Heat Transfer and Fluidization Characteristics of a High-Temperature Shallow Fluidized Bed
Toshio Aihara Shigenao Maruyama Institute of Fluid Science, Tohoku University, Sendal, Japan
Keiji Tanaka Jyun'ichi Yamaguchi Graduate School of Engineering, Tohoku University, Sendai, Japan
I
-,Experiments were carried out on the heat transfer and fluidization characteristics of a very shallow fluidized bed at high temperatures. Argon, helium, and air were used as working gases, and zirconia beads and silicon carbide particles were used as solid particles. In the experiment at a medium temperature range (bed temperature T b <_ 680 K), heat transfer characteristics with various particle materials and gases correlated well with the dimensionless variables calculated from the thermophysical properties at the temperature of gas jets from a multislit distributor. These dimension- less data showed good agreement with Andeen and Glicksman's empirical formula. As the operating temperature was increased (T b >_ 680 K), the fluidization deteriorated; then the heat transfer coefficient decreased very rapidly; finally, fluidization stopped completely. In some cases the fluidiza- tion recovered when the gas temperature was decreased. This phenomenon may be attributed to the impurities from thermal insulation material. It should be noted that the impurity content in working gas, even though only a small fraction, is very detrimental to the fluidization and heat transfer characteristics of a very shallow fluidized bed operated at a high tempera- ture.
Keywords: fluidized bed, fluidization, high temperatures
INTRODUCTION
Extensive research has been performed on fluidized bed heat exchangers at operating temperature ranging from a low temperature for an evaporator With frost formation [1] to a high temperature for a heat exchanger of a Stirling engine [2]. Fluidized bed heat exchangers were classified and extensively reviewed by Saxena [3]. Recently, the reduction of fossil fuel consumption and CO 2 production has become an important topic; thus high-performance heat transfer from a tube bundle immersed in a high-tem- perature fluidized bed is desired for fluidized bed combus- tors.
The mechanism of heat transfer between the fluidized bed and an immersed tube was investigated by Kurosaki et" al [4]; however, this mechanism has not been studied sufficiently at high temperatures. Most studies of high- temperature fluidized beds have treated fluidized beds With a comparatively high static bed height. The pressure loss of the conventional fluidized bed is large, and struc- tural and sealing problems may be expected because of the high pressure at the inlet chamber.
One method for reducing the pressure loss is to use lightweight particles [5]. We and our coworkers have de- veloped a low-pressure-loss fluidized bed heat exchanger with an extremely low static bed height and a multislit distributor [6]. We have applied the fluidized bed to air- cooled heat exchangers [7, 8] and to an evaporator for a
heat pump system by which defrosting and heat transfer enhancement were simultaneously achieved [9]. The low- pressure-loss fiuidized bed was also applied to fluidized bed combustors [10].
Many practical applications can be considered if one utilizes the very shallow fluidized bed heat exchanger with a bare tube bundle in a high temperature range. However, large differences in the mechanisms of heat transfer and fluidization are expected between a conventional fluidized bed with large static bed height and the very shallow fluidized bed. In particular, the heat transfer under a high heat flux and in high-temperature operation may be sub- jected to large variations in thermophysical properties of a working gas, and these variations may strongly affect the heat transfer and fluidization characteristics.
In the present study, experiments were carried out on heat transfer and fluidization characteristics of a very shallow fluidized bed under high-temperature conditions. A general correlation for predicting heat transfer charac- teristics in a medium temperature range and problems of a fluidized bed heat exchanger for high-temperature oper- ation are discussed.
VERY SHALLOW FLUIDIZED BED WITH A MULTISLIT DISTRIBUTOR
Before dealing with a high-temperature fluidized bed, let us consider the characteristics of a very shallow fluidized
bed with a multislit distributor. Most conventional flu- idized beds have a comparatively high static bed height, and the fluidization of the particles is due to the bubbling motion of the fluidized bed. The bubbling fluidized bed has excellent heat transfer performance; however, the pressure loss is too high to apply to a heat exchanger using a low-pressure fan. If the conventional high-pres- sure-loss fluidized bed is used at a high temperature, structural and sealing problems can be expected owing to the high pressure at the inlet chamber.
We have tried to improve the heat transfer perfor- mance of bare tubes by immersing them in a fluidized bed and reducing the pressure drop across the fluidized bed as much as possible to minimize the fan power. In order to improve performance, we developed a fluidized bed heat exchanger with a multislit distributor and a tube bank that produced stable fluidization and excellent heat transfer in spite of its extremely shallow static bed.
The experiments were carried out [6] with a fluidized bed heat exchanger composed of a single-row tube bundle, a multislit distributor whose slit is located just below a test tube, and glass beads. The distance between the center of the tubes and the top of a distributor l t was set as 10 mm. The experiments were performed with air at room tem- perature and with a small temperature difference.
Shallow fluidized beds have been studied by many re- searchers, and the characteristics have been discussed in classic textbooks on fluidized beds, such as that of Botter- ill [13]. The static bed height of the present very shallow fluidized bed is only 1.5-3 times as high as the diameter of the heat transfer tube. In such a shallow bed, the bubbling motion that is usually found in a conventional bubbling- type fluidized bed with high static bed height does not occur. The fluidization and heat transfer of the shallow bed are affected by the impinging jets from the multislit distributor.
In Fig. 1, the experimental results of the very shallow fluidized bed [6] are compared with those by Fukusako et al [11], which were obtained using a tube bank in a staggered arrangement with a similar geometry for hori- zontal tube spacing s t, d , a n d dp, but with a high static bed height l 0 = 150 mm. The average heat transfer coef- ficient is defined by the temperature difference between T s and inlet gas temperature Tg i, that is, h i = O / [ Z s (T, - TV)].
It can be seen from Fig. 1 that this very shallow flu- idized bed has excellent heat transfer performance com- parable with that of a conventional bubbling-type fluidized bed with high static bed height, except for s t = 12 mm. The solid curve in Fig. 1 is predicted from Andeen and Glicksman's empirical formula [12] for parameters d t = 10 mm, s t = 18 mm, and d_ = 195/zm.
F .
It should be noted that the experimental correlation of Andeen and Glicksman [12] was derived from their experi- mental data of shallow beds, and the approximate mini- mum static bed height of the beds was estimated as 20 mm. In spite of this, the correlation describes the heat transfer of conventional fluidized beds with a high static bed height [12].
The present very shallow fluidized bed is not a bubbling bed with high static bed height, and the mechanism of heat transfer is different from that of the bubbling bed; nevertheless, the heat transfer of the very shallow bed is similar to that of the bubbling bed, and the characteristics
Very Shal low F lu id ized Bed [6] F u k u s a k o et a l . [ l l ]
It = 10 m m , dt = 8 m m It = 80mm, dt = 1 0 m m
dp = 195 #m, 10 = 13 m m 10 = 1 5 0 m m
st m m st m m dp # m
o 12 + 15 115 z~ 18 Y 2 0 115 • 25.4 × 4 0 220
102 ' ' I , ×,y X--rx .... A
x y n_z~a~ ~ + ~ o
II EY ~ o Andeen & x / Glicksman [19.]
+ o
+
10 i i J i , i , i I J i J i L J i J
i 0 10 2 10 a R e = I td t /u
Figure 1. Comparison of heat transfer characteristics of a very shallow fluidized bed with those of a conventional bub- bling-type fluidized bed with large static bed height at room temperature.
can be roughly explained by the empirical formula. The decrease in heat transfer due to the limited particle mix- ing of the shallow bed is considered to be canceled out by the heat transfer enhancement by the impinging jets from the multislit distributor.
In spite of the very shallow static bed height, channeling did not occur in the very shallow bed, and stable fluidiza- tion could be achieved during the experiment using the multislit distributor. The pressure drop across the distrib- utor is one-sixth to one-fourteenth [6] that for the conven- tional fluidized bed with a static bed height of l 0 = 150 mm. A further decrease in pressure drop has been achieved by improving the distributor [7].
EXPERIMENTAL APPARATUS AND METHOD OF MEASUREMENT
High-Temperature Wind Tunnel
Figure 2 is a schematic diagram of the high-temperature wind tunnel of a closed type that was constructed specially for high-temperature working gases of various kinds. A diaphragm pump 1 was used to feed a compressed gas to catalyst tower 2 and dehumidification tower 3, where oxygen contained in the working gas was removed. The gas was heated by preheater 4 and main heater 5 and was fed into test section 6 through a contraction duct and an inlet chamber. After being cooled by the tube bundle immersed in the fluidized bed, the exhaust gas was cooled to room temperature by a shell-and-tube heat exchanger. The flow rate of the cooled gas was measured by an ultrasonic flow meter, and the gas was returned to the diaphragm pump.
The main heater section was made of tungsten wires and molybdenum walls, and specially designed radiation converters were used to heat the gas at a low velocity. The
284 Toshio Aihara et al
2200
2900
Figure 2. Schematics of a high-temperature wind tunnel. 1, Diaphragm pump; 2, catalyst tower; 3, dehumidification tower; 4, preheater; 5, main heater; 6, test section; 7, shell-and-tube heat exchanger; 8, working gas supply.
radiation converter was composed of a thick layer of a pebble bed of alumina beads (3 mm in diameter). The radiation emitted from the tungsten heater was absorbed by the pebble bed, and the energy was transmitted to the working gas passed through the pebble bed. Double-wall construction was adopted in the high-temperature sec- tions: the inner wall for the high-temperature condition, and the outer wall for the gastight condition. The inner walls of the main heater were made of molybdenum, and the outer walls were made of stainless steel. Ceramic fiber (Kaowool) filled the space between the two walls for high-temperature thermal insulation. The maximum oper- ating gas temperature Tg i at the inlet chamber was 1500 K.
Oxidization of the tungsten heater and molybdenum wall, owing to a very small content of oxygen in argon or helium (even 100 ppm), was critical for the high-tempera- ture operation of the present wind tunnel. Hence, hydro- gen was mixed with the inert gases to 3 vol % to remove the oxygen in the wind tunnel and thermal insulation layers; then water vapor produced by catalytic reaction in catalyst tower 2 was removed by a moisture absorbent in dehumidification tower 3. Air was used for an experiment in a low-temperature range.
Fluidized Bed Test Section
The cross-sectional view of the test section of the very shallow fluidized bed is shown in Fig. 3. The test section was composed of a square duct of 50 mm x 50 mm cross section. Four water-cooled stainless steel tubes 3 whose outside and inside diameters are 6.35 mm and 4.35 mm, respectively, were inserted in the fluidized bed. On the surfaces of heat transfer tubes 3, sheathed thermocouples (Type K) of 0.5 mm outer diameter were embedded.
To obtain stable fluidization with a very small static bed height, a multislit distributor 5 was used. The distributor was composed of two multislit plates with ceramic cloth (Nextel) sandwiched between them to prevent leakage of
particles. Each slit was located just below the heat trans- fer tubes, and the total opening area of the slits was 9% of the frontal area Aft of the duct. The distance between the upper surface of the distributor and the center of the tube bundle was set at 15 mm throughout the .experiment.
Zirconia beads 380 and 770 /~m in diameter (harmonic mean of sieve mesh opening) and silicon carbide particles 230 and 540 t~m in diameter were used as fluidizing particles. The particles were screened by standard sieves. The true density and minimum fluidizing velocity L/m~- of the zirconia and silicon carbide particles are listed in Table 1. The minimum fludizing velocities for air at room temperature were measured for a high static bed height of e d 50 mm. The value of Umf for a very shallow fluidized
cannot be determined accurately, as discussed in previous work [6]. The static bed height of the present
To Diaphragm Pump
f
" ~ F i ~ ' * ' ~ ) r~ To Diaphragm
\ I J
Figure 3. Cross-sectional view of test section. 1, Pressure taps; 2, sight glass; 3, heat transfer tube; 4, suction ther- mometers; 5, multislit distributor; 6, inlet chamber.
High-Temperature Shallow Fluidized Bed 185
Table 1. True Density tap and Minimum Fluidizing Velocity u m f in Air at Room Temperature
Particle dp ( / z m ) pp (kg/m 3) U,n f (m/s)
Zirconia 770 6000 0•65 SiC 540 3230 0.32
The estimated relative uncertainty of h b for most of the experimental runs was of the order of 11%.
RESULTS AND DISCUSSION
Heat Transfer in Medium Temperature Range and Generalized Correlation of Heat Transfer
fluidized bed l 0 was set at 15 mm in most of the experi- mental runs.
The test section has two side windows 2 made of fused silica and a window above the fluidized bed. These win- dows were used for observation and for taking video recordings to analyze fluidization characteristics•
Method of Measurement and Heat Transfer Coefficient
The total heat transfer rate of the tube bundle was determined from the temperature difference AT w of cool- ing water between the inlet and outlet, which was mea- sured by thermocouples (Type T) of 0.1 mm diameter• The error in the temperature measurement was _ 0.2 K. The net rate of heat transfer Q between the fluidized bed and the test tubes is obtained as
O = mwCpw ATw - Oleak (1)
where m w is the mass flow rate of cooling water, mea- sured by an electromagnetic flow meter with an accuracy of 0.5%, and Cpw is the specific heat of the cooling water• The heat leakage Qteak through the side walls of the test section and through the insulation layer was predicted with an empirical correlation determined from a prelimi- nary experiment on the heat leakage,
aleak = 0"31(Tm- ~ ) (2)
In the first stage of the experiment, heat transfer charac- teristics of the very shallow fluidized bed were measured in a medium temperature range (bed temperature T b < 680 K) for various kinds of gases and particles•
The heat transfer coefficient h o versus superficial veloc- ity u b based on the bed temperature T b is plotted in Fig. 4 for air, helium, and argon with various particles• The data were taken with the inlet temperature T_ i kept at 670, 870, and 1020 K, respectively. Under these condmons, the bed temperatures T b were below 680 K. Heat transfer data for each gas and particle material and size correlated relatively well independent of T_ i. However, the data for gases and pamcles tested do not all he on a single curve. As has been found from investigations of a conventional fluidized bed with a high static bed height, the smaller particle size and the higher thermal conductivity of gas result in a larger heat transfer coefficient. The value of h b of the present fluidized bed was as large as 1000 W / ( m z- K) for helium gas.
The measured values of the average heat transfer co- efficient h b are plotted in dimensionless form in Fig. 5, as expressed by
N u b = hbdt/Vb, Re b = Ubdt/A b (4)
where u b, Ab, and u o were evaluated at bed temperature T b. In order to evaluate the thermophysical properties of the mixture of inert gases and hydrogen, correlations [14]
where T m is the average temperature on the side wall and T s is the average surface temperature of the test tubes. The estimated uncertainty of aleak was of the order of 10% when T m < 600 K. The experiments in the medium temperature range were performed mostly within the tem- perature range T m < 600 K. Larger errors may be esti- mated in the higher temperature range due to thermal radiation when T m is much greater than 600 K.
The gas temperatures at the inlet chamber, Tgi, and at the exit of the test section were measured by suction thermometers (4 in Fig. 3) composed of K-type sheathed thermocouples of 0.5 mm diameter. As shown in Ref. 6 on a very shallow fluidized bed, the tube bundle is located in a so-called active bed section, where the gas-phase tem- perature varies rapidly on leaving the distributor and the determination of the bed temperature of the very shallow bed is extremely difficult• Consequently, the gas-phase temperature at the exit of the test section is assumed to be the bed temperature T b for simplicity. The errors in the measurement o f Tg i and T b are of the order of + 2 K and i 1 K, respectively• The heat transfer coefficient based on the bed temperature T b is defined as
h b = Q / A s ( T b - T~) (3)
2 x l O 3 , i i i q I
10 3
%
10 z
2 x l O ~
l o = 1 5 m m
0
i
lO-X
O o
fl) ®
A A + A x ~ - ~ o A
d, tzm
! 383 Zr0z 773
383
S i C 2 2 9
Gas 670! 870,1020 0 A Ar
• A He 470 670
x + Air
l L ;[ l l l
1 5
H b m / s
Figure 4. Heat transfer coefficients for various particles and where A s is the outside surface area of the tube bundle• working gases evaluated at bed temperature T b.
286 Toshio Aihara et al
10 2
II
10 ~
2 10 ~
O0 x A×+ 0
n -F A
0 ~
e 4 ° * °
®
0
dp r~ j K ,um 670 870 102(
383 o Zr0z 773 • •
383 ¢ 470 670
SiC 229 x
G a s
A Ar
z~ He
+ Air
l
10 z 5x10 2 hbd t = 900(1 - e )
Re b = U b d t /~ 'b ~.j Figure 5. Dimensionless heat transfer characteristics evalu- ated by the thermophysical values at bed temperature T b.
based on the semiempirical formulas of Wike were adopted for predicting the thermophysical properties of gases• The notations are the same as those in Fig. 4. The expressed data shown in Fig. 5 were scattered over a wide range because of differences in gases and particle sizes. There is less correlation between the data for various gases and particles. The values of Nu b for the same Re b differ by two- to three-fold for each case.
In previous reports [6-8] on very shallow fluidized bed heat exchangers, we and our coworkers proposed the use of the inlet gas temperature, that is, gas temperature at the exit of the slits, for estimation of the heat transfer coefficient• In the present experimental apparatus, the difference between the working gas temperature and the inner-wall temperature of the test section was substantial, and the heated gas was subjected to a relatively large temperature decrease when the gas passed through the multislit distributor, and a considerable difference existed between the gas temperature at the inlet chamber Tg i and that at the jet mouth of the multislit distributor•
Hence, the gas jet temperature Tag at the multislit distributor was evaluated from the exit gas temperature and the heat transfer rate from the gas to the tube bundle. Since T b was assumed to be the exit gas temperature, Tgj can be calculated from the equation
Tgj = T b - O/upgCpg (5)
where p and c__ are the density and specific heat of the g . t ~
gas, respectively. It should be noted that if there is no heat transfer from the wall of the inlet chamber and the multislit distributor, Tgj is equal to Tg i, as in the previous reports [6-8].
Heat transfer characteristics for various particle materi- als and gases are correlated with the Nusselt number and Reynolds number defined as follows, whose thermophysi- cal properties are evaluated at the temperature Tgj of gas jets from the multislit distributor:
Nuj = hjdt /Aj , hj = Q/As (Tg j - T~) (6)
R% = mgdt /Afr tzj (7)
where m is the mass flow rate of the working gas and A g . . . . J and /x, are the thermal conductivity and viscosity, respec-
• J • twely, of the gas at T Since the value of T was
• . . g J .
calculated from Q and ~b, the estimated uncertainties of Nuj and R% were of the order of 18% and 7%, respec- tively•
The data indicated in Fig. 5 are replotted in Fig. 6 in terms of these dimensionless variables• By introducing these dimensionless variables, the widely scattered data in Fig. 5 can be correlated very well with a single curve regardless of the particle size, material, and operating temperature for various kinds of gases•
The experimental data in Fig. 6 are compared with the empirical prediction of Andeen and Glicksman [12], which is expressed as follows:
• " 2 - "10.326 )0.3 [t mgdtPp )[ It) 1[ [~l,jCpgj L \ A , r p g , ~ , [ d , pi,UY-~-2g lJ 1 X-----~ (8)
where E is the void fraction of the fluidized bed and was estimated from Babu et al.'s correlation [15]
= 1 - (1 - Cnp( 14•341(u - Umf )x°738~l°°6ap p p 0 . 3 7 6 1
+ - ~
J Urn/ Pg
(9)
in which Umr is the minimum fluidizing velocity estimated by Wen and Yu's empirical formula [16] and emf is the void fraction at Umr. For estimating h b and e from Eqs. (8) and (9), the bed temperature T b is generally used. It should be mentioned that the thermophysical properties at Tgj were used for evaluating h b and Umf in the present
10 z
-<
- ,....~
10 ~
' ' ' ' ' ' 1
1 o =15ram ~ . , ~ ×
r.s K Gas 1 6vo 87o lo2c 383 0 A Ar
@ ZrOz 773 • 383 ¢ z~ He
470 670 229 x + A i r
2 . . . . . . . . . I . . . . 101 10 2 5x 10 2
Re i =rag d~/fA f r/zj)
Figure 6. Dimensionless heat transfer characteristics evalu- ated at Tgj and comparison with predicted value.
High-Temperature Shallow Fluidized Bed 287
prediction. The h b given by Eq. (8) is transformed to hj by the correlation
hb Ashb - - = 1 + (10) hj A f r Uj pgj Cpgj
The estimated values by Eq. (10) are plotted in Fig. 6 by solid lines; these lines correlate reasonably well with each other regardless of the gas, particle size, and particle material. As shown in Eq. (8), the prediction is a function of particle size; however, the effect of particle size is relatively small in the present explanation using the value at Tgj for the case of Fig. 6.
A~rnost all the data fall below the predicted values; however, relatively good agreement is observed with the scattered data in Fig. 5, which is based on bed tempera- ture T o. The results in Fig. 6 show that the heat transfer of the present very shallow fluidized bed heat exchanger in the medium temperature range can be estimated from the empirical formula of Andeen and Glicksman if the dimensionless parameters are evaluated at the tempera- ture of gas jets from the multislit distributor.
The above empirical correlation of heat transfer was derived from experimental data of shallow fluidized beds, and the correlation was in good accord with experimental data of conventional fluidized beds with high static bed height. The empirical correlation of the void fraction for a bubbling-type fluidized bed with high static bed height was used in estimating the void fraction of the present very shallow beds. In spite of this, the empirical correlation describes the heat transfer in the present bed without bubbling if the values are estimated at T : The same
• . . g l "
tendency was seen m Fig. 1, which can be explained as follows. The decrease in heat transfer due to the dilute phase of the shallow bed was canceled out by the heat transfer enhancement by the impinging jets from slits of the distributor located just below the heat transfer tubes. The present shallow bed is strongly affected by single- phase convective heat transfer of the impinging jets as discussed in previous work [7]. This is one of the reasons the experimental data correlate well using values at Tgj.
If one compares the experimental data and estimated values in Fig. 6, one observes a large difference between them. However, the empirical estimation differs by two- to three-fold from experimental data if one uses T b as the characteristic temperature. For the fluidized bed with a large temperature difference between the fluidized bed and heat transfer tubes, the choice of characteristic tem- perature for superficial velocity and thermophysical prop- erties is very important, and Tgj is suitable for the present system. The use of T b may cause a large difference between estimated and experimental heat transfer coef- ficients for the present fluidized beds. A more correct empirical correlation can be derived after the characteris- tic temperature is chosen, and the derivation of this correlation is a future objective.
Radiative heat transfer plays an important role in the high-temperature fluidized bed, and a broad review has been presented by Saxena [3]. Prior to the present experi- ment, the effect of radiative heat transfer was estimated, and we found that the contribution of radiative heat transfer was estimated to be less than 10% of the total heat transfer if T b < 700 K because convective heat trans-
fer of the fluidized bed is large compared with that of single-phase gas flow. Consequently, the effect of radiative heat transfer was not considered in the fluidized beds in the medium temperature range.
Radiative heat transfer becomes important at high tem- peratures, and larger errors may be estimated in the high temperature range owing to radiation. However, some peculiar phenomena that cannot be explained by the effect of radiation were observed. These phenomena are described as follows.
Heat Transfer and Fluidization at High Temperature
In the second stage of the experiment, fluidization and heat transfer characteristics were studied in a high tem- perature range (T b > 680 K) using argon gas with 3 vol % hydrogen as a working gas. The maximum operating gas temperature at the inlet chamber was 1500 K, but neither oxidation nor corrosion was observed in the high-tempera- ture section.
When the operating temperature is increased under a constant mass flow rate mg, the fluidization deteriorates in spite of an apparent increase in the volumetric superfi- cial velocity, owing to the thermal expansion of gas; conse- quently, channeling occurs. Figure 7 compares the heat transfer coefficients of the medium temperature range (T,i = 870 K) and those of the high temperature range
. o - - 3 with a constant mass flow rate of m_ = 1.5 × 10 k~/s. The bed temperature T b for the latter data is indicated in parentheses. The heat transfer coefficient decreases very rapidly when T b becomes higher than 680 K. Finally, fluidization stops completely, and the heat transfer behav- ior becomes similar to that of a fixed bed. The phe- nomenon was confirmed by the observation of fluidization by means of a video tape recorder.
The effect of bed temperature T b on the heat transfer coefficient is shown in Fig. 8, where h b was normalized by
10 3 i i i ) ) E i i i - -
ZrO 2
~ dp =383ffm (Tb=593.9K)
10=15mm / 1682.8)
/ o I (813 4 )
/ " "J;v v 2j /o Ar+3~_iz
102 o Tgi= 870K • m g = l . 5 x l 0 - 3 k g / s
5xlO i . . . . . . . . j 10 -1 1 3
L/b [II/S
Figure 7. Comparison of heat transfer characteristics be- tween medium and high temperature ranges.
288 Toshio Aihara et al
i 2 0 [ i i ZrOa t
¢ , . o o ! - -o o i
"~0.80 L- % \ Ar+3%Hz
0 . 6 0 o i n c r e a s i n g Tb ~'~o t d e c r e a s i n g T b [ ]
Figure 8. Effect of bed temperature on heat transfer coeffi- cient.
an average value of the data below T b = 700 K. These data were taken keeping the superficial velocity u i con- stant at 1.5 m/s . At medium or low temperatures, h b is almost constant. However, with increasing bed tempera- ture, fluidization deteriorates, the heat transfer coefficient also decreases, and channeling starts; finally, the fluidized bed becomes a fixed bed at T b = 870 K. In some cases, as shown by the square symbols in Fig. 8, the fluidization recovers when the gas temperature is decreased, and the heat transfer characteristics show hysteresis.
The hysteresis of heat transfer for various types of particles is shown in Fig. 9. The data were taken keeping mass flow rate m~ constant. Consequently, superficial gas velocity increases with increasing T b. T h e zlrconla beads of 380 /zm show an abrupt deterioration of heat transfer characteristics. As shown in Fig. 9, the channeling starts at T b between 700 and 900 K; it should be noted that the initiation temperature of heat transfer deterioration de- pends on the static bed height.
We made detailed observations of fluidization by means of video recordings. The onset of deterioration of flu- idization and its recovery are abrupt phenomena, and the decrease in gas density does not show a direct correlation with the onset of deterioration.
Figure 10 shows the heat transfer characteristics of the fluidized bed with high static bed height (l 0 = 50 mm). The data were taken keeping Tg i at 870 K and 1020 K.
4 0 0
: 2
300
z:~ 2 0 0
i 0 0
' . . . . ' . . . . J m e = l . 8 x 1 0 -3 k g / s ~
° ~ - - ~ x & l ° = l S m m t
...~ -q~-~ i3- \ '~ - ~ r, - - [5 : -~ , .'.?A 1
o Z r 0 z d p = 3 8 3 / z m z~ Z r 0 2 d p = 7 7 3 / z m D SiC d p = 5 4 3 / z m
0 . . . . J . . . . l . . . . 5 0 0 6 0 0 7 0 0
I , , , , I , , , ,
8 0 0 9 0 0 1 0 0 0
T b K
Figure 9. Effect of particle size and material on hysteresis of heat transfer.
Figure 10. Hysteresis of heat transfer for fluidized bed with large static bed height.
The bed temperature T~ was lower than 680 K, at which channeling and deterioration of heat transfer just occur in the present shallow fluidized bed; however, in the flu- idized bed with high static bed height, the hysteresis of heat transfer was observed at T b < 680 K. It is considered that mixing of the particles is not sufficient for a fluidized bed with high static bed height and that the deterioration of heat transfer starts earlier than in the present shallow fluidized bed with a multislit distributor.
Yamazaki et al [17] reported that the fluidizing particles may have adhesive force during high-temperature opera- tion, and this force affects the minimum fluidizing veloc- ity. However, the experimental evidence shows that the deterioration of fluidization and heat transfer occurred with much larger particles compared with the fluidized bed reported by Yamazaki et al [17] and that the deterio- ration started suddenly.
Problems in Practical Application of Very Shallow Fluidized Bed Heat Exchangers During High-Temperature Operation
The above-mentioned deterioration of fluidization and its recovery depend on operating temperature, particle size, and the duration of high-temperature operation of each run. Qualitative estimations of the phenomena and the contribution of each parameter have not yet been clari- fied.
To investigate the reason for the curious phenomena mentioned in the previous section, the surfaces of the test particles were examined, and their composition was ana- lyzed with a scanning electron microscope (SEM). The photographs of the surface of the zirconia beads are
shown in Fig. 11. Also, the surface composition of the beads was measured by SEM. Figures 12a and 12b show the typical results of composition analysis of the surface.
A zirconia bead that has not yet undergone high-tem- perature operation has a smooth surface, and no chemical
High-Temperature Shallow Fluidized Bed 289
composition other than zirconia is observed on its surface, as shown in Figs. 11a and 12a. After they are subjected to operation at a high temperature, observation of the zirco- nia particles (Figs. 11b and 11c) shows the deposition of some materials on the particle surfaces, and the surface condition is clearly different from that existing before high-temperature operation (Fig. 11a). In Fig. 12b the peaks show a large fraction of elements other than zirco- nium on the surface. The data represent some deposition material whose compositions are of silicon, sodium, potas-
~":-RR"/ L i v e : 100 --z F're_-':.et.: l r l r l s R e m a i n i n g : I ] s R e a l : 1 2izI s 1 7".; D,=-a,i
b
Figure II. (a) SEM photograph of a zirconia particle (dp = 380/zm) before high-temperature operation. (b) SEM photo- graph of a zirconia particle (dp = 3 8 0 / . tm) that has experi- enced high-temperature operation. (c) Close-up photograph of material deposition in (b).
A:! F i t ~J, I'i K K F F f,I l.I
F"'._--;= 8t:-':: 5 . il 2f l ~.: ~4.. 1 N. 1 ::: ,-_" h 512= 1 rl 7 ,-- t..-
I'1EI'11 Hr l . 3
a
X-RA'T' L i v e : 11".11"1E. F ' r e ~ e t . : 1 0 0 ~ F.:ernai n i r ig : O~ R e a l : 1 2 2 $ 1 :::'.'- Dead.
r,.
< -. 1 FS= 4F::: HEM1 : HO. 5
5 . O::-rl I-..: el...z 1 0 . 1 > ,-. h 51 2 = 7'9 c t. s
b
Figure 12. Surface composition of a zirconia particle (a) before and (b) after high-temperature operation.
290 Toshio Aihara et al
sium, and aluminum at some fractions. These formed the glass material that caused channeling at high tempera- tures. It should be noted that the softening point of soda glass agrees with the temperature at which channeling occurs. These impurities seem to originate from the ce- ramic fiber (Kaowool) thermal insulation material, which contains small fractions of sodium and potassium, because the seal of the inner wall of the wind tunnel was not tight enough.
From the above discussion, the mechanism of the hys- teresis of heat transfer characteristics and fluidization demonstrated in Figs. 6-8 can be explained as follows. When the bed temperature reaches the softening point of the deposit material, the coefficient of restitution of the particle decreases, so fluidization starts to deteriorate. When the bed temperature is reduced, the deposited material becomes brittle, and the particles separate from each other owing to the thermal stress between adhered particles and the fluid force from the working gas; then the fluidization recovers in some cases.
PRACTICAL SIGNIFICANCE
In order to estimate the heat transfer coefficient of a very shallow fluidized bed that is operated at a bed tempera- ture of less than 680 K, the empirical formula of Andeen and Glicksman can be used regardless of the gas, particle size and material, and gas temperature if the dimension- less parameters are evaluated at the temperature of the gas jets from the multislit distributor instead of the bed temperature.
For high-temperature operation of fluidized bed heat exchangers, oxidization of the tungsten heater and molyb- denum wall owing to a very small content of oxygen in argon or helium (even 100 ppm) was critical. Hence, mixing a small fraction of hydrogen in the inert gas was very effective in removing the oxygen from the wind tunnel and thermal insulation layers• The water vapor produced by catalytic reaction was removed by using a moisture absorbent• The present wind tunnel showed nei- ther oxidization nor corrosion after high-temperature op- eration (1500 K) at the inlet chamber•
However, it should be added that the Na or K impurity content of the working gas (which may come from the insulation layer), even though only a small fraction, has a highly detrimental effect on the fluidization and heat transfer characteristics of the fluidized bed operated at a high temperature. The control of these impurities is very important for the practical operation of high-temperature fluidized bed heat exchangers.
The above-mentioned deterioration of fluidization is a particular phenomenon that was observed with the pres- ent experimental setup• The deterioration is not a general phenomenon of fundamental fluid mechanics or heat transfer. However, the present detrimental effect on the fluidization can be found in a high-temperature fluidized bed in which the chemical reaction rate is very high and in which small amounts of impurities of gas and solid parti- cles, such as alkali and ash, can cause changes in the surface properties of particles• Consequently, the predic- tion of defluidization in high-temperature fluidized beds and its suppression are important on a practical basis for utilizing a high-temperature fluidized bed or a fluidized bed combustor.
As has been categorized by Geldart [18], a fluidized bed with small particles (category C in Ref. 18) can defluidize because of cohesive forces between particles. The particle size of the present fluidized bed was much larger than that of category C, and the present deterioration of fluidization cannot be explained by pure fluid dynamics or heat trans- fer in which change in the characteristics of the particles is not considered• It is important to remark that a practi- cal fluidized bed operated at high temperature, such as in the present case, can be subject to sudden defluidization under highly reactive circumstances.
CONCLUSIONS
Experiments were carried out on the heat transfer and fluidization characteristics of a very shallow fluidized bed at high temperatures. For this purpose, a specially de- signed high-temperature wind tunnel of a closed type was constructed. The maximum operating gas temperature was 1500 K at the inlet chamber. The test fluidized bed was composed of four water-cooled tubes and a multislit dis- tributor. Argon, helium, and air were used as the working gases, and zirconia beads and silicon carbide particles were used as solid particles• The results obtained are as follows•
1. As the operating temperature is increased (T b > 680 K), the fluidization deteriorates and channeling occurs. Then the heat transfer coefficient decreases very rapidly, and finally, fluidization stops completely. In some cases, the fluidization recovers when the gas temperature is decreased, and these heat transfer char- acteristics show hysteresis.
2. The above-mentioned decrease in heat transfer and deterioration of fluidization are mainly due to the deposition of impurities from the thermal insulation material• The present results demonstrate an example where the impurity in the working gas, even though only a small fraction, can have a highly detrimental effect on the fluidization and heat transfer characteris- tics of a high-temperature fluidized bed heat exchanger or combustor.
3. The heat transfer data for each gas and particle mate- rial correlated well with a single curve, independently of gas temperature for the case of bed temperatures T b < 680 K. However, not all the data for gases and particles lie on a single curve if the heat transfer coefficient hb and the superficial velocity and thermo- physical properties are evaluated at T b.
4. Heat transfer characteristics for various particle mate- rials and gases correlated well with the dimensionless variables that are calculated with the thermophysical properties at the temperature of gas jets from the multislit distributor. When all thermophysical proper- ties are evaluated at Z . , these dimensionless data show
• g
good agreement with ,~ndeen and Glicksman's empiri- cal formula, which was derived from the data on their shallow beds.
We wish to express our gratitude to Mr. M. Hongoh and Mr. T. Shimoyama, Institute of Fluid Science, Tohoku University; Mr. T. Itoh, Ishikawajima-Harima Heavy Industries Co., Ltd.; and Mr. S. Yuda, Toyota Motor Co., Ltd., for their assistance in the experi- ments. We also wish to express our thanks to Dr. T. Nakada, Dr. S.
High-Temperature Shallow Fluidized Bed 291
Enya, and Mr. K. Oohori of Ishikawajima-Harima Heavy Industries Co. Ltd., for their assistance in constructing the high-temperature wind tunnel. The research was carried out with a Grant-in-Aid for Scientific Research on Priority Areas 003-D01 of the Ministry of Education, Science and Culture of Japan.
NOMENCLATURE
A ~ frontal a rea o f test section, m 2 A s total outside surface area of test tubes, m 2 Cp specific heat at constant pressure, J / ( k g . K) dp harmonic mean d iameter of solid particles, /zm d t diameter of test tube, m g gravitational acceleration, m / s 2
h b average heat transfer coefficient based on Tb, Eq. (3), W / ( m 2. K)
hj average heat transfer coefficient based on Tgj, Eq. (5), W / ( m 2. K)
l 0 static bed height of solid particles, m I t distance between center of tubes and top of dis-
tr ibutor, m mg mass flow rate of working gas, k g / s rn w mass flow rate of cooling water, k g / s
Nuj average Nusselt number, Eq. (5), dimensionless Q net rate of heat transfer from gas to tube bundle,
Eq. (1), W Q*eak heat leakage through side walls and insulation
layer, W Rej Reynolds number, Eq. (6), dimensionless
st horizontal tube spacing, m T b bed temperature , K Tg i gas tempera ture at inlet chamber, K Tgj t empera ture of gas je t from multislit distributor, W T s average surface tempera ture of test tube, K
T m average tempera ture of side walls of test section, K ATw tempera ture difference of cooling water between
inlet and outlet of tube bundle, W u superficial gas velocity, m / s
Greek Symbols • void fraction
thermal conductivity, W / ( m . K) tz viscosity, P a - s v kinematic viscosity, m 2 / s p density, k g / m 3
Subscripts b value evaluated at T b g gas phase i at inlet chamber or value evaluated at Tg i j at gas je t or value evaluated at Tgj
p particle phase mf minimum fluidizing velocity
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Received October 3, 1991; revised November 2, 1992
