Difference Between Impulse and Reaction Turbine

By BYJU'S Exam Prep

Updated on: September 25th, 2023

The difference between Impulse and Reaction Turbines is a topic of great significance in the field of mechanical engineering. Turbines are used to convert the energy of a fluid, typically steam or water, into mechanical energy that can be used to generate electricity or perform other work. There are two main types of turbines used in power generation: impulse turbines and reaction turbines. Both types have their own unique characteristics and applications, and understanding the difference between them is critical for designing and operating efficient and reliable power systems.

Impulse turbines are characterized by the fact that the fluid flows through a set of fixed nozzles, which convert the pressure energy of the fluid into kinetic energy. The high-velocity fluid then strikes a set of stationary blades, known as buckets, which are shaped to redirect the flow of the fluid and extract its kinetic energy. Impulse turbines are typically used for high-head applications, where the fluid has a high velocity and low mass flow rate. This article provides the study notes about the “Difference Between Impulse and Reaction Turbine” topic of fluid mechanics.

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What is the Difference Between Impulse and Reaction Turbine?

Reaction turbines are characterized by the fact that the fluid flows through a set of stationary blades, known as stator blades, which are designed to redirect the flow of the fluid and increase its velocity. The high-velocity fluid then enters a set of moving blades, known as rotor blades, which are shaped to extract the kinetic energy of the fluid and convert it into mechanical energy. Reaction turbines are typically used for low-head applications, where the fluid has a low velocity and high mass flow rate.

While both impulse and reaction turbines serve the same basic function, their operational principles and design features are quite different. Understanding these differences is critical for selecting the right type of turbine for a given application and optimizing its performance. In the following sections, we will explore the key differences between impulse and reaction turbines in more detail, including their working principles, design features, and applications.

Impulse and Reaction Principles

Turbomachines are classified as impulse and reaction machines depending on the relative proportions of the static and dynamic heads involved in the energy transfer. To aid this, we define a term referred to as the degree of reaction Rd.

The degree of reaction Rd can be defined as the ratio of the static head to the total head in the energy transfer.


The degree of reaction can be zero, positive, or negative.

Rd=0 characterizes a close turbo machine for which a static head is equal to zero.

In the most general case, this will happen if U1 = U2 and Vr1 = Vr2.

These classes of turbomachines are referred to as impulse machines. In most practical situations Vr2 may be less than Vr1 even though r1 = r2.

This is generally due to frictional losses. Even then a machine is referred to as an axial flow turbine and pumps would have r1 = r2 and if Vr1 = Vr2, then they become examples of pure impulse machines.

Pelton Wheel and tangential flow hydraulic machines are also examples of impulse machines.

Radial Flow Pump and Compressors

Radial flow pumps and compressors are types of fluid machinery that are commonly used in various industrial and commercial applications. These machines work by transferring fluids or gases through a series of impellers or rotors that are arranged radially around a central shaft. Radial flow pumps are primarily used for pumping fluids such as water, oil, or chemicals, while radial flow compressors are used for compressing gases such as air, nitrogen, or helium. Both types of machines can be found in a wide range of industries, including oil and gas, power generation, chemical processing, and HVAC systems. Radial flow pumps and compressors offer several advantages, including high efficiency, low maintenance, and compact design, making them a popular choice for many applications.

General Analysis





Most of the turbo machines belong to this class. In general, they have a restricted flow area for a given rotor diameter and have low to medium specific speeds.

Significant Aspects

  1. Flow is outwards from the smaller to the larger radius of Euler’s turbine equation. i.e., P = m(Vw1U1 – Vw2U2)/t requires that Vw2U2 > Vw1U1 for pumps and compressors which are power-absorbing machines. For this sake radial flow compressors and pumps generally have fluid entering at a smaller radius and leaving at a larger radius.
  2. The absolute velocity at the inlet is oriented parallel to the axes of the shaft i.e., Va1 = V1, and hence there is no whirl component at the inlet i.e., Vw1 = 0.
  3. Since Vw1 = 0, the energy transferred is purely a function of the exit condition i.e. Significant

Head-capacity relationship


Expression for Degree of reaction in terms of rotor velocity and rotor blade angles

We know that the Degree of reaction is given by,


For a pump, it is generally acceptable to write the degree of reaction as



The degree of reaction is the ratio of the suction head to the total head. Which may be written as






General Analysis of Turbines

They are power-generating turbo machines, which run on both incompressible fluids such as water as well as compressible fluids such as gases. The efficiency of turbines may be defined as the ratio of actual work output to the fluid energy input. This involves 2 types of efficiencies:

  1. Hydraulic efficiency /isentropic efficiency.
  2. Mechanical efficiency.

Mechanical efficiency takes care of all losses due to energy transfer between mechanical elements. In the turbines, mechanical efficiency is very high and of the order of 98 to 99%.

Hydraulic efficiency takes care of losses during flow.

We realize that turbines must have a residual exit velocity so that flow is maintained.

However, this residual velocity so that flow is it represents a lot far as the rotor is concerned. Hence, even if we have idealized friction-free flow it is not possible to transfer all the energy in the fluids due to the need to have the final residual exit velocity.

Hence, hydraulic efficiency is a product of 2 terms and is given by

ηH = ε*ηV

ηV – where is referred to as vane efficiency and takes care of the frictional loss.

Utilization factor

Utilization Factor is defined as the ratio of the actual work transferred from the fluid to the rotor in an ideal condition to the maximum possible work that could be transferred in an ideal condition.

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