Thermodynamics and heat transfer. Methods of heat transfer and calculation. Heat transfer is ...

Today we will try to find the answer to the question "Heat transfer is ...?". In the article we will consider what the process is, what kinds of it exist in nature, and also learn what is the relationship between heat transfer and thermodynamics.

Definition

Heat transfer is a physical process, the essence of which is the transfer of heat energy. Exchange occurs between two bodies or their system. At the same time, the transfer of heat from warmer bodies to less heated bodies is a prerequisite.

Process Features

Heat transfer is the kind of phenomenon that can occur both in direct contact and in the presence of dividing partitions. In the first case, everything is clear, in the second case, bodies, materials, and media can be used as barriers. Heat transfer will occur in cases where a system consisting of two or more bodies is not in a state of thermal equilibrium. That is, one of the objects has a higher or lower temperature than the other. Then the transfer of heat energy takes place. It is logical to assume that it will end when the system comes to a state of thermodynamic, or thermal equilibrium. The process is spontaneous, as we can tell the second law of thermodynamics.

Kinds

Heat transfer is a process that can be divided into three ways. They will be of a basic nature, since within them one can distinguish real subcategories, which have their own characteristic features on a par with general laws. To date, it is common to distinguish three types of heat transfer. This is thermal conductivity, convection and radiation. Let's start with the first, perhaps.

Ways of heat transfer. Thermal conductivity.

This is how the property of this or that material body is called to perform the transfer of energy. In this case, it is transferred from the more heated part to the one that is colder. At the heart of this phenomenon lies the principle of chaotic motion of molecules. This is the so-called Brownian motion. The higher the body temperature, the more active the molecules move in it, since they have more kinetic energy. In the process of thermal conductivity, electrons, molecules, and atoms participate. It is carried out in bodies, different parts of which have an unequal temperature.

If the substance is capable of conducting heat, we can talk about the presence of a quantitative characteristic. In this case its role is played by the coefficient of thermal conductivity. This characteristic shows how much heat will pass through the unit of length and area per unit of time. In this case, the temperature of the body will change by exactly 1 K.

Previously it was believed that the exchange of heat in various bodies (including the heat transfer of the enclosing structures) is due to the fact that from one part of the body to another flows so-called heat. However, no one ever found any signs of his real existence, and when the molecular-kinetic theory developed to a certain level, everyone forgot to think about the heat, since the hypothesis turned out to be untenable.

Convection. Heat transfer of water

By this method of heat exchange, transmission by internal flows is understood. Let's imagine a kettle with water. As is known, more heated air flows rise upward. And the cold, heavier, fall down. So why should water be different? It's absolutely the same with her. And now in the process of such a cycle, all layers of water, no matter how many, will warm up before the onset of the state of thermal equilibrium. In certain conditions, of course.

Radiation

This method consists in the principle of electromagnetic radiation. It is due to internal energy. We will not go into the theory of thermal radiation , we simply note that the reason here is the arrangement of charged particles, atoms and molecules.

Simple problems of heat conduction

Now let's talk about how heat transfer calculation looks like in practice. Let's solve a simple task related to the amount of heat. Let's say that we have a mass of water equal to half a kilogram. The initial water temperature is 0 degrees Celsius, the final temperature is 100. Let's find the amount of heat we spent for heating this mass of matter.

For this we need the formula Q = cm (t ₂ -t ₁ ), where Q is the amount of heat, c is the specific heat of water, m is the mass of the substance, t ₁ is the initial, t ₂ is the final temperature. For water, the value of c is tabular. The specific heat capacity will be 4200 J / kg * C. Now substitute these values in the formula. Let's get that the amount of heat will be equal to 210000 J, or 210 kJ.

The first law of thermodynamics

Thermodynamics and heat transfer are related to each other by certain laws. They are based on the knowledge that changes in internal energy within the system can be achieved by two methods. The first - the fulfillment of mechanical work. The second is the message of a certain amount of heat. By the way, this principle is based on the first law of thermodynamics. Here is his wording: if the system has been informed of a certain amount of heat, it will be spent on doing work on external bodies or on incrementing its internal energy. Mathematical notation: dQ = dU + dA.

Pros or cons?

Absolutely all the quantities that enter into the mathematical notation of the first law of thermodynamics can be written with both a plus sign and a minus sign. And their choice will be dictated by the conditions of the process. Suppose that the system receives a certain amount of heat. In this case, the bodies in it are heated. Consequently, gas expansion takes place, which means that work is being done. As a result, the values will be positive. If the amount of heat is taken away, the gas cools down, work is performed above it. Values will take opposite values.

Alternative formulation of the first law of thermodynamics

Suppose that we have a periodically acting engine. In it, the working body (or system) performs a circular process. It is usually called a cycle. As a result, the system will return to its original state. It would be logical to assume that in this case the change in internal energy will be zero. It turns out that the amount of heat will equal the perfect work. These provisions make it possible to formulate the first law of thermodynamics in a different way.

From it we can understand that in nature there can not exist a perpetual motion machine of the first kind. That is, a device that does more work in comparison with the energy received from outside. In this case, actions must be performed periodically.

The first law of thermodynamics for isoprocesses

Let us first consider the isochoric process. With it, the volume remains constant. So, the volume change will be zero. Consequently, the work will also be zero. We drop this term from the first law of thermodynamics, after which we obtain the formula dQ = dU. Hence, in an isochoric process, all the heat supplied to the system goes to increase the internal energy of the gas or mixture.

Now let's talk about the isobaric process. The constant value in it remains the pressure. In this case, the internal energy will change in parallel with the work. Here is the original formula: dQ = dU + pdV. We can easily calculate the work done. It will be equal to the expression uR (T ₂ -T ₁ ). By the way, this is the physical meaning of the universal gas constant. In the presence of one mole of gas and a temperature difference of one Kelvin, the universal gas constant will be equal to the work performed in the isobaric process.