Re: What is the difference between a Venturi tube and a de Laval nozzle?

Hi,

Yes, that's the essential difference.  Both use the venturi effect to
accelerate the speed of gas flow by narrowing the path through which the
gas is flowing.  A generic venturi will do it so as to reduce the gas
pressure, allowing the injection of another fluid.  That's how gasoline was
mixed with air in carburetors, before the days of fuel injection.  

A venturi is basically a narrow part of a pipe, with gas flowing constantly
through it. The pipe is usually very much the same on either side of the
narrow portion.

A de Laval nozzle is a little more complicated, due to the goal of
accelerating hot gas to highly supersonic speeds.  Unlike many venturis,
the nozzle is not symmetric.  It has a chamber with hot gas on one side, a
narrow portion through which the hot gas flows, and then a flaring part
through which the gas exits into the outside air (or vacuum, for a rocket
in orbit).  

Hot gas begins by flowing subsonically through the entry of the nozzle.  As
the nozzle chokes down, the gas accelerates, until it is moving at the
sound speed.  

If that's all that happened, then when the nozzle expanded on the other
side, the gas would slow down again, and you'd just have a basic venturi. 
But, the gas still has a pressure well above that of the outside medium,
and so it continues to accelerate to supersonic speeds after passing
through the narrow part of the nozzle.  Maximizing the exit speed is a
matter of carefully shaping the conical part of the exit.

The end goal is to convert as much as possible of the thermal energy of the
hot gas into directed kinetic energy coming out of the nozzle.

Cheers,
Stephen