Todd Moss and Dr. Brent Webb, Mechanical Engineering
Impinging flame jets are used widely in industry and manufacturing because of their high heat transport characteristics and tight spatial control of the heating. The technique is used in heating or re-heating of stock in the metals and glass industries, and in welding and soldering operations. However, these impinging flame jets are not well understood, and few studies reporting local and average heat transfer characteristics exist in the open literature which might be used by designers (Baukal and Gebhart, 1995a, 199b). The purpose of this research is to experimentally characterize and correlate the local and average heat transfer behavior of impinging diffusion flame jets. As the proposed research will conclude in early 1998, the results of work thus far completed are given below, along with plans for future progress.
The design and construction of the experimental apparatus and the interfacing of the computerbased digital data acquisition to the measurement device have been the focus of current work. The collection of the experimental data, and its analysis and interpretation are to be completed in the near future. The experimental apparatus consists of three primary components: the flame enclosure, instrumented impingement plate, and burner nozzle. These components are now described.
The flame enclosure is designed to minimize the influence of air currents in the room on the impinging flame, support the water-cooled impingement plate, and give visual access to the flame itself. Inlet air is drawn through a number of fine-mesh screens at the bottom of the casing to minimize room air currents, and this air will feed the flame. A vent at the top of the apparatus prevents ambient air from entering the chamber. Plexiglas windows have been placed below the cooling plate to allow observation of the flame structure. The observed flame structure will be correlated with the measurements of local thermal transport to be described.
The impingement plate is cooled by circulating temperature-regulated water in channels machined in a solid aluminum plate. A gasket and cover plate is fastened to the top of the impingement plate to ensure that the cooling water will flow only through the desired channels. Fiberglass insulation will be affixed to the top of the cover plate to decrease the effects of heat loss during data collection. In the center of the impingement plate, a Vatell, Inc. Model HFM-6 high temperature (800(C) heat flux gage is mounted flush with the impingement surface to give near-instantaneous measurements of the local heat transfer rates. Coated (high emissivity) and uncoated (low emissivity) gage measurements will be used to quantify the partitioning between radiative and convective local heat flux.
The burner will be constructed of a stainless steel tube, with the diameter to be chosen so as to optimize the resolution capabilities of the instrumented impingement plate. The burner will be oriented perpendicular to the impingement plate, ensuring concentric radial flow and average heat transfer rates. With the local heat flux gage mounted at a fixed location in the impingement plate, the burner will be moved relative to the gage location permitting measurement of the radial variation in flame heat transfer to the cooled plate.
In constructing the test apparatus and acquiring the data acquisition system, various challenges have been encountered. The impingement plate required precision design to ensure proper heat transfer 154 and structural integrity. After reaching a design conclusion with the mentor professor, it was detennined that BYU’s Research Machine Shop had more than adequate facilities and personnel to construct the final product. Upon consultation with personnel, it was determined that National Instruments could provide the proper equipment to link the heat flux sensors to the computer system. However, upon installation, it was discovered that compatibility problems existed. This presents continuing challenges that are still in the process of being resolved with support from the manufacturer. Further plans to circumvent this aspect (such as a different form of data acquisition) are being considered.
The two photographs shown in Fig. 1, taken from a recent reference exploring the qualitative aspects of the reacting jet (Rigby and Webb, 1995), illustrate the difference in flame structure for two widely differing jet velocities. Lower jet velocities characteristic of laminar flame flow (Fig. la) result in ordered, annular flame rings which surround the stagnation point along the impingement plate, whereas higher velocities yielding a turbulent flame exhibit reaction “bubbles” (Fig. lb). The differences in flame structure and the corresponding local heat transfer along the impingement plate will be investigated in this study. Further, the temporal fluctuation characteristics of the heat transfer in the stagnation and radial flow zones will be studied. This information has not been heretofore available in the open literature.
When completed, the work will provide i) an improved understanding of the transport characteristics of impinging flame jets, and ii) an empirical correlation of the thermal transport characteristics for use by industrial designers. Materials required for the study and the experimental plan have been outlined.
References
- Baukal, C.E. and Gebhart, B., 1995a, “A Review of Flame Impingement Heat Transfer Studies: Part Experimental Conditions,” Combustion Science and Technology, Vol. 104, pp. 339-358.
- Baukal, C.E. and Gebhart, B., 1995b, “A Review of Flame Impingement Heat Transfer Studies: Part Measurements,” Combustion Science and Technology, Vol. 104, pp. 359-386.
- Rigby, J. and Webb, B.W., 1995, “An Experimental Investigation of Diffusion Flame Jet Impingement Heat Transfer,” Thermal Engineering – 1995, Vol. 3, eds. L.S. Fletcher and T. Aihara, ASME, New York, pp. 117-126.
