Heat Transfer in Mechanical Engineering, Part II, Convection

     Last week we talked about heat transfer by conduction, with an example that showed how heat flowed from hot to cold through a solid block of metal.  This week we’ll focus on another method of heat transfer known as convection.

     Convection occurs when heat is transferred between a surface and a moving fluid.  As with conduction, heat always wants to flow from a higher temperature to a lower temperature.  There are two types of convective heat transfer, natural and forced.

     An example of natural convection would be hot air rising off a blacktop highway on a sunny summer day.  The cool air near the ground picks up heat from the sun-baked blacktop.  As the air heats, its density decreases and it gets lighter.  The warmer, lighter air rises off the highway and more cool air rolls in to take its place.  This creates a continuous natural airflow that removes heat from the blacktop.  You can actually see this airflow as ripples in the air just above the highway.  It produces a mirage effect and almost appears to be water on the highway, particularly on very hot days.

     In natural convection we are concerned with heat transfer between a surface and a fluid moving over that surface.  To calculate the heat transfer, we need only consider the area of the surface and the temperature difference between the surface and the fluid.  There is no material thickness to consider like we saw in last week’s conductive heat transfer example, where heat was moving through a solid object.  So rather than working with a conductive heat transfer coefficient, we must work with a convective heat transfer coefficient, and our engineering reference guide will guide us to the correct convective coefficient to be used to calculate heat flow.

     With this said, we can calculate the natural convective heat flow to be: 

         Heat Flow = (The Convective Heat Transfer Coefficient) x

                                 (The Area That The Heat Is Flowing Through) x

                                     (The Difference In Temperature)

      Now let’s go back to the blacktop to see how this all works.  Suppose you have a parking lot and you want to know how much heat it is pumping into the atmosphere on a hot, sunny summer day with no wind.  We must first determine the area of the lot, and we measure it out to be 100 meters by 100 meters.  That’s 100 X 100, or 10,000 square meters of blacktop we’re talking about.  We now measure the surface temperature of the blacktop and find it to be 65°C, but the air temperature near the ground away from any source of blacktop is a cool 30°C.  Okay, so what is the heat transfer?

     First, you consult your friendly engineering reference manual.  It tells you that the convective heat transfer coefficient is 15 Watts/meter2K for still air over a horizontal surface.  The “K” represents temperature measured in degrees “Kelvin,” and this is calculated by simply adding 273.15 to any temperature measured in degrees Celsius, or °C.

     So, to get all of our units to match up in order to perform the calculation, the blacktop would be at a temperature of 65°C + 273.15 = 338.15K.  The cool air would be at a temperature of 30°C + 273.15 = 303.15K.  Our equation becomes:

Heat Flow = (15 W/m2K) x (10,000 m2) x (338.15K – 303.15K)

= 5,250,000 Watts 

     We have calculated that the air in the atmosphere picks up heat energy from the parking lot pavement at a rate of over 5 million watts.  This explains why it always seems to be warmer in cities compared to the surrounding countryside.  The presence of dark asphalt pavement and dark roofing materials absorb heat from the sun like any dark surface will, and this heat buildup then dissipates into the surrounding atmosphere through the process of natural convection.

     The other type of convection, the forced type, is just as its name implies.  It requires a powered device to move the fluid, that is to say, it does not rely on a natural source of energy like the sun.  An example of forced convection can be found in a hair dryer, which uses a small blower to move air over an electric heating element.  Another example of forced convection can be found in the water pump of your car.  This pump circulates water through the engine, absorbing heat as it goes, and then gives that heat up to the air which is flowing over the radiator fins.  This keeps the engine from overheating.

     Calculating heat transfer rates in forced convection problems can get extremely complicated, involving higher level mathematics and concepts of advanced fluid dynamics.  Some problems are so complex they can only be solved with the aid of specially written computer programs, so an example problem would be beyond the scope of the basic discussions in this series of articles.

     Next week we’ll analyze how the sun, which is separated from Earth by 93 million miles of the vacuum that we call Outer Space, is able to heat our blacktop pavement up from so great a distance.  It does this by the process of radiant heat transfer.


Tags: , , , , , , , ,

Comments are closed.