Turbulent flow and heat transfer in rectangular channel has an important significance in engineering. Conventional approach to caculate Nusselt number of rectangular channel approximately is to take the equivalent diameter as the characteristic length and use the classic circular channel turbulent heat transfer coefficient correlations. However, under these conditions, the caculation error of Nusselt number can reach to 14% and thus this approach can not substantially describe the variation of Nusselt number of rectangular cross-sections with different aspect ratios. Therefore, caculation by using equivalent diameter as the characteristic length in classic experiment formula needs to be corrected. Seven groups of rectangular channel models with different aspect ratios have been studied numerically in this paper. By using standard turbulence model, the flow and heat transfer law of air with varing properties has been studied in 4 different sets of conditions in Reynolds number. The simulation and experimental results are in good agreement. The simulation results show that with the increase of aspect ratio, the cross-sectional average Nusselt number increased, Nusselt number of circumferential wall distributed more evenly and the difference between the infinite plate channel and square channel went up to 25%. The effects of corner region and long\short sides on heat transfer have also been investigated in this paper. Results show that in rectangular channel, heat transfer in corner region is significantly weaker than it in other region. With the increase of aspect ratio, effect on the long side of heat transfer of the short side is gradually reduced, and then eventually eliminates completely in the infinite flat place. Based on the studies above, correction coefficient for rectangular channels with different aspect ratios has been proposed in this paper and the accuracy of the correction coefficient has been varified by numerical simulations. This can reflect the variation of Nusselt number under different aspect ratios more effectively and thus has current significance for project to calculate Nusselt number of heat transfer in rectangular channel.

Surface roughness is one of the most important factors to determine the flow and heat transfer characteristics of the microchannel. This paper experimentally and theoretically investigated the effects of surface roughness for the flow and heat transfer behavior within the circular microchannel. The stainless steel circular microchannels were fabricated by electrical spark-erosion perforating and drilling separately to control the relative roughness of the surface which is 1% for drilling method and 1.5% for electrical spark-erosion perforating method. Each test piece includes 44 identical circular microchannels in parallel with diameter of 0.4 mm. In the experiments, the air flowed through the circular microchannels with Reynolds number changing from 200 to 2600. The results showed that the surface roughness in microchannels has a remarkable effect on the performance of flow behavior and heat transfer within the circular microchannel. The values of Poiseuille number and Nusselt number are higher when the surface relative roughness is larger. At the same time, the flow behavior is inconsistent with the behavior within the macrochannel. For the flow behavior, Poiseuille number increases monotonously with the increase of Reynolds number, and is higher than the constant theoretical value. The Reynolds number for the transition from laminar to turbulent flow is between 1400 and 1600. For the heat transfer property, Nusselt number also increases as the increase of the Reynolds number.

In this work, we numerically study the effects of turbulence intensity at the fuel and oxidizer stream inlets on the soot aerosol nano-particles formation in a kerosene fuel-based combustor. In this regard, we study the turbulence intensity effects specifically on the thermal performance and nano-particulate soot aerosol emissions. To construct our computer model, we simulate the soot formation and oxidation using the Polycyclic Aromatic Hydrocarbons PAHs-inception and the hydroxyl concept, respectively. Additionally, the soot nucleation process is described using the phenyl route, in which the soot inception is described based on the formations of two-ringed and three-ringed aromatics from acetylene, benzene, and phenyl radical. We use the two-equation soot model in which the soot mass fraction and the soot number density transport equations are solved considering the evolutionary process of soot nanoparticles, where all the nucleation, coagulation, surface growth, and oxidation phenomena are suitable considered in calculations. For the combustion modeling part, we benefit from the flamelets library, i.e., a lookup table, considering a detailed chemical kinetic mechanism consisting of 121 species and 2613 elementary reactions and solve the transport equations for the mean mixture fraction and its variance. We take into account the turbulence-chemistry interaction using the presumed-shape probability density functions PDFs. We apply the two-equation high-Reynolds-number k -ε turbulence model with round-jet corrections and suitable wall functions in performing our turbulence modeling. Solving the transport equations of turbulence kinetic energy and its dissipation rate, the turbulence closure problem can be resolved suitably. Furthermore, we take into account the radiation heat transfer of soot and gases assuming optically-thin flame, in which the radiation heat transfer of the most important radiating species is determined locally through the emissions. To evaluate our numerical solutions, we first solve an available well-documented experimental test, which provides the details of a kerosene-fueled turbulent nonpremixed flame. Then, we compare the achieved flame structure, i.e., the distributions of mean mixture fraction, temperature, and soot volume fraction, with those measured in the experiment. Next, we change the turbulence intensities of the incoming fuel and oxidizer streams gradually. So, we become able to evaluate the effects of different turbulence intensities on the achieved temperature and soot aerosol concentrations. Our results show that using moderate turbulence intensities at both fuel and oxidizer stream inlets would effectively increase the maximum temperature inside the combustor and this would reduce the exhaust gases temperature. It also reduces the concentrations of soot in the combustor and its emission to the exhaust gases effectively.

This paper describes the CFD analysis of single rectangular microchannel for hydraulic diameter 319 μm. While CFD analysis the Nusselt number observed is 4 to 5 with different Reynolds Number variation for flow rate of 0.001 kg/sec to 0.012 kg/sec. The current work describes CFD analysis of single microchannels for length of 50 mm with water as a fluid medium with laminar flow. Computational Fluid dynamics analysis of Single rectangular microchannel Single rectangular microchannel of 319 μm hydraulic diameter is analyzed to study the flow characteristics in the inlet, microchannel test section and outlet test section with ANSYS CFX-11 for pressure drop, temperature drop, velocity counter of single micro-channel. For analyzing the weather the turbulence is created at inlet part of the microchannel a pressure drop analysis is carried for flow rate of 0.012 kg/sec with heat input 5.33 watt/cm 2 under laminar flow consideration. For analyzing the temperature profile across microchannel a for flow rate of 0.012 kg/sec with heat input 5.33 watt/cm 2 under laminar flow is considered.. For single microchannel the temperature rise of water is in range of 1 °K to 2 °K at center plane of microchannel. It is found that at leading edges or leaving edge the temperature rise in water is higher as compare to entering edge of microchannel. It is due to while entering to leaving of water particles in microchannel it collapse each other and try to increasing friction along each other so at outlet or leading edge the temperature rise is seen higher as compare to in let portion of single microchannel.

Numerical computations were performed to obtain for heat transfer characteristics of turbulent gas flow in micro-tubes with constant wall temperature whose temperature is lower than the inlet temperature (cooled case). The numerical methodology was based on Arbitrary-Lagrangian-Eulerinan (ALE) method to solve compressible momentum and energy equations. The Lam-Bremhorst Low-Reynolds number turbulence model was employed to evaluate eddy viscosity coefficient and turbulence energy. The tube diameter ranges from 100 μm to 400 μm and the aspect ratio of the tube diameter and the length is fixed at 200. The stagnation temperature was fixed at 300 K and the computations were done for wall temperature, which ranged from 250 K to 295 K. The stagnation pressure was chosen in such a way that the flow is in turbulent flow regime. The results in wide range of Reynolds number and Mach number were obtained. The bulk temperature based on the static temperature and the total temperature of the cooled case are compared with those of heated case and also with temperatures of the incompressible flow. The result shows that different heat transfer characteristics are obtained for each cooled and heated case. A correlation for the prediction of the heat transfer rate of the turbulent gas flow in a micro-tube is proposed.

Liquid-vapor oscillating flow and heat transfer in a vertically placed oscillating heat pipe (OHP) with a sintered particle wick structure inside are analyzed in this paper. The evaporation and condensation heat transfer coefficients are obtained by solving the microfilm evaporation and condensation on the sintered particles. The sensible heat transfer between the liquid slug and the channel wall are obtained by analytical solution or empirical correlations, depending on whether the liquid flow is laminar or turbulent. The effects of the maximum evaporation and condensation angles on the oscillatory flow, as well as sensible and latent heat transfer are analyzed.

Turbulent convective heat transfer and pressure drop of •-Al 2 O 3 /water and CuO/Water nanofluid inside a circular tube were experimentally investigated and compared. The nanofluids were very dilute. Results indicated that addition of small amounts of nanoparticles to the base fluid augmented heat transfer. Measurements showed that pressure drop for the dilute nanofluid was much greater than that of the base fluid. Experimental results were compared with existing correlations for nanofluid convective heat transfer coefficient in turbulent regime.

Present study tries to put light on hydrodynamics of nanofluids in microchannels. For the present hydrodynamic study, the microchannels of hydraulic diameters of 212 and 301 μm are used. Present study also uses nanofluids in microchannel. To observe the hydrodynamic effect of nanofluids in microchannel, the alumina nanoparticles with sizes 45 nm are chosen with the water as base fluid. The nanofluids with the dilute concentrations 0.25 vol% are used to observe the effect of volume fraction. From the study of base fluid flow in microchannel, it is found that the axial pressure drop is linear thus showing the incompressible behaviour of fluid. For all microchannels, early transition to turbulence was observed. Also for the same Re the pressure drop was higher for smaller channel. However, the usage of nanofluids in these microchannels shows different behavior from normal fluids. The axial pressure drop was again linear thus proving that even though these fluids are different from normal fluids; they follow the behaviour of incompressible Newtonian fluids. Surprisingly, the friction factor was similar for these fluids as compared to base fluids. This can be attributed to dilute concentration of nanofluids, which make them a homogeneous fluid. It suggests that the use of dilute nanofluids in microchannel results in no or little penalty in pressure drop. It also suggests that if nanofluids have to be used as a better coolant, the hydrodynamics and heat transfer characteristics has to be studied as higher concentrations.

The aim of the present study is to investigate the thermal fluid flow transport phenomenon of nanofluids in the heated horizontal circular tube. Consideration is given to the effects of volume fraction of the nanoparticle and Reynolds number on the turbulent heat transfer and pressure loss. Diamond, alumina (Al 2 O 3 ) and oxide copper (CuO) are employed here as nanoparticles. It is found that (i) the viscosity of nanofluids increases with an increase in the volume fraction of nanoparticles dispersed in the working fluid, (ii) the pressure loss of nanofluids increases slightly in comparison with that of pure fluid and (iii) enhancement heat transfer performance is caused by suspending nanoparticles except for the case of large particle aggregation.

The term of nanofluid refers to a solid-liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. Recent investigations on nanofluids indicate that the suspended nanoparticles markedly change the transport properties and heat transfer characteristics of the suspension. There are less published articles on deriving the forced convective heat transfer of nanofluids than articles on the effective thermal conductivity of nanofluids. Fully developed turbulent flow and heat transfer of two different nanofluids (Al 2 O 3 and TiO 2 ) in water flowing through a circular tube under constant heat flux condition have been experimentally studied. The results showed enhancement of convective heat transfer using the nanofluids. The Nusselt numbers of nanofluids were obtained for different nanoparticle concentrations as well as various Reynolds numbers. Experimental results emphasize the enhancement of heat transfer due to the nanoparticles presence in the fluid. Nusselt number increases by increasing the concentration of nanoparticles in nanofluid. And values of Nusselt number were calculated and these results have been introduced by experimental correlations for turbulent flow.

The disturbing of ultrafine particles on carrying phase is usually ignored in studies on dilute ultrafine particle systems. In a dense fractal-like agglomerate system, however, the effect of particle phase on carrier phase should be considered due to its unique heat and mass transfer while the corresponding mathematical model for this problem has not been established. By taking the agglomerate-laden suspension as a single pseudo fluid, we propose a two-way coupling model in which the developed governing equations for suspension and particle general dynamic equation (PGDE) for particle coagulation and breakage due to turbulence are simultaneously solved. The Taylor-expansion moment method has been applied to solve fractal-like aggregate process and dilute gas-to-particle conversion, and in this study it is further extended to close the PGDE equation by invoking fractal theory. The newly coupling model is not limited in dilute particle system, and thus it is expected to play important roles in studying dense fractal-like agglomerate synthesis or other particulate industrial applications.

The concept of head loss coefficients K for the determination of losses in conduit components is discussed in detail. While so far it has only been applied to fully turbulent flows it is extended here to also cover the laminar flow regime. Specific numbers of K can be determined by integration of the entropy production field (second law analysis). This general approach is discussed and illustrated with the specific example of a conical diffusor.

This paper aims to study the convective heat transfer behavior of aqueous suspensions of nano-diamond particles flowing through a horizontal tube heated under constant heat flux condition. Consideration is given to the effect of particle concentration and Reynolds number on heat transfer enhancement. It is found that (i) significant enhancement of heat transfer performance due to suspension of nano-diamond particles in the circular tube flow is observed in comparison with pure water as the working fluid, (ii) the enhancement is intensified with an increase in the Reynolds number and the nano-diamond concentration, and (iii) substantial amplification of heat transfer performance is not attributed purely to the enhancement of thermal conductivity due to suspension of nano-diamond particles.

A Numerical Simulation has been conducted to investigate the mixing of coaxial jets with zero-mass-flux synthetic jet actuators, two arrays of which are placed side by side around coaxial jets and in the exit plane of the outer primary jet. Both the temperature field of the primary jet and mixing processes are studied at given temperature ratio. High-frequency forcing leads to increases in the radial extent of the jet and turbulent intensity in the near-field and increased turbulent dissipation. During the analysis, the influence of adjacent synthetic jets is also considered. The comparison is made between the control effectiveness of two arrays of actuators and that of an array of actuators. The results show both of the actuator’s configurations can result in the significant mixing enhancement, but the former exhibits more desirable.