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Achieving axisymmetric boundary conditions in turbulent swirl flows and predicting thermo-fluidic behaviours within a nozzle is often a challenge due to the difficulties of measurement diagnostics. Adding nanoparticles to ase fluids exacerbates the complexity because of the regeneration of effective properties of the fluid medium. As such, this study numerically investigates the flow and thermal characteristics for the effect of swirl and nanofluids of incompressible turbulent liquid jets. The results show that the axial and tangential velocity profiles for water-based nanofluids resemble to those for water in no-to-high swirl flows. The surface Nusselt number for strong swirl flow reduces with axial distances at isothermal wall condition, and the Nusselt number is strongly dependent on types of nanofluid for isoflux condition. Water-based nanofluids cause higher heat transfer compared to Dowtherm+Al2O3 and Syltherm800+ Al2O3. The average Nusselt number and the pressure drop are found to be the largest for 10% volume fraction. The increase of thermal-hydraulic performance factor for nanofluids after nozzle convergence is steeper for highly swirling flow than for non-swirling and medium swirling cases. A strong turbulent kinetic energy is observed immediately after the nozzle converging section for all nanofluid cases and Dowtherm+Al2O3 shows relatively uniform distributions across the nozzle. [ABSTRACT FROM AUTHOR]

Bidirectional vortex combustors are among the most efficient devices in terms of pollutants emission, fuel residence time, an extended range of stable operation. Despite many papers devoted to flow structure in the bidirectional chamber, their heat transfer and wall temperature distributions are not well-studied experimentally as well as most available calculation results are based on adiabatic or isothermal wall conditions. The present paper reports on new experimental and numerical results on flow and flame structures and surface temperature distributions obtained for a combustor with a toroidal vortex chamber. These results show a strong effect of the combustor operation mode and geometry configuration on its thermal conditions. Based on experimental validation, a reliable numerical approach for fluid dynamics, combustion, and heat transfer simulations in the bidirectional vortex chamber is suggested. This approach could assist in determining the most efficient parameters of the bidirectional combustor in terms of energy and environmental indicators as well as wall thermal conditions and service life. [ABSTRACT FROM AUTHOR]

Heat transfer and flow resistance characteristics of a novel plate heat exchanger (PHE) with multi-chevron corrugate furrows have been studied experimentally and numerically for Reynolds Numbers (Re) ranging from 1200 to 5000. Numerical results obtained from 5 different turbulent models all show similar trends and good agreement with experimental values. Results indicate that the outlet temperature of the cold fluid, the heat transfer rate and pressure drops of both the cold and hot fluids decrease with an increase of Re. The opposite trends can be found for the outlet temperature of the hot fluid, while the friction factors of both the cold and hot fluids increase with increasing Re. Data obtained using the shear-stress transport k ∼ w (SST) model show the best agreement with experimental data and is recommended for PHE simulation. Hot temperature regions distribute mainly on the left side of the PHE as a result of the PHE flow arrangement, while cold temperature regions distribute on the right side; resulting in a "zigzag" flow pattern. Additionally, it also causes swirl flow to take place; creating an enhancement to heat transfer along with an increase to the pressure drop. [ABSTRACT FROM AUTHOR]

The present study considers the effects of the electromagnetic field and pulsating flow on nanofluids heat transfer and flow characteristics in the three-start helically fluted tube. The experiments are performed using anofluids with the Reynolds number varying from 8000 to 13000, and with an Fe3O4/water nanofluids (Ferrofluid) concentration of 0.005% by volume with and without an electromagnetic field effect. The measured results are verified with the predicted results from the proposed correlations and the published experimental results. As a result of the disturbed flow boundary layer, higher swirling Brownian motion, higher local thermal conductivity, and the mixing of turbulent intensity, the heat transfer performance also increases. For a given pulsating flow of 30 Hz, the Nusselt number enhancement increases significantly to 137.7% and 156.7% for the helical depth ratios of 0.028 and 0.093, respectively. The helically fluted tube with a helical depth ratio of 0.093 and a helical pitch ratio of 0.93 yields the highest Nusselt number improvement. The Nusselt number under the electromagnetic field effect increases by 19.0% and 14.4% for the pulsating flow of 30 Hz and 20 Hz, respectively. Finally, a comprehensive evaluation index is applied to estimate thermo-hydraulic performance. It is found that the comprehensive evaluation index increases with the increasing Reynolds number at first and then decreases as the Reynolds number increases further. [ABSTRACT FROM AUTHOR]

The dry ice sublimation process of CO2 is a unique technique in which temperature ranges below the triple point of −56 °C can be achieved in a CO2 refrigeration system. However, during the evaporation process of the actual refrigeration system, the dry-ice blockage maybe happens in the evaporator, which causes a risk of system failure in the suction of the compressor. In order to overcome this problem, in this study, an ultra-low temperature CO2 cascade refrigeration system with a novel tapered evaporator/sublimator was designed and constructed. The novel evaporator/sublimator included a swirl promoter, which induces the swirling flow of solid-gas two-phase flow. Experiments were conducted for the investigation of solid-gas two-phase flow heat transfer characteristics in the evaporator/sublimator. According to the experimental results, it is verified that the CO2 refrigeration system can operate consistently and steadily without dry ice blockage in the evaporator/sublimator. In addition, the dry ice particles are uniformly distributed along the inner wall of the evaporator/sublimator by the installation of the swirl promoter, and the heat transfer coefficient is considerably improved. [ABSTRACT FROM AUTHOR]

Heat transfer enhancement is an essential parameter of heat exchangers, that can increase by increasing the value of heat transfer coefficient. So, researchers used different types of inserts, in which, single strip helical screw tape showed excellent enhancement in heat transfer, due to the formation of high-intensity swirl flow. However, to increase more intensity of swirl flow, Double strip helical screw tape inserts (DS-HST) used in present work. An experimental analysis is carried out to analyze thermal and friction factor characteristics of fluid flow in heat exchanger tube, with DS-HST, at twist ratio of 1.5, 2.5, and 3, in the range of Reynolds number from 4000 to 16000, at constant heat flux condition. Experimental results showed that Nusselt number and friction factor achieved excellent enhancement with double strip helical screw tape insert than single strip helical screw tape insert (SS-HST) with decreasing value of twist ratio. The correlation developed for Nusselt number at a range of Reynolds number, twist ratio and a number of strips. Moreover, Thermal performance factor achieved maximum value at twist ratio of 2.5 and 3 for DS-HST, than SS-HST inserts in the higher values of Reynolds number, at constant pumping power. So this range of turbulent flow showed the suitability of DS-HST inserts, which can reduce the size of heat exchangers or thermal applications, utilize energy resources, and decrease pollution in the environment. [ABSTRACT FROM AUTHOR]

The existing numerical research is concentrated on studying pressure drop, various velocity components, and heat performance behavior of heat exchanger pipe fitted with dimple on the inner pipe wall. Three-dimensional computational calculations using the CFD technique are conducted to study the influence of four geometrical parameters, including the distance between dimples, number of dimples, dimple diameter, and dimple pitch on the enhancement of thermo-hydraulic heat transfer. Also, the effects of the following parameters are optimized using the design of experiments (DOE) approaches combined with both Taguchi and Response Surface Methods. The results revealed different patterns of flow field and heat performance due to the use of dimples on the inner pipe surface. Moreover, utilized dimples can rise heat performance due to the interactions between the dimpled wall surfaces and swirling flow; hence, that can rise in area of heat transfer. Detailed flow analysis between dimples and pipe wall is demonstrated to describe the mechanisms of heat performance and pressure drop change. The orthogonal experiment outcomes revealed that the optimal design of the dimpled pipe has improved by about 35.75% and 36.1%. The numerical outcomes indicate that high value for the overall evaluation factor (PEF) is higher than 1. Based on the above findings, it can be found that hydrodynamic analysis flow, enhancement of heat transfer performance, and dimple optimization are required for different design applications. [ABSTRACT FROM AUTHOR]

Flow distortions from upstream combustors generate various and significant changes to the patterns of endwall heat transfer for an aircraft engine. In this study, to model engine-representative oncoming flows into the high-pressure turbine, turbulence and swirl generators are carefully designed at the inlet of a linear turbine cascade. Detailed heat transfer characteristics over the endwall are measured by using a steady thermochromic liquid crystal (TLC) method for various Reynolds numbers. Comparisons with a baseline case that has no upstream turbulence or swirl generators show that both turbulence and inlet swirl alter the heat transfer levels and distribution patterns, but their mechanism of action differs from one another. The turbulence alters the heat transfer by thinning the inlet boundary layer, while the inlet swirl generates the changed heat transfer due to the sweep effects of the swirl on the endwall, resulting in a much higher heat transfer enhancement than that by the turbulence. Additionally, the highest enhancement for both cases is found at the low Reynolds number of 1.0 × 105 and higher Reynolds numbers yield to a reduced enhancement. Specially, compared to the turbulence case, the enhancement by the inlet swirl drops much more rapidly, indicating that the inlet swirl effects have stronger links to the Reynolds number. [ABSTRACT FROM AUTHOR]

Supercritical water (SCW) as fluid with enough potential abilities such as high thermal efficiency can be replaced in water cooling systems. However, helical pipes are the most common type of swirl generators used for heat transfer enhancement, and their geometrical parameters contribute significantly to the compactness and efficiency of heat exchangers. In this study, for the Reynolds ranges of 8000–20000, a numerical study is performed to investigate SCW flow within circular tube, twisted oval tube and twisted multi-lobe tubes. Effects of such parameters as cross-section shapes and lobed numbers on heat transfer performance and flow physics of different spiral tubes were investigated. Results indicated SCW has a contradictory behavior with ordinary water. The results showed that twisted tri-lobed tubes had higher average Nusselt number than circular tube and twisted oval tube, which was 53% and 41%, respectively. Also in these tubes the larger radius of connecting arc and the smaller length of major axis conclude the higher amount like 4%-6% of heat flux due to the enhancement of swirling fluid flow in the pipe and the heat flux. By increasing the number of lobes, the swirling flow rate that is bounded by the lobes will increase. [ABSTRACT FROM AUTHOR]

This research focuses on the evaluation of the potential of conical strip inserts in absorber tube. Results of these experiment help in enhancing the heat transfer, friction, and thermal properties of parabolic trough collector. The novel design of conical strip insert produces swirl flows that boost the thermal properties of the parabolic trough collector. The heat transfer effect was found to be twist ratio dependent. Minimum enhancement in pressure drop due to the inclusion of inserts is observed. [ABSTRACT FROM AUTHOR]

In the context of emission-free electricity generation, the authors developed a novel hydrogen-oxygen combustor, which is based on swirl-stabilized combustion technology and steam dilution. Following previous burner characterization, the current study investigates the heat transfer conditions in the combustion chamber wall. To this end, a carefully controlled co-flow of air is used to cool the combustion chamber in an annular duct, which surrounds it. Temperature measurements enable the evaluation of the heat flux from the combustor flow to the walls, local wall temperatures, and the Nusselt numbers on the hot and the cold side. The extremely high wall temperatures, caused by the H2/O2 flame, can be reduced by steam dilution down to approximately 900 K. Even better cooling could be reached by using the dilution steam as the coolant before it flows to the plenum. The Nusselt numbers at the inner combustion chamber wall are in the order of 10–50 and increase with the thermal power and the steam dilution ratio. [ABSTRACT FROM AUTHOR]

The swirling flow is used to increase mixing and decrease nitrogen oxide emission in the combustor. Meanwhile, the swirling incoming flow has a significant effect on both aerodynamic and heat transfer characteristics of the downstream first-stage turbine. This paper conducts a numerical investigation on the influences of the inlet swirl core on the film cooling performance of a leading-edge model. Besides, the trenched holes are employed to improve the cooling performance under conditions of different swirl numbers and blowing ratios. The results show that the inlet swirl benefits the generation of downstream detrimental vortex structures and increases the mixing between coolant and mainstream. Although the trenched holes cannot absolutely change the flow patterns of swirling mainstream, they can guarantee the lateral spread of coolant and suppress the downstream coolant rotation. At different swirl numbers, the trenched cases all provide a better cooling performance, especially on the rows away from the stagnation line. Besides, the trenched cases show incremental cooling performance with the blowing ratio increasing from 1.0 to 3.0. However, for the round holes, the highest averaged cooling effectiveness is obtained at blowing ratio of 2.0, which is still lower than the trenched cases. [ABSTRACT FROM AUTHOR]

Detailed flow field and heat transfer characteristics of an array of three corotating dual swirling impinging flames arranged in a triangular configuration have been investigated experimentally as well as numerically. Effect of varying separation distance and inter-jet spacing on interactions and impingement heat transfer has been studied at fixed Reynolds numbers under stoichiometric conditions. Adjacent co-swirling flames developed strong interactions resulting in high heat transfer due to increased mixing and turbulence. Bending of inner non-swirling flames takes place toward geometric center of the array due to asymmetric interactions. Numerical simulation also predicted asymmetric interactions resulting in the formation of large distorted recirculation bubble in each dual-swirling flame. Interacting co-swirling flames resulted in the formation of circulating patterns of flow at the geometric center of array. The effect of inter-jet spacing on average heat flux is observed to be more significant than separation distance. [ABSTRACT FROM AUTHOR]

This contribution presents the mathematical model to simulate the swirling turbulent gas-droplet flow in a sudden pipe expansion. The set of axisymmetrical steady-state Reynolds averaged Navier–Stokes equations (RANS) for the two-phase flow is utilized. The dispersed phase is modeled by the Eulerian approach. The flow swirl causes an increase in the intensity of heat transfer (more than 1.5 times compared with the nonswirling mist flow at other identical inlet conditions). Evaporation of the droplets leads to a significant increase in the heat transfer intensity in the swirling two-phase flow (more than 2.5 times compared with the single-phase flow). [ABSTRACT FROM PUBLISHER]

Turbulent fluid flows with a swirl occur in numerous engineering practice situations. Five widely recognized turbulence models were evaluated using experimental results. Among the RANS-based two-equation models, the SSTκ–ωmodel proved to be the most effective. The predictions obtained from a LES turbulence model were slightly better, however there was an enormous difference in the CPU time. The CPU time needed for the LES solution was 155.3 days, whereas it was only 14.2 days for the SSTκ–ωsolution. In this light, it is believed that the SSTκ–ωmodel is the most efficient of those investigated. An important input for the numerical simulation of turbulent flow and heat transfer is the turbulence intensity at the inlet of a solution domain. For the evaluation of turbulence models, use was made of the measured turbulence quantities from the verification experiments. More often, simulations are executed based on the uniform values of the turbulence intensity across the inlet. The errors in the heat transfer results due to this practice are evaluated and are shown to be significant. [ABSTRACT FROM PUBLISHER]

In this paper, flow and heat transfer of a swirl chamber that models an internal cooling passage for a gas turbine airfoil leading edge is studied with numerical simulation. The geometry consists of a circular pipe, and rectangular section inlets that lead inlet flow to impinge tangentially on the circular pipe. The effects of the ratio of jet spacing to swirl chamber radius and Reynolds numbers on swirl cooling performance are investigated. The results indicate how the pressure loss and globally averaged Nusselt number on the swirl chamber wall increase with increases of Reynolds number and the ratio of jet spacing to swirl chamber radius. A Nusselt number correlation on these parameters is suggested. Also shown is how Nusselt numbers on the swirl chamber surface increase with the ratio of jet spacing to swirl chamber radius. [ABSTRACT FROM AUTHOR]

This paper presents large eddy simulation (LES) results of convective heat transfer and incompressible-fluid flow around a square cylinder (SC) at Reynolds numbers in the range from 103to 3.5 × 105. The LES uses the swirling-strength based sub-grid scale (SbSGS) model. Several flow properties at turbulent regime are explored, including lift and drag coefficients, time-spanwise averaged sub-grid viscosity, and Kolmogorov micro-scale. Local and mean Nusselt numbers of convective heat transfer from the SC under isothermal wall temperature are predicted and compared with empirical results. [ABSTRACT FROM AUTHOR]

A comparative study of the flow field and heat transfer characteristics between swirl and impingement of mist/air cooling on blade leading edge is carried out to find a better cooling configuration for phase transition cooling. The Eulerian–Lagrangian particle tracking technique is used to investigate mist/air cooling. Comparisons are made between these two cooling forms in such aspects as vortex structure, heat transfer enhancement, pressure loss, and thermal uniformity with and without mist injection. The influences of mist ratio and Reynolds numbers on these parameters are studied in this paper. Results show that heat transfer is enhanced while pressure loss and thermal uniformity are improved by the swirl flow created by vortex impingement. The heat transfer performance increases by about 46.2 and 51.9%, respectively, for impingement and swirl cooling with 8% mist injection, and the pressure loss coefficient increases by 19%. The difference of heat transfer coefficient between swirl and impingement cooling with and without mist injection at high Reynolds number is larger than that at low Reynolds number. In addition, the heat transfer nonuniform coefficient of swirl cooling is about 15% lower than for impingement cooling. [ABSTRACT FROM PUBLISHER]

This study experimentally investigates the effect of conical injector type swirl generator inserts on heat and exergy transfer in a uniform heat flux tube for turbulence regime. One hundred twenty watts of heat are applied by coiling a flexible silicone heater on the tube (copper pipe) with an inside diameter of 62 mm and length of 1,200 mm. The conical injector type swirl generator has a conical shape, which is similar to a funnel, with angle (α) of 30°, 45°, and 60°. Circular holes are drilled on the conical injector type swirl generator as three different cross-sectional areas (Ah). The total areas (At= N.Ah) of the holes on the conical injector type swirl generator equal each other. Because of this, the three different holes numbers (N) are produced. Flow directors having three different angles (β = 30°, 60°, and 90°) to radial direction are attached to every one of the holes. This study is a typical example for decaying flow. Reynolds number (Re) was varied from 10,000 to 35,000, so the flow was considered as only a turbulent regime. All experiments were conducted with air accordingly; Prandtl number was approximately fixed at 0.71. The local and average heat transfer Nusselt number (Nuh) and exergy transfer Nusselt number (Nue) are calculated and discussed in this article. It is found that the Nuhincreases with an increase in Reynolds number, director angle (β), and director diameter (d) but with a decrease in the conical injector type swirl generator angle (α). However, Nuedecreases depending on the same independent parameters. [ABSTRACT FROM AUTHOR]