Module 3. Dimensional analysis and heat transfer

 

Lesson 15

HEAT TRANSFER BY FREE CONVECTION

15.1  Introduction

The relationship between dimensionless groups are usually presented as power functions e.g. Nu = CRen Prm, where c, n and m are abstract constants. Relationships of this type are purely empirical. They are employed only within the limits of independent variable fixed by experiment. They are not to be extrapolated for larger or smaller values of the argument.

Let us see how such relationships are practically found.

Consider Nu = CRen, Graphically this function can be expressed as a straight line on a log log paper. (Fig. 15.1)

i.e. log Nu = Log C + n log Re

Y = A + nx, denoting log Nu by Y, log Re by X and log c by A

n = tan φ, where φ is the angle the straight line makes with x axis.

The constant c is found from the relationship

holds for any point of the curve, if the test data is a curve, replace it by broken lines. Find c and n for different regions of broken line.

Experimental studies of heat transfer in the air moving within a tube have made possible the establishment of the following generalized relationship.

Nuf = 0.018 Re0.80

Heat transfer by free convection from different solids to fluid.

Relationship between the dimensionless groups can be represented by following power function.

Num = where m stand for mean

The vales of C and n are different for different conditions and are functions of independent variable (Gr Pr).

Table 15.1

C

n

Flow Conditions

1  10-3 to 5  10+2

1.18

1/8

Transient conditions

5 10+2 - 2  107

0.54

Flow in laminar

2  107 - 1  1013

0.135

1/3

Flow becomes turbulent

When (Gr Pr)m < 1, Num = 0.5 and remains constant, which means i.e. process of heating or cooling is fully determined by the thermal conductivity of the surroundings. This holds true only for heat transfer in film type conditions.

The above formula is applicable to any liquids and gases at Pr  0.7 and to bodies of any shape and size.

If the liquids and gases are free flow in horizontal tubes, we can use

For air, formula is simplified as , this is convenient for practical calculations.