How mass flow and pressure ratio is related in case of a Gas Turbine

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Discussion Overview

The discussion revolves around the relationship between mass flow and pressure ratio in gas turbines, particularly in the context of using evaporative coolers to lower inlet air temperature. Participants explore theoretical and practical implications of these changes on compressor performance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant observes that cooling the inlet air with an evaporative cooler increases compressor discharge pressure (PCD or P3) due to lower temperature and increased density, which leads to higher mass flow.
  • Another participant agrees with the initial observation and references the conservation of mass, stating that mass flow rate is constant and related to density, velocity, and flow area.
  • A later reply seeks clarification on the relationship between increased mass flow and pressure ratio, noting that the performance characteristic curve indicates an inverse relationship between pressure ratio and mass flow.
  • One participant requests more context to better understand the initial claims, suggesting that the discussion lacks detailed information.
  • Another participant expresses willingness to assist if more information and mathematical evidence are provided regarding the query.
  • A subsequent post clarifies the application context, describing a land-based aeroderivative gas turbine driving a centrifugal compressor and reiterating the observed increase in pressure ratio with increased mass flow.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between mass flow and pressure ratio, with some agreeing on the observations while others highlight the conflicting nature of the performance characteristic curve. The discussion remains unresolved regarding the exact nature of this relationship.

Contextual Notes

Participants have not fully explored the assumptions underlying the performance characteristic curve or the specific conditions under which the observed phenomena occur. There are also unresolved mathematical steps related to the claims made.

Ady707
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Hi,

As per my observation and experience, when we use evaporative cooler to cool the inlet air to the gas turbine, the compressor discharge pressure (i.e PCD or P3) rises. As, the temp. of the inlet air drops and the mass flow increases due to the increase in density, I believe the discharge pressure of the compressor also rises.

However, as per the performance characteristic curve of the compressor, the pressure ratio is inversely proportional to the mass flow.

Can please someone help me understand this
 
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Yes,your relation is absolutely right.
The conservation of mass (continuity) tells us that the mass flow rate mdot through a tube is a constant and equal to the product of the density r, velocity V, and flow area A:

Eq #1:

mdot = r * V * A
 
John Dalton said:
Yes,your relation is absolutely right.
The conservation of mass (continuity) tells us that the mass flow rate mdot through a tube is a constant and equal to the product of the density r, velocity V, and flow area A:

Eq #1:

mdot = r * V * A
I am interested in knowing that when the mass flow across the compressor rises what happen to the pressure ratio ?
 
Ady707 said:
Hi,

As per my observation and experience, when we use evaporative cooler to cool the inlet air to the gas turbine, the compressor discharge pressure (i.e PCD or P3) rises. As, the temp. of the inlet air drops and the mass flow increases due to the increase in density, I believe the discharge pressure of the compressor also rises.

However, as per the performance characteristic curve of the compressor, the pressure ratio is inversely proportional to the mass flow.

Can please someone help me understand this
Could you provide a bit more context ? You seem to know what you are describing, but we see only loose fragments.
 
If you provide more information and facts about your query,I am sure I will be able to help you out.
(By giving mathematical evidences).
 
BvU said:
Could you provide a bit more context ? You seem to know what you are describing, but we see only loose fragments.
It is a land based application of an aeroderivative gas turbine (twin shaft). Gas turbine is driving a centrifugal compressor to compress a natural gas to a certain pressure. We have installed an evaporative cooler to cool the inlet air to the gas turbine. Now, as the inlet air is cooled down, the air got dry, and the density of the air increases and so is the mass flow. As this happens, I observe that the pressure ratio across the integral compressor (gas generator) of the gas turbine increases. I want to understand why this is so that pressure ratio increases with the increase in mass flow ?

I hope I am pretty clear this time. :)

Thanks.
 

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