Abstract:

Pulse combustion has been used in a variety of ways since rst being discovered in 1877. This a combustion process that occurs under oscillatory conditions with changing state variables, such as pressure, temperature and velocity. This paper looks at the historic uses of pulse combustion, and it provides an overview of this unique process. Pulse combustion has been used to ampli thrust power with the German V-I rockets. Pulse combustion has been used to optimize ame ef ciencies, and it is now re­ emerging in many new industrial applications including some for Waste to Energy. 

 

What is Pulse Combustion?

Pulsating Combustion is a combustion process that occurs under oscillatory conditions. That means, that the state variables, such as pressure, temperature, velocity of combustion gases, etc., that describe the condition in the combustion zone, vary periodically with time.

Furthermore, acoustic oscillations generated in the combustion process are coupled with heat and released into the process at the same time.

The rst known example of pulse combustion is so called" singing ame" discovered by Dr. Higgins in 1 777. 

 

His studies were followed by Deluc, Chladni, Faraday, Tybdall, Rayleigh, which showed that sound of considerable amplitude can be generated when a gas flame is placed in a larger diameter tube. The observations also indicated that when a certain set of conditions is satis ed, an interaction

between the bu er-flame and generated acoustic oscillations can take place. 

 

The first known attempts to utilize pulse combustion process occurred at the tum of 20th century. The most well known application is German V-I rocket motor, or a buzz bomb. Another good example of a pulse combustion device is a Hel olz b er. The principle of this bu er is utilized by several boiler companies in their commercial products.

How Does Pulse Combustion Work?

All pulse burners can be divided in three sections: the inlet, the combustion chamber, and the exhaust section. The inlet section can consist, in case of the Helmholtz and Schmidt combustors of apper valves on air and fuel lines. In contrast, the other type of pulse butners (including our version), have no apper valves but consists of aerdynamic air inlet on the air side. 

 

 

 

To start the operation, the combustion reactants are delivered into the combustion chamber via air and fuel lines or apper valves, where a spark plug or a pilot ignites the mixture. The initiation of the combustion process creates a sudden pressure rise that closes apper valves and combustion gases move downstream towards the end of the exhaust tube. This movement, in turn, produces a negative pressure in the burner itself and allows apper valves to open and allow next portion of the air and fuel into the combustion chamber where it is re-ignited by the hot environment. The negative pressure created in the burner volume also creates reversal motion of the part of the exhaust gases back into the combustion chamber which facilitate ignition of fresh charge of the combustion reactants and next pressure rise in the combustion system. The combustion process now repeats itself indefinitely without spark plug and the operation is controlled by acoustic laws and takes place at a created single equency.

 

Pulse Combustion and Heat Transfer

High reversible velocities of combustion gases in pulse combustion have potential for improvement of heat transfer. This could be accomplished by introducing pulse combustion process in the area where the combined heat transfer process is present, for example convective radiant and conductive type. Reversible motion of combustion gases allows diminish boundary layer formed on the heat transfer surfaces, and, in tum, allow better heat transfer om combustion gases to the surfaces itself, resulting in increase of conductive heat transfer as well. Some works show improvements in heat transfer by the factor of 2.5 depending on application.

 

Pulse Combustion and Emissions.

In general, any combustion technology is characterized by its NOx and CO emissions, and bu er equivalence ratio or coef cient of excess of air. Also any combustion technology is dependent on As it is known om the classical theory of Zeldovich, there are three different type of NOx:

 

a) Thermal NOx formed during combustion process

b) Fuel NOx

c) Prompt NOx

 

Time. Term time really means residence time of combustion reactants to complete the combustion and"become" combustion gases. Typically residence time of a high velocity steady state bu er is about 20 milliseconds. The same residence time in a pulse bu er is 2-8 milliseconds. The short residence time, in tum, leaves less time for thermal NOx to be formed.

Turbulence: This is rather relates to advanced mixing formed by reversible flows of combustion gases. Pulse Combustion takes oxygen that is available for combustion and converts it into "usable" oxygen, thus allowing combustion process with very low coe cient of excess of air. In addition, the same high reversible velocities of the combustion gases are responsible for the high heat and mass transfer rates within the process. 

 

Temperature. This term relates to the temperature of the ame. Usually the thermal NOx are formed around stoichiometric temperature when the excess of air is low and the combustion is close to its complete conditions. Pulse combustion process creates reversible flow where esh portion of the combustion reactant is continuously and oscillatory is brought into the combustion zone, which decreases the average ame temperature, as the research show, by four hundred degrees and creates less favorable conditions for the thermal NOx formation.

 

Add ional factor that is present in pulse combustion and is attributive to its low emissions is Automatic Natural Flue Gas Re-circulation. 

 

As it was mentioned above, part of the exhaust gases is invited back into the combustion process. In addition to the reducing of the flame temperature, this part of the combustion gases creates natural automatic ue gas re-circulation, where NOx oxides are already formed,

thus, reduces rther formation of the NOx.

 

All of the factors above create very speci c conditions of the combustion that naturally creates lower level of NOx e l SSlOns. 

 

Current Applications

Today pulse combustion technology is success lly applied in boilers of commercial scale, spray, and conveyer drying. There were efforts to utilize this technology in cement pyro-processing, waste incineration and there are efforts to apply this technology to the waste-to­ energy systems. 

 

Potential Bene ts of Utilization of the Pulse Combustion Technology in the eld of Waste-To-Energy Systems 

The pulse combustion systems can be success lly applied as an alte ative to soot blowers. It generates a lot more power l level of sound (about 300 times) and can be tailor-designed to the speci cs of the area to be cleaned.

 

The pulse combustion systems can serve a power l heat transfer booster of"in­ situ" type where it appears necessary. The pulse combustion systems can serve as fast and very power l ignitor of the main combustion process. 

 

 

References:

 

 

  • Tyndall, J. Sound. D. Appleton & Company, New York
  • Keller, J. O. Pulse Combustion: The Mechanism of NOx Production. Combustion and Flame 80, 219-237, 1990.
  • P. A. Eibeck, et al. Pulse Combustion: Impinging Heat Transfer Enhancement.
  • Combustion Science and Technology Volume 94, 1-6, 1993
  • Pulse Combustion Boiler, Fulton Thermal Products Co, Product Brochure
  • C. Pope et al. Control of NOx Emissions in Con ned Flames by Oscillations.
  • Combustion and Flame 113 : 13-26, 1998
  • Plavnik, Z. et al. Pulse Combustion System and Method, US Patent 6,210,149 Bl, 2001

Pulsed Combustion

Fig 1. Discovery of a Singing Flame by Dr. Higgins, 1777

Fig. 2 An anned airplane with a pulse rocket engine: the V-I "buzz bomb." 

Fig 3. Schematic Diagram and Operation of a Pulse Burner

 

 

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