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Thread: Choked conditions through an orifice plate vs de laval nozzle

  1. #1
    Associate Engineer
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    Jul 2014
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    Choked conditions through an orifice plate vs de laval nozzle

    Hi all,
    It is clear to me that when looking at gas flow through pipe that both De laval nozzles (venturi) and orifice plates can become choked (mach 1 at/near throat) and accelerate flow. I understand that for large flow velocities,the de laval nozzle is much more appropriate for accelerating flow to super sonic velocities because it reduces turbulence and retains more energy. I read however that it is not possible to achieve super sonic conditions after an orifice plate choke. Can super sonic flow only be achieved by de laval nozzles or could you theoretically use an orifice plate despite its loss in efficiency?

    In other words yes I do understand that with de laval nozzles once the flow is choked, the velocity at the choke is always M1, and the only way to increase the velocity after the choke it to increase the pressure before the choke, as decreasing the pressure after the choke will have no effect. Is this also true with orifice plates?

    CB

  2. #2
    Lead Engineer
    Join Date
    Aug 2013
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    Houston TX USA
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    It is not possible to achieve a supersonic flow beyond a orifice plate due to the shock wave that is created at the orifice plate discharge point edge, it has nothing to do with the relative efficiencies of the orifice vs the nozzle. The diverging section of the nozzle is key element that allows the gas to make a smooth transition to supersonic velocities. With a standard orifice plate there is an immediate flow separation at the orifice back edge that results in the first of a series of sonic shock diamonds that dissipate energy as the pressure is successively reduced as heat until flow achieves the surrounding ambient pressure at the discharge.

    The shape of the entrance to the nozzle is not of importance in attaining a supersonic discharge flow; however, the De laval nozzle with the conical entrance will be more efficient from a flow vs. inlet pressure standpoint by reducing the flow constriction at the nozzle/orifice throat.
    The key issue in attaining a supersonic discharge is preventing flow separation from the nozzle wall in the discharge section.
    The two key elements in attaining a maximally efficient supersonic nozzle design are the correct nozzle interior angle and the length of the discharge cone. If the interior angle of the discharge is too large flow separation will occur if it is too small then an unnecessarily long nozzle is required. If the length of the nozzle is not matched to pressure differential between the nozzle throat discharge pressure and surrounding ambient (atmospheric, in the case of rockets) pressure then one of the two following conditions occurs. First, if the nozzle is too short then the maximum supersonic velocity possible will not be achieved and a shock wave will be created at the back edge of the nozzle to dissipate the remaining energy. Alternatively, if the discharge nozzle is too long then a shock wave will occur within the nozzle before the end of the nozzle is reached and the excess nozzle length results in additional engine weigh that is of no value. This is not to say that there will be no supersonic flow and thrust achieved in each of these cases; but, in the first case maximum power is not achieved and the second case the excess nozzle length results in additional engine weight that is of no value.

    In rocket design each rocket discharge nozzle must be designed for the specific altitude at which maximum thrust is desired.

    I hope this answers your question and a bit more while I was at it.

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