Simulation and verification of effect of throttle opening on temperature rise characteristic of Helmholtz pulsating burner

Abstract: Pulsating combustion has the advantages of high combustion intensity, high heat transfer coefficient, and low pollutant emission, and therefore it has been widely used to enhance the heat transfer process, such as aeromodelling's power, material drying, and pest control. In recent years, pulsating combustion heater, such as hot water boiler or steam boiler, has become another new application of pulsating combustion technology. Pulsating combustion heater employs the pulsating burner as the heat source, and the main components such as combustion chamber and tail pipe are immersed in heating medium (such as oil and water). When the pulsating burner works, heat is transferred to the heating medium through the high temperature outer wall of the combustion chamber and tail pipe, and then the heating medium is heated to the temperature required by the process. The efficiency and the economy of the pulsating combustion heater were related to the temperature increasing process of the heating medium. In order to investigate the characteristics of temperature distribution of airflow field outside combustion chamber of the pulsing burner, a two-dimensional unsteady numerical model was established. The increasing-temperature process in the central cross section of combustion chamber was simulated by Fluent software under the conditions of 5 different kinds of oil consumption (30?, 60?, 90?, 120?, and 150? throttle opening), employing a self-excited and self-suction Helmholtz pulsing burner as the heat source. Temperature increasing process of the pulsating burner in the heating medium was difficult to obtain, but it was easier to obtain in the air. The temperature increasing process of the heat source in the air was obtained by an infrared thermal cameral under all kinds of different oil consumption. Formula fitting was carried out for the increasing-temperature process. The fitted values and measured values were in good agreement. The maximum temperature difference between the fitted data and measured results was 5.2 ℃ and the maximum relative error was 2.2%. The temperature increasing law of the heat source was added to the numerical model by UDF (user-defined function) interface. A series of tests were performed to investigate the accuracy of the numerical model. It was found the simulation values were matched with the test data quite well, and the average relative error was between 2.68% and 5.54%. Temperature elevation of airflow field outside combustion chamber showed an "S" shape. In the beginning, air temperature increased rapidly. After a period of time, the temperature tended to be stable. With the increase of throttle opening, there was little difference in temperature increasing process for one single test point. The closer the distance to the combustion chamber in the same direction, the shorter the time it took to reach the maximum temperature, and the higher the temperature increasing speed and the maximum temperature. The test points above the combustion chamber got the higher temperature increasing speed and the higher maximum temperature than the test points which were at the right and the bottom of the combustion chamber when they were in the same distance to the combustion chamber. The 4 test points above the combustion chamber reached the highest temperature at 1.25 min, but the time required for the test points at the right and the bottom of the combustion chamber varied from 2 to 5 min when the throttle opening was 120?. The airflow outside the combustion chamber was driven upward from the bottom under the influence of density difference. Karman vortex street was observed when the airflow passed the combustion chamber, and the airflow temperature field above the combustion chamber oscillated periodically. As the vortices moved upward, their influence on the airflow temperature field was gradually weakened, and the temperature amplitude gradually decreased. This paper can be helpful to further understand the heating process of internal temperature field of the pulsating combustion heater when using oil or water as heating medium, and it also can provide references to optimize heat source structure design inside the pulsating combustion heater.Key words: temperature; models; computer simulation; pulsating combustion