Calculation of dynamic operating modes refrigeration machines with electric energy sources

. The method of calculating the dynamic operating modes of refrigerating machines with electric power sources is described. As an example, the results of the calculation and data of experimental studies of the processes of starting a refrigerating machine with an electric asynchronous motor are given.


Introduction
Refrigerating machines (RM) are part of the life support systems of the populated area.The efficiency and cost-effectiveness of the operation of these systems significantly depends on the technical condition of refrigeration machines.Calculation and modeling of dynamic operating modes of refrigerating machines is one of the most urgent and complex tasks of modern mechanical engineering.Despite the fact that a significant amount of work is devoted to the creation and improvement of methods of calculation, modeling, as well as experimental research of these machines, many tasks remain unresolved.This is due to the complexity of heat and moisture processes accompanying the operation of these machines, especially those that are part of the air conditioning systems of inhabited premises and vehicle salons.Specified systems air conditioning often works in difficult operating conditions.Severe operating conditions are understood here as conditions of significant dust, shock loads and vibration.In these conditions operate external units of air conditioning systems of vehicles and refrigerated trucks.Severe operating conditions significantly affect the technical condition of refrigerating machines and should be taken into account during their calculations and modeling.
The operating experience of the refrigerating machines studied here has shown the high efficiency of applying the principle of ampulization to them.The ampulization principle ensures maximum isolation of the working bodies of these machines from the environment.Ampullization is ensured by preliminary drying of working fluids, as well as by the use of sealed (welded) hydraulic lines for them.Ampullization significantly increases the service life and reliability of refrigeration machines, but at the same time, the complexity of the processes accompanying the operation of these machines in various modes of operation increases significantly.

The need for generalized modeling of refrigeration machines
The tasks of creating durable and cost-effective RMs require an analysis of their technical condition, including those under severe operating conditions.It should be noted that currently existing methods for assessing their technical condition, based on computer calculations, do not take into account the complexity of the processes of joint operation of refrigeration machines with electric motors and hydraulic machines, for example, RM compressors, as well as their interaction with each other, since do not consider them as single heat and electrohydraulic systems.This narrows the field of research of the dynamic modes of operation of the RM, reduces the accuracy and reliability of calculations of their parameters and dynamic characteristics, as well as the quality and adequacy of the constructed computer models of the RM to real processes and objects.The effectiveness of RM research is currently determined by the level of use of modern computer technologies that allow high quality modeling of the processes under study, as well as computer experiments and virtual tests.This requires more advanced generalized models of RM, namely, their technical, physical, mathematical and computer models combined with the models of the same name of electric energy sources.
The current level of knowledge of the features of dynamic operating modes of RM shows that the heat and mass exchange processes accompanying the operation of these machines are extremely complex.This is due to the fact that the working medium, which moves through the hydraulic channels of the refrigerating machine, changes its aggregate state several times during one cycle.At the same time coordinates of boundaries of phase transitions of working medium constantly change.It can be shown that it is the position of these boundaries that determines the efficiency of the RM.It follows that one of the main tasks of calculating the dynamic operating modes of RM is to determine the coordinates of the moving boundaries of phase transitions of working media.

Technical modeling
The method for calculating dynamic operating modes of refrigerating machines with electric energy sources described here was used to analyze the influence of the boundaries of phase transitions of working media of RM on the efficiency of their operation.The studies were performed on the experimental complex (Fig. 1b).Fig. 1.Experimental complex: a -the scheme of the complex; b -the appearance of the complex.
The experimental complex, the scheme of which is shown in Figure 1a, was created as a technical model of the air conditioning system of a passenger car.It is a refrigerating machine with a heat exchanger in the form of a surface air cooler.As part of the model (Figure 1a): asynchronous electric motor (AEM) 5, piston compressor 1, condenser 2, throttle 3, as well as evaporator 4.
In Figure 1, in addition to the specified elements and units, the movable boundaries of the phase transitions of the working medium of RM are indicated: 1 () xt -position of the movable boundary of the phase transition of the type "gas -vapor-liquid medium"; 2 () xt -position of the movable boundary of the phase transition of the type "liquid -vapor-liquid medium." The measurement system consisted of: pressure sensors such as BOCH REXROTH HM 13 10/250 (indicated by numbers 1-5 in circles), temperature sensors (indicated by numbers 1-2 in squares), as well as from the BOCH BT MAC 8-15/C -PM-4AX4 data collection system (Fig. 2).Fig. 2. Experimental complex with signal measurement system.At the experimental complex described here, the dynamic modes of operation of a refrigerating machine with an asynchronous electric motor as part of the air conditioning system were studied: during its start and stop, the operation of the system in the main and transient modes, as well as in the modes of operation with overloads.
Below, Figure 3 shows the data of the experimental studies performed here.For example, Fig. 3 shows curves characterizing the change in gas overpressure in the path at the inlet to the compressor, as well as at the outlet from the compressor.In addition, the same figure shows curves describing the nature of the temperature change in the condenser (curve) and in the evaporator (curve) of the air conditioning machine.

Features of physical modeling of the studied processes. Gas
It is known that the field of setting and solving problems of hydrodynamics with fluid boundaries is limited by the Euler approach traditionally used in industrial hydraulics, which is used to calculate the parameters of media with fixed boundaries.This circumstance is explained by the fact that sensors of parameters of media moving through hydraulic channels are installed at specific fixed points of the lines.Experience in solving hydromechanics problems shows that along with the application of the Euler approach, it is also necessary to apply the Lagrange approach, which is more general than the Euler approach, since it allows you to set and solve the problems of hydromechanics of continuous media with movable boundaries.Along with the use of Euler and Lagrange approaches, in calculating the characteristics of dynamic operating modes of refrigeration machines, Stefan's approach should also be used, which allows setting and solving problems with movable boundaries of phase transitions of liquid media.Stefan's classic task is to determine the mobile boundary of ice on the hard surfaces of cooled bodies.The problems of hydrodynamics of refrigerating machines are also complicated by the fact that here the boundaries of phase transitions of the type: "liquid -steam" constantly move, since at these boundaries a continuous process of evaporation and condensation of the substance of the working medium of the machine is carried out.This process is very complex, it is accompanied by ruptures in the continuity of the vapor-liquid medium flow, cavitation and turbulence.These processes significantly affect the technical condition of refrigerating machines, but they are little studied due to their physical complexity, difficulties in calculation and modeling, as well as due to limited opportunities for observing the features of these phenomena and measuring their physical parameters.These processes, despite their complexity, require thorough research, both in order to analyze their influences on the efficiency of refrigeration machines, and in order to It should be noted that the joint use of the principles of Euler, Lagrange and Steffan will make it possible to develop the fundamental principles of the mechanics of solid media with variable boundaries, in relation to the dynamic problems of hydraulic systems with movable boundaries of fluid phase transitions of working media.This will significantly expand the field of research, as well as the formulation and solution of problems for calculating the dynamic operating modes of simulated refrigerating machines.At the same time, the accuracy of calculation and the adequacy of modeling of refrigerating machines to real objects and processes accompanying their work will significantly increase.

Conclusion
As the results of technical and computer experiments have shown, the created method makes it possible to calculate dynamically operating modes of thermal-electrohydromechanical systems with movable boundaries of working liquid media of refrigerating machines with acceptable accuracy (15-20%).The application of this method can expand the field of studying the dynamic modes of operation of these systems based on their computer modeling, as well as the -of virtual computer tests.Considering the complexity of the processes of joint operation of electric and refrigerating machines with hydraulic systems studied here, especially in the modes of their start-up, regulation and overload, and taking into account the current level of knowledge about the actual characteristics of these processes.

Fig. 3 .
Fig. 3. Results of experimental studies.Starting of RM with AEM and axial piston compressor: 1change of medium temperature in the condenser; 2-change in the temperature of the medium in the evaporator; 3 -change of medium pressure in the condenser; RM; 4 -change of medium pressure in evaporator; 5 -change of electric current in AEM windings; 6-change of AEM shaft speed.