equilibrium state at any time, the process is uniform, slow and without any sudden
change, then the whole process can be regarded as a series of states very close to the
equilibrium state. The thermodynamic process is called quasi-static process.
In piston thermodynamic machinery, the piston speed is generally less than
10 m=s, but the velocity of pressure wave in gas is almost equal to that of sound,
usually hundreds of meters per second. Relatively speaking, the piston’s motion
speed is very slow. During the process, a series of force and heat equilibrium can be
established in time, which will not be far away from the equilibrium state.
Therefore, it can be used as quasi-static process calculation. Quasi-static process
provides convenience for theoretical research and is also the basis of numerical
calculation.
(3) Process of gas change in gas chamber is adiabatic process
The process of no heat exchange between the system and the outside world is
adiabatic process. When the state of the gas changes very quickly, the heat
exchange between the gas and the outside world can be neglected due to the short
time of change. The change of the gas in the front and rear chamber of DTH
hammer is very fast, and the numerical analysis by computer is based on the single
step of microelement time (e.g., 0.0001 s), so it can be regarded as an adiabatic
process in the analysis of the problem.
(4) Ignoring the influence of friction resistance and DTH hammer rotation
In large diameter DTH hammer, the big end of the piston contacts with the inner
wall of outer cylinder, and the small end contacts with the inner wall of inner
cylinder. Friction exists when piston moves up and down. Choosing reasonable
matching tolerance and lubricating the friction surface with lubricating paste when
working, the friction force is much smaller than that of the gas in the front and rear
chamber, which can be neglected. Similarly, the influence of rotary motion of DTH
hammer on gas state and piston motion is neglected.
11.4.2.2 Theoretical Model Equations of Pneumatic DTH Hammer
According to the model figure of DTH hammer shown in Fig. 11.16, the modeling
mainly involves thermodynamics and dynamics theory, including solving the
acceleration, velocity, and displacement of the piston in this time step and the
volume of the front and rear chambers according to the differential equation of
piston motion. The gas mass in the front and rear chamber entering and discharging
is calculated by gas flow equation, and the change of gas pressure in the front and
rear chamber entering and discharging is calculated by gas energy balance equation
according to the change of gas mass, and then the change of temperature is
calculated.
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11 Pneumatic Down-the-Hole Hammer
change, then the whole process can be regarded as a series of states very close to the
equilibrium state. The thermodynamic process is called quasi-static process.
In piston thermodynamic machinery, the piston speed is generally less than
10 m=s, but the velocity of pressure wave in gas is almost equal to that of sound,
usually hundreds of meters per second. Relatively speaking, the piston’s motion
speed is very slow. During the process, a series of force and heat equilibrium can be
established in time, which will not be far away from the equilibrium state.
Therefore, it can be used as quasi-static process calculation. Quasi-static process
provides convenience for theoretical research and is also the basis of numerical
calculation.
(3) Process of gas change in gas chamber is adiabatic process
The process of no heat exchange between the system and the outside world is
adiabatic process. When the state of the gas changes very quickly, the heat
exchange between the gas and the outside world can be neglected due to the short
time of change. The change of the gas in the front and rear chamber of DTH
hammer is very fast, and the numerical analysis by computer is based on the single
step of microelement time (e.g., 0.0001 s), so it can be regarded as an adiabatic
process in the analysis of the problem.
(4) Ignoring the influence of friction resistance and DTH hammer rotation
In large diameter DTH hammer, the big end of the piston contacts with the inner
wall of outer cylinder, and the small end contacts with the inner wall of inner
cylinder. Friction exists when piston moves up and down. Choosing reasonable
matching tolerance and lubricating the friction surface with lubricating paste when
working, the friction force is much smaller than that of the gas in the front and rear
chamber, which can be neglected. Similarly, the influence of rotary motion of DTH
hammer on gas state and piston motion is neglected.
11.4.2.2 Theoretical Model Equations of Pneumatic DTH Hammer
According to the model figure of DTH hammer shown in Fig. 11.16, the modeling
mainly involves thermodynamics and dynamics theory, including solving the
acceleration, velocity, and displacement of the piston in this time step and the
volume of the front and rear chambers according to the differential equation of
piston motion. The gas mass in the front and rear chamber entering and discharging
is calculated by gas flow equation, and the change of gas pressure in the front and
rear chamber entering and discharging is calculated by gas energy balance equation
according to the change of gas mass, and then the change of temperature is
calculated.
228
11 Pneumatic Down-the-Hole Hammer