Heat Transfer Driven by Temperature Differences: Applications and Distinctions in Engineering Thermodynamics and Heat Transfer

In the myriad worlds where we live, various processes occur constantly, one of the physical processes most closely related to human survival is the transfer of heat energy. From the central air conditioning in modern buildings to the formation of natural weather phenomena like frost, rain, and snow, from the thermal protection challenges of spacecraft re-entering the atmosphere to the effective cooling of electronic devices, from the seasonal changes in people’s clothing to the frozen storage of human food, all are closely related to the process of heat transfer.

Heat transfer studies are a discipline that investigates the laws of heat or energy transfer caused by temperature differences. The Second Law of Thermodynamics states: wherever there is a temperature difference, heat naturally transfers from a higher temperature object to a lower temperature object. This transferred heat is often referred to as thermal energy.

Temperature differences exist everywhere in nature and various fields of production technology, making heat transfer a very common physical phenomenon. For instance, winds in nature are caused by temperature differences between two places, causing air to flow from a higher temperature area to a lower temperature area, further influenced by the Earth’s rotation to form wind. Ocean currents and typhoons are also caused by temperature differences leading to the movement of water or air. In industrial production, using carbon fiber heating tubes to heat products also involves transferring the heat from the heating tubes to the heated material, which is another process of heat transfer.

The so-called laws of heat transfer primarily relate the amount of heat transferred per unit time to the corresponding temperature difference within an object. The first-level relation that reflects this law becomes the rate equation of heat transfer. In subsequent articles, I will discuss with you the rate equations of the three basic modes of heat transfer under certain simplified conditions. A deeper level of study is to find the temperature distribution at various points within an object under different conditions.

Heat transfer studies and engineering thermodynamics are both disciplines related to thermal phenomena. In China’s engineering education sector, these two courses are collectively referred to as thermal engineering courses. The fundamental difference between these two fields of science can be explained as follows: Engineering thermodynamics studies systems in equilibrium, where there are no temperature differences or pressure differences, whereas heat transfer studies precisely the opposite, the laws of heat transfer involving temperature differences. For example, consider the process of cooling a steel ingot from 1000°C in an oil bath to 100°C. Thermodynamics studies the heat lost per kilogram of steel ingot during this cooling process and the heat absorbed by the oil bath, but thermodynamics cannot tell us how long it takes for this temperature system to reach equilibrium. This time depends on the temperature of the oil bath, the movement of the oil, the physical properties of the oil, etc., which are precisely what heat transfer studies.

Furthermore, due to the fundamental difference mentioned above, there is a distinction in the physical parameters used in thermodynamics and heat transfer: in thermodynamics, physical quantities do not include time, whereas in heat transfer, the main physical quantities are denominated in time, i.e., heat transfer is more concerned with how much heat energy can be transferred per unit of time. On the other hand, heat transfer studies are closely related to engineering thermodynamics: the analysis of any heat transfer process must use the First Law of Thermodynamics, i.e., the law of energy conservation. The First Law of Thermodynamics can be applied in both closed and open systems, with each type of system having both steady and unsteady states. From the perspective of heat transfer, a so-called steady state process is one where the temperature at each point in the system does not change over time, whereas in an unsteady state process, the temperatures at each point change over time.

In future discussions on heat conduction in solids, we will use the First Law of Thermodynamics for closed systems, while the study of convective heat transfer requires the use of open systems. Moreover, when heat energy transfers from one medium to another, the principle of energy conservation must also be applied at the interface between the two media, such as the mentioned cooling process of a steel ingot in an oil bath, at the interface between the solid and the liquid, whether the heat transfer process is steady or unsteady, we believe that the heat transferred from the solid to the liquid and the heat absorbed by the liquid from the solid are equal at any given time. The concepts of energy balance and thermal balance mentioned in middle school physics are actually simple terms for the First Law of Thermodynamics.

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