【Mechanical Equipment Technology Exchange Edition】Mechanical Equipment 【Daily Question】20190919

The steam expands in the nozzle, with the pressure dropping from p0 to p1 and the flow velocity increasing from c0 to c1, thereby converting the thermal energy of the steam into kinetic energy. Once the steam enters the moving blade shroud, its direction of flow changes, generating a thrust force that causes the impeller to rotate and do work, thereby converting the kinetic energy of the steam into mechanical energy of the rotor. The speed of the steam as it leaves the moving blade cascade drops to c2. Since the steam does not expand in the rotor cascade, the pressure before and after the rotor cascade is equal, that is, p1 = p2.

The steam expands in the nozzle, with the pressure dropping from p0 to p1 and the flow velocity increasing from c0 to c1, thereby converting the thermal energy of the steam into kinetic energy. Once the steam enters the moving blade shroud, its direction of flow changes, generating a thrust force that causes the impeller to rotate and do work, thereby converting the kinetic energy of the steam into mechanical energy of the rotor. The speed of the steam as it leaves the moving blade cascade drops to c2. Since the steam does not expand in the rotor cascade, the pressure before and after the rotor cascade is equal, that is, p1 = p2.

When steam at a certain pressure and temperature enters the nozzle, due to the change in the cross-sectional shape of the nozzle along the direction of the steam flow, the pressure and temperature of the steam decrease, its specific volume increases, and its flow velocity rises. That is, the steam expands and accelerates within the nozzle, with thermal energy being converted into kinetic energy. Steam at high velocity flows out of the nozzle and enters the rotor blade channels, where it changes direction within the curved flow paths of these blades. The steam exerts a force on the blades, generating a torque that causes the blades to rotate, thereby driving the main shaft to spin and producing mechanical work by converting kinetic energy into mechanical energy.

A reaction turbine is a type of turbine in which steam expands only within the nozzles. In the rotor blades of a reaction turbine, the steam does not expand to do work; it merely changes direction of flow. In a single-stage turbine, when the specific enthalpy drop in the nozzles is large, the steam velocity at the nozzle exit is very high; as a result, the velocity c2 of the steam as it leaves the rotor blades is also high, which leads to significant losses and reduces the efficiency of the turbine. To reduce this loss, a row of guide vanes is installed behind the first row of moving vanes, causing the steam to change direction within the guide vanes before entering the second row of moving vanes on the same impeller to continue doing work. In this way, the kinetic energy of the steam stream exiting the first row of moving vanes is utilized in the second row of moving vanes, thereby reducing kinetic energy losses. If the steam stream exiting the second row of moving vanes still possesses significant kinetic energy, a second row of guide vanes and a third row of moving vanes can be installed. This type of stage, in which the kinetic energy generated by the expansion of steam in the nozzle is utilized in stages within the rotor blades, is called a velocity stage. A stage in which steam kinetic energy is utilized in two rows of vanes is typically called a two-row velocity stage, while a stage in which it is utilized in three rows of vanes is called a three-row velocity stage.

Working principle of impulse turbines: An impulse turbine is one in which steam primarily expands within nozzles; it expands little or not at all within the rotor blades, with the main change being in the direction of the flow. Modern impulse turbines have a certain degree of reaction in each stage, meaning that a small amount of expansion also occurs within the rotor blades, which helps to accelerate the steam flow. Nevertheless, they are still classified as impulse turbines. In a single-stage turbine, when the specific enthalpy drop in the nozzles is large, the steam velocity at the nozzle exit is very high; as a result, the velocity c2 of the steam as it leaves the rotor blades is also high, which leads to significant losses and reduces the efficiency of the turbine. To reduce this loss, a row of guide vanes is installed behind the first row of moving vanes, causing the steam to change direction within the guide vanes before entering the second row of moving vanes on the same impeller to continue doing work. In this way, the kinetic energy of the steam stream exiting the first row of moving vanes is utilized in the second row of moving vanes, thereby reducing kinetic energy losses. If the steam stream exiting the second row of moving vanes still possesses significant kinetic energy, a second row of guide vanes and a third row of moving vanes can be installed. This type of stage, in which the kinetic energy generated by the expansion of steam in the nozzle is utilized in stages within the rotor blades, is called a velocity stage. A stage in which steam kinetic energy is utilized in two rows of vanes is typically called a two-row velocity stage, while a stage in which it is utilized in three rows of vanes is called a three-row velocity stage.

New steam under certain pressure enters the nozzle through the throttle valve (which causes throttling losses). The steam expands and accelerates in the nozzle, with its pressure and temperature decreasing. Steam with high velocity enters the moving blades, changes the direction of the steam flow within those blades, and exerts a thrust on them. At the same time, the steam expands slightly before exiting the moving blades, resulting in a decrease in pressure and an increase in velocity, which exerts a reaction force on the moving blades. The steam converts most of its kinetic energy into mechanical energy in the moving blades. As the steam exits the moving blades, the pressure drops to Pi and the temperature drops to tl.

An impulse turbine is a type of turbine in which steam expands only within the nozzles. In the rotor blades of an impulse turbine, the steam does not expand to do work; it merely changes direction of flow.

The steam expands in the nozzle, resulting in a decrease in pressure and an increase in flow velocity; this converts the thermal energy of the steam into kinetic energy. Once the steam enters the blade row, its direction of flow changes, generating a thrust force that causes the impeller to rotate and perform work, thereby converting the kinetic energy of the steam into mechanical energy of the rotor. As the steam leaves the blade row, its velocity decreases.

The steam expands in the nozzle, with the pressure dropping from p0 to p1 and the flow velocity increasing from c0 to c1, thereby converting the thermal energy of the steam into kinetic energy. Once the steam enters the moving blade shroud, its direction of flow changes, generating a thrust force that causes the impeller to rotate and do work, thereby converting the kinetic energy of the steam into mechanical energy of the rotor. The speed of the steam as it leaves the moving blade cascade drops to c2. Since the steam does not expand in the rotor cascade, the pressure before and after the rotor cascade is equal, that is, p1 = p2.

A reaction turbine is one in which steam primarily expands within the nozzles; it expands little or not at all in the rotor blades, with the main change being in the direction of the flow. Modern reaction turbines have a certain degree of reaction in each stage, meaning that steam does expand to a small extent within the rotor blades as well, which helps to accelerate the steam flow. Nevertheless, they are still classified as reaction turbines. In a single-stage turbine, when the specific enthalpy drop in the nozzles is large, the steam velocity at the nozzle exit is very high; as a result, the velocity c2 of the steam as it leaves the rotor blades is also high, which leads to significant losses and reduces the efficiency of the turbine. To reduce this loss, a row of guide vanes is installed behind the first row of moving vanes, causing the steam to change direction within the guide vanes before entering the second row of moving vanes on the same impeller to continue doing work. In this way, the kinetic energy of the steam stream exiting the first row of moving vanes is utilized in the second row of moving vanes, thereby reducing kinetic energy losses. If the steam stream exiting the second row of moving vanes still possesses significant kinetic energy, a second row of guide vanes and a third row of moving vanes can be installed. This type of stage, in which the kinetic energy generated by the expansion of steam in the nozzle is utilized in stages within the rotor blades, is called a velocity stage. A stage in which steam kinetic energy is utilized in two rows of vanes is typically called a two-row velocity stage, while a stage in which it is utilized in three rows of vanes is called a three-row velocity stage.