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Friday, 11 October 2019

FUNCTION OF DIFFUSER IN CENTRIFUGAL COMPRESSOR

We were discussing the basic principle of operations, functions of various parts and performance characteristics of different types of fluid machines such as hydraulic turbinescentrifugal pumps and reciprocating pumps in our previous posts. We have seen there that working fluid for above mentioned fluid machines was liquid such as water.  

Now it’s time to discuss few other types of fluid machines such as centrifugal compressors, axial flow compressors, fans and blowers where working fluid will be steam, air or gases.  

We have already seen the working principle of centrifugal compressorvelocity diagram of centrifugal compressor, slip phenomenon and slip factor for centrifugal compressor and Mach number for centrifugal compressor in our last posts, where we were also discussing the various important parts of centrifugal compressor and their functions and we have also secured there the work done on air in centrifugal compressor. 

Today we will be interested here to discuss a very important part of centrifugal compressor in detail i.e. Diffuser with the help of this post i.e. Function of diffuser in centrifugal compressor.  

Function of diffuser in centrifugal compressor 

Impeller and diffuser are two very important parts of a centrifugal compressor. Energy will be added to the impeller and when working fluid i.e. air will come out from the impeller i.e. at the impeller outlet tip, it will gain the energy by transfer of angular momentum by the rotating action of impeller blades. 

When air will come out at the impeller tip, it will have energy in the form of kinetic energy and static pressure energy. 

Practical application of compressed air will be usually found in engines, gas turbine plants, turbojet engines etc., where this compressed air will be used in a combustion chamber to burn the fuel. 

When fuel will be burnt, there will be generation of high temperature or air will be heated to a high temperature by energy generated due to the burning of fuel. In order to secure the smooth and efficient burning of fuel, there will be requirement of the compressed air at high pressure with very small velocity as low as possible. 

High velocity in the combustion chamber will cause the instability of combustion and combustion could not be made efficiently. Therefore, we must note it here that we want air at the outlet of compressor with relatively very low velocity and high static pressure. 

Excess kinetic energy of the air, available at the outlet of impeller tip, will be converted in to static pressure energy in the diffuser. Therefore, diffuser converts the kinetic energy of the working fluid i.e. air in to static pressure energy and at the outlet of diffuser air will have relatively lower velocity and high static pressure. 

As we can see in figure, there will be vaneless space beyond the impeller tip and partial diffusion process will take place here in this vaneless space. Further a number of vanes will be available to execute the final diffusion process. 

Number of vanes will be provided to divide the air stream in to several channels to make effective control of the flow and simultaneously we will be able to secure the final diffusion i.e. rise in static pressure by conversion of kinetic energy in to static pressure within a short length. Therefore, number of vanes will be provided to direct the air stream in to different channels. 

Why are vaneless space in centrifugal compressor provided? 

This is a very important question that why vaneless space are required in a centrifugal compressor and why not these diffuser vanes are provided just after the outlet of impeller tip so that fluid coming out at the outlet of the impeller may go to diffuser directly. 

But it is not done ever and vaneless space are always provided between the impeller outlet and diffuser vanes. 

There are two basic and very important reasons behind the provision of vaneless space between the impeller outlet and diffuser vanes. 

First reason is that the mach number of flow need to be reduced before it enters to the diffuser vanes. In other words, we can say that velocity of fluid flow need to be reduced before fluid enters to the diffuser vanes. 

Therefore, there must be some space between the impeller outlet and diffuser vanes in order to reduce the velocity of flow of fluid before it enters to the diffuser vanes. This task of reduction of velocity of fluid flow, coming out from the impeller outlet tip, will be carried out by this vaneless space. 

Second reason is that if we will not provide this vaneless space, there will be an excessive circumferential variation of static pressure. If all diffuser vanes will be provided very close to the impeller outlet tip, there will be an excessive variation of circumferential stress and this stress will be propagated radially upstream inward to the impeller and will cause the vibration. 

If vaneless space will not be provided, impeller blades of centrifugal compressor might be damaged due to this vibration which will be caused by radially upstream inward propagation of circumferential stress. 

This vibration will be a function of relative velocity of fluid flow and number of impeller vanes. 

So, we have seen here the basic need and function of diffuser in the operation of a centrifugal compressor. We have also discussed here the importance of vaneless space in diffusion action in the operation of a centrifugal compressor. 

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Further we will find out, in our next post, different types of losses in centrifugal compressor.  

Reference:  

Fluid Machines, By Prof S.K. Som 
Image courtesy: Google   

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