СНТ
Stream-Niche Technology of Fuel Combustion
RUUAEN
102, Kyrylivska Str., Kyiv, Ukraine, 04080 +38 (044) 462-52-62
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  1. СНТ
  2. Stream-Niche Technology
Stream-Niche Technology

As is known, the main element of fire engineering equipment is a burner device (BD), whose working process determines the efficiency, reliability and environmental safety of the facility as a whole. The complex of aero-thermochemical processes, which underlies the work of BDs, is the most complex from a technical point of view, as well as insufficiently studied and cannot be accurately calculated. However, the needs of production are pushing for the search for appropriate approaches that would already allow the creation of BDs of the required efficiency. To do this, it is necessary to slightly change the attitude towards combustion technology as a set of physical and chemical processes that ensure the effective conversion of fuel chemical energy into combustion products of the required quality.

The term “combustion technology” is rarely used. And one is not likely to hear about the working process of BDs, the aerodynamic scheme, and stabilization of combustion, even in purely scientific circles. The aerodynamic flow structure (as studies show, it is the most important characteristic of BDs) has completely disappear off the radar of researchers of the BD working process.

An unclear perception of combustion basic principles and disregard for scientific approaches to solving this problem did not pass without a trace. Despite the fact that in modern world practice there are many hundreds of types of BDs of various companies (including Simmens, Wayshaupt, Riello, Girsh,...), unfortunately, we have to state the fact that there is no BD yet fully satisfying all modern requirements in terms of efficiency, environmental safety and reliability.

As a rule, an improvement in efficiency indicators is achieved due to deterioration in environmental performance, a decrease in the level of reliability and a narrowing operating adjustment range, etc.

In order to develop approaches to fuel combustion technology creation, first of all, it is necessary to clearly define current requirements for BDs:

1. Easy and reliable ignition at the lowest possible gas consumption (for “backfire-free” ignition of the boiler and smooth transition of the fire engineering facility (FEF) from “cold” to “hot” state, or drying the FEF);

2. Stable (backfire-free) combustion in a wide range of velocities of fuel and oxidizer (to prevent the loss of flame in case of sharp fluctuations in gas and air pressure);

3. The required adjustment range for power (Кр) and excess air ratio (L) (to ensure optimal drying modes of the lining and the thermal state of the FEF elements; the required quality of combustion products and their temperature level; the adjustment of the FEF power without disconnecting part of the BD);

4. The maximum possible completeness of fuel combustion (nг) in the FEF furnace volume;

5. Permissible level of emission of toxic substances (NOx, CO, SO2, etc.) in the entire range of loads;

6. The ability to adjust the length and luminosity of the flame, as well as its aerodynamic and concentration structure (to ensure the required intensity and uniformity of heat flow spreading; to reduce the likelihood of flame contact with FEF elements, as well as the formation of an oxidizing or reducing environment in combustion products);

7. The minimum possible resistance along the passages of the fuel and oxidizer (to ensure the possibility of operation at low gas and air pressures and to reduce power consumption for the drive of draft machines);

8. Reliability and ease of operating modes adjustment (to simplify automatics and ensure safety);

9. Possibility of reliable operation on buoyancy and in fanless mode at partial loads due to the discharging created by a smoke exhauster or a pipe, which is important in emergency shutdowns of draft means, and also allows to significantly save electricity;

10. Consistency of performance indicators during operation;

11. Low noise level;

12. Modularity, allowing the BDs to gain the required power from autonomous modules;

13. Technological effectiveness, ease of manufacture, low metal consumption and no need for expensive materials.

It should also be noted that the range of these requirements is constantly expanding, and the norms (environmental ones in particular) are becoming stricter. Currently, not a single BD, including the best examples of foreign firms, meets these requirements from a comprehensive perspective.

Long-term studies of the main components of the BD working process (aerodynamics of the flow of fuel, oxidizer and combustion products; chemical reaction of fuel and oxidizer; heat transfer processes) carried out in KPI (Kyiv Polytechnic Institute) combustion laboratory revealed the decisive role of aerodynamic processes, which made it possible to classify the types of BDs according to several gas-dynamic schemes of fuel and oxidizer supply (Fig. 1).

The analysis of BD aerodynamic structure with various aerodynamic schemes showed that the main reasons for their lack of efficiency when operating in variable modes are:

  • destruction of circulation zones of highly heated combustion products, providing aerodynamic stabilization of combustion;
  • violation of the uniformity of the fuel in the oxidizer flow distribution;

the exit of concentration of the fuel mixture in recirculation mixing zones (RMZ) beyond the ignition limits;

So, in order to create an efficient BD, it is necessary to provide a stable aerodynamic structure of the flow (the required fields of velocities and artificial turbulence; a system of stable vortices; the required depth of penetration of fuel streams into the oxidizer flow, etc.) of fuel, oxidizer and combustion products in a wide range of velocities with the required concentration field of the fuel mixture.

Analytical and experimental studies [3, 4, 5] showed that the BD regulating modern fuel combustion technology should provide:

  • rational initial distribution of fuel in the oxidizer flow;
  • high level of turbulence intensity in the area of fuel mixture formation;
  • stable controlled aerodynamic structure of the flow of fuel, oxidizer and combustion products with recirculation mixing zones in the flame stabilization area;
  • self-regulation of the fuel mixture in the recirculation mixing zone.

Fig. 2 shows the basic principles of creating an efficient combustion technology and methods of their implementation.

Fig.2 Basic principles of creating fuel combustion technologies.

Currently, the BD developers are trying to ensure a rational distribution of fuel in the oxidizer flow, turbulization of the fuel mixture and the creation of recirculation mixing zones in the flame stabilization area, but if they succeed, then this will only concern very narrow ranges of changes in operating factors. As a result, the full completeness of the requirements for the FEF will not be provided.

The new technology of fuel combustion is based on a gas-dynamic scheme, which provides for a transverse flow of fuel and the oxidizer crossflow in front of the vortex-creating device in the form of niches (stream-niche system) (Fig. 3).
The operating and geometric parameters of the study of such a gas-dynamic scheme varied in the following ranges

Fig. 3 Gas-dynamic scheme with fuel supply by a single-row stream system in front of a niche vortex-creating device.

A feature of the niche vortex-creating device is the generation of high-frequency pulsations of velocity, leading to the intensification of mixture formation (Fig. 4).

.

Fig. 4 Generation of velocity pulsations in the niche cavity.

The stream-niche system has a stable vortex structure with a variable volume of a stable circulation zone and a constant composition of the fuel mixture in the area of flame stabilization. Fig. 5 shows the formation of a stable vortex RMZ located below the boundary of zero velocities Wo = 0. With an increase in the rate of gas outflow from the openings, the size of the RMZ increases in direct proportion, providing the necessary intensity of mixture formation and stabilization of combustion.

 

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Fig. 5 Formation of stream-niche system circulation zones.

The way to implement an effective combustion technology is the proper location of the stream-niche system on an autonomous collector pylon (Fig. 6). Such a burner module closes all stages of the working process on itself: the distribution of fuel in the oxidizer flow, mixture formation to the required concentration level, ignition of the fuel mixture, flame stabilization and the formation of concentration, velocity and temperature fields of combustion products; the module can be actively self-cooled by an oxidizer and a fuel and, due to the self-regulation of the fuel mixture composition, does not require complex control automatics. Studies of the working process of such an autonomous module have shown the indisputable advantages of this scheme, i.e., a significant reduction in the flame length due to the two-way supply of the oxidizer.

Fig. 6 Stream-niche module.

 

Fig. 7 A grid made of stream-niche modules.

Kf = t/B = 0.15...0.5   (1)

Where Kf is a blockage coefficient
В = 10 mm t = 80 mm Н = mm

L=L/H=4    (2)

Studies of various modifications of grids made of stream-niche pylons (Fig. 7) showed a high burnup rate at the lowest possible flow blockage coefficients (Fig. 8), which is a prerequisite for low aerodynamic resistance of such systems and justifies the possibility of creating BDs based on them. Fig. 8 and 9 show the results of the study of natural gas burnout behind the pylon and behind the grid. It can be seen that the length of the flame behind the pylon system (ηг = 95%) is much shorter than behind a single pylon.

Fig. 8 Burnout of natural gas in a stream-niche system

where Lф is the flame length

Fig. 9 Burnout of natural gas behind the grid made of stream-niche modules.

t — step of the module’s location

Improvement of the burner stream-niche module with niche systems and an end niche, the correlation between operating and design parameters affecting the combustion process are shown in Fig. 10.

Fig. 10 Operating and design characteristics in the study of the stream-niche module.

The correlation between operating (a) and design (b) parameters and their influence on the characteristics of the module working process:

a)

i → quality of mixture formation

i1 → quality of mixture formation

i2 → quality of mixture formation

Н → В

L → i

L/H → combustion stabilization

d → Gr; ignition

B → tr; Kf; stabilization

С → mixture formation

S → Gr

d → Gr

Š → mixture formation; stabilization

βo → Prmin; ignition

Kf → Pbmin; aerodynamic resistance

b)

Wb → α

Wr → Gr 

q →

hc → Kf

iзу → flame stabilization

tr → ηг

tb → ηг

tст → reliability

α → ηг

Experimental and analytical studies of these relationships, as well as anomalous phenomena of aerodynamics and mixture formation in the stream-niche system made it possible to create a physical model of stable combustion in the stream-niche module (Fig. 11), which reflects the correlation between the flow structure and mixture formation in the vortex zones of the stream-niche pylon. With a narrowing of the gas flow rate (Gr q) over a wide range, vortex formation remains stable, and only when passing through the critical values of q, their structure and volume change. In this case, the concentration of the fuel mixture in the vortices is within the ignition range.

Moreover, only the implementation of the full complex of geometric factors in the module allows you to get the mode of work on the stream-niche technology. Even the slightest deviation of any of the dimensions resets the combustion mode from kinetic to diffusion or micro diffusion, which undermines the entire combustion technology.

Fig. 11 Physical model of stable combustion in a stream-niche module.

where Gг — fuel consumption through the system of openings,
q=ρгWгвWв — hydrodynamic parameter,
αЗОТ — coefficient of excess air in vortex zones,
Wг; Wв — velocity of fuel and oxide,
Wоб — velocity of the oxide flowing around the gas stream.

Studies of natural gas combustion showed that the flame behind each pylon has a stable aerodynamic structure (photo 12) and is self-similar in velocity, which makes it possible to collect and form burner devices of any capacity from them. Measurements of specific heat release in such flames at α → 1 showed that the combustion mechanism in them approaches the kinetic one, but at the same time provides wide control limits.

Fig. 12 Photo of combustion behind a system of two stream-niche modules.

The recirculation mixing zones, which are responsible for stabilizing combustion, are hundreds of times smaller in volume than ones in the most common BDs such as Weishaupt, RIELLO, Oilon! They are highly stable in a wide range of changes in the velocities of the fuel and oxidizer due to the constancy of the optimal fuel mixture composition in the recirculation mixing zone (Fig. 13).

Fig. 13 Limits of stable combustion in a stream-niche system B = 25 mm; kf = 0.3; d = 4 mm; L = 40 mm 

Fig. 14 Self-regulation of the fuel mixture in the recirculation mixing zone of the niche vortex generator.

Such properties of the module make it possible to compose BDs from them for almost any fire engineering facility.