4.2 Flow through a bed of grain Flow of air through a bed of grain is analogous to fluid flow through a network of pipes. (a) Fig. 4.17(a) shows a cross-section of a solid block pierced by n paral- lel tubes, each of radius a. As a is small, the flow is laminar (why?), in which case it may be shown that according to Poiseuille's law, the volume of fluid flowing through each tube is: nat 8u (dx (4.28) where u is the dynamic viscosity (see Review 2) and dø/dx is the pressure gradient driving the flow. Use this to show that the bulk fluid flow speed through the solid block of cross-section A, is:
4.2 Flow through a bed of grain Flow of air through a bed of grain is analogous to fluid flow through a network of pipes. (a) Fig. 4.17(a) shows a cross-section of a solid block pierced by n paral- lel tubes, each of radius a. As a is small, the flow is laminar (why?), in which case it may be shown that according to Poiseuille's law, the volume of fluid flowing through each tube is: nat 8u (dx (4.28) where u is the dynamic viscosity (see Review 2) and dø/dx is the pressure gradient driving the flow. Use this to show that the bulk fluid flow speed through the solid block of cross-section A, is:
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4.2 a, b
![4.2 Flow through a bed of grain
Flow of air through a bed of grain is analogous to fluid flow through a
network of pipes.
(a) Fig. 4.17(a) shows a cross-section of a solid block pierced by n paral-
lel tubes, each of radius a. As a is small, the flow is laminar (why?),
in which case it may be shown that according to Poiseuille's law, the
volume of fluid flowing through each tube is:
(4.28)
8u (ax
where u is the dynamic viscosity (see Review 2) and dp/dx is the
pressure gradient driving the flow. Use this to show that the bulk
fluid flow speed through the solid block of cross-section A, is:
Oal ea dp
total
A,
8 dx
(a)
(b)
Area
Ao
A
(c)
Ax
Fig. 4.17
For Problem 4.2:
a block pierced by paralel tubes;
b pores in a bed of grain;
e volume of grain bed.
where the porosity e is the fraction of the volume of the block which
is occupied by fluid, and Qo is the total volume flow through
the block.
(b) The bed of grain in a solar drier has a total volume Vood = A,Ax
(Fig. 4.17(c). The drier is to be designed to hold 1000 kg of grain of
bulk volume V= 1.3 m2. The grain is to be dried in four days (= 30
hours of operation). Show that this requires an air flow of at least
Q= 0.12 ms-1 (hint. refer to Worked example 4.1).](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2F85ff80cd-34a8-4471-9907-d9de1b52412a%2Fc8e4ec62-8354-4e54-a6b8-dfd4feb8f256%2Fm4j1h79_processed.jpeg&w=3840&q=75)
Transcribed Image Text:4.2 Flow through a bed of grain
Flow of air through a bed of grain is analogous to fluid flow through a
network of pipes.
(a) Fig. 4.17(a) shows a cross-section of a solid block pierced by n paral-
lel tubes, each of radius a. As a is small, the flow is laminar (why?),
in which case it may be shown that according to Poiseuille's law, the
volume of fluid flowing through each tube is:
(4.28)
8u (ax
where u is the dynamic viscosity (see Review 2) and dp/dx is the
pressure gradient driving the flow. Use this to show that the bulk
fluid flow speed through the solid block of cross-section A, is:
Oal ea dp
total
A,
8 dx
(a)
(b)
Area
Ao
A
(c)
Ax
Fig. 4.17
For Problem 4.2:
a block pierced by paralel tubes;
b pores in a bed of grain;
e volume of grain bed.
where the porosity e is the fraction of the volume of the block which
is occupied by fluid, and Qo is the total volume flow through
the block.
(b) The bed of grain in a solar drier has a total volume Vood = A,Ax
(Fig. 4.17(c). The drier is to be designed to hold 1000 kg of grain of
bulk volume V= 1.3 m2. The grain is to be dried in four days (= 30
hours of operation). Show that this requires an air flow of at least
Q= 0.12 ms-1 (hint. refer to Worked example 4.1).
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