R. H. Page

Heat transfer and combustion characteristics of an array of radial jet reattachment flames

Abstract Radial Jet Reattachment Combustion (RJRC) nozzle provided improved fuel/air mixing for use in impingement flame heating. Detailed heat transfer and combustion measurements are conducted to characterize the performance of the RJRC nozzles in an array configuration. The measurements indicated that the RJRC flames produce low NO x and CO emissions. However, their characteristics are highly dependent upon the between-nozzle spacing. At a small spacing, because of the high interaction among the RJRC nozzles, the sub-atmospheric region above the center nozzle vanishes, resulting in a large elliptic flame. In a moderately interactive spacing, high surface heat fluxes are achieved, while at a large spacing, the RJRC flames operate almost independently from each other.

Infrared imagery of an air/CO2 axisymmetric jet

This experiment uses an infrared imaging system to investigate a subsonic, non-isoenergetic, air/CO2 axisymmetric jet. The classical limitations of using IR imagery with hot gases are presented and a novel approach to overcome these limitations is proposed. The results suggest that radial and axial irradiant profiles measured with the IR imager, when non-dimensionalized, collapse onto curves of similarity. This behavior could allow temperature, velocity, and concentration profiles to be deduced from the IR image.

Heat Transfer Characteristics of a Slot Jet Reattachment Nozzle

A two-dimensional reattachment nozzle called the Slot Jet Reattachment (SJR) nozzle was designed and built with a zero degree exit angle. The heat transfer characteristics of this submerged nozzle were investigated by varying the Reynolds number, nozzle exit opening, and nozzle to surface spacing. The pressure distribution on the impingement surface for different Reynolds numbers and exit openings were measured. Correlations for location of the maximum local Nusselt number and local Nusselt number distribution along the minor axis of the SJR nozzle were determined. A nondimensional scheme for generalized representation of heat transfer data for two-dimensional separated/reattaching flows was developed. The local and average heat transfer characteristics along the minor axis of the SJR nozzle were compared to a conventional slot jet nozzle under identical flow power condition. The comparison showed that the peak local heat transfer coefficient for the SJR nozzle was 9 percent higher than that for a standard slot jet nozzle, while its average heat transfer coefficient was lower or at best comparable to the slot jet nozzle based on the same averaged area. The net force exerted per unit width by the SJR nozzle flow was 13 times lower than the slot jet nozzle flow under this criterion. Additional experiments were conducted to compare the SJR and slot jet nozzles under matching local peak pressures exerted by the jet flow on the impingement surface. The results indicated 52 percent increase in the peak local heat transfer coefficient, and a maximum enhancement of 35 percent in average heat transfer coefficient for the SJR nozzle over the slot jet nozzle based on the same averaged area under this criterion.

Comparison of Heat Transfer Characteristics of Radial Jet Reattachment Nozzle to In-Line Impinging Jet Nozzle

The heat transfer characteristics of three submerged radial jet reattachment (RJR) nozzles with exit angles of +45{degree}, 0{degree}, and {minus}10{degree} are compared to the heat transfer characteristics of a conventional submerged in-line jet (ILJ) nozzle. The comparisons are made under identical air flow power and at each nozzle`s favorable spacing from the impingement surface. The local and area averaged Nusselt numbers are presented. The results indicate that significant enhancements in local and area averaged Nusselt numbers can be achieved with the RJR nozzle over the conventional ILJ nozzle while being able to control the net exerted force on the impingement surface. Also a comparison is made between the ILJ and RJR nozzles on the basis of the same peak pressure exerted on the impingement surface. This comparison indicates that the RJR nozzle heat transfer characteristics are superior to the ILJ nozzle.

Heat transfer from a pair of radial jet reattachment flames

Flame jet impingement heat transfer for a pair of Radial Jet Reattachment Combustion (RJRC) nozzles has been studied for flames which were highly, moderately, and weakly interactive. The most uniform heat flux and temperature distributions occurred at the closest between-nozzle spacing, when the flames were highly interacting, while the highest heat flux and surface temperatures were measured when the two flame jets were moderately interacting at intermediate between-nozzle spacings. The optimal spacing for two nozzles was determined based on maximum heat flux and surface temperature. In addition, the percent overall heat transfer to the impingement surface decreased with increasing between-nozzle spacing. The results of this study provide valuable information for applying RJRC nozzles to industrial flame jet impingement heat-treatment processes.

Heat Transfer Characteristics of a Radial Jet Reattachment Flame

A Radial Jet Reattachment Combustion (RJRC) nozzle forces primary combustion air to exit radially from the combustion nozzle and to mix with gaseous fuel in a highly turbulent recirculation region generated between the combustion nozzle and impingement surface. High convective heat transfer properties and improved fuel/ air mixing characterize this external mixing combustor for use in impingement flame heating processes. To understand the heat transfer characteristics of this new innovative practical RJRC nozzle, statistical design and analysis of experiments was utilized. A regression model was developed which allowed for determination of the total heat transfer to the impingement surface as well as the NOx emission index over a wide variety of operating conditions. In addition, spatially resolved flame temperatures and impingement surface temperature and heat flux profiles enabled determination of the extent of the combustion process with regards to the impingement surface. Specifically, the relative sizes of the reaction envelope, high temperature reaction zone, and low temperature recirculation zone were all determined. At the impingement surface in the reattachment zone very high local heat flux values were measured. This study provides the first detailed local heat transfer characteristics for the RJRC nozzle.

Infrared images of jet impingement

Infrared images of jet impingement are used to evaluate the effectiveness of heat transfer due to an air jet from a nozzle striking a surface. A unique Heat Transfer Jet Impingement Facility (HJIF) has been designed and constructed for testing nozzles. The nozzles to be tested are mounted in the facility. The performance of the nozzles at various flow and operating conditions is evaluated from infrared images and computer codes. The experimental apparatus and the operational procedures for the utilization of the infrared images are described. This unique application of infrared imaging as a research and development tool for determining air nozzle performance at steady state conditions has been found to be generally superior to earlier approaches.

Measurement of impinging jet heat transfer utilizing infrared techniques

A heat transfer jet impingement facility (HJIF) was designed and developed in order to determine the overall surface heat transfer pattern due to the impingement of a turbulent jet. Turbulent jets of air in a standard air atmosphere were investigated for their surface impingement characteristics with various nozzle configurations. The HJIF was used to compare the heat transfer performance of the different impingement nozzles using infrared measurements of the impingement surface temperature. Rapid and accurate measurements utilizing this technique were possible.

COMBINED FLUID MECHANICS AND HEAT TRANSFER MEASUREMENTS IN NORMALLY IMPINGING SLOT JET FLOWS

ABSTRACT Flow field, surface pressure and heat transfer measurements of a submerged turbulent slot jet impingement flow at nozzle-to-surface spacings of 1 and 7 slot widths from the impingement surface are presented. Fluid mechanical data include laser Doppler anemometry (LDA) measurements of mean flow field, Reynolds averaged normal and shear stresses, and mean surface pressure and rms surface pressure fluctuation measurements along the nozzle minor axis. Detailed local heat transfer coefficients are calculated from surface temperature measurements performed using infrared (IR) thermography. The impingement region is seen to have a fairly high level of turbulence at the large nozzle-to-surface spacing. The peaks of mean and rms-averaged fluctuating surface pressure, and local heat transfer occur in the impingement region. At the close nozzle-to-surface spacing, the primary peak in heat transfer, which occurs in the impingement region, is followed by a region of local minimum, and a secondary peak, that occur at around 3.5 and 7.5 slot widths from the jet centerline, respectively. There is a good correlation between the locations of the secondary peak in heat transfer and peak near-wall streamwise turbulence. The peak mean surface pressure occurs in the impingement region; however, the rms-averaged surface pressure fluctuation profile exhibits a peak at the location of minimum heat transfer.

Infrared technique: heat transfer measurement

Impinging jets are used for heat and mass transfer in some industries. Previous impinging jet studies used thermocouples or liquid crystals to determine convection coefficients. These methods have inaccuracies that can be avoided with an infrared image temperature measurement. For this research, an infrared camera with a resolution of 0.025 degree(s)C was used to store images containing 52785 temperatures. The camera was checked for accuracy with a thermometer calibrated with a NIST standard. A facility for measuring surface heat transfer coefficients due to an impinging jet was developed. The infrared images obtained with the facility were used to identify impingement flow characteristics and to calculate local convection coefficients for in-line and radial jets. The results include photographs and convection coefficient graphs.