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Show combustion air, (4) compact design, (5) low system cost, and (6) low HOx emissions. The most significant advantages are the high heat transfer and low cost, although low NOx emissions could become important as environmental concerns increase(l}. Compact designs result from the high heat transfer and the fact that the combustor is also the heat exchanger. This feature is attractive when floor space, shipping, and installation are considered. BACKGROUND Several investigators have determined the heat-transfer augmentation resulting from pulsations in a pulse combustor. The degree of augmentation varies among the different experiments, but the overall results agree that higher heat transfer results from pulsations. Hanby(2} reported increases on the order of 100 percent with a straight tube fired with compressed air and propane. The tube measured 1880 mm (74 in) in length and was 50 mm (2 in) in diameter. Operating frequency was 100 Hz. In his paper, the results were reported on the basis of a dimensionless velocity ratio composed of an OSCillating velocity and a steady velocity based on overall firing rate and air/fuel ratio. Pressure amplitude was very high in the test apparatus, 34 kPa (5 Psig), which results in a high oscillating amplitude. Heat-transfer coefficient m~asured during th~ experiment was about 200 W/M -C (35 ~tu/Hr-Ft -F) dur~ng pulsations and 85 W/M -C (15 Btu/Hr-Ft -F) with steady flow. Moreover, Hanby showed that at some conditions of mild pulsations, the heat-transfer coefficient can actually be somewhat less than steady flow. Overall heat transfer was shown to increase linearly with pressure pulsation magnitude above this point however. Blomquist et al.(3} studied a Helmholtz type pulse combustor at Argonne National Laboratories, and measured the heat transfer on the surface of the burner. Overall heat transfer was augmented by 25 to 40 percent in the system. The pulse combustor fired at a rate of up to 59 Kw (200,000 Btu/Hr) on natural gas and operated at a frequency of approximately 70 Hz. Heat transfer was measured by calorimetric measurement of water flow rate and temperature rise. In fact, the burner was constructed of several "doughnuts", each water cooled. The heat-transfer coefficient was measured at about 50 mm (2 in) intervals along the combustion chamber and tail pipe. In some cases, a corebustor was inserted into the tailpipe to enhance heat transfer. Heat transfer in drying systems has been measured by Tamburello et al.(4). In this experiment, a pulse combustor was used to dry carrot slices inside a chamber. The drying time was measured as a function of dryer temperature. These results were compared to an electrically heated chamber operating without pulsations. The pulse combustor was an aerovalved design and operated with LPG at a frequency of 140 Hz. Fuel input rate was 139 Kw (475,000 Btu/Hr). Actual 40 rate of heat transfer is not given, instead the author reported the time required to reach)a I desired ratio of dried to wet weight (0.37. n the case of the pulse combustor, the time required was 10 minutes. With the elect~ical ly heated dryer, the time required was 35 mlnutes, although power consumption was not reported. MEASUREMENTS AT BATTELLE - Two experiments were conducted at Battelle to determine the heat transfer augmentation in pulsating combustion: These experiments involved a boiler and a drYlng process. The following sections summarize these results. Boiler Experi.ents - In 1982, Battelle completed an experimental program directed towards developing a high-efficiency commercial/industrial boiler(5). In this program, we measured the heat-transfer coefficient from two types of pulse combustors when operated as immersion heaters. An aerovalved design and a reed-valved design were considered. The results were compared to measurements taken from a conventional fire-tube boiler. Heat-transfer measurements in the combustion zone of a conventional fire-tube boiler show~d that the heat flu~ was approximately 85 Kw/M (27,000 Btu/Hr-Ft). An aerodynamically valved pulse combustor operating as an ~mmersion heater pr~duced a heat flux of 128 Kw/M (40,600 Btu/HrFt ), and a reed-valved2Pulse combustor pr~duced a heat flux of 184 Kw/M (58,200 Btu/Hr-Ft ). Stoichiometry, metal and water temperatures, and gas temperature were comparable for the three cases studied. The heat-transfer coefficient enhancement is reflected in the heat flux as a result. These tests showed that the aerovalved PIC augmented the heat transfer by 50 percent, and that the reed-valved system augmented heat transfer by 116 percent. These results agree favorably with Hanby's results in that the pressure amplitude in an aerovalved PIC is less than the amplitude in a mechanically valved system. Consequently, the heat transfer would be expected to be higher in the reed-valved system. Drying Experi.ents - There are no data available in the literature that show heat flux or heat-transfer coefficient in drying or directimpingement systems with pulsations. The data that are available discuss the rate of drying in terms of time or fuel required to reach a given moisture level in the product. While these data are impressive, the usefulness to deSigners is limited by the lack of numerical or design information. Battelle conducted an in-house study (unpublished) of the drying process as related to pulse-combustion drying to determine the heat-transfer rate in these types of systems. The experiment was conducted with a test specimen that was specially constructed to allow the heat-transfer parameters to be measured. Figure 2 shows the test setup and the specimen used. In these tests a tightly wound, fine-mesh screen was attached to the end of a small capillary tube. A thermocouple was inserted in the mesh windings to measure the temperature. |