Studies of single and multiple impinging hydrocarbon flame jets

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Studies of single and multiple impinging hydrocarbon flame jets

 

Author: Dong, Leilei
Title: Studies of single and multiple impinging hydrocarbon flame jets
Degree: Ph.D.
Year: 2002
Subject: Combustion
Jets -- Fluid dynamics
Heat -- Transmission
Liquefied petroleum gas
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: 1 v. (various pagings) : ill. (some col.) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1657641
URI: http://theses.lib.polyu.edu.hk/handle/200/4341
Abstract: The pre-mixed impinging flame jets have been used widely in both industrial and domestic applications. Extremely high convective heat transfer characterizes this impingement system for its important role in heating appliances. However, a full understanding of the flame jet impingement phenomenon is not yet obtained, such that most of this system are not used at their best performance. In addition, almost all the previous studies are focused on the methane or natural gas flame, and have not covered the other important gaseous fuels such as butane. To provide more information on the flame impingement heating process, the present investigation was carried out to investigate the heat transfer from pre-mixed butane/air impinging flame jets. Butane gas was selected because it is a relatively low-pressure gas and very suitable for domestic applications. An experimental study was undertaken in order to understand well the heat transfer characteristics from both single and multiple impinging flame jets to the target plate. It was found that for an impinging circular flame jet, the highest Nusselt numbers were obtained at the equivalence ratio of o=0.85 when the nozzle-to-plate distance was maintained at 5d. At the stoichiometric condition, the highest Nusselt number was obtained at the nozzle-to-plate distance of 6d. Non-dimensional correlations were developed from the experimental results to predict the maximum Nusselt number and average Nusselt number for laminar flame jets as a function of the nozzle-to-plate distance, Reynolds number of the jet and equivalence ratio of the fuel/air mixture. The effect of the flame impingement incidence on the heat transfer characteristics had also been determined. For the chosen inclination angles of 57o, 67o, 80o and 90o, it was found that location of the maximum heat flux would be shifted away from the geometrical impingement point by reducing the angle of incidence. Decreasing the angle of incidence also enhanced the maximum local heat flux, while reduced the average heat transfer rate. The effect of the burner nozzle shape on the heat transfer characteristics had also been examined by conducting similar experiments with an impinging slot jet, which was of the same effective nozzle diameter as that of the circular nozzle. It was found that the slot flame jet was able to produce more uniform heat flux profile and higher average heat flux than the circular flame jet. After obtaining the basic understanding of the impinging single flame jet, the investigation was then conducted to deal with the flame shape and the heat transfer characteristics of the multiple impinging flame jets. The emphasis was put on the influence of the between-jet interference on the heat transfer characteristics. For the dual jets system, it was found that the two jets were forced to separate from the impingement plate and went downwards after colliding at the midpoint between them due to the between-jet interference. Such interference increased with reducing S/d or H/d ratio. The interference reduced the heat transfer rate in the between-jet interacting zone for small S/d ratios, due to the separation of the high-temperature inner reaction zone of the flame from the impingement plate. For a row of three impinging flame jets, it was found that a positive pressure existed in the between-jet interacting zone to produce an asymmetric flame structure and heat transfer distribution of the side jet. The meeting point of the spreading wall jets of the central and side jets was not at the midpoint between the two neighboring jets, but was at some distance shifting outwards. The maximum local heat flux and the maximum area-averaged heat flux occurred at a nozzle-to-plate distance of 5d and a jet-to-jet spacing of 5d. The area-averaged heat flux reduced when both the jet-to-jet spacing and the nozzle-to-plate distance were decreased. To examine the fraction of radiation in the total heat flux, A Monte Carlo method was used to calculate the radiative heat transfer from the flame jet to the impingement plate. The calculation results showed that the radiation fraction in total heat flux was less than 10% for blackbody plate and less than 1% for polished copper plate. The present study provided detailed information and useful fundamental knowledge regarding the flame shape and the heat transfer characteristics of the single premixed butane/air impinging flame jet. A complete understanding of the heat flux distribution on the impingement plate obtained at different operating conditions, which had not been systematically reported previously, was achieved. Non-dimensional equations had been developed with the support of extensive experimental results, which enabled a more accurate prediction of the convective heat transfer coefficient at the flame/pate interface. The effort to investigate the flame shape and the heat transfer characteristics of the multiple flame jets system led to a significant contribution in exploring the science of this problem. In particular, information on the influence of the between-jet interference, which was almost absent in the available literatures, had been provided.

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