The design of mini/micro heat exchangers: A world of opportunities and constraints

Micro heat exchangers and heat sinks broadened their use in many technological fields during the last two decades. The reduction of the dimensions of the channels allows to obtain ultra-compact heat exchangers characterized by higher surface-to-volume ratio and overall heat transfer coefficients but, in general, with large pressure losses. Many imaginative configurations have been proposed and tested, by changing the geometry of the manifolds, the position of the inlet/outlet ports, the structure of the heat transfer core, the structural materials and others more. Unfortunately, these efforts were not coordinated and a complete overview of the results accumulated up to now is not available. However, some general conclusions can be made by using the published results and the main scope of this paper is to summarize these milestones. Some shared conclusion are the following: (i) the design of micro heat exchangers can be obtained by using the classical methods developed for conventional heat exchangers even if the presence of non- negligible scaling effects (i.e. compressibility effects, conjugate wall-fluid effects, viscous dissipation) must be always verified; (ii) the performances of micro heat exchangers and heat sinks is strongly influenced by the proper distribution of the flow rate within the heat transfer core and a series of different solutions is available in order to solve this problem, as summarized in this paper; (iii) the presence of strong conjugate wall-fluid heat transfer effects can become an opportunity for the use of miniaturized heat exchangers made with inexpensive materials having low thermal conductivity values, especially in presence of counter-current flow and cross-flow configurations

In-Situ Measurements in Microscale Gas Flows—Conventional Sensors or Something Else?

Within the last few decades miniaturization has a driving force in almost all areas of technology, leading to a tremendous intensification of systems and processes. Information technology provides now data density several orders of magnitude higher than a few years ago, and the smartphone technology includes, as well the simple ability to communicate with others, features like internet, video and music streaming, but also implementation of the global positioning system, environment sensors or measurement systems for individual health. So-called wearables are everywhere, from the physio-parameter sensing wrist smart watch up to the measurement of heart rates by underwear. This trend holds also for gas flow applications, where complex flow arrangements and measurement systems formerly designed for a macro scale have been transferred into miniaturized versions. Thus, those systems took advantage of the increased surface to volume ratio as well as of the improved heat and mass transfer behavior of miniaturized equipment. In accordance, disadvantages like gas flow mal-distribution on parallelized mini- or micro tubes or channels as well as increased pressure losses due to the minimized hydraulic diameters and an increased roughness-to-dimension ratio have to be taken into account. Furthermore, major problems are arising for measurement and control to be implemented for in-situ and/or in-operando measurements. Currently, correlated measurements are widely discussed to obtain a more comprehensive view to a process by using a broad variety of measurement techniques complementing each other. Techniques for correlated measurements may include commonly used techniques like thermocouples or pressure sensors as well as more complex systems like gas chromatography, mass spectrometry, infrared or ultraviolet spectroscopy and many others. Some of these techniques can be miniaturized, some of them cannot yet. Those should, nevertheless, be able to conduct measurements at the same location and the same time, preferably in-situ and in-operando. Therefore, combinations of measurement instruments might be necessary, which will provide complementary techniques for accessing local process information. A recently more intensively discussed additional possibility is the application of nuclear magnetic resonance (NMR) systems, which might be useful in combination with other, more conventional measurement techniques. NMR is currently undergoing a tremendous change from large-scale to benchtop measurement systems, and it will most likely be further miniaturized. NMR allows a multitude of different measurements, which are normally covered by several instruments. Additionally, NMR can be combined very well with other measurement equipment to perform correlative in-situ and in-operando measurements. Such combinations of several instruments would allow us to retrieve an “information cloud” of a process. This paper will present a view of some common measurement techniques and the difficulties of applying them on one hand in a miniaturized scale, and on the other hand in a correlative mode. Basic suggestions to achieve the above-mentioned objective by a combination of different methods including NMR will be given

Characterization of Emulsified Non-encapsulated Thermochromic Liquid Crystal Micro-particles

In this paper, the process for obtaining non-encapsulated Thermochromic Liquid Crystal (TLC) micro-particles from commercial bulk material (UN R25C10W) is described. The bulk material is analyzed in terms of morphology and rheological properties (i.e. viscosity, maximum shear rate). An experimental evaluation of surface tension values and contact angle measurements is made to complement the rheological data. On the basis of the obtained thermophysical values, an emulsification procedure is proposed and non-encapsulated TLC droplets with a dimension lower than 10 \u3bcm were acquired. Further, attention has been focused on the calibration process of TLC bulk material before and after the emulsification. A relation between the local temperature value, RGB and colour intensities (HSI) is obtained by analyzing the digital images with MATLAB Image Processing Toolbox. The obtained results indicate that the commercial bulk material UN R25C10W TLC can be used to obtain stable oil-in-water emulsion by proposed emulsification procedure in this paper

Reaction kinetics and mechanism of the molybdate-catalyzed epimerization of glucose in aqueous solutions using a microstructured reactor

In this research work the reaction kinetics of the molybdate-catalyzed epimerization of glucose in aqueous solutions has been mathematical modeled and investigated. Experiments were performed in a microstructured reactor in continuous flow regime. The results show high yield of mannose in the range between 21-27.1%, which exceeds the actual literature data with a ratio 3:1. The kinetic model is presented here as a four component systems, which describes the reaction mechanism of the molybdate-catalyzed epimerization of glucose in aqueous solutions using a microstructured reactor. Using this four component systems it is entirely possible to explain all the existing pathways of the reaction system. Furthermore, applying this kinetic model all reaction rate constants for every existing component in the examined system can be calculated................

Characterization of a Wireless Vacuum Sensor Prototype Based on the SAW-Pirani Principle

A prototype of a wireless vacuum microsensor combining the Pirani principle and surface acoustic waves (SAW) with extended range and sensitivity was designed, modelled, manufactured and characterised under different conditions. The main components of the prototype are a sensing SAW chip, a heating coil and an interrogation antenna. All the components were assembled on a 15 mm × 11 mm × 3 mm printed circuit board (PCB). The behaviour of the PCB was characterised under ambient conditions and in vacuum. The quality of the SAW interrogation signal, the frequency shift and the received current of the coil were measured for different configurations. Pressures between 0.9 and 100,000 Pa were detected with sensitivities between 2.8 GHz/Pa at 0.9 Pa and 1 Hz/Pa close to atmospheric pressure. This experiment allowed us to determine the optimal operating conditions of the sensor and the integration conditions inside a vacuum chamber in addition to obtaining a pressure-dependent signal

Numerical and Experimental Study of Microchannel Performance on Flow Maldistribution

Miniaturized heat exchangers are well known for their superior heat transfer capabilities in comparison to macro-scale devices. While in standard microchannel systems the improved performance is provided by miniaturized distances and very small hydraulic diameters, another approach can also be followed, namely, the generation of local turbulences. Localized turbulence enhances the heat exchanger performance in any channel or tube, but also includes an increased pressure loss. Shifting the critical Reynolds number to a lower value by introducing perturbators controls pressure losses and improves thermal efficiency to a considerable extent. The objective of this paper is to investigate in detail collector performance based on reduced-order modelling and validate the numerical model based on experimental observations of flow maldistribution and pressure losses. Two different types of perturbators, Wire-net and S-shape, were analyzed. For the former, a metallic wire mesh was inserted in the flow passages (hot and cold gas flow) to ensure stiffness and enhance microchannel efficiency. The wire-net perturbators were replaced using an S-shaped perturbator model for a comparative study in the second case mentioned above. An optimum mass flow rate could be found when the thermal efficiency reaches a maximum. Investigation of collectors with different microchannel configurations (s-shaped, wire-net and plane channels) showed that mass flow rate deviation decreases with an increase in microchannel resistance. The recirculation zones in the cylindrical collectors also changed the maldistribution pattern. From experiments, it could be observed that microchannels with S-shaped perturbators shifted the onset of turbulent transition to lower Reynolds number values. Experimental studies on pressure losses showed that the pressure losses obtained from numerical studies were in good agreement with the experiments (<4%)

Forced periodic temperature cycling of chemical reactions in microstructure devices

In this publication, several stainless steel microstructure reactors specially designed to obtain rapid and periodic temperature changes are presented. Different microstructure reactor designs have been manufactured and tested for their thermal behaviour and equally by running a test reaction under stationary and non-stationary temperature conditions. The devices were continuously electrically heated and periodically cooled by a deionized water flow. The objective of the experimental measurements was to demonstrate that non-stationary temperature conditions may lead to an increase in the reaction rate compared to the stationary conditions. The heterogeneously catalysed oxidation of CO was chosen as the test reaction. The catalyst used was a dispersion of platinum on a porous alumina support generated by sol–gel technology. The experiments realized under non-stationary temperature conditions with a temperature oscillation amplitude of 41K and a period duration of 21 s show an increase in the mean CO2 concentration of a factor 1.72 compared to the mean concentration under quasi-stationary temperature conditions. The simulations of a simple monomolecular reaction under non-stationary temperature conditions indicate that the presence of a transitional surface coverage generated by the temperature oscillations may be a possible explanation for the observed phenomenon

Novel windows for “solar commodities”: a device for CO2 reduction using plasmonic catalyst activation

Producción CientíficaA novel plasmonic reactor concept is proposed and tested to work as a visible energy harvesting device while allowing reactions to transform CO2 to be carried out. Particularly the reverse water gas shift (RWGS) reaction has been tested as a means to introduce renewable energy into the economy. The development of the new reactor concept involved the synthesis of a new composite capable of plasmonic activation with light, the development of an impregnation method to create a single catalyst reactor entity, and finally the assembly of a reaction system to test the reaction. The composite developed was based on a Cu/ZnO catalyst dispersed into transparent aerogels. This allows efficient light transmission and a high surface area for the catalyst. An effective yet simple impregnation method was developed that allowed introduction of the composites into glass microchannels. The activation of the reaction was made using LEDs that covered all the sides of the reactor allowing a high power delivery. The results of the reaction show a stable process capable of low temperature transformations

Design and Simulation of a Wireless SAW–Pirani Sensor with Extended Range and Sensitivity

Pressure is a critical parameter for a large number of industrial processes. The vacuum industry relies on accurate pressure measurement and control. A new compact wireless vacuum sensor was designed and simulated and is presented in this publication. The sensor combines the Pirani principle and Surface Acoustic Waves, and it extends the vacuum sensed range to between 10−4 Pa and 105 Pa all along a complete wireless operation. A thermal analysis was performed based on gas kinetic theory, aiming to optimize the thermal conductivity and the Knudsen regime of the device. Theoretical analysis and simulation allowed designing the structure of the sensor and its dimensions to ensure the highest sensitivity through the whole sensing range and to build a model that simulates the behavior of the sensor under vacuum. A completely new design and a model simulating the behavior of the sensor from high vacuum to atmospheric pressure were established