| OCR Text |
Show HEAT-TRANSFER ENHANCEMENT BY PULSE COMBUSTION IN INDUSTRIAL PROCESSES John M. Corliss, Abbott A. Putnam Battelle, Columbus Division Columbus, Ohio, USA ABSTRACT Heat transfer techniques that can be cost effectively applied to industrial processes have been of continuing interest to industry. Increasing the heat-transfer effectiveness of many processes would lead to greater productivity, efficiency, and in some cases, reduced equipment sizes. One such technique being studied at Battelle is the use of pulsecombustion systems in industry. Preliminary results have shown that the heat-transfer coefficient can be augmented by up to 100 percent with this technique, and that heating surface requirements can be reduced by half. Results also show that when coupled with mass-transfer processes, the use of pulse-combustion technology improves the process by up to a factor of 5. The heat-transfer enhancement in pulsecombustion systems is due to the oscillation of gas flow within the combustion system. This oscillation follows the periodic combustion process that takes place within the system. The periodic nature of the combustion processes results from the acoustic coupling of the system geometry with the natural tendency of flames to oscillate. The resulting synergism causes the system to operate at resonance with an increase in heat-transfer coefficient ranging from up to four times that of steady, nonpulsating systems. This paper describes the results of heat-transfer measurements and calculations performed at Battelle to determine the degree of beneficial effect of heat-transfer enhancement in pulse combustors in industrial processes. Three types of processes are considered. These are (1) immersion heaters, (2) radiant burners, and (3) direct-fired operations. Measurements are described for heat transfer in a pulse-combustion boiler and in a conventional fire-tube type boiler. In these experiments, heat-transfer coefficient enhancement of 100 percent was measured in the pulse-combustion system. 39 Measurements in a direct-impingement drying experiment are also described. Heat-transfer enhancement of up to 5 times was found in this system. INTRODUCTION THERE ARE TWO TYPES of pulse combustors in use today. These are the aerovalved and mechanically valved designs. In the aerovalved concept, there are no moving parts and system operation depends on fluid-mechanics effects. The mechanically valved design operates with the use of fastacting check valves to control the flows. Aerovalved systems have a further characteristic that allows some combustion gases to exit the system through the inlet, whereas mechanically valved systems effectively seal the inlet during the expansion portion of the cycle thereby eliminating the reverse flow of products. Pulse combustors described in this paper are of the Helmholtz-type design. An example of a mechanically valved pulse combustor of this type is shown in Figure 1. In operation, fuel and air are drawn into the combustor and ignited by recirculating hot and reacting gases. The fresh charge is ignited and the expanding gases cause the pressure inside to rise, closing the inlet valves and sending the combustion products rushing out the tailpipe. Due to the momentum of the high-velocity gases, a partial vacuum is formed inside the combustor after the burning is nearly complete. This vacuum opens the inlet valves and fresh fuel is brought into the system and the cycle repeats. In many cases, this process repeats up to 150 times per second. The oscillations in gas flow that result from the process are responsible for the heat-transfer enhancement and other benefits that will be discussed in this paper. ADVANTAGES OF PULSE-COMBUSTION SYSTEMS - Pulse combustors provide several advantages when used in industrial heating applications. These are (1) enhanced heat transfer, (2) pressure gain in the exhaust gases, (3) self-pumping of |
