rate, and as the secondary side flow rate increases, the entransy dissipation thermal resistance of the heat exchanger increases, indicating a decrease in heat transfer efficiency.However, for the mixed water heat exchanger, its entransy dissipation thermal resistance is still smaller than that of the traditional heat exchanger, indicating that the heat transfer efficiency of the mixed water heat exchanger is still higher than that of the traditional heat exchanger.
The variation of the secondary side flow rate affects the entransy dissipation thermal resistance.The simulation results verify that the entransy dissipation thermal resistance is minimized when the first and second side flow rates are matched, indicating that the heat transfer efficiency is highest under this condition.

The relationship between the temperature change of the primary and secondary side inlets and the entransy dissipation thermal resistance
When the secondary inlet temperature changes, the other parameters in the heat exchanger remain constant.The simulation data are shown in Table 2.  7-9, it can be seen that when the secondary inlet temperature remains constant, the traditional heat exchanger unit's entransy dissipation thermal resistance remains constant at 2.96×10 -6 K/W as the primary inlet temperature is raised from 348.15 K to 368.15 K, or when the primary inlet temperature remains constant, and the secondary inlet temperature is raised from 318.15 K to 338.15 K. Similarly, the mixed water heat exchanger unit's entransy dissipation thermal resistance remains constant at 2.88×10 -6 K/W.Currently, in the simulation control equation, the convective heat transfer coefficient is a constant of 3500 W/(m 2 ꞏK), and the effect of temperature changes on the convective heat transfer coefficient is not reflected in the current control equation.To study the influence of temperature changes on dissipative thermal resistance, it is necessary to further conduct system identification on actual heat exchanger units, fit the relationship between temperature and convective heat transfer coefficient with experimental data, and then use Modelica language to add this relationship to the existing simulation system for real-time heat transfer efficiency simulation analysis.

3.4.Relationship between Bypass Pipeline Regulating Valve Opening and entransy dissipation thermal resistance
When the bypass pipeline control valve is changed, the other parameters in the mixed water heat exchanger unit remain constant, and the simulation data are shown in Table 3.The simulation analysis of the mixed water heat exchange unit is as follows.When the opening of the bypass pipeline adjustment valve changes, and the valve opening is set at 10%-80%, the entransy dissipation thermal resistance changes with the valve opening and the bypass pipeline.The change curve of the opening of the regulating valve is shown in Figure 10, and the change curve of entransy dissipation thermal resistance is shown in Figure 11.
From Figures 10 and 11, it can be seen that as the bypass pipe regulating valve opening increases from 10% to 80%, the entransy dissipation thermal resistance decreases from 4.83×10 -5 K/W to a minimum of 2.12×10 -6 K/W, with a valve opening of 45%.As the valve opening continues to increase, the entransy dissipation thermal resistance increases to 4.28×10 -5 K/W.Therefore, the heat transfer efficiency of the mixed water heat exchanger gradually increases as the valve opening increases from 10% to 45%, and reaches a maximum heat transfer efficiency at a valve opening of 45% under this operating condition.When the valve opening increases from 45% to 80%, the heat transfer efficiency of the mixed water heat exchanger gradually decreases.

4.Conclusion
This paper is based on the Modelica language and uses the MWorks platform to analyze the entransy dissipation thermal resistance of traditional and mixed water heat exchangers under different conditions, including primary-side flow rate and inlet temperature, secondary-side flow rate and inlet temperature, and bypass valve opening.The results show that: 1) The simulation results verify that the entransy dissipation thermal resistance is minimized when the flow rates on the primary and secondary sides could be matched.
2) Under the simulation conditions where the convective heat transfer coefficient is constant, the entransy dissipation thermal resistance is not affected by the changes in the inlet temperatures of the primary and secondary sides.Real-time heat transfer coefficient identification shall be conducted accordingly.
3) The entransy dissipation thermal resistance of the mixed water heat exchanger unit and the traditional heat exchanger unit is different, which means that their real-time heat transfer efficiency is different.The mixed water heat exchanger unit can achieve optimal control of its real-time heat transfer efficiency through the optimization of the bypass valve opening.In other words, the bypass valve opening has a significant impact on the entransy dissipation thermal resistance.Under the given flow rate conditions in this paper, the opening degree of the valve is 45% when the entransy dissipation thermal resistance is at its lowest point, which is the optimal point for heat transfer efficiency.

Figure 1 .
Figure 1.Schematic diagram of traditional heat exchange unit.

Figure 2 .
Figure 2. Schematic diagram of mixed water heat exchange unit.

Table 2 .
Simulation parameter value table

Table 3 .
Simulation date value table