Practical Handbook of Thermal Fluid Science

1.1. INTRODUCTION TO THERMODYNAMICS, FLUID FLOW, AND HEAT TRANSFER 

Thermodynamics, fluid flow, and heat transfer not only play an important role in science and engineering but also in everyday life. For example, gasoline vehicles operate on the Otto cycle, residential air-conditioners are developed based on the vapor-compression refrigeration cycle, steam turbines are the central unit in traditional power plants, and flow meters and pipe flows are inherent in both industrial and residential applications.

2.1 INTRODUCTION 

In an experiment, one major task is to conduct measurements for collecting data. A large number of measurement data can be the direct outcome of experimental work. In general, the more data the better. Statistics is a popular valuable tool for experimentalists to conduct data analysis and eventually draw conclusions from data processing. The mean of a data sample is usually used as the final result for a measurement. The standard deviation of the sample measures the confidence of the final result and is usually used as additional information. In addition, each measurement needs to be independent so that the data set is equally weighed. Statistics may be used in experimental design and plan. Indeed, before conducting an experiment, several aspects need to be considered in preparation, including the selection of apparatus, relevant mathematical correlations, and uncertainty estimate of the final results. Selecting the proper apparatus is essential to any experimental work. For example, temperature measurement requires thermometers. There are several types of thermometers with various ranges and resolutions. A high-resolution apparatus is usually expensive and requires training before use. However, low-resolution apparatuses usually lead to a large standard deviation or uncertainty in the final result. In engineering applications, uncertainty needs to be within tolerance to avoid component mismatch or design failures. Understanding how the apparatus’ resolution is related to the measurement error or uncertainty and how the error or uncertainty propagates to the final value is thus fundamentally important for experimental design, which will be introduced in this chapter.

3.1. INTRODUCTION

Heat transfer is a subject that deals with the generation, use, conversion, and exchange of thermal energy between a system and its surroundings. At the molecular level, we can visualize thermal energy by the motion of particles and quantify it by the overall kinetic energy of all the particles. Heat transfer can then be explained as an exchange of the kinetic energy among particles via collisions or interactions. In a gas, the particles move faster, on average, under higher temperatures and transfer their kinetic energy to particles at lower temperatures through collisions. In a solid, the particles vibrate faster under higher temperatures and transfer their kinetic energy to particles at lower temperatures through molecular interactions. 

4.1. INTRODUCTION

Power plants or generating stations are facilities for generating electric power, which has become essential to our present lifestyle. Electric power supports many of our daily activities, including lighting, air circulation, HVAC, TV watching, cell phones, computers, elevators, etc. The generated power is delivered using complex systems of wires, towers, underground stations, transformers, control stations, etc., to consumer sites such as homes, apartments, stores, industry, and schools. These interconnected systems are known as grids.

5.1 INTRODUCTION

Flow and volume measurement of incompressible fluids are prevalent in many engineering processes. Flows in a pipe network, also called pipe flow, are widely encountered in industry and our daily activities. For example, water is delivered to homes via the pipelines of a water distribution infrastructure. In power plants, numerous pipe flows are used to transport liquid, vapor, and two-phase flows: steam produced in boilers is transported in pipes to steam turbines for energy conversion. Boiler feed water (BFW) is pumped to a boiler via a BFW pipe. 

6.1 INTRODUCTION

In thermodynamics, several major devices related to energy conversion are introduced, including the internal combustion engines (ICE), steam turbine, boiler, heat exchanger, and heat pump. Thermal efficiency is a dimensionless performance measure of these devices. In general, thermal efficiency is the fraction of energy addition in the form of heat or thermal energy converted to useful output, as shown in Fig. (6.1), given as a percentage value. The nominally Otto-cycle ICE in automobiles can reach about 30% efficiency at the flywheel. Rankine-cycle steam turbine thermal efficiency can be as high as 41%. In heat pump cycles where heat rejection in the high-temperature side is the useful output, the efficiency is usually defined as the ratio of the rejected heat to the compressor work input, commonly called the coefficient of performance (COP). Refrigeration moves heat from a confined space and dissipates it in the atmosphere. Heat pumps move heat from one spot (often from the atmosphere or underground) to a home of business. Residential refrigerators and air conditioners are based on a vapor-compression mechanical refrigeration cycle. The former generally has a COP over 1 in practice, while the latter may have a COP over 3 to 5. The COP of heat pumps is generally higher than that of their refrigerator counterparts. 

7.1 INTRODUCTION

Wind tunnels provide controlled air streams to study aerodynamic and fluid flow phenomena. They are frequently used to replicate the actions of an object flying or moving in an air stream, such as airplanes or vehicles. Table 7.1 lists several types of wind tunnels. Most wind tunnels are designed to have a uniform velocity profile in the test section at a low turbulence level. The testing object is placed in the test section for flow visualization and quantitative measurements such as lift and drag forces. To create an air flow, electric fans are usually used to blow air in or out of the tunnel. In large wind tunnels for testing real-size airplanes, rockets, or cars, powerful fans are required to generate a wind speed comparable with real conditions.

8.1. INTRODUCTION

Otto and Diesel cycle engines play an important role in our transportation and energy use. They are typically reciprocating heat engines that convert the thermal energy from fuel combustion to mechanical energy in the form of piston movement. The mechanical energy further drives a vehicle over a distance. The Otto and diesel cycle engines are the most common engine in passenger cars, light trucks, and other applications where small (10 Hp) to medium power (500 Hp) is required. Some large turbo supercharged radial aircraft engines reach 5,000 Hp. Applications of small power, such as lawnmowers and hand-held devices like trimmers and chain saws, require a level of 100-1,000 W power. Typical values of their thermal efficiency are 30-35% for Otto cycle engines and 30-40% for Diesel engines. Small utility-type engines may have ~20% efficiency due to simple design and control. While the basic principles of these reciprocating engines have not changed significantly since invention, advances in fuel induction, ignition systems, and exhaust emission controls have improved economy and performance and reduced pollution.

9.1. INTRODUCTION

Refrigerators and air conditioners are popular and important devices in our daily lives. The former provides a low-temperature environment to store food and drink, while the latter reduces the indoor temperature to a comfortable level. In nature, we observe that heat flows from a higher temperature toward a lower temperature. To reverse the heat flow from a low to high temperature, mechanical work needs to be added to enable mechanical refrigeration, by which a low-temperature environment is created for food saving or air conditioning. The most important feature of mechanical refrigeration is its high efficiency, usually called the coefficient of performance (COP): the COP of residential refrigerators is usually above one, and air conditioners can achieve even five. While other refrigeration methods have been developed, based on different physical principles, such as vortex tubes and thermoelectric coolers, their efficiencies, in general, are much less than one, and we use them for special applications where their other characteristics make them attractive.

10.1. OVERVIEW

To formally document a laboratory experiment, a written technical report is prepared. However, it should be noted that detailed lab notes compiled during the experiment are essential, so they can be used later in the full written report. The overall goal for any laboratory report is to summarize an experiment efficiently for an intended audience. Therefore, knowing who will read the laboratory report (e.g. researchers in a similar field in a technical journal, business leaders in a company as an internal white paper, or the general public for mass consumption) will dictate the style, the tone and the level of technical jargon needed in the written work. After identifying the target audience, the lab report should explain the purpose of the experiment, the research questions answered, the experimental procedures, the data observed, the analysis method and the results.