TITLE.PM5

852 ENGINEERING THERMODYNAMICS

dharm
\M-therm\Th15-4.pm5

dx = Thickness of body in the direction of flow, and

k = Thermal conductivity of the body.

3. Conduction of heat flow through slabs :

Q = At t

x

k

x

k

x

k

() 12

1

1

2

2

3

3

−

F ++

HG

I

KJ

......3 slabs.

4. Conduction of heat flow through pipe walls and lagging :

Q = ()

log

tt

lk

r

er

12

2

1

1

2

1

−

L

N

M

O

Q

π P

.

5. Conduction of heat through a hollow sphere :

Q =

tt

rr

kr r

12

21

(^412)
−
F −
HG
I
π KJ
.

6. Heat transfer by convection :
   Q = hA (t 1 – t 2 ).


7. Overall heat transfer co-efficient :

U = 111

h

L

hf++∑khcf

.

8. Heat transfer between two fluids separated by the walls of a composite tube of solid material :
   Q =

()

log log

tt

Lrh k

r

rk

r

rrh

hf cf

hf

ee

cf

−

+++

L

N

M

M

O

Q

P

P

1

2

11 1 1

11

2

12

3

π 24

9. A heat exchanger may be defined as an equipment which transfers the energy from a hot fluid to a cold
   fluid, with maximum rate and minimum investment and running cost.


10. The net heat transfer in case of grey bodies with emissivities ε 1 and ε 2 is given by :

Q = σ

εε

AT() 14 T 24

12

(^111)
−
F +
HG
I
KJ
−
In case of concentric or long co-axial cylinder,
Q = AT T
A
A
114 24
1
1
22
(^111)
σ
εε
()−
+−
F
HG
I
KJ

Objective Type Questions

Choose the Correct Answer :

1. The Fourier’s law of heat transfer by conduction is expressed as
   (a)Q = kA^2
   dt
   dx (b)Q = kA^

dt

dx

(c)Q = k^2 A

dx

dt (d)Q = k

(^3) A dx
dt.