IMPROVEMENT OF ENERGY EFFICIENCY IN THE OPERATION OF A THERMAL REACTOR WITH SUBMERGED COMBUSTION APPARATUS THROUGH THE CYCLIC INPUT OF ENERGY

Under contemporary conditions, 10–70 % of the cost of world product is the expenditures for energy resources. In connection with a total increase in the cost of energy resources in the world, much attention should be paid to the introduction of energy-efficient equipment, as well as technologies for the generation and consumption of energy, including thermal. Given this, of special relevance are the scientific and technical developments, directed toward the creation of modern energy-efficient ecologically clean technologies. Relevant methods include the generation and consumption of thermal energy based on the synergy of hardware-technological design of the process. The submerged combustion apparatuses (SCA) gained good reputation as a specific type of the heat-generating devices that operate based on the principle of combusting the gaseous or atomized liquid fuel directly in the heated medium. The thermal efficiency (performance efficiency, PE) of such apparatuses can be led almost to 100 % relative to the lowest heat of fuel combustion at the appropriate hardware-technological design and deep degree of the utilization of heat of waste gases. Possibilities of the intensification of heat-mass transfer in SCA as the means of improving their operational energy efficiency are far from exhausted. Therefore we find it relevant to develop and study the thermal reactor, equipped with SCA with the cyclic introduction of external energy, to determine its performance characteristics and their impact on the energy effectiveness of reactor as a whole.


Introduction
Under contemporary conditions, 10-70 % of the cost of world product is the expenditures for energy resources.In connection with a total increase in the cost of energy resources in the world, much attention should be paid to the introduction of energy-efficient equipment, as well as technologies for the generation and consumption of energy, including thermal.
Given this, of special relevance are the scientific and technical developments, directed toward the creation of modern energy-efficient ecologically clean technologies.Relevant methods include the generation and consumption of thermal energy based on the synergy of hardware-technological design of the process.
The submerged combustion apparatuses (SCA) gained good reputation as a specific type of the heat-generating devices that operate based on the principle of combusting the gaseous or atomized liquid fuel directly in the heated medium.The thermal efficiency (performance efficiency, PE) of such apparatuses can be led almost to 100 % relative to the lowest heat of fuel combustion at the appropriate hard-ware-technological design and deep degree of the utilization of heat of waste gases.
Possibilities of the intensification of heat-mass transfer in SCA as the means of improving their operational energy efficiency are far from exhausted.Therefore we find it relevant to develop and study the thermal reactor, equipped with SCA with the cyclic introduction of external energy, to determine its performance characteristics and their impact on the energy effectiveness of reactor as a whole.

Literature review and problem statement
Studies of many authors in recent years have addressed different methods for the intensification of heat-mass transfer in different technologies in thermal reactors, equipped with SCA, for the purpose of improving the energy effectiveness of their work [1][2][3].Designs of the evaporators for submerged combustion that operate on the liquefied natural gas are being improved [4].One of the new directions towards increasing the effectiveness of the technology of submerged combustion is the application of apparatuses that work at
There are the more economical methods for increasing the energy effectiveness in the SCA operation.These include the imposition of outside oscillations on the contacting phases [6][7][8], pulsation of turbulent jets of the combustion products that enter the volume of fluid.This contributes to the instability of bubbling and dissipation of the energy of a jet.
A considerable number of articles deal with experimental and theoretical studies of characteristics of the turbulent jets, fed into the volume of fluid [9][10][11].In this case, the methods of computer simulation and experimental measurements were employed to examine effect of the geometric parameters of a nozzle on the instability of a jet stream, the bubbling and dissipation of the jet energy [12,13].
Paper [14] explored influence of design factors on the burner operation with the twisting of streams on the work of an apparatus as a whole.
In the gas-liquid systems, a rapid damping of the imposed oscillations occurs already at the first stage of the contact between phases.This process is more intensive in comparison with the single-phase systems because of the friction between phase flows.Given this, one should recognize that there is no promise to solve the problems on the intensification of heat-mass transfer by creating the pulsations of gas phase at the input to the apparatus.It is much more promising to use for the intensification the frequency-modulation oscillations, which are practically not damped in the disperse systems.The creation of pulsations at each step of the contact is not rational either, since it unavoidably leads to a significant complication of the mass-exchange apparatus and increases energy costs.Taking this into account, the outside oscillations are expedient to impose not on the entire layer (this yields little effect and energetically inappropriate).It is expedient instead in to concentrate the oscillations in the places with the highest efficiency of the contact between phases -in the moment when gas enters the fluid, as well as in the moment of destruction or formation of the gas-liquid layer.
The gas-liquid apparatuses still have a capacity to employ, for the purposes of intensifying the mass exchange process, the low-frequency oscillations, created with the help of external sources of energy (pulsators) [15].In the pulsation reactors, the agitator is rigidly fixed inside a gas-liquid reactor.Pulses arrive at it from an autonomous oscillator -a pulsator.As a working medium in the creation of oscillations they use gas or liquid phases while the valve distributing mechanisms are widely applied as pulsators, as well as hydraulic devices with an electromagnetic actuator.The most frequently used is the pulsation reactor, which is a tank, inside which there is a full shaft with a disc, which makes reciprocating movement from an electromagnetic actuator [16].
One of the most promising trends is the intensification of mass exchange in the capacitive gas-liquid reactors, equipped with the submerged combustion apparatuses, built-in agitators (pulsators).

The aim and tasks of the study
The aim of present study is to improve energy efficiency of the work of thermal reactor with the submerged combustion apparatuses through the intensification of heat-mass transfer with the help of pulsator.
To achieve the set aim, the following tasks had to be solved: -to examine the intensity of turbulent pulsations and their influence on the effectiveness of mass transfer as a whole, to determine performance characteristics of device with the cyclic input of external energy; -to design a device for the cyclic input of external energy based on the step principle; -to fabricate a device for the cyclic input of external energy, to assemble it under conditions of laboratory reactor with the system "gas -fluid"; -based on the obtained research results, to develop a thermal reactor with the submerged combustion apparatuses and the cyclic input of external energy and to conduct its energy-technological tests.

Materials and methods for examining a thermal reactor with the cyclic input of external energy
Article [16] examines a thermal reactor (contact-modular thermal system) with the built-in submerged combustion apparatuses.An increase in its operational energy effectiveness is possible through the intensification of heat-mass transfer, agitating the interphase boundary gas -fluid by the cyclic input of energy into the interphase zone.
A cyclic input of energy is accomplished by changing the step ratio of a screw agitating device (SAD), which revolves at constant angular velocity w (Fig. 1).The device is placed on the lateral side of reactor.Fig. 2 shows a chart of change in the introduced power by the screw agitating device.The effect of increasing the speed of mass transfer at the cyclic input of external energy is achieved due to the additional turbulization of contacting phases gas -fluid.Heat dissipation in this case is described by a criterial equation of the following form [17]: where α is the heat transfer coefficient of blades; D is the diameter of the agitating device; λ is the coefficient of thermal conductivity of the agitated medium; Re, Pr are the Reynolds and Prandtl criteria, respectively, for the process of agitation.
Constant c and powers of A and B depend on the conditions of the process, the type of agitating device, its dimensions and are determined experimentally.Consequently, heat emission in the thermal reactor grows proportionally to the increase in Re.
The Reynolds criterion for the agitating process is determined from relationship where n e =k×n is the effective number of revolutions of the agitating device; k is the experimental coefficient; d is the diameter; ρ is the medium density; h is the dynamic coefficient of viscosity.Thus, at other conditions being equal, with the rise in the effective number of revolutions of the agitating device, the large-scale turbulent eddies are generated, created by the stepped variation in the introduced power.In this case, the energy effectiveness of a thermal reactor as a whole improves.A change in the relative pulse duration (frequency and directions of oscillations) was accomplished with the help of methods, which provide cyclic regimes of interaction between fluid and gas using the microprocessor [17].The microprocessor was assigned with the appropriate period, a pulse from the microprocessor arrived at a diaphragm valve.The diaphragm valve shifted the screw device at a specified step.In this case, the power of the screw agitating device changed and, as a result, the mass transfer.The research conducted on the laboratory reactor with a capacity of 50 l established that the cyclic input of external energy increases the process energy effectiveness in the liquid phase in 2-2.5 times.In this case, the optimal amplitude-frequency characteristics of the process are ensured.

Fig. 2. Chart of change in the input power
Fig. 2 shows that the energy, transferred by pulsations (Fig. 2, selected area), grows with an increase in the introduced power.

Results of examining the intensity of turbulent pulsations
The estimation of an increase in the intensity of turbulent pulsations at cyclic input of external energy was conducted guided by the following considerations.In the apparatuses with SAD, the dependence of the introduced power N on the step ratio t takes the form [18]: where N 1 , N 2 , t 1 , t 2 are the introduced power and step relations, respectively.Thus, at cyclic variation in the step relation t, the power changes that is spent for the agitation and the interphase turbulent transfer of substance.In this case, at the same cyclic recurrence, the hydrodynamic situation in the apparatus changes (hydrodynamic head P, developed by SAD, medium consumption Q and other indicators).
Energy E, introduced into the reactor by the periodic law with period 2ℓ, is determined by the integral of Fourier series: where C n is the speed of device rotation at number of revolutions n, i is the step relation; n is the number of revolutions.
Then, the energy, transferred by pulsations (Fig. 2, selected region), is determined as: With a stepped variation in the step relation, hydrodynamic head P and consumption Q changes: where K P , K Q are the coefficients of head and consumption at different step relations, respectively.The power, transferred by the pulsations in this case, is determined from expression: By using a linear dependence of the coefficients of head on the coefficients of consumption that exists in region 0≤K P ≤1,4 and 0,6≤K Q ≤1 (Fig. 3 we find from equation ( 6) the value of ΔK p at a cyclic variation in the introduced power: A change in the averaged speed of turbulent motion U is determined by formula: ( ) Resulting expression (10) makes it possible to estimate an increase in the intensity of turbulent pulsations in the apparatuses with cyclic input of energy: where U is the averaged speed of turbulent flow over the period t 2 .≥ Fig. 3 shows how with an increase in step relations i the coefficients of head and consumption grow accordingly, and, as a result, the power transferred by pulsations.Fig. 3. Relation between coefficients of head K P and consumption K Q at different step relations i

Discussion of results of examining the obtained results
In the examined submerged combustion apparatuses the energy spectrum of eddies enhances the macro-and micro-agitation, as well as the mass transfer.In the SAD zone, there form both eddies with the large wave numbers that dissipate in the region of action of mechanical device and the large-scale turbulent pulsations with low wave numbers.For the large eddies, degeneration velocity of total kinetic energy of turbulence, expressed through the relative speed of its change, is substantially less than that for the eddies that correspond to large wave numbers.They pass significant distance in the device with SCA until, as a result of energy losses, they dissipate into small eddies.Eddies, which correspond to wave number K d , have a characteristic linear dimension ℓ k , commensurate to the sizes of gas pockets.By actively interacting with the latter, they turbulize the phase contact surface, which contributes to an increase in the rate of transfer.These processes proceed far from the zone of direct action of SAD, which contributes to the more uniform supply of external energy into the zone of action of SCA and its more effective use.
In the installations with built-in SCA and with the cyclic input of energy, the velocity of mass transfer can be additionally increased by creating the counter flows of gas and fluid (Fig. 4).
Shaft 1, hub 5, rigidly connected with it, with blades 4, fixed on it, and rod 2 make a rotational motion.Rod 2 with the help of a drive of translational motion makes a reciprocating movement with the programmed frequency and amplitude.Forward motion of rod 2 with the help of slider 3 and eccentric fingers 6, kinematically connected to it, and blades 4 is converted into continuous rotational motion of blades around own axis.Thus, according to the preset program, the magnitude and direction of speed of the axial flows change, as well as specific power, introduced to the heating installation with SCA.Air is fed through the annular space between shaft 1 and rod 2 inside a hollow hub.Through the radial channels in the necks of blades and drilling in the hub it enters upper or lower collector 7, depending on the angle of blade rotation.In the position of blades, shown in Fig. 4, the air is fed to the lower collector and is dispersed towards the flow of liquid phase (feed water at the input to the system) created by the blades.Countercurrent interaction of the contacting phases is realized at the turning of blades to angle j 1 +j 2 and a change in the direction of axial flow.The air, preheated in the heating installation, which operates with SCA according to the principle of a "vapor pump" [14], enters the upper collector.
In addition to the high productivity and effectiveness of using the external energy, installations with built-in SCA and with the cyclic input of energy make it possible to conduct the controlled processes.They are easily reconfigured depending on the requirements to a technological regime.
In order to perform energy-technological tests of the designed device, the experimental installation with built-in SCA was equipped with a screw agitating device (SAD) (Fig. 5).

a b
Fig. 4. Agitator of the reactor with the cyclic input of energy and counter motion of gas phase: a -general view of device; b -cross-section along axis A-A; 1 -hollow shaft; 2 -rod; 3 -slider; 4 -blades of the screw agitating device; 5 -detachable hub; 6 -eccentric fingers; 7 -gas collectors; 8 -gas-distributing openings The installation operates in the following way: cooled water from the heating system through a branch pipe (6) enters tank (1) with installed SCA (2) and SAD (3).The shaft of SAD with the blades mounted on it is placed on the mark of the phase boundary, formed by the work of SCA (Fig. 5).Partitions (5) separate SCA and SAD.When entering the zone of SAD action, which operates according to the set program, the fluid is turbulized.Large-scale turbulent pulsations, moving in the direction of the action zone of working SCA, dissipate into small eddies, which contributes to the intensification of the heat-mass transfer process as a whole.
Flue gases (II), cooled to 100-120 °С, enter heat exchanger (5), then heat the air that enters the air collector of SAD to 50-60 °С, which is dispersed towards the flow of liquid phase created by the blades.Table 1 gives selected indicators of the testing.
An analysis of results of the performed thermo-technical tests of a thermal reactor with SCA and the cyclic input of energy with the help of a screw agitating device reveals that energy efficiency of the thermal system corresponds to high energy-technological requirements.Performance efficiency of the thermal reactor by direct balance equals 98.6 %.

Conclusions
1. We examined influence of the turbulent pulsations on the effectiveness of mass transfer.It is established that the cyclic input of external energy increases mass transfer by 2-2.5 times, as well as energy efficiency of the process as a whole.
2. A design of the device for the cyclic introduction of pulses of external energy is proposed for the intensification of heat-mass transfer and improvement of energy effectiveness of the work of thermal reactors with the built-in submerged combustion apparatuses.A distinctive feature of the thermal reactor with the cyclic input of energy is in the fact that air, preheated in the

Fig. 1 .
Fig. 1.Screw agitating device with a variable step ratio: 1 -shaft; 2 -screw device; 3 -blades of the external agitating device into account the equation of energy (3), a change in pulse speed U′ is determined from dependence:

Fig. 5 .
Fig. 5. Schematic of the screw agitating device, built into a thermal reactor equipped with submerged combustion apparatus: 1-tank filled with a heat carrier; 2 -submerged combustion apparatus; 3 -screw agitating device; 4 -heat exchanger; 5 -partition; 6 -feed of fluid to the system; 7 -output of the heated heat carrier to the consumer; material flows: I -heat carrier from the circuit; II -heated heat carrier; III -combustion products; IV -heated air

Table 1
Results of the thermo-technical testing of the screw agitating device built into the system