IR temperature measurements in monomode microwave cavity for soot-trap filter regeneration Anna Angela Barbaa, Gennaro Cuccurullo*b, Matteo d’Amorea a Dept. of Chemical and Food Engineering; bDept. of Mechanical Engineering, University of Salerno,Fisciano (SA), Italy ABSTRACT The study of microwaves assisted soot-trap filter regeneration has been performed in a single mode microwave cavity by use of an IR thermographic system. The thermal history of the soot loaded filter during the microwave treatment has been followed on line. A simple one-dimensional transient mathematical model has been proposed, which takes into account the generation term due to the interactions between matter and electromagnetic field. Keywords: IR measurements; microwave; single mode cavity; soot-trap filter.
1. INTRODUCTION Microwaves as a tool for thermal processes are attracting increasing interest from both scientific community and industries for the advantages they offer, such as rapid and selective heating, excellent process control. Applications, even if often on a preliminary basis, can be found in a number of fields1, such as soil remediation2, chemical synthesis3, toxic waste inertization4, materials processing with special reference to polymers5,6 (curing) and ceramics6,7. Actually, there is a querelle in the scientific community about the real extent of the chemical effect microwaves have in a process, being on the contrary their thermal influence widely examined. It could be postulated that selectively heating a material deposited on a support is by itself a sort of chemical effect, since this has a fall-out on the reaction kinetics. Recently, the regeneration via microwaves of a ceramic soot filter has been object of studies, directed to investigate the feasibility of the project, with a special concern to the advantages that microwave would give in such an application8. Actually, ceramic monolith has been generally accepted as the most suitable choice as a soot trap-filter. As the monolith gets progressively blocked, raise in the total backpressure penalises engine’s performance so periodic cleaning of the filter is necessary. Howbeit, self-regeneration is not possible because of the high ignition temperature of diesel soot, typically in the range of 500-650 °C, to be compared to the temperature of the exhaust, generally lower than 400°C9. An additional methodology is needed to purify the filter from the trapped soot. One of the possible methods is thermal incineration by fuel burners or electrical heaters to bring the collected soot up to the ignition temperature, but high temperatures required for the soot combustion and non-uniformity heating of the filter can lead to the filter breakage or melting10. Microwave irradiation has been proposed as an effective mean of particulate trap regeneration, since this can overcome some of the difficulties encountered with other regeneration methods. In particular, instantaneous penetration of microwaves into the filter and their selective absorption make soot burn whereas the ceramic filter remains undisturbed10. Is has to be noted that the electromagnetic field inside microwave cavities interacts with metal probes usually available for temperature measurements, so much so that experimental arrangements which include infrared radiometers become necessary. As a matter of fact, in many industrial processes radiation thermometry is widely used as it allows non-intrusive temperature measurements with a short response time11. In this work, attention has been focused on the feasibility of the temperature resolution in time and space during the regeneration of a ceramic soot trap in a monomode microwave cavity, using an IR thermographic system. Results of temperature measurements obtained are then transferred into a specially developed mathematical model of the regeneration process to clarify the physics of the phenomena.
*
[email protected]; phone +39 089964316; fax +39 089964037; http://www.dimec.unisa.it/dimec/fistec/cuccurullo/index.htm; University of Salerno, via Ponte don Melillo, 84084 Fisciano (SA), Italy.
2. EXPERIMENTAL 2.1 Materials and Equipments Diesel soot. Soot powder deposition on the trap-filter is carried out at the exhaust of a gas-oil burner. A commercially avaible gas-oil is used; the soot generator apparatus is described in details elsewhere 12. Soot-trap filter. A ceramic foam with 92% in porosity is used as a inert trap-filter for diesel soot (DS). The trap-filter has a cylindrical shape (76 mm in diameter and 15 mm in thickness). So is the shape of the microwave oven realized. Insulating material. Wool quartz, with a maximum allowable temperature of 2000°C is used to cohibent the ceramic foam disk. Monomode cavity. A specially designed monomode (or single mode) microwave cylindrical cavity is used as a oven for the regeneration process. Monomode cavities are characterized by having only one excited mode, so that the spatial distribution of the electromagnetic field can be fully predicted. The design of the oven has been developed to obtain a selected propagation, mode namely TM010, in a cavity whose section is circular in shape. From the treatise on single mode cylindrical cavities13, 14 we obtain the fundamental design parameter, i.e. the internal radius (b = 46.8 mm), while the cavity length is imposed (200 mm) by the needs of the laboratory scale of the experiments (manaeable apparatus, easy manipulation of the heating bodies in the cavity etc…). The microwave source is a magnetron operating at 2.45 GHz in frequency and 900 W in power. The oven is built-up in steel for the good electrical conductivity, the chemical inertness toward the reactions to be performed and, finally, the smoothness and cleanability of the machined surface. Leakage and safety controls are performed measuring the levels of the electromagnetic strength inside the laboratory. IR thermographic system. The IR thermography system (INFRAMETRICS SC 1000) which allows temperature sensitivity of 0.07°C and scan rate of 50 Hz. Measurements, image storages and image presentations are totally computer assisted. In the present experimental arrangement the detectable temperature range is -10 to 450 °C. The experimental set-up is schematically sketched in Fig. 1. 2.2 Methodology The cavity is connected with the outside through two different ways. The first one is in correspondence of the microwave source; the second one, facing the generator, is near the IR system. The foam disk was placed in the microwave cavity coaxial with the cavity axis. Being, as above outlined, the electromagnetic field in a single mode cavity independent of the axial coordinate, the filter was placed as much near to the IR system as possible (see Fig. 2).
Fig. 1: Experimental set-up: microwave oven and IR thermographic system.
A squared stainless steel net cover allowing electromagnetic oven insulation has been specially designed, such as to have unaltered microwaves propagation inside the cavity together with optical accessibility to the irradiated filter consistently with the spatial resolution of the IR system. The protective grid illumination produces an external EM field generating electronics disturbances on the IR system. To avoid the addressed problem, a metallic box wraps the IR camera, which looks at the target through a hole 25mm diameter, compatible with the camera optical system.
Starting of temperature recording and microwaves generation are synchronized. Runs are performed in air entering the cavity at room temperature. Reaction can be considered as performed in a differential reactor, since the excess air allowed. Run ends when temperature decays to the initial room value since the soot has been burned out. Temperature records are then analysed to work out temperature profiles evolution during the run.
Fig. 2: (a) Soot trap-filter (radius: b°) and insulating material; (b) single mode cavity with the trap-filter placed in.
3. MODELING For sake of simplicity the proposed model is based on one space coordinate, although variability along the cavity axis of the parameters involved in the regeneration can be expected. The choice of a single mode field distribution allows to postulate that only radial temperature and soot concentration gradients are significant as due to the microwaves effect. The heat transfer at the surface exposed to the IR camera is disregarded. 3.1 Balance equations With reference to both the insulating and the filter spatial domain, the transient one-dimensional energy balance can be written as follows15: 1 ∂ ∂T (t , r ) & ∂T (t , r ) (1) (1 − p) ρc p = (1 − p)k r + Qg (t , r ) + rc (t , r )∆H ∂t ∂r r ∂r the generation term being zero in the former domain. In eq.(1) t and r are the time and the space variable, respectively, while T(t, r) is the unsteady temperature field. Porosity p, density ρ, specific heat cp and conductivity k are the parameters (in the following subscript wq e f are referred to insulating and ceramic filter, respectively). Eq. (1) also accounts for the heat generation Q& g (t , r ) due to microwaves, and for the heat release rc(t, r)∆H from the soot oxidation reaction. The equation must be coupled with the mass balance equation. This latter, if only accounting for reaction kinetics, can be written as: ∂CDS (t , r ) (2) = rc (t , r ) ∂t where CDS(t, r) is the solid reactant concentration, i.e. the mass of the solid particulate distributed in the total volume of the trap Vf. The function rc(t, r) is the kinetics rate of oxidation of the diesel soot. The initial conditions are for uniform temperature and concentration: 0 (3) T (0, r ) = T0 ; C DS (0, r ) = C DS while boundary conditions can be written as: B.C. 1
∀ t > 0, @ r = 0
B.C. 2
∀ t > 0, @ r = b
∂T =0 ∂r ∂T − k wq = h(T )(T − Ta ) ∂r
The coupling conditions between ceramic filter and wool quartz are: @ r = b°
T b°− = T b° +
In the eq. (5) h(T) is the total heat transfer coefficient.
kf
∂T ∂T = k wq ∂r ∂r
(4) (5)
(6)
3.2 Heat generation by microwaves The generation term which takes into account the interactions between electromagnetic field and matter is developed as a volumetric heat generation rate1, 13, 14:
Q& g (t, r ) =
ωε 0ε "(t, r ) 2 E (r ) 2
(7)
where ω is the angular frequency, 2πf, ε0 is the vacuum permittivity, ε” is the loss factor (imaginary part of dielectric constant) and E(r) is the field strength. Note that the loss factor appearing in eq. (7) at the actual space coordinate depends on the soot concentration, having as a minimum value the ceramic foam permittivity8,17. In a TM010 single mode resonant cylindrical cavity, the electric field E is described by eq. (8): x (8) E (r ) = E max J 0 01 r b where, Emax is the maximum field strength, x01 is the first root of the J0, the 0-th Bessel function of first kind, and b is the radius of the cylindrical closed cavity. 3.3 Heat regeneration due to soot oxidation reaction. The contribution of the chemical reaction to the heat generation term in eq (1) can be calculated as follows. Equation (2) is the mass balance which accounts for the soot consumption due to oxidation, and gives the volumetric rate of reaction. The soot oxidation kinetics has been calculated by the expression obtained in previous combustion experiments on the same soot18: (9) rc (t , r ) = K C (T ) PO2 where KC is the kinetic constant and PO2 is the oxygen partial pressure. KC is expressed by the Arrhenius’ law: E (10) K C = K C 0 exp − att RT 0 Having defined C DS = m0 V f as the initial soot volumetric concentration and C DS = m V f as the soot actual concentration, then: ∂C DS (t , r ) E = − K C' 0 exp − att ∂t RT
(11)
' where KCO becomes K CO since it includes the oxygen partial pressure and the initial soot volumetric concentration.2
4. RESULTS AND DISCUSSION
1.0
450
0.9
400
0.8
45 s
350
30 s
0.7
temperature, °C
electrical field strength (E/Emax)
Figure 3 reports the electrical field strength in the built-up single mode cavity as a function of the cavity radius, as worked out by the design equations of the cavity.
0.6 0.5 0.4 0.3
300 250
150
0.2
100
0.1
50
0.0
0 -50
-40
-30
-20
-10
0
10
20
30
40
50
cavity radius, mm
Fig. 3: Electrical field strength as a function of the cavity radius.
15 s
200
ceramic filter 0
10
20
30
radius, mm
insulation 40
50
Fig. 4: Evolution with time of the filter temperature as a function of radius obtained during the heating phase of the regeneration.
In Figure 4 shows the evolution with time of the filter temperature as a function of radius obtained during the heating phase of the regeneration. The heating is in the orders of the tenth of seconds, thus extremely fast when compared to a conventional filter regeneration process, which lasts 10 times as much. The same conclusions can be drawn by the analysis of temperature maps taken during the run at various exposure times to microwave reported in Fig.5. Here the non perfect axial symmetry can be also appreciated: this probably due to non-homogeneity of the soot concentration in the filter volume because of collection operated in a real engine.
(a)
(b)
(c) (d) Fig.5: Temperature maps taken during the run at various exposure times to microwave: (a) 15 s; (b) 30 s; (c) 45 s; (d) 90 s. 450
8
400
15 s 30 s
7
45 s
350
45 s
5 3
250 200
CDS, kg/m
temperature, °C
6
30 s
300
15 s
4
3
150 2
100
1
50
ceramic filter
insulation
ceramic filter
0
insulation
0 0
10
20
30
radius, mm
40
Fig. 6: Model results: temperature profiles.
50
0
10
20
30
radius, mm
40
50
Fig. 7: Model results: residual concentration profiles.
Model results are reported in the Figs. 6-9. Figure 6 shows the temperature profiles evolution during the heating phase worked out by the model equations. A comparison between the experimental results and the corresponding model predictions evidence a similar behaviour for temperature profiles, the discrepancies being probably due to nonhomogeneity of the soot concentration in the filter volume, as before observed. In particular in Figs. 8-9, it can be noted the relevance of the contribution of the soot oxidation rate to the temperature increase above 500°C. This results in a sudden consumption of soot as soon as the temperature overcomes 500°C, i.e. at times above 30 seconds in the model calculations. Figure 7 shows zero soot concentration at 45 seconds in the central zone of the filter , where temperature has first reached the higher values. 2500
8
7
48 s
2000
1500
44 s
5
45 s
3
47 s
CDS, kg/m
temperature, °C
6
46 s 45 s 1000
46 s 4
47 s 3
44 s 2
500
insulation
ceramic filter
1
ceramic filter
48 s
0
0 0
10
20
30
40
50
insulation
0
10
20
30
40
50
radius, mm
radius, mm
Fig. 8: Model results: temperature profiles at larger times.
Fig. 9: Model results: residual concentration profiles at larger times.
Figures 8 and 9 outline the contribution of microwaves to the filter heating. In the figures are reported temperatures and soot concentration profiles in the heating phase at larger times. Where soot concentration has decreased due to reaction, a lesser amount of microwaves energy can be dissipated and the temperature increasing rate slows down. As reaction proceeds, maximum of the temperature profiles moves to larger radius. As a consequence the soot concentration decreases at the corresponding space coordinates. The thermal flux thus reverses and the filter center can be kept at high temperature, until the soot is burned out.
Fig. 10: Partially regeneration of the soot loaded filter.
In Fig. 10 is a snapshot of a during the microwaves treatment at the conditions predicted by the model as reported in Fig. 9, when the regeneration has been partially performed and the soot concentration has decreased to zero in the central area.
5. CONCLUSIONS IR thermography appears to be the only technique capable of time- and space-resolving the temperature evolution of a soot loaded filter during microwaves treatment in a cavity. By the technique, phenomenology of the regeneration process could be clearly observed and followed. The proposed mathematical model quite satisfyingly depicts heat and mass transport phenomena induced by dielectrical heating and may be helpful in designing power control sy stems to a process optimization. Results obtained confirm the suitability of microwaves as a new powerful tool in the industrial engineering. Future developments of the present work require to optimize the heating process inspecting the effect on temperature response due to different cavity geometries and working frequency. Furthermore, a more involved model could be introduced taking into account the axial temperature variation.
AKNOWLEDGMENTS Authors are indebted to Ing. Gregorio Farina for its great contribution in the collection and analysis of the experimental data.
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