Narrow Results

In this paper, forced convection of a turbulent flow of pure water, Al₂O₃/water nanofluid and Al₂O₃–Cu/water hybrid nanofluid (a new advanced nanofluid composited of Cu and Al₂O₃ nanoparticles) through a uniform heated circular tube is numerically analyzed. This paper examines the effects of these three fluids as the working fluids, a wide range of Reynolds number (10 000 ≤ Re ≤ 10 0000) and also the volume concentration (0% ≤ ϕ ≤ 2%) on heat transfer and hydrodynamic performance. The finite volume discretization method is employed to solve the set of the governing equations. The results indicate that employing hybrid nanofluid improves the heat transfer rate with respect to pure water and nanofluid, yet it reveals an adverse effect on friction factor and appears severely outweighed by pressure drop penalty. However, the average increase of the average Nusselt number (when compared to pure water) in Al₂O₃–Cu/water hybrid nanofluid is 32.07% and the amount for the average increase of friction factor would be 13.76%.

A membrane humidifier with porous media flow field (metal foam) can provide more water transfer, low manufacturing complexity and low cost in comparison with the conventional humidifier. In this study, a two-dimensional numerical model is developed to investigate the performance of the humidifier with porous metal foam. The results indicate that the dew point increases with a decrease in the permeability, but at permeabilities lower than 10^-8 the pressure drop increases extremely. At all ranges of pressures, temperatures and flow rates of humidifier inlet, the pressure drop in humidifier with porous media flow field is only about 0.5 kPa higher than that of the conventional humidifier, which is not significant and it can be ignored. An increase in the pressure at dry side inlet and wet side inlet of the humidifier results in a better humidifier performance. Humidifier performs better at high flow rates and temperatures of humidifier wet side inlet. At all ranges of pressures, flow rates and temperatures humidifier with porous metal foam indicates better performance.

The author of this paper has conducted the experimental study and optimal computing for the condenser of automobile air conditioner. Especially studied and computed the properties of heat transfer and flow at the side of air when using louvered fin. Analyzed the influence of several combinations of fin's geometrical dimensions to heat transfer and pressure drop. Furthermore, designed a program to conduct optimal computing, and the computing results are basically consistent with experimental results.

The performance of evaporator is of importance in cryogenic and refrigeration systems. In this paper, evaporative heat transfer and pressure drop characteristics of refrigerant R134a boiling inside a new geometry microfinned helically coiled tube is experimentally studied. Compared with corresponding smooth helically coiled tube, the heat transfer enhancement factors of microfinned helically coiled tube are up to 2.2 with the varieties of mass flux and heat flux. The frictional pressure drop values are obtained by subtracting acceleration pressure drop and gravitational pressure drop from the measured total pressure drop. The frictional pressure drop data can be well correlated using Lockhart–Martinelli method.

In PEM fuel cells, during electrochemical generation of electricity more than half of the chemical energy of hydrogen is converted to heat. This heat of reactions, if not exhausted properly, would impair the performance and durability of the cell. In general, large scale PEM fuel cells are cooled by liquid water that circulates through coolant flow channels formed in bipolar plates or in dedicated cooling plates. In this paper, a numerical method has been presented to study cooling and temperature distribution of a polymer membrane fuel cell stack. The heat flux on the cooling plate is variable. A three-dimensional model of fluid flow and heat transfer in cooling plates with 15 cm × 15 cm square area is considered and the performances of four different coolant flow field designs, parallel field and serpentine fields are compared in terms of maximum surface temperature, temperature uniformity and pressure drop characteristics. By comparing the results in two cases, the constant and variable heat flux, it is observed that applying constant heat flux instead of variable heat flux which is actually occurring in the fuel cells is not an accurate assumption. The numerical results indicated that the straight flow field model has temperature uniformity index and almost the same temperature difference with the serpentine models, while its pressure drop is less than all of the serpentine models. Another important advantage of this model is the much easier design and building than the spiral models.

Lubricating the engine reciprocating components effectively against various engine speeds is significant for a proper lubrication system. This paper presents the lubrication system optimization of a twin cylinder 700 c.c. turbocharged engine. A couple of modifications were tested including correction of ducts misalignment, consistence diameter of ducts and replacement of a low flow resistance oil cooler. Compared to the base engine, experimental results prove that differential pressure (DP) between the oil pump and main oil gallery has been decreased to a minimum 19% and maximum 54% at engine speed between 2000 and 7000 rpm. The lower the pressure drop, the lower the flow resistance. Thus, mechanical loss has also been improved.

The desalination process using membrane distillation (MD) has recently attracted wide attention in the last few years around the world. Especially, membranes that have an asymmetric geometry, for their performance to filtrate the salt water and the high salt rejection. In this paper, the Langevin dynamics model was adopted as a simulation method to investigate the transport of salt water through the sloping membrane under a pressure drop. The surface of the used membrane is considered as a hydrophilic sloping surface. Thus, the pressure drop effect on the fluid flux was observed, which leads to attracting the salt water into pores that are randomly distributed. The influence of deposition and accumulation of the ions into pores, the incline angle of the membrane surface, and the thickness of the formed layer on the surface were investigated. In addition, the impact of biofouling is caused by the accumulation and the variation of the fluid velocity as a function of pressure drop values. The obtained results show that the relationship between the fluid velocity and the pressing force is a power law. Moreover, the increase in fluid flow velocity in the porous medium is severe in the earlier time regime, but it becomes almost constant in the second regime. However, the time desalination process increases linearly with the pressure drop. Moreover, the accumulation and deposition of ions into the pores cause a decrease in the water flow through the pores resulting in a higher pressure drop in the less inclined direction. Finally, the influence of deposition and accumulation of the salt phase into the pores on the membrane performance was remarked, resulting in a high desalination rate. The obtained results explain the salt water behavior through a porous membrane, which provides ideas for making a high membrane performance.

Convective heat transfer process between fractal tube bank and coolant water was simulated by using the finite element method. Both rectangular and circular tube banks were investigated in detail. For the relationship between Nusselt number and Reynolds number, the fractal circular tube bank shows the same trend as the rectangular tube one. Based on simulation results, the empirical heat transfer correlations for different stage fractals were regressed. The exponents of correlations increase with the fractal stage. Almost linear correlations between Nu and Re for fourth stage fractal were obtained, indicating a good heat transfer enhancement with the increase in fractal stages. Compared to the uniform tube bank, the fractal tube banks show better overall performance in enhanced heat transfer effect. This research would be helpful to understand the heat exchange mechanism of the multi-scale structure.

Assessment of the pressure drop that occurs in cable-in-conduit conductors (CICCs) requires a reliable predictive model. In this paper, a fractal model to predict the pressure drop of helium flows in CICCs is proposed based on the porous media analogy and fractal theory. A fractal friction factor correlation is also derived using the proposed pressure drop model. The good agreement between the predictions and the experimental data for conductors with $n_1=3$ verifies the validity of both the fractal pressure drop model and the friction factor correlation for such configuration conductors. The fractal correlation indicates that the friction factor of the cable bundle in a CICC is related not only to the void fraction but also to the average hydraulic diameter, the tortuosity of the fluid flow paths, the maximum pore size and the fractal dimension of the cable cross-section. This theoretical correlation does not contain any fitting constants and each parameter has a clear physical meaning.

This paper proposes a kind of tree-like convergent microchannel heat sink (TCMCHS) and numerically investigates effects of the outlet-to-inlet width ratio $\alpha$ of the convergent microchannel, the width ratio of the inlet of the daughter microchannel to the outlet of the parent microchannel $\gamma$ (D-P width ratio), the channel length ratio $\beta$, the initial branching number $N$, and the branching levels $m$ on the maximum temperature $T_{max}$, the total thermal resistance $R_T$, the pressure drop $\mathrm{\Delta }P$ and the coefficient of performance (COP) of the TCMCHS with rectangular channel cross-section under the size limitations of fixed total channel volume. The results display that both the maximum temperature and the total thermal resistance drop with the decrease of the outlet-to-inlet width ratio $\alpha$; however, the pressure drop $\mathrm{\Delta }P$ of TCMCHS first drops and then increases with the rise of the outlet-to-inlet width ratio $\alpha$. The different influences of the outlet-to-inlet width ratio $\alpha$ on the hydraulic and thermal performances lead to first increased and then decreased COP with the growth of the outlet-to-inlet width ratio $\alpha$. Moreover, the COP declines with the increasing length ratio $\beta$, initial branching number $N$ and the branching level $m$, but first rises when $\alpha$ is small and then declines when $\alpha$ is large with the rising D-P width ratio $\gamma$. The findings of this paper provide a feasible scheme for the improvement of the thermal and hydraulic performances of MCHS by using tree-like convergent microchannel.

Micro-channel heat sinks have been demonstrated to disperse heat at an extremely higher rate than the traditional devices because of huge heat transfer surface-to-volume ratio. This paper proposed to study the hydrothermal performance of hybrid combination of micro-channel heat sink (HMCHS) with secondary oblique channels in alternating directions and rectangular ribs. To enhance the heat transfer characteristics of the proposed hybrid model, we optimize three geometrical parameters such as secondary channel width, relative rib width and angle between the secondary channels. Primarily, numerical modeling is used to find the ranges of parameters by way of using the continuity, momentum and energy equations. Then the proposed model utilizes opposition-based antlion optimization (OALO) algorithm for selecting the optimal ranges of geometrical parameters to improve the system model. The optimal range will enhance the heat transfer characteristics of micro-channel and reduce the pressure drop when compared to the ones in the existing literature. Further, as a result of higher pressure drop, the micro-channel heat sink with secondary channels and ribs continuously loses its favorable position as an effective heat transfer enhancement technique at higher Reynolds number.

The micromixer is an important prerequisite for realizing the function of the microfluidic chip system. Micromixers are mainly used in protein crystallization, biological reactions, drug delivery and other fields. Efficient micromixers enable fast bioreaction processes. In this paper, according to the principle of plane geometry, micromixers with three different structural obstacles, circular, equilateral triangle and square, are designed to study how to improve the mixing efficiency of the micromixer. In this paper, the influence of the obstacle structure on the mixing efficiency and pressure drop of the micromixer is analyzed. Through numerical simulation, the results show that when Re = 100, the mixing efficiency of the circular obstacle micromixer is up to 97.4%, and the pressure drop is 15,789.597 Pa. In addition, this paper analyzes that the use of cylindrical obstacles can make the fluid generate horizontal and vertical vortex flow, break the usual laminar flow, and significantly improve the mixing efficiency.

Tortuous saphenous vein graft (SVG) hemodynamics was investigated using computational fluid dynamics (CFD) techniques. Computed tomography (CT) technology is used for noninvasive bypass graft assessment seven days after surgery. CT investigation shows two regions with severe shape remodeling, one is an angle type contortion and the other one is a sharp curvature with tortuous area reduction. The numerical analysis carefully examines the effect of an SVG geometry remodeling through flow separation, particle deposition, and wall shear stress (WSS). During the cardiac cycle, overall pressure drop increases from 2.6mmHg to 4.4mmHg. In the accelerating part of the systolic phase, particles released in the inlet section move downstream toward the first narrowed part (elbow type contortion) with a helical motion. WSS range along the cardiac cycle varies from 2Pa to 42Pa, enough to damage the endothelial cells. Vessel torsion induced helical flow can reduce the flow disturbance and separation. Additionally, in the distal end of the graft, the high particle concentrations can promote the inflammatory processes in the vessels.

Ejection of blood from the left ventricle to the aorta is studied using two-dimensional Navier–Stokes equations, the work-energy equation and the magnetic resonance imaging of a normal ventricular motion. Vortex shedding in the sinuses of Valsalva is dominated by the aortic jet, flow acceleration and valve motion. Momentums produced by ventricular contraction are in concert with vortices in the ventricle for blood ejection. Shear stresses and net pressures on the aortic valve are calculated during valve opening and closing. The rate of work done by shear and the energy dissipation in the ventricle are small. The Bernoulli energy flux delivered to blood from ventricular contraction is practically balanced by energy flux at the aortic root and the rate change of kinetic energy in the ventricle.

Computer simulations of the blood flow through right coronary arteries with two stenoses in the same arterial segment are carried out to investigate the interactions of serial stenoses, especially the effect of the distal stenosis. Various mathematical models are developed by varying the location of the distal stenosis. The numerical results show that the variation of the distal stenosis has significant impact on coronary hemodynamics, such as the pressure drop, flow shifting, wall shear stress and flow separation. Our simulations demonstrate that the distal stenosis has insignificant effect on the disturbed flow pattern in the regions of upstream and across the proximal stenosis. In a curved artery segment with two moderate stenoses of the same size, the distal stenosis causes a larger pressure drop and a more disturbed flow field in the poststenotic region than the proximal stenosis does. A distal stenosis located at a further downstream position causes a larger pressure drop and a stronger reverse flow.

The pressure drop across a laboratory-scale catalyst packed bed with mechanical failure of catalyst pellets has been examined. It was found that the increased pressure drop can be described by a simplified model deduced from Ergun's equation. The pressure drop is determined mainly by the term of viscous energy loss.

An optimum design approach to reverse-flow cyclones based on the concept of optimizing cyclone dimensions is introduced in this paper. This approach involves optimizing cyclone dimensions by categories, calculating cyclone performance by correlating similitude numbers and optimizing the combination of four cyclone parameters, D, K_A, and v_i, which has been proven to be applicable not only for single-stage cyclone, but also for multistage cyclone separators. Applications of the designed cyclones in FCC units and acrylonitrile reactors are also presented.

This paper presents a review of differences and similarities of in-tube heat transfer and pressure drop between ammonia (NH₃) and carbon dioxide (CO₂) from the perspective of the design of heat exchangers for NH₃ two-stage and CO₂/NH₃ cascade refrigeration systems. The focus is on differences in thermophysical properties and thus different characteristics of heat transfer and pressure drop. A brief summary of published literatures about CO₂/NH₃ cascade refrigeration systems is provided and literature review of available correlations and developed correlations are presented for flow boiling and condensation heat transfer and pressure drop. Because of large deviation of calculated values with exiting correlations from measured results, a new correlation to predict flow condensation heat transfer coefficients was developed based on experimental results for CO₂ at -15°C. From comparison of measured and predicted values, it is shown that some correlations, previously published in open literature, can be used to calculate flow boiling heat transfer coefficients for NH₃ at -20°C, if a flow pattern can be appropriately determined for a flow condition. Also, it is presented that existing correlations can predict well the heat transfer coefficients for CO₂ flow boiling at -15 and -30°C. It is shown that some correlations can predict pressure drop relatively well for NH₃ and CO₂ two-phase flow. The NH₃ and CO₂ flow evaporation heat transfer and pressure drop characteristics at -40°C are compared with predicted values.

This study presents experimental frictional pressure drop for R410A/oil mixture flow boiling in small horizontal smooth tubes with inside diameters of 4.18 mm and 2.0 mm. Experimental conditions cover nominal oil concentrations from 0 to 5%. The test results show that the presence of oil enhances two-phase frictional pressure drop about 0–120% and 0–90% at present test conditions for 4.18 mm I.D. smooth tube and 2.0 mm I.D. smooth tube, respectively, and the enhanced effect is more evident at higher vapor qualities where the local oil concentrations are higher. A new correlation to predict the local frictional pressure drop of R410A/oil mixture flow boiling inside conventional size and small smooth tubes is developed based on local properties of refrigerant–oil mixture, and the experimental data of 4.18 mm I.D. and 2.0 mm I.D. smooth tubes and that of 6.34 mm I.D. smooth tube (Hu et al., 2008) are well-correlated with the new correlation.

Heat transfer and pressure drop characteristics of the welded plate heat exchangers are experimented to apply the high- and low-temperature solution heat exchanger (SHX) of absorption systems. Two different SHXs were made using the seam and tig welding method. In this paper, the welded embossing type plate heat exchangers were tested by controlling mass flow rate and inlet/outlet temperatures. It was found that heat transfer and pressure drop performance increased with increasing Reynolds number. It was also found that the pressure drop from the present W-embossing type plate heat exchanger was much lower than that from the brazed type, as low as 1/7 times. The experimental correlations for Nusselt number and Fanning friction factor were developed with the error bands of ± 20% and ± 25%, respectively. These results provide a guideline to apply the welded plate heat exchanger for the solution heat exchanger of absorption systems.