Chapter 1. Review of Thermodynamics and Heat Transfer
pedro badillo
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This chapter provides a comprehensive review of the principles of thermodynamics and heat transfer, focusing on the laws governing energy transfers and the classification of systems. It discusses the first and second laws of thermodynamics, emphasizing the conservation of energy and the concept of entropy. The chapter also covers practical applications, including calculations related to energy balances and psychrometric analysis for HVAC systems.
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Chapter 1 INTRODUCTION AND BASIC CONCEPTS Thermodynamics and Heat Transfer
1-1C Thermodynamics deals with the amount of heat transfer as a system undergoes a process from one equilibrium state to another. Heat transfer, on the other hand, deals with the rate of heat transfer as well as the temperature distribution within the system at a specified time. 1-2C (a) The driving force for heat transfer is the temperature difference. (b) The driving force for electric current flow is the electric potential difference (voltage). (a) The driving force for fluid flow is the pressure difference. 1-3C The caloric theory is based on the assumption that heat is a fluid-like substance called the "caloric" which is a massless, colorless, odorless substance. It was abandoned in the middle of the nineteenth century after it was shown that there is no such thing as the caloric. 1-4C The rating problems deal with the determination of the heat transfer rate for an existing system at a specified temperature difference. The sizing problems deal with the determination of the size of a system in order to transfer heat at a specified rate for a specified temperature difference. 1-5C The experimental approach (testing and taking measurements) has the advantage of dealing with the actual physical system, and getting a physical value within the limits of experimental error. However, this approach is expensive, time consuming, and often impractical. The analytical approach (analysis or calculations) has the advantage that it is fast and inexpensive, but the results obtained are subject to the accuracy of the assumptions and idealizations made in the analysis. 1-6C Modeling makes it possible to predict the course of an event before it actually occurs, or to study various aspects of an event mathematically without actually running expensive and time-consuming experiments. When preparing a mathematical model, all the variables that affect the phenomena are identified, reasonable assumptions and approximations are made, and the interdependence of these variables are studied. The relevant physical laws and principles are invoked, and the problem is formulated mathematically. Finally, the problem is solved using an appropriate approach, and the results are interpreted. 1-7C The right choice between a crude and complex model is usually the simplest model which yields adequate results. Preparing very accurate but complex models is not necessarily a better choice since such models are not much use to an analyst if they are very difficult and time consuming to solve. At the minimum, the model should reflect the essential features of the physical problem it represents. PROPRIETARY MATERIAL.
Chapter 2 BASIC CONCEPTS OF THERMODYNAMICS Systems and Properties
2-1C The radiator should be analyzed as an open system since mass is crossing the boundaries of the system. 2-2C A can of soft drink should be analyzed as a closed system since no mass is crossing the boundaries of the system. 2-3C Intensive properties do not depend on the size (extent) of the system but extensive properties do. State, Process, Forms of Energy 2-4C In electric heaters, electrical energy is converted to sensible internal energy. 2-5C The forms of energy involved are electrical energy and sensible internal energy. Electrical energy is converted to sensible internal energy, which is transferred to the water as heat. 2-6C The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame. The microscopic forms of energy, on the other hand, are those related to the molecular structure of a system and the degree of the molecular activity, and are independent of outside reference frames. 2-7C The sum of all forms of the energy a system possesses is called total energy. In the absence of magnetic, electrical and surface tension effects, the total energy of a system consists of the kinetic, potential, and internal energies. 2-8C The internal energy of a system is made up of sensible, latent, chemical and nuclear energies. The sensible internal energy is due to translational, rotational, and vibrational effects. 2-9C Thermal energy is the sensible and latent forms of internal energy, and it is referred to as heat in daily life. 2-10C For a system to be in thermodynamic equilibrium, the temperature has to be the same throughout but the pressure does not. However, there should be no unbalanced pressure forces present. The increasing pressure with depth in a fluid, for example, should be balanced by increasing weight. 2-11C A process during which a system remains almost in equilibrium at all times is called a quasi-equilibrium process. Many engineering processes can be approximated as being quasi-equilibrium. The work output of a device is maximum and the work input to a device is minimum when quasi-equilibrium processes are used instead of nonquasi-equilibrium processes. 2-12C A process during which the temperature remains constant is called isothermal; a process during which the pressure remains constant is called isobaric; and a process during which the volume remains constant is called isochoric. 2-13C The state of a simple compressible system is completely specified by two independent, intensive properties. 2-14C Yes, because temperature and pressure are two independent properties and the air in an isolated room is a simple compressible system. 2-15C A process is said to be steady-flow if it involves no changes with time anywhere within the system or at the system boundaries.
BASICS OF HEAT TRANSFER
While teaching heat transfer, one of the first questions students commonly ask is the difference between heat and temperature. Another common question concerns the difference between the subjects of heat transfer and thermodynamics. Let me begin this chapter by trying to address these two questions. 1.1 Difference between heat and temperature In heat transfer problems, we often interchangeably use the terms heat and temperature. Actually, there is a distinct difference between the two. Temperature is a measure of the amount of energy possessed by the molecules of a substance. It manifests itself as a degree of hotness, and can be used to predict the direction of heat transfer. The usual symbol for temperature is T. The scales for measuring temperature in SI units are the Celsius and Kelvin temperature scales. Heat, on the other hand, is energy in transit. Spontaneously, heat flows from a hotter body to a colder one. The usual symbol for heat is Q. In the SI system, common units for measuring heat are the Joule and calorie.
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Heat transfer is a mature science, and so is thermodynamics. They are almost 200 years old having developed largely independently until the 1980s. Maturity comes from the usefulness and success of the thermal sciences. This review uses the thermodynamics of heat transfer to focus on aspects that are usually not discussed in physics: performance, purpose, function, objective and direction of evolutionary design. The article illustrates the unity of the thermal sciences discipline (heat transfer +thermodynamics + constructal law), and uses the opportunity to correct a few recent interpretations of the thermodynamics of heat transfer regarding dissipative engines and energy storage.
Module -5: Heat, Temperature and First Law of Thermodynamics
Thermodynamics: Thermodynamics (Greek: thermos = heat and dynamic = change) is the study of the conversion of energy between heat and other forms, mechanical in particular. All those problems that are related to the inter-conversion of heat energy and work done are studied in thermodynamics. In thermodynamics, we discuss different cycles such as Carnot cycle, Rankine cycle, Otto cycle, diesel cycle, refrigerator, compressors, turbines and air conditioners. Thermal equilibrium and Temperature: The central concept of thermodynamics is temperature. Temperature is familiar to us all as the measure of the hotness or coldness of objects. We shall learn afterwards that temperature is a measure of the average internal molecular kinetic energy of an object. It is observed that a higher temperature object which is in contact with a lower temperature object will transfer heat to the lower temperature object. The objects will approach the same temperature, and in the absence of loss to other objects, they will then maintain a constant temperature. They are then said to be in thermal equilibrium. Thermal equilibrium is the subject of the Zeroth Law of Thermodynamics. Temperature and Heat: If you take a can of cola from the refrigerator and leave it on the kitchen table, its temperature will rise-rapidly at first but then more slowly – until the temperature of the cola equals that of the room(the two are then in thermal equilibrium). In generalizing this situation, we describe the cola or coffee as a system (with temperature T S) and the relevant part of the kitchen as the environment (with temperature T E) of that system. Our observation is that if T S is not equal to T E , then T S will change until the two temperatures are equal and thus thermal equilibrium is reached. Such a change in temperature is due to the transfer of energy between the thermal energy of the system and the system's environment. It may be mentioned that thermal energy is an internal energy that consists of the kinetic and potential energies associated with the random motions of the atoms , molecules and other microscopic bodies within an object. The transferred energy is called heat and is symbolized Q. Heat is positive when energy is transferred to a system's thermal energy from its environment (we say that heat is absorbed). Heat is negative when energy is transferred from a system's thermal energy to to its environment (we say that heat is released or lost). We are then led to this definition of heat: " Heat is the energy that is transferred between a system and its environment because of a temperature difference that exists between them. " Recall that energy can also transferred between a system and its environment as work W via a force acting on a system. Heat and work, unlike temperature, pressure, and volume, are not intrinsic properties of a system. They have meaning only as they describe the transfer of energy into or out of a system. Let us now look into the Molecular Theory of Matter for an explanation of heat and temperature. Molecular Theory of Matter states that matter is made up of tiny particles called molecules. These particles are in constant motion within the bounds of the material. Since the relationship between kinetic energy of an object and its velocity is: KE = ½ mv 2 , which means that the more energy an object has, the faster it is traveling (or vice versa). Thus, when you provide extra energy to an object, you cause its molecules to speed up. Those molecules, in turn, can cause other molecules to speed up. The sum effect of the speed or energy of these molecules is PAGE 14
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The First Law of Thermodynamics represents the principle of energy conservation applied to the interaction between different macroscopic systems. The traditional mathematical description of the First Law (e.g. dU=TdS-PdV) is rather simplistic and lack universal validity, as it is only valid when several implicit assumptions are met. For example, it only considers mechanical work done associated with a change in volume of a system, but completely neglects other types of work. On the other hand, it employs the concept of entropy which is not only ambiguous but also implies only heat associated with a temperature difference, neglecting other types of heat transfer that may take place at mesoscopic and/or microscopic levels. In addition, it does not consider mass transfer effects. In the previous report of this series, a more general representation of the First Law is obtained considering different conditions and different types of interactions between the systems. In this report, the expression previously obtained is applied to different representative examples, involving macroscopic systems with no volume change, gas systems with volume change, and even a case where mass transfer between the systems takes place.
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The First Law of Thermodynamics is the Principle of Conservation of Energy applied to the interaction between Systems. Such interaction is partially observed at a macroscopic scale, in the form of Work. The remaining interaction, taking place at the microscopic scale and not observed as macroscopic work, is denoted as Heat. Thus, the change in energy of a system can be interpreted as the sum of energies transferred in the form of (macroscopic) Work and (microscopic) Heat. However, there are different types of heat. The most common type of heat is proportional to the temperature difference between the systems, but there are other types which are independent of the systems temperatures. To avoid the incorrect use of the First Law, it is important to clearly understand the concepts of Heat and Work. In the first part of these series, these fundamental concepts are discussed in detail, and a general formulation of the First Law is presented. In the second part of the series, this general formulation is applied to a wide variety of representative interacting systems.
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