1. Introduction

The recent increase in the cost of energy, has added incentive to the development of efficient techniques for power generation by the use of available fossil fuels in Jordan. One of the most important fuels in Jordan is oil shale. An extensive work is going on to develop fluidized bed in which the combustion of solid fuels, coal and oil shale occurs because the fluidized bed combustion technology has many attractive features such as: relatively effective emission control, fuel flexibility, excellent solid- gas mixing, and temperature homogeneity[1-4]. However, further NOx and SOx reduction is still needed in order to have a more stringent emission limit.

For proper design of fluidized bed reactors, it is sufficient to understand the mechanism of heat transfer and the effect of operating parameters on the heat transfer process.

Several models have been proposed to explain the aforementioned heat transfer behavior, as was summed up by[5, 6]. Although there are still doubts about the procedure, many authors approximate the overall heat transfer coefficient for particle convention, gas convention and thermal radiation. A simple and reliable prediction of heat transfer analysis between fluidized bed and immersed surfaces. The correlations have been shown to give reasonable conditions. One of these empirical correlations was given by Al-Busoul[7]:

(1)

Another empirical correlation was given by Ganza which represented the conductive and convective components of particle Nuselt, Nu as[8]:

(2)

(3)

(4)

Where the constant C is determined experimentally.

There is large number of variables affecting the heat transfer rate, which means that most of proposed correlations are of limited use. Several measurements have been reported for the heat transfer to or from a tube immersed horizontally in a fluidized bed[9-16].

Consistent with the results of this study would be variation in the heat transfer with respect to the orientation of a flat plate immersed in a fluidized bed. Fillippovski and Baskakov[16] have measured this variation for a large heated plate, which would correspond in size to the exterior wall of their fluidized bed.

The Kelloge et al[17] have used a small heated plate to investigate the effect of orientation on heat transfer coefficient. However, the fluid velocities used in the fluidized bed were relatively low and only one particle size was used.

This study helps in gaining a better understanding of the heat transfer in a fluidized bed from angled surface, a series of tests were carried out for a small-immersed plate surface. These experiments were conducted using a wide range of fluid velocities, different particle sizes and for different heater radial positions.

2. Experimental Apparatus and Procedure

The experimental apparatus used in this study has been described previously[18]. It’s formed from a Plexiglas column 142mm I.D. and 1m in height. The specifications of the air supply system, instrumentation flow meters, pressure drop measurement and power supply and thermocouples have been described in a detail in a previous study[18].

For the heat transfer measurement an electrical plate heater shown in fig 1 is used. The heater is formed from a copper plate of (12 cm²) in area. The minco electrical resistance used is (50Ω). This resistance is housed in a Teflon cover, which in the same time fasten the copper plate and limits the heat losses. A thermocouple was used to measure the surface temperature and another thermocouple was used to measure the bed temperature.

The heating power was constant at (6 w). For different particle size oil shale were used as the fluidized bed. The properties of the oil shale particles are shown in table 1. The value of U_mf found in the table was determined experimentally by using the following relation Δp = f (u).

Figure 1. Electrical heater

Table 1. Materials Used and Their Physical Properties Material Particle fraction (um) Mean diameter (um) Density kg/m3 Umf m/s Oil shale A 50-350 132 1500 0.035 Oil shale B 75-425 234 1500 0.063 Oil shale C 90-600 357 1500 0.092 Oil shale D 500-1250 856 1500 0.165

The height of fixed bed was constant in all the experiments at (20cm). The height of the center of the heater was also constant at (12cm) from distributor. The heater position was changed in the radial direction to three positions or (r/R = 0.0, 0.3, 0.7. the gas velocity was changed from 0.035 m/s at ambient temperature was about 20C. The concentration of solids in fluidized bed was calculated using the equation of Andersson[3]:

(5)

Where the concentration of particles in static bed is related to the density ratio as:

(6)

The convective heat transfer coefficient can be determined using the following equation:

(7)

3. Results and Discussion

Figures 2, 3, 4 and 5 show the variation of heat transfer coefficient with the angle of inclination of the heater surface from the direction of fluid flow, for various values of u/u_mf and r/R = 0. Each figure represents experiment results for certain particle size.

Figure 2. The effect of heater angle on h at r/R=0

Figure 3. The effect of heater angle on h

Figure 4. The effect of heater angle on h at r/R=0

Figure 5. The effect of heater angle on h at r/R=0

It could be seen in these figures that for u/umf=1, h values increase as the angle increase from 0 to 60 where reaches a maximum value called h_max, then h values start to decrease to reach h_min at = 180. This behavior could be attributed to the fact that at in the range 0 to 60 the particle movement is more intensive and the heater has a large effective surface area for heat transfer. On the other hand, as increases beyond 60. The effective surface area for heat transfer decrease and the intensity of particles motion near the heater decreases, too. This will increase the contact time between the particles and the heater surface and as a result h decreases at =180, h has a minimum value, because the particles are stationary at the heater surface and are not affected by the gas flow. The heat transfer mode in this case is only by conduction.

For at 120 and 150 the values of h are relatively higher compared with its value at = 180 because of this angles particles slippage occurs at the inclined heater surface. It is evident that the value of h for the same value of increases with increasing the rate u/u_mf until reaches 4. However, the angle where h reaches a maximum increases as u/u_mf increases until it reaches 90 at u/u_mf = 4. These results are in agreement with[16] but they are not in an agreement with[18] because the maximum rate of heat transfer coefficient at angle was about 120^o when the u_mf = 1 or 2.

As u/u_mf increases, the flow reign in the bed changes from homogenous fluidization of the bed to the bubbling regime. In addition, as u/u_mf increases in the range of 1-4 the angle corresponding to u/u_mf changes from 60 to 90 because the heater surface area projected to the bubbles decreases as changes from 60 to 90. Furthermore, the extent of h increase is higher for > 90 than those values corresponding to > 90.

Comparing the behavior in figures 3, 4 and 5. It could be seen that as the particle diameter increases the values of u/u_mf correspond to u/u_mf decrease. These results are in agreement with those of[19].

As u/u_mf value increases beyond 4, h values decrease for all values owing to the large expansion occurring to the bed with the consequence of a decrease in the solid concentration around the heater. This causes a reduction in h. In addition, as u/u_mf increases; the angle corresponding to u/u_mf is more than 90. It increases until it reaches 180 at values of u/u_mf equal 20. The bed flow reign changes in these conditions from bubbling to turbulent flow reign. The intensity of particle movement on the heater surface increases as increase from 90 to 180. These results are in agreement with[7, 20, and 21] but are not in agreement with[18].

Figure 6 shows the variation of h values with when r/R = 0.3 and for three different values of u/u_mf. It is clear that the behavior of the relation between h and is similar to the case where r/R = 0, However, at u/u_mf = 1 or 2, the values h are smaller than those for r/R = 0. On the other hand, at u/u_mf = 4 h values are larger than those for r/R = 0. This agrees with these results of[18, 20, and 21].

Figure 6. The variation of h with the probe angle for r/R=3

Figure 7. The variation of h with the probe angle for r/R=3

Figure 8. Variation of h with probe angle at r/R=0 and U/Umf =1

Figure 7 Shows the variation of h with near the wall at r/R= 0.7 for three different values of u/u_mf. For certain value of it could be seen that the angle at which, h increases rapidly is smaller near the wall than that at center of the bed. Owing to the difference of the intensity of the particle motion. In addition, h values at the center are higher than those values near the wall for the same reason. This agrees with the results found in[18, 21] but it is not in agreement with the results found in[20].

The variation of h values with at u/u_mf =1 and r/R=0 for different particle sizes is shown in figure 8. It could be noting that as d_p increase h decrease as demonstrate by many researchers.

4. Conclusions

An experimental study has been performed to investigate the effect of the orientation of a flat heater on the heat transfer coefficient in a gas – solid fluidized bed. The effect of the flat heater and the gas flow were examined by changing the angle from 0^o – 180^o, the particle diameter, the gas velocity and the radial heater positions. The experimental results showed the following conclusions:

1- at u/u_mf = 1, h_max is the same for all particle sizes and occurs at = 60 .

2- as u/u_mf increases from 1 to 4, corresponding to h max changes from 60 to 90 and the maximum occurs at 180^o.

3- at u/u_mf = 4, h_max occurs at 90, and increases with u/u_mf until it reaches 180 when u/u_m reaches 20-30.

4- h values at the axis are higher than those values near the wall.

5- as d_p increase, u/u_mf corresponding to u/u_mf increase.

Finally, it is recommended to conduct more research to find out the effect of different variables on the heat transfer coefficient in a gas- solid fluidized bed.

List of Symbols

A : area of heat transfer surface (m²).

Ar : Archimedes number.

C : constant in correlation number (4).

d_p : average particle diameter mm .

h : heat transfer coefficient w/m².k

H : fluidized bed height (m).

H_o : static bed height (m).

Nu : Nuselt number.

Nu_cond : conductive component of particle Nuselt.

Nu_conv : convective component of particle Nuselt.

Pr : Prandtl number.

Q : heat transfer rate (w).

Re : Reynolds number.

T_b : bed temperature (k).

T_w : wall temperature (k).

U : fluid velocity.

u/u_mf : fluid velocity at minimum fluidization m/s.

ρ_p : density of solid particle (kg/m³).

ρ_u : apparent density of solid (kg/m³).

: Probe angle.

1-ε : concentration of solid particle in fluidized bed.

1-ε_o : concentration of particles in static bed.