Abstract:In this paper, four kinds of refrigeration working fluids are selected to analyze the influence of node temperature difference on the performance coefficient of automobile air-conditioning refrigeration system under different evaporator outlet air temperature and condenser outlet air temperature, and the total heat transfer area of parallel flow heat exchanger is optimized with the system performance coefficient per unit heat exchange area as the objective function. The results show that with the increase of the node temperature difference, the performance coefficient of the refrigeration system decreases, the area of the heat exchanger decreases, and the objective function value increases first and then decreases, that is, there is an optimal node temperature difference to make the economic performance of the refrigeration system the bestUnder the set working conditions, the temperature difference of the optimal condenser node corresponding to R245FA, R1234Ze, R134A and R123 is respectively When the R134A refrigerant is used, when the air temperature at the outlet of the evaporator is 15 and the temperature of the outlet air of the condenser is 45, the system performance coefficient per unit of heat exchange area is the largest.
With the development of social economy, it has become particularly important to save energy, reduce energy consumption and achieve sustainable development. At present, there are still many automotive air conditioners at home and abroad that still adopt traditional design methods, and do not carry out the optimal matching design of refrigeration systems from the perspective of energy saving and material saving.
The nodal analysis method is a method to comprehensively analyze the heat transfer effect of the heat exchanger and the maximum energy obtained in the energy utilization system, which is suitable for the optimization of the organic Rankine cycle (ORC) system and the optimization of the refrigeration system. On the air side of the evaporator, the node temperature difference has an important impact on the condensation of moist air, the moist air first contacts with the cold wall surface to achieve cooling, and when the temperature drops below the ** temperature, the water vapor on the wall surface begins to condense. It is found that the cold wall temperature is the decisive factor affecting the convective mass transfer of moist airIn Ref. [6], the influence of moist air velocity on the condensation heat exchange process was obtained through numerical models and experimental studies of heat and moisture exchangeIn the literature, the effect of moist air velocity on the condensation heat transfer law of condenser was studied by experimental methods.
In this paper, for different refrigeration working fluids R245FA, R1234Ze, R134A, and R123, the heat transfer area of the condenser and evaporator of the automobile air conditioning and refrigeration system is optimized by node analysis, and the custom function F=ccop AT, that is, the coefficient of performance (COP) of the refrigeration system per unit area is established In order to optimize the objective function, the matching relationship between the heat exchanger area and the performance coefficient of the refrigeration system was studied, so as to determine the optimal working fluidThe influence of node temperature difference on the economic performance of heat exchange of the system under different conditions such as evaporator outlet air temperature, condenser outlet air temperature, and evaporator inlet air velocity of the preferred working fluid was analyzed.
The structure of the shutter fin of the parallel flow heat exchanger is shown in Figure 1, the refrigerant flows in the porous flat tube, and the flow on the refrigerant side is divided into several processes through the manifold and the partition, and the corrugated louver fin is used on the air side.
FigParallel flow heat exchanger louver fin structure
In Figure 1: FP is the fin spacing;ll is the length of the shutter;fl is the height of the fins;fd is the width of the fins;lp is the shutter spacing;for the angle of the blinds.
The thermal physical properties of refrigeration working fluid are one of the key factors affecting the performance of refrigeration system, and R245FA, R1234Ze, R134A, and R123 working fluids were selected as research objects.
The thermophysical parameters of the four working fluids are listed in Table 1. The physical property data of each refrigerant used in this paper are from the Refprop database. Table 1: ODP is the ozone-depleting potential;GWP stands for Global Warming Potential.
TableThermophysical parameters of working fluids
The cyclic enthalpy (p-h) curve of refrigeration in an automobile air conditioner is shown in Figure 2a, and the temperature entropy (t-s) curve is shown in Figure 2b. In Figure 2a: 1 2 is the compression process;2 5 is the condensation process;5 6 is the throttling process;6 1 is the evaporation process. The minimum heat transfer temperature difference between the air temperature and the working fluid temperature is the node temperature difference. Generally, the temperature difference of the evaporator node is between the air outlet and the refrigerant inlet, which is expressed by ΔTEP, ΔTEP=TA6-T6, and the temperature difference of the condenser node is between TA3 and T3, which is expressed by ΔTCP, ΔTCP=T3-TA3. The nodal temperature difference not only affects the coefficient of performance of the refrigeration system, but also affects the heat exchange area of the heat exchanger. Taking the temperature difference of the node as the control variable, the thermophysical parameters of the working fluid are called to calculate the heat exchange area of the air side and the refrigerant side. The calculated working conditions are: evaporator air inlet temperature ta1=26;Condenser air inlet temperature ta5=35;Superheat tsup = 10;Supercooling degree tsub=5. By changing the node temperature difference δtep and δtcp, evaporator air outlet temperature ta6 and condenser air outlet temperature ta3 of the working fluid, the variation law of thermodynamic parameters with the temperature difference of evaporator node was studied, and the performance coefficient per unit heat exchanger area was optimized to achieve the best performance matching of the refrigeration system.
FigDiagram of the refrigeration cycle of an automobile air conditioner
The temperature difference of the evaporator node is:
tep=ta6-t6=ta6-te
The temperature difference of the condenser node is:
tcp=t3-ta3=tc-ta3
The air flow is:
me=va(ho+fl)ln ρ
Wherein: VA is the air inlet wind speed of the evaporator;ho is the outer height of the flat tube;fl is the height of the fins;l is the length of the flat tube;n is the total number of flat tubes of the evaporator;for the density of the air.
6 7 1 The heat produced by the evaporation process is:
qe=mec(ta1-ta6)
The refrigerant flow rate is:
1 2 The work done during the compression process is:
w=mf(h2-h1)
Nussel number nu is:
nu=0.026 5re0.8pr0.333
Where: re is the Reynolds number;pr is the Plante number.
The heat transfer coefficient of the refrigerant side surface is:
Wherein: d is the hydraulic diameter of the inner hole of the flat pipe;For the thermal conductivity of the refrigerant, call refprop to obtain.
The heat transfer coefficient of the air-side surface is:
The average surface heat transfer coefficient is:
Where: ari (i=1,2,3,4) is the heat transfer coefficient of the refrigerant side surface of each process;Ni(i=1,2,3,4) is the number of flat tubes per process, respectively.
The heat transfer coefficient is:
Wherein: ar is the inner surface area of the long flat pipe per meter;aa is the total external surface area per meter of tube length;Ra is the thermal resistance of the dirt on the air side, and the value in this document is 0000 3 m2·k/w。
The average logarithmic temperature difference in the single-phase zone of the evaporator is:
The heat exchange area of the single-phase zone of the evaporator is:
The average logarithmic temperature difference in the single-phase zone of the condenser is:
In the same way, the heat exchange area of the single-phase zone of the condenser is as follows:
The heat exchange area of the two-phase zone is:
The total heat exchange area is:
at=ae1+ae2+ac1+ac2
The refrigeration system COP value is:
ccop=qe/w
The custom function F, i.e., the COP per unit area is:
f=ccop/at
Among them: MF and ME are the mass flow rates of working fluid and heat source, respectivelyhi is the specific enthalpy of each corresponding state point;Ke and KC are the heat transfer coefficients.
Figure 3 shows the calculation process of the system.
FigThe system calculates the process
When the air inlet temperature of the evaporator is ta1=26 and the air inlet temperature of the condenser is ta5=35, the performance of the refrigeration system and the area of the heat exchanger and the custom function f of the four working fluids are shown in Fig. 4 and Fig. 6.
As can be seen from Figure 4a, the Cop values of R245FA, R1234Ze, R123 and R134A decrease gradually as ΔTEP increases from 2 to 30, and the CoP values of R245FA and R1234Ze change very similarlyAs the ΔTEP increases from 0 to 16, the COP values for the same ΔTEP are R134A, R123, R1234Ze, and R245Fa. It can be seen from Figure 4b that with the increase of δTCP from 2 to 30, the COP values of R245FA, R1234Ze and R123 all decrease gradually, and the COP value of R134A decreases first and then increasesFor the same δTCP, the size of the COP values is R134A, R123, R1234Ze, and R245FaFor R134a, the maximum COP value is 44% greater than the minimum COP value. From the COP value, it can be seen that R134A working fluid has better thermal performance in the refrigeration cycle of automobile air conditioning.
FigA kind of working fluidcopThe change in value with the temperature difference of the node
As shown in Fig. 5a, as the AT values of R245FA, R1234Ze, R123 and R134A decrease gradually and then gradually decrease, as the ΔTEP increases from 2 to 30For the same δTep, the AT values are R245FA, R123, R134A, and R1234Ze. As can be seen from Fig. 5b, the AT values of R245FA, R1234Ze, R123 and R134A decrease gradually as the δTCP increases from 2 to 30For the same δTCP, the AT values are R245FA, R123, R134A, and R1234Ze, and when ΔTCP=2 and TCP=30, the AT values of R245FA are more than those of R134A. For R134A, the maximum AT value is 65% greater than the minimum AT value. Because the heat exchange area is too small, the heat exchange efficiency will be reduced, and if the heat exchange area is too large, the heat exchanger consumables will increase, so it is necessary to choose a moderate heat exchange area.
It can also be seen from Figure 5 that the temperature difference of the evaporator node has a greater impact on the AT value of the total heat exchange area than the temperature difference of the condenser node.
FigA kind of working fluidatChanges with node temperature difference
As can be seen from Fig. 6a, as the δTep increases from 2 to 30, the F values of R245FA, R1234Ze, R123 and R134A all increase rapidly and then decrease slowly, but the F value at ΔTEP=30 is larger than that at ΔTEP=2R245Fa has an inflection point at ΔTEP=22, R1234Ze and R134A have an inflection point at ΔTEP=20, and R123 has an inflection point at ΔTEP=24. As can be seen from Figure 6b, the F values of R245FA, R1234Ze and R123 increase first and then decrease as ΔTCP=4 increases from 2 to 30, and R245FA and R123 reach the inflection point at ΔTCP=4 and R1234Ze reach the inflection point when ΔTCP=6R134A first reaches the first inflection point at ΔTCP=4, gradually rises to the second inflection point at ΔTCP=22, and then increases rapidly in F-value.
It can also be seen from Figure 6 that the temperature difference of the evaporator node has a greater influence on the f-value than the condenser node temperature difference.
Considering the characteristics of the refrigeration system, such as the COP value, the total area of heat exchange, the custom function F, and the environmental friendliness of R134A, it is reasonable to choose R134A as the refrigerant for automobile air conditioning. By analyzing the influence of the temperature difference of the condenser node and the temperature difference of the evaporator node on the COP value, AT and F, it can be seen that the temperature difference of the evaporator node has a great influence.
FigA kind of working fluidfChanges with node temperature difference
2.2.1 Analysis of results at different outlet air temperatures Ta6.
The influence of the temperature difference of the evaporator node on the performance coefficient f of the refrigeration system per unit area under different air outlet temperatures Ta6 of the working fluid R134A is shown in Figure 7. It can be seen from Fig. 7 that when Ta6 is , as the temperature difference of the evaporator node δTep increases from 2 to 30, the F value increases first and then decreases, and the δTep shows an inflection point atThe f-value at δtep=2 and ta6=20 is lower than that at ta6Ta6 increased from 5 to 20, and the F-value showed a gradual increasing trend. When δTep is 2 4, the same f value is reached, and the temperature difference of the node required for Ta6=10 is minimal;When δTep is 4 6, the temperature difference of the node required by Ta6=15 is the smallest;When δtep is 2 8, the temperature difference of the node required by Ta6=20 is the largest;When δtep is 6 22, the temperature difference of the node required by Ta6=20 is the smallestWhen δTep is 22 30, the F value at Ta6=15 is much greater than the F value of Ta6, which is due to the fact that the change of COP value is faster than the change of heat exchange area, the COP value increases, the required heat exchange area decreases, and the economic performance increases. It can be seen that the increase of the temperature difference of Ta6 heat exchange will be improved, and then the heat transfer of the evaporator will be improved. Therefore, the air outlet temperature of the evaporator is not easy to be too high, and the economic performance is better when TA6=15.
Figdifferentta6downfChanges with the temperature difference of the evaporator node
2.2.2 Analysis of results at different air outlet temperatures Ta3.
The influence of the temperature difference of the evaporator node on the performance coefficient f of the refrigeration system per unit area under different air outlet temperatures Ta3 of the working fluid R134A is shown in Figure 8.
Figdifferentta3downfChanges with the temperature difference of the evaporator node
It can be seen from Figure 8 that when Ta3 is , as the temperature difference of the evaporator node δtep increases from 2 to 30, the F value appears at an inflection point when δtep isTa3 increases from 43 to 49, and the f-value decreases gradually, and the size of the f-value is similar. For the same f value, the node temperature difference required at TA3=43 is the smallest, and the node temperature difference required at TA3=49 is the largest. Therefore, the air outlet temperature of the condenser is not easy to be too high, and the outlet temperature is about 10 times higher than the inlet temperature, and the heat exchange effect is better. It can also be seen from Figure 8 that the effect of different air outlet temperatures Ta3 of the condenser on the f-value of the performance coefficient of the refrigeration system per unit area is not obvious.
In this paper, the influence of node temperature difference on the performance of refrigeration system under different working fluids, different evaporator outlet air temperatures and condenser outlet air temperatures is simulated, and the following conclusions are drawn:
1) For the selected working fluid, there is an optimal node temperature difference, with the increase of the node temperature difference, the system COP value decreases, the heat exchanger area decreases, and the objective function value increases first and then decreases, that is, there is an optimal node temperature difference to make the economic performance of the system the best.
2) The temperature difference of the optimal condenser node corresponding to R245FA, R1234Ze, R134A, and R123 is respectively, and the temperature difference of the optimal evaporator node is respectively
3) Considering the thermal power and economy, a custom function is adopted, that is, the performance coefficient of the refrigeration system per unit area is f, the larger the f value, the better the comprehensive performance of the system, and the best performance of R134A under the set working conditions. When the air temperature at the outlet of the evaporator is 15 and the air temperature at the outlet of the condenser is 45, the temperature difference of the corresponding evaporator node is 20.
Authors: Tang Jingchun1, Wang Xiaoqian1, Zhang Xiuping21.School of Automotive and Traffic Engineering, Hefei University of Technology.
2.Hefei General Machinery Research Institute***