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Mixed Convection in a Vertical Parallel Plate Microchannel

In this study, fully developed mixed convective heat transfer of a Newtonian fluid in an open-ended vertical parallel plate microchannel is analytically investigated by taking the velocity slip and the temperature jump at the wall into account. The effects of the mixed convection parameter, Gr/Re, the Knudsen number, Kn, and the ratio of wall temperature difference, r T , on the microchannel hydrodynamic and thermal behaviors are determined. Finally, a Nu=f(Gr/Re,Kn,r T ) expression is developed.

Mixed Convection in a Vertical Parallel Plate Microchannel With Asymmetric Wall Heat Fluxes

In this study, exact analytical results are presented for fully developed mixed convective heat transfer of a Newtonian fluid in an open-ended vertical parallel plate microchannel with asymmetric wall heating at uniform heat fluxes. The velocity slip and the temperature jump at the wall are included in the formulation. The effects of the modified mixed convection parameter, Gr q /Re, the Knudsen number, Kn, and the ratio of wall heat flux, r q =q 1 /q 2 , on the microchannel hydrodynamic and thermal behaviors are determined. Finally, a Nu= f(Gr q /Re,Kn,r q ) expression is developed. For, the limiting case of Kn=0, the results are found to be in an excellent agreement with those in the existing literature.

Mixed Convection in a Vertical Microannulus Between Two Concentric Microtubes

In this study, fully developed mixed convective heat transfer of a Newtonian fluid in a vertical microannulus between two concentric microtubes is analytically investigated by taking the velocity slip and the temperature jump at the wall into account. The effects of the mixed convection parameter Gr/Re, the Knudsen number Kn, and the aspect ratio r* on the microchannel hydrodynamic and thermal behaviors are determined. Finally, a Nu = f (Gr/Re, Kn,r*) expression is developed. It is disclosed that increasing Gr/Re enhances heat transfer while rarefaction effects considered by the velocity slip and the temperature jump in the slip flow regime decreases it.

Analysis of Micro-Graetz Problem in a Microtube

A theoretical analysis is presented for the problem of hydrodynamically developed, thermally developing, steady, laminar forced convective heat transfer of a Newtonian fluid in the entrance region of a microtube. The viscous dissipation effect, the velocity slip, and the temperature jump at the wall are taken into consideration. Two different thermal boundary conditions are considered: the constant heat flux (H1-type) and the constant wall temperature (T-type). Either wall heating (the fluid is heated) or wall cooling (the fluid is cooled) is considered. The downstream variation and the asymptotic value of the Nusselt number are determined as a function of the Brinkman number and the Knudsen number. For some limiting cases, the results are compared with those existing in the literature and an excellent agreement is observed.

Second-law analysis of heat and fluid flow in microscale geometries

In this study, the second law analysis of thermodynamics is applied to two different microgeometries: microtube and microduct, between two parallel plates. Hydrodynamically and thermally fully developed flow with constant properties is examined. Microscale effects are included in the analysis in terms of the viscous dissipation, the velocity slip and temperature jump. Using the previously obtained velocity and temperature profiles, a parametric study is carried out to determine the combined effects of the Brinkman number, Br, and the Knudsen number, Kn, on the entropy generation. Entropy generation is shown to decrease with an increase in Kn while increasing Br results in increasing entropy generation.

Prediction of Heat Transfer Coefficient in Saturated Flow Boiling Heat Transfer in Parallel Rectangular Microchannel Heat Sinks: An Experimental Study

ABSTRACT This study focuses mainly on the prediction of saturated flow boiling heat transfer in microchannels. A wide range of experiments has been carried out with de-ionized water to obtain a comprehensive data set. Experiments of mass fluxes of 51–728.7 kg/m2s, wall heat fluxes of 36–221.7 kW/m2, vapor qualities of 0.01–0.69, liquid Reynolds number of 7.72–190, aspect ratios of 0.37–5.00 (with a constant hydraulic diameter of 100 µm) and hydraulic diameters of 100–250 µm (for constant aspect ratio = 1). A new correlation including the aspect ratio effect is proposed to predict the heat transfer coefficient for saturated flow boiling in microchannels. The proposed correlation shows very good predictions with an overall mean absolute error of 16.9% and 86.4%, 96.2% and 99.5% of the predicted data falling within ±30, ±40 and ±50% error bands, respectively.

Exergy analysis of a counter–flow Ranque–Hilsch vortex tube having different helical vortex generators

In this study, thermodynamic analysis of a counter–flow vortex tube is made. Equations are derived for cold mass fraction (yr), entropy generation and irreversibility. Also, efficiency terms are defined based on the Carnot cycle by considering vortex tubes as both a cooling machine and a heat pump. Experiments are performed for various values of design parameters and working conditions. Experimental data obtained is then analysed thermodynamically. It is disclosed that irreversibility increases with an increase in the length of the helical vortex generator and in the inlet pressure. Both cooling and heating efficiencies are shown to decrease with an increase in the length of the helical vortex generator.

Frictional Pressure Drop for Gas Flows in a Microchannel Between Two Parallel Plates

In this study, air flow through a microchannel between two parallel plates of height ranging from 100 to 710 μm was investigated experimentally. Each channel was made of Plexiglas and had a large cross-sectional aspect ratio to supply the microplaneduct geometry. The flow rate and pressure drop across the microchannel were measured at steady state to obtain the friction factor. The Reynolds number ranged from 30 to 2300. The experimental friction factor values were found in good agreement with an existing analytical solution for an incompressible, fully developed, laminar flow under no-slip boundary conditions.

Saturated flow boiling characteristics in single rectangular minichannels: effect of aspect ratio

ABSTRACT In this study, saturated flow boiling characteristics of deionized water in single rectangular minichannels are investigated experimentally. A special attention is paid to the effect of aspect ratio (channel width to depth, Wch/Hch) on the heat transfer and total pressure drop. Experiments are conducted for various values of the mass flux and the wall heat flux. Flow visualization is used as a complementary technique for a deeper physical understanding of flow phenomena. The results show that the channel aspect ratio has a significant effect on both the local two-phase heat transfer coefficient and the total pressure drop. In general manner, the aspect ratio of 1 presents the highest heat transfer coefficients, while the aspect ratio of 0.25 demonstrates the lowest ones. On the other hand, the lowest values of the pressure drop are obtained at the extreme values of the aspect ratio (0.25 and 4).

Prediction of Pressure Drop for Flow Boiling in Rectangular Multi-Microchannel Heat Sinks

ABSTRACT In this study, two new correlations are developed to predict pressure drop for the flow boiling in micro systems with low mass flux. The correlations developed rely on extensive experimental results. Experiments are conducted for flow boiling in nine different silicon multichannel heat sinks with deionized water. In the experiments, mass fluxes of 51–324 kg⋅m−2⋅s−1, wall heat fluxes of 36–121.8 kW⋅m−2, exit vapor qualities of 0.04–0.81, liquid-only Reynolds number of 20.3–89.4, aspect ratios of 0.37–5.00 and hydraulic diameters of 100–250 µm are tested. At first, validation tests for the single phase have been conducted. Then, some of the well-known existing correlations developed for the prediction of two phase pressure drop are used for comparison of the experimental results obtained. Finally, two new empirical correlations are developed for low mass flux conditions. The first one is for frictional pressure drop component, which is obtained by following a general procedure. The second one is for the prediction of total pressure drop (a dimensionless pressure drop correlation). The latter has been shown to predict better with an overall mean absolute error of 14.5% and, 87.8%, 94.8% and 96.5% of the predictions falling within ±30, ±40 and ±50% error bands, respectively.