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In this paper, a literature review of the recent research advances and applications of microchannel (from 1 μm to 1 mm) heat exchangers (μHXs) is presented. After a brief introduction of basic definitions, the advantages and limitations of μHXs are summarized with examples. Recent applications of these exchangers are reported. Although much in-depth research work still needs to be done, great potential has already been shown for large-scale applications of μHXs in process intensification of various industrial areas. Moreover, in order to solve the key issue of the link between the micro-scale and the macro-scale, multi-scale methodology such as fractal and constructal approaches for the design optimization of μHXs are introduced. Finally, the concept of constructal heat exchanger is presented. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines experimentally the effects of constructal distributors or collectors, built on a binary pattern of pores, on flow equidistribution in a multi-channel heat exchanger. Thermal performance and pressure drop have been determined with different assembly configurations of constructal distributors, conventional pyramid distributors and a mini crossflow heat exchanger (MCHE). Experimental results show that the integration of constructal distributors/collectors could homogenize the fluid flow distribution and consequently lead to a better thermal performance of the MCHE, but it also results in higher pressure drops. Among all tested assembly configurations, the configuration where the inlet is equipped with a conventional pyramid distributor, and the outlet is equipped with a constructal collector (Apec) shows a relatively better thermal performance as well as low pressure drops under experimental conditions considered. [Copyright &y& Elsevier]

Parallel micro or mini-channels are widely used in various devices of process and energy engineering including micro-reactors, compact heat exchangers and fuel cells. Nevertheless, the flow maldistribution due to the improper design of distributor/collector is usually observed, leading to globally poor performances of these devices. The objective of this study is to optimize the shape of the distributor/collector pipes so as to achieve a uniform flow distribution among an array of parallel mini-channels. A Z-type ladder fluid network with 10 mini-channels in parallel having square section is introduced and investigated. Two methods are used to optimize the shape of distributor/collector pipes: an optimized discrete stairway shape and a continuous tapered shape with an inclined angle varying from 0° to 30°. 3D-CFD simulations are carried out using the ANSYS FLUENT code. Numerical results obtained show that a relatively uniform flow distribution may be reached by the discrete stairway shape or by the continuous tapered shape distributor/collector under very low flow-rate conditions. Larger inclined angle or fewer channels in parallel are favorable for more uniform flow distribution under higher flow-rate conditions. Nevertheless the distributor and the collector pipes occupy a large volume so that the entire device is less compact. [ABSTRACT FROM AUTHOR]

Abstract: A multi-functional heat exchanger-reactor comprising arborescent (tree-like) distributors and collector, 16 mini-channels in parallel and T-mixers is introduced in this paper. Flow distribution property, pressure drop and heat exchange performance of proposed heat exchanger-reactor are tested and discussed. Firstly, flow distribution uniformity is characterized by CFD simulation and then qualitatively confirmed by visualization experiment. Results show that for total flowrates ranging from 5 mL s−1 to 20 mL s−1, good distribution uniformity is obtained, with maximum flowrate deviation less than 10%. Then, experiments of heat exchange between hot and cold water are carried out. High values of overall heat transfer coefficient ranging from 2000 to 5000 W m−2 °C−1 are obtained under our working conditions. The volumetric heat exchange capability (UA/V) is found to be around 200 kW m−3 °C−1, showing a high heat exchange capability with compact design. The roles of end-effect and non-established flow are discussed and are supposed to be responsible for efficient heat transfer. Finally a typical fast exothermic reaction, neutralization between acid and basic solutions, is carried out to test the thermal control capability of the studied heat exchanger-reactor. Results indicate that isothermal condition could be realized by circulating appropriate flowrate of coolant through the heat exchanger. The design of heat exchanger-reactor with arborescent distributor and collector makes possible the application of multi-channel systems. This paper introduces systematically the successful integration of heat exchanger-reactor and its performance evaluation. [Copyright &y& Elsevier]

Abstract: A mini reactor with 16 channels and with channel diameter of 1mm or 2mm is designed, fabricated and studied experimentally. Two fluids are divided into 16 channels through distributor, then contacted at T-mixers, and finally collected. Both distributor and collector are designed following the arborescent shape. Visualization with pH indicator bromothymol blue (BTB) is used to qualitatively observe the global mixing; while for the micromixing performance assessment, we used competitive iodide/iodate (Villermaux/Dushman) reactions. Segregation index and micromixing time, with respect to energy dissipation rate are used to characterize micromixing performance. Visualization with pH indicator shows that complete global mixing at the collector by liquid impingement is obtained under our tested conditions. Iodide/iodate reaction shows a rapid mixing at molecular level as the flowrate increases and impingement being enhanced. Micromixing time estimated by IEM (Interaction by Exchange with the Mean) model is compared with other micro-structured mixers in the literature. Comparable mixing performance is obtained using our millimetric reactor with that of micro-structured designs. The design of multi-channel reactor with arborescent distributor and collector makes possible the numbering-up of multiple channels. Process intensification is highlighted by high throughput and compact size, thus may be attractive for industrial application. [Copyright &y& Elsevier]

Abstract: This paper introduces an optimization method for improving thermoelectric heat pump performance by operating condition management of the thermoelectric modules (TEMs) and design optimization of the heat exchangers linked to the TEMs. The device studied, corresponding to an original configuration of the thermoelectric heat pump, comprises two commercial thermoelectric modules and two mini-channel heat sinks through which water flows, in contact with both sides of the TEMs. The objective function is the maximization of the device’s coefficient of performance (COP), including the electrical and mechanical consumption of the thermoelectric modules and the circulating auxiliaries. First, the optimization variables are the number and the diameter of mini-channels, and the mass flows for both heat sinks (hot and cold sides). The results show that similar results are obtained by minimization of the entropy generation in the device. Finally, the hot thermal power demand is included in the optimization variables for complete optimization of the device. The results of full optimization converge with those obtained with the previous partial optimization. [Copyright &y& Elsevier]

Abstract: With a view towards better efficiency in renewable energy utilisation, particularly solar energy, the authors study a long-term solar thermal energy storage based on water absorption by a lithium bromide aqueous solution. After a description of the process, the system dynamic simulation model is detailed and used to investigate the influence of certain parameters (heat exchanger size, solution flow rate, absorption percentage) and operating conditions (heat supply temperature to the building, crystallisation ratio, heat need) on the system performance (storage density, thermal efficiency, etc.). The analysis of simulations made for a low-consumption building in Chambéry shows that the solution flow rate is a critical parameter in the process performance. It also appears that crystallisation in the solution storage tank increases the storage density more than three times. The simulation results are used in the design of a prototype that is under experimentation for validation of the model. [Copyright &y& Elsevier]

Abstract: This paper describes an experimental study and the numerical model of a system that cools or warms an airflow by means of thermoelectric modules (TE modules). The device consists of four TE modules in contact with fins within a circulating airflow. The opposite sides of TE modules are in contact with water box heat exchangers. The experimental results are in good agreement with the theoretical thermoelectricity equations, and confirm the feasibility of cooling or heating air through TE modules with a good COP. The numerical model presented afterward is based on an electric analogy and calculates temperatures along the flow. The results are very close to those obtained during the experiment, thereby reasonably validating this model. [Copyright &y& Elsevier]

The critical importance of heat exchangers (HXs) on energy systems has been widely recognized, which can largely determine the overall efficiency. With the rise of the topology optimization (TO) and additive manufacturing techniques, it is anticipated that the TO may became a leading optimization technique in designing HX structures for the heat transfer intensification. Various algorithms for TO of HXs are dispersed in the literature, while a comprehensive and comparative review on their features, advantages, disadvantages, and limitations, is still lacking. Therefore, this paper aims at filling the literature gap by providing a comprehensive state-of-the art review on the TO for HXs over the past decades, so as to indicate the most promising technology roadmap. Each stage of the TO, i.e. the design parametrization, the heat transfer modeling, the optimization, and the final realization, is analyzed carefully in the corresponding section, highlighting the major pros, cons and challenges. Our statistics demonstrate that the current TO, though well-developed and fast improved, still have numerous limitations in handling the industrial HXs that hold the complicate structures and flow patterns. Eventually, three emerging schemes, i.e. machine learning, model order reduction, and moving morphable components, aimed to improve the efficiency of TO are also discussed. • Recent researches on TO of heat exchangers are reviewed following its workflow. • Current TO is found to be not powerful enough to handle industrial heat exchangers. • Various TO algorithms are compared to indicate the promising tech roadmap. • Emerging schemes to improve TO's efficiency are discussed. [ABSTRACT FROM AUTHOR]

Abstract: This paper gives a review on performances of compact heat exchangers (CHEs), including well-established devices, some relative newcomers to the market and also designs still being tested in the laboratory. The structures of the CHEs are briefly introduced, and their heat transfer enhancement mechanisms, as well as their advantages and limitations, are summarized. Then, different heat transfer enhancement technologies in CHEs are compared and their thermo-hydraulic performances are analyzed on the basis of available correlations for heat transfer and friction factor developed by various investigators quoted in the open literature. Finally, the technologies that may fit the specifications for a new generation of solar receiver, which is a critical component of the Concentrated Solar Power (CSP) system, are proposed. It is concluded, among others in the review, that solar receivers based upon CHE technology have been rarely reported, and therefore, more work is needed in this field for a comprehensive understanding and to improve the uses of new energy sources and contribute to sustainability. [Copyright &y& Elsevier]

Abstract: The general problem of describing the flow distribution in manifolds feeding parallel channels has been addressed in a number of earlier publications. Such systems are used for example as heat exchanger plates, as distributors for fuel cells, or as solar collectors. We address here the "inverse problem", i.e. of finding a design, which provides uniform flow distribution among the channels, for which much less literature is available. We approach that problem by considering the steady-state flow-rates in the branches of 2D rectangular lattice networks with a single inlet and a single outlet, of which the classical parallel channels (ladder) network can be viewed as a special case. The purpose of this paper is to show how to organize the flow resistances on the periphery of the lattice so as to achieve flow uniformity in all inside channels. This is done under the assumption of a resistive network, with linear flow/pressure behavior (pure viscous Stokes flow, neglecting the inertial terms, neglecting or linearizing the effect of branching singularities) allowing completely analytical solutions and comparison of different configurations. The compatibility of uniform flow distribution and uniform Residence-Time-Distribution is discussed. Examples of new conceptual designs of heat exchanger plates, of fluid distributors and of mixers are proposed, based on this possibility of uniform distribution. [Copyright &y& Elsevier]