### Newtons Law of Cooling

• State Newton’s law of cooling.

• Calculate rate of heat flow by convection using Newton’s law of cooling.

• Understand the governing equation relevant to natural convection.

Convection is expressed by Newton’s law of cooling as:

$\dot{Q} = h A_{s}(T_{s} - T_{\infty}) = \rho V C_P \dfrac{dT}{dt}$

Where h is the convection heat transfer coefficient
$A_{s}$ is the surface area through which convection heat transfer takes place
$T_{s}$ is the surface temperature
$T_{\infty}$ is the temperature of the fluid sufficiently far from the surface

### Solved Example:

#### 81-1-01

When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by: (SSC JE ME paper 6- March 2017 Morning)

### Solved Example:

#### 81-1-02

When heat is transferred by molecular collision, it is referred to as heat transfer by:

### Solved Example:

#### 81-1-03

Heat transfer in liquid and gases takes place by: (UPPCL JE March 2021-Shift I)

### Solved Example:

#### 81-1-04

In convection heat transfer from hot flue gases to water tube, even though flow may be turbulent, a laminar flow region (boundary layer of film) exists close to the tube. The heat transfer through this film takes place by:

### Solved Example:

#### 81-1-05

Consider a hot boiled potato kept on a plate and cooled by natural convection in air. During the first minute, the temperature drops by 10$^{\circ}\mathrm{C}$. During the second minute, will the temperature drop be more than, less than or same as that during the first minute?

### Solved Example:

#### 81-1-06

Air at 300$^\circ$C flows over a plate of dimensions 0.50 m, by 0.25 m. if the convection heat transfer coefficient is 250 $W/m^2.K$; determine the heat transfer rate from the air to one side of the plate when the plate is maintained at 400$^\circ$C.

Solution:
\begin{align*} q &=h A\left( T_{\infty }-T_{s}\right)\\ &= 250\left( 0.25 \times 0.50\right) \left(300-40\right)\\ &=8125\ W \end{align*}