Patent Application
20210372611
Effective design of a combustion burner requires control of flame properties, most notably the flame length. Precise knowledge of flame length is particularly important since many burners are installed in combustion chambers that afford no visual access to verify the flame properties during operation. While experimentation and engineering guidelines are helpful for predicting flame length based on burner and combustion chamber design and operational parameters, it is known that prediction and control of flame length is more reliable for turbulent flames than for laminar flames. While many factors can influence the transition point from a laminar to a turbulent flame, a non-dimensional Reynolds Number, Rej, based on the properties of the flow and geometric properties of the burner's central jet is conventionally employed. This is defined as follows in equation 1:
Rej=ρVjDj/μ (1)
where ρ is the central jet density at the nozzle exit plane; V is the central jet average nozzle exit velocity, D is the nozzle diameter and μ is the central jet dynamic viscosity, again determined by conditions at the nozzle exit plane.
As pointed out in a seminal paper by Hawthorne et al (“Mixing and Combustion in Turbulent Gas Jets”, Third Symposium on Combustion, Flame and Explosion Phenomena, pp. 266-288, 1949.), the characteristically different response of flame length to gas flow rate in laminar and turbulent flows is rooted in the inherent difference between molecular and eddy diffusivity. Mixing in a laminar flame is governed by molecular diffusivity, a material property of the gases being mixed, while mixing in turbulent flames is driven by turbulent “eddies.” Unlike molecular diffusivity, so-called eddy diffusivity is not a material property, but scales with the product of flame jet velocity and nozzle exit plane diameter. The resultant effect on flame length associated with these disparate modes of mixing is summarized in
It is further evident from
Finally, in most practical combustion applications the furnace internal environment is generally not quiescent. Long, laminar flames tend to be relatively difficult to control in these situations since much of the fuel stream remains unreacted while its momentum has dissipated. By contrast, the turbulent jet flame mixes rapidly and the stability of the flame jet thereby benefits from increased velocity of the expanding hot gases. Consequently, laminar flames are much more prone to deflection by furnace gas currents and incomplete combustion than turbulent flames, and these deficiencies often lead to performance and operational shortfalls related to reduced and/or mis-directed flame-to-load heat transfer and reduction in process fuel efficiency.
A burner is described herein comprising at least two bluff bodies in the burner gas supply conduit at different axial positions upstream of the exit plane. The use of multiple bluff bodies facilitates the diffusion of turbulence by initiation of eddies at multiple locations in the flow field, while the longitudinal spacing promotes amplification of turbulent effects emanating from each upstream bluff body via flow field interactions.
Each bluff body may be introduced from a different circumferential position around the inner surface of the gas supply conduit. This orientation generates eddy vorticity having distinct axes (for vorticity is a vector quantity) which subsequently results in more efficient distribution of turbulence throughout the flow field.
Aspect 1: A burner gas supply apparatus for increasing flame turbulence, the apparatus comprising a conduit having a characteristic width, W, defined by an inner surface having a circumferential direction and an axial direction, the axial direction terminating in a nozzle defining a nozzle exit plane having a characteristic dimension, d, where d<=W; and a first bluff body having characteristic dimension, Dbb-1, projecting a length, L1, into the conduit from the inner surface, wherein 0.5<=L1/W<=1, the first bluff body being spaced apart from the nozzle exit plane by an axial distance of X1, wherein X1/Dbb-1<=30; and a second bluff body having a characteristic dimension, Dbb-2, projecting a length, L2, into the conduit from the inner surface, wherein 0.5<=L2/W<=1, the second bluff body being further from the nozzle exit plane than the first bluff body and spaced apart from the first bluff body by an axial distance of X2, wherein X2/Dbb-2<=30; a third bluff body having a characteristic dimension, Dbb-3, projecting a length, L3, into the conduit from the inner surface, wherein 0.5<=L3/W<=1, the third bluff body being further from the nozzle exit plane than the second bluff body and spaced apart from the second bluff body by an axial distance of X3, wherein X3/Dbb-3<=30; wherein the first bluff body and the second bluff body are separated in the circumferential direction by a first spacing angle; and wherein the second bluff body and the third bluff body are separated in the circumferential direction by a second spacing angle.
Aspect 2: A burner gas supply apparatus according to Aspect 1, wherein 0.5*(1+d/W)<=L1/W<=1; wherein 0.5*(1+d/W)<=L2/W<=1; and wherein 0.5*(1+d/W)<=L3/W<=1.
Aspect 3: A burner gas supply apparatus according to Aspect 1 or Aspect 2, wherein 0.8<=L1/W<=1; wherein 0.8<=L2/W<=1; and wherein 0.8<=L3/W<=1.
Aspect 4: A burner gas supply apparatus according to any of Aspects 1 to 3, wherein 0.6<=d/W<=0.7.
Aspect 5: A burner gas supply apparatus according to any of Aspects 1 to 4, wherein X1/Dbb-1<=5; wherein X2/Dbb-2<=5; and wherein X3/Dbb-3<=5.
Aspect 6: A burner gas supply apparatus according to any of Aspects 1 to 5, wherein the first bluff body and the conduit are sized so that a first bluff body Reynolds number, Rebb-1=ρVinletDbb-1/μ, is equal to or greater than 50; and wherein the nozzle characteristic dimension, d, is sized so that a jet Reynolds number, Rej=ρVjd/μ, is equal to or greater than the first bluff body Reynolds number, Rebb-1.
Aspect 7: A burner gas supply apparatus according to any of Aspects 1 to 6, wherein the first bluff body has a circular cross-section.
Aspect 8: A burner gas supply apparatus according to any of Aspects 1 to 7, wherein the first spacing angle and the second spacing angle are each greater than 60 degrees and less than 180 degrees.
Aspect 9: A burner gas supply apparatus according to any of Aspects 1 to 8, wherein the first spacing angle and the second spacing angle are each from 110 degrees to 130 degrees.
Aspect 10: A burner gas supply apparatus according to any of Aspects 1 to 9, wherein the first spacing angle and the second spacing angle are each from 80 degrees to 100 degrees.
Aspect 11: A burner gas supply apparatus according to any of Aspects 1 to 10, further comprising a fourth bluff body having a characteristic dimension, Dbb-4, projecting a length, L4, into the conduit from the inner surface, wherein 0.5<=L4/W<=1, the fourth bluff body being further from the nozzle exit plane than the third bluff body and spaced apart from the third bluff body by an axial distance of X4, wherein X4/Dbb-4<=30; wherein the third bluff body and the fourth bluff body are separated in the circumferential direction by a third spacing angle.
Aspect 12: A burner gas supply apparatus according to Aspect 11, wherein the third spacing angle is from 80 degrees to 100 degrees.
Aspect 13: A burner comprising a burner gas supply apparatus as in any of Aspects 1 to 12; and a reactant conduit surrounding the burner gas supply apparatus; wherein the burner gas supply apparatus is configured and arranged to supply one of a fuel and an oxidant; and wherein the reactant conduit is configured and arranged to supply the other of a fuel and an oxidant.
The present invention will hereinafter be described in conjunction with the appended figures wherein like numerals denote like elements:
Forehearths are employed in many glass manufacturing facilities to provide slow but controlled cooling of the molten glass prior to forming in the cold end. Precise control of cooling rates, which occurs due to a balance between heat loss through the forehearth walls and heat input via burners, is critical to preventing formation of glass defects. Since a single forehearth may employ several hundred burners firing across narrow channels (typically 2-3 ft wide), post-installation inspection of burner flames is not practical. Moreover, oxy-fuel burner firing rates are relatively low and flame jet Reynolds numbers are often in a range where low momentum laminar flames prevail. Further, the influence of flame disturbance due to cross-flow of combustion gases within the forehearth channel can be significant. The deleterious effect of a multitude of laminar flames in such an environment is illustrated in
Due to the very low firing rate requirement in a forehearth, oxy-fuel burners in a forehearth frequently produce jet-like flames that are laminar or transitional (in the process of changing from laminar to turbulent). An undesirable aspect of laminar and transitional flames is that their flame length and stability changes very significantly with increasing or decreasing firing rate. This is prone to cause problems in forehearth operation that may go undetected as it is very difficult to observe flames from outside the forehearth.
Therefore, as described herein, burner features have been identified to generate transition to a turbulent flame at lower Reynolds numbers than would naturally occur in a typical burner, while also avoid any unnecessary increase in pressure drop. This design results in the ability to operate oxy-fuel burners in a forehearth with a turbulent flame that provides better control and reliability.
Further, the rapid mixing associated with the fully turbulent regime significantly reduces soot formation and flame temperature stratification, which assists in precise temperature control within the forehearth. Other features may include variability in the central nozzle axial position within the forehearth burner block and the ability for on-the-fly removal and replacement of the entire central nozzle assembly.
In the embodiments described herein, the central nozzle flows a fuel gas and is surrounded by an annular oxidant flow that typically comprises oxygen-enriched air or oxygen. Flowing fuel gas through the central nozzle typically reduces the amount of unburned fuel. However, the same enhanced mixing benefits can be achieved in a burner wherein the central nozzle flows oxidant and is surrounded by an annular fuel gas flow.
The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the invention. Various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention, as set forth in the appended claims.
Various turbulence generators are used in combustion and related arts. Oftentimes these devices enhance mixing via the creation of an array of high velocity jets via N small nozzles in a burner tip as depicted in
ΔPT,actual/ΔPT,baseline=(A/Ad)2 (2)
Rej,actual/Rej,baseline=(A/Ad)0.5 (3)
These relationships are plotted in
To be of practical industrial value, a forehearth oxy-fuel burner should be capable of generating a turbulent flame at low Reynolds number while minimizing the loss of total pressure. The inventors have determined that an effective device for enabling this low Reynolds number laminar to turbulent flame transition is a truncated bluff body within the flow conduit of the center jet upstream of a nozzle exit plane. As used herein, the term “truncated” means that one end of the bluff body emanates from a flow boundary while the other end is terminated within the flow field. The term “upstream” means a location that is further from the nozzle exit plane, as opposed to a “downstream” location closer to the nozzle exit plane.
In an alternative embodiment, the bluff bodies may be non-truncated, i.e. attached at both ends to the inner surface of the nozzle. Note, by comparison, that a non-truncated bluff body possesses the horseshoe vortices at each end of the cylinder, but does not possess either the tip or trailing vortex. Moreover, the arch vortex is replaced by dual vortex tubes whose axes are aligned with that of the cylinder axis. While the flow structures of the non-truncated cylinder do not possess the same degree of three-dimensionality as the truncated cylinder, the inventors have found that the non-truncated cylinder can still be used advantageously in a burner for the application described herein.
An embodiment of an oxy-fuel burner described herein is based on a strategic arrangement of multiple bluff bodies to amplify the effects of the vorticity formed at each bluff body and, in so doing, catalyzes the low-Reynolds Number laminar-to-turbulent transition of the burner flame. Accordingly, the present burner comprises at least two bluff bodies in the central conduit at different axial positions upstream of the exit plane as illustrated in
There are five important features of the inventive burner that can be used separately, and preferably in combination with one another:
First, the Reynolds number of the gas flow past each bluff body should be greater than the minimum Reynolds number for formation of a Karman vortex street in its wake, as this produces a flow unsteadiness from which turbulence develops. For the burner described herein, a minimum bluff body Reynolds number (Rebb-i) of 50 was used. For Rebb-i and Dbb-i, i refers to the numerical index for each bluff body as can be seen in
Rebb-i=ρVinletDbb-i/μ (4)
where the velocity, Vinlet, and length scale, Dbb-i, pertain to the average free-stream gas velocity approaching the bluff body and dimension of the i-th bluff body in a plane that is perpendicular to the principal direction of flow. In the case of a non-circular bluff body, the length scale, Dbb-i, represents the breadth of the i-th bluff body in the plane of the flow as illustrated in
Second, the Reynolds number of the gas flow past each bluff body should also be less than the Reynolds number of the gas jet flow exiting the nozzle, as calculated in equation 1 above where Dj is defined as d in
Accordingly, combining the first and second features yields the following expression for the bluff body Reynolds number in equation 5:
50≤Rebb-i≤Rej (5)
Third, each bluff body should traverse a length, Li, between 0.5 and 1.0 times the characteristic width, W, of the gas flow plane as shown in
0.5≤Li/W≤1.0 (6)
Note that in the edge case of Li/W=1.0, the i-th bluff body would completely cross the central conduit and be non-truncated.
Fourth, the streamwise bluff body separation, defined as the axial distance between two neighboring or adjacent bluff bodies divided by the upstream bluff body length scale, Xi/Dbb-i (see
Fifth, the normalized streamwise separation between the farthest downstream bluff body and the nozzle exit plane, X1/Dbb-1 (see
In one embodiment of an oxy-fuel burner for use in forehearth applications, each bluff body is introduced from a different circumferential position around the boundary of the central conduit as illustrated in
Preferably, four bluff bodies with circumferential spacing angles between adjacent bluff bodies of about 90 degrees are employed, where about is defined as ±10 degrees (i.e., the spacing between each adjacent pair of bluff bodies can be from 80 degrees to 100 degrees). In addition, more preferably, the streamwise bluff body separation is less than or equal to 5 and the bluff bodies extend across the diameter of the exit nozzle, d, as in
0.5×(1+d/W)≤Li/W≤1.0 (7)
Equations 4, 5, 6, and 7 are presented for the case in which each bluff body may have different dimensions and spacing. In the case where it is advantageous for all bluff bodies to have the same dimensions and/or spacing, the terms Dbb, L, X, and Rebb may replace Dbb-i, Li, Xi, and Rebb,i for simplicity.
The intention is to amplify the interaction between the wake flow fields of adjacent bluff bodies, with the expectation of a resultant increase in turbulence generation.
An experimental comparison was made of several embodiments of the oxy-fuel burner including design principles as set forth herein against four other configurations: a baseline nozzle having no upstream turbulence generating devices (
Oxy-fuel combustion tests were conducted wherein fuel was introduced through the central nozzle designs listed in Table 1 inserted within an oxygen plenum as shown in
First, the laminar-to-turbulent “transitional” Reynolds number, ReTR, of the central jet. Determination of ReTR was made by the initial appearance of distortion and temporal movement of the flame interface between the fuel and oxygen streams. This type of flame front appearance, sometimes referred to as a turbulent brush or flamelets, are depicted schematically in
Second, the total fuel pressure loss normalized by the kinetic energy head loss at the nozzle exit plane; i.e. ΔPtotal/½ρVexit2.
Third, the length of the flame as a function of the central nozzle flow rate. Flame length measurements reported herein were made by image analysis of flame photographs. The basis of the flame length determination was identification of the interface between the largely unreacted orange portion of the central fuel jet produced by soot formation and the adjacent light blue color associated with non-equilibrium, high-temperature emission of C2, the so-called Swan band at the highly reactive portion of the flame tip.
It was expected that the 2-BB nozzle would have had a greater reduction in transitional Reynolds number, more similar to 3-BB than 1-BB. The inventors speculate, without being bound by theory, that the 3-BB and 4-BB configurations may be more effective because the turbulent effects are amplified to a greater extent when the spacing angle between adjacent bluff bodies are about 120 degrees (i.e., from 110 degrees to 130 degrees) in the 3-BB case or about 90 degrees (i.e., from 80 degrees to 100 degrees) in the 4-BB case. In the 2-BB configuration one or more of the vortices generated by the upstream bluff body may have been better aligned with the downstream bluff body due to the 180 degree spacing angle, leading to less amplification. For this reason a 2-BB nozzle may be more effective at reducing the transitional Reynolds number if the circumferential spacing angle between the two adjacent bluff bodies is more than about 0 degrees and less than about 180 degrees, or more than about 60 degrees and less than about 180 degrees.
Moreover, of the inventive nozzles tested, the one employing the longer bluff bodies, 4-BB-L, demonstrated the greatest reduction in transitional Reynolds number for a given pressure loss. Further to this point,
Although all experiments took place with fuel in the nozzle and oxygen through the annulus, it is evident that similar beneficial results could have been achieved by flowing oxygen through the nozzle and fuel through the annulus. Ensuring that the gas flowing through the nozzle exit plane is turbulent is sufficient to promote mixing and achieve a turbulent flame.
While the principles of the invention have been described above in connection with preferred embodiments, it is to be clearly understood that this description is made only by way of example and not as a limitation of the scope of the invention.
