Blower fitted with De Laval Nozzle

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Discussion Overview

The discussion revolves around the feasibility of achieving supersonic airflow using a blower fitted with a De Laval nozzle. Participants explore the conditions necessary for creating supersonic flow, including the pressure ratios and specific parameters of the nozzle design. The conversation includes technical aspects of compressible flow and the implications of various nozzle configurations.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether a blower can create supersonic flow with a De Laval nozzle, given a specific airflow speed of 100 m/s and a throat to inlet area ratio of 1:4.
  • Another participant suggests that achieving supersonic flow depends on the pressure ratio between the back pressure and supply pressure, indicating that the blower must support this ratio.
  • Clarification is sought regarding the term "pressure ratio" in the context of blowers, with a focus on the blower's ability to generate sufficient pressure to initiate flow through a convergent-divergent nozzle.
  • Participants discuss the necessary starting pressure for a CD nozzle and how it can be calculated, with references to isentropic pressure ratios at design Mach numbers.
  • One participant requests specific examples of CD nozzles and their starting pressures, expressing a desire for practical illustrations of the concepts discussed.
  • Another participant emphasizes the importance of understanding compressible flows and the relationship between inlet and exit pressures and temperatures in determining exit velocities.
  • Concerns are raised about the adequacy of the blower's specifications, particularly regarding its ability to achieve the necessary pressure for supersonic flow.
  • Discussions include the implications of different expansion ratios and their corresponding Mach numbers, as well as the temperature ratios that affect exit velocities.

Areas of Agreement / Disagreement

Participants express differing views on the capabilities of the blower and the conditions required for achieving supersonic flow. There is no consensus on whether the specified conditions can yield a maximum velocity of 400 m/s, and the discussion remains unresolved regarding the specific performance of the blower and nozzle combination.

Contextual Notes

The discussion highlights the complexities of compressible flow, including the dependence on specific parameters such as pressure ratios and temperature, which are not fully defined in the initial query. The relationship between the design of the nozzle and the performance of the blower is also a critical factor that remains underexplored.

  • #91
boneh3ad said:
The question is efficiency.
If the recovery is 75%, do you consider that efficient or not.
 
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  • #92
Depends on the mach number. As Boneh3ad said earlier, efficient pressure recovery becomes more difficult with increasing mach number, so 90%+ is trivial at a bit over mach 1, but even 75% would be incredibly difficult when hypersonic.
 
  • #93
T C said:
If the recovery is 75%, do you consider that efficient or not.

I think you need to study gas dynamics a bit because it is clear from your questions that you are not familiar with the topic and it is making this very difficult to discuss.

Any supersonic flow that started at atmospheric pressure must, at some point, be slowed back down to reach atmospheric pressure. If no diffuser is used, a normal shock typically forms and causes a certain amount of total pressure loss. By fitting various forms of diffuser geometries to the outlet of such a device, we can try to improve upon the performance of a normal shock with varying degrees of success. Therefore, diffuser efficiency is typically measured by comparing it with the efficiency of a normal shock since the pressure recovery is highly dependent on Mach number.

So, if you are asking whether 75% pressure recovery is good, I'd go back to the answer I gave you last time you asked that. That would be pretty great if you had a Mach 5 flow, pretty trivial if you had a Mach 2 flow, and you really just made things worse if you have a Mach 1.5 flow.
 
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