Conjugate turbulent heat transfer in a square cavity with a solar control coating deposited to a vertical semitransparent wall

This paper presents a numerical study of the conjugate heat transfer (natural convection, surface thermal radiation and conduction) in a square cavity with turbulent flow. The cavity has one vertical isothermal wall, two horizontal adiabatic walls and one vertical semitransparent wall with a selective coating applied to the inner side to control the solar radiation transmission. Later on the semitransparent wall is replaced with another one without the selective coating. The mathematical model for the turbulent flow in the cavity was solved using the finite volume method. The system had the following conditions: the uniform temperature in the isothermal wall was 21 °C, the external ambient temperature was fixed at 35 °C and on the semitransparent wall the direct normal solar irradiation of 750 W/m² was considered constant. The Rayleigh number was varied in the range of 10⁹ ? Ra ? 10¹² by changing the lengths of the cavity from 0.70 m to 6.98 m, respectively. The results show that, even though the air temperature of the cavity with the solar control film coating semitransparent wall (case A) is higher compared with the one without solar film coating (case B), the total amount of heat going through the cavity is lower compared to the one going through the cavity without solar control film. The total amount of energy transferred to the air in cavity for the case A was 41.98% less than for the case B. A set of correlations for the Nusselt number was obtained for both cases considering the conjugate heat transfer.     

Lagrangian slug flow modeling and sensitivity on hydrodynamic slug initiation methods in a severe slugging case

Severe slugging is a dynamic two-phase flow phenomenon with regular liquid accumulation and blow-out in flow-line riser geometries. This paper discusses the applicability of a slug tracking model on a case where hydrodynamic slug initiation in a horizontal part of the pipeline upstream the riser base affects the severe slugging cycle period. The given experimental case is from the Shell laboratories in Amsterdam: air–water flow in a 100 m long pipe (65 m horizontal and 35 m −2.54° downwards) followed by a 15 m long vertical riser.A Lagrangian slug and bubble tracking model is described. A two-fluid model is applied in the bubble region and the slug region is treated as incompressible flow, with an integral momentum equation. Slug initiation from unstable stratified flow can be captured directly by solving the two-fluid model on a fine grid (a hybrid capturing and tracking scheme). Alternatively, slug initiation can be made from sub grid models, allowing for larger grid sizes. The sub grid models are based on the two established flow regime transition criteria derived from the stability of stratified flow and from the limiting solution of the unit cell slug flow model.Sensitivity studies on hydrodynamic slug initiation models on the severe slugging characteristics are presented. No hydrodynamic slug initiation (e.g. large grid size in the capturing scheme) overestimates the severe slug period compared with the experiments. Slug capturing and sub grid initiation models both give good predictions for small grid sizes (provided the detailed inlet configuration is included in the capturing case). Good predictions are also shown for larger grid sizes (factor of 50) and sub grid initiation models.The numerical tests show that correct prediction of the severe slugging cycle is sensitive to the initiation of upstream hydrodynamic slugs, but less sensitive to the local structure of the slug flow (frequencies and lengths) in the upstream region.     

Numerical analysis of unsteady cavitating turbulent flow and shedding horse-shoe vortex structure around a twisted hydrofoil

Cavitating turbulent flow around hydrofoils was simulated using the Partially-Averaged Navier–Stokes (PANS) method and a mass transfer cavitation model with the maximum density ratio (ρ_l/ρ_v,clip) effect between the liquid and the vapor. The predicted cavity length and thickness of stable cavities as well as the pressure distribution along the suction surface of a NACA66(MOD) hydrofoil compare well with experimental data when using the actual maximum density ratio (ρ_l/ρ_v,clip = 43391) at room temperature. The unsteady cavitation patterns and their evolution around a Delft twisted hydrofoil were then simulated. The numerical results indicate that the cavity volume fluctuates dramatically as the cavitating flow develops with cavity growth, destabilization, and collapse. The predicted three dimensional cavity structures due to the variation of attack angle in the span-wise direction and the shedding cycle as well as its frequency agree fairly well with experimental observations. The distinct side-lobes of the attached cavity and the shedding U-shaped horse-shoe vortex are well captured. Furthermore, it is shown that the shedding horse-shoe vortex includes a primary U-shaped vapor cloud and two secondary U-shaped vapor clouds originating from the primary shedding at the cavity center and the secondary shedding at both cavity sides. The primary shedding is related to the collision of a radially-diverging re-entrant jet and the attached cavity surface, while the secondary shedding is due to the collision of side-entrant jets and the radially-diverging re-entrant jet. The local flow fields show that the interaction between the circulating flow and the shedding vapor cloud may be the main mechanism producing the cavitating horse-shoe vortex. Two side views described by iso-surfaces of the vapor volume fraction for a 10% vapor volume, and a non-dimensional Q-criterion equal to 200 are used to illustrate the formation, roll-up and transport of the shedding horse-shoe vortex. The predicted height of the shedding horse-shoe vortex increases as the vortex moves downstream. It is shown that the shape of the horse-shoe vortex for the non-dimensional Q-criterion is more complicated than that of the 10% vapor fraction iso-surface and is more consistent with the experiments. Further, though the time-averaged lift coefficient predicted by the PANS calculation is about 12% lower than the experimental value, it is better than other predictions based on RANS solvers.     

IR measurements of the thermodynamic effects in cavitating flow

The understanding of the thermodynamic effects of cavitating flow is crucial for applications like turbopumps for liquid hydrogen LH2 and oxygen LOx in space launcher engines. Experimental studies of this phenomenon are rare as most of them were performed in the 1960s and 1970s. The present study presents time resolved IR (Infra-Red) measurements of thermodynamic effects of cavitating flow in a Venturi nozzle.Developed cavitating flow of hot water (95 °C) was observed at different operating conditions – both conventional high speed visualization and high speed IR thermography were used to evaluate the flow parameters.Both the mean features of the temperature distributions and the dynamics of the temperature field were investigated. As a result of evaporation and consequent latent heat flow in the vicinity of the throat a temperature depression of approximately 0.4 K was measured. In the region of pressure recuperation, where the cavitation structures collapse, the temperature rise of up to 1.4 K was recorded. It was found that the temperature dynamics closely follows the dynamics of cavitation structures.Finally experimental results were compared against a simple model based on the Rayleigh–Plesset equation and the thermal delay theory and plausible agreement was achieved.Experimental data is most valuable for further development of numerical models which are, due to poor ensemble of existing experimental results, still at a very rudimentary level.     

Flow boiling heat transfer of R134a,R236fa and R245fa in a horizontal 1.030 mm circular channel

This research focuses on acquiring accurate flow boiling heat transfer data and flow pattern visualization for three refrigerants, R134a, R236fa and R245fa in a 1.030 mm channel. We investigate trends in the data, and their possible mechanisms, for mass fluxes from 200 to 1600 kg/m²s, heat fluxes from 2.3 kW/m² to 250 kW/m² at T_sat = 31 °C and ΔT_sub from 2 to 9 K. The local saturated flow boiling heat transfer coefficients display a heat flux and a mass flux dependency but no residual subcooling influence. The changes in heat transfer trends correspond well with flow regime transitions. These were segregated into the isolated bubble (IB) regime, the coalescing bubble (CB) regime, and the annular (A) regime for the three fluids. The importance of nucleate boiling and forced convection in these small channels is still relatively unclear and requires further research.     

Saturated critical heat flux in a multi-microchannel heat sink fed by a split flow system

An extensive experimental campaign has been carried out for the measurement of saturated critical heat flux in a multi-microchannel copper heat sink. The heat sink was formed by 29 parallel channels that were 199 μm wide and 756 μm deep. In order to increase the critical heat flux and reduce the two-phase pressure drop, a split flow system was implemented with one central inlet at the middle of the channels and two outlets at either end. The base critical heat flux was measured using three HFC Refrigerants (R134a, R236fa and R245fa) for mass fluxes ranging from 250 to 1500 kg/m² s, inlet subcoolings from ?25 to ?5 K and saturation temperatures from 20 to 50 °C. The parametric effects of mass velocity, saturation temperature and inlet subcooling were investigated. The analysis showed that significantly higher CHF was obtainable with the split flow system (one inlet–two outlets) compared to the single inlet–single outlet system, providing also a much lower pressure drop. Notably several existing predictive methods matched the experimental data quite well and quantitatively predicted the benefit of higher CHF of the split flow.     

主动射流控制水翼空化的数值模拟与分析

为了改善高速流动工况下水翼吸力面上流场的空化特性,提出了水翼表面主动射流对绕水翼周围流动加以控制的方法.基于密度分域滤波的FBDCM混合湍流模型联合Zwart-Gerber-Belamri空化模型,分析了来流空化数为0.83,来流攻角为8°,射流位置距水翼前缘为x=0.19c时,主动射流对于水翼吸力面上流动的空化特性和水动力特性影响.对回射流的强度进行了量化分析,以探究回射流与流场空化特性的关系.数值分析结果表明,在射流水翼吸力面上的时均空泡体积为原始水翼的1/15,使得流场内空化流动由云空化状态转变为较为稳定的片空化状态,显著地削弱了云空化的发展.此外,射流极大地改善了水翼的水动力性能,使得水翼的升阻比较原始水翼提高了22.9%,空泡的脱落频率减少了26.2%,空泡脱落所引起的振幅减小了9.1%.射流大幅降低了水翼吸力面上低压区面积,水翼吸力面上流体的逆向压力减小,回射流强度降低;同时,射流使水翼吸力面上的边界层减薄,增强了流动的抗逆压梯度能力,一定程度上阻挡了回射流向水翼前缘的流动,这也从机理上分析了主动射流抑制空化的原因.      

Hydraulic characterization of high temperature hydrocarbon liquid jets

An experimental study has been conducted to investigate the hydraulic characteristics of a plain orifice nozzle issuing pressurized high-temperature liquid hydrocarbon, in order to simulate injection of aviation fuel after being used as coolant in an active cooling system in a hypersonic flight vehicle. The fuel was heated to 553 K (280°C) using an induction heater, at an upstream pressure of up to 1.0 MPa, and injected to atmospheric pressure conditions through a sharp-edged orifice of diameter 0.7 mm and length 4.3 mm. It has been observed that the isothermal lines on the plane of the mass flow rate versus the square root of the pressure drop (ΔP) were clearly affected by increased fuel temperatures, and the discharge coefficient (C_d) decreased sharply with increasing fuel injection temperature (T_inj) above the fuel boiling point of 460 K. The Reynolds number (Re) for three ΔPs with respect to T_inj reached maxima and then began to decrease as T_inj increased for each ΔP case, and the fuel temperature of maximum Re at a given pressure condition increased as ΔP increased. The effects of cavitation on the hydraulic characteristics of the high temperature fuel were explored by representing C_d with respect to three cavitation numbers and dissipation efficiency. The behaviors of C_d showed a clear dependency on cavitation number, and all of the results collapsed to a single curve, regardless of ΔP. In addition, the curve indicated that the C_d characteristics was divided into non-cavitating and cavitating regions by the critical cavitation numbers near the fuel boiling point, and a sharp decrease in C_d was found to be typical in the cavitating region. The relationship between C_d and Re showed that when T_inj exceeded the boiling point the high temperature liquid jets experienced a sharp decrease in C_d at a determined Reynolds number, due to the collapse of the mass flow rate induced by the choked cavitaiton.     

Effect of particle size distribution on pressure drop and concentration profile in pipeline flow of highly concentrated slurry

The experiments were conducted in 54.9 mm diameter horizontal pipe on two sizes of glass beads of which mean diameter and geometric standard deviation are 440 μm & 1.2 and 125 μm & 1.15, respectively, and a mixture of the two sizes in equal fraction by mass. Flow velocity was up to 5 m/s and overall concentration up to 50% by volume for each velocity. Pressure drop and concentration profiles were measured. The profiles were obtained traversing isokinetic sampling probes in the horizontal, 45° inclined and vertical planes including the pipe axis. Slurry samples of the mixture collected in the vertical plane were analyzed for concentration profiles of each particle batch constituting the mixture. It was found that the pressure drop is decreased for the mixture at high concentrations except 5 m/s and a distinct change of concentration profiles was observed for 440 μm particles indicating a sliding bed regime, while the profiles in the horizontal plane remains almost constant irrespective of flow velocity, overall concentration and slurry type.     

Une technique de suivi d'interfaces pour la modélisation de la cavitation par poche

A tracking method is presented for the modeling of partial and supercavitation. The velocity and pressure fields in the cavitating flow are computed by a Navier–Stokes solver using a pseudo-compressibility method. The cavity flow is computed from the velocity field by a tracking method based on a volume of fluid technique (VOF). The method is illustrated by several computations, two cases of partial cavitation on a hydrofoil and a case of a cavitating body emerging at a free surface.     

Resolving the stratification discrepancy of turbulent natural convection in differentially heated air-filled cavities. Part III: A full convection–conduction–surface radiation coupling

The present study concerns an air-filled differentially heated cavity of 1 m × 0.32 m × 1 m (width × depth × height) subject to a temperature difference of 15 K and is motivated by the need to understand the persistent discrepancy observed between numerical and experimental results on thermal stratification in the cavity core. An improved experiment with enhanced metrology was set up and experimental data have been obtained along with the characteristics of the surfaces and materials used. Experimental temperature distributions on the passive walls have been introduced in numerical simulations in order to provide a faithful prediction of experimental data. By means of DNS using spectral methods, heat conduction in the insulating material is first coupled with natural convection in the cavity. As heat conduction influences only the temperature distribution on the top and bottom surfaces and in the near wall regions, surface radiation is added to the coupling of natural convection with heat conduction. The temperature distribution in the cavity is strongly affected by the polycarbonate front and rear walls of the cavity, which are almost black surfaces for low temperature radiation, and also other low emissivity walls. The thermal stratification is considerably weakened by surface radiation. Good agreement between numerical simulations and experiments is observed on both time-averaged fields and turbulent statistics. Treating the full conduction–convection–radiation coupling allowed to confirm that experimental wall temperatures resulted from the coupled phenomena and this is another way to predict correctly the experimental results in the cavity.     

Fluid dynamics of horizontal air–water slug flows through a dividing T-junction

Experimental data from horizontal air–water slug flows were obtained in a test facility which was a 34 mm internal diameter, 10 m long Plexiglas pipe connected to the 90° branch arms from a T-junction. The test points were located on the flow pattern map in the proximity of the transition lines which separates different flow patterns. Capacitive probes with helical and concave plate sensors were used to quantify the dynamic liquid holdup in each branch. They were combined with Venturi nozzles + differential pressure transmitters in each outlet branch for measuring the two-phase mass flow rates. The dynamic characteristics of the slug flow splitting in a T-junction were studied from the acquired signals. Diaphragm straight-through type valves were used in the run and in the lateral branch arms to imitate equipments consuming the two-phase flow after the T-junction. This assembly can also be used as a gas–liquid separation system. The results showed different mechanisms acting on the slug flow division phenomenon. Liquid accumulation into the run branch, between the TJ and the control valve, caused more gas to come to the lateral branch.     

Experimental study of the blockage boundary for dense-phase pneumatic conveying of powders through a horizontal slit

The estimation of the blockage boundary for pneumatic conveying through a slit is of significant importance. In this paper, we investigate the characteristics for blockage of powder (48 μm average diameter) through a horizontal slit (1.6 m × 0.05 m × 0.002 m). The results show that the required critical solid mass flow rate increases as the superficial air velocity increases superficial air velocity. The solid loading ratio and superficial air velocity displayed a decreasing power law relationship. This finding agrees with existing theory and experimental results. However, a minimum inlet solid loading ratio exists. When the air velocity is greater than the corresponding air velocity of the minimum solid loading ratio, the solid loading ratio exhibits an increasing trend in power law. We also found that when the inlet conveying pressure increased, the critical solid mass flow rate required for blockage, the inlet solid loading ratio, and the minimum inlet solid loading ratio increased.     

Two-phase flow behavior inside a header connected to multiple parallel channels

The main objective of this work is to examine the flow distribution of two-phase mixture to parallel channels and to investigate the flow behavior at header-channel junctions simulating the corresponding parts of compact heat exchangers. The cross-section of the header and the channels were fixed to 14 mm × 14 mm and 12 mm × 1.6 mm, respectively. The mass flux and the mass quality ranges were 70–165 kg/m² s and 0.3–0.7, respectively. Air and water were used as the test fluids. The flow distribution at the fore part of the header (region A) is affected only by the upstream flow configuration and the rate of liquid flow separation decreased a flowing downwards. On the other hand, in the rear part, the downstream effect predominates over the upstream effect due to strong flow recirculation near the end plate. In this part, the liquid separation increased (region B) and then decreased (region C) as the mixture proceeds downwards. The validity of the existing models for branching flows at parallel T-junction was tested, and turned out to be appropriate for region A. However, the models were not applicable to the rear part due to a strong flow recirculation. Moreover, the effect of the membranes in channels was investigated, but that was minor.     

Flow around submerged structures subjected to shallow submergence over plane bed

The results of an experimental investigation on the flow field around submerged structures on horizontal plane beds, measured by an acoustic Doppler velocimeter (ADV), are presented. Experiments were conducted for various conditions of submergence, having submergence factors ranging from 1.0 to 2.0 and average flow velocity ranging from 0.25 to 0.51 m/s. The Froude number and the Reynolds number of the approaching flow for different runs are in the range of 0.18–0.42 and 50 000–76 500, respectively. The vertical distributions of time-averaged three dimensional velocity components and turbulence intensity components at different radial distances from the submerged structures are plotted. Deceleration and acceleration of the approaching flow around the submerged body are evident from the vertical distributions of the horizontal velocity component, whereas the lifting and diving nature of the flow are indicated by the vertical velocity component distributions. The vertical distributions of the horizontal velocity component indicate reduction of 30% of the non-dimensional time-averaged horizontal velocity component magnitude for the cylinder of diameter 11.5 cm in comparison to the cylinder of diameter 10 cm. Also, there is an increase of 10–25% in the horizontal velocity component at different radial sections. The flow is three dimensional in the downstream of the submerged structure. The velocity and the turbulent intensity components are also well predicted by FLUENT. The flow characteristics in the wake and the induced bed shear stress are also analyzed with FLUENT.The profiles of non-dimensional shear velocity deviate from the log law in the wake and the far downstream directions. The scour prone regions may be identified from the profiles of the induced bed shear stress around the submerged structure.     

Gas dispersion in horizontal pulp-fibre-suspension flow

Dispersion of gas into pulp-suspension horizontal flow was investigated downstream of 90° tees for ranges of fibre mass concentrations (0–3.0%), superficial liquid/pulp velocities (0.5–5.0 m/s) and superficial gas velocities (0.11–0.44 m/s) based on a gas mixing index, derived from the standard deviation of cross-sectional local gas holdup obtained from electrical resistance tomography. Mixing for dilute suspensions was similar to that for water, but differed significantly for higher suspension concentrations. Mixing worsened with increasing fibre mass concentration for the bubble flow regime, likely due to dense fibre networks in the core of the pipe causing bubbles to congregate near the wall. When buoyancy was significant, gas uniformity improved with increasing pulp concentration, since robust fibre networks caused liquid/pulp slugs to flow at the top of the pipe, whereas stratified flow was approached at lower concentrations. Mixing was less dependent on superficial liquid/pulp velocity at higher pulp concentrations, due to less variation in flow regimes.     

Evolution of three-dimensional cavitation following water entry of an inclined cylinder

The water entry of an inclined cylinder is firstly studied experimentally for low Froude number. The cylinder is 50 mm in diameter and 200 mm in length, with a moderate length to diameter ratio. As it is submerged below the water surface, the cavity is fully three-dimensional. Due to the rotation of the cylinder caused by the initial inclined impact, the cavity evolution is quite complicated and a new phenomenon is revealed. The cylinder moves along a curved trajectory in water, which greatly affects the evolution of the cavities. The cavity breaks up into two sub-cavities, and finally collapses because of hydrostatic pressure.     

Two-phase pressure drop and flow visualization of FC-72 in a silicon microchannel heat sink

The rapid development of two-phase microfluidic devices has triggered the demand for a detailed understanding of the flow characteristics inside microchannel heat sinks to advance the cooling process of micro-electronics. The present study focuses on the experimental investigation of pressure drop characteristics and flow visualization of a two-phase flow in a silicon microchannel heat sink. The microchannel heat sink consists of a rectangular silicon chip in which 45 rectangular microchannels were chemically etched with a depth of 276 μm, width of 225 μm, and a length of 16 mm. Experiments are carried out for mass fluxes ranging from 341 to 531 kg/m² s and heat fluxes from 60.4 to 130.6 kW/m² using FC-72 as the working fluid. Bubble growth and flow regimes are observed using high speed visualization. Three major flow regimes are identified: bubbly, slug, and annular. The frictional two-phase pressure drop increases with exit quality for a constant mass flux. An assessment of various pressure drop correlations reported in the literature is conducted for validation. A new general correlation is developed to predict the two-phase pressure drop in microchannel heat sinks for five different refrigerants. The experimental pressure drops for laminar-liquid laminar-vapor and laminar-liquid turbulent-vapor flow conditions are predicted by the new correlation with mean absolute errors of 10.4% and 14.5%, respectively.     

Dynamic mode decomposition of turbulent cavity flows for self-sustained oscillations

Self-sustained oscillations in a cavity arise due to the unsteady separation of boundary layers at the leading edge. The dynamic mode decomposition method was employed to analyze the self-sustained oscillations. Two cavity flow data sets, with or without self-sustained oscillations and possessing thin or thick incoming boundary layers (Re_D = 12,000 and 3000), were analyzed. The ratios between the cavity depth and the momentum thickness (D/θ) were 40 and 4.5, respectively, and the cavity aspect ratio was L/D = 2. The dynamic modes extracted from the thick boundary layer indicated that the upcoming boundary layer structures and the shear layer structures along the cavity lip line coexisted with coincident frequency space but with different wavenumber space, whereas structures with a thin boundary layer showed complete coherence among the modes to produce self-sustained oscillations. This result suggests that the hydrodynamic resonances that gave rise to the self-sustained oscillations occurred if the upcoming boundary layer structures and the shear layer structures coincided, not only in frequencies, but also in wavenumbers. The influences of the cavity dimensions and incoming momentum thickness on the self-sustained oscillations were examined.