The piston pump provides liquid supply from the working chamber to the gas cap via connecting pipe 2 . When performing the calculations, we assume that the reduction in the delivered liquid volume due to leakage, underfilling of the working chamber and compressibility is negligible. Then, the liquid delivered to the lid can be determined as:
2.1. Mathematical model of the gas cap working process
We consider the most general case where there is no separating element between the gas and liquid phases in the gas cap.
The calculation of the gas phase thermodynamic parameters of the gas cap is similar to the calculation of the changes in thermodynamic parameters during the compression and expansion processes of a liquid piston reciprocating compressor.
At present, three methods are mainly used to calculate the working process of reciprocating compressors: multi-party approximation, lumped parameter mathematical model and distributed parameter mathematical model. [29].
The gas cap is the first stage of a reciprocating compressor with a liquid piston. Mathematical models with distribution parameters determine the main thermodynamic parameters at any point in the volume of a compressible gas. However, their application is limited by changing the computational grid every moment, which requires time to implement the approach and the effort to compile the program. Since the velocity of the liquid piston is low compared to the speed of sound through which pressure fluctuations propagate, the resulting uneven distribution of the thermodynamic parameters quickly disappears and, therefore, their values tend to zero.For this purpose, when calculating the working process of a reciprocating compressor, a mathematical model with lumped parameters is used [29]. The developed mathematical model of the gas cap working process is based on the basic equations of energy conservation, mass conservation and motion conservation, and takes into account many factors not considered in other models, namely: first-order phase change (condensation-evaporation), gas and gas cap Changes in the heat transfer coefficient between the walls with time, and between the liquid and the gas cap wall.
The main assumptions made are considered and analyzed in detail [29]. Among the assumptions made, the following points can be distinguished: the working process is balanced and reversible, the compressible gas is homogeneous, and there are no uneven distributions of temperature and pressure in the compressible gas. The mathematical model of lumped parameters is based on the general equations of energy conservation, mass conservation, volume conservation and equation of state.
Taking into account the negligible kinetic energy of the added and separated masses, in the presence of first-order phase changes and gas solubility, the energy conservation equation translates into the first law of variable-mass thermodynamics for open thermodynamic systems. Then,
Basic external heat transfer is determined according to Newton-Lachmann’s law as follows
Where (); is the average temperature of the inner wall surface of the cap ( It is determined through experiments [29]).
The determination of the heat transfer coefficient in the reciprocating compressor cylinder is based on the experiment of a single-acting reciprocating compressor with a diameter of (0.1÷0.22) m; the number of revolutions (1000 ÷ 1500) rpm, the widely used form of the Prilutsky-Fotin formula is [30]: Where is the Nusselt number; is the Reynolds number.
The free surface velocity in the gas cap can be determined using the continuity equation for the liquid phase.
Where It is the mass of liquid entering the gas cap from the pump cylinder.
Considering (7),
It is determined as
The deformation work dL is determined as
The basic change of the gas phase volume in the cover during dτ can be determined as
In-cap gas phase mass: Changes in the gas phase mass within the cap through leakage through the cap without external leakage are due to condensation or evaporation of the working fluid (first-order phase change). In this case, mass transfer occurs through concentration diffusion, thermal diffusion, and pressure diffusion. Given the accepted assumption that pressure and temperature are constant throughout the gas cap, there will be no thermal or pressure diffusion.
Considering that the mutual motion of the phases in the gas cap is very small and can be ignored, in order to calculate the mass flow rate during the concentration diffusion process, we use Fick’s first law:
values
can be determined as
Where is the vapor elastic pressure at the liquid surface (a function of liquid temperature and interface curvature, determined by the Clausius-Cleperon equation [31]).
mass transfer coefficient
is determined based on [32]
It should be noted that the solubility of gases in liquids is generally described by Henry’s equation and increases with increasing pressure.
Therefore, considering the above situation, the gas phase mass conservation equation in the cap can be written as:
Equation of state: It is well known that air follows the equation of state of an ideal gas at pressures up to 10 MPa and temperatures up to 600 K.In this case we also consider that the specific internal energy and specific enthalpy of an ideal gas depend only on the temperature, we have
If the gas cap pressure is greater than 10 MPa, you need to introduce a compressibility factor in the equation of state and use one of the existing equations of state for ideal gases: van der Waals, Berthelot, Dupre, Clausius or Vukalovich-Kirilin. It must be remembered that for real gases, and Where It’s specificon.
If there are separated units, then no mass transfer process will occur and the system of equations to calculate the changes in thermodynamic parameters will be written as
Liquid phase: Liquid phase pressure is defined as
Overvoltage Δ
p Due to the elasticity of the dividing elements:
If there are no split elements then .
To determine the temperature of the liquid in the lid, without the dividing element we use the law of conservation of energy.
Studies conducted have proven that pressure fluctuations within the cap do not exceed 5% [10] from the average pressure in the lid. The liquid enters the air cap and is compressed in the pump. We calculate the relative change in volume of a liquid due to its compressibility. We assume that the nominal pressure at the outlet of the plunger pump is 40 MPa. Therefore, the pressure increase in the gas cap will be 2MPa.Then, the relative change in volume will be equal to
= 0.1%, that is, the relative change in volume is much less than one percent and can be ignored without affecting the accuracy of the results.
Considering that the liquid compression in the gas cap is negligible, then The deformation work is equal to 0.The total internal energy can be determined as
Considering that the fluid is incompressible and
convert equation (25) into:
Where ——Specific heat capacity of liquid.
Taking (27) into account, the equation determining the temperature change of the liquid can be written as:
For a gas cap with a dividing element, Equation (28) is converted to:
In Equations (28) and (29), the specific enthalpies are determined as
The elementary external heat transfer
is determined as
The heat transfer coefficient
for convective heat transfer depends on the flow mode for a round pipe and is defined as [33] For laminar flow:
and for turbulent flow mode:
where is the Reynolds number; is the Prandtl number; and is the Prandtl number at wall temperature.
The heat exchange surface with a cylindrical shape of the cap is determined as
Mass Conservation Equation: if there is no dividing element in the cap, it is determined by Equation (7); in another case we have
2.2. Mathematical Model of Liquid Flow in the Pipeline from the Gas Cap
Currently, various models are used to describe the flow of liquid in a pipeline, ranging from the simplest ones based on the energy conservation equation (Bernoulli), both without taking into account inertial pressure losses and taking them into account, to complex ones using two-parameter turbulence models: k-ε, k-ω, SST and others.
When developing a mathematical model of fluid flow in a connecting pipeline, we use the principle of hierarchy and consider the calculation of the flow based on the Bernoulli equation and the unsteady one-dimensional flow of a viscous incompressible fluid.
In accordance with [4,20]the integral of Bernoulli’s equation can be written as
Where — Head loss caused by local resistance along the length and hydraulic resistance.
The solution of this equation is in [34].
Without considering inertial forces, the equation for determining the velocity of liquid in a pipe is:
The coefficient of friction along the length is a function of the Reynolds number [35] And, accordingly, . Therefore, Equation (37) must be solved at each time step by the successive approximation method.
The system of differential equations used to describe unsteady one-dimensional flow of a viscous incompressible liquid can be written as a system of equations of motion and continuity [36,37,38]:
We propose a “feature” approach to this system.
The boundary conditions at the end of the pipe are adjacent to the gas cap on the one hand and the consumption end of the liquid in the form of pressure on the other hand: in the gas cap –pw; at the user of the liquid –pd.
defined
or
the value of
It is determined as