1. Introduction

Heat exchangers are a fundamental tool in the thermal engineering fields, such as refrigeration, power systems cooling, electronics cooling, and air conditioning. Enhanced heat transfer (EHT) techniques provide:


© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.


The development of new kinds of heat exchangers is ever in progress, both for seeking to reduce the volume of the heat exchanger and to enhance the performances in terms of pressure drop or heat transfer capacity. Clearly, studies in these fields also consider the methods of design, the numerical tools to investigate the performances of a heat exchanger, and also the times that such a tool requires for a reasonable design. Rohsenow et al. [1], Kays and London [2], and Bejan and Lorente and Bejan et al. [3, 4] have largely investigated these issues. Yang et al. [5], You et al. [6], Caputo et al. [7] have given valid examples of optimization and design techniques related to heat exchangers. The process of design of aviation heat exchangers occurs with an increase in the power stress and servicing energy systems that must be cooled. At moderate flight speeds, the systems of aviation engines, lubricants, and different equipment and optional energy systems can be cooled with heat exchangers using free air or other carriers. The design should provide both the satisfaction of such constructional requirements as the compensation of different thermal extensions of heat exchange surfaces and the body, the heat exchanger compactness and the possibility of assembling heat exchangers with the use of the existing equipment in a reliable manner. These requirements are often contradictory. That is why, when designing heat exchangers, it is very important to determine the optimality criteria in each particular case. Heat exchangers may be classified according to the following main criteria:


First of all, heat exchangers can be distinguished considering either an intermediate storage or a direct transfer of heat (see also Figure 1). In a regenerator, the heat coming from the primary medium is first stored in a medium playing as a reservoir and then regenerated from that mass by the secondary medium. The reservoir material can be the one of the ducts or a porous medium through which the primary and the secondary flows are driven.

In a recuperator both the media are separated by a wall through which heat is transferred directly. Further an intermediate medium can carry heat from the primary medium in a first heat exchanger to the secondary medium in a second heat exchanger.

The regenerator needs unfortunately an intermediate storage material that is a good heat conductor for the storage function. This generates high heat conduction levels in the flow direction producing a substantial loss of effectiveness (<<90%). In a recuperator, instead, the only fundamental loss is the heat conduction through the wall in the flow direction, which however can be reduced to less than a per mille by using materials with low thermal conductivity such as plastics. If we want to achieve the required effectiveness, only a recuperator can be used.

Figure 1. HE classification based on flow arrangements [2].

• Compound enhancement—use of two or more methods

• Type of application (two-fluid HE vs. single fluid HE)

exchangers may be classified according to the following main criteria:

• Geometry of construction: tubes, plates, and extended surfaces

medium through which the primary and the secondary flows are driven.

heat exchanger to the secondary medium in a second heat exchanger.

• Transfer processes: direct contact and indirect contact

• Heat transfer mechanisms: single phase and two phase • Flow arrangements: parallel, counter, and cross-flows

• Recuperators and regenerators

• Single or two phase flow, free or forced convection, laminar, or turbulent flow

The development of new kinds of heat exchangers is ever in progress, both for seeking to reduce the volume of the heat exchanger and to enhance the performances in terms of pressure drop or heat transfer capacity. Clearly, studies in these fields also consider the methods of design, the numerical tools to investigate the performances of a heat exchanger, and also the times that such a tool requires for a reasonable design. Rohsenow et al. [1], Kays and London [2], and Bejan and Lorente and Bejan et al. [3, 4] have largely investigated these issues. Yang et al. [5], You et al. [6], Caputo et al. [7] have given valid examples of optimization and design techniques related to heat exchangers. The process of design of aviation heat exchangers occurs with an increase in the power stress and servicing energy systems that must be cooled. At moderate flight speeds, the systems of aviation engines, lubricants, and different equipment and optional energy systems can be cooled with heat exchangers using free air or other carriers. The design should provide both the satisfaction of such constructional requirements as the compensation of different thermal extensions of heat exchange surfaces and the body, the heat exchanger compactness and the possibility of assembling heat exchangers with the use of the existing equipment in a reliable manner. These requirements are often contradictory. That is why, when designing heat exchangers, it is very important to determine the optimality criteria in each particular case. Heat

First of all, heat exchangers can be distinguished considering either an intermediate storage or a direct transfer of heat (see also Figure 1). In a regenerator, the heat coming from the primary medium is first stored in a medium playing as a reservoir and then regenerated from that mass by the secondary medium. The reservoir material can be the one of the ducts or a porous

In a recuperator both the media are separated by a wall through which heat is transferred directly. Further an intermediate medium can carry heat from the primary medium in a first

The regenerator needs unfortunately an intermediate storage material that is a good heat conductor for the storage function. This generates high heat conduction levels in the flow direction producing a substantial loss of effectiveness (<<90%). In a recuperator, instead, the only fundamental loss is the heat conduction through the wall in the flow direction, which however can be

• Mode of heat transfer and flow regime

150 Heat Exchangers– Advanced Features and Applications

When classified according to the transfer process involved, heat exchangers can be a directcontact type or an indirect-contact type. The most common type employed is the indirectcontact heat exchanger. In direct-contact exchangers, heat transfer between fluids occurs through direct interaction, ideally without mixing or leakage. The fluids come into direct contact, exchange heat, and are then separated. Advantages of these include a low cost and a lack of fouling (absence of transfer surface), the major drawback being the fact that applications are limited to situations in which direct contact of fluid streams is viable. They are particularly useful in applications involving mass transfer in addition to heat transfer, obtained through fluid phase change; heat transfer involving only sensible heat is rare for this type of exchanger. Due to the increased enthalpy, latent heat transfer is responsible for the greater portion of energy transferred in this process.

The selection of the flow arrangement influences the overall performance of the heat exchanger and is influenced by available pressure drops, permissible velocities, and thermal stresses, flow paths, required temperature levels, amongst others. The following diagram establishes a classification of heat exchangers based on available fluid stream flow types.

In the single-pass setup, fluids enter the exchanger and come into thermal contact once before proceeding to exit the device. Amongst single-pass exchangers, the counter-flow configuration is the most efficient, producing the highest temperature change in each fluid. In these, the fluids flow parallel to each other, but in opposing directions. The parallel flow type, in which fluid streams enter together at the end, is the least efficient of the single-pass devices. A more unique flow arrangement is the cross-flow, fluids flowing in normal directions to each other, most common in extended transfer surfaces, leading to two-dimensional temperature variations.

As for the multipass arrangement, the fluids essentially transit through the heat exchanger on more than one occasion, using two or more passages for each fluid, in order to achieve this. Multipass design is particularly useful in situations requiring extreme exchanger length or low fluid velocities. A great advantage of this type of exchangers is the increase in overall efficiency that results from increasing the effectiveness in each pass, resulting the greater thermal transfer load. As would be expected, at some stage of the fluid trajectory, reversal must take place. This task is typically accomplished by the introduction of U-bends in the fluid passages, which dismisses the requirement of additional external power sources. The multipass configuration can be further classified in order of construction type: extended surface (in which fluid cross-flow is the typical arrangement), shell and tube (common domain for U-bend employment), and plate exchangers [8–10].

To save the consumption of fuel and for efficient cooling, one needs to keep the heat exchangers clean for smooth functioning. An aircraft for the transport of passengers has often a pleasant cabin environment for a comfortable journey. Heat exchangers are commonly used to cool hydraulics, rammed air, auxiliary power units, gearboxes, and many other components that consist of an aircraft. Although temperature is the main feature associated with liquid cooling, when heat exchanger services are used at high altitudes air density and pressure are additional features considered. In order to guarantee a sufficient airflow, heat exchanger's fan must be carefully selected based on the ambient pressure. At high altitudes, the density of air is drastically lower, so it takes more airflow to remove the same amount of heat. Liquid cooling can provide notably better performance than air cooling, along with a quieter behavior and not vulnerable to altitude. They also reduce weight and power consumption avoiding the need for large fans or the need for wide spacing for placing components. Heat exchangers, liquid cooled chassis, and cold plates are used to provide thermal solutions to cool aircraft fluids and electronic equipment. Also, air is significantly colder than at sea level on high altitudes. Novel compact heat exchanger (CHE) solutions are needed in aerospace environmental control, avionics, and engine oil cooling systems, see Figure 2.

Figure 2. Heat exchangers in a typical aircraft air conditioning (ACS) pack—a typical single-pass, cross-flow, plate fin heat exchanger.

Heat exchangers are compact generally when the heat transfer area per unit volume is >700 m2 /m3 . Nowadays, the compact heat exchangers are increasingly used in a large number of industries. The application determines the material of construction, fabrication, and development of the compact heat exchangers. Inside the compact heat exchangers, fluids interact with a much larger surface area which provides higher heat transfer rates and large effectiveness. These heat exchangers are particularly suitable for the aerospace applications due to their less weight, greater compactness, and higher performances due to the improved heat transfer surfaces. In aerospace industries, furthermore, attention is paid on size and weight without compromising on performance aspects, and these compact heat exchangers are principally utilized. A compact heat exchanger also includes thin plates and fins which are stacked together and are normally brazed or welded. The aircraft heat exchangers during their operation also can meet adverse ambient conditions [11].

The aircraft heat exchangers experience arduous and extreme working conditions during their operation. Hence, the mechanical integrity and endurance life of heat exchanger need to be estimated before leading to flight clearance. The compact heat exchanger (CHE) is characterized by a small volume and a high rate of energy exchange between two fluid streams by employing intricate flow passages. Thermohydraulic performances of compact heat exchangers are strongly depended upon the prediction of performance of various types of heat transfer surfaces, such as offset strip fins, wavy fins, rectangular fins, triangular fins, triangular, and rectangular perforated fins in terms of colburn "j" and fanning friction "f" factors. Earlier, these data could be generated only through a dedicated experimental test rigs.

Now, the numerical methods play a major role for analysis of compact plate-fin heat exchangers, which are cost effective and fast. The aerospace applications—microchannel heat exchangers have the following characteristics:

Figure 3. Examples of microchannels.

task is typically accomplished by the introduction of U-bends in the fluid passages, which dismisses the requirement of additional external power sources. The multipass configuration can be further classified in order of construction type: extended surface (in which fluid cross-flow is the typical arrangement), shell and tube (common domain for U-bend employment), and plate

To save the consumption of fuel and for efficient cooling, one needs to keep the heat exchangers clean for smooth functioning. An aircraft for the transport of passengers has often a pleasant cabin environment for a comfortable journey. Heat exchangers are commonly used to cool hydraulics, rammed air, auxiliary power units, gearboxes, and many other components that consist of an aircraft. Although temperature is the main feature associated with liquid cooling, when heat exchanger services are used at high altitudes air density and pressure are additional features considered. In order to guarantee a sufficient airflow, heat exchanger's fan must be carefully selected based on the ambient pressure. At high altitudes, the density of air is drastically lower, so it takes more airflow to remove the same amount of heat. Liquid cooling can provide notably better performance than air cooling, along with a quieter behavior and not vulnerable to altitude. They also reduce weight and power consumption avoiding the need for large fans or the need for wide spacing for placing components. Heat exchangers, liquid cooled chassis, and cold plates are used to provide thermal solutions to cool aircraft fluids and electronic equipment. Also, air is significantly colder than at sea level on high altitudes. Novel compact heat exchanger (CHE) solutions are needed in aerospace environmental control,

Heat exchangers are compact generally when the heat transfer area per unit volume is >700 m2

Nowadays, the compact heat exchangers are increasingly used in a large number of industries. The application determines the material of construction, fabrication, and development of the compact heat exchangers. Inside the compact heat exchangers, fluids interact with a much larger surface area which provides higher heat transfer rates and large effectiveness. These heat exchangers are particularly suitable for the aerospace applications due to their less weight, greater compactness, and higher performances due to the improved heat transfer surfaces. In aerospace industries, furthermore, attention is paid on size and weight without compromising

Figure 2. Heat exchangers in a typical aircraft air conditioning (ACS) pack—a typical single-pass, cross-flow, plate fin

/m3 .

exchangers [8–10].

152 Heat Exchangers– Advanced Features and Applications

heat exchanger.

avionics, and engine oil cooling systems, see Figure 2.


The features of the most commonly used heat exchangers in aviation are listed in Table 1. It shows that the materials most often used are aluminum, copper, and carbon steel, while the typical sizes range between 100 mm and 132 cm.


Table 1. Materials and commonly used fluids and sizes in aviation heat exchangers.
