| OCR Text |
Show A schematic ot a Helmholtz pulse combustor investigated at Georgi,a, Tech under a GRI sponsored research program l14 ) is shown in Figure 2. It differs from the quarter wave combustor by having a larger diameter combustion chamber. In addition, this design, which resembles that of the Lennox pulse furnace, also contains a mixer head which had been originally introduced to provide a space for mixing the incoming fuel and air char~I~) Recent studies conducted at Georgia Tech have shown, however, that combustion is initiated as soon as the fuel and air jets impinge upon one another and it is practically completed within the mixer head. The extent to which combustion proceeds into the larger diameter combustion chamber depends upon the fuel loading of the system. A coal burning Rijke t ype(8 §ula, combustor developed at Georg i a Tech " is shown in Fi gure 3. It is based upon the principles of the acoustic Rijke tube which consists of a vertical pipe open at both ends and having a heated metal gauze in the middle of 11~) lower half. In the acoustic Rijke tube heat transferred from the metal gauze to the adj acent gas mol ecu 1 es results in the spontaneous excitation of the fundamenta 1 acoustic mode of the tube whose pressure di stri but i on has a max i mum at the center of the tube and minima at the two open ends, see Figure 3. In the Rijke combustor, the heated metal gauze is replaced by a combustion process which provides the energy required for exciting the combustor oscillations. The interaction of the combustion process with its periodic flow environment establishes a complex feedback loop through which the combustion process provides the energy necessary for maintaining the oscillations. In the combustor shown in Figure 3, unpulverized coal is supplied by an auger to a metal grid located at the distance L/4 from the entrance to the combustor. The combustion air is supplied through the lower decoupling chamber and it reacts with the coal as it passes through the combustion zone above the metal grid. Interestingly, observations of this combustion process revea 1 ed that the presence of acoustic velocity oscillations in the combustion bed region apparently causes (or contributes to) a spouting-like motion of the coal particles in the bed. The latter may be responsible for the excellent performance of this combustor. Finally, recent studies conducted at Georgia Tech have shown that liquid and gaseous fuels can also be burned in Rijke type pulse combustors. APPLICATIONS To date, pulse combustors ranging in capacity from .Ot ~ 25 MBTU/HR have been developed II,! -21). The units at low end of this range have been utilized in various 57 domestic space and water heating applications and the 1 arger un its have been investigated for industrial processes or power generation. The designs of most of these combustors were based on those of either the quarter wave tube or the HelTholtz resQnator and they burned ~afr£u~2~2r' 4,17, i:::l) 1 i qui d (17, Its) and ~011d' ) fuels. Interestingly, practlcally all of the smaller capacity units were self aspirating while the larger units used fans to move the air and combustion products through the system. However, self aspirating gas burning pulse combustors with capac it i es of up to 2 MBTU/HR have recent 1 y been developed and units with capacities between 2 a~51 MBTU/HR are currently under development. In heating applications the water or air is heated by contact wi th the wall s of the combustor, its ta i 1 pipe, and one or more add it i ona 1 heat exchangers wh i ch are often located downstream of the exhaust decoupling chamber, see Fi g. 2. The rna in advantages of these units is their compactness, low NO formation, and self aspiration capabilit§ which eliminates the need for using a chimney to exhaust the combustion products out of the building. Consequently, most of the energy produced duri ng combusti on can be recovered by cool i ng the combustion products to near room temperature pri or to exhaustion out of the system. This results in high thermal efficiencies (around 95%) and considerable fue 1 savi ngs. The cool i ng process produces, however, some corrosive condensates in the exhaust flow which requires that noncorrosive materials be used in the final heat removal stage. In space heating applications, a fan is emp 1 oyed to move the heated air past the pulse combustor surfaces while in water heating and steam raising applications the pulse combustor is immersed in the heated fluid. When a surface or a particle is subj ected to osc ill atory flow conditions it experiences both pressure and velocity oscillations. The magnitude of the oscillatory velocity is often larger than the magni tude of the steady vel oci ty component resulting in periodic reversal of the flow direction next to the surface under consideration. These periodic flow reversals, the associated flow accelerations and dec 1 erat ions and the 1 ~~1 pressure oscillations are believed (although without a conclusive proof) to disturb (or break up) the quiescent boundary layers which are established around immersed particles or adj acent to soli d surfaces. The break-up of such a boundary layer reduces the resistance to heat and mass transfer to and from the surface under cons i derati on, thus enhancing the process. For example, in dryi ng applications, the presence of pulsations in the drying volume is believed to enhance the rates of moi sture heatup, vapor i zat i on and |
