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About this book

Encompassing both practical applications and recent research developments, this book takes the reader from fundamental physics, through cutting-edge new designs of ejectors for refrigeration. The authors’ unique vision marries successful design, system optimization, and operation experience with insights on the application of cutting-edge Computational Fluid Dynamics (CFD) models. This robust treatment leads the way forward in developing improved ejector technologies. The book covers ejectors used for heat powered refrigeration and for expansion work recovery in compression refrigerators, with special emphasis on two-phase flows of “natural” fluids within the ejector, i.e. steam and carbon dioxide. It features worked examples, detailed research results, and analysis tools.

Table of Contents

Frontmatter

Chapter 1. Introduction

Ejectors are part of a vast family of equipment that may collectively be named “jet devices.” These devices may convey gases, liquids, or even solids (fine particles), but all share two main features:
  • Have no moving parts
  • Use a relatively high-pressure stream to convey a low-pressure stream toward an intermediate pressure receiver
Jet devices have a long history and may be used for a wide range of applications. They are produced on custom specification or in small series by a relatively small number of companies in Europe and the USA. Their use as fluid-driven compressors in refrigeration systems is known since the early twentieth century but has received increased attention from the scientific community in the last two decades, due to the potential as a simple and effective option for heat-powered cooling. When compared to absorption or adsorption cooling, the use of ejectors is competitive only in niche markets, but the situation could change if significant performance increase was registered.
This chapter gives a brief explanation of the ejector working principle and shows how a refrigeration system can be built on it. A short history of the ejector follows. Finally, some applications within and outside of the refrigeration field are reviewed.
Giuseppe Grazzini, Adriano Milazzo, Federico Mazzelli

Chapter 2. Physics of the Ejectors

The global behavior of the ejector results from a combination of complex flow features including shock trains, turbulent mixing layers bounded by wall regions, shock-induced separations, boundary layers subject to adverse pressure gradients, non-equilibrium phase change, etc. It is because of this complexity that ejector design and performance have thus far been difficult to characterize and optimize.
In what follows, a brief review of the main physical phenomena occurring in a supersonic ejector will be described, and some relations will be introduced. These findings will be further exploited in Chap. 3 when some design tools will be described.
Giuseppe Grazzini, Adriano Milazzo, Federico Mazzelli

Chapter 3. Ejector Design

Ejector design may be performed at various levels of complexity. In many cases, ejectors are designed in a rather empirical way, and the only elements of the ejector geometry that receive a calculation effort are the main flow sections. Other elements, like the mixing zone length or the angle of the secondary flow inlet, are left to the experience of the designer. The effect of other details, like the presence of fillets between conical and cylindrical parts, is also neglected.
Indeed, a detailed analysis of the influence of geometrical details on the supersonic flow is not feasible with analytical tools. The only way to get a complete picture of the flow field is to analyze the ejector by a Computational Fluid Dynamics (CFD) approach. However, it must be stressed that CFD is not a design tool. The complete geometry of the ejector must be known in advance before any CFD analysis is attempted. Eventually, the design may be modified in order to mitigate any problem that could be revealed by the CFD results, but there is no way to state that all possible design options have been explored.
Probably, a hybrid approach combining a first scrutiny of possible configurations and a subsequent CFD analysis could be an answer. In the following sections, a few simple design tools will be presented, while the potential offered by up-to-date CFD techniques will be resumed in the following chapter.
Giuseppe Grazzini, Adriano Milazzo, Federico Mazzelli

Chapter 4. Ejector CFD Modeling

The prediction of the supersonic ejector dynamics implies the accurate description of all the complex flow features discussed in Chap. 2. Unfortunately, the theoretical modeling approach necessitates a number of simplifying assumptions and empirical constants that introduce significant uncertainty and reduce the capability of capturing a number of relevant flow features. In this respect, computational fluid dynamics may represent a tool to overcome these difficulties and analyze the flow details of arbitrary ejector geometries.
In this chapter, we will try to overview some of the main features that should be considered in order to set up a reliable CFD scheme for ejector flow studies.
Giuseppe Grazzini, Adriano Milazzo, Federico Mazzelli

Chapter 5. Experimental Activity

The importance of an extensive and accurate experimental activity has been recalled many times in this book, for example, in connection with the implementation of reliable numerical tools. In this chapter, we will present some experimental results related to ejectors, refrigeration systems containing ejectors, or physical processes which are relevant for ejectors development.
Unfortunately, the huge amount of data routinely collected by the industries that produce ejectors (for a few names, see Chap. 1) are unavailable or confidential and hence cannot be included here. Therefore, we will deal only with experimental data coming from laboratory prototypes built for research purpose. A caveat on these results concerns the size: in most cases prototypes tend to be small. In principle, this should pose a penalty on the measured performance. Even without citing real data from industry, we may at least certify that, invariably, steam ejectors used for industrial use show a clear decreasing trend of specific consumption of motive steam as size increases.
Giuseppe Grazzini, Adriano Milazzo, Federico Mazzelli

Chapter 6. Concluding Remarks

Many issues concerning ejectors and ejector chillers have been left out of this book. Some of them were cut on purpose. For example, our direct experience tells us that the noise produced by an ejector varies in a quite unpredictable fashion, and we decided to postpone any analysis on this point. This doesn’t mean that the noise is not a relevant problem. Actually, it could prevent the diffusion of ejectors in some applications.
Giuseppe Grazzini, Adriano Milazzo, Federico Mazzelli

Backmatter

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