M.M. Awad

A review of entropy generation in microchannels

In this study, a critical review of thermodynamic optimum of microchannels based on entropy generation analysis is presented. Using entropy generation analysis as evaluation parameter of microchannels has been reported by many studies in the literature. In these studies, different working fluids such as nanofluids, air, water, engine oil, aniline, ethylene glycol, and non-Newtonian fluids have been used. For the case of nanofluids, “nanoparticles” has been used in various kinds such as Al2O3 and Cu, and “base fluid” has been used in various kinds such as water and ethylene glycol. Furthermore, studies on thermodynamic optimum of microchannels based on entropy generation analysis are summarized in a table. At the end, recommendations of future work for thermodynamic optimum of microchannels based on entropy generation analysis are given. As a result, this article can not only be used as the starting point for the researcher interested in entropy generation in microchannels, but it also includes recommendations for future studies on entropy generation in microchannels.

Review and Modeling of Two-Phase Frictional Pressure Gradient at Microgravity Conditions

First, a detailed review of two-phase frictional pressure gradient at microgravity conditions is presented. Then, a simple semi-theoretical method for calculating two-phase frictional pressure gradient at microgravity conditions using asymptotic analysis is presented. Two-phase frictional pressure gradient is expressed in terms of the asymptotic single-phase frictional pressure gradients for liquid and gas flowing alone. In the present model, the two-phase frictional pressure gradient for x ≅ 0 is nearly identical to single-phase liquid frictional pressure gradient. Also, the two-phase frictional pressure gradient for x ≅ 1 is nearly identical to single-phase gas frictional pressure gradient. The proposed model can be transformed into either a two-phase frictional multiplier for liquid flowing alone (φl2) or two-phase frictional multiplier for gas flowing alone (φg2) as a function of the Lockhart-Martinelli parameter, X. Comparison of the asymptotic model with experimental data at microgravity conditions is presented.Copyright

Heat transfer and pressure loss in narrow channels with corrugated walls

The convective heat transfer and pressure drop characteristics in corrugated channels were experimentally investigated. Experiments were performed on channels of uniform wall temperature and of fixed corrugation ratio, gamma (2A/L=0.2) over a range of Reynolds number, 3220 lesReles9420. Corrugated channels of 0deg, 90deg and 180deg phase shift and with different spacing are considered. The effects of these geometrical parameters on the heat transfer and pressure drop are discussed. The obtained results showed a significant heat transfer enhancement and pressure drop penalty associated with corrugated channels. The average heat transfer coefficients and pressure drops exceeded by about 2.6 to 3.2 and 1.9 to 2.6 times those for parallel plate channel, respectively, depending upon both the spacing and phase shift. The effect of spacing variations on heat transfer and pressure drop was more pronounced than that of phase shift variation, especially at high Reynolds number. Comparing results of the tested channels by considering the flow area goodness factor (j/f), it was better for corrugated flow channel with geometrical parameters of 2 les epsiv les 3, and Phi les 90deg.

Heat transfer for laminar thermally developing flow in parallel-plates using the asymptotic method

Heat transfer for laminar, thermally developing flow in parallel-plates is investigated using the asymptotic method. There are two asymptotes for local, and mean Nusselt numbers (Nuz and Num) in the parallel-plates thermal entrance problem at both uniform wall temperature (UWT) and uniform heat flux (UHF). The first asymptote corresponds to very small value of dimensionless axial coordinate (z∗). The second asymptote corresponds to very high value of dimensionless axial coordinate (z∗). Using the methods discussed by Churchill and Usagi (1972, “General Expression for the Correlation of Rates of Transfer and Other Phenomena,” AIChE J., 18(6), pp. 1121–1128), the fitting parameter in the proposed model can be determined. Comparisons of the asymptotic model with the analytical and numerical solutions available in the literature are presented.

Bounds on Two-Phase Frictional Pressure Gradient and Void Fraction in Circular Pipes

Simple rules are developed for obtaining rational bounds for two-phase frictional pressure gradient and void fraction in circular pipes. The bounds are based on turbulent-turbulent flow assumption. Both the lower and upper bounds for frictional pressure gradient are based on the separate cylinders formulation. For frictional pressure gradient, the lower bound is based on the separate cylinders formulation that uses the Blasius equation to represent the Fanning friction factor while the upper bound is based on the separate cylinders equation that represents well the Lockhart-Martinelli correlation for turbulent-turbulent flow. For void fraction, the lower bound is based on the separate cylinders formulation that uses the Blasius equation to predict the Fanning friction factor while the upper bound is based on the Butterworth relationship that represents well the Lockhart-Martinelli correlation. These two bounds are reversed in the case of liquid fraction (1-α). For frictional pressure gradient, the model is verified using published experimental data of two-phase frictional pressure gradient versus mass flux at constant mass quality. The published data include different working fluids such as R-12, R-22, and Argon at different mass qualities, different pipe diameters, and different saturation temperatures. The bounds models are also presented in a dimensionless form as two-phase frictional multiplier (ϕ l and ϕ g ) versus Lockhart-Martinelli parameter (X) for different working fluids such as R-12, R-22, and air-water and steam mixtures. For void fraction, the bounds models are verified using published experimental data of void fraction versus mass quality at constant mass flow rate. The published data include different working fluids such as steam, R-12, R-22, and R-410A at different pipe diameters, different pressures, and different mass flow rates. It is shown that the published data can be well bounded for a wide range of mass fluxes, mass qualities, pipe diameters, and saturation temperatures.

A Robust Asymptotically Based Modeling Approach for Two-Phase Flows

A simple semitheoretical method for calculating two-phase frictional pressure gradient in horizontal circular pipes using asymptotic analysis to develop a robust compact model is presented. Two-phase frictional pressure gradient is expressed in terms of the asymptotic single-phase frictional pressure gradients for liquid and gas flowing alone. The proposed model can be transformed into either a two-phase frictional multiplier for liquid flowing alone (ϕl2) or two-phase frictional multiplier for gas flowing alone (ϕg2) as a function of the Lockhart-Martinelli parameter, X. Single-phase friction factors are calculated using the Churchill model which allows for prediction over the full range of laminar-transition-turbulent regions and allows for pipe roughness effects. The proposed model is compared against published data to show the asymptotic behavior. Comparison with other existing correlations for two-phase frictional pressure gradient such as the Chisholm correlation, the Friedel correlation, and the Müller-Steinhagen and Heck correlation, is also presented. Comparison with experimental data for both ϕl and ϕl versus X is also presented. At the end of the paper, the present asymptotic model is also extended to minichannels and microchannels.

Modeling of Interfacial Component for Two-Phase Frictional Pressure Gradient at Microscales

A simple approach for calculating the interfacial component of frictional pressure gradient in two-phase flow at microscales is presented. This approach is developed using superposition of three pressure gradients: single-phase liquid, single-phase gas, and interfacial pressure gradient. The proposed model can be transformed in two different ways: first, two-phase interfacial multiplier for liquid flowing alone ( ϕ l , i 2 ) as a function of two-phase frictional multiplier for liquid flowing alone ( ϕ l 2 ) and the Lockhart-Martinelli parameter, X, and, second, two-phase interfacial multiplier for gas flowing alone (ϕg,i2) as a function of two-phase frictional multiplier for gas flowing alone (ϕ g 2) and the Lockhart-Martinelli parameter, X. This proposed model allows for the interfacial pressure gradient to be easily modeled. Comparisons of the proposed model with experimental data for microchannels and minichannels and existing correlations for both ϕ l and ϕ g versus X are presented.

A Note on Mixture Density Using the Shannak Definition

In this study, a note on mixture density using the Shannak definition of the Froude number is presented (Shannak, B., 2009, “Dimensionless Numbers for Two-Phase and Multiphase Flow,” Proceedings of the International Conference on Applications and Design in Mechanical Engineering (ICADME), Penang, Malaysia, Oct. 11–13, 2009). From the definition of the two-phase Froude number, an expression of the two-phase density is obtained. The definition of the two-phase density can be used to compute the two-phase frictional pressure gradient using the homogeneous modeling approach in circular pipes, minichannels, and microchannels. We cannot have gas density≤two-phase density≤liquid density for 0≤mass quality≤1. Therefore, attention must be paid when using the obtained expression of the two-phase density in this note at any x value.

Two-Phase Flow Modeling in Oil and Gas Applications

In the current study, two-phase flow modeling in oil and gas applications using asymptotic analysis is presented. Examples of two-phase liquid-liquid flow in pipes, two-phase gas-liquid flow in fractures, and two-phase gas-liquid flow in porous media are presented. In the present study, a simple semi-theoretical method for calculating the two-phase frictional pressure gradient in oil and gas applications using asymptotic analysis is presented. The proposed model can be transformed into two-phase frictional multiplier as a function of the Lockhart-Martinelli parameter, X. The advantage of the new model is that it has only one fitting parameter (p). Therefore, calibration of the new model to experimental data is greatly simplified. The new model is able to model the existing multi parameters correlations by fitting the single parameter p. Comparison with experimental data for two-phase frictional multiplier versus the Lockhart-Martinelli parameter (X) is presented.Copyright

A Critical Review on the Determination of Convective Heat Transfer Coefficient During Condensation in Smooth and Enhanced Tubes

Heat exchangers using in-tube condensation have great significance in the refrigeration, automotive and process industries. Effective heat exchangers have been rapidly developed due to the demand for more compact systems, higher energy efficiency, lower material costs and other economic incentives. Enhanced surfaces, displaced enhancement devices, swirl-flow devices and surface tension devices improve the heat transfer coefficients in these heat exchangers. This study is a critical review on the determination of the condensation heat transfer coefficient of pure refrigerants flowing in vertical and horizontal tubes. The authors’ previous publications on this issue, including the experimental, theoretical and numerical analyses are summarized here. The lengths of the vertical and horizontal test sections varied between 0.5 m and 4 m countercurrent flow double-tube heat exchangers with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The measured data are compared to theoretical and numerical predictions based on the solution of the artificial intelligence methods and CFD analyses for the condensation process in the smooth and enhanced tubes. The theoretical solutions are related to the design of double tube heat exchangers in refrigeration, air conditioning and heat pump applications. Detailed information on the in-tube condensation studies of heat transfer coefficient in the literature is given. A genetic algorithm (GA), various artificial neural network models (ANN) such as multilayer perceptron (MLP), radial basis networks (RBFN), generalized regression neural network (GRNN), and adaptive neuro-fuzzy inference system (ANFIS), and various optimization techniques such as unconstrained nonlinear minimization algorithm-Nelder-Mead method (NM), non-linear least squares error method (NLS), and Ansys CFD program are used in the numerical solutions. It is shown that the convective heat transfer coefficient of laminar and turbulent condensing film flows can be predicted by means of theoretical and numerical analyses reasonably well if there is a sufficient amount of reliable experimental data. Regression analysis gave convincing correlations, and the most suitable coefficients of the proposed correlations are depicted as compatible with the large number of experimental data by means of the computational numerical methods.Copyright