A Hybrid Numerical Methodology Based on CFD and Porous Medium for Thermal Performance Evaluation of Gas to Gas Micro Heat Exchanger

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 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 μm. Microchannels have a rectangular cross section of 200 μm x 200 μm with a wall thickness of 100 μm 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

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%)

ADVANCED CFD METHODOLOGY TO INVESTIGATE HIGH- TEMPERATURE COMPLEX WIRE NET MICRO HEAT EXCHANGER PERFORMANCE

International audienceThe objective of this paper is to predict micro heat exchanger performance for a micro Combined Heat and Power systems by a detailed modelling of complex microchannels and a new CFD methodology to assess the entire heat exchanger characteristics based on reduced order modelling. CFD methodology comprises of Conjugate Heat Transfer models and Reduced order models. This could reduce the computational size to a considerable large extent (billion cells to few million cells) with good accuracy. The porous medium model, based on Darcy-Forchheimer law is modified (Constant Integration Method) to account for the temperature evolution. They have been implemented and verified. The best-revised methodology allows obtaining pressure losses with less than three percent error with respect to the 3D CFD CHT modelling. Higher order turbulence models were used to investigate the influence of mesh on heat exchanger performance. A parametric study was conducted to study the influence of microchannel parameters on heat exchanger performance

A Hybrid Numerical Methodology Based on CFD and Porous Medium for Thermal Performance Evaluation of Gas to Gas Micro Heat Exchanger

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

Advanced Numerical Methodology to Analyze High-Temperature Wire-Net Compact Heat Exchangers For a Micro-Combined Heat and Power System Application: Wire-net heat exchangers

International audienceThe objective of this paper is to predict compact heat exchanger (CHE) performance for a miniaturized combined heat and power system by detailed modeling of the complex microchannels and assessing the collector performance using a new reduced-order modeling (ROM). The ROM was introduced to decrease the computational size and predict the collector performance with reasonable accuracy. The CHE is assembled as a stack of counterflow passages with optimized thickness and an isotropic wire-net (to provide required stiffness and enhance the mixing) which separates the thin partition foils. Computational fluid dynamics (CFD) methodology comprises of conjugate heat transfer (CHT) analysis for a microchannel section and ROM to analyze the entire CHE performance based on the collector performance. The porous medium model, based on the Darcy-Forchheimer law, is modified (constant integration method) to account for the temperature evolution and localized turbulence effects. The resulting microchannel characteristics from a series of threedimensional CFD-CHT analysis are used to calculate the inertial and viscous coefficients using the constant integration method. These characteristics have been implemented and verified numerically as well as experimentally. The best-revised methodology allows obtaining pressure drop with less than three percent error with respect to the CHT model

Advanced CFD methodology to investigate heat exchanger performance and experimental validation of a reduced model for a micro-CHP application

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A Porous Media Model for a Double-Layered Gas-to-Gas Micro Heat Exchanger operating in Laminar Flow Regime

International audienceIn 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 reduced order model based on porous medium approximation is developed for microchannels. With the help of this model, a detailed flow analysis through individual microchannels can be avoided by studying the device as a whole at considerably less computational cost. A cocurrent flow micro heat exchanger with 133 parallel microchannels (average hydraulic diameter of 200µm) is employed for current study. Hot and cold streams are separated by a stainless steel partition foil having thickness of 100µm. Rectangular microchannels have a cross section of 200µm × 200µm with wall thickness of 100µm 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 close to the laminar regime. Inertial and viscous coefficients extracted from this study are then utilized to model pressure and temperature drops within porous medium. In order to cater for density dependence of inertial and viscous coefficients due to high pressure drop in microchannels, a modified formulation of Darcy-Forchheimer law is adopted. A complete model of a double layer micro heat exchanger with collecting and distributing manifolds where microchannels are modeled as porous medium is finally developed and used to estimate the overall heat exchanger effectiveness of theinvestigated micro heat exchanger. A comparison of computational results using current model with previously published experimental results of the same micro heat exchanger showed that adopted methodology can predict the the heat exchanger effectiveness within the experimentaluncertainty for the range of mass flows considered in the current study

NUMERICAL AND EXPERIMENTAL INVESTIGATION OF HEAT EXCHANGER PERFORMANCE FOR A MICRO-CHP APPLICATION

International audienceThe objective of this paper is to investigate in detail the complex microchannel (with wire-net and S-shaped perturbators) performance and using a new CFD methodology to assess the entire heat exchanger characteristics based on reduced-order modelling. Localized turbulence enhances the heat exchanger performance along with an increased pressure loss. Shifting the critical Reynolds to lower Reynolds number by using perturbators will control the pressure losses and enhance the thermal efficiency to a considerable extent. Here we consider the microchannel performance (based on thermal efficiency and pressure losses) for two different types of perturbators, Wire-net [1, 2, 3] and the S-shape [4]. The wire-net heat exchanger (Fig.1a, 1b and 1c)) is assembled as a stack of counterflow passages with optimized thickness separated by thin foils. A metallic wire mesh is inserted in the flow passages (Fig.1c) to provide the required stiffness and enhance the microchannel efficiency (localized turbulence). The S-shaped heat exchanger model consists of S-shaped protrusions (Fig.1d and 1e) along the counter-flow passages with an integrated collector

Numerical Study of Perturbators Influence on Heat Transfer and Investigation of Collector Performance for a Micro-Combined Heat and Power System Application

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Numerical and experimental investigation of a wire-net compact heat exchanger performance for high-temperature applications

International audienceThe 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 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 efficiency. 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 stiffness and enhance the microchannel efficiency. A parametric study was conducted on various heat exchanger parameters to optimise the heat exchanger size, thermal effectiveness 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 effect 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 efficiency