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By now, having exposure to thermodynamics, we should have a novice idea of what heat transfer means. We are surrounded by a lot many daily life experiences that deal with the science of heat transfer. From finding a way to sip hot tea to attempting to cook Maggi in under 2 minutes using a cooker (no matter how hard you try, two minutes is still a mystery), from turning the AC on getting inside a car parked right under the sun to wrapping complete with a blanket on cold nights from seeing a mirage on a warm road to choked pipes on cold winters, there are plenty of examples. But do you know what are the different ways in which heat transfer takes place?
Table of Contents
Introduction
Before beginning the discussion on heat transfer, let us consider the following scenarios
Scenario 1 - Your tea/coffee cup is too hot and you are getting late for your work/ class, you blow air over your drink, or try putting your drink over a saucer or use another cup to transfer the content of one cup to another and do this repeatedly till the drink is comfortable enough to sip.
Scenario 2 - After running for a while even on a cold day, your body starts feeling warm automatically. Or getting into a car parked on a hot sunny day, makes the interior of the car warm and it becomes too difficult to get into the car and the first thing you do after getting into the car is turn on the air conditioner.
Now, if we happen to access both scenarios, we can observe, we are either trying to raise or lower the temperature of the system under consideration. In short, we are causing the heat to transfer between the system and surroundings. In scenario 1, with different techniques discussed, the temperature change is either sped up or slowed down. This speed or the rate at which heat transfer takes place is the complete science we are interested in, in this blog. Also, this forms the basis to distinguish effectively between thermodynamics and heat transfer. While in thermodynamics, we are more concerned with the "amount" of heat transfer during a process, the latter is more conclusive about the rate of heat transfer. A simple application to this would be manufacturing a thermos flask, there is going to be temperature fall or heat transfer between the flask and the liquid inside but here a designer would be more interested in slowing down this heat transfer so that the thermos can be used effectively as well as efficiently. But obvious, heat transfer will not take place if the two bodies are at the same temperature.
The following are a few of the major applications where heat transfer plays a crucial role:
Heat furnaces
Heaters
Radiators
Refrigerators
Air conditioners
Boilers
Types/modes of heat transfer
The mechanism behind any mode of heat transfer is that the heat transfer will take place only from an object at a higher temperature to an object at a lower temperature. No heat transfer takes place once both the objects reach the same temperature. The different modes of heat transfer are as follows :
Conduction
Convection and
Radiation
Next, let's seek to learn more about each of these.
Conduction
Conduction is the mode of heat transfer that takes place as a result of interaction between two adjacent particles having a difference of energy. The heat transfer takes place from higher energy particles to lower energy particles. The process of conduction can happen in solids (through vibration) as well as fluids (through collision and diffusion). The rate of heat transfer (Q) is given as,
where, k is defined as a constant called the thermal conductivity, A is the area exposed for heat transfer, delta T is the difference in temperature, and delta x is the thickness. When delta x tends to zero, the above equation takes a differential form which is more popularly referred to as Fourier's law of heat conduction. This can be simply illustrated, if the exposed area is large, the heat transfer is higher, that's the reason tea cools faster in a saucer as compared to the teacup. Again, a large difference in temperatures allows more provision of heat transfer. The lesser the thickness, the more heat transfer as less thickness here indicates lower levels of insulation. This is the reason why we wear thick sweaters in winters. The thermal conductivity here measures the ability of the object to conduct heat. A high value of 'k' indicates a good conductor while a lower value of k refers to an insulator
Convection
When the heat transfer takes place between a solid and an adjacent moving fluid particle, the mode of heat transfer is referred to as convection. The presence of fluid motion is very important for convection, without the moving fluid, the heat transfer mode would simply be conduction. The more the speed of moving fluid, the more is the convection. Thus, on stirring the tea in a teacup, the heat transfer can be enhanced, or transferring the tea between two cups also sets the fluid in motion, thereby increasing the heat transfer rate. Based on the way the fluid motion is caused, the mode of convection is further divided into two subcategories:
Free or natural convection: In free convection, the cause of fluid motion is due to the buoyant force-induced due to the density differences caused by temperature differences within the fluid
Forced convection: In forced convection, the flow of fluid is caused by external means such as a fan.
Again coming to the example of hot tea in the teacup, free convection causes the heat transfer when the teacup is left as it is while blowing over the tea is an example of forced convection. Further, the heat transfer taking place while there is a change of phase like in the formation of bubble on liquid boiling or condensation of the liquid droplet during cooling is also identified as a convective mode of heat transfer. Convective heat transfer is governed by Newton's law of cooling which states the rate of heat transfer is proportional to the area exposed and the excess temperature of the body over the surrounding temperature. The mathematical form is given as:
here, 'h' is a constant called a convection heat transfer coefficient. This constant needs to be experimentally determined by considering all of the different parameters such as the geometrical surface, nature of fluid motion, fluid properties as well as the velocity of the fluid.
Radiation
The last mode of heat transfer is due to the emission of electromagnetic waves from the object. This mode of heat transfer is the quickest and does not require any medium for its propagation. We receive the sunlight from the sun with this mode of heat transfer. The maximum rate of heat transfer is governed by the Stefan–Boltzmann law, which states the heat transfer is directly proportional to the area and the fourth power of absolute temperature. This is an idealized case and the body emitting this maximum heat is often termed as a black body. This mode of heat transfer takes place with all the phases of matter. The mathematical form of Stefan–Boltzmann law is given as
where sigma is the Stefan–Boltzmann constant. For all the real surfaces, a factor called the emissivity power (epsilon) occurs in the formulation above. The value lies between 0 and 1 and for emissivity power equals unity, the body will behave like a black body.
Simultaneous multiple modes of heat transfers
In general, all three modes of heat transfer cannot co-exist simultaneously. In solids, there can be heat transfer due to conduction and radiation. But, in the case of a fluid flow over a surface that is being heated up, convection mode takes place outside the solid, and conduction mode takes place within the solid.
In the absence of any radiation, for a still fluid, the mode of heat transfer is conduction while for flowing fluid, it is convection. In the presence of radiation, in the respective cases, radiation modes will have to be added up. Furthermore, in a vacuum, since there is no medium, no other mode of heat transfer is possible except the radiation mode.
History
A French chemist Lavoisier in an attempt to understand the subject of heat proposed the caloric theory. According to his theory, the heat was to be treated similar to a fluid-like substance that has no mass, no color, no taste, and no odor. Like fluid, heat can be poured from one object to another, on the addition of caloric to an object, the temperature of the object increases while on the removal of caloric to the object, a decrease in temperature takes place. When no more heat can be added, the object is said to have reached a saturation state. However, this theory faced a lot of criticism as the theory assumed heat to be a substance that can neither be created nor destroyed. Benjamin Thompson proved the generation of heat in a continuous mode through friction. Further, James Joule was the one to disprove heat to be a substance.
Summary
We looked into the difference between thermodynamics and heat transfer
Further, we studied the different modes of heat transfer
References
Cengel, Yunus A., and Afshin J. Ghajar. "Heat and mass transfer." A practical approach (2007).