Thermal properties of food, thermal diffusivity and newtons law of cooling

1. FPE – 601
ENGINEERING AND THERMAL
PROPERTIES OF FOODS
T H E R M A L D I F F U S I V I T Y
A N D
N E W TO N ’ S L AW O F
C O O L I N G

2. THERMAL DIFFUSIVITY
 Physical property associated with transient heat
flow
 m2/s in the SI system
 Measures the ability of a material to conduct
thermal energy relative to its ability to store
thermal energy
 Larger the thermal diffusivity, quick will be the
response to thermal variation
 Ratio of heating times of two materials of the same
thickness will be inversely proportional to their
respective diffusivities

3. METHODS
Indirect
Predictio
n
Method
Direct
Measuremen
t Methods

4. THE TEMPERATURE HISTORY METHOD
Transient temperature history charts giving Fourier
number as a function of temperature ratio for different
Biot numbers are available and they are used to
determine the thermal diffusivity

5. FLASH METHOD
A small, thin disc
specimen is subjected
to a high intensity
short duration radiant
energy pulse
The thermal diffusivity value is calculated from the
specimen thickness and the time required for the rear
face temperature rise and to reach its maximum value

6. DICKERSON METHOD
• In this method, product is loaded in tightly sealed sample
tube
• Insert the thermocouple and clamp the end plate of the
tube
• Place the assembly in water bath
• Record the center temperature (T0) and surface
temperature (TR) at 2 minute intervals until the surface
temperature reaches to 800C

7. a =
A.(R)
4(TR -T0 )
2
Where,
A = slope of the heating curve, 0C/min
R = radius of cylinder, cm
TR = water bath temperature at t, 0C
T0 = cylinder center temperature at t, 0C
α = thermal diffusivity, cm2/min

8. NEWTON’S LAW OF COOLING
Temperature difference in any system results in
energy flow into a system or from a system to
surroundings
States that the rate of energy flow in or from system is
proportional to the difference between the
temperature of the body and that of the surrounding
medium
Same is useful for studying water heating because it
can tell us how fast the hot water in pipes cools off

9. Where,
T1 = initial temperature
T = temperature after time t
T2 = final temperature
K = positive proportionality constant
(It depends upon the surface properties
of the material being cooled)
dT
dt
= -k(T -T2 )

10. At Initial conditions, T=T1 at t=0
- kt = log (T - T2) + log C
T – T2 = Ce-kt ..(1)
Applying initial conditions,
C = T1 – T2
Now substituting value of C in equation (1)
This equation represents the
Newton’s law of cooling
T = T2 + (T1 – T2) -kt

11. The graph drawn between the temperature of the
body and time is known as cooling curve.
The slope of the tangent to the curve at any point
gives the rate of fall of temperature.

12. THANK YOU