The objective of this paper is to have a detailed investigation of the microchannel performance of a complex wire-net compact heat exchanger and investigate its collector performance experimentally for a micro combined heat and power system. Localised turbulence can enhance the heat exchanger performance. Besides, this will increase the pressure losses also. Shifting the Reynolds critical to smaller Reynolds number by using perturbators will control the pressure losses and enhance the microchannel thermal e ciency. In this paper, we consider microchannels with wire-net perturbators. The wire-net micro heat exchanger is assembled as a stack of counterflow flow passages with optimised thickness separated by thin foils. A metallic wire-net mesh is inserted in the flow passages to provide the required sti↵ness and enhance the microchannel e ciency. A parametric study was conducted on various heat exchanger parameters to optimise the heat exchanger size, thermal e↵ectiveness and pressure losses for a micro-CHP system. Besides, a detailed investigation of the wire-net flow physics was made using a higher order Reynolds stress turbulence model to obtain the full velocity gradient tensor. This could detail the e↵ect of anisotropic flow physics in the isotropic wire-net microchannels. Lambda 2 criteria was implemented to investigate the flow mixing of the centrally convected non-disturbed mass flow. Furthermore, the analysis of the turbulence production terms provided a deeper insight into flow attachment and detachment near the wire-net intersections. The heat exchanger was experimentally tested, and it was found that the collector pressure losses are not su ciently low as compared to the microchannel pressure losses. The microchannel conjugate heat transfer thermal e↵ectiveness is in good agreement with the overall experimental e ciency.
The objective of this article is to analyze complex micro-channels with wire-net and Sshaped perturbators and implement a reduced order modeling (ROM) approach to assess the entire heat exchanger performance and validate through experiments. Shifting the critical Reynolds number to lower values using perturbators decreases the pressure losses and enhance the thermal efficiency. There is an optimum mass flow (for both perturbators) where the thermal efficiency reaches maximum. The thermal efficiency of the wire-net perturbator is relatively high compared to S-shaped perturbators. The S-shaped perturbators induces strong wall-normal velocity fluctuations and enhances the heat transfer. Furthermore, the turbulence production term provides a deeper insight into flow attachment and detachment near the wire net intersections. The computational fluid dynamics approach (conjugate heat transfer models and ROM) was introduced to reduce the computational grid size and predict the collector performance. The secondary collector performance is determined by considering the microchannels as porous mediums. Apparently, the primary collector performance is determined by considering both secondary collectors and microchannels as porous mediums. The cylindrical secondary collectors contribute nearly 40-50% of the pressure drop. Experimental validation showed that the ROM predicts the heat exchanger performance with a good (<4.4%) accuracy.
In micro heat exchangers, due to the presence of distributing and collecting manifolds as well as hundreds of parallel microchannels, a complete conjugate heat transfer analysis requires a large amount of computational power. Therefore in this study, a novel methodology is developed to model the microchannels as a porous medium where a compressible gas is used as a working fluid. With the help of such a reduced model, a detailed flow analysis through individual microchannels can be avoided by studying the device as a whole at a considerably less computational cost. A micro heat exchanger with 133 parallel microchannels (average hydraulic diameter of 200 μ m) in both cocurrent and counterflow configurations is investigated in the current study. Hot and cold streams are separated by a stainless-steel partition foil having a thickness of 100 μ m. Microchannels have a rectangular cross section of 200 μ m × 200 μ m with a wall thickness of 100 μ m in between. As a first step, a numerical study for conjugate heat transfer analysis of microchannels only, without distributing and collecting manifolds is performed. Mass flow inside hot and cold fluid domains is increased such that inlet Reynolds number for both domains remains within the laminar regime. Inertial and viscous coefficients extracted from this study are then utilized to model pressure and temperature trends within the porous medium model. To cater for the density dependence of inertial and viscous coefficients due to the compressible nature of gas flow in microchannels, a modified formulation of Darcy–Forschheimer law is adopted. A complete model of a double layer micro heat exchanger with collecting and distributing manifolds where microchannels are modeled as the porous medium is finally developed and used to estimate the overall heat exchanger effectiveness of the investigated micro heat exchanger. A comparison of computational results using proposed hybrid methodology with previously published experimental results of the same micro heat exchanger showed that adopted methodology can predict the heat exchanger effectiveness within the experimental uncertainty for both cocurrent and counterflow configurations.
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%).
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