Design Of Micro Heat Exchangers Research Articles

Assessment of micro-scale heat exchangers efficiency using lattice Boltzmann method and design of experiments

The study is focused on the use of nanofluids in a micro-open tall cavity, which is a type of micro heat exchanger (MHE). The cavity is heated from the bottom sidewall in a sinusoidal pattern, and the effects of four input parameters (Ra, Ha, Kn, and Vf) on heat transfer and irreversibility are investigated using numerical simulations based on Lattice Boltzmann Method (LBM). The findings of the study suggest that the local heat transfer on the bottom sidewall is strongly influenced by Ra and Ha, while the surface distribution of entropy generation is mainly dependent on Kn. The study also shows that the optimization of the magnitude and wavelength of the sinusoidal temperature can improve both local heat transfer and surface distribution of entropy generation. The results of the study provide valuable insights into the design of micro heat exchangers and suggest that the optimization of micro-porous geometries using DOE could lead to increased energy efficiency. The study contributes to our understanding of the complex interactions between input parameters in micro heat exchangers and highlights the importance of considering multiple parameters in the design process.

Experimental validation of a two equation RANS transitional turbulence model for compressible microflows

Laminar-to-turbulent flow transition in microchannels can be useful to enhance mixing and heat transfer in microsystems. Typically, the small characteristic dimensions of these devices hinder in attaining higher Reynolds numbers to limit the total pressure drop. This is true especially in the presence of a liquid as a working medium. On the contrary, due to lower density, Reynolds number larger than 2000 can be easily reached for gas microflows with an acceptable pressure drop. Since microchannels are used as elementary building blocks of micro heat exchangers and micro heat-sinks, it is essential to predict under which conditions, the laminar-to-turbulent flow transition inside such geometries can be expected. In this paper, experimental validation of a two equations transitional turbulence model, capable of predicting the laminar-to-turbulent flow transition for internal flows as proposed by Abraham etal. (2008), is presented for the first time for microchannels. This is done by employing microchannels in which Nitrogen gas is used as a working fluid. Two different cross-sections namely circular and rectangular are utilized for numerical and experimental investigations. The inlet mass flow rate of the gas is varied to cover all the flow regimes from laminar to fully turbulent flow. Pressure loss experiments are performed for both cross-sectional geometries and friction factor results from experiments and numerical simulations are compared. From the analysis of the friction factor as a function of the Reynolds number, the critical value of the Reynolds number linked to the laminar-to-turbulent transition has been determined. The experimental and numerical critical Reynolds number for all the tested microchannels showed a maximum deviation of less than 12%. These results demonstrate that the transitional turbulence model proposed by Abraham etal. (2008) for internal flows can be extended to microchannels and proficiently employed for the design of micro heat exchangers in presence of gas flows.

Average Friction Factors of Choked Gas Flow in Microtubes

A micro-tube passage is a basic and important element in the design of micro heat exchangers and for this reason during the last decade a series of investigations have been made with the aim to clarify the main scaling effects playing an important role in microtubes. In this paper, a combined analysis of numerically and experimentally obtained average friction factors in microtubes under the situation of under-expanded (choked) gas flow is presented. The working fluid (nitrogen) passes through the microtube and discharges into the atmosphere under an increasing inlet pressure. Experiments and numerical computations are performed for microtubes with 249 and 528.9 mm in diameter, by varying the aspect ratio (i.e. length/diameter) from 100 to 200. The numerical methodology to solve the governing equations is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. In order to capture the under-expansion characteristics of the flow during choking, the computational domain is extended in the downstream region beyond the microtube outlet. Both experimental and numerical results were obtained for a wide range of Mach number and Reynolds number. In the previous study, it was demonstrated how the outlet Mach number can be expressed as a function of the tube diameter under choked conditions. In this paper, a data reduction procedure for the estimation of the average friction factor between the inlet and the outlet of the microtube is proposed for choked flows in which the outlet gas temperature and pressure are obtained by using the outlet Mach number calculated numerically as a function of the microtube diameter. It is demonstrated how this data reduction method allows an accurate calculation of the average friction factors in microtubes by using a limited number of parameters which are easy to measure. The results obtained in this way are in good agreement with the numerical predictions as well as with the most common empirical correlations.

Numerical Studies of Flow and Temperature Distribution in a Micro-heat Exchanger

In the design of micro-heat exchangers, the flow and temperature uniformities are essential parameters for achieving better performance. In this work, uniformity of flow and temperature distributions was numerically studied in parallel micro-channels. Carboxymethyl cellulose (CMC) aqueous solution (0.5 wt.%) was used as a cooling (working) fluid. The rheological behavior of this solution is as power-law non-Newtonian fluid. Different manifold structures were used, and the effect of them on the flow and temperature distributions was studied. Results found that rectangular structure has lower values for temperature difference and uniformity parameter than other cases. So, with the rectangular structure of manifold, temperature and flow distributions are more uniform. In the next consideration, the direction of the inlet flow to micro-channels was studied. Three directions (vertical (90°), horizontal (0°) and oblique (45°)) were used. With different directions, various velocity profiles in the micro-channels were shown. As, with horizontal inlet flow, velocity in middle micro-channel is higher than other micro-channels, while, with vertical inlet flow, the first micro-channel has maximum velocity and velocity decreases in next micro-channels. Also, between these three directions, horizontal inlet flow has better uniformity of flow and temperature.

Effects of temperature-dependent thermo-physical properties on hydrodynamic swirl decay in microtubes

In this paper, the influence of temperature-dependent thermo-physical properties is investigated on the decay of swirl in a laminar swirling flow of liquid through a heated microtube. The conservation equations for mass, momentum, and energy are numerically solved to obtain the velocity and temperature fields. The decay of swirl is quantified by the variation of the ratio of swirl number at a section to that at the inlet ( S/ S0) along the flow. The results reveal that constant property simulations result in faster swirl decay as compared to the variable-property simulations. Further, the implications of slip length, inlet swirl number, inlet swirl velocity profile, and Reynolds number on the swirl decay are presented. The results of the present analysis bear significant technological consequences in the design of micro-heat exchangers with intrinsic rotationalities.

Performance evaluation of multi-layered porous-medium micro heat exchangers with effects of slip condition and thermal non-equilibrium

Porous-medium micro heat exchanger can be employed for dissipating heat in high heat flux applications. In this work, thermal performance of multi-layered micro heat exchangers with porous medium is theoretically studied. Forced convective heat transfer in porous microchannel is analytically investigated by considering the velocity/thermal slips at the wall. With the local thermal non-equilibrium (LTNE) model, analytical solutions for velocity and solid/fluid temperatures are obtained. Thermal resistance network method is presented for thermal transport in porous micro heat exchangers with the temperature jump thermal resistance. Based on the analytical solution, thermal performance of multi-layered porous-medium micro heat exchangers is evaluated for counter-flow (CF) and parallel-flow (PF) arrangements in terms of the local temperature distribution, log mean temperature difference, and effectiveness. It is found that the Nusselt number of porous microchannel first increases and then decreases with an increase in Knudsen number. The performance evaluation criteria (PEC), integrating flow resistance and heat transfer, is employed for evaluating thermal performance of porous microchannel. A maximum PEC, corresponding to an optimal pore density, exists with the increased pore density. Thermal performance of micro heat exchanger with porous medium is much better than that without porous medium, and the CF heat transfer is superior to the PF heat transfer. This work is useful for analysis in porous microchannels and design of micro heat exchangers with porous materials.