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Show This device was then placed in the tailpipe of a pulse combustor at a point 1/4 of the distance back from the outlet as shown in Figure 2. The pulse combustor operated at a firing rate of 73 Kw (250,000 Btu/Hr) and a frequency of 100 Hz. A conventional, retention-type burner was adapted to fit into the inlet region of the pulse combustor such that the system could be operated at the same firing rate and air/fuel ratio, but without pulsations. During the experiment, water was fed to the specimen while the combustor was operated. When an insufficient supply of water was present, the temperature of the specimen was very high. In cases where too much water was supplied, the specimen temperature was low. When the water flow matched the evaporation rate, specimen temperature was very close to boiling temperature. Measurements were made with the system pulsing and at steady flow. Data are shown in Figure 3. These results show that the heattransfer coefficient is increased by up to a factor of five in the pulsing system. It should be noted that the overall heat transfer shown in Figure 3 is composed of convection and mass transfer so that these results are more closely related to impingement heat transfer when vaporization or other mass-transfer process is simultaneously occurring. PIC APPLICATIONS TO INDUSTRIAL SYSTEMS Three systems that can realize a benefit from pulse combustors are {1} immersion heaters such as bath heaters or boilers, {2} radiant tubes, and {3} direct impingement heaters. These systems are discussed in the following sections. Each section begins with a discussion of conventional technology and the heat transfer process in the system as presently designed. The same system is then reconsidered with the use of pulse-combustion technology. In many cases, actual industrial experience is cited in the case of conventional technology, but in the case of pulse combustion, the discussion is theoretical or based on experimental results because these systems are not presently used in industry. IMMERSION HEATERS - This type of burner is used in applications such as boilers and heating chemical baths etc. Primary mode of heat transfer from the combustion gases to the load is convection first from the flue products to the burner wall, then convection from the burner outside surface to the liquid. The system is limited by the gas-side convection coefficient, and as previously discussed, the pulse combustor can have a beneficial impact on this coefficient. A cursory heat balance has been performed to estimate the benefits of using pulse-combustion technology in this application as described below. Thg systems considered are fired at 293 Kw (1 x 10 Btu/Hr) with 10 percent excess air. Flue gas leaves the system at 260 C (500 F) yielding an efficiency of 80 percent. The thermal load is oil being heated to 120 C (250 41 F). Conventional Technology - A potential immersion heater design is shown schematically in Figure 4. A conventional combustor is employed and flue products are directed into several tubes compriSing the submerged heat exchanger. Heat transfer is governed by turbulent convection inside the submerged tubes according to the relation known as Reynolds' analogy(6}: f k h = 8 0 Pr Re Where: h = heat-transfer coefficient f = friction factor D = hydraulic diameter k = thermal conductivity Pr = Prandtl number Re = Reynolds number The heat-transfer coefficient on the outside of the immersion heater would be significantly higher because a liquid is being heated, so the gas-side heat transfer controls the process. An energy balance for a typical heater system in~icates t2at a total heat-transfer area of 12.7 M {137 Ft } would be needed. This could be provided with 20 50 mm {2 in} diameter tubes 3 M {10 Ft} in length, along with a combustion chamber measuring 305 mm (12 in) in diameter and 305 mm (10 Ft) in length. Pressure drop through the tubes would be less than 25 mm (1 in) H20 based on the Darcy friction factor and entrance and exit effects(7}. Pulse-eo.bustion Technology - The introductory section of this paper has already outlined some of the direct benefits made possible by pulse-combustion technology for this application. Namely the heat-transfer coefficient inside the combustor is enhanced by up to 100 percent. A second, more salient feature of pulse combustors that works to enhance this application is the ability of the pulse combustor to impart a pressure gain on the combustion products. This pressure gain or boost is usually on the order of 100 to 200 mm (4 to 8 in) H20. By properly deSigning the system, the augmented heat transfer and pressure-gain characteristics of the system can be advantageously used. A pulse-combustion system suitable for this application is shown schematically in Figure 5. In this system, a combustion chamber having dimensions of 250 mm (10 in) diameter and 460 mm {18 in} in length is used. Several tailpipes are connected to the combustor. The use of multiple tailpipes is required to increase the heattransfer area of the combustor and take advantage of the heat-transfer enhancement. Twenty tailpipes measuring 50 mm (2 in) in diameter and 1.5 M (60 in) in length are envisioned. The |