Module 2. Convection heat transfer

Lesson 7

HEAT TRANSFER BY CONVECTION

7.1  Introduction

The concept of heat transfer by convection embraces the process of transfer of heat from a liquid or gas to a solid through direct contact. The heat is transferred by conduction and convection simultaneously and this combined phenomenon is known as ‘heat transfer by convection’. As in case of solids, heat conduction in liquids and gases are fully determined by thermal conductivity and temperature gradient, it is different with convection, the second elementary mode of heat propagation because here the process of heat transfer is inseparably bound to the transfer of the environment itself. Convection therefore is possible only in liquids and gases because their particles can be easily displaced.

The displacement of fluids particles i.e. convection process depends on many factors and particularly on origin and condition of the flow. Kind and physical properties of the fluid, the shape and the size of surface of solid, therefore heat transfer by convection is a very complex process.

7.2  Main Factors Which Influence, Convection Mode of Heat Transfer

7.2.1  Origin of convection

There are two types of convection by origin

(a)    Natural (free) convection

(b)   Forced convection

In natural convection, the free motion of fluid is due to difference in the densities of the heated and cold particles of the fluid. The origin and intensity of free convection are fully determined by the thermal condition of the process and depend on the kind of the fluid, temperature potential and volume of the space where the process takes place.

Forced motion of fluid or forced convection occurs under the influence of some external agency such as wind, pump and agitation fan etc. Forced convection depends on the kind and physical properties of the fluid, its temperature, flow, velocity, the shape and size of the passage in which forced flow of liquid occurs. In general, forced convection is accompanied by free convection and relative influence of free convection increases with difference in temperature of individual particle of the fluid. The influence of free convection decreases with increase in velocity of forced flow. Therefore the influence of natural convection is insignificant at high velocities of flow.

7.2.2  Types of fluid flow

Hydrodynamics shows that there are two types of fluid flow:

(a)    Laminar flow

(b)   Turbulent flow

In the first case, particles of the fluid move in a direction parallel to the wall of passage, where as in case of turbulent flow particles move at random. (Fig. 7.1)

The change in laminar to turbulent flow occurs, when fluid velocity is equal to or exceeds critical velocity. Critical velocity is not constant. It is different for different fluids and also depends on geometry of flow. In turbulent flow, not all the particles of the fluid move irregularly. At the wall, limiting the flow, there is always a thin layer of fluid where the flow remains laminar due to viscosity of fluid. This layer is defined as boundary layer. The thickness of boundary layer depends on mean velocity of flow and decreases with increase in velocity of flow. Therefore, type of flow, play significant role in process of heat transfer because it determines its mechanism. In laminar flow, heat is transferred in direction normal to the wall mainly by conduction. Therefore, its rate is determined by thermal conductivity. In turbulent flow, heat is transferred by intensive intermixing of particles. In such conditions of flow, the intensity of heat transfer for certain heat transfer agents such as gas, water, alcohol are various oils is determined mainly by thermal resistance of boundary layer because thermal resistance of fluid in turbulent flow is of secondary importance. Greatest variation in temperature occurs within the boundary layer of the wall.

7.2.3  Physical properties of liquid

The most diverse substance such as air, water, gases, oil, crude oil, benzene, gasoline, kerosene, alcohol, mercury, molten matters etc are now employed as heat transfer agent. The process of heat transfer depends on physical properties of fluid/substance. These properties directly affect the process are:

·         Thermal Conductivity

·         Specific heat

·         Density

·         Thermal Diffusivity

·         Viscosity

These properties have different magnitudes for each substance and are usually function of temperature and some of them of pressure. The physical properties of water, air, stream and certain other substance, employed in engineering design are given in Appendices.

7.2.3.1  Thermal conductivity

 Thermal conductivity characterizes the ability of a substance to conduct heat and its value determines the quantity of heat passing per unit time per unit area at a temperature difference of 1oC per unit length.

           

7.2.3.2  Specific heat

Amount of heat required to raise the temperature of 1 kg of substance by 1oC. The unit of specific heat is kcal/kgoC.

·         Heat capacity at constant pressure is denoted by Cp(kcal/kg-oC)

·         Heat capacity at constant volume is denoted by Cv(kcal/kg- oC)

7.2.3.3  Specific gravity

It is the weight of unit volume of substance. Its unit is kg/m3. It is denoted by    

The reciprocal of specific gravity is specific volume

7.2.3.4  Density

Density of a substance is its mass per unit volume

           

7.2.3.5  Thermal diffusivity

Sometimes, the above listed physical properties of substance do not effect heat transmission individually but they do so as a complex of properties. One of such is thermal diffusivity.

Thermal diffusivity characterizes the rate of change in temperature in transient heat transfer processes. The higher the thermal diffusivity of a substance, the higher is the rate of temperature propagation.

           

7.2.3.6  Viscosity

All real liquids are viscous; a force of internal friction offering resistance to flow always arises between the particles or layers of fluid moving past one another at different velocities. According to Newton’s law, this force, applied to unit area, is proportional to the velocity gradient, namely,

The factor µ of this equation is called the internal friction factor or the viscosity factor; its unit is kg-sec/ m2. At  ; hence the viscosity factor is the friction force per unit contact area between two layers of liquid sliding past one another, provided flow velocity changes one unit per unit length of the normal to the sliding surface.

Equations of hydrodynamics and heat transfer often contain the ratio of the viscosity factor µ to the density ϱ and the product of the viscosity factor by the acceleration of gravity g. The first ratio is called the hydrodynamic kinematic viscosity and is denoted by ν:

The second value is defined as the coefficient of dynamic viscosity and is denoted by η