6.4 Investigation of heat transfer
6.4.4 Variation of wall-to-bed heat transfer coefficient with weight
Figures 6.53 through 6.59 present the variation of heat transfer coefficient at a height of 1.57 m from the distributor plate at 2.5 %, 7.5 %, 15.0 % and 20 % blending of sawdust in sand. The comparisons were made at two different weight composition ratios and at two different superficial velocities of 5 and 7 m/s. From these figures, it has been found that, the heat transfer coefficient increases with the increase in operating pressures. At superficial velocity of 5 m/s, the heat transfer coefficient is found to be higher (120-135 W/m2-K) at 7.5 % blending (Fig.6.54) with weight composition ratio 30 g: 400 g as compared to the other three percentage blending. At superficial velocity of 7 m/s, the heat transfer coefficient is found to be higher (between 120-130 W/m2-K) at 15 % blending (Fig.6.58) with weight composition ratio (B/S) of 90 g: 600 g as compared to the other three percentage blending. The heat transfer coefficient is found to be lowest (90-105 W/m2-K) at 20 % biomass blending (Figs.6.55 and 6.59) among the other blending at both the superficial velocities i.e. at 5 and 7 m/s. This may be due to the lowest solid circulation rate observed in both the cases as compared to the other percentage blending. The values of the solid circulation rate at the superficial velocities of 5 and 7 m/s and at four different percentage blending is shown in the Table-6.1.
100 105 110 115 120 125 130 135 140 145
20 30 40 50 60 70
Suspension density (ρs), kg/m3 Heat Transfer coffecient, W/m2-K
12.5 % blend, Usup= 6 m/s, Particle size = 307 µm
Fig.6.53 Variation of heat transfer coefficient at Usup = 5 m/s and at 2.5 % blending
Fig.6.54 Variation of heat transfer coefficient at Usup = 5 m/s and at 7.5 % blending
Fig.6.55 Variation of heat transfer coefficient at Usup = 5 m/s and at 20.0 % blending
Fig.6.56 Variation of heat transfer coefficient at Usup = 7 m/s and at 2.5 % blending
Fig.6.57 Variation of heat transfer coefficient at Usup = 7 m/s and at 7.5 % blending
Fig.6.58 Variation of heat transfer coefficient at Usup = 7 m/s and at 15.0 % blending
90 100 110 120 130 140 150
0 2 4 6
B/S=10/400 B/S=15/600
Operating pressure, bar h, W/m2-K
Usup= 5 m/s, 2.5 % blending
90 100 110 120 130 140 150
0 2 4 6
B/S=30/400 B/S=45/600
Operating pressure, bar h, W/m2-K
Usup= 5 m/s, 7.5 % blending
90 100 110 120 130 140 150
0 2 4 6
B/S=80/400 B/S=120/600
Usup= 5 m/s, 20 % blending
Operating pressure, bar h, W/m2-K
90 100 110 120 130 140 150
0 2 4 6
B/S=10/400 B/S=15/600
Operating pressure, bar h, W/m2-K
Usup= 7 m/s, 2.5 % blending
90 100 110 120 130 140 150
0 2 4 6
B/S=30/400 B/S=45/600
h, W/m2-K
Operating pressure, bar Usup= 7 m/s, 7.5 % blending
90 100 110 120 130 140 150
0 2 4 6
B/S=60/400 B/S=90/600
Operating pressure, bar h, W/m2-K
Usup= 7 m/s, 15 % blending
Fig.6.59 Variation of heat transfer coefficient at Usup = 7 m/s and at 20.0 % blending
Figure 6.60 show the variation of heat transfer coefficient along the heat transfer probe at the operating pressure of 5 bar and at the superficial velocity of 5 m/s. It is observed that, the heat transfer coefficient increases from the bottom to the top of the heat transfer probe. This is a representative figure for percentage blending of biomass in sand at varied pressure conditions.
The similar variation of heat transfer coefficient without blending of biomass is demonstrated by Gupta and Nag (2002).
Fig.6.60 Comparison of variation of heat transfer coefficient along the heat transfer probe
90 100 110 120 130 140 150
0 2 4 6
B/S=80/400 B/S=120/600
Operating pressure, bar h, W/m2-K
Usup= 7 m/s, 20 % blending
90 100 110 120 130 140 150
1.3 1.4 1.5 1.6 1.7 1.8 1.9
2.5 % blend (B:S) 10:400 2.5 % blend 15:600 (B:S) 7.5 % blend 30:400 (B:S) 7.5 % blend 45:600 (B:S) 15.0 % blend 60:400 (B:S) 15.0 % blend 90:600 (B:S) 20.0 % blend 80:400 (B:S)
Usup= 5 m/s, P = 5 bar h, W/m2-K
Riser height from the distributor, m
Table-6.1 Solid circulation rate, Gs (kg m-2s-1) data with pressure
Figures 6.61 through 6.63 present the variation of heat transfer coefficient along the heat transfer probe at three different operating pressures of 1, 3 and 5 bar respectively. The comparisons were made at four different blending of biomass (sawdust) in sand with two different sets of weight compositions. In all the three figures, similar variations of heat transfer coefficient have been observed i.e. heat transfer coefficient increasing toward the riser exit. Lowest values of the heat transfer coefficient along the heat transfer probe have been found at 20 % sawdust blending in sand in both the weight compositions at all the three operating pressures. This may be due to the lower solid circulation rate (0.0817 kg m-2 s-1 for P = 1 bar, 0.1388 kg m-2 s-1 for P = 3 bar and 0.3033 kg m-2 s-1 for P = 5 bar) at this condition, which results lower suspension density at the riser exit. As the operating pressure changes, the heat transfer coefficient associated with the percentage blending of biomass in sand and weight composition also changes. At P = 1 bar and P = 3 bar, the heat transfer coefficient is found to be higher at 7.5 % sawdust blending with weight composition ratio of 30 g: 400 g followed by 2.5 % with weight composition ratio of 15 g: 600 g respectively. At these conditions, solid circulation rates are found to be 0.946 and 1.276 kg m-2 s-1, respectively. At operating pressure of 5 bar, the heat transfer coefficient is found to be higher as compared to the other two operating pressures and the maximum values of heat transfer coefficient is found at 15.0 % sawdust blend at a weight composition ratio of 60 g: 400 g. The corresponding solid circulation rate is 1.637 kg m-2 s-1. Moreover, at this condition very close variations of heat transfer coefficient has been observed at the blending ratios of 2.5 %, 7.5 % and 15.0 % at both the weight composition ratios. The heat transfer coefficient varies from 85.18 to 147.75 W/m2-K in the entire range of biomass blending. More uniform and higher heat transfer has been observed as the operating pressure increases. Based on the requirement and
P, bar
2.5 % blend 7.5 % blend 15.0 % blend 20.0 % blend
(B/S) 10:400
(B/S) 15:600
(B/S) 30:400
(B/S) 45:600
(B/S) 60:400
(B/S) 90:600
(B/S) 80:400
(B/S) 120: 600 Usup = 5 m/s
1 0.816 1.381 0.946 1.072 0.773 0.912 0.205 0.0817
3 1.164 1.086 1.623 0.753 1.094 1.762 0.926 0.1388
5 0.952 1.455 1.798 1.350 1.637 1.327 0.989 0.3033
Usup = 7 m/s
1 1.296 1.406 1.565 1.261 1.137 1.451 1.064 0.1921
3 1.275 1.123 2.089 1.205 1.434 1.861 1.352 0.2609
5 1.093 1.455 2.012 1.260 1.389 1.650 1.332 0.3533
operating conditions the optimum weight composition may be maintained for maximum heat transfer and gas yield.
Fig.6.61 Variation of heat transfer coefficient at P = 1 bar
Fig.6.62 Variation of heat transfer coefficient at P = 3 bar
80 90 100 110 120 130 140 150 160 170
1.3 1.4 1.5 1.6 1.7 1.8 1.9
2.5 % blend 10:400 (B:S) 2.5 % blend 15:600 (B:S) 7.5 % blend 30:400 (B:S) 7.5 % blend 45:600 (B:S) 15.0 % blend 60:400 (B:S) 15.0 % blend 90:600 (B:S) 20.0 % blend 80:400 (B:S) 20.0 % blend 120: 600 (B:S)
Usup= 5 m/s, P = 1 bar
Riser height from the distributor, m h, W/m2-K
80 90 100 110 120 130 140 150 160 170
1.3 1.4 1.5 1.6 1.7 1.8 1.9
2.5 % blend 10:400 (B:S) 2.5 % blend 15:600 (B:S) 7.5 % blend 30:400 (B:S) 7.5 % blend 45:600 (B:S) 15.0 % blend 60:400 (B:S) 15.0 % blend 90:600 (B:S) 20.0 % blend 80:400 (B:S) 20.0 % blend 120: 600 (B:S)
Usup= 5 m/s, P = 3 bar
Riser height from the distributor, m h, W/m2-K
Fig.6.63 Variation of heat transfer coefficient at P = 5 bar
6.4.5 Variation of heat transfer coefficient along the radial direction without biomass