**3. Other complementary sources**

also developed to accurately estimate the mixing time in a DCHE. L2‐star discrepancy (UC‐ LD)–based uniformity coefficient (UC) method presented for assessing the uniformity and mixing time of bubbles behind the viewing windows in a DCHE was found to be effective. Furthermore, they have shown some advantages including rotation invariance (reflection invariance), permutation invariance, and the ability to measure projection uniformity. Their experimental results revealed that UC‐CD gives more sensitive performance than uniformity coefficient based on wrap‐around discrepancy (UC‐WD) and thus, the UC‐CD method is more appropriate for industry. Nonetheless, authors believe that the complexity of the bubble swarm patterns can be reduced, their mechanisms can be clarified, and the heat transfer per‐

Author Jaremkiewicz described a method for measuring the transient temperature of the flowing fluid in heat exchangers based on time‐temperature changes of the thermometer, which was considered as an inertial system of first and second order. To reduce the influence of random errors in the temperature measurement, the local polynomial approximation based on nine points was used. As a result, the first and second derivatives of a temperature that indicates how the temperature of the thermometer varies over time were determined very accurately. Then, the time constant was defined as a function of fluid velocity for sheathed thermocouples with different diameters. The applicability of their method was validated with real experimental data. They inferred that this method is mostly suitable for measuring the transient temperature of gases in the exchangers, and it can also be used for the online moni‐

A detailed description and comprehensive review of the transient effectiveness methodology for heat exchanger analysis are reported by authors Tianyi and co‐workers. Three important applications for transient effectiveness methodology are reported that include (i) character‐ ization of heat exchanger dynamic behaviors, (ii) characterization of the transient response of closed coupled cooling/heating systems with multiple heat exchanger units, and (iii) devel‐ opment of compact transient heat exchanger models. For studying heat exchangers' tran‐ sient characteristics, authors introduced novel transient effectiveness methodologies, which were found very useful for thermal dynamic characterization of heat exchangers as well as development of compact CFD transient models. The transient effectiveness curves capture the transient response and the impact of thermal capacitance of each heat exchanger unit. The two CFD compact modeling methodologies (i.e., a full transient effectiveness methodology and a partial transient effectiveness methodology) were also developed and validated in their study. These models were found to be accurate and fast and can be integrated into large‐scale

Salcedo and co‐authors carried out numerical simulations to study the unsteady laminar flow and mixed convection heat transfer characteristics around two identical isothermal semicyl‐ inders arranged in tandem and confined in a channel. Simulations were performed using the control‐volume method on a nonuniform orthogonal Cartesian grid. The immersed‐bound‐ ary method was employed to identify the semicylinders inside the channel. The variation of the mean and instantaneous nondimensional velocity, vorticity and temperature distribu‐ tions with Richardson number were presented along with the nondimensional oscillation

formance in a DCHE can be elucidated.

4 Heat Exchangers– Design, Experiment and Simulation

toring of fluid temperature change with time.

models as well.

Many areas and features of heat exchangers that are not covered or not detailed in the above‐ introduced contributions can be found in many other available reference sources. For exam‐ ple, among many books and sources, very popular and useful sources of knowledge on the design of heat exchangers and application of related theories and modeling of designs are the two popular and comprehensive books by Thulukkanam [4] and Shah and Sekulic [2]. The second edition of heat exchanger design handbook by Thulukkanam [4] also provides current advances in heat exchanger technology particularly design and modes of operation. The book by Rao and Savsani [5] describes research works that explore different advanced optimization techniques. It also includes algorithms and computer codes for various advanced optimiza‐ tion techniques that can be useful to the readers. In a study, Lee et al. [6] reported numerical methodologies of the fluids flow and heat transfer analysis in various types of heat exchang‐ ers. They also proposed an analysis method for the conjugate heat transfer between hot flow‐ separating plate and cold flow of a plate heat exchanger. More detail on recent development on the numerical simulations of the heat exchangers and advances in numerical heat transfer can also be found in a very recent book edited by Minkowycz and other co‐editors [7].

Over the past few decades, a large number of research efforts have been devoted to enhance the heat transfer performance of heat exchangers by various methods, which have been dis‐ cussed in a recent compressive review on double pipe heat exchangers by Omidi et al. [8]. Generally, the heat transfer enhancement methods are classified as active method, passive method, and compound method. While active and compound methods are less popular and used, passive methods are widely employed to improve the heat transfer of heat exchangers. Among passive methods, extended surface (e.g., fins), twisted tape insert, and wired coils are commonly used particularly in double pipe heat exchangers [8]. The book edition on the heat transfer enhancement of heat exchangers by Kakac et al. [9] is a good source of knowledge and references.

For compact heat exchangers, the second edition of a popular book by Hesselgreaves et al. [10] is a complete reference, which compiles all aspects of theory, design rules, operational issues, and the most recent developments and technological advancements in these heat exchangers. A comprehensive review on performance of compact heat exchangers was also reported by Li et al. [11]. Among other books and works, the book published by Sundén and Faghri [12] is a good source for numerical simulations in compact heat exchangers.

In recent years, there are large numbers of research works reported on improving the design and performance of heat exchangers to meet the cooling demands of modern devices and industries. This book provides topic‐wise detailed and state‐of‐the‐art information on the development of design, experiments, and numerical simulations on heat exchangers. It is noted that various advanced features and applications of heat exchangers are provided in our other volume of this book [13].
