Search

In this work, the effect of using baffles and vortex generators on the cooling performance of a gas turbine combustion chamber is numerically investigated. The RNG k-ɛ model is utilized in order to simulate the flow turbulence. The effects of baffles and vortex generator pitch, hydraulic diameter and bypass ratio on the Nusselt number and friction coefficient at different Reynolds number are studied. The cooling performance of applying baffles and vortex generators are also evaluated and compared based on the Performance Evaluation Criterion (PEC). The numerical results are verified with available experimental data. The results show that the pitch increment results in the decrement of the Nusselt number and the wall friction coefficient due to the lower vortices generation and the turbulence kinetic energy dissipation. Also, the Nusselt number is increased by increasing both the bypass ratio and the hydraulic diameter. It is also found that vortex generators and baffles improve the cooling performance of the gas turbine combustion chamber by 18.8% and 26% respectively through decreasing the external wall temperature.

The MARS code has been assessed for the prediction of onset of flow instability (OFI) in a vertical channel. For assessment, we built an experiment database that consists of experiments under various geometry and thermal-hydraulic condition. It covers pressure from 0.12 to 1.73 MPa; heat flux from 0.67 to 3.48 MW/m2; inlet sub-cooling from 39 to 166 °C; hydraulic diameters between 2.37 and 6.45 mm of rectangular channels and pipes. It was shown that the MARS code can predict the OFI mass flux for pipes reasonably well. However, it could not predict the OFI in a rectangular channel well with a mean absolute percentage error of 8.77%. In the cases of rectangular channels, the error tends to depend on the hydraulic diameter. Because the OFI is directly related to the subcooled boiling in a flow channel, we suggest a modified subcooled boiling model for better prediction of OFI in a rectangular channel; the net vapor generation (NVG) model and the modified wall evaporation model were modified so that the effect of hydraulic diameter and heat flux can be accurately considered. The assessment of the modified model shows the prediction of OFI mass flux for rectangular channels is greatly improved.

The microchannel is one of the essential components for the construction of various micro systems. However, it has been reported that the flow and heat transfer behavior in a microchannel deviates from predictions based on the conventional assumption generally accepted in macro-scale channels. Frictional characteristics of nitrogen and helium flows in three different microchannels (hydraulic diameter range 3-10 w m) have been investigated experimentally. The frictional resistance of gaseous flow in a trapezoidalcross-section microchannel was observed to be smaller than that in the conventional-sized channel. The reduced frictional resistance in microchannels is caused by the rarefaction effect due to extremely small dimensions of flow passages. By using Maxwell's first-order slip boundary condition, we can well predict the mass flow rate through microchannels and the friction constant.

The present paper reports the thermal behavior of flat and curved surfaces with impinging jets employing circular and elliptical nozzle for identical equivalent diameter (de). Tests are performed w...

The suction force on high-sphericity seeds is important to the design of air-suction seed-metering devices. However, little systematic research has been conducted on the suction force. A self-designed suction force measurement device and idealised model were used to conduct CFD simulation, single factor experiments, and orthogonal experiments to improve the model of the suction force on seeds with sphericity over 90.34% and equivalent diameter 6.63–7.16 mm under the following conditions: suction hole diameter 4–6 mm, suction hole vacuum 3–5 kPa, seed-filling angle ±π/8, and horizontal distance from the seed centre to the suction hole 3.5–5.5 mm. The verification experiment conducted using the seeds of soybean, pea, and Panax notoginseng showed that the accuracy was higher for the improved suction force model. Simulation of flow behaviours under different factors revealed that the changes in the air velocity gradient and pressure gradient are the main causes of the change in the suction force. The relationship between the drag force model and the suction force model was studied by comparing the predicted value of the drag force and the measured value of the suction force under the same conditions.

This work is aimed at investigating the effects of the refrigerant charge, expansion device restriction, compressor speed, and their interactions on the performance of a household refrigerator. To this end, a micrometric valve was installed in series with the capillary tube of a frost-free refrigerator to vary the expansion restriction within a range equivalent to capillary tubes with inner diameters spanning from 0.55 to 0.85 mm. The refrigerant charge was manually changed from 35 to 65 g whereas the compressor speed was adjusted between 2500 and 4500 rpm. In total, 294 steady-state energy consumption tests were conducted, being 147 at a surrounding temperature of 32 °C, and the rest at 16 °C. It was noticed that low energy consumptions can be achieved for several combination of refrigerant charge and expansion restriction, albeit the minimal energy consumption figures were always achieved for the highest charges and restrictions. It was also verified that the refrigerator operates in a sub-optimum region so that increasing the charge from 42 g to 52 g while decreasing the capillary tube diameter from 0.64 mm to 0.60 mm led to a 3% decrease on the energy consumption. It was also found that a single combination of refrigerant charge and equivalent diameter provided satisfactory performance levels regardless of the compressor speed. Finally, a method to reduce the number of tests was proposed, in such a way that only 9 datapoints are required to generate a full energy consumption map for a given compressor speed and surrounding temperature.

Multi-holed Orifices (MOs) are widely used in swirling, cavitating and developing flows to reduce pressure losses. This study focuses on analysis of MOs for developing turbulent flows. A large number of experiments were performed, exploring the number of holes (n), Equivalent Diameter Ratio (EDR), compactness ratio (C), plate thickness ratio (s/d) and upstream developing length (L/D) in the Reynolds number (ReD) range of 24,500–55,500. An empirical correlation of pressure loss coefficient and flow rate is formulated. It is observed that pressure loss coefficient decays with the fourth power of EDR. A comparative study of the experimental data with the literature is presented to ensure consistency and reliability.

A three-dimensional computational model, coupled with the Large Eddy Simulation (LES) model and Volume of Fluid (VOF) multiphase model, was established to investigate the two-phase flow and slag entrainment during steel bloom continuous casting. The influence of the in-mold electromagnetic stirring (M-EMS), casting speed, and submerged entry nozzle (SEN) immersion depth on the flow pattern and slag entrainment were evaluated. The number, size, velocity, and spatial distribution of slag droplets were counted and the location, velocity and probability of the slag entrainment on the meniscus were exported in real time through a user defined function. The result shows that the slag entrainment mainly occurred around the 1/4 width of the mold. The equivalent diameter of most slag droplets was 2–6 mm under the current casting condition. A formula for the equivalent diameter distribution of slag droplets was proposed according to the statistical result.

The flow field in a semi-circular duct is simulated by Large Eddy Simulation (LES) and its particle field is simulated by Lagrange particle tracking method. Reynolds number Reb (based on bulk velocity and hydraulic diameter) is 80,000 and Reτ (based on friction velocity and hydraulic diameter) is 3528. Particle diameter dp is chosen as 10, 50, 100, 500 μm corresponding to St as 0.10, 2.43, 9.72, 243.05. The results show that the intensity of the secondary flow near the ceiling is less than that near the floor because the ceiling is curved and able to inhibit the secondary flow. It is found that the difference between the semi-circular duct and the square duct is that the secondary flow in a corner of the semi-circular duct is not symmetrical along the diagonal although they have the same generation mechanism. Regarding the particles, small particles (dp ≤ 10 μm) are found to uniformly distribute in the duct, while large particles (dp ≥ 50 μm) preferentially distribute in the corner and floor center. The maximum particles (dp = 500 μm) fall on the floor quickly and their dispersion mainly depends on the secondary flow near the floor. Particle deposition in the corner depends on particle size due to the effect of secondary flow and gravity. The effect of lift force on particles becomes more significant for 50 and 100 μm particles in comparison with other smaller particles. In the end, the effect of secondary flow is found to be more significant to dominate particle behavior than that of flow fluctuation.

The secondary atomization of droplets is one of the means to improve the efficiency of diesel fuel injection atomization. As a promising biomass fuel, emulsified biodiesel showed a good prospect in improving the atomization effect of diesel engines. In this study, a high-temperature and pressure-resistant evaporator was designed to simulate diesel-like conditions, and the evaporation and combustion experiments of emulsified biodiesel droplets were carried out. The morphological changes in the droplets were dynamically captured using a high-speed camera. According to the collected images, the evaporation characteristic parameters, the dynamic parameters of droplet motion, and the correlation between the original and secondary droplets were quantitatively analyzed. The gain effect of secondary atomization for droplets on the diesel engine spray was evaluated. The results showed that the emulsified biodiesel underwent nucleation, agglomeration, puffing, and explosion during the evaporation process, while the classic d2 law only meets a few cases of this fuel. The effect of high temperature was reflected in reducing the normalization time of droplet agglomeration and explosion, while the higher pressure inhibited the expansion of the droplets, thus slowing down the expansion rate and restricting the droplet volume. Driven by the water content, the time of droplet explosion was closer to the droplet lifetime. The evaporation process of the secondary droplet was similar to that of the original droplet over a reduced scale of 1-2 orders of magnitude. (Dd /Dd0)2 ≈ 1 was a necessary condition for the secondary droplets to be produced in large quantities. The average equivalent diameter of the droplets was distributed in the range of 80-140 μm, and the secondary atomization caused expansion of the spray range by 20-40%. Expansion of the range of secondary droplets was beneficial to shortening the ignition delay and increasing the combustion rate.

Two-dimensional time-resolved particle image velocimetry (TR-PIV) measurements were carried out in confined and non-confined channels with a backward facing step (BFS) while the inlet flow was an oscillating jet from a double feedback fluidic oscillator. In addition, different Reynolds (Re) numbers in the range of 2500–10,000 were applied (based on the hydraulic diameter and mean velocity of jet throat) and the velocity distribution of the oscillating jet was measured in external domains. The effects of the aspect ratio (AR) of the expanded region and the expansion ratio (ER) were studied. Typical time-averaged structures of velocity statistics and the Proper Orthogonal Decomposition (POD) method were used to analyze the flow dynamics. The results show that the oscillation frequency is independent of the external domain and is almost constant for different ER or AR. In contrast, the spreading angle of the fluidic oscillator is much larger value when increasing Re number or decreasing AR. Additionally, a pair of vortices is induced because of the entrainment effect in confined external domain, and the velocity fluctuations can also be increased. Furthermore, the POD analysis revealed that the first two modes are predominant and represent the main characteristics of the flow.

Flow boiling experiments in microchannels under low mass fluxes have rarely been reported in the literature. In this study, flow boiling critical heat flux (CHF) for a parallel straight copper microchannel heat sink with 41 channels and hydraulic diameter of 420 µm (width: 300 µm; height: 700 µm) is reported under low mass fluxes(< 100 kg/m2s). The flow patterns reveal that the inertial force will be too low to flush the bubble downstream. The heat transfer coefficient was observed to decrease with an increase in the heat flux or vapor quality, which pointed toward convective boiling dominancy inside the channels. When approaching the CHF, the vapor flowed back to the heat sink’s inlet plenum, resulting in a flow blockage, which is the main factor for the CHF occurrence.

In the current study, three-dimensional models were established to simulate the transient formation and dripping of droplets during Electroslag Remelting (ESR) of steel, combining the κ-e two-equation turbulence model, Volume of Fraction (VOF) multiphase model, and Magnetohydrodynamics (MHD) model. The influence of the interfacial tension on the varied size distribution and the transient dripping velocity of droplets were calculated and statistically summarized. The absolute velocity distribution of droplets showed that all the fluid inside the droplet flowed downward, while the relative velocity distribution illustrated a recirculation inside the droplets, indicating that the flow of molten steel inside the droplet have two kinds of motion: a downwards flow due to the gravity and a recirculation inside the droplet which led to the deformation of droplets.With a melting rate of 0.03 kg s−1, the equivalent diameter and the dripping frequency in functions of the interfacial tension were D = 0.0119 σ0.23 and f = 1.45 σ −2.37, respectively.

In this work, the optimization of a monolith reactor 3-dimensional model is carried out, for a first order reaction, isothermal conditions and fully developed laminar flow. The obtained results are consistent with the optimal reactor design obtained under plug flow conditions, namely a maximum conversion reactor features maximum channel width, minimum hydraulic diameter, and maximum fluid velocity determined by an upper bound that is a function of the channel’s width, height, and the capital cost to compression cost ratio.

The present study focuses on the energy performance, entropy generation, and irreversibility of the use of nanofluid in an industrial Peugeot automobile radiator (size: 250 x 301 x 60.4 mm, channel number: 34 and hydraulic diameter: 1.923 mm) with a louvered fin type. 50:50 EG - water which is widely used in existing automobile radiators and a new generation of EG - water - Al2O3 (0.5%) nanofluid have been compared. In order to examine the effect of variable operating conditions on thermal performance for both fluid mixtures, experiments have been performed at variable air velocity (4-5 m/s), variable coolant flow rate (10-15 - 20 It/min), and an inlet temperature of 95 degrees C (real automobile conditions). Thermohydraulics calculations such as entropy generation, irreversibility, effectiveness, NTU, heat transfer rate, pumping power have been obtained from experimental data. Among conducted experiments, with increasing coolant flow rate, entropy generation increases on the airside, while decreasing on the coolant side. When the entropy generation due to temperature and pressure difference is compared, entropy production due to pressure difference can be neglected. It has been evaluated that the use of nanofluid increases the heat transfer rate by 9.52%, reducing the irreversibility by about 68% at the 4 m/s air velocity and 10 It/min coolant flow. (C) 2020 Karabuk University. Publishing services by Elsevier B.V.

This paper discusses the use of volumetric enhancers in ducts of heat exchangers for high-viscosity liquid media, which ensure the transition from a laminar flow to a turbulent one. Small random metal packings with a large free volume and an equivalent diameter of 5 to 25 mm were used as volumetric enhancers. It has been found that volumetric enhancers provide an increase in the heat transfer coefficient by a factor of 15 or more when industrial oil is heated. For a comparative assessment of the thermal hydraulic effectiveness of various packings, a modified expression of the Kirpichev energy coefficient has been obtained, which is associated with the thermal effectiveness of the process. The results of calculations are presented and conclusions are drawn about more effective designs of enhancers and operating modes.

The article presents an experimental study on heat transfer and friction behaviors in a solar air duct fitted with multiple V-shaped ribs on the absorber and delta-grooves on the back plate. Measurements were carried out in the test duct having a cross-section of width, W = 300 mm and height, H = 27 mm. The air flow rate inside the duct was varied to have Reynolds numbers based on the duct hydraulic diameter from about 7000 to 30,000. Two vortex flow devices: rib and groove turbulators, were introduced to generate the vortex flows along the duct. In the test duct, the upper/absorber plate having a constant heat-flux was mounted repeatedly by multiple V-shaped thin ribs with an attack angle (α) of 45° relative to main flow direction while the lower/back plate was grooved periodically in the delta/triangular shape with an attack angle (θ) of 60°. In the present investigation, the geometrical parameters of the ribs included three different rib- to duct-height ratios (e/H = BR = 0.108, 0.162 and 0.217) and three rib-pitch to duct-height ratio (P/H = PR = 1.0, 1.5 and 2.0). The experimental results have shown that the duct with the V-ribbed absorber at BR = 0.217, PR = 1 in conjunction with the delta-grooved back plate has the greatest heat transfer and pressure loss. However, the use of the combined devices with PR = 1, BR = 0.108 leads to the highest thermal performance and also provides greater heat transfer and thermal performance than employing the V-rib or the delta-groove alone.

In present work, numerical simulation has been performed using the volume of fluid method under steady-state condition to find the effect of various operating parameters, viz. mass flux, vapor quality and effective thermal conductivity on heat transfer coefficient using refrigerants R1234ze and R1234yf. The numerical simulation has been carried out using various turbulence models, viz. the standard k − e model, the standard k − ω model and the low-Re SST k − ω model. The model having minimum deviation with experimental results is considered for subsequent study. The hydraulic diameter and length of mini-channel considered are 1.085 and 420 mm, respectively. The vapor quality has been varied from 0.14 to 0.95, and mass flux has been varied from 200 to 800 kg/m2 s keeping saturation temperature of 313 K. The heat transfer coefficient results of hydrofluoroolfine refrigerants R1234yf and R1234ze have been compared with hydrofluorocarbons refrigerant R134a in order to check the suitability of new hydrofluoroolfine refrigerants as a replacement to hydrofluorocarbons refrigerant. It has been observed from the flow regime map proposed by Sardesai et al. (Chem Eng Sci 36:1173–1180, 1981) that majority results of heat transfer coefficient for refrigerants R1234ze and R1234yf fall in the annular flow regime. It has been observed that the effective thermal conductivity may play vital role in the local heat transfer rate compared with the liquid film thickness.

Experiments are conducted to study the heat transfer and friction in a rectangular duct roughened by arrays of alternate ribs (square-, triangle-. and circle-shaped ribs). The Reynolds number Re based on duct hydraulic diameter rages from 12.000 to 60.000. whereas the rib pitch-to-height ratio (P/h) varies from 6 to 18. To understand the mechanism of heat transfer enhancement the measurements of streamwise mean velocities are also conducted in the square-ribbed duct. The results show that the flow of air through the ribbed duct becomes periodically fully developed after the third rib from the duct inlet. The data indicate also that the triangular type ribs have a substantially higher heat transfer performance than any other ribs in the studied range. The experimental results show that the heat transfer enhancement factors are greater than the friction factor ratios associated with the use of rib-turbulators in the rectangular ducts. Also, the rib conductivity affects significantly on the heat transfer enhancement factor. Correlations for friction and heat transfer in square-ribbed ducts are developed to account for rib spacing to height ratio and flow Reynolds number.

This paper is focussed on investigating into some key aspects relating to design of an integrated proton exchange membrane fuel cell. Analysis of all currently utilised storage mediums was undertaken. High and low inlet operating conditions were studied by determining the distributions of over potentials, velocity distribution, and pressure. At entrance and exit, a triangle channel flow-field was predicted to have better polarization performance and regular velocity distribution, whereas for rectangular that was performance differences among four serpentine flow-fields for hydrogen and air. The pressure in the triangle channels were monitored to have periodically similar plots and showed important low pressure drop. Considering low pressure drop of rectangular channel has mid effect between triangles and rectangular for the same dimension that depends on the hydraulic diameter, these flow-fields would be beneficial for the larger inlet channel area. The study found that the triangular channels produced a better performance than their square flow path counterparts.

The current study is based on the DEM computer simulation of three experimental test devices with different dimensions to determine the difference in the results of the formation of shear and repose angles that the particles experience when grouped under the action of the gravitational force. In this respect, the experimental test devices with different height, width, and depth were geometrically modeled with iron pellet particles using morphology and a granulometric variation from 6 mm to 9 mm of equivalent diameter in its spherical shape. Depending on the results obtained, a reliable size of the experimental test device will be available to obtain the necessary data for a correct adjustment of the calibration parameters for the DEM simulation of mining-metallurgical processes that use granulated material of iron pellet.

The article presents measurements and calculations of selected gas phase parameters in a liquid metal-gas flow in a magnetic field. The gas phase were argon bubbles, while the liquid metal was the GaInSn eutectic. The flow velocities of the gas bubbles, their equivalent diameter, and drag coefficient values were calculated for different argon inlet nozzles. The boundaries between the rise of individual bubbles and the chain flow were also indicated. The ultrasonic pulse echo method was chosen as the measurement method.

In order to design a more compact small nuclear reactor, liquid metal gallium can be selected as the reactor coolant. Gallium is rarely used in reactor as liquid metal coolant, however, due to its excellent thermal conductivity and low melting point, research on the flow and heat transfer characteristics of liquid metal gallium in channels can provide a theory basis for nuclear reactor design optimization and relax reactor safety limits. The flow heat transfer characteristics of liquid gallium are obtained by calculating the surface heat transfer coefficient of liquid gallium in various thermal and structural parameters. The calculation results show that the heat transfer between liquid gallium and the pipe wall can be enhanced by the increase of the inlet flow velocity, the increase of the wall heat flux density, or the reduction of the pipe size. When the hydraulic diameter of each pipe is the same, the heat exchange effect of liquid gallium flow in tubes is: triangle tube>square tube>round tube. The flow heat exchange effect of liquid gallium is better than that of lead-bismuth fluid.

The objective of this research is to study flame structure and heat transfer characteristics for the premixed flame jet from the swirling chamber. In this study, LPG and air was utilized as gas fuel and oxidizer for a premixed flame. The equivalence ratios () of LPG and air were considered at 0.8, 1.0, and 1.2 under a Reynolds number Re = 4,000. The swirl flame was generated by double tangential inlets in cylindrical chamber. The diameter of chamber was fixed at D = 20 mm and the hydraulic diameter of the inlet was Dh = 5 mm. In this study, the effect of chamber geometry on flame structure was investigated by varying the chamber from H = 2.2Dh to 7.0Dh. The structures and temperature of the free flame jet was recorded with camera and measured with a thermocouple. The heat transfer rate of impinging flame jet was also measured at distance from chamber outlet to flame impingement surface varying from L = 4Dh to 10Dh. The results show that the maximum of flame temperature occurs at =1.2. Impinging flame jet for case of chamber height at H = 4.6Dh and impingement distance at L = 4Dh give the highest heat transfer for all equivalence ratios due to the reaction zone of combustion reached to approach near the heat transfer surface.

The aim of this work is at investigating the applicability of electronic expansion valves (EEVs) in domestic refrigerators. To this end, a valve with an orifice of 0.19 mm of hydraulic diameter was installed in a conventional refrigerator to correlate the flow restriction with the overall energy consumption spanning different operating conditions. Tests with the same refrigerator mounted with a capillary tube were also performed. Also, a mathematical model was devised to simulate the effect of the refrigerant charge and the expansion device restriction on the system performance. More than a thousand different conditions were simulated. The results revealed that zero superheating is not necessarily the best-case scenario in terms of energy consumption. In addition, it was noticed that the lowest energy consumptions were achieved for the highest refrigerant charges and expansion restrictions. Furthermore, it was observed that, when the surrounding temperature or the compressor speed were changed, the energy consumption could be reduced by varying the restriction of the expansion device. The system with the EEV, nonetheless, presented higher energy consumptions than the system with the capillary tube for most conditions, which was mostly related to the extra thermal load generated by the valve. Nevertheless, by increasing the internal heat exchanger length and mitigating the heat dissipated by the EEV, it was found that the system with the EEV consumed between 4 and 9% less energy than the system mounted with capillary tube under operating conditions different from that the capillary tube has been sized for.

To solve the problem of non-uniform refrigerant distribution among multi-tubes of micro-channel heat exchangers (MCHXs), a novel embedded-clapboard distributor is presented in this study, in which the embedded clapboards have a spray hole to transform the irregular flow pattern of inlet refrigerant into a dispersed bubble flow, and to form a cyclic channel to maintain the regular layout of dispersed bubble flow. The design method for the distributor is illustrated, including the calculation method for the spray hole diameter deduced from the transition criteria of dispersed bubble flow at both the horizontal and the vertical installations, and the calculation method for the hydraulic diameter of cyclic channel deduced based on the Bernoulli Equation for the circulation of dispersed bubble flow. The validation of refrigerant distribution performance includes the comparison of two-phase flow pattern between the proposed distributor prototype and conventional distributor prototype manufactured with transparent material, and the test of distribution unevenness among the outlet tubes for these two types of distributor prototypes. The experimental results show that the embedded-clapboard distributor forms the regular dispersed bubble flow at all installation patterns, and the average unevenness of the proposed distributor at the vertical, the sloped and the horizontal installations is 42.5%, 45.9% and 41.4% lower than that of the conventional cylinder-type distributor, respectively.

Nowadays, there is a large demand for many electronic devices such as the laptop, and cell phone. The heat generated by such electronic component increases due to continuous functioning. Implementing microchannel could be a good solution despite the heat from microminiature refrigerators, microelectronics, micro heat pipe spreader, fuel processing biomedical, and aerospace. Therefore, several investigations have been done to improve the performance of such continuous operating electronic devices by dissipating heat with the use of microchannels. In this study, an experimental and numerical investigation is done for the circular microchannel having the hydraulic diameter of 253 µm and 63 mm in length under the condition of constant wall temperature by submerging the microchannels in the oil at constant temperature and water is forced to pass through total 5 microchannels. Experimental conducted for various flow rates shows that the microchannel has a significant impact on the heat transfer rate for the considered flow rate. Numerical results through COMSOL 5.1 software show good agreement with the experimental results. It is observed that the heat transfer coefficient increases with the Reynolds number whereas friction factor decreases with Reynolds number. Based on numerical and experimental results, empirical correlations for friction factor and Nusselt number are suggested to provides a reasonable estimate of heat transfer in the microchannel.

In this paper, an experimental study on the effect of the low aspect ratio of microchannel on the critical heat flux (CHF) during saturated flow boiling of water is presented. Multiple parallel silicon microchannels with different aspect ratios, hydraulic diameter, and number of channels in the range of 0.2–0.5, 154.0–184.8 μm, and 3–9 respectively are fabricated, and the experiments are performed in the range of mass fluxes 24.1–806.5 kg m−2 s−1. It is found that the CHF is caused by the wall dryout near the microchannel exit. The effect of microchannel aspect ratio with the same hydraulic diameter on the CHF is found to be insignificant. Twenty-five CHF data points are obtained for microchannel flow boiling of water and are compared with the CHF predictive methods reported in the literature. A new CHF correlation is proposed, which predicts the measured CHF with the mean absolute deviation of 12.1%, which is substantially better than available CHF predictive methods. The findings from this study provide guidelines for designing multiple parallel microchannel heat sinks in two-phase flow for the thermal management of electronic equipment, spacecraft, satellites, energy conversion devices, and biomedical engineering.

Particle-resolved, three-dimensional, time-dependent simulations of rigid and flexible cylinders fluidized by a liquid flow in fully periodic domains have been performed by means of the lattice-Boltzmann method supplemented with immersed boundaries. The solids volume fraction ranges from 0.10 to 0.48 and the length-over-diameter aspect ratio of the cylinders from 4 to 12. The bending stiffness of the cylinders is the third major input parameter. The resulting Reynolds numbers based on the average slip velocity of the cylinders and their equivalent diameter range from 6 to 70. It is shown that increasing the flexibility—that is, decreasing the bending stiffness—reduces the Reynolds number, an effect that is most pronounced for low solids volume fractions and long cylinders. As for rigid cylinders, the distribution of the orientation relative to the direction of gravity of the flexible cylinders is a pronounced function of the solids volume fraction and the aspect ratio. Flexibility tends to somewhat randomize the orientation distribution, which could explain the effect of flexibility on the slip velocity and thus the Reynolds number.

In this study, refrigerant R134a experiments were carried out to observe two-phase flow in an oscillating horizontal rectangular minichannel conduit at refrigerant mass flux of approximately 30 kg•m − 2•s − 1 to 300 kg•m − 2•s − 1, saturation temperature of approximately 30 °C, and average vapor quality in the test section inlet of approximately 0.05 to 0.9. The test section was made from quartz and consists of a visible rectangular minichannel with a hydraulic diameter of 0.84 mm. The test section was oscillated vertically using a reciprocating motor. The effects of oscillation on slug and stratified flow patterns were investigated. The experimental observation was compared with an existing flow pattern map. The thin liquid film around the vapor of slug flow mostly moved up and down when the test section oscillated. As a result, a dry surface was not observed around the vapor due to the lower heat transfer coefficient. When the test section oscillated, the stratified flow changed to annular flow, and a dry surface was also not observed on the upper side of the minichannel conduit.

We study a two-dimensional flow over a cylinder and three ellipses with the aspect ratio $$a / b = 2.04$$ , $$3.65$$ , and $$5$$ at a Reynolds number $$Re = 100$$ based on the inflow velocity and hydraulic diameter using a direct numerical simulation (DNS) and linear stability analysis. The DNS shows that increase in $$a / b$$ leads to stabilization of the flow due to decrease in the recirculation zone. The linear stability analysis based on the time-averaged velocity field shows that the modes of interest describe a Karman vortex street. The oscillation frequency reconstructed from the linear stability analysis is in excellent agreement with the DNS. Solving the linearized adjoint equations made it possible to identify the flow area where direct and adjoint modes overlap. These areas of the field $$S_w$$ , usually called “wavemaker,” change their shape with increase in $$a / b$$ . Non-zero values of $$S_w$$ tend to approach the bottom part of the “tail” of the ellipse due to the asymmetry of the recirculation region, attached to the top side of the ellipse.

In the current study, thermal boundary conditions are considered in a rotating smooth channel with a square cross-section to investigate the secondary flow and compare it to that of the same channel without heating. The measurement is conducted at three streamwise planes (X=445 mm, 525 mm, 605 mm). The flow parameters are the Reynolds number (Re=4750, which was based on the average longitudinal or primary velocity U and the hydraulic diameter D of the channel cross-section), the rotation number (Ro=ΩD/U, where Ω is the rotational velocity, ranging from 0 to 0.26), and the aspect ratio of the channel cross-section (AR=1, which is calculated by dividing the channel height by the channel width). The leading and trailing walls are heated under a constant heat flux qw=380 W/m2, and the top and bottom walls are isothermal at room temperature. This work is in a series with our previous work without thermal boundary conditions. Based on the experimental data, we obtained a four-vortex regime. There is a counter-rotating vortex pair near the leading side and the trailing side. Because the leading and trailing walls are heated, the buoyancy force increases the relative vertical position of the vortex pair near the trailing side from 5% to 12.5% of the hydraulic diameter. When moving upstream along the streamwise direction, the upper vortex near the trailing wall becomes weaker, whereas the lower vortex becomes stronger. As the rotational speed increases, the vortex pair near the trailing side is inhibited by the Coriolis force. Under heated thermal boundary conditions, the vortex pair near the trailing side reappears due to the effect of buoyancy force. These results indicate that the buoyancy force has a substantial effect on the secondary flow regime under thermal boundary conditions.

Numerical simulation of velocity and temperature fields with laminar liquid flow in a laboratory experimental loop thermosiphon (natural circulation loop) has been made. A vertical electrically heated pipe served as a lift section. Calculations were carried out for the case of heating the entire length of the pipe under conditions of constant heat flux density on the wall. The construction of the loop is considered, in which local pressure losses and friction losses of pressure in the descending pipe are negligibly small in comparison with friction losses of pressure in the ascending section. The object of the analysis is the laminar mode of flow. In this regime, in the flows originating exclusively under the action of thermogravitational forces, the skin-friction coefficient varies in the most sophisticated way along the flow. Using the results of calculation of velocity and temperature fields, the longitudinal changes of the skin-friction coefficients and of heat transfer coefficients are determined. As a whole, according to the results of 2D numerical simulation, the skin-friction coefficients depend in the main on the change of the velocity gradient on the wall along the flow as a result of practically continuous rearrangement of velocity profile along the entire heating zone. The form of the velocity profiles and the degree of their deformation depend in turn on the heat flow density on the wall and on the hydraulic diameter. It is shown that in a single-phase loop of natural circulation (i.e., under conditions where the liquid moves exclusively under the action of thermogravitational forces) the shear stresses on the wall change along the heating zone in a complex way, and the skin-friction coefficient cannot be described by the simple dependence of the form ξ = a/Reb for being used in one-dimensional calculations. In all of the calculated regimes, including the one with the least (of those considered) heat flux density on the wall, the Nusselt numbers exceeded the stabilized values under the conditions of forced flow with constant thermophysical properties. With increase in distance from the entrance into the zone of heating, the Nusselt numbers first decrease monotonically, attain minimum values, and then begin to increase.

The current study presents an experimental investigation of heat transfer and flow characteristic for subcooled flow boiling of deionized water in the microchannel heat sink. The test section consisted of a single microchannel having 300μm wide nominal dimensions and 300μm height (hydraulic diameter of 300μm). The test section formed of oxygen-free copper with 72mm length and 12mm width. Experimental operation conditions spanned the heat flux (78-800) kW/m2, mass flux (1700 and 2100) kg/m2.s at 31˚C subcooled inlet temperature. The boiling heat transfer coefficient is measured and compared with existing correlations. Also, the experimental pressure drop is measured and compared with microscale pressure drop correlations. The results showed that higher mass flux leads to higher boiling heat transfer coefficient, and the dominant mechanism is convective boiling. Also, the experimental pressure drop decrease with increasing heat flux in a single-phase region while it increases in a two-phase region. Comparing the experimental results in the experimental condition range, showed that an existing correlation provides a satisfactory prediction of heat transfer coefficient and pressure drop.

This work attempts to determine the influence of the integration of a combination of roughness elements on the heat dissipation and frictional losses occurring within the airflow channel pathways of a counterflow double-pass solar thermal air heater. The investigation considers three configurations, namely (1) circular rings with a relative width ratio of 4 (Case A), (2) circular rings with a relative width ratio of 6 (Case B), and (3) combination of circular rings and ribs (Case C). All three configurations were studied at different operating conditions of the Reynolds number (in the range of 4000 to 10,000) and relative roughness height (0.026 and 0.043). During the experiments, the operating parameters considered as constants for comparison were heat input (1000 W m−2), aspect ratio (20), and hydraulic diameter (0.048). The resultant observations of the three configurations were also compared with that of the smooth-plate absorber configuration. The observations revealed that with an increase in relative roughness height, there was only a marginal improvement in Nusselt number. The experimental findings reported that Case C resulted in maximum improvement in Nusselt number with a penalty of increase in friction factor among the different configurations. For Case C, the calculated maximum value of the Nusselt number ratio and friction factor ratio obtained corresponding to relative roughness height of 0.043 was 3.19 and 1.46, respectively. The maximum enhancement obtained in the thermohydraulic performance parameter for Case C configuration corresponding to the relative roughness height of 0.026 and 0.043 was 2.91 and 3.01, respectively.

The gas-solids flow in high-density downers with FCC particles was simulated by Computational Particle Fluid Dynamics (CPFD) approach. A drag model was proposed based on the equivalent diameter of cluster with the analysis of force balance on particle cluster. A global sensitivity analysis method was used to analyze the influencing factors of cluster size. The drag model was incorporated into the three-dimensional (3D) CPFD approach to simulate the gas-solids flow in two different downers operating at various conditions. 3D CPFD simulation provided detailed trajectories and distributions of solids. Good agreements of the predicted solids holdup and velocity with experimental data were achieved at both low and high-density conditions, which verified the rationality and applicability of the present drag model considering the characteristics of particle cluster in the downer. Finally, the flow and clustering behavior in the high-density downer were adequately analyzed and compared with that in low-density downer.

In this reported study, a new Q criterion was proposed to analyze the vortex in three cyclones that had various inlet structures. The Reynolds stress model (RSM) was used to simulate the gas flow, and the Phase Doppler Particle Analyzer (PDPA) was used to measure the fluid velocity. The results showed that the vortex structure was intuitive according to the new isovortex surface that was obtained using the Q criterion. The isovortex surface was found to be distorted and not distributed around the center axis. With the development of flow, the equivalent diameter of the isovortex surface gradually decreased. The magnitude of the vorticity and the vortex lines of axisymmetrical volute inlet (AVI) structure were more uniform and symmetrical than those found into the single tangential inlet (STI) structure and the single volute inlet (SVI) structure.

A series of experiments were carried out to investigate the effect of ullage height, i.e. the distance between fuel surface and the pool upper rim, on flame characteristics of heptane and ethanol pool fires. The pool diameters included 10 cm, 15 cm, and 20 cm. For each pool, ullage height was increased from zero to the value that the flame self-extinguished. During each test, the ullage height was kept constant by a fuel level maintaining device. Results show that ullage height has a significant effect on flame characteristics. When the ullage height equals zero, there is a stable conical structure near the pool upper rim, above which the unstable plume develops. Under lower ullage height conditions, plume puffing happens. The unstable plume begins to initiate around the pool upper rim, then rolling and expanding upward. At the same time, its beneath flame is stretched thinner and thinner and eventually breaks from the bulk. When the ullage height further increases, plume puffing becomes less evident. While for the mass loss rate, it generally decreases with the increase of h/D. It was proved in literature that the previous flame height model is inapplicable to pool fires with large ullage height. In this paper, to consider the effect of ullage height on plume flow structures, an equivalent hydraulic diameter, Deq., is proposed to establish one new flame height correlation. By comparing previous model predictions with the current experimental data, it is found that conventional flame oscillation models are unsuitable to pool fires with large ullage height. Based on Deq., two new dimensional and dimensionless flame oscillation models were proposed according to the current experimental data. The correlated flame height model and proposed flame oscillation models were validated and agreed well with reference data.

In the present work, heat transfer characteristics and flow field from a heated elliptic cylinder immersed in fluidized bed is studied numerically using CFD package. The flow pattern and heat transfer fields around the heated cylinder is predicted using a two fluid Eulerian-Eulerian model coupled with kinetic theory of granular flow (KTGF) with heat transfer. Runs were carried out under uniform heat flux condition, with air as a fluidizing gas and pulverized coal as a bed material. The Reynolds number based on the equivalent cylinder hydraulic diameter, Dh, was ranged from 941to 1263. The simulation results of the present study showed that, for the range of gas velocity considered, at minimum fluidization velocity, Re = 914, the Nuθ is maximum at the sides of the cylinder and minimum at the stagnation, θ=0o, and top, θ=180o, of the cylinder. The average Nusselt number for the elliptic cylinder increases with the increase of the fluidization velocity. The present data is compared with a previous well-known correlation from the literature for a circular cylinder and a reasonable deviation is found.

This experiment measured the instantaneous temperature and velocity field synchronously in non-isothermal turbulent boundary layer in a rotating straight channel with a parallel-array hot-wire probe. The Reynolds number based on the bulk mean velocity (U) and hydraulic diameter (D) is 19000, and the rotation numbers are 0, 0.07, 0.14, 0.21 and 0.28. The mean velocity u and mean temperature T as well as their fluctuating quantity u’ and T’ were measured at three streamwise locations (x/D = 4.06, 5.31, 6.56). A method for temperature-changing calibration with constant temperature hot-wire anemometers was proposed. It achieved the calibration in operational temperature range (15.5 °C–50 °C) of the hot-wire via a home-made heating section. The measurement system can obtain the velocity and temperature in a non-isothermal turbulent boundary layer at rotating conditions. The result analysis mainly contains the dimensionless mean temperature, temperature fluctuation as well as its skewness and flatness and streamwise turbulent heat flux. For the trailing side, the rotation effect is more obvious, and makes the dimensionless temperature profiles lower than that under static conditions. The dimensionless streamwise heat flux shows a linear decrease trend in the boundary layer. It is hoped that this research can improve our understanding of the flow and heat transfer mechanism in the internal cooling passages of turbine rotor blades.

An experimental study was carried out to investigate the flame characterization and temperature profile for single and multiple pool fire with the influence of cross wind. There were 13 test cases in total, categorized into circle and rectangle fuel pans, with diameter (or equivalent diameter) ranged from 50 mm to 300 mm. Kerosene was used for the fuel of pool fire. Some K-type thermocouples were arranged around the flame to monitor the flame temperature, while the flame tilt angle was measured based on the photograph of flame for different case. Firstly, it can be found that there are three phases, including preheating, steady burning and extinguishing phase, during the flame evolution. The maximum temperature near the fuel surface is ∼1040 K, which is higher than that of flame plume (∼600 K), in the steady burning phase of circle single pool fire (D=300 mm), while the average burning rate is ∼1.525 g/s. In addition, the burning rates of all cases were measured and compared with the current predicted method. Typically, the flame morphology of single/multiple pool fire at different cross wind speed (ranging from 0 to 3.5 m/s) was analyzed, and it is found that the results for single pool fire agree with Thomas model and AGA model well, which are not suitable for multiple pool fire. Finally, the temperature profile of different case was measured with various wind speed.

Accurate experimental characterization of longitudinal vortices induced by rectangular winglets is necessary to develop and validate numerical models that can predict these flows. For this aim, an experimental investigation is conducted to examine the characteristics of longitudinal vortices generated downstream rectangular winglets in fully developed turbulent channel flow. A row of rectangular winglet pairs is implemented in a parallel plate flow. Stereoscopic Particle Image Velocimetry (SPIV) is used to harvest three-dimensional (3D) instantaneous velocity fields for a Re = 4400 based on the channel’s hydraulic diameter and mean flow velocity. 3D velocity components, velocity streamlines, topologies of Γ2 function and normalized turbulent kinetic energy downstream the winglets are analyzed. It is shown that the flow properties are disturbed, and a main vortex pair is generated in addition to induced vortices downstream the rectangular winglets. Moreover, this work provides an SPIV benchmark of the complex flow developing downstream rectangular winglet pairs, where this configuration has been widley studied using numerical simulations.

Heat transfer, local distributions of Nusselt number, flow structure, and friction characteristics of twisted square ducts are presented. Numerical analysis was carried out to investigate the influence of the twist ratio (TR = p/D = 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0) on the thermal-hydraulic performance of twisted square ducts under constant wall heat flux condition for Reynolds numbers based on the hydraulic diameter of the twisted square duct ranging from 3000 to 20 000. The straight square duct was also analyzed for comparison. The numerical results showed that the twisted square ducts were more efficient in heat transfer than the straight square ducts because the swirl flow helped to increase fluid mixing and reduce thermal layer boundary thickness. The decrease of the twist ratio led to the increase in the Nusselt number and friction factor due to the higher frequency of swirl flow. As compared to the straight square duct, the twisted square ducts with TR = 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0 improved heat transfer by 52, 49.82, 45.85, 42.22, 39.54, 35.41, and 31.77 %, respectively. Among the studied twisted ducts, the ones with twist ratio TR = 3.5 offered the maximum thermal enhancement factor of 1.42 at Re = 3000. In addition, the results also revealed that the twisted square ducts are thermo-hydraulically superior to the straight square ducts.

The objective was to study the effect of change in swirl intensities, S=0.4, 0.7 & 1 of the annular swirling flow on the exit flow field of an unconfined annular swirl burner operated at isothermal (only dry air) and reacting flow (premixed methane air mixture) conditions. Reynolds number at the burner’s annular exit based on its hydraulic diameter (D) was kept constant at 4000. Exit flow field at isothermal conditions was measured using planar particle image Velocimetry rig and processed using commercial software. The percentage decay in the magnitude of peak value of axial velocity obtained from its radial profile at a height of 4D from the burner exit with the change in swirl intensity of 1, 0.7, 0.4 and 0 was 65%, 55%, 47.2% and 13.5%. The jet spreading angle was 6.5o for S=0, 8.4o for S=0.4, 9.8 for S= 0.7 and 14.2o for S=1. Recirculation zone was observed only for S=0.7 and 1.0. The width of the recirculation zone was 3D (S=0.7) and 3.4D (S=1) respectively. The normalized reverse mass flow rates estimated were 0.027 for S = 0.7 and 0.058 for S = 1.0. The magnitude of turbulence intensities at wake shear layer was much higher than the jet shear layer due to the presence of recirculation zones for S= 0.7 and 1.0. The integral length scales calculated were varied in the range of 0.06D0.18D for all swirl intensities. Reaction front was identified by deconvoluting the time mean OH* chemiluminescence using Abel inversion method. The flame became shorter and wider with increase in swirl number which was in consonance with the observation of increase in size of recirculation flow in the isothermal flow. The equivalence ratios at which the lean blow out observed were 0.58, 0.6 and 0.62 for S=0.4, 0.7 and 1.

The objective of this work is to evaluate the capability of different combinations of a turbulence model and a Lagrangian particle tracking (LPT) model integrating a particle-wall interaction (PWI) model to predict particle-laden flow in 90-deg bends, as well as the impact of the PWI model on the prediction of the referred flow. The experimental data from Kliafas and Holt (1987) (LDV measurements of a turbulent air-solid two-phase flow in a 90° bend. Experiments in Fluids, 5: 73-85) concerning a vertical to horizontal square-sectioned duct with a hydraulic diameter of 0.1 m that are connected by a 90-deg bend with a curvature ratio of 3.52, served as the benchmark for the aimed analysis. Air with glass spheres of 50 μm diameter flows in the experimental duct system with a Reynolds number of 3.47×105. The airflow was modelled by four different turbulence models: a low Reynolds number k-e model, the SST k-ω model, the v2-f model, and the RSM SSG model. The particle-phase was modelled by a LPT formulation, and the particle-wall interaction was calculated using four different models: Brauer, Grant & Tabakoff, Matsumoto & Saito and Brach & Dunn PWI models. The 3D simulation results of mean streamwise velocities from the sixteen RANS-LPT/PWI combinations were compared qualitatively and quantitatively to experimental and numerical data available in the literature. The four turbulence models produced errors for the gas-phase in the order of 8%. Concerning the particle-phase, the errors produced by all RANS-LPT/PWI combinations were below 4% for bend angles up to 15° and up to 18% for bend angles higher than 30°. The best results for the particle-phase were obtained with the SST k-ω and v2-f model combined with the LPT/Brauer or LPT/Brach & Dunn PWI models, which produced errors inferior to 14%.

Summary Two-phase flow in large annular ducts is very common in industrial applications. Nevertheless, many two-phase-flow phenomena in such a geometry have not been fully understood. This article addresses an investigation on the effects of duct geometry and liquid viscosity on vertical upward annular-duct two-phase flows. Compressed air and tap water or mineral oil were the working fluids. Three oil viscosity ranges were tested, from 30 to 400 mPa·s. A 10.0-m-long inclinable experimental setup was designed and built for the experiments conducted in two equivalent geometries. The first is made of a 95-mm-inner-diameter circular pipe (called the reference pipe), and the second is a concentric annular duct with 95-mm hydraulic diameter, the same diameter as the reference pipe. The radial geometries were similar to those observed in oil wells. Flow patterns, total pressure drop, and in-situ volumetric fractions were obtained. According to the results, the standard hydraulic diameter might not be appropriate for the modeling of vertical gas/liquid flow in large annular ducts.

Various studies dealing with decay characteristics of circular and noncircular supersonic jets were conducted by previous researchers. But in these studies due emphasis was not given to the hydraulic diameter (Dh), shape factor (ζ) & the nozzle lip parameters which have significant impact on the characteristics of noncircular supersonic jet. In this study, it has been shown that these parameters played a significant role on supersonic core decay characteristics [2, 3, 6] of the jet. The scope of this study included supersonic core length (Lc), decay pattern, due to noncircular shaped nozzle. In the literature, the supersonic jet characterization and the related experimental correlation are available for optimum expansion conditions whereas for other expansion (under and over) conditions the experimental correlation is barely available. While investigating experimentally, new empirical relations were obtained which were the improved forms of earlier correlations for supersonic core length [4]. For experiments, six different types of nozzles (circular, hexagon, square, triangular, elliptical and rectangular) with the same exit to throat area ratio, convergent length and divergent length were used. The results obtained from the experimentally developed correlations were coherent with numerical results, experimental data and flow visualization.

This paper investigated effects of slip length and hydraulic diameter on the hydraulic entrance length of laminar flow in superhydrophobic microchannels. Numerical investigations were performed for square microchannels with Re ranging between 0.1 and 1000. It is found that superhydrophobic microchannels have a longer hydraulic entrance length than that of conventional ones by nearly 26.62% at a low Re. The dimensionless hydraulic entrance length slightly increases with the increasing slip length at approximately Re

The hydraulic performance of thin-wall lay-flat polyethylene pipes with non-coaxial emitters were evaluated in this study. Two experiments were performed to measure the dimensional variation of commercially available lay-flat drip irrigation pipes. A model for estimating the corresponding local head loss was developed and validated using dimensional analysis. The model was then implemented in a finite element algorithm to analyse the hydraulic performance of this type of pipes. Finally, a design example was provided. The results revealed that, for a wall thickness of 0.13 and 0.20 mm, the pipe height and width tended to be equal above a relatively low pressure, and the pipe cross-section rapidly inflated from a quasi-rectangular form to circular. For thicker walls, the equivalent diameter (four times the hydraulic radius) increased continuously with increasing pressure head but it hardly reached its nominal diameter (16 mm). The proposed model indicated that the local head loss was affected by the equivalent diameter, emitter geometry, and flow velocity. The model had high accuracy in determining the local head loss. The estimated results were in close agreement with the measured data. According to the numerical simulation, the ratio of local head loss to total loss along a pipe was up to 74.71%. The design results, when using an earlier model, demonstrated a longer maximum lateral length, since the model did not consider further contraction of the flow caused by pipe deformation. However, the model proposed here could be used in the design of systems for thin-wall lay-flat polyethylene pipes.

Cohesion may have a significant effect on particle fluidization. Strong cohesive forces may result in a non-fluidized bed of particles. The effect of cohesive forces mainly depends on the magnitude of the forces and the particle weight. Geldart's classification places materials into four groups based on particle diameter and density difference between the particles and the fluid. Each group has different fluidization characteristics caused by the cohesive forces; specifically, in Geldart's experiments, these were Van der Waals forces (VdWFs). This study presents an investigation of the cohesive forces in fluidized beds. Using Geldart's classification, it attempts to predict the classification of moist particulate materials, affected by the liquid bridge forces (LBFs) and fluidized in air. In order to do so, a model which calculates the LBFs was developed. The classification of the wet particles is predicted by calculating an equivalent diameter and using it in the Geldart chart.