Convection Heat Transfer – Definition and Calculation

Heat transfer is a very important area in engineering. Also, they develop lots of kinds of useful systems for heat transfer calculations. There are three main mechanisms of heat transfer. These mechanisms are conduction, radiation, and convection. Here we explain the convection heat transfer. So, you can find how to calculate convective heat transfer and what are the main aspects of it.

What is the Convection Heat Transfer?

Firstly, convection heat transfer is the type of heat transfer where the heat transfer takes place by the motion of the fluid. So, various factors affect the convection heat transfer.

So, convection heat transfer has lots of common with fluid flow. The characteristics of fluid are very important for convection heat transfer.

How to Calculate the Convection Heat Transfer?

Convection heat transfer is a very complex topic. But in the most general aspects, the convection heat transfer is directly related to Newton’s law of cooling. Newton’s law of cooling is like this;

In this formula;

h = Heat transfer coefficient of the environment with the unit of W/(m2°C) [Btu/(hr-ft2°F)].

A = The area of the surface where convection takes place. The unit is m2 or ft2.

Tsurface = Temperature of the surface in K or °F.

Tambient: Temperature of the ambient in K or °F.

You can use this calculation for static fluids.

Nusselt Number

Also, the Nusselt number is a very important dimensionless number in convection heat transfer calculations. We use the Nusselt number to see the degree of convection in a heat transfer. We can calculate the Nusselt number with this equation;

In this equation,

h = Heat transfer coefficient of the environment with the unit of W/(m2°C) [Btu/(hr-ft2°F)].

Lc = Characteristic length which you can calculate by dividing the volume by area.

k = Thermal conductivity of the fluid which has the unit of BTU/(hft⋅°F) or (W/mK).

So, with the increasing Nusselt number, the rate of the convection heat transfer is higher if we compare it with the conduction. And if the Nusselt number is 1, all the heat transfer is convection in a heat transfer mechanism.

Forced Convection vs. Natural Convection

There are two types of this heat transfer mechanism; Forced convection and natural convection.

In the forced convection heat transfer, the motion of fluid takes place with an external factor. For example, cooling fans are forcing the fluid which is air to the materials to cool them. So, there is a forced convection heat transfer phenomenon here.

Also in natural convection, there is no external factor for the fluid flow. Fluids are free to convect the heat energy. For example, heat transfer between the hot object and the cold medium is an example of the natural convection phenomenon.

Fluid Properties that Affect to the Convection

As we stated above, different kinds of fluid properties are affecting the convection heat transfer. Around these fluid properties;

• Dynamic viscosity
• Thermal conductivity
• Density
• Specific heat
• Fluid velocity

Effect of Dynamic Viscosity on Convection

The dynamic viscosity of a fluid is the force to overcome the internal friction of the fluid. With the increasing dynamic viscosity, the ease of flow decreases. For example, lubricants that are used in machines have very high dynamic viscosities. Also, water has very low viscosity if we compare it with other fluids.

Dynamic viscosity has a tremendous effect on the flow characteristics. So, there is a direct relationship between convection and dynamic viscosity. With the increasing dynamic viscosity, convection decreases because of the decreasing translation of fluid.

Thermal Conductivity

The thermal conductivity of material means the ease of conducting heat through itself. If we think about the total heat transfer, with the increased thermal conductivity, the total heat transfer increases. So, there is a direct proportion between the thermal conductivity and the convection heat transfer.

Density of Fluid

In general, fluids with high densities, are very hard to make flow. So, convection heat transfer will be harder with dense fluids.

Specific Heat

Specific heat of fluid means the total unit capacity to absorb heat energy. Fluids with high specific heat tend to absorb more heat energies. So, according to the fluid flow, fluids with higher specific heat will transfer much more heat by convection.

Fluid Velocity

With the higher fluid velocities, the total heat transfer will increase. Because fluid transfers the heat energy much more rapidly without dissipating it.

Along with the fluid properties, classes and flow regimes of fluids are also very important in convection.

Types of Flow Regimes in Convection Heat Transfer

Classifications of fluids are also very important in convective heat transfer. Different types of fluid flow classifications that we have. Take a look at these classifications and the effects of convection.

One-Dimensional, Two-Dimensional, and Three-Dimensional Flows

This classification is the calculation approach to flow regimes. One-dimensional flows are much easier to calculate, and two-dimensional and three-dimensional flows are much harder to calculate.

According to the different kinds of engineering analyses, you need to select the proper regime to calculate. It is very important in convection problems.

Steady and Unsteady Flow Regimes in Convection

In general engineering systems, we are making calculations of steady flows. Steady flow means the regime of the fluid flow is always the same. Unsteady flow means the regime of flow changes with the changing time.

Convection calculations for steady flows are much easier if we compare them with unsteady flows. Lots of engineering systems include steady flows such as intercooler systems in automobiles. So, steady flow convection calculations are very common in heat transfer.

Natural and Forced Flows

So, it is directly related to natural and forced convection. The logic is the same. If we obtain the fluid flow with the help of an external device or force, we call it forced flow. The reverse is a natural flow.

Both the natural and forces flows are the topic of engineering. There are different kinds of engineering systems to calculate natural and forced flows. For example, there is a natural flow around the heat transfer between the thermos bottle and the surroundings.

Another example is, that we need to make forced flow convection calculations for flow through the pipes.

Laminar and Turbulent Flow

Laminar and turbulent flows have an extreme effect on convection. In turbulent flows, the convection takes place better if we compared it with laminar flows. So, with the increasing Reynolds number, the convection heat transfer increases.

For aerodynamical systems, we would like to obtain laminar flows for low coefficient of drags. But for higher convection heat transfers, we want to obtain more turbulent flows if the convection is desired.

Compressible and Incompressible Flow Regimes

It is not a correct viewpoint on the compressible and incompressible flows that affect the convection. These two approaches are directly related to the calculations about flow regimes.

Internal and External Flows in Convection

Also, this is a very important classification. There are different kinds of engineering problems related to internal and external flows in convection. In the internal flow, fluid flows inside a tube or pipe enclosed inside the walls. Also in external flows, fluid flows over a wall or plane.

The internal or external flow regimes affect the calculations of convection heat transfers. There are various engineering systems that internal and external heat transfer regimes that they use.

Inviscid and Viscous Flows in Convection

Inviscid and viscous flows are not a classification. There is an inviscid or viscous flow in fluid flows in tubes or on planes. So, in the contact region between the plane and the fluid, there is a layer on the plane the total velocity of the fluid is zero. With the increasing distance between this layer, the velocity of the fluid increases. We call this flow region as viscous flow region.

After a distance, the flow is fully-developed. Also, in this fully-developed region, we call this fluid flow inviscid flow. There are no viscous effects in this region.

Inviscid and viscous flows affect the convection calculations. So, we need to know whether the flow is inviscid or viscous in the convection calculations.

Conclusion

So, as you see above, convection heat transfer has a very important place in engineering. We can calculate it with Newton’s law of cooling formula. Also, the Nusselt number is a very important factor in calculations.

Fluid flow characteristics are also very important in convective heat transfer. And also, the fluid flow regime type has a tremendous effect on the convection.

Finally, this is the general information about convective heat transfer.

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FAQs About Convection