Flow characteristics of refrigerant in a hermetic compressor

The refrigeration system simulation technology was proposed from the late 1970s and has a history of more than 30 years. During this period, with the rapid development of refrigeration technology and computer technology, the refrigeration system simulation technology has gradually formed and further improved in practice. In theory, many basic problems have been solved, and today's refrigeration system simulation technology is gradually turning to practical use.

The Institute of Refrigeration and Cryogenic Engineering of Shanghai Jiaotong University began to engage in digital simulation of refrigeration systems since the early 1980s. After nearly 20 years of development, Shanghai Jiaotong University has gradually established a dynamic and steady-state simulation platform for large, medium and small-sized refrigeration units, such as the “room air conditioner intelligent simulation” jointly developed with Chunlan Group.

The software has been appraised by the Shanghai Science and Technology Commission, and the “large screw unit performance simulation” software commissioned by UTC Corporation (United Technologies Corporation of the United States) has also passed the acceptance of UTC.

The purpose of the refrigerator is to bring the temperature inside the box to the desired value. The control method is to control the compressor to start and stop by setting the upper and lower limits of the temperature of the refrigerating compartment. The refrigerator system consists of two parts, a cooling module and a heat load module. Cooling modules (mainly compressors, condensers, capillaries, evaporators) generate cooling. However, for a certain amount of cooling, how the temperature inside the box changes depends on the heat load of the box. Figure 2 shows the mutual coupling relationship between the cooling module and the cabinet. In order to obtain the dynamic cooling capacity of the refrigeration module, it is necessary to know the air temperature in the tank, and the determination of the air temperature in the tank must be based on the cooling capacity.

Therefore, the dynamic simulation of the refrigerator should be combined into a system model based on the above coupling relationship after establishing dynamic models for the two parts.

The refrigerator system simulation should be able to predict the general working conditions of the refrigerator and the national standard test (cooling speed test, refrigeration capacity test, power consumption test, load temperature recovery test, storage temperature test, ice making capacity test). Parameters to be concerned include: air temperature in the tank, test pack temperature, condensing pressure, evaporation pressure, compressor flow, capillary flow rate, compressor operating rate, power consumption, refrigeration capacity, ice making capacity, load temperature recovery time, etc.

Modeling ideas and module division. Compressor Model Modeling Ideas Most small refrigeration units use fully enclosed compressors, which are more complicated in construction.

Journal of System Simulation 1 Compressor 2 Condenser 3 Drying Filter 4 Capillary 5 Freezing Evaporator 6 Freezer Room 7 Refrigeration Evaporator 8 Refrigeration Room 9 Intake Pipe Refrigeration System Cabinet Evaporator Cooling Capacity Cooling Heat Dissipator Compressor Heat Dissipating Box Internal air temperature, ambient temperature and structural parameters, ambient temperature and structural parameters, load module, air temperature, refrigeration simulation, the purpose of research on the compressor is not to optimize its own structure, but to meet the requirements of system simulation optimization, the main need to be more accurate Knowing the import and export parameters, the model should be built to ensure a certain accuracy, while reducing the calculation time as much as possible. Therefore, compressor displacement, power, exhaust temperature or enthalpy is a parameter of concern for refrigerator system simulation.

According to the flow characteristics of the refrigerant in the fully enclosed compressor, the compressor model can be divided into two parts: the cylinder compression section and the shell side heat exchange section. Since the compressor frequency is about 50 Hz, it is much faster than the temperature of the air in the tank to be concerned. Therefore, it can be considered that the influence of the flow rate on the boundary conditions of the compressor is instantaneous, and the equation of the steady state can be used to describe the characteristics of the compressor. . For the heat exchange of the shell side (including the shell, refrigerant and oil), the temperature change of the shell is equivalent to the change of the temperature inside the tank, so a dynamic model is needed.

Condenser Model Modeling Thought In the simulation of the refrigerator system, the dynamic characteristics of the condenser are mainly reflected in the process of opening and stopping the device. The refrigerant in the condenser changes considerably during the start and stop of the compressor. When the machine is shut down for a long time, the refrigerant in the condenser is superheated gas. When the device is started, the compressor discharges a large amount of refrigerant superheated gas into the condenser, and the pressure of the condenser rises rapidly, when the pressure rises to the saturation corresponding to the local temperature. At pressure, the refrigerant liquid begins to condense in the condenser, followed by a subcooled liquid zone. During most of the operation of the condenser, the refrigerant therein contains three parts: superheated, two-phase, and supercooled. When the compressor is just shut down, the refrigerant in the condenser continues to flow through the capillary to the evaporator, the condenser pressure gradually decreases, and the condenser and evaporator pressures quickly reach equilibrium. The outlet state of the condenser also changes from supercooling to two phases and gradually becomes superheated gas. Moreover, due to the large amount of refrigerant in the condenser and the slow flow rate, the response time of the outlet state to the inlet disturbance is long, which is usually greater than the time step of the dynamic simulation of the refrigerator. It can be seen from the above analysis that in the dynamic simulation of the refrigerator system, the condenser should adopt a dynamic model, and in the component condenser component module, the working condition of the condenser should be divided into four stages (starting the complete superheated gas phase, starting the liquid condensation phase). Modeling is performed separately, with a liquid condensation phase during shutdown and a fully superheated gas phase with shutdown.

The condenser dynamic model can be divided into three types: centralized parameter model, distributed parameter model and phased concentration parameter model. It is difficult to distinguish the heat transfer in different states by the lumped parameter model, and the model error is large. In theory, the distributed parameter model has the highest accuracy. However, the accuracy of the model is also related to the accuracy of the heat transfer coefficient. At present, the formula of the local heat transfer coefficient of the heat exchanger is large, so the high precision of the distributed parameter model is difficult to guarantee. At the same time, the distributed parameter model has a large amount of calculation and the stability of the calculation is not good. Since system simulation requires both accuracy and program stability and speed, it is appropriate to use a phase-separated parameter model.

The evaporator model modeling idea is the same as that of the condenser. The evaporator also uses a dynamic phase separation centralized parameter model.

When the refrigerator is shut down for a long time, the refrigerant in the evaporator is superheated gas. After the start-up, as the compressor continuously draws refrigerant from the evaporator, the evaporator pressure drops rapidly. As the capillary two-phase refrigerant is charged, the evaporator exhibits a two-phase region. During most of the operation of the evaporator, the refrigerant therein includes two phases, two phases of superheating. When the compressor is shut down, the refrigerant in the condenser continues to flow through the capillary to the evaporator, the evaporation pressure gradually rises, and is balanced with the rapidly decreasing condensing pressure. When the compressor is turned on again, the evaporator is still in a gas-liquid two-phase state, which is different from the initial start-up.

For the different working conditions of the above evaporator, the evaporator can be separately modeled according to the initial start-up phase, the shutdown phase and the restart phase. Throttling Module Modeling Ideas Refrigerators typically use a capillary as a throttling element. Since the capillary has less refrigerant and high flow rate, the response time of the capillary outlet state to the inlet disturbance is very short, which is much smaller than the time constant of the evaporator and the condenser. Therefore, the capillary can adopt a steady state model. In addition, the capillary is usually partially welded to the return air tube, and the capillary exchanges heat with the return air tube, which makes the capillary model quite complicated. For the sake of simplicity, the capillary model can be divided into two parts: the heat exchange section and the adiabatic section, and the heat exchange amount of the capillary is equivalent to the "effective enthalpy difference" of the capillary inlet to realize the decoupling of the two links.

When the refrigerator is operated under stable conditions, the capillary inlet refrigerant is generally a supercooled liquid. After the refrigerant enters the capillary, the pressure drop caused by the flow friction sequentially undergoes a liquid phase flow region and a gas-liquid two-phase flow region (ignoring the metastable region).

In the liquid phase flow zone, the refrigerant is considered to be incompressible, and the thermal performance is independent of the pressure. For the adiabatic capillary, the liquid phase zone can be regarded as an isotropic throttling process, so the refrigerant temperature does not change. In the gas-liquid two-phase flow zone, the refrigerant pressure drops rapidly, the refrigerant flow rate rises rapidly, and can approach or even reach the local sonic velocity, causing the clogging flow phenomenon; at this time, the refrigerant pressure at the capillary outlet Ding Guoliang, etc.: household refrigerator The analysis and modeling of the part model will be greater than or equal to the capillary back pressure (the capillary back pressure is the inlet pressure of the evaporator) and the refrigerant flow will no longer be affected by the capillary back pressure. Therefore, the case of choking should be considered in capillary modeling.

Load module modeling idea The heat load of the refrigerator can be divided into the heat load of the refrigerator compartment and the heat load of the freezer compartment, including: environment, insulation layer, air inside the box, cargo in the freezer compartment and electric heater in the refrigerator compartment. The heat transfer relationship is shown in Figure 3. As shown in the figure, the insulation layer is the hub for heat transfer between the cabinet and the environment, the cabinet and the cooling components, and the air temperature inside the tank is the output of the load module.

Freezer compartment insulation layer freezer compartment air refrigerator compartment insulation compartment refrigerator compartment air refrigerator compartment freezer diagram is the cooling capacity of the freezer evaporator; is the cold volume of the refrigerator compartment evaporator; is the ambient temperature; is the heat dissipation of the condenser; The amount of cold that is transmitted to the freezer compartment by the wall of the freezer; the amount of cold that is transmitted to the air in the refrigerator compartment by the wall of the refrigerator; the temperature of the air around the compressor; the temperature of the air in the freezer compartment and the refrigerator compartment.

The characteristics of the thermal insulation layer are that the physical property parameters and the inner and outer heat transfer coefficients are approximately constant, the outer ambient temperature is basically constant, and the inner boundary conditions are quite complicated, not only the complex geometric structure but also the internal heat source and the door joint leakage heat are considered. . For these characteristics of the insulation layer, a state space model based on modern control theory can be used to express different forms of boundary conditions and internal heat sources in a unified form, and the physical meaning is clear. However, the shortcomings of this model are that the calculation is large and the storage capacity is large, and it is difficult to synthesize the heat transfer links related to multiple inputs and outputs through the state space model. Therefore, this model is suitable for the initial model. By retransforming the state space model into a transfer function model, the above shortcomings of the state space model can be basically solved. In addition, since the thermal insulation layer includes a plurality of wall surfaces, the directly obtained thermal insulation layer model contains a plurality of transfer functions, and the form is still relatively complicated. By the corresponding combination, a simple synthetic transfer function model can be obtained.

The air in the box is the same as the insulation layer. The physical properties are approximately constant. The transfer function model can also be used and can be combined with the transfer function of the insulation layer.

The goods in the box are different from the insulation layer. The phase change occurs during freezing and has typical nonlinear characteristics. Therefore, the cargo model should not adopt the transfer function model. The heat transfer of goods in the refrigerator is a boundary condition asymmetry, phase change, three-dimensional, non-steady state heat conduction problems, but also affected by the location of the goods and the type of goods. Considering that the accuracy of the cargo model is limited by the inaccurate understanding of the heat transfer boundary conditions and thermal property parameters of the cargo, and the calculation speed of the cargo model, this paper uses the simplified model of the literature <13>.

The environment and electric heaters can be easily processed into one input because they remain essentially unchanged. There are also some minor thermal loads inside the refrigerator, such as racks, thermocouple leads, etc. Since these objects have a small heat capacity and have little effect on the air temperature inside the box, they are all included in the air model inside the box. The load module includes three parts: the air model in the box, the insulation layer model and the cargo model.

Refrigerant charge calculation The refrigerant charge is closely related to the operating characteristics of the refrigeration unit. The refrigerant has three states in the refrigeration system, namely, vapor, liquid and two phases coexist. The harder to calculate is the quality of the refrigerant in the two-phase zone. Therefore, it is important to select a suitable model of the bubble coefficient for the two-phase region. Existing studies have shown that the Premoli Cavitation Coefficient Model is more accurate for domestic refrigerators. Therefore, the Premoli Cavitation Coefficient Model is used to calculate the refrigerant quality in the two-phase zone.