Studies of the pre-mixed butane/air impinging flame jets

Abstract

A comprehensive experimental study was conducted to investigate the flame and thermal characteristics of the pre-mixed butane/air laminar flame jet, which was impinging vertically upwards upon a horizontal rectangular plate. There were three kinds of flame jet investigated: a single slot (rectangular) jet, three slot jets arranging in series and a single round (circular) jet. Effects of the major parameters including the nozzle-to-plate distance (H/d), jet Reynolds number (Re), air/fuel mixture equivalence ratio (?) and jet-to-jet spacing (s/d) on the thermal performance of the flame jet system under consideration were fully examined. There were four major tasks performed successfully to achieve the main objective of the present study. In the 1st task, the flame and thermal characteristics of an impinging pre-mixed butane/air slot flame jet operating at laminar condition were experimentally investigated. Effects of Re, ? and H/d on the flame shape and length, temperature and heat flux distributions on the impingement plate were fully explored. The slot jet was found to have rather different thermal performance along its width (i.e. x-direction) and length (i.e. y-direction) due to the different mass fluxes. An idea to obtain a matching between the length of the inner reaction cone of the flame and the nozzle-to-plate distance to achieve the best heat transfer was proposed. Provision of such information, which was not available in the literature, became one significant contribution of the present study. In the 2nd task, the same experimental investigations were repeated with the round flame jet. In additional to a better understanding of the flame and thermal characteristics of the impinging round flame jet, effect of the shape of nozzle (i.e. rectangular or circular) was identified through comparison between the results with those obtained in the 1st task. The round jet had a very high but concentrated heat flux at the stagnation region, but it decreased very rapidly in the wall jet region. Whereas the slot jet was able to produce more uniform and on avenge higher heat flux. It was another significant contribution of the present study because such comparison was rarely reported in the previous studies. In the 3rd task, similar experimental investigations were performed to fully study the flame and thermal characteristics of the impinging multiple slot flame jets. In addition to those important parameters as mentioned in the 1st task, another important geometric parameter, s/d, was involved in the study. Another significant contribution was made in the present study by providing very valuable results to facilitate the understanding of this flame jet system, which had not been systematically investigated previously. A very significant suppression was obtained between two adjacent flame jets when they were putting too close together, which led to the formation of the distinctive "W-shape" flame and a detachment of the flame front from the impingement plate around the wall jet region. Comparison between the results obtained from the single and multiple impinging slot flame jets provided more foresights about the interaction between adjacent jets, which was an extremely important parameter in the design of multiple flame jets thermal system such as the domestic cooker. In the 4th task, several methods were examined and compared, in order to choose the most suitable method to analyse and handle the obtained experimental results, such that they could be used more conveniently and systematically by other researchers or designers. Empirical equations were developed via generalisation of the obtained experimentally results for predictions of both the stagnation point and area-averaged Nusselt numbers for the single impinging pre-mixed butane/air laminar slot flame jet, with effects of H/d, Re and ? involved. It was another significant contribution of the present study because equation for the prediction of area-averaged Nusselt number had not been reported in the previous studies. The present study provided very valuable information and useful fundamental knowledge to facilitate much better understanding of the flame and thermal characteristics of the round and slot pre-mixed impinging laminar flame jets utilizing a butane/air mixture. In addition, suggestions were made to provide useful guidelines for future work, which will be conducted along the same direction of the present study.